Claims:WE CLAIM

1. Lean alloyed steel composition adapted for variable thermal processing conditions for generating properties of the Third Gen Advanced high strength steel with tensile toughness between 25 and 30 GPa.%. comprising of 0.1 to 0.25 wt. % carbon preferably 0.17%C, Mn 1.5 to 1.2 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.

2. The lean alloyed steel composition as claimed in claim 1 adapted for processing including under 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 ferroalloy addition, followed by casting the same through continuous casting in slab caster, hot rolling of the cast slab to produce steel strip for application with generation of hot rolled steel between 2 to 5 mmwith tensile strength in the range 800 to 865 MPa and ductility in the range of 11 to 12 %.

3. A process for manufcature of third gen advanced high strength steel with tensile toughness between 25 and 30 GPa.%. from lean alloyed steel compostion as claimed in anyone of claims 1 or 2 comprising:

involving said selective lean alloyed steel compostion comprising of 0.1 to 0.25 wt. % carbon preferably 0.17%C, Mn 1.5 to 1.2 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 processing the same into hot rolled steel strip and thereafter subjecting to variable stepped autenization process including folowing:
Initial austenitization in the temperature range of 780 and 910 for a period of 1 to 25 min;
subsequent temperature rise to a range of 925 and 950 for 1 to 10 min;
holding temperature for 400 and 450 for 1 to 10 minutes;
final water quenching for bainitic microstructure for desired high strength and tensile toughness.

4. The process as claimed in claim 3 wherein said lean alloyed steel composition is processed into hot rolled steel strip and thereafter was subjected to stepped austenitization process involving initial austenitization between 860 and 910 oC and typically at 900 oC and held for a time duration between 1 to 25 min and typically at 20 min. in the first stage followed by stepping up to a temperature rise between 925 and 950 and typically at 920 oC and held for a duration between 1 and 10 min and typically at 5 min followed by holding at a temperature range between 400 and 450 and typically at 425oC and held for a time between 1 to 10 min and typically 5 min followed by final water quenching to achieve a bainitic microstructureand involving seelctively a yield strength of 437 MPa , tensile strength of about 993 MPa, elongation of 25% , tensile elongation 25.02 GPa.%, Yield ratio 0.44 and work hardening coefficient 0.18.

5. The process as claimed in claim 3 wherein said lean alloyed steel composition was processed into hot rolled steel strip and therefater was subjected to stepped austenitization process involving initial austenitization between 820 and 840 oC and typically at 830 oC and held for a time duration between 1 to 25 min and typically at 20 min. in the first stage followed by stepping up to a temperature rise between 925 and 950 and typically at 930 oC and held for a duration between 1 and 10 min and typically at 5 min followed by holding at a temperature range between 400 and 450 and typically at 425oC and held for a time between 1 to 10 min and typically 5 min followed by final water quenching such as to achieve a bainitic microstructure and involving seelctively a yield strength of 431MPa , tensile strength of about 974 MPa, elongation of 26.46 %, tensile elongation 25.77 GPa.%, Yield ratio 0.44 and work hardening coefficient 0.20.

6. The process as claimed in claim 3 wherein said lean alloyed steel composition is processed into hot rolled steel strip and thereafter subjected to stepped austenitization process involving initial austenitization between 800 and 820 oC and typically at 810 oC and held for a time duration between 1 to 25 min and typically at 20 min. in the first stage followed by stepping up to a temperature rise between 925 and 950 and typically at 930 oC and held for a duration between 1 and 10 min and typically at 5 min followed by holding at a temperature range between 400 and 450 and typically at 425oC and held for a time between 1 to 10 min and typically 5 min followed by final water quenching such as to achieve a bainitic microstructure and involving seelctively a yield strength of 397 MPa , tensile strength of about 971 MPa, elongation of 27.67 % , tensile elongation 26.87 GPa.%, Yield ratio 0.41 and the work hardening coefficient 0.21.

