Microstructure Characteristics and Mechanical Properties of In-situ Composite Steel Processed by Severe Cold-rolling and Subsequent Annealing

In this paper, Q235 steel was investigated in order to manufacturing ultra-high strength material. The process of severe cold-rolling and low temperature annealing of lath martensite effectively reduced the crystal size from about 300 nm to 20 nm, and introduced mass weak interfaces in steel, has been demonstrated a new promising technique for producing in-situ composite multi-nanolayer steel with ultra-high strength (sigma(b) 2112 MPa). Cold rolling and subsequent annealing have great impact on microstructure evolution as well as material mechanical properties. In the as-rolled state, the strength is approximately four times increased than as-received material (hot-rolled state, at, 515 MPa), which is attributed to work hardening and grain refining during cold rolling. As the cold-rolled sample subjected to further annealing below 500 degrees C, deformed microstructure underwent further recovery and recrystallization, finally became refined equiaxed grains, microstructure characteristics along rolling direction arrangement was decreased; In addition to ultrafine ferrite grains, nano-carbides precipitated uniformly in the specimen annealed at 500 degrees C, total elongation increased to 16%, the corresponding yield strength was 1208MPa, much higher than that of as-received samples. The phenomenon of fracture delamination was observed from the specimens, which were cold-rolled and annealed at 500 degrees C and the delamination plane was parallel to the rolling plane. In-situ composite weak interfaces effect has great impact on the fracture surface.