As a new type of ultra-fine grained high-strength steel, UF340 has the potential to replace HC180BD in automotive exterior panels. This paper investigated its mechanical properties and formability. The results indicate that UF340 and HC180BD have similar mechanical properties in terms of plasticity, processing hardening index, and anisotropy, but UF340 does not exhibit significant bake hardening properties. The formability of the two materials is similar. There is certain difference between the forming limit curve calculated by the Keeler model and the forming test results. The forming limit diagram obtained by linear segment fitting data has a higher degree of agreement with the experiment, the forming limit curve obtained by this method can provide a reference on the forming application of new materials.

As the pace of product updates accelerates, the flexible roll forming process has garnered widespread attention within the industry. However, due to the inherent complexity of its forming process, inevitable defects arise. To address this, numerical simulation and analysis of the forming process of variable cross-section panels along a three-point bending curve were conducted, examining the impact of material, thickness, and flange height on warping. The results indicate that the warping defect in the panels is related to longitudinal strain, with the minimum warping defect occurring when the longitudinal strain distribution is uniform. Orthogonal simulation design analysis determined the primary and secondary order of process parameters affecting warping height as thickness, flange height, and panel material. The conclusion is that compared to aluminum alloy materials, the warping height is reduced when using high-strength steel with a high yield strength ratio, whereas the warping height increases when using high-strength steel with a low yield strength ratio.

Failure analysis of the 34MnB5 hollow stabilizer bar fracture occurred in the road test of a mass production vehicle was carried out using laser confocal microscopy, scanning electron microscope and other equipment. It was confirmed that the insufficient shot blasting process time and the existence of axial original cracks caused early fatigue failure and fracture of the stabilizer bar. In order to further clarify the root cause of the failure and fracture, this article investigated and analyzed the manufacturing process of the stabilizer bar, determined that the main cause for the fracture of the stabilizer bar was insufficient shot blasting time, thus evaluated the subsequent impact of this fracture failure accident. Finally, the article identified the direction of rectification for the risk points during the investigation process, and proposed optimization measures to further prevent the occurrence of such problems.

To address the issue of batch cracking of seat crossbeam parts stamped with 2 GPa hot forming material after welding, this article analyzed and found that this part was cracked caused by hydrogen embrittlement. Through tracing the parts process, it was found that hydrogen embrittlement cracking was caused by factors such as high heating temperature, long heating time, and long part transfer time. Through orthogonal design, different hot stamping process combinations of parts were improved to verify the prototyped parts, it was finally confirmed that for the 2 GPa hot formed parts with hot stamping process window in hot-formed austenitizing furnace, dew point temperature shall be under -5 ℃, heating time of 3~13 min, and heating temperature of 880~940 ℃.

The effects of pre-strain and baking on the microstructure and mechanical properties of QP980 cold rolled sheet were studied by means of universal tensile testing machine, pendulum impact testing machine, Scanning Electron Microscope (SEM) and X-Ray Diffraction (XRD). The results show that with the increase of pre-strain, the ultimate bending properties and impact properties of QP980 cold rolled sheet decrease After baking at 170 ℃ for 20 min, the impact properties of the material increase, and the Bake Hardening (BH) properties increase with the increase of pre-strain; X-ray diffraction analysis shows that baking does not cause phase transformation, and the carbon content in residual austenite decreases; Scanning analysis shows that the ductile fracture area in the impact fracture increases significantly after baking treatment.

The effect of cooling water between rolling mills on the surface microstructure uniformity of UF340 was investigated using hot rolling experiments and full-process tracking. The results show that the cooling water between rolling mills has certain effect on the organization uniformity of the hot rolled coil along the transverse direction, and will be partially inherited to the final UF340 strip organization. When the cooling water between rolling mills is fully open, a small amount of non-recrystallized grain is retained on the surface of the final UF340 strip located at about 1/4 from the edge. When the latter part of water between rolling mills is closed, due to the full recrystallization during hot rolling, the final product has a better consistency of the organization along the transverse direction, presenting a fine and fully recrystallized tissue on the surface, which is beneficial to improving surface Distinctness of Image (DOI) of the material. The extraordinary uniformity of the microstructure makes the waviness of the UF340 after painting and deforming is as low as 0.169 μm, and DOI value reaches 92, which fully meets the requirements of the surface quality for the intercoat-free coating process.

