A method for eliminating extremely fine structure on the surface of GCr15 bearing steel bar

Abstract

The invention discloses a method for eliminating the extremely fine structure on the surface of a GCr15 bearing steel bar. The method comprises the following steps: performing two-stage cooling during controlled cooling after rolling; in stage I, water cooling is performed in a water-penetrating cooling area using a water tank; after water cooling, the surface temperature of the bar is controlled at 610-650°C; and then the bar enters a second air-cooling area; after air cooling, the surface temperature of the bar is controlled at 550-600°C; the bar is then slowly cooled on a cooling bed and then air-cooled. The method not only satisfies the carbide control requirements of the bearing steel bar, but also avoids the formation of an extremely fine structure layer on the surface of the bar due to excessive cooling intensity during direct water-penetrating cooling, thereby improving the surface quality of the hot-rolled GCr15 bearing steel bar.

Description

Technical Field

[0001] The invention belongs to the technical field of bearing steel, and in particular relates to a method for eliminating extremely fine structures on the surface of a GCr15 bearing steel bar. Background Art

[0002] Currently, hot-rolled GCr15 bearing steel bars in my country are typically cooled with water or water tanks after rolling. This method, however, is difficult to precisely control the water flow rate, which inevitably leads to the formation of extremely fine microstructures on the surface of the hot-rolled bearing steel bars. This also creates a complex microstructure, such as tempered martensite, formed after rapid cooling and self-tempering, and carbide precipitation that cannot be precisely controlled. The presence of these microstructures obscures the decarburization layer, hindering its measurement.

[0003] Chinese patent CN104359726A discloses a method for rapidly measuring the decarburization layer of GCr15 bearing steel after it has penetrated water, where the ultrafine structure covers the decarburization layer. This method effectively helps metallographic evaluators quickly assess the decarburization layer, eliminating the need for normalizing heat treatment to obtain a hot-rolled structure for decarburization layer measurement. This method allows laboratories to quickly and efficiently complete metallographic testing, saving energy during heat treatment and benefiting the environment. It also saves labor, reduces measurement costs, shortens the inspection cycle, and improves production efficiency. However, manual testing is still subject to haphazardness and cannot accurately determine the specific depth of the decarburization layer. Therefore, a thorough solution is needed to address the problem of ultrafine structure covering the decarburization layer. Summary of the Invention

[0004] In order to solve the above technical problems, the present invention provides a method for eliminating the extremely fine structure on the surface of GCr15 bearing steel bars. After final rolling, the hot-rolled bearing steel bars are cooled in two stages, first by water cooling and then by air cooling. This method not only meets the carbide control requirements of the bearing steel bars, but also avoids the formation of an extremely fine structure layer on the surface of the bar due to excessive cooling intensity of direct water cooling, thereby improving the surface quality of the hot-rolled GCr15 bearing steel bars.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A method for eliminating extremely fine surface microstructure in GCr15 bearing steel bars involves two-stage cooling during controlled cooling after rolling. Stage I involves water cooling in a water-through cooling area using a water tank. After water cooling, the bar surface temperature is controlled at 610-650°C. The bar then enters stage II, an air-cooling area, where the surface temperature is controlled at 550-600°C. The bar is then placed on a cooling bed for insulation and then air-cooled. The extremely fine microstructure consists of tempered bainite and degraded pearlite.

[0007] When cooling through water, the average cooling rate of the bar surface is 40-50℃ / s.

[0008] When air-cooled, the average cooling rate of the bar surface is 10-20°C.

[0009] The slow cooling time on the cooling bed is 10 to 20 minutes.

[0010] The surface of the GCr15 bearing steel bar does not have an extremely fine structure, and the level of the network carbide after spheroidizing annealing + quenching and tempering is ≤ level 2.5.

