Enhanced mechanical properties of a high-carbon martensite steel processed by heavy warm rolling and tempering

Abstract

The strength-ductility trade-off in steel, especially high-carbon steel, has been a long-standing and troubling issue; steel with increased mechanical properties has attracted much attention in recent years. In this study, an ultrafine-grained ultrastrong high-carbon steel with good ductility was produced by stabilizing retained austenite (RA) and accelerating carbide precipitation by warm rolling, quenching and tempering. The ultimate tensile strength reaches approximately 2550 MPa, with a total elongation of 5.1% after tempering at 220 °C. The corresponding microstructure was composed of stable high-carbon RA and ultrafine-grained tempered martensite with carbide precipitates. The thermodynamic stability of RA was increased by carbon partitioning during quenching. The volume fraction was optimized to 6%. As the grain size of the primary austenite was refined to approximately 3 μm or smaller by warm rolling along the normal direction, the substructure of the high-carbon martensite was characterized as ultrafine twins and high-density dislocations. Carbide precipitations from the martensite matrix are accelerated at lower tempering temperatures. The main strengthening mechanism was still the strengthening of the solid solution and dislocations. Good tensile ductility was mainly derived from the effective weakening of the solid solution of martensite, whose carbon content decreased from 0.86 wt% to 0.61 wt%. Additionally, the greater ductility was attributed to the transformation-induced plasticity effect of larger amounts of high-carbon RA. These results may help in the design and production of ultrahigh strength and ductile high-carbon steel.