The methodology of phase diagrams and that of alloy design were reviewed. The construction of a thermodynamic database for multicomponent systems by the CALPHAD method are shown, where the first principles calculations play a significant role in the estimation of thermodynamic properties of metastable phases. Some examples of alloy design for advanced materials using phase diagrams are shown in Pb-free machinable steels, high strength steels with low density, superelastic Fe-based and Cu-based alloys, magnetic recording media of Co-Cr-based alloys and Co-based superalloys.

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After ausforming appeared as the first thermomechanical processing of steels in the first half of the 1960 s, various thermomechanical processings have been developed for the improvement of mechanical properties over the last fifty years. Their application was mainly to martensitic steels in the 1960 s such as ausforming and TRIP, and moved to ferrite (+ pearlite) structures by the development of controlled rolling and accelerated cooling of HSLA steels in the 1970~1980 s. However, recently, interest has returned to martensite (and also bainite) because of the demand for higher strength, and the ausforming and TRIP have been revived and successfully applied to commercial practice. Very recently, severe plastic deformation (SPD) is the focus of attention as a new method of producing a very fine-grained structure with grain size of less than 1 μm. By the application of SPD, dynamic phenomena such as dynamic recrystallization and dynamic ferrite transformation occur in the process. We need more systematic studies on such phenomena for the development of new type of thermomechanical processing in steels.

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A short historical review is given for the dislocation theory in the special reference to the mechanism of metal strengthening. Until the mid of 1950’s, the basic formalism of the dislocation theory has been completed. However, the further development has been confronted by various difficulties in the application of the theory to elucidate the strengthening mechanism, although some progress in problems, such as the visualization of individual dislocations and the measurement of the dislocation velocity, has been made. The present status of this research field, inclusive of recent developments, is described. The solution hardening, the work hardening, the low temperature strength and others are discussed in some details. Recent experimental treatises to examine the strengthening mechanisms are also overviewed.

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Progress of corrosion science and corrosion engineering of steels has been reviewed. Case histories of atmospheric corrosion including research and development of weathering steels, and environmental degradation such as stress corrosion cracking, hydrogen embrittlement and corrosion fatigue of steels were explained in detail. Finally, future subjects that should be done for better understanding of corrosion and corrosion control of steels were proposed.

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Controlled rolling in intercritical region (austenite and ferrite two phase region) is one of the effective processes both to strengthen the steel plate and to improve toughness. In this study mechanical properties of experimentally controlled rolled heavy thickness steel plate were investigated. In case of heavy thickness steel plate, toughness enhancement by controlled rolling in intercritical region was small, because ferrite grain refinement is insufficient due to limitation of amount of thickness reduction in controlled rolling. In addition, Charpy transition temperature at the 1/4 t position increased as lowering the finishing rolling temperature in intercritical region. Toughness dependence on thickness position was considered in terms of texture. The main component of texture at 1/2 t position is {001} <110> and {113} <110>, whereas that at 1/4t position is {110} <001>, which is formed by shear strain in controlled rolling. The effect of texture on toughness difference in thickness position and anisotropy was analyzed using crystallite orientation distribution function. Calculated brittleness parameter was evaluated, which is relationship between direction of stress axis that induces brittle fracture and (100) cleavage plane distribution. It was revealed that texture is an important factor to control the toughness of heavy thickness controlled rolled steel plates.

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(0.2-0.6)%C-2%Si-1%Cr-1%Mo steels were quenched and tempered at 773 K and then deformed by multi-pass caliber rolling (i.e.,warm tempforming) with a rolling reduction of 78% to obtain ultrafine elongated grain (UFEG) structures. Tensile and Charpy impact properties of the warm tempformed (TF) steels were investigated to make it clear the influence of the carbon content on toughening in the UFEG structures. The TF samples consisted of UFEG structures with a strong <110>//RD fiber deformation texture. Transverse grain size and aspect ratio in the UFEG structure tended to reduce with increasing the carbon content while carbide particle size slightly became larger. The increase in carbon content resulted in an increase in yield strength from 1.68 to 1.95 GPa at room temperature, while it was accompanied by a loss of tensile ductility. In contrast to quenched and tempered samples exhibiting ductile-to-brittle transitions, the TF samples exhibited inverse temperature dependences of the impact toughness due to the delaminations, where the cracks branched in the longitudinal direction (//RD) of the impact test bars. The upper-self energy of the TF sample was enhanced as the carbon content decreased, and the higher absorbed energy was also obtained through occurrence of the delamination at lower temperature. The delamination was found to be controlled not only by the transverse grain size, the grain shape, the <110>//RD fiber deformation texture but also by the carbide particle distribution in the UFEG structure.

