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 :
A LEAN ALLOYED ADVANCED HIGH STRENGTH STEEL(AHSS) AND A PROCESS FOR ITS PRODUCTION THROUGH MODIFIED SINGLE STAGE QUENCH AND PARTITIONING 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 INVENTION
The present invention relates to a lean alloyed advanced high strength steel (AHSS) from C-Mn-Si steel microalloyed with Nb-Ti produced through single stage quench and partitioning(Q&P) heat treatment followed by air cooling whereby the modified Q&P treatment helps to convert the remaining retained austenite largely to bainite. The steel in its hot rolled condition with the thickness range of 1 to 5 mm by austenitizing above the Ac3 temperature for a brief time of 2 minutes followed by salt bath quenching in the temperature range of 330 to 360oC resulted in development of a unique microstructure containing bainite in the range of 40-50%, martensite in the range 5-10%, retained austenite in the range of 3-6% with rest of the phase being ferrite. The steel having ultimate tensile strength of >1000MPa with total elongation>25% to achieve tensile toughness close to or greater than 30GPa. %conforming to third generation AHSS. The steel has a low yield ratio of 0.40, with a great potential for energy absorption in plastic deformation in crash condition along with weight reduction of the automotive components.

BACKGROUND OF THE INVENTION
Weight reduction of the automotive components is a prime requirement of automotive industry for the improvement of fuel efficiency and reduction in CO2 emission to reduce environmental pollution. At the same time safety of the passengers is of great importance. Hence ultra high strength steels with excellent ductility are growing demand in the automotive industry. As cost of the automotive components increases with addition of alloying elements lean alloy steels are preferred for the cost effectiveness of the automotive components.

Prior Art
CN113061698A prior art report of the invention of Q&P steel clearly shows that it is different than the present invention. The present invention differs in chemical composition from said prior art w.r.t several alloying elements such as Mn, Cr, Ni, Mo and V. The reference prior art has used alloying elements like Cr, Ni and Mo and processed through Q&P treatment to achieve the required mechanical properties. However, in the proposed invention these alloying elements are not present for developing Q&P steel. Moreover the processing conditions in both the processes are entirely different. The prior art proposed 2 stage heat treatment process as mentioned in detail: rapidly transferring the sample from the salt bath at 770 ? to an oil bath at 130 ?, rapidly cooling the sample to 130 ?, heating the sample to 400 ?, preserving the heat for 500s, and then cooling to room temperature. However in the proposed invention our methodology adoption is different as shown in figure-1. So, the prior art CN113061698Adiffers in both chemical composition of the steel material as well as the processing conditions to achieve Q&P steel. Moreover, the mechanicalproperties in the prior art is 1600-2000MPa where as in the present invention 1040-1180MPa.

CN111440987Aprior art reports invention of Q&P steel has a chemical composition clearly different with the proposed inventions. The composition of the steel is close to the present composition, however, the prior art invention contains V whereas in the present invention Nb is the micro-alloying which differs from the prior art invention. In this prior art the inventors mentions the use of 2 stage heat treatment where they anneal the steel materials at 820-930 ? followed by slowly cooling to 660-700 ? at a cooling speed of 5-10 ?/s which further cools rapidly to 200-300 ? at a cooling speed of not lessthan 60 ?/s which further heated to 360-460 ? for 150-550 s to distribute the temperature of 360-460 ? for 150-550 s and finally cooled to room temperature. However in the proposed invention our methodology adoption for Q&P heat treatment is different as shown in figure-1. So, the prior art CN111440987Adiffers in both chemical composition of the steel material as well as the processing conditions to achieve Q&P steel. Moreover, the mechanicalproperties reported in the prior art is close to 980MPa tensile strength whereas in the present invention it is quite superior in the range of 1040-1180MPa.

