Control method for frame segregation of gear steel bar produced based on rectangular section continuous casting billet

Abstract

The invention discloses a control method for frame segregation of a gear steel bar produced on the basis of a rectangular-section continuous casting billet, which comprises the following steps of: rapidly reducing the surface temperature of the billet through a cooling device to form temperature field distribution with a hard shell and a soft core; during subsequent high-pressure rolling, deformation force is easier to permeate into a core part with high temperature and low deformation resistance, so that the size of a segregation frame of the core part is more effectively adjusted, and the shape of the segregation frame tends to be square; and meanwhile, core looseness can be improved easily under continuous large pressure, and the steel density is improved. The method has the beneficial effects that the width-to-height ratio of a segregation frame can be stably controlled to be smaller than or equal to 1.10 in the limited rolling pass of large-specification and small-compression-ratio gear steel, and the requirement of high-end gear steel is met; the dependency on a specific rolling pass can be reduced while a better segregation control effect is achieved, and the process adaptability is higher; and meanwhile, the core looseness can be improved, and the steel compactness is improved.

Description

Technical Field

[0001] This invention belongs to the field of special steel bar production technology in the metallurgical industry, and is a method for controlling frame segregation of gear steel bars produced from rectangular cross-section continuous casting billets. Background Technology

[0002] Steel used in gear production has strict requirements regarding quenching performance and end-hardenability bandwidth, steel purity and grain size, machinability, and uniform microstructure and composition. During solidification, the steel billet forms a fine-grained zone on the surface, a columnar zone in the middle, and an equiaxed zone in the core, from the surface inwards. The equiaxed zone in the core is referred to in the industry as the frame-shaped segregation zone. Influenced by the inherited frame-shaped segregation of the billet, the gear steel bars produced from this continuously cast billet also exhibit a frame-shaped segregation zone in their core. If the segregation frame of the round steel is rectangular, it will inevitably cause inconsistent deformation in different directions after heat treatment due to compositional differences in different directions. This is the significance of frame-shaped segregation control.

[0003] For rectangular cross-section continuously cast billets, the equiaxed crystal zone in the core is also rectangular, which is an inherent property of continuously cast billets. Although the size of the equiaxed crystal zone in the core can be adjusted by changing the superheat of continuous casting, casting speed, and specific water content, the adjustment capability is limited. Manufacturers using rectangular cross-section continuously cast billets to produce gear steel have also conducted a lot of research on improving frame segregation.

[0004] Chinese Patent Application No. 202310198941.X discloses a production method for improving the segregation of large-section rectangular bearing steel frames. By optimizing key elements in the chemical composition of steel, improving the production process of continuous casting, and optimizing the rolling process, the segregation frame ratio of the final rolled material is no more than 30%, and the width-to-height ratio is no more than 1.1, thereby obtaining a round steel bar with a low degree of center segregation.

[0005] Chinese Patent Application No. 202410587351.0 discloses a rolling method for controlling frame segregation in steel. By controlling the number of passes with a reduction rate of ≥13% and the rolling speed with a reduction rate of ≥25% in each direction, the morphology and proportion of frame segregation in the continuously cast billet after rolling are effectively controlled.

[0006] Chinese Patent Application No. 202411201961.9 discloses a production method for improving the segregation of the frame shape of large-section rectangular gear steel. The main methods include optimizing the electromagnetic stirring parameters of the crystallizer to minimize its intensity; appropriately increasing the casting speed while ensuring low billet quality; optimizing the secondary cooling water to make the segregation frame as square as possible instead of rectangular; and optimizing the rolling speed and rolling pass reduction to improve the size and width-to-narrowness ratio of the segregation frame in large-section rectangular gears. Through these four optimizations, the resulting round steel segregation frame length is <1 / 2d (d is the diameter of the round steel), and the width-to-narrowness ratio is controlled between 1.05 and 1.15.

[0007] All the technologies disclosed in the above patents employ "surface rolling" as a means of controlling the aspect ratio of segregation frames during the rolling process. However, "surface rolling" technology mainly achieves this by adjusting the reduction rate of different faces. For large-size gear steels with low compression ratios, the number of rolling passes is limited. "Surface rolling" relying solely on deformation distribution is insufficient to adequately improve the segregation frame morphology within the limited deformation penetration, thus failing to meet the stringent requirements of high-end gear steels (segregation frame aspect ratio ≤ 1.10). Therefore, other methods are needed to meet user needs.

