Claims:WE CLAIM :

1. Lean alloyed steel of compostion adapted for austenite reverse transformation heat treatment cyclecomprising 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 and which is variable austenite reverse transformation (ART) heat treatment cycle based high strength steel with tensile toughness in the range of 20 to 36 GPa% including Third Gen Advanced high strength steel with tensile toughness greater than 30 GPa.%,.
2. The Lean alloyed steel as claimed in claim 1 which is austenite reverse transformation (ART) heat treatment cycle based on cast slab produced steel strip between 2 to 5 mm and the properties in the range of 800-865MPa and elongations 11 to 12 %.
3. The Lean alloyed steel as claimed in anyone of claims 1 or 2 including selectively(i) tensile toughness of 20 to 25 preferably 21 GPa.% which is based on a microstructure of tempered martensite with reverted austenite(ii) tensile toughness in the range of 28 to 32 preferably about 29.62 GPa.% which is based on a microstructure of bainite with reverted austenite and (iii) and tensile toughness of 30-36 GPa.% which is based on a microstructure of bainite with reverted austenite.
4. A process for manufcature of the lean alloyed steel as claimed in anyone of claims 1 to 3 comprising :
(iii) base steel produced 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 %.;and
(iv) subjceting to said selectively variable austenite reverse transformation (ART) involving seelctive microstruture for desired tesile toughness.
5. The process as claimed in claim 4 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 which is subjected to austenite reverse transformation (ART) involving initial austenitization between 890-910 oC and typically at 900 oC and held for a time duration between 1 to 5 min and typically at 2 min, therefater the steel is quenched in water and put at temperature of 770-790oC for 2-10min and typically 5 min and again quenched in water to get yield strength of 620 MPa, tensile strength of 760 MPa, total elongation of 27.75%, work hardening exponent of 0.16 and yield ratio of 0.81 and tensile toughness of 21 GPa.% with a microstructure of tempered martensite with reverted austenite.
6. The process as claimed in claim 4 wherein said lean alloyed steel composition is processed into hot rolled steel strip and thereafter subjected to austenite reverse transformation (ART) involving initial austenitization between 890-910oC and typically at 900 oC and held for a time duration between 1 to 5 min and typically at 2 min. thereafter the steel is quenched in salt bath at 440 to 460oCpreferably 450 oC 2-10 min and typically 5 min and water quenched and put at temperature of 740-760oC for 2-10min and typically 5 min and again quenched in water to get yield strength of 578 MPa, tensile strength of 750 MPa, total elongation of 39.50%, work hardening exponent of 0.18 and yield ratio of 0.77 and tensile toughness of 29.62 GPa.% with a microstructure of bainite with reverted austenite.
7. The process as claimed in claim 4 wherein said lean alloyed steel composition is processed into hot rolled steel strip and thereafter subjected to austenite reverse transformation (ART) involving initial austenitization between 890-910 oC and typically at 900 oC and held for a time duration between 1 to 5 min and typically at 2 min thereafter the steel is quenched in salt bath at 440 to 460oC preferably about 450 oC for 2-10 min and typically 5 min and water quenched and put at temperature of 770-790oC for 2-10min and typically 5 min and again quenched in water to get yield strength in the range of 410-511MPa,tensile strength of 721-767 MPa, total elongations of 44-47%, yield ratio of 0.53-0.71, work hardening exponent in the range of 0.21-0.27 and tensile toughness of 31.86-36.08 GPa.% with a microstructure of bainite with reverted austenite.
8. The process as claimed in claim 5 or 6 wherein seecltively (a) when said lean steel and after water quenching when subjected to reverted austenite transformation in the inter critical temperature regime at 780 oC with 5 min holding gave a tensile strength of 760 MPa with 27.75% elongation which corresponded to a tensile toughness of 21.09 GPa.%E, with strain hardening exponent, n was ~0.16 and (b) when
the lean steel and converted to martensite was subjected to an intermediate isothermal holding at 450 oC with 5 min holding in a salt bath subjected to reverted austenite heat treatment at 750 oC for 5 min gave 750 MPa tensile strength with 39.5% elongation, which corresponds to a tensile toughness of 29.62 GPa.%. the n value was 0.18 and the microstructure corresponding to this obtained was bainitic ferrite with 1.6% retained austenite
9. The process as claimed in claim 5 or 6 wherein the lean steel converted to martensite was subjected to an intermediate isothermal holding at 450 oC with 5 min holding in a salt bath and thereafter subjected to reverted austenite heat treatment at 780 oC for 5 min to obtain tensile strength value between 721 and 767 MPa with an elongation between 44 and 47%. The strain hardening exponent, n was 0.21 to 0.27 the tensile toughness varied between 32 and 36% and the microstructure of the steel attained bainitic ferrite with retained austenite content of about 2.2% and wherein the heat treatment condition provided for a highly plastic steel suitable for high formability.

