Effect of austenite grain structure on the strength and toughness of direct-quenched martensite

The effect of prior austenite grain structure on the microstructure and properties of two low alloyed hot-rolled and direct-quenched martensitic steels was investigated. Strength properties were determined using uniaxial tensile testing, while toughness properties were characterized by using Charpy-V and fracture toughness tests. Microstructures were characterized using OM, SEM, EBSD and a novel EBSD-based image quality (IQ) technique. It was found that an increase in the rolling reduction in the non-recrystallization temperature regime of austenite was an effective way to improve the strength, impact and fracture toughness without a significant decrease in uniform elongation. In addition, this approach also decreased the in-plane anisotropy of the tensile and toughness properties. Refinement of the equiaxed austenite grain structure during rolling in the recrystallization temperature regime (roughing) also improved the strength and toughness properties, but the effect was found to be weaker than the effect of rolling in the non-reaystallization regime. In toughness testing, a correlation was found between the Charpy-V 28J transition temperature and the fracture toughness characteristic temperature T-0. However, the correlation differs significantly from that reported in the literature for lower strength ferritic steels. In all cases, the steel microstructures consisted of mainly auto-tempered martensite and lower bainite. A brief discussion of the microstructural features controlling the strength and toughness properties is given. (C) 2012 Elsevier B.V. All rights reserved.