CN103820613A prior art reports invention of Q&P steel from C-Mn-Al series TRIP 590 steel whereas in the present invention C-Mn-Si containing steel having Ti and Nb as micro alloying element is used. The tensile toughness reported in the prior art is 21-22 GPa% whereas in the present invention it is quite superior in the range of 22.5-32GPa%. In the prior art they used 2 stage heat treatment and their heat treatment processes include pre-processed TRIP590 cold-roll steel sheets were annealed at 1000-1150 °C for 2-5 min followed by rapidly cooling the steel sheets to 200-220 °C for 10-30 s followed by quenching. The quenched steel was then rapidly heated to 350-370 °C for 30-60 s and finally, water quenched. The austenization temperature used in the prior art invention is 1000 to 1150oC which is much higher compared to the present invention (900-950 and preferably 925oC). However in the proposed invention our methodology adoption for Q&P heat treatment is different as shown in figure-1. So, the prior art CN103820613A differs in both chemical composition of the steel material as well as the processing conditions to achieve Q&P steel in third generation AHSS regime.

KR101694875B1 prior art reports invention of Q&P steel from a chemical composition clearly different than the proposed invention. The composition of the steel is close to the invented steel except the additional Nb micro alloy addition as compared to the prior art invention. The heat treatment process used in prior art is heating the rolled pieces at a temperature of 800 to 900 ° C are rapidly cooled to 500 to 600 ° C at a cooling rate of 50 ° C / s, air-cooled for 5 to 10 seconds followed by cooling to Ms-Mf temperature range at a cooling rate of more than 50 ° C to obtain a structure of pro-eutectoid ferrite, martensite and retained austenite. Finally, after reeling, the steel is slowly cooled to room temperature to obtain a high strength hot rolled Q & P steel. However, in the proposed invention the steel is heated in a range of 900 – 950 °C and preferably 925oC followed by cooling below 400 ° C which is different than the temperature of 500-600 °C cited in the reference prior art. The detailed heat treatment process of the proposed invention is shown as a graphical form in Figure 1.The properties reported in the prior art invention with yield strength 700MPa, tensile strength 1300MPa and total elongation 10% or more with 20% retained austenite whereas in the proposed invention the properties such as yield strength 450-530MPa, ultimate tensile strength 1040-1180MPa and total elongation in the range 22 to 30% with retained austenite content in the range of 3-5% with bainite in the range of 30-50%.The prior art production process is completely different compared to the present invention.

CN102766818B prior art reports invention of Q&P steel from a chemical composition different than the proposed invention. The chemical composition of steel cited in the reference prior art differs with the proposed invention w.r.t the Mn, Al, Cr, Mo and Nb content. Mn and Al content in the prior art is significantly different than the proposed invention. The Cr and Mo is intentionally added to the cited prior art whereas Nb is an intentionally added alloying element in the proposed invention. The cited reference patent have boron content in the range of 0.002-0.004%. The Si & Al is added in such a way that its total content should be greater than 1% (Si + Al > 1%). In the heat treatment of the cited prior art, steel sample is heated to 950 ? in salt bath furnace for 30 minutes duration for complete austenitizing followed by oil quench to room temperature. The quenched steel is heated again to a temperature of 450 ? for 8-10 seconds for carbon partitioning followed by quenching at a cooling rates of 38 ?/s. The chemical composition and Q&P process cited in reference prior art is significantly different than the proposed invention. The mechanical properties are also superior in the present invention compared to the prior art.

CN103805851Aprior art reports invention of Q&P steel from a chemical composition different than the proposed inventions. The chemical composition of steel cited in the reference prior art is almost similar except the Ti and Nb content of the proposed invention. Ti is added intentionally to a higher value than the cited prior art to restrict the grain growth of austenite phase and thus achieve finer martensite with enhanced properties. Nb is also added intentionally to the proposed invention. In the claimed prior art, the Q&P steel is manufactured through hot rolling process where heating Strand or ingot heating, 1100 ~ 1200 ? of Heating temperatures, soaking time 1 ~ 2 hour followed by hot rolling 1000 ~ 1070? of start rolling temperatures, more than 950 ? and accumulative total deflection >=50%, subsequently intermediate blank treat temperature to 800 ~ 850 ?, then carry out last 3 ~ 6 passage rollings followed by press quenching Finish to gauge finishes rear steel plate and is cooled to 150 ~ 250 ? with the cooling rate of >50 ?/s, obtain martensite+residual austenite body tissue, after final batching, slowly cool to room temperature. The UTS reported >1400MPa and elongation >10% in the prior art whereas in the present invention UTS in the range of 1040-1180MPa and elongation in the range of 22 to 30%%. Based on the above mentioned details the proposed patent is different than the prior art cited document.

