High-grade decorative ferritic stainless steel and manufacturing method thereof

Abstract

A high-grade decorative ferritic stainless steel and its manufacturing method. The chemical composition of the ferritic stainless steel is as follows: C: ≤0.02wt%, Si: ≤0.1wt%, Mn: ≤0.1wt%, P: ≤0.04wt%, S: ≤0.005wt%, Cr: 18-20wt%, Mo: 1.2-1.5wt%, N: ≤0.02wt%, Nb: 0.4-0.6wt%, Al: 0.1-0.2wt%, O: ≤0.002wt%, with the remainder being Fe and unavoidable impurities. The manufacturing method comprises: 1) Steelmaking and Casting: Steelmaking is performed using an electric furnace-AOD-VOD process according to the above chemical composition. The soft stirring time of the molten steel is greater than 30 minutes, the molten steel stabilization time is greater than 20 minutes, the molten steel superheat is 40-50°C, the continuous casting billet casting speed is ≤1.0m / min, and the billet surface is ground and removed by 3mm. Compared with the existing technology, the present invention can improve the surface quality of the material.

Description

Technical Field

[0001] The invention belongs to the technical field of stainless steel manufacturing, and particularly relates to high-grade decorative ferritic stainless steel and a manufacturing method thereof. Background Art

[0002] Ferritic stainless steel has broad development prospects in my country. From the perspective of consumption, my country is the world's largest hardware manufacturing and export base, as well as a major producer of home appliances. The hardware and home appliance industries are major users of ferritic stainless steel (such as grades 410 and 430). For example, the drum of a washing machine uses 430 ferritic stainless steel. With the development of the domestic automobile industry, automobile exhaust systems are also an important area for the use of ferritic stainless steel (such as grades 409 and 439). Ferritic stainless steel also has a huge application space in indoor and outdoor decoration, such as elevators (such as grades 443). In industrial fields such as petrochemicals and environmental protection, which require better corrosion resistance, ultra-pure ferritic stainless steel will become the choice in these fields (such as grades 444). In housing construction, especially roofing, the use of ferritic stainless steel is more advantageous (such as grades 445).

[0003] Ultra-pure medium-chromium ferritic stainless steel (Cr content of approximately 16-20% by weight) is a newly emerging steel grade. This steel improves its corrosion resistance by reducing carbon and nitrogen contents, and enhances its cold working and welding properties by adding microalloying elements (such as niobium, titanium, and vanadium) that stabilize carbon and nitrogen. These trace elements also improve the surface quality after cold working, for example, by reducing wrinkling after cold forming. This steel boasts superior corrosion resistance and cold forming properties compared to 304 austenitic stainless steel, while also offering low cost, making it widely used in industries such as elevators, automobiles, finished goods, and electrical appliances. Currently, many steel companies both domestically and internationally are mass-producing this steel, under designations such as 436. This steel is primarily used to replace more expensive austenitic stainless steel and conventional medium-chromium ferritic stainless steel, which has less corrosion resistance. It has been adopted in numerous industries and enjoys significant market demand.

[0004] In some applications, such as building decorative panels and automotive exterior parts, the requirements for material surface quality and corrosion resistance are becoming increasingly higher, requiring steel companies to continuously develop new products. However, ultra-pure medium-chromium ferritic stainless steel often contains large, hard oxide inclusion particles. These particles easily fall off the surface of the material, causing defects such as peeling, which affects the surface quality. Especially for cold-rolled thin plates, ultra-pure medium-chromium ferritic stainless steel often suffers from surface defects such as peeling, and the surface brightness and smoothness also decrease. However, in environments with very high surface quality requirements, such as building decorative panels and automotive exterior parts, such surface defects are not allowed to occur. Therefore, for ferritic stainless steel with very high surface quality requirements, it is necessary to minimize the number of oxide inclusions, and reducing the oxygen content is one of the most effective control methods.

[0005] Table 1 lists the chemical composition of previous ultra-pure medium-chromium ferritic stainless steel patents. It can be seen from the table that the carbon and nitrogen elements in most patents are controlled at a low level, and contain a certain amount of elements such as niobium, titanium, vanadium and aluminum that stabilize carbon and nitrogen. Some patents also add a certain amount of nickel and copper. Such chemical composition designs are all intended to improve the corrosion resistance and mechanical properties of the material. However, none of the patents in Table 1 give the requirements for controlling the oxygen content in the alloy, nor do they give methods for controlling the lower oxygen content. For example, the first patent in Table 1 adds a higher niobium element, which has certain benefits in improving the mechanical properties of the product, but the patent does not impose strict restrictions on the oxygen content, so it is difficult to ensure high surface quality requirements. In addition, those skilled in the art used to believe that adding elements such as aluminum and titanium to stainless steel would increase the number of oxide inclusions, increase defects such as peeling on the surface of the material, and thus reduce the surface quality of the material.

