Masayuki Wakita

Ultrafine Grained Steels due to Super Short Interval Multi-Pass Rolling in Stable Austenite Region

Ultrafine-grained steel sheets with the chemical composition of 0.15%C-0.74%Mn- 0.01%Si have been prepared using a laboratory rolling mill by Super Short Interval Multi-pass Rolling (SSMR) process, in which the inter-pass time is extremely shortened to enhance the cumulative strain. The SSMR process with a finish rolling around Ae3 leads to ultrafine equiaxed ferrite structure with 1μm in average grain size. In order to clarify the grain refinement mechanism in the SSMR process, the deformation substructure in deformed austenite was simulated using 70%Ni-30%Fe, which was a fcc alloy with equivalent stacking fault energy to C-Mn steels. TEM observations have shown that the dislocation substructure in the Ni-Fe alloy hot-rolled by SSMR process mainly consists of dislocation cells, of which size are refined to less than 1μm with shortening inter-pass time. It is concluded that the SSMR process can accumulate the deformation strain in austenite enough to densely nucleate ferrite inside austenite grains.

Crystallography and Mechanical Properties of Ultrafine TRIP-Aided Multi-Phase Steels

This study aims at examining thermomechanical controlled process to realize ultrafine TRIP-aided multi-phase microstructures in low carbon steels. Heavy deformation at a supercooled austenite region was found to lead the formation of 2 μm ferrite as well as retained austenite with high volume fraction. The morphology of retained austenite was changed from film-like shape to granular shape with lowering finish rolling temperature in austenite field. This ultrafine TRIP-aided multi-phase steel showed good balance of tensile strength with total elongation, ie. 1080MPa and 26.9%. A novel in-situ neutron diffraction measurement demonstrated that the retained granular austenite transformed to martensite at a relatively large strain compared with the retained film austenite. The therein-underlying mechanism of the good mechanical properties was discussed from the view points of the morphological and thermodynamical stabilization of retained austenite.

A Variant Selection Rule in Transformation in Steel and Prediction of Transformation Texture

Quantitative prediction of transformation textures in steel becomes possible if a variant selection rule is taken into account, in which ferrite nucleating on austenite grain boundaries prefers to have orientation relationship with two neighboring austenite grains at the same time. The mathematical model of the variant selection rule is described and some examples of simulation in transformation textures in hot-rolled steel sheets are presented using the textures of retrained austenite and the misorientation distribution function method. An excellent agreement is attained between the predicted and experimental ferrite textures.

The formation of ultrafine ferrite through static transformation in low carbon steels

A novel approach was used to produce an ultrafine grain structure in low carbon steels with a wide range of hardenability. This included warm deformation of supercooled austenite followed by reheating in the austenite region and cooling (RHA). The ultrafine ferrite structure was independent of steel composition. However, the mechanism of ferrite refinement changed with the steel quench hardenability. In a relatively low hardenable steel, the ultrafine structure was produced through dynamic strain induced transformation, whereas the ferrite refinement was formed by static transformation in steels with high quench hardenability. The use of a model Ni-30Fe austenitic alloy revealed that the deformation temperature has a strong effect on the nature of the intragranular defects. There was a transition temperature below which the cell dislocation structure changed to laminar microbands. It appears that the extreme refinement of ferrite is due to the formation of extensive high angle intragranular defects at these low deformation temperature that then act as sites for static transformation.