An Optimized Method for Duplex Heat Treatment of Medium-Crferritic Stainless Steel

Abstract

The application discloses an optimization method of medium-chromium ferrite stainless steel dual-phase zone heat treatment, which adopts an annealing pickling line online heat treatment mode to optimize the alpha and gamma dual-phase zone heat treatment of the medium-chromium ferrite stainless steel, and comprises the following steps: in the steelmaking stage, the percentage content of C element is set to be lower than 0.035%, and the percentage content of N element is set to be lower than 0.030%, so as to reduce the austenite content in the dual-phase zone annealing process and optimize the alpha and gamma dual-phase zone proportion in the high-temperature section; in the continuous annealing stage, the heating furnace is heated by using different temperatures, the low-temperature alpha single-phase zone annealing mode after the high-temperature alpha+gamma dual-phase zone heat treatment is adopted, the overline speed of the A3 line is controlled in a preset speed range, and the martensite organization in the rolled state organization is quickly and sufficiently eliminated; in the pickling stage, the medium-chromium ferrite stainless steel subjected to the hot rolling annealing is pickled by using different pickling solutions with different concentrations, so that the medium-chromium ferrite stainless steel has good toughness and plasticity and is not prone to stamping cracking during use.

Description

Technical Field

[0001] This invention relates to the field of stainless steel manufacturing technology, and in particular to an optimized method for heat treatment of the duplex region of medium chromium ferritic stainless steel. Background Technology

[0002] Medium-chromium ferritic stainless steel is a type of ferritic stainless steel with good corrosion resistance and oxidation resistance. Taking 430 series stainless steel as an example, it is mainly used in construction, decoration, home appliances, kitchenware and other fields. In these fields, 430 stainless steel is widely recognized and used due to its excellent performance and reasonable price.

[0003] Currently, the annealing process for medium-chromium ferritic stainless steels, primarily the 430 series, widely employs all-hydrogen bell-type annealing. However, due to the inherent limitations of the bell-type furnace structure (such as the all-hydrogen bell-type furnace structure...), the annealing process is limited by the bell-type furnace structure itself. Figure 1 As shown in the figure, this results in long annealing times (over 40 hours) and uneven temperature distribution, ultimately leading to a series of problems on the surface of hot-rolled stainless steel coils, such as annealing color differences, uneven performance, and uneven strip thickness after rolling. Currently, many stainless steel manufacturers have developed online continuous annealing methods, but due to the short annealing time (around 5 minutes), the toughness and plasticity of the products are relatively low, making them prone to stamping cracks during use. Summary of the Invention

[0004] To address some or all of the technical problems existing in the prior art, this invention provides an optimized method for heat treatment of the duplex region of medium-chromium ferritic stainless steel. By realizing a continuous annealing process in the duplex region of medium-chromium ferritic stainless steel, the annealing time is shortened, there is no annealing color difference, and the problems of low toughness and plasticity of hot-rolled coils of medium-chromium ferritic stainless steel after continuous annealing, uneven thickness of strip after rolling, inconsistent performance, and easy stamping cracking are improved.

[0005] The technical solution of the present invention is as follows:

[0006] This invention provides an optimized method for heat treatment of the duplex region of medium-chromium ferritic stainless steel, characterized by optimizing the heat treatment of the α and γ duplex regions of medium-chromium ferritic stainless steel using an online heat treatment method via annealing and pickling line, including:

[0007] During the steelmaking stage, the percentage content of carbon element is set to less than 0.035%, and the percentage content of nitrogen element is set to less than 0.030% to reduce the austenite content during the duplex annealing process, optimize the ratio of α and γ two-phase regions in the high-temperature section, so that the ferrite region in the duplex region of medium-chromium ferritic stainless steel is located directly above the austenite region, and the ferrite region formed by ferrite and the austenite region formed by austenite do not intersect, and there is a transition region between the numerical regions of ferrite region and austenite region, reducing the area of ​​the austenite region and ferrite surface, shortening the temperature range of ferrite and austenite, reducing the martensite phase content in hot-rolled strip steel, and achieving the effect of softening strip steel;

[0008] During the continuous annealing stage, the furnace is heated at different temperatures simultaneously. The process involves high-temperature α+γ dual-phase heat treatment followed by low-temperature α single-phase annealing. The speed of the A3 line is controlled within the preset speed range to quickly and fully eliminate the martensite structure in the rolled structure.

