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304 stainless steel cold rolling and vacuum annealing process

Stainless steel has excellent quality and characteristics, so its application range is getting wider and wider, and the production and demand of stainless steel around the world have been showing a trend of continuous growth. As a general-purpose corrosion-resistant material, 304 stainless steel sheet can be cold-rolled to obtain high-performance products, which can be used in many fields including food industry equipment, kitchen utensils, and electronic industries. Different uses in various fields have different performance requirements for cold-rolled stainless steel 304. Especially when the thickness of the stainless steel strip is small, it is extremely important to seek its comprehensive cold working performance. This paper analyzes the influence of rolling and vacuum annealing process on its properties and microstructure, and studies the influence of production process parameters on its microstructure and properties, so as to provide experimental data for field optimization of production and performance improvement.

1.Experimental materials and methods

1.1 Experimental materials

The experimental material is 304 stainless steel produced by a factory, and its chemical composition (mass fraction, %)

It is: 0.0528C, 0.5166Si, 0.03P, 1.1983Mn, 0.0016S, 17.016Cr, 8.0061Ni, 0.1989Cu, 0.083Mo, 0.0087Sn.

1.2Experimental method

Vacuum annealing at different temperatures (1060, 1080 and 1100 °C) for different times (2, 5 and 8 min) was carried out in a resistance furnace. The metallographic structure of the samples after vacuum annealing was observed on an optical microscope, and the microstructure characteristics of the field annealed state and the microstructure characteristics of the cold-rolled state with a thickness of 0.60 mm after annealing in the laboratory were analyzed. The tensile test was carried out on the electronic universal testing machine, the strength was measured and the n and r values were calculated; and the hardness was measured.

2.Experimental results and analysis

2.1 Influence of cold rolling process on microstructure and r value

The thickness of the hot-rolled stock with a thickness of 2.0 mm is 0.45 mm and 0.38 mm after cold rolling, the corresponding reduction ratios are 77.5% and 80%, and the annealing temperature is 1080 °C. Its structure is shown in Figure 1. The properties obtained in the tensile test are shown in Table 1.

It can be seen from Figure 1 that with the increase of cold rolling reduction, the grains have a significant growth trend, and the conventional mechanical properties have no obvious change. When the deformation rate was increased from 77.5% to 80%, the tensile strength decreased by only 5.3%, while the r value increased by 12.4%. Combined with the annealed structure, it can be seen that increasing the cold rolling reduction rate, the grains grow significantly. This is due to the increase of the deformation storage energy, which makes the recrystallization driving force larger and promotes the growth of the grains. The result is beneficial to Improve the stamping properties of the material.

2 Influence of vacuum annealing process on microstructure and properties

2.2.1 Effect of vacuum annealing process on microstructure

The cold-rolled 0.6mm material was taken on site and annealed at different temperatures (1060, 1080 and 1100 °C) for different times (2, 5 and 8 min) in the laboratory, and its microstructure is shown in Figures 2 to 4. It can be seen that under the same annealing temperature, with the prolongation of the annealing time, the grains grow; under the same annealing time (5 min), the grains grow gradually with the increase of the annealing temperature. It can be seen that during the annealing process, increasing the annealing temperature and prolonging the annealing time, the grains tend to grow, and a higher r value can be obtained, which is beneficial to improving the stamping performance.

2.2.2 Effect of annealing process on mechanical properties and r value

The cold-rolled material with a thickness of 0.6 mm was subjected to a tensile test after vacuum annealing, and the variation law of r is shown in the figure. It can be seen that the r value tends to decrease with the prolongation of annealing time. The change trend is small at 1060 and 1080 °C; however, the change is larger at 1100 °C. When the annealing time is 2-5 min, the yield ratio decreases and r increases gradually, which can improve the stamping performance; while the annealing time is 5 At -8min, the yield ratio increases and the r value decreases gradually, which is not conducive to the improvement of stamping performance. When the annealing time is 5 and 8 min, r increases gradually with the increase of annealing temperature. Comprehensive analysis of microstructure characteristics and performance changes, when the temperature is 1100 ℃ and the time is 5min, the grains are coarse and uniform, the r value is the largest, reaching 1.39, and the yield ratio is the smallest, which is 0.36, which can improve the stamping performance and meet the material properties. Require.

2.2.3 Hardness test

The hardness test results of 304 stainless steel are shown in the table. It can be seen that as the annealing temperature increases and the annealing time increases, the hardness of the material decreases. From the analysis of metallographic photos, it can be seen that with the increase of annealing temperature and the prolongation of annealing time, the grains are coarser, so the hardness decreases. However, after annealing at 1100°C x 5min, the grains are larger and uniform, so the hardness is the smallest, and the r value is the largest.

3 Conclusions

(1) With the increase of cold rolling reduction rate, the grain size becomes larger and the r value increases. Therefore, increasing the cold rolling reduction rate can improve the deep drawing performance of the material.

(2) With the increase of the annealing temperature and the prolongation of the holding time, the grain size increases, the yield ratio decreases, and the grain size is uniform, which helps to enhance the deep drawing performance of the material.

(3) According to the analysis results of this experiment, in order to obtain a higher r value without losing other properties, the more suitable vacuum annealing process in production is 1100℃×5 min.