Heat treatment method for eliminating grain clusters in ultra-pure ferritic stainless steel recrystallization structure

Abstract

The invention discloses a heat treatment method for eliminating grain clusters in a recrystallized structure of ultra-pure ferritic stainless steel, and belongs to the technical field of heat treatment. The ultra-pure ferritic stainless steel cold-rolled sheet has grain clusters in the existing annealing process. Due to the existence of grain clusters after recrystallization annealing, in the stretching or deep drawing machining process of ultra-pure ferritic stainless steel, due to anisotropic flow during plastic deformation, wrinkling stripe defects parallel to the rolling direction are generated on the surface of a workpiece. According to the invention, a duplex heat treatment process of recrystallization annealing and secondary annealing is adopted, and the processability of the ultra-pure ferritic stainless steel is improved by improving the uniformity of recrystallized grains and reducing or eliminating grain clusters.

Description

Technical Field

[0001] The invention relates to the technical field of heat treatment of ultrapure ferrite stainless steel, in particular to a heat treatment method for eliminating grain clusters in a recrystallized structure of ultrapure ferrite stainless steel. Background Art

[0002] In recent years, the tight supply and demand relationship of nickel resources and the sharp fluctuations in nickel prices have seriously restricted the application and development of austenitic stainless steel. Ultrapure ferritic stainless steel has gradually replaced austenitic stainless steel and become the inevitable choice of stainless steel companies around the world because it contains no nickel or a small amount of nickel and has low cost. At the same time, this type of stainless steel has the characteristics of low thermal expansion coefficient (60% of austenitic stainless steel), high thermal conductivity (1.5 times that of austenitic stainless steel), insensitivity to stress corrosion, and excellent welding performance. Its extremely low carbon and nitrogen content, as well as the addition of stabilizing elements such as niobium and titanium, can significantly improve the corrosion resistance, welding performance, and forming performance of ferritic stainless steel. Compared with austenitic stainless steel, these excellent properties of ultrapure ferritic stainless steel (FSS) have made it widely used in the automotive mobile industry, petrochemical industry, and manufacturing industry.

[0003] During the forming process of ultra-pure ferritic stainless steel cold plate, its surface is very likely to produce "wrinkle stripes" parallel to the rolling direction, which leads to serious quality problems after deep drawing and deep punching of ferritic stainless steel cold plate, which has become a common problem of ferritic stainless steel. The wrinkle defects of ultra-pure ferritic stainless steel cold plate during the forming process not only damage the appearance of the product, but also increase the difficulty of polishing in the subsequent processing, greatly increasing the production and processing costs.

[0004] The purpose of the present invention is to eliminate the grain clusters caused by plane strain during the rolling process of ultra-pure ferritic stainless steel through a double heat treatment process of "recrystallization annealing + secondary annealing", increase the proportion and uniformity of γ texture, reduce wrinkling streak defects in subsequent processing and forming, and improve and enhance the processing performance of ultra-pure ferritic stainless steel. Summary of the invention

[0005] The purpose of the present invention is to provide a heat treatment method for eliminating grain clusters in the recrystallized structure of ultra-pure ferrite stainless steel in view of the above problems.

[0006] The object of the present invention is achieved in this way: a heat treatment method for eliminating grain clusters in a recrystallized structure of ultrapure ferrite stainless steel comprises the following steps: Step 1: Recrystallization annealing process: The ultra-pure ferritic stainless steel sheet is subjected to recrystallization annealing at 1040-1080°C, with a holding time of 1-1.5 min / mm, and the heating furnace atmosphere is air, hydrogen or hydrogen + nitrogen atmosphere, and then cooled to 930-960°C at a cooling rate of 45-50°C / s; Step 2: Secondary annealing heat treatment process: Heat the fully recrystallized ultra-pure ferritic stainless steel sheet at 930-960°C for 3-5 min / mm in an atmosphere of air, hydrogen or hydrogen + nitrogen, and water cool to room temperature.

[0007] The chemical composition mass percentage of ultra-pure ferritic stainless steel cold-rolled plate is: C: ≤0.02%; Si: ≤1.00%; Mn: ≤1.00%; S: ≤0.03%; Cr: 17.0~22.0%; Mo: 0~2%; N: ≤0.04%; Ti+Nb: 0.20~1.0% and ≥6 (C+N); the balance is Fe and unavoidable impurities.

[0008] The beneficial effect of the present invention is that the technical problem to be solved by the present invention is to achieve the purpose of reducing the wrinkle streak defect in the forming process of ultra-pure ferritic stainless steel through the double heat treatment process of "recrystallization annealing + secondary annealing". Expanding the application scope of ultra-pure ferritic stainless steel is conducive to the promotion of ultra-pure ferritic stainless steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The present invention will be further described below in conjunction with the accompanying drawings.