JP2019504202A prior art reports invention of Q&P steel from a chemical composition is different than the proposed invention. The chemical composition of steel cited in the reference prior art differs with the proposed invention w.r.t the Cr, Mo and V content. The Cr, Mo and V is intentionally added to the cited prior art which is different than proposed invention. In the claimed prior art, the Q&P steel is manufactured through hot rolling process where as in the proposed invention the Q&P process adopted is a heat treatment through salt bath single stage heat treatment process, as mention in figure-1. Based on the above mentioned details the proposed patent is different than the prior art cited document.

In applicant’s co-pending patent (202321055976 dated 21.08.2023) having almost similar chemical composition is used. However the heat treatment cycle has been modified with a different salt bath temperature and the final cooling as air cooling to promote bainite formation in the present invention to improve the ductility compared to the water quenched condition to promote martensite and retained austenite in the earlier invention.

In one of the publications (Quenching and Partitioning Steel Heat Treatment, Li Wang and John G. Speer, Metallogr. Microstruct. Anal. (2013) 2:268–281, DOI 10.1007/s13632-013-0082-8) mentioned the Q&P treatment with a different chemical composition than the proposed invention. The chemical composition of steel cited in the reference prior art differs with the proposed invention w.r.t the Ti and Nb content. The Ti and Nb is intentionally added in the proposed invention to achieve Q&P steel which differs from the cited prior art. In the claimed prior art, the Q&P steel is manufactured throughheat treatment where steel is heated to a temperature above Ac3 (annealing temperature) followed by slow cooling to a temperature below Ar3 (~ 740 ?) followed by quenching to a temperature between Ms and Mf with a cooling rate higher than 50 ?/s.Based on the above mentioned details the proposed patent is different than the prior art cited document.

In one of the publications (Thermal Stability of Austenite and Properties of Quenching &Partitioning (Q&P) Treated AHSS, R. M. Wu, L. Wang and X. J. Jin, Physics Procedia 50 (2013) 8 – 12) mentioned the Q&P treatment with a different chemical composition than the proposed invention. The chemical composition of steel cited in the reference prior art differs with the proposed invention w.r.t the Ti and Nb content. The Ti and Nb is intentionally added in the proposed invention to achieve Q&P steel which differs from the cited prior art. In the claimed prior art, the Q&P steel is manufactured throughheat treatment where steel was austenitized at 900? for 5 minutes, followed by quenching into salt bath at 320? for 60 seconds, then furtherquenched into water at room temperature. However in the present invention the hot rolled steel was austenitized in the temperature range of 900-950oC and preferably at 925oC for a brief time of 2 minute and then quenched in salt bath in the temperature range of 290-390oC for 5 minute for partitioning and then air cooled to transfer the retained austenite to bainite phase. Based on the above mentioned details the proposed patent is different than the prior art cited document.

In one of the publications (The effect of pre-quenching process on microstructure and mechanicalproperties in aNb-microalloyed low carbon Q-P steel, Jun Zhang, Hua Ding, Jingwei Zhao, University of Wollongong, Research Online) mentioned the Q&P treatment with a different chemical composition than the proposed invention. The chemical composition of steel cited in the reference prior art differs with the proposed invention w.r.t the Ti content. The Ti micro alloying element is intentionally added in the proposed invention to achieve Q&P steel which differs from the cited prior art. In the claimed prior art, the Q&P steel is manufactured through heat treatment process where two different routes are adopted to achieve Q&P steel and thus named as two groups based on heat treatment process (group 1 & 2). The first group was austenitized at 910 ? for 3 min, and then quenched to 220 ?. Subsequently, these quenched specimens were held at 400 ? in a time range from 5s to 500s. Finally, a second quenching process was carried out to room temperature. The second group was treated by a pre-quenching process, from 910 ? to room temperature. Then the samples were austenitized at 850 ? for 3 min followed by quenched to 220 ? and partitioned at400 ? from 5s to 500s. Finally, the partitioned samples were quenched to room temperature. The heat treatment process adopted in the proposed invention is quite different than the cited prior art. Based on the above mentioned details the proposed patent is different than the prior art cited document.

