• Corrosion Science and Technology
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• v.6 no.3
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• pp.83-89
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• 2007

It is expected that advanced high strength steels (AHSS) would be widely used for vehicles with better performance in automotive industries. One of distinctive features of AHSS is the high value of carbon equivalent (Ceq), which results in the different properties in formability, weldability and paintability from those of common grade of steel sheets. There is an exponential relation between Ceq and electric resistance, which seems also to have correlation with the thickness of electric deposition (ED) coat. Higher value of Ceq of AHSS lower the thickness of ED coat of AHSS. Some elements of AHSS such as silicon, if it is concentrated on the surface, affect negatively the formation of phosphates. In this case, silicon itself doesn't affect the phosphate, but its oxide does. This phenomenon is shown dramatically in the welding area. Arc welding or laser welding melts the base material. In the process of cooling of AHSS melt, the oxides of Si and Mn are easily concentrated on the surface of boundary between welded and non‐welded area because Si and Mn could be oxidized easier than Fe. More oxide on surface results in poor phosphating and ED coating. This is more distinctive in AHSS than in mild steel. General results on paintability of AHSS would be reported, being compared to those of mild steel.

• Corrosion Science and Technology
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• v.6 no.4
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• pp.193-197
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• 2007

It is well known that the development of Advanced High Strength Steels (AHSS) is very important for the automotive industry in order to improve fuel efficiency and the reduction of material costs. However, it is particularly difficult to improve the surface quality of AHSS because the high amount of Si, Al, Mn and Ti etc. in AHSS promote selective oxidation, resulting in surface defects. The reheating process in the hot strip mill would cause severe oxidation because it is carried out at elevated temperatures under aggressive environments. In this study a reheating furnace simulator was developed to investigate oxidation phenomena in the reheating process. The environmental gas for the reheating furnace was made by burning coke oven gas with air in the simulator. The air/fuel ratio is precisely controlled by MFC. Ti oxides are easily formed on grain boundaries and Mn and Si oxides are usually formed in inner grains near the steel surface with a small round shape.

• Journal of Welding and Joining
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• v.33 no.3
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• pp.75-80
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• 2015

Self-piercing riveting is an joining method of advanced high strength steels (AHSS) and other dissimilar materials. It has attracted considerable interest from the automotive industry. The SPR has become an interesting alternative joining technique for difficult to weld materials such as steels and aluminium alloys. In this paper, self-piercing rivet and anvil for SPR were designed for the joining conditions with AHSS and aluminium alloy. Various conditions of SPR were simulated for the design of rivets and anvils. The simulated results were in good agreement with experimental ones. As a result, over HV500 rivet is desirable to joint SPFC780 AHSS and aluminum alloy.

• Transactions of the Korean Society of Automotive Engineers
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• v.17 no.2
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• pp.104-111
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• 2009

In this paper, optimization for frontal crashworthiness is carried out for the weight reduction design of an auto-body member with the advanced high strength steels(AHSS) such as 780TRIP and 780DP. The frontal crashworthiness is evaluated in order to optimize thicknesses for the front rail member of the ULSAB-AVC, Thicknesses of the front rail member with AHSS are optimized by comparison of crushing distance, absorbed energy and the deceleration for the auto-body with the response surface methodology. The results demonstrate that the crashworhiness of the front rail member with the optimum thicknesses of the AHSS is similar to analysis results obtained from the ULSAB-AVC project. The results also show that the weight reduction design is performed by substituting the AHSS for conventional structural steels such as 440E in the auto-body members.

• Corrosion Science and Technology
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• v.9 no.6
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• pp.254-258
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• 2010

Due to the global warming and economic crisis, automakers are currently focusing on the development of high fuel-efficiency vehicles. To accord with these requirements, steelmakers have been trying to develop advanced high-strength steels with improved automotive-related properties. In addition, galvanizing technologies have been developed to improve coating properties for AHSS (Advanced High Strength Steel) such as pre-oxidation and pre-coating, as well as roll dent prevention. In this paper, newly developed products and technologies for automotive galvanized steel sheet are reviewed.

• Transactions of Materials Processing
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• v.24 no.5
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• pp.354-360
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• 2015

AHSS (Advanced High Strength Steels) has been increasingly employed by global automotive OEMs in order to satisfy strengthened regulations and reduce weight for fuel efficiency. Hot stamping using boron steels in AHSS increases not only formability but also strength. The typical hot-stamped automotive part is the center pillar that is critical for vehicle side impact. However, the hot-stamped part can be risky for the passenger safety caused by brittle fracture under a vehicle collision. The high power diode laser is suitable for the heat treatment giving AHSS increased elongation that prevents brittle fracture in car crash. Therefore, local softening by laser heat treatment for energy absorption area on the hot-stamped part improves crash-worthiness.

