Claims:We Claim:
1. Spherodized soft bearing steel of 100Cr6 or equivalent grade comprising
C: 0.70 to 1.3 preferably 0.98wt%;
Si: 0.15.to.0.35 preferably 0.29wt%;
Mn: 0.30 to 0.45. preferably 0.40wt%;
Cr: 1.35 to 1.50 preferably 1.38wt%;
Mo: preferably less than 0.01wt%;
Ni: preferably less than 0.02wt%;
P: preferably less than 0.03wt%;
S: preferably less than 0.04 wt%;
Al: preferably less than 0.02wt%;
Cu: preferably less than 0.07wt%;
Ti: preferably less than 0.003wt%;
and rest is iron; having warm forging induced spherodization structure with a degree of spherodization varying between 85 and 100%; with hardness in the range of 248 to 170BHN preferably 217 BHN, and % elongation in the range of 24 to 35facilitating machining or cold forging operation.

2. Spherodized soft bearing steel as claimed in claim 1 comprising mechanical properties including
Degree of spherodization: 85 to 100%
Hardness: 248 to 170 BHN
YS:.620.to.568 MPa
UTS: 859 to 568 MPa
%Elongation: 24 to 35%
% Reduction in area: 51 to 58%
CVN Impact Strength at 25oC:20 to 50J
CVN Impact strength at -60°C = 7.1 to 13.1J

3. Spherodized soft bearing steel as claimed in anyone of claims 1 or 2 wherein said warm forging induced spherodization is attained from initial fully or high pearlite content microstructure.

4. Spherodized soft bearing steel as claimed in anyone of claims 1 to 3 having variable degree of spherodized structure and softness is based on the level of warm forging induced spherodization.

5. Spherodized soft bearing steel as claimed in anyone of claims 1 to 4 which is obtained from 100Cr6 grade or equivalent bearing steel in as hot rolled or as-forged steel at room temperature as starting material comprising an initial fully pearlitic microstructure prior to reheating for warm forging.

6. A process for producing spherodized soft bearing steel as claimed in anyone of claims 1 to 5 comprising:

subjecting the steel having initially fully pearlite microstructure to step of warm forging based deforming to thereby impart forging induced spherodization for desired softness to the steel.

7. A process for producing spherodized soft bearing steel as claimed in claim 6 comprising step of controlling the degree of spherodized structure to be in the range of 85 to 100%and softness by controlled inducing of said warm forging spherodization.

8. A process for producing spherodized soft bearing steel as claimed in anyone of claims 6 or 7 wherein reduction ratio during warm forging is increased for achieving more hardness and strength and the room temperature impact toughness is significantly improved.

9. A process for producing spherodized soft bearing steel as claimed in anyone of claims 6 to 8 comprising carrying out prior cold forging before warm forging for improved softening.

10. A process for producing spherodized soft bearing steel as claimed in anyone of claims 6 to 9 comprising carrying out more than one reheating during said warm deformation.

11. A process for producing spherodized soft bearing steel using warm deformation as claimed in anyone of claims 6 to 10 comprising
(i) providing as hot rolled or as forged 100Cr6 grade or equivalent bearing steel at room temperature as starting material having
composition by wt% comprising
C: 0.70 to 1.3 preferably 0.98wt%;
Si: 0.15.to.0.35 preferably 0.29wt%;
Mn: 0.30 to 0.45. preferably 0.40wt%;
Cr: 1.35 to 1.50 preferably 1.38wt%;
Mo: preferably less than 0.01wt%;
Ni: preferably less than 0.02wt%;
P: preferably less than 0.03wt%;
S: preferably less than 0.04 wt%;
Al: preferably less than 0.02wt%;
Cu: preferably less than 0.07wt%;
Ti: preferably less than 0.003wt%; and rest is iron; andhaving an initial fully pearlitic microstructure prior to reheating for warm forging;
(ii) reheating said starting steel material to heated to warm deformation/forging temperature of 730 to 810oC;
(iii) subjecting said reheated steel to warm deformation/forging for selective percent reduction of cross section in stages, with or without inter stage reheating, to generate forging induced spherodization, which softens the steel.

12. A process as claimed in claim 11 comprising initially heating the hot rolled steel bar at a forging temperature between A1 and Acm and holding for sufficient time so that the cross section of the preform achieves uniform temperature which ensures some dissolution of pre-existing pearlite.

13. A process as claimed in anyone of claim 11 or 12 wherein steel has to be heated as per a schedule with a minimum holding time comprising heating the steel bar from room temperature to 650oC, holding for 5 min. And further raising the temperature rapidly to the warm forging temperature in the range of 730-810°C and hold for 1.5 h/inch [90 min for 25 mm] thickness to enable some amount of pearlite dissolution which aids further breakage of spherodized microstructure.

14. A process as claimed in anyone of claim 11 to 13 wherein steel has to be held at the chosen temperature before warm forging as per the following equation,
Time of holding in minutes
= 0.0023 (temperature of warm forging)2 -3.9529 (temperature of warm forging) +1688.7.
15. A process as claimed in anyone of claim 11 to 14 wherein said warm forging is applicable for closed die and open die warm forging, and post forging, in all cases the steel bar or component is air cooled, which softens the material for improved machinability.

16. A process as claimed in anyone of claims 11 to 15 wherein during warm deformation strains have to be higher in the starting stage with as high as 30% reduction in area and in the case of open die forging, after repeated blows, re-soaking at the same forging temperature is applied followed by another 30% minimum reduction in area, with at least one reheating ensures very high degree of spherodization.

17. A process as claimed in anyone of claims 11 to 16 wherein in case of closed die forging, higher the degree of metal working in the die greater is the degree of spherodization possible, so that higher degree of reduction gives higher degree of softness or lower hardness.

18. A process as claimed in anyone of claims 11 to 17 wherein direct warm forging introduced spherodization softens the material without heat treatment and makes it amenable for better machinability.

19. A process as claimed in anyone of claims 11 to 18 wherein lower the warm forging temperature better is the degree of spherodization and lower the hardness.

20. A process as claimed in anyone of claims 11 to 19 wherein warm forging induced spherodization is a relatively fast process compared to conventional spherodize annealing heat treatment involving very long times, thus improving productivity.

21. A process as claimed in anyone of claims 11 to 20 which is applicable for other through hardened hyper-eutectoid steels such as rail steels in general.

22. A process as claimed in anyone of claims 11 to 21 wherein warm deformation induced spherodization is an effective process that is also applicable in other warm deformation processes such as warm rolling, warm extrusion etc.

Dated this the 29th day of May, 2019 Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199
, Description:FIELD OF THE INVENTION
The present invention relates to a process for producing soft bearing steels using warm deformation during high carbon through hardeningbearing steel obtained thereof facilitating subsequent machining and cold forging operation to near net shapes. More particularly, the present invention is directed to a process, wherein, to soften near eutectoid and hyper eutectoid bearing steels having high pearlitic content in the matrix, a new warm forging processing technique. This enables production of the steel during warmforming, post steel making, to achieve conditions, where soft steel can be obtained. Alternatively, any hot worked bearing steel can be additionally warm forged to get softness instead of regular spherodizing the steel, needing longannealing heat treatment at temperatures close to A1 temperatures for prolonged time periods. The mechanical properties in the warm deformed condition (forged or rolled) are equivalent to that in the spherodize annealed condition.

BACKGROUND OF THE INVENTION
Through hardened bearing steels are high carbon near eutectoid or hypereutectoid steel. The steel in the hot rolled condition exhibits often predominantly pearlitic microstructures. The near eutectoid and hyper eutectoid bearing steels have high pearlitic content in the matrix. Hence, they have high strengths and are difficult to machine or cold deform by cold forging to near net shapes. In order to soften the material for further processing, these high-carbon near-eutectoid and hyper-eutectoid steels are subjected to prolonged annealing heat treatment at temperatures close to A1 temperatures for prolonged time periods. This treatment involves holding the bar near A1 temperature for periods varying between 25 to 40 hours depending on the steelgrade and its diameter.

There are published literatures and patents which talk about softening a bearing steel by spherodize annealing. The treatment includes holding the steels at close to A1 temperatures for long durations. The times range for periods as long as 25 to 40 hours depending on prior microstructures. There is few published information on warm deformation at laboratory scale and hot rolling which talk about warm rolling as a means to spherodize the steel.

1. A study on Gleeble machine by Guo-hui ZHU and Gang ZHENG showed warm upset on laboratory test sample on a bearing steel of composition in wt.%[0.96 %C- 0.21 %Si- 0.31 %Mn- 1.46 %Cr- 0.01 %Mo- 0.014 %P- 0.011 %S- 0.09 %Ni-0.036 %Al- 0.25 %Cu] showed four different processing condition. The as rolled bar was, (i) heated to 950 °C for 20 min+ 840°C rolling, followed by cooling to 650°C at 500°C/h which resulted in 336 HB hardness; (ii) Heated to 880 °C/held for 5 min + rolled in 4 passes with 12% pass reduction + cooled at 500°C/h+ followed by quenching to room resulted in 326 HB hardness(iii) Heated to 840°C/held for 5 min+rolled for 4 passes with 12% pass reduction+cooled at 500°C/h+ followed by quenching to room temperature resulted in 303 HB hardness;(iv) Heated to 840°C/held 5 min+rolled for 4 passes with 12% pass reduction+cooled at 300°C/h+ followed by quenching to room temperature that resulted in 253 Hardness. The study is conforming to hot rolling that resulted in softening. Best spherodization could be obtained at 840 oC at 300 °C/h, whereas the above study is a lab scale study.It is not pertaining to warmforging, as in thepresent invention. The processing conditions with respect to temperature of holding and cooling schedule are completely different from the present study. The property range is also not fully established, as in the present invention.

2.Patent KR101304670B1 has suggested a means to manufacture spherodized steel without spherodizing heat treatment. A steel with composition [(0.9 to 1.4) %C-(1.0 to 3.0) %Si-(0.3 to 0.7) %Mn-(1.0 to 2.0) %Cr- (0.001 to 0.01) %B- (0.01 to 0.05) %Ti] was warm rolled. The process involved reheating a billet at 900-1100°C followed by rolling between A1 and Acm (typically at 900oC) followed by cooling therolled steel to temperature between A1 and A1+20°C at the cooling rate of 3-20°C/s which results inan inter particular spacing of 20microns. The present invention deals with hot forging operation, where the strains and strain rates or the deformation parameters are significantly different from that in the above study.

3.In another study on microstructure of warm rolling and pearlitic transformation of ultrafine-grained GCr15 bearing steel by Jun-Jie Sun, Fu-Liang Lian, Hong-Ji Liu, Tao Jiang, Sheng-Wu Guo , Lin-Xiu Du and Yong-Ning Liu of composition [0.98 %C–0.2 %Si–.34 %Mn–1.5 %Cr–0.01 %Mo–0.08 %Ni–0.013 %P–0.003 %S ] laboratory sample of dimension 100 mm × 70 mm × 52 mm, rolled at temperature between 850 to 900 oC and holding for times between 1.5 to 9 min, with a 65% reduction of followed by air cooling to room temp. The sample was further warm rolled with 75% reduction at 600oC followed by air cooling to room temp and followed by annealing at 650 C with 2 h holding followed by air cooling to room temperature gave ultra-fine grained steel with 1-micron grain size.Holding at 800oC between 2 to 9min showed hardness between 37 and 42 HRC. The present study is an industrial scale forging operation, as against the above laboratory scale study, which established a moderate hardness and is focused on ultra-fine grain development. The study states that after deformation a final annealing after warm deformation gives finer structure. Details on deformation related spherodization was not well established in hot forging operation as in the present invention.

4. Patent CN107138660A and Patent WO2019001262A1reports warm ring rolling of a high-carbon-chromium bearing steel of composition [GCr15] and [GCr15SiMn]. Steel grade GCr15 was heated to 780oC [Above A1] with 10 min holding and cooled at a rate 200oC/h to 650oC [Below A1] and ring rolled to 40% reduction followed by air cooling, which resulted in spherodization, with 0.25-micron average carbide size. Steel grade GCr15SiMn was heated to 790oCand held for 10 min followed by cooling at the rate of 200 oC/h to 710oC followed by ring rolling with 70% reduction followed by air cooling. This resulted in spherodization with 0.46 micron. The above study emphasizes deformation induced spherodization for warm ring rolling and suggests a temperature schedule where after initial heating sharp cooling is required unlike the present invention where no such complex temperature scheduling is involved. Further, the process claims, reduction by ring rolling which has shallow strain penetration than warm forging process which has deeper strain penetration.

5. In another experimental study by Dongsheng Qiana, Jun Yanga, Huajie Maoa and Lin Huabon warm ring rolling of 52100 bearing steel,that result in spherodized microstructure,Gleeble simulation was used for finding the processcondition for warm ring rolling. The dimension of Gleeble sample was 8mm diameter and 15 mm height and sample for rolling was 110mm x 46mm x 32 mm plate steel was subject to warm rolling. There are different test conditions, the steels were heated from room temperature to, an austenizing temperature of 1050 and 800oC in two different schedules and held for 5 min. This was followed by furnace cooling to 700°C with 60% reduction. This was followed by air cooling from 650oC. The sample with lower austenizing temperature of 800oC gave softening to 227 HV with 0.25-micron average carbide size. Gleeble simulation on ring rolling of a plate warm rolled was studied at 800 oC/30 min holding followed by furnace cooling to 700 with 40% reduction and subsequent air cooling from 500oC. This resulted in a hardness of 229HV and spherodized carbide of 0.35 micron.The present invention is not a laboratory simulation as reported in the above study, where specified deformation and holding temperature and cooling rate requirement sequence are reported for warm rolling.

6. In another study by Tao He, YuanmingHuo, Xiaojun Shi and Shoushuang Chen, as hot rolled 52100 Bearing Steel was subjected to warm forming conditions using a Gleeble simulator. Hot rolled bearing steel of composition in wt.% [1.01 %C- 0.25 %Si-0.36 %Mn -1.52 %Cr-0.018 %P-0.003 %S] of dimension, 8mm diameter and 15 mm height was subjected to a temperature between (650°C or 700° C or 750° C) at a temperature rise 10oC/s for 3 min. The test sample was subjected to strain between 0.2 to 0.8 at controlled strain rate between 0.1 to 10.0/s followed by water Quenching. The microstructure showed spherodized structure that resulted in carbide size varying between 0.43 to 0.74 microns at true strain varying between 0.2 and 0.6. This study is another laboratory scale investigation where specific range of deformation parameters has been established for hot deformation where deformation induced spherodization take place. This does not refer to industrial scale hot forging to get a deformation induced spherodization as disclosed in the present invention.

7. In another study by YuanmingHuo, Tao He, ShoushuangChen and Riming Wu, on the mechanical behaviour and microstructure evolution of bearing steel 52100 during Warm Compression, the steel of composition [1.01 %C- 0.25 %Si-0.36 %Mn -1.52 %Cr-0.018 %P-0.003 %S] of dimension 8mm diameter and 15 mm height, was subject to Gleeble simulation for rolling. Samples were heated at a rate of 10 oC/s to a temperature between 650 oC and 850 oC, held for 3 min and subjected to deformation at a true strain of 0.8 and strain rate between 0.1 to 10 /s followed by water quenching. The spherodization study showed that at 750oC and strain rates between 0.1 to 10/s the spherodized carbide size varied between 0.41 to 0.51 microns. Higher temperature gave pearlite formation. The above study deals with a warm compression simulation on a laboratory scale operation to ensure direct deformation induced spherodization. It does not refer to industrial scale warm forging spherodization, which can be attained in a range of temperatures as given in the present invention.

