Motivation

A growing demand for long-term corrosion protection in the automotive industry has caused an increased use of double-side galvanized sheets. These materials in particular show an intensified abrasion of the zinc-coat on press tools made from gray cast iron. Defects, increased cost caused by rejects as well as impairment of the component quality and process security are direct consequences. This effect occurs especially with coated sheets made from high-tensile steels.

Objectives

It is the aim of this project to optimize wear- and abrasion-behaviour of tool surfaces. The potential of innovative surface solidification processes as hard rolling and machine hammer peening are to be estimated and the results transferred to put into practice. This way the wear of tools can be reduced in a sense of a tribologic process optimization and the shaping of high-tensile steel sheets can be improved.

Approaches

The PtU`s combined strip-drawing test stand allows imaging real press-plant conditions very detailed. A practical oriented examination of the abrasion-behaviour of galvanized sheets as well as abrasion- and wear-behaviour of the used drawing tools is being guaranteed. Thus all relevant factors of abrasion can be analyzed in a laboratory and criteria for avoiding zinc-abrasion can be derived. A previous project has shown that especially the open pores of cast tools made from GGG 70L (nodular cast iron) cause the abrasion. Edge layer hardening processes for the pressing tools, such as smooth or hard rolling or hammering, promise good results. Tools that have been treated that way show a much smoother and wear-resistant surface, so a decrease of the abrasion phenomena with good results can be expected.

Results

Preliminary examinations indicate a surface smoothing as well as a possible hardening enhancement close to the surface. A first wear test on hammered tools shows that zinc abrasion still appears; however compared to standard grinded tools the abrasion is clearly reduced.

Outlook

The processes of the mechanical surface hardening offer some potential because they are applicable after the last milling operation in the same setting. Both processes are to be optimized in a way to save grinding and hardening operations on press tools.

Acknowledgments

The PtU would like to thank the following companies for their support for the realization of this project: