Abstract
Most structural components undertake cyclic loads in engineering and failures always cause catastrophic economic losses and casualties. In the present work, the phase evolution of Al-Si coating of high-strength boron steel during hot stamping was investigated. Two types of 1500 MPa grade boron steel sheets, one with Al-Si coating and the other without, were studied to reveal the effect on the high-cycle fatigue behavior. The as-received continuously hot-dip Al-Si coating was composed of α(Al), eutectic Al-Si and τ5. After hot stamping at 1193 K, three phases formed in this coating: β2, Fe(Al,Si)2 and α(Fe). The experimental results showed that the endurance limit of the coated steel sheet was 370 MPa under 107 fully reversed tension-compression loading cycles as opposed to 305 MPa in the uncoated sheet. Both the coated and the uncoated specimens showed surface-induced transgranular fatigue fractures. In the uncoated sheet, the fatigue cracks were generated from the decarburization surface, but the Al-Si coating effectively prevented the occurrence of near-surface decarburization during high-temperature hot stamping, and the only cracks in the coated steel sheet were initiated at wire-cutting surfaces.
Highlights
Demands for increasing safety, reducing the weight of automobile bodies and fuel consumption in automotive manufacturing have promoted technological innovation [1,2,3,4]
It was inevitable that bare steel sheets were thermally oxidized in earlier hot stamping technology and the oxidized scale had to be removed by shot blasting or shot peening [14]
Our work indicated that the Al-Si coating could significantly improve the fatigue life
Summary
Demands for increasing safety, reducing the weight of automobile bodies and fuel consumption in automotive manufacturing have promoted technological innovation [1,2,3,4]. For the direct hot stamping process, the steel sheet is heated firstly, transferred to the press and subsequently formed and quenched in a closed tool [8]. A pre-formed part is cold-stamped firstly in case of the indirect hot stamping process, and only quenching and calibration are performed after austenitizing in the press [9]. Boron steel sheets have been widely used for the production of high strength panels of car bodies and vehicle chassis structures due to its excellent formability [10,11]. The ultimate tensile strength of hot-stamping boron steel sheets would reach up to 1500 MPa with a full martensite microstructure [12,13]. The attempt to use lubricant oils was abandoned due to complex
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