Title of Invention

IMPROVED PROCESS FOR PPRODUCING HIGH STRENGTH STEEL PLATES WITH IMPROVED IMPACT VALUES USING RELAXED ROLLING CONDITION.

Abstract

The present invention relates to a process for the production of high strength steel plates with improved impact values which comprises; (a) providing a molten steel having the following alloy steel composition comprising in wt.% C: 0.07-0.10% Mn: 1.40-1.50% Si: 0.25-0.35% S: 0.01%max P. 0.02% max Al 0.025-0.045% Nb: 0.06-0.09% Ti:0.01-0.02% (b) subjecting the same to Calcium treatment by adding Calcium silicate during refining, whereby the calcium treatment is done to modify the inclusion population in steel; (c) casting into a slab, (d) avoiding re-oxidation during casting using argon injection as necessary; (e) then subjecting the steel slab to controlled rolling wherein the slabs in a reheating furnace which serve as heating and buffer zone , the casts slabs are heated up to pre determined rolling temperature in the furnace for the efficient rolling. The schedule of the controlled rolling of the steel slab is as follows: Soaking temp. And time : 125O+ 10°C/ 51/2hrs. Rough rolling : 1150-1050 °C (10 passes) Finish rolling :1000-850 °C (5 passes) Cumulative reduction in the finish zone : > 65% Reduction in final pass : > 10% such that there is obtained a low reduction per pass, high finish rolling start temperature and absence of accelerated cooling followed by; (f) subjecting the finish rolled plate to air cooling wherein the finish rolling is preferably carried out at temperature in the range of 830-880 °C.

