Title of Invention	"A PROCESS FOR PRODUCTION OF NB TI TREATED TOUGH & DUCTILE ULTRA HIGH STRENGTH LOW ALLOY MARTENSITIC (USLAM) STEEL"
Abstract	There is disclosed a process for production of Nb-Ti treated tough & ductile Ultra-High Strength Low Alloy Martensitic (USLAM) steels, comprising the following steps: - Melting and casting a. Selecting necessary alloy compositions keeping the contents of alloying elements within the following limits, %c %Mn %Si %Cr %Ni %Mo % V %AI %S %P 0.18 1.00 0.90 1.10 0.65 0.25 0.06 0.04 0.015 0.020 to to to to to to to to Max. Max. 0.22 1.30 1.10 1.70 0.75 0.35 0.12 0.09 With Nb: 0.06% max & Ti: 0.02% max as micro-alloying elements. b. Melting the selected materials under careful conditions to achieve the desired chemistry. c. Subjecting the melt so obtained to complete de-oxidation using necessary quantities of one or more of agents selected from Fe-Si, Fe-Mn & Al. d. Monitoring and analyzing the melt, e. Effecting necessary final addition of alloying elements in any conventional manner, preferably by ladle/VAD treatment, for effective alloying of the elements, so as to, obtain the final alloy having the desired composition. f. And finally subjecting the final alloy to ingot casting by bottom pouring technique. g. Thermo-mechanically controlled processing finish rolling temperature in the range of 760°-950°C, complied with >15% deformation. h. Suitable combination of heat treatment parameters like temperature and holding/ soaking petrol with reference to Normalizing, Hardening, Low Temperature Tempering, High Temperature Tempering and Aging. 16

Title of Invention

"A PROCESS FOR PRODUCTION OF NB TI TREATED TOUGH & DUCTILE ULTRA HIGH STRENGTH LOW ALLOY MARTENSITIC (USLAM) STEEL"

Abstract

There is disclosed a process for production of Nb-Ti treated tough & ductile Ultra-High Strength Low Alloy Martensitic (USLAM) steels, comprising the following steps: - Melting and casting a. Selecting necessary alloy compositions keeping the contents of alloying elements within the following limits, %c %Mn %Si %Cr %Ni %Mo % V %AI %S %P 0.18 1.00 0.90 1.10 0.65 0.25 0.06 0.04 0.015 0.020 to to to to to to to to Max. Max. 0.22 1.30 1.10 1.70 0.75 0.35 0.12 0.09 With Nb: 0.06% max & Ti: 0.02% max as micro-alloying elements. b. Melting the selected materials under careful conditions to achieve the desired chemistry. c. Subjecting the melt so obtained to complete de-oxidation using necessary quantities of one or more of agents selected from Fe-Si, Fe-Mn & Al. d. Monitoring and analyzing the melt, e. Effecting necessary final addition of alloying elements in any conventional manner, preferably by ladle/VAD treatment, for effective alloying of the elements, so as to, obtain the final alloy having the desired composition. f. And finally subjecting the final alloy to ingot casting by bottom pouring technique. g. Thermo-mechanically controlled processing finish rolling temperature in the range of 760°-950°C, complied with >15% deformation. h. Suitable combination of heat treatment parameters like temperature and holding/ soaking petrol with reference to Normalizing, Hardening, Low Temperature Tempering, High Temperature Tempering and Aging. 16

