Influence of Hot Rolling Process on Microstructure and Properties of Low-strength IF Steel

Abstract

On the basis of the successful developed ferrite zone rolling process technology in a hot rolling plant production line， a comparative study was conducted to investigate the effects of two hot rolling processes， on the metallographic microstructure， precipitation phases， macroscopic texture and mechanical properties of low-strength IF steel. OM， TEM and XRD diffraction were used to compare and analyse the differences in metallographic microstructure， precipitation phases and macroscopic texture of IF steel in hot-rolled and cold-rolled annealed condition under the two rolling processes， and at the same time， the three-way tensile mechanical properties of IF steel were tested by the ZWICK tensile tester after cold-rolled annealing. The results show that the microstructure of hot-rolled IF steels in the austenitic zone was equiaxed grains， while the grains of hot-rolled IF steel in the ferrite zone are elongated and fibrous along the rolling direction； Compared with the austenitic zone hot rolling process， the optimum process parameters for ferrite zone rolling are heating temperature 1 150 ℃+final rolling temperature 800 ℃+coiling temperature 720 ℃， in which the yield strength Rp0.2 is significantly reduced， and the size of precipitated phases is overall larger by 70-85 nm， but there is no significant difference in the size of TiN precipitated phases； In terms of texture， cold-rolled annealed IF steel γ texture is stronger， and the orientation distribution of each component in the main texture is more concentrated with smaller differences in strength.The corresponding r value increases by 0.64， while the △r value decreases by 0.35， significantly improving its deep drawing formability. This provides a technical reference for promoting the application of ferrite rolling technology.

Keywords

• low-strength if steel； ferrite rolling； microstructure property； precipitate； texture