A production process for improving the compactness and grain size of the core of high-carbon bearing steel

Abstract

The present application relates to the technical field of bearing steel, and particularly relates to a production process for improving the heart density and grain size of high-carbon bearing steel. The production process comprises the following steps: (1) continuous casting; (2) heating of the casting blank; and (3) single-side large reduction isothermal rolling. The present application uses the continuous casting + heating + single-side large reduction isothermal rolling process to manufacture the bearing steel, so that the heart of the bearing steel is free of shrinkage holes under the acid pickling macroscopic examination, the steel macroscopic ultrasonic flaw detection result is above the B level, and the grain size is above the 9th level. The isothermal single-side large reduction rolling + heavy reduction technology is used to improve the work efficiency, the continuous casting speed is increased when the heart density of the steel reaches the general level, the continuous casting efficiency is improved, the bearing steel macroscopic quality is better than that of the bearing steel under single reduction at high speed, and the improper heavy reduction position in the continuous casting process can be prevented to avoid the internal crack of the casting blank.

Description

Technical Field

[0001] This invention relates to the field of bearing steel technology, and more particularly to a production process for improving the core density and grain size of high-carbon bearing steel. Background Technology

[0002] High-carbon bearing steel is an important raw material in high-end manufacturing, widely used in industries such as automobiles, high-speed rail, wind power, precision machine tools, aerospace, and mining machinery. With the continuous development of industrial technology, the quality requirements for high-carbon bearing steel are becoming increasingly stringent, especially its core density and grain size.

[0003] However, traditional casting processes often fail to meet the core density requirements of high-end manufacturing for large square billets of bearing steel. As the cross-section widens and thickens, the internal cooling conditions deteriorate significantly, columnar crystals develop in the solidification structure, and the residual molten steel rich in solute segregation elements between dendrites tends to flow in equilibrium, leading to more severe defects such as segregation, porosity, and shrinkage cavities in the billet. These defects are difficult to eliminate effectively during subsequent heating and rolling processes, thus affecting the quality of the final product. Currently, researchers both domestically and internationally mostly employ light or heavy reduction techniques, especially heavy reduction at the end of the solidification of the continuously cast billet, which squeezes out the solute segregated molten steel and simultaneously welds the central shrinkage cavity, truly transferring deformation to the center of the billet, thereby effectively improving core density. However, heavy reduction places strict requirements on equipment and the reduction position; otherwise, cracks are easily generated. Electromagnetic stirring at the end of solidification can also improve the density of the billet core. However, it is important to note that the selection of parameters for the end electromagnetic stirring (such as stirring intensity, frequency, and installation position) has a significant impact on the stirring effect. If the parameters are not selected properly, negative effects may occur, such as the formation of defects like "bright white bands".

[0004] Furthermore, related studies have shown that as the size of bearing steel increases, it becomes difficult to guarantee the grain size, leading to a significant decrease in fatigue life. Current methods to improve grain size include controlling alloying elements. Patent CN109457081A discloses a rare-earth microalloyed bearing steel and its preparation method, which controls and inhibits grain growth by controlling the Nb content. Patent CN113755766A discloses a large-size, long-life high-carbon chromium bearing steel bar, which significantly refines the original austenite structure and carbide size by adding Nb, V, Mo, and Zr microalloying elements to the bearing steel, thereby significantly improving the contact fatigue life of large-size bearing steel and meeting the quality and life requirements of large bearings for large-size bearing steel bars. However, the above-mentioned existing technologies all rely on controlling alloying elements and adding rare-earth microalloying elements to refine the grain size, which is not only costly but also has a low yield.

[0005] Therefore, it is of great significance to develop new processes and technologies to improve the core density and grain size of bearing steel. Summary of the Invention

[0006] The purpose of this invention is to provide a production process for improving the density and grain size of high-carbon bearing steel cores. The process employs continuous casting + heating + single-sided high-reduction isothermal rolling to manufacture bearing steel, ensuring that the bearing steel core is free of shrinkage cavities under low-magnification acid immersion inspection, and that the steel achieves a grade B or higher result in low-magnification ultrasonic flaw detection, with a grain size of grade 9 or higher.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows:

[0008] This invention provides a manufacturing process for improving the density and grain size of high-carbon bearing steel cores, comprising the following steps:

[0009] (1) Continuous casting

[0010] The tundish temperature is 1471–1481℃, the reduction is 32–40 mm, the pulling speed is maintained at 0.40–0.46 m / min, the secondary cooling water ratio is 0.1725–0.1883 L / kg, the crystallizer electromagnetic stirring current is 300–500 A, the frequency is 1–3 Hz, and the end electromagnetic stirring current is 500–650 A, the frequency is 5–9 Hz.

