Title of Invention	LASER WELDING METHOD FOR HOT-ROLLED STEEL SHEETS AND APPARATUS THEREFOR"
Abstract	A laser welding method for hot-rolled steel sheets in a continuous production process for coils, and an apparatus therefor. Two hot-rolled steel sheets subject to low-temperature transformation are butted against each other. Butted portions of the hot-rolled steel sheets are laser-welded to form a weld. The weld of the hot-rolled steel sheets is compressed with a compressor. The invention allows a weld to be Less hardened in its microstructure and ensures a stable quality for the weld.

Title of Invention

LASER WELDING METHOD FOR HOT-ROLLED STEEL SHEETS AND APPARATUS THEREFOR"

Abstract

A laser welding method for hot-rolled steel sheets in a continuous production process for coils, and an apparatus therefor. Two hot-rolled steel sheets subject to low-temperature transformation are butted against each other. Butted portions of the hot-rolled steel sheets are laser-welded to form a weld. The weld of the hot-rolled steel sheets is compressed with a compressor. The invention allows a weld to be Less hardened in its microstructure and ensures a stable quality for the weld.

Full Text	LASER WELDING METHOD FOR HOT-ROLLED STEEL SHEETS ..AND APPARATUS THEREFOR CLAIM OF PRIORITY [0001] This application claims the benefit of Korean Patent Application No. 2005-130248 filed on December 27, 2005 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. BACKGROUND OF THE INVENTION Field of the Invention [0002] The present invention relates to a laser welding method for hot-rolled sheets in a continuous production process for coils, in which two hot-rolled steel sheets are welded together. More particularly, the present invention relates to a laser welding method in which hot-rolled steel sheets are welded together and a weld is less hardened in its microstructure, thereby guaranteeing stable welding quality. Description of the Related Art [0003] The present invention relates to a las-er welding method for hot-rolled steel sheets in a continuous production process for coils, in which two hot-rolled steel sheets are welded together. More particularly, the present invention relates to a laser welding method for hot rolled steel sheets in a continuous production process for coils which ensures less hardening in a weld of steel subject to low-temperature transformation in a laser welding process involving abrupt heating and cooling, thereby guaranteeing stable welding quality. Description of the Related Art [0004] A technological field of producing sheet metal has faced a strong demand for a continuous production process which leads to higher productivity and quality, and bigger-sized products. [0005] Such a continuous production process has found its application broadened to high quality steel such as electrical steel or ferritic stainless steel. [0006] The continuous production process for coils is represented by pickling and tandem cold rolling mill (PCM) processes that are conducted in synchronization. A cold coil obtained from a hot coil may be manufactured by conducting pickling and tandem cold rolling mill (TCM), respectively. However, alternatively, the cold rolled coil can be produced by PCM. The PCM process enhances productivity significantly over the respective implementation of the pickling and TCM processes, thus growing in its application recently. [0007] A great importance in the TCM field lies in a technique of: bonding a rear end of a preceding rolled sheet and a following rolled sheet together . The bonding technology for the TCM includes solid state bonding and welding. [0008] In the case of welding, the rear end of the preceding rolled sheet is welded with the following rolled sheet at an entrance of a TCM Line to form a weld and then passes through a following cold rolling mi 11 line. Here, the weld, if in poor quality, is fractured while passing through the following cold rolling mill line, potentially halting an overall process. Therefore, for the TCM line, it is pivotal to attain a high quality weld in hot and cold coils. Especially, the PCM line is longer in the production line and greater in the number of loopers compared with the existing pickling and TCM lines, thereby requiring more rigorous quality standard for welding than the existing lines. [0009] Examples of welding for the TCM line includes flash butt: weJding in which short circuit and flashing are repeated, and a laser welding for utilizing a high-density heat source. [0010] The flash butt welding is big in heat input ,-thus facing an obstac 1 e in selecting a welding material. For example, electrical steel, ferritlc stainless steel, and high carbon steel are not sufficient in bonding strength, also occasionally ruptured during cold rolling. Especially, the high carbon steel is high in C content and thus considered il 1-suited for the flash butt welding. Also, when the welding is repeated under given schedules and conditions, welds each demonstrate non-uniform quality arid thus pose a problem to reproducibility. [0011] Laser welding has higher energy density requiring smaller heat input than the existing flash butt welding, thus achieving superb welding quality. [0012] However the laser welding, when conducted on the high carbon steel for the TCM process, causes pores and pin holes in weld metal arid cracks in the weld metal and a heat-affected zone (HAZ). [0013] The pores and pin holes are closely related to the C content of the welding material. It is known that carbon in molten metal reacts with oxygen in the air during welding, creating CO gas. Thus the CO gas trapped inside remains during solidification of the molten metal, thus resulting in pores. [0014] Therefore, it is of importance to lower C content of the molten metal and notably a suitable welding material may be adopted to diminish occurrence of pores. [0015] Rupture of the welds is associated with hardening of microstructures thereof. The high carbon steel suffers rupture in the: weld chiefly owing to martensite microstructures created in the abrupt-heating and cooling processes during welding. Here, the weld meta] and heat-affected zone are concurrently hardened in their microstructures, which thus necessitates complicated and various ways to remedy the prob Lem. [0016] The following is conventional technologies for conducting TCM for steel which is hardened it its microstructure. [0017] First, Japanese Laid-Open Patent Application No. H 5 50276 discloses heat treatment of a weld, in which a fixed heat source is employed to keep the weld at a specific heat treatment temperature during a certain duration according to C content of a hot-rolled steel sheet. However, this increases an overall welding time depending on the heat treatment duration. [0018] Another conventional technology is taught in Japanese Laid Open Patent Application No. H 5-132719. This technology pertains to laser-welding a weld and then heat treating the same at a temperature of at least 400°C and up to Acl point within a minute. However, welding should be performed during quite a long time to fully eliminate a hardened microstructure at a temperature of at least 400°C and up to Acl point. Also, in case of abrupt cooling which is required in the laser welding, the weld should be abruptly heated within few minutes after welding Cor heat treatment, thereby considerably complicating the heat treatment process. [0019] Japanese Laid-Open Patent Application No. H 8-57502 discloses further another conventional welding technology, in which low carbon steel with superior weldability is inserted between joints of high carbon steel. This more than doubles the number of welding processes over other welding methods and requires preparation of a leader strip every time, thereby not suitable for mass-production. [0020] Japanese Laid-Open Patent Application No. H 8^215872 teaches further another technology, in which a laser weld is cooled while passing through a mixed area of ferrite and pearlite to be heat treated. In this laser welding, heating and cooling occur more abruptly than in Arc weldi ng , and thus the weld is hardly transformed into the mixed area of ferrite and pearlite. Especially, the high carbon steal, if welded, suffers hardening severely. [0021] Further another conventional technology is taught in Japanese Laid-open Patent Application No. 2000-317642, in which bonding portions are laser welded and then a hot-rolled steel sheet is heat treated. However, here, due to laser welding, a weld is abruptly cooled and transformed into a martensite microstructure before heat treatment, thereby leading to microcracks. Therefore, this technology is hardly applicable to a production line such as PCM that needs high quality welding. [0022] Japanese Laid-open Patent Application No. 2001-353587 proposes further another conventional technology, in which a filler wire is utilised in a joint between heterogeneous materials of high carbon steel and low carbon steel. In this method, heat treatment is not employed, arid a laser beam is irradiated onto the low carbon to prevent cracks in the weld. Yet this technology fails to eliminate a hardened microstructure produced in a heat-affected zone of the high carbon which is not melted. [0023] Further another conventional technology is disclosed in Japanese Laid-open Patent Application No. 2000-317642. This technology concerns flash butt welding for heat treating a weld. Similar technologies for heat treating welds, however by different methods, are suggested in Japanese Patent Application Publication Nos. H 5-132719 arid 2000-317642 and 2004-76159. However, these methods do not ensure stable quality for the weld of high carbon steel which contains at least 0.5% C. [0024] The aforesaid technologies for bonding steel sheets for TCM, although significant in their number, are merely applicable to high carbon steel with relatively low C content or a production line riot requiring high welding quality. [0025] Consequently, there has been a demand for a technology for securing good quality for a weld joint to carry out TCM on high carbon steel containing at least 0.5% C and steel having a strength of at least. 4bOMPa, e.g. , high strength steel for cars, which has a hardening microstructure such as martensite or bainite after being laser "welded and then cooled down. SUMMARY OF THE INVENTION [0026] The present invention has been made to solve the foregoing problems of the prior art and therefore an aspect of the present invention is to provide a laser welding method in which a laser weld is less hardened in its microstructure to assure stable welding quality, thereby significantly improving productivity for a continuous production process . [0027] Another aspect of the invention is to provide a laser welding apparatus for a continuous production by which a laser weld is less hardened in its microstructure to assure stable welding quality, thereby significantly improving productivity for the continuous production process. [0028] According to an aspect of the invention, the invention provides a laser welding method for hot-rolled steel sheets in a continuous , production process for coils, including: butting two hot-rolled steel sheets subject to low-temperature transformation against each other; laser-welding butted portions of the hot-rolled steel sheets to form a weld; and compressing the weld of the hot-rolled steel sheets with a compressor. [0029] Preferably, the weld of the hot-rolled steel sheets is compressed at a temperature of ACT to Ac3. Preferably, the weld is compressed at a compression force of 75MPa or less. Preferably, the compressed weld is reduced in thickness at a ratio of 5.8% or less. Preferably, the weld is compressed vertically with respect to the hot-rolled steel sheets via a planishing roll. •A [0033] The hot-rolled steel sheets subject to low-temperature transformation comprise one selected from a group consisting of a high carbon steel containing at least 0.5wt% C, a dual phase (DP) steel, a transformation induced plasticity (TRIP) steel and a composite phase (CP) steel. The high carbon steel consists of at least 0.5wt% C, 0.1 to 0.5wt% Si, 0.3 to 0.6wt% Mn, up to 0.05wt% P, up to 0 . 05wt% S, up to 0.5wt% Cu, up to 3wt% Ni, 0.05 to 0.5wt% Cr, at least 0.05wt% Al, the balance being Fe and unavoidable impurities. [0031] Preferably, a material for the welding comprises a carbon steel containing up to lwt% C and 0 to 1.22 wt% Cr or an Ni alloy containing up to O.lwt% C and 0 to 1.22wt% Cr. The welding material comprises one selected from a group consisting of a wire, a powder and a film. [0032] Preferably, before the laser-welding, the butted portions of the hot-rolled steel sheets are pre-heated to a temperature of 600°C to 800°C. Preferably, after the laser-welding, the weld of the butted portions is post-heated to a temperature of 800°C to 1100°C. [0034] The continuous production process for the coils comprises one selected from a group consisting of pickling and tandem cold rolling mill line, pickling and oiling line, annealing and pickling line, pickling line and tandem cold rolling mill line. [0035] According to another aspect of the invention, the invention provides a laser-welded sheet comprising hot-rolled steel sheets subject to low temperature transformation by laser welding and having a hardness difference of 90Hv or less between a weld and portions adjacent to the weld. [0036] According to further another aspect of the invention, the invention provides a laser welding apparatus for a continuous production process, adapted to laser-weld hot-rolled steel sheets subject to low-temperature transformation. The laser welding apparatus includes a laser welding device for welding the hot-rolled steel sheets; apre-heater for pre-heating a weld of the hot-rolled steel sheets at a front end of the welding device; a post-heater for post-heating the weld of the hot-rolled steel sheets at a rear end of the welding device,- and a compressor for compressing the weld of the hot-rolled steel sheets at a rear end of the post-heater. BRIEF DESCRIPTION OF THE DRAWINGS [0037] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: [0038] FIG. 1 is a graph illustrating hardness distribution of a laser weld according to the invention; [0039] FIG . 