Influence of ultrasonic surface rolling on tensile properties of high carbon low alloy quenching-partitioning-tempering steel

Abstract

The effect of ultrasonic surface rolling process on tensile properties of a novel Fe-0.6C-2.0Al-1.5Mn-0.04 Nb quenching-partitioning-tempering steel was investigated. Optical microscope, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy were brought out to characterize the microstructure. Compared with quenching-partitioning-tempering steel, the strength and plasticity of the ultrasonic rolling specimen were increased simultaneously, reaching 1329.1 MPa and 23.6%, respectively, and the product of strength and elongation reached the range of the third generation Advanced High Strength Steels. The plasticity improvement mechanism of the USRP1specimen is mainly stemmed from two aspects: First, obvious strain gradients during the tensile process was found in the USRP specimens, which not only avoids the strain localization, but also generates a biaxial stress state. The geometrically necessary dislocations induced by the strain gradient and the dislocations induced by the biaxial stress state promoted extra work hardening, thus ensuring high plasticity of the material. Second, there was a moderate content and small size of twinned martensite in the USRP1 specimen. The improvement in yield strength of the USRP specimens primarily results from grain refinement strengthening, dislocation strengthening, phase transformation strengthening, and back stress strengthening. Among them, phase transformation strengthening contributed the most.