Full Text	Introduction to the Field of the Invention This Invention relates to a process for production of Nb-Ti treated tough & ductile Ultra-High Strength tow Alloy Martensitic (USLAM) steels. Necessity of the invention It is already known to prepare the above mentioned USLAM steels by defined technique in use. These USLAM steels are prepared because Ultra Strength Low Alloy Martensitic (USLAM) steels find wide application in several fields including, aviation/automobile and other engineering sectors as shafts, rotors, sheets, etc. due to their extremely high strength/density ratio in the quenched & tempered condition coupled with cost-effectiveness. Prior Art and Draw backs However, these steels generally suffer from toughness & ductility related deficiencies i.e. low toughness & temper-embrittlement and poor ductility. The low tensile ductility & impact toughness at ambient are clearly revealed from the Table 1, which depicts the chemistry & properties of some existing USLAM steels. Moreover, these steels are highly susceptible to temper-embrittlement phenomena. Table 1, shows low impact toughness of some of these alloys when subjected to tempering/aging at around 500° C either the steels have poor toughness as the source of strength is primarily from medium carbon content i.e. 0.23%-0.5%, as shown for grades AISI 4340 (Si-modified), D-6a/D-6ac, Hy-TUF, AISi-4130, AISM140, AISI-4340 (Table-1) or they become expensive due to i, Addition of costly alloying elements Ni & Mo in substantial amount. ii. Additional processing cost towards refining in order to make the steels ultra Clean. Both these above mentioned steps are restored to for achieving a higher toughness e.g. BP-25 (Table-1). The following may be noted from the Table-1. *AII existing USLAM steels with strength>1500 MPa & CVN>50 J contain high amount of Ni & Mo. 2 Steels containing lower Ni & Mo content exhibit poor CVN vafues-sometimes nothing is specified. This reveals that combination of low Ni & Mo could not impart adequate impact toughness to USLAM steels. AISI-4130 & AISI-4140 are economical & strong but brittle, making it unsuitable for many critical & strategic applications. Objects of the Invention It is therefore a principal object of the Invention to propose improved USLAM steels and a method for the preparation of USLAM steels, which will have high toughness and good ductility. It is another object to propose such an improved process, which will enable the preparation of an improved USLAM steels, which will have carbon, nickel and molybdenum in smaller amounts than possible so far and yet have satisfactory properties. It is yet another object to propose such a method, which will enable use of Mn, Si & Cr for optimum utilization. It is a further object to propose such a method, which will enable use of micro-alloying elements selected from V, Nb, Ti & Al to achieve required properties. These and other objects of the Invention will be more clearfy understood from the following paragraphs. 3 Summary Of The Invention: Thus according to the basic aspect of the present invention there is provided a process for production of Nb -Ti treated tough & ductile Ultra-High Strength Low Alloy Martensitic (USLAM) steel comprising the following steps: a. melting selective EDD/Extra low Sulphur & Phosphorus scrap such as to achieve a selective composition comprising of : C = 0.18 to 0.22 %;Mn.= 1.00 to 1.30 %;Si. = 0.90 to 1.10 %;Cr. = 1.10 to 1.70 %;Ni. = 0.65 to 0.75 %;Mo. =0.25 to,0.35 %;V = 0.06 to 0.12 %;AI. = 0.04 to 0.09 %;S = upto0.015 %; P = upto 0.020 ; Nb. = upto 0.06 % and Ti = upto 0.02 % and balance Fe. after final addition of alloying & micro-alloying elements. b. subjecting the melt to complete de-oxidation using one or more of de- oxidisers selected from Fe-Si, Fe-Mn and At; c. effecting necessary final addition of any alloying elements in any conventional manner , preferably by ladle/VAD treatment for effective alloying of the elements such as to maintain the final alloy having said desired composition; d. subjecting the final alloy thus obtained to ingot casting by bottom- pouring technique; e. subjecting the ingot to selective thermo-mechanically controlled processing comprising finish rolling temperature in the range of 760° to 950° along with > 15% deformation; followed by, f) carrying out heat treatment operations with controlled parameters like temperature and holding/soaking period selectively involving Normalizing, Hardening, Low temperature Tempering ,High Temperature Tempering and Aging. Following the above process of the invention it is thus possible to provide the alloy having selective combination of the following elements :- i) C, Mn. Cr, Ni & Mo in small amounts; and ii) V, Nb, Ti & Al as micro-alloying elements. Importantly, the above method of the invention and in particular the process involving the stated Thermo-Mechanical Controlled Processing in the selective finish rolling temperature in the range of 760°-950° C alongwith the selective > 15 % deformation enable achieving Yield Strength (YS) in the range of 1010 - 1360 Mpa and Tensile Strength (UTS) in the range of 1410-1535 Mpa, Impact Strength (J) of 49/0-145.0 J at 200C and an elongation of 11.0-17.0 %. -4- It is found that by way of the above selective Thermo-Mechanically Controlled processing,it is possible to achieve higher Yield Strength (YS) and Tensile Strength (UTS) much more than that could be achieved by the proposed forging at a temperature of 1800.degree. to 2300.degree. F. and directly quenching said forging in the prior US'492 which could achieve an Yield strength of only about 620.6- 1137.7 Mpa and Tensile Strength (UTS) of about 827.4 - 1447.95. The above clearly and sufficiently establishes the advantages in the selective Thermo-mechanically Controlled processing including the finish rolling temperature in the range of 760°- 950° C along with the > 15 % deformation followed by the present process. It is thus