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A highly anti-corrosive organic-inorganic hybrid paint for automotive steel parts has been developed. The inorganic component included in the paint is silicon dioxide (SiO2), which has the capability to passivate zinc. By application of the paint on a trivalent chromatetreated zinc-plated steel sheet or a trivalent chromate-treated zinc-nickel-plated steel sheet, high anti-corrosion protection can be provided to steel materials. Particularly in the case of application over a zinc-nickel-plated steel sheet, 0 mm corrosion depth after a cyclic corrosion test (CCT) of 450 cycles was demonstrated.

Vibration damping steel sheets (VDSS), which have sandwich structures with intermediate layers of resin, have been studied. The most important characteristics of VDSS for inner panels of automotive vehicles are the vibration damping properties, press formability and spot weldability. Vibration damping properties, which are quantified by loss factor,η, were influenced by both tanδ, which indicates damping capacity of resins, and elastic modulus of core resin. From a view point of vibration damping properties, resins with larger tanδ and relatively lower elastic modulus were favorable. Because these mechanical characteristics vary considerably with temperature, it is important to select the most suitable resin for the service temperature range. The relationship between noise reduction effect and loss factor of VDSS were also studied. It was experimentally confirmed that noise reduction effect of VDSS is proportional to the logarithm of their loss factor.

This paper describes the development of a simplified fracture finite element (FE) model for resistance spot welds (RSW) of ultra-high-strength steel (UHSS) that can be incorporated into large-scale vehicle FE model. It is known that the RSW of UHSS generates two types of fracture modes: heat-affected zone (HAZ) and nugget zone fractures. Lap shear and peeling coupon tests using UHSS sheets found that the different RSW fracture modes occurred at different nugget diameters. To analyze this phenomenon, detailed simulated coupon tests were carried out using solid hexahedral elements. The analytical results revealed that RSW fractures are defined by both the application of plastic strain on the elements and the stress triaxiality state of the elements. A detailed model incorporating a new fracture criteria model recreated the different UHSS RSW fracture modes and achieved a close correlation with the coupon test results.

A technique for induction-hardening local portions of vehicle body reinforcements press-formed of thin sheet steel has been developed, with the aim of ensuring occupant safety in a side collision. This technique for increasing the tensile strength of sheet steel was practically applied to the front floor cross member and center pillar reinforcement. Owing to this method, the weight of body reinforcements can be decreased. New induction-hardening systems have also been developed for the present technique. One is an apparatus which allows induction-hardening a part with a three-dimensionally curved surface. Another is a straightening quench technique used to retain the same dimensional accuracy as the original press-formed part.

1 The principal characteristics required of sealing materials used in the body shop have focused on their adhesion to oily steel sheets and quick curing performance. Means for attaining these characteristics have been narrowed down to a basic resin system and a curing system. Various techniques have been studied to ensure proper anti-corrosion performance at the sealer application boundaries and thin application areas. They include the addition of anti-corrosion fillers, the provision of conductivity (through electro-deposition), and the application of a micro foam film over the application boundaries. Thus, prospects for attaining the same level of anti-corrosion performance as existing materials have been achieved.

Fluoroelastmers are well known for their resistance to heat and fluids, and have become major material for crankcase oil seals. On the other hand, new additive formulations are developed for engine lubricants used for fuel economic gasoline engines. In this paper, the effects of those additives on properties of fluoroelastmers are investigated. The results of the immersion tests of both test plaques and oil seal products indicate that dithiocarbamates, friction modifier, have hardening effects on fluoroelastmers. The fluoroelastmer deterioration mechanism is determined by analysis of elastmer samples after immersion in oil.

This paper describes the lap welding of Zinc-coated steel sheet using a high power continuous wave YAG laser. The well-known problem of welding the Zinc-coated sheet is related to the low boiling point of zinc compared with the melting point of steel. During lap welding, zinc coating at the interface vaporize rapidly and causes defects1)2). In this study, therefore, lap welding was performed by YAG laser. The effects of type of coating layer, welding conditions, tensile strength and corrosion resistance after electro-deposition was examined. It was found that the weldability of coated steel is different by type of coating. Zn-Ni coated steel showed good weldability, but galvanealed steel inevitably pore pits with no gap set up. These defects not only lower the strength of joint, but also produce irregular bead where easily corroded after electro-deposition.

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.

Galvannealed steel sheet (GA) is very extensively used for vehicle panels. However ζ-phase (FeZn13) in GA coat causes poor stamping formability. Previously, there were no easy methods to evaluate the influence of ζ-phase on the frictional characteristics other than the X-ray diffraction method. This study will discuss the development of a new testing method: Dr. STAMP Method that is both efficient and convenient with pin-on-disc tester.

Tensile testing technique for the small sample was newly developed. Small tensile specimens with gage length of 1mm were taken from spot welds of high tensile strength steel sheets, and stress-strain relationships and ductility of base metal, heat affected zone including corona bond and nugget were individually measured. Finite element analyses of spot-welded joints under the conditions of static and dynamic tensile-shear loading were carried out with these local mechanical properties to predict the fracture mode and strength of the joints. It was clarified that the effects of both nugget diameter and class of steels were evaluated with good accuracy.