2 is a conceptual view illustrating an exemplary 1 aser we 1 d Lng apparatus according to the invention; [0040] FIG. 3 is a graph illustrating a thermal cycle of a laser weJd according to the invention; [0041] FIG. 4 is a graph illustrating change in hardness in accordance with change in compression force which is applied after laser welding according to the invention; and [0042] FIG. 5 is a picture illustrating a laser weld after a PCM process for SK85 steel according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT [0043] Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. [0044] In this specification, "hot-rolled steel sheets subject to low-temperature transformation" denote hot-rolled steel sheets whose welds are transformed in their microstructures at a low temperature when the hot-rolled steel sheets are bonded together to form the welds by laser welding and cooled down. The low-temperature transformation microstructure denotes a microstructure containing martensite andbainite . Also, the hot-rolled steel sheets subject to low temperature transformation after laser welding include high carbon steel or high strength steel. [0045] The high carbon steel designates steel having at least O.bwt% C. By way of a representative example, the high carbon steel consists of at least 0.5wt% C, 0.1 to 0.5wt% Si, 0.3 to 0.6wt% Mn, up to 0.05wf% P, up to 0.05wt% S, up to 0.5wt% Cu, up to 3wt% Ni, 0.05 to 0.5wt% Cr, at least 0.05wt% Al, the balance being Fe and unavoidable impurities, in the present invention, % represents wt% unless specified otherwise . Steei containing at least 0.5% C is construed to fall within the high carbon steel according to the invention. Here, an alloy element such as Mo, V, Ti, W, B, Nb and Sb may be added to impart a specific function to the high carbon steel. [0046] The high strength steel is designed to have a tensile strength of 450MPa chiefly for use in cars, and exemplified by steel subject to low-temperature transformation. Examples of the steel subject to low-temperature transformation include a dual phase (DP) steel, a transformation induced plasticity (TRIP) steel, and a composite phase (CP) steel. The high strength steel is typically referred to as steel subject to low-temperature transformation. The DP steel has a dual phase with two characteristics of ductile ferrite and strong martens.i te. The DP steel ensures superior processabilty and high strength with a small alloy element. Meanwhile, the TRIP steel is composed of ductile fe.rri.te, strong bainite and austenite meta-stable at a high temperature. In the TRIP steel, the meta-stable austenite phase is transformed into very strong martensite phase. The CP steel has precipitation in the martens.i tc or bainite microstructure. The steel subject to low--temperature transformation as just described is phase'-transformed in its microstructure after laser welding. [0047] As described above, in the high carbon steel or steel subject to low-temperature transformation, a weld is phase-transformed in its microstructure at a low-temperature into e.g. , martensite or bariite with strong brittleness, when the steel is laser welded and cooled down. In this fashion, the low-temperature transformation microstructure causes crack or rupture in the weld. [0048] Also, in this specification, "a weld" refers to a bonding portion formed by laser welding a rear end of a preceding hot coil and a following hot coil in a continuous production process for coils. Here, the weld includes molten metal which is melted by laser and solidified, and a heat affected zone (HAZ) influenced by a heat source of the laser. [0049] Moreover, in this specif ication, "a continuous production process for coils" denotes continuously producing coils in a hot or cold rolling line. Here, the continuous production process includes a polishing process in which the hot-rolled steel sheets are bonded together to be polished, or all continuous production processes such as a molten zinc plating process or an annealing process. [0050] First, according to an aspect of the invention, the reason for hardening in a weld will be examined based on a microstructure test conducted on a laser weld with low temperature transformation microstructure, and an Erichsen test. [0051] The inventors of this invention confirmed that the weld meta 1 and heat-affected zone, respectively, suffered hardening for which martens.i te, bainite and carbide were mainly responsible. [0052] Such a hardening microstructure can be removed to a certain level by employing a compression technique in the weld and controlling chemical composition and heat treatment of the weld to a certain level. [0053] This invention suggests methods for compressing the weld in Later process, reducing C content of a weld metal by using a welding mater laJ with lower C content than the hot-rolled steel sheets, and heat treating the weld to reduce hardness. [0054] The quality of the weld which is hardened in its microstructure is affected not only by hardness of the weld as just described but also an overall hardness distribution thereof. [0055] That is, for example, the weld of high carbon steel may be lowered in hardness to a certain extent by heat treatment. However, as shown in FIG. 1, a notch in hardness occurs between the weld and the hot rolled steel sheets, thereby hardly achieving a weld quality that can satisfy the standard. [0056] Accordingly, according to the invention, the weld is heat, treated and compressed as well to be less hardened. Also, this alleviates overaJ 1 hardness distribution of the weld as indicated with arrows in FIG. 1, thereby assuring a stable weld quality for steel subject to low -temperature transformation. [0057] Also, in a continuous production process such as PCM, welding and heat treatment, if carried out independently, prolong a welding time, thereby decreasing an overall production rate. Moreover, an increase in C content in the hot-rolled steel sheets leads to an increase in heat treatment time. [0058] As a result, according to the invention, to solve these problems, as shown in FIG. 1, a heat treatment device and a compressor are configured integral with a welding device so that a weld of hot-rolled steel sheets can be welded, and heat treated and compressed. [0059] In an explanation with reference to FIG. 2, the welding apparatus of the invention is largely constructed of the welding device 10, the heat treatment device 20 and the compressor 30. These three devices are formed integral. FIG. 2 is a schematic view illustrating the welding apparatus according to the invention, in which thus these devices are not shown configured integrally. But a technique for configuring these devices integrally can be worked by a known art and thus will be explained in no more detail. [0060] The welding device 10 of the invention includes a laser generator 12 for generating laser and a filler supplier 16 for supplying a welding material. The heat treatment device 20 adopts as a heat source all moans for heating moving hot-rolled steel sheets speedily. A preferable heat source is a high frequency induction coil. The heat treatment device 20 is constructed of a pre-heater 22 installed upstream of the welding device 10 to heat hot-rolled steel sheets 40 prior to welding and a post-heater 24 for heating hot-rolled steel sheets 50 after welding. Further, the compressor 30 compresses the hot-rolled steel sheets just after welding via a planishing roll. [0061] In a case where the moving hot-rolled steel sheets are welded together via the welding apparatus of the invention, first a front end of the following hot coil 40 and a rear end of a preceding hot coii bO are butted against each other and welded by the welding device 10 after being heated by the pre-heater 22. A weld 60 with the hot-rolled steel sheets bonded together is post-heated by the post heater 24. Then, the weld passing through the post-heater 24 is cooled down and passes through the compressor 30 installed just downstream of the post-heater 24 , thereby undergoing Post Welding Hot Deformation (PWHD) or compression. [0062] In this invention, heat treatment and compression of the weld are preferably conducted together but may be carried out independently or selectively. [0063] The moving hot-rolled steel sheets, when welded via the welding apparatus of the invention, also move the welding apparatus. Here, the hot-rolled steel sheets may move in the same or opposite direction with respect to the welding apparatus. In a case where the hot-rolled steel sheets move in the same direction as the welding apparatus, preferably, the hot-rolled steel sheets move at the same or faster rate with respect to the welding apparatus. [0064] FIG. 3 is a schematic view illustrating a thermal cycle history for welds of hot-rolled steel sheets and a subsequent microstructure status thereof when welding is performed by a movable welding apparatus according to an embodiment of the invention. [0065] The invention suggests a method for controlling hardening of the welds to conduct a continuous production process for the hot-rolled steel sheets with low-temperature transformation microstructures . The method involves controlling chemical composition of the welds, heat treating and compressing the welds. [0066] These methods will be explained hereunder. [0067] First, a method for controlling chemical composition of the welds will be explained. [0068] This method pertains to controlling a welding material which constitutes most of molten metal. Preferably, the welding material is supplied from a filler suppler 16 to a weld 60 and basically contains up to 0.1% C and 0 to 1.22% Cr. Such a welding material adopts carbon steel and an Ni alloy with high tensile strength. The carbon steel is preferable since it assures more stable welding quality. [0069] A welding material composed of stainless steel and an Ni alloy is degraded in wettability with high carbon steel, i.e., the base material, and has base material elements not completely diluted therein in a case where an optimum welding parameter is not derived. This may render the weld brittle occasionally. However, the invention is not limitative of the Ni alloy. [0070] Preferably, according to the invention, the weldmetal is regulated to contain up to 0.4% C. This is because in the laser welding, a tiny amount of the welding material is melted and filled in the weld so that the base material dilutes at a greater rate compared with typical Arc welding. [0071] For example, in a case where the hot-rolled steel sheets adopt high carbon steel containing at least 0.85% C, the welding material should contain up to about 0 . lwt% C in order to keep C content of the weld metal at up to 0.4% with a dilution rate set to maximum 30%. [0072] Further, Cr reacts with carbon in the hot-rolled steel sheets to form chromium carbides in the vicinity of the weld metal and heat affected zone. Thus preferably, the welding material should contain up to 1 .22 % Cr. [0073] As a result, according to the invention, preferably, the welding material supplied from the welding device is carbon steel or an Ni alloy containing up to 0 . 1 % C and up to 0 to 1.22 % Cr. Main composition of the carbon steel and Ni alloy is Fe and Ni, respectively. The welding material applicable according to the invention can be typical carbon steel or Ni alloy that satisfies the aforesaid C and Cr conditions. The welding material is preferably shaped as a wire, but a powder or a f.i Irn is also applicable. [0074] Then an explanation will be given about a method for heat treating the weld. [0075] In this invention, heat treatment of the weld breaks down into pre-heating and post-heating. The pre-heating is carried out prior to welding to prevent cracks in a weld joint and the post-heating is performed after welding to relieve hardening of the weld joint. [0076] In a case where the hot-rolled steel sheets subject to low-temperature transformation is laser welded and the weld undergoes only post-heating, the weld after welding is abruptly cooled down before being post-heated, thus potentially causing fracture in the weld. [0077] Consequently, it is preferable to pre-heat the welding material subject to low temperature transformation in order to alleviate abrupt cooling resulting from laser welding. [0078] In a case where the movable heat treatment device is employed as in an embodiment of the invention, the welding material does not achieve sufficient pre-heating effect just above a martensite transformation temperature (Ms) , and thus preferably should be pre-heated to a. temperature higher than that. [0079] Therefore, according to an embodiment of the invention, preferably, the weld of the high carbon steel is pre-heated to a temperature of 600°C to 800°C. A preheating temperature of up to 600°C does not ensure a sufficient time for pre-heating the moving hot-rolled steel sheets, thereby leading to unsatisfactory quality for the weld. On the other hand, a preheating temperature of at least 800°C deforms the weld due to excessive heat input, thus rendering the weld far from solid. [0080] Moreover, according to the invention, post-heating of the weld is carried out by two concepts. [0081] The first concept pertains to tempering in which the wold is post-heated at Acl or less during a relatively long time to change its martensite microstructure into a tempered martensite, thereby attaining ductility. [0082] The second concept is to actively control a cooling thermal cycle in laser welding to transform the weld into ferrite and pearl ite microstructures. [0083] The tempering method requires relatively long heat treatment and subsequently ensures sufficient ductility. Yet, in a coil production lino with a fast yield rate, this type of tedious post-heating may degrade productivity. Therefore, in the laser welding system using a movable heat source, the second concept of relieving a cooling cycle after laser welding is more preferable. [0084] The weld is post-heated preferably to a temperature of 800°C to 1100°C, and more preferably 950°C to 1100°C. Also, according to an embodiment of the invention, preferably, the weld is post-heated without. holding time and naturally cooled down. [0085] A post-heating temperature of up to 800°C creates a marterisite microstructure in the weld after cooling due to lack of heat input, thus ineffective in reducing hardness. On the