Microstructure analysis of hot-rolled NM500/Q345/NM500 composite interface

Abstract

Rolling reduction rate is one of the important factors affecting the quality of the bonding interface of the composite plate. This paper studies the influence of rolling reduction on the interface microstructure of the NM500/Q345R/NM500 composite plate. Using optical microscope, scanning electron microscope and other means, the interface microstructure, interface element distribution and defect components of the composite plates with different reduction ratios of 30%, 50%, and 70% were analyzed. With the increasing in the reduction rate, the interface grains became fine, the difference in the size of the two sides of the organization decreased, the distribution of ferrite and pearlite was more even, and the content of pearlite increased. When the reduction rate was 30% and 50%, holes and granular defects exist at the interface of the composite plate. The defects were oxides and partial sulphides of Si and Mn. The interface oxides gradually were broke with the increasing of the rolling rate. The oxides number decreased, and the oxides size becames smaller and smaller. The composite plate with a reduction ratio of 70% had uniform interface structure and elements, and basically no obvious hole defects. During the rolling process of composite plate, there was a discontinuous dynamic recrystallization mechanism at the interface of the composite plate, and there was no obvious preferred orientation. When the rolling ratio reaches 70%, the grains of NM500 and Q345R formed nearly the same small-sized equiaxed crystal.