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 :
ADVANCED HIGH STRENGTH STEEL AND METHOD OF PRODUCING THE SAME BY DUAL STABILIZATION HEAT TREATMENT.



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 present invention relates to development of a thermal cycle of dual stabilization heat treatment on lean alloyed steel to realize the third Gen AHSS properties. The steel with low carbon content of 0.17 %C along with 1.72%Mn and 1.35% Si as major alloying elements, additionally micro alloyed with 0.02%Nb and 0.04%Ti (compoisition in wt.%) is a TRIP assisted Steel. The steel subjected to intercritical holding at 790oC, 810oC and 830oC for a holding time of 5 min to enable carbon partition between the ferrite and austenite, which enriches the austenite with carbon rejected by ferrite. The enrichment of austenite with carbon tends to lower the Ms temperature, which is the first stabilization treatment. This is followed by rapid quenching and holding the steel in salt bath at the isothermal holding temperature of 450oC for 30 sec. The isothermal bainitic transformation of the austenite fraction, promote ferritic bainite phase formation partly. During the bainitic transformation, the steel is quenched to below Ms temperature to transform martensite and retained austenite similar to a quench partitioning. From this lowest temperature, the steel is reheated to bainitic holding temperature, which enables the carbon from the martensite to partition to the retained austenite stabilizing the steel.

The dual stabilized steel showed 45 to 47% ferrite, 21-37%bainite, 14 to 26% martensite and 3.3 to 9.5 % retained austenite. The mechanical properties showed yield strength between 460 and 480MPa, tensile strength between 960 and 1010MPa, with total elongations between 27 and 28 % and tensile toughness above 27GPa.% conforming to third generation advanced high strength steel.

BACKGROUND OF THE INVENTION
Third generation advanced high strength steelsare being increasingly produced for achieving the weight reduction to improve the fuel efficiency, reduction of CO2 emissions with greater safety, with excellent formability and energy absorption characteristics. To meet such a requirement the properties of the steel should have been ultra high strength and excellent ductility. The product of tensile strength and elongation (tensile toughness) exceed greater than 20GPa.%. To attain such properties a wide variety of steels composition have been developed. This includes TRIP assisted steel, carbide free bainitic steel, quench & partitioned steel, austenite reverted transformation with widely varying microstructure. Another emerging steel aimed at such properties is the dual stabilization (DS) heat treatment cycle that has been carried out in a lean alloyed steel that gave excellent combination of strength and ductility with tensile toughness exceeding 27 GPa.%.

High mechanical properties are achieved when the microstructure has bainitic ferrite along with film form of retained austenite intertwinned with ferrite. The presence of Si or Al tends to suppress the carbide formation and stabilizes the austenite. The austenite can be stabilized by alloying with elements with Mn, Ni, Mo etc. But carbon is the most inexpensive element in steel, that can stabilize austenite. Dual stabilization involves a two stage carbon partitioning to the austenite.

State of Prior Art
Hao Qu et al., (HaoQu, Gary M. Michal and Arthur H Heuer, Third Generation 0.3C-4.0Mn Advanced High Strength Steels Through a Dual Stabilization Heat Treatment: Austenite
Stabilization ThroughParaequilibrium Carbon Partitioning, Metallurgical and Materials Transactions A, 27 Feb. 2014) produced third generation AHSS in two steel compositions with a base composition 0.3% C, 0,5% Cr and 4% Mn content. The first steel has to the extent of 1.8% Al and 0.6% Si and the second steel has 2.1%Al and 1.5% Si. Both elements Al and Si suppress carbide formation and promotes retained austenite. The processing involves austenitization at 900 and 835 C followed by isothermal bainitic holding briefly followed by quenching between 80 to 125 C to form martensite. This is followed by reheating to isothermal bainitic condition where carbon gets partitioned. The phase constitution involves more than 30% retain austenite along with bainitic ferrite and martensite. The first steel had tensile strength in the range 1100 to 1450 MPa and total elongation in the range 8 to 14 pct, and the steel has tensile strength from 1400 to 1650 MPa and total elongation from 16 to 22 pct. While the prior art involves a medium Mn steel, the current steel invented is a low carbon lean alloyed steel sand the properties realized are targetted at 1000 MPa class with more than 26% elongation and tensiole toughness 26 to 29 GPa.%.

Jing et al., (Cainian Jing, Xiaoyun Ding, Daomin Ye, Jingrui Zhao, Tao Lin, and ShuboXu, Effect of Different Isothermal Time on Microstructure and Mechanical Property of the Low alloy Carbon Steel Treated by Dual-Stable C-Mn Partitioning Process, Hindawi Scanning, Volume 2020, Article ID 5931721, 11 pages, https://doi.org/10.1155/2020/5931721) reported a dual stabilization where initially the first stabilization takes place in the intercritical temperature range. The second stage is a single stage quench partitioning where the second partitioning takes place. The steel has a composition of 0.11C-1.5Mn-1.16Si-0.043Al-0.024Cr-0.01Ti-0.001S-0.008P-0.003B. The present investigation defers from the prior art where the second stage does not involve single stage partitioning in martensite but a reheating to the isothermal bainitic holding temperature post quenching. The properties realized in the prior art has tensile strength close to 875-910MPa and elongation 20-24%, with tensile toughness 21GP.%. The present invention shows tensile strength above 960 MPa and with more than 26% ductility and tensile toughness close to 29 GPa.%.

US patent US2020332385A1and CA Patent CA3138889A1(Publication/Issue Date: 22-Oct-2020 Filing/Application Date: 06-May-2020 Estimated Expiry Date: 06-May-2040 Earliest Priority Date: 10-May-2016 Earliest Publication Date: 16-Nov-2017 Priority Date: 06-May-2020;19-Aug-2019;10-May-2017;10-May-2016;19-Sep-2016;07-May-2019 Priority Year: 2016) and (Publication/Issue Date: 12-Nov-2020 Filing/Application Date: 06-May-2020 Estimated Expiry Date: 06-May-2040 Earliest Priority Date: 10-May-2016 Earliest Publication Date: 16-Nov-2017 Priority Date: 07-May-2019;06-May-2020 Priority Year: 2019), having a closest prior art on methods of producing high strength continuously cast hot rolled steel sheet products with tensile toughness greater than 25GPa.% comprising 0.15 to 0.5 weight percent C, from 1 to 3 weight percent Mn, and from 0.8 to 3 weight percent of a combination of Si and Al matches with the present invention whereas the processing is different compared to the present invention. The prior art invention governs hot rolling the continuously cast steel slab including a finish rolling step on a hot strip mill with a finish rolling temperature of at least 820° C to form a hot rolled steel sheet product on the hot strip mill; quenching the hot rolled steel sheet product on the hot strip mill to form a predominantly martensitic microstructure; subjecting the quenched hot rolled steel sheet product to a thermal cycling step comprising soaking the sheet product in an intercritical regime at a temperature of from 720 to 850° C. followed by holding the sheet product at a temperature of from 360 to 445° C.; and quenching the thermally cycled sheet product to room temperature, wherein the steel sheet product comprises ferrite and retained austenite grains. Whereas in the present invention after inter critical annealing the steel was held for 30sec in the bainitic regime and water quenched followed by second bainitic holding for 30 sec and water quenching to achieve microstructure of 45 to 47% ferrite, 21-37% bainite, 14 to 26% martensite and 3.3 to 9.5 % retained austenite with tensile toughness greater than 27GPa% to produce third generation advanced high strength steel.

Another prior art patent No. WO2004022794A1, where two stages quench partitioning treatment is done on low carbon steel to stabilize the retained austenite whereas in the present invention dual stabilization heat treatment is made to stabilize the retained austenite along with the ferrite and bainitic phase to achieve third generation AHSS properties.
It is seen that very few prior art is available on the said field and there is significant difference between the invented process against the prior art.

OBJECTS OF THE INVENTION
The basic object of the present invention is directed to development of lean steel composition that could effectively be used as a third generation advanced high strength steels by a dual stabilization heat treatment, that involves carbon enriched austenite in the first stage stabilization and two stage quench partitioning in the second stage.

Another object of the invention is to produce a steel with a complex microstructure consisting of ferrite, bainite, retained austenite and martensite.

