A type of cold-rolled stainless steel sheet for painted doors

Abstract

This invention discloses a cold-rolled stainless steel sheet for painted doors, the chemical composition of which includes C: 0.060%~0.10%; Si: 0.30%~0.70%; Mn: 8.0%~10.0%; S: 0.000%~0.010%; P: 0.000%~0.060%; Cr: 15.0%~16.5%; Ni: 1.10%~1.50%; Cu: 1.0%~2.0%; N: 0.16%~0.20%, with the balance being Fe and unavoidable impurities; and the difference (MD) value between 580-520*C%-2*Si%-16*(Mn%+Cr%)-23*Ni%-300*N%-26*Cu% is controlled between 10 and 40; through composition design and adjustment of the cold rolling process, this invention ensures that the produced 200 series stainless steel cold-rolled sheet meets the requirements for stainless steel door panels for painted doors.

Description

Technical Field

[0001] This invention relates to the field of stainless steel smelting, and in particular to a cold-rolled stainless steel sheet for painted doors. Background Technology

[0002] Currently, the main materials used for entrance doors are galvanized cold-rolled steel sheets, zinc-iron alloy sheets, aluminum sheets, and 304 stainless steel cold-rolled sheets. The manufacturing process requires embossing the door panel to create complex surface patterns, followed by a baking paint process to produce an entrance door that is both aesthetically pleasing and practical.

[0003] Among them, galvanized cold-rolled steel sheets and zinc-iron alloy sheets have problems such as zinc vapor pollution and harm to employees' health due to welding during the manufacturing process, and short service life in coastal outdoor areas due to insufficient corrosion resistance. They are gradually being phased out. Meanwhile, aluminum sheets and 304 stainless steel cold-rolled sheets are expensive and lack market competitiveness.

[0004] Currently, low-nickel 200 series stainless steel is used for door panels, which is more cost-effective than aluminum plates and 304 stainless steel cold-rolled plates, and is more environmentally friendly and corrosion-resistant than galvanized cold-rolled plates and zinc-iron alloy plates. However, 200 series stainless steel also has many drawbacks: its high hardness and high surface smoothness make it difficult to polish, resulting in poor paint adhesion and easy paint film peeling; its high strength also leads to insufficient three-dimensionality of patterns during embossing, resulting in poor aesthetics. In addition, 200 series stainless steel induces a certain amount of martensitic phase transformation during the embossing deformation of door panels. The formation of martensite significantly increases the material strength (especially yield strength) and reduces plasticity and toughness, resulting in uneven embossing deformation, uneven door panel surface, and insufficient three-dimensionality of patterns, making it difficult to meet usage requirements.

[0005] In summary, there is an urgent need to develop a low-cost 200 series stainless steel material that combines excellent performance in pressing and forming complex patterns with good paint adhesion to meet the production requirements of stainless steel painted entrance doors. Summary of the Invention

[0006] The purpose of this invention is to provide a special stainless steel cold-rolled sheet for painted doors.

[0007] The technical solution to achieve the first objective of this invention is: a stainless steel cold-rolled sheet for painted doors, the chemical composition by weight percentage as follows: C: 0.060%~0.10%; Si: 0.30%~0.70%; Mn: 8.0%~10.0%; S: 0.000%~0.010%; P: 0.000%~0.060%; Cr: 15.0%~16.5%; Ni: 1.10%~1.50%; Cu: 1.0%~2.0%; N: 0.165~0.20%, with the balance being Fe and unavoidable impurities; simultaneously, the difference MD value of 580-520*C%-2*Si%-16*(Mn%+Cr%)-23*Ni%-300*N%-26*Cu% must be controlled between 10 and 40; The preparation process of the stainless steel cold-rolled sheet for painted doors includes a cold rolling process. During the cold rolling process, the roughness of the roughing roll is controlled at 1.0 to 1.4 μm, the roughness of the finishing roll is controlled at 1.2 to 1.4 μm, and the surface roughness Ra value of the slab is controlled at 0.70 to 1.2. In the cold rolling annealing stage of the cold rolling process, the annealing temperature is controlled at 1080 to 1120℃, and the annealing speed is controlled at 60 m / min to 80 m / min.