• Journal of Welding and Joining
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• v.33 no.6
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• pp.55-59
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• 2015

Self-piercing riveting (SPR) is a sheet-joining method that can be used for materials that are difficult or unsuitable for weld, such as aluminum alloys and other steel sheet metals. The increased application of lightweight materials has initiated many investigations into new SPR conditions for riveting dissimilar materials. However, buckling of the semi-tubular rivet occurs during the riveting of AHSS. In this study, a helical SPR was designed for the riveting of AHSS and Al-alloy. In addition, the reinforced helical SPR which has straight parts was designed. The riveting of AHSS and Al-alloy was simulated. Simulated results were verified by comparison with experimental ones.

• Transactions of the Korean Society of Automotive Engineers
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• v.14 no.6
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• pp.112-119
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• 2006

This paper is concerned with the light-weight design of a center-pillar assembly for the high-speed side impact of vehicle using advanced high strength steels(AHSS). Steel industries continuously promote the ULSAB-AVC project for applying AHSS to structural parts as an alternative way to improve the crashworthiness and the fuel efficiency because it has the superior strength compared to the conventional steel. In order to simulate deformation behavior of the center-pillar assembly, a simplified center-pillar model is developed and parts of that are subdivided employing tailor-welded blanks(TWB) in order to control the deformation shape of the center-pillar assembly. The thickness of each part which constitutes the simplified model is selected as a design parameter. Factorial design is carried out aiming at the application and configuration of AHSS to simplified side-impact analysis because it needs tremendous computing time to consider all combinations of parts. In optimization of the center-pillar, S-shaped deformation is targeted to guarantee the reduction of the injury level of a driver dummy in the crash test. The objective function is constructed so as to minimize the weight and lead to S-shape deformation mode. Optimization also includes the weight reduction comparing with the case using conventional steels. The result shows that the AHSS can be utilized effectively for minimization of the vehicle weight and induction of S-shaped deformation.

• Corrosion Science and Technology
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• v.10 no.1
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• pp.1-5
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• 2011

The major purpose of the present study is to evaluate the performance of various galvanized (GI) or galvannealed (GA) mild steels and AHSS in stamping applications. Finite Element Analysis (FEA) of selected stamping operations was conducted to estimate the critical pressure boundary conditions that exist in practice. Using this information, laboratory tribotests, e.g. Twist Compression (TCT), Deep Drawing (DDT) and Strip Drawing (SDT) Tests, were developed to evaluate the performance of selected lubricants and die materials/coatings in forming galvanized steels of interest. The sheet materials investigated included mild steels and AHSS (e.g. DP600 GI/GA, DP780 GI/GA, TRIP780 GA and DP980 GI/GA). Experimental results showed that galvanized material resulted in more galling, while galvannealed material showed more powdering and flaking. The surface roughness and chemical composition of galvanized sheet materials affected the severity of galling under the same testing conditions, i.e. lubricants and die materials/coatings. The results of this study helped to determine the critical interface pressure that initiates lubricant failure and galling in stamping selected galvanized sheet materials. Thus, to prevent or postpone the critical interface conditions, the results of this study can be used to select the optimum combination of galvanized sheet, die material, die coating and lubricant for forming structural automotive components.

• Transactions of Materials Processing
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• v.29 no.2
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• pp.76-88
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• 2020

The need for advanced high strength steel (AHSS) forming technology is increasing as interest in light weight and safe automobiles increases. Multipoint dieless forming (MDF) is a novel sheet metal forming technology that can create any desired longitudinal and transverse curvature in sheet metal. However, since the springback phenomenon becomes larger with high strength metal such as AHSS, predicting the required MDF to produce the exact desired curvature in two directions is more difficult. In this study, a prediction model using artificial neural network (ANN) was developed to predict the springback that occurs during AHSS forming through MDF. In order to verify the validity of model, a fit test was performed and the results were compared with the conventional regression model. The data required for training was obtained through simulation, then further random sample data was created to verify the prediction performance. The predicted results were compared with the simulation results. As a result of this comparison, it was found that the prediction of our ANN based model was more accurate than regression analysis. If a sufficient amount of data is used in training, the ANN model can play a major role in reducing the forming cost of high-strength steels.