Yet another object of the invention is to develop high strength steel having ultimate tensile strength between 950 and 1000 MPa and ductility greater than 26%, such that the tensile toughness achieved more than 27GPa%.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to a steel compsotion adapted for dual stabilization heat treatment based high stregth steel comprising :
carbon between 0.03 and 0.3 % and preferably around 0.17% C; Mn content varying between 1.5 and 2.5% preferably about 1.7%; Si content varying between 1.0 to 2.5 % preferably about 1.35%; Nb content between 0.01 and 0.06 preferably about 0.02%; Ti content between 0.03 and 0.05% preferably 0.04%; Al content between 0.02 and 0.08% preferably about 0.06% with S content less than 0.01 and P content < 0.03%.

A further aspect of the present invention is directed to a process for manufcature of dual stabilization heat treatment based high stregth steel involving the steel compostion comprising :
(i) providing said steel slabs having compsotion of lean chemistry comprisng of carbon between 0.03 and 0.3 % and preferably around 0.17% C; Mn content varying between 1.5 and 2.5% preferably about 1.7%; Si content varying between 1.0 to 2.5 % preferably about 1.35%; Nb content between 0.01 and 0.06 preferably about 0.02%; Ti content between 0.03 and 0.05% preferably 0.04%; Al content between 0.02 and 0.08% preferably about 0.06% with S content less than 0.01 and P content < 0.03% (ii) subjceting to hot rolling after reheating the slab at 1250oC, with roughing rolling at 1210oC, and finishing rolling at 890oC to thickness of 2-5mm and coiling at 570oC, to form a hot-rolled steel sheet on the hot strip mill; which is then (iii) subjceted to a dual stabilization heat treatment to produce third generation AHSS properties.
A still further aspect of the present invention is directed to said process wherein said step of subjecting to a dual stabilization heat treatment included dual stabilization heat treatment cycle as per heating in the intercritical temperature to introduce inter critical ferrite, between 790 and 830 oC held for 2-10 min typically 5 min for first stabilization; followed by a second stabilization treatment by quenching in salt bath between 440-460oC and typically 450oC for time betwwen 20-40sec and typically 30 sec and quenching in water to form martensite and austenite followed by an isothermal holding leading to carbon partitioning between 440-460oC and typically 450oC for time between 20-40sec and typically 30 sec and finally water quenched to prodcue said dual stabilization heat treatment based high stregth steel.
A still further aspect of the present invention is directed to said dual stabilization heat treatment based high stregth steel obtained of steel composition as described above having a microstructure with ferrite content in the range of 45 to 47% with 21-37% bainite, 14 to 26% martensite and 3.3 to 9.5 % retained austenite
Another aspect of the present invention is directed to said dual stabilization heat treatment based high stregth steel having an yield strength in the range of 461 to 484 MPa, with a tensile strength in the range of 964 to 1007 MPa and with total elongation between 27.4 and 28.10 % and the tensile toughness maintained around 27 GPa.% conforming to the third generation Advanced High Strength Steels.

The above aspects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non-limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1: Phase diagram of the steel as function of carbon content.
Fig.2: Dual stabilization heat treatment cycle for the steel.
Fig.3: Optical Microstructure of the steelsubjected to dual stabilization heat treatment.
Fig.4: SEM micrograph of the steel subjected to dual stabilization heat treatment cycle at different magnifications.
Fig.5: XRD of the steelsubjected to dual stabilization heat treatment.
Fig.6: Stress-strain diagram of the steel subjected to dual stabilization heat treatment cycle.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS
The present invention is directed to the development of a lean alloyed steel composition range and typical composition as per Table 1, adapted for a dual stabilization heat treatment cycle, to achievethe Third Gen Advanced High Strength Steel with tensile toughness greater than 25 GPa.%. The steel used is comprised of of 0.03 to 0.30 wt. % carbon preferably 0.17%C; 1.5 to 2 wt. % Mn, preferably 1.72% Mn; 1.0 to 1.5% Si with Si content of 1.35% ; microalloyed with 0.01 to 0.04% Nb and 0.03 to 0.05 wt.% Ti, and N<600 ppm. The residual Al varied between (0.04 and 0.06)%Al. The residual element Al is maintained in the range 0.04 to 0.08 wt. % Al, and 0.002 to 0.004 wt.% S and 0.015 to 0.017 wt.%P.