Dated this the 24th day of March, 2022
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 :
LEAN ALLOY STEEL COMPOSITION AND A PROCESS TO PRODUCE THIRD GENERATION ADVANCED HIGH STRENGTH STEEL THEREFROM BY AUSTENITE REVERSE TRANSFORMATION.



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 lean alloyed steel composition and a process to develop a third generation advanced high strength steel by austenite reverse transformation (ART) heat treatment cycle. More particularly,the present invention proivdes a process for manufacture of the lean alloyed steel including processing said lean alloyed steel composition adapted for variable thermal processing wherein lean alloyed steel comprises 0.17 wt. % C, 1.35 wt. % Si and 1.72 wt. % Mn with micro alloying elements 0.02 wt. %Nb and 0.04 wt. %Ti that could give yield strength in the range of 410-620MPa, tensile strength of 721-767 MPa and excellent ductility in the range of 28-47%. In three different ART cycles adopted, tensile toughness greater than 20GPa.% was realized. The ART cycle consisting of austenitization at 900oC for 2min followed by isothermal holding at 450oC for 5min followed by water quenching was reuastenitized at 780oC held for 5min followed by water quenching gave best combination of mechanical properties with yield strength (YS) in the range of 410-510MPa, tensile strength (TS) in the range of 721-767, total elongations (EL) in the range of 44-47%. The strain hardening exponent (n) was in the range of 0.21-0.27 and strength constant (K) was in the range of 1178-1359 and tensile toughness in the range of 30-36 GPa.%. Microstructure of tempered martensite in the conventional ART cycle or bainitic ferrite with reverted austenite was found in the modified ART cycle of bainitic holding. As the steel is having high strength with exceptional ductility, it could be a very good formable product for automotive applications. The high ductility enable energy absorption during impact.

BACKGROUND OF THE INVENTION
Third generation advanced high strength steels (3GAHSS) are being increasingly produced for achieving the weight reduction to improve the fuel efficiency, reduction of CO2 emissions with greater safety or with excellent formability and energy absorption characteristics. The 3G AHSS
include steels such as TRIP aided Bainitic Ferrite (TBF), Medium Mn TRIP, Delta TRIP, Quench & Partitioning (Q&P), Dual Stabilization (DS) and Austenite Reverse Transformation (ART) is falling in the 3GAHSS category. In the present invention, a lean alloyed steel has been subjected to ART heat treatment cycle, to achieve an excellent combination of strength and ductility where tensile toughness as high as 34 GPa.% was realized that enhances formability or strength or a combination of both.

State of Prior Art
Sun et al. studied Austenite reverted austenite transformation in a 0.2%C-5% Mn steel. There is the formation of martensite laths on quenching. This is followed by reverted austenite treatment at 650 oC at 1, 10 and 360 min. The treatment gave a high retained austenite content of 18.4 to 33.6% which gave a tensile elongation varying between 25.316 GPa.% to 44.496 GPa.%. The present invention is different from this prior art as the present steel is a lean steel with 1.7%Mn against the medium Mn steel(5%Mn) of the prior art. The retained austenite is not significant but under specific processing condition the present invention shows highly superior properties.
(SUN Rong-min, LI Guo-yang, WANG Hui, WANG Cun-yu, WANG Yu-hui,CAO Wen-quan, “Microstructure and properties of Fe–Mn–C medium-Mn steel processed by ART-annealing”, DOI:10.13228/j.boyuan.issn0449-749x.20200558)

