Substructure Development and Damage Initiation in a Carbide-Free Bainitic Steel upon Tensile Test

Abstract

Carbide-free bainitic (CFB) steels belong to the family of advanced high strength steels (AHSS) that are struggling to become part of the third-generation steels to be marketed for the automotive industry. The combined effects of the bainitic matrix and the retained austenite confers a significant strength with a remarkable ductility to these steels. However, CFB steels usually show much more complex microstructures that also contain MA (Martensite–Austenite) phase and auto-tempered martensite (ATM). These phases may compromise the ductility of CFB steels. The present work analyzes the substructure evolution during tensile tests in the necking zone, and deepens into the void and crack formation mechanisms and their relationship with the local microstructure. The combination of FEG-SEM imaging, EBSD, and X-ray diffraction has been necessary to characterize the substructure development and damage initiation. The bainite matrix has shown great ductility through the generation of high angle grain boundaries and/or large orientation gradients around voids, which are usually found close to the bainite and MA/auto-tempered martensite interfaces or fragmenting the MA phase. Special attention has been paid to the stability of the retained austenite (RA) during the test, which may eventually be transformed into martensite (Transformation Induced Plasticity, or TRIP effect).

Advanced Multiphase Steels

“…The manuscripts gathered in this Special Issue are committed to a variety of steel grades, embracing DP (Dual Phase) steels [1], medium-Mn steels [2,3], CFB (Carbide Free Bainitic steels) [4], nano-bainitic cast steel [5], δ-TRIP (TRansformation Induced Plasticity) [6] and precipitation hardened ferritic stainless steel [7].…”

“…A wide range of topics are researched that comprise the alloy design [2,6,7], optimization of both the hot working process [2] and the final heat treatment [5], and the final mechanical properties [1,3,4,6].…”

“…The optimization of the final bainitic heat treatment is analyzed for nano-bainitic cast steel [5] In references [3][4][5][6], the mechanical stability of the retained austenite during tensile testing is studied deeply, whereas its thermal stability upon final thermomechanical or thermal treatment is analyzed in [2,6]. It is a pivotal phase to optimize the mechanical behavior of various multiphase steels.…”

Influence of Si Content on the Microstructure and Tensile Properties of Weathering Bridge Steel Produced via Thermal Mechanical Control Process

“…Figure 6 shows the inverse pole figures (IPFs) determined using EBSD. The white and black lines represent the low-angle boundary (2 • ≤ θ ≤ 15 • ) and high-angle boundary (θ > 15 • ), respectively [14,16]. The mean equivalent diameter (MED) of the matrix structure with a misorientation tolerance angle (MTA) from 2 • to 30 • was measured.…”

Copper-Induced Strengthening in 0.2 C Bainite Steel

“…Monophase steels generally exhibit better values [ 7 ]. Even the presence of retained austenite has been proven to be detrimental to the fracture toughness of high strength steel sheets [ 8 , 9 , 10 ]. This is important for the group of bainitic steels because a carbide-free bainite (i.e., mixture of bainitic ferrite and retained austenite) is intensively studied as a promising structure of high strength steels [ 11 , 12 , 13 ].…”