A method for high-grain forging and rolling forming of large-diameter austenitic stainless steel rods

Abstract

The application discloses a large-diameter austenitic stainless steel bar high-grain-degree forging-rolling forming method and belongs to the technical field of austenitic stainless steel bar production, and comprises the following steps: S1, heating a steel billet to 1200±20 DEG C and keeping the temperature; S2, continuously performing upsetting and elongating twice on the steel billet; S3, after the steel billet is forged, surface defect inspection is performed, and defects are treated; S4, heating the billet to 1000±10 DEG C and keeping the temperature; then rapidly heating the square billet to 1250±10 DEG C and keeping the temperature; S5, feeding the square billet into a rough rolling mill group to perform rough rolling, and repeatedly performing rough rolling; S6, through a plurality of groups of longitudinal and transverse alternating forming rolls of a finishing rolling mill group, the square billet is rolled into a bar with a diameter of D; and S7, immediately performing spray blowing cooling after the bar rolling is completed; the application can realize the production and manufacturing of large-diameter austenitic stainless steel bars with high-grain-degree requirements, and simultaneously solves the problems of coarse grains in local forging and insufficient grain-degree grade in the core of rolling.

Description

Technical Field

[0001] This invention relates to the field of austenitic stainless steel bar production technology, specifically a high-grain-size forging and rolling method for large-diameter austenitic stainless steel bars. Background Technology

[0002] For austenitic stainless steel bars, due to their material properties, it is difficult to effectively improve the grain size through heat treatment processes such as solution treatment. For bars requiring high grain size grades, a higher grain size grade must be achieved during forging or rolling.

[0003] Forged bars can be produced in a wide range of diameters, but the forging process is inefficient, and the billet deformation during forging is uneven. This results in large bars having a high overall grain size after forging, but with localized coarse grains and mixed grains, leading to products that do not meet requirements. Furthermore, forged bars have poor straightness, large machining allowances in the diameter direction, low material utilization, high processing costs, and are generally only produced in lengths of 5 meters or less.

[0004] Rolled bars offer high production efficiency, high bar straightness, large rollable length, precise allowance control, and low processing volume. However, due to the limitations of rolling production characteristics, the rolling mill primarily causes surface deformation of the billet, resulting in low rolling penetration. Rolled bars exhibit a characteristic of stratified grain size variation along the cross-sectional diameter. From near the surface to the core, the grain size gradually decreases while the grain size gradually increases. For austenitic stainless steel bars with smaller diameters and lower grain size requirements, the rolling force can effectively penetrate to the core due to the small cross-sectional diameter, allowing the core to meet grain size requirements. However, for large-diameter (D = Φ150–200 mm) austenitic stainless steel bars with high grain size requirements (overall cross-sectional grain size ≥ 5), while the surface grain size may meet the requirements, the core grain size is difficult to achieve. Summary of the Invention

[0005] The purpose of this invention is to provide a high-grain-size forging and rolling method for large-diameter austenitic stainless steel bars, so as to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for forging and rolling large-diameter austenitic stainless steel bars with high grain size includes the following steps:

[0008] S1. Heat the steel billet to 1200±20℃ and hold for 2-4 hours;

[0009] S2. After the steel billet is heated evenly, it is quickly taken out of the furnace. The steel billet is then rapidly and continuously subjected to two upsetting and two drawing processes by a hydraulic press to draw the billet into a square billet with a cross-sectional width of D1. During the forging process, the forging pressure ratio of each upsetting and each drawing process should be ≥2. After two upsetting and two drawing processes, the steel billet fully and effectively compacts and breaks the as-cast structure, and the internal grains of the steel billet are refined for the first time.

[0010] S3. After the steel billet forging is completed, surface defects are inspected and addressed accordingly.

[0011] S4. When the billet is put into the furnace, first heat the billet to 1000±10℃ and hold it for 2 to 3 hours; then quickly heat the billet to 1250±10℃ and hold it for 0.5 to 1 hour.

[0012] S5. The billet is quickly discharged from the furnace and sent to the roughing mill for roughing. The rolling amount per pass is 10-15mm. After multiple reciprocating roughing, the billet is rolled into a square billet with a cross-sectional width of D2.

[0013] S6. The rough-rolled billet is quickly sent to the finishing mill. Through multiple sets of alternating longitudinal and transverse forming rolls of the finishing mill, the deformation of each set of rolling mills is 15% to 20%, and the billet is rolled into a bar with a diameter of D in one pass, wherein the diameter D of the bar is Φ150 to 200 mm.

[0014] S7. Immediately after the bar is rolled, spray air cooling is performed to quickly cool the bar and prevent the bar grains from coarsening or becoming mixed crystals due to high residual heat.

[0015] As a further aspect of the present invention, in step S2, D1 = (2.3~2.7)D.

[0016] As a further aspect of the present invention, in step S3, when the surface defect inspection is carried out after the steel billet forging is completed, if forging cracks or folding defects appear on the surface of the steel billet, the surface is ground and cleaned; if crack defects appear on the surface of the steel billet, the defects are removed by carbon arc gouging. During the defect removal process, care should be taken to avoid forming deep cleaning pits, and the width to depth ratio of the cleaning pits should be controlled between 4 and 8.