8. In another study by Dong-Xu Han, Lin-Xiu Du, Bin Zhang and Raja Devesh Kumar Misra ondeformation induced carbides in bearing steels, warm rolling of a laboratory scale plate 150 mm x950 mm x 930 mm of steel of composition [1 %C-0.22 %Si-0.3 %Mn-1.51 %Cr- 0.05 %Mo-0.18 %Ni-0.003 %P-0.002 %S-0.02%Al- 0.08 %Cu- 0.005 %Ti] was subjected to heating to 964oC with a holding time of 0.5h followed by rolling in three deformation conditions,
(i) 5 passes with a reduction of 20%in each pass
(ii) performing double pass rolling (with 40% reduction per pass)
(iii) performing double pass rolling (with reduction of 20% per pass followed
Post rolling the samples were cooled to 720oC and held for 2 to 6 h followed by air cooling. The spherodization was observed in the microstructure with hardness varying between 200 to 290 VHN and the average spherodized carbide size was varying between 0.2 to 0.7 microns. The lowest lamellar pearlite content of 4% and average carbide size of 0.2 micron was obtained in the (i) trial at 780oC warm rolling. The above study involves hot rolling of the plate and from above austenization temperature of 964oC. The present invention pertains to warm forging within inter critical temperatures with totally different set of processing condition.
[References:
1. Zhu GH and Zheng G. Directly spheroidizing during hot deformation in GCr15 steels. Frontiers of Materials Science in China. 2008 Mar 1;2(1):72-5.
2. LEE JAE SEUNG and KIMKWAN HO Patent KR101304670B1, 2013
3. Sun JJ, Lian FL, Liu HJ, Jiang T, Guo SW, Du LX and Liu YN. Microstructure of warm rolling and pearlitic transformation of ultrafine-grained GCr15 steel. Materials Characterization. 2014 Sep 1; 95:291-8.
4. (i) QIAN DONGSHENG YANG,JUN HUA LIN LIU and QINGLONG WANG FENG, Patent CN107138660A, 2017 and
(ii)QIAN DONGSHENG, YANG JUN, HUA LIN, LIU QINGLONG and WANG FENG, Patent WO2019001262A1, 2019
5. Qian D, Yang J, Mao H and Hua L. Experiment study on warm ring rolling of 52100 bearing steel coupling microstructure spheroidization. Procedia engineering. 2017 Jan 1; 207:1224-9.
6. He T, Huo Y, Shi X and Chen S. Modelling of Carbide Spheroidization Mechanism of 52100 Bearing Steel Under Warm Forming Conditions. Metallurgical and Materials Transactions A. 2019 Feb 15;50(2):936-946.
7. Huo Y, He T, Chen S and Wu R. Mechanical Behaviour and Microstructure Evolution of Bearing Steel 52100 During Warm Compression. JOM. 2018 Jul 1;70(7):1112-1117.
8. Han DX, Du LX, Zhang B and Misra RD. Effect of deformation on deformation-induced carbides and spheroidization in bearing steel. Journal of Materials Science. 2019 Feb 1;54(3):2612-27.]

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide a process for producing spherodized soft bearing steels using warm deformation during hot forming and bearing steel obtained thereof facilitating subsequent machining and cold forging operation to near net shapes.

A further object of the present invention is directed to provide a process for producing spherodized soft bearing steels using warm deformation during hot forming, wherein, as hot rolled near eutectoid and hyper eutectoid bearing steels having high pearlitic content in the matrix can be effectively warm forged to form completely spherodized microstructure by warm deformation to achieve softness required for subsequent processing.

A further object of the present invention is directed to a process for producing soft bearing steels using warm deformation during hot forming wherein hot rolled steel samples at room temperature having microstructure comprising fully pearlitic are subjected to reheating prior to warm forging at a temperature between A1 and Acm, and holding/soaking for selective time so that the cross section of the preform achieves uniform temperature and samples so soaked are subject to warm forging.

A still further object of the present invention is directed to a process for producing soft bearing steels using warm deformation during hot forming wherein warm forging can be done on closed die forged samples and as well as open die forged components with deformation strains have to be higher in the starting stage as high as 30% reduction in area.

A still further object of the present invention is directed to a process for producing soft bearing steels using warm deformation during hot forming wherein in the case of open die forging re-soaking at the same temperature is required followed by another 30% minimum reduction in area and higher degree of reduction gives higher degree of softness or lower hardness.

Another object of the present invention is directed to a process for producing soft bearing steels using warm deformation during hot forming, wherein direct warm forging introduced spherodization softens the material without heat treatment and makes it amenable for better machinability.

Yet another object of the present invention is directed to a process for producing soft bearing steels using warm deformation during hot forming wherein lower the warm forging temperaturein between the inter-critical temperature, better is the degree of spherodization.

A further object of the present invention is directed to a process for producing soft bearing steels using warm deformation during hot forming wherein even very large diameter bars can be effectively warm forged to form completely spherodized microstructure by warm deformation and air cooling to room temperature.

A further object of the present invention is directed to a process for producing soft bearing steels using warm deformation, wherein, for warm forging induced spherodization, the starting microstructure should be high pearlite content.
SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to provide spherodized soft bearing steel of 100Cr6 or equivalent grade comprising
C : 0.70 to 1.30 preferably 0.98wt%;
Si: 0.15.to.0.35preferably 0.29wt%;
Mn: 0.30 to 0.45 preferably 0.40wt%;
Cr: 1.35 to 1.50preferably 1.38wt%;
Mo: preferably less than 0.01wt%;
Ni: preferably less than 0.02wt%;
P: preferably less than 0.03wt%;
S: preferably less than 0.04 wt%;
Al: preferably less than 0.02wt%;
Cu: preferably less than 0.07wt%;
Ti: preferably less than 0.003wt%;
N: preferably less than 0.0070wt.% and rest is iron; having warm forging induced spherodization structure with a degree of spherodization varying between 85 and 100%; with hardness in the range of 248 to170BHN preferably 217BHN and % Elongation in the range of 24 to 35 facilitating machining or cold forging operation.