7. The process as claimed in claim 3 wherein said lean alloyed steel composition is processed into hot rolled steel strip and thereafter was subjected to stepped austenitization process involving initial austenitization between 780 and 800 oC and typically at 790 oC and held for a time duration between 1 to 25 min and typically at 20 min. in the first stage followed by stepping up to a temperature rise between 925 and 950 and typically at 930 oC and held for a duration between 1 and 10 min and typically at 5 min followed by holding at a temperature range between 400 and 450 and typically at 425oC and held for a time between 1 to 10 min and typically 5 min followed by final water quenching such as to achieve a bainitic microstructure and involving selctively a yield strength of 412 MPa, tensile strength of about 1021 MPa, elongation of 29.10 %. tensile elongation 29.71 GPa.%, the Yield ratio 0.40 and the work hardening coefficient 0.21.

Dated this the 1st day of January, 2022
Anjan Sen
Of Anjan Sen & Associates
(Applicants’ 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 :
ULTRA HIGH STRENGTH LOW-CARBON-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 present invention relates to ultra high strength low-carbon-bainitic steel and more particularly development of thermal processing cycle on a lean alloyed steel (0.17 wt. % C, 1.35 wt. % Si and 1.72 wt. % Mn) with micro alloying (0.02 wt. %Nb and 0.04 wt. %Ti) that could give ultra-high strength (about 1000 MPa) with good ductility (> 25%). The thermal processing was conducted with stepped austenitization treatment where the steel is intercritically heated or heated above A3 temperature for 20 min followed by a higher A3 temperature for full austenitization for 5 min holding followed by isothermal bainitic holding in a salt bath at 425oC to produce bainitic microstructures with retained austenite =3%. The steels thus processed shows yield strength in the range of 397 to 437 MPa; tensile strength in the range 971to1021MPa and elongation between 25.2 and 29.1%. The work hardening coefficient, n was found to be in the range 0.18 and 0.21 and the yield ratio was in the range 0.40 and 0.44. The product of UTS and % Elongation (tensile toughness), was found to be in the range 25.02 to 29.71GPa.%, which implies that the condition developed gives third generation advanced high strength steel (AHSS) properties. Using innovative thermal processing lean alloyed transformation induced plasticity (TRIP) steel was converted to give third generation AHSS steel.

BACKGROUND OF THE INVENTION
Automotive steel is continuously working on the development of steels of ultra high strength with excellent elongations to reduce weight of auto body parts by wall thinning for the improved fuel efficiency, reduction of CO2 emission and very good crash resistance properties for safety. Various steels are for the purpose are delta TRIP steel, medium manganese steel, TRIP aided bainitic steel (TBF) and quench and partitioning (Q& P) steels. The aim of such development is to achieve the strength level >1GPa and excellent elongations and tensile elongation (UTS*%Elongations) in the range of 20 to 40GPa. % for the development of third generation advanced high strength steels. The steels with ultra high strength and good ductility will not only help in weight reduction while better formability of the steels and energy absorption.
State of Prior Art
Liansheng et al., (Acta Metallurgica Sinica, Vol.51, No.5, 2015) reported effect of Mn pre-partitioning on carbon partitioning and retained austenite of Q&P steel. In the intercritical annealing process, Mn improves the stability of the austenite by partitioning from ferrite to austenite and the enrichment of Mn in austenite can also impact on the diffusion of C element from martensite to retained austenite in partitioning process. Based on C partitioning, Mn partitioning can further improve the product of strength and elongation, and has no negative effect on weldability of the low carbon high strength steel. Although step austenization is reported in the said literature for the Q&P, in the present study a series of step austenization followed by bainitic holding for achieving the higher strength and ductility in a lean alloyed steel.

Krizan et al, (19th International scientific conference, Transfer 2018) reported development of third generation advanced high strength steels for automotive applications where TBF steel with a cycle of austenization above A3 followed by bainitic holding is reported for the achievement of better properties, however in the present study a series of step heat treatment followed by bainitic holding is reported to achieve excellent combination of strength and elongation in the third generation regime.

Q&P and TBF steels with 1000-1500MPa strength with 10-20% elongations is reported by Fekhreddine et al, (IRJET, Vol. 8, Issue.3, 2021), while in the present invention a lower strength (about 1000MPa) with greater than 25 percentage of elongation is achieved by a series of step austenization heat treatment followed by bainitic holding.

