Ultra-fine grain continuous rolling method for ultra-pure ferrite stainless steel wire

Abstract

The ultra-pure ferrite stainless steel wire ultra-fine grain continuous rolling method comprises the following operation steps that S1, pretreatment is conducted, specifically, an ingot type raw material is heated through a preheating section, a first heating section, a second heating section, a third heating section and a soaking section, the heating temperature of the preheating section is smaller than or equal to 850 DEG C, the temperature of the first heating section is 900-950 DEG C, the temperature of the second heating section is 990-1030 DEG C, and the temperature of the third heating section is 950-950 DEG C; the temperature of the third heating section is 990-1030 DEG C, and the temperature of the soaking section is 990-1030 DEG C; s2, rolling is conducted, specifically, the product A is subjected to continuous round rolling forming, the finish rolling temperature is 790-820 DEG C, and a product B is obtained; and S3, post-treatment: completely cooling the product B, and then carrying out stress relief annealing to obtain a finished product. By controlling the heating temperature and the finish rolling temperature, the ultra-pure ferritic stainless steel ultra-fine grain ferritic structure is directly prepared, and the method has the beneficial effects that the working procedures are reduced, the cost is reduced, and the preparation efficiency is improved.

Description

Technical Field

[0001] This invention relates to a method for rolling stainless steel wire, and more particularly to a method for continuous rolling of ultra-pure ferritic stainless steel ultrafine-grained wire. Background Technology

[0002] Ultra-pure ferritic stainless steel refers to a composite material of iron, chromium, and nickel. It belongs to the high-end category of ferritic stainless steel. By drastically reducing interstitial elements such as carbon (≤0.02%) and nitrogen (≤0.015%), it solves the problems of insufficient toughness and easy intergranular corrosion during welding inherent in traditional ferritic stainless steel. Compared with ordinary stainless steel, it has higher strength and better corrosion resistance, and also features strong weldability, ease of processing, and good wear resistance. It is an important material widely used in aerospace, manufacturing, and instrumentation equipment.

[0003] However, during hot working, especially rolling, ultrapure ferritic stainless steel tends to coarsen rapidly, resulting in poor room temperature plasticity and insufficient toughness. This makes it highly susceptible to brittle fracture during subsequent cold working (such as cold drawing and cold rolling), severely impacting product yield and processing efficiency. Furthermore, conventional processes typically require annealing followed by cold drawing and deformation to achieve the ultrafine grain structure of stainless steel wire, increasing the number of steps, costs, and efficiency. Summary of the Invention

[0004] The purpose of this invention is to provide a method for continuous rolling of ultra-fine-grained ferritic stainless steel wire. This invention directly produces ultra-fine-grained ferritic microstructure of ultra-pure ferritic stainless steel by controlling the heating temperature and final rolling temperature, which features reduced processes, lower costs, and improved preparation efficiency.

[0005] The technical solution of this invention: a method for continuous rolling of ultra-fine grains of ultrapure ferritic stainless steel wire, comprising the following steps: S1. Pretreatment: The ingot-shaped raw material is heated through a preheating section, a first heating section, a second heating section, a third heating section, and a soaking section. The heating temperature of the preheating section is ≤850℃, the temperature of the first heating section is 900~950℃, the temperature of the second heating section is 990~1030℃, the temperature of the third heating section is 990~1030℃, and the temperature of the soaking section is 990~1030℃, to obtain product A. S2. Rolling: Roll product A into a round shape continuously at a final rolling temperature of 790~820℃ to obtain product B. S3. Post-processing: After product B is completely cooled, stress-relief annealing is performed to obtain the finished product.

[0006] In the aforementioned method for continuous rolling of ultra-fine ferritic stainless steel wire, in step S2, when rolling wire with a diameter of 6~12mm, the finished product linear speed is 21~31m / s.

[0007] In the aforementioned method for continuous rolling of ultra-fine grains of ultrapure ferritic stainless steel wire, in the post-processing step S3: during annealing, the temperature is first raised to 780℃~800℃, and then lowered to 680~700℃.

[0008] In the aforementioned method for continuous rolling of ultra-fine grains of ultrapure ferritic stainless steel wire, in step S3, the heating rate is ≤80℃ / h and the holding time is 0.5~2h.

[0009] In the aforementioned method for continuous rolling of ultra-fine grains of ultrapure ferritic stainless steel wire, in step S3, the heating rate is 50~60℃.

[0010] In the aforementioned method for continuous rolling of ultra-fine ferritic stainless steel wire, in step S3, the cooling rate is ≤80℃ / h, and the holding time is 0.5~2h.

