Method for producing medium-high chromium, nickel-reduced austenitic stainless steel strip

Abstract

The application discloses a production method of medium-high-chromium nickel-reduced austenitic stainless steel strip, which comprises the steps of continuous casting, heating in a heating furnace, rough rolling, finish rolling and the like, wherein the process parameters of the continuous casting step are adjusted to lay a foundation for rough rolling, and the temperature settings of each stage in the heating furnace ensure that the temperature inside and outside the slab is uniform; and because the heating speed is moderate, the slab will not be cracked due to excessive local thermal stress; the target temperature T in the second heating section is calculated according to a formula according to the actual composition; the setting of the target temperature can ensure that the continuous casting blank is heated away from the high-temperature two-phase zone temperature, and the growth of ferrite in high-temperature production is avoided, thereby reducing the risk of edge cracking and ensuring that the high-nitrogen continuous casting blank obtains the necessary heating temperature, and the phenomenon of being unable to be rolled in the rolling process is avoided. Through the whole-process process adjustment and optimization, the medium-high-chromium nickel-reduced austenitic stainless steel coil obtained is free of edge cracking, has good surface quality, and only has sporadic peeling defects or no peeling defects.

Description

Technical Field

[0001] This invention relates to iron and steel smelting and rolling processes, and more particularly to a method for producing medium-high chromium nickel-saving austenitic stainless steel strip. Background Technology

[0002] Medium-high chromium, high-nitrogen, nickel-saving austenitic stainless steel (specific composition range is shown in Table 1) has better corrosion resistance than the most widely circulated nickel-saving austenitic stainless steel 201 / 202. Due to its high nitrogen content, it even has higher pitting corrosion resistance than 304 with the same Cr content. Therefore, it can become a substitute for traditional Cr-Ni austenitic stainless steel in many industries.

[0003] Despite this, the microstructure of medium-high chromium, high-nitrogen, and nickel-reducing austenitic stainless steel presents significant challenges to production due to its unique composition design. As shown in Table 1, the composition system of medium-high chromium, high-nitrogen, and nickel-reducing austenitic stainless steel inevitably leads to the formation of a high-temperature ferrite phase during solidification. This results in severe compositional segregation in the two-phase microstructure. Once the temperature in the furnace reaches a certain level, the high-temperature ferrite structure rapidly grows and increases, easily causing edge cracks during hot rolling, which can occur even in the initial rolling stage. Simultaneously, the large difference in high-temperature plasticity between the two-phase microstructures caused by severe segregation leads to cracking along phase boundaries during hot rolling. Oxidation of the grain boundaries causes the oxidized grain boundary regions to detach from the matrix during subsequent rolling, resulting in surface peeling defects. Furthermore, the high nitrogen content results in high steel strength, and the heating temperature cannot be too low; otherwise, the rolling pressure will be too high, exceeding the mill load and triggering an alarm, making the rolling process difficult. This not only reduces production efficiency but also affects the strip yield. If the above problems are not effectively resolved, the pressure from process costs will directly affect the promotion and application of this type of steel.

[0004] Table 1

[0005] Element C Si Mn P S Cr Ni Cu N （%） 0.04～0.1 ≤0.60 5.0～10.0 ≤0.045 ≤0.006 18.0～23.0 0.5～3.0 0.5～2.0 0.18～0.25 Summary of the Invention

[0006] The purpose of this invention is to provide a method for producing medium-high chromium nickel-saving austenitic stainless steel strip.

[0007] The technical solution to achieve the objective of this invention is: a method for producing medium-high chromium nickel-retaining austenitic stainless steel strip, which includes the following steps:

[0008] (1) The medium-high chromium, high-nitrogen, nickel-saving austenitic stainless steel is smelted according to its chemical composition, wherein the weight percentage of the chemical composition of the medium-high chromium, high-nitrogen, nickel-saving austenitic stainless steel is as follows: C: 0.04~0.1%; Si ≤0.6%; Mn: 5.0~10.0%; P: ≤0.045%; S: ≤0.006%; Cr: 18.0~23.0%; Ni: 0.5~3.0%; Cu: 0.5~2.0%; N: 0.18~0.25%, with the balance being iron and unavoidable impurities;

[0009] (2) Continuous casting: The superheat of molten steel is required to be 25℃~35℃, the casting speed is 1.0m / min~1.2m / min, the equiaxed crystal ratio of the billet is required to reach more than 50%, the water flow rate of the wide face of the continuous casting machine crystallizer is 2000l / min~2500l / min, and the water flow rate of the narrow face is 320l / min~350l / min; after the billet is cast and cut, it forms a slab.