[0010] Figure 1 This is a three-dimensional morphology image of the sample in the embodiment obtained by laser confocal microscopy testing.

[0011] Figure 2 This is the metallographic structure diagram of the sample in the embodiment.

[0012] Figure 3 This is the ODF image obtained by EBSD test of the sample in the embodiment.

[0013] Figure 4 This is a three-dimensional morphology image obtained by laser confocal microscopy testing in the comparative example.

[0014] Figure 5 This is the metallographic structure diagram of the sample in the comparative example.

[0015] Figure 6 This is the ODF image obtained from the EBSD test of the sample in the comparative example. DETAILED DESCRIPTION

[0016] The technical problem to be solved by the present invention is to eliminate "grain clusters" and improve the proportion and uniformity of γ texture through a double heat treatment process of "recrystallization annealing + secondary annealing", so as to achieve the purpose of reducing wrinkling streak defects in the forming process of ultra-pure ferritic stainless steel. The plane strain caused by the rolling deformation of ultra-pure ferritic stainless steel will cause large differences in grain size after recrystallization annealing to form "grain clusters". The strain of the surface layer is mainly caused by the shear strain generated by the friction between the rollers and the steel strip. The strain of the center layer is mainly caused by plane strain. In addition, during the rolling process, there will be a strong texture gradient along the thickness direction of the plate, which is easy to form an unevenly distributed deviated γ texture {111} <112> Therefore, these band-like grain clusters and uneven textures will lead to "wrinkling defects" on the workpiece surface.

[0017] The present invention firstly controls the recrystallization annealing system to fully recrystallize the deformed grains after cold rolling; secondly, a reasonable secondary annealing treatment system is selected: the temperature is 930-960°C, and the holding time is 3-5min / mm. In this temperature range, the secondary annealing stage can make the grains of different sizes after recrystallization uniform, and the uniformity of grain size is conducive to eliminating "grain clusters". According to the Hall-Petch formula σ s =σ0+Kd -1 / 2 The smaller the grain, the higher the yield strength. If the grain size is uneven, the yield strength will be different everywhere, showing plastic anisotropy, which is manifested in the form of wrinkling on a macro scale. At the same time, secondary annealing can control the difference in energy storage of grains of different sizes, so that small-sized recrystallized grains with {111} orientation grow preferentially in the secondary annealing stage and can swallow up other oriented grains with lower energy storage. By controlling the uniformity of grain size, the effect of eliminating grain clusters and increasing the proportion and uniformity of γ texture can be achieved, thereby improving the processing performance of ultra-pure ferritic stainless steel. The degree of wrinkling of ultra-pure ferritic stainless steel is evaluated by measuring the roughness of the surface of the 15% tensile specimen.

[0018] The technical solution adopted by the present invention is: the ultra-pure ferritic stainless steel of the present invention contains the following chemical components, and the mass percentage is: C: ≤0.02%; Si: ≤1.00%; Mn: ≤1.00%; S: ≤0.03%; Cr: 17.0-22.0%; Mo: 0-2%; N: ≤0.04%; Ti+Nb: 0.20-1.0% and ≥6 (C+N); the balance is Fe and unavoidable impurities.

[0019] The present invention provides a dual heat treatment process of ultrapure ferrite recrystallization annealing process + secondary annealing, which reduces or eliminates grain clusters by improving the uniformity of recrystallized grains. The invention solves the technical problem that in the process of drawing or deep drawing of ultrapure ferrite stainless steel, due to the existence of grain clusters after recrystallization annealing, the strip crystal texture flows anisotropically during plastic deformation, which eventually causes wrinkle stripe defects on the surface of the workpiece parallel to the rolling direction. The dual heat treatment process of "recrystallization annealing + secondary annealing" includes the following steps: Step 1: Recrystallization annealing process: The ultra-pure ferritic stainless steel cold-rolled sheet is subjected to recrystallization annealing at 1040-1080°C, with a holding time of 1-1.5 min / mm. The heating furnace atmosphere is air, hydrogen or hydrogen + nitrogen atmosphere, and then cooled to 930-960°C at a cooling rate of 45-50°C / s. The purpose of this step is to fully recrystallize the cold-rolled sheet. If the recrystallization annealing temperature is lower than 1040°C, it will lead to insufficient recrystallization; if the recrystallization annealing temperature is higher than 1080°C, the grains will grow rapidly, and it is difficult to obtain good mechanical properties.

