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Technical Paper

Heat-Resistant Ferritic Stainless Steel with High Formability for Automotive Exhaust Gas Systems

Improved formability and higher heat-resistance are increasingly demanded to materials used for hot end parts of automotive exhaust gas system in order to meet stricter emission regulations. A heat-resistant ferritic stainless steel and pipe with high formability was developed by refining the microstructure of hot rolled and annealed sheets. The newly developed steel showed a remarkable improvement in various forming properties such as limit drawing ratio, limit expansion ratio of pipe, and thickness reduction ratio of pipe after bending in comparison with conventional steels.
Technical Paper

Development of Hot-Rolled Sheet Steel with the Significant Increase in Tensile Strength Induced by Strain Age Hardening

A new type of bake-hardenable high strength hot-rolled sheet steel was developed, which shows a remarkable increase in tensile strength as well as yield strength after strain age hardening without addition of special elements. The new sheet possesses excellent crashworthiness and high fatigue strength, and also shows good formability, equal to that of conventional high strength sheets. This unique combination of properties makes it possible to reduce the weight of the car body by using thinner gauge material when the new sheet is applied to crash-resistant parts and underbody parts.
Technical Paper

Development of High Corrosion Resistant Stainless Steels for Automotive Mufflers Based on Condensate Corrosion Test and Field Investigation

Automotive exhaust gas condensate causes severe corrosion inside mufflers. Corrosive condensate became a problem following the introduction of three-way catalytic converters in exhaust systems. The authors previously established a new laboratory test method that simulates condensate corrosion. This test method contributed to the development of new stainless steels for automotive mufflers. The addition of molybdenum (Mo) and copper (Cu) to ferritic stainless steels was found to be effective in improving condensate corrosion resistance, and the effect was arranged using the index [%chromium (Cr)+3×%Mo+1.5×%Cu]. A field test was carried out to investigate the corrosion behavior of mufflers under actual conditions. Aluminized steels showed severe damage by general corrosion. On the other hand, 19%Cr-Cu ferritic stainless steel exhibited excellent corrosion resistance. The corrosion behavior in the field test showed good agreement with the results of the laboratory test.
Technical Paper

Properties of Precoated Galvannealed Steel Sheets for Lead-Free Automotive Fuel Tank Use

The goal of this study was to identify a pre-coated steel sheet that will satisfy press formability, seam weldability, spot weldability, brazing property, external corrosion resistance and internal fuel resistance. We established an experimental method for anti-fuel resistance measurement by applying synthetic spoiled gasoline by investigating the composition of spoiled gasoline and corrosion products in the fuel tank, which was collected in North America after 160,000 kilometers' running. Newly developed lead-free pre-coated steel sheet shows excellent anti-fuel corrosion resistance in comparison with that of usual terne coated steel sheet according to this method.
Technical Paper

Fatigue Properties of Nb-Bearing Stainless Steels for High Temperature Applications in Automobile Exhaust Gas System

Exhaust manifolds have been fabricated from stainless steels such as type 409 in place of cast iron in order to reduce the weight and to increase performance of the engine. In this paper, thermal fatigue and high temperature fatigue resistances of Nb-bearing ferritic stainless steels and type 409 have been investigated. The results obtained are as follows: Nb-bearing steels were superior to type 409 in high temperature fatigue resistance. Thermal fatigue resistance depended on a restraint ratio. The fatigue life of Nb-bearing steels was longer than that of type 409 in the condition of a low restraint ratio which might be near to the actual automobile exhaust system.
Technical Paper

High Strain Rate Deformation of High Strength Sheet Steels for Automotive Parts

The effects of strain rate on the deformation behavior of steels were investigated to find the most appropriate micro structure of steel for anti-crash parts of automobiles, such as front-side-members. The dual phase steel absorbed a higher amount of energy during dynamic deformation than other steel with the same static yield strength. The increase of volumetric fraction of the austenite phase in the dual phase steel deteriorates the dynamic deformation behavior. The FEM analysis for crash test of member also showed the superior performance of the dual phase steel.
Technical Paper

Evaluation of the Corrosion Rate of Zn Coated Steel Sheets for Automotive Body Use

The corrosion mechanism of zinc coated steel sheets in automotive bodies was studied in field vehicle tests and several types of accelerated tests. Perforation corrosion starts in unpainted areas of lapped parts, and proceeds in the following steps: i) galvanic protection by the Zn coating, ii) protection by corrosion products, and iii) corrosion of the steel substrate and perforation. Although the corrosion processes were the same in all the cases tested, the corrosion rate depended significantly on the environment, such as atmospheric exposure conditions and the part of the automotive body. In accelerated corrosion environments, Zn coating is largely ineffective against perforation corrosion because galvanic protection and protection by corrosion products cannot be maintained over the long term.
Technical Paper

Medium-Heat-Resistant Bearing Steel

Operating temperatures in many bearing applications exceed 100 °C. Conventional bearing steel (SAE52100,SAE5120,Etc.) will begin to experience a reduction in life due to dimensional instability, and reduced surface hardness, at these temperatures. High speed tool steel (such as AISI-M50) can operate in temperatures up to 300°C but is very costly. SAE52100 can be heat stabilized up to 300°C to improve the dimensional stability, but this process can severely decrease the surface hardness and fatigue life. There are many applications where the high operating temperature is greater than 100°C but less than 200°C. A new material has been developed, based on SAE52100, with improved dimensional stability, and surface hardness, for this medium operating temperature range.
Technical Paper

