|Title of Invention||
METHOD OF PRODUCING A HOT ROLLED HIGH STRENGTH DUCTILITY STEEL TO GENERATE ARTICLES AND ARTICLES SO PRODUCED THEREOF
|Abstract||This invention relates to a method of producing a hot rolled high strength ductile steel with low yield to tensile strength ratio to generate articles comprising making a steel slab of compositions in wt% consisting of carbon 0.08 to 0.18, silicon 0.30 to 0.70, manganese 1.00 to 1.70, phosphorus 0.005 to 0.03, sulphur 0.005 to 0.03, nitrogen less than 0.005, aluminium 0.02 to 0.06, chromium 0.05 to 0.15, vanadium 0,05 to 0.15, molybdenum 0.05 to 0.20 and the balance being iron and incidental impurities, the said slab being hot rolled to steel article at finishing roll at a temperature 870°C - 900°C, cooling the article at a temperature in the range of 730°C - 780°C, retaining the article at this temperature for a while, cooling the article to 450°C - 520°C at a cooling rate of 30°C to 35°C/sec and coiling the article at the same temperature followed by air cooling to room temperature to result multiphase microstructure consisting of ferrite, bainite and martensite.|
|Full Text||HELD OP INVENTION
The present invention relates to a method of producing high strength steel. More specifically the invention relates to development of high tensile strength steel and articles including sheets, rods, bars and other articles produced thereof in hot rolled condition having a compatible ductility maintained through elongation and low yield strength/ultimate tensile strength ratio characteristics.
BACKGROUND OF THE INVENTION
The structural parts of automotive (long member, cross member, structural panels etc.) body require high strength. There have been a number of steels developed for this purpose. However, elongation of these high strength steels is not adequate, and these cannot be formed easily. In order to have good formability, the yield strength has to be low at the same time tensile strength of these steels must be high enough to improve crash worthiness.
In the known art High strength ferrite-martensite dual phase steels have been developed keeping the above requirements in mind. For the said requirements hot roiled ferrite-martensite dual phase steels have to be coiled at rather low temperatures in order to form martensite. In many conventional hot railing mills low coiling temperature is quite difficult to achieve. Therefore there is a long need to produce comparable steels using the high coiling temperature option in this field of applications.
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To solve the prior art difficulties the present invention proposes to develop high strength steel with superior ductility and low yield to tensile ratio that can be produced by hot rolling.
Additions of alloying elements such as nickel, manganese, cobalt and others sharply increase austenite stability. On the other hand chromium, Vandium, Tungsten etc are known as ferritic stabiliser elements.
Role of constituents added as alloying elements in production of steel and heat treatment of the present steel is enumerated as follows:-
Si: Si is an element effective for solid solution strengthening. It increases the strength of ferrite, and when used in conjunction with other alloys can help increase the toughness.
Mn: Manganese plays an important role as it lowers the temperature at which austenite transforms into ferrite, thus avoiding cementite precipitation at ferrite grain boundaries, and by refining the resulting pearlite structures. When the cooling process is accelerated by quenching, austenite transforms into structures with high strength such as bainite and martensite. Manganese improves the response of steel to quenching by its effect on the transformation temperature. Manganese is also weak carbide former. Another important property of manganese is its ability to stabilise the austenite in steel. The effect of manganese in forming austenite can be reinforced by combining it with nitrogen, which is also an austenite-forming element. Manganese also increases hardenability rate, used to significant advantage, depending on the steel type and the end product, to improve mechanical properties. The content of Mn should be 0.5% or higher from the viewpoint of solid solution strengthening.
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P: P is effective for solid solution strengthening. S: A lower content of 5 is more desirable.
Al: A1 is added as a deoxidizer. If the content of A1 is higher than 0.1% both of the elongation and the stretch flangeability deteriorate. Therefore, the content of Al should be 0.1% or lower.
N: A lower content of N is more desirable. If the content of N is higher than 0.006%, coarse nitrides increase, so that the stretch flangeability degrades. Therefore, the content of N should be 0.006% or lower.