[0006] Table 1 Chemical composition of conventional ultrapure medium chromium ferritic stainless steel (wt%)

[0007] Summary of the Invention

[0008] The first technical problem to be solved by the present invention is to provide a high-grade decorative ferritic stainless steel in view of the current status of the above-mentioned prior art, so as to improve the surface quality of the material.

[0009] The second technical problem to be solved by the present invention is to provide a method for manufacturing the above-mentioned high-grade decorative ferritic stainless steel.

[0010] The technical solution adopted by the present invention to solve the first technical problem mentioned above is: a high-grade decorative ferritic stainless steel, characterized in that the chemical composition of the ferritic stainless steel is: C: ≤0.02wt%, Si: ≤0.1wt%, Mn: ≤0.1wt%, P: ≤0.04wt%, S: ≤0.005wt%, Cr: 18-20wt%, Mo: 1.2-1.5wt%, N: ≤0.02wt%, Nb: 0.4-0.6wt%, Al: 0.1-0.2wt%, O: ≤0.002wt%, and the remainder is Fe and unavoidable impurities.

[0011] Carbon: It is an element that increases the strength of the alloy, but it should not be too much, otherwise the material's plasticity and corrosion resistance will be lost. It is best to control it within 0.02%.

[0012] Silicon: Considering that excessive silicon content will reduce plasticity, it should be kept as low as possible, and it is more appropriate to control it below 0.1%.

[0013] Manganese: Considering that manganese will reduce plasticity and corrosion resistance, it is necessary to keep the content as low as possible, and it is more appropriate to control it below 0.1%.

[0014] Phosphorus and sulfur: For thermoplasticity and corrosion resistance considerations, these two elements should be kept as low as possible, and P should be controlled to be ≤ 0.04% and S ≤ 0.005%.

[0015] Chromium: An important element to improve corrosion resistance. When it is less than 18%, the corrosion resistance is poor and cannot meet the use requirements; but when it exceeds 20%, it will increase the difficulty of production and easily cause hot rolling roller sticking defects, thereby reducing the surface quality of the material. It is best to control it at 18-20%.

[0016] Molybdenum: An important element for improving corrosion resistance. When the content is less than 1.2%, the corrosion resistance is poor and cannot meet the use requirements. However, when the content exceeds 1.5%, it will increase the production difficulty and the raw material cost.

[0017] Nitrogen: It can increase strength. The requirement of less than 0.02% is to ensure that the material has excellent plasticity and corrosion resistance.

[0018] Niobium: Adding a certain amount of niobium stabilizes the carbon and nitrogen content of the material, refines the microstructure, and improves plasticity and surface smoothness. Considering the characteristics of the alloy's chemical composition and the effects of niobium on material properties and surface quality after combining with carbon and nitrogen, a range of 0.4-0.6% is ideal.

[0019] Aluminum: It is added as an excellent deoxidizer. Low aluminum content results in suboptimal deoxidation, and oxide inclusions are more likely to form in the material, leading to surface defects such as peeling. High aluminum content also increases the risk of aluminum oxide inclusions, causing surface defects such as peeling. This also complicates production control during steelmaking and continuous casting, leading to surface color variations such as streaky color variations. Given the unique chemical composition of this alloy, controlling the aluminum content to 0.1-0.2% not only keeps the oxygen content low, but also minimizes the amount of oxide inclusions, thereby preventing surface color variations.

[0020] In addition, it is also necessary to control the impurity element O: ≤0.002%. By controlling the oxygen content, the number of large-sized and high-hardness oxide inclusions in the material can be reduced, and defects such as peeling on the surface of the material can be reduced, thereby improving the surface quality of the material.

