Research On Rules And Control Methods Of Sheet Surface Damage In Forming Of Auto Sheet Steels

The demands for lightweight security and corrosion-resistance in automotiveindustry have focused the attention of automotive manufacturers on the use of highstrength steels and galvanized steels. At the same time, the application of highstrength steels and galvanized steels presents many challenges in sheet metal forming.For high strength steels problems with surface damages are of major interest, sinceforming pressures are high, as well as blank holder forces. For galvanized steels,powdering, abscission and crack of coatings are popular in the forming processes.Surface damage is one of the major problems in the forming of high strength steelsand galvanized steels, which not only decreases the corrosion resistance and surfacequalities of products but also decrease the stability of the forming process. It canextend the use field and proportion of the high strength steels and galvanized steelsand shorten the time of new product design if the surface damages of high strengthsteels and galvanized steels are decreased.Based on the comparison and evaluation of the surface damages of the highstrength steels and galvanized steels, sheet surface damage in sheet metal forming areinvestigated systemically. The experimental and evaluating methods on the surfacedamage in sheet metal forming are established through the analysis of thecharacteristics of sheet deformation, the friction between the blank and the tool. Andthen the effects of the forming tool and process parameters on sheet surface damageare investigated and the model of sheet surface damage in the forming of highstrength steels is established. Aimed at the forming process of different high strengthsteels, the hardness matching window of sheet material and forming tool is defined.At last the effects of tool hardness and surface roughness on surface damages of thegalvanized steels and the influence of galvanized process on surface damage areinvestigated. And the characteristics and mechanisms of surface damages in theforming of hot-dip galvanized, galvannealed and electro-galvanized steels are studied.To fulfill the above research objectives, the following four aspects of efforts areperformed: (1) Experimental and evaluating system of sheet surface damages for the sheetmetal stampingThe frictional characteristics between the die and the sheet blank in sheet metalforming are different with the traditional contact friction between two solid due to theplastic deformation of sheet material and complexity of sheet deformation. Therefore,the contact characteristics between the die and the sheet blank in sheet metal formingare analysed firstly in this chapter and a complicated part that have typical sheetdeformations in stamping is designed to study the sheet surface damages in sheetmetal forming. To study the effects of the forming tools and sheet materials on sheetsurface damages, U-channel forming experiment with draw-bead is selected tosimulate sheet surface damage in the forming of sheet steels under tension-bending.Finally, based on the producing conditions in automobile industry, the experimentalproject is established and the parameter to evaluate sheet surface damage and themethod to measure the parameter are studied. This chapter is the foundation of theexperiments described in the followed chapters.(2) The rules and numerical model of sheet surface damage for the forming ofhigh strength steel under tension-bendingThe effects of tool materials Cr12MoV, Mo-Cr alloy cast iron, nodular iron andgray cast iron on sheet surface damages are studied using the high strength steelB170P1. It is demonstrated that Mo-Cr alloy cast iron is the best tool material amongthe investigated materials for sheet metal forming. Through the U-channel formingtests, the effects of tool hardness, tool surface roughness, blank holder force andlubrication on blank surface damage are investigated. It is indicated that sheet surfacedamages can be decreased through increasing tool hardness, surface roughness anddecreasing blank holder force. Based on the experimental data, the model for thesurface damage in the forming of high strength steels is established through non-linearcurve fitting. The reliability of the equation was validated. Using the results of thischapter, the technical measures can be established considering the effects of formingtools and process parameters on sheet surface damages and the sheet surface damageswill be decreased obviously.(3) Hardness matching window of the blank and die aimed at surface damages ofhigh strength steelsTool hardness is one of the important factors that influence sheet surface damage insheet metal forming and it also affects tool wear, tool life and the cost of tool manufacture. With increasing of sheet strength, heavy sheet surface damages and toolabrasion are happied. Thus, for the forming of the high strength steels with differentstrength, the forming tools should be heat treated with different mothods to getdifferent tool hardness. In this chapter, using the high strength steels B170P1, DP590and DP780the matching of sheet strength and tool hardness is researched. The effectsof tool hardness on different high strength steels are investigated based on theBox-Behnken test design and then the hardness matching window of sheet materialand forming tool is established. The selection of tool materials and prediction of toollife can be done through the matching window, and then the sheet surface damagesand part rejection rate can be decreased.(4) Effects and mechanism of galvanized coatings on surface damageHot-dip galvanized, galvannealed and electro-galvanized process are the typicalgalvanized processes and the surface damages of the three types of galvanized steelsare different with each other. Firstly, the effects of tool hardness and tool surfaceroughness on surface damages of galvanized steels are investigated. And then theeffects of galvanized processes on the types of sheet surface damages in the formingof galvanized steels are studied and the methods of decreasing surface damages ofgalvanized steels are proposed. Finally, the mechanisms of sheet surface damages ofdifferent galvanized steels are investigated through the analyses of coating hardness,surface topography and chemical composition.Through this research, the sheet surface damages in the forming of high strengthsteels and galvanized steels are studied comprehensively. The characteristics,simulating methods and evaluation of sheet surface damage in sheet metal forming areanalysised. The effects of the forming tool, process parameters and sheet materials onsheet surface damages are studied. The research will extend the usages of highstrength steels and galvanized steels in automobile industry.