Combination of Al, Nb and Mo in different processing conditions achieved a similar properties reported by Sugimoto et al (ISIJ Int., Vol. 47, No. 9,2007) however in the present invention a simple step heat treatment cycle with stepped austenitization treatment is used in a lean alloyed hot rolled steel without the expensive alloying element such as Mo for the achievement of ultra high strength with superior ductility.

OBJECTS OF THE INVENTION

The objective of the invention is directed to ultra-high strength steel with strength level close to 1000MPa and elongation greater than 25 percentages for use in automobile structures.

A further objective of the present invention is directed to a process to produce said ultra-high strength steel comprises thermal processing of lean alloyed steel including stepped austenitization treatment followed by isothermal bainitic holding in a salt bath with selected parameters to produce bainitic microstructures with retained austenite =3%, ensuring desired strength and ductility.

A further objective of the present invention is directed to provide an innovative thermal processing applied to a lean alloyed TRIP steel to convert to third generation AHSS steel.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to lean alloyed steel composition adapted for variable thermal processing conditions for generating properties of the Third Gen Advanced high strength steel with tensile toughness between 25 and 30 GPa.%. comprising of 0.1 to 0.25 wt. % carbon preferably 0.17%C, Mn 1.5 to 1.2 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 %..

A further aspect of the present invention is directed to a process for manufcature of third gen advanced high strength steel with tensile toughness between 25 and 30 GPa.%. from lean alloyed steel compostion as described above comprising:

involving said selective lean alloyed steel compostion comprising of 0.1 to 0.25 wt. % carbon preferably 0.17%C, Mn 1.5 to 1.2 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 processing the same into hot rolled steel strip and thereafter subjecting to variable stepped autenization process including folowing:
Initial austenitization in the temperature range of 780 and 910 for a period of 1 to 25 min;
subsequent temperature rise to a range of 925 and 950 for 1 to 10 min;
holding temperature for 400 and 450 for 1 to 10 minutes;
final water quenching for bainitic microstructure for desired high strength and tensile toughness.

Another 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 was subjected to stepped austenitization process involving initial austenitization between 860 and 910 oC and typically at 900 oC and held for a time duration between 1 to 25 min and typically at 20 min. in the first stage followed by stepping up to a temperature rise between 925 and 950 and typically at 920 oC and held for a duration between 1 and 10 min and typically at 5 min followed by holding at a temperature range between 400 and 450 and typically at 425oC and held for a time between 1 to 10 min and typically 5 min followed by final water quenching to achieve a bainitic microstructure and involving seelctively a yield strength of 437 MPa , tensile strength of about 993 MPa, elongation of 25% , tensile elongation 25.02 GPa.%, Yield ratio 0.44 and work hardening coefficient 0.18.

A further aspect of the present invention is directed to said process wherein said lean alloyed steel composition was processed into hot rolled steel strip and therefater was subjected to stepped austenitization process involving initial austenitization between 820 and 840 oC and typically at 830 oC and held for a time duration between 1 to 25 min and typically at 20 min. in the first stage followed by stepping up to a temperature rise between 925 and 950 and typically at 930 oC and held for a duration between 1 and 10 min and typically at 5 min followed by holding at a temperature range between 400 and 450 and typically at 425oC and held for a time between 1 to 10 min and typically 5 min followed by final water quenching such as to achieve a bainitic microstructure and involving seelctively a yield strength of 431MPa , tensile strength of about 974 MPa, elongation of 26.46 % , tensile elongation 25.77 GPa.%, Yield ratio 0.44 and work hardening coefficient 0.20.

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 stepped austenitization process involving initial austenitization between 800 and 820 oC and typically at 810 oC and held for a time duration between 1 to 25 min and typically at 20 min. in the first stage followed by stepping up to a temperature rise between 925 and 950 and typically at 930 oC and held for a duration between 1 and 10 min and typically at 5 min followed by holding at a temperature range between 400 and 450 and typically at 425oC and held for a time between 1 to 10 min and typically 5 min followed by final water quenching such as to achieve a bainitic microstructure and involving seelctively a yield strength of 397 MPa , tensile strength of about 971 MPa, elongation of 27.67 % , tensile elongation 26.87 GPa.%, Yield ratio 0.41 and the work hardening coefficient 0.21.