[0021] The technical solution adopted by the present invention to solve the second technical problem is: a method for manufacturing the high-grade decorative ferritic stainless steel as described above, characterized by the following steps:

[0022] 1) Steelmaking and casting: Steelmaking is carried out by electric furnace-AOD-VOD according to the above chemical composition, with the soft stirring time of molten steel greater than 30 minutes, the stabilization time of molten steel greater than 20 minutes, the superheat of molten steel at 40-50°C, the casting speed of continuous casting billet ≤ 1.0 m / min, and the surface of the billet is ground and removed by 3 mm;

[0023] 2) Hot rolling: initial temperature is 1200-1220℃, final temperature is 900-950℃;

[0024] 3) Annealing and pickling: annealing temperature is 1000-1020℃, holding time is 5-10s, pickling is performed after annealing;

[0025] 4) Cold rolling: cold rolling reduction rate ≥80%;

[0026] 5) Bright annealing after cold rolling: Bright annealing temperature is 1000-1020℃, and holding time is 5-10s.

[0027] In the above-mentioned manufacturing method, the soft stirring and settling time of the molten steel in step 1) of steelmaking is longer than that of conventional medium-chromium ultra-pure ferritic stainless steel; the molten steel superheat is relatively high; and the casting speed of the continuous casting billet is relatively low. These process adjustments are intended to ensure that oxide inclusions such as alumina in the molten steel can fully float up, thereby significantly reducing the number of oxide inclusions in the molten steel, preventing defects such as surface peeling, and thereby improving the surface quality of the material. The surface of the continuously cast billet is ground down by 3mm to remove oxide inclusions accumulated on the billet surface.

[0028] Step 2) The initial hot rolling temperature is higher than that of conventional medium chromium ultrapure ferritic stainless steel, which is 1200-1220°C, and the final temperature is 900-950°C. This is to ensure a sufficiently high hot-rolled plate temperature and avoid or reduce the occurrence of defects such as hot-rolled roll sticking.

[0029] Step 4) During the cold rolling process, the thinning rate is controlled to be ≥80%. The large deformation cold rolling process is to ensure that sufficient deformation energy is stored in the cold-rolled sheet, so that good metallographic structure and performance can be obtained in the subsequent annealing process, and the surface quality of the product can be improved.

[0030] The annealing process after hot rolling and the bright annealing process after cold rolling both use high temperature and short time annealing to enable the steel plate to obtain good metallographic structure and performance.

[0031] Preferably, after step 5) bright annealing, step 4) cold rolling and step 5) post-cold rolling bright annealing are performed again. That is, the hot-rolled and annealed sheet undergoes two cold rolling and annealing processes. This can produce a more uniform and fine grain structure, thereby ensuring that the material has better plasticity and strength, and also improves surface quality.

[0032] Compared with the existing technology, the advantages of the present invention are: under the premise of ensuring that the material has the basic characteristics of conventional stainless steel, the present invention improves the plasticity, corrosion resistance and other properties of the material by controlling the carbon and nitrogen content at a low level and adding a certain amount of niobium and molybdenum content; by adding appropriate aluminum elements and combining with special steelmaking processes, it is possible to control the oxygen content at a low level and the amount of oxidized impurities in the material at a low level, reducing defects such as surface peeling, thereby improving the surface quality of the material; in addition, by subjecting the material to large-scale cold rolling processing and high-temperature short-time annealing treatment, it is possible to ensure that the grain size of the material is uniform and small, thereby further improving the surface quality and performance of the product. That is, the manufacturing process of the stainless steel product of the present invention is simple and effective; the stainless steel product has excellent surface quality; and the stainless steel product has excellent corrosion resistance. The stainless steel of the present invention can be processed into products of various shapes and applied to fields such as transportation and architectural decorative parts. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a microstructure diagram of the cold-rolled sheet after annealing in an embodiment of the present invention. DETAILED DESCRIPTION

[0034] The present invention will be described in further detail below with reference to the accompanying drawings and embodiments.

[0035] The chemical composition of the stainless steel used in each example and comparative example is shown in Table 2. According to the chemical composition in Table 2, the steel was smelted using an electric furnace, AOD, and VOD process. The molten steel was softly stirred for 35 minutes, cooled for 25 minutes, and superheated to 45°C. The continuous casting billet was cast at a speed of 0.9 m / min, with a billet thickness of 200 mm. The billet surface was ground to remove 3 mm of thickness, hot rolled to 5 mm, annealed and pickled, then cold rolled to 0.5 mm and bright annealed. The manufacturing process parameters are shown in Table 3. The cold-rolled product was then tested for mechanical properties, corrosion resistance, and surface quality. The results are shown in Table 4.

