Production process for improving density and grain size of high-carbon bearing steel core

Abstract

The invention relates to the technical field of bearing steel, in particular to a production process for improving the density and grain size of a high-carbon bearing steel core. Comprising the following steps: (1) continuous casting; (2) casting blank heating; and (3) single-side high-pressure isothermal rolling is carried out. According to the method, the bearing steel is manufactured and produced by adopting the processes of continuous casting, heating and single-side large-pressure isothermal rolling, so that the core part of the bearing steel has no shrinkage cavity under acid leaching macroscopic inspection, the macroscopic ultrasonic flaw detection result of the steel is above grade B, and the grain size reaches above grade 9. By adopting the isothermal and single-sided large reduction rolling and heavy reduction technology, the working efficiency is improved, the continuous casting speed can be increased and the continuous casting efficiency can be improved when the compactness of the core part of the steel meets the requirement of a common level, the macroscopic quality of the bearing steel at a high pulling speed is superior to that of the bearing steel at a single reduction, and the inner crack of a casting blank due to an improper heavy reduction position in the continuous casting process can be prevented.

Description

Technical Field

[0001] The present invention relates to the technical field of bearing steel, and in particular to a production process for improving the core density and grain size of high-carbon bearing steel. Background Art

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

[0003] However, traditional casting processes often fail to meet the requirements of high-end manufacturing for the core density of large square billets of bearing steel. As the cross-section widens and thickens, its internal cooling conditions deteriorate significantly, columnar crystals develop in the solidification structure, and the flow of residual molten steel rich in solute segregation elements between dendrites tends to be balanced, resulting in more serious defects such as segregation, porosity and shrinkage in the billet. These defects are difficult to effectively eliminate during the subsequent heating and rolling processes, thus affecting the quality of the final product. At present, researchers at home and abroad mostly use light reduction or heavy reduction technology, especially heavy reduction at the end of solidification of the continuous casting billet to squeeze out the solute segregated molten steel, and at the same time weld the central shrinkage cavity to truly transfer the deformation to the center of the billet, thereby effectively improving the core density. However, heavy reduction has strict requirements on equipment and the reduction position, otherwise cracks are likely to occur. The density of the core of the ingot can also be improved by electromagnetic stirring at the end of solidification, but it is also necessary to pay attention to the parameter selection of the end electromagnetic stirring (such as stirring intensity, frequency, installation position, etc.) which has a great influence on the stirring effect. If the parameters are not selected properly, negative effects may occur, such as the formation of defects such as "white bright bands".

[0004] In addition, relevant studies have shown that as the size of bearing steel increases, the grain size of bearing steel is difficult to guarantee, and the fatigue life is greatly reduced. The current method for increasing grain size includes alloy element control. Patent CN109457081A discloses a rare earth microalloyed bearing steel and a preparation method thereof, which controls and inhibits grain growth by controlling the Nb content. Patent CN113755766A discloses a large-scale, long-life high-carbon chromium bearing steel bar. By microalloying the bearing steel with Nb, V, Mo and Zr, the original austenite structure and carbide size are greatly refined, thereby achieving a significant improvement in the contact fatigue life of large-scale bearing steel and meeting the quality and life requirements of large-scale bearing steel bars for large-scale bearings. However, the above-mentioned prior arts all refine the grains by controlling the alloy elements and adding rare earth microalloying elements, which is not only costly but also has a low yield.

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

[0006] The purpose of the present invention is to provide a production process for improving the density and grain size of the core of high-carbon bearing steel. The bearing steel is manufactured by adopting a continuous casting + heating + single-sided large-pressure isothermal rolling process, so that the core of the bearing steel has no shrinkage cavity under low-magnification inspection of acid immersion, the low-magnification ultrasonic flaw detection result of the steel is above grade B, and the grain size reaches above grade 9.

[0007] In order to achieve the above object, the technical solution of the present invention is as follows:

[0008] The present invention provides a production process for improving the core density and grain size of high-carbon bearing steel, comprising the following steps:

[0009] (1) Continuous casting

[0010] The tundish temperature is 1471-1481°C, the reduction is 32-40 mm, the pulling speed is maintained at 0.40-0.46 m / min, the secondary cooling water volume 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 ingot

[0012] The furnace temperature is 1180-1200°C, the total heating time is 660-760 minutes, the preheating section temperature is 600-650°C, the second section temperature is 750-850°C, the first section temperature is 1200-1260°C, and the holding time is greater than 210 minutes. Ensuring the high-temperature diffusion time can ensure that the carbide liquid precipitation and carbide banding of the bearing steel meet the standard requirements. The soaking section temperature is 1190-1230°C, and the holding time is greater than 120 minutes. At the same time, the soaking section controls the blast furnace and coke oven gas ratio to 3:7 to avoid oxidation and decarburization of the steel.