The base steel produced in the present invention through primary steel making process using hot metal from blast furnace in a EAF or BOF furnace, followed by secondary steel making process using a Ladle steel making route, where the desired composition was achieved with ferro alloy addition, followed by casting the same through continuous casting in slab caster, hot rolling the cast slab to produce steel strip between 2 to 5 mm and the properties in the range of 800-865MPa and elongations 11 to 12 %.

The present invention is also directed to provide a process for manufacture of the lean alloyed steel including :processing said lean alloyed steel composition adapted for variable thermal processing comprising of 0.03 to 0.30 wt. % carbon preferably 0.17% C, Mn 1.5 to 2 wt. % preferably 1.72% Mn, Si 1 to 1.5 wt.% preferably 1.35%Si, Nb 0.01 to 0.03 wt. %, 0.03 to 0.05 wt.% Ti, and less than 60 ppm N, 0.04 to 0.08 wt. % Al, 0.002 to 0.004 wt.% S and 0.015 to 0.017 wt%P under primary steel making process, followed by secondary steel making process including ferroalloy addition, followed by casting the same through continuous casting in slab caster to produce hot rolled strip of 2-5mm thickness.

The present invention further provides the design of heat treatment cycle where the dual stabilization heat treatment involving first stage stabilization by inter critical annealing where the carbon is partitioned between ferrite and austenite. The intercritical austenitization between 770oC to 850oC and typically at 790oC, 810oC and 830oC and held for a time duration chosen between 2 to 10 min and typically at 5 min,tends to form different proportion of ferrite and austenite. As the ferrite has a low solubility, the carbon from ferrite enriches austenite which decreases the Ms temperature of austenite that provides the first stage stabilization. The lower the inter critical temperature, higher the ferrite content which results in higher carbon content in the lower fraction of austenite in equilibrium. The austenitization is followed by quick transfer to a salt bath at isothermal bainitic in a salt bath at typicxally 450 oC, and held for a duration between 20 to 40 sec and typically 30 sec followed by water quenching tends to transform the residual austenite to bainite from the first stage stabilization. The steel is rapidly quenched to a temperature lower than Ms temperature to form martensite and residual austenite the fraction of which which can be estimated using Koinstein-Maurberger equation. This is followed by reheating the steel to the same isothermal hold temperature of 450 oC. The carbon from the martensite partitions to the residual austenite providing the secod stage stabilization. Such dual stabilization is carried out for Medium Mn steels. However, the present invention, shows that the second stabilization treatment is a two stage quench partition type treatment, gives consistently good strengthand ductility. At the final stage the isothermally held sample is water quenched.

The steel as per the lean chemistry in Table 1 when subjected to the above thermal cycle gives an yield strength varying between 461and 484MPa, tensile strength varying between 964 and 1007MPa, with a total elongation between27.4 and 28.10 %. The tensile toughness in the lean steel shows values is around 27GPa.%, qualifying the steel as a Third Generation AHSS.

The nital etched optical microstructure of the steel under various thermal processing conditions is shown in Fig.3. The microstructure shows bainitic and bainitic microstructure. The fraction of ferrite content is about 45 to 47%and the rest is bainitic fraction. The optical microstructure of the steel etched with Lepera reagent shows predominant fraction of bainite and martensite with ferrite as shown in Fig.4. The SEM microstructure of the steel after the dual stabilization heat treatment at low and high magnifications is shown in Fig.5. The individual phases are shown in Fig.5. The ferrite fraction is clearly visible and it decreases with increasing intercritical austenitization temperature. The ferritic bainite phase is surrounded by a rim of retained austenite.

Table 1: Chemical composition (wt. %) and critical temperatures (oC) of the steel.
C Mn Si Nb Ti N Al S P AC1 AC3 Bs Ms
Range 0.03-0.30 1.5-2.5 1.0-2.5 0.01-0.06 0.03-0.05 < 0.006 0.04-0.08 0.002-0.004 0.015-0.017 - - - -
Actual 0.17 1.72 1.35 0.02 0.04 0.005 0.06 0.003 0.016 731 875 557 417