possible following the above method to impart the Selective following combination of properties in the Ultra High Strength Low-Alloy Martensitic (USLAM) steel of the invention: i. Y.S. in the range of 750 Mpa to 1350 Mpa or more; ii. UTS in the range of 840 Mpa to 1540 Mpa or more ; iii. Impact toughness (CVN) at room temperature in the range of 48J to 145J or more; iv. Increase in impact toughness at room temperature to the tune of 10J (20%) compared to the existing /conventional alloys in case of Low Temperature Tempering (LTT); v. Remarkable enhancement (more than two-fold) in the imact toughness at R.T. after High Temperature Tempering (HTT) to the tune of 100J (220%) with a corresponding loss of 700 Mpa (82%) in UTS; vi. An almost fully ductile mode of fracture with fine & deep dimple network coupled with small micro-voids, the plausible sites for initiation of fracture; vii. Control of segregation/co-segregation of P,Mn,Cr,Ni etc. viii. Lesser susceptibility to both Temper Matensite Embrittlement and Tempered Embrittlement compared to similar steels. -5- This Invention will now be more fully described with reference to the following examples. Melting and casting a. Necessary alloy compositions were selected keeping the contents of alloying elements within the following limits: - %c %Mn %Si %Cr %Ni %Mo %V %AI %S %P 0.18 1.00 0.90 1.10 0.65 0.25 0.06 0.04 0.015 0.020 to to to to to to to to Max. Max. 0.22 1.30 1.10 1.70 0.75 0.35 0.12 0.09 With Nb: 0.06% max & Ti: 0.02% max as micro-alloying elements. b. The selected materials were melted under careful conditions to achieve the desired chemistry. c. The melt was then subjected to complete de-oxidation using necessary quantities of one or more of agents selected from Fe-Si, Fe-Mn & Al. d. The melt was well monitored and analyzed. e. Necessary final addition of alloying elements were effected in any conventional manner. However, ladle/VAD treatment was preferred for effective alloying. f. The final alloy so prepared had the following compositions: - i) Carbon Content = 0.19% ii) Nickel Content = 0.72% 6 iii) Molybdenum Content = 0.30% iv) Vanadium Content = 0.082% v) Sulphur Content 0.015% vi) Niobium Content 0.03-0.06% vii) Titanium Content 0.005-0.017% viii) Phosphorous Content 0.020% ix) Silicon Content 0.98% x) Manganese Content 1.20% xi) Chromium Content 1.66% xii) Aluminium Content 0.07% g. The final alloy was subjected to ingot casting. The material produced was well suited for hot working, cold shaping and heat treating. Hot working Rolling trials were conducted at various temperatures in order to assess the hot workability. Apart from working on the as-cast structure the suitable hot-working temperature was found to be in the range of 760°-950°C. Heat treatment Experimentation was conducted for all heat treatment operations viz. Annealing, Hardening, Tempering & Aging with various sets of parameters like temperatures,period of soaking etc. for the experimental heats and the ranges were established. 7 In further detail, the material, thus produced, behaves satisfactorily as can be seen from the following details. Hot working: Initial slow heating of as cast stock, if the stock input temperature is below 500° C. i. Forging/Rolling: a) Primary rolling of ingots/as-cast stock above 1000°C, b) Forging/rolling of blooms/billets/slabs above 900° C, c) Finish forging/rolling temperature in the range of 760°C-950°C. ii. Annealing: Annealing of stock of suitable size at Selection of the actual annealing temperature depends upon the shape & size of stock, as annealed hardness required, subsequent cold shaping/processing operations, type of furnace, end use envisaged etc. Cold Shaping of Stock: Undertaking of suitable cold shaping operations e.g. Bending, Machining, Cutting, Shearing etc. Heat treatment:. Heat treatment in three steps i.e. Normalizing, Hardening & Tempering. i) Normalizing: a. Air cooling from a range of 880°-950° C depending upon the composition, shape and size of the stock. b. Controlled furnace atmosphere preferable. 8 ii. Hardening: a. Solution treatment of the stock at austenitizing temperature (880°-950° C) b. Controlled furnace atmosphere preferable. c. Quenching in oil/water. iii. Tempering: a. Tempering temperature: 200°-350°C or 600°-650°C. b. Tempering time: 0.5-3 hr. Final Shaping: Grinding/precision machining/bending/shaping etc. The product properties were generally as follows: Product Properties: i) CE ii) Y.S. in the range of 750 Mpa to 1350 Mpa or more. iii) UTS in the range of 840 Mpa to 1540 Mpa or more,. iv) Impact toughness (CVN) at room temperature in the range of 48 J to 145 J or more. v) % elong in the range of 12% to 17% or more. 9 Moreover, the positive effects of the Thermo-Mechanically Controlled Processing (TMCP) and in particular the finish rolling temperature in the temperature of 760°-950° C and degree of deformation > 15 % on the Yield Strength (YS)ensile Strength (TS) and % Elongation were studied and noted as detailed hereunder: Effect of TMCP: FRT (º0C) Degree of deformation (%) Heat treatment Q: Quenching T: Tempering YS (Mpa) UTS (Mpa) Elongation (%) 902 15.5 Q:900Cx30 min T:300Cxl20 min 1200 1410 13.0 795 17.0 -do- 1090 1426 15.6 884 22.0 -do- 1220 1430 15.6 g. The hot worked product is normalized, oil/water quenched in accordance with the shape & size of the final product. The normalizing/solution treatment/ austenitization temperature & soaking periods are worked out on the basis of chemistry, shape & size of the product, heating rate etc. h. The final product is then tempered at selected temperature(s) and time period. The final heat treatment parameters are selected from amongst the following on the basis of application vis-a-vis performance related property requirements. Normalizing temperature: 880°-950°C, soaking period: 15-60 min. Hardening temperature: 880°-950° C, soaking period: 15-60 min. Tempering temperature: 220°-350º/600°-650°C. i. The heat treatment product is then finally shaped by grinding, precision machining final bending etc. c) Economics: This steel will be cheaper compared to similar existing grades due to • lower Ni & Mo contents • Lower hot-working temperature • Lower heat-treatment temperature. However, actual quantum of benefit in terms of processing cost will depend on many factors, some are as enumerated below: - a. Which particular USLAM or any other steel will be replaced by this steel? b. What are the absolute costs of ingredients like Ni, Mo, Si, Cr, Mn vis-a-vis their ferro-alloys at the plant of production. c. Yield/recovery of the above mentioned alloying elements with respect to the sources of addition. 