Study on the Formation of Ferrite-Cementite Microstructure by Strain Induced Dynamic Transformation in Medium carbon Steels

摘要

Cold heading quality (hereafter CHQ) steels are widely used in automobile and construction industries. The various processes involved in the production of wire rod from CHQ steels are drawing, spheroidization annealing, forming, and quenching/tempering. Especially, the spheroidization annealing is inevitably used to soften the wire rod before severe forming. However, due to the economical constraints, many efforts have been made to shorten or avoid the spheroidization annealing process. One of the possible ways is to produce the ferrite-cementite (FC) microstructure directly after the drawing (rolling) process without performing any subsequent spheroidization annealing. In the present study, the effect of SIDT (Strain Induced Dynamic Transformation) on the microstructure of medium carbon steels was investigated to develop spheroidized annealing-free steel wire rods. When 0.45%C steels were hot deformed by heavy reduction at low temperatures conditions, deformed microstructures were quite different from conventional ferrite-pearlite structure. For example, the microstructure containing 57 vol.% of ferrite, which is far beyond the equilibrium ferrite fraction of 43 vol.% of 0.45%C steel was obtained by deformation at 700°C. Moreover, as deformation temperature decreased to 675°C, the pearlite structure disappeared completely and new microstructure composed of ferrite and cementite appeared. It was considered that this FC microstructure was obtained owing to small retained austenite grains existing between fine SIDT ferrites prefer to transform to cementite and ferrite instead of pearlite during cooling. Finally, the optimum conditions for producing the ferrite-cementite microstructure have been identified.