Mo; Mo forms fine composite carbides, and thus strengthens the steel while the high elongation and the excellent stretch flangeability are maintained. In the case of a steel sheet having tensile strength of around 780 MPa, the content of Mo should be in the range of 0.05 to 0.6%, and in the case of a steel sheet having tensile strength of around 950 MPa, the content of Mo should be in the range of 0.3 to 0.7%.
V: V is effective in making the structure fine, and also form composite precipitates together with Mo, which contributes to the increase in elongation and stretch flangeability. However, if the content of V is higher than 0.15%, the elongation decreases.
By various choice of addition of allowing elements and their behaviour characteristics during steel making and during heat treatment and cooling rates employed to increase hardenability of steel by solution heat treatment it is
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known in development of structural steel, tools steels and steels of special physical properties producing steels of dual and multiphase by influencing decomposition characteristics of austenite to precipitate carbides trotstite, acicular trotsite (bainite) by employing different course of heat treatment methods viz quenching, martempering, austempering as known in the art.
Bainite is formed as a result of austempering of carbon steels in which austenite decomposition is fully completed in the intermediate zone. Such phase distribution ensures a very high strength with sufficient toughness.
Such tough steel attempted and developed so far to decrease chances of breaking for high strength steel with high hardness does require for further improvement in the structural parts of automotive body which require high strength but at the same time require characteristics to prevent breaking, heat resistant and wear resistant during high coiling temperature as in the conventional hot rolling mill low coiling temperature is difficult to be employed due to formation of martensite at this low coiling temperature.
According to one objective of the invention the low alloy steel is proposed to be developed on thorough studies and experiments on behaviour of addition of low alloying elements and opting for course of hardenability to produce micro-structure consisting in combination of ferrite, bainite and martensite phase in the resulted steel structure.
According to another objective of the invention it is proposed to develop a high tensile strength steel with compatible ductility by reducing high amount of martensite and strengthening ferrite phase resulted through precipitation during solution hardening.
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According to still another objective of the invention it is proposed to avoid yield point phenomenon by maintaining limited amount of martensite in the resulted steel but without sacrificing the tensile strength characteristics by strengthening ferrite phase during heat treatment.
According to still further objective of the invention producing high strength steel with superior ductility is proposed by maintaining low yield to tensile strength ratio of the resultant steel,
According to yet another objective of the invention it is proposed to develop a high strength with compatible ductility steel structure to be produced by hot roiling during on line production of the same in the working floor.
According to yet further objective of the invention it is proposed to produce a low yield to tensile strength ratio by producing a steel slab from low alloyed carbon steel having added alloying elements from the group of silicon, manganese, phosphorous, sulphur, nitrogen, aluminium, cromium, vanadium and molybdenum.
The present invention is directed to the production of low yield to tensile strength ratio, high strength and ductility hot rolled steels having compositions of 0.08 to 0.18 wt.% of C, 0.3 to 0.7 wt% Si, about 1.7 wt% or less of Mn, 0.02 wt% or less P, 0.03wt% or less S, less than 0.005 wt% N, about 0.04wt% of Al, 0.15wt% or less Cr, 0.15 wt% or less V, 0.2 wt% or less Mo, the balance being substantially Fe and incidental impurities.
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According to one aspect of the present invention it involves the method of manufacturing a low yield to tensile ratio, high strength and ductile hot rolled steel having a structure consisting of ferrite, bainite and martensite. The method comprises manufacturing steel slab of the above described composition as a raw material, hot rolling the steel slab, finish roiling at a temperature of about 870 deg C or higher, retaining the steel sheet in the range of temperature of about 730 deg to 780 deg C, cooling the steel at a cooling rate of about 30 deg C /sec or higher and coiling the resultant steel sheet at a temperature in the range of 450°C-520°C.