[0011] (2) Heating of the billet

[0012] The furnace exit temperature is 1180–1200℃, the total heating time is 660–760 min, the preheating section temperature is 600–650℃, the second heating section temperature is 750–850℃, the first heating section temperature is 1200–1260℃, and the holding time is >210 min. This high-temperature diffusion time ensures that the carbide precipitation and carbide banding of the bearing steel meet the standard requirements. The soaking section temperature is 1190–1230℃, and the holding time is >120 min. At the same time, the ratio of blast furnace gas to coke oven gas in the soaking section is controlled at 3:7 to avoid oxidation and decarburization of the steel.

[0013] (3) Single-sided high-reduction isothermal rolling

[0014] The rolling process employs a primary rolling mill and a continuous rolling mill, using two isothermal operations. The first isothermal operation involves isothermalizing the billet on the heating furnace roller table after it has been heated and tapped, with an isothermal time of 2.5–3.5 minutes. The primary rolling mill uses a single-sided large reduction rolling process. The second isothermal operation involves isothermalizing the billet before it enters the fourth stand of the continuous rolling mill, with an isothermal time of 2–3 minutes. When the temperature is ≤930℃, it enters the finishing mill and is then kept warm in a heat preservation pit. After the billet is opened by the primary rolling mill, an "isothermal operation" is performed before the continuous rolling mill. Utilizing the difference in cooling rates between the surface and core of the intermediate billet, the surface temperature of the intermediate billet is lower than the core temperature after the "isothermal operation." This allows the rolling force to be transferred to the core during subsequent rolling, improving the core density of the steel.

[0015] In the above technical solution, further, in step (1), the entire process of casting is protected, and the mold protective slag is made of high carbon steel.

[0016] In the above technical solution, further, in step (1), the size of the casting billet is 335~385mm×400~480mm.

[0017] In the above technical solution, further, in step (2), heating is carried out using a walking beam furnace.

[0018] In the above technical solution, further, in step (3), the initial rolling temperature is 1000~1100℃.

[0019] In the above technical solution, further, in step (3), the primary rolling mill adopts a reversible primary rolling mill, and the primary rolling mill adopts a 9-pass reduction process. The reduction amounts of the 1st to the 9th passes are 40-70mm, 50-80mm, 40-60mm, 50-80mm, 5-35mm, 5-35mm, 80-120mm, 5-35mm, and 50-80mm, respectively. In the 7th pass, a single-sided large reduction rolling is performed, and the steel flipping operation is performed in the 1st, 3rd, 5th, and 9th passes.

[0020] In the above technical solution, further, in step (3), during the rolling process, the billet is kept warm by using red steel pads, and red steel is pressed on top to ensure straightness. The heat preservation time is ≥30h, and the red steel temperature is ≥400℃ to ensure the heat preservation effect. The weight of the upper red steel is used to prevent the lower steel from deforming and the temperature difference between the inside and outside. The heat preservation pit and red steel are used to release stress and avoid stress cracks in the steel.

[0021] The beneficial effects of this invention are as follows:

[0022] 1. This invention utilizes continuous casting under heavy pressure technology and electromagnetic stirring technology at the end of solidification to ensure that the bearing steel core is free of shrinkage cavities under low magnification acid immersion inspection, and the ultrasonic flaw detection results reach level B or above.

[0023] 2. This invention adopts isothermal, single-sided high-reduction rolling + heavy reduction technology, which improves working efficiency. When the core density of the steel reaches the required level, the continuous casting speed can be increased, and the continuous casting efficiency can be improved. Moreover, the low magnification of bearing steel at high casting speed is better than that of bearing steel at low magnification under single reduction. It can also prevent internal cracks in the billet caused by improper placement of heavy reduction during continuous casting.

