The changes of microstructural evaluation are reviewed from the view point of fusion of “material science” and “science of shape”. In addition, the status of microstructural evaluation in the three-dimension is described in detail.

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Stress-strain curve is essentially important to evaluate microstructural dependence of mechanical properties in steels and has been widely used in materials researches. Hence lots of experimental and theoretical studies have been made so far. Historical progress on this issue is briefly reviewed focusing on empirical relationships between microstructure and mechanical properties, theoretical modeling using continuum mechanics, deformation mechanism revealed by neutron diffraction, and modeling based on thermodynamics of dislocation motion. Mutual correspondence among these four topics is highlighted.

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Computational materials science is an exciting field which holds much future potential. In this article, at first, the dramatic advances of the computational methodologies are briefly summarized at scales from the atomistic to macroscopic levels. Then, each coauthor introduces the three research fields in detail, (1) CALPHAD modeling supported by key experiments and first-principles calculation, (2) Studies on the phase transformation in alloys based on the MD simulations, and (3) Predictions of the microstructure evolution and the mechanical properties based on the phase-field method, where remarkable progresses have been attained.

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The progress of measurement technologies in the Japanese steel industry over the last 100 years is reviewed. In the early period, the main objects of measurement were temperature in the blast furnace, steelmaking process, etc., using thermocouples and radiation thermometers. Through the years, instrumentation technologies have been developed with progress in electronics, and various techniques, including ultrasonic, image processing, laser technology, and others, have been applied.
Several remarkable in-line measurement technologies published in “Tetsu-to-Hagané” are reviewed in the history below, and the future direction is considered.

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The effect of pearlite on the X-ray diffraction peak reflected from ferrite phase in ferrite-pearlite steel was investigated using normalized carbon steels with different volume fraction of pearlite and a hypereutectoid steel with various pearlite lamellar spacing. The lattice strain in ferrite phase, which causes the broadening of X-ray diffraction peak, was increased in proportion to both of the volume fraction of pearlite and the inverse of pearlite lamellar spacing. As a result, the lattice strain in ferrite-pearlite steel can be simply formulated as functions of them. On the other hand, TEM observation reveals that pearlite has low-density dislocation in ferrite phase. This result suggests that the misfit between ferrite and cementite in pearlite generates the significant amount of elastic strain, which leads to the increasing in lattice strain. Therefore, the dislocation density must be overestimated in carbon steels with pearlite, if it is estimated from the experimental lattice strain directly.

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A standard elastoplastic constitutive model of metallic materials is extended to describe the stress-strain relationship including yield-point phenomenon. Based on a general framework of finite strain elastoplasticity, two constitutive models are formulated in this study. One of them is a phenomenological constitutive equation added one scaler internal variable. Another is a constitutive model based on a representative characteristic length defined as a dominant strengthening mechanism in some competing strengthening mechanisms including dislocation accumulations. The feature of these constitutive models is discussed by reproducing an experimental stress-strain relationship.

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For a better understanding the relation between the heterogeneous microscopic strain caused by metallographic microstructure and the macroscopic strain concentration responsible for ductile fracture, digital image correlation (DIC) method was applied to the digital images obtained by a scanning electron microscope (SEM) to analyze a strain distribution in a low carbon dual-phase steel in multiple scales. As well as grain boundaries, a substructure in martensite grains and a corrosion shade in ferrite matrix were clearly observed in SEM image and they enhance the random nature of SEM digital image. Therefore, the combination of SEM image and DIC method (SEM-DIC method) is able to evaluate a heterogeneous strain distribution in metal in multiple scales. The development of strain distribution looks different depending on the magnification of SEM observation due to the difference in pixel-dependent spatial resolution; micro-scale strain map indicated that strain is localized in ferrite matrix, while macro-scale strain map revealed that strain seems to exist over a width of tensile specimen. This result suggests that macroscopic strain concentration is accompanied by a percolation of microscopic localized strains. In addition, the strain in martensite grains is increased considerably as the hardness of martensite is reduced by tempering treatment, which leads to the reduction in the strain gap between ferrite and martensite. As a result, the developments of not only microscopic localized strain but also macroscopic strain concentration are retarded, leading to the significant recover of ductility in DP steel.