From the viewpoint of measures for environmental issues, the amount of solvents in paint for aluminum wheels needs to be minimized. Environmentally friendly powder coatings have been used widely for primer coating and clear coating, but there is no precedent for its use for base coating. This time, we optimized the condition of surface treatment of pigment and hardening behavior of constituent resin in the melting process and succeeded in developing a metallic powder coating for aluminum wheels that fulfills the appearance and the quality requirements of aluminum wheels.

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.

Two types of TS 450 to 600 MPa grade hot rolled high strength steels have been developed whose strength increases by a post-deformation heat treatment. One is a nitrogen added steel, produced by a low temperature coiling method. This steel shows an increase in tensile strength by about 60 to 90 MPa after a conventional baking treatment. The other is a Cu-bearing steel, produced also by a low temperature coiling method. This steel shows much larger increase in tensile strength by about 200 MPa due to the precipitation hardening of copper, but it requires an additional heat treatment at a relatively high temperature such as 600° C.

Since wear resistance and fatigue strength are key requirements for chassis components, induction hardening is widely used to apply compressive stress for controlling crack growth. Therefore, it is crucial that the influence of defects is examined with compressive residual stress applied to parts. In this report, the relationship between crack depth and compressive residual stress is evaluated using a cylindrical specimen and a torsional fatigue test. The test results were found to be consistent with CAE simulations performed in advance. In the future, it will be necessary to make this method applicable to product design to further improve vehicle safety performance.

In an effort to reduce mass, future automotive bodies will feature lower gage steel or lighter weight materials such as aluminum. An unfortunate side effect of lighter weight bodies is a reduction in sound transmission loss (TL). For barrier based systems, as the total system mass (including the sheet metal, decoupler, and barrier) goes down the transmission loss is reduced. If the reduced surface density from the sheet metal is added to the barrier, however, performance can be restored (though, of course, this eliminates the mass savings). In fact, if all of the saved mass from the sheet metal is added to the barrier, the TL performance may be improved over the original system. This is because the optimum performance for a barrier based system is achieved when the sheet metal and the barrier have equal surface densities. That is not the case for standard steel constructions where the surface density of the sheet metal is higher than the barrier.

A wider usage of hot rolled high strength steels (HRHSSs) are intensively studied for more complicated underbody structural parts of cars now. With an emphasis placed on the improvement of sheared-edge flangeability, we have developed new types of HRHSSs ranging from 440N/mm2 to 690N/mm2 in ultimate tensile strength (UTS). The microstructure and chemistry of these steels were controlled: The hard second phase in the polygonal ferrite matrix was controlled both by adjusting the chemical composition, mainly, silicon-content, and by applying thermomechanical treatment to a hot strip mill. Compared to conventional high strength steels such as a high-carbon, low-silicon steel and a niobium-bearing steel, various formability-performances of the developed steels were found to be good.

In order to investigate the corrosion resistance of organic composite-coated steel sheets ( OCS ) in a real automotive environment, many kinds of corrosion tests were performed on test pieces and real automotive doors. Tests with a corrosive solution including iron rust were introduced to simulate the real corrosive environment of automotive doors. The relationship between the components of OCS and the corrosion resistance in the rust-including tests was examined. In addition, electrochemical studies were performed. Results indicate OCS has much better corrosion resistance than plated steel sheets with heavier coating weight in all tests. OCS shows excellent corrosion resistance in rust-free corrosive solution, however, some types of OCS do have corrosion concerns in rust-including tests. It became clear that these OCS types have an organic coating with lower cross-linking.

Recently, many kinds of coated steel sheets are developed and used for automobile panels. Some of them show higher coefficient of friction and poorer formability than a cold rolled steel sheet. New polymer lubricating films are developed and proved to improve frictional characteristics of coated steel sheets in this study. The developed polymer lubricating films are applied to Electro Galvanized steel sheet and Hot Dip Galvannealed steel sheet, and coefficient of friction and formability of them are measured by using model dies. Different coefficients of friction are obtained by a change in a combination of components in a polymer lubricating film. As a result, the most suitable coefficient of friction is able to chose in forming conditions by the developed polymer lubricating films.

In search for thinner auto-body panels, investigation has been made on the level of yield strength of high strength steel sheets in view of the fact that the dent resistance must be at least equal to that of today's mild steel panels. Dent resistance should be evaluated for slight dent, and loss of the dent resistance caused by thinning is most remarkably seen in this case. Rephosphorized aluminum killed steel with bake-hardenability is most favourable, taking the necessary resistance to slight dent and the formability into consideration.

The residual stresses produced by static and progressive induction hardening processes were investigated using different diameter bars and different heating conditions. The x-ray diffraction method of stress measurement was used. Compressive stresses were observed at and slightly below the surface in the hardened zone, while lower compressive stresses or tensile stresses were seen at the ends of this zone. These stresses depended on the shapes and sizes of the heated zone and the cooling patterns. Shot peening was seen to convert the tensile stress to a compressive value.