A further aspect of the present invention is directed to spherodized soft bearing steel comprising mechanical properties including
Degree of spherodization: 85 to 100%
Hardness: 248 to 170 BHN
YS:.620.to.568 MPa
UTS: 859 to 568 MPa
%Elongation: 24 to 35%
% Reduction in area: 51 to 58%
CVN Impact Strength at 25oC:20 to50J
CVN Impact strength at -60°C = 7.1 to 13.1J

A still further aspect of the present invention is directed to spherodized soft bearing steel, wherein the said warm forging induced spherodization is attained from the initial fully pearlite microstructure.
A still further aspect of the present invention is directed to spherodized soft bearing steel having variable degree of spherodizaed structure and softness is based on the level of warm forging induced spherodization.
A still further aspect of the present invention is directed to spherodized soft bearing steel which is obtained from 100Cr6 grade or equivalent bearing steel in as hot rolled or as-forged steel at room temperature as starting material comprising an initial fully pearlitic microstructure prior to reheating for warm forging.
Another aspect of the present invention is directed to a process for producing spherodized soft bearing steel as described abovecomprising:

subjecting the steel having initially fully pearlite microstructure to the step of warm forging based deforming to thereby impart forging induced spherodization for desired softness to the steel.

Yet another aspect of the present invention is directed to a process for producing spherodized soft bearing steel comprising step of controlling the degree of spherodized structureto be in the range of 85 to 100% and softness by controlledwarm forging inducedspherodization.
A further aspect of the present invention is directed to a process for producing spherodized soft bearing steel, wherein reduction ratio during warm forging is increased for achieving more hardness and strength and the room temperature impact toughness is significantly improved.
A still further aspect of the present invention is directed to a process for producing spherodized soft bearing steel comprising carrying out prior cold forging before warm forging for improved softening.
A still further aspect of the present invention is directed to aprocess for producing spherodized soft bearing steel comprising carrying out more than one reheating during said warm deformation.
Another aspect of the present invention is directed to aprocess for producing spherodizedsoft bearing steel using worm deformation comprising
(i) providing as hot rolled or as forged 100Cr6 grade or equivalent bearing steel at room temperature as starting material having
composition by wt% comprising

C: 0.70 to 1.3 preferably 0.98wt%;
Si: 0.15.to.0.35 preferably 0.29wt%;
Mn: 0.30 to 0.45. preferably 0.40wt%;
Cr: 1.35 to 1.50 preferably 1.38wt%;
Mo: preferably less than 0.01wt%;
Ni: preferably less than 0.02wt%;
P: preferably less than 0.03wt%;
S: preferably less than 0.04 wt%;
Al: preferably less than 0.02wt%;
Cu: preferably less than 0.07wt%;
Ti: preferably less than 0.003wt%;
and rest is iron; and
having an initial fully pearlitic microstructure prior to reheating for warm forging;

(ii) reheating said starting steel material to heated to warm deformation/forging temperature of 730 to 810oC;
(iii) subjecting said reheated steel to warm deformation/forging for selective percent reduction of cross section in stages, with or without inter stage reheating, to generate forging induced spherodization, which softens the steel.
A still further aspect of the present invention is directed to aprocess comprising initially heating the hot rolled steel bar at a forging temperature between A1 and Acm and holding for sufficient time so that the cross section of the preform achieves uniform temperature which ensures some dissolution of pre-existing pearlite.
Yet another aspect of the present invention is directed to saidprocess,wherein steel has to be heated as per a schedule with a minimum holding time comprising heating the steel bar from room temperature to 650oC, holding for 5 minand further raising the temperature rapidly to the warm forging temperature in the range of 730-810°C and hold for 1.5 h/ inch [90 min for 25 mm] thickness to enable some amount of pearlite dissolution which aids further breakage of spherodized microstructure.
A further aspect of the present invention is directed to aprocess, wherein, the steel has to be held at the chosen temperature before warm forging as per the following equation,
Time of holding in minutes
= 0.0023 (temperature of warm forging)2-3.9529 (temperature of warm forging) +1688.7.
A still further aspect of the present invention is directed to aprocess wherein the said warm forging is applicable for closed die and open die warm forging, and post forging, in all cases the steel bar or component is air cooled, which softens the material for improved machinability.
A still further aspect of the present invention is directed to aprocess wherein during warm deformation strains have to be higher in the starting stage with as high as 30% reduction in area and in the case of open die forging, after repeated blows, re-soaking at the same forging temperature is applied followed by another 30% minimum reduction in area, with at least one reheating ensures very high degree of spherodization.
A still further aspect of the present invention is directed to aprocess wherein in case of closed die forging, higher the degree of metal working in the die greater is the degree of spherodization possible, so that higher degree of reduction gives higher degree of softness or lower hardness.
A still further aspect of the present invention is directed to aprocess wherein direct warm forging introduced spherodization softens the material without heat treatment and makes it amenable for better machinability.
Another aspect of the present invention is directed to aprocess wherein lower the warm forging temperature within the intercritical temperaturebetter is the degree of spherodization and lower the hardness.
Yet another aspect of the present invention is directed to aprocess wherein warm forging induced spherodization is a relatively fast process compared to conventional spherodize annealing heat treatment involving very long times, thus improving productivity.
A further aspect of the present invention is directed to aprocess which is applicable for other through hardened hyper-eutectoid steels such as high carbon steels, tool steels, rail steels etc. in general.
A still further aspect of the present invention is directed to aprocess wherein warm deformation induced spherodization is an effective process also applicable in other warm deformation processes such as warm rolling, warm extrusion etc.
The above and other objects and advantages of the present invention are described hereunder with reference to following accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1: illustrates Microstructure of the bearing steel 100Cr6 shows complete pearlitic microstructure in as hot rolled bar at mid radius
Fig.2: illustrates Microstructure of the bearing steel 100Cr6 shows spherodized carbide microstructure after conventional spherodize annealing process with 170 BHN hardness.
Fig.3: illustrates microstructure with Spherodization of the steel obtained after warm forging at 800oC without reheating with a hardness level of 255 BHN hardness.
Fig.4: illustrates graphically the Mechanical properties of the bearing steel as a function of warm forging done without reheating.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS AND EXAMPLES