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Deformation twinning behavior in Fe-17Mn-0.6C, Fe-17Mn-0.8C, and Fe-18Mn-1.2C (wt.%) twinning-induced plasticity (TWIP) steels was investigated by atomic force microscopy (AFM) and electron backscatter diffraction pattern (EBSD) analyses. The AFM-based surface relief analysis combined with the EBSD measurements was employed to determine active twinning direction as well as deformation twin fraction in specific crystallographic orientations. A carbon addition is known to increase the stacking fault energy; however the deformation twin fraction in the <144> tensile orientation did not change against carbon concentration. On one hand, the <111> tensile orientation grains showed suppression of deformation twinning with increasing carbon concentration. These results imply that another factor in addition to the stacking fault energy-based criteria is required to interpret the deformation twinning behavior of carbon-added TWIP steels.

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In the previous paper, carbon concentration dependence of deformation twinning behavior in twinning-induced plasticity steels had been investigated, which clarified that the deformation twin fraction in the <144> tensile orientation did not change against carbon concentration. Additionally, in this paper, twinning deformation occurred in the Fe-18Mn-1.2C steel at 473 K with relatively high stacking fault energy of 55 mJ/m². To explain these experimental results, dynamic strain aging of Shockley partials dislocations was proposed as an additional contributing factor to assist the deformation twinning in high carbon-added austenitic steels. Most abnormalities about deformation twinning such as the high stacking fault energy in Fe-Mn-C austenitic steels were interpreted by considering the influence of dynamic strain aging.

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The plastic deformation behavior in a uniaxial tensile test was examined by applying the nano-indentation hardness test to the ferrite-austenite duplex stainless steel. Each phase revealed the same hardness and a different work hardening ratio. The effect of the hardness difference in each phase on the void nucleation process was investigated by means of EBSD analysis.
From the EBSD analysis, deformation twinning and deformation-induced martensitic transformation were observed in the austenite phase near the fracture surface. The hardness of austenite rose to a higher degree than that of ferrite, and showed an extremely-high value due to martensitic transformation. The void appeared mainly at the interface between the martensite induced by deformation and the ferrite phases. Thus the void nucleation was enhanced by the martensitic transformation which occurred at the location of higher equivalent strain of 1.5.

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The effect of Mn addition on a brittle-to-ductile transition (BDT) in Ti added ultra-low carbon steels was investigated comparing with the effect of Ni on it. The effects of Mn on the temperature dependences of 0.2% proof stress, activation volume and effective stress in Ti added ultra-low carbon steels were nearly the same as those of Ni. The decrease in the activation energy of dislocation glide with Mn content was also nearly the same amount as that with Ni, indicating that dislocation mobility relating to thermally activated process was increased with Mn addition. The temperature dependence of absorbed impact energy showed that the BDT temperature increases with the Mn content. The fraction of inter-granular fracture surface increased with Mn content suggesting that Mn decreases the surface energy for inter-granular fracture, which controls the BDT temperature. Those results suggest that Mn has a potential to improve low temperature toughness when grain boundaries are strengthened enough. The effect of Mn addition on the temperature dependence of absorbed fracture energy in no Ti-added ultra-low carbon steels was also presented.

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The authors proposed a three dimensional model to explain microscopic behavior of cleavage crack propagation in steel, which was validated by experimental results, in the previous paper. Purpose of this paper is to explore further into microscopic cleavage crack propagation behavior in steels having anisotropy using the proposed model. Charpy impact testing and small size crack arrest testing were conducted for observing anisotropy of cleavage fracture toughness and it was confirmed that R direction has lower toughness than LT and TL directions. And simulation was also conducted using the data of crystal orientation, which was derived from Electron Back Scattering Diffraction analysis. The comparison between the experiment and the simulation showed good agreement and the lowest toughness in the R direction was explained by a concentration of {100} planes in that direction.

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Numerical simulation of microscopic stress distribution in weld metal considering anisotropy of crystal orientation observed in weld solidification microstructure was performed. A finite element model of columnar grain aggregate was generated and anisotropic crystal orientation considering the characteristics of austenitic stainless steel weld metal was defined. Microscopic stress distribution occurred in the columnar grain model under tensile loading was investigated by a series of numerical simulations incorporating crystal plasticity theory.
From the case where tensile displacement was applied perpendicular to the longitudinal direction of columnar grains, the following results were obtained: The maximum microscopic stress occurred in the columnar grain model was almost independent of the angle between the longitudinal direction of columnar grains and <1 0 0> direction, and it was 1.59 times to 1.75 times higher compared to the macroscopically applied stress. From the case where tensile displacement was applied parallel to the longitudinal direction of columnar grains, the following results were obtained: The maximum microscopic stress occurred in the columnar grain model increased with increasing angle between the longitudinal direction of columnar grains and <1 0 0> direction. The above two major simulation results showed that higher microscopic stress compared to the macroscopically applied stress could be observed when tensile load was applied perpendicular to the longitudinal direction of columnar grains.