Table 2: Mechanical properties of the steel subjected to dual stabilization heat treatment cycle.
Dual Stabilization heat treatment condition YS (0.2 %)
MPa UTS
MPa TE
% YR UTS*TE
GPa.%
790oC/5min/450oC/30sec/WQ/450oC/30sec/WQ 461+13 979 + 4 27.7 + 0.015 0.47 27.12
810oC/5min/450oC/30sec/WQ/450oC/30sec/WQ 474 + 6 1007+ 6 27.4 + 1 0.47 27.59
830oC/5min/450oC/30sec/WQ/450oC/30sec/WQ 483.5+1 964.5+3.5 28.1 + 0 0.50 27.10

The chemical composition of the steel is shown in Table-1. The composition of the steel can be mapped from the phase diagram shown in Fig.1. The pseudo binary phase diagram gives the composition helps to estimate the equilibrium phases in the inter critical temperature range by Lever Rule. The heat treatment cycle applied to the steel is shown in Fig.2. The optical microstructure of the dual stabilized steel is shown in Fig.3 and the SEM microstructure in Fig.4. The XRD of the steel at the dual stabilization cycles is shown in Fig.5. The engineering stress-strain diagram of the steel subjected to various dual stabilization heat treatments is shown in Fig. 6. The mechanical properties of the steel are summarized in Table-2.

The dual stabilized steel showed 45 to 47% ferrite, 21-37%bainite, 14 to 26% martensite and 3.3 to 9.5 % retained austenite. The mechanical properties showed yield strength between 460 and 480MPa, tensile strength between 960 and 1010MPa, with total elongations between 27 and 28 % and tensile toughness above 27GPa.% conforming to third generation advanced high strength steel.
, Claims:WE CLAIM
1.Steel compsotion adapted for dual stabilization heat treatment based high stregth steel comprising :
carbon between 0.03 and 0.3 % and preferably around 0.17% C; Mn content varying between 1.5 and 2.5% preferably about 1.7%; Si content varying between 1.0 to 2.5 % preferably about 1.35%; Nb content between 0.01 and 0.06 preferably about 0.02%; Ti content between 0.03 and 0.05% preferably 0.04%; Al content between 0.02 and 0.08% preferably about 0.06% with S content less than 0.01 and P content < 0.03%.
2. A process for manufcature of dual stabilization heat treatment based high stregth steel involving the steel compostion as claimed in claim 1 comprising :
(i) providing said steel slabs having compsotion of lean chemistry comprisng of carbon between 0.03 and 0.3 % and preferably around 0.17% C; Mn content varying between 1.5 and 2.5% preferably about 1.7%; Si content varying between 1.0 to 2.5 % preferably about 1.35%; Nb content between 0.01 and 0.06 preferably about 0.02%; Ti content between 0.03 and 0.05% preferably 0.04%; Al content between 0.02 and 0.08% preferably about 0.06% with S content less than 0.01 and P content < 0.03% (ii) subjceting to hot rolling after reheating the slab at 1250oC, with roughing rolling at 1210oC, and finishing rolling at 890oC to thickness of 2-5mm and coiling at 570oC, to form a hot-rolled steel sheet on the hot strip mill; which is then (iii) subjceted to a dual stabilization heat treatment to produce a third generation AHSS properties.
3. The process as claimed in claim 2 wherein said step of subjecting to a dual stabilization heat treatment included dual stabilization heat treatment cycle as per heating in the intercritical temperature to introduce inter critical ferrite, between 790 and 830 oC held for 2-10 min typically 5 min for first stabilization; followed by a second stabilization treatment by quenching in salt bath between 440-460oC and typically 450oC for time betwwen 20-40sec and typically 30 sec and quenching in water to form martensite and austenite followed by an isothermal holding leading to carbon partitioning between 440-460oC and typically 450oC for time between 20-40sec and typically 30 sec and finally water quenched to prodcue said dual stabilization heat treatment based high stregth steel.
4. The dual stabilization heat treatment based high stregth steel obtained of steel composition as claimed in claim 1 having a microstructure with ferrite content in the range of 45 to 47% with 21-37%bainite, 14 to 26% martensite and 3.3 to 9.5 % retained austenite
5. The dual stabilization heat treatment based high stregth steel as claimed in claim 4 having an yield strength in the range of 461 to 484 MPa, with a tensile strength in the range of 964 to 1007 MPa and with total elongation between 27.4 and 28.10 % and the tensile toughness maintained around 27 GPa% conforming to the third generation Advanced High Strength Steels.

Dated this the 16th day of June, 2021
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199