The present invention is directed to a process for producing soft bearing steelproviding a suitable means to achieve the desired softness by a new warm forging processing technique. This enables production of the steel during warmforming, post steel making itself, to achieve conditions, where soft steel can be obtained. Alternatively, any hot worked bearing steel can be additionally warm forged to get softness instead of regular spherodizing the steel. The mechanical properties in the warm deformed condition (forged or rolled) are equivalent to that in the spherodize annealed condition.
In the present invention, bearing steel was made by primary and secondary steel making process route that involves Blast furnace ?Energy Optimizing Furnace ?Ladle Refining Furnace ?Vacuum Degasser?Continuous Casting Machine?Reheating furnace?Hot rollingand followed by air cooling to room temperature. The microstructure of the steel at this condition is fully pearlitic with pro-eutectoid cementite. The hot rolled steel samples at room temperature, are subjected to reheating to warm forging at a temperature between A1 and Acm. The deformation in a specified manner below ensures softening of the steel by deformation on cooling to room temperature with a micro structure comparable to that obtained by prolonged annealing in a furnace close to A1 temperature for prolonged time periods.
The critical temperature for the given grade, involves A1 = 755oC and ACm= 850oC. Warm Forging for direct spherodization is carried out by initially heating the steel at a temperature between A1 and Acm. Sufficient holding time has to be given so that the cross section of the preform achieves uniform temperature. The holding temperature at different warm forging temperature may differ. Higher the warm forging temperature within the specified range a shorter duration of holding is maintained, which ensures some dissolution of pre-existing pearlite. Higher the temperature, the dissolution of the pearlite into matrix increases. Hence, the steel has to be held at the chosen temperature before warm forging heated as per the enclosed equation,
Time of holding in minutes
= 0.0023 (temperature of warm forging) 2-3.9529 (temperature of warm forging) +1688.7
After holding the steel as per the specified holding time after uniform temperature has been achieved in the bar. Conventional norms of 1 hour/25 mm thickness of the sample have to be calculated followed by the addition of the holding time as mentioned above.
The samples so soaked are subject to warm forging. The warm forging can be done on closed die forged samples and as well as open die forged components. The deformation strains have to be higher in the starting stage as high as 30% reduction in area. After repeated blows, in the case of open die forging re-soaking at the same temperature is required followed by another 30% minimum reduction in area. At least one reheating ensures very high degree of spherodization. In the case of closed die forging,higher the degree of metal working in the die greater is the degree of spherodization possible. In general, higher degree of reduction gives higher degree of softness or lower hardness. Direct warm forging introduced spherodization softens the material without heat treatment and makes it amenable for better machinability. In Open die forging, without any size limitation the forging can be done with minimum one reheating at the warm forging condition to get a spherodized structure. Post forging, in all cases the steel bar or component is air cooled.
Forging temperature is a very important parameter. Lower the warm forging temperaturewithin the inter-critical temperature, better is the degree of spherodization. The microstructure after forging deformation shows highly spherodized microstructure.
The biggest advantage of the present process is that even very large diameter bars can be effectively warm forged to form completely spherodized microstructure by warm deformationwithin a shorter duration. A series of forging trials carried out is brought out as illustrated by following examples:
Example:
Experimental Trials Carried Out by way of present invention:
A series of forging trials were carried out to achieve the direct warm forging induced direct spherodization in a through hardening bearing steel 100Cr6. A 70 mm diameter bar of 100Cr6 as per chemistry in Table 1 was chosen for the study. The steel had a completely pearlitic microstructure as shown in Fig.1
The steel bar was subjected to hot forging by heating the bar as per schedule as in Table 2. It was subjected to forging as per details in Table 3.
The steel was subjected to warm forging by re-heating the conventionally hot rolled steel. The conventional steel can also be in the as-forged condition. The starting microstructure in this condition is fully pearlitic with pro-eutectoid cementite.
a) Bench mark process testing:
One typical starting as-hot rolled sample was subjected to annealing and spherodize annealing. The mechanical properties are shown in Table 4 for comparison of properties in the warm forged condition. Hardness is a critically measured parameter. It was 160 to 255 BHN. The microstructure as observed in SEM showed spherodized microstructure as in Fig.2. It is seen that the room temperature mechanical properties show softening improved ductility and impact toughness. The impact toughness at -60oC showed lower valuesthan the warm forged samples in the invention.

