Claims:We Claim:
1.Low carbon steel comprising low carbon (C<0.25%) alloyed with Mn 2.5 to 3.2% preferably 3 %Mnwhich is heat treatable including selected from annealing, normalizing, austempering and hardening and tempering heat treatments having microstructures including bainite, ferrite, martensite, and retained austenite with selectively variable tensile strength between 746 to 1298MPa ,ductility in the range of8 to 25.2% elongation.
2.The low carbon steel as claimed in claim 1 with 3%Mnand including selective microalloying elements Ti and Nb and having a composition and having microstructure bainiticferrite, martensite, and retained austenite based mixed microstructures.
C Si Mn Ti Al Nb Cr P S N
0.07 to 0.25 preferably 0.088 0.2 to 0.6 preferably 0.227 2.5 to 3.2 preferably 3.09 0.01 to 0.04
preferably 0.025 0.01 to 0.08 preferably 0.06 0.02 to 0.06
preferably 0.029 0 to 0.05 preferably 0.025 0.001 to 0.04
preferably 0.015 0.001 to 0.006 preferably 0.002 0.0040 to 0.0070 preferably 0.0046

3.The low carbon steel with Mn 2.5 to 3.2% preferably 3 %Mnas claimed in anyone of claims 1 or 2,which whensubject to a annealingheat treatmenttypically at 850 to 950 oCfor 2 min to 60 min preferably 5 min holding attain properties in the high strength rangewith tensile strength between 863 to 924 MPa, ductility in the range of 15.6 to 16% elongation and yield ratio between 0.55 to 0.59 and with microstructure includes a mixture offerrite, bainite and small amount of retained austenite;
4. The low carbon steel with Mn 2.5 to 3.2% preferably 3 %Mnas claimed in anyone of claims 1 to 3 which when subjected to normalizing heat treatment above Ac3 temperaturetypically at 850 to 950 oC held for 2 to 60 min, preferably 5 min, attain tensile strength varies between 899 to 958 MPa and ductility around 17 to 17.4%, with lower temperature giving better strength,yield ratio between 0.54 to 0.6 andwith a micro structure that includes ferrite, bainite with retained austenite and isolated regions of martensite/ austenite constituents;
5. The low carbon steel with Mn 2.5 to 3.2% preferably 3 %Mnas claimed in anyone of claims 1 to 4, which when subjected to austempering heat treatment with austenization in the intercritical temperaturerange between 700 to800 oC with 2 to 60 min, preferably 5 min holding followed by austempering at temperatures between 350 and 450 oC, attain properties that are consistently superior to annealed or normalized conditions of the steelincluding tensile strength (746 to 1081 MPa),excellent ductility (15.9 to 25.1 %E), yield ratio in the range 0.46 to 0.55 and with microstructure that consisted of a mixture of ferrite, bainite, retained austenite and martensite/austenite constituents.
6.The low carbon ~3% Mnsteel as claimed in anyone of claims 1 to 5, which whensubjected to the austenite holding temperature above AC3temperature at 850 to 900 oC and held for times between 2 to 60 min, preferably 5 min followed by austempering at 350 to 400 oC held for 5 to 60 min preferably 5 min time, have excellent tensile strength 844 to 984 MPa and ductility ( 16.7 to 19.8% E ) and Yield ratio attained between 0.54 to 0.7with a microstructure that consists of dense bainitic ferrite, with evenly distributed and bright white retained austenite and isolated zones of martensite / austenite constituent.
7.The low carbon ~3% Mnsteel as claimed in anyone of claims 1 to 6 which when subjected to hardening heat treatment by quenching the steel from an austenitization heat treatment in the inter critical range 750 to 800 oC,held for 5 to 60 min preferably 5 min time followed by water quenching to make the steel with ferrite and martensite dual phase structure which on further tempering at 150 oC held for 15 to 60 min preferably 30 min time gives tensile strength in the range (836 to 1125 MPa) and with a ductility of 14 to 20% elongation and yield ratio of 0.60 to 0.66 with a final microstructure consisting of ferrite and tempered martensite.
8.The low carbon ~3% Mnsteel as claimed in anyone of claims 1 to 7 which when subjected to hardening heat treatment by quenching the steel from an austenitization heat treatment above Ac3 temperature range typically at 850 and 900 oCheld for 5 to 60 min preferably 5 min timefollowed by water quenching to make fullymartensitemicrostructure which on further tempering at 150 oCheld for 15 to 60 min preferably 30 min time gives tensile strength in the range (1187 -1298 MPa) and with a ductility of 8 to 10.27% elongation and yield ratio of 0.78 to 0.87 with a final microstructure consisting of ferrite and tempered martensite which attain inferior properties when the hardening temperature was above Ac3 compared to other conditions.
9. The low carbon steel as claimed in anyone of claims 1 to 8 which when subjected to a tempering heat treatment at 150 oC moderately lowers the strength and marginally improves the ductility, with good strength and ductility combination on water quenched from 750 oC, with steel tempered at 150 oC for 30 min in steel hardened from inter critical range at 750 oCattaining moderately lower strength but with significant ductility wherein the yield ratio varied from 0.6 to 0.8 post tempering and having microstructure of the steels including lath martensitic microstructure in the as-quenched steels and tempered martensitic microstructure on tempering and wherein In both the cases with presence of residual austenite in the matrix and wherein with the inter critically treated steel attaining ferrite additionally.

