The cold rolling strain effect on the tensile properties of a high nitrogen alloyed austenitic steel was systemically investigated. Quasi-static tensile experiments were performed on the samples with different cold rolling strain. The material possessed good balance between strength and ductility in the entire rolling strain range. Mechanical property and microstructure of the nitrogen alloyed austenitic steel were greatly affected by cold rolling strain. With the increase in cold rolling strain, the strength increased sharply but the ductility declined. That is related to the gradual changes of microstructure induced by the cold rolling process. The cold rolling process leads to substantial microstructural change, from the appearance of slip bands and twins at low cold rolling strain; the microtwins formation at intermediate cold rolling strain and followed by sequences of their bending, breaking and disappearance; and finally to the formation of drossy twins at the high cold rolling strain.
The plastic deformation and fracture behaviour of a high-nitrogen nickel-free austenitic stainless steel were examined by performing tensile testing at room temperature and at a wide strain rate range. The tensile testing demonstrated that this steel shows a significant strain rate dependence of the strength and ductility. With increasing strain rate from 10−4 to 1 s−1, the yield strength increases from 673 to 801 MPa and the ultimate tensile strength increases from 958 to 1003 MPa; the uniform elongation decreases from 75 to 44% and the total elongation decreases from 86 to 58%. The analysis of the stress–strain curve by using the Ludwigson equation showed that this steel exhibits a two-stage strain hardening behaviour, the strain hardening exponents at low and high strain regions ([Formula: see text] and [Formula: see text]) and the transition strain ([Formula: see text]) decrease with increasing strain rate. Based on the analysis results of the stress–strain curve, the transmission electron microscopy characterisation of the microstructure and the scanning electron microscopy observation of the deformation and fracture surfaces, the significant strain rate dependence of the strength and ductility of this steel was discussed and connected with the variation in the dislocation activity with strain rate.
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