Microstructure and strength of a bainitic steel for underground mine equipment

Abstract

To develop high strength steel for underground mine equipment, the microstructure and mechanical property at two different finish cooling temperatures were investigated. The primary microstructures were massive granular bainite. Coarse martensite-austenite constituents as well as a small amount of quasi-polygonal ferrite and degenerated pearlite formed at the finish cooling temperature of 630°. As the finish cooling temperature was lowered to 580°, the steel showed higher yield and ultimate tensile strength of 605 MPa and 870 MPa, respectively. Besides grain coarsening, the strength reduction due to increasing finish cooling temperature is mainly attributed to the existence of the small amount of quasi-polygonal ferrite based on the analysis for available strength models. The effect of a relatively higher proportion of coarse martensite-austenite constituents on tensile property is weakened when small amounts of quasi-polygonal ferrite in granular bainitic steel appear. Thus, a new yield strength model is proposed for granular bainitic steel. The steel also showed superior high temperature strength at 500°, which exceeded 70 pct of that at room temperature.