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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.

Vibration Damping Steel Sheet (VDSS) for automotive use, which has a three layer structure of steel/viscoelastic resin/steel, has been studied. For automotive body panels, VDSS is required to have not only high vibration damping capability but also other properties such as bonding strength, formability, weldability and durability. In this research, the effect of resin layer on these properties was studied. It is found that VDSS which satisfies these properties can be made from thermosetting resin involving metal particles.

Weight reduction of automobiles is one of the most highlighted subjects in automobile industry from the energy saving and clean environment points of view. A typical approach for the purpose is to use high strength steel sheets as well as optimizing designs and using low density materials. It is not, however, easy to apply high strength steel sheets to automotive panels because of their strict requirement for the shape-fixability although a high dent resistance is also required. Besides the use of bake hardening steels, two different high strength steel sheets, which are a continuously annealed extra-low carbon titanium-added IF steel and a low carbon TRIP steel which contains about six volume% of austenite, were assessed for the application to automotive panels in combination with a tension-controled press-forming technique and showed as good shape-fixability as a conventional box-annealed aluminum-killed DDQ steel sheet when high blank-holding-forces were applied.

The purpose of this study is twofold: to clarify the factors governing the torsional strength of surface induction hardened parts and, to present a method for strengthening automotive shaft parts for their weight reduction. The torsional strength against Mode III fracture can be expresssed by a new indicator, “equivalent hardness” defined as an average hardness weighted with the radius squared. If the equivalent hardness is continuously increased, the fracture mode change from Mode III to Mode I. The torsional strength against Mode I fracture is governed by grain boundary strength. Accordingly, the key-points in increasing the torsional strength of surface induction hardened parts are to raise the equivalent hardness and increase the grain boundary strength of the steel. By application of this method, the torsional strength of steel can be raised by 50%, which, in turn, enables about a 25% weight reduction for shaft parts.

Aluminum alloy sheets are used for automotive body-panels, but their small Young's modulus results in inferior shape-fixability than conventionally-applied steel sheets with similar strengths. Smaller radius of curvature, indicating better shape-fixability, is found at the center of a panel press-formed with higher blank holder force (BHF). Higher force can be applied for press-forming of alloy sheets with larger strain-hardening exponent (n value) induced by an increased addition of Mg. Recently-developed 5000 series alloy sheets containing 5.5 pct Mg and 0.3 pct Cu have an elongation over 33 pct at an ultimate tensile strength of 270 MPa and can be press-formed with better shape-fixability.

The newly developed sheet steel lightly coated with an organic composite is as follows. Zn-Ni alloy plated sheet steel with a coating weight of 30 g/m2 and average Ni concentration of 11.5 ∼12.0 % is chromated through electrolysis. The coating weight of chromate film is 50 ∼90 mg/m2 in Cr. Furthermore, emulsified olefin-acrylic acid copolymer resin mixed with colloidal silica of particle size 7 ∼8 nm applied to a thickness of 1.0 ∼1.8 μm. Olefin-acrylic acid copolymer resin and colloidal silica are mixed at the rate of 100 and 30 (parts by weight). It maintains excellent corrosion resistance even after forming, C-ED paint corrosion resistance and paint adhesion. Furthermore, it has excellent perforation resistance. The product has excellent weldability and is well suited to continuous forming, too.

We have taken notice of Zn-Fe alloy electroplating with an eye to developing new corrosion-resistant steel sheets for automotive use with both cosmetic corrosion resistance and perforating corrosion resistance, and as a result of investigations into its paintability and corrosion resistance over the whole range of its compositions, we have come to a conclusion that steel sheets with two-layer Zn-Fe alloy electroplating that consists of a thin upper layer with a 75 to 85% Fe content and a lower layer with a 10 to 20% Fe content is the best choice.

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.

In order to clarify the effect of design and materials of the hem as well as the climatic factors on perforation corrosion of the automobile doors, field car and laboratory investigation has been carried out Field car investigation revealed that corrosion of the hem can be minimized by using two side galvanized steel plus adhesives. The ratio of wet/dry environment was evaluated in laboratory on hemmed sample, and it was found that the design of the hem in conjunction with the various wet/dry ratio affected the corrosion rate differently.

The application of tailored blanks to autobody parts has progressed because of its numerous advantages. The forming of tailored blanks has, however, a lot of technical problems. Among the problems, weld-line movement and formability deterioration are the most significant ones in case of deep drawing. The weld-line movement and formability change were examined experimentally as a function of weld-line location in square cup deep drawing. The weld-line movement of tailored blank consists of two sources. One is the geometrical reason, and the other is due to the hardening of weld bead. The formability of tailored blank is inferior to that of an original blank by the existence of hardened weld region. The mode of fracture changes from wall breakage to a fracture adjacent to punch radius when the weld-line was close to the punch corner.

The main operations for the manufacturing of auto parts are the cutting of the flange and the stamping. In order to perform accurate fatigue calculation it is necessary to have the material properties for each point of the structure. Usually, only the fatigue curve obtained on the flat sheet with polished edges is used because it represents the basic metal behaviour. The real edge quality decreases the fatigue limit while the hardening induced by the stamping increases it. To take these effects into account allows a better fatigue calculation of the structural part.