[0011] The GCr15 bearing steel bar contains, by weight percentage, the following: C: 0.95-1.05%, Si: 0.25-0.35%, Mn: 0.25-0.45%, Cr: 1.43-1.60%, Al≤0.030%, Ni≤0.10%, Cu≤0.10%, Mo≤0.08%, As≤0.030%, Sn≤0.020%, Sb≤0.0050%, Pb≤0.0020%, Ti≤0.0015%, Ca≤0.0010%, P≤0.020%, S≤0.020%, O≤0.0008%, N≤0.0060%, H≤0.00015%, and the rest is Fe and other inevitable impurities.

[0012] The production process of the GCr15 bearing steel bar is: electric furnace smelting → LF refining → RH refining → 250mm×250mm square billet continuous casting → bar controlled rolling and controlled cold rolling → Φ16~40mm hot-rolled round steel.

[0013] During continuous casting, induction heating of the tundish is adopted, the temperature of the molten steel in the tundish is controlled at 15 to 25°C above the liquidus temperature, two-stage electromagnetic stirring and light and heavy pressure processes are used.

[0014] The continuous casting billet is put into the furnace by hot charging process, with the temperature of the first heating section being 900-1080℃, the temperature of the second heating section being 1220-1250℃, and the soaking section being 1230-1250℃; the residual oxygen is controlled to be ≤4%; and the total heating time is 5-6h.

[0015] During controlled rolling of bars, the starting rolling temperature is 1100-1200°C. After rough rolling, intermediate rolling, pre-finishing rolling, finishing rolling and KOCKS rolling mill, the final rolling temperature is controlled at 780-820°C, and the final rolling deformation is ≥35%. After deformation in the two-phase zone, the carbides precipitated first and the unrecrystallized austenite are simultaneously subjected to large deformation plastic processing, which increases the deformation bands within the austenite grains, creates conditions for the precipitation of dispersed carbides, improves the carbide network to a certain extent, and creates favorable conditions for rapid cooling after rolling.

[0016] In the method for eliminating the extremely fine structure on the surface of GCr15 bearing steel bars provided by the present invention, two-stage cooling is performed immediately after rolling. I: Rapid water cooling in the water cooling zone with an average cooling rate of 40 to 50°C / s. After water cooling, the surface temperature of the bar is controlled between 610 and 650°C. The purpose of this rapid cooling is to suppress the precipitation of carbide networks. The reduction in bar temperature reduces the diffusion rate of C and Cr elements, thereby reducing the driving force for the aggregation and growth of carbides at the austenite grain boundaries. Therefore, during the rapid cooling process, there are not enough C and Cr elements at the grain boundaries to accelerate the precipitation and growth of carbide networks. The C and Cr content in the carbide network decreases, while the C and Cr content in the matrix pearlite increases. The nucleation and precipitation of the carbide network are closely related to the diffusion of C and Cr elements in austenite. As the cooling rate increases, the diffusion of carbide-forming alloying elements such as C and Cr to the grain boundaries becomes difficult, which suppresses the precipitation of carbide networks at the grain boundaries. II: Air Cooling: While the rapid water-through cooling in stage I reduces the nucleation driving force for the carbide network and avoids the temperature range of rapid carbide network formation, a small amount of carbide network precipitation still occurs at this temperature. Furthermore, the higher bar temperature at this point is not conducive to grain refinement and pearlite interlamellar spacing. However, continued water-through cooling at this point would easily reduce the instantaneous surface temperature of the bar to the martensitic transformation temperature range of the bearing steel, resulting in the formation of an extremely fine surface structure. Therefore, further cooling is required using slower air cooling, with an average cooling rate of 10-20°C. The surface temperature of the bar after cooling is controlled between 550-600°C. This slower cooling method not only prevents the GCr15 bearing steel bar from experiencing excessively low surface temperatures, resulting in an extremely fine structure, but also facilitates the adjustment of the final cooling temperature, ensuring that the bar completes the pearlite transformation within the appropriate temperature range. After cooling, the bar is placed on a cooling bed for 10-20 minutes before being removed from the bed for air cooling.