Full Text

2. IMPROVED PROCESS FOR PRODUCING HIGH STRENGTH STEEL PLATES WITH IMPROVED IMPACT VALUES USING RELAXED ROLLING CONDITION INTRODUCTION TO THE FIELD OF INVENTION This invention relates to an improved process for producing high strength steel plates with improved impact values. The high strength steel plates with improved impact values are achieved by using modified alloy steel composition, and roiling at temperatures higher than the conventional 800°C, which can be up to 880 °C. This invention more particularly relates to a process for producing high strength steel plates with improved impact values, using modified alloy steel composition combined with relaxed rolling conditions. The alloy steel composition is modified by addition of specific quantity of Nb and Ti, which helps in grain refinement thereby increasing the strength and toughness of the steel. PRIOR ART AND DRAWBACKS The major limitation of underpowered mills in rolling high strength steel plates e.g. higher grades of API are the inability to give higher reductions per pass and to roll at low finish rolling temperature (800°C). In addition, the absence of accelerated cooling adds to the problem of achieving desired grain size of 4-8 micron, which is optimum with respect to strength, toughness and ductility. OBJECTS OF THE INVENTION It is therefore an object of this invention to propose an improved process for producing high strength steel plates by using a modified steel alloy composition. z It is another object of this invention to combine the use of modified steel alloy composition with modified raffing conditions. It is thus another object of this invention to propose dual improved conditions in the manufacture of high strength steel plates in underpowered mills. BACKGROUND OF THE INVENTION After intensive studies, experiments and evaluation tests, it was found that the roiling at the temperatures higher than the conventional 800°C even up to 8S0°C it is possible to get desired properties. We have also studied the behavioral characteristics of the various alloying elements inter reactions, which was possible only after intensive research and experiments. We have also been able to achieve less deformation per pass at higher rolling temperature, which is unique for producing high strength steel plates and has not been anywhere at all. Furthermore, our studies have been oriented to achieve restricted austenite grain size during slab reheating. Our studies were oriented towards evaluating the behaviors of Nb and Ti and it has been found that Niobium upto 0 05% increases strength and toughness through grain refinement. Beyond 0.05% Nb increases the toughness of the steel and facilitates relaxed rolling of plates (higher finishing temperature). It is made possible as austenite non-crystallization temperature (Tnr) is raised due to Nb addition and it maximize % reduction below Tnr which is prerequisite of thermomechanical control processing (TMCP). 4- Titanium combines with Nitrogen to form TiN precipitates, which restrict austenite grain size during slab reheating. It also reduces the free Nitrogen in the steel and improves toughness property. BRIEF STATEMENT OF THE INVENTION Thus according to this invention there is provided a process for the production of high strength steel plates with improved impact values which comprises: (a) providing a molten steel having the following alloy steel composition comprising in wt. % C: 0.07-0.10% Mn: 1.40-1.50% Si: 0.25-0 35% S: 0.01%max P: 0.02% max Al: 0.025-0.045% Nb: 0.06-0.09% Ti:0.01-0.02% (b) subjecting the same to Calcium treatment by adding Calcium silicate during refining, whereby the calcium treatment is done to modify the inclusion population in steel; . (c) casting into a slab; (d) avoiding re-oxidation during casting using argon injection as necessary; (e) then subjecting the steel slab to controlled rolling wherein the slabs in a reheating furnace which serve as heating and buffer zone , the casts slabs are heated up to pre determined rolling temperature in the furnace for the efficient rolling. The schedule of the controlled rolling of the steel slab is as follows: Soaking temp. And time : 1250+ 10°C/ 51/2hrs. Rough rolling : 1150-1050°C (10 passes) Finish rolling : 1000-850°C (5 passes) Cumulative reduction in the finish zone : > 65% Reduction in final pass : > 10% s such that there is obtained a low reduction per pass, high finish rolling start temperature and absence of accelerated cooling followed-by; (f) subjecting the finish rolled plate to air cooling wherein the finish rolling is preferably carried out at temperature in the range of 830-880 °C With low reduction per pass, high finish rolling start temperature and absence of accelerated coling, ferrite grain size of 4-8 micron was achieved and mechanical properties obtained upto 15mm thickness were; YS: 480Mpa min; UTS: 540 Mpa min.; % EL: 33 min and Charpy impact: 150 J min at 0°C DETAILED DESCRIPTION OF THE INVENTION In thermomechamcal treatments of low carbon microalloyed steels, the main goal is to promote austenite grain refinement in the recrystallization temperature range, combined with straining at temperatures below the startup of