A method for reducing transverse creases in cold-rolled ferritic stainless steel sheets

Abstract

This invention discloses a method for reducing transverse wrinkles in cold-rolled ferritic stainless steel sheets, comprising: controlling the annealing time of hot-rolled coils; for hot-rolled coils with a thickness ≥3.5mm, the annealing time is controlled to be 2-8 hours longer than the annealing time for hot-rolled coils with a thickness <3.5mm; controlling the total deformation rate of cold rolling; for cold-rolled sheets with a thickness of 1.0mm-1.5mm, the total deformation rate is 75%-80%; for cold-rolled sheets with a thickness of 1.51mm-2.0mm, the total deformation rate is 70%-75%; for cold-rolled sheets with a thickness of 2... For cold-rolled sheets with a thickness of 0.01mm to 2.5mm, the total deformation rate is 65% to 70%; for cold-rolled sheets with a thickness of 2.51mm to 3.5mm, the total deformation rate is 60% to 65%; for cold-rolled sheets with a thickness of 3.51mm and above, the total deformation rate is 40% to 60%. In terms of finishing process control, for cold-rolled sheets with a thickness of 1.0mm to 1.5mm, the leveling process requires 2 passes; for cold-rolled sheets with a thickness of 1.51mm and above, the leveling process requires 1 pass. The tension of the leveling unit is controlled at 8 to 16 tons. This invention reduces the incidence of transverse crease defects in ferritic stainless steel cold-rolled sheets to below 1.2%, thereby improving the surface quality of the product.

Description

Technical Field

[0001] This invention belongs to the field of ferritic stainless steel cold-rolled sheet production technology, and more specifically relates to a method for reducing transverse creases in ferritic stainless steel cold-rolled sheets. Background Technology

[0002] With increasingly fierce competition in the stainless steel market in recent years, users' requirements for product quality have been continuously rising, especially in the home appliance and decoration markets where the requirements for product surface quality are more stringent. Ferritic stainless steel, represented by 430, often exhibits transverse crease defects on the surface of cold-rolled sheets during cold rolling or leveling. These creases are 90° to the rolling direction and are irregularly distributed along the width of the stainless steel cold-rolled sheet. Sometimes the creases run the entire width of the sheet, and sometimes they appear within a certain range; they are visible to the naked eye and can be felt by touch. There is a clear correlation between the thickness of the stainless steel cold-rolled sheet and the crease defect. Statistics show that the incidence of transverse creases is approximately 4.5% for stainless steel cold-rolled sheets with a thickness ≥1.0mm, and approximately 1.25% for those with a thickness <1.0mm. The presence of transverse crease defects restricts further improvement in the quality of ferritic stainless steel cold-rolled sheets and fails to meet market requirements.

[0003] In existing technologies, research on stripe-like defects on the surface of ferritic stainless steel mainly focuses on "Lisin lines" parallel to the rolling direction. Control measures primarily involve increasing the proportion of equiaxed grains in continuously cast billets, reducing the finishing rolling temperature, and increasing the rolling deformation rate. The aim is to improve the γ-fiber texture, increase the plastic strain ratio (r), and enhance the material's wrinkle resistance. Additionally, existing technologies also address the "orange peel" defect on the surface of cold-rolled ferritic stainless steel sheets. This "orange peel" defect is caused by excessively large grain size and mixed grains. Control measures primarily involve optimizing the cold wire heat treatment process to obtain fine, uniform grains. However, transverse folding defects in cold-rolled ferritic stainless steel sheets differ significantly from the aforementioned surface defects in appearance and size, and their formation mechanisms are completely different. Currently, there are no effective solutions for transverse folding defects in cold-rolled ferritic stainless steel sheets. Summary of the Invention

[0004] To address the aforementioned technical problems in the prior art, the present invention provides a method for reducing transverse wrinkles in cold-rolled ferritic stainless steel sheets, the method comprising:

[0005] (1) Annealing time control of hot-rolled coils: The stainless steel hot-rolled coils used as raw materials for ferritic stainless steel cold-rolled sheets are annealed in a bell-type furnace. For hot-rolled coils with a thickness ≥ 3.5 mm, the annealing time is controlled to be 2 to 8 hours longer than that of hot-rolled coils with a thickness < 3.5 mm.