10.A process for manufacture of Low carbon 3%Mn steel selectively heat treated for selective variable microstructure combination, thatincludesphases of bainitic, ferritic, martensitic, tempered martensitic, retained austenite, with versatile tensile strength between 746 to 1298MPaand ductility in the range of 8 to 25.2 % elongation.
subjecting the low carbon (C<0.25%) steel alloyed with Mn 2.5 to 3.2 % preferably 3 % selectively to annealing, normalizing, austempering and hardening and tempering comprising to attain variable properties involving anyone or more of:
C Si Mn Ti Al Nb Cr P S N
0.07 to 0.25 preferably 0.088 0.2 to 0.6 preferably 0.227 2.5 to 3.2 preferably 3.09 0.01 to 0.04
preferably 0.025 0.01 to 0.08 preferably 0.06 0.02 to 0.06
preferably 0.029 0 to 0.05 preferably 0.025 0.001 to 0.04
preferably 0.015 0.001 to 0.006 preferably 0.002 0.0040 to 0.0070 preferably 0.0046

a) annealing heat treatment between 850 to 950 oC for 2 min to60 min preferably 5 min holding to attain properties in the high strength range including tensile strength between 863 to 924 MPa , ductility in the range of 15.6 to 16.1% elongation and yield ratio between 0.55 to 0.59 and with microstructure including ferrite, acicular ferrite, bainite and small amount of retained austenite;
b) normalizing heat treatment between 850 to 950 oC held for2 min. to 60 min preferably 5 min, to attain tensile strength varies between 899 to 958 MPa and ductility around 17% elongation, with lower temperature giving better strength,yield ratio between 0.54 to 0.60 andmicro structure including ferrite, bainite with retained austenite and isolated regions of martensite/ austenite constituents;
c) austempering heat treatment with austenization in the inter critical temperature range typically between 700 to 800 oC with 2 min. to 60 min preferably 5 min holding, followed by austempering at temperatures between 350 and 450 oCwith 2 min. to 60 min preferably 5 min, to attain properties that are consistently superior to annealed or normalized conditions of the steel including tensile strength (746 to 1081 MPa) , ductility (15.9 to 25.2 %E), yield ratio in the range 0.46 to 0.55with a mixed microstructure consisting of ferrite, bainite, retained austenite , martensite/austenite constitutent.
d) austenite holding temperature exceeding AC3temperature typically at 850 and 900 oCwith 2 min. to 60 min, preferably 5 min holding followed by austemperingat range between 350 and 400 oCwith 2 min. to 60 min, preferably 5 min holdinghaving excellent ductility having microstructure of dense bainitic ferrite, evenly distributed bright white retained austenite and isolated zones of martensite / austenite constituent
e) hardening heat treatment by water quenching the steel from an austenitization heat treatment in the inter critical temperature range preferably at 750 or 800 oC to have a ferrite and martensite microstructure which on a further tempering heat treatment at typically 150 C for a hold time 15 to 60 min preferably at 30 min holding forms a combination of ferrite and tempered martensitic microstructure with (836 to 1125) MPa strength and (14.4 to 20.5 )% elongation and yield ratio 0.60 to 0.66.
f) hardening heat treatment by water quenching the steel from an austenitization heat treatment above Ac3 temperature range preferably at 850 or 900 oCfor a hold time between 2 to 60 min preferably 5 min, to form a fully martensite microstructure which on further tempering heat treatment at typically 150 C for a hold time 15 to 60 min preferably at 30 min forms a fully tempered martensitic microstructure and some residual austenite with (1187 to 1298) MPa strength and (8.19 to 10.27 )% elongation and yield ratio 0.78 to 0.87.
g) While the hardened steel in (e) & (f) conditions can be tempered at various tempering temperature between 50 and 650 oC, typical tempering at 150 oCensures high strength retention with reasonable ductility.

11.The process as claimed in claim 10 wherein the said low carbon steel used have solid solution elements of 3 % Mn and including selective microalloying elements Ti of 0.01 to 0.04% and Nbof 0.02 to 0.06% and has a typical preferred composition as below
C Si Mn Ti Al Nb Cr P S N
0.07 to 0.25 preferably 0.088 0.2 to 0.6 preferably 0.227 2.5 to 3.2 preferably 3.09 0.01 to 0.04
preferably 0.025 0.01 to 0.08 preferably 0.06 0.02 to 0.06
preferably 0.029 0 to 0.05 preferably 0.025 0.001 to 0.04
preferably 0.015 0.001 to 0.006 preferably 0.002 0.0040 to 0.0070 preferably 0.0046

12.The process as claimed in anyone of claims 10 or 11 wherein the low carbon steel subjected to selective heat treatment is obtained involving primary steel making by basic oxygen furnace followed by secondary steel processing Ladle furnace followed by vacuum degassing to achieve the desired steel composition followed by hot and cold rolling to desired thickness and subject to the thermal processing conditions as per the heat treatment conditions evolved.

Dated this the 17th day of September, 2021
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199
, Description:FORM 2
THE PATENT ACT 1970
(39 OF 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

1 TITLE OF THE INVENTION :
HIGH STRENGTH AND ULTRA HIGH STRENGTH LOW-CARBON-MEDIUM-MANGANESE BAINITIC STEEL AND A PROCESS FOR ITS MANUFACTURING.


2 APPLICANT (S)

Name : JSW STEEL LIMITED.

Nationality : An Indian Company incorporated under the Companies Act, 1956.

Address : JSW CENTRE,
BANDRA KURLA COMPLEX,
BANDRA(EAST),
MUMBAI-400051,
MAHARASHTRA,INDIA.



3 PREAMBLE TO THE DESCRIPTION

COMPLETE

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
The invention pertains to development of a low carbon steel (<0.25%C) alloyed with medium Mn content (approx.3%) to ensure solid solution strengthening and optionally alloyed with microalloying elements to refine the grain size, ensures the formation of a fine grained steel with bainitic phase as a major constituent, that is usually produced in austempered condition. Suitable alloying with simple heat treatment such as annealing and normalizing itself produces complex micro structures that are obtained in the austempered condition. Attractive range of properties could be derived by annealing and normalizing the steel to achieve microstructure and properties that could be obtained after austempering heat treatments. The steel showed further improved properties and associated microstructure when austempering was performed on the steel. The steel responds to the hardening and tempering heat treatment with attractive properties in the as-hardened condition, when the austenitization temperature was maintained in the inter critical range. The microstructure in the hardened condition shows lath martensite in ferrite. Tempering resulted in a loss of strength but improved the ductility. The properties in the conventional hardened conditions, austenitinzing above Ac3 and tempering did not yield superior properties. Unlike the requirement of a critical processing condition for the development of complex microstructures to meet the first generation high strength properties, the alloying condition is such that the steel is fine grained and at most processing condition it exhibits a fully bainitic matrix including annealing and normalizing conditions.