REN Yongqiang et al. performed ART treatment followed by quench partitioning on 0.23%C, 1.8%Mn and 1.35 % Si steel. The process gave greater than 26 GPa.%. The process involves enrichment of C&Mn in ART and the Q&P treatment gave further enrichment of carbon in retained austenite. The strength values were between 600 and 900 MPa. And elongation above 16%. Unlike this prior art, the present steel superior properties were realized by single stage ART treatment without quenching and partitioning. Hence, in a lean steel Advanced high strength properties with tensile elongation between 30 to 36% GPa.% is realized.
Ren Yongqiang, Xie Zhenjia, Shang Chengjia J. Acta Metallurgica Sinica, 2012; 48 (9): 1074-1080. (In Chinese))

Baifeng An et al. tried ART, tempering post ART and ART with Q&P treatments in 0 0,22%C, 2.3%Mn, 0,5 %Cr , 1.8%Si steel. Tensile elongation greater than 35 GPa.% was realized at tensile strength of 1230 MPa and 26% elongation. A mixed microstructure of ferrite, martensite , bainite and retained austenite enhanced the properties. ART with tempering showed 22.219 GPa.%. Q&P showed 27.748 GPa.%. When ART & Q&P is followed subsequently, the strength is as high as 1232 MPa tensile strength with 28.6% elongation. The present invented process involves lower level processing to realize the properties compared to the prior art. High retained austenite is obtained by carbon partitioning of the intercritical treatment, Carbon partition during Q&P and the formation of carbide free bainite.
(Baifeng An, Guhui Gao, Xiaolu Gui, Zhunli Tan, and Bingzhe Bai, “Enhanced mechanical properties of a 0.22C-Mn-Si-Cr low alloyed steel treated by ART and Q&P processes”, Advanced Materials Research Vols. 1004-1005 (2014) pp 203-208 Submitted: 2014-06-06 © (2014) Trans Tech Publications, Switzerland Accepted: 2014-06-08 doi:10.4028/www.scientific.net/AMR.1004-1005.2)

Hasegawa et al. studied a (0.03-0.35)%C- (3.5-10)% Mn-(0.5-3)%Si-<0.1%P-<0.02%S- (0.01 to 2)%Al with at least one of (0.005 to 2.00)% Cr, (0.005 to 2.00)%Mo, (0.005 to 2.00)%V, (0.005 to 2.00)%Ni and (0.005% to 2.00)%Cu, (0.005-0.2%)Ti or Nb and 3 to 50 ppm B steel which was Ca treated. The steel was hot rolled and cold rolled. The cold rolled steel was subjected to austenite reversion treatment by heating to intercritical temperature followed by annealing and galvanizing showed 980 MPa with 25% elongation (24.5 GPa.%). Unlike the prior art where austenite reversion was carried out post cold rolling, tne present invention deals with quenching followed by austenite reversion. The properties in the present invention are achieved at much leaner Mn containing steel and with much superior mechanical properties.
(HASEGAWA, Hiroshi, and KAWASAKI, Yoshiyasu,“Method for producing high-strength steel sheet having superior workability”, EP 2 772 556 B1, 2018.)

Yi Honliang et al. studied a steel (0.22-0.48) %C- (5-9.5) %Mn- (0.5-3) % (Si+Al) steel which was hot rolled followed by cold rolling and annealing. The steel was heated to an inter critical temperature of 650 to 780 oC for 0.5 to 60 min time where 30% of austenite reversion takes place. This was hot stamped to achieve in the stamped product a tensile strength between 1 and 1.5 GPa and tensile elongation of 25 GPa.%. Unlike this prior art where austenite reversion was followed hot stamping, the invented process does not involve stamping process. While the strength levels are very high in the prior art, the ductility levels are very high in the invented process. In addition, this prior art deals with medium to high Mn steel while the invented process is a very lean steel with 1.7%Mn. The tensile elongation of the invented steel exceed 30 GPa.%E unlike the prior art.
(Yi Hongliang, Du Pengju and Yang Dapeng, “Steel material for stamping, forming, formed member and heat treatment method thereof, Patent No. 2017/092104 A1)