[0017] As a further embodiment of the present invention, in step S5, D2 = (1.1~1.2)D.

[0018] Compared with the prior art, the beneficial effects of the present invention are: the forging and rolling forming technology provided by the present invention can realize the production and manufacturing of large-diameter austenitic stainless steel bars with high grain size requirements. In particular, for stainless steel bars with a cross-sectional diameter D of Φ150~200mm and a grain size requirement of ≥5, the disadvantages of coarse grains in the forging process and insufficient grain size in the rolling core can be solved at the same time. Attached Figure Description

[0019] Figure 1 This is a simplified schematic diagram illustrating the high-grained forging and rolling method for large-diameter austenitic stainless steel bars according to an embodiment of the present invention.

[0020] Figure 2 This is a grain size grade diagram of a full-section sample taken from a large-diameter bar after forging and rolling, according to an embodiment of the present invention.

[0021] Figure 3 This is a grain morphology diagram of a large-diameter bar after forging and rolling according to an embodiment of the present invention. Detailed Implementation

[0022] The technical solution of this patent will be further described in detail below with reference to specific embodiments.

[0023] A method for forging and rolling large-diameter austenitic stainless steel bars with high grain size includes the following steps:

[0024] S1. Heat the steel billet to 1200±20℃ and hold for 2-4 hours;

[0025] S2. After the steel billet is heated evenly, it is quickly removed from the furnace. A hydraulic press is then used to rapidly and continuously perform two upsetting and two drawing operations on the billet, drawing it into a square billet with a cross-sectional width of D1, where D1 = (2.3~2.7)D. During the forging process, the forging pressure ratio for each upsetting and drawing operation should be ≥2. After two upsetting and two drawing operations, the steel billet's fragmented as-cast structure is fully and effectively compacted, resulting in the first overall refinement of the internal grains. This invention effectively compacts the fragmented as-cast structure of the steel billet through two upsetting and two drawing operations, while simultaneously improving the overall grain size of the billet. Experiments and production verification show that after two upsetting and two drawing operations, the overall grain size of the steel billet can reach grade 4-5. Drawing the billet to D1 = (2.3~2.7)D ensures a total rolling ratio of approximately 7-9 in the subsequent rolling process, ensuring sufficient deformation and further effectively improving the grain size of the forging after rolling.

[0026] The size of the forged square billet, D1, should be controlled within the range of (2.3 to 2.7)D to ensure a total rolling ratio of approximately 7 to 9 during the rolling process. This ensures sufficient deformation to effectively improve the grain size of the bar. On the other hand, for large-diameter bars with a diameter of Φ150 to 200 mm, the billet size should not be too large to avoid situations such as skipping during rolling, which could affect the quality of the formed bar or prevent rolling from progressing, leading to reduced roughing efficiency, extended rolling time, and billet cooling, which could affect subsequent finish rolling.

[0027] S3. After the steel billet forging is completed, surface defects are inspected and addressed accordingly.

[0028] S4. The billet is fed into the furnace and first heated to 1000±10℃, held for 2-3 hours; then rapidly heated to 1250±10℃, held for 0.5-1 hour. Since subsequent rolling will involve large-diameter bars with a high rolling ratio, the billet has high deformation resistance. Heating and holding at this temperature improves the billet's plasticity and reduces the load requirements on the rolling equipment. Furthermore, initial internal homogenization at a lower temperature increases the overall temperature and plasticity of the billet. At this temperature, grain growth is very slow, and significant grain growth does not occur. Rapid, short-duration heating at the high temperature stage further improves the billet's plasticity while preventing significant grain growth. Since subsequent rolling will further improve the bar grain size, the impact is controllable.

[0029] S5. The billet is rapidly discharged from the furnace and fed into the roughing mill for rough rolling. Each pass involves a rolling depth of 10-15 mm. After multiple reciprocating rough rolling passes, the billet is rolled into a square billet with a cross-sectional width of D2, where D2 = (1.1-1.2)D. The billet size after rough rolling is controlled to D2 = (1.1-1.2)D. On the one hand, this aims to make the cross-sectional area of ​​the billet after rough rolling as close as possible to the final bar cross-sectional area, reducing the difficulty of forming the billet from a square to a circular cross-section in the finishing mill, reducing equipment load, and improving rolling efficiency. On the other hand, during rough rolling, there is a certain degree of difficult-to-deform zone at the corners of the square billet cross-section, with relatively less strain. To ensure the grain size of the entire bar cross-section, a deformation of ≥30% is required during finishing rolling to ensure sufficient strain across the entire bar cross-section, effectively triggering dynamic recrystallization and improving the bar grain size grade.

[0030] S6. The rough-rolled billet is quickly sent to the finishing mill. Through multiple sets of alternating longitudinal and transverse forming rolls of the finishing mill, the deformation of each set of rolling mills is 15% to 20%, and the billet is rolled into a bar with a diameter of D in one pass, wherein the diameter D of the bar is Φ150 to 200 mm.

