The effect of cold-rolling prior to the inter-critical heat treatment on microstructure and mechanical properties of 4340 steel with ferrite - Martensite microstructure

This study investigates the effect of cold rolling prior to inter-critical annealing on microstructure and mechanical properties of ferrite-martensite dual phase steel. Samples were heated to 850 degrees C for 1 h followed by oil quenching, then the steel sheet were cold rolled by 0%,10%,15% and 20% reduction in thickness. The intercritical annealing treatment (750 degrees C, 120min) was performed to generate a ferrite-martensite microstructure. Microstructural studies showed that increasing the applied cold rolling, leading to increase in volume fraction of martensite and decrease in ferrite grain size. Mechanical properties of dual phase steel were measured by tensile, impact and hardness tests. Results showed that ultimate tensile strength, yield strength, micro-hardness and toughness (e.g. total elongation, uniform elongation and impact energy) increased with increasing the applied cold rolling. Improvement of mechanical properties were related to increase in martensite volume fraction and ferrite grain refinement. Analysis of strain hardening behavior of DP steels, by Hollomon analysis, showed two stages of strain hardening corresponding to ferrite deformation and co-deformation of ferrite and martensite, respectively. The strain hardening exponent of first stage (n(I)) increased with increasing volume fraction of martensite. The energy absorption capacity (UE x UTS) increased with increasing cold rolling deformation.