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 :
Ti AND Mo ALLOYED LOW ALLOY STEEL AND METHOD OF PRODUCING THE SAME INVOLVING THERMAL PROCESSING CONDITIONS TO ACHIEVE SUPERIOR DUCTILITY AND ADVANCED HIGH STRENGTH STEEL PROPERTIES.



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 a Ti, Mo containing micro alloyed steel that is commercially manufactured by traditional BOF-LF- Hot rolling- air cooled route with a carbon content below 0.1 %C, (1.5 to 2.0) % Mn; (0.1 to 0.13) % Ti, (0.14 to 0.20)% Mo –(0.03 to 0.06)% Nb, and more particularly to a method to develop thermal processing conditions targeting steel so manufactured by hot rolling and air cooling to provide mechanical properties at various thermal processing conditions directed to advanced high strength steels with superior ductility conforming to third generation advanced high strength steel (AHSS).

BACKGROUND OF THE INVENTION
Advanced high strength steels are a class of steels where there is superior strength and elongation that enables stronger and highly formable grades of steels. Micro alloyed steels are steels with additions of Nb, V and Ti which promotes the formation of nano inter phase carbide precipitates that enhances the strength of the steel with simple processing that involves hot deformation followed by fast air cooling. Such steels have ferrite pearlite matrix and in the ferrite regions there is the formation of nano carbide precipitate that additionally strengthen the steel. The presence of pearlite in the steel however deteriorates the ductility and toughness in micro-alloyed steel. To obviate the loss in ductility and toughness a new micro-alloyed steel grade was evolved which has low carbon levels, that promote higher ferrite fraction and lower pearlitic content. The steel is strengthened mainly by addition of Tiand Mo as a major alloying elementswhich tend to promote the formation of a (TiMo)2C type inter phase precipitate that strengthen the matrix, while retaining good ductility. In this condition, the steel shows excellent range of mechanical properties of good strength and ductility. The product of tensile strength and ductility (tensile toughness) of the steel usually correspond to 15 GPa.%. The present invention has come up with a series of modified thermal processing routes, where the steel exhibits a versatile range of mechanical properties which are superior to the bench mark steel. The properties belong to the lower end of the third generation advanced high strength steels.

Prior Art
Some of the prior arts based on steel containing Ti and Mo alloyed low carbon steel family and their comparative features are summarized below.

Patent No. W 2013/044640 A1: A steel with (0.05 to 0.08) % C; (0.15 to 0.30)% Si; (1.55 to 1.85)% Mn. With P < 0.015%; S <0.005%; (0.015-0.04) %Al-(0.015-0.025) %Nb – (0.01 to 0.02) %Ti-(0.20-0.40) %Cr- (0.18-0.30) Mo -N<0.006%; O < 0.004 % and (0.0015 to 0.005) % Ca and Ni <0.4% with a Ca/S ratio greater than 1.5 has been reported. The steel gave high toughness with low yield ratio. This steel deals with strengthening associated with (TiMo)C nano precipitate which gives good toughness and strength. The invented steel has lower Si content <0.1% while the prior art steel has higher Si content; In addition, the invented steel has boron alloying to enhance hardenability, which is not the case with prior art steel. The prior art steel is calcium treated while the invented steel is not Ca treated. While the invented steel responds to the heat treatment in the prior art, it develops an entirely new range of mechanical properties at different heat treatment conditions that includes acicular ferrite formation, austempering heat treatment above Ac3 and from inter critical treatment, as quenched treatment above Ac3 and within intercritical range also hardened and tempered heat treatment. The invented steel meets automotive requirement with the product of tensile strength and elongation in the range 20 to 26 GPa.%. The ductility values were high at high strength levels. (Zhang Aiwen, Jiao Sihai, Yuan Xiangqian and Chen Yushan, Patent No. W 2013/044640 A1)

Patent No.BR112014000376B1: A wear resistant steel with (0.205 to 0.25) % C; (0.20 to 1.00)% Si; (0.2 to 1.00)%Mn; P < 0.015%; S <0.01%; (0.02-0.04) %Al; (0.01 to 0.02) %Ti; 0.70 %Cr; 0.30 %Mo; -N<0.006%; was produced by ingot or continuous casting and hot rolled at 1150 to 1250 oC with 70% reduction and finish rolling temperature of 860 oC rolled to thickness of 6 to 25 mm. The air-cooled steel show martensite with 5 to 10% retained austenite and the strength was 1000 MPa with 18% elongation. While the prior art steel shows retained austenite that cause TRIP effect, the invented steel has different chemistry and the steel invented has multiple heat treatment response responding to a wide variety of properties. The invented steel has much lower Si content and it is not designed for TRIP effect. The invented steel has boron alloying to enhance hardenability. The Cr and Mo range is also very much lower than prior art steel. The invented steel in the as-hot rolled and air-cooled condition relies on nano carbides. The steel invented responds to austempering develops bainitic ferrite, acicular ferrite and tempered martensite with different distinct mechanical properties, which is not reported. The carbon content of the invented steel is much lower to ensure good formability and weldability. The invented steel has high values of tensile strength and ductility, the product of which exceeds 20 to 26 GPa.%. The tensile strength is restricted to about 850 MPa. (Zhang Aiwen, Guodong Wang and Jiao Sihai Patent No. BR112014000376B1)

Patent No.BR112013032424B1: A high strength high tenacity steel sheet with (0.03 to 0.06) % C; 0.3%Si; (1.00 to 1.50)% Mn; 0.02%P; 0.01% S; (0.02-0.05) %Al; (0.005 to 0.025) %Ti; 0.30 %Mo; -N<0.006%; 0.005% Ca; 0.40 %Ni was manufactured by vacuum degassing, continuously cast slab or bar; heated to 1100 to 1250 oC in multi-pass or single pass rolling to 70% reduction with finish rolling temperature of 860 oC. The steel is water cooled at a cooling rate of 15 to 50 oC to final temperature of 200 to 300 oC and air cooled for 5 to 60 s. The 6 to 25 mm thick plates are further reheat treated at a speed of 1 to 10 oC/s to 450 to 550 oC, quench for 15 to 45 s followed by air cooling. The steel develops 700 MPa tensile strength with 50% elongation. The steel in the present invention has much lower Si content, Ti level in the invented steel is higher than in the prior art steel and the Mo level is lower, there is boron in the invented steel to enhance the hardenability and there is no Ni in invented steel. The invented steel responds to much wider range of heat treatments than in the prior art and the invented steel developed microstructures consisting of acicular ferrite, bainitic ferrite, tempered martensite and some with the combination of polygonal ferrite. The range of properties developed in the inveneted steel is much wider than the patented steel. (Zhang Aiwen, Guodong Wang and Jiao Sihai Patent No. BR112013032424B1)
CN103147005B: The patent describes a Thermomechanical processing of steel with (0.04 to 0.12) % C; (0.1-0.5) %Si; (1.40 to 1.60)%Mn; <0.018%P; <0.01% S; (0.02-0.07) %Al; (0.01 to 0.03) %Ti; (0.01-0.04) %Nb; (0.005-0.02) %Mo; (0.1-0.30) %Ni was produced and thermo-mechanically processed to achieve low temperature (-40 & -60 oC) toughness at low costs. The invented steel has much lower carbon and higher content of Ti and Mo that can precipitate as carbides. The Si content, Ti content and Mo content is much lower in the invented steel. The invented steel has no Ni and B content but the prior art steel has Ni content. The invented steel responds to a wide variety of heat treatments and exhibits versatile mechanical properties. The heat treatment includes full and partial austenitization followed by acicular ferrite formation, bainitic ferrite formation, tempered martensite formation. (Xiao Y, Wang Xin, Tang Zhigang, Wei Fanjie, Zheng Jianhua, Huang Weitao, Meng Gang and Mao JiQiang Patent No. CN103147005B)

