Yoshihiro Hosoya

Effect of Retained Austenite Stability on Mechanical Properties of 590MPa Grade TRIP Sheet Steels

Industrial low alloy TRIP sheet steels contain blocky and lath-shaped retained austenite. In the present study, transformation behaviour of blocky and lath-shaped retained austenite during straining was investigated to clarify its effect on mechanical properties. Two types of TRIP steels containing almost the same amount but the different morphology of retained austenite were used. A steel containing large amount of lath-shaped retained austenite exhibits superior ductility, and sustains high work-hardenability in a high strain region. On the contrast, a steel containing large amount of blocky retained austenite exhibits low ductility. 　The work-hardenability increased steeply to the maximum at a low strain region, and then reduced in a high strain region. The stability of the blocky austenite has been found to be poor with respected to martensite transformation. The lath-shaped retained austenite remains until a high strain region whereas the blocky retained austenite transformed into martensite in a low strain region. Carbon content was higher in the lath-shaped retained austenite than in the blocky retained austenite. Stability of retained austenite is, however, inexplicable only by the carbon content, and would be affected by the different morphology and the resulting restraint conditions.

Effect of Niobium Addition on the Texture Formation of High Strength Cold-Rolled Low Carbon Steel Sheets

The effect of niobium (Nb) addition on recrystallization texture formation in cold-rolled low carbon steel sheets containing 2% manganese (Mn) was investigated. The microstructures of hot-bands were significantly refined by Nb addition, which led to the development of the cold-rolling texture in both the γ-fiber (<111>//ND-fiber) and the α-fiber (<110>//RD-fiber). Recrystallization was retarded by Nb addition, in particular, the growth of <110>//ND grains was retarded. However, the γ-fiber and {112}<110> grains developed during annealing even in the intercritical (α-γ) region. Consequently, the r-value increased as the content of Nb was increased due to the development of the intensity ratio of the <111>//ND texture to the <100>//ND texture, which is desirable for deep-drawable high strength steel sheets.

State-of-Art High Strength Steel Sheets for the Automotive Application

New type of IF cold-rolled high strength steels (HSSs) with the strength level of 390 and 440MPa have been developed under the chemistry of the extra-low carbon steel containing around 60ppm C with an intentional addition of niobium by hybridizing the precipitation hardening with niobium carbides and the supplemental solid-solution hardening. In this steel, Precipitation Free Zone (PFZ) nearby recrystallized grain boundaries forms during continuous annealing. This structure leads to unique mechanical properties such as lower yielding and superior anti-secondary-work embrittlement under fine grain structure strictly required for the exposed panels in Body-in-White. Principles of the unique mechanical properties of the steel are introduced related with the formation of PFZ during annealing, and the results of further approach to improve them as the state-of-the-art product, which is widely used for the exposed panels in Body in White, are introduced in the paper.

Phase-Field Simulation of Cooperative Growth of Pearlite

Cooperative growth of pearlite is simulated for eutectoid steel using the multi-phase field method. This allows to take into account diffusion of carbon not only in γ phase, but also in α phase. The lamellar spacing and growth velocity are estimated for different undercoolings and compared with experimental results from literature and theoretical results from analytical models. It is predicted, that diffusion in ferrite and growth of cementite from the ferrite increase the kinetics of pearlitic transformation by a factor of four as compared to growth from austenite only, which is assumed by the classical Zener-Hillert model. Further on the effect of stress due to inhomogeneous carbon distribution in austenite and due to transformation strain is discussed shortly.

DEVELOPMENT OF IF HIGH STRENGTH STEEL WITH FINE GRAIN STRUCTURE FOR EXPOSURE PANEL

New type of IF high strength steel (HSS) has been developed by hybridizing the grain refinement and the supplemental solid-solution hardening. Grain refinement was achieved by the fine distribution of carbide under the appropriate combination of the higher carbon content near 60ppm and niobium. Although the grain refinement is an effective method to improve the toughness of steel, this method has formability. While this steel has the fine grain structure, yield ratio is decreased due to the unique carbide distribution, and r-value is improved by the favourable texture formation. As the result, it has been clarified that a new type of 440MPa grade grain-refined IF-HSS improved the press-formability and exhibited a superior secondary work-embrittlement to the conventional IF-HSS. (A) For the covering abstract see ITRD E121867.