A method for preparing a bearing steel by precarburization

Abstract

The present application belongs to the technical field of bearing steel, and particularly relates to a preparation method of pre-carburization treatment bearing steel. The method comprises the following steps: steel billet finishing, pre-carburization mixed agent smearing, steel billet heating furnace heating, wire rod rolling and cooling. The present application proposes that a plurality of carbon compounds are used to pre-carburize the billet in the heating furnace before heating and rolling, and a specific heating temperature and heating rhythm are used for rolling, so that the effect of pre-carburization treatment of the rolled material surface is realized, the generation of the secondary decarburization layer in the spheroidizing annealing in the subsequent processing is reduced, the turning depth of the finished bearing part is reduced, the production cost control is lower, the surface hardness of the bearing part is improved, the bearing life is improved, and the market competitiveness is stronger.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of bearing steel, and particularly relates to a preparation method of bearing steel with pre-carburization treatment. BACKGROUND

[0002] Bearing steel is steel used to manufacture balls, rollers and bearing rings. Bearings bear great pressure and friction when working, so bearing steel is required to have high and uniform hardness and wear resistance, and a high elastic limit. Therefore, the uniformity of the chemical composition of bearing steel, the content and distribution of non-metallic inclusions, the distribution of carbides and the like are all very strict.

[0003] At present, the domestic bearing steel production process generally adopts billet skinning, anti-decarburization coating and the like to control the decarburization layer depth, such as the patent CN109112280A "control method for decarburization depth of bearing steel wire rod" and the patent CN111687209A "rolling process of medium-carbon high-sulfur alloy steel wire rod", which control the decarburization layer depth by ① reducing the original decarburization layer of the billet and ② adopting anti-decarburization coating to reduce the in-furnace oxidation behavior. Although the surface decarburization layer of the finished material can be controlled to about 0.5% D, it is only a temporary solution, and the material surface still has an obvious decarburization layer. Customers will inevitably adopt spheroidizing annealing process in subsequent bearing steel wire rod processing. Under the protection of the atmosphere, the surface decarburization layer cannot be avoided to be generated again, and the service life of the finished steel ball will be reduced by 30% after being processed into the finished steel ball. SUMMARY

[0004] In view of the bearing steel decarburization layer control and the deficiencies of the existing process, the application proposes to adopt a pre-carburization mixture (adopting a plurality of carbon compounds, and pre-carburizing the billet in the heating furnace) before heating and rolling, and to adopt a specific heating temperature, heating rhythm and the like to finally realize the effect of pre-carburization treatment of the surface of the rolled material.

[0005] The bearing steel hot-rolled material produced by the method has a carburization layer on the surface, reduces the generation of the secondary decarburization layer in the subsequent spheroidizing annealing process, reduces the turning depth of the finished part, has lower production cost, improves the surface hardness of the bearing part, prolongs the service life of the bearing, and has stronger market competitiveness.

[0006] The application realizes the purposes of the application through the following technical solutions:

[0007] The preparation method of the pre-carburizing treated bearing steel comprises the following steps: billet finishing, pre-carburizing mixture coating pretreatment, billet heating furnace heating, wire rod rolling and cooling.

[0008] The specific preparation method steps are as follows:

[0009] (1) flame cleaning treatment is performed on the surface of the billet, the flame cleaning depth is controlled to be 2.8-3.4 mm, and the corner of the billet is treated by chamfering, and the chamfer width is controlled to be 20-25 mm;

[0010] (2) the surface of the billet treated in step (1) is mixed and sprayed with pre-carburizing mixture and water glass according to a mass ratio of 1:2, the main components of the pre-carburizing mixture are carbon powder 8-10%, SiC 4-7%, Na2CO3 5-15%, CaCO3 5-13%, SiO2 38-48% and Cr2O3 7-18%, the coating thickness is controlled to be 1.5-1.8 mm, and after the surface is dried, the billet is heated in a heating furnace.