CN 109536846 B: The patent describes a steel (0.06 to 0.10) % C; <0.1 %Si; (1.20 to 1.80)% Mn; <0.012%P; <0.006% S; 0.04 %Al;<0.004 %N; (0.06 to 0.12) %Ti; (0.1-0.3) %Mo; (0.1-0.30) %Ni, which has 700 MPa yield strength in a 1.5 to 14 mm thick plates with tensile strength between 780 and 850 MPa with elongation 20 to 25% and impact toughness 192 to 256 J at -40 oC. The invented steel has Nb addition for recrystallization during high temperature deformation and it has also boron for improved hardenability unlike the prior art steel. In addition, the invented steel responds to a wide variety of heat treatments that responds to a versatile range of microstructures and mechanical properties not in the prior art. (Duan Zhengtao, Pei Xinhua, GuoYuanyuan, and Sun MingJun, Patent no. CN 109536846 B)

Patent no. CN 110621794 A : The patent describes a steel for automotive frame structure that consists of (0.05 to 0.15)% C; (0.05-0.5) %Si; (1.00 to 2.00)% Mn;(2-35)ppm B; <0.04%P; <0.008% S;(0.005-0.1) %Al; (6-65) ppm N; (0.01 to 0.14) %Ti; (0.01-0.10 )Nb; (0-0.75)%Cu- (0.1-1.2)% Cr-(0.05-0.7) %Mo; (0-0.30) %Ni, (0.1-0.4)% V where in the as hot rolled air cooled condition a yield strength between 586 and 877 MPa ; tensile strength between 725 and 968 MPa and elongation between 9.5 and 14.8 %. The same steel in cold rolled in 1 mm thickness annealed condition gave a yield strength between 428 and 737 MPa; tensile strength between 730 and 922 MPa and elongation between 6.5 and 19 %. Compared to the prior art steel, the invented steel is leaner than the patented steel has consists of Cu, Cr, Ni and V while these are not present in the invented steel. The invented steel has a different set of heat treatment and microstructure and different range of mechanical properties. (Chen Shangpeng, and Mostert Richard, Patent no. CN 110621794 A)

Patent No. EP 3 147 381 B1: This patent shows a hot-rolled high-strength roll-formable steel sheet with excellent stretch-flange formability. The steel consists of (0.05 to 0.15)% C; (0-0.1) %Si; (1.00 to 2.00)% Mn; <0.003 %B; <0.012%P; <0.005% S;<0.1 %Al; 0.02% N; - (0.1-0.4)% V-(0.5-1.2) %Cr-(0.01-0.1) %Nb or(0.01-0.14) %Ti-(0.05 -0.70)% Mo – Ca treated hot-rolled high-strength roll-formable steel sheet with excellent stretch-flange formability has polygonal ferrite and bainitic ferrite is precipitation-strengthened with fine composite carbides and/or carbo-nitrides of V and/or of Mo and optionally of Ti and/or of Nb, and at most 5 vol.% of martensite and retained-austenite mixture. The yield ratio is at least 0.7 and a hole-expansion ratio (?) is greater than 40%. Compared to the prior art the invented steel has Compared to the prior art steel, the invented steel is leaner than the patented steel has consists of Cu, Cr, Ni and V while these are not present in the invented steel. The invented steel is not calcium treated like the prior art steel. The invented steel has a different set of heat treatment and microstructure and different range of mechanical properties. (Rijkenberg Rolf Arjan, and Aarnts Maxim Peter, Patent No. EP 3 147 381 B1)

Patent ES2670008T3: Steel sheet with low creep ratio and high toughness by Vacuum degassing continuous cast hot worked between 1150 to 1250 C with 80% reduction and a finish rolling temperature of 850 C followed by water quench at rate of 15 to 50 C/s. The Bs temperature is 60 to 100 C, then air cooled and bainitic isothermally heat treated by in line induction heating. The steel used are (0.05 to 0.08)% C; (0.15-0.30) %Si; (1.55 – 1.85)% Mn; <0.003 %B; <0.015%P; <0.005% S;(0.015-0.04) %Al; <0.006% N; (0.2-0.4) %Cr;(0.015-0.04) %Nb; (0.01-0.02) %Ti; (0.20 -0.40)% Mo; (0.0015-0.0050) Ca -0.004O-< 0.4 Ni, where Ca/S>1.5 in plates 10 to 25 mm could give Yield strength >500MPa; Yield ratio <0.75 and elongation greater than 20% and impact toughness at -60 C greater than 200 J. The invented steel has higher Si content, Lower Ti and Mo content in addition it is not Ca treatment compared to Prior art steel. The invented steel responds to a range of heat treatment including isothermal bainitic holding, where a versatile range of mechanical properties could be obtained. The bainitic heat treatment gives a set of better mechanical properties (Zhang Aiwen, Jiao Sihai, Yuan Xiangqian, Chen Yushan, Patent: ES2670008T3).

Patent: ES2895456T3: A high strength steel manufacturing is reported with (0.02 to 0.05)% C; (0.10-0.60) %Si; (1.1 – 2.0)% Mn; (0.01-0.15) %Al; (0.02-0.06) %Nb; <0.005 %B; <0.015%P; <0.005% S; (<0.5) %Cr; (0.01-0.08) %Ti; (0.20 -0.40)% Mo; (0.0015-0.0050)- <0.05Cu- <0.7% Ni,<0.1%V has 40 to 80% polygonal ferrite ; 20 to 40 % bainite along with <20% pearlite and martensite which gives a tensile strength greater than 500 MPa and Charpy V impact strength greater than 150 J in the -50 C. The invented steel has lower Si content. In addition, the steel is alloyed with Ni, V, Cu, higher level of Cr, lower Mo. The microstructure is a mixed microstructure after rolling and air cooling. In comparison the invented steel has structured heat treatments that gives a versatile range of microstructure and mechanical properties. (TastJouni, PikkarainenTeppo, LiimatainenTommi, and Rytinki Kati, Patent: ES2895456T3).

