Mechanical Property of Ultrafine Elongated Grain Structure Steel Processed by Warm Tempforming and Its Application to Ultra-High-Strength Bolt

Abstract

Synopsis : Our strategy to enhance fracture properties of ultra-high-strength low-alloy steel with a yield strength of 1.4 GPa or over is to arrest the propagation of brittle crack in a hierarchical, anisotropic and ultrafine grain structure designed to be fail-safe, in addition to suppressing the crack initiation. The present article reviews strength, ductility, toughness and delayed fracture resistance of ultra-high-strength low-alloy steel with an ultrafine elongated grain structure that was processed by deformation of a tempered martensitic structure at an elevated temperature (warm tempforming). The evolution of heterogenous microstructure during warm tempforming using multi-pass caliber rolling and the microstructural factors controlling the strength and the fracture properties of the warm tempformed steels are discussed. Furthermore, we introduce the application of the warm tempformed steel with an ultrafine elongated grain structure to ultra-high-strength bolt.

Effect of Ultrasonic Treatment on Microstructure and Properties of 2000 MPa Ultra-High-Strength Steel-Welded Joints

“…Currently, research on the performance enhancement of ultra-high-strength steel primarily focuses on improving the steel's microstructure and refining grain size. Kimura Y et al [7] studied the mechanical properties such as strength, ductility, toughness, and resistance to the delayed fracture of ultra-fine grain martensitic high-strength steel and analyzed the effect of high-strength steel grain size on mechanical properties. Dolzhenko P et al [8] analyzed the explanation for grain structure modifications in ultrafine-grained steel during plastic deformation.…”

Mechanisms of Grain Structure Evolution in a Quenched Medium Carbon Steel during Warm Deformation

“…For instance, according to Reference [6], an as-quenched Fe-1.2wt.%C steel had better workability in comparison with that in an initially spheroidized condition. One of the attractive strategies of warm deformation to receive an excellent balance of strength, ductility and toughness in a wide range of temperatures is the formation of an elongated fine-grained (EFG) structure with nanoscale carbide particles and a strong deformation texture [7], that has recently been applied in industry [8]. This microstructure can be obtained using warm deformation of as-quenched MCLA steels with the martensitic structure [7][8][9][10]-this working is sometimes defined as tempforming [11].…”

“…One of the attractive strategies of warm deformation to receive an excellent balance of strength, ductility and toughness in a wide range of temperatures is the formation of an elongated fine-grained (EFG) structure with nanoscale carbide particles and a strong deformation texture [7], that has recently been applied in industry [8]. This microstructure can be obtained using warm deformation of as-quenched MCLA steels with the martensitic structure [7][8][9][10]-this working is sometimes defined as tempforming [11]. During warm deformation, various processes, including precipitation and/or spheroidization of carbide particles, dynamic recovery, dynamic recrystallization, elongation of ferritic grans along the deformation direction and texture formation, can develop in MCLA steels with the martensitic structure [11][12][13].…”