Kobe Steel, mass producing high-productivity, hot stamping steel sheet

February 17, 2017

Kobe Steel, Ltd.

Kobe Steel, Ltd. announces that it has developed a hot stamping steel sheet (1470 MPa class after hardening) with outstanding productivity during press operations. Receiving an order from Toyota Motor Corporation, Kobe Steel is mass producing the steel, which is being used as a structural body part of the Prius. This is Kobe Steel’s first order for the hot stamping steel sheet.

By mainly adjusting the chemical composition of the steel, Kobe Steel was able to make considerable improvements to the hardenability of the hot stamping steel. Productivity during press operations can be improved about two to four times, in comparison to conventional hot stamping steel sheet. The steel is also characterized by the lower occurrence of strength deficiency from uneven quenching. Kobe Steel aims to expand the use of this steel, appealing to customers on the high productivity and stable quality of the material.

As CO2 emission regulations grow stricter year by year, automobile manufacturers have been facing the challenge of the increasing need for lighter cars while improving the strength of vehicle bodies to meet stronger collision safety regulations in Japan, the United States and Europe. To resolve these two issues, Kobe Steel has been working on making steel sheet thinner and stronger. It has developed ultra high-strength steel with tensile strengths of 780 MPa and higher, as well as hot stamping steel sheet. The use of these steel products depends on the strength required for the automotive parts.

In general, the structure of steel sheet hardens and the sheet becomes stronger when it undergoes heat treatment—namely heating the material to over a certain temperature followed by rapid quenching. Ultra high-strength steel sheet achieves its tensile strength when it undergoes heat treatment during production at the steelworks by the steelmaker.

For hot stamping steel on the other hand, it is the customer who heats, press-forms and quenches the steel in a mold to increase the strength of the material. Steel for hot stamping is characterized by its excellent workability and outstanding dimensional accuracy owing to hot press forming. In addition, in comparison to ultra high-strength steel, parts of higher strength can be made from hot stamping steel. In recent years, hot stamping parts have been increasingly used in Europe as well as Japan.

However, conventional hot stamping steel sheet, after hot-pressing, must undergo in-mold quenching for a certain length of time. As a result, in comparison to ultra high-strength steel that has been cold-pressed, conventional hot stamping steel suffers from low productivity. Furthermore, when uneven quenching arises, the steel is prone to strength deficiency. The hot stamping steel sheet developed by Kobe Steel is an innovative product that significantly shortens in-mold quenching time, as well as reduces uneven quenching. This steel has already been patented.

In addition to the high functionality of ultra high-strength steel, hot stamping steel, aluminum and other materials, Kobe Steel offers solution technology centered on the joining of steel and aluminum. Through multi-materials, Kobe Steel aims to meet the weight reduction needs of automakers.

Process

Conventional hot stamping steel:
Coil → Blanking → Heating → Pressing → Long in-mold quenching
Kobe Steel’s hot stamping steel:
Coil → Blanking → Heating → Pressing → Shape fixability through in-mold quenching

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