[0006] (2) Control of total cold rolling deformation rate: For ferritic stainless steel cold-rolled sheets with a finished thickness of 1.0mm to 1.5mm, the total cold rolling deformation rate is controlled at 75% to 80%; for ferritic stainless steel cold-rolled sheets with a finished thickness of 1.51mm to 2.0mm, the total cold rolling deformation rate is controlled at 70% to 75%; for ferritic stainless steel cold-rolled sheets with a finished thickness of 2.01mm to 2.5mm, the total cold rolling deformation rate is controlled at 65% to 70%; for ferritic stainless steel cold-rolled sheets with a finished thickness of 2.51mm to 3.5mm, the total cold rolling deformation rate is controlled at 60% to 65%; for ferritic stainless steel cold-rolled sheets with a finished thickness of 3.51mm and above, the total cold rolling deformation rate is controlled at 40% to 60%.

[0007] (3) Finishing process control: For ferritic stainless steel cold-rolled sheets with a finished thickness of 1.0mm to 1.5mm, the leveling passes are adjusted to 2 passes; for ferritic stainless steel cold-rolled sheets with a finished thickness of 1.51mm and above, the leveling passes are adjusted to 1 pass; the tension of the leveling unit is controlled to 8 to 16 tons.

[0008] Furthermore, the above-mentioned method for reducing transverse creases in cold-rolled ferritic stainless steel sheets also includes control of the chemical composition of the steel grade, wherein the chemical composition of the ferritic stainless steel is controlled as follows by mass percentage: C≤0.040%, Si≤1.0%, Mn≤1.0%, P≤0.040%, S≤0.040%, Cr: 11.0%~20.0%, Ni≤0.75%, N≤0.040%, Nb≤1.0%, Ti≤1.0%, Mo≤1.0%, with the remainder being Fe and unavoidable impurities.

[0009] As a specific implementation method, the above-mentioned method for reducing transverse creases in cold-rolled ferritic stainless steel sheets is applied to medium-chromium ferritic stainless steel. The finished thickness of the cold-rolled ferritic stainless steel sheet is 1.2 mm, and the total cold-rolling deformation rate is 76%. A 5.0 mm thick stainless steel hot-rolled coil is selected as the raw material for the cold-rolled ferritic stainless steel sheet. In the annealing time control of the hot-rolled coil, after reaching the set holding temperature of 850°C, the holding time is increased to 24 hours. In the finishing process control, the leveling passes are 2, and the tension of the leveling unit is set to 13 tons. The typical chemical composition of the medium-chromium ferritic stainless steel, by mass percentage, is:

[0010] C Si Mn P S Cr N Nb Ti Mo 0.040 0.32 0.37 0.028 0.001 16.15 0.038 ≤0.01 ≤0.01 ≤0.01 0.035 0.31 0.34 0.025 0.001 16.05 0.032 ≤0.01 ≤0.01 ≤0.01 0.038 0.29 0.35 0.032 0.001 16.24 0.028 ≤0.01 ≤0.01 ≤0.01 0.036 0.30 0.41 0.030 0.001 17.21 0.034 ≤0.01 ≤0.01 0.12

