Masaaki Fujioka

Effect of Molybdenum Content on Hardenability of Boron and Molybdenum Combined Added Steels

The upper limit of the Mo-B combined effect on hardenability was investigated in 0.15%C-B added steels containing 0 to 1.5%Mo. The hardenability of Mo-B steels increases up to 0.75%Mo suppressing the precipitation of Fe23(C,B)6. In contrast, the effect decreases over 0.75%Mo where Mo2FeB2 precipitates instead of Fe23(C,B)6. By thermodynamic calculation, it is suggested that Mo2FeB2 precipitates during reheating and the precipitation of Mo2FeB2 decreases the solute B content in reheating, which determines the limit of the Mo-B combined effect.

Estimation of Solute Carbon Concentration by Electrical Resistivity Method in Low-Carbon Martensitic Steel

The concentration of solute carbon in as-quenched tempered low-carbon martensitic steels (Fe-2%Mn-0.3%C) were estimated from the electrical resistivity. It was found that the electrical resistivity decreased gradually with the increase of the tempering period, and its decreasing rate was enlarged by raising the tempering temperature. The decrement in electrical resistivity was mainly due to the decrease in the amount of solute carbon caused by carbide precipitation. An empirical equation was then applied to convert the electrical resistivity to the solute carbon concentration, where the densities of dislocation and that of grain boundary were also taken into account. Quantitative analysis for a specimen tempered at 373 K for 3.0 ks revealed that the concentration of solute carbon was decreased by 0.005 mass% during the tempering. This estimated value agreed well with the amount of precipitated carbide (Fe2.5C) measured by TEM observation. As a result, it was concluded that the solute carbon concentration could be estimated quantitatively from the electrical resistivity measurement in as-quenched and tempered martensitic steel.

The Effects of Prior-γ Grain Boundary Segregation of Phosphorus, Manganese and Molybdenum on Intergranular Fracture Stress in Low Carbon Martensite Steels

Influence of prior-γ grain boundary segregation of alloy elements on intergranular fracture stress is important for the mechanism of temper embrittlement. There are a few efforts based on pure-iron [1], but no report on low carbon martensitic steels. In this study, the effect of segregation of phosphorus (P), manganese (Mn) and molybdenum (Mo) was investigated. The samples were melted by changing the amount of P, Mn and Mo based on the base Fe-0.1%C-3%Mn-90 ppmP. The martensitic steels with coarse prior-gamma (γ) were made by quenching and tempering. The segregation was measured by Auger electron spectroscopy, and the intergranular fracture stress was regarded as the yield strength at ductile brittle transition temperature of Charpy V-notch test. This study revealed that the segregation of P weakened the fracture stress mostly in the order of P and Mn, and that of Mo strengthened the fracture stress quantitatively. Mn-P co-segregation was not observed. The segregation of P was decreased by the addition of Mo.

Modelling physical metallurgy of steel products and its application to commercial processes

The present paper will describe the research activities for modelling of micro-structural evolution and properties of steel products. An integrated model in which the metallurgical phenomena and mechanical properties of steel plates produced by Thermo–mechanical control process are calculated will be introduced as the most advanced example. This model is in operation as a part of quality design system of commercial production line after the examination with a wide range of experimental data. Research for expanding applicable products and processes and for improving accuracy is in progress. Monte Carlo simulation for describing inhomogeneity of microstructures is being developed as an advanced modelling technique.

COMPUTATIONAL MODEL FOR PREDICTING MICROSTRUCTURES AND MECHANICAL PROPERTIES IN MANUFACTURING OF HEAVY STEEL PLATES

A computational model for predicting microstructural changes at any stage of manufacturing by Thermo-Mechanical Control Process and mechanical properties of heavy steel plates has been developed. Optimum combination of chemical composition and process conditions can be calculated by using this model.