Patent No. US 5716464 reported a process for hot rolled steel strip with very high yield point as per composition, which is control cooled at a rate between 20 and 100 oC/s. The steel had a composition, ~0.1%C; <0.3 %Si; (1.0 – 2.0) % Mn; (0.01-0.1) %Al; (0.04-0.06) %Nb; <0.03%P; <0.01% S; %Ti; (0.1 -0.2) % Mo. The microstructure was consisted of ferrite and granular bainite, hardened by the precipitation of Nb and Ti carbides. The yield point was about 740 MPa and elongation between 15 and 20 and ductile to brittle transition below -50 oC. Compared to the patented steel, the invented steel has boron as an alloying element to enhance hardenability. The invented steel responds to a wide variety of heat treatment that includes partial and full austenitization followed by the formation of acicular ferrite, bainitic ferrite, and tempered martensite. The focus of the invented steel is on getting high product of strength and elongation in the range 20 to 26 GPa.%. The yield point in the invented steel is lower than the prior art steel. Xavier Bano, Istres; Maurice Malta, Martigues, Stéphane Fiori, Patent No. US 5716464

Patent No: US 9856548 B2: The patent describes a process for the manufacture of a hot rolled steel sheet with 1200 MPa tensile strength, yield ratio <0.75 and elongation > 10%. The steel had a composition (0.1 to 0.25) % C- (1 to 3)% Mn; > 0.015 % Al; <1.98% Si; <0.3% Mo; Cr< 1.5 %; <0.015 % S; <0.1%P; <1.5% Co and B< 0.005 %. The steel had 75% bainite; 5 % retained austenite and 2 % martensite. Compared to the prior art patent, the invented steel is not a high Si TRIP steel. The invented steel is devoid of expensive Co metal and the Cr range is much lower. The microstructure of the invented steel shows a wide variety of microstructure totally different as the thermal processing conditions are different and a totally different range of mechanical properties are obtained (Sebastien Allain, Audret Couturier, Thierry Iung, and Christine Colin, Patent No: US 9856548 B2)

Patent No. WO 2013/044640 A1: The Patent deals with manufacture of a steel plate with low yield ratio high toughness. The steel has a composition (0.05-0.08) %C; (0.15-0.3)%Si; (1.55-1.85) %Mn; <0.15 % P; <0.005 %S; (0.01-0.02) %Ti; (0.02 – 0.4) %Cr; (0.18-0.3) %Mo; < 0.006 %N; < 0.004 %O and (0.0015 o 0.0050) % Ca, with Ca/S ration greater than 1.5. The invented steel has much lower Si content and is additionally alloyed with B and Nb. The invented steel is not Ca treated unlike the prior art steel. The Cr levels are higher in the patented steel. The prior art steel showed 535 to 583 MPa Yield strength; 760 to 805 MPa tensile strength with 21 to 28 % elongation. The invented steel on the other hand has boron alloying additive, and Cr content is much lower. The Ti level is higher in the invented steel. [ Zhang Aiwen, Jiao Sihai, and Yuan Xiangqian, Patent No. WO 2013/044640 A1]

Patent WO 2013 /044641 A1: A high strength high toughness steel plate was developed by vacuum degassing, continuous cast billet, followed by deforming at 1100 to 1250 oC deformed by 70% reduction in multipass or single pass rolling with a finish rolling temperature > 860 oC, followed by water cooling to 200 to 300 oC at 15 to 50 oC/s. followed by air cooling for 5 to 60 s. The cooled plates are reheated in a inline furnace at 450 to 550 oC at 10 oC/s. This is with a tempering for 15 to 45 s and air cooled. in lates with 6 to 25 mm thickness. The steel had a composition (0.03-0.06) %C- <0.3 % Si- (1.0 – 1.5) % Mn- < 0.02 % P- <0.01 % S- (0.02-0.05) %Al- (0.005-0.025) %Ti- < 0.006 %N- < 0.005 Ca. Along with <0.75 %Cr - < 0.4 % Ni and < 0.3% Mo. The invented steel has slightly higher Mn content, higher Ti content, in addition it is not alloyed with Ni and the Cr level are lower. The invented steel has B as an alloying element which is not the case with Prior art steel. The Prior art deals with hardened and tempered condition, the invented steel includes several heat treatment responses with partial and full austenitization followed by isothermal holding to form acicular ferrite, bainitic ferrite and tempered martensite with a versatile range of mechanical properties. [ Zhang Aiwen, Jiao Sihai and Zhang Quingfeng, Patent WO 2013 /044641].

Patent WO 2013 /075473 A1: The patent deals with a steel plate manufacture with high strength and abrasion resistance and hardness > 420 HB. The steel has (0.205 to 0.25)%C; (0.2 to 1.0)%Si; (1.0-1.5)%Mn- < 0.015 %P; < 0.01 %S; (0.02 to 0.04) % Al ; (0.01 – 0.03) %Ti; < 0.006 % N; <0.005 Ca. In addition, the steel has at least one among < 0.7 % Cr or 0.5 %Ni or 0.3% Mo The steel is continuously cast, hot rolled from 1150 to 1250 oC with >70% reduction and finish rolling temperature of 860 oC followed by cooling below Ms with water quench below Ms temperature at a cooling rate 50 oC/s followed by air cooling. The steel shows martensite with 5 to 10% retained austenite to give yield strength > 1000 MPa with 18% elongation and 27 J Charpy V notch toughness at -40 oC. The invented steel is not having high Si content as the prior art steel and is not based on TRIP mechanism. The invented steel has born addition to improve hardenability. The Ti levels are higher and the additions Cr to the higher level or Ni are not added. While a single bainitic heat treatment was carried out in the prior art steel, the invented steel responds to several diferent het treatment and developes different microstructures that give a much versatile range of mechanical properties. [ Zhang Aiwen, Wang Guodong and Jiao Sihai Patent WO 2013 /075473 A1].

Patent No. WO 2016119500 A1: The patent reports a high crack arresting plate made of ( 0.04-0.12)% C; (0.1 to 0.3) % Si; (0.6 to 1.90) % Mn; < 0.015 % P; < 0.004 % S; (0.01 to 0.4)% Cr- (0.1 to 0.7)% Ni; ( 0.001 to 0.4 ) % Mo; ( 0.001 to 0.5 ) % Cu; ( 0.01 to 0.06 ) % Al; ( 0.01 to 0.06 ) % Nb ; ( 0.01 to 0.06 ) % Ti; < 0.06 % N; < (0.0001 to 0.0045) % Ca and the Ca/S ratio between 1 and 2. The continuously cast as slab, billet hot worked with controlled rolling and control cooling. The invented steel on the other hand does not have expensive Ni and Cu; the Si, Ti and Cr content is much lower. The prior art steel is calcium treated while it is not so in the invented steel. The yield strength of the plate was greater than 400 MPa with 64 J Charpy V-notch toughness at control cooled condition. The invented steel on the other hand shows different type of microstructure due to varied heat treatment involving the formation of acicular ferrite, bainitic ferrite and tempered martensite. The property range of invented steel is mucha more versatile than the invented steel. [Li Beng, Zhang Caiyi and Gao Shan, Patent No. WO 2016119500]

SETO Kazuhiro et al. in the publication introduces an inter phase precipitation strengthened low carbon steel with single phase ferrite microstructure with (Ti,Mo)C nano precipitates that gave yield strength 745 MPa; UTS 805 MPa ; elongation 20% with 100% hole expansion ratio. The steel is low carbon 0.04%C with Si in trace amount and alloyed with Ti and Mo. The present steel is different as it is additionally alloyed with Nb and B. It is also processed witnseveral different heat treatment where a wide variety of microstructure in introduced in the steel. SETO Kazuhiro , FUNAKAWA Yoshimasa*2 KANEKO Shinjiro*3, JFE TECHNICAL REPORTNo. 10,(Dec. 2007, p.19-25]

It is generally seen that the Ti, and Mo added steels are hot rolled and control cooled to get interphase precipitate which strengthen the matrix with inter phase precipitate. The present investigation brings forth that such steels can be further processed under different heat treatment conditions to get a wide variety of microstructure and excellent range of mechanical properties.

