Effect of Interphase Hardness and Elastic Modulus on Fracture Behavior during Hole Expansion Testing of Hot‐Rolled Steels with Low‐Temperature Transformation Microstructures

Abstract

This study investigates the fracture behavior during hole expansion testing of a 980 MPa hot‐rolled Fe–Cr–Ti–B steel, with particular focus on analyzing the microvoid initiation and crack propagation during plastic deformation. The microstructure of the steel consists of a tempered martensite matrix and granular bainite, with each constituent phase being identified by field emission scanning electron microscopy, transmission electron microscopy, micro Vickers hardness, and nanoindentation. Microvoids are observed at the intergranular boundaries, transgranular regions, and at the interface of precipitates/matrix and non‐metallic inclusions/matrix. The main crack is generated at the intergranular interface between the tempered martensite and granular bainite, an area exhibiting a large deviation in hardness and elastic modulus. The tempered martensite has a greater hardness of 385 Hv and elastic modulus of 287 GPa, as opposed to the 326 Hv and 257 GPa of the granular bainite. Due to this difference in elastic modulus, the granular bainite plastically deforms at a lower applied stress than the tempered martensite. The main crack obtained from the hole expansion test generates a fractography mixed with dimple and quasi‐cleavage fracture pattern. Therefore, the crack initiated at the interface of the tempered martensite and granular bainite is propagated along the transgranular regions.

Evaluation of Strengthening Mechanisms in Novel Fully Ferritic Advanced High‐Strength Steels

“…[6][7][8][9][10][11][12] Excellent strength and flangeability properties can be achieved in the complex-phase (CP) steels and in a steel consisting of tempered martensite and granular bainite; however, that required a strict thermomechanical process control together with a well-balanced steel chemistry. [2,13] Besides the abovementioned steels, the fully ferritic steel grades typically have excellent formability but a relatively low strength; thus, Funakawa et al [14] introduced a method to increase their strength substantially by nanometer-sized interphase precipitates (IP) that were formed during the austenite-to-ferrite phase transformation. They developed a new 0.04%C-1.5%Mn-0.09%Ti-0.2%Mo steel in which the fine (∅ = 3 nm) interphase carbides in rows existed within the fully ferritic matrix.…”

“…[ 6–12 ] Excellent strength and flangeability properties can be achieved in the complex‐phase (CP) steels and in a steel consisting of tempered martensite and granular bainite; however, that required a strict thermomechanical process control together with a well‐balanced steel chemistry. [ 2,13 ]…”