Microstructure-Based Modelling of Flow and Fracture Behavior of Tailored Microstructures of Ductibor® 1000-AS Steel

Abstract

Emerging grades of press-hardening steels such as Ductibor® 1000-AS are now commercially available for use within tailor-welded blanks (TWBs) to enhance ductility and energy absorption in hot-stamped automotive structural components. This study examines the constitutive (hardening) response and fracture limits of Ductibor® 1000-AS as functions of the as-quenched microstructure after hot stamping. Three different microstructures consisting of bainite and martensite were obtained by hot stamping with die temperatures of 25 °C, 350 °C, and 450 °C. Mechanical characterization was performed to determine the hardening curves and plane-stress fracture loci for the different quench conditions (cooling rates). Uniaxial-tension and shear tests were conducted to experimentally capture the hardening response to large strain levels. Shear, conical hole-expansion, plane-strain notch tension, and Nakazima tests were carried out to evaluate the stress-state dependence of fracture. A mean-field homogenization (MFH) scheme was applied to model the constitutive and fracture behavior of the mixed-phase microstructures. A dislocation-based hardening model was adopted for the individual phases, which accounts for material chemistry, inter-phase carbon partitioning, and dislocation evolution. The per-phase fracture modelling was executed using a phenomenological damage index based upon the stress state within each phase. The results revealed that the 25 °C hot-stamped material condition with a fully martensite microstructure exhibited the highest level of strength and the lowest degree of ductility. As bainite was formed in the final microstructure by quenching at higher die temperatures, the strength decreased, while the ductility increased. The predicted constitutive and fracture responses in the hot-stamped microstructures were in line with the measured data. Accordingly, the established numerical strategy was extended to predict the mechanical behavior of Ductibor® 1000-AS for a broad range of intermediate as-quenched microstructures.

A Novel 1000 MPa Grade Ultrafine-Grained Dual-Phase Press Hardening Steel with Superior Oxidation Resistance and High Ductility

“…PHS is widely used for automobile structural component due to its impressive mechanical properties and no springback after forming [1,2]. Based on the application, PHS is mainly divided into anti-intrusion PHS and high energy-absorbing PHS [3][4][5]. The former is usually required to possess high yield strength and ultimate tensile strength.…”

“…However, its bending toughness tends to be limited [3,4]. The latter usually exhibits impressive bending toughness, which endows it with extraordinary energy-absorption ability during collision [5,6]. A typical example is 1000 MPa grade low-carbon martensitic PHS [5].…”

“…The latter usually exhibits impressive bending toughness, which endows it with extraordinary energy-absorption ability during collision [5,6]. A typical example is 1000 MPa grade low-carbon martensitic PHS [5]. In actual automotive components, these two PHSs are usually required to be combined using laser welding followed by integral hot stamping, to obtain gradient performance parts, which can realize the combination of high strength-toughness [7].…”