The influence of a heating rate up to 1000 C/s on recrystallization behavior and texture formation of an extralow carbon steel sheet was investigated. To discuss the experimental results of this investigation, a mathematical model for predicting recrystallization behavior was developed. An analysis using the model shows that the heating rate influences the apparent thermal activation energy of recrystallization. At a higher heating rate, C dissolved from cementite and AlN precipitated during heating decreases and as a consequence the apparent thermal activation energy is lower. This can also be a valid explanation for the fact that recrystallization of a steel sheet coiled at low temperature is retarded.The main orientations of the recrystallization texture are weakened if the heating rate is increased. This tendency has already been observed at the initial stage of recrystallization. If the specimen is heated slowly up to the initial stage of recrystallization, the decrease in the intensity of the main orientations is reduced even though the heating rate subsequently increases. This result indicates that, concerning formation of the recrystallization texture, the heating rate mainly influences the nucleation process.

The orthorhombic 00 martensite was formed in Ti-8 mass%Mo alloy by quenching from 1223 K. The purpose of this study was to investigate phase transformation of the 00 martensite structure by isothermal aging. In differential scanning calorimetry curve of the quenched specimen, an exothermic peak that indicated decomposition from the 00 martensite to and phases was observed near 780 K, so that isothermal aging was performed at 723 K and 923 K for 9.0 ks. Optical microscopy, X-ray diffraction and transmission electron microscopy were performed to these specimens. Band-like products that were composed of the single variant of ! phase were observed in the quenched 00 martensite structure. On the other hand, ð111Þ 00 twins were observed in the 723 K-aged 00 martensite structure. The quenched 00 martensite structure indicated low elastic incline and good ductility, whereas the 723 K-aged 00 martensite structure indicated high yield stress and brittleness. It was pointed out that the ! products were formed to relax the volume expansion from the phase to the 00 martensite, and the ð111Þ 00 twins were formed during the isothermal aging at 723 K with the extinction of the ! products.

The orthorhombic a″martensite was formed in Ti 8 massMo alloy by quenching from 1223 K. The purpose of this study was to investigate phase transformation of the a″martensite structure by isothermal aging. In differential scanning calorimetry curve of the quenched specimen, an exothermic peak that indicated decomposition from the a″martensite to a and b phases was observed near 780 K, so that isothermal aging was performed at 723 K and 923 K for 9.0 ks. Optical microscopy, X ray diffraction and transmission electron microscopy were performed to these specimens. Band like products that were composed of the single variant of v phase were observed in the quenched a″martensite structure. On the other hand, (111)a″twins were observed in the 723 K aged a″martensite structure. The quenched a″martensite structure indicated low elastic incline and good ductility, whereas the 723 K aged a″martensite structure indicated high yield stress and brittleness. It was pointed out that the v products were formed to relax the volume expansion from the b phase to the a″martensite, and the (111)a″twins were formed during the isothermal aging at 723 K with the extinction of the v products.

For pure molybdenum nitrided at 1373 K in NH 3 gas, microstructural observations of a molybdenum nitride layer were conducted through a transmission electron microscope. The molybdenum nitride layer consisted of two molybdenum nitrides: an outer one of ␥-Mo 2 N and an inner one of ␤-Mo 2 N. A great number of (011)[01 1] type twins were observed in the ␤-Mo 2 N phase, whereas the microstructure of the ␥-Mo 2 N phase was almost free from lattice defects except for slightly observed {111}͗112͘ type twin.

To develop ultrahigh-strength steels for automotive impact safety parts, the effects of the microstructure and nitrogen content on the fatigue properties of ultrahigh-strength low alloy transformation-induced plasticity (TRIP)-aided steels with martensite (TM), bainitic ferrite-martensite (TBM), and bainitic ferrite (TBF) matrices were investigated. Compared to TBF steels, both the TM and TBM steels achieved high tensile strength, of more than 980 MPa, and excellent fatigue properties. This results from the suppression of crack propagation due to the effective TRIP of the relatively stable interlath retained austenite and the increase in tensile and yield strengths attributed to the low isothermal transformation treatment. The fatigue strengths of the ultrahigh-strength low alloy TRIP-aided steels were slightly increased by the addition of 100 ppm of nitrogen. The increase in fatigue strength of TM, TBM, and TBF steels with 100 ppm of nitrogen was caused by the fine and uniform martensite and bainitic ferrite matrices and retained austenite, along with the increase in carbon concentration in the retained austenite due to the precipitation of AlN.