BACKGROUND OF THE INVENTION
Medium Mn steel, with Mn content between 3 to 10% Mn are being evolved as third generation advanced high strength steels (AHSS) with the development of Twin induced plasticity type mechanism. However, the effect of high Mn alloying on promoting bainitic structure has not been well exploited. The present research does not have any significant additions of Si or Al, along with high Mn to suppress the formation of larger quantities of retained austenite in a matrix of bainite and or martensite, in spite of which, excellent bainitic microstructure is developed, unlike TRIP aided bainitic ferrite steels of equivalent properties.
There is hardly any investigation on using the lower end of medium Mn content steels that promotes bainite without any complex processing to develop the microstructure. Simple processing such as annealing or normalising itself promote the formation of bainite that has attractive strength and ductility. The present investigation is aimed at developing a range of bainitic microstructures that gives attractive range of strength and ductility, with simple alloying of the steel with 3% Mn, along with about 0.03% each of the microalloying elements Ti and Nb, in a steel manufactured by Al killing.
The available knowledge in the 3% Mn containing steel, is limited. It is the lowest level Mn mentioned in medium manganese bainitic steels, but the actual experimental data is limited. The following prior art brings out 3%Mn containing steels with some form of bainitic structure.
State of Prior Art
Prior art-1: A series of study was reported by Grajcaret. al.(Archieves of Mat.Sci. and Eng. Vol. 49, Issue-1, pp.5-14, 2011,J. of Achievements in Mater.and Manufac. Eng. Vol.54, Issue-2, pp.168-177, 2012,Metals,Vol.8, Issue- 929, pp. 1-19,2018) on the development of medium manganese steels with Mn in the range of 3 to 5%.; Al content between 1.6 and 1.7%, along with expensive alloying element; Mo of 0.22 to 0.23%. Unlike the steel in prior art, the present steel is lean alloyed with just 3% Mn and micro alloyed with 0.03% Ti and Nb and the steel is Al killed. The present development does not have such expensive alloying elements or the necessity to have high Al or Si content in the steel. Whereas the prior art talks about Vacuum-melted in argon atmosphere, cast was hot forged, rolled in four pass to 9 mm, TMP to 4.5 mm at 850oC followed by controlled cooling, the present invention was made in simple route at industrial scale. The steel was made by Basic oxygen furnace followed by secondary ladle steel making followed by industrial scale hot rolling, followed by heat treatments. The steel in the present invention is flexible enough to be processing under annealed, air cooled, Austempered and harden-tempered condition, which is not the case with the prior art. The properties achieved by the steel in the present invention shows a tensile strength of 960 MPa and elongation of 15.5%. At some of the other heat treatment conditions tensile strength above 1000MPa with a maximum of elongation 25% is achieved in the present invention.
Prior art-2: Nina Fonstein(Technical contribution to the 50th Rolling Seminar – Processes, Rolled and Coated Products, November, 18th to 21st, 2013, OuroPreto, MG, Brazil) reported medium Mn steels with 4-10% Mn, also containing, Si or Al and microalloying elements such as Nb. After inter critical annealing these steels were achieved different levels of strengths (TS from 1000-1800MPa) and maximum elongation of 19%. However, the present invention contains lower Mn (2.5-3.5), lower carbon (<0.25) along with grain refining alloying elements Nb and Ti still achieve strength level > 1000MP and high elongation. The present invented steel manufacture, the composition design philosophy and the versatile range of heat treatment is significantly different from that of the prior art.
Prior art-3: Indian patent application No. IN201631009627A, 2017, A Novel Method of Stabilization of Austenite in High Strength Steels,Pampa Ghosh, reported a novel method of stabilization of austenite in high strength steels. The Steel ingot of the composition comprising, in weight per cent 0.1-0.2 C, 4 - 5 Mn, 0.3 – 1.0 Si, 0.3-0.5 Al and rest being substantially iron and incidental impurity first cast in the laboratory. The reported cast ingot in the prior art is homogenized at 1250 °C for 4 hours followed by forging into 30 mm X 30 mm cross-section billet in the temperature range of 1200 °C to 900 °C, the forged ingot is subsequently air cooled to room temperature. The forged ingot is austenitized at 1200 °C for 2 hours prior to laboratory hot rolling to 6-7 mm. This material is further heated to Ac3 + 50 °C and quench either in water or oil or in air. The material is further heated to the Intercritical temperature and a deformation of 30-50% is given continuously. The starting temperature of deformation is 50% austenite and 50% ferrite. After inter- critical deformation, the material is cooled to room temperature with a cooling rate varying from 1 °C/sec to 60 °C/sec. This process results in an austenite fraction of 20-40% in the final structure. Unlike the above prior art, the present invention, has a lower Mn content (2.5-3.5%) and carbon lower than 0.25%, but has grain refining addition of about 0.03% each of Nb and Ti for grain refinement. Thus, the alloy design philosophy is different. The present invention is made at industrial scale unlike lab scale and the processing is totally different. The prior art talks about inter critical deformation and achieves 30 to 50% austenite. In the present invention, there is no deformation and only a wide variety of heat treatment carried out. Unlike the prior art, where 30 to 50% austenite is there in the matrix, the present invention has predominantly bainitic ferrite with < 10 % retained austenite. The properties are also different between the present invention compared to the prior art.