[0011] (3) the billet is heated at a temperature of 1250-1280 DEG C for 10-13.5 h (of which, the high-temperature section is 8-10.3 h), the heating section 1 is 1200-1250 DEG C (2-3 h), the heating section 2 is 1250-1280 DEG C, and the soaking section is 1250-1280 DEG C (the total time of the second section and the soaking section is 8-10.3 h), the furnace atmosphere is controlled, the air-fuel ratio of the heating section 1 is 0.4-0.8, the air-fuel ratio of the heating section 2 is 0.6-0.8, the air-fuel ratio of the soaking section is 0.6-0.8, and the opening rolling temperature is 1100-1180 DEG C;

[0012] (4) the wire rod is produced on a wire rod production line through multi-pass rolling, the final rolling temperature is controlled to be 800-880 DEG C by using a water tank, the wire rod is quickly cooled by a Stelmor air cooling line, the cooling rate reaches 8-15 DEG C / S, and the final cooling temperature is controlled to be 500-550 DEG C.

[0013] Beneficial effects

[0014] In the present application, a plurality of carbon compounds are used to perform pre-carburizing treatment on the billet in the heating furnace before heating and rolling, the addition of the plurality of carbon compounds in the treatment agent causes the carbon element concentration difference, and the effect of carburizing is achieved; the produced bearing steel surface is zero-decarburized layer, and the carburized layer is formed on the hot-rolled material surface, the generation of the secondary decarburized layer in the spheroidizing annealing in the subsequent processing is reduced, the turning depth of the finished part is reduced, the production cost of the bearing steel produced by using the process of the present application is lower, the surface hardness of the bearing part is improved, the bearing service life is improved, and the market competitiveness is stronger. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 It is the microscope graph of the bearing steel organization prepared by example 1 for Φ11 mm specification GCr15.

[0016] Figure 2 Microscope photograph of the bearing steel organization prepared in Example 2 for Φ11mm specification GCr15.

[0017] Figure 3 Microscope photograph of the bearing steel organization prepared in Example 3 for Φ11mm specification GCr15.

[0018] Figure 4 Microscope photograph of the bearing steel organization prepared in Example 4 for Φ11mm specification GCr15.

[0019] Figure 5 Microscope photograph of the bearing steel organization prepared in Comparative Example 1 for Φ11mm specification GCr15.

[0020] Figure 6 Microscope photograph of the bearing steel organization prepared in Comparative Example 2 for Φ11mm specification GCr15.

[0021] Figure 7 Microscope photograph of the organization prepared in Comparative Example 3 for Φ11mm specification 55 steel. DETAILED DESCRIPTION

[0022] The present application is further described below in combination with the production of bearing steel GCr15:

[0023] Billet finishing→pre-carburizing mixed agent spraying→billet heating furnace heating→wire rod rolling and cooling.

[0024] Example 1

[0025] The bearing steel GCr15 is produced by the present method, and the specific operation steps are as follows:

[0026] (1) The surface of the billet is treated by flame cleaning, the flame cleaning depth is controlled at 3.0mm, and the corner of the billet is treated by rounding, and the chamfer width is controlled at 22mm;

[0027] (2) The surface of the billet treated in step (1) is mixed and sprayed with pre-carburizing mixed material and water glass according to a mass ratio of 1:2, and the coating thickness is controlled at 1.6mm, and after the surface is dried, it is heated into a heating furnace;

[0028] The pre-carburizing powder is composed of carbon powder (9.3%), SiC (6.7%), Na2CO3 (12.2%), CaCO3 (10.5%), SiO2 (44.2%), and Cr2O3 (17.1%).

[0029] (3) the billet is heated at a temperature of 1265℃ for 13.2h (9.3h in the high-temperature section), the first heating section is at 1220℃, the second heating section is at 1265℃, the soaking section is at 1265℃, the atmosphere in the furnace is controlled, the air-fuel ratio in the first heating section is 0.8, the air-fuel ratio in the second heating section is 0.7, the air-fuel ratio in the soaking section is 0.7, and the rolling starting temperature is 1180℃;

[0030] (4) the wire rod is produced through multi-pass rolling in a wire rod production line, the final rolling temperature is controlled at 880℃ by using a water tank, the wire rod is rapidly cooled through a Stelmor air cooling line, the cooling rate reaches 13℃ / S, and the final cooling temperature is controlled at 520℃;

[0031] The bearing steel GCr15 produced through the above process has a decarburized layer depth of 0mm and a carburized layer (depth of 0.06mm Figure 1 ) with a surface Rockwell hardness of 37.5HRC and a rated service life L 10 of 1.98*10 7 .