[0011] In the aforementioned method for continuous rolling of ultra-fine grains of ultrapure ferritic stainless steel wire, in step S3, the cooling rate is 50~60℃.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention, by controlling the heating temperature and adjusting the rolling process, can roll ultrafine ferritic wire with uniform microstructure and finished specifications of φ6-φ12mm. The resulting ultrapure ferritic stainless steel wire has a metallographic grain size of 10-12 and a grain size difference of <1.5, eliminating the need for subsequent cold working, shortening the preparation time, improving the preparation efficiency, and reducing the preparation cost.

[0013] Furthermore, this method employs a two-stage heat treatment after rolling: first, rapid recrystallization annealing is performed to soften the material, followed by low-temperature stabilization annealing to inhibit grain growth. This results in stable performance, with a room temperature yield strength ≥350MPa, tensile strength ≥500MPa, and elongation after fracture ≥30%. This significantly improves the cold working plasticity of stainless steel wire, making it suitable for manufacturing precision parts with strict ductility requirements. It solves the problem of high brittleness and poor plasticity in subsequent cold working of ultrapure ferritic stainless steel due to coarse ferrite grains. Moreover, this method has high production efficiency and produces products with stable performance. Attached Figure Description

[0014] Figure 1 This is a metallographic image of the grain size of the product in Example 1.

[0015] Figure 2 This is a metallographic image of the grain size of the product in Comparative Example 1.

[0016] Figure 3 This is the metallographic image of the grain size of the product in Comparative Example 2. Detailed Implementation

[0017] The present invention will be further described below with reference to embodiments, but these embodiments are not intended to limit the scope of the invention.

[0018] Example 1: Taking the ultra-fine grain of ultrapure ferritic stainless steel 446 wire with a rolled diameter of 6mm as an example: Its chemical composition by weight percentage is: C≤0.015%, N≤0.015%, Cr: 23.0-27.0%, Mn≤1.0%, Si≤0.75%, P≤0.040%, S≤0.015%, with the remainder being Fe and unavoidable impurities.

[0019] Its continuous rolling method includes the following steps: S1. Pretreatment: Ingot-shaped raw materials of 150*150*4000~6000mm are heated in a walking beam furnace. The heating process is divided into a preheating section, a first heating section, a second heating section, a third heating section, and a soaking section. The temperature of the preheating section is ≤850℃, the temperature of the first heating section is 900~950℃, the temperature of the second heating section is 990~1030℃, the temperature of the third heating section is 990~1030℃, and the temperature of the soaking section is 990~1030℃, yielding product A. The heating time of each stage is maintained at more than 1.5 hours to ensure heating uniformity.

[0020] The furnace body is 28 meters long and 6.6 meters wide. The furnace is divided into 5 zones, of which heating zone 1, heating zone 2, heating zone 3 and soaking zone all use upper and lower burners.

[0021] S2. Rolling: Product A is continuously rolled into round shape through 4 550 continuous rolling mills, 6 450 continuous rolling mills, 6 380 continuous rolling mills, 6 280 continuous rolling mills, and 10 finishing mills. The finished product line speed is 29~31m / s, and the final rolling temperature is 790~820℃ to obtain Product B. After testing, the grain size of the metallographic structure of Product B is 10-12, the structure is uniform, there are no mixed crystals, and the grain size difference is 1 level.

[0022] S3. Post-processing: After the B product is completely cooled, stress-relief annealing is carried out. First, the furnace temperature is raised to 780℃~800℃ at a rate of 50~60℃ / h and held for 1 hour. Then, the temperature is lowered to 680~700℃ at a rate of 50~60℃ / h and held for 1.5 hours. After being removed from the furnace, it is air-cooled to obtain a finished round bar with a specification of Φ6mm, which improves the ductility of the product in subsequent cold working.

[0023] The main purpose of post-treatment is to eliminate the deformation stress and rapid cooling stress generated during the rolling process, while avoiding the precipitation of harmful phases. This is primarily to prevent the cooling process from taking too long in the brittle phase range of 400-600℃ (leading to the precipitation of chromium-rich σ' phase). During this recrystallization annealing process, the 780-800℃ range mainly aims to eliminate the deformation stress and rapid cooling stress generated during rolling, softening the microstructure and replacing the broken and elongated grains caused by rolling deformation with new, stress-free, fine equiaxed grains. Then, the temperature is lowered to 680-700℃ for low-temperature stabilization treatment, avoiding the brittle phase region of 400-600℃, to inhibit grain growth, prevent phase transformation, maintain grain size, and improve subsequent cold working plasticity.