[0017] This invention utilizes low-temperature rolling and post-rolling two-stage cooling, and controls the surface cooling intensity to prevent the formation of extremely fine microstructure on the surface of GCr15 bearing steel bars due to excessively low temperatures. It also inhibits the precipitation and aggregation of carbide networks at grain boundaries, ensuring a network carburization level of ≤2.5 after spheroidizing annealing and quenching and tempering. This meets the bearing industry's requirements for high-quality bearing steel. All performance indicators of the GCr15 hot-rolled round steel produced by this invention meet the requirements of the national standard GB / T 18254-2016. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is the microstructure of the hot-rolled round steel obtained by the post-rolling controlled cooling process in Example 1. It can be seen that there is no extremely fine structure on the surface;

[0019] Figure 2 This is the microstructure diagram of the hot-rolled round steel after spheroidizing annealing in Example 1, with a network carbide level of 2.0;

[0020] Figure 3 This is the microstructure of the hot-rolled round steel obtained by the post-rolling controlled cooling process in Example 2. It can be seen that there is no extremely fine structure on the surface;

[0021] Figure 4 This is the microstructure diagram of the hot-rolled round steel after spheroidizing annealing in Example 2, with a network carbide level of 2.0;

[0022] Figure 5 This is the microstructure of the hot-rolled round steel obtained by the post-rolling controlled cooling process in Example 3. It can be seen that there is no extremely fine structure on the surface;

[0023] Figure 6 This is the microstructure of the hot-rolled round steel after spheroidizing annealing in Example 3, with a network carbide level of 2.0;

[0024] Figure 7 This is the microstructure of the hot-rolled round steel obtained by the post-rolling controlled cooling process in Example 4. It can be seen that there is no extremely fine structure on the surface;

[0025] Figure 8 This is the microstructure diagram of the hot-rolled round steel after spheroidizing annealing in Example 4, with network carbide level 2.0;

[0026] Figure 9 This is the microstructure of the hot-rolled round steel obtained by the post-rolling controlled cooling process in Example 5. It can be seen that there is no extremely fine structure on the surface;

[0027] Figure 10 This is the microstructure diagram of the hot-rolled round steel after spheroidizing annealing in Example 5, with network carbide level 2.0;

[0028] Figure 11 This is the microstructure of the hot-rolled round steel obtained by the controlled cooling process after rolling in Comparative Example 1. It can be seen that there is no extremely fine structure on the surface;

[0029] Figure 12 This is the microstructure of the hot-rolled round steel in Comparative Example 1 after spheroidizing annealing, with a network carbide level of 2.0;

[0030] Figure 13 This is the microstructure of the hot-rolled round steel obtained by the controlled cooling process after rolling in Comparative Example 2. It can be seen that there is no extremely fine structure on the surface;

[0031] Figure 14 This is the microstructure of the hot-rolled round steel in Comparative Example 2 after spheroidizing annealing, with network carbide level 2.0;

[0032] Figure 15 Schematic diagram of controlled cooling after rolling in the present invention. DETAILED DESCRIPTION

[0033] The present invention provides a method for eliminating the extremely fine structure on the surface of a GCr15 bearing steel bar. The production process of the GCr15 bearing steel bar is: electric furnace smelting → LF refining → RH refining → 250mm×250mm square billet continuous casting → bar controlled rolling and controlled cold rolling → Φ16-40mm hot-rolled round steel.

[0034] During continuous casting, induction heating of the tundish is adopted, the temperature of the molten steel in the tundish is controlled at 15 to 25°C above the liquidus temperature, two-stage electromagnetic stirring and light and heavy pressure processes are used.

[0035] The continuous casting billet is put into the furnace by hot charging process, with the temperature of the first heating section being 900-1080℃, the temperature of the second heating section being 1220-1250℃, and the soaking section being 1230-1250℃; the residual oxygen is controlled to be ≤4%; and the total heating time is 5-6h.

[0036] During controlled rolling of bars, the starting rolling temperature is 1100-1200°C. After rough rolling, intermediate rolling, pre-finishing rolling, finishing rolling and KOCKS rolling mill, the final rolling temperature is controlled at 780-820°C, and the final rolling deformation is ≥35%.

