Effect of heat treatment on microstructure and mechanical properties of trip seamless steel tube

Abstract

A low-carbon TRIP seamless steel tube, which is expected to be used in the hydroforming process, was successfully fabricated using piercing, cold-drawing and two-stage heat treatment process. In this study, to maximize the volume fraction and stability of retained austenite as well as to obtain a TRIP seamless steel tube with good combination of strength and ductility, the optimal heat treatment conditions (intercritical annealing "IA" and isothermal bainite treatment "IBT") were investigated. The influence of heat treatment on the microstructure of the TRIP seamless steel tube was studied via optical microscopy, TEM and XRD. The mechanical properties in axial and circumferential directions of TRIP seamless steel tube were also evaluated by conventional tensile test and the ring tensile test developed by the author, respectively. The results show that for a particular set of IA and IBT temperature, the volume fraction of retained austenite increased with increasing IA holding time but decreased with the increase of IBT holding time in the set of time range in this study. As a result, the high retained austenite volume fraction of 10.26% was obtained with IA holding time of 5 min at IA temperature of 810°C, IBT holding time of 6 min at temperature of 410°C. And this sample possessed a UTS of 542 MPa, YS of 355 MPa and EL of 35% in the circumferential direction obtained by the ring tensile test. © 2012 The Japan Institute of Metals OVERVIEW.