[0011] As a specific implementation method, the above-mentioned method for reducing transverse creases in cold-rolled ferritic stainless steel sheets is applied to low-chromium ferritic stainless steel. The finished thickness of the cold-rolled ferritic stainless steel sheet is 2.0 mm, and the total cold-rolling deformation rate is 71%. A 6.85 mm thick hot-rolled stainless steel coil is selected as the raw material for the cold-rolled ferritic stainless steel sheet. In the annealing time control of the hot-rolled coil, after reaching the set holding temperature of 830°C, the holding time is increased to 30 hours. In the finishing process control, the leveling pass is 1 pass, and the tension of the leveling unit is set to 15 tons. The typical chemical composition of the low-chromium ferritic stainless steel, by mass percentage, is:

[0012] C Si Mn P S Cr N Nb Ti Mo 0.032 0.32 0.31 0.028 0.001 13.18 0.031 ≤0.01 ≤0.01 ≤0.01 0.025 0.35 0.38 0.029 0.001 13.25 0.028 ≤0.01 ≤0.01 ≤0.01 0.035 0.25 0.34 0.026 0.001 13.52 0.032 ≤0.01 ≤0.01 0.15

[0013] The method for reducing transverse wrinkles in cold-rolled ferritic stainless steel sheets of the present invention has the following advantages and beneficial effects:

[0014] This invention targets ferritic stainless steel cold-rolled sheets with a thickness ≥1.0mm. By optimizing the hot-rolled coil annealing process, cold-rolling process, and finishing process, the incidence of transverse crease defects in ferritic stainless steel cold-rolled sheets with a thickness ≥1.0mm can be reduced from 4.5% in the prior art to below 1.2%, thereby achieving the goal of reducing the proportion of transverse crease defects and improving the surface quality of the product. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0016] The method for reducing transverse creases in cold-rolled ferritic stainless steel sheets proposed in this invention is particularly suitable for cold-rolled ferritic stainless steel sheets with a thickness ≥1.0mm. By optimizing the hot-rolled coil annealing process, cold-rolling process, and finishing process, the method aims to reduce the proportion of transverse crease defects and improve the surface quality of the product. Specifically, the method for reducing transverse creases in cold-rolled ferritic stainless steel sheets of this invention includes:

[0017] (1) Control of annealing time of hot rolled coil: The stainless steel hot rolled coil, which is used as raw material for ferritic stainless steel cold rolled plate, is annealed in a bell furnace. For hot rolled coil with a thickness ≥ 3.5 mm, the annealing time is controlled to be 2 to 8 hours longer than that of hot rolled coil with a thickness < 3.5 mm.

[0018] In existing bell-type furnace annealing processes, the annealing time for hot-rolled stainless steel coils of different thicknesses is the same. This results in a lower heat treatment effect for thicker hot-rolled coils compared to thinner ones under the same annealing time, leading to poorer carbonitride and microstructure uniformity and affecting material deformation. Therefore, in this invention, the annealing time for hot-rolled coils with a thickness ≥3.5mm is controlled to be 2–8 hours longer than that for coils with a thickness <3.5mm. This improves the uniformity of material microstructure and properties, and further reduces the occurrence of transverse crease defects in ferritic stainless steel cold-rolled sheets.

[0019] (2) Control of total cold rolling deformation rate: For ferritic stainless steel cold-rolled sheets with a finished thickness of 1.0 mm to 1.5 mm, the total cold rolling deformation rate is controlled at 75% to 80%; for ferritic stainless steel cold-rolled sheets with a finished thickness of 1.51 mm to 2.0 mm, the total cold rolling deformation rate is controlled at 70% to 75%; for ferritic stainless steel cold-rolled sheets with a finished thickness of 2.01 mm to 2.5 mm, the total cold rolling deformation rate is controlled at 65% to 70%; for ferritic stainless steel cold-rolled sheets with a finished thickness of 2.51 mm to 3.5 mm, the total cold rolling deformation rate is controlled at 60% to 65%; and for ferritic stainless steel cold-rolled sheets with a finished thickness of 3.51 mm and above, the total cold rolling deformation rate is controlled at 40% to 60%.