Olli Ojha et al., studied a steel (0.05-0.3)%C, (0.9-6)Mn, 5% or less Si, 3 or less Al, 2% or less Ni, 0.8% or less Cr, 0.3% or less V, 01% or less Nb, 0.2% or less Ti, 0.005%or less B, 0.006% or less Ca, 0.01% or less N, 0.05% or less P, 0.01% or less S with Fe bal., microstructure consists of 45-95% Martensite, 4-55% retained austenite, 10% or less bainite, has a product of tensile strength and total elongation between 10-50GPa.%. To achieve the microstructure inter critical holding is carriedout after quenching and then again quenched to room temperature or bainitic holding is carriedout after the inter critical holding and final water quenching. Whereas in the present invention bainitic holding is made after initial austenization to get a bainitic microstructure and reverted austenite transformation is carried out in the inter critical regime and quenched in water subsequently.A relatively higher retained austenite (4-55%) is reported in the refered patent while in the present invention the retained austenite content is below 4%. No costly alloying elemeents is used as compared to the refered invention while 32-36 GPa.% of tensile toughness could be achieved with the innovative approach of ART cycle.
(Olli Ojha, Erik Nymann, Petri Jussila, Jenny Fritz, Medium Manganeese steel product and method of manufacturing the same, Patent No. WO2021089851A1)

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide a lean steel composition that could effectively be used as a third generation advanced high strength steel.

A further object of the present invention is directed to providea third generation advanced high strength steel where the austenite reverted transformation takes place post water quenching to form initially martensite where 20 GPa.% is realized.

A still further object of the present invention is directed to provide a modified innovative heat treatment cycle that involved bainitic holding before reverted austenite annealing to achieve tensile toughness greater than 30 GPa. %.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a lean alloyed steel of compostion adapted for austenite reverse transformation heat treatment cycle 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 and which is variable austenite reverse transformation (ART) heat treatment cycle based high strength steel with tensile toughness in the range of 20 to 36GPa% including Third Gen Advanced high strength steel with tensile toughness greater than 30 GPa.%,.

A further aspect of the present invention is directed to said lean alloyed steel which is austenite reverse transformation (ART) heat treatment cycle based on cast slab produced steel strip between 2 to 5 mm and the properties in the range of 800-865MPa and elongations 11 to 12 %.

A still further aspect of the present invention is directed to saidlean alloyed steel including selectively (i) tensile toughness of 20 to 25 GPa.% preferably 21 GPa.% which is based on a microstructure of tempered martensite with reverted austenite (ii) tensile toughness in the range of 28 to 32GPa.% preferably about 29.62 GPa.% which is based on a microstructure of bainite with reverted austenite and (iii) and tensile toughness of 30-36GPa.% which is based on a microstructure of bainite with reverted austenite.

Another aspect of the present invention is directed to a process for manufcature of the lean alloyed steel as described above comprising :
(i) base steel produced 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 %.;and
(ii) subjceting to said selectively variable austenite reverse transformation (ART) involving seelctive microstruture for desired tesile toughness.

Yet another aspect of the present invention is directed to said process involving 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 which is subjected to austenite reverse transformation (ART) involving initial austenitization between 890-910 oC and typically at 900 oC and held for a time duration between 1 to 5 min and typically at 2 min, therefater the steel is quenched in water and put at temperature of 770-790oC for 2-10min and typically 5 min and again quenched in water to get yield strength of 620 MPa, tensile strength of 760 MPa, total elongation of 27.75%, work hardening exponent of 0.16 and yield ratio of 0.81 and tensile toughness of 21 GPa.% with a microstructure of tempered martensite with reverted austenite.

A further aspect of the present invention is directed to said process wherein said lean alloyed steel composition is processed into hot rolled steel strip and thereafter subjected to austenite reverse transformation (ART) involving initial austenitization between 890-910oC and typically at 900 oC and held for a time duration between 1 to 5 min and typically at 2 min. thereafter the steel is quenched in salt bath at 440 to 460oC preferably 450 oC for 2-10 min and typically 5 min and water quenched and put at temperature of 740-760oC for 2-10min and typically 5 min and again quenched in water to get yield strength of 578 MPa, tensile strength of 750 MPa, total elongation of 39.50%, work hardening exponent of 0.18 and yield ratio of 0.77 and tensile toughness of 29.62 GPa.% with a microstructure of bainite with reverted austenite.

