Abstract
The microstructural mechanisms providing delamination toughness in high-strength low-alloyed steels are briefly reviewed. Thermo-mechanical processing methods improving both the strength and impact toughness are described, with a close relation to the microstructures and textures developed. The effect of processing conditions on the microstructure evolution in steels with different carbon content is discussed. Particular attention is paid to tempforming treatment, which has been recently introduced as a promising processing method for high-strength low-alloyed steel semi-products with beneficial combination of strength and impact toughness. Tempforming consists of large strain warm rolling following tempering. In contrast to ausforming, the steels subjected to tempforming may exhibit an unusual increase in the impact toughness with a decrease in test temperature below room temperature. This phenomenon is attributed to the notch blunting owing to easy splitting (delamination) crosswise to the principle crack propagation. The relationships between the crack propagation mode, the delamination fracture, and the load-displacement curve are presented and discussed. Further perspectives of tempforming applications and promising research directions are outlined.
Highlights
Carbon steels are widely used structural materials
Fact, the grain refinement increases both the the brittle fracture stress ductile fracture can be expanded toward low temperatures due to the increase in brittle fracture effective yield stress
This beneficial effect of grain size on mechanical properties has been encouraging the development of processing methods involving the grain refinement
Summary
Carbon steels are widely used structural materials. One of drawbacks of such steels is their relatively high temperature of ductile-brittle transition in the hardened state that makes the steels brittle at temperatures just below room temperature and, correspondingly, restricts their applications at lowered temperatures. A decrease in the effective flow stress at the crack tip can be achieved in the microstructure, which spontaneously delaminates ahead of the crack crosswise to the crack propagation direction and, blunts the crack tip Both approaches, i.e., the grain refinement and the delamination, have been utilized through thermo-mechanical treatment that is commonly known as ausforming, under the conditions of stable or meta-stable austenite. The proposed method consists in the formation of a submicrocrystalline lamella type microstructure with a uniform dispersion of secondary phase particles by means of warm rolling under conditions of tempering Such thermo-mechanical treatment, which has been referred to as tempforming, provides an outstanding combination of mechanical properties in low-alloyed carbon steels.
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