[0020] The greater the total cold rolling deformation rate, the greater the deformation energy storage. During the subsequent annealing process of ferritic stainless steel cold-rolled sheets, the material is more prone to recrystallization, resulting in better microstructure uniformity. Therefore, in this invention, according to the above-described method, the corresponding total cold rolling deformation rate is precisely controlled based on the different finished thicknesses of the ferritic stainless steel cold-rolled sheets, thereby reducing the generation of transverse crease defects in the ferritic stainless steel cold-rolled sheets.

[0021] (3) Finishing process control: For ferritic stainless steel cold-rolled sheets with a finished thickness of 1.0mm to 1.5mm, the leveling passes are adjusted from the conventional 3 passes to 2 passes. For ferritic stainless steel cold-rolled sheets with a finished thickness of 1.51mm and above, the leveling passes are adjusted to 1 pass. The tension of the leveling unit is controlled at 8 to 16 tons.

[0022] To ensure good sheet shape after annealing, ferritic stainless steel cold-rolled sheets generally require leveling or tension straightening. During leveling, the ferritic stainless steel cold-rolled sheets are subjected to tensile stress from the tension of the leveling or tension straightening unit and compressive stress from the work rolls. Under these tensile and compressive stresses, the ferritic stainless steel cold-rolled sheets undergo 0.5% to 3% plastic deformation. Research shows that yield elongation is reduced under lower tensile and compressive stresses. Therefore, this invention, by controlling the tension of the leveling unit and the number of leveling passes, reduces the number of leveling passes and lowers the tension of the leveling unit by 15% to 25% compared to conventional finishing processes, thereby further reducing the occurrence of transverse crease defects in ferritic stainless steel cold-rolled sheets.

[0023] Furthermore, the method for reducing transverse creases in cold-rolled ferritic stainless steel sheets of the present invention also includes control of the chemical composition of the steel grade, wherein the chemical composition of the ferritic stainless steel is controlled as follows by mass percentage: C≤0.040%, Si≤1.0%, Mn≤1.0%, P≤0.040%, S≤0.040%, Cr: 11.0%~20.0%, Ni≤0.75%, N≤0.040%, Nb≤1.0%, Ti≤1.0%, Mo≤1.0%, with the remainder being Fe and unavoidable impurities.

[0024] Annealed ferritic stainless steel cold-rolled sheets exhibit a distinct yield plateau and clearly defined upper and lower yield points. The appearance of these yield points is due to the Cotillard atmosphere formed by C and N atoms, which hinders dislocation movement. The presence of these yield points causes textures distributed along a specific direction to appear on the surface of the ferritic stainless steel cold-rolled sheet once deformation begins in a localized area. Therefore, in this invention, the C and N elements in the ferritic stainless steel are controlled to reduce the hindrance of C and N atoms to dislocation movement, thereby further reducing the occurrence of transverse crease defects in the ferritic stainless steel cold-rolled sheet.

[0025] The following detailed description, in conjunction with specific embodiments, illustrates the method of reducing transverse wrinkles in ferritic stainless steel cold-rolled sheets according to the present invention.

[0026] Actual example 1

[0027] The method for reducing transverse folds in ferritic stainless steel cold-rolled sheets according to Embodiment 1 of the present invention is applied to medium-chromium ferritic stainless steel with the chemical composition shown in Table 1 below. The finished thickness of the ferritic stainless steel cold-rolled sheet is 1.2 mm, and the total cold rolling deformation rate is 76%. Therefore, a stainless steel hot-rolled coil with a thickness of 5.0 mm is selected as the raw material for the ferritic stainless steel cold-rolled sheet.

[0028] Table 1 shows typical chemical compositions (wt%) of chromium ferritic stainless steel.

