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New type of IF high strength steel (HSS) has been developed by hybridizing the grain refinement and the supplemental solid-solution hardening. Grain refinement was achieved by the fine distribution of carbide under the appropriate combination of the higher carbon content near 60ppm and niobium. Although the grain refinement is an effective method to improve the toughness of steel, this method has not been taken into consideration in view of formability. While this steel has the fine grain structure, yield ratio is decreased due to the unique carbide distribution, and r-value is improved by the favorable texture formation. As the result, it has been clarified that a new type of 440MPa grade grain-refined IF-HSS improved the press-formability and exhibited a superior secondary work-embrittlement to the conventional IF-HSS.

Suitability of ultra high strength steels (UHSS) for mechanical joining was investigated. When mechanical joining was applied to conventional 980N/mm2-class UHSSs having dual phase microstructure (ferrite and martensite), surface cracking occurred. Additionally cleaving inside mechanical joints was also observed in cases involving joining of dual phase steel by tools with larger clearance than designed. This cleaving causes a serious decrease in joint strength. Resistance to surface cracking and inside cleaving depends on the metallurgical structure of UHSS. Single martensite phase is superior for preventing the occurrence of defects in mechanical joining. This is because of its excellent local formability, which can be evaluated by the hole-expanding test using a machined hole specimen. Besides superior resistance to cracking and inside cleaving, single martensite phase steel exhibited higher mechanical joint strength than dual phase steel in cross tensile test.

Finite element modeling based on both 3-D shape measurement and experimental stress-strain relationship was applied to the FEM simulation for estimating the crashworthiness of automobile parts. Compared with the result dynamic crash test using hat-square-column type specimen made of 300 - 590MPa grade steels, the accuracy of the FEM simulation was evaluated for various modeling methods. It was revealed that the modeling of corner radius, bulging of specimen wall and strain rate sensitivity of materials played important roles in predicting the actual dynamic deformation process and the force-stroke relationship.

In order to estimate and improve the crashworthiness of practical automotive parts, dynamic crash test was conducted by using double hat specimen composed of the materials with different strength and thickness. Estimation method of the average force of the specimen was discussed. From the test results, it was clear that absorbed energy of the specimen composed of the materials with different strength and thickness of plate can be evaluated from the linear mixture law for the relations of the absorbed energy and the thickness of the materials used. And the “effective width theory” is useful way to estimate the average force of the parts.