Ultra High-strength Sheet Steels Research Articles

Investigation of tool damage in shear cutting of ultra-high strength sheet steels

Proportions of high- and ultra-high strength steels are constantly increasing in modern car bodies. In the press shop there are uncertainties concerning the application of steels with strength higher than 1000MPa particularly concerning shear cutting processes. To support the steel user regarding a tool material and coating and the wear behavior, long-term shear cutting test had been carried out. The effects of various shear cutting conditions on tool damage were investigated by continuous shear cutting test with an amount of cutting cuts up to 100,000 times. Tested sheet materials were 780, 1’180 and 1’470MPa grade steels. The changed cutting conditions were sheet steel strength, tool material, cutting clearance, and rake angle. Measurements by laser and confocal microscopes were carried out in order to reveal tool damage mechanisms. The features of tool damage were classified as wear, chipping, and plastic deformation. The amount of plastic deformation rises with the increase of sheet steel strength. Not only plastic deformation but also chipping in the tested tool was observed in the cutting test of 1180MPa sheet steel by standard tool steel (DIN 1.2379) after 40’000 cuts. Large plastic deformation in the tested tool was observed in a cutting test of 1470MPa sheet steel by high-speed steel (DIN 1.3343) after 100’000 cuts although no chipping was detected. Under a rake angle of 5°, serious chipping and tool wear were observed especially near the final sheet-tool contact position. Regarding clearance, the amount of plastic tool deformation increased with the decrease of the clearance. This effect of the clearance can be explained by vertical force and by sheet rotation during cutting.

張出し形状を有する低剛性部品の寸法精度不良要因

The dimensional defects in low stiffness automotive panels with convex shape such as waving are severe problem in the press forming of ultrahigh-strength sheet steels. The main countermeasure to dimensional defects is the reduction in springback. However, conventional countermeasures to springback are often ineffective for the waving of low stiffness panels. In this study, the dimensional defects in low stiffness model panels with convex shape were investigated by press forming experiments and numerical simulations. The results indicate that there are four main factors for the occurrence of waving in low stiffness panels: (1) bending moment resulting from the stress difference between circumferential compression around the convex shape and radial tension in the convex bottom, (2) out-of-plane deformation caused by circumferential compression around the convex shape, (3) bending moment resulting from the stress distribution in the thickness direction at punch and die shoulders, and (4) excessive material around the convex shape. On the basis of these results, it can be considered that preforming in which the four factors can be reduced by applying tensile stresses to an entire panel is effective in reducing waving.

Net section tension strength of bolted connections in ultra-high strength sheet steel during and after fire

This paper investigates the net section tension strength of bolted connections in ultra-high strength sheet steel (yield stress greater than 1000 MPa) under various fire scenarios through laboratory testing. The specimens are tested at room (ambient) temperature and elevated temperatures up to 700 °C, and are subjected to air or water cooling from temperatures as high as 1000 °C. The degradations of the tensile strength at elevated temperatures are much more severe than design code predictions. Substantial recovery of the tensile strength is feasible using water cooling if the steel has been exposed to 1000 °C, by almost 90%. Based on finite element analyses incorporating tensile fracture simulation, the paper determines that elongation at fracture is the most relevant parameter of material ductility for a structural steel connection. Tension fractures in very low ductility materials tend to be inclined to the loading direction of the bolted connection. This paper also examines the alternative formulae for determining the net section tension strength of a stagger bolted connection. Their accuracy is verified through the use of a certain geometry, which eliminates any potential errors due to the use of incorrect material tensile strength in the calculations. Cochrane's original formula is confirmed to be significantly more accurate than the simplified version used in the literature and some design codes, for the tested specimens.

Bending Strength of Laser-Welded Sandwich Steel Panels of Ultra-High Strength Steel

The study was performed to investigate the bending resistance of laser-welded sandwich panels (Vf-core). The main aim of the study was to determine the effect of the tensile strength on bending strength of the panel structures. Panels were manufactured using an ultra-high strength (UHS) and low strength (LS) steels with yield strengths of 1200 and 200 MPa, respectively. Secondly, the bending strength of the panel structures was compared with the conventional sheet steels to estimate the possibilities for weight reduction. Results showed that the UHS steel panels had significantly higher bending strength than panels of the LS steel. The bending strength in the weakest loading direction of the UHS panel was approximately four times higher than the one of LS steel panel. The panels made with UHS steel faceplates and LS steel cores had better bending strength than LS steel panels. In comparison to UHS sheet steel, 30% weight saving is estimated by using the geometry optimized UHS steel panel.

Hydrogen embrittlement of DP-1000 flat steel sheet: Influence of mechanical properties, specimen geometry, pre-damaging and electrolytically zinc galvanizing

Studies towards the impact of solute hydrogen on the mechanical properties of ultra-high strength sheet steel grades (UHSS) for the automotive industry have been increasingly conducted in the past years. This paper focuses on the pre-damaging of a dualphase DP-1000 automotive sheet steel, as found in the manufacturing of automotive body parts, in connection with hydrogen embrittlement. It is shown that the kind of pre-damaging of the specimens has a severe influence on the occurrence of hydrogen embrittlement. Hydrogen embrittlement resistance under corrosive conditions was determined through constant load tests. Tensile specimens, manufactured from electrolytically zinc galvanized DP-1000 steel, were immersed in a 5% sodium chloride solution. The impact of material properties, pre-damaging and specimen geometry are critically discussed. Furthermore a discussion is given about the material testing methods most suitable to indicate high or low hydrogen embrittlement resistance if constant load testing is performed.

IMP-08: Effects of Bake Hardening Property on Dynamic Yield Strength of Ultra High Strength Sheet Steels(IMP-I: IMPACT BEHAVIOR OF MATERIALS AND STRUCTURESG)

IMP-08: Effects of Bake Hardening Property on Dynamic Yield Strength of Ultra High Strength Sheet Steels(IMP-I: IMPACT BEHAVIOR OF MATERIALS AND STRUCTURESG)

Recent advances in ultrahigh-strength sheet steels for automotive structural use: Yamazaki, K., Oka, M., Yasuda, H., Mizuyama, Y. and Tsuchiya, H. Nippon Steel Technical Rep. (1995) 64, 37–44

The phenomenon of strain localization is considered the most effective cause of failure in dual phase steels. In the present study, we have performed in-situ tensile tests to investigate the strain localization in ferrite phase. In order to quantify the level of microscopic deformation, an image processing code was used and the the strain map was subsequently superimposed onto the scanning electron microscopy (SEM) image obtained from the in-situ test. The SEM image that was captured during the in-situ tensile test was further used in a finite element dislocation density model, based on which the similarities and differences between the experimental and simulation results were discussed. The amount of localized deformation in the ferrite phase was found to be much higher in the regions between martensite islands. A parametric study was then performed to gain deeper insights on the effect of martensite grain size on the strain localization of the neighboring ferrite. Furthermore, using the experimental results, the localization of strain inside the high deformational fields before the final failure was discussed.