other hand, a post heating temperature of at least 1100°C coarsens the microstructure of the weJ.d due to excessive heat input or partially regenerates the martensite that is a hardened microstructure, thereby deteriorating physical properties of the weld. [0086] Finally, a method for compressing the weld will be explained. [0087] A high-temperature area of the weld, if compressed as shown in the invention, remarkably reduces hardness over heat treatment. [0088] Compression is conducted for the weld that is being cooled down at a temperature of Acl to to Ac3 . The compression conducted at such a temperature reduces size of austenite and easily transforms the weld into ferrite and pearlite microstructures. [0089] Also, the compression performed at Acl or less work-hardens the heat-affected zone to increase hardness to a certain level, thereby alleviating overall hardness distribution of a joint. [0090] Preferably, the hot-rolled steel sheets are compressed at a compression force of 75MPa or less and at a temperature of Acl to Ac3 during cooling. A compression force of at least 75MPa deforms the weld and causes minor fractures in a low-temperature area thereof. [0091] Moreover, in a case where compression is carried out for steel whose weld is transformed into martensite or bainite microstructures during cooling after welding, the weld is reduced in thickness at a ratio of 5.8% or less. [0092] As just described, in an optical method for control Ling hardening of the weld of high carbon steel according to the invention, a Vickers difference (Hv) of 90Hv or less exists between the weld and the base materia L, i.e., the hot-rolled sheets. The hot-rolled steel sheets, if with the hardness difference of 90Hvas just described, can be continuously produced without suffering rupture in the continuous production process requiring high-strength. This also does not effect deformation in the weld. This invention is preferably applicable to the hot-rolled steel sheet having a thickness of 0.5 to 6mm. [0093] The laser welding of the invention is applicable to all methods for continuously producing coils which include,-for example, pickling and tandem cold rolling mill (PCM) line, pickling and oiling line (POL), annealing and pickling line (APL), pickling line (PL) and tandem cold rolling mill (TCM) line. [0094] Preferred embodiments of the invention will be explained hereunder by way of Examples. [0095] The Examples of the invention will be explained in relation to high carbon steel, which is one of steels hardened in the microstructure. However it is easily understood that the scope of the invention is not limited to the high carbon steel. Examples [0096] The Examples adopted hot-rolled steel sheets of high carbon steel having a composition noted in Table 1. The hot-rolled steel sheets were 2 . 0 mm thick and welded to each other via a CO2 laser welding device having a maximum output of 12KW. The Examples utilized as a welding material a wire filler (0 . 9mm) having a chemical composition of low carbon as noted in Table 1. Table 1 (Table Remove) [0097] The hot-rolled steel sheets were laser welded via the laser welding device under the condition that the weld was free from welding defects such as a pore underfill. Here, the laser had an output of 8.4kW and a welding rate of 4.5m/min, and a joint had a spacing of 0.15mm. [0098] To heat treat the weld, a high frequency induction furnace having a heat source of 20wx2001mm moved along a weld line by changing an output. [0099] The weld was heat treated at a heating rate of about lOOoC/s, pre-heated to 723°C and post-heated to 1005°C, respectively, and then naturally cooled down (air-cooled). [00100] In heat treating the weld, an R-type pyrometer was spot welded on a melting boundary to measure the temperature history of the weld heated by the high wave induction furnace. A highest attainable temperature was obtained from the temperature history curve to determine as a heat treatment temperature. [00101] Compression was carried out by a moving roll having a width of 20mm installed downstream of the post-heating treatment device . The weld was compressed after welding and heat treatment. [00102] The weld obtained according to above-mentioned conditions passed the standard for the PCM line that was Erichsen height of 4mm or more. Thus, quality was evaluated with an Erichsen tester. To evaluate the quality of the weld, measurement was taken on a plastically deformed he:i ght of the weld until the occurrence of cracks. [00103] First, Table 2 shows quality evaluation results of SK85 steel containing 0.85% C. Table 2 (Table Remove) Notes) HC Heat treatment condition PC Compression force (MPa) HH* Highest hardness of weld (Hv) HD** Hardness difference (Hv) (Weld-base material) EH*** Erichsen height (mm) [00104] As noted in Table 2, in a case where the weld of SK85 steel was not heat treated, the weld suffered cracks just after welding, thereby failing to achieve a weld joint that could satisfy the standard. [00105] Moreover, in a case where only one of pre-heating and post heating was carried out, the weld could not attain a quality enough to be passable;. [00106] In contrast, in a case where pre-heating and post-heating were both performed as in this invention, the weld was found to be improved in its quality compared with when only one of them was performed. [00107] The weld processed as just described was compressed by varying a compression force. [00108] FIG. 4 demonstrates hardness distribution along a longitudinal direction of the weld when the weld of SK85 steel was compressed. [00109] Increase in the compression force led to increase in KrLchsen height and quality. [00110] As shown in FIG. 4, with compression, the highest; hardness and hardness difference of the weld were reduced, thus resulting in more moderate hardness distribution in the overall weld. [00111] The compression force in a range of 75MPa or less linearly increased Erichsen height. However, the compression force in a range of more than 75MPa severely deformed a weld joint and excessively hardened a low-temperature area, causing microcracks. [00112] Also, as shown in FIG. 4, a compression force of OMPa noticeably increased hardness in molten metal and relatively diminished hardness in a heat-affected zone. But it is found that with increase of the compression force from 30MPa to 75MPa, the weld was less steep in hardness difference, thus exhibiting more moderate hardness distribution along a longitudinal direction thereof. [00113] Table 3 notes quality evaluation results for S50C steel containing 0.5% C. Table 3 (Table Remove) Notes) HT* Heat treated PC Compression force (MPa) TRR* thickness reduction ratio of weld HH** Highest hardness of weld (Hv) HD* Hardness difference (Hv) (Weld-base EH**** Erichsen height (mm) [00114] As seen in Table 3, the laser weld of S50C steel overall showed superior quality to SK85 steel regardless of the welding materials and heat treatment methods. [00115] Moreover as noted in Table 3, steel whose weld is transformed into martensite or bainite microstrucutres, was found to be superior in welding quality when it was compressed at a temperature of Acl to Ac3 while cooled down after welding and reduced in its thickness to 5.8% or less. [00116] This is apparently attributed to less hardening resulting from lower C content in the steel. [00117] This steel, when welded and then compressed, is increased .in the Erichsen height and reduced in the highest hardness and hardness difference like SK85, thereby remarkably improving welding quality. [00118] Preferred embodiments of the invention were explained hereinabovc . However this invention is not limited to welding conditions of the continous production process for high carbon steel as manifested in the Examples. The invention is applicable to various welding conditions necessary for the continuous production process which encompasses the scope of the invention. [00119] As set forth above, according to exemplary embodiments of the invention, a laser welding for a continuous production process provides welding conditions that have not been adopted, thereby continuously producing steel sheets with a low temperature transformation microstructure. [00120] Also, in order to ensure a secure laser weld joint free from welding imperfections, hot-rolled steel sheets are -welded together to continuously produce steel containing at least 0.5% C as in the Examples. Furthermore, as shown in FIG. 5, the invention enables continuous production without causing fracture in a laser weld. [00121] In addition, according to the invention-, the welding time is shortened to 25 seconds, which are required for general steel, irrespective of C content of steel. This noticeably enhances product! vi ty in the continuous production process. [00122] Further, steel subject to low temperature transformation under the welding conditions according to the invention can withstand strong compressive load applied in the continuous production process and tensile load imposed between stands, thereby carrying out continuous production with the weld free from rupture. [00123] While the present invention has been shown and described in connection with the preferred embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without, departing from the spirit and scope of the invention as defined by the appended claims. What Is Claimed Is; 1. A laser welding method for hot-rolled steel sheets in a continuous production process for coils, comprising: butting two hot-rolled steel sheets subject to low-temperature transformation against each other; laser-welding butted portions of the hot-rolled steel sheets to form a weld; and compressing the weld of the hot-rolled steel sheets with a ; compressor. 2. The laser welding method according to claim 1, wherein the weld ; of the hot-rolled steel sheets is compressed at a temperature of Ac, to 'Ac3. 3. The laser welding method according to claim 1, wherein the weld is compressed at a compression force of 75MPa or less. 4 . The laser welding method according to one of claims 1 to 3 , wherein the compressed weld is reduced in thickness at a ratio of 5.8% or less. 5. The laser welding method according to claim 1, wherein the weld is compressed vertically with respect to the hot-rolled steel sheets via a planishing roll. 6. The laser welding method according to claim I, wherein the hot-rolled steel sheets subject to low-temperature transformation comprise one selected from a group consisting of a high carbon steel containing at least 0.5wt% C, a dual phase (DP) steel, a transformation induced plasticity (TRIP) steel and a composite phase (CP) steel. 7. The laser welding method according to claim 6, wherein the high carbon steel consists of at least 0 . 5wt% C, 0 .1 to 0 .5wt% Si, 0.3 to 0 .6wt% Mn, up to 0.05wt% P, up to 0.05wt% S, up to 0.5wt% Cu, up to 3wt% Ni, 0 .05 to 0 . 5wt% Cr, at least 0 . 05wt% Al, the balance being Fe and unavoidable impurities. 8. The laser welding method according to claim 1 or 7, wherein a material for the welding comprises a carbon steel containing up to lwt% C and 0 to 1.22 wt% Cr or an Ni alloy containing up to O.lwt% C and 0 to 1.22wt% Cr. 9 . The laser welding method according to claim 8 , wherein the welding material comprises one selected from a group consisting of a wire, a powder :and a film. 10 . The laser welding method according to claim 1 or 7 , wherein before the laser-welding, the butted portions of the hot-rolled steel sheets are pre-heated to a temperature of 600°C to 800°C. 11. The laser welding method according to claim 1, wherein after the laser-welding, the weld of the butted portiojis is post-heated to a temperature of 800°C to 1100°C. 12. The laser welding method according to claim 10, wherein after the laser-welding, the weld of the butted portions is post-heated to a temperature of 800°C to 1100°C. 13 . The laser welding method according to claim 12 , wherein the weld is compressed at a compression force of 75MPa or less and at a temperature of Aci to Ac3. 14. The laser welding method according to claim 1, wherein the continuous production process for the coils comprises one selected from a group consisting of pickling and tandem cold rolling mill line, pickling and oiling line, annealing and pickling line, pickling line and tandem cold rolling mill line. 15. A laser-welded sheet comprising hot-rolled steel sheets subject. to low temperature transformation by laser welding and having a hardness difference of 90Hv or less between a weld and portions adjacent to the weld. 16. The laser-welded sheet according to claim 15, wherein the hot-rolled steel sheets subject to low-temperature transformation comprise one selected from a group consisting of a high carbon steel containing at least 0.5wt%, a dual phase (DP) steel, a transformation induced plasticity (TRIP) steel and a composite phase (CP) steel. 17. The laser-welded sheet according to claim 16, wherein the high carbon steel consists of at least 0 . 5wt% C, 0 .1 to 0 . 5wt% Si , 0 . 3 to 0 . 6wt% Mn, up to 0.05wt% P, up to 0.05wt% S, up to 0.5wt% Cu, up to 3wt% Ni, 0 . 05 to 0 . 5wt% Cr, at least 0 . 05wt% Al, the balance being Fe and unavoldabi e impurities. 18. A laser welding apparatus for a continuous production process, adapted to laser-weld hot-rolled steel sheets subject to low-temperature transformation, comprising: a laser welding device for welding the hot-rolled steel sheets,- a pre-heater for pre-heating a weld of the hot-rolled steel, sheets at a front end of the welding device; a post-heater for post-heating the weld of the hot-rolled steel sheets at a rear end of the welding device; and a compressor for compressing the weld of the hot-rolled steel sheets at a rear end of the post-heater. 19. A laser welding method for hot-rolled steel sheets in a continuous production process for coils substantially as herein described with reference to the foregoing description, examples, tables arid the accompanying drawings. 20. A laser-welded sheet substantially as herein described with reference to the foregoing description, examples, tables and the accompanying drawings. 21. A laser-welded apparatus for a continuous production process substantially as herein described with reference to the foregoing description, examples, tables and the accompanying drawings.

