Influence of strain-induced martensite on tensile properties of metastable duplex stainless steels consisting of Fe-Cr-Mn-Ni and Fe-Cr-Mn-N

The effects of Ni or N both on the austenite stability and the tensile properties of duplex stainless steels were investigated at various temperatures. Two series of duplex stainless steel sheets consisting of Fe-(19-22)%Cr-5%Mn-(4-7)%Ni and Fe-(19-22)%Cr-5%Mn-(0.19-0.34)%N were employed. 20%Cr-5%Mn-5%Ni steel and 20%Cr-5%Mn-0.25%N steel indicated maximum improvement in elongation under tensile tests at 293 K among each series of specimens. The amount of strain-induced martensite was measured, indicating that there is the optimum transformation rate of strain-induced martensite with strain to obtain the maximum elongation under transformation-induced plasticity (TRIP). 20%Cr-5%Mn-0.25%N steel exhibited both extremely high elongation at room temperature equivalent to the conventional austenitic stainless steels of SUS304, and high tensile strength equivalent to the conventional duplex stainless steels of SUS329J4L. The total elongation of 20%Cr-5%Mn-0.25%N steel was larger than that of 20%Cr-5%Mn-5%Ni, though there is little difference between them in the average amount of strain-induced martensite introduced during the uniform deformation. The difference in elongations between 20%Cr-5%Mn-0.25%N and 20%Cr-5%Mn-5%Ni could be due to the difference in the hardness of the strain-induced martensite. The strain-induced mar-tensite in 20%Cr-5%Mn-0.25%N steel was extremely hardened by the nitrogen concentrated to the austenite phase at annealing. Such hard martensite maintained high strain-hardening rate in a wide range of strain and increased the uniform elongation with high tensile strength.