[0008] Furthermore, the manufacturing process of the stainless steel cold-rolled sheet for painted doors also includes leveling the cold-rolled coil after cold rolling and annealing, and controlling the unevenness within 10mm per 2000mm length of the cold-rolled coil through tension straightening. Sheet shape control methods typically include two types: leveling roller shape control and tension straightening. The inventors found that if leveling rollers are used for sheet shape control, the pressure of the leveling rollers on the surface of the cold-rolled sheet will reduce the surface roughness, which is detrimental to the paint adhesion effect. Therefore, tension straightening was chosen to control the sheet shape.

[0009] Furthermore, the following alloying elements are specified: C: 0.070%–0.09%; Si: 0.40%–0.60%; Mn: 8.5%–8.8%; S: 0.000%–0.010%; P: 0.000%–0.060%; Cr: 16.25%–16.5%; Ni: 1.10%–1.30%; Cu: 1.05%–1.15%; N: 0.17%–0.18%, which better suits the control requirements of large-scale production. Limiting the alloying elements in stainless steel to a narrow, preferred range compresses the compositional fluctuation range, reduces smelting and solidification segregation, stabilizes the microstructure and grain size, weakens the inter-element interaction sensitivity effect, significantly reduces batch variations, and meets the process tolerance, performance consistency, and controllable yield requirements of large-scale continuous industrial production.

[0010] In the technical solution described in this invention, the composition design principle of the stainless steel cold-rolled sheet for painted doors is as follows: 1) The Cr content is controlled at 15.0%–16.5%; Mn at 8.0%–10.0%; Ni at 1.10%–1.50%; and N at 0.16%–0.20%. Simultaneously, the MD value, which affects the product's microstructure and properties, is controlled between 10 and 40. This effectively reduces work hardening caused by increased deformation martensite formation during embossing, ensuring the strength and three-dimensionality of the door panel while maintaining good economic efficiency. The MD value is calculated using an empirical formula; an excessively low MD value is detrimental to cost reduction and good processing performance.

[0011] 2) Regarding the carbon element design, considering that carbon is an austenitic element, it can produce age-hardening and solid-solution strengthening effects to improve the strength of steel. However, the carbon content should not be too high, as excessive carbon content will lead to a decrease in the toughness of the steel. Furthermore, increased carbon content will promote the formation of twinned martensite, resulting in increased processing hardness, which is detrimental to the surface embossing of steel used in door manufacturing. Therefore, this invention controls the carbon content at 0.06%–0.10% to ensure that the steel has a high level of strength and toughness, while controlling the nitrogen content at 0.16%–0.20% to ensure the austenitic structure ratio and reduce the amount of precious metal Ni, which is also an austenitic element, thus lowering production costs.

[0012] 3) In the design of the Cu element, the special requirements of stainless steel for door manufacturing must be considered, which necessitates improving surface roughness and paint adhesion. This invention controls the Cu content in the steel to 1.0%–2.0% to reduce the surface hardness of the cold-rolled sheet. This facilitates the use of the work rolls to press the surface roughness of the cold-rolled sheet closer to that of the work rolls at the end of the cold rolling process, thereby achieving the goal of improving surface roughness.

[0013] To achieve suitable yield strength and elongation, and improve material plasticity, in addition to compositional factors, controlling the internal microstructure of the material is also crucial. The inventors discovered in their experiments that excessively low cold rolling annealing temperatures, insufficient annealing, and excessively small grain sizes lead to increased yield strength, causing localized rebound on the embossed surface and insufficient pattern definition. While increasing the cold rolling annealing temperature helps reduce yield strength, excessively high temperatures and excessive grain growth negatively impact material plasticity, i.e., elongation, leading to cold working deformation and cracking. This invention, by controlling the cold rolling annealing temperature at 1080–1120℃ and the annealing speed at 60 m / min–80 m / min, achieves sufficient solution treatment and recrystallization, thereby controlling the grain size between 8.0 and 9.0 grades. Precise grain size control results in a yield strength of 370–410 MPa and an elongation of 52%–58%, thus improving material plasticity and enhancing the three-dimensionality and aesthetics of the embossed pattern.