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As a method for producing ultrahigh strength automotive steel components with high gauge accuracy, the hot stamping technology draws attention and the amount of hot stamped automotive components has exponentially increased in the last decade. The concerns about the potential danger of embrittlement and delayed fracture, however, limit the strength of conventional hot stamped components to 1500 MPa. If these concerns are eliminated, automotive steel components with higher strength which is demanded by the automotive industry can be further commercialized.
In this study, microstructures and mechanical properties of V and/or Nb bearing ultrahigh strength hot stamped steel components have been investigated. The V addition improves the resistance to delayed fracture by forming VC whose interface with matrix is supposed to act as preferential hydrogen trap site. The Nb addition refines the microstructure, which improves both the toughness and the resistance to delayed fracture. Proper hot stamping conditions for these steels has been discussed to create an ultrafine martensite microstructure and finely dispersed precipitates. The results indicate that 1900 MPa class steel components with good toughness and high resistance to delayed fracture can be produced.

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We investigated the factors affecting static strain aging under stress in a Fe-22Mn-0.6C twinning-induced plasticity steel at room temperature. The magnitude of strengthening by the static strain aging was estimated by tensile strain holding and subsequent re-loading. Strain holding time, pre-strain, strain rate, external stress, and diffusible hydrogen content were varied to clarify their effects on static strain aging, and the present static strain aging was found to be affected by all of these factors. In this paper, we show the phenomenological laws of the relationship among the factors and the stress increase due to the static strain aging.

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Al effects on strain aging and resistance against hydrogen embrittlement were examined in Fe-18Mn-0.6C-based twinning-induced plasticity steels deformed at different strain rates. The Fe-18Mn-0.6C steel showed hydrogen-induced fracture when it had been pre-deformed at a strain rate of 1.7×10^–6 s^–1. The hydrogen-induced fracture was suppressed by increasing strain rate and increasing Al content. From the viewpoint of material strengthening by strain aging, we found two important factors improving the resistance to the hydrogen embrittlement; (1) suppression of dynamic strain aging by increasing strain rate and Al content, and (2) suppression of static strain aging under loading by the Al addition.

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The effects of Ni or N both on the austenite stability and the tensile properties of duplex stainless steels were investigated at various temperatures. Two series of duplex stainless steel sheets consisting of Fe-(19~22)%Cr-5%Mn-(4~7)%Ni and Fe-(19~22)%Cr-5%Mn-(0.19~0.34)%N were employed. 20%Cr-5%Mn-5%Ni steel and 20%Cr-5%Mn-0.25%N steel indicated maximum improvement in elongation under tensile tests at 293 K among each series of specimens. The amount of strain-induced martensite was measured, indicating that there is the optimum transformation rate of strain-induced martensite with strain to obtain the maximum elongation under transformation-induced plasticity (TRIP). 20%Cr-5%Mn-0.25%N steel exhibited both extremely high elongation at room temperature equivalent to the conventional austenitic stainless steels of SUS304, and high tensile strength equivalent to the conventional duplex stainless steels of SUS329J4L. The total elongation of 20%Cr-5%Mn-0.25%N steel was larger than that of 20%Cr-5%Mn-5%Ni, though there is little difference between them in the average amount of strain-induced martensite introduced during the uniform deformation. The difference in elongations between 20%Cr-5%Mn-0.25%N and 20%Cr-5%Mn-5%Ni could be due to the difference in the hardness of the strain-induced martensite. The strain-induced martensite in 20%Cr-5%Mn-0.25%N steel was extremely hardened by the nitrogen concentrated to the austenite phase at annealing. Such hard martensite maintained high strain-hardening rate in a wide range of strain and increased the uniform elongation with high tensile strength.