deformation-induced precipitation. The latter process is very effective because, during Sow temperature hot deformation, precipitates such as carbonitride particles stabilize the dislocation substructure and retard recrystaliization, thus facilitating ferrite refinement. The main purpose of this steel processing is to produce the fine and homogeneous ferrite grains as well as a high volume fraction of carbide/nitride precipitates during or after austenite-ferrite transformation. This process results in superior mechanical properties such as high strength, toughness, and good ductility and weidability. These purposes can be achieved by controlling the processing stages as schematically described below: 1. The reheating prior to rolling; 2 The controlled rolling of austenite in the recrvstaHization region, T > Tnr (non-recrystallization temperature); 3, The controlled rolling of austenite in the non-recrystallization region, T (non-recrystallization temperature); 6 4.- The controlled cooling after rolling. All the above processes interact in determining the final steel properties, and widely known as Thermo mechanical Controlled Processing (TMCP). Recent high strength structural steel plates are produced by applying thermomechanical controlled processing (TMCP), which includes controlled rolling followed by accelerated cooling (AcC) or quenching and tempering (Q-T). Especially, advance in the accelerated cooling process has enabled higher strength by transformation strengthening under higher cooling rate. Low carbon steels are usually used for the steels that are required of high strength' and higher levels of base metal toughness and HAZ (Heat Affected Zone) toughness, as well as good weldability. Micro-alloying elements, such as Nb and Ti, are added for preventing grain coarsening during heating at austenization temperature by pinning effect of carbo-nitride particles. Nb is also a quite important element in controlled rolling process because solute Nb can increase the non re-crystallization temperature and insure the grain refining of * transformed bainite micro structure after controlled rolling and accelerated cooling. Strength and toughness of steel is strongly affected by its transformation and precipitation behavior, therefore, controlling those phenomena based on complete understanding of transformation and precipitation mechanisms should be a key issue for materials designing of high strength steels. EFFECTS OF MICRO ALLOYING ELEMENTS Carbon: Most of the microalloyed steels developed for forging have carbon contents ranging from 0.07 to 0.10%, which is enough to form a large amount of pearlite. The pearlite is responsible for substantial strengthening. This level of carbon also decreases the solubility of the micro alloying constituents in austenite. 7 Niobium and Titanium: Formation of carbonitride precipitates is the other major strengthening mechanism of microalloyed forging steels. Niobium and titanium enhance strength and toughness by providing control of austenite grain size Niobium Microalloyed Steels: like vanadium, niobium increases yield strength by precipitation hardening; the magnitude of the increase depends on the size and amount of precipitated niobium carbides. However, niobium is also a more effective grain refiner than vanadium. Thus, the combined effect of precipitation strengthening and ferrite grain refinement makes niobium a more effective strengthening agent than vanadium. The niobium addition is 0.06 to 0.09%, which is about one-third the optimum vanadium addition. Strengthening by niobium is 35 to 40 MPa per 0.01% addition. Titanium Microalloyed Steels: Titanium in low-carbon steels forms into a number of compounds that provide grain refinement, precipitation strengthening, and sulfide shape control. However, because titanium is also a strong deoxidizer, titanium can be used only in fully killed (aluminum deoxidized) steels so that titanium is available for forming into compounds other than titanium oxide. Commercially, steels precipitation strengthened with titanium are produced in thicknesses up to 9.5 mm in the minimum yield strength range from 345 to 550 MPa with controlled rolling required to maximize strengthening and improve toughness. Titanium-Niobium Microalloyed Steels: Although precipitation-strengthened titanium steels have limitations in terms of toughness and variability of mechanical properties, research has shown that an addition of titanium to low-carbon niobium steels results in an overall improvement in properties. Titanium increases the efficiency of niobium because it combines with the nitrogen-forming titanium nitrides, thus preventing the formation of niobium nitrides. Manganese: Manganese is used in relatively large amounts (1.4 to 1.5%) in many microalloyed-forging steels. It tends to reduce the cementite plate thickness while maintaining the interlamellar spacing of pearlite developed; thus high manganese levels require lower carbon contents to retain the large amounts of pearlite required for high hardness. Manganese also provides substantial solid solution strengthening, enhances the solubility of vanadium carbonitrides, and lowers the solvus temperature for these phases. 