[0032] Example 2

[0033] The bearing steel GCr15 is produced by using the method, and the specific operation steps are as follows:

[0034] (1) the surface of the billet is treated by flame cleaning, the flame cleaning depth is controlled at 3.2mm, and the corner of the billet is treated by rounding, and the rounding width is controlled at 24mm;

[0035] (2) the surface of the billet treated in step (1) is mixed and sprayed by using pre-permeation mixed material and water glass according to a mass ratio of 1:2, the coating thickness is controlled at 1.5mm, and after the surface is dried, the billet is heated in a heating furnace;

[0036] The pre-permeation carbon powder has the following composition: carbon powder (8.2%), SiC (6.9%), Na2CO3 (12.8%), CaCO3 (12.5%), SiO2 (46.3%), and Cr2O3 (13.3%).

[0037] (3) the billet is heated at a temperature of 1258℃ for 11.5h (8.2h in the high-temperature section), the first heating section is at 1210℃, the second heating section is at 1258℃, the soaking section is at 1258℃, the atmosphere in the furnace is controlled, the air-fuel ratio in the first heating section is 0.4, the air-fuel ratio in the second heating section is 0.6, the air-fuel ratio in the soaking section is 0.6, and the rolling starting temperature is 1145℃;

[0038] (4) the wire rod is produced through multi-pass rolling in a wire rod production line, the final rolling temperature is controlled at 880℃ by using a water tank, the wire rod is rapidly cooled through a Stelmor air cooling line, the cooling rate reaches 13℃ / S, and the final cooling temperature is controlled at 520℃;

[0039] The bearing steel GCr15 produced by the above process has a decarburized layer depth of 0 mm, accompanied by a carburized layer of 0.05 mm. Figure 2 ), surface Rockwell hardness 37.8 HRC, rated life L 10 1.86*10 7 .

[0040] Example 3

[0041] The specific operating steps for producing bearing steel GCr15 using this method are as follows:

[0042] (1) The surface of the billet is flame cleaned, the flame cleaning depth is controlled at 3.4mm, and the corners of the billet are rounded, with the chamfer width controlled at 24mm.

[0043] (2) The surface of the steel billet after step (1) is mixed with precarburizing mixture and water glass at a mass ratio of 1:2 and sprayed. The coating thickness is controlled at 1.8 mm. After the surface is dry, it is heated in the heating furnace.

[0044] The composition of the pre-carburized powder is as follows: carbon powder (9.8%), SiC (4.3%), Na2CO3 (10.9%), CaCO3 (12.5%), SiO2 (47.5%), and Cr2O3 (15.0%).

[0045] (3) The billet is heated at 1278℃ for 11.5h (8.2h in the high-temperature section), the first heating section is 1230℃, the second heating section is 1278℃, the furnace atmosphere is controlled, the air-fuel ratio of the first heating section is 0.5, the air-fuel ratio of the second heating section is 0.7, the air-fuel ratio of the soaking section is 0.7, and the initial rolling temperature is 1150℃.

[0046] (4) The wire rod is rolled into finished products through multiple passes on the wire rod production line. The final rolling temperature is controlled at 810℃ using a water tank. The wire rod is rapidly cooled by the Steyrmore air cooling line with a cooling rate of 8℃ / S and the final cooling temperature is controlled at 500℃.

[0047] The bearing steel GCr15 produced by the above process has a decarburized layer depth of 0 mm, accompanied by a carburized layer of 0.05 mm. Figure 3 ), surface Rockwell hardness 37.6 HRC, rated life L 10 1.96*10 7 .

[0048] Example 4

[0049] The specific operating steps for producing bearing steel GCr15 using this method are as follows:

[0050] (1) The surface of the billet is flame cleaned, the flame cleaning depth is controlled at 3.4mm, and the corners of the billet are rounded, with the chamfer width controlled at 24mm.

[0051] (2) The surface of the steel billet after step (1) is mixed with precarburizing mixture and water glass at a mass ratio of 1:2 and sprayed. The coating thickness is controlled at 1.8 mm. After the surface is dry, it is heated in the heating furnace.

[0052] The composition of the pre-carburized powder is as follows: carbon powder (8.1%), SiC (6.8%), Na2CO3 (7.9%), CaCO3 (11.5%), SiO2 (47.8%), and Cr2O3 (17.9%).

