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New highly ductile AHSS steel grades with tensile strength greater than 980 MPa have been developed with the aim of combining high strength and excellent formability. The new jetQ-Family offers high local and global ductility while still fulfilling standards for resistance towards hydrogen embrittlement and weldability. These improved properties are based on their specifically engineered microstructure, which utilize the TRIP-mechanism in a strengthened matrix. This work shows how the microstructure plays a significant role for the tensile testing as well as hole-expansion. Based on the increased yield strength a better crash performance compared to conventional DP steel grades can be attained. The local ductility is demonstrated with excellent hole expansion ratios and high resistance to sheared edge failure. In combination with improved bending angles and thickness strain at fracture a robust process for manufacturing of components can be achieved.

A fundamental study on the ductility of high strength steels under impact deformation is carried out to investigate the effect of the local ductility of various materials on crash performance. In this study, newly developed 980 and 1180 MPa grade steels are investigated to clarify their advantages in term of crash performance compared to conventional DP (Dual Phase) steels. The features of the developed steel, named as jetQ are higher yield strength and higher local ductility due to an optimized microstructure by the quenching and partitioning process (QP) [1, 2]. The bending test according to VDA 238-100 is performed while observing the fracture propagation during the bending test. Fracture strain in the tensile tests is evaluated by a three-dimensional shape measurement technique for the fracture surface. Both three-point bending tests and axial impact tests are performed to evaluate the crashworthiness of different types of steel.

An automotive body is made by joining over 500 components made from steel sheets. Since the joining locations for spot-welding are decided by the designer of each component, the number of spot-welding points tends to be either excessive or inadequate for the required automotive body stiffness. In this study, a topology method which is able to select effectively from design space was applied to optimization of spot-welding locations for vehicle stiffness performance by using a full vehicle model. Static stiffness using constraint of nodes cannot sufficiently express deformation during driving. Torsional deformation occurred in all parts of the body in the mode in which one point of the front bilateral suspension parts was forced and the other three points were constrained in the general static stiffness mode.

A simple testing method is proposed in order to investigate ductile fracture in crashed automotive components made from advanced high strength steels. This type of fracture is prone to occur at spot-welded joints and flange edges. It is well known that the heat affected zone (HAZ) is a weak point in high strength steel due to the formation of annealed material around the spot-welded nugget, and the flange edge also has low ductility due to the damage caused by shearing. The proposed method is designed to simulate a ductile fracture which initiates from a spot-welded portion or a sheared edge in automotive components which are deformed in a crash event. Automotive steel sheets with a wide range of tensile strengths from 590MPa to 1470MPa are examined in order to investigate the effect of material strength on fracture behavior. The effects of material cutting methods, namely, machining and shearing, are also investigated.

In order to develop a new carburizing steel material that realizes an intermediate heat treatment-free process in parts manufacturing, the cold forgeability of the as-rolled steel and suppression of abnormal grain growth of austenite were studied. It was shown that adjustment of addition amount of Si, Mn and Cr, suppression of dynamic strain aging during cold forging, and an increase of ferrite fraction by controlled rolling contribute to the reduction of deformation resistance. However, Nb precipitation control by fully utilizing mill manufacturing processes was also necessary for suppression of abnormal grain growth of austenite. A new steel for carburizing was developed by integrating these technologies, making it possible to eliminate annealing before cold forging and normalizing before carburizing simultaneously. Thus, the developed steel is an important innovation in the parts manufacturing process.

Although reduction of the thickness of materials used in the automobile body is important for weight reduction, reducing the thickness of outer panels deteriorates dent resistance and surface distortion. To investigate the potential for weight reduction, the factors which influence the surface distortion and dent resistance properties were evaluated quantitatively with the aim of securing these properties. The materials used in these experiments were a tensile strength (TS) 340MPa grade bake hardenable (BH) steel sheet, which is often used in door outers, and a TS 440MPa grade BH steel sheet for outer panels. Surface distortion increases as a result of higher yield point (YP). It is possible to suppress the increase in surface distortion by increasing the blank holding force (BHF) in press forming. However, because this reduces the BHF range to the forming limit, application of low YP material is considered to be more advantageous for suppressing surface distortion.

A new topology analysis method was developed to optimize part shapes and the configuration of automotive components. Only solid elements are used in the conventional topology optimization method. The key point of the new method is to embed solid elements in a model made of shell elements. In this study, stiffness optimizations were carried out for a simple cylindrical model, automotive floor model and full vehicle model. Specifically, optimized automotive components were a center tunnel, a side-sill and a joint linking a side-member and a cross-member, which are made of steel sheets and have rectangular cross sections. The results show that the newly-developed topology optimization method is valuable in the optimization of automotive components which are made of steel sheets and have rectangular cross sections.

Since galvannealed steel sheets (GA) are widely used for automobile body parts, they require excellent features such as press formability, resistant spot weldability and phosphatability. We have focused on improving the press formability of GA since the late 1990s, and have developed a new type of surface modified GA which has a lower friction coefficient than conventional GA. The developed surface modified GA based on mild steel is now used by all automakers in Japan, especially for those parts such as side panels that are difficult to form. This paper describes the features of the surface modified GA.

Sheet hydro-forming was applied to hydro-form a door outer panel using different steel grades. The effect of mechanical properties and the forming conditions on panel properties such as thickness profile and cross-sectional shape accuracy were investigated by both experimental sheet hydro-forming and FEM forming analysis. 590MPa T.S. steel grade was successfully formed with improved dent resistance compared to the conventional 340MPa T.S. steel grade. On the other hand, the results of the FEM forming process analysis showed that the pre-forming conditions were important in controlling the fracture formation during forming and to improve dent resistance, which successfully led to the best forming condition.

A method of fatigue life prediction of spot welded joint under multi-axial loads has been developed by fatigue life estimation working groups in the committee on fatigue strength and structural reliability of JSAE. This method is based on the concept of nominal structural stress ( σ ns) proposed by Radaj and Rupp, and improved so that D value is not involved in stress calculation. The result of fatigue life estimation of actual vehicle with nominal structural stress which was calculated through newly developed method had very good correlation with the result of multi-axial loads fatigue test carried out with test piece including high strength steel.