Prior art-4:US patent No. 3,783,040, Low Carbon High Strength Steel, John B. Balance, Bedford Heights, Stephen J. Matas, and Konrad J. A.,reported an invention of steel designed to give bainitic microstructure on direct air cooling from fully austenitizing treatment above AC3 1450F Hot rolled finish temperature, to give an yield strength greater than 95 ksi (>655 MPa) with > 12 ftLb impact energy. Whereas the steel in this prior art involves continuous cooling, the present invention undergoes a series of heat treatment post rolling that involves heating in the intercritical temperature range and above AC3temperature followed by 180 F/min that results in a much wider range of mechanical properties. Hence, the processing and properties range is totally different in the present invention. The composition range of the invented steel is leaner than the composition suggested in prior art, where the range reported is <0.25% C- <1% Mo or B or both with 5 to 1000 ppm B - <3% Mn, 0.5 to >1 %Cr, 0.8 to 2.5% Ni,0.3 to 0.5%Cu, <0.8%Si, 0.2% Nb. The invented steel has Mn content around 3% and only microalloyed with Nb and Ti. Hence, composition, processing and properties are significantly different although the microstructure claimed is bainitic structure in the prior art.
Prior art-5: Chinese patent No. CN110408861A, 2019,Cold-rolling medium Mn steel with low Mn content and high product of strength and ductility, and preparation method for same, Yang Feng, Han Yun, Jiang Yinghua, Liu Huasai, XieChunqian, QiuMusheng, Pan Limei , BaiXue, TengHuaxiang, Chen Bin, Cao Jie, Zhang Jun, Zhu Guosen, reported a cold rolled steel with (0.15 to 0.6)%C- (3 to 6) % Mn - (1 to 3)% Al-2%Si – < 5% Cr-<2%Ni is processed by continuous casting followed by multi-pass hot rolling to a plate and air cooled. This is followed by roll bending and soft annealing, pickling followed by cold rolling and annealing yield strengths greater than 600 MPa and elongation is as high as 45 to 70% leading to the product of strength and ductility > 67 GPa%. Unlike this patent with high level of alloying elements the present steel deals with lowest Mn content in the medium Mn steel of 3% Mn. The types of heat treatment in the present invention deals with cold rolled steel subjected to intercritical and full austenitization heat treatments followed by salt bath heat treatments that have a variety of bainite containing microstructures. Hence, the present investigation is different in terms of alloy design philosophy, composition, microstructure and properties to obtain high bainite content. The prior art, deals with austenite and martensitepresent in the steel, that yield high strength and ductility.
Prior art-6: Chinese patentNo.CN111041373A, 2020, Medium Mn steel for chain wheel and preparation method and application thereof, Zhu Xiuguang, Liang Xiaokai, XuWeiguo, Kang Shaoguang, Zhang Yanbin, Wang Lijun, Zong Changjiang, NieJunxian, Guo Yunpeng, Yin Xueqin, Liu Weidong, reported a low carbon (0.08-0.40) % C, (0.10-0.40) % Si- < 0.01% P, <0.01% S, (2.50-4.50) %Mn, < 3.0% Cu, < 5.0% Ni and (0.30-0.80)% Mo, < 5.0% Cr, (0.1-1.0%) V steel. The steel was a centrifugal electroslag cast component. Unlike a much higher range of other alloying elements in this prior art, the invented steel is wrought steel and has low carbon along with 3%Mn and microalloyed with Nb and Ti. Hence, the composition and processing is different although the steel has 3%Mn as common alloying element. While the invented steel is on maximizing bainitic steel, the prior art is about a mixture of bainite with martensite base unlike bainitic microstructure with ferritic matrix in the present invention.
Thus, the prior art compiles steel where 3%Mn have been claimed. The prior art either have additional alloying elements that is significantly different from the steel studied in terms of additional alloying elements, different processing and different range of properties compared to the invented steel.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to the development of high strength steel having a versatile range of mechanical properties in a low carbon (<0.25%C) and about 3% Mn containing steel by various heat treatments that generates bainitic or martensitic or tempered martensitic microstructure in the matrix and a process to produce the same.
A further object of the present invention is directed to said high strength steel and a process to produce the same wherein the heat treatment involves annealing, normalising, austempering, hardening and tempering the steel. Simple heat treatments such as annealing and normalizing or continuous cooling treatment itself produce a fully bainitic steel with properties in the range of advanced high strength steels.
A still further object of the present invention is directed to said high strength steel and a process to produce the same wherein attractive range of properties could be derived by annealing and normalizing the steel to achieve microstructure and properties that could be obtained after austempering heat treatments.
A still further object of the present invention is directed to said high strength steel and a process to produce the same wherein the steel showed further improved properties and associated microstructure when austempering was performed on the steel.
A still further object of the present invention is directed to said high strength steel and a process to produce the same wherein the steel responds to the hardening and tempering heat treatment with attractive properties in the as-hardened condition, when the austenitization temperature was maintained in the inter critical range.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to the development of a low carbon steel comprising low carbon (C<0.25%) alloyed with Mn2.5 to 3.2 % preferably 3 % and microalloyed with 0.01 to 0.04%Ti and 0.02 to 0.06% Nb, which is heat treatable including selected from annealing, normalizing, austempering and hardening and tempering heat treatments and having microstructures including bainite, ferrite, martensite, and retained austenite with selectively variable tensile strength between 748 to 1298MPa ,ductility in the range of 8.2 to 25.2 % elongation.
A further aspect of the present invention is directed to said low carbon steel with about 3% Mn content and including selective microalloying elements Ti and Nb and having a composition and having microstructure bainite, ferrite, martensite, and retained austenite based mixed microstructure.

C Si Mn Ti Al Nb Cr P S N
0.07 to 0.25 preferably 0.088 0.2 to 0.6 preferably 0.227 2.5 to 3.2 preferably 3.09 0.01 to 0.04
preferably 0.025 0.01 to 0.08 preferably 0.06 0.02 to 0.06
preferably 0.029 0 to 0.05 preferably 0.025 0.001 to 0.04
preferably 0.015 0.001 to 0.006 preferably 0.002 0.0040 to 0.0070 preferably 0.0046