[0014] On the other hand, the surface roughness of conventional stainless steel cold-rolled sheets is typically controlled within 0.3μm. The roughness requirement for the finishing rolls is 0.2–0.3μm, and for the roughing rolls, it is 0.5–0.6μm. Stainless steel cold-rolled sheets produced using these process parameters do not require painting, thus eliminating concerns about paint peeling during high-temperature baking. However, for stainless steel cold-rolled sheets specifically designed for painted doors, the surface roughness must be controlled to achieve good paint adhesion and a beautiful, aesthetically pleasing finish after painting, meeting the requirements of door manufacturers. Methods for controlling surface roughness include wire drawing and sanding processes. While these reprocessing steps on the cold-rolled surface can achieve the required roughness, they add an extra step, increasing manufacturing costs and reducing production efficiency. Furthermore, this reprocessing increases surface hardness, affecting the subsequent embossing effect.

[0015] Through in-depth research and extensive experimental verification, the inventors discovered a close correlation between the surface roughness of cold-rolled steel sheets and the initial roughness of the raw material, the reduction rate of the cold-rolled steel sheets {(initial thickness of cold-rolled steel sheets - finished product thickness) / initial thickness of cold-rolled steel sheets}, the surface hardness of the cold-rolled steel sheet material, and the surface roughness of the rolls that contact the cold-rolled steel sheets. The rolls are in direct contact with the cold-rolled steel sheets, and their surface roughness is directly reflected on the surface of the cold-rolled steel sheets. The effect of this roughness reflection is related to the reduction rate of the cold-rolled steel sheets and the surface hardness of the material. The original surface roughness of the cold-rolled steel sheets after pickling is around 2.0 μm. A lower reduction rate helps maintain the original surface roughness, but cold-rolled steel sheets used for finished door panels require a certain strength, so the reduction rate cannot be too low. This invention controls the surface roughness of the finished product after cold rolling to 0.7–1.2 μm by controlling the Cu composition and the reduction rate. This ensures good paint adhesion without compromising the gloss of the painted surface due to excessive roughness.

[0016] Therefore, through component design and adjustment of the cold rolling process, this invention ensures that the produced 200 series stainless steel cold-rolled sheet has good embossing performance and surface paint adhesion, meeting the requirements for stainless steel door panels for painted doors. Detailed Implementation

[0017] The preferred embodiments of the present invention will be described in detail below. Example 1

[0018] A type of cold-rolled stainless steel sheet specifically for painted doors, with the following weight percentage of its chemical composition: C: 0.078%; Si: 0.43%; Mn: 8.65%; S: 0.006%; P: 0.045%; Cr: 16.32%; Ni: 1.12%; Cu: 1.17%; N: 0.176%, balance being Fe and unavoidable impurities; Wherein, MD value = 580 - 520 * C% - 2 * Si% - 16 * (Mn + Cr) - 23 * Ni% - 300 * N% - 26 * Cu; MD value is 30.1; The preparation process of the stainless steel cold-rolled sheet for painted doors includes a cold rolling process. During the cold rolling process, the roughness of the roughing roll is controlled at 1.1 μm and the roughness of the finishing roll is controlled at 1.2 μm. In the cold rolling annealing stage of the cold rolling process, the annealing temperature is controlled at 1100℃ and the annealing speed is controlled at 78 m / min. The cold coil after cold rolling annealing is leveled by tension straightening.

[0019] The performance indicators of the finished cold-rolled sheet obtained in Example 1 are as follows: Mechanical properties: yield strength 395MPa, tensile strength 782MPa, elongation 57%.

[0020] Surface roughness: Measured Ra values ​​along the width of the plate are 0.76, 0.78, and 0.87 μm.

[0021] 3) Plate shape: The unevenness reaches 5mm within a length of 2000mm.

[0022] The three-dimensionality of the pattern in the pressed patterned door panels, as well as the warping and unevenness of the panel surface, all met the requirements of the customers for painted doors. During the production of the finished painted doors, the door panels did not warp or peel off the paint, thus meeting the needs of painted door users.

[0023] Examples 2-6

[0024] Examples 2-6 use the same preparation process as Example 1, the difference being the different component design and process parameters.

[0025] Comparative Examples 1 and 2

[0026] This invention also provides Comparative Example 1 and Comparative Example 2. Comparative Example 1 uses conventional 200 series stainless steel J3 (BN1D3) with the same composition and preparation process as Comparative Example 1. Comparative Example 2 has the same composition as Comparative Example 1, but different component proportions and a different preparation process. The weight percentages (in %) of each chemical component in the composition design of Examples 1-6, Comparative Examples 1 and 2 are shown in Table 1, and the balance is Fe and unavoidable impurity elements. The key process parameters are shown in Table 2.