[0037] Two-stage cooling is carried out during controlled cooling after rolling. In stage I, a water tank is used for water cooling in the water cooling area. After water cooling, the surface temperature of the bar is controlled at 610-650°C. Then it enters the air cooling area of ​​stage II. After air cooling, the surface temperature of the bar is controlled at 550-600°C. Then it is put on the cooling bed for slow cooling for 10-20 minutes and then air-cooled.

[0038] When cooling through water, the average cooling rate of the bar surface is 40-50℃ / s.

[0039] When air-cooled, the average cooling rate of the bar surface is 10-20°C.

[0040] The GCr15 bearing steel bar contains, by weight percentage, the following: C: 0.95-1.05%, Si: 0.25-0.35%, Mn: 0.25-0.45%, Cr: 1.43-1.60%, Al≤0.030%, Ni≤0.10%, Cu≤0.10%, Mo≤0.08%, As≤0.030%, Sn≤0.020%, Sb≤0.0050%, Pb≤0.0020%, Ti≤0.0015%, Ca≤0.0010%, P≤0.020%, S≤0.020%, O≤0.0008%, N≤0.0060%, H≤0.00015%, and the rest is Fe and other inevitable impurities.

[0041] The present invention is described in detail below with reference to the embodiments.

[0042] Example 1

[0043] A method for producing hot-rolled bars with a diameter of Φ16.0 mm is provided. The method can control the carbide network level and eliminate the surface fine structure layer. The chemical composition of the hot-rolled bars is: C 0.98%, Si 0.26%, Mn 0.33%, Cr 1.45%, P 0.007%, S 0.001%, Ni 0.02%, Mo 0.01%, Al 0.021%, Cu 0.03%, Ti 0.0010%, O 0.0005%, As 0.003%, Pb 0.0002%, Ca 0.0002%, and the remainder is Fe and unavoidable impurities.

[0044] The method comprises the following steps: electric furnace smelting → LF refining → RH refining → 250mm×250mm square billet continuous casting → bar controlled rolling and controlled cold rolling → Φ16-40mm hot-rolled round steel;

[0045] The continuous casting stage uses tundish induction heating with a superheat of 15°C, two-stage electromagnetic stirring, and light and heavy reduction processes;

[0046] The mill was subjected to multiple rolling passes, with the final rolling temperature controlled at 820°C and the final rolling deformation of 52.4%.

[0047] Two-stage cooling after rolling: I: rapid water cooling, the average surface cooling rate is 40℃ / s, and the surface temperature of the steel is cooled to 650℃; II: air cooling, the average surface cooling rate is 10℃ / s, and the surface temperature of the steel is cooled to 600℃. In this way, the highest temperature at which the surface of the steel returns to red can avoid the temperature range of 700-850℃ where a large amount of carbide network precipitation occurs in austenite. After the final temperature is returned to normal, the cross-section temperature of the bar is controlled at 610℃, and then it is slowly cooled on the cooling bed for 11 minutes, and then air-cooled outside the cooling bed.

[0048] Example 2

[0049] A method for producing hot-rolled bars with a diameter of Φ20.0 mm is disclosed. The method can control the carbide network level and eliminate the surface fine structure layer. The chemical composition of the hot-rolled bars is: C 0.98%, Si 0.23%, Mn 0.36%, Cr 1.44%, P 0.008%, S 0.002%, Ni 0.03%, Mo 0.01%, Al 0.020%, Cu 0.04%, Ti 0.0012%, O 0.0006%, As 0.004%, Pb 0.0002%, Ca 0.0002%, and the remainder is Fe and unavoidable impurities.

[0050] The method comprises the following steps: electric furnace smelting → LF refining → RH refining → 250mm×250mm square billet continuous casting → bar controlled rolling and controlled cold rolling → Φ16-40mm hot-rolled round steel;

[0051] The continuous steel casting stage uses tundish induction heating with a superheat of 16°C, two-stage electromagnetic stirring, and light and heavy reduction processes;

[0052] The mill is subjected to multiple rolling passes, with the final rolling temperature controlled at 790°C and the final rolling deformation being 48.3%.