[0008] "Face-recognition rolling" requires precise allocation of the reduction amount on different faces in different passes, which is complex in process design and has limited effectiveness when the number of passes is limited. Conventional "differential temperature rolling" is mostly used in the plate or final forming stage to control the shape or performance of the plate. There has never been any existing technology that inspired its application in the rectangular billet opening stage to solve the geometric morphology problem of frame segregation. Summary of the Invention

[0009] In order to meet the requirements of high-end gear steel for the aspect ratio of segregation frames based on existing technology, this invention provides a method for controlling frame segregation of gear steel bars produced from rectangular cross-section billets.

[0010] The technical solution of this invention: A method for controlling frame segregation in gear steel bars produced from rectangular cross-section continuous casting billets includes the following steps: a) Rapid surface cooling: The rectangular continuous casting billet is rapidly cooled to reduce its surface temperature to 1000-1020℃, so as to form a temperature gradient of more than 80℃ in the radial direction of the billet. b) Draft rolling: Immediately after cooling in step a), the billet is subjected to multi-pass draft rolling, wherein each pass uses a large reduction of ≥19% per pass.

[0011] Furthermore, a) rapid surface cooling: the aspect ratio of the rectangular continuous casting billet is 1.2 to 1.3.

[0012] Furthermore, a) Rapid surface cooling: Surface cooling is achieved through multiple high-pressure water descaling processes.

[0013] Furthermore, b) Billet rolling: large reduction rolling is adopted, with a single-pass reduction rate ≥19%.

[0014] Furthermore, the gear steel bar produced by the method has a segregation frame aspect ratio ≤ 1.10.

[0015] Furthermore, the final gear steel bar has a center porosity rating of ≤1.5.

[0016] The beneficial effects of this invention are: it can stably control the aspect ratio of the segregation frame to ≤1.10, or even lower (e.g., 1.05), within a limited number of rolling passes for large-size, low-compression-ratio gear steel, thus meeting the requirements of high-end gear steel; compared with "surface rolling", the method of this invention can achieve better segregation control while reducing dependence on specific rolling passes, making it more adaptable to the process; and it helps to improve core porosity and increase the density of the steel. Attached Figure Description

[0017] Figure 1 This is a low-magnification microstructure diagram of the rectangular cross-section continuously cast billet used in this invention.

[0018] Figure 2 This is a low-magnification microstructure diagram of the round steel used in Comparative Example 1 of the present invention.

[0019] Figure 3 This is a low-magnification microstructure diagram of the round steel used in Comparative Example 2 of the present invention.

[0020] Figure 4 This is a low-magnification microstructure diagram of a round steel bar according to an embodiment of the present invention. Detailed Implementation

[0021] The present invention will be further described below with reference to specific comparative examples and embodiments to help those skilled in the art to further understand the present invention, but without limiting the present invention in any way.

[0022] Before implementation, we used simulation software to simulate the rolling process of rectangular cross-section continuous casting billets under different reduction schedules and temperature fields. By setting tracking points, we observed the changes of the segregation frame in each rolling pass, thus providing valuable reference for process optimization.

[0023] Figure 1 As shown: The rectangular cross-section continuously cast billet used in this invention has a low magnification microstructure, with a billet cross-sectional size of 342mm × 435mm, a steel grade of 18CrNiMo, and a segregation frame size of 160mm × 240mm. To improve the microsegregation and temperature uniformity of the billet, a high-temperature diffusion process is adopted for heating. The total heating time of the billet is 360–480 min, the heating temperature is 1180–1240℃, the soaking temperature is 1200–1240℃, and the soaking time is ≥180 min. After exiting the furnace, the billet is descaled by high-pressure water (pressure ≥24MPa), rolled into intermediate billets, and then rolled into round bars in an elliptical-round pass section on a continuous rolling mill.

[0024] Comparative Example 1: After being descaled by high-pressure water, the billet immediately enters the billet mill for rolling in 9 passes. The control method of "surface rolling" is adopted. The 1st, 3rd, 6th and 9th passes are for the narrow side of the billet and are rolled with a large reduction. The 5th and 8th passes do not turn the billet. The size of the intermediate billet is 205mm×205mm. The actual rolling temperature is 1120~1140℃. The billet mill reduction procedure is shown in Table 1.

[0025] Table 1 Comparative Example 1: Rolling Down Procedure of the Billet Mill .

[0026] After four passes of continuous rolling, the intermediate billet is rolled into finished round steel with a diameter of 170 mm. Its low-magnification microstructure is as follows: Figure 2 As shown. Testing revealed that the aspect ratio of its segregation frame was 87 / 75≈1.16, which, according to GB / T1979, indicates a central porosity level of 2.0.