Full Text

LASER WELDING METHOD FOR HOT-ROLLED STEEL SHEETS ..AND APPARATUS THEREFOR
CLAIM OF PRIORITY
[0001] This application claims the benefit of Korean Patent Application No. 2005-130248 filed on December 27, 2005 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION Field of the Invention
[0002] The present invention relates to a laser welding method for hot-rolled sheets in a continuous production process for coils, in which two hot-rolled steel sheets are welded together. More particularly, the present invention relates to a laser welding method in which hot-rolled steel sheets are welded together and a weld is less hardened in its microstructure, thereby guaranteeing stable welding quality.
Description of the Related Art
[0003] The present invention relates to a las-er welding method for hot-rolled steel sheets in a continuous production process for coils, in which two hot-rolled steel sheets are welded together. More particularly, the present invention relates to a laser welding method for hot rolled steel sheets in a continuous production process for coils which ensures less hardening in a weld of steel subject to low-temperature transformation in a laser welding process involving abrupt heating and cooling, thereby guaranteeing stable welding quality.
Description of the Related Art
[0004] A technological field of producing sheet metal has faced a strong demand for a continuous production process which leads to higher productivity and quality, and bigger-sized products.
[0005] Such a continuous production process has found its application broadened to high quality steel such as electrical steel or ferritic stainless steel.
[0006] The continuous production process for coils is represented by pickling and tandem cold rolling mill (PCM) processes that are conducted in synchronization. A cold coil obtained from a hot coil may be manufactured by conducting pickling and tandem cold rolling mill (TCM), respectively. However, alternatively, the cold rolled coil can be produced by PCM. The PCM process enhances productivity significantly over the respective implementation of the pickling and TCM processes, thus

