Microstructure and Mechanical Properties of Ultra-high Strength Steel with Different Cooling Process

Abstract

Influences of different colling processes on microstructure transformation and mechanical properties and microstructure of ultra-high strength steels(UHSS) were investigated. microstructure and distribution characteristics were obtained. Samples were proceeded by a controlled rolling (CR) followed by a second step of the direct air cooling (AC), controlled cooling (CC) and direct quenching(DQ), respectively. Experimental results of CR+AC process showed that hot rolling microstructure was 0.2-0.3μm slender bamboo-like lath bainitic ferrite. CR+CC process acquired higher strength σ0.2=915MPa, σb=1640MPa, better tenacity and lower yield ratio σ0.2/σb=0.56, αkv=52.13Jcm1/2. ductile fracture happened during CR+AC. Multiple rupture mechanisms of quasi- cleavage and ductile fracture existed in CR+CC process. blockish structure and volume fraction of retained austenite decreased, while carbon content increased compared with CR+AC process.The fracture of the alloys was observed to be in quasi-cleavage failure mode during CR+DQ process, blocky structure in retained austenite disappeared basically, volume fraction and carbon content lower than that in CR+CC process.