Material advance

ThyssenKrupp Steel and Japan's second-biggest steel producer JFE Steel Corporation have jointly developed new multiphase steel for the automotive industry.

ThyssenKrupp claims the steel has a minimum strength of 780 megapascals (MPa), similar to that of the advanced ultrahigh-strength steels CP-W 800 from ThyssenKrupp Steel and NANO 780 from JFE.

The steel is said to possess significantly improved forming properties because it achieves up to 40 per cent higher elongation compared with conventional 780MPa high-strength steel.

The two companies have jointly filed a patent on the newly developed material, which they have called TP-N 68/78.

ThyssenKrupp Steel and JFE have been working since 2002 under a cooperation agreement aimed at securing the global availability of high-performance steel materials for the automotive customers of both partners.

Results so far include the definition of common specifications for high-strength automotive steels and the conclusion of a cross-licensing agreement for the steel grades CP (complex phase) from ThyssenKrupp Steel and NANO HITEN from JFE. Under the agreement, both companies can produce the other partner's material.

The high strength and good formability of the newly developed 780MPa high-strength steel is attributable to a bainitic-ferritic microstructure with embedded retained austenite in combination with nanometre-sized precipitations.

During forming, the austenite content is transformed into hard martensite, which means that the steel only reaches its final strength during fabrication to the finished part. To achieve these specific properties, the materials developers at ThyssenKrupp Steel and JFE developed a new alloying concept and also employed new temperature control methods during hot rolling and cooling.

The newly developed steel is available as hot-rolled coil, and coated on request. Typical applications include crash-relevant automotive body structural parts.