growing in its application recently.
[0007] A great importance in the TCM field lies in a technique of: bonding a rear end of a preceding rolled sheet and a following rolled sheet together . The bonding technology for the TCM includes solid state bonding and welding.
[0008] In the case of welding, the rear end of the preceding rolled sheet is welded with the following rolled sheet at an entrance of a TCM Line to form a weld and then passes through a following cold rolling mi 11 line. Here, the weld, if in poor quality, is fractured while passing through the following cold rolling mill line, potentially halting an overall process. Therefore, for the TCM line, it is pivotal to attain a high quality weld in hot and cold coils. Especially, the PCM line is longer in the production line and greater in the number of loopers compared with the existing pickling and TCM lines, thereby requiring more rigorous quality standard for welding than the existing lines.
[0009] Examples of welding for the TCM line includes flash butt: weJding in which short circuit and flashing are repeated, and a laser welding for utilizing a high-density heat source.
[0010] The flash butt welding is big in heat input ,-thus facing an obstac 1 e in selecting a welding material. For example, electrical steel, ferritlc stainless steel, and high carbon steel are not sufficient in bonding strength, also occasionally ruptured during cold rolling. Especially, the high carbon steel is high in C content and thus considered il 1-suited for the flash butt welding. Also, when the welding is repeated under given schedules and conditions, welds each demonstrate non-uniform quality arid thus pose a problem to reproducibility.
[0011] Laser welding has higher energy density requiring smaller heat input than the existing flash butt welding, thus achieving superb welding quality.
[0012] However the laser welding, when conducted on the high carbon steel for the TCM process, causes pores and pin holes in weld metal arid cracks in the weld metal and a heat-affected zone (HAZ).
[0013] The pores and pin holes are closely related to the C content of the welding material. It is known that carbon in molten metal reacts with oxygen in the air during welding, creating CO gas. Thus the CO gas trapped

inside remains during solidification of the molten metal, thus resulting in pores.
[0014] Therefore, it is of importance to lower C content of the molten metal and notably a suitable welding material may be adopted to diminish occurrence of pores.
[0015] Rupture of the welds is associated with hardening of microstructures thereof. The high carbon steel suffers rupture in the: weld chiefly owing to martensite microstructures created in the abrupt-heating and cooling processes during welding. Here, the weld meta] and heat-affected zone are concurrently hardened in their microstructures, which thus necessitates complicated and various ways to remedy the prob Lem.
[0016] The following is conventional technologies for conducting TCM for steel which is hardened it its microstructure.
[0017] First, Japanese Laid-Open Patent Application No. H 5 50276 discloses heat treatment of a weld, in which a fixed heat source is employed to keep the weld at a specific heat treatment temperature during a certain duration according to C content of a hot-rolled steel sheet. However, this increases an overall welding time depending on the heat treatment duration.
[0018] Another conventional technology is taught in Japanese Laid Open Patent Application No. H 5-132719. This technology pertains to laser-welding a weld and then heat treating the same at a temperature of at least 400°C and up to Acl point within a minute. However, welding should be performed during quite a long time to fully eliminate a hardened microstructure at a temperature of at least 400°C and up to Acl point. Also, in case of abrupt cooling which is required in the laser welding, the weld should be abruptly heated within few minutes after welding Cor heat treatment, thereby considerably complicating the heat treatment process.
[0019] Japanese Laid-Open Patent Application No. H 8-57502 discloses further another conventional welding technology, in which low carbon steel with superior weldability is inserted between joints of high carbon steel. This more than doubles the number of welding processes over other welding methods and requires preparation of a leader strip every time, thereby not suitable for mass-production.
[0020] Japanese Laid-Open Patent Application No. H 8^215872 teaches

further another technology, in which a laser weld is cooled while passing through a mixed area of ferrite and pearlite to be heat treated. In this laser welding, heating and cooling occur more abruptly than in Arc weldi ng , and thus the weld is hardly transformed into the mixed area of ferrite and pearlite. Especially, the high carbon steal, if welded, suffers hardening severely.
[0021] Further another conventional technology is taught in Japanese Laid-open Patent Application No. 2000-317642, in which bonding portions are laser welded and then a hot-rolled steel sheet is heat treated. However, here, due to laser welding, a weld is abruptly cooled and transformed into a martensite microstructure before heat treatment, thereby leading to microcracks. Therefore, this technology is hardly applicable to a production line such as PCM that needs high quality welding.
[0022] Japanese Laid-open Patent Application No. 2001-353587 proposes further another conventional technology, in which a filler wire is utilised in a joint between heterogeneous materials of high carbon steel and low carbon steel. In this method, heat treatment is not employed, arid a laser beam is irradiated onto the low carbon to prevent cracks in the weld. Yet this technology fails to eliminate a hardened microstructure produced in a heat-affected zone of the high carbon which is not melted.
[0023] Further another conventional technology is disclosed in Japanese Laid-open Patent Application No. 2000-317642. This technology concerns flash butt welding for heat treating a weld. Similar technologies for heat treating welds, however by different methods, are suggested in Japanese Patent Application Publication Nos. H 5-132719 arid 2000-317642 and 2004-76159. However, these methods do not ensure stable quality for the weld of high carbon steel which contains at least 0.5% C.
[0024] The aforesaid technologies for bonding steel sheets for TCM, although significant in their number, are merely applicable to high carbon steel with relatively low C content or a production line riot requiring high welding quality.
[0025] Consequently, there has been a demand for a technology for securing good quality for a weld joint to carry out TCM on high carbon steel containing at least 0.5% C and steel having a strength of at least. 4bOMPa, e.g. , high strength steel for cars, which has a hardening microstructure such as martensite or bainite after being laser "welded and then cooled down.

SUMMARY OF THE INVENTION
[0026] The present invention has been made to solve the foregoing problems of the prior art and therefore an aspect of the present invention is to provide a laser welding method in which a laser weld is less hardened in its microstructure to assure stable welding quality, thereby significantly improving productivity for a continuous production process .
[0027] Another aspect of the invention is to provide a laser welding apparatus for a continuous production by which a laser weld is less hardened in its microstructure to assure stable welding quality, thereby significantly improving productivity for the continuous production process.
[0028] According to an aspect of the invention, the invention provides a laser welding method for hot-rolled steel sheets in a continuous
*,
production process for coils, including:
butting two hot-rolled steel sheets subject to low-temperature transformation against each other;
laser-welding butted portions of the hot-rolled steel sheets to form a weld; and
compressing the weld of the hot-rolled steel sheets with a compressor.
[0029] Preferably, the weld of the hot-rolled steel sheets is compressed at a temperature of ACT to Ac3. Preferably, the weld is compressed at a compression force of 75MPa or less. Preferably, the compressed weld is reduced in thickness at a ratio of 5.8% or less. Preferably, the weld is compressed vertically with respect to the hot-rolled steel sheets via a planishing roll.
•A
[0033] The hot-rolled steel sheets subject to low-temperature transformation comprise one selected from a group consisting of a high carbon steel containing at least 0.5wt% C, a dual phase (DP) steel, a transformation induced plasticity (TRIP) steel and a composite phase (CP) steel. The high carbon steel consists of at least 0.5wt% C, 0.1 to 0.5wt% Si, 0.3 to 0.6wt% Mn, up to 0.05wt% P, up to 0 . 05wt% S, up to 0.5wt% Cu, up to 3wt% Ni, 0.05 to 0.5wt% Cr, at least 0.05wt% Al, the balance being Fe and unavoidable impurities.
[0031] Preferably, a material for the welding comprises a carbon steel

containing up to lwt% C and 0 to 1.22 wt% Cr or an Ni alloy containing up to O.lwt% C and 0 to 1.22wt% Cr. The welding material comprises one selected from a group consisting of a wire, a powder and a film.
[0032] Preferably, before the laser-welding, the butted portions of the hot-rolled steel sheets are pre-heated to a temperature of 600°C to 800°C. Preferably, after the laser-welding, the weld of the butted portions is post-heated to a temperature of 800°C to 1100°C.
[0034] The continuous production process for the coils comprises one selected from a group consisting of pickling and tandem cold rolling mill line, pickling and oiling line, annealing and pickling line, pickling line and tandem cold rolling mill line.
[0035] According to another aspect of the invention, the invention provides a laser-welded sheet comprising hot-rolled steel sheets subject to low temperature transformation by laser welding and having a hardness difference of 90Hv or less between a weld and portions adjacent to the weld.
[0036] According to further another aspect of the invention, the invention provides a laser welding apparatus for a continuous production process, adapted to laser-weld hot-rolled steel sheets subject to low-temperature transformation. The laser welding apparatus includes a laser welding device for welding the hot-rolled steel sheets; apre-heater for pre-heating a weld of the hot-rolled steel sheets at a front end of the welding device; a post-heater for post-heating the weld of the hot-rolled steel sheets at a rear end of the welding device,- and a compressor for compressing the weld of the hot-rolled steel sheets at a rear end of the post-heater.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
[0038] FIG. 1 is a graph illustrating hardness distribution of a laser weld according to the invention;
[0039] FIG . 2 is a conceptual view illustrating an exemplary 1 aser we 1 d Lng apparatus according to the invention;