OBJECTIVES OF THE INVENTION
1. The basic objective of the invention is to develop a Ti, Mo containing micro alloyed steel that is commercially manufactured by traditional BOF-LF- Hot rolling- air cooled route with a carbon content less than 0.1 %C, (1.5 to 2.0) % Mn; (0.1 to 0.13) % Ti, (0.14 to 0.20)% Mo –(0.03 to 0.06)% Nb and the steel so manufactured by hot rolling and air cooling processed through a range of thermal processing condition targeting to ensure mechanical properties of advanced high strength steel at various thermal processing condition.
2. A further object of the present invention is directed to said Ti, Mo alloyed steel subjected to forced air cooling where the bench mark microstructure and mechanical properties as per traditional properties known in the prior art has been achieved.
3. A still further object of the present invention is directed to said steel is further subjected to a inter critical holding for austenitization followed by a Forced air cooling treatment to achieve a mixed microstructure.
4. A still further object of the present invention is directed to said steel subjected to austenitization temperature 875 oC, above Ac3 temperature followed by bainitic holding at a temperature close to Bs temperature in a salt bath whereby this treatment gives bainitic ferrite uniformly, where the mechanical properties and microstructures were obtained.
5. The steel subjected to a intercritical austenitization followed by holding isothermally at a temperature close to Bs temperature in a salt bath and above Ms temperature gives a carbon rich bainitic steels with inter critical ferrite with unique properties.
6. The steel subjected to full austenitization at 875 oC followed by water quenching and holding which tend to form a single stage quench partitioning process
7. The steel subjected to inter critical austenite followed by water quenching which tend to form a dual phase type of heat treatment where martensite with ferrite is formed.
8. The steel subjected to full austenitization at 875 oC followed by water quenching to form martensite followed by a low temperature tempering to form a hardened and tempered steel.
9. The steel subjected to intercritical austenitization at 800oC, where inter critical ferrite co-exist with austenite, which on water quenching tend to form ferrite and martensite, that on a low temperature tempering forms a soft ferrite containing tempered martensite steel.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to Ti and Mo alloyed steel having composition comprising,
carbon between 0.05 and 0.08 % and preferably 0.065% ; manganese between 1.5 and 2.0 %, preferably 1.65%; Ti ranging between 0.10 and 0.15 % preferably 0.11%; Mo ranging between 0.14 and 0.20 % preferably 0.17%; Nb ranging between 0.03 and 0.06 % preferably 0.04%; Boron between 0.002 and 0.005% preferably 0.005% and residual elements S less than 0.005%; P less than 0.02% ; Si less than 0.1%; Cr less than 0.05% with varied superior ductility and high strength properties including product of strength and ductility (tensile toughness) ranging from 15.58 to 26.03GPa% and tensile strength ranging from 834. to 608 MPa.

A further aspect of the present invention is directed to said Ti and Mo alloyed steel comprising thermally processed said selectively Ti and Mo microalloyed steel having selectively variable microstructure selected from (i)ferrite with nano-metric (TiMo) C interphase precipitates strengthening that show 820 MPa tensile strength with 19% elongation and yield ratio of 0.93 and tensile toughness was corresponding to 15.58 GPa%, (ii)intercritical ferrite and polygonal ferrite with interphase carbide having 814 MPa tensile strength and elongation of 29.4% with a yield ratio of 0.63, tensile toughness of 23.87 GPa.% (iii) self-tempered martensite having tensile strength of 834 MPa , elongation of 30.9% with an yield ratio of 0.67,tensile toughness of 25.77GPa.% (iv) free intercritical polygonal ferrite along with regions of martensite having tensile strength of 829 MPa , elongation of 31.4%,tensile toughness of 26.03 GPa % and the yield ratio 0.76 (v) fully tempered martensite having tensile strength of 702 MPa with an elongation of 25.2 % , yield ratio of 0.54, tensile toughness corresponding to 17.68 GPa.% (vi) intercritical ferrite along with regions of tempered martensite having tensile strength of 820 MPa with an elongation of 30.1% , yield ratio of 0.54, tensile toughness corresponding to 24.68 GPa.% (vii) complete acicular ferrite having 618 MPa tensile strength, elongation of 33.7% with an yield ratio of 0.62, tensile toughness of 20.8 GPa% (viii) soft carbide free intercritical polygonal ferrite and acicular ferrite having 818 MPa tensile strength,elongation of 30.2% with a yield ratio of 0.65, tensile toughness of 24.7GPa% (ix) granular bainitic ferrite having 608 MPa tensile strength and elongation of 33.4%,tensile toughness of 20.3GPa% and the yield ratio 0.52(x) intercritical polygonal ferrite along with granular bainite having 796 MPa tensile strength, elongation of 30.6% with an yield ratio of 0.8, tensile toughness of 24.3GPa%.

A still further aspect of the present invention is directed to a process for the manufacture of Ti and Mo alloyed steel as described above comprising:

(i) providing the steel composition comprising of carbon between 0.05 and 0.08 % and preferably 0.065% ; manganese between 1.5 and 2.0 %, preferably 1.65%; Ti ranging between 0.10 and 0.15 % preferably 0.11%; Mo ranging between 0.14 and 0.20 % preferably 0.17%; Nb ranging between 0.03 and 0.06 % preferably 0.04%; Boron between 0.002 and 0.005% preferably 0.005% and residual elements S less than 0.005%; P less than 0.02% ; Si less than 0.1%; Cr less than 0.05% ;

(ii) melting the said steel composition involving a primary steel making using BOF furnace, followed by secondary steel making in RH degasser followed by Ladle furnace treatment, followed by casting the liquid metal into a continuous cast slab preferably about 220 mm thickness;

(iii) subjecting the thus cast steel to hot rolling at 1230 to 980 oC preferably about 1190 oC initially through a roughing mill and finally through finishing mill to get a final hot rolled steel thickness of 1 to 6 mm preferably about 2.6 mm followed by picking hot rolled coils and subjecting to cold rolling to a final thickness of the wrought strip of 2 to 0.8mm preferably about 1.6 mm; and

(iv) selectively thermally processing the steel thus obtained Ti and Mo micro-alloyed steel such as to achieve varied superior ductility and high strength properties including tensile toughness ranging from 15.8 to 26.03GPa.% and tensile strength ranging from 834 to 608 MPa.

A still further aspect of the present invention is directed to said process wherein said step of selectively thermally processing the thus obtained Ti and Mo micro-alloyed steel comprises subjecting to heating to a temperature of 875 oC for full austenitization, followed by air cooling where the steel showed 820 MPa tensile strength with 19% elongation and had a yield ratio of 0.93. The tensile toughness was corresponding to 15.58 GPa.%, and generating a microstructure comprising of ferrite with nano-metric (TiMo)C interphase precipitate in the hot rolled and air cooled as-received condition.

Another aspect of the present invention is directed to said process wherein said step of selectively thermally processing the thus obtained Ti and Mo micro-alloyed steel comprises subjecting to heating to an austenitization temperature of 800 oC for 5 min duration for partial austenitization and inter critical ferrite, followed by air cooling, which gave 814 MPa tensile strength and elongation of 29.4% with a yield ratio of 0.63 with the tensile toughness of 23.87 GPa.% and having microstructure comprising of intercritical ferrite and polygonal ferrite with interphase carbide.