OBJECTS OF THE INVENTION
The basic object of the present invention is directed to provide a lean alloyed advanced high strength(AHSS) steel comprising C-Mn-Si steel microalloyed with Nb-Ti, produced through a process including modified quench and partitioning (Q&P) heat treatment cycle.

A further object of the present invention is directed to said lean alloyed AHSS wherein said steel in its hot rolled condition is subjected to Q&P heat treatment by austenitizing above the Ac3 temperature for a brief time of 2 minutes followed by salt bath quenching in the temperature range of 330 to 360oC followed by air cooling resulted in microstructure comprising, bainite, martensite, retained austenite and ferrite in selective proportion to achieve ultimate tensile strength of >1000MPa with total elongation>25%, tensile toughness close to or greater than 30GPa. % and a low yield ratio of 0.40 favouring energy absorption in plastic deformation in crash condition along with weight reduction of the automotive components.

A further object of the present invention is directed to said lean alloyed AHSS wherein superior strength and ductility properties are achieved due to the higher content of bainitic phase in the steel (>40%) in association with the TRIP effect due to the presence of small amount retained austenite.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to a lean alloyed advanced high strength steel (AHSS) having composition comprising
Carbon : 0.03 to 0.20 wt. % preferably 0.17%;
Mn: 1.5 to 1.8 wt. % Mn, preferably 1.73%;
Si: 1.0 to 1.5% Si; preferably 1.35% ;
microalloy including Nb 0.01 to 0.04% and Ti 0.03 to 0.05 wt.%
N: 0.003-0.006%;
residual Al 0.04 and 0.08% ;
S: 0.002 to 0.004 wt.%;
P: 0.015 to 0.017 wt.%; and balance is iron, and having ultimate tensile strength of >1000MPa in the range of 1050 to 1150MPawith total elongation>25% in the range of 26 to 30% to achieve tensile toughness close to or greater than 30GPa. % in the range of 30to 32GPa.%.

A further aspect of the present invention is directed to said lean alloyed advanced high strength steel(AHSS) as steel sheet having thickness in the range of 1-5mm having ultimate tensile strength in the range of 1050to 1150MPa, total elongation in the range of 26 to 30%, tensile toughness in the range of 30 to 32GPa. % and a low yield ratio of 0.40, with a great potential for energy absorption in plastic deformation in crash condition.

A further aspect of the present invention is directed to said lean alloyed advanced high strength steel(AHSS) having microstructure containing by volume fraction bainite in the range of 40-50%, martensite in the range 5-10%, retained austenite in the range of 3-6% with rest of the phase being ferrite.

A still further aspect of the present invention is directed to a process for producing lean alloyed advance high strength steel as described above comprising the steps of
(i)producing said steel through primary steel making process using hot metal from blast furnace in a EAF or BOF furnace, followed by secondary steel making process including a Ladle steel making routewith ferro alloy addition, to provide desired lean alloy steel composition comprising
C: 0.03 to 0.20 wt. % preferably 0.17%;
Mn :1.5 to 1.8 wt. % preferably 1.73%;
Si: 1.0 to 1.5%preferably 1.35% ;
Microalloys incluidng Nb 0.01 to 0.04% and Ti 0.03 to 0.05 wt.%,
N: 0.003-0.006%;
residual Al 0.04 and 0.08% ;
S: 0.002 to 0.004 wt.%;
P: 0.015 to 0.017 wt.%; and balance is iron;
(ii) casting slab from the said steel through continuous casting in slab caster;
(iii) hot rolling the cast slab to produce steel strip having thickness between 1 to 5 mm with yield strength 670-690 MPa, ultimate tensile strength in the range of 840 to 930MPa, total elongations in the range of 15 to 17 %, tensile toughness (product of ultimate tensile strength to total elongation) between 14 to 16 GPa%;
(iv)subjecting said hot rolled steel to single stage quench and partitioning (Q&P) heat treatment cycle to achieve said advanced high strength steel properties.

A still further aspect of the present invention is directed to said process wherein said hot rolling comprising
Slab Reheating temperature 1250oC
Roughing Mill temperature: 1210oC
Finishing temperature: 890oC
Coiling Temperature: 570oC
No of passes: = 6

Another aspect of the present invention is directed to said process wherein said Q & P heat treatment cycle comprising heating said hot rolled steel to temperature above Ac3 temperature in the range of 900-950oC and preferably 925oC; holding for 1 to 5 minute and preferably 2 min; followed by salt bath quenching in the temperature range of 330-360oC for a brief time of 2 to 10 minute and preferably 5minute for the single stage quench and partitioning and then air cooling to convert the remaining carbon enriched retained austenite to bainite.

