Advanced High Strength Steel Sheets for Automotive Application

Hiroshi Takechi
Fukuoka Institute of Technology

Advanced High Strength Steel Sheet (HSS) for automobile and its application technology developed in Japan are introduced. TRIP steel containing Si and Mn tends to increase the quantity of retained austenite and improves elongation and n value of product. However, those elemnts deteriorate adhesive property of Zn by the formation of oxide film on the sheet surface. Nippon Steel developed Ni and Cu containing TRIP steel applicable to galvannealing treatment, which improves the wettability of Zn and stabilises retained austenite at the galvannealing temperature.

The reason why TRIP steel has an excellent deep drawability even though the mean r value is almost 1.0 is solved by considering the relation between the volume expansion at the transformation from austenite to martensite and the stress condition at flanging area or die wall area where a breakage takes place generally. The new mechanism shows that deep drawability is not always governed by only crystallographic texture.

A new IF based HSS containing higher C (~0,005%) and Nb(~0.07%) is developed by JFE Steel. A characteristic microstructure called precipitates free zone (PFZ) is formed along grain boundaries. PFZ is based on Ostwald ripening mechanism worked on NbC located at grain boundary, and it brings about lower YS and larger n value to product. Grain refining in hot rolled steel sheet due to finely dispersed NbC improves r value of product and increases the resistant property against cold working embrittlement.

Ultra fine grained steel has been developed by Nakayama Steel with using a new hot rolling strip mill. The hot rolling mill is characteristic of having single roll driving with different work roll diameter, which is called SRDD system. The SRDD system can apply strong shearing force to steel sheet during rolling, resulting in saving 30% of rolling force and 20% of rolling torque. Large reduction at a roll stand during hot rolling can be available and the cooperation with strong water cooling brings about very fine grain structure as much as 3 microns of diameter. The very fine grained steel sheet has a better relation between tensile strength and elongation than conventional one, and those having 500and 600 MPa of tensile strength have excellent Charpy impact value as compared with conventional SS400 and SM490B grade. Remarkable softening at HAZ cannot be observed, and concluded that the steel sheet having such a lower carbon equivalence is not inferior to the conventional one at bonded strength, when tensile strength of base metal is the same.

It should be very useful for us to press stronger HSS at elevated temperature or to quench it in the die (die quenching) after hot pressing, because pressing load and spring back can be remarkably reduced.

Four kinds of press forming process namely hot pressing, preformed hot pressing, warm pressing and partial quenching are being developed as well as studying on the material condition. In the case of hot pressing, B containing steel can be formed easily when tensile strength after hot pressing is approximately 1,500 MPa. The partial quenching process can give tensile strength as high as 1,250MPa after press forming, when the formability is good because the base metal is fairly soft. Various examples are described in this paper.

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