Yet another aspect of the present invention is directed to said process wherein the said step of selectively thermally processing the thus obtained Ti and Mo microalloyed steel comprises subjecting to heating to a austenitization temperature of 875 oC for 5 min duration for complete austenitization, followed by water quenching the steel to room temperature to form a soft lath martensitic matrix to steel having tensile strength of 834 MPa and elongation of 30.9% with an yield ratio of 0.67, with tensile toughness of25.77GPa.% wherein the microstructure corresponds to self-tempered martensite.

A further aspect of the present invention is directed to said process wherein said step of selectively thermally processing the thus obtained Ti and Mo micro-alloyed steel comprises subjecting to heating to a austenitization temperature of 800 oC for 5 min duration for partial austenitization, followed by water quenching to room temperature, where a tensile strength of 829 MPa and elongation of 31.4% was realized with tensile toughness of 26.03 GPa % and the yield ratio 0.76 with the final microstructure corresponding to free intercritical polygonal ferrite along with regions of martensite.

A further aspect of the present invention is directed to said process wherein said step of selectively thermally processing the thus obtained Ti and Mo micro-alloyed steel comprises subjecting to heating to a temperature of 875oC for 5 min duration for complete austenitization, followed by water quenching and tempering at 150 oC for 15 min duration to a tensile strength of 702 MPa with an elongation of 25.2 % and yield ratio of 0.54 with tensile toughness corresponding to 17.68 GPa.%. and having the microstructure corresponds to fully tempered martensite.

A still further aspect of the present invention is directed to said process wherein said step of selectively thermally processing the thus obtained Ti and Mo micro-alloyed steel comprises subjecting to heating to a temperature of 800oC for 5 min duration for partial austenitization, where intercritical ferrite forms, followed by water quenching and tempering at 150 oC for 15 min duration gave a tensile strength of 820 MPa with an elongation of 30.1% and yield ratio of 0.54, tensile toughness corresponding to 24.68 GPa.% with the microstructure corresponds to intercritical ferrite along with regions of tempered martensite.

A further aspect of the present invention is directed to said process wherein said step of selectively thermally processing the thus obtained Ti and Mo micro-alloyed steel comprises subjecting to heating to a austenitization temperature of 875 oC for 5 min duration for complete austenitization, followed by holding the steel at a temperature range above Bs temperature and below Ac1 temperature at 630 oC to get 618 MPa tensile strength and elongation of 33.7% with an yield ratio of 0.62, tensile toughness 20.8 GPa% with the microstructure of the steel consists of complete acicular ferrite.

A still further aspect of the present invention is directed to said process wherein said step of selectively thermally processing the thus obtained Ti and Mo micro-alloyed steel comprises subjecting to heating to a austenitization temperature of 800oC for 5 min duration for partial austenitization, followed by holding the steel isothermally at a temperature range above Bs temperature and below Ac1 temperature. at 630 oCto get 818MPa tensile strength and elongation of 30.2% with a yield ratio of 0.65. The tensile toughness was 24.7GPa.%. The microstructure consists of soft carbide free intercritical polygonal ferrite and acicular ferrite.

A still further aspect of the present invention is directed to said process wherein said step of selectively thermally processing the thus obtained Ti and Mo micro-alloyed steel comprises subjecting to heating to a austenitization temperature of 875 oC for 5 min duration for complete austenitization, followed by holding the steel isothermally at a temperature below Bs temperature and above MS temperature at 575oC to obtain 608 MPa tensile strength and elongation of 33.4% with tensile toughness of 20.3GPa% and the yield ratio 0.52 and with microstructure corresponding to granular bainitic ferrite.

A further aspect of the present invention is directed to said process wherein said step of selectively thermally processing the steel thus obtained Ti and Mo micro-alloyed steel comprises subjecting to heating to a austenitization temperature of 800oC for 5 min duration for partial austenitization, followed by isothermally holding the steel isothermally at a temperature range above Bs temperature and below Ac1 temperature at 575oC to obtain 796 MPa tensile strength and elongation of 30.6% with an yield ratio of 0.8, tensile toughness of 24.3GPa.% and the yield ratio was 0.80 with the microstructure corresponding to free intercritical polygonal ferrite along with granular bainite.

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 ACCOMPANYING DRAWINGS
Fig.1: Various thermal cycles for the heat treatment carried out on the Ti, Mo alloyed steel according to present invention.
Fig.2: TTT diagram corresponds to the steel as evaluated using MUCG83 software of Cambridge University.
Fig.3: Optical microstructures of the steel at different heat treatment conditions.
Fig.4:SEM microstructures of the steel at the different heat treatment conditions.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS

The steel in the present invention was made at JSW Steel, Vijayanagar using basic oxygen furnace followed by RH degassing and ladle furnace refining. The steel made was cast as slab of 300 mm. The cast slab was hot rolled in the JSW steel mill to 5 mm thick hot rolled coil and air cooled. The steel so obtained was subjected to a series of thermal processing treatments as shown in Fig. 1.
The steel studied in the present invention has a carbon content varying between 0.05% to 0.08%. This level of carbon promotes predominantly ferritic structure with extremely low pearlite fraction, which results in improved ductility and toughness of the steel. The steel has 1.5 to 2.0 % Mn content, which reduces the austenite forming temperature and enhances the austenite content during inter critical austenization treatment. In addition, the Mn content enhances hardenability of the steel and solid solution strengthening of the steel. The steel has Nb content 0.03 to 0.06%, which reduces TnR temperature that aids dynamic recrystallization and promotes fine prior austenite grain size in the steel. The steel has a Ti content varying between 0.10 and 0.15% and a Mo content varying between 0.14 and 0.20, both of which promote a nano (TiMo)2C inter phase precipitates in the steel that significantly enhances the strength of the steel in the austenitized and fast air cooled condition. The steel has < 50 ppm B which enhances the hardenability fo the steel and conditions the grain boundaries. The steel is aluminium killed and has a residual Al content less than 0.05% that combine with N and refines the grain structures. Cr is a residual element which is < 0.05%. Sulphur is maintained < 0.005% to reduce sulphide inclusions that impart anisotropy and decrease in toughness properties. The steel has P content < 0.02%, which is a residual element and prevents formation of embrittling phases.

Table 1 Chemical composition range of the steel developed and one typical composition of the steel produced
Steel C Mn S P Si Al Ti N B Nb Mo Cr
JSWNANO1
Range 0.05-0.08 1.5-2.0 < 0.005 <0.02 < 0.1 <0.05 0.10-0.15 <0.007 0.002-0.005 0.03-0.06 0.14-0.20 < 0.05
Typical 0.065 1.65 0.0033 0.018 0.050 0.055 0.110 0.006 0.005 0.040 0.17 0.02

Fig.1 shows the heat treatment carried out on the given steel (a) FAC: inter critically austenitized followed by forced air cooling (b) Water quenched from full austenite at 875 oC (c) Water quenched from full austenite at 875 oC (c) Water quenched from inter critical partial austenitization at 800 oC (d) Water quenched from full austenite at 875 oC and tempered at 150 oC (e) Water quenched from inter critical partial austenitization at 800 oC and tempered at 150 oC (f) Isothermal holding at 630 oC for acicular ferrite above Bs and below Widmanstatten start temperature post full austenitization at 875 oC (g) Isothermal holding at 630 oC for acicular ferrite above Bs and below Widmanstatten start temperature post partial austenitization at 800 oC within the inter critical temperatures. (h) Isothermal holding at 570oC for bainitic ferrite range below Bs and above Ms, post full austenitization at 875oCabove Ac3 (i)Isothermal holding at 570oC for bainitic ferrite range below Bs and above Ms, post partial austenitization at 800oCabove Ac3