According to the invention there is provided a method of producing a hot rolled high strength ductile steel with low yield to tensile strength ratio to generate articles comprising making a steel slab of compositions in wt% consisting of carbon 0.08 to 0.18, silicon 0.30 to 0.70, manganese 1.00 to 1.70, phosphorus 0.005 to 0.02, sulphur 0.005 to 0.03, nitrogen less than 0.005, aluminium 0.02 to 0.06, chromium 0.05 to 0.15, vanadium 0.05 to 0.15, molybdenum 0.05 to 0.20 and the balance being iron and incidental impurities, the said slab being hot rolled to a steel article at finishing roll at a temperature 870°C - 900°C, cooling the article at a temperature in the range of 730°C - 780°C, retaining the article at this temperature for a while, cooling the article to 450°C - 520°C at a cooling rate of 30°C to 35°C/sec and coiling the article at the same temperature followed by air cooling to room temperature to result multiphase microstructure consisting of ferrite, bainite and martensite.
The invention will be better understood by the following description with reference to the accompanying drawings in which
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Figure 1 represents a stress - strain diagram prepared from stee! specimens of steei prepared according to the present invention and
Figure 2 represents a typical microstrueture consisting of ferrite, bainite and martensite phase, the specimen being etched with La Pera reagent.
Ten steeL slab compositions were prepared to conduct the different experiments for the present invention. The compositions consisting essentially m wt % of 0.08 to 0.18 wt% of C, 0.30 to 0.70 Si, 1.00 to 1.70 Mn, 0.005 to 0.03 P, 0.005 to 0.03 S, less than 0.005 Nr 0.02 to 0.06 Al, 0.05 to 0.15 Cr, 0.05 to 0.15 V, 0.05 to 0.20 Mo, the balance being substantially fe and incidental impurities. All the steel slabs were hot-rolled under various conditions to manufacture steel sheets each having a thickness of 3.00 mm. Mechanical properties and microstructural characteristics were determined for the hot-rolled steels. One typical chemistry and the respective tensile properties are listed in the following tables:
Table 1 . Chemical composition (wt%)
C Mn Si Cr V Mo P S Al N
0.168 1.47 0.426 0.11 0.05 0.17 0 .019 0 .012 0 .037 0.0048
Table 2. Tensile properties
YS (MPa) UTS (MPa) %EI Ratio of YS/UTS Hntshion Roll Temperature
312 668 26 0.47 870
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As is apparent from Table 2, the steel of the present invention has a tensile strength of 668 MPa, has a low yield to tensile ratio and good ductility. Presence of Cr and Mo suppress the formation of pearlite and hence promotes bainitic transformation, chromium being known as highest promoting hardenabifity. The steel produced is apt for oil or air hardening, the alloying element - Cr reduces critical cooling rate required for martensite formation, increases hardenability and thus improves the aptitude for heat treatment. Mo while present in steel in more than 0.05 wt% forms very fine precipitates, and retards the pearlite formation. Vanadium increases strength of ferrite by forming precipitates of vanadium carbide and through grain refinement.
Fig 1. shows the engineering stress-strain diagram of one of the invented steels produced according to the invention. This plot essentially shows continuous yielding, high rate of strain hardening and very good ductility. These are the features of conventional dual phase steels. These properties have been achieved in the steels produced by the invention due to the appropriate phase formation of ferrite, bainite and martensite. By test datas from stress-strain curve for various compositions of the steel it has been found that tensile strength in the range of 500 MPa to 700 MPa is possible. Total elongation and YS/UTS ratio being observed as 24-28% and 0.45 to 0.60 respectively.
Fig. 2 shows a typical microstructure of one of the composition according to the invented steel. Presence of a mixture of ferrite, bainite and martensite is the special feature of this microstructure. A small amount of martensite is sufficient to suppress the yield point phenomenon. In the microstructure shown ferrite
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phase is the matrix, bainite is shown as dark contrast and martensite is shown as white streaks. By standard metallurgical test methods grain sizes and grain counts are carried out and it is observed that the ferrite, bainite and martensite phase in volume % are in the ranges of 40-55%, 3045% and 5-15% respectively.
