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Effect of Vacuum Annealing Temperature on Stainless Steel Cold Rolled Sheet
1. Experimental materials and methods
Using 430 stainless steel cold-rolled sheet as the raw material, eight annealing samples with the size of 160 mm × 40 mm were taken from three directions of 0°, 45° and 90° respectively from the rolling direction to simulate the vacuum annealing experiment. In the actual production process, the 430 ferritic stainless steel cold-rolled sheet adopts a two-stage heating annealing process. First, the strip is heated at a heating rate of 15 ℃·S-1 to an intermediate temperature of 560 ℃, and then at a heating rate of 6.9 ℃. · Heating rate of S-1 to a temperature of around 870 °C and rapid cooling to avoid the formation of unfavorable structures and textures. Since the softening process mainly occurs in the heating stage II, the heating rate and the maximum heating temperature (Tmax) of the heating stage II are the two main factors that determine the continuous annealing process of the 430 stainless steel cold-rolled sheet.
In order to simulate the actual process, in the vacuum annealing experiment, the heating rate and intermediate temperature of heating stage I and the heating rate of heating stage II were kept unchanged, which were 15 and 6.9℃·S-1, respectively, and the maximum annealing temperature was 620, 660, and 690, respectively. , 730, 770, 810, 840 and 880 ℃. Two tensile specimens of the same specification (parallel area width of 12 mm and gauge length of 50 mm) were cut from the vacuum annealed specimen, and the average value of the two specimens was taken as the tensile test result.
The tensile test was carried out on a micro-controlled electronic universal tensile testing machine. When the elongation was 12%, the width of the parallel zone was measured to calculate the plastic strain ratio r (Lankford value), and the values of 0°, 45° and 0° under different vacuum annealing processes were obtained. After r0, r45 and r90 of the 90° thin plate specimen, the average plastic strain ratio r and the plane anisotropy exponent Δr were calculated using. In order to analyze the influence of the vacuum annealing process on the strength and elongation of the sheet, the average value of the strength and elongation in the three directions of 0°, 45° and 90° was taken as the strength and plasticity of the sheet under the same annealing process. , take a small sample of 6 mm × 10 mm × 0.4 mm (rolling length × transverse length × thickness) from the annealed sample to measure the structure and hardness. After grinding and polishing the section of the sample in the rolling direction, it was etched with FeCl3 hydrochloric acid solution, the microstructure was observed under an optical microscope, and the hardness of the sheet under different annealing conditions was measured with a digital Vickers hardness tester.
2 Experimental results
2.1 Influence on the microstructure and hardness of the thin plate
The internal microstructure of 430 stainless steel cold-rolled sheet changes with annealing temperature. Due to the high stacking fault energy of ferritic stainless steel, there are less stacking faults in the stacking layer of grains, so the plastic deformation is mainly caused by dislocations. The slip mechanism is the main mechanism, and a large number of intragranular deformation bands will appear after a large cold rolling reduction. For the 430 ferritic stainless steel cold-rolled sheet, when the maximum annealing temperature is 620 ℃, the internal structure of the material basically retains the characteristics of the cold-rolled state, and when the maximum annealing temperature is 690 ℃, some fine particles appear in the deformation zone The recrystallized grains show that the nucleation phenomenon of recrystallization just begins to appear at this temperature, and the recrystallized grains tend to nucleate inside the deformation zone; when the maximum annealing temperature reaches 730 ℃, the recrystallization inside the material has already been formed. To a certain extent, there are still some obvious deformation bands and some larger deformed grains; when the maximum annealing temperature is around 840 °C, the material is almost completely recrystallized; when the maximum annealing temperature reaches 880 °C, it is A uniform equiaxed ferrite structure can be obtained, and the average grain size is about 13.8 μm. During the annealing process, the softening of the cold-rolled sheet can be divided into two stages: the first stage is the recovery softening mechanism, which mainly occurs at 600 ~ 690 ℃ Within the temperature range of 200°C, the microstructure of the material does not change significantly in the recovery stage, while the hardness decreases from HV 256 and stabilizes at about HV 220; the second stage is the recrystallization softening mechanism, which occurs at temperatures above 690 °C Within the annealing temperature range of 690-810 ℃, the hardness of the sheet decreases rapidly, and when the temperature reaches above 810 ℃, the hardness basically does not change, and maintains around HV 145. For the two-stage heating vacuum annealing process of 430 stainless steel cold rolled sheet, the intermediate temperature can be set at around 600 °C. When the temperature is below 600 °C, the microstructure and mechanical properties of the sheet will not change significantly, so the heating rate of heating stage I does not need to be precisely controlled, as long as the heating rate of heating stage II and the maximum heating temperature Tmax are precisely controlled. , that is, the 430 stainless steel sheet that meets the requirements can be obtained.
2.2 Influence on the strength and plasticity of the sheet
Through the tensile test of the material under different annealing conditions, the relationship between the strength and elongation of 430 stainless steel cold-rolled sheet with the annealing temperature is obtained. It gradually decreased and finally stabilized at about 450 MPa. At the same time, the yield strength also showed a two-stage decreasing trend with the increase of annealing temperature. There is a sharp increase in the rate, followed by a gradual slowdown, and the yield strength finally stabilizes at around 320 MPa. The elongation of the sheet has an obvious “S”-shaped change with the increase of the annealing temperature. When the annealing temperature is lower than 700 ℃ When the annealing temperature is increased, the elongation of the sheet is basically maintained below 2.5%, although the strength of the sheet decreases significantly, and the elongation increases rapidly with the further increase of the annealing temperature. The tensile stress-strain curve was processed to obtain the value of the work hardening exponent n of the 430 cold-rolled sheet under different annealing states. After the temperature is higher than 810 ℃, the n value is basically stable at about 0.26. During the annealing process of the 430 stainless steel cold-rolled sheet, with the increase of the annealing temperature, the residual strain inside the material decreases, which makes the work hardening rate of the sheet gradually increase. When the annealing temperature is higher than 810 °C, the sheet is almost completely softened, and the n value remains basically unchanged.
(1) A reasonable vacuum annealing process system for 430 stainless steel cold-rolled sheet was obtained through simulated annealing experiments: the heating rate of the heating section I could not be precisely controlled, and the intermediate temperature was selected as 600 °C; the heating rate of the heating section II was maintained at 6.9 °C·S -1, the maximum annealing temperature is 840 ℃.
(2) With the increase of the maximum annealing temperature, the yield strength and hardness of the 430 stainless steel cold-rolled sheet show an obvious two-stage decreasing trend, and the elongation shows an “S”-shaped increasing trend; the maximum annealing temperature has a significant effect on the average plastic strain ratio r The effect is small, and the r value is maintained at about 1.25; the anisotropy index Δr of the rolling plane decreases with the increase of the highest annealing temperature, indicating that a higher annealing temperature is beneficial to increase the wrinkle resistance of the sheet.
Annealing Equipment Selection: RVA vacuum annealing furnace produced by SIMUWU is an excellent choice for completing this type of process. Its process performance can fully meet the needs of such thermal processing, with good temperature control accuracy, temperature uniformity and annealing uniformity. High process repeatability, stable production, quality output can be guaranteed.
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