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

[0014] The rolling adopts the blooming mill and the continuous rolling mill group, and adopts two isothermal processes, among which: the first isothermal process: after the billet is heated and tapped, it is isothermalized on the roller table of the heating furnace, and the isothermal time is 2.5 to 3.5 minutes. The blooming mill adopts the single-sided large reduction rolling process for rolling. The second isothermal process: the billet is isothermalized before entering the fourth continuous rolling mill, and the isothermal time is 2 to 3 minutes. When the temperature is ≤930℃, it is subjected to finishing rolling, and then put into the holding pit for insulation; after the billet is opened by the blooming mill, "isothermal operation" is carried out in front of the continuous rolling mill. By utilizing the difference in cooling rate between the surface and the core of the intermediate billet, the surface temperature of the intermediate billet is made lower than the core temperature after the "isothermal operation", so that the rolling force can be transmitted to the core in the subsequent rolling process, thereby improving the core density of the steel.

[0015] In the above technical solution, further, in step (1), the whole process is protected casting, and the crystallizer protective slag uses high carbon steel protective slag.

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

[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°C.

[0019] In the above technical solution, further, in step (3), the initial rolling mill adopts a reversible initial rolling mill, and the initial rolling 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, 50-80mm respectively, and single-sided large reduction rolling is performed in the 7th pass, and steel turning operations are performed in the 1st, 3rd, 5th and 9th passes.

[0020] In the above technical solution, further, in step (3), during the rolling process, the ingot is insulated with red steel as the bottom, and the red steel is pressed on top to ensure flatness. The insulation time is ≥30h, wherein the red steel temperature is ≥400℃ to ensure the insulation effect. The weight of the upper red steel is used to prevent the deformation of the lower steel and the temperature difference between the inside and the outside, and the insulation pit and the red steel insulation are used to release stress to avoid stress cracks in the steel.

[0021] The beneficial effects of the present invention are:

[0022] 1. The present invention uses continuous casting heavy pressure technology and solidification end electromagnetic stirring technology to ensure that the core of the bearing steel has no shrinkage cavity under low-magnification acid immersion inspection, and the ultrasonic flaw detection results reach Class B or above.

[0023] 2. The present invention adopts isothermal, single-sided large reduction rolling + heavy reduction technology to improve work efficiency. When the density of the steel core reaches the normal level, the continuous casting speed can be increased, and the continuous casting efficiency can be improved. The low-multiple bearing steel at high drawing speed is also better than the low-multiple bearing steel quality at single reduction. It can also prevent internal cracks in the ingot due to improper heavy reduction position during continuous casting.

[0024] 3. The present invention adopts controlled rolling and controlled cooling to control the grain size, without the need to add Nb and gold elements or adjust the heat treatment process as in traditional methods, and the grain size reaches above level 9. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1This is a low-magnification photograph of the bearing steel pickling in Example 1, a is the longitudinal direction, b is the transverse direction;

[0026] Figure 2 This is a photo of the metallographic microstructure of the bearing steel of Example 1;

[0027] Figure 3 This is a photo of the metallographic microstructure of the bearing steel of Example 2. DETAILED DESCRIPTION

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

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

[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%, and the rest are Fe elements.

[0031] Example 1

[0032] (1) Continuous casting

[0033] The tundish temperature is 1471°C, 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 casting, the crystallizer protection slag uses special high-carbon steel protection slag, the billet insulation uses red steel as the bottom, the red steel temperature is required to be ≥400°C, the upper and lower layers of red steel are 100 tons each, and the insulation time is 32h to ensure the insulation effect. The weight of the upper red steel is used to prevent the deformation of the lower steel and the temperature difference between the inside and the outside, which may cause stress cracks. The insulation pit and the red steel insulation are used to release stress. The billet size is 350×480mm;

