Methods for producing large-size high-carbon chromium bearing steel bars with low compression ratios

Abstract

The application discloses a method for producing large-size high-carbon chromium bearing steel rod with small compression ratio, which comprises continuous casting and continuous rolling processes, wherein the continuous casting process comprises the following steps: full-process protection pouring, the temperature of the molten steel in the tundish is 1465-1485 DEG C; the current of the electromagnetic stirring of the crystallizer is 330-350 A, and the frequency is 3-4 Hz; the current of the electromagnetic stirring of the secondary cooling end is 340-360 A, and the frequency is 8-9 Hz; the drawing speed of the continuous casting billet is 0.8-1.0 m / min, the solidification is carried out through secondary cooling, and the secondary cooling intensity is 0.2-0.3 L / kg; the center porosity of the obtained continuous casting square billet is not more than 2.0 level, the center shrinkage hole is not more than 1.5 level, and the cross-section size is 197-205*197-205 mm 2 ; the continuous rolling process comprises the following steps: the continuous casting square billet is loaded into a steel rolling heating furnace for heating; the continuous casting billet is discharged into a rough rolling machine after being heated, and the outlet speed of the finished product with a round steel φ70-90 mm specification is 0.90-1.5 m / s; and the round steel with a diameter of 70-90 mm is rolled. The method can produce large-size rod with small compression ratio, expand the production capacity of equipment, and improve the utilization rate under the existing process equipment.

Description

Technical Field

[0001] This invention relates to the manufacturing technology of alloy steel in the metallurgical industry, and in particular to a method for producing large-size high-carbon chromium bearing steel bars with a small compression ratio. Background Technology

[0002] As early as the 1980s, foreign countries had already adopted continuous casting technology to produce high-carbon chromium bearing steel. Domestic special steel enterprises began researching the smelting and continuous casting technology of high-carbon chromium bearing steel in the 1990s. Due to incomplete supporting process equipment, problems such as center segregation and center porosity easily occur. Therefore, both domestic and foreign special steel enterprises generally adopt a continuous casting-rolling production method with large square billets, using a single or double casting process to improve the internal quality of the material by utilizing a larger metal compression ratio.

[0003] In recent years, the metallurgical industry has vigorously developed continuous casting technology, leading to a continuous reduction in the overall compression ratio of hot-rolled bars. However, for the continuous casting process, one key objective is to minimize central porosity and segregation to ensure high-quality continuously cast billets. Even while ensuring billet quality, a certain compression ratio rolling process is still necessary to bring the finished steel to the required performance. Given the current level of continuous casting technology, the size of continuously cast billets cannot infinitely approximate the size of the finished product, especially for alloy steel continuously cast square billets. The necessary compression ratio must be considered based on the billet's density.

[0004] High-carbon chromium bearing steel GCr15 is a hypereutectoid steel with a carbon content as high as 1%, resulting in a relatively large temperature difference between its liquidus and solidus phases. This leads to greater shrinkage during solidification and cooling compared to typical low-carbon steels. The volume shrinkage during solidification of GCr15 billets, along with the volume shrinkage caused by the continued cooling of the solidified central portion of the billet due to heat transfer, cannot be compensated for by the molten steel, resulting in periodic, intermittent shrinkage cavities in the center of the billet. These shrinkage cavities in the center of the continuously cast billet cannot heal during rolling, leading to shrinkage cavities on the round steel and severely affecting the steel's properties. According to relevant literature, the compression ratio (the ratio of the billet's cross-sectional area to the steel's cross-sectional area) of continuously cast high-carbon chromium bearing steel billets is typically around 20, with better ratios around 15, and the lowest compression ratio in China is 14.

[0005] For rolled bars, the compression ratio is a key process parameter determining the microstructure and properties of the rolled product. The compression ratio is expressed as the ratio of the cross-sectional area of ​​the rolled product before rolling to the cross-sectional area of ​​the rolled product after rolling. In the past, it was generally believed that the quality requirements for continuously cast alloy steel were high and difficult to control and guarantee, so the compression ratio was often used to control product quality. A higher compression ratio results in greater deformation of the billet; a lower compression ratio means less deformation of the billet. Especially when rolling large-section billets, the rolling deformation is primarily concentrated on the surface and subsurface layers of the large-section billet, making it difficult for deformation to penetrate to the core. This makes it difficult to improve the core microstructure of the final rolled product, and metallurgical defects such as porosity are difficult to compact and weld, consequently making it difficult to stabilize the mechanical properties of the bar product. Therefore, from the perspective of improving the performance of steel products, a higher compression ratio is better.