[0053] Two-stage cooling after rolling: I: rapid water cooling, the average surface cooling rate is 40℃ / s, and the surface temperature of the steel is cooled to 630℃; II: air cooling, the average surface cooling rate is 15℃ / s, and the surface temperature of the steel is cooled to 580℃. In this way, the highest temperature of the steel surface returning to red is within the temperature range of 700-850℃ where a large amount of carbide network precipitation occurs in austenite. After the final temperature is returned to the bar, the cross-section temperature is controlled at 610℃, and then it is slowly cooled on the cooling bed for 15 minutes, and then air-cooled outside the cooling bed.

[0054] Example 3

[0055] A method for producing hot-rolled bars with a diameter of Φ25.0 mm is disclosed. The method can control the carbide network level and eliminate the surface fine structure layer. The chemical composition of the hot-rolled bars is: C 0.97%, Si 0.26%, Mn 0.36%, Cr 1.45%, P 0.008%, S 0.001%, Ni 0.02%, Mo 0.01%, Al 0.017%, Cu 0.03%, Ti 0.0013%, O 0.0006%, As 0.004%, Pb 0.0003%, Ca 0.0004%, and the remainder is Fe and unavoidable impurities.

[0056] The method comprises the following steps: electric furnace smelting → LF refining → RH refining → 250mm×250mm square billet continuous casting → bar controlled rolling and controlled cold rolling → Φ16-40mm hot-rolled round steel;

[0057] The continuous steel casting stage uses tundish induction heating with a superheat of 18°C, two-stage electromagnetic stirring, and light and heavy reduction processes;

[0058] The steel was rolled in multiple passes on the rolling mill, with the final rolling temperature controlled at 800°C and the final rolling deformation of 42.4%.

[0059] Two-stage cooling after rolling: I: rapid water cooling, the average surface cooling rate is 45℃ / s, and the surface temperature of the steel is cooled to 620℃; II: air cooling, the average surface cooling rate is 15℃ / s, and the surface temperature of the steel is cooled to 580℃. In this way, the highest temperature of the steel surface returning to red is in the temperature range of 700-850℃, where a large amount of carbide network precipitation of austenite is avoided. After the final temperature is returned to the bar, the cross-section temperature is controlled at 620℃, and then it is slowly cooled on the cooling bed for 15 minutes, and then air-cooled out of the cooling bed.

[0060] Example 4

[0061] A method for producing hot-rolled bars with a diameter of Φ30.0 mm is disclosed. The method can control the carbide network level and eliminate the surface fine structure layer. The chemical composition of the hot-rolled bars is: C 0.98%, Si 0.25%, Mn 0.33%, Cr 1.45%, P 0.005%, S 0.002%, Ni 0.03%, Mo 0.01%, Al 0.018%, Cu 0.04%, Ti 0.0011%, O 0.0005%, As 0.004%, Pb 0.0002%, Ca 0.0005%, and the remainder is Fe and unavoidable impurities.

[0062] The method comprises the following steps: electric furnace smelting → LF refining → RH refining → 250mm×250mm square billet continuous casting → bar controlled rolling and controlled cold rolling → Φ16-40mm hot-rolled round steel;

[0063] The continuous steel casting stage uses tundish induction heating with a superheat of 15°C, two-stage electromagnetic stirring, and light and heavy reduction processes;

[0064] The mill is subjected to multiple rolling passes, with the final rolling temperature controlled at 790°C and the final rolling deformation of 38.7%.

[0065] Two-stage cooling after rolling: I: rapid water cooling, the average surface cooling rate is 40℃ / s, and the surface temperature of the steel is cooled to 630℃; II: air cooling, the average surface cooling rate is 20℃ / s, and the surface temperature of the steel is cooled to 570℃. In this way, the highest temperature of the steel surface returning to red is in the temperature range of 700-850℃, which is a large amount of network carbide precipitation in austenite. After the final temperature is returned to the bar, the cross-section temperature is controlled at 630℃, and then it is slowly cooled on the cooling bed for 18 minutes, and then air-cooled on the cooling bed.