Yet another aspect of the present invention is directed to said process leading to said Q&P heat treated advanced high strength steel (AHSS) having microstructure comprising bainite in the range of 40-50%, martensite in the range 5-10%, retained austenite in the range of 3-6% with rest of the phase being ferrite; and properties comprising
ultimate tensile strength of >1000MPa with total elongation>25% to achieve tensile toughness close to or greater than 30GPa. %; and
a low yield ratio of 0.40, with a great potential for energy absorption in plastic deformation in crash condition and weight reduction of automotive components obtained therefrom.

A further aspect of the present invention is directed to said process as claimed in anyone of claims 4 to 7 comprising selectively maintaining higher content of bainitic phase in the steel (>40%) in association with the TRIP effect due to the presence of retained austenite for achieving superior strength (ultra high strength>1000MPa) and ductility (>25%).

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: Single stage modified Q&P heat treatment cycle of the steel(Marked red is the regime of patent claimed).
Fig. 2: Engineering stress-strain diagram of the steel at various Q&P cycles.
Fig. 3: Optical microstructure of the steel at various Q&P cycles.
Fig. 4: SEM micrograph of the steel at various Q&P cycles.
Fig. 5: EBSD analysis of the steel in as-received condition.
Fig. 6: EBSD analysis of the steel subjected to single stage Q&P heat treatment 925oC/2min salt bath quench at 291oC/5min followed by air cooling.
Fig. 7: EBSD analysis of the steel subjected to single stage Q&P heat treatment 925oC/2min salt bath quench at 354oC/5min followed by air cooling.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS
The present invention is directed to the provide a lean alloyed steel composition range and typical composition as per Table 1, adapted for a quench and partitioning heat treatment cycle, to achieve the Third Generation Advanced High Strength Steel with tensile toughness greater thanor equal to 30 GPa% when subjected to austenitization above Ac3 temperature (925oC) for a brief time of 2 minutes followed by salt bath quenching in the temperature range of 330 to 360oC for a brief time of 5minute followed by air coooling. The steel used is comprised of of 0.03 to 0.20 wt. % carbon preferably 0.17% C; 1.5 to 1.8 wt. % Mn, preferably 1.73% 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 0.003-0.006% N. The residual Al varied between 0.04 and 0.08% 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 steel is a low carbon steel preferably with 0.17%C, 1.72% Mn, 1.35% Si as major alloying elements along with microalloying elements Ti and Nb that forms finer grain size by the formation of carbonitrides. The steel is aluminium killed and has 0.06% Al that refines the grain size by the formation of AlN phase. The rest of the residual elements in the steel include 0.02% Cr, 0.008% each of Ni and Cu, Mo content of 0.001 and V content of 0.002%. The Mn content enlarges the austenite field, lowers the A1 temperature. The Si content suppresses the formation of cementite and has a tendency to enable the formation of austenite.

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 rolled the cast slab to produce steel strip between 2 to 5 mm with yield strength 670-690 MPa, ultimate tensile strength in the range of 840 to 930MPa, total elongations in the range of 15 to 17 %, tensile toughness (product of ultimate tensile strength to total elongation) between 14 to 16 GPa%.

The present invention is directed to a process for manufacture of the lean alloyed steel having composition adapted for variable thermal processing comprising of 0.03 to 0.20 wt. % carbon preferably 0.17% C, Mn 1.5 to 1.8 wt. % preferably 1.73% Mn, Si 1 to 1.5 wt.% preferably 1.35%Si, Nb 0.01 to 0.04 wt. %, 0.03 to 0.05 wt.% Ti preferably 0.04%Ti, and 0.003-0.006% 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 and subjected to hot rolling to produce hot rolled strip between 1-5mm thickness.