[0024] 3. The present invention uses controlled rolling and controlled cooling to control its grain size, without the need to add Nb or gold elements or adjust the heat treatment process as in traditional methods, and the grain size reaches level 9 or above. Attached Figure Description

[0025] Figure 1These are low-magnification photographs of the bearing steel after acid immersion in Example 1, where a is the longitudinal direction and b is the transverse direction.

[0026] Figure 2 The image shows the metallographic microstructure of the bearing steel in Example 1.

[0027] Figure 3 This is a metallographic microstructure image of the bearing steel in Example 2. Detailed Implementation

[0028] The following examples are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way.

[0029] Taking bearing steel GCr15 as an example, in the following embodiments, the bearing steel is composed of the following chemical elements by weight percentage:

[0030] C: 0.98%, Si: 0.22%, Mn: 0.32%, Cr: 1.49%, Mo: 0.01%, Ni: 0.02%, Cu: 0.03%, Alt: 0.005%, P: 0.009%, S: 0.005%, O: 0.0006%, Ti: 0.0016%, As+Sn+Sb: 0.008%, Pb: 0.001%, with the remainder being Fe.

[0031] Example 1

[0032] (1) Continuous casting

[0033] The tundish temperature is 1471℃, the reduction is 40mm, the casting speed is maintained at 0.40m / min, the secondary cooling water ratio is 0.1725~0.1883L / kg, the crystallizer electromagnetic stirring current is 500A, the frequency is 2Hz, the end electromagnetic stirring current is 500A, the frequency is 7Hz, the whole process is protected during casting, the crystallizer protective slag uses special high carbon steel protective slag, the billet insulation uses red steel bottom, the red steel temperature is required to be ≥400℃, the upper and lower layers of red steel are 100 tons each, the insulation time is 32h to ensure the insulation effect, the weight of the upper red steel is used to prevent the lower steel from deforming and the internal and external temperature difference from generating stress cracks, and the insulation pit and red steel insulation are used to release stress. The billet size is 350×480mm;

[0034] (2) Heating of the billet

[0035] The furnace exit temperature is 1220℃, the total heating time is 745min, the preheating section temperature is 600℃, the second heating section temperature is 800℃, the first heating section temperature is 1210℃, and the holding time is 250min. The high-temperature diffusion time ensures that the carbide precipitation and carbide banding of the bearing steel meet the standard requirements. The soaking section temperature is 1210℃, and the holding time is 150min. At the same time, the ratio of blast furnace gas to coke oven gas is controlled at 3:7 in the soaking section to avoid oxidation and decarburization of the steel.

[0036] (3) Single-sided high-reduction isothermal rolling

[0037] The rolling process employs a primary rolling mill and a continuous rolling mill, using a two-stage isothermal process. The first isothermal stage involves isothermal treatment of the cast billet on the heating furnace roller table after tapping, with an isothermal time of 2.75 minutes. The BD primary rolling mill uses a single-sided large reduction rolling process, with reductions of 59mm, 71mm, 49mm, 63mm, 14mm, 10mm, 100mm, 14mm, and 81mm for passes 1 through 9, respectively. A steel-turning operation is performed on passes 1, 3, 5, and 9. The primary rolling temperature... The second isothermal treatment is performed at 1050℃: the billet is isothermally treated before entering the fourth continuous rolling mill for 2.5 minutes, and enters the finishing mill at 928℃. It is then placed in the insulation pit for insulation to obtain Φ180mm finished steel. After the billet is opened by the primary rolling mill, it is subjected to "isothermal operation" before the continuous rolling mill. By utilizing the difference in cooling rate between the surface and core of the intermediate billet, the surface temperature of the intermediate billet is lower than the core temperature after the "isothermal operation". In the subsequent rolling process, the rolling force can be transmitted to the core, which improves the core density of the steel.

[0038] Example 2

[0039] The production process is similar to that of Example 1, except that the end electromagnetic stirring current is 650A and the pulling speed is 0.46m / min, while the other parameters are the same as those of Example 1.

[0040] Example 3

[0041] The production process is similar to that of Example 1, except that the end electromagnetic stirring current is 650A, while the other parameters are the same as those of Example 1.

[0042] Comparative Example 1

[0043] The production process is similar to that of Example 1, except that the traditional rolling process is used, and the steel directly enters the BD roughing mill after exiting the heating furnace. The other parameters are the same as those of Example 1.