A still further aspect of the present invention is directed to said low carbon steel with 2.5 to 3.2% Mn preferably 3%Mn content which when subjected to annealing heat treatment typically at 850 to 950 oC for 2 min to 60 min preferably 5 min holding, attain properties in the high strength range including tensile strength between 863 to 924 MPa, ductility in the range of 15.6 to 16% elongation and yield ratio between 0.55 to 0.59 and with microstructure including bainitic ferrite in the matrix along with ferrite with small amount of retained austenite.
A still further aspect of the present invention is directed to said low carbon steel with 2.5 to 3.2% Mn preferably 3%Mn content which when subjected to normalizing heat treatment above Ac3 temperature typically at 850 to 950 oC held for 2 min to 60 min preferably 5 min, attain tensile strength varies between 899 to 958 MPa and ductility around 17 to 17.4%, with lower temperature giving better strength, yield ratio between 0.54 to 0.60 and microstructure including ferrite, bainite with retained austenite and isolated regions of Martensite/ austenite constituents.
Yet another aspect of the present invention is directed to said low carbon steel with 2.5 to 3.2% Mn preferably 3%Mn content, which when subjected to austempering heat treatment with austenization in the intercritical temperature range between700 to900 oC with 2 min to 60 min and preferably 5 min holding followed by austempering at temperatures between 350 and 450 oC, attain properties that are consistently superior to annealed or normalized conditions of the steel including tensile strength (746 to 1081 MPa) , ductility (15.9 to 25.1 %E), yield ratio in the range 0.46 to 0.55 in the inter critically treated condition exhibiting a microstructure of dense bainitic ferrite, evenly distributed bright white retained austenite and isolated zones of martensite/austenite constituents.
A further aspect of the present invention is directed to said low carbon steel with ~3%Mnwhich when subjected to the austenite holding temperature exceeding AC3typically 850 and 900 oC held for 2 min to 60 min preferably 5 min, followed by austempering at 350 and 400 oC held for 2 min to 60 min preferably 5 min, attain a tensile strength (844 to 984) MPa with (16.4 to 19.8) % elongation and yield ratio between 0.54 to 0.60 with a microstructure showing bainite with higher retained austenite content along with ferrite, martensite/austenite constituents and wherein the properties at 350 oC austempering and 400 oC austempering with prior intercritical austenitization at 700 to 750 oC attain extremely good strength and ductility combination.
A still further aspect of the present invention is directed to said low carbon steel with ~3%Mn which when subjected to hardening heat treatment by quenching the steel from an austenitization heat treatment above Ac3 temperature typically at 850 and 900oC, followed by water quenching to form low carbon martensite followed by a low temperature tempering treatment typically at 150 oC held for 15min to 60 min preferably 30 min gives a strength of (1187 to 1298) MPa and ductility range (8.19 to 10.27) %elongation and yield ratio of 0.78 to 0.87, while the microstructure consisted of fully tempered martensite.
A still further aspect of the present invention is directed to said low carbon steel with ~3%Mn which when subjected to hardening heat treatment by quenching the steel from an intercritical austenitization heat treatment typically at 750 and 800oC, followed by water quenching to form ferrite and low carbon martensite which on low temperature tempering treatment, typically at 150 oC held for 15 min to 60 min preferably 30 min gives a strength of (836 to 1125) MPa and ductility range (14.4 to 20.5) %elongation and yield ratio of 0.60 to 0.66, while the microstructure consisted of fully ferrite and tempered martensite.

A still further aspect of the present invention is directed to a process for manufacture of Low carbon selectively heat treated steel for selectively variable microstructure including bainitic microstructure, tensile strength between 746, to 1081 MPa, ductility in the range of 15.9 to 25.2% elongation by subjecting the low carbon (C<0.25%) steel alloyed with Mn 2.5 to 3 % preferably 3 %Mn selectively to annealing, normalizing, austempering and hardening and tempering comprising to attain variable properties involving anyone or more of:
C Si Mn Ti Al Nb Cr P S N
..0.07 to 0.25 preferably 0.088 0.2 to 0.6 preferably 0.227 2.5 to 3.2 preferably 3.09 0.01 to 0.04
preferably 0.025 0.01 to 0.08 preferably 0.06 0.02 to 0.06
preferably 0.029 0 to 0.05 preferably 0.025 0.001 to 0.04
preferably 0.015 0.001 to 0.006 preferably 0.002 0.0040 to 0.0070 preferably 0.0046

a) annealing heat treatment between 850 to 950 oC for 2 min to 60 min preferably 5 min holding to attain properties in the high strength range including tensile strength between 863 to 924 MPa, ductility in the range of 15.6 to 16.1% elongation and yield ratio between 0.55 to 0.59 and with microstructure including polygonal ferrite, acicular ferrite and bainite with small amount of retained austenite;
b) normalizing heat treatment between 850 to 950 oC for 2 min to 60 min preferably 5 min, to attain tensile strength varies between 899 to 958 MPa and ductility around 17%, with lower temperature giving better strength, yield ratio between 0.54 to 0.6 and microstructure including ferrite, bainite with retained austenite and isolated regions of martensite/ austenite constituents;
c) austempering heat treatment with austenization in the inter critical range typically at 700,750 and 800 oC, for 2 min to 60 min and preferably 5 min holding time followed by austempering at temperatures between 350 and 450 oC holding for 2 min to 60 min and preferably 5 min, to attain properties that are consistently superior to annealed or normalized conditions of the steel including tensile strength (746 to 1081 MPa) , ductility (15.9 to 25.2 %E), yield ratio in the range 0.46 to 0.96 in the inter critically treated condition.
d) austempering heat treatment with austenization in the temperature above Ac3 at 850 and 900 oC for 2 min to 60 min and preferably 5 min hold time followed by austempering at temperatures between 350 and 450 oC holding for 2 min to 60 min and preferably 5 min, to attain properties that are consistently superior to annealed or normalized conditions of the steel including tensile strength (746 to 1081 MPa) , ductility (15.9 to 25.2 %E), yield ratio in the range 0.46 to 0.55.
e) hardening heat treatment by water quenching the steel from an austenitization heat treatment in the inter critical temperature range preferably at 750 or 800 oC to have a ferrite and maretnsite microstructure which on a further tempering heat treatment at typically 150 C for a hold time 15 to 60 min preferably at 30 min forms a ferrite and tempered martensitic microstructure with (836 to 1125) MPa strength and (14.4 to 20.5 )% elongation and yield ratio 0.60 to 0.66.
f) hardening heat treatment by water quenching the steel from an austenitization heat treatment above Ac3temperature range preferably at 850 or 900 oCfor a hold time between 2 to 60 min preferably 5 min, to form a fully maretnsite microstructure which on further tempering heat treatment at typically 150 C for a hold time 15 to 60 min preferably at 30 min forms a ferrite and tempered martensitic microstructure and some residual austenite with (1187 to 1298) MPa strength and (8.19 to 10.27 )% elongation and yield ratio 0.78 to 0.87 .