This article studied laser welding of the coating-free press hardened steels (CFPHS) with low carbon footprint and laser welding of aluminum-silicon coating press hardened steels (CFPHS). The study shows that both CFPHS-to-CFPHS and CFPHS-to-AlSi-coated PHS do not require filler wire and laser ablation. Both weldments demonstrate good tensile ductility, with fracture occurring away from the weld seam. Besides, CFPHS-to-AlSi-coated PHS shows high Charpy V-notch impact toughness of 86% of bulk metal. Furthermore, microstructure analysis was conducted on CFPHS-to-AlSi-coated PHS: Microstructure of weld zone is mainly martensite with very small amount of ferrite, consistent with the hardness being as high as bulk metal. Finally, thermodynamics and phase transformation kinetics were used to study the weld seam between CFPHS and AlSi-coated PHS: (1) CFPHS reduces the Al and Si content from coating contamination during weld fusion, avoiding or reducing the formation of brittle AlSi-rich ferrite at high temperatures; (2) Cr alloying improves the hardenability of the weld seam, thus full martensitic microstructure can be obtained during die quenching of hot forming, which guarantees the high hardness of the weld seam.

Research on the fatigue performance and failure modes of H340LAD+Z (1.2 mm)-H340LAD+Z (1.2 mm) spot welding cross joints and spot welding stripping joint was conducted. The study identifies that the maximum force of the spot welding cross joint tensile test is 9.0 kN, the maximum force of the spot welding stripping joint tensile test is 1.9 kN, and the maximum hardness of the spot welding joint is 348 HV 1 located at the nugget area, the spot welding cross joint and the maximum force-fatigue life curve of the spot welded stripping joint show a linear relationship under the logarithmic coordinate axis, and the fatigue performance of the spot welding cross joint is better than that of the spot welding stripping joint. Finally, the fatigue failure modes and fatigue fracture morphology characteristics of the spot welding cross joint and the spot welding stripping joint were observed. The failure modes of the two joints are slightly different, and the fatigue fracture morphology is composed of crack sources and expansion zones.

In order to reduce stamping cost from perspective of material cost control, this article discussed the cost control strategy for automotive stamping material from three aspects: improving material utilization rate, applying new sheet metal manufacturing processes, and selecting materials rationally, combined with specific application cases. Firstly, material utilization can be improved by product collaborative development, stamping process optimization, and reusing stamping waste materials. Secondly, in the application of new sheet metal manufacturing processes, processes such as roller pressing and integrated heat forming can be used to replace the process of single stamping or multiple separate stamping followed by spot welding. Finally, in terms of rational material selection, principles for selecting steel with the same strength, using bake-hardening steel instead of interstitial-free steel, and recommendations for the application of high-strength steel are provided.

It is difficult to solve the problem of exterior quality control of PVC bubbles in doors and covers in the production process. To eliminate the drawbacks in PVC bubble quality by empirical qualitative management, a free sliding wooden plug closed space model for PVC bubbles generation was constructed, and combined with the Ideal Gas Law, the quantitative formula for characterizing the quality state of PVC bubbles was attained, and the formation mechanism of PVC bubbles was analyzed from the perspective of mathematical models. Based on mathematical model and quantitative characterization formulas, the impact of factors including original cavity volume of the sheet metal fold, the highest temperature of PVC drying room, PVC materials, and other environmental factors on the positive or negative correlation and degree of influence of PVC bubbles can be objectively reflected, thereby the direction of quality control of PVC bubbles is more explicit from design and process.

The aluminum alloy vacuum high-pressure die-casting technologies are gradually applied in the automotive industry. This paper studied and analyzed the material, process, product and the key issues at present stage of the vacuum high-pressure die-casting, summarized the physical characteristics, chemical compositions and mechanical properties of heat treatment materials and heat treatment free materials in detail, elaborated the technical process, production line, process design, process parameter and formability properties of vacuum high-pressure die casting, analyzed the main issues at present stage including product performance, thickness, dimensions, connection, repair and die. The paper also summarized the advantages of vacuum high pressure die-casting technologies and forecasted its development prospects.