[0066] Example 5

[0067] A method for producing a hot-rolled bar with a diameter of Φ40.0 mm is provided. The method can control the carbide network level and eliminate the surface fine structure layer. The chemical composition of the hot-rolled bar is: C 0.96%, Si 0.27%, Mn 0.32%, Cr 1.43%, P 0.006%, S 0.002%, Ni 0.03%, Mo 0.01%, Al 0.020%, Cu 0.04%, Ti 0.0011%, O 0.0005%, As 0.004%, Pb 0.0002%, Ca 0.0005%, and the remainder is Fe and unavoidable impurities.

[0068] The method comprises the following steps: electric furnace smelting → LF refining → RH refining → 250mm×250mm square billet continuous casting → bar controlled rolling and controlled cold rolling → Φ16-40mm hot-rolled round steel;

[0069] The continuous steel casting stage uses tundish induction heating with a superheat of 15°C, two-stage electromagnetic stirring, and light and heavy reduction processes;

[0070] The steel was rolled in multiple passes on the rolling mill, with the final rolling temperature controlled at 800°C and the final rolling deformation of 35.8%.

[0071] Two-stage cooling after rolling: I: rapid water cooling, the average surface cooling rate is 50℃ / s, and the surface temperature of the steel is cooled to 610℃; II: air cooling, the average surface cooling rate is 20℃ / s, and the surface temperature of the steel is cooled to 550℃. In this way, the highest temperature of the steel surface returning to red is in the temperature range of 700-850℃, where a large amount of carbide network precipitation of austenite is avoided. After the final temperature is returned to the bar, the cross-section temperature is controlled at 630℃, and then it is slowly cooled on the cooling bed for 20 minutes, and then air-cooled out of the cooling bed.

[0072] Comparative Example 1

[0073] The other parts are the same as Example 1, except that the post-rolling cooling process is water-cooled, the average surface cooling rate is 50°C / s, the surface final cooling temperature is 550°C, and the surface of the bar has a very fine structure, the center network carbide level is 2.0, and the metallographic structure is shown in FIG. Figure 11 、 12 .

[0074] Comparative Example 2

[0075] The other aspects are the same as those in Example 1, except that a two-stage cooling process is adopted after rolling. The average surface cooling rate of the water cooling section is 60°C / s, the average surface cooling rate of the air cooling section is 30°C / s, and the final cooling temperature is 550°C. An extremely fine structure appears on the surface of the bar, and the central network carbide level is 2.0. The metallographic structure is shown in FIG. Figure 13 、 14 .

[0076] The metallographic structures of the hot rolled round steel obtained by controlled cooling after rolling in the above embodiments are as follows: Figure 1 、 3 As shown in Figures 5, 7 and 9, it can be seen that the surface layer of the GCr15 bearing steel hot-rolled round steel produced by the production method of the present invention does not have extremely fine structure.

[0077] The metallographic structures of the hot rolled round steel obtained by controlled cooling after rolling in Comparative Examples 1 and 2 are as follows: Figure 11 、 13 As shown, it can be seen that the surface layer of the GCr15 bearing steel hot-rolled round steel produced by the production method in the comparative example has extremely fine structure.

[0078] The metallographic structures of the hot rolled round steel in the above embodiments after spheroidizing annealing and quenching and tempering are as follows: Figure 2 、 4, 6, 8, and 10, the level of the network carbide of the round steel in each embodiment is ≤ level 2.5, and the metallographic structures of the hot-rolled round steel in Comparative Examples 1 and 2 after spheroidizing annealing + quenching and tempering are as follows: Figure 12 、 14 As shown, the grade of the network carbides of the round steel in each comparative example is ≤ Grade 2.5. The network carbides are graded according to GB / T18254-2016. The spheroidizing annealing method is as follows: the bar is heated to 790°C over 4.5 hours, held at this temperature for 5 hours, then slowly cooled to 720°C over 1.5 hours and held at this temperature for 4.5 hours. Finally, it is slowly cooled to 650°C over 2 hours and air-cooled after being removed from the furnace. The quenching and tempering process after spheroidizing annealing is as follows: hold at 840°C for 30 minutes, then oil quench, then hold at 160°C for 90 minutes, then air-cool.

