In recycling a lot of low grade ferrous scraps, we cannot remove tramp elements, such as copper and tin, which are inevitably dissolved in molten iron. Accordingly, the thermodynamic data between copper and alloying elements in molten iron are necessary to know the influence of copper on the property of steel. However, the available data are not enough because of the difficulty of the measurements. One of the purposes of this study is to develop a method to measure the thermodynamic data of tramp elements in iron. In this work, we have tried to measure the oxidation equilibria of copper in molten iron by using silver as an intermediate phase. By applying this method, the interaction between Cu and M(M: B, Co or Ni) in molten iron has been investigated at 1873 K.

In recent years, the treatment of inverse-transformation has been adopted in the HCR (Hot Charge Rolling) process of CC (Continuous Casting)-Blooming for steel production to prevent surface cracking on blooming. However, the quenching of bloom in this treatment occasionally causes some troubles, such as distortion of bloom and quenching crack. On the behavior of deformation of bloom and the generation of stress in cross section of bloom, the effects of cooling conditions of immersion cooling (quenching), steel grade and size of austenite grain were analyzed in order to prevent these troubles by the model of metallo-thermo-mechanics in this study. The results obtained from this analysis are as follows: The behavior of deformation of bloom and the distribution of stress in cross section of bloom by quenching is affected by steel grade and size of austenite grain caused by the difference in the distribution of amount of both diffusion and no diffusion transformation accompanying expansion and heat generation by latent heat in cross section of bloom.

Neutron engineering diffraction is a powerful technique which provides the information of the micro structure of steels in bulk-average, while X-ray diffraction or Electron backscatter diffraction can provide information only from the surface layer. However, such measurement using neutron diffraction is typically performed in a large facility such as a reactor and a synchrotron, while a compact neutron source has never been used for this purpose. Authors have recently developed a neutron diffractometer installed in Riken Accelerator driven compact Neutron Source (RANS) and succeeded in the measurement of texture evolution of a steel sheet. In this study, we made an attempt to measure the volume fraction of retained austenite by RANS. Background noise was carefully eliminated in order to detect as many diffraction peaks as possible with low flux neutrons. The volume fraction was estimated by Rietveld analysis. The accuracy of the measurement result was discussed by comparing with those obtained by a large neutron facility (J-PARC TAKUMI). The volume fraction obtained by RANS with reasonable measurement time, i.e. 30-300 min, showed only 1-2% discrepancies with those obtained in J-PARC. These comparisons suggest that neutron diffraction by RANS is capable of quantitative analysis of the volume fraction of crystal phases, showing the possibility of practical use of an in-house compact neutron source in the industry.

Brittle crack arrest toughness in high strength steel plates with a thickness of 25 mm containing various amounts of C, Mn and Ni manufactured by several controlled-rolling (CR) conditions in a laboratory was investigated using ESSO tests to quantitatively clarify the metallurgical factors affecting arrest property. Arrest toughness of the steel plates was improved by low temperature and high reduction of CR. Optical and scanning electron microscopies revealed that arrest toughness correlates with the refinement of the effective grain size. The addition of Ni was confirmed to improve arrest toughness. Since grain refinement is not recognized in Ni bearing steels, it is attributed to the intrinsic effect of Ni, which is probably manifested as the increase in ductile fracture surface and randomness of crack propagation directions. Considering these effects, a simple equation predicting T_Kca4000 was derived by linearly combining the effects of the Ni content and the effective grain size.

In order to develop a simplified evaluation method for brittle crack arrest toughness, correlation between the result of ESSO tests and that of Charpy impact tests was investigated. Charpy impact test specimens with three notch shapes were cut from various steel plates manufactured in a laboratory and a plate mill. The transition temperature determined by the chevron-notch Charpy test correlated better to the results of the ESSO test than those of V-notch or pressed-notch Charpy tests. To determine the cause, an instrumented Charpy test was performed using the V-notch and chevron-notch specimens. As a result, fracture initiation energies of chevron-notch specimens were considerably low irrespective of materials. This indicates that the chevron-notch Charpy test is suitable for evaluating arrest toughness. Furthermore, the transition temperature is linearly related to the square root inverse of the cleavage facet size measured by SEM. Therefore, it is concluded that the chevron-notch Charpy test can be applied to estimate arrest toughness in heavy thick steel plates produced in a plate mill.

Multiple slip deformation in dispersion hardening steels are numerically analyzed by a crystal plasticity finite element technique and macroscopic strain hardening characteristics are discussed. The critical resolved shear stress (CRSS) for slip system is given by the extended expression of the Bailey-Hirsch type model which include contributions from lattice friction, Taylor hardening and also the Orowan stress to give a size effect from microstructure. Strain hardening of slip systems are estimated by the density of statistically stored dislocations (SSDs) through the Taylor term. Increment of the SSD density is evaluated by slip strain and the dislocation mean free path. Models for the CRSS and dislocation mean free path for the primary and secondary slip systems are discussed. Analysis results show that macroscopic yield stress agrees well with experimental one but strain hardening ratio is overestimated when the CRSS and dislocation mean free paths for secondary systems are given the same characteristics of scale dependency to that for the primary one. Some new models for the CRSS and mean free path for secondary systems are studied and analysis results with these models show to agree well not only for the yield stress but also for the strain hardening behavior.

It is well known that brittle crack arrest toughness can be improved by grain refinement. The relationship between the fracture surface and the cross-sectional microstructure of the ESSO test pieces by means of EBSD was investigated to clarify the effective grain size of high-strength steel plates. Taking into consideration the fact that a big difference in brittle crack propagation resistance sometimes takes place even if the misorientation between neighboring grains is the same, a new index denoting the propagation resistance referred to crack propagation deviation angle was proposed. The index is defined as an angle between two vectors, which are projected on the ND plane, of the normal vectors of cleavage {100} planes closest to the RD direction of neighboring two grains. It was found that the boundary with the crack propagation deviation angle of 25° or more corresponds to the boundary of the cleavage facet size. The effective grain size evaluated by applying the intercept method on the EBSD data with a high crack propagation deviation angle of the base metal is almost equal to the cleavage facet size. Therefore, the average effective grain size can be estimated without performing any tests to evaluate arrest toughness.