A still further aspect of the present invention is directed to said process wherein said lean alloyed steel composition is processed into hot rolled steel strip and thereafter subjected to austenite reverse transformation (ART) involving initial austenitization between 890-910 oC and typically at 900 oC and held for a time duration between 1 to 5 min and typically at 2 min thereafter the steel is quenched in salt bath at 440 to 460oC preferably about 450 oC for 2-10 min and typically 5 min and water quenched and put at temperature of 770-790oC for 2-10min and typically 5 min and again quenched in water to get yield strength in the range of 410-511MPa,tensile strength of 721-767 MPa, total elongations of 44-47%, yield ratio of 0.53-0.71, work hardening exponent in the range of 0.21-0.27 and tensile toughness of 31.86-36.08 GPa.% with a microstructure of bainite with reverted austenite.

A still further aspect of the present invention is directed to said process wherein seecltively (a) when said lean steel and after water quenching when subjected to reverted austenite transformation in the inter critical temperature regime at 780 oC with 5 min holding gave a tensile strength of 760 MPa with 27.75% elongation which corresponded to a tensile toughness of 21.09 GPa.%E, with strain hardening exponent, n was ~0.16, and
(b) when the lean steel and converted to martensite was subjected to an intermediate isothermal holding at 450 oC with 5 min holding in a salt bath subjected to reverted austenite heat treatment at 750 oC for 5 min gave 750 MPa tensile strength with 39.5% elongation, which corresponds to a tensile toughness of 29.62 GPa.%. the n value was 0.18 and the microstructure corresponding to this obtained was bainitic ferrite with 1.6% retained austenite

A still further aspect of the present invention is directed to said process wherein the lean steel converted to martensite was subjected to an intermediate isothermal holding at 450 oC with 5 min holding in a salt bath and thereafter subjected to reverted austenite heat treatment at 780 oC for 5 min to obtain tensile strength value between 721 and 767 MPa with an elongation between 44 and 47%. the strain hardening exponent, n was 0.21 to 0.27the tensile toughness varied between 32 and 36% and the microstructure of the steel attained bainitic ferrite with retained austenite content of about 2.2% and wherein the heat treatment condition provided for a highly plastic steel suitable for high formability.

The above and other 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: Heat Treatment cycle applied to the steel.
Fig.3: Optical Microstructure of the steel at various ART heat- treated cycles.
Fig.4: SEM micrograph of the steel at various ART heat- treated cycles at different magnifications.
Fig.5: XRD of the heat treated sample with quantification of retained austenite .
Fig. 6: Stress-strain diagram of the steel at various heat -treated cycles.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS

The present invention is directed to provide a lean alloyed steel composition adapted for austenite reverse transformation heat treatment cycle, to achieve Third Gen Advanced high strength steel with tensile toughness between greater than 30 GPa.%, 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.

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 %..

Another aspect of the present invention is directed to 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 was subjected to austenite reverse transformation (ART) involving initial austenitization between 890-910oC and typically at 900 oC and held for a time duration between 1 to 5 min and typically at 2 min. The steel is then quenched in water and put at temperature of 770-790oC for 2-10min and typically 5 min and again quenched in water to get yield strength of 620 MPa, tensile strength of 760 MPa, total elongation of 27.75%, work hardening exponent of 0.16 and yield ratio of 0.81 and tensile toughness of 21 GPa.% with a microstructure of tempered martensite with reverted austenite.

A still further aspect of the present invention is directed to said process wherein said lean alloyed steel composition is processed into hot rolled steel strip and thereafter subjected to austenite reverse transformation (ART) involving initial austenitization between 890-910oC and typically at 900 oC and held for a time duration between 1 to 5 min and typically at 2 min. The steel is then quenched in salt bath at 450 oC 2-10 min and typically 5 min and water quenched and put at temperature of 740-760oC for 2-10min and typically 5 min and again quenched in water to get yield strength of 578 MPa, tensile strength of 750 MPa, total elongation of 39.50%, work hardening exponent of 0.18 and yield ratio of 0.77 and tensile toughness of 29.62 GPa.% with a microstructure of bainite with reverted austenite.

