The Effect of Initial Microstructure on Microstructure Evolution and Mechanical Properties of Intercritically Rolled Low-Carbon Microalloyed Steel Plates

Steel plates rolled in the intercritical region have ultrafine and elongated-grained microstructures, and excellent toughness. The effect of the initial microstructure on microstructure evolution and the mechanical properties of intercritically rolled (IR) low-carbon microalloyed steel plates is investigated herein. Coarse-grained initial microstructure mainly containing lath bainite and fine-grained initial microstructure mainly containing quasi-polygonal ferrite are produced by austenitization treatments at different temperatures. The results show that the coarse-grained initial microstructure restrains continuous dynamic recrystallization during intercritical rolling, which leads to a less fraction of high-angle grain boundaries (HAGBs). However, initial microstructure has little influence on the average size of ferrite grains surrounded by HAGBs. Moreover, coarse-grained initial microstructure results in a relatively higher strength improving about 15 MPa due to the lower fraction of HAGBs. Fine-grained initial microstructure is beneficial to the Charpy impact toughness such that the upper shelf energy is improved from 108 to 118 J, and low-temperature toughness is also enhanced. However, the effect of the initial microstructure on the microstructure and mechanical properties of IR rolled steel plates is not great, which indicates that coarse-grained initial microstructure can also be used to develop this kind of steel plates with excellent toughness.