[0031] S7. Immediately after the bar is rolled, spray air cooling is performed to quickly cool the bar and prevent the bar grains from coarsening or becoming mixed crystals due to high residual heat.

[0032] In this embodiment of the invention, in step S3, when the surface defect inspection is carried out after the steel billet forging is completed, if forging cracks or folding defects appear on the surface of the steel billet, the surface is ground and cleaned; if crack defects appear on the surface of the steel billet, the defects are removed by carbon arc gouging. During the defect removal process, care should be taken to avoid forming deep cleaning pits, and the width to depth ratio of the cleaning pits should be controlled between 4 and 8.

[0033] The specific implementation method is as follows:

[0034] As one embodiment of the present invention, please refer to Figure 1 , Figure 1The medium dimension is a technical parameter in the embodiments of this invention. A method for forging and rolling large-diameter austenitic stainless steel bars with high grain size includes the following steps:

[0035] S1. The material to be cut is 316H steel billet with a specification of Φ550×1300mm.

[0036] S2. Rapid out-of-furnace process: The steel billet is rapidly and continuously upset twice and drawn twice to forge a square billet with dimensions of □450×1490mm.

[0037] S3. After the billet forging is completed, a surface inspection is carried out. If there are defects such as forging cracks or flaking, the surface is ground or cleaned by carbon arc gouging to ensure the surface quality of the billet before rolling.

[0038] S4. The billet is put into the furnace and first heated to 1000±10℃ for 2 hours. Then it is rapidly heated to 1250±10℃ for 0.5 to 1 hour.

[0039] S5. The billet is quickly discharged from the furnace and sent to the roughing mill for repeated roughing. The rolling amount per pass is 10-15mm. After multiple reciprocating roughing passes, the billet is rolled into a square billet with a cross-sectional width of 220mm.

[0040] S6. The rough-rolled billet is quickly sent to the finishing mill. Through multiple sets of alternating longitudinal and transverse forming rolls of the finishing mill, the deformation of each set of rolling mills is 15% to 20%, and the billet is rolled into a bar with a diameter of Φ190 in one pass.

[0041] S7. After the bar is rolled, immediately spray it with air to cool it to room temperature.

[0042] After forging and rolling according to the above steps, samples of large-diameter austenitic stainless steel bars were taken for full-section grain size testing. The analysis results showed that although the bar still showed a certain trend of gradually decreasing grain size from the surface to the core, for large-diameter austenitic stainless steel bars with a diameter of Φ190, the grain size of the entire cross section reached level 5 to 7. The grain size level was high and the grain morphology distribution was generally uniform, which achieved the technical objective of the present invention.

[0043] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0044] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for forging and rolling large-diameter austenitic stainless steel bars with high grain size, characterized in that, Includes the following steps: S1. Heat the steel billet to 1200±20℃ and hold for 2-4 hours; S2. After the steel billet is heated evenly, it is quickly taken out of the furnace. The steel billet is then rapidly and continuously subjected to two upsetting and two drawing processes by a hydraulic press to draw the billet into a square billet with a cross-sectional width of D1. During the forging process, the forging pressure ratio of each upsetting and each drawing process should be ≥2. After two upsetting and two drawing processes, the steel billet fully and effectively compacts and breaks the as-cast structure, and the internal grains of the steel billet are refined for the first time. S3. After the steel billet forging is completed, surface defects are inspected and addressed accordingly. S4. When the billet is put into the furnace, first heat the billet to 1000±10℃ and hold it for 2 to 3 hours; then quickly heat the billet to 1250±10℃ and hold it for 0.5 to 1 hour. S5. The billet is quickly discharged from the furnace and sent to the roughing mill for roughing. The rolling amount per pass is 10-15mm. After multiple reciprocating roughing, the billet is rolled into a square billet with a cross-sectional width of D2. S6. The rough-rolled billet is quickly sent to the finishing mill. Through multiple sets of alternating longitudinal and transverse forming rolls of the finishing mill, the deformation of each set of rolling mills is 15% to 20%, and the billet is rolled into a bar with a diameter of D in one pass, wherein the diameter of the bar is Φ150 to 200 mm. S7. Immediately after the bar is rolled, spray air cooling is performed to quickly cool the bar and prevent the bar grains from coarsening or becoming mixed crystals due to high residual heat.

2. The method for high-grained forging and rolling of large-diameter austenitic stainless steel bars according to claim 1, characterized in that, In step S2, D1 = (2.3~2.7)D.

3. The method for forging and rolling large-diameter austenitic stainless steel bars with high grain size according to claim 1, characterized in that, In step S3, when the surface defect inspection is carried out after the steel billet forging is completed, if forging cracks or flaking defects appear on the surface of the steel billet, the surface is ground and cleaned; or, if forging crack defects appear on the surface of the steel billet, the defects are removed by carbon arc gouging. During the defect removal process, care should be taken to avoid forming deep cleaning pits, and the width to depth ratio of the cleaning pits should be controlled between 4 and 8.

4. A method for forging and rolling large-diameter austenitic stainless steel bars with high grain size according to any one of claims 1-3, characterized in that, In step S5, D2 = (1.1~1.2)D.

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