[0006] In addition, S. Ekrond proposed a unique semi-empirical formula based on his research and experimental data, pointing out that the deformation rate during the rolling process has a certain impact on the internal quality of the rolled material. However, when used to calculate the average pressure in hot rolling, this formula has a certain applicable range: the billet rolling temperature is ≥800℃, the material is carbon steel (Mn≤1%, Cr≤2~3%), and the rolling speed is not greater than 20m / s, etc.; its formula is shown in equation (1):

[0007] (1)

[0008] In formula (1):

[0009] C is (0.9682 + 0.00656v), and after correction, its speed range can reach 5 to 60 m / s;

[0010] R is the radius of the roll groove in mm;

[0011] v is the rolling line speed in m / s;

[0012] Thickness of H and h rolled pieces before and after rolling;

[0013] B and h are the front and rear widths of the rolled piece, respectively.

[0014] △h = H – h is the absolute reduction.

[0015] The empirical formulas for calculating K and η are shown in equations (2) and (3):

[0016] K=9.8×(14-0.01t)(1.4+C+Mn) MPa (2)

[0017] η=0.1×(14-0.01t) MPa·S (3)

[0018] In equations (2) and (3): t—rolling temperature, °C; C—carbon content expressed as %; Mn—manganese content expressed as %

[0019] μ is calculated using the following formula (4):

[0020] μ = a(1.05-0.0005t) (4)

[0021] In equation (4): for steel rolls a=1, for cast iron rolls a=0.8; t is the rolling temperature.

[0022] When the temperature t ≥ 800℃ and the manganese content is 1.0%, the average pressure is directly proportional to the viscosity coefficient and the average deformation rate. The viscosity coefficient is related to the rolling speed; the higher the rolling speed, the lower the viscosity coefficient, and therefore the lower the average unit pressure. From the above analysis, it can be concluded that reducing the rolling speed increases the average unit pressure.

[0023] The relevant patent applications concerning compression ratio control of continuously cast high-carbon chromium bearing steel are described below.

[0024] Chinese patent application CN200410089358.2 provides a production method for reducing and refining Class D inclusions in high-carbon chromium bearing steel; although it mentions a compression ratio >10 in the document, it does not mention control methods related to rolling.

[0025] Chinese patent application number CN200710046394.4 uses a size of 220×220mm. 2 The continuous casting billet is rolled into 20-75mm (including 70mm) continuous casting bearing steel round bars with a minimum compression ratio of 10.96. During the continuous casting billet production process, the superheat of the molten steel in the tundish is controlled at 20-30℃, the continuous casting speed is controlled at 0.70-1.10m / min, the secondary cooling strength is controlled at 0.30-0.50 L / kg, the maximum heating temperature is 1220-1250℃, and the total heating time is more than 270min. However, the process parameters of the rolling process are not mentioned.

[0026] Chinese patent application CN02112241.5 uses a small square billet (120×120mm) for continuously cast bearing steel. 2 140×140mm 2 160×160mm 2 This method describes the control of the steelmaking process, including a superheat of 20–40°C, a casting speed of 1.9–2.5 m / min, a specific water content of 0.5–0.9 L / kg, and electromagnetic stirring, to control the low-compression ratio of the cast billet and thus meet the quality requirements of the bearing steel product under a low compression ratio. However, it does not mention the process parameters of the rolling process. Summary of the Invention

[0027] The technical problem to be solved by the present invention is to provide a method for producing large-size high-carbon chromium bearing steel bars with a small compression ratio, so as to ensure that the low-magnification microstructure of the bars meets the requirements of the GB / T18254-2016 high-carbon chromium bearing steel quality steel standard.

[0028] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: it includes continuous casting and bar rolling processes. The continuous casting process involves: full-process protective casting; the temperature of the molten steel in the tundish is 1465–1485℃; the current of the electromagnetic stirring in the crystallizer is 330–350A, and the frequency is 3–4Hz; the current of the electromagnetic stirring at the end of the secondary cooling stage is 340–360A, and the frequency is 8–9Hz; the billet drawing speed is 0.8–1.0 m / min, and solidification is achieved through secondary cooling with a secondary cooling intensity of 0.2–0.3 L / kg; the resulting continuous casting square billet has a central porosity of no more than grade 2.0, a central shrinkage cavity of no more than grade 1.5, and a cross-sectional dimension of 197–205 × 197–205 mm. 2 ;

[0029] The bar rolling process is as follows: the continuously cast square billet is loaded into the steel rolling heating furnace for heating; after the continuously cast billet exits the furnace, it enters the roughing mill, and the exit speed of the finished round steel with a specification of φ70~90mm is 0.90~1.5m / s; it is rolled into round steel with a specification of φ70~90mm.