11 d. The facilities available at the production unit or the process route followed compared to existing similar commercial grades of USLAM steel e.g. secondary refining, heat treatment furnace type & size etc. The benefit will depend upon whether secondary refining techniques like VAD, ladle degassing etc. is resorted to or by-passed. Benefit at the heat treatment operations will depends on the type of furnace, their sizes & capabilities. e. The desired final properties (application specific) which will ultimately decide all the processing parameters. Hence any quantification of the benefit at this stage is difficult. d) Impact in Industry : • USLAM steels can be made with above mentioned properties for application in the following areas as cheaper substitute, -Aviation industry for aircraft under carriages -Pressure vessels -Engineering components like high-strength bolts/fasteners, -Springs, high-speed rotors/shafts e:tc. • The steels can be processed/hot worked to desired shapes with out any difficulty. 12 WE CLAIM: 1. A process for production of Nb -Ti treated tough & ductile Ultra-High Strength Low Alloy Martensitic (USLAM) steel comprising the following steps: a. melting selective raw materials like EDO/Extra low Sulphur & Phosphorus scrap & other elements such as to achieve a selective composition comprising of: C = 0.18 to 0.22 %;Mn.= 1.00 to 1.30 %;Si. = 0.90 to 1.10 %;Cr. = 1.10 to 1.70 %;Ni. = 0.65 to 0.75 %;Mo. =0.25 to 0.35 %;V = 0.06 to 0.12 %;AI. = 0.04 to 0.09 %;S = upto 0.015 %; P = upto 0.020 ; Nb. = upto 0.06 % and Tj = upto 0.02 % and balance Fe. after final addition of alloying/ microalloying elements. b. subjecting the melt to complete de-oxidation using one or more of the de- oxidizers selected from Fe-Si, Fe-Mn and Al; C. effecting necessary final addition of any alloying elements in any conventional manner , preferably by ladle/VAD treatment , for effective alloying of the melt such as to maintain the final alloy having said desired composition; d. subjecting the final alloy thus obtained to ingot casting by bottom pouring technique; e. subjecting the ingot to selective thermo-mechanically controlled processing comprising finish rolling temperature in the range of 760° to 950° along with > 15% deformation; followed by, f) carrying out heat treatment operations with controlled parameters like temperature and holding/soaking period selectively involving Normalizing, Hardening, Low temperature Tempering ,Hlgh Temperature Tempering and Aging. 2.A method as claimed in claim 1 wherein the alloy produced has selective combination of the following elements:- - 13- i. C, Mn. Cr, Ni & Mo in small amounts; and ii. V, Nb, Ti & Al as micro-alloying elements. 3.A method as claimed in anyone of claims 1 or 2 wherein the alloy produced has the following combination of properties: i. Y.S. in the range of 750 Mpa to 1350 Mpa ; ii. UTS in the range of 840 Mpa to 1540 Mpa ; iii. Impact toughness (CVN) at room temperature in the range of 48J to 145J; iv. Increase in impact toughness at room temperature to the tune of 10J (20%) compared to the existing /conventional alloys in case of Low Temperature Tempering (LTT); v. Remarkable enhancement (more than two-fold) in the impact toughness at R.T. after High Temperature Tempering (HTT) t_ the tune of 100J (220%) with a corresponding loss of 700 Mpa (82%) in UTS; vi) An almost fully ductile mode of fracture with fine & deep dimple network coupled with small micro-voids, the plausible sites for initiation of fracture; - 14- vi. Control of segregation/co-segregation of P,Mn,Cr,Ni etc. vii. Lesser susceptibility to both Temper Matensite Embrittlement ment and Tempered Embrottlement compared to similar steels. 4.A method for the preparation of an alloy substantially as herein described with reference to the accompanying examples. There is disclosed a process for production of Nb-Ti treated tough & ductile Ultra-High Strength Low Alloy Martensitic (USLAM) steels, comprising the following steps: - Melting and casting a. Selecting necessary alloy compositions keeping the contents of alloying elements within the following limits, %c %Mn %Si %Cr %Ni %Mo % V %AI %S %P 0.18 1.00 0.90 1.10 0.65 0.25 0.06 0.04 0.015 0.020 to to to to to to to to Max. Max. 0.22 1.30 1.10 1.70 0.75 0.35 0.12 0.09 With Nb: 0.06% max & Ti: 0.02% max as micro-alloying elements. b. Melting the selected materials under careful conditions to achieve the desired chemistry. c. Subjecting the melt so obtained to complete de-oxidation using necessary quantities of one or more of agents selected from Fe-Si, Fe-Mn & Al. d. Monitoring and analyzing the melt, e. Effecting necessary final addition of alloying elements in any conventional manner, preferably by ladle/VAD treatment, for effective alloying of the elements, so as to, obtain the final alloy having the desired composition. f. And finally subjecting the final alloy to ingot casting by bottom pouring technique. g. Thermo-mechanically controlled processing finish rolling temperature in the range of 760°-950°C, complied with >15% deformation. h. Suitable combination of heat treatment parameters like temperature and holding/ soaking petrol with reference to Normalizing, Hardening, Low Temperature Tempering, High Temperature Tempering and Aging. 16