[0044] The acid leaching process used for low-magnification testing is as follows: hot etching with a 1:1 hydrochloric acid aqueous solution for 20 minutes. The test results are as follows. Figure 1 As shown, the low-magnification rating is: longitudinally qualified, no shrinkage cavities; transversely: general porosity 0, central porosity 0.5, segregation 0, shrinkage 0.

[0045] Metallographic microstructure testing method GB / T13299-1991.

[0046] Evaluation of average grain size of metals GB / T6394-2017.

[0047] Ultrasonic flaw detection assessment GB / T226.

[0048] Table 1. Process parameters and low-magnification, metallographic, and ultrasonic testing results of Examples 1-3 and Comparative Example 1 of the present invention.

[0049]

[0050] In Table 1, Example 1 uses isothermal single-sided high-reduction rolling, with a low casting speed of 0.40 m / min. The optimal flaw detection and grain size levels are achieved when the end-stage electromagnetic stirring is set to 500 A. Examples 2 and 3 also use isothermal high-reduction rolling, but changing the continuous casting parameters—increasing the casting speed and electromagnetic stirring intensity—both lower the flaw detection level. However, the flaw detection results for the large-diameter bearing steel Φ180mm reach level B, still meeting general user requirements. The comparative examples use traditional rolling methods, and the grain size levels are lower than in Examples 1-3.

[0051] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the implementation. The scope of protection of the present invention should be determined by the scope defined in the claims. Other variations or modifications can be made based on the above description. Obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A production process for improving the core density and grain size of high-carbon bearing steel, characterized in that, Includes the following steps: (1) Continuous casting The tundish temperature is 1471~1481℃, the reduction is 32~40mm, the pulling speed is maintained at 0.40~0.46m / min, the secondary cooling water ratio is 0.1725~0.1883L / kg, the crystallizer electromagnetic stirring current is 300~500A, the frequency is 1~3Hz, and the end electromagnetic stirring current is 500~650A, the frequency is 5~9Hz. (2) Heating of the billet The furnace exit temperature is 1180~1200℃, the total heating time is 660~760min, the preheating section temperature is 600~650℃, the second heating section temperature is 750~850℃, the first heating section temperature is 1200~1260℃, the holding time is >210min, the soaking section temperature is 1190~1230℃, and the holding time is >120min; (3) Single-sided large reduction isothermal rolling The rolling process employs a primary rolling mill and a continuous rolling mill, with two isothermal processes. The first isothermal process involves isothermalizing the billet on the roller table of the heating furnace after it is heated and tapped, with an isothermal time of 2.5 to 3.5 minutes. The primary rolling mill uses a single-sided large reduction rolling process. The second isothermal process involves isothermalizing the billet before it enters the fourth continuous rolling mill, with an isothermal time of 2 to 3 minutes. When the temperature is ≤930℃, the billet enters the finishing mill and is then kept warm in the heat preservation pit. In step (3), the primary rolling mill adopts a reversible primary rolling mill, and the primary rolling mill adopts a 9-pass reduction process. The reduction amounts of the 1st to 9th passes are 40~70mm, 50~80mm, 40~60mm, 50~80mm, 5~35mm, 5~35mm, 80~120mm, 5~35mm, and 50~80mm, respectively. In the 7th pass, a single-sided large reduction rolling is performed, and the steel flipping operation is performed in the 1st, 3rd, 5th, and 9th passes. In step (3), the billet is kept warm during the rolling process by using red steel pads and pressing red steel on top to ensure flatness. The heat preservation time is ≥30h, and the temperature of the red steel is ≥400℃.

2. The production process for improving the core density and grain size of high-carbon bearing steel according to claim 1, characterized in that, In step (1), the entire casting process is protected, and high-carbon steel protective slag is used for the crystallizer.

3. The production process for improving the core density and grain size of high-carbon bearing steel according to claim 1, characterized in that, In step (1), the dimensions of the cast billet are 335~385mm×400~480mm.

4. The production process for improving the core density and grain size of high-carbon bearing steel according to claim 1, characterized in that, In step (2), heating is performed using a walking beam furnace.

5. The production process for improving the core density and grain size of high-carbon bearing steel according to claim 1, characterized in that, In step (3), the initial rolling temperature is 1000~1100℃.

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