[0009] During the pickling stage, different pickling solutions of varying concentrations are used to pickle the hot-rolled and annealed medium-chromium ferritic stainless steel, which gives the medium-chromium ferritic stainless steel good toughness and plasticity, making it less prone to stamping cracks during use.

[0010] Furthermore, during the continuous annealing stage, multiple heating furnaces are used to heat the medium-chromium ferritic stainless steel. The temperature of furnace sections 1-7 in the multiple heating furnaces is controlled at 900℃~1200℃; the temperature of furnace sections 8-18 is controlled at 650℃~950℃.

[0011] Furthermore, the preset speed range for the crossing speed of line A3 is 10~20m / min.

[0012] Furthermore, the pickling process targets a mixed acid section, and the pickling solution used is HF and HNO3.

[0013] Furthermore, when using pickling solution for pickling in the mixed acid section, the HF concentration is 5~10 g / L and the HNO3 concentration is 60~80 g / L.

[0014] The main advantages of the technical solution of this invention are as follows:

[0015] The optimized method for heat treatment of the duplex region of medium-chromium ferritic stainless steel of the present invention optimizes the heat treatment of the α and γ duplex regions of medium-chromium ferritic stainless steel during the steelmaking stage, the continuous annealing stage, and the pickling stage, respectively. By controlling the steelmaking composition, optimizing the heat treatment method, optimizing the temperature distribution in the continuous heating furnace, optimizing the line speed, and adjusting the pickling process in the pickling section, the continuous annealing process of the duplex region of medium-chromium ferritic stainless steel of the present invention shortens the annealing time and improves the problem of low toughness and plasticity and easy stamping cracking of hot-rolled coils of medium-chromium ferritic stainless steel after continuous annealing. Attached Figure Description

[0016] The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and constitute a part of this invention, illustrate exemplary embodiments of the invention and, together with their description, serve to explain the invention and do not constitute an undue limitation thereof. In the drawings:

[0017] Figure 1 This is an example of an optimized method for heat treatment of the duplex region of chromium ferritic stainless steel in an embodiment of the present invention, using the existing conventional 430 stainless steel phase diagram.

[0018] Figure 2 An optimized phase diagram of 430 stainless steel obtained by employing the dual-phase heat treatment of chromium ferritic stainless steel in this invention is provided as an embodiment of the present invention.

[0019] Figure 3 The phase diagram of 430 stainless steel with the optimized phase diagram line A3 is provided for an embodiment of the present invention, which adopts the dual-phase heat treatment of chromium ferritic stainless steel in the present invention. Detailed Implementation

[0020] 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 and corresponding drawings. 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.

[0021] The technical solutions provided by the embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0022] Example 1

[0023] As attached Figure 1-3 As shown, this embodiment of the invention provides an optimized method for heat treatment of the duplex region of medium-chromium ferritic stainless steel. The method optimizes the heat treatment of the α and γ duplex regions of medium-chromium ferritic stainless steel using an online heat treatment method within an annealing and pickling line. The optimized method includes:

[0024] During the steelmaking stage, the percentage content of carbon element is set to less than 0.035%, and the percentage content of nitrogen element is set to less than 0.030% to reduce the austenite content during the duplex annealing process, optimize the ratio of α and γ two-phase regions in the high-temperature section, so that the ferrite region in the duplex region of medium-chromium ferritic stainless steel is located directly above the austenite region, and the ferrite region formed by ferrite and the austenite region formed by austenite do not intersect, and there is a transition region between the numerical regions of ferrite region and austenite region, reducing the area of ​​the austenite region and ferrite surface, shortening the temperature range of ferrite and austenite, reducing the martensite phase content in hot-rolled strip steel, and achieving the effect of softening strip steel;