[0040] FIG. 3 is a graph illustrating a thermal cycle of a laser weJd according to the invention;
[0041] FIG. 4 is a graph illustrating change in hardness in accordance with change in compression force which is applied after laser welding according to the invention; and
[0042] FIG. 5 is a picture illustrating a laser weld after a PCM process for SK85 steel according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0043] Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings.
[0044] In this specification, "hot-rolled steel sheets subject to low-temperature transformation" denote hot-rolled steel sheets whose welds are transformed in their microstructures at a low temperature when the hot-rolled steel sheets are bonded together to form the welds by laser welding and cooled down. The low-temperature transformation microstructure denotes a microstructure containing martensite andbainite . Also, the hot-rolled steel sheets subject to low temperature transformation after laser welding include high carbon steel or high strength steel.
[0045] The high carbon steel designates steel having at least O.bwt% C. By way of a representative example, the high carbon steel consists of at least 0.5wt% C, 0.1 to 0.5wt% Si, 0.3 to 0.6wt% Mn, up to 0.05wf% P, up to 0.05wt% S, up to 0.5wt% Cu, up to 3wt% Ni, 0.05 to 0.5wt% Cr, at least 0.05wt% Al, the balance being Fe and unavoidable impurities, in the present invention, % represents wt% unless specified otherwise . Steei containing at least 0.5% C is construed to fall within the high carbon steel according to the invention. Here, an alloy element such as Mo, V, Ti, W, B, Nb and Sb may be added to impart a specific function to the high carbon steel.
[0046] The high strength steel is designed to have a tensile strength of 450MPa chiefly for use in cars, and exemplified by steel subject to low-temperature transformation. Examples of the steel subject to low-temperature transformation include a dual phase (DP) steel, a transformation induced plasticity (TRIP) steel, and a composite phase (CP) steel. The high strength steel is typically referred to as steel subject to low-temperature transformation. The DP steel has a dual phase

with two characteristics of ductile ferrite and strong martens.i te. The DP steel ensures superior processabilty and high strength with a small alloy element. Meanwhile, the TRIP steel is composed of ductile fe.rri.te, strong bainite and austenite meta-stable at a high temperature. In the TRIP steel, the meta-stable austenite phase is transformed into very strong martensite phase. The CP steel has precipitation in the martens.i tc or bainite microstructure. The steel subject to low--temperature transformation as just described is phase'-transformed in its microstructure after laser welding.
[0047] As described above, in the high carbon steel or steel subject to low-temperature transformation, a weld is phase-transformed in its microstructure at a low-temperature into e.g. , martensite or bariite with strong brittleness, when the steel is laser welded and cooled down. In this fashion, the low-temperature transformation microstructure causes crack or rupture in the weld.
[0048] Also, in this specification, "a weld" refers to a bonding portion formed by laser welding a rear end of a preceding hot coil and a following hot coil in a continuous production process for coils. Here, the weld includes molten metal which is melted by laser and solidified, and a heat affected zone (HAZ) influenced by a heat source of the laser.
[0049] Moreover, in this specif ication, "a continuous production process for coils" denotes continuously producing coils in a hot or cold rolling line. Here, the continuous production process includes a polishing process in which the hot-rolled steel sheets are bonded together to be polished, or all continuous production processes such as a molten zinc plating process or an annealing process.
[0050] First, according to an aspect of the invention, the reason for hardening in a weld will be examined based on a microstructure test conducted on a laser weld with low temperature transformation microstructure, and an Erichsen test.
[0051] The inventors of this invention confirmed that the weld meta 1 and heat-affected zone, respectively, suffered hardening for which martens.i te, bainite and carbide were mainly responsible.
[0052] Such a hardening microstructure can be removed to a certain level by employing a compression technique in the weld and controlling chemical composition and heat treatment of the weld to a certain level.

[0053] This invention suggests methods for compressing the weld in Later process, reducing C content of a weld metal by using a welding mater laJ with lower C content than the hot-rolled steel sheets, and heat treating the weld to reduce hardness.
[0054] The quality of the weld which is hardened in its microstructure is affected not only by hardness of the weld as just described but also an overall hardness distribution thereof.
[0055] That is, for example, the weld of high carbon steel may be lowered in hardness to a certain extent by heat treatment. However, as shown in FIG. 1, a notch in hardness occurs between the weld and the hot rolled steel sheets, thereby hardly achieving a weld quality that can satisfy the standard.
[0056] Accordingly, according to the invention, the weld is heat, treated and compressed as well to be less hardened. Also, this alleviates overaJ 1 hardness distribution of the weld as indicated with arrows in FIG. 1, thereby assuring a stable weld quality for steel subject to low -temperature transformation.
[0057] Also, in a continuous production process such as PCM, welding and heat treatment, if carried out independently, prolong a welding time, thereby decreasing an overall production rate. Moreover, an increase in C content in the hot-rolled steel sheets leads to an increase in heat treatment time.
[0058] As a result, according to the invention, to solve these problems, as shown in FIG. 1, a heat treatment device and a compressor are configured integral with a welding device so that a weld of hot-rolled steel sheets can be welded, and heat treated and compressed.
[0059] In an explanation with reference to FIG. 2, the welding apparatus of the invention is largely constructed of the welding device 10, the heat treatment device 20 and the compressor 30. These three devices are formed integral. FIG. 2 is a schematic view illustrating the welding apparatus according to the invention, in which thus these devices are not shown configured integrally. But a technique for configuring these devices integrally can be worked by a known art and thus will be explained in no more detail.
[0060] The welding device 10 of the invention includes a laser generator 12 for generating laser and a filler supplier 16 for supplying a welding

material. The heat treatment device 20 adopts as a heat source all moans for heating moving hot-rolled steel sheets speedily. A preferable heat source is a high frequency induction coil. The heat treatment device 20 is constructed of a pre-heater 22 installed upstream of the welding device 10 to heat hot-rolled steel sheets 40 prior to welding and a post-heater 24 for heating hot-rolled steel sheets 50 after welding. Further, the compressor 30 compresses the hot-rolled steel sheets just after welding via a planishing roll.
[0061] In a case where the moving hot-rolled steel sheets are welded together via the welding apparatus of the invention, first a front end of the following hot coil 40 and a rear end of a preceding hot coii bO are butted against each other and welded by the welding device 10 after being heated by the pre-heater 22. A weld 60 with the hot-rolled steel sheets bonded together is post-heated by the post heater 24. Then, the weld passing through the post-heater 24 is cooled down and passes through the compressor 30 installed just downstream of the post-heater 24 , thereby undergoing Post Welding Hot Deformation (PWHD) or compression.
[0062] In this invention, heat treatment and compression of the weld are preferably conducted together but may be carried out independently or selectively.
[0063] The moving hot-rolled steel sheets, when welded via the welding apparatus of the invention, also move the welding apparatus. Here, the hot-rolled steel sheets may move in the same or opposite direction with respect to the welding apparatus. In a case where the hot-rolled steel sheets move in the same direction as the welding apparatus, preferably, the hot-rolled steel sheets move at the same or faster rate with respect to the welding apparatus.
[0064] FIG. 3 is a schematic view illustrating a thermal cycle history for welds of hot-rolled steel sheets and a subsequent microstructure status thereof when welding is performed by a movable welding apparatus according to an embodiment of the invention.
[0065] The invention suggests a method for controlling hardening of the welds to conduct a continuous production process for the hot-rolled steel sheets with low-temperature transformation microstructures . The method involves controlling chemical composition of the welds, heat treating and compressing the welds.
[0066] These methods will be explained hereunder.

