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Initiation and propagation of ductile fractures in crashed automotive components made from high strength steels are investigated in order to understand the mechanism of fracture propagation. Fracture of these components is often prone to occur at the sheet edge in a strain concentration zone under crash deformation. The fracture then extends intricately to the inside of the structure under the influence of the local stress and strain field. In this study, a simple tensile test and a 3-point bending test of high strength steels with tensile strengths of 590 MPa and 1180 MPa are carried out. In the tensile test, a coupon having a hole and a notch is deformed in a uniaxial condition. The effect of the notch type on the strain concentration and fracture behavior are investigated by using a digital imaging strain measurement system.

We have developed a new resistance spot welding process with “pulsed current pattern”, which consists of short-time high-current post-heating and short-time cooling to improve joint strength of ultra high strength steel (UHSS) sheets with a tensile strength over 980MPa. The high-current post-heating reheats the heat affected zone (HAZ) near the electrodes and that near the sheet-sheet interface rather than the center of the nugget, and this pulsed current pattern utilizes this procedure to improve the toughness of the nugget. In the case of 980MPa grade steel with a 1.6mm thickness, the pulsed current pattern improved cross tension strength (CTS) from 7.2kN to over 10kN and its failure mode from interface failure to plug failure.

It has been widely recognized that the accurate estimation of spot welded joint fracture are needed to improve the accuracy of car crash FE analyses. The criteria model must be developed to realize the consideration of spot welded joint fracture under various kinds of load conditions. Conventionally, only tensile shear test and cross-tensile test have been usually performed for estimation of the strength characteristic of spot-welded joints. However, the spot-welded strength under complex loads can not be adequately estimated by using the conventional test methods. In this study, newly developed tests, “Inclined Cross Tensile Tests”, loading both shear and axial force on spot welded joint were conducted to understand fracture strength and build up its criteria. Especially, in the inclination of cross-tensile test, special attachment for the specimen to prevent bending deformation was developed.

Environmental issues require auto makers to improve the fuel consumption of motor vehicles. One way of saving energy is to reduce the total weight of the vehicle by using high strength steels. However, with conventional steels, increasing tensile strength (TS) reduces elongation and stretch flange formability. JFE Steel developed a new TS/780MPa grade of hot-rolled high strength steel with excellent stretch flange formability in which the ferrite matrix is strengthened by ultra-fine, nano-sized precipitates. Transmission electron microscopy (TEM) reveled that the precipitates in ferrite grains are TiMoC2 and their average diameter is less than 10 nm. The steel has an extremely high YS/TS ratio and an excellent balance of elongation and the hole expansion ratio, superior to that of TS/780MPa grade dual phase and HSLA steels. Moreover, because of its Si-free alloy chemistry, the new steel has good coatability and can be used as a substrate for galvanized or galvannealed steel sheets.