[0010] (3) The slab formed by continuous casting in step 2 is sent to the hot rolling furnace. The heating process in the furnace involves a preheating section, a first heating section, a second heating section, and a soaking section, where the temperature is slowly increased. In the preheating section, the slab is required to be heated to 600℃±20℃ within 60-70 minutes. In the first heating section, the slab is required to be heated to 1000℃±20℃ within 50-60 minutes. In the second heating section, the slab is required to be heated to the target temperature T±20℃ within 40-50 minutes. The target temperature T is maintained in the soaking section for 30-40 minutes. The target temperature T is determined by the actual steel composition according to the formula.

[0011] The result is calculated as T = 1290 - (60C% + 8Si% + 5Mn% + 25P% + 25S% + 5.5Cu% + 1.5Cr% + 3.5Ni% + 85N%).

[0012] (4) Rough rolling: 5 to 7 rolling passes, intermediate billet thickness 20 mm to 30 mm, and final rolling temperature not lower than 1040℃;

[0013] (5) Finish rolling: 5 to 7 rolling passes, with a final rolling temperature of not less than 980℃, a rolling coil temperature of not less than 750℃, and a hot-rolled coil thickness of 2.5mm to 5mm to obtain black-skinned coils.

[0014] Furthermore, it also includes step (6) annealing and pickling to obtain a white strip. In order to eliminate the linear high-temperature ferrite phase in the steel strip as much as possible, the annealing temperature is controlled at 1050℃~1150℃, and the grain size after annealing is required to be 7.5~8. The pickling process can refer to conventional 300 series stainless steel, such as 304.

[0015] Furthermore, in step (2) of the continuous casting process, a roller electromagnetic stirrer is used in the second cooling stage to reduce segregation during the solidification process of molten steel and ensure that the equiaxed crystal ratio of the billet is above 50%.

[0016] Furthermore, the slab size specifications in step (2) are required to be 180-200mm * 8000-9000mm. The cooled slab is then transferred to the heating furnace. If the slab is too short, it will reduce production efficiency and material yield; for medium-high chromium, nickel-saving, and high-nitrogen austenitic stainless steel, if the slab is too long, the temperature during the hot rolling process cannot be guaranteed.

[0017] This invention provides a high-quality continuously cast billet in step (4) by adjusting the process parameters of the continuous casting process. This billet has good surface quality, high equiaxed crystal ratio, and light compositional segregation. The billet will not have poor surface quality or edge cracks during rough rolling due to poor billet quality. The temperature settings in each stage of the heating furnace in step (3) ensure that the temperature inside and outside the billet is uniform. Because the heating rate is moderate, there will be no excessive local thermal stress that causes micro-cracks in the billet. The target temperature T in the second heating section is calculated by the actual composition according to the formula. By setting the target temperature, the continuously cast billet can avoid the high-temperature two-phase region temperature heating, avoid the continued growth of ferrite formed at high temperature, thereby reducing the risk of edge cracks. At the same time, it ensures that the high-nitrogen continuously cast billet receives the necessary heating temperature, so that it will not be unable to be rolled during the rolling process. Thus, through the adjustment and optimization of the whole process, the obtained medium-high chromium nickel-reducing austenitic stainless steel coil has no edge cracks, good surface quality, and only sporadic peeling defects or no peeling defects. Detailed Implementation

[0018] The specific implementation method of the high-chromium nickel-saving austenitic stainless steel strip of the present invention is described in detail below: Example 1