[0025] During the continuous annealing stage, the furnace is heated at different temperatures simultaneously. The process involves high-temperature α+γ dual-phase heat treatment followed by low-temperature α single-phase annealing. The speed of the A3 line is controlled within the preset speed range to quickly and fully eliminate the martensite structure in the rolled structure.

[0026] During the pickling stage, different pickling solutions of varying concentrations are used to pickle the hot-rolled and annealed medium-chromium ferritic stainless steel, which gives the medium-chromium ferritic stainless steel good toughness and plasticity, making it less prone to stamping cracks during use.

[0027] The online heat treatment method of the annealing and pickling line of this invention optimizes the heat treatment of the α and γ duplex regions of medium chromium ferritic stainless steel. By controlling the steelmaking composition, optimizing the heat treatment method, optimizing the temperature distribution in the continuous heating furnace, optimizing the line speed, and adjusting the pickling process in the pickling section, the continuous annealing process of the duplex region of medium chromium ferritic stainless steel of this invention not only shortens the annealing time, but also improves the problem of low toughness and plasticity and easy stamping cracking of hot-rolled coils of medium chromium ferritic stainless steel after continuous annealing.

[0028] It should be noted that the medium chromium in the optimized method for heat treatment of medium chromium ferritic stainless steel in the duplex region of this invention is 17 series, that is, included in the range of 13-17, and the maximum thickness of the stainless steel produced is 8mm.

[0029] Specifically, in an optimization method for heat treatment of ferrochromic stainless steel in the duplex region of the present invention, the content of C and N elements is controlled at a low level of less than 0.035% during the steelmaking process. The purpose is to reduce the proportion of austenite in the α and γ duplex annealing process, thereby optimizing the proportion of α and γ two-phase regions in the high-temperature section.

[0030] like Figure 2As shown, based on actual heat treatment data and experiments, in some optional implementations, the stainless steel phase diagram obtained by the heat treatment optimization method of this invention, compared with the conventional stainless steel phase diagram, makes the ferrite region in the dual-phase region of medium-chromium ferritic stainless steel located directly above the austenite region, and the ferrite region formed by ferrite and the austenite region formed by austenite do not intersect. There is also a transition region between the numerical regions of the ferrite region and the austenite region, reducing the area of ​​the austenite region and the ferrite surface, shortening the temperature range of ferrite and austenite, reducing the martensite phase content in hot-rolled strip steel, and achieving the effect of softening the strip steel.

[0031] In some alternative implementations, the range of the aforementioned transition region, i.e., the range between the lowest point of the ferrite region and the highest point of the austenite region in the stainless steel phase diagram after heat treatment, is at least 20 wt%. Compared with the stainless steel phase diagram without heat treatment, the temperature range of the austenite and ferrite regions is significantly reduced, and the overall area of ​​the region is also significantly reduced.

[0032] Specifically, in the heat treatment optimization method of the present invention, by adopting an optimized heat treatment method, the martensite in the chromium ferrite body during the heat treatment process is fully eliminated and the carbides are homogenized, thereby improving a series of problems such as low toughness and plasticity of the product and easy cracking.

[0033] Specifically, by optimizing the temperature distribution in the continuous heating furnace and optimizing the linear speed in the heat treatment optimization method of the present invention, it is further ensured that the martensite structure in the rolled structure is eliminated quickly and completely.

[0034] Specifically, by adjusting the pickling process in the pickling section of the heat treatment optimization method of the present invention, medium chromium ferritic stainless steel has good toughness and plasticity, and is not prone to stamping cracks during use.