Another aspect of the present invention is the design of heat treatment cycle where the single stage quench and partitioning (Q&P) is carriedoutby austenizattizing the steel above Ac3 temperature between 900 to 950oC preferrably at 925oC for 1 to 5 min and preferrably 2min followed by quenching in salt bath between 330 to 360oC preferrably at 334 and 354 for a time period of 1-10min preferrably 5 min and then air cooled.
The fraction of martensite formed (fQTm ) as a function of the quench temperature (QT) is given by Koinsteinen-Maurberger equation as follows;
fQTm = 1–exp (-1.1 x 10-2 (Ms-QT)) ----- Eq.(1)

The temperatures selected for salt bath holding are 291, 334, 354, 375 and 391oC for the calculated target martensite content of 75, 60, 50, 40, 25%. Optical and SEM micrographs were obtained for the microstructure evaluation. The phases were evaluated through the EBSD study. It was found that the superior strength (ultra high strength>1000MPa) and ductility (>25%) were achieved due to the higher content of bainitic phase in the steel (>40%) in association with the TRIP effect due to the presence of retained austenite.

The steel composition, processing parameters, properties and phases in microstructure achieved are presented in following Table 1 to 4.

Table-1: Chemical composition of the steel(wt.%)
Elements C Mn Si Nb Ti N Al S P AC1 AC3 Bs Ms
Trial range 0.03-0.2 1.0-1.8 1-1.5 0.01-0.04 0.03-0.05 0.003-0.006 0.040-0.08 0.002-0.004 0.015-0.017
Specific sample 0.17 1.73 1.35 0.02 0.04 0.005 0.06 0.003 0.016 731 875 557 417

Table-2: Steel Making and Hot Rolling Parameters
Steel making Making Blast furnace Hot metal ? 180 ton Basic oxygen furnace ? Ladle steel making
Continuous casting Slab size (Length *Width*thick=7250*1270*220 mm3)
Hot rolling Slab Reheating temperature 1250oC
Roughing Mill temperature: 1210oC
Finishing temperature: 890oC
Coiling Temperature: 570oC
No of passes: = 6

Table-3: Mechanical properties of the steel
Heat Treatment Condition Yield Strength, MPa Ultimate Tensile Strength, MPa Total Elongation, % Yield Ratio UTS*TE, GP. %
As-Received 489 927 17.35 0.53 16.09
925oC/2min/Salt bath quenching
291oC/5min/Air cooled 477 1178 24.27 0.40 28.59
334oC/5min/Air cooled 455 1149 26.08 0.39 29.96
354oC/5min/Air cooled 463 1073 29.96 0.43 32.14
375oC/5min/Air cooled 456 1072 24.77 0.42 26.54
391oC/5min/Air cooled 530 1039 21.66 0.51 22.50

Table-4: Phase Fraction Evaluated through EBSD
Heat Treatment Condition Ferrite (a) Bainite (aB) Martensite(a’) Retained Austenite (?R)
As-received 45.9 37.2 12.8 4.0
925oC/2min/Salt bath quenching at 291oC/5min/Air cooled 51.6 30.6 15.0 2.8
925oC/2min/Salt bath quenching at 354oC/5min/Air cooled 40.9 46.0 8.7 4.3

