Microstructure Refinement of Structural Steel in China
In Oct. 1998 Chinese government opened national program: “Fundamental Research on New Generation of Iron and Steel Materials in China” (N.G. Steels). The progress of on-going project has been briefly reported in this paper. In order to get microstructure refinement in structural steels, the following 5 ways have been studied:
For Ferrite+Pearlite microstructure in plain carbon steels and low-alloy steels,
(1) Purified steel-making→Fully equiaxed continuous casting→Rough-mill with higher Zener-Hollman parameter (γ-DRX refinement)→Finishing-mill rolling by Deformation Induce Ferrite Transformation (DIFT) technology.
(2) For thin-slab continuous casting and rolling (CSP) process, ultra-fine sulfides (20-60 nm) and oxides (5-20nm) controlling technology.
(3) For Low Carbon Bainite (and Ultra Low Carbon Bainite) microstructure (LCB/ULCB) in microalloyed steels, combination of Deformation Induced Precipitation (DIP) and subsequent middle temperature phase transformation controlling.
For quench-temperated martensite microstructure in alloy structural steels:
(4) Ultra-fine γ grains by innovated heat treatment and alloy design;
(5) Carbide-free bainite/martensite microstructure with retained γ films along lath/sub-lath boundaries.