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The application of high-strength steel sheets to car bodies is expanding to improve the crashworthiness and achieve weight reduction [1, 2]. Conversely, in recent years, the occurrence of liquid metal embrittlement (LME) cracks has been discussed in resistance spot welding using a Zn-based coated high-strength steel [3-5]. This study examined the factors causing LME cracks and identified the locations of LME cracks found in resistance spot welds using a Zn-coated high-strength steel sheet. Furthermore, through an analytical approach using a scanning electron microscopy (SEM) and transmission electron microscopy (TEM), for a joint with an LME crack, it was found that (1) grain boundary fracture occurred at LME crack portion and its fracture surface was covered with Zn, (2) Zn penetrated into prior-austenite grain boundaries near the LME crack, and (3) Zn concentration decreased toward the tip of the Zn-penetrated site.

This paper proposes a rolling machine that forms fine corrugated section patterns for thin sheets. A prototype of the machine was made and the performance of the machine was tested. As compared with press forming, rolling has the advantages of the high forming limit, the low forming reaction force, the easy control of the thin sheet's curve and high productivity. We confirmed these four advantages by using finite element analyses and the prototype rolling machine. Stainless steel sheets and titanium sheets, which were one of the materials with a low forming limit, were used. Firstly, the rolling showed a 1.3-times higher forming limit than the press forming in the case that a fine corrugated section pattern was formed in a stainless steel sheet of 22-mm square sizes. Secondly, the forming reaction force of the rolling was about one-twentieth of the press forming without coining, and the experimental results agreed with the finite element simulation.

In this study, with the purpose of applying to the metal catalyst substrates, we have examined the feasibility of improving the light-off performance of a catalytic converter by enhancing heat transfer in the cells with heat-transfer promoters. Experimental and CFD analyses have been conducted to estimate heat transfer rates and pressure losses of the model cells with hemispherical protrusions. The analyses show that, by enhancing heat transfer of the cells, the cell density can be reduced keeping the catalytic performance in the steady state at the same level as that of conventional ones. As a result, the thermal mass of the substrate can be also reduced effectively without an increase of the pressure loss, and consequently the light-off performance of the catalytic converter can be improved noticeably.

New 440MPa class high-strength steel, which had high r-value(1.6) and elongation(38%), was applied to outer-panel for the first time in the world. In this development FEM simulation was carried out to clarify the necessary steel properties, and the production conditions in strip mill were established. 10-kg weight reduction was realized by using this steel.

In order to quantitatively evaluate mechanical durability of metal substrates for catalytic converters under heat cycles, thermal stresses and strains were simulated by FEM elastic-plastic analysis. Flat and corrugated sheets constituting honeycomb structures were directly modeled by thick-shell elements without replacing the structures with equivalent solid elements. It was reported that an asymmetric joint structure with “Strengthened Outer Layer” could provide metal substrates with high mechanical durability against heat cycles and the results of analysis in this study could show their high durability. It is important for improvement of mechanical durability to control the location of initial cracks generation and the direction of their propagation.

In order to examine the compatibility of improvement of crashworthiness with weight saving of automobiles by using high strength steel, a combination analysis of Finite Element Method and Dynamic Mechanical Properties has been established. Material properties used in this analysis have been measured by “one bar method” high velocity tensile tests, which can examine the deformation behavior of materials at a bend crush speed range (∼55km/h). It was confirmed that the strength of steel measured by one bar method was raised remarkably after press and hydro forming of high strength steels. It was also confirmed by FEM analysis and load drop test that absorbed energy of bend crush was improved by pre-strain effect. Further, we proved that absorbed energy of bend crush was also improved by appropriate design of thickness and the ratio of bend span and plate length. These effects are applicable to respective high strength steels.

In order to examine the compatibility of improvement of crashworthiness with weight-saving of automobiles by using high strength steel, a combination analysis of Finite Element Method and Dynamic Mechanical Properties has been established. The material properties used in this analysis have been measured by “one bar method” high velocity tensile tests, which can examine the deformation behaviour of materials at an actual crash speed range (∼55km/h). As for the accuracy of this system, comparison between experiments and FEM simulation both of this test machine and other high-velocity-tensile-test machines have clarified the feature of one bar method and the metallurgical features of high velocity deformation. It was confirmed that the stress-strain curve measured by the one bar method agreed with that measured by the modified Split Hopkinson pressure bar method.

This paper describes the deformation analysis of hemispherical punch stretching and square shell drawing, using 3-D finite element program “ROBUST”. The effects of material properties and process factors on cup height to punch force relation, and strain distributions on formed parts were investigated. The calculated values give considerably good agreement with experimental measurements from LDH, FLD and square shell tests. The results can be expected to contribute to predictive evaluation of forming limits using computer simulation.