Another aspect of the present invention is directed to said process wherein said low carbon steel with about 3%Mn used and selective alloying with micro alloying element Ti between 0.01 and 0.04 % and Nb between 0.02 and 0.06 and preferably 0.029 % subjected to selective heat treatments described (a) to (h) involve the manufacture of the steel involving primary steel making by basic oxygen furnace followed by secondary steel processing Ladle furnace followed by vacuum degassing to achieve the desired steel composition followed by hot and cold rolling to desired thickness.
The above and other objects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non limiting drawings and exemplary embodiments.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1: Equilibrium phase diagram of Fe-3%Mn steel computed by Thermo-Calc along with incorporated T0 line.
Fig.2 : (a) The TTT diagram of the steel studied using MUCG 83 (b) The T0’ temperature of the steel studied.
Fig. 3: Various heat treatment conditions of the steels evaluated (a) Annealing (b) normalizing (c) austempering.
Fig. 4: Microstructure of samples in the fully annealed condition.
Fig.5: XRD studies on samples in the annealed condition shows ferrite as the major phase.
Fig.6: Microstructure of samples in the fully Normalized condition.
Fig. 7: Bainitic structure developed by austensitising at different conditions and austempered in a salt bath at 350 oC.
Fig.8: Bainitic structure developed by austensitising at different condition and austempered at 400 oC.
Fig. 9: Bainitic structure developed by austensitising at different condition and austempered at 450 oC.
Fig. 10: XRD of the selected samples in the salt bath austempered condition.
Fig.11: The mechanical property of the steels as a function of austempering temperature and austenization temperature.
Fig. 12: Structure developed by austensitising at different condition for 5 min followed by WQ.
Fig. 13: Structure developed by austensitising at different condition for 5 min followed by WQ and tempered at 150oC for 30 min.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS
The present invention is directed to the development of high strength steel having a versatile range of mechanical properties in a low carbon (<0.25%C) and about 3% Mn containing steel by various heat treatments that generates bainitic or martensitic or tempered martensitic microstructure in the matrix and a process to produce the same.
The steel composition used in the present invention is melted in Applicant’s (JSW steel Ltd.), Vijayanagar unit, using primary steel making by Basic oxygen furnace followed by secondary steel processing Ladle furnace followed by vacuum degassing to achieve the desired steel composition. The steel was hot rolled to 3 mm hot band. The steel is further cold rolled to 2 mm thickness.
The steel has low carbon content which ensures good weldability and formability in the steel. The steel has low silicon content. The steel has a medium Mn content of 3%, which provides solid solution strength to the steel. The Mn further shifts the C-curve to the right associated with higher alloying elements, which ensures development of fast cooled microstructures at lower cooling rates. The Mn content promotes bainitic microstructures at normal cooling rates. The steel has a micro alloying addition of Nb of 0.03% and Ti of 0.025%, which increases the no recrystallization temperature TnR. This pancakes the grains during hot rolling and promotes the formation of finer grains in the steel that increases the strength by Hall-Petch relationship and improves toughness. Ti in the steel promotes formation of cuboidal TiN precipitates during solidification which refines aides the grain refinement. The steel is deoxidized with Al and has a residual level of 0.06%. The residual Al in the steel promotes AlN which is again a grain refiner. The P level is 0.015% to ensure that embrittlement associated with it is minimized. The S level is maintained at 0.002% which minimizes sulphide inclusions that affect the toughness. From the composition, the estimated transformation temperatures are AC1= 645oC; AC3 = 791 oC; Bs= 522 oC; Ms = 380 oC.
Table 1 Chemical analysis (in wt.% ) of low carbon 3% Mnalloyed steel
C Si Mn Ti Al Nb Cr P S N
0.07 to 0.25 preferably 0.088 0.2 to 0.6 preferably 0.227 2.5 to 3.2 preferably 3.09 0.01 to 0.04
preferably 0.025 0.01 to 0.08 preferably 0.06 0.02 to 0.06
preferably 0.029 0 to 0.05 preferably 0.025 0.001 to 0.04
preferably 0.015 0.001 to 0.006 preferably 0.002 0.0040 to 0.0070 preferably 0.0046

The phase diagram and T0 temperature of the steel computed through Thermocalc software is given in Fig.1 (a) & (b) respectively.
The TTT- diagram for the steel was developed using Cambridge University software MUCG 83. The results are shown in Fig.2(a). The diagram shows that the ferrite transformation is delayed significantly and the C-curve is shifted to the right while the bainitic transformation is shifted to right but has a prominent projection. Even slow cooling tends to directly pass through the bainitic regime. Hence, annealing and normalizing both is expected to show bainite. The T0’ curve for the steel is given by the Fig. 2(b). The bainitic transformation is limited by the composition at the T0’ temperatures till which bainitic transformation proceeds.
The steel developed was subjected to different processing conditions in the laboratory scale to develop a wide range of mechanical properties. The processing conditions developed are annealed, normalized, austempered and hardened-tempered. A variation in austenitizing condition was also carried out. The austempering was carried out in a laboratory salt bath. The process conditions are shown in Figs. 3(a) to (d).

Properties of the steel in furnace Annealed and Normalized conditions
The steel strip extracted from the cold rolled steel, was subjected to annealing heat treatment cycle as per Fig.3(a).The strips were reheated to austenitizing temperatures of 850, 900 and 950oC, soaked for 5 min followed by slow cooling in the furnace to achieve the annealed condition. The microstructures of the steel were examined in the annealed condition in optical microscope and scanning electron microscope, which shows a bainitic microstructure as shown in Fig.4.The high magnification micrograph shows bainitic ferrite with small amount of retained austenite, which is less than 5% and hence XRD could not clearly decipher as shown in Fig. 5.
Another set of steel strips were extracted from the cold rolled strip and was subjected to austenitizing at temperatures 850, 900 and 950 oC, held for 5 min, followed by cooling the sample in air as per cycle in Fig. 3(b) to get normalized condition. The microstructure of the samples, thus normalized, is shown in Fig. 6, where bainitic ferrite structure is observed at all austenitising temperature in the normalized condition. Bright white dispersion along with the bainitic ferrite seen in the high magnification microstructure is the retained austenite, the content of which is less than 5% .
The mechanical properties of the steel in the annealed and normalized conditions are shown in Table 2. The mechanical properties of the steel in the annealed condition show high strength range (856 to 928 MPa) with reasonably good ductility (%E= 15.4 to 16.1) and yield ratio of 0.56 to 0.59 (Table 2). The mechanical properties studied as a function of increasing holding time during annealing showed a marginal variation in strength and ductility. Higher holding time shows an improvement in yield strength and tensile strength especially after holding for 60 min. The effect of austenitizing temperature (850, 900 and 950 oC) during annealing, was studied. It is seen that the strength, in general, increases moderately with increasing strength and ductility simultaneously. The microstructure remains completely bainitic in the annealed condition itself with small amount of retained austenite. The high Mn content in the steel shifts the continuous cooling transformation (CCT) curve to the right associated with higher alloying element which enables formation of displacive transformation products even with the slowest cooling rate in annealing. As the variation is not significant in terms of holding time or temperature, the lowest holding time of 5 min is adequate and an austenitizing temperature of 850oC is sufficient.
The mechanical properties of the steel austenitized at varying austenitizing temperature in the normalized condition, is shown in Table 2. It is observed that with increasing austenitizing temperature, there is a decrease in strength while the ductility remains constant in normalized steel. The yield ratio in normalized steel varies between 0.54 to 0.59. Compared to the properties in the annealed condition, the properties in the normalized condition show both strength and ductility are enhanced. The microstructure in the normalized condition shows bainitic structure with small amount of retained austenite and martensite/retained austenite product. During normalizing, austenitizing at a lower temperature is preferable for highest strength and ductility.
The properties of the 3% Mn steel in both the annealed and normalized conditions show high strength range with good ductility with a full bainitic matrix. It is important to state that the properties are as good as achieved with steels such as a dual phase steel or a TRIP steel. Thus, the high Mn steel with simplest processing route gives high strength properties equivalent to higher end of the first-generation advanced high strength steels.
The laboratory scale evaluation can be correlated with the industrial operation. It is possible to implement the cycle in the actual production. The cold rolled steels can be directly subjected to batch annealing to achieve the properties in the annealed condition. Even the lowest cooling rate gives bainitic structure with good strength and ductility. The normalizing condition can be simulated by heating the cold rolled coil to desired temperature cooling to below Ac3 and subjected to coiling at the lowest possible temperature. The cycle can be implemented in the hot rolled steel by moderate cooling in the ROT simulating the normalizing condition.