The TTT Diagram of the steel was determined using the Cambridge University software MUCG83 as shown in Fig.2. The critical transformation points are shown. The steel as per the TTT diagram responds to a series of thermal processing conditions that give the steel a versatile range of mechanical properties some of which conform to the third generation advanced high strength steels. The heat treatments involved are shown in Fig.1. The heat treatment was intended to develop unique microstructural characteristics at each heat treatment conditions. The microstructures after the heat treatment were carried out using optical metallography as in Fig.3(a)-(h) and SEM microstructure Fig.4(a)-(h). Forced air cooling after inter critical treatment gives inter critical ferrite and polygonal ferrite with nano interphase carbides (Figs.3(a) & 4(a)). Water quenching from 875 oC directly produces self-tempered lath martensite like single stage quench partition (Figs. 3(b) &4(b)). Very fine martensite packets are seen along grain boundaries. The full austenitization at 875 oC, followed by quenching gives completely tempered martensite (Figs. 3(c) & 4(c)). The inter critical austenitization at 800 oC followed by quenching gives ferrite along with self-temperedmartensite (Figs. 3(d) &4(d)). The steel martensite formed in water quenching may be tempered at low temperature of 150 oC where a fine tempered martensite is formed. The fully austenitized sample form fully tempered martensite as in (Figs. 3(e) & 4(e)). The steel on holding isothermally in the acicular ferrite bay below Bs and Ms temperature at 630 oC gives fully acicular ferrite (Figs. 3(f) & 4(f)), when the austenitization is full at 875 oC. When the steel is partially austenitized at 800 oC and held isothermally in the acicular ferrite bay below Bs and Ms temperature at 630 oC gives soft inter critical ferrite and fully acicular ferrite (Fig.3(g)- 4(g)). The steel on holding in the bainitic bay below Bs and Ms temperature gives fully bainitic ferrite,when the austenitization is full when holding at 875 oC (Fig.3(h)- 4(h)). With intercriticalaustentization at 800 oC, followed by isothermal bainitic holding the steel isothermally below Bs and above Ms ensures the formation of intercritical ferrite along bainitic ferrite (Fig. 3(i)-4(i)).

Fig.2 shows the TTT diagram corresponds to the steel as evaluated using MUCG83 software of Cambridge University.
Fig.3 shows the optical microstructures of the steel at different heat treatment conditions(a) inter critical austenitization + Forced air cooled (b) Full austenitization + water quenching (c) Intercritical austenitization + water quenching (d) Full austenitization + water quenching+ tempering at 150 oC (e) Intercritical austenitization + water quenching+ tempering at 150 oC (f) Full austenitization +Isothermal holding in the acicular ferrite range at 630 oC + water quenching (g) Intercritical austenitization + Isothermal holding in the acicular ferrite range at 630 oC + water quenching (h) Full austenitization +Isothermal holding in the bainitic range at 570 oC + water quenching (i) Intercritical austenitization + Isothermal holding in the bainitic range at 570 oC + water quenching

Fig.4 shows the SEM microstructures of the steel at the different heat treatment conditions (a) inter critical austenitization + Forced air cooled (b) Full austenitization + water quenching (c) Intercritical austenitization + water quenching (d) Full austenitization + water quenching+ tempering at 150 oC (e) Intercritical austenitization + water quenching+ tempering at 150 oC (f) Full austenitization +Isothermal holding in the acicular ferrite range at 630 oC + water quenching (g) Intercritical austenitization + Isothermal holding in the acicular ferrite range at 630 oC + water quenching (h) Full austenitization +Isothermal holding in the bainitic range at 570 oC + water quenching (i) Intercritical austenitization + Isothermal holding in the bainitic range at 570 oC + water quenching

Table The typical mechanical properties of the steel
S No
Heat treatment Cycle Microstructure
Target 0.2%
Yield
Strength
MPa Tensile
Strength
MPa Total
Elongation
% Yield
Ratio UTS.%E
GPa.%
1 TM09BES
875 oC Hot deformed + Air cooled Ferrite +interphasecarbide
768
820
19

0.93

15.58
2. 800 oC Hot deformed + Forced Air cooled Ferrite +interphase carbide 509 814 29.4 0.63 23.87
3. 875 oC/5min + WQ Martensiteself tempered 558 834 30.9 0.67 25.77
4. 800 oC/5min + WQ Ferrite + martensite+ bainite 631 829 31.4 0.76 26.03
5. 875 oC/5min + WQ+150 oC/ 15 min Tempered martensite 382 702 25.2 0.54 17.68
6. 800 oC/5min + WQ +150 oC/15 min Ferrite + Tempered martensite 446 820 30.1 0.54 24.68
7. 875 oC/5 min +630 oC/15 min+WQ Accicular ferrite 389 618 33.7 0.62 20.8
8. 800 oC/5 min +630 oC/15 min+WQ Intercritical ferrite + Accicular ferrite 529 818 30.2 0.65
24.7
9. 875 oC/5 min +575oC/15 min Bainitic ferrite 316 608 33.4 0.52 20.3
10. 800 oC/5 min +575 oC/15 min Inter critical ferrite +
Bainitic ferrite 634 796 30.6 0.80 24.3

The steel is subjected to several different heat treatments, where the steel shows a wide variety of mechanical properties many of which makes the steel attractive as a third generation advanced high strength steel for automotive applications. The traditional heat treatment of the steel, involves hot forming followed by controlled cooling which promotes interphase carbide precipitate in a ferrite matrix. The properties correspond to 820 MPa tensile strength with 15.58% elongation with tensile toughness of16 GPa.%.
The present invention has come up with a series of thermal processing treatments, which shows that the steel can develop a versatile range of mechanical properties which third generation advanced high strength range.
Thermal Processing Route 1: Intercritical treatment + forced air cooling
The steel strip in this condition is subjected to inert critical austenitization at 800 oC for 5 min soaking time, where there is equilibrium ferrite and austenite is formed. As the ferrite has low carbon content, the carbon rejected by ferrite enriches austenite. The intercritical ferrite with carbon rich austenite is subjected to air cooling. The carbon rich austenite undergoes transformation to ferrite and interphase carbide precipitation in the austenite fraction. Thus, the microstructure consists of soft intercritical ferrite and harder ferrite with interphase carbide phase. As the ausatenite is enriched with carbon, the carbide fraction is likely to be higher than steel subject to full austenitization followed by air cooling. The steel in this condition shows a tensile strength of 814 MPa with 29.4 % elongation with a yield ratio of 0.63 and tensile toughness corresponds to 23.87 GPa.%.
Thermal Processing Route 2: Water quenched from full austenitization
The steel strip in this condition is heated to a temperature well above Ac3 at 875 oC where full austenitization is ensured. This is followed by directly quenching the steel in water to room temperature. As the steel is a very low carbon steel, soft lath martensite tends to form in the steel. The processing is similar to single stage quench partitioning, where the steel self-tempers with carbon partition to residual retained austenite. The steel in this condition, shows 834 MPa tensile strength with 30.9% elongation and tensile toughness is as high as 25.77 GPa.%. High strength is associated with lath martensite with high dislocation density. High ductility is observed as the carbide contents that make the steel brittle are extremely low. The yield ratio of the steel is 0.67.