[0067] First, a method for controlling chemical composition of the welds will be explained.
[0068] This method pertains to controlling a welding material which constitutes most of molten metal. Preferably, the welding material is supplied from a filler suppler 16 to a weld 60 and basically contains up to 0.1% C and 0 to 1.22% Cr. Such a welding material adopts carbon steel and an Ni alloy with high tensile strength. The carbon steel is preferable since it assures more stable welding quality.
[0069] A welding material composed of stainless steel and an Ni alloy is degraded in wettability with high carbon steel, i.e., the base material, and has base material elements not completely diluted therein in a case where an optimum welding parameter is not derived. This may render the weld brittle occasionally. However, the invention is not limitative of the Ni alloy.
[0070] Preferably, according to the invention, the weldmetal is regulated to contain up to 0.4% C. This is because in the laser welding, a tiny amount of the welding material is melted and filled in the weld so that the base material dilutes at a greater rate compared with typical Arc welding.
[0071] For example, in a case where the hot-rolled steel sheets adopt high carbon steel containing at least 0.85% C, the welding material should contain up to about 0 . lwt% C in order to keep C content of the weld metal at up to 0.4% with a dilution rate set to maximum 30%.
[0072] Further, Cr reacts with carbon in the hot-rolled steel sheets to form chromium carbides in the vicinity of the weld metal and heat affected zone. Thus preferably, the welding material should contain up to 1 .22 %
Cr.
[0073] As a result, according to the invention, preferably, the welding material supplied from the welding device is carbon steel or an Ni alloy containing up to 0 . 1 % C and up to 0 to 1.22 % Cr. Main composition of the carbon steel and Ni alloy is Fe and Ni, respectively. The welding material applicable according to the invention can be typical carbon steel or Ni alloy that satisfies the aforesaid C and Cr conditions. The welding material is preferably shaped as a wire, but a powder or a f.i Irn is also applicable.
[0074] Then an explanation will be given about a method for heat treating

the weld.
[0075] In this invention, heat treatment of the weld breaks down into pre-heating and post-heating. The pre-heating is carried out prior to welding to prevent cracks in a weld joint and the post-heating is performed after welding to relieve hardening of the weld joint.
[0076] In a case where the hot-rolled steel sheets subject to low-temperature transformation is laser welded and the weld undergoes only post-heating, the weld after welding is abruptly cooled down before being post-heated, thus potentially causing fracture in the weld.
[0077] Consequently, it is preferable to pre-heat the welding material subject to low temperature transformation in order to alleviate abrupt cooling resulting from laser welding.
[0078] In a case where the movable heat treatment device is employed as in an embodiment of the invention, the welding material does not achieve sufficient pre-heating effect just above a martensite transformation temperature (Ms) , and thus preferably should be pre-heated to a. temperature higher than that.
[0079] Therefore, according to an embodiment of the invention, preferably, the weld of the high carbon steel is pre-heated to a temperature of 600°C to 800°C. A preheating temperature of up to 600°C does not ensure a sufficient time for pre-heating the moving hot-rolled steel sheets, thereby leading to unsatisfactory quality for the weld. On the other hand, a preheating temperature of at least 800°C deforms the weld due to excessive heat input, thus rendering the weld far from solid.
[0080] Moreover, according to the invention, post-heating of the weld is carried out by two concepts.
[0081] The first concept pertains to tempering in which the wold is post-heated at Acl or less during a relatively long time to change its martensite microstructure into a tempered martensite, thereby attaining ductility.
[0082] The second concept is to actively control a cooling thermal cycle in laser welding to transform the weld into ferrite and pearl ite microstructures.
[0083] The tempering method requires relatively long heat treatment and subsequently ensures sufficient ductility. Yet, in a coil production lino

with a fast yield rate, this type of tedious post-heating may degrade productivity. Therefore, in the laser welding system using a movable heat source, the second concept of relieving a cooling cycle after laser welding is more preferable.
[0084] The weld is post-heated preferably to a temperature of 800°C to 1100°C, and more preferably 950°C to 1100°C. Also, according to an
embodiment of the invention, preferably, the weld is post-heated without.
holding time and naturally cooled down.
[0085] A post-heating temperature of up to 800°C creates a marterisite microstructure in the weld after cooling due to lack of heat input, thus ineffective in reducing hardness. On the other hand, a post heating temperature of at least 1100°C coarsens the microstructure of the weJ.d due to excessive heat input or partially regenerates the martensite that is a hardened microstructure, thereby deteriorating physical properties of the weld.
[0086] Finally, a method for compressing the weld will be explained.
[0087] A high-temperature area of the weld, if compressed as shown in the invention, remarkably reduces hardness over heat treatment.
[0088] Compression is conducted for the weld that is being cooled down at a temperature of Acl to to Ac3 . The compression conducted at such a temperature reduces size of austenite and easily transforms the weld into ferrite and pearlite microstructures.
[0089] Also, the compression performed at Acl or less work-hardens the heat-affected zone to increase hardness to a certain level, thereby alleviating overall hardness distribution of a joint.
[0090] Preferably, the hot-rolled steel sheets are compressed at a compression force of 75MPa or less and at a temperature of Acl to Ac3 during cooling. A compression force of at least 75MPa deforms the weld and causes minor fractures in a low-temperature area thereof.
[0091] Moreover, in a case where compression is carried out for steel whose weld is transformed into martensite or bainite microstructures during cooling after welding, the weld is reduced in thickness at a ratio of 5.8% or less.
[0092] As just described, in an optical method for control Ling hardening

of the weld of high carbon steel according to the invention, a Vickers difference (Hv) of 90Hv or less exists between the weld and the base materia L, i.e., the hot-rolled sheets. The hot-rolled steel sheets, if with the hardness difference of 90Hvas just described, can be continuously produced without suffering rupture in the continuous production process requiring high-strength. This also does not effect deformation in the weld. This invention is preferably applicable to the hot-rolled steel sheet having a thickness of 0.5 to 6mm.
[0093] The laser welding of the invention is applicable to all methods for continuously producing coils which include,-for example, pickling and tandem cold rolling mill (PCM) line, pickling and oiling line (POL), annealing and pickling line (APL), pickling line (PL) and tandem cold rolling mill (TCM) line.
[0094] Preferred embodiments of the invention will be explained hereunder by way of Examples.
[0095] The Examples of the invention will be explained in relation to high carbon steel, which is one of steels hardened in the microstructure. However it is easily understood that the scope of the invention is not limited to the high carbon steel.
Examples
[0096] The Examples adopted hot-rolled steel sheets of high carbon steel having a composition noted in Table 1. The hot-rolled steel sheets were 2 . 0 mm thick and welded to each other via a CO2 laser welding device having a maximum output of 12KW. The Examples utilized as a welding material a wire filler (0 . 9mm) having a chemical composition of low carbon as noted in Table 1. Table 1

(Table Remove)

[0097] The hot-rolled steel sheets were laser welded via the laser welding device under the condition that the weld was free from welding defects
such as a pore underfill. Here, the laser had an output of 8.4kW and a welding rate of 4.5m/min, and a joint had a spacing of 0.15mm.
[0098] To heat treat the weld, a high frequency induction furnace having a heat source of 20wx2001mm moved along a weld line by changing an output.
[0099] The weld was heat treated at a heating rate of about lOOoC/s, pre-heated to 723°C and post-heated to 1005°C, respectively, and then naturally cooled down (air-cooled).
[00100] In heat treating the weld, an R-type pyrometer was spot welded on a melting boundary to measure the temperature history of the weld heated by the high wave induction furnace. A highest attainable temperature was obtained from the temperature history curve to determine as a heat treatment temperature.
[00101] Compression was carried out by a moving roll having a width of 20mm installed downstream of the post-heating treatment device . The weld was compressed after welding and heat treatment.
[00102] The weld obtained according to above-mentioned conditions passed the standard for the PCM line that was Erichsen height of 4mm or more. Thus, quality was evaluated with an Erichsen tester. To evaluate the quality of the weld, measurement was taken on a plastically deformed he:i ght of the weld until the occurrence of cracks.
[00103] First, Table 2 shows quality evaluation results of SK85 steel containing 0.85% C. Table 2
(Table Remove)

Notes)
HC* Heat treatment condition
PC** Compression force (MPa)
HH*** Highest hardness of weld (Hv)
HD**** Hardness difference (Hv) (Weld-base material)
EH***** Erichsen height (mm)
[00104] As noted in Table 2, in a case where the weld of SK85 steel was not heat treated, the weld suffered cracks just after welding, thereby failing to achieve a weld joint that could satisfy the standard.
[00105] Moreover, in a case where only one of pre-heating and post heating was carried out, the weld could not attain a quality enough to be passable;.
[00106] In contrast, in a case where pre-heating and post-heating were both performed as in this invention, the weld was found to be improved in its quality compared with when only one of them was performed.
[00107] The weld processed as just described was compressed by varying a compression force.
[00108] FIG. 4 demonstrates hardness distribution along a longitudinal direction of the weld when the weld of SK85 steel was compressed.
[00109] Increase in the compression force led to increase in KrLchsen height and quality.
[00110] As shown in FIG. 4, with compression, the highest; hardness and hardness difference of the weld were reduced, thus resulting in more moderate hardness distribution in the overall weld.
[00111] The compression force in a range of 75MPa or less linearly increased Erichsen height. However, the compression force in a range of more than 75MPa severely deformed a weld joint and excessively hardened a low-temperature area, causing microcracks.

