The Role of Microstructural Constituents on Strength–Ductility–Local Formability of a Transformation‐Induced‐Plasticity‐Aided Bainitic Steel

Abstract

The microstructural evolution for varied austenitizing temperature is investigated in a transformation‐induced‐plasticity (TRIP)‐aided bainitic steel. Special attention is given to the effect of microstructural constituents on mechanical properties and local formabilities (stretch flangeability and bendability). With the decreasing of austenitizing temperature, proeutectoid ferrite is formed due to the refinement of prior austenite grain size, which yields the increased amount of large blocky martensite/austenite (M/A) constituents. The existence of ferrite and the TRIP effect provided by blocky retained austenite contributes to the higher strain hardening rate in the early stage of deformation as well as the more gradual decreasing of the strain hardening rate, leading to higher uniform elongation (UEL) for decreased austenitizing temperature. Whereas, the post uniform elongation (PEL) shows the opposite trend. Both hole expansion ratio (HER) and bending ratio (BR) have a linear relationship with PEL as well as yield ratio (YR). Replacing the multiphase microstructure with uniform bainitic microstructure would increase the balance of strength and local formability, in which the role of PEL is consistent with YR.

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

“…[1][2][3][4] The relatively high-volume fraction of martensite and/or bainite, which is often required to achieve a high TS value in the steel, unfortunately tends to have an adverse effect on the stretch flangeability of the dual-phase (DP), multiphase (MP), and low-alloyed, transformation-induced plasticity (TRIP) 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.…”

“…[ 1–4 ] The relatively high‐volume fraction of martensite and/or bainite, which is often required to achieve a high TS value in the steel, unfortunately tends to have an adverse effect on the stretch flangeability of the dual‐phase (DP), multiphase (MP), and low‐alloyed, transformation‐induced plasticity (TRIP) steels. [ 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 ]…”