| Low nickel austenitic stainless steels are Cr-Mn-N stainless steels developed by using manganese and nitrogen instead of scarce and expensive nickel element. Because of its good corrosion resistance, high and low temperature properties, it took place of 300 series stainless steel on many occasions. However, low stability and strain-induced α’-martensite of low nickel stainless steels were not conductive to drawing, and fracture behavior of sheet metals that suffered of delayed cracking after deep drawing hindered its widespread use. Therefore, the main purpose of this study was to find the most economical and effective way to solve delayed cracking. 201 stainless steel was chosen as the research material, the following works were carried out:(1) Martensitic transformation affected the mechanical properties of the material. Material in different situations exhibited different mechanical properties because of martensite was sensitive to strain rate and temperature. Therefore, this paper studied the mechanical properties of 201 stainless steel at the strain rate of 0.001~0.1s-1 and temperature within the range of 25~200 ℃ by uniaxial tensile test. The results shown that true strain and strain-induced α’-martensite had positive correlation. Martensitic transformation gave rise to the secondary hardening of the stress-strain curve. The elongation of the material increases and the martensite reduced with the rise of temperature. when the deformation temperature reached 100℃, there would be no longer martensitic transformation. The largest elongation would be obtained in each strain rate under 50℃. Yield strength, tensile strength and yield ratio increased with the increase of strain rate, martensitic transformation decreased as well. The ductility was best at 0.001s-1 at each temperature.(2) The reversion law of martensite was studied by isothermal annealing to provide theory guidance for the annealing process research. Tests shown that the critical temperature of strain-induced α’-martensite reversion was about 550℃. With the increase of holding time, the reversion starting temperature decreased, and that a complete reversion process could be finished within 15~20s at 800℃, within 10 s at 900℃. After two-stage deep drawing of 201 stainless steel, martensite in the wall was linear distribution. After annealing of the flange part delayed cracking occurred in the region with high martensite volume fraction. The cup did not suffer from delayed cracking with annealing the flange and the wall.(3) The finite element model of the differential temperature drawing was established to analyze forming process by ABAQUS. Effect of die and punch temperature on forming quality was analyzed from the temperature and thickness distribution of the cup. The results shown that the surface temperature of drawing part presented the same gradient changes at different die temperature. The maximum reduction rate increased with increasing die temperature; When the die temperature was constant, the rise of punch temperature would increase the maximum reduction rate. In order to enable the smooth progress of differential temperature drawing, punch temperature should be control below 40℃.(4) Designed and built test platform of the differential temperature drawing. The effect of temperature on the forming quality was studied. The results shown that differential temperature drawing could improve the forming limit of 201 stainless steels. The increase of forming temperature would reduce the martensitic transformation. The incubation time to fracture and the number of cracks depended chiefly on the amount of strain induced phase transformation. Forming at 100℃ and above, no phase transformation and delayed cracking would occur. The result of orthogonal test shown that the influence of the forming temperature had become the main factor in differential temperature drawing. And obtained the optimization working parameters were as follow: forming temperature 100℃, punch velocity 10mm/s, no insulation. |
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