Search Results

In order to save more weight using steel sheet, Thyssen Stahl has recently completed research work on creating a very light-weight composite steel material. Two thin steel sheets currently as low as 0.30 mm in thickness are joined by local welds of a certain spacing. Prior to joining, locally formed, hemispherically shaped domes are used to keep the cover sheet metals at the intended distance. These depressions can be applied either to one, or even to both cover sheet metals. The total thickness of the final compound steel sheet varies from 1.0 mm to 1.8 mm. The material weight per unit area is equal to 0.7 mm, yet it is able to serve for certain mechanical properties of about 1.0 mm solid steel. A weight saving of 30 % seems possible for specific panel applications. Because of the similarity between the depression shape and a bubble, the material is designated as “Bubble Steel Sheet Metal”.

The steel industry has been offering high-strength steel grades for decades to fulfil various customer demands. Our new approach is a high strength hot-dip galvanized steel aiming at a strength level of about 500 N/mm2. This material combines high corrosion resistance and high-strength with an increased potential for further material gauge reduction and hence weight savings. As an example, the designing, stamping and testing of a complicatedly shaped bumper bar are discussed. This features, specifically, the shape control technique used to overcome springback and sidewall curl effects. These springback effects are the result of an unbalanced bending and unbending process while the sheet metal passes over the draw radius. The appropriate balancing of the forming tools has contributed to an extremely robust stamping process, which is desensitised tremendously against the variation in yield strength.

The benefits of bake-hardening steels for body applications in the automotive industry are well known since several years. These steels offer the potential of weigth savings without major loss of formability. Thus this group of high-strength steels is the first to find broad application for exposed panels. This paper sums up the different concepts of producing cold-rolled electrogalvanized and hot-dip galvanized bake-hardening steels. These concepts are critically discussed from the point of view of stable production. Important aspects for batch annealed or continuously annealed steels are the control of grain size and temper rolling. New hot-dip galvanized bake-hardening steels have been developed; as a result of degassing to ultra low carbon contents in modern vacuum treatment facilities. Beside bake-hardening effects special emphasis is given to the strain-hardening behaviour and to the ageing resistance of bake-hardening steels.

The quality of a final product in particular made from hot-rolled sheet metal, is dependent upon the initial blanking, applied prior to forming the part. The material thickness and strength are the most important factors influencing the shearing parameters to be chosen. High-strength grades offer certain specific advantages when combined to consolidated blanks. Those type of tailored blanks requires a special practice and knowledge to shear the material independent of the grade and thickness used. The shearing in particular, guillotine shearing of various materials and thicknesses were studied in order to carry out a high quality cut for laser-beam welding purposes. In addition, the stamping of those materials, with special regard to high-strength and mild steel grades mostly used to stamp complex parts for automotive applications, are discussed as well.

Tailored blanks represent a basis for solving current problems in the automotive industry and are thus a product both for the present and for the future. The concept enables new ways of design which allows the manufacture of lighter, simpler and more cost effective automotive components. Tailored blanks are capable of eliminating production steps at stamping and assembly plants of our customers and thus reducing capital investments and expenditures of work.

In the past three years 1.5 million laser-welded sheets have been formed to deep drawn floor panels very successfully. Further developments of the applied laser-welding process enables joining of different sheet materials, as mild steel, HSLA, stainless and coated steel. Forming trials show the high remaining capacity of the welded blanks. The possibility of joining also different sheet thicknesses with ductil welds provides the design of lighter and stronger drawing parts or integration of several parts into one “tailored blank” reducing assembly cost. Application examples will be discussed.