[0035] Specifically, multiple heating furnaces are used to heat the medium-chromium ferritic stainless steel during the continuous annealing stage.

[0036] In some optional implementations, taking 18 heating furnaces as an example, the temperature of furnace sections 1-7 is controlled at 900℃~1200℃; the temperature of furnace sections 8-18 is controlled at 650℃~950℃.

[0037] It should be noted that when there are multiple heating furnaces, during the sequential heating process, the furnaces that are earlier in the order of their sequence are heated at higher temperatures, while the furnaces that are later in the order are heated at lower temperatures. The specific heating temperature and number of furnaces depend on the actual heating situation, and the above heating method can be divided into multiple heating temperatures for different heating ranges of different furnaces, allowing for alternating high and low temperature heat treatment.

[0038] Specifically, such as Figure 3 As shown, the preset speed range for the A3 line is 10~20 m / min. This setting reduces the line speed, and by using different furnace temperatures for heating, it further ensures the rapid, uniform, and thorough elimination of martensite in the rolled structure within a single furnace, resulting in superior performance of the medium-chromium ferritic stainless steel.

[0039] Specifically, the pickling process targets the mixed acid section, and the pickling solution used is HF and HNO3.

[0040] Specifically, when using pickling solution for pickling in the mixed acid section, the HF concentration is 5~10g / l and the HNO3 concentration is 60~80g / l.

[0041] With this setup, the strip's feed rate is reduced after the feed rate is adjusted, thus avoiding a series of problems such as over-pickling and pickling marks during the pickling process.

[0042] In summary, the optimized heat treatment method for the duplex region of medium-chromium ferritic stainless steel according to the present invention optimizes the heat treatment of the α and γ duplex regions of medium-chromium ferritic stainless steel as a whole during the steelmaking stage, the continuous annealing stage, and the pickling stage. By controlling the steelmaking composition, optimizing the heat treatment method, optimizing the temperature distribution in the continuous heating furnace, optimizing the line speed, and adjusting the pickling process in the pickling section, the continuous annealing process for the duplex region of medium-chromium ferritic stainless steel according to the present invention shortens the annealing time and improves the problem of low toughness and plasticity and easy stamping cracking of hot-rolled coils of medium-chromium ferritic stainless steel after continuous annealing.

[0043] Example 2

[0044] The method of Example 1 was adopted, with 430 as the main implementation object. First, the steelmaking composition was optimized, with C content below 0.035% and N content below 0.030%. At the same time, the temperature of the hot rolling pickling line heating furnace was adjusted, controlling the temperature of furnace sections 1-7 of the continuous pickling line heating furnace at 900℃~1200℃; and the temperature of furnace sections 8-18 at 650℃~950℃.

[0045] A total of 54 rolls were tested in this batch, and the mechanical properties and surface conditions obtained are shown in Table 1.

[0046] Table 1. Average mechanical properties and surface condition data at each stage after implementation.

[0047]

[0048] Comparing groups 1 and 3, and groups 2 and 4 in Table 1 above, it was found that reducing the C and N content significantly improved the material's ductility and toughness, with the yield strength decreasing by more than 20 MPa and the elongation increasing by about 2%. Comparing groups 1 and 2, and groups 3 and 4, it was found that the improved duplex heat treatment process reduced the yield strength by more than 30 MPa and increased the elongation by more than 4%. This indicates that reducing the C and N content and adopting a duplex annealing process can effectively improve the ductility and toughness of medium-chromium ferritic stainless steel. However, the proportion of over-pickled stainless steel also increased significantly.

[0049] Secondly, to address the issue of a significantly increased over-pickled ratio, the pickling concentration was adjusted. The process used a mixed acid section with an HF concentration of 5-10 g / L and an HNO3 concentration of 60-80 g / L. 23 tests were conducted, and the results are shown in Table 2.

[0050] Table 2. Average mechanical properties and surface condition data at each stage after implementation.