Full Text

Introduction to the Field of the Invention
This Invention relates to a process for production of Nb-Ti treated tough & ductile Ultra-High Strength tow Alloy Martensitic (USLAM) steels.
Necessity of the invention
It is already known to prepare the above mentioned USLAM steels by defined technique in use. These USLAM steels are prepared because Ultra Strength Low Alloy Martensitic (USLAM) steels find wide application in several fields including, aviation/automobile and other engineering sectors as shafts, rotors, sheets, etc. due to their extremely high strength/density ratio in the quenched & tempered condition coupled with cost-effectiveness.
Prior Art and Draw backs
However, these steels generally suffer from toughness & ductility related deficiencies i.e. low toughness & temper-embrittlement and poor ductility. The low tensile ductility & impact toughness at ambient are clearly revealed from the Table 1, which depicts the chemistry & properties of some existing USLAM steels. Moreover, these steels are highly susceptible to temper-embrittlement phenomena. Table 1, shows low impact toughness of some of these alloys when subjected to tempering/aging at around 500° C either the steels have poor toughness as the source of strength is primarily from medium carbon content i.e. 0.23%-0.5%, as shown for grades AISI 4340 (Si-modified), D-6a/D-6ac, Hy-TUF, AISi-4130, AISM140, AISI-4340 (Table-1) or they become expensive due to
i, Addition of costly alloying elements Ni & Mo in substantial amount.
ii. Additional processing cost towards refining in order to make the steels ultra Clean.
Both these above mentioned steps are restored to for achieving a higher toughness e.g. BP-25 (Table-1).
The following may be noted from the Table-1.
*AII existing USLAM steels with strength>1500 MPa & CVN>50 J contain high amount of Ni & Mo.
2

Steels containing lower Ni & Mo content exhibit poor CVN vafues-sometimes nothing is specified.
This reveals that combination of low Ni & Mo could not impart adequate impact toughness to USLAM steels.
AISI-4130 & AISI-4140 are economical & strong but brittle, making it unsuitable for many critical & strategic applications.
Objects of the Invention
It is therefore a principal object of the Invention to propose improved USLAM steels and a method for the preparation of USLAM steels, which will have high toughness and good ductility. It is another object to propose such an improved process, which will enable the preparation of an improved USLAM steels, which will have carbon, nickel and molybdenum in smaller amounts than possible so far and yet have satisfactory properties.
It is yet another object to propose such a method, which will enable use of Mn, Si & Cr for optimum utilization.
It is a further object to propose such a method, which will enable use of micro-alloying elements selected from V, Nb, Ti & Al to achieve required properties.
These and other objects of the Invention will be more clearfy understood from the following paragraphs.
3

Summary Of The Invention:
Thus according to the basic aspect of the present invention there is provided a process for production of Nb -Ti treated tough & ductile Ultra-High Strength Low Alloy Martensitic (USLAM) steel comprising the following steps:
a. melting selective EDD/Extra low Sulphur & Phosphorus scrap such as
to achieve a selective composition comprising of :
C = 0.18 to 0.22 %;Mn.= 1.00 to 1.30 %;Si. = 0.90 to 1.10
%;Cr. = 1.10 to 1.70 %;Ni. = 0.65 to 0.75 %;Mo. =0.25 to,0.35
%;V = 0.06 to 0.12 %;AI. = 0.04 to 0.09 %;S = upto0.015 %;
P = upto 0.020 ; Nb. = upto 0.06 % and Ti = upto 0.02 % and
balance Fe. after final addition of alloying & micro-alloying
elements.
b. subjecting the melt to complete de-oxidation using one or more of de-
oxidisers selected from Fe-Si, Fe-Mn and At;
c. effecting necessary final addition of any alloying elements in any
conventional manner , preferably by ladle/VAD treatment for
effective alloying of the elements such as to maintain the final alloy
having said desired composition;
d. subjecting the final alloy thus obtained to ingot casting by bottom-
pouring technique;
e. subjecting the ingot to selective thermo-mechanically controlled
processing comprising finish rolling temperature in the range of 760°
to 950° along with > 15% deformation; followed by,
f) carrying out heat treatment operations with controlled parameters like temperature and holding/soaking period selectively involving Normalizing, Hardening, Low temperature Tempering ,High Temperature Tempering and Aging.
Following the above process of the invention it is thus possible to provide the alloy having selective combination of the following elements :-
i) C, Mn. Cr, Ni & Mo in small amounts; and
ii) V, Nb, Ti & Al as micro-alloying elements.
Importantly, the above method of the invention and in particular the process involving the stated Thermo-Mechanical Controlled Processing in the selective finish rolling temperature in the range of 760°-950° C alongwith the selective > 15 % deformation enable achieving Yield Strength (YS) in the range of 1010 - 1360 Mpa and Tensile Strength (UTS) in the range of 1410-1535 Mpa, Impact Strength (J) of 49/0-145.0 J at 200C and an elongation of 11.0-17.0 %.
-4-