[0053] (3) The billet is heated at 1278℃ for 11.5h (8.2h in the high-temperature section), the first heating section is 1230℃, the second heating section is 1278℃, the furnace atmosphere is controlled, the air-fuel ratio of the first heating section is 0.5, the air-fuel ratio of the second heating section is 0.7, the air-fuel ratio of the soaking section is 0.7, and the initial rolling temperature is 1150℃.

[0054] (4) The wire rod is rolled into finished products through multiple passes on the wire rod production line. The final rolling temperature is controlled at 810℃ using a water tank. The wire rod is rapidly cooled by the Steyrmore air cooling line with a cooling rate of 8℃ / S and the final cooling temperature is controlled at 500℃.

[0055] The bearing steel GCr15 produced by the above process has a decarburized layer depth of 0 mm, accompanied by a carburized layer of 0.05 mm. Figure 4 ), surface Rockwell hardness 37.1 HRC, rated life L 10 It is 1.89*10 7 .

[0056] Example 5

[0057] The specific operating steps for producing bearing steel GCr15 using this method are as follows:

[0058] (1) Flame cleaning treatment is performed on the surface of the billet, with the flame cleaning depth controlled at 3.0 mm. The corners of the billet are rounded, with the chamfer width controlled at 22 mm.

[0059] (2) The surface of the steel billet after step (1) is mixed with precarburizing mixture and water glass at a mass ratio of 1:2 and sprayed. The coating thickness is controlled at 1.6 mm. After the surface is dry, it is heated in the heating furnace.

[0060] The pre-carburized powder has the following composition: carbon powder (9.8%), SiC (4.3%), Na2CO3 (10.9%), CaCO3 (12.5%), SiO2 (47.5%), and Cr2O3 (15.0%).

[0061] Step (3) and step (4) are the same as in Example 1.

[0062] The bearing steel GCr15 produced by the above process has a decarburized layer depth of 0 mm, accompanied by a carburized layer of 0.04 mm, a surface Rockwell hardness of 36.8 HRC, and a rated service life L 10 of 1.79*10 7 .

[0063] Example 6

[0064] The bearing steel GCr15 is produced by the method, and the specific operation steps are as follows:

[0065] Step (1) and step (2) are the same as in Example 1.

[0066] (3) The billet is heated at a temperature of 1250℃ for 13.2h (high temperature section 9.3h), heating section 1 1200℃, heating section 2 1250℃, soaking section 1250℃, control the atmosphere in the furnace, heating section 1 air-fuel ratio 0.8, heating section 2 air-fuel ratio 0.7, soaking section air-fuel ratio 0.7, opening rolling temperature 1180℃;

[0067] (4) The wire rod is produced by the wire rod production line through multi-pass rolling, the terminal rolling temperature is controlled by the water tank at 880℃, the wire rod is quickly cooled by the Stelmor air cooling line, the cooling rate reaches 13℃ / S, and the final cooling temperature is controlled at 520℃;

[0068] The bearing steel GCr15 produced by the above process has a decarburized layer depth of 0 mm, accompanied by a carburized layer of 0.06 mm, a surface Rockwell hardness of 37.9 HRC, and a rated service life L 10 of 1.93*10 7 .

[0069] Example 7

[0070] The bearing steel GCr15 is produced by the method, and the specific operation steps are as follows:

[0071] Step (1) and step (2) are the same as in Example 1.

[0072] (3) The billet is heated at a temperature of 1280℃ for 10h (high temperature section 9.3h), heating section 1 1250℃, heating section 2 1280℃, soaking section 1280℃, control the atmosphere in the furnace, heating section 1 air-fuel ratio 0.4, heating section 2 air-fuel ratio 0.7, soaking section air-fuel ratio 0.7, opening rolling temperature 1100℃;

[0073] (4) The wire rod is produced through multi-pass rolling, the final rolling temperature is controlled at 800°C by using a water tank, the wire rod is rapidly cooled by a Stelmor air cooling line, the cooling rate reaches 8°C / s, and the final cooling temperature is controlled at 520°C;

[0074] The bearing steel GCr15 produced by the above process has a decarburized layer depth of 0 mm, and is accompanied by a carburized layer of 0.06 mm, a surface Rockwell hardness of 36.9 HRC, and a rated service life L 10 of 1.78*10 7 .