[0027] Comparative Example 2: After being descaled by high-pressure water, the billet immediately enters the billet mill for rolling in 7 passes. Each pass uses a large reduction rolling, and the billet is turned over after each pass. The size of the intermediate billet is also 205mm×205mm. The actual rolling temperature is 1120~1140℃. The billet mill reduction procedure is shown in Table 2.

[0028] Table 2. Rolling down procedure of the billet mill in Comparative Example 2 and Example 3 .

[0029] After four passes of continuous rolling, the intermediate billet is rolled into finished round steel with a diameter of 170 mm. Its low-magnification microstructure is as follows: Figure 3 As shown. Testing revealed that the aspect ratio of its segregation frame was 88 / 73≈1.21, which, according to GB / T1979, indicates a central porosity level of 1.5. Example

[0030] The billets exiting the furnace undergo multiple high-pressure water descaling processes, reducing the surface temperature to 1000–1020°C within one minute, while the core temperature remains above 1100°C, creating a radial temperature gradient greater than 80°C. The billet is rolled in seven passes on a billet mill, each with a large reduction. The billet is flipped after each pass, and the intermediate billet size remains the same at 205mm × 205mm. The billet mill reduction schedule is the same as in Comparative Example 2, as shown in Table 2.

[0031] After four passes of continuous rolling, the intermediate billet is rolled into finished round steel with a diameter of 170 mm. Its low-magnification microstructure is as follows: Figure 4 As shown. Testing revealed that the aspect ratio of its segregation frame was 82 / 78≈1.05, which, according to GB / T1979, indicates a central porosity level of 1.0.

[0032] The comparison of test data for different rolling schemes is shown in the table below.

[0033] Field tests verified the feasibility of differential temperature rolling combined with continuous high reduction in improving the frame segregation of gear steel bars produced from rectangular cross-section continuously cast billets. Furthermore, compared to Comparative Example 1 ("surface rolling", segregation frame width-to-height ratio 1.16), the method of this invention achieves superior segregation frame control (differential temperature rolling, segregation frame width-to-height ratio 1.05) with fewer rolling passes on the billet mill (7 passes), demonstrating the significant advantage of differential temperature rolling in solving the frame segregation problem within a limited number of passes. Compared to Comparative Example 2 (conventional high reduction rolling, segregation frame width-to-height ratio 1.21), the method of this invention, under the same billet mill reduction procedure, more effectively adjusts the core segregation frame size by implementing differential temperature rolling, making its morphology more square.

[0034] Although "differential temperature rolling" is a known concept, its specific application in controlling the aspect ratio of frame segregation during the billet opening stage of rectangular cross-section continuous casting billets, achieving unexpected technical results, is the innovation of this invention. Differential temperature rolling combined with continuous high reduction can obtain a more ideal aspect ratio for frame segregation compared to "surface rolling." A reasonable temperature field for the reshaped billet is a fundamental condition for ensuring deformation penetration. Based on the basic principles of differential temperature rolling, this invention combines differential temperature rolling with continuous high reduction rolling to effectively improve the frame segregation of large-size gear steel.

[0035] Table 3 Comparison of test data for different rolling schemes .

Claims

1. A method for controlling frame segregation in gear steel bars produced from rectangular cross-section continuous casting billets, comprising the following process steps: a) Rapid surface cooling: The rectangular continuous casting billet is rapidly cooled to reduce its surface temperature to 1000-1020℃, so as to form a temperature gradient of more than 80℃ in the radial direction of the billet. b) Initial rolling: Immediately after cooling in step a), the billet is subjected to multiple passes of initial rolling, wherein... Each pass uses a large reduction of ≥19% for rolling.

2. The method for controlling frame segregation of gear steel bars produced from rectangular cross-section continuous casting billets according to claim 1, characterized in that... a) Rapid surface cooling: The aspect ratio of the rectangular continuous casting billet is 1.2 to 1.

3.

3. The method for controlling frame segregation of gear steel bars produced from rectangular cross-section continuous casting billets according to claim 1, characterized in that... a) Rapid surface cooling: Surface cooling is achieved through multiple high-pressure water descaling processes.

4. The method for controlling frame segregation of gear steel bars produced from rectangular cross-section continuous casting billets according to claim 1, characterized in that... b) Rolling of billets: Large reduction rolling is adopted, with a single-pass reduction rate of ≥19%.

5. The method for controlling frame shape segregation of gear steel bars produced from rectangular cross-section continuous casting billets according to claim 1, characterized in that: The final gear steel bar has a segregation frame aspect ratio ≤ 1.

10.

6. The method for controlling frame shape segregation of gear steel bars produced from rectangular cross-section continuous casting billets according to claim 1, characterized in that: The final gear steel bar has a central porosity rating of ≤1.5.

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