[0051]

[0052] The surface of the steel coils processed by the process data in groups 2 and 5 and groups 4 and 6 in Table 2 above was inspected and compared. It was found that after optimizing the pickling process, the proportion of surface over-pickling was significantly improved.

[0053] Therefore, the optimized method for heat treatment of medium-chromium ferritic stainless steel in the duplex region of this invention significantly improves the toughness and plasticity of medium-chromium ferritic stainless steel through the above process. In the above example using 430 as an example, the elongation increased from 26% to 30%, and the proportion of stamping cracks was significantly reduced. This invention solves the problem of extremely long annealing times in conventional bell-type furnaces (up to 40 hours or more). By using the optimized heat treatment method of this invention and employing an online heat treatment line for annealing and pickling, the heat treatment time can be greatly shortened to about 20 minutes. It also solves the problem of high strength, low toughness and plasticity, and easy stamping cracks in existing online continuous annealing patents due to the use of a single temperature range in a hot-rolled pickling line. The two-stage, synergistic heat treatment method of the hot-rolled pickling line in this invention improves the toughness and plasticity after annealing without affecting heat treatment efficiency. Furthermore, it addresses the issue of insufficient adjustment of the pickling process after annealing in the annealing and pickling line. The method of this invention adjusts and optimizes the pickling process to a certain extent, improving the surface quality of the stainless steel after pickling. This not only shortens the annealing time but also improves the problem of low toughness and plasticity of hot-rolled coils of medium-chromium ferritic stainless steel after continuous annealing, which makes them prone to stamping cracks.

[0054] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are 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 a process, method, article, or apparatus. Additionally, the terms "front," "back," "left," "right," "upper," and "lower" in this document refer to the placement shown in the accompanying drawings.

[0055] Finally, it should 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 spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for optimizing the heat treatment in the duplex region of a medium-chromium ferritic stainless steel, characterized in that, The annealing pickling line is used for online heat treatment of the medium chromium ferritic stainless steel in the alpha and gamma two-phase region, including: In the steelmaking stage, the percentage content of C element is set to be lower than 0.035%, and the percentage content of N element is set to be lower than 0.030%, so as to reduce the austenite content in the two-phase region annealing process, optimize the alpha and gamma two-phase region ratio in the high temperature section, make the ferrite region in the medium chromium ferritic stainless steel two-phase region be located just above the austenite region, and the ferrite region formed by the ferrite and the austenite region formed by the austenite are not intersected, and there is a transition region between the numerical regions of the ferrite region and the austenite region, the area of the austenite region and the ferrite curved surface is reduced, the temperature interval of the ferrite and the austenite is shortened, the martensite phase content in the hot-rolled strip steel is reduced, and the function of softening the strip steel is achieved; In the continuous annealing stage, the heating furnace is heated at different temperatures, the low-temperature alpha single-phase region annealing after the high-temperature alpha+gamma two-phase region heat treatment is used, the A3 line passing speed is controlled in the preset speed range, the martensite organization in the rolled organization is quickly and fully eliminated; In the pickling stage, the hot-rolled annealed medium chromium ferritic stainless steel is pickled by using different pickling solutions with different concentrations, so that the medium chromium ferritic stainless steel has good toughness and plasticity, and is not easy to crack during stamping; The preset speed range of the A3 line passing speed is 10-20 m / min; The process targeted in the pickling process is the mixed acid section, and the pickling solution used is HF and HNO3; When the mixed acid section is pickled by using the pickling solution, the HF concentration is 5-10 g / l, and the HNO3 concentration is 60-80 g / l.

2. A method of optimizing the heat treatment in the duplex region of a medium chromium ferritic stainless steel according to claim 1, characterized in that, In the continuous annealing stage, the medium chromium ferritic stainless steel is heated by using multiple heating furnaces, the temperature of 1-7# furnace section in the multiple heating furnaces is controlled at 900-1200℃, and the temperature of 8-18# furnace section is controlled at 650-950℃.

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