[0075] Comparative Example 1

[0076] The bearing steel GCr15 is produced, and the specific operation steps are as follows:

[0077] (1) The surface of the billet is subjected to scarfing treatment, the scarfing depth is controlled at 3.0 mm, and the corner portion of the billet is subjected to chamfering treatment, and the chamfering width is controlled at 22 mm;

[0078] (2) The billet is heated at a temperature of 1258°C for 13.2 h (9.3 h in the high-temperature section), the first heating section is at 1220°C, the second heating section is at 1265°C, the soaking section is at 1265°C, the atmosphere in the furnace is controlled, the air-fuel ratio in the first heating section is 0.8, the air-fuel ratio in the second heating section is 0.7, the air-fuel ratio in the soaking section is 0.7, and the starting rolling temperature is 1180°C;

[0079] (3) The wire rod is produced through multi-pass rolling, the final rolling temperature is controlled at 880°C by using a water tank, the wire rod is rapidly cooled by a Stelmor air cooling line, the cooling rate reaches 13°C / s, and the final cooling temperature is controlled at 520°C.

[0080] The bearing steel GCr15 produced by the above process has a decarburized layer depth of 0.11 mm Figure 5 , a surface Rockwell hardness of 34.1 HRC, and a rated service life L 10 of 0.95*10 7 .

[0081] Comparative Example 2

[0082] The bearing steel GCr15 is produced, and the specific operation steps are as follows:

[0083] (1) The surface of the billet is subjected to flame cleaning treatment, the flame cleaning depth is controlled at 3.0 mm, and the corner portion of the billet is subjected to chamfering treatment, and the chamfering width is controlled at 22 mm;

[0084] (2) The surface of the billet treated in step (1) is mixed and sprayed with a pre-carburizing mixture and water glass in a mass ratio of 1:2, and the coating thickness is controlled at 1.6 mm, and after the surface is dried, the billet is heated in a heating furnace;

[0085] The composition of the pre-carburized powder is as follows: carbon powder (2.2%), SiC (15.9%), Al2O3 (12.6%), MgO (10.4%), SiO2 (24.1%), ZnO (17.5%), and CaO (17.3%).

[0086] (3) The billet is heated at 1265℃ for 13.2h (9.3h in the high-temperature section), the first heating section is 1220℃, the second heating section is 1265℃, the second heating section is 1265℃, the atmosphere in the furnace is controlled, the air-fuel ratio in the first heating section is 0.8, the air-fuel ratio in the second heating section is 0.7, the air-fuel ratio in the soaking section is 0.7, and the initial rolling temperature is 1180℃;

[0087] (4) The wire rod is rolled into a finished product through multiple passes on the wire rod production line. The final rolling temperature is controlled at 880℃ by a water tank. The wire rod is rapidly cooled by the Steyrmore air cooling line with a cooling rate of 13℃ / S and the final cooling temperature is controlled at 520℃.

[0088] The decarburized layer depth of the bearing steel GCr15 produced by the above process is 0.08 mm. Figure 6 ), surface Rockwell hardness 35.2 HRC, rated life L 10 0.99*10 7 .

[0089] Comparative Example 3

[0090] The specific operating steps for producing 55 steel are as follows:

[0091] (1) The surface of the billet is flame cleaned, the flame cleaning depth is controlled at 3.4mm, and the corners of the billet are rounded, with the chamfer width controlled at 24mm.

[0092] (2) The surface of the steel billet after step (1) is mixed with precarburizing powder and water glass at a mass ratio of 1:2 and sprayed. The coating thickness is controlled at 1.8 mm. After the surface is dry, it is heated in the heating furnace.

[0093] The composition of the pre-carburized powder is as follows: carbon powder (8.1%), SiC (6.8%), Na2CO3 (7.9%), CaCO3 (11.5%), SiO2 (47.8%), and Cr2O3 (17.9%).

[0094] (3) The billet is heated at 1278℃ for 11.5h (8.2h in the high-temperature section), the first heating section is 1230℃, the second heating section is 1278℃, the furnace atmosphere is controlled, the air-fuel ratio of the first heating section is 0.5, the air-fuel ratio of the second heating section is 0.7, the air-fuel ratio of the soaking section is 0.7, and the initial rolling temperature is 1150℃.