[0029] C Si Mn P S Cr N Nb Ti Mo 0.040 0.32 0.37 0.028 0.001 16.15 0.038 ≤0.01 ≤0.01 ≤0.01 0.035 0.31 0.34 0.025 0.001 16.05 0.032 ≤0.01 ≤0.01 ≤0.01 0.038 0.29 0.35 0.032 0.001 16.24 0.028 ≤0.01 ≤0.01 ≤0.01 0.036 0.30 0.41 0.030 0.001 17.21 0.034 ≤0.01 ≤0.01 0.12

[0030] In the method for reducing transverse folds in ferritic stainless steel cold-rolled sheet according to Embodiment 1 of the present invention, in the control of the annealing time of hot-rolled coil, a bell-type furnace is used to anneal the stainless steel hot-rolled coil. After reaching the set holding temperature of 850°C, the holding time is increased from 20 hours in the conventional process to 24 hours.

[0031] In the method for reducing transverse folds in ferritic stainless steel cold-rolled sheet in Embodiment 1 of the present invention, in the finishing process control, a leveling unit is used to improve the shape of the ferritic stainless steel cold-rolled sheet, the leveling passes are 2, and the tension of the leveling unit is set to 13 tons, which is 20% lower than the conventional finishing process of the prior art.

[0032] Using the method for reducing transverse folds in ferritic stainless steel cold-rolled sheets as described in Example 1, a total of 85 rolls of medium-chromium ferritic stainless steel cold-rolled sheets were produced, of which 1 roll had transverse fold defects, and the proportion of transverse fold defects was only 1.18%.

[0033] Actual example 2

[0034] The method for reducing transverse folds in ferritic stainless steel cold-rolled sheets according to Embodiment 2 of the present invention is applied to low-chromium ferritic stainless steel with the chemical composition shown in Table 2 below. The finished thickness of the ferritic stainless steel cold-rolled sheet is 2.0 mm, and the total cold rolling deformation rate is 71%. Therefore, a stainless steel hot-rolled coil with a thickness of 6.85 mm is selected as the raw material for the ferritic stainless steel cold-rolled sheet.

[0035] Table 2 Typical chemical composition of low-chromium ferritic stainless steel (mass percentage, wt%)

[0036] C Si Mn P S Cr N Nb Ti Mo 0.032 0.32 0.31 0.028 0.001 13.18 0.031 ≤0.01 ≤0.01 ≤0.01 0.025 0.35 0.38 0.029 0.001 13.25 0.028 ≤0.01 ≤0.01 ≤0.01 0.035 0.25 0.34 0.026 0.001 13.52 0.032 ≤0.01 ≤0.01 0.15

[0037] In the method for reducing transverse folds in ferritic stainless steel cold-rolled sheet in Embodiment 2 of the present invention, in the control of the annealing time of hot-rolled coil, a bell-type furnace is used to anneal the stainless steel hot-rolled coil. After reaching the set holding temperature of 830°C, the holding time is increased from 24 hours in the conventional process to 30 hours.

[0038] In the method for reducing transverse folds in ferritic stainless steel cold-rolled sheet in Embodiment 2 of the present invention, in the finishing process control, a leveling unit is used to improve the shape of the ferritic stainless steel cold-rolled sheet, the leveling pass is 1 pass, and the tension of the leveling unit is set to 15 tons, which is 18% lower than the conventional finishing process of the prior art.

[0039] Using the method for reducing transverse folds in ferritic stainless steel cold-rolled sheets in Example 2, a total of 192 rolls of medium-chromium ferritic stainless steel cold-rolled sheets were produced, of which 2 rolls had transverse fold defects, and the proportion of transverse fold defects was only 1.04%.

[0040] In summary, the method for reducing transverse creases in ferritic stainless steel cold-rolled sheets of the present invention targets ferritic stainless steel cold-rolled sheets with a thickness ≥1.0mm. By optimizing the hot-rolled coil annealing process, cold-rolling process, and finishing process, the incidence of transverse crease defects in ferritic stainless steel cold-rolled sheets with a thickness ≥1.0mm can be reduced from 4.5% in the prior art to below 1.2%, thereby achieving the purpose of reducing the proportion of transverse crease defects and improving the surface quality of the product.

