土田 紀之

In order to develop the steels bearing low environmental burdens as well as high performances, the present situation in Japan has been analyzed from several points of view. The issues related with steel production include 1) potentiality of the domestic contribution to reducing the global amount of CO₂ emission and 2) possible recipes in practical process to lower the environmental load with high performance. A sector occupying a huge share in the industry field can reduce the CO₂ emission effectively and the steel industry is such the sector. A sophisticated use of thermo-mechanical process in steel production can improve both the environmental and the service performance. By lowering the heating temperature in the thermo-mechanical process, the CO₂ emission can be decreased and the yield strength can be increased by ultrafine-grain strengthening.

Tensile tests below room temperature were conducted to investigate effects of temperature, strain rate and ferrite grain size on tensile flow stress for ferrite-pearlite (FP) steels with ferrite grain sizes between 3.6 and 46.2 μm. The temperature and strain rate sensitivity of flow stress for the FP steels is almost independent of ferrite grain size. The flow stress for the FP steels is expressed by Hall-Petch equation; σ=σ₀+KD^-1/2, where D refers to ferrite grain size and σ₀ and κ constants. The slope κ of the equation is almost identical being independent of strain, temperature and strain rate. By summarizing the experimental results, it is possible to describe the flow stress of the FP steels by the following equation, σ=σ_t+182+5.8D^-1/2+900ε where σ_t and ε mean thermal component of stress and true strain, respectively; σ_t is influenced by ε, temperature and strain rate.

Elasto-plastic flow curves of dual phase steels consisting of martensite and ferrite have been predicted by using the secant method proposed by W_<ENG>. The predicted flow stress for a given volume fraction of martensite becomes the upper bound when the matrix is considered to be martensite, whilst the predicted flow stress becomes the lower bound when the matrix is ferrite. This prediction is compared with experimental results obtained from Mn-Si-C martensite-ferrite steels where the volume fraction and morphology were altered by changing heat treatment. It is found that the prediction is in good agreement with the experimental results when the matrix structure can be identified by means of optical microscopy. In the case where the matrix structure is percolated and is not distinguishable, the flow stress curves lie between those two bounds.