Improving toughness and strain-hardening of a silicon-enriched ferritic/martensitic (F/M) steel via an austenite isothermal hot-rolling (AIHR) process

Abstract

Si-enriched ferritic/martensitic (F/M) heat resistance steels are emerged as structural materials in advanced thermal and nuclear power stations attributed to their satisfactory corrosion resistance to coolants. However, the low toughness from high Si content over 0.5 wt% due to the formation of weaking δ-ferrite phase and the weak bonding between ferrite and martensite has been a tough trouble for application. Long time annealing at austenitizing temperature could alleviate ferrite phase but at the sacrifice of coarsening carbides and grains. Here we present an austenite isothermal hot-rolling (AIHR) strategy in which multi-pass rolling and limited annealing time are conducted at isothermal states above the austenitizing temperature. This increased toughness by almost 700% (from 7 to 56 J/cm2) meanwhile the same yield strength (787 MPa) compared to the normal quenching + tempering treatment (NT, 757 MPa). The control of element diffusion and dynamic recrystallization (DRX) by AIHR accelerated the dissolution of ferrite within short process time, coupled with the reduced {100} cleavage plane & texture and the reduction of grain size and M23C6 phases, which are responsible for the improved toughness. Besides, higher strain hardening exponent accompanying refined microstructure and primary dislocation density implies deformation stability of the steel via AIHR process. These results provided a possible way of tailoring both the strength/toughness trade-off and the hardening ability on a Si-enriched F/M steel.