Table 2 Mechanical properties of the samples in the annealed and normalized conditions
Sample Condition Yield
Strength
(0.2%)
MPa Ultimate
tensile
Strength, MPa %E
(50
GL) Yield
Ratio TS*E
( MPa %) XRD, Ret. Aust.%
Annealed condition
Austenitized at 850oC/held for 5 min /
Furnace cooled ( annealed) 492 891 15.9 0.55
14167
<4
Austenitized at 850oC/held for 30 min /
Furnace cooled ( annealed) 529 894 15.8 0.59 14125
Austenitized at 850oC/held for 60 min /
Furnace cooled 527 917 15.6 0.57 14305

Austenitized at 900oC/held for 5min /
Furnace cooled (annealed) 524 924 16.1 0.57
14876
<4
Austenitized at 950oC/held for 5min /
Furnace cooled ( annealed) 512 863 16.1 0.59
13894

Normalized condition from above Ac3 temperature
Austenitized at 850oC/held for 5min /
Air cooled ( normalized/ continuously cooled) 516 958 17.3 0.54 16573
Austenitized at 900oC/held for 5min /
Air cooled ( normalized/ continuously cooled) 530 927 17.4 0.57 16130
Austenitized at 950oC/held for 5min /
Air cooled ( normalized/ continuously cooled) 545 899 17 0.60 15283

Properties of the steel in the austempered condition
One portion of the base steel was subjected to austempering cycle using a salt bath heat treatment process as shown in Fig.3(c). The study was carried out by subjecting the various samples extracted from the cold rolled samples, as per the treatments shown. The steel strips were subjected to initial austenitizing treatment at various temperatures (700, 750, 800, 850, 900 oC) and held for 5 min. This was followed by quenching each of the steel sample in a salt bath held at three different austempering temperatures (350, 400 and 450 oC) for 5 min followed by air cooling to room temperature. The austenitizing temperature of 700 and 750 oC corresponds to a regime where it is within intercritical temperature range. The 800 oC austenitizing temperature, is a composition almost on AC3temperature. Two other temperatures of 850 and 900oC is well above AC3 temperature. Thus, a series of microstructure and mechanical properties was obtained as in Fig. 7 to Fig.9 at the three different austempering conditions. The microstructure of the steels in all the tested condition showed bainite in a ferrite matrix, with minor constituents of martensite and retained austenite. The details are shown in Table 3. Typical samples were analysed for phase constitution by XRD as in Fig. 10. The microstructure shows the presence of significant fraction of retained austenite which was estimated between 4 and 10%.
The mechanical properties of the steels in austempered condition is shown in Table 4. Austempering after lower inter critical austenitization temperature at 750 oC gave 1016 MPa with 25 % elongation. The values are still attractive in all other austenization condition in the inter critical range. Austemperingtemperature for properties depends on the austenitization temperature. The properties at 800oC austeniuzation and 450oC austempering gives similar properties as that of 700 oC austenitization with 400 oC austempering.
The steel in the present study, when processed by austenitizing at 750 oC followed by austempering at 350 oC, shows exceptional property, where tensile strength above 1 GPa is obtained with 25% elongation. The yield ratio is 0.65. This property range takes the steel to 3rd Generation advanced high strength steel. The effect of austenitization temperature on tensile strength, yield strength and elongation is brought out in Fig.9. It is seen that the tensile strength peaks and yield strength is lowered at around 800oC at each austempering temperature. The elongation is highest in the inter critical austenitizing temperature [Fig.9(c)].
It is slightly lower at the lower austenitizing temperature and higher austenitizing temperature. It is also seen that yield,tensile strength is, in general, highest at 450 oCaustempering although the differences among the values are moderate. The yield strength wise 350oC austempering show highest values. Hence yield ratio is affected based on austempering condition. Ductility wise, higher values are seen with 350 oCaustempering. The mechanical properties of the steel studied at different austenite holding temperature and austempering temperature is shown in Fig.11. It is seen that the tensile strength peaks at around 800 oC while the yield strength pass through a minima. Thus, the yield ratio is significantly affected depending on the austempering temperature. The ductility shows a mixed trend, the 350oC, austempering shows a peak while 400 and 450oC austempering shows a minimum value.