A still further aspect of the present invention is directed to said process wherein said lean alloyed steel composition is processed into hot rolled steel strip and thereafter subjected to austenite reverse transformation (ART) involving initial austenitization between 890-910oC and typically at 900 oC and held for a time duration between 1 to 5 min and typically at 2 min. The steel is then quenched in salt bath at 450 oC 2-10 min and typically 5 min and water quenched and put at temperature of 770-790oC for 2-10min and typically 5 min and again quenched in water to get yield strength in the range of 410-511MPa,tensile strength of 721-767 MPa, total elongations of 44-47%, yield ratio of 0.53-0.71, work hardening exponent in the range of 0.21-0.27 and tensile toughness of 31.86-36.08 GPa.% with a microstructure of bainite with reverted austenite.
The composition and the mechanical properties of the invented steel grades are presented in following Table 1 and Table 2.

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 at different ART cycles.
Heat Treatment Condition Mechanical properties
YS
MPa UTL
MPa TE
% n K UTS*TE
GPa.%
900/2-WQ-780/5-WQ 620 760 27.75 0.16 1208 21.09
900/2-450/5-WQ-750/5-WQ 578 750 39.50 0.18 1192 29.62
900/2-450/5-WQ-780/5-WQ 410 to 511 721 to 767 44.18 to 47.05 0.21 to 0.27 1178 to 1359 31.86 to 36.08

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. At the inter critical temperature of 780 oC, the ferrite fraction is 67 % and the austenite fraction is 34%. Similarly at 750 oC austenitization temperature the equilibrium austenite fraction is 36% austenite and the rest is ferrite phase. These are the two temperature 750 and 780 oCat which the reverted austenite transformation was carried out.
The heat treatment cycle applied to the steel is shown in Fig.2. There are three different cycles that was carried out. In all the cycles, the steel was austenitized at 900 oC and held for 2 min. This is followed by three different cooling cycles. In the first cooling cycle the steel is quenched in water before reverted austenite transformation. In the second cycle the steel is subjected to an isothermal bainitic holding heat treatment in salt bath at 450 oC for 5 min. The partition of carbon from ferrite to austenite takes place as the bainitic transformation is an incomplete transformation reaction and retains carbon rich austenite during bainitic holding. The steel so quenched with martensite or bainitic microstructure was subjected to reverted austenite transformation at 780 oC.

The optical microstructure of the steel at various holding treatments is shown in Fig.3 and the SEM microstructure in Fig.4. It is seen that the steel quenched to room temperature from 900 oC followed by reverted austenite treatment at 780 oC shows tempered martensite with bright white spots of reverted austenite decorating the martensite block boundaries (SEM). The steel subjected to bainitic holding followed by reverted austenite heat treatment shows bainitic ferrite at 450 oC for 5 min, the subsequent reverted austenite transformation annealing was carried out at 750 oC and 780 oC as well. It is seen that reverted austenite microstructure decorates the bainitic sheaves. The properties varied with the cycle as shown in Table 2. Highly plastic properties were obtained in the isothermally held steel at 450 oC followed by reverted austenite formation at 780 oC. The XRD patterns of the steel are shown in Fig.5. The reverted austenite content at the three different heat treatment varied between 1 to 2.2% retained austenite. The steel isothermally held followed by reverted austenite transformation gave the highest retained austenite content of 2.2% which gave the best elongation in the steel. This condition was repeated more than once and gave the best 3rd Gen AHSS properties with tensile elongation varying between 32 and 36 GPa.%. The steel with isothermal holding and lower reverted austenite holding temperature of 750 oC gave 1.66% retained austenite and had lower ductility had a tensile toughness of 29 GPa.%. The same steel processed through the traditional reverted austenite transformation gave 21 GPa.%. Accompanying Fig. 6 shows Stress-strain diagram of the steel at various heat -treated cycles