8 Silicon: The silicon content .of most commercial microalloyed forging steels is about 0.30%. some grades contain up to 0.70%. Higher silicon contents are associated with significantly higher toughness, apparently because of an increased amount of ferrite relative to that formed in ferntE-pearlite steels with lower silicon contents. Sulfur: It is undesirable as it reduces impact properties. Phosphorus: Phosphorous is considered undesirable because they reduce elevated temperature ductility and hence affect the stress rapture strength and thermal fatigue life. An excessive content of Phosphorous in steel makes it impossible to prevent defects form being formed. Aluminum: Aluminum is important for austenite grain size control in micro alloyed steels The mechanism of aluminum grain size control is the formation of aluminum nitride particles. Aluminium is an element of deteriorating the index of cleanliness of steel. Moreover, Al causes a nozzle to be stopped when a base material is produced in the continuous casting Therefore it is necessary to set the Aluminium content not more than 0.05%'. On the other hand, Aluminium is effective for deoxidation, so that the Aluminium content should be preferably not less than 0.02%; PROCESSING DETAILS Steel making: The steel is made through the Basic Oxygen Process, where pure Oxygen is blown into a bath of molten blast-furnace iron and scrap. The Oxygen initiates a series of intensively exothermic reactions, including the oxidation of such impurities as Carbon, Silicon, Phosphorous and Manganese. Most of the alloying additions and deoxidation carried out at Converter shop. The crude steel tapped from the converter contains impurities, which are not desired in the final steel chemistry; hence, the steel is processed through Steel Refining Unit where final additions are made to have desired chemistry Steel is purged by argon to facilitate floatation of inclusions and uniformity of chemistry. Argon bubbling is applied to homogenize the steel composition and for avoiding re- 9 oxidation during casting. Calcium Silicate is added for inclusion modification. Calcium forms compounds with Al2O3, which are liquid and float to the top. Sulphur forms Calcium Sulphide, which are of globular, shape. The adverse affect of Sulphur is taken care by this. Rolling: The normal controlled rolling process consists of three steps. The first step is known as the roughing stage and is characterized by the application high strain degrees to the rolling stock; this is possible due to the high rolling temperature. The second step is known as the holding stage; at that time the rolling stock is only cooled at the roller table, without being submitted to hot rolling until its temperature reaches an ideal value to resume the forming process. That is, there is no hot rolling at all during this step The third step consists in the finishing stage of the rolling stock, which now is under a lower temperature As the hot strength is very high, the strain degree applied in each pass became very low. During the holding stage of a rolling stock other slabs are roughened. This concomitant process, known as tandem rolling, increases the productivity of the piate mill line. Formerly the rolling stock thickness after the first controlled rolling stage was defined accordingly to the final plate thickness. The control of grain size at high austenitizing temperatures requires as fine a grain boundary precipitate, as possible, and one which will not dissolve completely in the austenite. even at the highest working temperatures (120Q-1300°C). The best grain refining elements are very strong carbide and nitride formers, such as niobium, titanium and vanadium, also aluminum that forms only a nitride. As both carbon and nitrogen are present in control-rolled steels, and as the nitrides are even more stable than the carbides it is likely that the most effective grain refining compounds are the respective carbo-nitrides, except in the case of aluminum nitride. As a result of the combined use of controlled rolling and fine dispersions of carbo-nitrides in low alloy steels, it has been possible to obtain ferrite grain sizes between 5 and 10 micron. 1O The effect of the finishing temperature for rolling is important in determining the grain size and, therefore, strength level reached for particular steel. It is now becoming common to roll through the transformation into the completely ferritic condition, and so obtain fine sub grain structures in the ferrite, which provide an additional contribution to strength. Alternatively, the rolling is finished above the y/a transformation, and increasing the cooling rate alters the nature of the transformation. Slow rates of cooling obtained by coiling at a particular temperature will give lower strengths than rapid rates imposed by water spray cooling following rolling. Hence, in the modern control-rolled micro-alloyed steels, there arc at least three strengthening mechanisms, which contribute to the final strength achieved. The relative contribution from each is determined by the composition of the steel and, equally important, the details of the thermomechanical treatment to which the steel is