Investigation on Mechanical Properties and Oxidation Behavior of 1.2 and 1.7 GPa Grades Coating-Free Press-Hardened Steels

Abstract

Al-Si-coated boron-alloyed steels are the most widely used press-hardened steels (PHSs), which offers good oxidation resistance during hot forming due to the presence of the near eutectic Al-Si coating. In this study, a recently developed novel un-coated oxidation resistant PHS, called coating-free PHS (CF-PHS), is introduced as an alternative to the commercial Al-Si coated PHSs. With tailored additions of Cr, Mn, and Si, the new steel demonstrates superior oxidation resistance with a sub-micron oxide layer after the conventional hot stamping process. Hence, it does not require shot blasting before the subsequent welding and E-coating process. Two CF-PHS grades have been developed with ultimate tensile strengths of approximately 1.2 and 1.7 GPa, respectively. Both grades have a total elongation of 8–9%, exceeding the corresponding Al-Si-coated PHS grades (1.0 GPa/6–7%, 1.5 GPa/6–7%). Furthermore, the bendability of CF-PHS was similar to the corresponding Al-Si PHS grades. On the other hand, performance evaluations relevant to automotive applications, such as weldability, the E-coat adhesion, and tailor-welded hot stamp door ring, were also conducted on the CF-PHS steel to satisfy the requirements of manufacturing.

High‐Temperature Oxidation Behavior of Cr‐Si Alloyed Press‐Hardened Steel in Comparison with 22MnB5 Steel

“…The CF–PHS exhibits an oxide scale of less than 1 μm during the hot‐forming process at 900–950 °C, demonstrating excellent oxidation resistance and promising application prospects. [ 19 ] Some researchers have shown that adding Cr and Si to steel results in synergistic antioxidant effects, which improve the high‐temperature oxidation resistance of steel. Lim [ 20 ] reported that with the addition of Si to high‐Cr (12 wt%) Fe–Cr alloys, Si could serve as an efficient diffusion barrier by forming a localized Si‐oxide network beneath the Cr‐rich oxide layer.…”

A Hot‐Stamped Medium‐Mn Steel with Superior Strength–Ductility–Bendability Combination and Thin Oxidation Layer

“…The structural parts of automobile body usually deform and fracture under a stress state close to plane strain bending during vehicle crash. [15][16][17] Therefore, plane strain bendability is an important material property for hot-stamped steels. At present, the three-point bending test according to VDA 238-100 standard is widely accepted to assess the plane strain bendability of hot-stamped steels.…”

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

“…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]. Such integrated hot stamping of anti-intrusive PHS with high energy-absorbing PHS is widely used in tailor hot-stamped, laser-welded door rings [8].…”

“…Moreover, this hot stamping also exhibits good ductility of ~11.2% due to the introduction of the ferrite phase. Unfortunately, almost all studies on high-temperature oxidation resistance have focused on 1500~2000 MPa grade anti-intrusion PHSs, while related studies on energy-absorbing PHSs are extremely lacking, especially 1000 MPa high energyabsorbing PHSs, which are usually matched with 1500~2000 MPa anti-intrusion PHSs for integrated hot stamping to form gradient performance parts, such as tailor hot-stamped laser-welded door rings [7,8]. Therefore, to improve the overall antioxidant performance of the part, the oxidation resistance of 1000 MPa high energy-absorption PHS also needs to be emphasized.…”

“…Therefore, to improve the overall antioxidant performance of the part, the oxidation resistance of 1000 MPa high energy-absorption PHS also needs to be emphasized. It is worth mentioning that, in addition to low oxidation resistance, the microstructure of conventional 1000 MPa high energy-absorbing PHS is usually dominated by low-carbon martensite, which exhibits excellent toughness; the ductility, however, tends to be limited, about 6% [8].…”