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Influence of casting conditions and chemical compositions on ductile-to-brittle transition (DBT) behavior in duplex stainless steels was studied. Cleavage fracture of ferrite grain causes DBT in spite of ductile fracture of austenite grans at low temperature. Most influential factors on DBT temperature (DBTT) is chemical compositions, ferrite volume fraction and purity of cast steel. Compared with forged steels with similar fracture unit, the present cast steels show much lower DBTT, which is attributed to dispersed austenite grains that hinder the cleavage crack initiation and propagation. By using a serial sectioning 3D method, a group of austenite grains with an identical variant observed with a conventional 2D method was revealed to be connected each other.

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Creep of carbon free Fe-20Cr-30Ni-2Nb (at.%) steels strengthened by Fe₂Nb Laves phase on grain boundaries has been examined in air and steam atmospheres at 1073 K/70 MPa. Sufficiently covering grain boundaries by stable Laves phase can reduce the creep rate, resulting in a significant extension of the creep rupture life without a loss of creep ductility in steam atmosphere as well as air. This effect becomes remarkable in higher area fractions of Laves phase on grain boundaries (ρ) than 80%. The creep rupture strain slightly decrease by steam atmosphere, which would be associated with the interglanular oxidation and its related cracks propagation along grain boundaries enhanced by steam atmosphere in the creep acceleration stage. Its effect can be suppressed by covering grain boundaries by Laves phase (increasing ρ). Therefore, Grain Boundary Precipitation Strengthening by stable Laves phase is a key mechanism for use of materials for boiler tubes in the innovative advanced ultra-super critical (A-USC) thermal power plants to be operated above 973 K.

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Fe-25Cr-1N-0, 2, 5Mn mass% alloys were subjected to isothermal heat treatment, and their microstructure formation and phase transformation behavior were investigated in order to clarify the effect of Mn addition on the transformation mechanism of high nitrogen austenite. Microstructure observation for the 1073 K heat-treated specimens revealed that the Fe-25Cr-1N alloy exhibited (α+Cr₂N) lamellar eutectoid structure, while the Fe-25Cr-1N-2Mn and -5Mn alloys did finer (α’(or retained γ)+Cr₂N) lamellar structure as well as (α+Cr₂N) lamellar eutectoid structure. It was suggested that the (α’+Cr₂N) lamellar structure had been formed through γ₁→γ₂+Cr₂N cellular precipitation followed by martensitic transformation of γ₂ on cooling to ambient temperature. Nitrogen concentration in untransformed austenite in the Fe-25Cr-1N-2Mn and -5Mn alloys was continuously decreased with progressing of (γ+Cr₂N) cellular precipitation due to nitrogen long-range diffusion from untransformed austenite to (γ+Cr₂N) cellular structure. As a result of decreased nitrogen concentration in untransformed austenite, transformation mechanism switched from (γ+Cr₂N) cellular precipitation to (α+Cr₂N) eutectoid transformation.

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We have examined the growth kinetics of Fe-Zn intermetallics layers formed on Fe-1.5Mn (α-Fe single phase) and Fe-3.5Mn-0.1C (γ-Fe+α-Fe two phase) steel sheets dipped in pure Zn melt at 460 ºC. In an early stage of dipping, FeZn₁₃-ζ phase layer is initially formed and its thickness slightly increases with increasing dipping time till about 10 s, and then all Fe-Zn intermetallics layers (ζ, δ₁, Γ) grow following a parabolic law. The growth kinetics can be recognized in both steels. This result demonstrates a slight effect of steel microstructure on the formation of Fe-Zn intermetallics layers. The composition analysis revealed a significant Fe dissolution into pure Zn melt in an early stage of dipping. The supersaturated Fe in Zn melt would provide the driving force for the crystallization of ζ phase layer on steel sheets. The continuous layer of ζ phase would control Fe diffusion for the following growth of the other Fe-Zn intermetallics layers.

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