It is found that by way of the above selective Thermo-Mechanically Controlled processing,it is possible to achieve higher Yield Strength (YS) and Tensile Strength (UTS) much more than that could be achieved by the proposed forging at a temperature of 1800.degree. to 2300.degree. F. and directly quenching said forging in the prior US'492 which could achieve an Yield strength of only about 620.6- 1137.7 Mpa and Tensile Strength (UTS) of about 827.4 - 1447.95.
The above clearly and sufficiently establishes the advantages in the selective Thermo-mechanically Controlled processing including the finish rolling temperature in the range of 760°- 950° C along with the > 15 % deformation followed by the present process.
It is thus possible following the above method to impart the Selective following combination of properties in the Ultra High Strength Low-Alloy Martensitic (USLAM) steel of the invention:
i. Y.S. in the range of 750 Mpa to 1350 Mpa or more;
ii. UTS in the range of 840 Mpa to 1540 Mpa or more ;
iii. Impact toughness (CVN) at room temperature in the range of 48J to 145J or more;
iv. Increase in impact toughness at room temperature to the tune
of 10J (20%) compared to the existing /conventional alloys in
case of Low Temperature Tempering (LTT);
v. Remarkable enhancement (more than two-fold) in the imact toughness at R.T. after High Temperature Tempering (HTT) to the tune of 100J (220%) with a corresponding loss of 700 Mpa (82%) in UTS;
vi. An almost fully ductile mode of fracture with fine & deep dimple network coupled with small micro-voids, the plausible sites for initiation of fracture;
vii. Control of segregation/co-segregation of P,Mn,Cr,Ni etc.
viii. Lesser susceptibility to both Temper Matensite Embrittlement and Tempered Embrittlement compared to similar steels.
-5-

This Invention will now be more fully described with reference to the following examples.
Melting and casting
a. Necessary alloy compositions were selected keeping the contents of alloying elements within the following limits: -

%c %Mn %Si %Cr %Ni %Mo %V %AI %S %P
0.18 1.00 0.90 1.10 0.65 0.25 0.06 0.04 0.015 0.020
to to to to to to to to Max. Max.
0.22 1.30 1.10 1.70 0.75 0.35 0.12 0.09
With Nb: 0.06% max & Ti: 0.02% max as micro-alloying elements.
b. The selected materials were melted under careful conditions to achieve the desired
chemistry.
c. The melt was then subjected to complete de-oxidation using necessary quantities of one or
more of agents selected from Fe-Si, Fe-Mn & Al.
d. The melt was well monitored and analyzed.
e. Necessary final addition of alloying elements were effected in any conventional manner.
However, ladle/VAD treatment was preferred for effective alloying.
f. The final alloy so prepared had the following compositions: -
i) Carbon Content = 0.19%
ii) Nickel Content = 0.72%
6

iii) Molybdenum Content = 0.30%
iv) Vanadium Content = 0.082%
v) Sulphur Content 0.015%
vi) Niobium Content 0.03-0.06%
vii) Titanium Content 0.005-0.017%
viii) Phosphorous Content 0.020%
ix) Silicon Content 0.98%
x) Manganese Content 1.20%
xi) Chromium Content 1.66%
xii) Aluminium Content 0.07%
g. The final alloy was subjected to ingot casting.
The material produced was well suited for hot working, cold shaping and heat treating.
Hot working
Rolling trials were conducted at various temperatures in order to assess the hot workability. Apart from working on the as-cast structure the suitable hot-working temperature was found to be in the range of 760°-950°C.
Heat treatment
Experimentation was conducted for all heat treatment operations viz. Annealing, Hardening, Tempering & Aging with various sets of parameters like temperatures,period of soaking etc. for the experimental heats and the ranges were established.

