Hydrogen Absorption and Desorption Behavior on Aluminum-Coated Hot-Stamped Boron Steel during Hot Press Forming and Automotive Manufacturing Processes

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.

Interpretation of surficial shear crack propagation mechanisms in bending for Zn or AlSi coated hot press forming steels

The bending angle at the peak load is regarded as the most important parameter for evaluating bending properties of hot-press-forming (HPF) steels. However, it is not a mechanics-based parameter for the bending criterion, and the data interpretation is difficult because bending criteria in relation with microstructures and associated bending mechanisms have not been verified yet. In this study, effects of coating and baking treatments on bending angles at the peak load of three kinds of 1470 MPa-grade HPF steels were investigated by interrupted three-point bending tests coupled with direct microstructural observation. According to direct observations of sequential cracking processes of V-shaped crack (V-crack), bending procedures were classified into four stages: (1) formation of small V-crack, (2) increase in number and size of V-cracks, (3) initiation of shear-crack propagation from the V-crack tip, and (4) further propagation and opening of the shear crack. The minimum bending angle required for initiating the shear-crack propagation from the V-crack tip was defined as a critical angle, which meant the boundary between the 2nd and 3rd stages. The present bending behavior related with critical bending angle and V-cracking could be interpreted similarly by the fracture-mechanics concept, i.e., the initiation of shear-crack propagation.

Fabrication of 9,9′-Bis(aryl)fluorene-modified nanocellulose bamboo fiber composite

A green composite was fabricated using bamboo fiber and 9,9′-bis(aryl)fluorene-modified cellulose nanofiber (FCNF). Cellulose nanofiber (CNF) and finely crushed bamboo fiber (CBF) were also used as binders. The mechanical properties of the composites were compared. It was found that the FCNF-bamboo fiber composite had the maximum flexural strength among these binders. This result was likely due to strong bonding by chemical reactions among fibers and the FCNF. The effect of fiber orientation accuracy on the mechanical properties of the composites was also investigated. When the bamboo fibers were carefully aligned, without fibers crossing each other, the mechanical properties increased by two times, compared to the composites with fibers crossing each other. In the accurately aligned bamboo composites, the cross sections of the fibers were largely deformed by compression stress during hot-press forming. Thus, the gaps among fibers decreased, and interfacial adherence was improved. The effect of fabrication temperature on the mechanical properties of the FCNF-bamboo composite was also examined. It was found that the maximum flexural modulus and strength of the composites were at approximately 250 °C, and the mechanical properties rapidly decreased above 270 °C due to thermal degradation of the bamboo fiber.

An Infrared Local-Heat-Assisted Cold Stamping Process for Martensitic Steel and Application to an Auto Part

The automotive industry has tried to employ ultra-high-strength steel (UHSS), which has a higher strength with a thinner thickness. However, because of its low formability, there is a limit to the use of UHSS in industrial applications. Even though the hot-press-forming method can resolve the formability problem, elevated-temperature conditions lead to side effects—heat transfer and productivity issues. This work presents the concept of an infrared local-heat-assisted cold stamping process. Before the forming process, local areas, where the formability problem occurs, are locally heated by the gathering of infrared rays and cooled to room temperature before delivery. Since the heat treatment is completed by the material supplier, the stamping companies can conduct cold stamping without new investments or the productivity issue. In this work, a heat-assisted cold V-bending test was conducted with a martensitic (MS) 1.5 GPa steel, the CR1470M steel provided by POSCO. The heating effects on the microstructure, hardness, and local ductility were also observed. Finally, a commercial door impact beam was successfully manufactured with the present method. In this application, only a targeted small area was heated. The results show that the present method can improve the formability and springback problems of MS steel in the stamping process.