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For the improvement in the frontal crash resistance of automotive body, the application of high-strength sheet steels are considered. At first the authors conducted the high speed tensile test by Split Hopkinson Bar Method to clarify the mechanical property of various sheet steels at a high strain rate. Secondary the authors conducted dynamic and static compression tests of the columns of various sheet steels to evaluate the energy absorption at the frontal crash. Results clarified that the energy absorption by column collapse is estimated only by tensile test. It was also clarified in comparison with the sheet steels with the same tensile strength, that the absorbed energy can be increased 5 - 15% by the adoption of a proper sheet steel. In case of the sheet steels with tensile strength of 590MPa, a tri-phase sheet steel of ferrite, bainite and retained austenite exhibited the largest energy absorption than others of similar tensile strength.

For the automotive exposed panels, several types of 350N/mm2 grade bake-hardenable sheet steel have been developed and actually applied. However for further weight reduction of automotive body panels, especially inner panels, a 450N/mm2 grade sheet steel with excellent formability has been required. For this demand a new 450N/mm2 grade sheet steel has been developed. As the result it was found, that by the co-addition of Mn and P to ultra-low carbon interstitial free steel the recrystallization texture favorable for deep drawability can be formed, accompanied with the increase in tensile strength, when hot band coiled temperature is lower than 773K. In order to improve the property of the 450N/mm2 grade steel, the effect of Si content has been studied. It was found that the deep drawability is not deteriorated by the addition of Si into the Mn and P co-added ultra-low carbon IF-steel.

Various types of bake-hardenable steels have been developed and applied to automotive body panels for a weight reduction since 1981. Recently for the improvement in the cosmetic corrosion resistance of the exposed panels, an ultra-low carbon bake-hardenable galvannealed steel was developed and has been produced in a large quantity. The main metallurgical features of the ultra-low carbon bake-hardenable steel are follows, (1) adjustment of bake-hardenability by controlling the total carbon content between 15 and 25 wt.ppm, (2) suppression of TiC formation by controlling Ti content between 48/14 ·N and 48·(N/14+S/32). (3) optimisation of temper rolling condition to eliminate the stretcher strain.