Inverse Temperature Dependence of Toughness in an Ultrafine Grain-Structure Steel

Abstract

Materials are typically ductile at higher temperatures and become brittle at lower temperatures. In contrast to the typical ductile-to-brittle transition behavior of body-centered cubic (bcc) steels, we observed an inverse temperature dependence of toughness in an ultrahigh-strength bcc steel with an ultrafine elongated ferrite grain structure that was processed by a thermomechanical treatment without the addition of a large amount of an alloying element. The enhanced toughness is attributed to a delamination that was a result of crack branching on the aligned {100} cleavage planes in the bundles of the ultrafine elongated ferrite grains strengthened by nanometer-sized carbides. In the temperature range from 60 degrees to -60 degrees C, the yield strength was greater, leading to the enhancement of the toughness.

Influence of Carbon Content on Toughening in Ultrafine Elongated Grain Structure Steels

“…[4][5][6] It was clear that the transverse grain size, the grain shape and the <110>//RD fiber texture in the UFEG structure were the dominant microstructural factors in controlling the delamination. [4][5][6][7] Furthermore, the carbon content may influence the microstructural features of the UFEG structures and the occurrence of delamination for the warm tempformed steels; however systematic investigation on this subject has not yet been conducted.…”

“…[4][5][6][7]21) Brittle fracture occurs when the maximum tensile stress (σt) at the notch root or crack tip exceeds the brittle fracture stress (σBF). The σt is proportional to the σy.…”

“…When the WTF was performed on medium-carbon low-alloy steels such as 0.39%C-2%Si-1%Cr-1%Mo steel 4,5) and 0.6%C-2%Si-1%Cr steel, 6) using multi-pass caliber rolling at 773 K with a rolling reduction of 78%, UFEG structures with strong <110>//RD fiber textures were formed. The steels with the UFEG structures demonstrated an inverse temperature dependence of toughness; the Charpy absorbed energy (vE) increased with decreasing test temperature in the subzero temperature range, in which ultrahigh-strength low-alloy steels usually undergo a ductile-to-brittle transition (DBT).…”

“…The present authors [4][5][6] have developed a warm tempforming (WTF) process in order to achieve the toughening of ultrahigh-strength low-alloy steel bars. When the WTF was performed on medium-carbon low-alloy steels such as 0.39%C-2%Si-1%Cr-1%Mo steel 4,5) and 0.6%C-2%Si-1%Cr steel, 6) using multi-pass caliber rolling at 773 K with a rolling reduction of 78%, UFEG structures with strong <110>//RD fiber textures were formed.…”

Deformation‐Induced Martensite: A New Paradigm for Exceptional Steels

“…The four major diffraction peaks, {110}, {200}, {211} and Steel is the dominant structural engineering material with currently more than 1.6 billion tons produced every year. [ 1 ] Its enormous success is rooted in its wealth of properties, including ferromagnetism, high stiffness, corrosion resistance, [ 2,3 ] and, most importantly, its outstanding mechanical properties, [4][5][6] which can be extensively varied by manipulating the microstructure, making use of phase transformation and metastable phases. Among the various types of steel, plain carbon steels, i.e., Fe-C with minor additional alloying elements, are of a particular interest to the steel industry due to a good balance of properties and price.…”