[0020] Step 2: Secondary annealing heat treatment process: Heat the fully recrystallized ultra-pure ferritic stainless steel plate at 930-960℃ for 3-5min / mm, and cool it to room temperature in an atmosphere of air, hydrogen or hydrogen + nitrogen. This step can make the grains of different sizes after recrystallization uniform, reduce the yield strength difference between grains, and eliminate "grain clusters". In addition, the secondary annealing heat treatment will make the small-sized recrystallized grains with {111} orientation grow preferentially in the secondary annealing stage, and can swallow up other oriented grains with lower energy storage, so as to improve the proportion and uniformity of γ texture. Finally, the surface "wrinkling defect" of ultra-pure ferritic stainless steel is reduced. If the secondary annealing temperature is lower than 930℃ or the holding time is too short (<3min), it is difficult to achieve the effect of uniform grains; but if the temperature is too high (>960℃) or the holding time is too long (>5min), it will lead to abnormal secondary growth of grains. Example 1

[0021] Taking SUS443 ultra-pure ferrite stainless steel 1mm cold plate as an example, the chemical composition is C: 0.013%; Si: 0.31%; Mn: 0.20%; S: 0.001%; ​​Cr: 21.1%; N: 0.011%; Ti: 0.25%; Nb: 0.15%; the remainder is Fe and unavoidable impurities. The specific implementation steps are as follows: Step 1: The ultrapure ferritic stainless steel SUS443 cold-rolled sheet is subjected to recrystallization annealing at 1050°C for 1 min in an air atmosphere, and then cooled to 930°C at a cooling rate of 50°C / s.

[0022] Step 2: The secondary heat treatment temperature is 930°C, the holding time is 5 minutes, the heating furnace atmosphere is air, hydrogen or hydrogen + nitrogen atmosphere, and water-cooled to room temperature. Comparative Example 1

[0023] Only step 1 is performed: the ultrapure ferritic stainless steel SUS443 cold-rolled sheet is subjected to recrystallization annealing at 1050°C for 1 min in an air furnace atmosphere, and then cooled to room temperature at a cooling rate of 50°C / s.

[0024] The samples treated in the examples and comparative examples were subjected to a 15% tensile test according to the national standard (GB / T228.2002) "Room Temperature Tensile Test Method for Metallic Materials" to simulate the forming test of ultra-pure ferritic stainless steel. The wrinkling defect was observed by laser confocal microscopy, and the surface roughness value of each sample after stretching was tested as a quantitative indicator of the wrinkling defect. The grain size was observed by metallographic microscopy, and the ODF results obtained by EBSD test were compared with the peak intensity of the γ-oriented texture.

[0026] from Figure 1 It can be seen that in the embodiment treated by the double annealing process, the surface of the sample is flat and has no obvious wrinkling phenomenon, while the surface of the sample in the comparative example has obvious wrinkling defects such as Figure 4 From the R of the embodiments and comparative examples in Table 1 a The test results also show the same pattern. Figure 2 It was observed that the grain size of the sample in the embodiment was more uniform. Figure 3 It can be observed that the sample in the embodiment has only γ-oriented texture, and the peak intensity is relatively high. The double annealing process significantly increases the proportion of γ-oriented texture. Figure 6 ) It can be seen that in addition to the γ-oriented texture, there are also strong α-oriented texture and cubic texture.

[0027] The results of the examples and comparative examples show that after being treated by the method of the present invention, the surface roughness value (as shown in Table 1) of the ultrapure ferritic stainless steel after a 15% tensile test is significantly reduced, and the wrinkling degree is reduced.

[0028] The above description is only a specific embodiment of the present invention, but the structural features of the protection scope of the present invention are not limited thereto. Any changes or modifications made by any technician in the field of the present invention are all covered by the patent scope of the present invention.

Claims

1. A heat treatment method for eliminating grain clusters in the recrystallized structure of ultrapure ferrite stainless steel, characterized in that: The following steps are involved: Step 1: Recrystallization annealing process: The ultra-pure ferritic stainless steel sheet is subjected to recrystallization annealing at 1040-1080°C, with a holding time of 1-1.5 min / mm, and the heating furnace atmosphere is air, hydrogen or hydrogen + nitrogen atmosphere, and then cooled to 930-960°C at a cooling rate of 45-50°C / s; Step 2: Secondary annealing heat treatment process: Heat the fully recrystallized ultra-pure ferritic stainless steel sheet at 930-960°C for 3-5 min / mm in an atmosphere of air, hydrogen or hydrogen + nitrogen, and water cool to room temperature.

2. The heat treatment method for eliminating grain clusters in the recrystallized structure of ultrapure ferrite stainless steel according to claim 1, characterized in that: The chemical composition mass percentage of ultra-pure ferritic stainless steel cold-rolled plate is: C: ≤0.02%; Si: ≤1.00%; Mn: ≤1.00%; S: ≤0.03%; Cr: 17.0~22.0%; Mo: 0~2%; N: ≤0.04%; Ti+Nb: 0.20~1.0% and ≥6 (C+N); the balance is Fe and unavoidable impurities.

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