Abstract
Hot forming combined with austempering and quenching and partitioning (QP) processes have been used to shape two cold rolled high silicon steel sheets into hat profiles. Thermal simulation on a Gleeble instrument was employed to optimize processing variables to achieve an optimum combination of strength and ductility in the final parts. Microstructures were characterized using optical and scanning electron microscopy and X-ray diffraction. Tensile strengths (Rm) of 1190 and 1350 MPa and elongations to fracture (A50mm) of 8.5 and 7.4%, were achieved for the two high-silicon steels having 0.15 and 0.26 wt % C, respectively. Preliminary results show that press hardening together with a QP heat treatment is an effective method of producing components with high strength and reasonable tensile ductility from low carbon containing steels that have the potential for carbide free bainite formation. The QP treatment resulted in faster austenite decomposition during partitioning in the steels in comparison with an austempering treatment.
Highlights
Press hardening of boron alloyed steels has been used since the 1980s [1] to produce beams, pillars, and safety-related components for cars [2]
This paper reports an account of the mechanical properties obtained after press hardening experiments to produce hat-shaped profiles using quenching and partitioning (QP) heat treatments for high-silicon steels, in comparison with those of commercial 22MnB5 profiles
Even though the steel with a carbon content of 0.15 wt % gave yield and tensile strength values lower than those of conventional press hardened boron steel, the steel with the carbon content of 0.26 wt % resulted in a yield strength comparable with that of the boron steel, with a lower tensile strength
Summary
Press hardening of boron alloyed steels has been used since the 1980s [1] to produce beams, pillars, and safety-related components for cars [2]. Strength levels achievable in boron steels in as quenched conditions are considered excellent (Rm ≈ 1500 MPa) but the ductility is often limited (A50 mm ≈ 6% or lower) as a result of the essentially martensitic microstructure of the steels [5]. Tailor-welded blanks and differentiation of heat treatment are the methods that can be used to tailor and optimize the properties in different parts of a component [6]. Both ductility and toughness may be enhanced in these steels with the formation of carbide free bainitic (CFB) microstructures through austempering process and/or subjecting these steels to a novel concept of quenching and partitioning (QP) thermal treatment as described below. Formation of CFB microstructures can be facilitated in specially tailored steel compositions containing high levels of Si and/or Al (about 1.5–3 wt %), through austempering
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