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Technical Paper

Development of 780 MPa grade high-strength, hot-rolled steel sheet

Increase of load capacity for trucks by weight reduction of the body has been required to cut down transportation costs. The weight reduction of truck body is feasible in terms of the application of high-strength steel sheet to body. Especially, the application to truck frame is important because it is heavy. The required properties of steel sheet for truck frame use are press formability and fatigue strength. Press formability can be optimized based on the previous investigation results. On the other hand, investigations on fatigue strength are relatively few, and the value of fatigue strength is not sufficient. Accordingly, the fatigue strength of high-strength steel sheet for truck frame use was investigated in this study. As for fatigue strength, the fatigue strength of steel sheet with scale is significant because the steel sheet is painted without pickling to remove the surface scale.
Technical Paper

Hydroforming Formability of Electric Resistance Welded Tubes for Automobiles

From the viewpoint of applying ERW (Electric Resistance Welded) steel tubes for automotive structural parts by means of hydroforming technology, a series of free-bulge experiments is carried out. Four kinds of mild and high tensile strength steel tubes are provided. Initial stress ratio, αm that represents an axial force level is adopted as an experimental parameter. Expansion limit increases with decrease of αm. The bulge shape and the expansion limit are also affected by the mechanical properties of tubes. The strain hardening property in bulge deformation is approximated by a parabolic equation.
Technical Paper

A New Paint-Bake-Hardenable Aluminum Alloy for Auto Body Sheet Applications

A new 5xxx-series aluminum alloy, which was designed to have an excellent paint-bake-hardenability with practically no room-temperature aging, has been developed for automobile body sheets. The alloy contains 2∼3%Mg with additional amounts of Cu and Si. Yield strength in the as-shipped condition is very low, 80∼100MPa, but it can be increased by as much as about 60MPa through 2% stretch and paint-bake at 170°C. Such an excellent response to paint-bake was revealed to result from Guinier-Preston-Bagaryatskii(GPB) zone formation and enhanced by the optimum addition of Si. Density of vacancies in the alloy was reduced through a special annealing process to suppress room-temperature aging. Press-forming for fender outer and engine hood inner panels was done, and the alloy was demonstrated to have formability superior to 6xxx-series alloys and Al-4.5%Mg alloys, due to the reduced initial yield strength which also suppressed susceptibility to spring back.
Technical Paper

Fatigue Properties of Fillet Welded Lap Joints of High Strength Steel Sheets for Automobiles

The effect of a concentration of stress at a welded joint on the fatigue limit was investigated from the point of view of the effective design of automotive parts made of high strength hot rolled steel sheets. The stress concentration at the welded joint was evaluated using FEM analysis and the effect of stress concentration on endurance limit was investigated by fatigue tests using specimens of practical welded joints and specimens with the simulated configuration of welded joints. It is pointed out that another effect besides stress concentration and residual stress should be considered to evaluate the endurance limits for high tensile strength materials of over 590 N/mm2.
Technical Paper

Structural Performance and Weight Reduction of Steel, Aluminum and Stampable Sheet for Bumper Beams

This paper presents the results of experimental analysis on the structural performance of steel, aluminum alloy and stampable sheets for bumper beams and the calculation results on the weight reduction of those bumper beams. Bending tests of straight beams, which were made of the three kinds of materials, were carried out in order to examine their structural performance. In consideration of failure modes, it was found that the maximum load and the absorbed energy of those beams can be evaluated by their material strength, their side-wall thickness and the height of their cross sections. Based on this evaluation method, the weight and the absorbed energy of the constant displacement were calculated using a common cross section. The weight reduction of aluminum alloy and stampable sheets is superior to 590MPa high tensile strength steel sheets.
Technical Paper

Corrosion Resistance of Electrodeposited Zinc-Silica Composite Coated Steel

Corrosion resistance of Zn-SiO2 composite coated steel has been investigated both in the wet/dry cyclic corrosion test and in the accelerated atmospheric corrosion test (modified Volvo test). In this investigation, perforation corrosion test were carried out using cathodic electrocoated lapped-panel models. Zn-SiO2 composite coated steel has shown good perforation resistance as well as good cosmetic corrosion resistance. Polarization measurements for the Zn-SiO2 composite coating have indicated the inhibition of both anodic and cathodic reaction. The inhibition of these reactions becomes more marked as corrosion of the coating progresses. From the microstructural analysis of the coating after the corrosion tests, excellent corrosion resistance of Zn-SiO2 composite coated steel is attributed to the chemically stable corrosion product consisting of zinc hydroxide and SiO2 particles.
Technical Paper