[0041] It should be noted that, in this document, the term "comprising" or any other variation thereof is intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0042] It should also be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the present invention.

Claims

1. A method for reducing transverse creases in cold-rolled ferritic stainless steel sheets, characterized in that, The method includes: (1) Control of annealing time of hot rolled coil: The stainless steel hot rolled coil, which is used as raw material for ferritic stainless steel cold rolled plate, is annealed in a bell furnace. For hot rolled coil with a thickness ≥ 3.5 mm, the annealing time is controlled to be 2 to 8 hours longer than that of hot rolled coil with a thickness < 3.5 mm. (2) Control of total cold rolling deformation rate: For ferritic stainless steel cold-rolled sheets with a finished thickness of 1.0mm~1.5mm, the total cold rolling deformation rate is controlled at 75%~80%; for ferritic stainless steel cold-rolled sheets with a finished thickness of 1.51mm~2.0mm, the total cold rolling deformation rate is controlled at 70%~75%; for ferritic stainless steel cold-rolled sheets with a finished thickness of 2.01mm~2.5mm, the total cold rolling deformation rate is controlled at 65%~70%; for ferritic stainless steel cold-rolled sheets with a finished thickness of 2.51mm~3.5mm, the total cold rolling deformation rate is controlled at 60%~65%; for ferritic stainless steel cold-rolled sheets with a finished thickness of 3.51mm and above, the total cold rolling deformation rate is controlled at 40%~60%. (3) Finishing process control: For ferritic stainless steel cold-rolled sheets with a finished thickness of 1.0mm~1.5mm, the leveling passes are adjusted to 2 passes; for ferritic stainless steel cold-rolled sheets with a finished thickness of 1.51mm and above, the leveling passes are adjusted to 1 pass; the tension of the leveling unit is controlled at 8~16 tons. The method also includes the control of the chemical composition of the steel: the chemical composition mass percentage of the ferritic stainless steel is controlled as follows: C≤0.040%, Si≤1.0%, Mn≤1.0%, P≤0.040%, S≤0.040%, Cr: 11.0%~20.0%, Ni≤0.75%, N≤0.040%, Nb≤1.0%, Ti≤1.0%, Mo≤1.0%, and the remainder is Fe and unavoidable impurities.

2. The method for reducing transverse wrinkles in cold-rolled ferritic stainless steel sheets as described in claim 1, characterized in that: The method is applied to medium-chromium ferritic stainless steel. The finished thickness of the ferritic stainless steel cold-rolled sheet is 1.2 mm, and the total cold rolling deformation rate is 76%. A stainless steel hot-rolled coil with a thickness of 5.0 mm is selected as the raw material for the ferritic stainless steel cold-rolled sheet. In the control of annealing time for hot-rolled coils, after reaching the set holding temperature of 850℃, the holding time is increased to 24 hours; In the finishing process control, the leveling passes are 2, and the tension of the leveling unit is set to 13 tons. The typical chemical composition of the medium-chromium ferritic stainless steel, by mass percentage, is as follows: 。 3. The method for reducing transverse wrinkles in cold-rolled ferritic stainless steel sheets as described in claim 1, characterized in that: The method is applied to low-chromium ferritic stainless steel. The finished thickness of the ferritic stainless steel cold-rolled sheet is 2.0 mm, and the total cold rolling deformation rate is 71%. A stainless steel hot-rolled coil with a thickness of 6.85 mm is selected as the raw material for the ferritic stainless steel cold-rolled sheet. In the control of annealing time for hot-rolled coils, after reaching the set holding temperature of 830℃, the holding time is increased to 30 hours; In the finishing process control, the leveling pass is 1 pass, and the tension of the leveling unit is set to 15 tons. The typical chemical composition of the low-chromium ferritic stainless steel, by mass percentage, is as follows: 。

Images