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 was subjected to stepped austenitization process involving initial austenitization between 780 and 800 oC and typically at 790 oC and held for a time duration between 1 to 25 min and typically at 20 min. in the first stage followed by stepping up to a temperature rise between 925 and 950 and typically at 930 oC and held for a duration between 1 and 10 min and typically at 5 min followed by holding at a temperature range between 400 and 450 and typically at 425oC and held for a time between 1 to 10 min and typically 5 min followed by final water quenching such as to achieve a bainitic microstructure and involving selctively a yield strength of 412 MPa , tensile strength of about 1021 MPa, elongation of 29.10 %. tensile elongation 29.71 GPa.%, the Yield ratio 0.40 and the work hardening coefficient 0.21.

The present invention thus relates to development of thermal processing condition to achieve ultra-high strength steel (close to 1000 MPa tensile strength) and excellent ductility (elongation between 25 and 30 %) from a lean alloyed hot rolled steel product of 3mm thickness. The thermal processing involved austeniztion of the steel above A3 for 20 min or intercritical treatments (20 min holding) followed by increasing the temperature above A3with a higher temperature for holding for a small time duration (5min) followed by isothermal holding in the bainitic zone (5min) in a laboratory made salt bath and then water quenching to develop bainitic microstructure that resulted in yield strength in the range of 397 to 437 MPa, tensile strength in the range of 971 to1021MPa, elongation in the range of 25.2 to 29.1%. The yield ratio of the steel was in the range of 0.40 to 0.44 and the work hardening coefficient in the range of 0.18 to 0.21.The tensile elongation is achieved in the range of 25.02 to 29.71GPa.% qualifying the Third Generation Advanced high strength steels. The steel is expected to give weight reduction in structures along with meeting formability requirements.
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: Heat Treatment cycle applied to the steel.
Fig.3: Optical Microstructure of the steel at various heat- treated cycles.
Fig.4: SEM micrograph of the steel at various heat- treated cycles.
Fig.5: High magnification SEM micrograph of the steel at various heat- treated cycles.
Fig.6: XRD of the heat treated sample with retained austenite =3%.
Fig.7: Stress-strain diagram of the steel at various heat -treated cycles.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS
The ultra high strength steel produced according to present invention is a lean alloyed low carbon steel with 0.10 to 0.25% carbon with typically 0.17%C, 1.0 to 1.5 wt.% Silicon with typically 1.35%Siand 1.5 to 2.0 wt.% manganese with typically 1.72% Mn as major elements. The steel is further alloyed with microalloying elements0.01 to 0.03 wt. % Niobium with typically 0.02% Nband0.03 to 0.05 wt.% titanium with typically 0.04%Nb. The residuals in the steel are 0.002 to 0.004 wt. % sulphur, 0.015 to 0.017 wt. % Phosphorous, 0.04 to 0.08 wt. % Aluminium. The nitrogen content is maintained < 60 ppm. as given in Table-1.The low carbon in the steel ensures good formability and weldability. The manganese in the steel enlarges austenite field at lower intercritical temperatures, in addition to, solid solution strengthening and enhancing the steel hardenability. The silicon content in the steel suppresses the cementite formation and promotes retained austenite, which transforms to martensite deformation by TRIP effect to provide the improved strength and ductility. The microalloying elements Ti, Nb, along with residual Al promote finer grain size in the steel. The phase diagram of the steel obtained is shown in Fig.1.
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 0.15-0.25 1.5-2.0 1.0-1.5 0.01-0.03 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