[0024] Testing revealed that the finished product has a grain size of 10-12, with no mixed crystals, and a grade difference of 1.5. Figure 1 It is an ultra-fine grain structure of 446 ultrapure ferritic stainless steel with a grain size of grade 10, a room temperature yield strength of 380 MPa, a tensile strength of 550 MPa, and an elongation of 32%.

[0025] Comparative Example 1: This comparative example is basically the same as Example 1, except that the heating temperature of heating stage 2, heating stage 3, and soaking stage is all 940-980℃; the metallographic image of the final ultrapure ferritic stainless steel wire is as follows. Figure 2 As shown, the original grains were not uniformly broken and were mixed crystals, which cannot be rated.

[0026] Comparative Example 2: This comparative example is basically the same as Example 1, except that the heating temperature of heating section 2, heating section 3, and soaking section is 1040~1080℃; the grain size of the finally obtained ultrapure ferritic stainless steel wire is grade 4-8, with mixed crystals, and the grade difference is 4. Figure 3 Metallographic diagram of the obtained ultrapure ferritic stainless steel wire with a grain size of grade 6.

[0027] As shown in Examples 1, 1, and 2, when the temperature is too low or too high, mixed crystals will occur, and large temperature differences will also appear. Therefore, the optimal heating temperatures for the second and third heating stages and the soaking stage of this ingot are 990-1030℃.

[0028] Comparative Example 3: This comparative example is basically the same as Example 1, except that: the finished product line speed is 25-28m / s, the final rolling temperature is 750~780℃, and the final ultrapure ferritic stainless steel wire is mixed crystal, which cannot be rated. The yield strength is 450MPa, the tensile strength is 650MPa, and the elongation is 18%.

[0029] Comparative Example 4: This comparative example is basically the same as Example 1, except that the finished product line speed is 32-35m / s, the final rolling temperature is 830-860℃, and the resulting ultra-pure ferritic stainless steel wire has a large core temperature rise due to large deformation, resulting in microstructure growth, grain size of 7-10, mixed crystals, large range, yield strength of 300MPa, tensile strength of 480MPa, and elongation of 26%.

[0030] As can be seen from Example 1 and Comparative Examples 3-4, the change in linear velocity leads to the change in final rolling temperature. After testing, it was found that different final rolling temperatures have different effects on the properties and microstructure of ultra-pure ferritic stainless steel wire. When the final rolling temperature is too low, it leads to a mixed crystal state. When the final rolling temperature is too high, there will be a large difference in crystal size and a mixed crystal phenomenon. Therefore, the optimal final rolling temperature is 790-820℃.

[0031] It should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Those skilled in the art can modify the technical solutions described in the above embodiments, or make equivalent substitutions for some of the technical features; and all such modifications and substitutions should fall within the protection scope of the appended claims of the present invention.

Claims

1. A method for continuous rolling of ultra-fine grains of ultrapure ferritic stainless steel wire, characterized in that: The following steps are included: S1. Pretreatment: The ingot-shaped raw material is heated through a preheating section, a first heating section, a second heating section, a third heating section, and a soaking section. The heating temperature of the preheating section is ≤850℃, the temperature of the first heating section is 900~950℃, the temperature of the second heating section is 990~1030℃, the temperature of the third heating section is 990~1030℃, and the temperature of the soaking section is 990~1030℃, to obtain product A. S2. Rolling: Roll product A into a round shape continuously at a final rolling temperature of 790~820℃ to obtain product B. S3. Post-processing: After product B is completely cooled, stress-relief annealing is performed to obtain the finished product.

2. The method for continuous rolling of ultra-fine grains of ultrapure ferritic stainless steel wire according to claim 1, characterized in that: In step S2, during the rolling process, when rolling wire with a diameter of 6~12mm, the finished product linear speed is 21~31m / s.

3. The method for continuous rolling of ultra-fine grains of ultrapure ferritic stainless steel wire according to claim 1, characterized in that: In step S3 post-processing: during annealing, first raise the temperature to 780℃~800℃, then lower it to 680~700℃.

4. The method for continuous rolling of ultra-fine grains of ultrapure ferritic stainless steel wire according to claim 3, characterized in that: In step S3, the heating rate is ≤80℃ / h, and the holding time is 0.5~2h.

5. A method for continuous rolling of ultra-fine grains of ultrapure ferritic stainless steel wire according to claim 4, characterized in that: In step S3, the heating rate is 50~60℃.

6. The method for continuous rolling of ultra-fine grains of ultrapure ferritic stainless steel wire according to claim 3, characterized in that: In step S3, the cooling rate is ≤80℃ / h, and the holding time is 0.5~2h.

7. The method for continuous rolling of ultra-fine grains of ultrapure ferritic stainless steel wire according to claim 6, characterized in that: In step S3, the cooling rate is 50~60℃.

Images