Wheel disk manufacture using Nb-bearing TRIP steels

TRIP (TRansformation Induced Plasticity) steels have been studied frequently because of its superior strength and ductility. The high values of tensile strength and elongation are mainly attributed to the strain induce martensite transformation (SIMT) phenomena of retained austenite. Because of the excellent tensile property combination, TRIP steels have many advantages when the steels are used for automotive applications. To produce as-hot-rolled TRIP steels with better microstructure and mechanical properties, an appropriate chemical composition together with the exact control of cooling in run-out table is major parameter. A 3-step cooling pattern in run-out table (ROT) should be employed to secure sufficient amount of ferrite and resultant carbon enrichment in untransformed austenite. Effective carbon enrichment in untransformed austenite is very important to increase stability of austenite in lower temperature region. Therefore, a precise temperature control model should be prepared to produce TRIP steels commercially. In this paper, a temperature model considering a heat generation during phase transformation is introduced. The developed temperature model was implanted in POSCO #1 hot strip mill for actual production. A mill trial using the model for the production of Nb-containing TRIP steels is successful and resultant tensile strength and elongation levels are 80 kg/mm² and 30%, respectively. The steel was subsequently employed for the lightweight wheel disk, and the hub hole can be fabricated without any cracks. FEA analysis of the disk manufacturing was performed, and the results predict well the actual forming process.