The typical chemistry of the steel conforming to the above specified range of steel composition in Table 1 is shown as the actual chemistry.
The steel is produced through conventional steel making involving blast furnace hot metal to basic oxygen furnace for primary steel making followed by secondary steelmaking to make the desired composition with ferro alloy additions. The material so produced was continuously cast as slabs and was hot rolled to 3 mm thick hot rolled steel strip. A detailed description of the production of the steel is summarized in Table2.
Table 2: Steel Making and Hot Rolling Parameters

Steel Making Blasite 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

The 3 mm thick steel is then subjected to a series of thermal processing cycles in a laboratory muffle furnace and laboratory salt bath furnace consisting of four different heat treatment cycles.
• Cycle-1: 900oC/20min (muffle furnace)-930oC/5min(muffle furnace)-425oC/5min(Salt bath)-WQ (Water Bath)
• Cycle-2: 830oC/20min(muffle furnace)-930oC/5min(muffle furnace)-425oC/5min(Salt bath)-WQ (Water Bath)
• Cycle-3: 810oC/20min(muffle furnace)-930oC/5min(muffle furnace)-425oC/5min(Salt bath)-WQ (Water Bath)
• Cycle-4: 790oC/20min(muffle furnace)-930oC/5min(muffle furnace)-425oC/5min(Salt bath)-WQ (Water Bath)
The thermal processing cycles are shown in Fig.2. The steels holding temperature increased in the muffle furnace after the inter critical or full austenization at lower austenization temperature to higher austenization temperature followed by salt bath quenching, holding for small time and water quenching. At 900 the sample was fully austenitized. At the inter critical austenization the steel is having austenite content calculated from Lever rule from Fig.1 as 43, 53 and 71 % at inter critical temperatures 790, 810 and 830 oC respectively and holding at such temperature lead to partitioning of carbon from ferrite to enrich the austenite. When the steel temperature is increased to 930oC again carbon tries to move from the austenite to ferrite for homogenization, however lower time lead to incomplete of the homogenization. For the different heat treatment cycle the optical microstructure of the steel is shown in Fig.3 with its corresponding SEM micrographs at low and high magnifications is shown in Fig.4 & Fig.5 respectively. The microstructure of the steel is predominantly bainitic in nature with small amount of martensite-retained austenite product and retained austenite. The retained austenite obtained through XRD is shown in Fig. 6 with the retained austenite content in the range of =3%.

Very good combination of strength and ductility were found for the heat treatment cycles, the stress- strain diagram of the steels is shown in Fig. 7 and the mechanical properties of the steel are summarized in Table 3. It is found that the tensile strength of the steels is in the range of 970MPa to 1021MPa with the elongations in the range of 25 to 29%. The tensile toughness is in the range of 25GPa. % to 29.71GPa, indicating that the processing conditions qualify the third generation advanced high strength steel property range. In one of the condition, the steel is almost close to 30GPa.%. Hence with a lean composition it is possible to achieve a third generation advanced high steel close to 30GPa%. It can also be found that the better properties came at lower inter critical austenization temperature (at 790oC for 20 min) followed by increasing the temperature to above A3 temperature (930oC) followed by bainitic holding at 425oC and then water quenched. With the increase in inter critical austenization temperature the properties found to be deteriorated however the tensile elongations does not fall below 25GPa. %.

Table 3Mechanical Properties of the steel at various heat treated conditions
Heat Treatment Condition n K YS, MPa UTS, MPa EL, % YR UTS*%E
900oC /20 min -930oC /5 min -425oC /5 min-WQ 0.18 1678 437 993 25.20 0.44 25.02
830oC /20 min -930oC /5 min -425oC /5 min–WQ 0.20 1685 431 974 26.46 0.44 25.77
810oC /20 min -930oC /5 min -425oC /5 min–WQ 0.21 1724 397 971 27.67 0.41 26.87
790oC /20 min -930oC /5 min -425oC /5 min–WQ 0.21 1820 412 1021 29.10 0.40 29.71

It was observed that the steel obtained under steel making and hot rolling only as stated above does not give ultra high strength (800-865 MPa tensile strength) properties and does not give elongation greater than 25%(actual11-12%). However, by adopting the step heat treatment cycles only lead to achieve the ultra high strength properties (close to 1000MPa tensile strength) and greater than 25% elongation.