7

In further detail, the material, thus produced, behaves satisfactorily as can be seen from the following details.
Hot working:
Initial slow heating of as cast stock, if the stock input temperature is below 500° C.
i. Forging/Rolling:
a) Primary rolling of ingots/as-cast stock above 1000°C,
b) Forging/rolling of blooms/billets/slabs above 900° C,
c) Finish forging/rolling temperature in the range of 760°C-950°C. ii. Annealing:
Annealing of stock of suitable size at Selection of the actual annealing temperature depends upon the shape & size of stock, as annealed hardness required, subsequent cold shaping/processing operations, type of furnace, end use envisaged etc.
Cold Shaping of Stock:
Undertaking of suitable cold shaping operations e.g. Bending, Machining, Cutting, Shearing etc.
Heat treatment:.
Heat treatment in three steps i.e. Normalizing, Hardening & Tempering.
i) Normalizing:
a. Air cooling from a range of 880°-950° C depending upon the composition, shape and size of
the stock.
b. Controlled furnace atmosphere preferable.
8

ii. Hardening:
a. Solution treatment of the stock at austenitizing temperature (880°-950° C)
b. Controlled furnace atmosphere preferable.
c. Quenching in oil/water.
iii. Tempering:
a. Tempering temperature: 200°-350°C or 600°-650°C.
b. Tempering time: 0.5-3 hr.
Final Shaping:
Grinding/precision machining/bending/shaping etc.
The product properties were generally as follows:
Product Properties:
i) CE ii) Y.S. in the range of 750 Mpa to 1350 Mpa or more.
iii) UTS in the range of 840 Mpa to 1540 Mpa or more,.
iv) Impact toughness (CVN) at room temperature in the range of 48 J to 145 J or more.
v) % elong in the range of 12% to 17% or more.
9

Moreover, the positive effects of the Thermo-Mechanically Controlled Processing (TMCP) and in particular the finish rolling temperature in the temperature of 760°-950° C and degree of deformation > 15 % on the Yield Strength (YS)ensile Strength (TS) and % Elongation were studied and noted as detailed hereunder:
Effect of TMCP:

FRT (º0C) Degree of deformation
(%) Heat treatment
Q: Quenching
T: Tempering YS (Mpa) UTS
(Mpa) Elongation (%)
902 15.5 Q:900Cx30 min
T:300Cxl20 min 1200 1410 13.0
795 17.0 -do- 1090 1426 15.6
884 22.0 -do- 1220 1430 15.6

g. The hot worked product is normalized, oil/water quenched in accordance with the shape & size of the final product. The normalizing/solution treatment/ austenitization temperature & soaking periods are worked out on the basis of chemistry, shape & size of the product, heating rate etc.
h. The final product is then tempered at selected temperature(s) and time period. The final heat treatment parameters are selected from amongst the following on the basis of application vis-a-vis performance related property requirements.
Normalizing temperature: 880°-950°C, soaking period: 15-60 min.
Hardening temperature: 880°-950° C, soaking period: 15-60 min.
Tempering temperature: 220°-350º/600°-650°C.
i. The heat treatment product is then finally shaped by grinding, precision machining final bending etc.
c) Economics:
This steel will be cheaper compared to similar existing grades due to
• lower Ni & Mo contents
• Lower hot-working temperature
• Lower heat-treatment temperature.
However, actual quantum of benefit in terms of processing cost will depend on many factors, some are as enumerated below: -
a. Which particular USLAM or any other steel will be replaced by this steel?
b. What are the absolute costs of ingredients like Ni, Mo, Si, Cr, Mn vis-a-vis their ferro-alloys at
the plant of production.
c. Yield/recovery of the above mentioned alloying elements with respect to the sources of
addition.
11

d. The facilities available at the production unit or the process route followed compared to
existing similar commercial grades of USLAM steel e.g. secondary refining, heat treatment
furnace type & size etc. The benefit will depend upon whether secondary refining techniques
like VAD, ladle degassing etc. is resorted to or by-passed. Benefit at the heat treatment
operations will depends on the type of furnace, their sizes & capabilities.
e. The desired final properties (application specific) which will ultimately decide all the
processing parameters.
Hence any quantification of the benefit at this stage is difficult.
d) Impact in Industry :
• USLAM steels can be made with above mentioned properties for application in the
following areas as cheaper substitute,
-Aviation industry for aircraft under carriages
-Pressure vessels
-Engineering components like high-strength bolts/fasteners,
-Springs, high-speed rotors/shafts e:tc.
• The steels can be processed/hot worked to desired shapes with out any difficulty.
12

WE CLAIM:
1. A process for production of Nb -Ti treated tough & ductile Ultra-High Strength Low Alloy Martensitic (USLAM) steel comprising the following steps:
a. melting selective raw materials like EDO/Extra low Sulphur &
Phosphorus scrap & other elements such as to achieve a selective
composition comprising of:
C = 0.18 to 0.22 %;Mn.= 1.00 to 1.30 %;Si. = 0.90 to 1.10 %;Cr. = 1.10 to 1.70 %;Ni. = 0.65 to 0.75 %;Mo. =0.25 to 0.35 %;V = 0.06 to 0.12 %;AI. = 0.04 to 0.09 %;S = upto 0.015 %; P = upto 0.020 ; Nb. = upto 0.06 % and Tj = upto 0.02 % and balance Fe. after final addition of alloying/ microalloying elements.
b. subjecting the melt to complete de-oxidation using one or more of the
de- oxidizers selected from Fe-Si, Fe-Mn and Al;
C. effecting necessary final addition of any alloying elements in any conventional manner , preferably by ladle/VAD treatment , for effective alloying of the melt such as to maintain the final alloy having said desired composition;
d. subjecting the final alloy thus obtained to ingot casting by bottom
pouring technique;
e. subjecting the ingot to selective thermo-mechanically controlled
processing comprising finish rolling temperature in the range of 760°
to 950° along with > 15% deformation; followed by,
f) carrying out heat treatment operations with controlled parameters like temperature and holding/soaking period selectively involving Normalizing, Hardening, Low temperature Tempering ,Hlgh Temperature Tempering and Aging.
2.A method as claimed in claim 1 wherein the alloy produced has selective combination of the following elements:-
- 13-

