Abstract
Our study mainly focused on diffusible hydrogen in aluminum–silicon-coated hot-stamped boron steel during a hot press forming process and in pre-treatment sequential lines of the automotive manufacturing process using a thermal desorption spectroscopy (TDS) technique. First, in the hot stamping procedure, as the soaking time increased in the heating furnace at a specific dew point when austenitizing, a high concentration of diffusible hydrogen was absorbed into the hot-stamped boron steel. Based on the TDS analysis of hydrogen absorbed from hot stamping, the activation energy value of hydrogen trapping in 1.8 GPa grade steel is lower than that of 1.5 GPa grade steel. This means that diffusible hydrogen can be more easily diffused into defective sites of the microstructure at a higher level of the tensile strength grade. Second, in sequential pre-treatment lines of the automotive manufacturing process, additional hydrogen did not flow into the surface, and an electro-deposition process, including a baking procedure, was effective in removing diffusible hydrogen, which was similar to the residual hydrogen of the as-received state (i.e., initial cold rolled blank). Based on these results, the hydrogen absorption was facilitated during hot press forming, but the hydrogen was sequentially desorbed during automotive sequential lines on aluminum-coated hot-stamped steel parts.
Highlights
Hot stamping parts, which are extensively applied as ultra-high-strength steel, are in the spotlight due to their excellent formability and strength
The hot-stamped boron steel is a single-structure steel that has a microstructure composed of a hard martensite phase by heating a 600 MPa cold rolled grade steel sheet, which is composed of a ferrite phase and pearlite at a high-temperature austenizing temperature of 1173 K or higher, and rapidly cooling the steel sheet by using a metallic mold
The present study investigates the behavior of the diffusible hydrogen of two kinds of grade steel plates that are aluminum-coated, which have tensile strengths of 1.5 GPa and 1.8 GPa, and were studied with different carbon contents in the specific hot stamping conditions
Summary
Hot stamping parts, which are extensively applied as ultra-high-strength steel, are in the spotlight due to their excellent formability and strength. A hydrogen delayed fracture is caused by the diffusion and accumulation of hydrogen that penetrates through the surface of the steel during the manufacturing process of the steel or in a corrosion environment when in usage of the vehicle. These hydrogen atoms can accumulate in the defective sites in the microstructure. These accumulated hydrogens induce the initiation and propagation of local cracks under stress states, and will eventually cause failure of the components
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