[00112] Also, as shown in FIG. 4, a compression force of OMPa noticeably increased hardness in molten metal and relatively diminished hardness in a heat-affected zone. But it is found that with increase of the compression force from 30MPa to 75MPa, the weld was less steep in hardness difference, thus exhibiting more moderate hardness distribution along a longitudinal direction thereof.
[00113] Table 3 notes quality evaluation results for S50C steel containing 0.5% C. Table 3

(Table Remove)

Notes)
HT* Heat treated
PC** Compression force (MPa)
TRR*** thickness reduction ratio of weld
HH**** Highest hardness of weld (Hv)
HD***** Hardness difference (Hv) (Weld-base
EH****** Erichsen height (mm)
[00114] As seen in Table 3, the laser weld of S50C steel overall showed
superior quality to SK85 steel regardless of the welding materials and
heat treatment methods.
[00115] Moreover as noted in Table 3, steel whose weld is transformed into martensite or bainite microstrucutres, was found to be superior in welding quality when it was compressed at a temperature of Acl to Ac3 while cooled down after welding and reduced in its thickness to 5.8% or less.
[00116] This is apparently attributed to less hardening resulting from lower C content in the steel.
[00117] This steel, when welded and then compressed, is increased .in the Erichsen height and reduced in the highest hardness and hardness difference like SK85, thereby remarkably improving welding quality.
[00118] Preferred embodiments of the invention were explained hereinabovc . However this invention is not limited to welding conditions of the continous production process for high carbon steel as manifested in the Examples. The invention is applicable to various welding conditions necessary for the continuous production process which encompasses the scope of the invention.
[00119] As set forth above, according to exemplary embodiments of the invention, a laser welding for a continuous production process provides welding conditions that have not been adopted, thereby continuously producing steel sheets with a low temperature transformation microstructure.
[00120] Also, in order to ensure a secure laser weld joint free from welding imperfections, hot-rolled steel sheets are -welded together to continuously produce steel containing at least 0.5% C as in the Examples. Furthermore, as shown in FIG. 5, the invention enables continuous production without causing fracture in a laser weld.
[00121] In addition, according to the invention-, the welding time is shortened to 25 seconds, which are required for general steel, irrespective of C content of steel. This noticeably enhances product! vi ty in the continuous production process.
[00122] Further, steel subject to low temperature transformation under the welding conditions according to the invention can withstand strong compressive load applied in the continuous production process and tensile load imposed between stands, thereby carrying out continuous production with the weld free from rupture.
[00123] While the present invention has been shown and described in connection with the preferred embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without, departing from the spirit and scope of the invention as defined by the appended claims.

What Is Claimed Is;
1. A laser welding method for hot-rolled steel sheets in a continuous
production process for coils, comprising:
butting two hot-rolled steel sheets subject to low-temperature transformation against each other;
laser-welding butted portions of the hot-rolled steel sheets to form a weld; and
compressing the weld of the hot-rolled steel sheets with a ; compressor.
2. The laser welding method according to claim 1, wherein the weld
; of the hot-rolled steel sheets is compressed at a temperature of Ac, to
'Ac3.
3. The laser welding method according to claim 1, wherein the weld
is compressed at a compression force of 75MPa or less.
4 . The laser welding method according to one of claims 1 to 3 , wherein the compressed weld is reduced in thickness at a ratio of 5.8% or less.
5. The laser welding method according to claim 1, wherein the weld
is compressed vertically with respect to the hot-rolled steel sheets via
a planishing roll.
6. The laser welding method according to claim I, wherein the
hot-rolled steel sheets subject to low-temperature transformation
comprise one selected from a group consisting of a high carbon steel
containing at least 0.5wt% C, a dual phase (DP) steel, a transformation
induced plasticity (TRIP) steel and a composite phase (CP) steel.
7. The laser welding method according to claim 6, wherein the high
carbon steel consists of at least 0 . 5wt% C, 0 .1 to 0 .5wt% Si, 0.3 to 0 .6wt%
Mn, up to 0.05wt% P, up to 0.05wt% S, up to 0.5wt% Cu, up to 3wt% Ni,
0 .05 to 0 . 5wt% Cr, at least 0 . 05wt% Al, the balance being Fe and unavoidable
impurities.
8. The laser welding method according to claim 1 or 7, wherein a
material for the welding comprises a carbon steel containing up to lwt%
C and 0 to 1.22 wt% Cr or an Ni alloy containing up to O.lwt% C and 0

to 1.22wt% Cr.
9 . The laser welding method according to claim 8 , wherein the welding
material comprises one selected from a group consisting of a wire, a powder
:and a film.
10 . The laser welding method according to claim 1 or 7 , wherein before
the laser-welding, the butted portions of the hot-rolled steel sheets
are pre-heated to a temperature of 600°C to 800°C.
11. The laser welding method according to claim 1, wherein after
the laser-welding, the weld of the butted portiojis is post-heated to a
temperature of 800°C to 1100°C.
12. The laser welding method according to claim 10, wherein after
the laser-welding, the weld of the butted portions is post-heated to a
temperature of 800°C to 1100°C.
13 . The laser welding method according to claim 12 , wherein the weld is compressed at a compression force of 75MPa or less and at a temperature of Aci to Ac3.
14. The laser welding method according to claim 1, wherein the
continuous production process for the coils comprises one selected from
a group consisting of pickling and tandem cold rolling mill line, pickling
and oiling line, annealing and pickling line, pickling line and tandem
cold rolling mill line.
15. A laser-welded sheet comprising hot-rolled steel sheets subject.
to low temperature transformation by laser welding and having a hardness
difference of 90Hv or less between a weld and portions adjacent to the
weld.
16. The laser-welded sheet according to claim 15, wherein the
hot-rolled steel sheets subject to low-temperature transformation
comprise one selected from a group consisting of a high carbon steel
containing at least 0.5wt%, a dual phase (DP) steel, a transformation
induced plasticity (TRIP) steel and a composite phase (CP) steel.
17. The laser-welded sheet according to claim 16, wherein the high
carbon steel consists of at least 0 . 5wt% C, 0 .1 to 0 . 5wt% Si , 0 . 3 to 0 . 6wt%
Mn, up to 0.05wt% P, up to 0.05wt% S, up to 0.5wt% Cu, up to 3wt% Ni,
0 . 05 to 0 . 5wt% Cr, at least 0 . 05wt% Al, the balance being Fe and unavoldabi e

impurities.
18. A laser welding apparatus for a continuous production process,
adapted to laser-weld hot-rolled steel sheets subject to low-temperature
transformation, comprising:
a laser welding device for welding the hot-rolled steel sheets,-
a pre-heater for pre-heating a weld of the hot-rolled steel, sheets at a front end of the welding device;
a post-heater for post-heating the weld of the hot-rolled steel sheets at a rear end of the welding device; and
a compressor for compressing the weld of the hot-rolled steel sheets at a rear end of the post-heater.
19. A laser welding method for hot-rolled steel sheets in a
continuous production process for coils substantially as herein described
with reference to the foregoing description, examples, tables arid the
accompanying drawings.
20. A laser-welded sheet substantially as herein described with
reference to the foregoing description, examples, tables and the
accompanying drawings.
21. A laser-welded apparatus for a continuous production process
substantially as herein described with reference to the foregoing
description, examples, tables and the accompanying drawings.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=/FK2vDMqqhXYhzsVdIRG8Q==&loc=+mN2fYxnTC4l0fUd8W4CAA==

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Patent Number

270276

Indian Patent Application Number

2785/DEL/2006

PG Journal Number

50/2015

Publication Date

11-Dec-2015

Grant Date

08-Dec-2015

Date of Filing

22-Dec-2006

Name of Patentee

POSCO

Applicant Address

I KOEDONG-DONG,NAM-KU,POHANG,KYUNGSANGBOOK-DO 790-300,REPUBLIC OF KOREA.

Inventors:

#	Inventor's Name	Inventor's Address
1	WOO,IN-SU	C/O POSCO WORKS,5 DONGCHON-DONG,NAM-KU,POHANG,KYUNGSANGBOOK-DO,REPUBLIC OF KOREA
2	PARK,JOON-SIK	C/O POSCO WORKS,5 DONGCHON-DONG,NAM-KU,POHANG,KYUNGSANGBOOK-DO,REPUBLIC OF KOREA
3	JEONG,BO-YOUNG	C/O POSCO WORKS,5 DONGCHON-DONG,NAM-KU,POHANG,KYUNGSANGBOOK-DO,REPUBLIC OF KOREA
4	KIM,JEONG-KIL	C/O POSCO WORKS,5 DONGCHON-DONG,NAM-KU,POHANG,KYUNGSANGBOOK-DO,REPUBLIC OF KOREA
5	LEE,JONG-BONG	C/O POSCO WORKS,5 DONGCHON-DONG,NAM-KU,POHANG,KYUNGSANGBOOK-DO,REPUBLIC OF KOREA

PCT International Classification Number

B23K33/00; B23K15/02

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	10-2005-0130248	2005-12-27	Republic of Korea