The Impact of Strain Heterogeneity and Transformation of Metastable Austenite on Springback Behavior in Quenching and Partitioning Steel

Abstract

Multiple strengthening methods, such as high dislocation density, high twin density, small grain size, and metastable austenite phase can give high strength to ultra-high strength steels (UHSSs). However, the high strength of UHSSs often results in a greater tendency for springback when applied in manufacturing vehicle components. In the present study, two types of UHSSs, dual-phase (DP) steel and quenching and partitioning (QP) steel are investigated to study the springback behavior during the bending process. Results indicated that both the strain heterogeneity and the transformation of retained austenite impacted the springback behavior. The springback angle of the DP steel increased with the increase in bending angle, which was caused by the increasing degree of strain heterogeneity. However, the springback angle of the QP steel decreased to a 14.75 • when QP specimens were strained at a 104 • bending angle due to the inhibiting effect of the phase transformation. This indicated that there was preferential phase transformation in the thickness direction in the retained austenite of the outer and inner zones. The phase transformation caused low strain heterogeneity, which resulted in a lower tendency for springback. The results suggested that QP steel could possess lower springback at a proper bending angle.

Performance of Mechanical Properties of Ultrahigh-Strength Ferrous Steels Related to Strain-Induced Transformation

“…One review and nine research articles have been published in this Special Issue of Metals. The subjects of this Issue are roughly divided into four categories: (1) strain-induced transformation behavior [1][2][3], (2) deformation mechanism [4,5], (3) formability [6,7] and (4) mechanical properties [8][9][10] of the ultrahigh-strength ferrous steels. The abovementioned (1) and (2) give important information for improving (3) and (4).…”

“…Kaar et al [6] studied the structure-ductility relationship in cold-rolled medium manganese bainite/martensite steels subjected to quenching and partitioning processes. Yang et al [7] investigated the springback behavior during the bending process for a quenching and partitioning steel.…”

Hydrogen Embrittlement Behavior of a Commercial QP980 Steel

“…It should be also noted that a heterogenous distribution of strain (i.e., strain localization) can exist during the deformation of QP steel owing to the inherent mechanical property differences among various constituent phases [20], and cracking is apt to occur at the region with high localized strain. Under the attack of hydrogen, the strain distribution may be affected and strain localization can be intensified through the HELP mechanism owing to the different hydrogen-trapping capacities of various phases, making an early failure across martensite, ferrite or along martensite/austenite phase boundaries.…”