By using a steel with standardized chemical composition and conventional manufacturing processes for flat-rolled steel strip, a 1 500 MPa class stainless steel sheet, whose product of yield strength (YS) and total elongation (El) exceeds 30 000 MPa%, was developed and its mass production was established. Besides the excellent YS-El balance, the developed steel sheet has excellent performance for not only an anti-secondary work embrittlement but also high cycle fatigue endurance. Core technology of the developed method is composed of a combination of high precision cold-rolling and isothermal partitioning treatment in a batch furnace, named as a rolling and partitioning (R&P) method. By the R&P method, the microstructure of steel is controlled to the mixture of a strain-induced martensite as the matrix phase, and an optimum amount of retained austenite as the second phase which is dispersed in isolation and surrounded by the transformed martensite. In this paper, the microstructure formation during the R&P process and the deformation mechanism that would bring about the excellent strength-ductility balance are discussed based on the results obtained from the in situ neutron diffraction measurement. The results have revealed that the typical Lüders-like stress-strain curve of R&P steel is caused by competitive plastic flow between austenite and martensite, and an effective transformation induced plasticity phenomenon.

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The effect of Nb addition of less than 0.05 mass% on the quenching and tempering behavior of spheroidized eutectoid steel, which has usually being applied to knitting needles, was investigated. The results obtained are as follows. 1) Hardenability upon brief heating was markedly improved by 0.01 mass% Nb addition. 2) Both quenching elongation and its standard deviation decreased with 0.01 mass% Nb addition compared with those of Nb-free steel. 3) While hardly any effect of Nb addition on the hardness was observed during low-temperature tempering, not only the impact toughness but also the fatigue durability was improved by 0.01 mass% Nb addition. 4) Atom probe tomography (APT) analyses revealed that the precipitation of carbon in solution directly resulted in the formation of ε -and/or θ -carbides with carbon contents of around 25 at% without the formation of clusters with 10-15 at% C upon the addition of a trace amount of Nb. 5) For the same P content, the average bulk concentration of P in the martensite phase markedly increased with the addition of up to 0.05 mass% Nb. 6) Regarding the optimum Nb content of 0.01 mass% for various mechanical properties upon the low-temperature tempering of martensite, it is considered that the mechanical properties are dominated by the balance between the positive effect of promoting carbide precipitation during low-temperature tempering by Nb addition and the negative effect of deteriorating the toughness with increasing bulk concentration of P in the martensitic phase upon the addition of more than 0.02 mass% Nb.

Synopsis : Effect of Nb addition less than 0.05 mass% on the quench and tempering behavior of spheroidized eutectoid steel, which has been usually applied to knitting needles, was investigated. The results obtained are as follows. 1) Hardenability with brief heating was markedly improved by 0.01 mass% Nb addition. 2) Both quenching elongation and its standard deviation decreased with 0.01 mass% Nb addition compared to those of Nb free steel. 3) While the effect of Nb addition on the hardness change during low temperature tempering was hardly observed, not only the impact toughness but also the fatigue durability were improved with 0.01 mass% Nb addition. 4) APT (Atom Probe Tomography) analyses indicated that the precipitation of carbon in solution proceeded directly to the ε and/or θ carbides with carbon contents of higher than 25 at% by Nb addition without going through a clustering process up to 10~15at% during low temperature tempering. 5) In spite of the same content of P, the average bulk concentration of P in the martensite phase markedly increased with the addition of Nb up to 0.05 mass%. 6) Regarding the optimum content of 0.01 mass% Nb on the various mechanical properties under the low temperature tempering of martensite, it is considered that they are dominated by the sum of the positive effect for promoting carbide precipitation during low temperature tempering with Nb addition and the negative effect for deteriorating the toughness with increasing bulk concenteration of P in the martensitic phase with addition of Nb higher than 0.02 mass%.