subjected. Firstly, there are the solid solution strengthening increments from manganese, silicon and uncombincd nitrogen. Secondly, the grain size contribution to the yield stress is shown as a very substantial component, the magnitude of which is very sensitive to the detailed thermomechanical history. Finally, a typical increment is dispersion strengthening. The total result is a range of yields strengths between about 350 and 500 MPa.. Micro-alloyed steels produced by controlled rolling arc a most attractive proposition in many engineering applications because of their relatively Jow cost, moderate strength; and very good toughness and fatigue strength, together with their ability to be readily welded. They have, to a considerable degree, eliminated quenched and tempered steels in many applications. ADVANTAGES With low reduction per pass, high finish rolling start temperature and absence of accelerated coling, ferrite grain size of 4-8 micron was achieved and mechanical properties obtained upto 15mm thickness were: YS: 480Mpa min; UTS: 540 Mpa min., % El: 33 min and Charpy impact: 150 J min at 0°C. We Claim: (1) A process for the production of high strength steel plates with improved impact values which comprises. (a) providing a molten steel having the following alloy steel composition comprising in wt.% C: 0.07-0.10% Mn: 1.40-1.50% Si: 0.25-0.35% S: 0.01%max P: 0.02% max Ah 0.025-0.045% Nb: 0.06-0.09% Ti:0.01-0.02% (b) subjecting the same to Calcium treatment by adding Calcium silicate during refining, whereby the calcium treatment is done to modify the inclusion population in steel, (c) casting into a slab; (d) avoiding re-oxidation during casting using argon injection as necessary; (e) then subjecting the steel slab to controlled rolling wherein the slabs in a reheating furnace which serve as heating and buffer zone , the casts slabs are heated up to pre determined rolling temperature in the furnace for the efficient rolling. The schedule of the controlled rolling of the steel slab is as follows: Soaking temp. And time : 1250+ 10°C/ 51/2hrs. Rough rolling : 1150-1050 °C (10 passes) Finish rolling : 1000-850 °C (5 passes) Cumulative reduction in the finish zone : > 65% Reduction in final pass : > 10% such that there is obtained a low reduction per pass, high finish rolling start temperature and absence of accelerated cooling followed by; \1 (f) subjecting the finish rolled plate to air cooling wherein the finish rolling is preferably carried out at temperature in the range of 830-880 °C. (2) A process as claimed in claim 1, wherein, the air-cooling stage produces a steel plate with the following characteristics: low reduction per pass, high finish rolling start temperature and absence of accelerated coling, ferrite grain size of 4-8 micron was achieved and mechanical properties obtained upto 15mm thickness were: YS: 480Mpa min; UTS: 540 Mpa min.; % El: 33 min and Charpy impact". 150 J min at 0°C. To, The Controller of Patents, The Patent Office, Kolkata. (3) A process for the production of high strength steel plates with improved impact values substantially as herein described. The present invention relates to a process for the production of high strength steel plates with improved impact values which comprises; (a) providing a molten steel having the following alloy steel composition comprising in wt.% C: 0.07-0.10% Mn: 1.40-1.50% Si: 0.25-0.35% S: 0.01%max P. 0.02% max Al 0.025-0.045% Nb: 0.06-0.09% Ti:0.01-0.02% (b) subjecting the same to Calcium treatment by adding Calcium silicate during refining, whereby the calcium treatment is done to modify the inclusion population in steel; (c) casting into a slab, (d) avoiding re-oxidation during casting using argon injection as necessary; (e) then subjecting the steel slab to controlled rolling wherein the slabs in a reheating furnace which serve as heating and buffer zone , the casts slabs are heated up to pre determined rolling temperature in the furnace for the efficient rolling. The schedule of the controlled rolling of the steel slab is as follows: Soaking temp. And time : 125O+ 10°C/ 51/2hrs. Rough rolling : 1150-1050 °C (10 passes) Finish rolling :1000-850 °C (5 passes) Cumulative reduction in the finish zone : > 65% Reduction in final pass : > 10% such that there is obtained a low reduction per pass, high finish rolling start temperature and absence of accelerated cooling followed by; (f) subjecting the finish rolled plate to air cooling wherein the finish rolling is preferably carried out at temperature in the range of 830-880 °C.

Documents:

00147-kol-2003-abstract.pdf

00147-kol-2003-claims.pdf

00147-kol-2003-correspondence.pdf

00147-kol-2003-description(complete).pdf

00147-kol-2003-form-1.pdf

00147-kol-2003-form-13.pdf

00147-kol-2003-form-18.pdf

00147-kol-2003-form-2.pdf

00147-kol-2003-form-26.pdf

00147-kol-2003-form-3.pdf

00147-kol-2003-letters patent.pdf

00147-kol-2003-p.a.pdf

00147-kol-2003-reply f.e.r.pdf

147-KOL-2003-(08-02-2012)-FORM 27.pdf

147-KOL-2003-FORM 27.pdf

147-kol-2003-granted-abstract.pdf

147-kol-2003-granted-claims.pdf

147-kol-2003-granted-description (complete).pdf

147-kol-2003-granted-form 2.pdf

147-kol-2003-granted-specification.pdf