Table 1: Composition of the 100Cr6 steel chosen for warm forging in wt.%
C Si Mn Cr Mo Ni P S Al Cu Ti
0.98 0.29 0.40 1.38 0.01 0.02 0.01 0.005 0.02 0.01 0.003

Table 2: Heating schedule prior to warm forging the steel
Input bar dimension 70 mm diameter and length 100 mm
Furnace used Electrically fired muffle furnace
Heating schedule Heat the bar from room temperature to 650oC. Hold for 5 min followed by rising to the warm forging temperature
Temperature of warm forging 820 or 800 or 760oC
Temperature cycle Heat from room temperature to 650oC ; hold for 5 min. Raise the temperature rapidly to the warm forging temperature and hold for 1.5 h/ inch [ 90 min for 25 mm ] thickness

Table 3: Warm forging schedule
Forging Hammer capacity 250 kg hammer
Holding Held by tongs
Initial sample cross section, mm 70 mm dia [ 3846.5 mm2area ]
Final sample cross section, mm 33x33 mm square [ 1098.9 mm2 area]
Reduction ratio 2.4 and 3.5
Forging temperature, oC 760, 800 and 820
No of Reheating Some samples were reheated once and another set of samples were forged without any reheating

Table 4:The mechanical properties in the as-hot rolled and in the conventional spherodize anneal condition
Forging
Condition
Degree of
Spherodization (%) Hardness
BHN YS
(MPa) UTS
(MPa) %E
%RA
CVN
(25 oC)
CVN
(-60 oC)

As Hot Rolled
70 mm dia
0 341 903 1094 5.01 13 5.3 5.3
As Rolled+ soft Annealed*
73 255 679 907 20.48 22 5.1 3.7
As Rolled +Bar sph.Annealing#
100 170 359 568 33.36 58 15.4 5.3

*Soft annealing cycle: (RT to 550oC)/8.5 h+(550 to 745oC)/4h+ 745°C/holding/2 h + (745 to 705oC)/2.5h + (705 to 725oC)/2.5h + (725 to 705°C)/2.5h + (705 to 500oC)/8h + (500 to 200oC)/3.5h Total No of hours = 33.5 h
#Spherodize annealing cycle: (RT to 650oC)/0.7 h+650 oC/holding 1.5h+ 730o°C/holding 1.5 h + 780oC)/holding 4h + 745oC/holding 7h +700oC/holding 1.5h +650oC/holding 1h+ 650 to 100oC)/12.3h Total No of hours = 29.5 h

b) Effect of Warm forging temperature
Warm forging was done after reheating the steel from room temperature as per description earlier. After the required holding time at the warm forging temperature, the steel is subjected to various reduction ratios as in Table5. The typical microstructure of the steel after warm forging at 800oC, without reheating shows forging induced spherodized structure as shown in Fig.3. Again during deformation, it was found forging with minimum one reheating at the warm forging temperature gave better spheroidal carbide than without reheating. The mechanical properties varied accordingly. It is observed that the softening of the steel is better with reduced warm forging temperature. It is also observed that as the reduction ratio increases at the same warm forging condition the mechanical properties show softening. Significant improvement in impact toughness is also observed which is superior to conventional spherodization heat treatment. Hence, direct warm forging induced spherodization shows superior impact toughness than conventional spherodization. The mechanical properties as a function of warm forging temperature plotted typically for sample warm forged without reheating is shown in Fig.4. When compared with conventional spherodized microstructure as in Fig.2, the warm forged microstructure shows finer spherodized carbides, hence there is a moderately higher hardness. However, this hardness can be further reduced by lowering the warm forging temperature with more than two reheat and followed by slow cooling post warm forging to achieve hardness as low as 170 BHN.

Table 5: The mechanical properties at various warm forged temperature with and without reheating
Warm forge/ temperature,oC No of
reheat
during
forging Reduction
Ratio
Degree of
Spherodization
(%) Hardness
BHN YS
(MPa)
UTS
(MPa)
%E
%RA
CVN (25 oC)
CVN
(-60 oC)

Effect of Warm Forging at Low Reduction Ratiowithout reheating
760 0 1.8 100 229 587 785 28.2 57 40.6 9.3
820 0 1.8 36 302 868 1061 15.8 16 8.8 6.4

Effect of Warm Forging without reheating
760 0 3.5 82 255 680 873 33.3 42 38.8 13.2
800 0 3.5 73 277 739 952 19.2 33 22.0 9.1
820 0 3.4 9 321 890 1058 15.7 30 10.0 5.2

Effect of Warm Forging at Medium Reduction Ratio with reheating
760 1 2.5 100 229 610 772 26 56 56 6.5
800 1 2.4 100 217 576 782 23.7 52 49.2 13.1
820 1 2.5 48 269 612 897 20 43 13.2 10.2

Effect of Warm Forging at high Reduction Ratio with reheating
760 1 4.9 92 229 643 849 25 56 17.8 10.4
800 1 4.9 85 248 620 859 26 51 19.2 7.1
820 1 4.9 65 269 652 971 20 40 22.6 7.7

c) Effect of Warm Forging Reduction Ratio
The effect of reduction ratio during warm forging was assessed as in Table 6. With increasing reduction ratio during warm forging, there is more hardness and strength increase and the room temperature impact toughness is significantly improved. Reheating during forging imparts higher impact toughness even though hardness is moderately higher than lower forge reduction, which implies hardening is observed. High hardness level is observed at 820oC forging without reheating.