Thermal Processing Route 3: Water quenched from Intercritical austenitization
The steel strip in this condition is heated to an intercritical austenitization temperature at 800 oC and held for 5 min, where the steel tends to form ferrite and carbide. As the carbon solubility of ferrite phase is low, the carbon from ferrite enriches the austenite. The steel from the intercritical temperature range is quenched rapidly to room temperature. The steel in this condition, forms softer intercritical ferrite, along with cabon rich lath martensite. The steel under this condition shows 829 MPa with elongation of 31.4 % and tensile toughnessof26.03 GPa.%. The yield ratio is 0.76%.
Thermal Processing Route 4: Hardening & Tempering
The steel strip in this condition is subjected to a hardening and tempering heat treatment. The 1.6 mm thick steel is subjected to full austenitization at 875 oC, for 5 min followed by water quenching to form martensite. The steel is subjected to a low temperature tempering at 150 oC for 15 min. The steel gave a decrease in strength from 820 MPa to 702 MPa strength but the ductility enhanced from 19 to 25.2 MPa with tensile toughness of 17.7 GPa.%. The yield strength is considerably lowered to 382 MPa. There is decrease in yield ratio to 0.56 on tempering. The steel in this condition has better work hardening characteristics. The steel is expected to have better hole expansion ratio.

Thermal Processing Route 5:Inter-critical Hardening and Tempering
The steel strip in this condition is heated to inert critical temperature range at 800 oC for partial austenitization where austenite is in equilibrium with ferrite. Due to low carbon solubility in ferrite the austenite fraction is enriched with carbon rejected by ferrite. The steel with partial ferrite and austenite is water quenched to form ferrite in martensite. This is followed by a low temperature tempering temperature of 150 oC for 15 min. The steel shows a tensile strength of 820 MPa which is equivalent to the bench mark heat treatment. There is a drastic improvement in elongation of 30.1%, The yield ratio is still low with 0.54. The yield strength is lower at 446 MPa. The tensile toughness is 24.68 MPa.

Thermal Processing Route 6: Isothermal Holding at Accicular Ferrite Range
The steel strip in this condition is subjected to austenitization above Ac3 temperature to form fully austenitic structure at 875 oC for 5 min. From the fully austenitic condition, the steel is cooled to a temperature of 630 oC which is above Bs temperature and below Ac1and held isothermally at this temperature using a salt bath to form acicular ferrite. The steel showed a lowered tensile strength of 618 MPa with 33.7% elongation, with a yield ratio of 0.62. The tensile toughness shows a value of 20.8 GPa.%. It is seen that the yield strength dips to 388 MPa. Hence, during acicular ferrite formation the contribution of interphase precipitation to strength is not effective.
Thermal Processing Route 7: Intercritical austenitization followed by Isothermal Accicular Ferrite holding
The steel strip in this condition is subjected to an inter critical austenitization at 800 oC for 5 min to form intercritical ferrite and austenitic microstructure. As the carbon solubility is lower in ferrite, the austenite gets enriched with carbon. The steel is then quenched in a salt bath at 630 oC, which is above Bs temperature and below Ac1 temperature. The austenite fraction held isothermally at 630 oC undergoes acicular ferrite formation. This mixed microstructure shows a strength of 818 MPa strength with 30.2% elongation that correspond to tensile toughness of 24.7 GPa.%. The yield ratio of the steel is 0.65. the excellent formability at 800 MPa benefits energy absorption and local formability.
Thermal Processing Route 8: Austempered BainiticFerritic Treatment
The steel strip in this condition is heated to a temperature above Ac3 to form a fully austenitic matrix. The steel is quenched rapidly in a molten salt bath maintained at 575 oC, which is below Bs temperature and held for 15 min. duration. The steel forms granular bainitic ferrite microstructure. The steel showed a tensile strength of 608 MPa with an elongation of 33.4%, with tensile toughness of 20.3 GPa.%. The yield ratio of the steel is 0.52. The steel shows lowest elongation of 315.5 MPa, which implies that there is no contribution of the interphase precipitate to strength.

Thermal Processing Route 9: Inter-critically held &Austempered Steel
The steel strip in this condition is heated to an intercritical austenitization temperature of 800 oC, where there is the formation of ferrite and austenite phases are in equilibrium. The carbon rejected by ferrite enriches the austenite. The steel from 800 oC, is quenched to a salt bath at 575 oC, which is below the Bs temperature. The carbon rich austenite undergoes transformation to bainitic ferrite. The soft intercritical ferrite enhances the ductility while the carbon rich austenite forms bainitic ferrite. The steel in this condition shows 796 MPa tensile strength with 30.6 % elongation. The tensile toughness is 24.3 GPa.% and the yield ratio is 0.8.

Thus, in the different conditions studied a versatile range of properties could be generated from the base steel. In most thermal processing conditions, the product of strength and elongation is more than 20 and it is as high as 26 GPa.%. Hence, the present invention shows that the thermal processing adopted enables achievement of high properties than the bench mark properties.

, Claims:We Claim:
1. Ti and Mo alloyed steel having composition
comprising of, carbon between 0.05 and 0.08 % and preferably 0.065% ; manganese between 1.5 and 2.0 %, preferably 1.65%; Ti ranging between 0.10 and 0.15 % preferably 0.11%; Mo ranging between 0.14 and 0.20 % preferably 0.17%; Nb ranging between 0.03 and 0.06 % preferably 0.04%; Boron between 0.002 and 0.005% preferably 0.005% and residual elements S less than 0.005%; P less than 0.02% ; Si less than 0.1%; Cr less than 0.05% with varied superior ductility and high strength properties including the product of strength and ductility (tensile toughness) ranging from 15.8 to 26.03 GPa.% and tensile strength ranging from834 to 608 MPa.

2. The Ti and Mo alloyed steel as claimed in claim 1 comprising thermally processed said selectively Ti and Mo micro-alloyed steel having selectively variable microstructure selected from (i)ferrite with nano-metric (TiMo) C interphase precipitate having 820 MPa tensile strength with 19% elongation and a yield ratio of 0.93 and tensile toughness corresponding to 15.58 GPa.% (ii)intercritical ferrite and polygonal ferrite with interphase carbide having 814 MPa tensile strength and elongation of 29.4% with a yield ratio of 0.63 andtensile toughness 23.87 GPa.% (iii) self- tempered martensite having tensile strength of 834 MPa , elongation of 30.9% with an yield ratio of 0.67 andtensile toughness 25.77GPa.% (iv) free intercritical polygonal ferrite along with regions of martensite having tensile strength of 829 MPa , elongation of 31.4% ,yield ratio 0.76 and tensile toughness of 26.03 GPa. % (v) fully tempered martensite having tensile strength of 702 MPa with an elongation of 25.2 % , yield ratio of 0.54 andtensile toughness corresponding to 17.68 GPa.% (vi) intercritical ferrite along with regions of tempered martensite having tensile strength of 820 MPa with an elongation of 30.1% , yield ratio of 0.54 and tensile toughness corresponding to 24.68 GPa.% (vii) complete acicular ferrite having 618 MPa tensile strength, elongation of 33.7% with an yield ratio of 0.62 and tensile toughness of 20.8 GPa% (viii) soft carbide free intercritical polygonal ferrite and acicular ferrite having 818 MPa tensile strength,elongation of 30.2% with a yield ratio of 0.65 andtensile toughness of 24.7GPa.% (ix) granular bainitic ferrite having 608 MPa tensile strength and elongation of 33.4%, yield ratio of 0.52 and tensile toughness of 20.3GPa.% (x) intercriticalpolygonal ferrite along with granular bainite having 796 MPa tensile strength , elongation of 30.6% with an yield ratio of 0.8 and tensile toughness of 24.3GPa%.
3. A process for the manufacture of Ti and Mo alloyed steel as claimed in anyone of claims 1 or 2 comprising:

(i) providing the steel composition comprising of, carbon between 0.05 and 0.08 % and preferably 0.065% ; manganese between 1.5 and 2.0 %, preferably 1.65%; Ti ranging between 0.10 and 0.15 % preferably 0.11%; Mo ranging between 0.14 and 0.20 % preferably 0.17%; Nb ranging between 0.03 and 0.06 % preferably 0.04%; Boron between 0.002 and 0.005% preferably 0.005% and residual elements S less than 0.005%; P less than 0.02% ; Si less than 0.1%; Cr less than 0.05% ;

(ii) melting the said steel composition involving a primary steel making using BOF furnace, followed by secondary steel making in RH degasser followed by Ladle furnace treatment, followed by casting the liquid metal into a continuous cast slab preferably about 220 mm thickness;

(iii) subjecting the thus cast steel to hot rolling at 1230 to 980 oC, preferably about 1190 oC initially through a roughing mill and finally through finishing mill to get a final hot rolled steel thickness of .1 to .6 mm preferably about 2.6 mm followed by picking hot rolled coils and subjecting to cold rolling to a final thickness of the wrought strip of 2 to 0.8 mm preferably at about 1.6 mm; and

(iv) selectively thermally processing the thus obtained Ti and Mo microalloyed steel such as to achieve varied superior ductility and high strength properties with tensile toughness ranging from 15.8 to 26.03 GPa.% and tensile strength ranging from 834 to 608 MPa.

4. The process as claimed in claim 3 wherein said step of selectively thermally processing the thus obtained Ti and Mo micro-alloyed steel comprises subjecting to heating to a temperature of 875 oC for full austenitization, followed by air cooling where the steel showed 820 MPa tensile strength with 19% elongation and had a yield ratio of 0.93. The tensile toughness was corresponding to 15.58 GPa.%, and generating a microstructure comprising of ferrite with nano-metric (TiMo)C interphase precipitate in the hot rolled and air cooled as-received condition.

5. The process as claimed in claim 3 wherein said step of selectively thermally processing the thus obtained Ti and Mo micro-alloyed steel comprises subjecting to heating to an austenitization temperature of 800 oC for 5 min duration for partial austenitization and inter critical ferrite, followed by air cooling, which gave 814 MPa tensile strength and elongation of 29.4% with a yield ratio of 0.63 andtensile toughnessof23.87 GPa.% with microstructure comprising of intercritical ferrite and polygonal ferrite with interphase carbide.

6. The process as claimed in claim 3 wherein said step of selectively thermally processing the thus obtained Ti and Mo micro-alloyed steel comprises subjecting to to heating to a austenitization temperature of 875 oC for 5 min duration for complete austenitization, followed by water quenching the steel to room temperature to form a soft lath martensitic matrix to steel having tensile strength of 834 MPa and elongation of 30.9% with an yield ratio of 0.67, with tensile toughnessof25.77GPa.% wherein the microstructure corresponds to self-tempered martensite.

7. The process as claimed in claim 3 wherein said step of selectively thermally processing the thus obtained Ti and Mo micro-alloyed steel comprises subjecting to heating to a austenitization temperature of 800 oC for 5 min duration for partial austenitization, followed by water quenching to room temperature, where a tensile strength of 829 MPa and elongation of 31.4% was realized with tensile toughness of 26.03 GPa % and the yield ratio 0.76 with the final microstructure corresponding to free intercritical polygonal ferrite along with regions of martensite.

8. The process as claimed in claim 3 wherein said step of selectively thermally processing the thus obtained Ti and Mo micro-alloyed steel comprises subjecting to heating to a temperature of 875oC for 5 min duration for complete austenitization, followed by water quenching and tempering at 150 oC for 15 min duration to a tensile strength of 702 MPa with an elongation of 25.2 % and yield ratio of 0.54 with tensile toughness of 17.68 GPa.%. and having the microstructure corresponds to fully tempered martensite.

9. The process as claimed in claim 3 wherein said step of selectively thermally processing the thus obtained Ti and Mo microalloyed steel comprises subjecting to heating to a temperature of 800oC for 5 minduration for partial austenitization, where intercritical ferrite forms, followed by water quenching and tempering at 150 oC for 15 min duration gave a tensile strength of 820 MPa with an elongation of 30.1% and yield ratio of 0.54 , tensile toughness of24.68 GPa.% with the microstructure corresponds to intercritical ferrite along with regions of tempered martensite.

10. The process as claimed in claim 3 wherein said step of selectively thermally processing the thus obtained Ti and Mo microalloyed steel comprises subjecting to heating to a austenitization temperature of 875 oC for 5 min duration for complete austenitization, followed by holding the steel at a temperature range above Bs temperature and below Ac1 temperature at 630 oC to get 618 MPa tensile strength and elongation of 33.7% with an yield ratio of 0.62, tensile toughness of 20.8 GPa% with the microstructure of the steel consists of complete acicular ferrite.

11. The process as claimed in claim 3 wherein said step of selectively thermally processing the thus obtained Ti and Mo microalloyed steel comprises subjecting to heating to a austenitization temperature of 800oC for 5 min duration for partial austenitization, followed by holding the steel isothermally at a temperature range above Bs temperature and below Ac1 temperature. at 630 oCto get 818MPa tensile strength and elongation of 30.2% with a yield ratio of 0.65,tensile toughnessof 24.7GPa%. The microstructure consists of soft carbide free intercritical polygonal ferrite and acicular ferrite.

12. The process as claimed in claim 3 wherein said step of selectively thermally processing the thus obtained Ti and Mo microalloyed steel comprises subjecting to heating to a austenitization temperature of 875 oC for 5 min duration for complete austenitization, followed by holding the steel isothermally at a temperature below Bs temperature and above MS temperature at 575oC to obtain 608 MPa tensile strength and elongation of 33.4% with tensile toughness of 20.3GPa% and the yield ratio 0.52 and with microstructure corresponding to granular bainitic ferrite.

13. The process as claimed in claim 3 wherein said step of selectively thermally processing the thus obtained Ti and Mo microalloyed steel comprises subjecting to heating to a austenitization temperature of 800oC for 5 min duration for partial austenitization, followed by isothermally holding the steel isothermally at a temperature range above Bs temperature and below Ac1 temperature at 575oC to obtain 796 MPa tensile strength and elongation of 30.6% with an yield ratio of 0.8, tensile toughness of 24.3GPa.% with the microstructure corresponding to free intercritical polygonal ferrite along with granular bainite.

Dated this the 6th day of September, 2022
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199