Takehide Senuma

Model for Predicting Recrystallization Behavior of Cold Rolled Extralow Carbon Steel Sheets

Microstructual observations indicate that the recrystallization of cold rolled extralow carbon steel sheets occurs due to the abnormal growth of selected subgrains in recovered subgrain microstructures. The authors measured the orientations of a recovered microstructure of a cold rolled extralow carbon steel sheet by SEM-EBSP and classified the deformed grains into several types due to the orientation and its scattering degree. In this study, a model for predicting the recrystallization behavior of cold rolled extralow carbon steels has been developed by applying the model of Humphreys modified to the grain of each type.A good agreement between the experimental and calculated results using the model developed was obtained.

Precipitation and Precipitation Hardening Behavior of V and/or Cu Bearing Middle Carbon Steels

In this study, the precipitation and precipitation hardening behavior of a 0.3%V and 2%Cu bearing middle carbon steel has been investigated in comparison with that of a 0.3%V bearing steel and a 2%Cu bearing steel. The precipitation treatment was carried out isothermally at 600°C.The amount of the precipitation hardening of the 0.3%V and 2%Cu bearing steel is nearly equal to the sum of the precipitation hardening of the 0.3%V bearing steel and the 2%Cu bearing steel In the 0.3%V bearing steel, precipitates were observed in rows, which indicates the occurrence of the interphase precipitation while precipitates observed in the 2%Cu bearing steel were randomly dispersed. In the V and Cu bearing steel, randomly dispersed precipitates were not observed where there were aligned precipitates. In the paper, the different precipitation behavior of the three steels is discussed.

Influence of Thermal History on Microstructure and Mechanical Properties of Steels for Hot Stamping

Hot stamping is an attractive method to produce extra high strength automotive components. In the conventional hot stamping, the furnace heating is employed and the heating rate is quite low. To improve the productivity of the hot stamping technology, the reduction of time for the heating process is required. In this study, the influence of the heating rate in a range up to 200°C/s, heating temperatures between 650°C and 950°C and cooling condition on microstructure and mechanical properties of 0.22% C -3%Mn steel has been investigated. The steel is a promising material for the highly productive new hot stamping technology because this steel transformed into martensite from austenite even at cooling in free air. The specimens heat-treated at a high heating rate and for short holding time at the heating temperature just above Ac3 show significantly fine martensite microstructure and a good strength-toughness balance. In this paper, the α→ γ transformation behavior and the γ→ α transformation behavior after inter-critical annealing are discussed to explain the evolution of the microstructures and mechanical properties.

Influence of Alloying Elements on Precipitation Behavior of VCN in Middle Carbon Steels

For lightening the hot forged automotive components such as connecting rods, crank shafts etc. the increase in their yield strength is an important technical issue. Recent developments indicate that it is a promising way to increase the yield strength of the components using the ferrite-pearlite microstructure strengthened by precipitation hardening of VC. In this study, the influence of alloying elements, cooling rate and aging temperature on the precipitation hardening behavior of V containing middle carbon steels was investigated. The precipitation hardening is very sensitive to cooling rate and aging temperature. The addition of Si reduced the sensitivity of the cooling rate. The deformation in the austenite region slightly decreases the precipitation hardening. From a detailed analysis, it was found out that the precipitation hardening is strongly influenced by the γ→α transformation behavior, which indicates that the interphase precipitation plays a significant role for the precipitation hardening.

Nitriding behavior and strengthening mechanism of Ti added steels

The nitriding process is one of the common methods for surface hardening, and consists of heat treatment in a furnace for many hours. The nitriding behavior and strengthening mechanism of Ti added steels in the nitriding process, which is applicable to a high temperature and rapid process such as the continuous annealing of steel strip, were investigated. The sheets were hardened only near the surface. The hardening of the surface layer is due to the formation of clusters or fine precipitates with disc-like shape consisting of titanium and nitrogen. The maximum hardness depends on the content of Ti in the steel while the annealing time and the concentration of NH3 influence the depth of the hardened zone affected by nitriding. It is thought that the hardening only near surface improve bending stiffness without significant increase of yield stress. So there is a possibility that the surface hardening improves the dent resistance of automobile outer panels without significant worsening of the surface deflection. To study these behaviors theoretically, a model for predicting the precipitation behavior due to nitriding has been developed. The experimental results can be reasonably explained by the model calculations. And also, the estimation of the amount of strengthening was carried out. It indicated that the strengthening mechanism is mainly the precipitation hardening of TiN that could be Ti nitrides or Ti-N clusters.

Effects of hydrogen on the vacancy formation in magnesium

Quenching and annealing experiments with electric resistivity measurements were applied to magnesium to investigate the formation of thermal vacancies. Two specimens made from materials differing in impurity contents were examined. One of the specimens that was quenched into a methanol bath at -80°C from elevated temperatures ranging from 160 to 500°C revealed a significant decrease in electrical resistance during subsequent annealing for ten minutes in the bath. This decrease is attributed to the presence of hydrogen in solution, on the basis of annealing behaviors at low temperatures (-100~-60°C) after the quenching from 200°C. Another specimen, presumably containing smaller amounts of hydrogen, was quenched into iced water from elevated temperatures (200~560°C), which yielded results characterized by two thermal activation processes. These processes have the activation energies, 54.1 kJ/mol (0.56 eV) and 89.8 kJ/mol (0.93 eV) for lower and higher quenching temperature range, respectively. The former is ascribed to the formation energy of a vacancy interacting with hydrogen and the latter the intrinsic formation energy of a vacancy. The difference of these energies, namely 35.7 kJ/mol (0.37 eV), can be identified as the binding energy between a vacancy and a hydrogen atom.

Mathematical Model for Predicting Microstructure and Strength of Hot Rolled Steels

A mathematical model for predicting microstructural evolution in hot rolling processes and the strength of hot rolled steels has been developed. Application examples are given for hot strip rolling. An accurate online prediction of microstructure and tensile strength is achieved in the whole length of hot strip. Furthermore, an accurate prediction of the resistance of hot deformation at hot strip mill is also realized using the model.