[0079] The above-mentioned detailed description of a method for eliminating the extremely fine structure on the surface of GCr15 bearing steel bars with reference to the embodiment is illustrative rather than restrictive. Several embodiments can be listed according to the limited scope. Therefore, changes and modifications without departing from the overall concept of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for eliminating the extremely fine structure on the surface of GCr15 bearing steel bar, characterized in that: Two-stage cooling is carried out during controlled cooling after rolling. In stage I, water cooling is carried out in a water tank in the water cooling area. After water cooling, the surface temperature of the bar is controlled at 610~650℃. Then the bar enters the air cooling area in stage II. After air cooling, the surface temperature of the bar is controlled at 550~600℃. The bar is then put on a cooling bed for slow cooling and then air cooling. When cooling through water, the average cooling rate of the bar surface is 40~50℃ / s; When air-cooled, the average cooling rate of the bar surface is 10~20℃ / s.

2. The method for eliminating the extremely fine structure on the surface of GCr15 bearing steel bar according to claim 1, characterized in that: The slow cooling time on the cooling bed is 10 to 20 minutes.

3. The method for eliminating the ultrafine structure on the surface of GCr15 bearing steel bar according to claim 1 or 2, characterized in that: The surface of the GCr15 bearing steel bar does not have an extremely fine structure, and the level of the network carbide after spheroidizing annealing + quenching and tempering is ≤ level 2.

5.

4. The method for eliminating the ultrafine structure on the surface of GCr15 bearing steel bar according to claim 1 or 2, characterized in that: The GCr15 bearing steel bar contains, by weight percentage, the following: C: 0.95~1.05%, Si: 0.25~0.35%, Mn: 0.25~0.45%, Cr: 1.43~1.60%, Al≤0.030%, Ni≤0.10%, Cu≤0.10%, Mo≤0.08%, As≤0.030%, Sn≤0.020%, Sb≤0.0050%, Pb≤0.0020%, Ti≤0.0015%, Ca≤0.0010%, P≤0.020%, S≤0.020%, O≤0.0008%, N≤0.0060%, H≤0.00015%, and the rest is Fe and other inevitable impurities.

5. The method for eliminating the ultrafine structure on the surface of GCr15 bearing steel bar according to claim 1 or 2, characterized in that: The production process of the GCr15 bearing steel bar is: electric furnace smelting → LF refining → RH refining → 250mm×250mm square billet continuous casting → bar controlled rolling and controlled cold rolling → Φ16~40mm hot-rolled round steel.

6. The method for eliminating the ultrafine structure on the surface of GCr15 bearing steel bar according to claim 1 or 2, characterized in that: During continuous casting, induction heating of the tundish is adopted, the temperature of the molten steel in the tundish is controlled at 15~25℃ above the liquidus temperature, two-stage electromagnetic stirring and light and heavy pressure processes are used.

7. The method for eliminating the ultrafine structure on the surface of GCr15 bearing steel bar according to claim 1 or 2, characterized in that: The continuous casting billet is put into the furnace using a hot charging process, with the temperature of the first heating section at 900~1080℃, the temperature of the second heating section at 1220~1250℃, and the soaking section at 1230~1250℃; the residual oxygen is controlled at ≤4%; the total heating time is 5~6h.

8. The method for eliminating the ultrafine structure on the surface of GCr15 bearing steel bar according to claim 1 or 2, characterized in that: During controlled rolling of bars, the starting rolling temperature is 1100~1200℃. After rough rolling, intermediate rolling, pre-finishing rolling, finishing rolling and KOCKS rolling mill, the final rolling temperature is controlled at 780~820℃, and the final rolling deformation is ≥35%.

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