[0036] The tensile properties of the cold-rolled stainless steel sheets produced in the examples and comparative examples of the present invention were tested (test standard: GB / T 228-1987 Metal Tensile Test Methods). The results are shown in Table 4. Table 4 shows that the chemical composition of the stainless steel in each example meets the requirements of this patent, with elongation exceeding 33%, yield strength exceeding 320 MPa, and tensile strength exceeding 450 MPa. These examples exhibit excellent plasticity and meet the performance requirements of the decoration industry. It can be seen from each example that a high Nb content results in better surface quality and strength. It can also be seen that the Al content in Comparative Example 1 is lower than the scope of the present invention, resulting in an O content higher than the scope of the present invention, which leads to a high degree of surface peeling defects on the finished cold-rolled sheet, indicating that low Al content results in poor surface quality. Comparative Example 2, in which the Al content is significantly higher than the scope of the present invention, results in a high degree of surface peeling and color difference defects on the finished cold-rolled sheet, indicating that high Al content results in poor surface quality.

[0037] The corrosion resistance of the cold-rolled stainless steel sheets of the present invention and the comparative example was tested according to the ferric chloride pitting corrosion test method for stainless steel (test standard: GB / T 17897-1999). The test results are shown in Table 4. The corrosion resistance of the stainless steel of the present invention example is significantly improved by more than 20% compared to conventional 304 austenitic stainless steel.

[0038] The surface quality of the products of the present invention and the comparative examples was tested (Baosteel high-grade decorative stainless steel strip surface quality judgment standard: every 1m on the steel strip surface 2 No defects larger than 2mm are allowed within the scope; every 1m on the surface of the steel strip 2 No more than 2 defects with a size of 1 to 2 mm are allowed; 2No more than five defects smaller than 1 mm are permitted; the overall defect rate on the steel strip surface is not permitted to exceed 5%. The test results are shown in Table 4. The results show that the surface quality of the product in the embodiment of the present invention is excellent, meeting the surface quality requirements for high-grade decorative panels. The product in Comparative Example 1 exhibited numerous peeling defects and poor surface quality, failing to meet the surface quality requirements for high-grade decorative panels. The product in Comparative Example 2 exhibited numerous peeling and color differences, resulting in poor surface quality, failing to meet the surface quality requirements for high-grade decorative panels.

[0039] Table 2. Chemical composition of stainless steel in various examples and comparative examples (wt%)

[0040]

[0041]

[0042] Note: The italicized bold text in the above table indicates the content values ​​of ingredients that are beyond the scope of this patent.

[0043] Table 3. Manufacturing process parameters

[0044]

[0045] Table 4. Mechanical properties, corrosion resistance and surface quality of stainless steel products obtained in various embodiments and comparative examples

[0046]

[0047] Note: The italicized and bold text in the above table are surface quality characteristics that do not meet the requirements of this patent.

Claims

1. A high-grade decorative ferritic stainless steel, characterized by: The chemical composition of the ferritic stainless steel is: C: ≤0.02wt%, Si: ≤0.1wt%, Mn: ≤0.1wt%, P: ≤0.04wt%, S: ≤0.005wt%, Cr: 18-20wt%, Mo: 1.2-1.5wt%, N: ≤0.02wt%, Nb: 0.4-0.6wt%, Al: 0.1-0.2wt%, O: ≤0.002wt%, and the rest is Fe and unavoidable impurities; The manufacturing method of the high-grade decorative ferritic stainless steel comprises the following steps: 1) Steelmaking and casting: Steelmaking is carried out by electric furnace-AOD-VOD according to the above chemical composition, with the soft stirring time of molten steel greater than 30 minutes, the stabilization time of molten steel greater than 20 minutes, the superheat of molten steel at 40-50°C, the casting speed of continuous casting billet ≤ 1.0 m / min, and the surface of the billet is ground and removed by 3 mm; 2) Hot rolling: initial temperature is 1200-1220℃, final temperature is 900-950℃; 3) Annealing and pickling: annealing temperature is 1000-1020℃, holding time is 5-10s, pickling is performed after annealing; 4) Cold rolling: cold rolling reduction rate ≥80%; 5) Bright annealing after cold rolling: Bright annealing temperature is 1000-1020℃, and holding time is 5-10s.

2. The high-grade decorative ferritic stainless steel according to claim 1, characterized in that: After the bright annealing in step 5), step 4) cold rolling and step 5) bright annealing after cold rolling are performed again.

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