Table 6: Effect of Reduction ratio on the mechanical properties in the warm rolled condition, with and without reheating.
Warm forging temperature, oC No of
reheat
during
forging Reduction
Ratio Degree of spherodization (%) Hardness
BHN YS
(MPa)
UTS
(MPa)
%E
%RA
CVN (25 oC)
CVN
(-60 oC)

Effect of Reduction Ratio at 760 C without reheating
760 0 1.8 100 229 587 785 28.20 57 40.6 9.3
760 0 3.5 82 255 680 873 33.33 42 38.8 13.2

Effect of Reduction Ratio at 760 C with reheating
760 1 2.5 100 229 610 772 26 56 56 6.5
760 1 4.9 92 229 643 849 25 56 17.8 10.4

Effect of Reduction Ratio at 800 C with reheating
800 1 2.37 100 217 738 782 23.72 52 49.2 13.1
800 1 4.90 85 248 620 859 26 51 19.2 7.1

Effect of Reduction Ratio at 820 C without reheating
820 0 1.8 36 302 868 1061 15.80 16 8.8 6.4
820 0 3.4 9 321 890 1058 15.67 30 10.0 5.2

Effect of Reduction Ratio at 820 C with reheating
820 1 2.5 48 269 612 897 20 43 13.2 10.2
820 1 4.9 65 269 652 971 20 40 22.6 7.7

d) Effect of prior cold forging before Warm forging
The effect of cold forging prior to warm forging and the corresponding mechanical properties made by additional trials is shown in Table 7. There is softening associated with prior cold deformation. This is normally used in cold rolling of wire rod, where prior cold deformation is followed by spherodization heat treatment. In the present study, a small amount prior cold forging before warm forging gave good amount of softening. Hence, cold deformation followed warm forging aids spherodization further.

Table 7: Effect of prior cold working and prior soft annealing before warm forging on the mechanical properties
Warm forge Condition/ temperature, oC No of
reheat
during
forging Reduction
Ratio Degree of
Spherodization (%) Hardness
(BHN) YS
(MPa)
UTS
(MPa)
%E
%RA
CVN
(25 oC)
CVN
(-60 oC)

As Rolled+ Soft annealing*+ Cold forging (1.05 RR) +Warm forged at 800 0 2.4

90 229 621 812 13.3 47 24.8 5.7

*Soft annealing cycle: (RT to 550oC)/8.5 h+(550 to 745oC)/4h+ 745o°C/holding/2 h + (745 to 705oC)/2.5h + (705 to 725oC)/2.5h + (725 to 705)/2.5h + (705 to 500)oC/8h + (500 to 200) oC/3.5h Total No of hours = 33.5 h

f) Effect of prior Soft annealing before Warm forging:
In another trial, analysis was made whether softannealed steel on warm forging showed any other improvement in mechanical properties as shown in Table 8. The results showed that there is no further softening of the steel or enhancement of mechanical properties.

Table 8:Effect of prior soft annealing before warm forging on the mechanical properties
Warm Forging Condition/temperature,oC No of
reheat
during
forging Reduction
Ratio Degree of spherodization
(%) Hardness
(BHN) YS
(MPa) UTS
(MPa) %E
%RA
CVN
(25 oC) CVN
(-60 oC)
As Rolled+ Soft annealing* + Warm forged at 800oC 0 2.30
88 239 610 811 13.4 48 24.2 6.2

* Soft annealing cycle: (RT to 550oC)/8.5 h+(550 to 745oC)/4h+ 745o°C/holding/2 h + (745 to 705oC)/2.5h + (705 to 725oC)/2.5h + (725 to 705)/2.5h + (705 to 500oC/8h + (500 to 200oC/3.5h Total No of hours = 33.5 h

g) Effect of Warm forging directly after hot forging operation:
In another trial, the steel which was hot forged in conventional temperature range of 1200oC, after deformation is brought down to warm forging temperature of 800oC and held for 3 hours before warm forging at 800oC. The properties did not show significant softening. Hence, from the starting austenite phase, when we drop the sample temperature to warm forging temperature to 800oC, followed by deformation, the spherodization is not effective. Hence, for warm forging induced spherodization, the starting microstructure should be pearlite with proeutectoid cementite.

Table 9: Effect of warm forging on a steel hot worked at regular hot deformation in austenite at 1200oC followed by warm forging at 800oC.

Warm forging
ConditionWarmforge /temperature,oC No of
reheat
during
forging Reduction
Ratio Degree of
Spherodization (%) Hardness
BHN YS
(MPa) UTS
(MPa)
%E
%
RA
CVN
(25 oC)
CVN
( -60 oC)

As Rolled + conventional forged @ 1200°C+ warm forged at 800°C/ 3 hours 0 3.5
5 341 939 1207 6.8 10 5.8 5.6

g) Effect of hardening and tempering heat treatment on Warm forge induced spherodization heat treatment:
A comparison of mechanical properties was made in the steel samples conventionally spherodize annealed and that warm forged at 800oC where the hardness and micro structure was similar as shown in Table 10. Normally the heat treatment may involve lower tempering temperatures. In the present case, a higher tempering temperature is deliberately chosen to ensure all carbide precipitation is completed after tempering. The hardening and tempering showed tempered martensite with comparable mechanical properties after heat treatment. Thus, the warm forged bearing steel is an effective means of softened steel equivalent to that in spherodize annealed condition.

Table 10:Effect of hardening and tempering heat treatment on warm forged steel compared with conventional
Warm forging / Heat treatment Condition. oC No of
reheat
during
forging
Reduction
Ratio Hardness
BHN YS
(MPa) UTS
(MPa)
%E
%RA
CVN
(25 oC)
CVN
(-60 oC)

As Rolled+Warm forge at
800°C+840oC/Oil quench + tempered at 600oC 0 2.4 363 1091 1237 17.7 28 12.8 6.9
Conventional As Rolled+ Annealed+ 840 oC/oil quench + Tempered at600oC 0 2.4 358 984 1256 13.2 44 10.4 6.5