While comparing and studying the test results of the steeL specimens according to the present invention it has been strikingly observed that the low yie)d/tensik strength ratio is maintained with the phase distribution formed on heat treatment of the low alloy steel compositions developed by the present invention. The reason of high strength characteristics with ductility is attributed due to the fact of ferrite phase being strengthened by the precipitation of Vanadium carbide and solid solution hardening by the alloying constituents Si, Mn and Mo, beside the fact that the steel is basically strengthened due to presence of martensite and bainite.
The invention as disclosed herein and illustrated should not be read in a restrictive manner as various adaptations, modifications and changes are possible as encampused within the scope of the appended claims.
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1. A method of producing a hot roiled high strength ductile steel with low
yield to tensile strength ratio to generate articles comprising making a steel slab
of compositions in wt% consisting of carbon 0.08 to 0.18, silicon 0.30 to 0.70,
manganese 1.00 to 1.70, phosphorus 0.005 to 0.03, sulphur 0.005 to 0.03,
nitrogen less than 0.005, aluminium 0.02 to 0.06, chromium 0.05 to 0.15,
vanadium 0.05 to 0.15, molybdenum 0.05 to 0.20 and the balance being iron
and incidental impurities, the said slab being hot rolled to steel article at finishing
roll at a temperature 870°C - 900°C, cooling the article at a temperature in the
range of 730°C - 780°C, retaining the article at this temperature for a while,
cooling the article to 450°C - 520°C at a cooling rate of 30°C to 35°C/sec and
coiling the article at the same temperature followed by air cooling to room
temperature to result multiphase microstructure consisting of ferrite, bainite and
2. A method as claimed in claim 1 wherein the steel has elongation and yield
strength/ultimate tensile strength (UTS) ratio characteristic as 24-28% and 0.45
to 0.60 respectively.
3. A method as claimed in claim 1 wherein ferrite phase of the resultant steel
is strengthened by vanadium carbide precipitation and solution hardened by
addition of Silicon, Manganese and Molybdenum.
4. A method as claimed in the preceeding claims wherein ferrite, bainite and
martensite phases formed are in the ranges by volume % of 40-55%, 3045%
and 5-15% respectively.
5. A method as claimed in the preceeding claims wherein the steel produced
and solution hardened has UTS in the range of 500 MPa to 700 MPa.
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6. A method as claimed in the preceeding claims wherein coiling temperature
of the resultant article from the steel is maintained at 450° to 520°C and the
article is coiled without any difficulty at the same temperature range.
7. A method as claimed in claim 4 wherein the yield point phenomenon of
the resultant article produced from the steel is avoided due to presence of
limited amount of martensite in the resulted article.
8. A hot rolled high strength ductility with low yield/tensile strength ratio
steel produced according to method claim 1 having compositions in wt% 0.08 to
0.18 wt.% of C, 0.30 to 0.70 wt% Si, 1.00 to 1.70 Mn, 0.005 to 0.03 P, 0.005 to
0.Q3 S, less than .0.005 N, 0.02 to 0.06 Al, 0.05 to 0.15 Cr, 0.05 to 0.15 V, 0.05
to 0.20 Mo and the balance being substantially Fe and incidental impurities.
9. A method of producing hot rolled high strength steel as claimed in the
preceeding claims wherein the article resulted is coiled steel sheets or bar, rod or
other steel articles adapted to be coiled.
10. A method of producing a hot rolled high strength ductile steel with low
yield to tensile strength ratio to generate articles as described herein and
654-kol-2006-granted-reply to examination report.pdf
654-KOL-2006-REPLY TO EXAMINATION REPORT.pdf
|Indian Patent Application Number||654/KOL/2006|
|PG Journal Number||18/2009|
|Date of Filing||03-Jul-2006|
|Name of Patentee||TATA STEEL LIMITED|
|PCT International Classification Number||C22C 38/04|
|PCT International Application Number||N/A|
|PCT International Filing date|