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Appropriate hydrogen charging methods were investigated to simulate hydrogen absorption from actual environments where high strength steels are used. Both absorbed hydrogen concentrations and hydrogen desorption profiles were compared between various hydrogen charging tests - immersion tests, cathodic charging tests, cyclic corrosion tests (CCT) and exposure tests in high pressure gaseous hydrogen. Absorbed hydrogen was analyzed using thermal desorption analysis (TDA) after the hydrogen charging tests. It was confirmed that acid immersion tests or cathodic charging tests in NaCl solution were appropriate to simulate hydrogen absorption from CCT. Fédération Internationale de la Précontrainte (FIP) tests or cathodic charging tests in NaCl - NH₄SCN solution could be used to simulate hydrogen absorption in high pressure hydrogen gas environments. Comparisons in TDA profiles showed that the ratio of strongly-trapped hydrogen and weakly-trapped hydrogen changed depending upon the total amount of absorbed hydrogen. This result means the state of hydrogen in steels, which affects hydrogen embrittlement, can change according to the amount of absorbed hydrogen. The above-mentioned hydrogen charging tests and hydrogen analysis were conducted in 15 laboratories to check the scatter of measured hydrogen concentration.

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Resistance to hydrogen embrittlement of low alloy steels was evaluated based on their critical hydrogen content and critical stress. Constant load test, Slow Strain Rate Technique (SSRT) and Conventional Strain Rate Technique (CSRT) were carried out using JIS-SCM435 and V-added steels in six laboratories. It was confirmed that the same test results were obtained in different laboratories under the same test conditions. Furthermore, the relationships between the diffusible hydrogen content and nominal fracture stress obtained by means of CLT and by SSRT were similar to each other. In SSRT and CSRT, fracture surfaces showed Quasi-cleavage mode under small hydrogen content, while they showed Inter-granular fracture under large hydrogen content. In order to compare the three methods considering the concentration of hydrogen in stress field, locally accumulated hydrogen content under the same fracture stress was calculated. The order of the locally accumulated hydrogen content at a given fracture stress is as follows; SSRT < CLT < CSRT in JIS-SCM435, and CSRT < CLT ≒ SSRT in V-added steels. The difference of the evaluation results for JIS-SCM435 is presumably attributed to the dependence of the interaction between hydrogen and dislocations on the strain rate.

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To standardize the evaluation technique of hydrogen embrittlement properties of ultra high-strength steel sheets, hydrogen embrittlement properties of SCM435 and V added steel sheets with tempered martensitic matrix were investigated and compared by using 4-Point Bending Cathode Charging Technique, 4-point Bending Constant Displacement Compression Technique and Conventional Strain Rate Technique which was tensile tests. In the SCM435 and the V added steels, it was confirmed that little deterioration of maximum stress was observed at a low diffusible hydrogen concentration region while rapid deterioration of maximum stress was indicated at a high diffusible hydrogen region in all of the testing techniques. Similar hydrogen embrittement properties of the both steels evaluated by 4-Point Bending Cathode Charging Technique and 4-Point Bending Constant Displacement Compression Technique were obtained when maximum tensile stress and accumulated diffusible hydrogen concentration of outside at the center in length of specimens were estimated by FEM analysis. Hydrogen embrittlement properties obtained by both of 4-Point Bending Technique of SCM435 and V added steels were higher than those properties evaluated by Conventional Strain Rate Technique. This might be caused by the difference in the fracture morphologies between 4-Point Bending Technique and Conventional Strain Rate Technique. In 4-Point Bending Technique, intergranular fracture with plastic deformation was occurred in the prior austenite grain near initiation point of cracks, even though intergranular fracture without plastic deformation was observed in the prior austenite grain near the edge of specimen in Conventional Strain Rate Technique.

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Applications of ultra high strength steel sheets to automotive body have been expanding year after year. Hydrogen embrittlement (HE) is one of the problems of ultra high strength steels. Various methods are used as the evaluation method of HE resistance. In this study, the critical HE conditions obtained by SSRT, CSRT and 4-point bending test were compared by using the same materials, which have tempered martensite microstructure with the composition of a SCM435 or an V added steel with many hydrogen trapping precipitations. Specimens were charged with hydrogen by the cathodic charging method. The specimen used in the SSRT and the CSRT was machined with notches on the both sides of a parallel part and the stress concentration factor (Kt) of the specimens was 4.26 or 1.76. The specimen used in the 4-point bending test was coupon shape. The critical HE conditions evaluated with the average applied stress and the average hydrogen content of the specimen were different depending on the test methods. HE conditions were also evaluated with the local stress and the local accumulated diffusible hydrogen content at the fracture initiation point. The critical condition evaluated by the 4-point bending test was located in the higher stress and higher hydrogen content region compared with the critical conditions obtained by the CSRT and the SSRT.