It is thus possible by way of the present invention to provide a lean alloyed AHSS steel produced through conventional steel making process followed by hot rolling to produce sheet thickness in the range of 1-5mm, which is subjected to Q&P heat treatment including heating to temperature above Ac3 temperature in the range of 900-950oC and preferably 925oC holding for 1 to 5 minute and preferably 2 min followed by salt bath quenching in the temperature range of 330-360oC for a brief time of 2 to 10 minute and preferably 5minute for the single stage quench and partitioning and then air cooling to convert the remaining carbon enriched retained austenite to bainite, whereby a unique microstructure containing bainite in the range of 40-50%, martensite in the range 5-10%, retained austenite in the range of 3-6% with rest of the phase being ferrite, is achieved resulting in ultimate tensile strength of >1000MPa with total elongation>25% to achieve tensile toughness close to or greater than 30GPa. %, and a yield ratio of 0.40, with a great potential for energy absorption in plastic deformation in crash condition coupled with weight reduction for automobile components.
, Claims:We Claim
1. A lean alloyed advanced high strength steel (AHSS) having composition comprising
Carbon : 0.03 to 0.20 wt. % preferably 0.17%;
Mn: 1.5 to 1.8 wt. % Mn, preferably 1.73%;
Si: 1.0 to 1.5% Si; preferably 1.35% ;
microalloy including Nb 0.01 to 0.04% and Ti 0.03 to 0.05 wt.%
N: 0.003-0.006%;
residual Al 0.04 and 0.08% ;
S: 0.002 to 0.004 wt.%;
P: 0.015 to 0.017 wt.%; and balance is iron, and having ultimate tensile strength of >1000MPa in the range of 1050 to 1150MPa with total elongation>25% in the range of 26 to 30% to achieve tensile toughness close to or greater than 30GPa. % in the range of 30 to 32GPa.%.
2. The lean alloyed advanced high strength steel (AHSS) as claimed in claim 1as steel sheet having thickness in the range of 1-5mm having ultimate tensile strength in the range of 1073 to 1149MPa, total elongation in the range of 26 to 29.9%, tensile toughness in the range of 29.9 to 32GPa.% and a low yield ratio of 0.40, with a great potential for energy absorption in plastic deformation in crash condition.
3. The lean alloyed advanced high strength steel(AHSS) as claimed in anyone of claim 1 or 2 having microstructure containing by volume fraction bainite in the range of 40-50%, martensite in the range 5-10%, retained austenite in the range of 3-6% with rest of the phase being ferrite.

4. A process for producing lean alloyed advance high strength steel as claimed in anyone of claims 1 to 3 comprising the steps of
(i) producing said steel through primary steel making process using hot metal from blast furnace in a EAF or BOF furnace, followed by secondary steel making process including a Ladle steel making routewith ferro alloy addition, to provide desired lean alloy steel composition comprising
C: 0.03 to 0.20 wt. % preferably 0.17%;
Mn :1.5 to 1.8 wt. % preferably 1.73%;
Si: 1.0 to 1.5%preferably 1.35% ;
Microalloys incluidng Nb 0.01 to 0.04% and Ti 0.03 to 0.05 wt.%,
N: 0.003-0.006%;
residual Al 0.04 and 0.08% ;
S: 0.002 to 0.004 wt.%;
P: 0.015 to 0.017 wt.%; and balance is iron;
(ii) casting slab from the said steel through continuous casting in slab caster;
(iii) hot rolling the cast slab to produce steel strip having thickness between 2 to 5 mm with yield strength 670-690 MPa, ultimate tensile strength in the range of 840 to 930MPa, total elongations in the range of 15 to 17 %, tensile toughness (product of ultimate tensile strength to total elongation) between 14 to 16 GPa%;
(iv)subjecting said hot rolled steel to single stage quench and partitioning (Q&P) heat treatment cycle to achieve said advanced high strength steel properties.

5. The process as claimed in claim 4 wherein said hot rolling comprising
Slab Reheating temperature 1250oC
Roughing Mill temperature: 1210oC
Finishing temperature: 890oC
Coiling Temperature: 570oC
No of passes: = 6

6. The process as claimed in anyone of claim4 or 5, wherein said Q & P heat treatment cycle comprising heating said hot rolled steel to temperature above Ac3 temperature in the range of 900-950oC and preferably 925oC; holding for 1 to 5 minute and preferably 2 min; followed by salt bath quenching in the temperature range of 330-360oC for a brief time of 2 to 10 minute and preferably 5minute for the single stage quench and partitioning and then air cooling to convert the remaining carbon enriched retained austenite to bainite.

7. The process as claimed in claim 4 to 6 leading to said Q&P heat treated advanced high strength steel (AHSS) steel having microstructure comprising bainite in the range of 40-50%, martensite in the range 5-10%, retained austenite in the range of 3-6% with rest of the phase being ferrite; and properties comprising
ultimate tensile strength of >1000MPa with total elongation>25% to achieve tensile toughness close to or greater than 30GPa. %; and
a low yield ratio of 0.40, with a great potential for energy absorption in plastic deformation in crash condition and weight reduction of automotive components obtained therefrom.

8. The process as claimed in anyone of claims 4 to 7 comprising selectively maintaining higher content of bainitic phase in the steel (>40%) in association with the TRIP effect due to the presence of retained austenite for achieving superior strength (ultra high strength>1000MPa) and ductility (>25%).

Dated this the 13th day of September, 2023
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199