[0019] A method for producing medium-high chromium nickel-retaining austenitic stainless steel strip includes the following steps:

[0020] (1) Smelting; The process of smelting molten steel using electric furnace-argon-oxygen decarburization furnace (AOD)-LF furnace is adopted. The composition of the molten steel in the LF furnace meets the required composition. After standing for 10 minutes, the molten steel can be transferred to the continuous casting platform through the ladle. The mass fraction (%) of each element in the molten steel composition is as follows:

[0021] Element C Si Mn P S Cr Ni Cu N 0.045 0.3 8.0 0.04 0.002 18 1.5 1.5 0.23

[0022] (2) Continuous casting: The superheat of molten steel is required to be 28℃, the casting speed is 1.2m / min, the slab length is 8000mm, the water flow rate of the wide face of the continuous casting machine crystallizer is 2000l / min~2500l / min, and the water flow rate of the narrow face is 320l / min~350l / min; after the slab is cast and cut, it forms a slab.

[0023] (3) After the billet is cast and cut, it is immediately sent to the hot rolling furnace. The heating process in the furnace is slowly heated through the preheating section, the first heating section, the second heating section, and the soaking section. In the preheating section, the billet is heated to 580°C within 65 minutes. In the first heating section, the billet is heated to 1000°C within 55 minutes. In the second heating section, the billet is required to be heated to the target temperature of 1190°C within 40 to 50 minutes. In the soaking section, the target temperature of 1184°C is maintained for 35 minutes.

[0024] (4) Rough rolling: 5 rolling passes, intermediate billet thickness 250mm, final rolling temperature 1050℃;

[0025] (5) Finish rolling: 7 rolling passes, with a final rolling temperature of 1000℃, a rolling coiling temperature of 750℃, and a hot-rolled coil thickness of 3mm to obtain black-skinned coils.

[0026] (6) Annealing and pickling to obtain white strip. In order to eliminate the linear high-temperature ferrite phase in the steel strip as much as possible, the annealing temperature is controlled at 1080℃, and the grain size after annealing is required to be grade 7.5.

[0027] Examples 2-6 and Comparative Examples 1-4 use the same process route as the examples, the difference being the steel composition and process parameters of each step. The mass fraction (%) of each element in the steel composition is shown in Table 2.

[0028] Table 2

[0029]

[0030] Table 3

[0031]

[0032] The process parameters inside the heating furnace in step (3) are shown in Table 4:

[0033] Table 4

[0034]

[0035] The process parameters for steps 4-6 are shown in Table 5:

[0036] Table 5

[0037]

[0038] The steel coils produced according to the design requirements of Examples 1-6 above were free of edge cracks, had a good surface, and showed almost no peeling defects. However, in Comparative Example 1, the high superheat in the continuous casting process (exceeding the temperature range of 25℃~35℃) and the lack of electromagnetic stirring resulted in a slab dominated by columnar crystals with severe segregation. Furthermore, the heating furnace temperature and soaking zone temperature were excessively high, exceeding the range of T±20℃. These unreasonable continuous casting and heating processes ultimately led to a large amount of high-temperature ferrite within the slab, resulting in edge cracks during hot rolling and severe peeling defects after pickling. In Comparative Example 2, the excessively low superheat of the molten steel and poor slag formation resulted in scale formation on the surface of the continuously cast slab and slag inclusions under the slab surface. Although the parameters of subsequent processes were within the required range, serious defects such as heavy scale and mountain scale appeared on the surface of the steel coil after rolling. In Comparative Example 3, the heating temperature of the second stage and the temperature of the soaking stage both exceeded the required temperature. Although other process parameters were within the design range, the slab still developed edge cracks during hot rolling. It can be seen that the heating temperature of the second stage and the temperature of the soaking stage play a very important role in the smooth production process of this type of steel. In Comparative Example 4, the slab length exceeded the design requirements, and the temperature dropped significantly after rough rolling, falling below 1040℃. The slab was hard, resulting in the inability to roll during the finishing rolling process.