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Hydrogen embrittlement has become a crucial issue with the promotion of high-strength steel. Many studies have been conducted on the mechanism of hydrogen embrittlement. Because the elucidation of the state of hydrogen is important to understand the mechanism, the states of hydrogen in the steels investigated were controlled. In the present study, 0.35 mass% C and 0.8 mass% C steels annealed in the hydrogen atmosphere followed by quenching from the austenite region together with drawn pearlitic steel of 0.8 mass% C were used to analyze the state of the hydrogen contributing to the emission peak, in particular, at about 300 ºC in the Thermal Desorption Analysis (TDA) curve. The peak at 300 ºC was significant for quenched 0.8 mass% C steel with low Ms temperature; however, the peak decreased with aging at room temperature. However, in 0.35 mass% C steel with high Ms temperature, the peak at 300 ºC was no longer observed. Moreover, in the hydrogen charged as drawn 0.8 mass% pearlitic steel, the peak at 300 ºC did not change with aging at room temperature because of no significant carbon in solid solution, while the peak at 100 ºC decreased with the increase in aging time. Taking into account the competitive phenomenon of hydrogen trapping at the dislocation core and C segregation to dislocations during room temperature aging or during quenching from Ms temperature, it was concluded that the hydrogen peak at about 300 ºC is hydrogen trapped in the dislocation core, while the other hydrogen peak at 100 ºC is attributed to the hydrogen trapped by the stress field generated by dislocation.

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The effects of carbon content on tetragonality and magnetic moment of bcc iron have been evaluated by first-principles calculation. Three kinds of supercells, Fe₅₄C₁, Fe₅₄C₂ and Fe₁₂₈C₁ (which correspond to Fe-0.40C, Fe-0.79 and Fe-0.17C mass%, respectively) are used for the calculation of tetragonality and magnetic moment of Fe-C system. Main results obtained are as follows. (1) The total energy and mechanical energy of the Fe-C system with carbon atom at the octahedral sites are smaller than those with carbon atom at the tetragonal sites. The carbon atom at octahedral site produces fairly large expansion in one direction. (2) Tetragonality of Fe-C system obtained by first-principles calculation increases linearly with increasing carbon content and agrees well with experimental results. The average magnetic moment of an Fe atom increases with increasing carbon content. (3) The magnetic moment of an Fe atom at the nearest neighbor of carbon atom is lower than that of pure iron and increases with increasing distance between the iron and carbon atoms. The projected density of states shows a hybridization with main contributions from Fe d and C p states which leads to the above mentioned decrease of the magnetic moment of an Fe atom. (4) In Fe₅₄C₂, tetragonality and magnetic moment of iron atom change with the distance between two carbon atoms. The value of tetragonality is either 0.981, 1.036 or 1.090. When the dumbbell structure which consists of the first carbon atom and its two nearest neighbor iron atoms is perpendicular to the second dumbbell structure which consists of the second carbon atom and its two nearest neighbor iron atoms, the tetragonality is 0.981 and does not agree with experimental value. The mechanical energy is relatively large. On the other hand, when the first dumbbell structure is parallel to the second dumbbell structure, the tetragonality is 1.036 which agrees well with experimental data. The mechanical energy is relatively small. When straight C-Fe-C pair is formed, tetragonality is 1.090. (5) In Fe₅₄C₂, formation enthalpy is relatively low when the calculated tetragonality is 1.036, and the existence probability under the assumption of Boltzmann distribution is high. In other cases, the existence probability is nearly zero. (6) The average magnetic moment of an Fe atom is proportional to volume, but not in a clear relation with tetragonality. It is considered that the increase of magnetic moment of an Fe atom by the addition of carbon atom is mainly due to the magneto-volume effect but not due to the tetragonality effect.

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This study develops an eddy current testing probe whose signals due to a surface breaking crack appearing in a ferromagnetic material exhibit clear change with respect to the depth of the crack. Experimental verifications are conducted using a type 430 ferritic stainless plate specimen with five artificial slits whose depths are 1, 3, 5, 10, and 15 mm. Although exciting frequencies adopted were 25, 50 and 100 kHz, there is clear difference between signals due to the 10 mm deep slit and those due to the 15 mm deep one. Subsequent finite element simulations are conducted to discuss the physical background of the difference on the basis of the reciprocity theorem and electromagnetic fields.

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