[0030] During the bar rolling process described in this invention, the maximum temperature of the furnace gas during heating is controlled at 1200-1250℃, and the total heating time is not less than 240 minutes.

[0031] In the bar rolling process described in this invention, the reduction amounts of the first three roughing mills are 37-45 mm, 53-61 mm, and 75-83 mm, respectively.

[0032] The high-carbon chromium bearing steel described in this invention is GCr15.

[0033] The beneficial effects of adopting the above technical solution are as follows: By controlling the superheat of molten steel in the tundish, the continuous casting speed, and the intensity of secondary cooling, and in conjunction with auxiliary equipment such as electromagnetic stirring in the crystallizer and electromagnetic stirring at the end, this invention produces continuously cast square billets with a central porosity of no more than 2.5 and a central shrinkage cavity of no more than 2.0, with cross-sectional dimensions of 197~205×197~205mm. 2This invention produces round bars with a diameter of 70-90mm by reducing the rolling speed during bar rolling, achieving a compression ratio of 6.29-10.40. Ultimately, it enables the production of large-diameter bearing steel bars from continuously cast billets with a low compression ratio, and the low-magnification microstructure meets the requirements of the GB / T18254-2016 standard for high-carbon chromium bearing steel. This invention ensures the performance of bearing steel while producing large-diameter bars with a low compression ratio, thus expanding equipment production capacity. It also increases the capacity of the continuous casting machine-rolling production line, improving the utilization rate of existing process equipment. Furthermore, it enables the production of larger-diameter continuously cast bearing steel round bars from small-diameter continuously cast billets, overcoming the existing challenge of controlling the compression ratio in continuously cast high-carbon chromium bearing steel. Attached Figure Description

[0034] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0035] Figure 1 This is a low-magnification microstructure diagram of the round steel obtained in Example 1 of the present invention;

[0036] Figure 2 This is a low-magnification microstructure diagram of the round steel obtained in Example 2 of the present invention;

[0037] Figure 3 This is a low-magnification microstructure diagram of the round steel obtained in Example 3 of the present invention;

[0038] Figure 4 This is a low-magnification microstructure diagram of the round steel obtained in Example 4 of the present invention;

[0039] Figure 5 This is a low-magnification microstructure diagram of the round steel obtained in Example 5 of the present invention. Detailed Implementation

[0040] This method for producing large-size high-carbon chromium bearing steel bars with a low compression ratio employs a converter primary refining process, LF refining, RH vacuum treatment, continuous casting, and continuous rolling; the process is described below.

[0041] (1) Converter primary refining, LF refining and RH vacuum treatment: Blast furnace hot metal and scrap steel are used as raw materials. After deoxidation and alloying of the molten steel in the converter primary refining, it undergoes deep deoxidation and degassing to remove inclusions through LF refining and RH vacuum treatment. Finally, the chemical composition of the molten steel meets the requirements, and the temperature of the molten steel leaving the RH station is controlled at 1500-1510℃. The chemical composition and weight percentage of the obtained GCr15 steel molten steel are as follows: C 0.95-1.05%, Si 0.15-0.35%, Mn 0.25-0.45%, S≤0.020%, P≤0.020%, Cr 1.40–1.65%, Al≤0.0050%, Ti≤0.0030%, Ca≤0.0010%, Cu≤0.10%, Mo≤0.10%, Ni≤0.10%, As≤0.04%, Pb≤0.002%, As+Sn+Sb≤0.075%, O≤0.0012%, with the remainder being Fe and unavoidable impurities.