[0034] (2) Heating of the ingot

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

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

[0037] The rolling adopts the blooming mill and the continuous rolling mill, and adopts two isothermal processes, wherein: the first isothermal process: the billet is heated and then tapped out and then isothermalized on the roller table of the heating furnace, the isothermal time is 2.75min, the BD blooming mill adopts the single-sided large reduction rolling process for rolling, the reductions of the 1st to 9th passes are 59mm, 71mm, 49mm, 63mm, 14mm, 10mm, 100mm, 14mm, 81mm respectively, and the steel turning operation is performed in the 1st, 3rd, 5th and 9th passes, and the blooming temperature is 2.75min. The second isothermal process is: the steel billet is isothermalized before entering the fourth continuous rolling mill, the isothermal time is 2.5min, and the steel billet is finished rolled at a temperature of 928℃, and then put into the holding pit for insulation to obtain Φ180mm finished steel; after the billet is opened by the primary rolling mill, an "isothermal operation" is carried out in front of the continuous rolling mill, and the difference in cooling rate between the surface and the core of the intermediate billet is utilized. After the "isothermal operation", the surface temperature of the intermediate billet is lower than the core temperature, and the rolling force can be transmitted to the core during the subsequent rolling process, thereby improving the density of the core of the steel.

[0038] Example 2

[0039] A production process similar to that of Example 1 was adopted, with the following differences: the end electromagnetic stirring current was 650 A, the pulling speed was 0.46 m / min, and the other parameters were consistent with those of Example 1.

[0040] Example 3

[0041] A production process similar to that of Example 1 was adopted, with the difference that the terminal electromagnetic stirring current was 650 A, and the other parameters were consistent with those of Example 1.

[0042] Comparative Example 1

[0043] A production process similar to that of Example 1 is adopted, except that a traditional rolling process is adopted, and the steel directly enters BD rough rolling after leaving the heating furnace. The other parameters are consistent with those of Example 1.

[0044] The acid leaching process used for low-magnification detection is: hot corrosion with 1:1 hydrochloric acid aqueous solution for 20 minutes. The test results are as follows: Figure 1 As shown, it can be seen that the low-magnification rating is qualified in the longitudinal direction, with no shrinkage cavities, and in the transverse direction: general porosity 0, central porosity 0.5, segregation 0, and shrinkage cavities 0.

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

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

[0047] Ultrasonic flaw detection assessment GB / T226.

[0048] Table 1 Process parameters and macroscopic, metallographic and ultrasonic inspection 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, a low pulling speed of 0.40 m / min, and an end-stage electromagnetic stirring of 500 A, achieving the best flaw detection and grain size grades. Examples 2 and 3 also use isothermal high-reduction rolling, but change the continuous casting parameters to increase the pulling speed and electromagnetic stirring intensity, respectively. Both reduce the flaw detection grade, but the flaw detection results of large-size bearing steel Φ180 mm reach Class B, still meeting general user requirements. The comparative examples use traditional rolling methods, and the grain size grades are all lower than those of Examples 1-3.

[0051] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. The scope of protection of the present invention shall be determined by the scope defined in the claims. Other variations or modifications may be made based on the above description. Obvious variations or modifications derived therefrom shall remain 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: The following steps are involved: (1) Continuous casting The tundish temperature is 1471-1481°C, the reduction is 32-40 mm, the pulling speed is maintained at 0.40-0.46 m / min, the secondary cooling water volume 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; (2) Heating of the ingot The furnace 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 greater than 210min, the soaking section temperature is 1190-1230℃, and the holding time is greater than 120min; (3) Single-sided high-pressure isothermal rolling The rolling adopts the blooming mill and the continuous rolling mill, and adopts two isothermal processes, wherein: the first isothermal process: after the billet is heated and cast, it is isothermalized on the roller of the heating furnace, and the isothermal time is 2.5 to 3.5 minutes. The blooming mill adopts the single-sided large reduction rolling process for rolling. The second isothermal process: the billet is isothermalized before entering the fourth continuous rolling mill, and the isothermal time is 2 to 3 minutes. When the temperature is ≤930℃, it enters the finishing rolling and then enters the holding pit for insulation.

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 whole process is protected casting, and high carbon steel protective slag is used as the crystallizer protective slag.

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 size of the ingot is 335-385 mm x 400-480 mm.

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°C.

6. 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), a reversible blooming mill is used for blooming, and a 9-pass reduction process is used for blooming. 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, 50-80mm, respectively. Single-sided large reduction rolling is performed in the 7th pass, and steel turning operations are performed in the 1st, 3rd, 5th and 9th passes.

7. 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), red steel is used as a bottom layer for heat preservation of the ingot during rolling, and the red steel is pressed on top to ensure the straightness. The heat preservation time is ≥30h, and the temperature of the red steel is ≥400℃.

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