[0095] (4) the wire rod is produced by multi-pass rolling, the final rolling temperature is controlled at 810℃ by using a water tank, the wire rod is rapidly cooled by a Stelmor air cooling line, the cooling rate reaches 8℃ / S, and the final cooling temperature is controlled at 500℃;

[0096] The decarburized layer depth of the 55 steel produced by the above process is 0.10mm Figure 7 ).

[0097] Comparative Example 4

[0098] The bearing steel GCr15 is produced by using the method, and the specific operation steps are as follows:

[0099] (1) the surface of the billet is treated by flame cleaning, the flame cleaning depth is controlled at 3.4mm, the corner of the billet is treated by rounding, and the rounding width is controlled at 24mm;

[0100] (2) the surface of the billet treated in step (1) is mixed and sprayed by using pre-carburizing mixed material and water glass according to a mass ratio of 1:1, the coating thickness is controlled at 1.8mm, and after the surface is dried, the billet is heated in a heating furnace;

[0101] The pre-carburizing powder is composed of carbon powder (8.1%), SiC (6.8%), Na2CO3 (7.9%), CaCO3 (11.5%), SiO2 (47.8%), and Cr2O3 (17.9%).

[0102] (3) the billet is heated at a temperature of 1278℃ for 11.5h (8.2h in the high-temperature section), the heating section 1 is at 1230℃, the heating section 2 is at 1278℃, the atmosphere in the furnace is controlled, the air-fuel ratio of the heating section 1 is 0.5, the air-fuel ratio of the heating section 2 is 0.7, the air-fuel ratio of the soaking section is 0.7, and the rolling temperature is 1150℃;

[0103] (4) the wire rod is produced by multi-pass rolling, the final rolling temperature is controlled at 810℃ by using a water tank, the wire rod is rapidly cooled by a Stelmor air cooling line, the cooling rate reaches 8℃ / S, and the final cooling temperature is controlled at 500℃;

[0104] The decarburized layer depth of the bearing steel GCr15 produced by the above process is 0.03mm, the surface Rockwell hardness is 34.3HRC, and the rated service life L 10 is 0.96*10 7 .

Claims

1. A method for manufacturing a bearing steel subjected to pre-carburization treatment, comprising billet finishing, pre-carburization mixture application, billet heating furnace heating, wire rod rolling and cooling, characterized in that, The preparation method comprises the following steps: (1) flame cleaning treatment is performed on the surface of the steel billet, and the corner of the billet is treated by rounding; (2) the surface of the steel billet treated in step (1) is mixed and smeared with pre-carburizing mixture and water glass, and then is heated in a heating furnace after the surface is dried; The pre-carburizing mixture comprises the following components: carbon powder 8-10%, SiC 4-7%, Na2CO3 5-15%, CaCO3 5-13%, SiO238-48%, and Cr2O3 7-18%; the mass ratio of the pre-carburizing mixture to the water glass is 1:2; (3) the steel billet is heated at a temperature of 1250-1280 DEG C, the air-fuel ratio in the furnace is controlled, and the open rolling temperature is 1000-1150 DEG C; (4) the steel billet is rolled into a wire rod on a wire rod production line, the final rolling temperature is controlled by a water tank, and the wire rod is rapidly cooled by a Stelmor air cooling line.

2. The method of producing a pre-carburized bearing steel according to claim 1, wherein The flame cleaning depth in step (1) is controlled to be 2.8-3.4 mm, and the width of the billet rounding is controlled to be 20-25 mm.

3. The method of producing a pre-carburized bearing steel according to claim 1, wherein The pre-carburizing mixture in step (2) is smeared to a thickness of 1.5-1.8 mm.

4. The method of producing a pre-carburized bearing steel according to claim 1, wherein In step (3), the steel billet is heated for 10-13.5 h, wherein the steel billet is heated at a temperature of 1250-1280 DEG C for 8-10.3 h, and the air-fuel ratio in the furnace is controlled as follows: the air-fuel ratio in the first heating section is 0.4-0.8, the air-fuel ratio in the second heating section is 0.6-0.8, and the air-fuel ratio in the soaking section is 0.6-0.

8.

5. The method of producing a bearing steel for pre-carburizing treatment according to claim 1, wherein In step (4), the final rolling temperature is 800-880 DEG C, the cooling rate is 8-15 DEG C / S, and the final cooling temperature is controlled to be 500-550 DEG C.

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