In continuous annealing, accelerated cooling methods have recently found practical application in addition to conventional gas jet cooling or water jet quenching method. Moderate cooling rate of gas jet cooling or accelerated cooling makes it possible to utilize new high-strength steels with excellent properties. The faster cooling rate deteriorates ductility of steels. These newly developed steels are improved rephosphorized steels with high r̄-value and high bake hardenability, dual phase steels with superior ductility, Ti-stabilized high-strength steels with very high r̄-value, grain boundary hardening steels employing low temperature annealing, and unique ultra high-strength steels. This paper describes properties and process factors of these steels, and their applications to automotive.

Carburizing heat treatment is one of the automobile component manufacturing steps, which consumes a large amount of energy. Raising the carburizing temperature can shorten the carburizing time and save the energy, but involves the risks of grain coarsening and attendant property deterioration. The authors have clarified the precipitation behavior of aluminum nitride (A1N) in the automobile gear manufacturing process and the optimum precipitation of A1N in as-rolled steel bars to prevent the grain coarsening. Through the application of the controlled rolling technique to ensure the optimum precipitation of A1N in continuously cast steel of uniform chemical composition, the authors have substantially saved energy while maintaining high quality, and developed a high-temperature carburizing steel expected to minimize and stabilize quenching strains.

Improvement of material characteristic values, adjustment of forming conditions as well as introduction of new forming technics are necessary to promote wide application of aluminum alloy sheets into automotive parts. 5000 series and 6000 series aluminum alloy sheets are concerned about the relationship between material characteristic values and fundamental forming ability required to apply them to automobile body parts as well as the effect of lubricant on their formability. The hardening parameters, n values, of them are larger than those of cold-rolled steel sheets. However, the r values and the local elongations are extremely small. The improvement of stretch formability owing to increase of n value is smaller than that of the steel sheets. Inferior deep drawability of the aluminum alloy sheets is due to low fracture resistance force caused by low r value.

In this paper the fatigue life of welded steel sheet structures is predicted by using FE-Fatigue, which is one of fatigue analysis software tools on the market, and these predicted results are evaluated by reference to corresponding experimental results. Also, we try to predict these structures by using two fatigue life prediction theories established by the JSAE fatigue and reliability committee to compare prediction results. It was confirmed that spot welds fatigue life predictions agree qualitatively with corresponding experimental results and arc welds fatigue life predictions are in good agreement with corresponding experimental results in cases where the SN curve database is modified appropriately.

Extremely formable cold sheet steel with an ultra-high Lankford value of more than 2.5 and an n value of more than 0.27 has been developed. This steel is obtained due to the following factors; using extremely pure IF (Interstitial free) steel, immediate rapid cooling upon completion of rolling in the hot rolling process, a high reduction in the cold rolling process, and a high soaking temperature in the continuous annealing process. This steel sheet shows excellent deep drawability and stretch formability compared with conventional steel sheet (former IF steel and low carbon aluminum-killed steel) as a result of evaluating the limiting drawing ratio and limiting dome height, respectively. This excellent formability is also shown by the model forming tests for simulating the actual stamping of an oilpan and a side-panel. Furthermore, this steel shows the same spot-weldability as that of former IF steel, and zinc phosphatability similar to that of low carbon aluminum-killed steel.

This paper describes the development of alloy cast iron that can be used for the cutting edges of the trimming die of a press die. Usually, a block of tool steel or steel casting is inserted at the cutting edge of the trimming die of a press die. However, we unified the structure part and the cutting-edge part of a press die with alloy cast iron. As it can''t bear as the cutting edge in this state, the cutting edge is processed by flame-hardening. After the flame- hardening, we developed the alloy cast iron so that enough hardness may be obtained by natural air cooling. Thereby, the machining of the installation seat of the cutting edge decreased and the expense of dies has been reduced.

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.

This paper describes about development of the alloy cast iron which can be used as the cutting edges of a trimming die of press die. Usually, the block of a tool steel or steel casting is inserted to the cutting edge of the trimming die of a press die. However, we unified the structure part and the cutting-edge part of a press die with alloy cast iron. Of course, in this state, since it can't bear as the cutting edge, the cutting edge is processed in flame-hardening. After the flame hardening, we developed the alloy cast iron so that hardness enough by the natural air cooling might be obtained. Thereby, the machining of the installation seat of the cutting edge decreased and the expenses on dies has been reduced.

As a demand for vehicles of higher functionality grows, automakers and material suppliers are devoting increasing efforts to develop technologies for greater safety, lighter weight, higher corrosion resistance, and enhanced quietness. The resin-sandwiched vibration damping steel sheet (VDSS), developed as a highly functional material for reducing vehicle vibration and noise, has been used for oil pans1) and compartment partitions2). First applied for a structural dash panel of the new Mazda 929, a Zn-Ni electroplated VDSS which allows direct electric welding has contributed to greater weight reduction as well as improved quietness.