[0039] In addition, if the superheat of the molten steel in step (2) of this invention is too large, for example, above 35°C, strong cooling will result in very developed columnar crystals in the billet, causing severe compositional segregation. If weak cooling is used, steel leakage may occur. If the superheat is too low, for example, below 25°C, it is not conducive to the formation of protective slag in the crystallizer and will deteriorate the surface quality of the billet. Since the steel involved in this invention has a high N content, it is more prone to work hardening than ordinary austenitic stainless steel, making it difficult to roll smoothly. Therefore, the temperature design is very critical. Step (4) The design of the thickness of the intermediate billet in the roughing process and the design of the final rolling temperature should minimize the difficulty of the finishing process. Therefore, the final rolling temperature is designed to be relatively high, not lower than 1040℃. The temperature drop between the continuous casting billet leaving the furnace and the completion of the roughing process is fully considered to ensure the smooth progress of the subsequent finishing rolling. The steel grade involved in this invention usually has carbide precipitation at 600℃ to 980℃. The precipitation of carbides at the grain boundaries reduces the grain boundary bonding force and increases the risk of edge cracking. Therefore, step (5) requires the final rolling temperature to be not lower than 980℃. The coiling temperature is not lower than 750℃ based on the coil shape consideration.

[0040] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent process transformations made using the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for producing medium-high chromium nickel-saving austenitic stainless steel strip, characterized in that: It includes the following steps: (1) The medium-high chromium, high-nitrogen, nickel-saving austenitic stainless steel is smelted according to its chemical composition, wherein the weight percentage of the chemical composition of the medium-high chromium, high-nitrogen, nickel-saving austenitic stainless steel is as follows: C: 0.04~0.1%; Si ≤0.6%; Mn: 5.0~10.0%; P: ≤0.045%; S: ≤0.006%; Cr: 18.0~23.0%; Ni: 0.5~3.0%; Cu: 0.5~2.0%; N: 0.18~0.25%, with the balance being iron and unavoidable impurities; (2) Continuous casting: The superheat of molten steel is required to be 25-35℃, the casting speed is 1.0-1.2m / min, the equiaxed crystal ratio of the billet is required to reach more than 50%, the water flow rate of the wide face of the continuous casting machine crystallizer is 2000l / min-2500l / min, and the water flow rate of the narrow face is 320l / min-350l / min; after the billet is cast and cut, it forms a slab. (3) The slab obtained in step (2) is immediately sent to the hot rolling furnace. The heating process in the furnace involves a preheating section, a first heating section, a second heating section, and a soaking section, where the temperature is slowly increased. In the preheating section, the slab is required to be heated to 600℃±20℃ within 60-70 minutes. In the first heating section, the slab is required to be heated to 1000℃±20℃ within 50-60 minutes. In the second heating section, the slab is required to be heated to the target temperature T±20℃ within 40-50 minutes. In the soaking section, the target temperature T is maintained for 30-40 minutes. The target temperature T is determined by the actual steel composition according to the formula. The result is calculated as T = 1290 - (60C% + 8Si% + 5Mn% + 25P% + 25S% + 5.5Cu% + 1.5Cr% + 3.5Ni% + 85N%). (4) Rough rolling: 5 to 7 rolling passes, intermediate billet thickness 20 mm to 30 mm, and final rolling temperature not lower than 1040℃; (5) Finish rolling: 5 to 7 rolling passes, with a final rolling temperature of not less than 980℃, a rolling coil temperature of not less than 750℃, and a hot-rolled coil thickness of 2.5mm to 5mm to obtain black-skinned coils.

2. The method for producing medium-high chromium nickel-saving austenitic stainless steel strip according to claim 1, characterized in that: It also includes step (6) annealing and pickling to obtain white leather rolls.

3. The method for producing medium-high chromium nickel-saving austenitic stainless steel strip according to claim 1, characterized in that: In step (2), a roller electromagnetic stirrer is used in the secondary cooling stage of the continuous casting process.

4. The method for producing medium-high chromium nickel-saving austenitic stainless steel strip according to claim 1, characterized in that: The slab size requirements in step (2) are 180-200mm * 8000-9000mm.

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