Properties of New Organic-Silicate Composite Coated Steel Sheet for Automotive Outer Panel

In response to the increasing demand for organic-silicate composite coated steel sheet with bake hardenability, we developed EZN-UCII, which allowed the coating to be cured in the temperature range where bake hardenability could be maintained. It was said that improved perforation corrosion resistance of automotive body panels was achieved through the development of EZN-UCII. In recent years, the most important objective with respect to the corrosion resistance of automotive body panels has been to improve cosmetic corrosion resistance of the outer panel. The cosmetic corrosion resistance can be improved by applying an organic-silicate composite coating on Zn-Ni plated steel sheet. EZN-UCII, however, could not be applied to the automotive outer panel because the inferior appearance of the cationic e-coat made it worthless for use on the exposed side of the outer panel.
Technical Paper

Development of Vibration Damping Steel Sheet Using Thermosetting Resin

Vibration Damping Steel Sheet (VDS), which is composed of two steel-sheet skin layers and a middle layer of viscoelastic polymer resin, has been applied to automotive parts. For application to body panels, VDS needs several superior characteristics, such as, high damping capacity at room temperature and spot weldability as well as press formability. VDS developed by sandwiching a thermosetting resin has the loss factor of 0.2 at room temperature, and can be press formed and spotwelded under the same conditions as for an conventional steel sheet.
Technical Paper

Effects of Microstructures on Formability in Hot Rolled High Strength Steel Sheets

The Effects of microstructures on stretch flangeability and stretchability in hot rolled high strength steel was investigated. Stretch flangeability of multi-phase steels was inferior to that of single phase steels at a given tensile strength, although stretchability of the former was superior to that of the latter. In multi-phase steel, the main factor affecting stretch flangeability is the hardness ratio of ferrite to hard phase, on the other hand, that which most affects stretchability is the volume fraction of ferrite. According to these results, several types of high strength hot rolled steel sheets with excellent formability were developed.
Technical Paper

Development of Zn-Electroplated Steel Sheet Having Good Formability

The formability of electrodeposited Zn-SiO2 composite coating has been studied. Flaking of the coating and die pick-up were hardly observed during the draw bead simulation test. Powdering resistance of the coating were also better than that of galvannealed steel. Good formability of Zn-SiO2 composite coatings is attributed, primarily to the low friction coefficient of the coating, and secondly to the microcracking effect which release the forming stress.
Technical Paper

Effect of Coating Structure on Powdering Resistance of Galvannealed Steel Sheet

Galvannealed steel sheets containing 10wt%Fe in coated layer were prepared in various galvannealing temperature and cooling rate. Powdering resistances were evaluated by a draw bead test. X-ray diffraction examination revealed an attribution of the coating structure to powdering resistance. Higher galvannealing temperature(500∼550°C) deteriorates powdering resistance, but slower cooling from higher temperature improves it. At low galvannealing temperature(440°C), powdering resistance is rather good independent of cooling rate. Г1-phase(Fe5Zn21) is formed along the interface between coating and steel when samples are galvannealed at lower temperature(≦500°C) or cooled at slower cooling rate from higher galvannealing temperature. The results suggest that the increase of Г1 ratio [Г1/(Г+Г1)] in the coating is inclined to improve powdering resistance.
Technical Paper

Evaluation of Anti-Cratering Properties of Precoated Steel Sheets Using Current-time Profile and Mechanism of Cathodic Electrocoat Primer Cratering

A new method of evaluating the anti-cratering properties of precoated steels has been developed which detects minute differences among materials. The anti-cratering properties of precoated steel sheets are evaluated by measuring the variation of current with electrodeposition time (current-time profile) at the initial stage of cathodic electrocoat deposition. Although some current peaks were observed in the current-time profiles, the slight differences of anti-cratering properties in precoated steel sheets were able to be evaluated by measuring the last current peak height. In the case of cratering at high applied voltage, the peak height of the last current peak was high and the variation of the current was vigorous. On the other hand, when few craters were generated, the last current peak height was low.