[0042] (2) Continuous casting: The continuous casting process is carried out under full protection during pouring. The temperature of the molten steel in the tundish is 1465-1485℃. The current of the electromagnetic stirring in the crystallizer is 330-350A and the frequency is 3-4Hz. The current of the electromagnetic stirring at the end of the secondary cooling is 340-360A and the frequency is 8-9Hz. The continuously cast billet is drawn at a speed of 0.8-1.0m / min and solidified by secondary cooling. The intensity of the secondary cooling, i.e., the secondary cooling water ratio, is 0.2-0.3L / kg. Finally, a bearing steel continuously cast square billet with a central porosity of no more than 2.0 grade and a central shrinkage cavity of no more than 1.5 grade is produced, with a cross-sectional size of 197-205×197-205mm. 2 .

[0043] (3) Bar rolling: The continuously cast square billet is loaded into the steel rolling heating furnace, and the maximum furnace gas temperature is controlled at 1200-1250℃ and the total heating time is not less than 240min; after the continuously cast billet exits the furnace, it enters the roughing mill, and the reduction of the first three roughing mills is 37-45mm, 53-61mm and 75-83mm respectively; after finishing rolling, the round steel product is obtained, and the exit speed of the finished product is 0.90-1.5m / s, which is 62-70% of the conventional process; the specifications of the round steel product are φ70-90mm, and the compression ratio, that is, the ratio of the cross-sectional area of ​​the billet to the cross-sectional area of ​​the final round steel product, is 6.29-10.40.

[0044] (4) The obtained round steel is tested by low magnification and meets the requirements of GB / T18254-2016 high carbon chromium bearing steel high quality steel standard.

[0045] Examples 1-5: The method for producing large-size high-carbon chromium bearing steel bars with a small compression ratio adopts the following specific process.

[0046] (1) After converter primary refining, LF refining and RH vacuum treatment in each embodiment, molten steel of GCr15 steel is obtained.

[0047] (2) The process parameters of each embodiment are shown in Table 1; the molten GCr15 steel is continuously cast and rolled into bar stock to obtain bar stock-round steel.

[0048] Table 1: Process parameters for each embodiment

[0049]

[0050] In Table 1, the roughing reduction is the reduction of the first three roughing mills, which is the height difference between the original billet and the rolled piece.

[0051] (3) The round steel produced in Examples 1-5 was tested according to the standard requirements of GB / T18254-2016. The test results met the standard requirements for high-carbon chromium bearing steel and high-quality steel. The results are shown in Table 2. Typical low-magnification photographs of the round steel obtained in each example are shown in Table 2. Figure 1-5 .

[0052] Table 2: Low-magnification inspection results of the obtained round steel bars

[0053]

[0054] Depend on Figure 1-5 As shown in Table 2, this method enables the rolling of large-diameter bearing steel bars from continuously cast billets with a small compression ratio, and the low-magnification microstructure meets the requirements of the GB / T18254-2016 standard for high-carbon chromium bearing steel. Therefore, this method expands the utilization limit of bearing steel billet production into bars, improves equipment utilization, and solves the production organization dilemma faced by enterprises.

Claims

1. A method of producing a large-size high-carbon chromium bearing steel bar with a small compression ratio, which includes a continuous casting and bar rolling process, characterized in that, The continuous casting process is as follows: full protection casting, the temperature of the molten steel in the tundish is 1465-1485 DEG C; the current of the electromagnetic stirring of the crystallizer is 330-350 A, and the frequency is 3-4 Hz; the current of the electromagnetic stirring of the secondary cooling end is 340-360 A, and the frequency is 8-9 Hz; the drawing speed of the continuous casting blank is 0.8-1.0 m / min, the secondary cooling solidification is carried out, and the secondary cooling intensity is 0.2-0.3 L / kg; the center porosity of the obtained continuous casting square blank is not more than 2.0 level, the center shrinkage hole is not more than 1.5 level, and the cross-sectional size is 197-205*197-205 mm 2 ​ The bar rolling process comprises the following steps: the continuous casting billet is loaded into a rolling heating furnace for heating, the maximum temperature of the furnace gas is controlled to be 1200-1250 DEG C during the heating, and the total heating time is not less than 240 min; after the continuous casting billet is discharged from the furnace, the billet enters the rough rolling mill, the reduction of the first three rough rolling mills is 37-45 mm, 53-61 mm and 75-83 mm respectively, and the outlet speed of the round steel with a diameter of 70-90 mm is 0.90-1.5 m / s; the round steel with a diameter of 70-90 mm is rolled, and the compression ratio is 6.29-10.

40.

2. The method of producing large size high carbon chromium bearing steel bar stock at low reduction ratio as claimed in claim 1 wherein: The high-carbon chromium bearing steel is GCr15.

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