Corrosion Behavior of a Vehicle Submitted in Service in Subtropical Marine Environment of Okinawa Island for Eight Years

A passenger vehicle in the subtropical marine environment of Okinawa Island for eight years was submitted for corrosion investigation.The precoated sheet steels such as Zn-Ni and Zn-Fe showed excellent corrosion resistance in the door hem flange and a scribed outer panel, compared to cold rolled steel. Cosmetic corrosion of a North America vehicle was more severe than that of a Okinawa vehicle, but a similar corrosion rate for perforation was shown in door hem flange. Basic zinc chloride and β-FeOOH were observed by using micro X-ray diffraction method in the initial stage of corrosion, and then these changed to ZnO and α-FeOOH, Fe304.
Technical Paper

Dynamic Deformation Behavior of Steel Sheet for Automobile

The effects of strain rate on the deformation behavior of several steels were investigated to find the most appropriate material for anti-crash parts of automobiles, such as front-side-members. Dual phase steel, TRIP steel, and work hardened steel showed higher dynamic to static yield strength ratios than solid-solution hardened steel. The dual phase steel absorbed a higher amount of energy during dynamic deformation than other steels with the same static yield strength due to its high strain rate sensitivity and high work hardening coefficient during dynamic deformation.
Technical Paper

Improvements in Formabibity and Spot Weldability of Galvannealed Steel Sheet

Modifications in the properties of galvannealed steel sheet have been made to meet the needs of the automotive industry. The press formability (powdering and flaking) of the galvannealed steel sheet is very sensitive to the phase composition and coating weight of the coating. The allowable coatings for a satisfactory anti-powdering property may fail to provide the anti-flaking property, showing a high coefficient of friction due to the existence of a soft phase, ζ, at the surface. In this study, the effects of an inorganic film on the surface of the galvannealed coating were studied improving press formability. It was found that a thin borax film was effective to minimize surface friction and to improve unti-flaking properties. Short electrode life is another problem, as it limits continuous spot welding of the galvannealed steel sheet. During welding, electrode tips (Cu) are consumed by an interaction with the coating and the steel substrate.
Technical Paper

Reduction in Weight of Steel Wheels by Development of 780 MPa-Grade Hot Rolled Steel Sheets

Two new types of high-strength 780 MPa-grade hot rolled steel sheets were developed for use in wheel discs and rims, which are the structural parts of the wheel. The fundamental composition of the disc material is 0.08%C-1.5%Si-1.75%Mn-0.1%Ti. The basic material is a dual phase, ferrite-martensite steel, with precipitation strengthening of the soft ferrite phase by TiC. The aim of hardening the ferrite phase is to suppress the difference in hardness between the ferrite and martensite phases, and thus secure good stretch flangeability. The fundamental composition of the rim material is 0.09%C-1.7%Mn-0.05%Nb-0.15%Ti. This steel is based on a ferrite-bainite material and is precipitation strengthened using NbC and TiC. Although 590 MPa-grade steel had been considered to represent the limit of formability for wheels, a 30% increase in strength was achieved in the newly developed materials, while maintaining good formability.
Technical Paper

Corrosion Behavior of Organic Composite Coated Steel Sheet

Organic composite coated steel sheets retain their excellent corrosion resistance during cyclic corrosion tests (CCT). To clarify the corrosion behavior of these sheets during CCT, variations in corrosion products and coating components were examined. Moreover, the contribution of the corrosion products, organic composite coating, and chromate film to corrosion resistance was examined by AC impedance measurements. Formation of crystalline ZnCl2·4Zn(OH)2 and amorphous zinc carbonate were detected by X-ray diffraction (XRD) and Fourier transformation infrared spectroscopy (FT-IR). Crystalline ZnCl2·4Zn(OH)2 is formed during CCT on and under the organic composite coating. The corrosion products formed on the coating contain silicates from the silica in the organic composite coating. Consequently, the contents of zinc and silica in the coating decrease, while nickel and chromium in the chromate film and carbon in the coating remain constant during CCT.
Technical Paper

Development of Vibration Damping Steel Sheets for Automobile Oil Pans

Recently, Vibration Damping Steel Sheet (VDSS) has been increasingly used for automobile parts such as dash boards and partition panels. VDSS is also used in oil pans, but the rquirements in this application are much stricter than those in other auto applications. The material must remain sound (show no wall break) and not delaminate during deep drawing press forming. In service, it must be oil tight at the welded drain nut and must maintain good adhesion characteristics at high temperatures of 60-120°C. The VDSS with a thermoplastic resin core used to date in oil pans provides poor baking durability and heat durability in high temperature service. For use in oil pans, the authors therefore developed a new VDSS material comprising two extra deep drawing class steel skin sheets (of ultra low carbon steel) and a thermosetting polyester resin core layer containing metal powder.
Technical Paper

Characteristics of Al2O3 Dispersed Zn-Co-Cr

Al2O3 -dispersed Zn-Co-Cr alloy electroplated has been developed for automobile use. Optimum coating composition ranges from 0.5 to 2.0wt% for Co, 0.3 to 1.0wt% for Cr, and 0.1 to 2.0wt% for Al2O3. The coating showed excellent corrosion resistance with and without paint. Superior anti-corrosive property of the coating results from the formation of the mechanically and chemically stable corrosion products, ZnCl2 −4Zn (OH)2, and the passivation in anodic polarization. The coating also showed good stone chipping resistance as well as paintability, formability, and weldability. Accordingly Al2O3 -dispersed Zn-Co-Cr alloy electroplated is suitable for application to both unexposed and exposure parts, including interior and exterior surfaces.