Table 3 Microstructural constitution of the steel at various austemperedcondition
Austenitizing
Austempering Ferrite
(%) Martensite+ retained austenite(%) Bainite
(%)
Temperature oC time/ min Temperature oC time/ min
700 5 350 5 59.46 8.43 32.11
700 5 400 5 27.67 19.35 52.98
700 5 450 5 46.36 10.02 43.62
750 5 350 5 15.18 10.27 74.55
750 5 400 5 26.97 12.29 60.74
750 5 450 5 39.36 6.01 54.63
800 5 350 5 39.88 5.43 54.69
800 5 400 5 37.73 9.37 52.9
800 5 450 5 43.92 4.86 51.22
850 5 350 5 36.51 16.08 47.41
850 5 400 5 44.18 8.19 47.63
850 5 450 5 9.00 16.15 74.85
900 5 350 5 35.43 10.37 54.2
900 5 400 5 19.74 24.95 55.31
900 5 450 5 27.44 12.59 59.97

Table 4(a) Mechanical Properties of the samples in the differentaustempered conditions after inter critical temperature austenitization
Sample Condition Yield
Strength
(0.2%)
MPa Ultimate
tensile
Strength
, MPa %E
(50GL) Yield Ratio TS*E
(GPa %) XRD, Ret. Aust.%
Austempered at 350 C at various austenitizing temperature
Austenitized at 800 oC / 5 min +
austempered at 350 oC/ 5min 516 894 16.9 0.58 15.108
Austenitized at 750 oC / 5 min +
austempered at 350 oC/ 5min 664 1016 25.2 0.65 25.603 <4
Austenitized at 700 oC / 5 min +
austempered at 350 oC/ 5min 732 802 20.8 0.91 16.682
Austempered at 400 C at various austenitizing temperature
Austenitized at 800 oC / 5 min +
austempered at 400 oC/ 5min 552 1009 15.9 0.55 16.043
Austenitized at 750 oC / 5 min +
austempered at 400 oC/ 5min 443 922 16.9 0.48 15.582
Austenitized at 700 oC / 5 min +
austempered at 400 oC/ 5min 714 746 25.1 0.96 18.725 10
Austempered at 450 C at various austenitizing temperature
Austenitized at 800 oC / 5 min +
austempered at 450 oC/ 5min 577 1081 16.7 0.53 18.053 <4
Austenitized at 750 oC / 5 min +
austempered at 450 oC/ 5min 430 931 19.1 0.46 17.782
Austenitized at 700 oC / 5 min +
austempered at 450 oC/ 5min 714 755 22.5 0.94 16.987

Table 4(b) Mechanical Properties of the samples in the different austempered conditions after austenitization at above Ac3 temperature
Sample Condition Yield
Strength
(0.2%),MPa Ultimate
tensile
Strength, MPa %E
(50GL) Yield Ratio TS*E
(GPa %)
Austenitized at 850 oC
Austenitized at 850 oC / 5 min +
austempered at 350 oC/ 5min 541 894 19.8 0.605 17.701
Austenitized at 850 oC / 5 min +
austempered at 400 oC/ 5min 482 893 18.3 0.54 16.342
Austenitized at 850 oC / 5 min +
austempered at 450 oC/ 5min 548 984 16.7 0.56 16.433
Austenitized at 900 oC
Austenitized at 900 oC / 5 min +
austempered at350 oC/ 5min 681 894 13.7 0.76 12.248
Austenitized at 900 oC / 5 min +
austempered at 400 oC/ 5min 595 844 17.5 0.70 14.770
Austenitized at 900 oC / 5 min +
austempered at 450 oC/ 5min 507 901 16.4 0.56 14.776

Properties of the steel in the hardened and tempered condition
In general steels subjected to hardening and tempering heat treatment enable achievement of good mechanical properties. Hence, the steels were subjected to hardening as per schedule shown in Fig. 3(d). The steels austenitized at 750, 800,850,900 oC for 5 min were water quenched to room temperature. This is followed by tempering at 150 oC/30min. This tempering temperature would retain an ultra high strength range in the steel. The properties were evaluated in the as-quenched condition and in the hardened and tempered conditions. The microstructure in the hardened condition is shown in Fig.12 and that after tempering in Fig.13. The steel shows tempered martensite after tempering. Along with the martensite, bright white film type retained austenite tends to form along the martensite islands. The inter critically austenitized and quenched steel is actually the dual phase heat treatment.
The mechanical properties of the steels in the as hardened condition and subsequent tempering are shown in Table 5. The strength is generally increases with austenitization temperature while the ductility value decreases. At the intercritical austenitized temperature at 750 and 800 oC shows attractive mechanical properties. The tensile strength at 750 oC is 858 MPa and at 800 oC is 1125 MPa with corresponding ductility was 17.7 and 14.3 % respectively. Tempering essentially improved the ductility. The strength increases at 810 oC austenitization significantly with a subsequent loss in ductility in comparison to quenching from 750 oC. The ductility value decrease from 18 to 14 %.This is because the austenite content is more with higher austenitizing temperature. There is remnant soft intercritical ferrite that enhances ductility. The properties of the steel at the austenitizing temperature of 850, and 900 oC show fully martensitic matrix without ferrite. This forms lath martensite in the steel. The tensile strength is very high at 1218 MPa and 1295 MPa while the ductility is lower at 8 and 9 % at austenitizing temperature of 850 and 900 oC. The tempering of the steel improves the ductility of the steel. Thus attractive combination of properties is obtained in the inter critically treated steel with good ductility compared to conventional hardening and tempering treatment. Best combination of strength and ductility is realized by DP steel treatment with inter critical holding at 750 oC followed by water quenching. This may be followed by a brief tempering treatment at low temperature which improves ductility with marginal loss in strength.
Table 5 Mechanical Properties of the samples in the various austenization temperature followed by WQ and tempering after WQ.
Condition
Yield Strength, 0.2% Ultimate Ttensile Strength, MPa %E
(50GL) YR TS*E
GPa.%
Austenitized above Ac3
900-WQ 1024 1298 8.19 0.79 10.632
850-WQ 1063 1218 8.77 0.87 10.689
900-WQ + 150 Temper 1004 1281 9.13 0.78 11.694
850-WQ + 150 Temper 1014 1187 10.27 0.85 12.185
Intercriticaly austenitized
800-WQ
[ DP steel] 726 1125 14.39
0.65 16.182
750-WQ
[DP steel] 514 858 17.72 0.60 15.208
800-WQ + 150 Temper 718 1086 15.12 0.66 16.423
750-WQ + 150 Temper 521 836 20.53 0.62 17.164