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

Abstract

Our strategy is to enhance the fracture property of ultra-high-strength low-alloy steels with a yield strength of 1.4 GPa or over by arresting the propagation of brittle cracks in hierarchical, anisotropic, and ultrafine-grained structures. This provides a fail-safe design in addition to suppressing crack initiation. The present article reviews the strength, ductility, toughness, and delayed fracture resistance of ultra-high-strength low-alloy steels with ultrafine elongated grain structures processed by the deformation of tempered martensitic structures at elevated temperatures (referred to as warm tempforming). The evolution of heterogeneous microstructures during warm tempforming using multi-pass caliber rolling is discussed, as are the microstructural factors controlling the strength and fracture properties of warm tempformed steels. Furthermore, we apply warm tempformed steels with ultrafine elongated grain structures to the fabrication of ultra-high-strength bolts.

Relationship between yield strength, σ_ys, and V-notch Charpy absorbed energy, vE, at room temperature for various steels; ultrafine grain ferritic steel,³⁰⁾ JIS-low-alloy,³¹⁾ AISI/SAE-low-alloy,³²⁾ 0.34%C-2%Si-1%Cr-3%Ni,³³⁾ ausformed 0.2%C-3%Ni-3%Mo,³⁴⁾ HY130,³⁵⁾ HY180,²⁵⁾ AF1410,²⁶⁾ high-purity 18%Ni,²⁷⁾ 18%Ni (250),²⁸⁾ and 18%Ni (350)²⁹⁾ maraging steels. Data for 0.4%C-2%Si1%Cr-1%Mo steels that were quenched and tempered at a temperature of 500°C (QT) and tempformed at a temperature of 500°C with an equivalent strain, ε_eq, of 1.75 (TF),⁵⁾ are also shown. (Online version in color.)