The effects of substituting Ti for V and post-rolling treatments on mechanical properties in weathering type high-strength low-alloy steels were investigated. The post controlled-rolling heat treatments were air-cooling, accelerated cooling by water-spraying, and normalizing.
Accelerated cooling greatly enhanced the strength and impact toughness in the Ti-bearing steel. Ti-bearing steels also exhibited better isotropic impact behavior. Normalized Ti-bearing steel showed lower tensile strength, but higher impact toughness than air-cooled steel.
V-bearing steel maintained its strength and toughness after normamlizing, and this phenomenon was attributed to re-precipitation of V-carbide after normalizing.
Fully reversed axial strain controlled low cycle fatigue tests were conducted using Instron servohydraulic test machine. All of the steels cyclically hardened, but the amount of cyclic hardening varied with post-rolling treatments. Water-spraying produced initial cyclic hardening and softening, whereas air-cooling or normalizing yielded cyclic hardening modes. The water-sprayed Ti-bearing steel exhibited better strain life fatigue resistance than that of the air-cooled Ti-steel. Normalized Ti-bearing steel displayed the best strain-life resistance. The fatigue resistance was increased by adding titanium, compared with that of C-Mn steel. At 2.0% total strain, the fatigue life was doubled by alloying with 0.1 wt.% Ti. The increase of fatigue resistance by accelerated cooling was attributed to increase of material ductility. The increase of fatigue lives at normalizing treatment was attributed to slip dispersal enhanced by coarse TiC particles. The fatigue resistance of V-bearing steel, however, was similar regardless of post-rolling treatments.
High cycle fatigue tests were conducted using R.R. Moore type rotating bending fatigue test machine. The fatigue strength of air-cooled Ti-bearing steel was 18\% higher than that of air-cooled V-bearing steel. This was attributed to the increase of tensile strength of Ti-steel.