[0027] Table 1

[0028] Table 2

[0029] The performance indicators of the finished cold-rolled sheets obtained in Examples 1-6 and Comparative Examples 1-2 are shown in Table 3: Table 3

[0030] The stainless steel cold-rolled sheets for painted doors obtained in Examples 1-6 of this invention all meet the requirements for pattern three-dimensionality, warping, and unevenness of the pressed patterned door panels. During the production of finished painted doors, the door panels do not warp or peel off the paint, thus meeting the needs of painted door users.

[0031] Comparative Example 1 uses J3 (BN1D3), a common deep-drawing stainless steel for 200 series stainless steel currently on the market, with a Cu content of 0.53%. It adopts the same preparation process as Example 1. Its mechanical properties are 20% higher in strength than those of Example 1. However, due to the high strength of the material, the roughness Ra value does not reach the lower limit of 0.7 required for painted doors. When the actual material is pressed into a patterned door panel, the paint adhesion is poor, and some peeling occurs. In addition, as shown in Table 3, due to the higher overall yield and tensile strength of the material, the elongation of Comparative Example 1 is not as good as that of Example 1. When the door panel is actually embossed, the three-dimensionality of the pattern in Comparative Example 1 does not meet the requirements. In particular, after the complexity of the embossed pattern is increased, cracking occurs, indicating that the cold working performance of the material is insufficient.

[0032] Comparative Example 2 used the same composition system as Example 1, but the preparation process differed, mainly in the control of the surface roughness of the rolls. Comparative Example 2 used conventional cold rolling roll surface roughness and employed leveling rolls for shaping after cold rolling. As shown in Table 3, although the overall mechanical properties of the material in Comparative Example 2 were relatively close to those in Example 1, the surface roughness Ra value of the finished product was significantly reduced to below 0.3 due to the difference in roll roughness and leveling process. This resulted in insufficient paint adhesion, paint peeling, and an increase in material strength of nearly 30 MPa after leveling, which was also unfavorable for pressing and molding complex patterns on painted doors.

[0033] Furthermore, the cost of the stainless steel cold-rolled sheet obtained by this invention is about 7,800 yuan / ton, which is about 4,200 yuan / ton lower than the cost of traditional 304 stainless steel (about 12,000 yuan / ton). This results in lower production costs, significant economic benefits, and a prominent cost advantage.

[0034] 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 cold-rolled stainless steel sheet specifically for painted doors, characterized in that: Its chemical composition by weight percentage is as follows: C: 0.060%~0.10%; Si: 0.30%~0.70%; Mn: 8.0%~10.0%; S：0.000%～0.010%； P：0.000%～0.060%； Cr: 15.0%~16.5%, Ni: 1.10%~1.50%; Cu: 1.0%~2.0%; N: 0.16%~0.20%, with the balance being Fe and unavoidable impurities; Simultaneously, the difference (MD) value between 580-520*C%-2*Si%-16*(Mn%+Cr%)-23*Ni%-300*N%-26*Cu% must be controlled between 10 and 40. The preparation process of the stainless steel cold-rolled sheet for painted doors includes a cold rolling process. During the cold rolling process, the roughness of the roughing roll is controlled at 1.0 to 1.4 μm, and the roughness of the finishing roll is controlled at 1.2 to 1.4 μm, ensuring that the surface roughness of the cold-rolled sheet reaches Ra value control of 0.70 to 1.

2. In the cold rolling annealing stage of the cold rolling process, the annealing temperature is controlled at 1080 to 1120℃, and the annealing speed is controlled at 60 m / min to 80 m / min.

2. The stainless steel cold-rolled sheet for painted doors according to claim 1, characterized in that: The manufacturing process of the stainless steel cold-rolled sheet for painted doors also includes leveling the cold-rolled coil after cold rolling and annealing, and controlling the unevenness within 10mm per 2000mm length of the cold-rolled coil through tension straightening.

3. The stainless steel cold-rolled sheet for painted doors according to claim 1, characterized in that: Its chemical composition by weight percentage is as follows: C: 0.070%–0.09%; Si: 0.40%–0.60%; Mn: 8.5%–8.8%; S：0.000%～0.010%； P: 0.000%~0.060%; Cr: 16.25%~16.5%, Ni: 1.10%~1.30%; Cu: 1.05%~1.15%; N: 0.17%~0.18%.

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