i. C, Mn. Cr, Ni & Mo in small amounts; and
ii. V, Nb, Ti & Al as micro-alloying elements.
3.A method as claimed in anyone of claims 1 or 2 wherein the alloy produced has the following combination of properties:
i. Y.S. in the range of 750 Mpa to 1350 Mpa ;
ii. UTS in the range of 840 Mpa to 1540 Mpa ;
iii. Impact toughness (CVN) at room temperature in the range of 48J to 145J;
iv. Increase in impact toughness at room temperature to the tune of 10J (20%) compared to the existing /conventional alloys in case of Low Temperature Tempering (LTT);
v. Remarkable enhancement (more than two-fold) in the impact toughness at R.T. after High Temperature Tempering (HTT) t_ the tune of 100J (220%) with a corresponding loss of 700 Mpa (82%) in UTS;
vi) An almost fully ductile mode of fracture with fine & deep dimple network coupled with small micro-voids, the plausible sites for initiation of fracture;
- 14-
vi. Control of segregation/co-segregation of P,Mn,Cr,Ni etc. vii. Lesser susceptibility to both Temper Matensite Embrittlement ment and Tempered Embrottlement compared to similar steels. 4.A method for the preparation of an alloy substantially as herein described with reference to the accompanying examples.
There is disclosed a process for production of Nb-Ti treated tough & ductile Ultra-High Strength Low Alloy Martensitic (USLAM) steels, comprising the following steps: -
Melting and casting
a. Selecting necessary alloy compositions keeping the contents of alloying elements within the following limits,

%c %Mn %Si %Cr %Ni %Mo % V %AI %S %P
0.18 1.00 0.90 1.10 0.65 0.25 0.06 0.04 0.015 0.020
to to to to to to to to Max. Max.
0.22 1.30 1.10 1.70 0.75 0.35 0.12 0.09
With Nb: 0.06% max & Ti: 0.02% max as micro-alloying elements.
b. Melting the selected materials under careful conditions to achieve the desired
chemistry.
c. Subjecting the melt so obtained to complete de-oxidation using necessary quantities
of one or more of agents selected from Fe-Si, Fe-Mn & Al.
d. Monitoring and analyzing the melt,
e. Effecting necessary final addition of alloying elements in any conventional manner,
preferably by ladle/VAD treatment, for effective alloying of the elements, so as to,
obtain the final alloy having the desired composition.
f. And finally subjecting the final alloy to ingot casting by bottom pouring technique.
g. Thermo-mechanically controlled processing finish rolling temperature in the range of
760°-950°C, complied with >15% deformation.
h. Suitable combination of heat treatment parameters like temperature and holding/ soaking petrol with reference to Normalizing, Hardening, Low Temperature Tempering, High Temperature Tempering and Aging.
16

Documents:

00016-cal-2001 abstract.pdf

00016-cal-2001 claims.pdf

00016-cal-2001 correspondence.pdf

00016-cal-2001 description(complete).pdf

00016-cal-2001 form-1.pdf

00016-cal-2001 form-13.pdf

00016-cal-2001 form-18.pdf

00016-cal-2001 form-2.pdf

00016-cal-2001 form-3.pdf

00016-cal-2001 letters patent.pdf

00016-cal-2001 p.a.pdf

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Patent Number

203832

Indian Patent Application Number

16/CAL/2001

PG Journal Number

11/2007

Publication Date

16-Mar-2007

Grant Date

16-Mar-2007

Date of Filing

11-Jan-2001

Name of Patentee

STEEL AUTHORITY OF INDIA LTD.,

Applicant Address

RESEARCH AND DEVLOPMENT CENTRE FOR IRON AND STEEL P.O-DORANDA , RANCHI -834002

Inventors:

#	Inventor's Name	Inventor's Address
1	SAHA BISHNU BHUSAN	RESEARCH AND DEVLOPMENT CENTRE FOR IRON AND STEEL P.O-DORANDA , RANCHI -834002
2	SEN SUSHIL KUMAR	RESEARCH AND DEVLOPMENT CENTRE FOR IRON AND STEEL P.O-DORANDA , RANCHI -834002

PCT International Classification Number

C 21 D 8/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA