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EBSD mapping was used for phase analysis of multi-phase steels. Some products of austenite decomposition could be discriminated using a range of grain-average functions and properties derived from the map data. In DP steels, martensite could be extracted from the ferrite matrix using the average band contrast or a set of geographic information. Bainite in TRIP steel was identified with the local variations of band contrast and orientation inside a grain. As the proposed method reproduced a phase map, a visual confirmation of the result and additional analyses were possible.

In a transformation-induced plasticity (TRIP)-assisted multiphase steel, the phases around a micro-crack at the initial stage of cracking during wire drawing were observed by a focused ion beam (FIB), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). A novel FIB method, which is termed as “In-plane lift-out” technique, was applied to obtain an overall plan-view of the TEM specimen containing the micro-crack. The equilibrium concentration ratio of Mn in austenite and ferrite was calculated for the alloy and compared with the EDS line profile of Mn concentration across the micro-crack in the TEM specimen. The interface between the ferrite and martensite was identified as the crack initiation site during drawing of this multi-phase steel.

Tensile tests were conducted in order to study the effects of nitrogen on the mechanical properties of the 0.2C–1.5Mn–1.5Si–0.04Al–(0.003–0.015)N steels often annealing at 800–830°C, followed by the austempering at 400–450°C. The results show that both tensile strength and elongation increase and the balance of tensile strength×elongation are improved upon nitrogen addition to the TRIP steel. It was found that the density of AlN precipitates increases with addition of nitrogen. The volume fraction of retained austenite increases due to AlN precipitation, because the precipitates retard the transformation of austenite during cooling and austempering. As a larger volume fraction of austenite remained after annealing has been transformed to martensite during deformation, elongation as well as strength has increased in the nitrogen added steel. It is also observed that the average grain size of ferrite and bainite decreases because of AlN precipitation that hinders grain growth. The refinement of ferrite and bainite by AlN precipitates also contributes to the increase in the strength of nitrogen added steels.

A study has been made of nitriding of interstitial free (IF) steel in the potassium-nitrate salt bath at temperatures ranging from 400 to 650°C. The salt is decomposed to generate nitrogen and oxygen. Nitrogen diffuses into steel, or steel is nitrided, while oxygen reacts on steel surface to form the oxide scale. The oxide scale thickness is much smaller than the nitriding thickness. Most of nitrogen resides in steel as a form of interstitial solid-solution. For nitriding at higher temperatures, nitride precipitates (γ'-Fe 4 N and ζ-Fe 2 N) exist mostly in grain boundaries and partly in grains of the steel. The nitrate nitriding gives rise to much larger nitriding depth than other nitriding methods at similar nitriding temperature and time. The nitrate nitriding of steel substantially increase its tensile strength as well as hardness, e.g., an IF steel specimen nitrided at 650°C for 1.5 h shows a tensile strength of 916 MPa, which is 2.2 times higher than that of non-nitrided IF steel specimen, and an elongation of 20% at 70°C. Severe serrations are observed in flow curves of nitrided steel specimens, mainly due to dynamic strain aging that occurs because of interaction between dissolved nitrogen and moving dislocations. The effective diffusion coefficient of nitrogen D N obtained from the nitriding data, D N =D 0 expl(-Q/RT) with D 0 =3.789x10 -7 m 2 .s -1 and Q=76.62 kJ mol -1 , is approximately the same as that for diffusion of nitrogen in α-iron.

In order to analyze the phenomenon of mold level hunching (MLH) during thin slab casting process, the variation of the mold level and the bulging of strand were measured and analyzed using Fast Fourier Transform (FFT) spectrum analysis. Both of mold level hunching and bulging had the same frequency and the specific frequency corresponded to the main roll pitch of the thin slab caster. The unsteady bulging was found to be a main reason of mold level hunching. The unsteady bulging profile through the full range of the caster and the height of mold level hunching were calculated using a heat transfer model and a continuous beam model. The effects of process variables on mold level hunching and the process condition of reducing mold level hunching were studied. The amount of mold level hunching increased as unsteady bulging increased due to the hunching of casting speed. An aperiodic roll pitch between segments could be effective in reducing mold level hunching.

Fe-4.5wt%Si strips were produced using a vertical type twin-roll strip casting process. The effect of superheat on the microstructure and texture was studied through thickness direction of the as-cast strips. The heterogeneity of texture anti microstructure through thickness direction of the strips was observed, In the case of superheat 20°C, Goss texture evolved between the subsurface and the middle layer and major rolling texture with minor solidification texture evolved in the center layer. In the case of superheat 30°C, solidification texture of (100) fiber evolved from the surface to the center layer.

Flow curves of ferrite/pearlite two-phase steels were simulated using finite element method (FEM) with regression equations for flow curves of each ferrite and pearlite phase proposed by Hiramatsu et al. and Furukawa et al. The calculated flow strength of ferrite/pearlite two-phase steel was lower than the experimentally measured one mainly due to the underestimation of flow strength of ferrite phase according to the yield elongation. To improve the simulation, the flow curves in homogeneous deformation range were considered. Microscopic observations revealed that most deformation was accumulated in the ferrite phase during yield elongation. Flow curves of ferrite phase were re-evaluated based on this observation and flow curves of ferrite/pearlite steels were also re-calculated. Re-calculated yield strength of ferrite/pearlite steels showed good agreement with measured ones, however the work hardening rate of re-calculated flow curves is still lower than that of measured one. The simplicity of pearlite morphology for FEM analysis is thought to be responsible for lower work hardening behavior of calculated flow curve by FEM than that of measured one.

Fe-4.5wt%Si strips were made using vertical type twin-roll strip casting process, and the microstructure and texture of as-cast strips were studied through thickness direction. The heterogeneity of the texture acid microstructure through thickness direction was observed. Between the subsurface layer and the middle layer, Goss texture, with the subgrains of low angle grain boundary was evolved by the shear deformation. In the center layer, major rolling and recrystallization textures with minor solidification texture were evolved The deformation behavior during strip casting process was simulated by hot rolling of Al alloy with temperature gradient through thickness direction. And the shear and normal strains distribution through thickness direction were measured. The heterogeneity of the texture and the microstructure through thickness direction resulted from the strength gradient, which originated from the temperature gradient through thickness direction.

Tensile strength and ductility of carbon steels have been measured in the temperature range of mushy zone by the in-situ melting tensile test technique with Gleeble system. The specimen was melted and cooled to the test temperature before the tensile deformation in order to get the mechanical properties subject to the continuous casting process. During hot tensile test, a ceramic fiber tube was used to reduce the radial temperature gradient in the heated specimen. Tensile strength of carbon steels in the temperature range of mushy zone increased with decreasing test temperature, and was well described by the modified yield criterion for porous metals. The measured zero strength temperature (ZST) and zero ductility temperature (ZDT) were related to the solid fractions evaluated by the numerical simulation of microsegregation developed by Ueshima et al. The characteristic solid fractions of ZST and ZDT which corresponded to 0.75 and 0.99, respectively, were well described by the prediction equation on ZST and ZDT at given steel compositions and cooling rates.

Using a 2-dimensional finite element, the effects of carbon content, slab width, narrow face taper and casting speed on solidification cracking during continuous casting of slabs were analyzed. The possibility of solidification cracking during continuous casting of slabs was predicted using the Specific Crack Susceptibility, s S c . The new proposed parameter, s S c , can represent the averaged possibility of solidification cracking of the strand during continuous casting in the mold. The surface crack formed near corner region of strand at the initial stage of solidification, and the internal crack was found in the internal regions of wide center, corner and off-corner at the middle and final stage of solidification. The carbon range sensitive to solidication cracking is between 0.1 wt% C and 0.14 wt% C steels. At the slab widths of 1900 mm and 2 150 mm, the possibility of solidification cracking increased in the regions of wide center, corner and off-corner in comparison with the slab width of 1 600 mm. With increasing narrow face taper, the possibility of solidification cracking decreased along the region of wide face due to the mechanical compression imposed by the narrow face taper. With increasing casting speed, the possibility of solidification cracking increased in the regions of wide center, corner and off-corner, because the shell thickness is largely decreased due to decrease of dwelling time of strand within the mold. These predictions at various casting conditions were in good agreement with the experimental observations in industry.

A two-dimensional thermo-elasto-plastic finite element model for the analysis of thermo-mechanical behavior of strand in continuous casting process has been developed. The model incorporates the effect of microsegregation of solute elements on hot tears using a thermo-mechanical model of mushy zone and δ and γ phases. A finite element technique for the liquid region of the slice of strand, which can take the ferrostatic pressure due to gravity force into account, was proposed. The model successfully analyzed the thermo-mechanical behavior of the solidifying shell of slab in the mold during the solidification. The calculated results, such as the deformed geometries, the temperature history, the stress distribution and the formation of air gap between solidifying shell and mold in the continuous casting process of slab, were obtained. These were compared with the reported observations.

Stress-strain relations of carbon steels in the γ phase and δ phase regions have been analyzed through the high temperature tensile tests at various temperatures from 1100 to 1450°C and strain rates from 10–4 to 10–2/s using Gleeble system. During hot tensile test, a ceramic fiber tube was used to reduce the radial temperature gradient in the heated specimen. The flow stresses of carbon steels varied not only with the test temperatures and strain rates, but also with the process condition of cooling rate. The measured flow stress of δ phase was lower than that of γ phase. The degree of strain hardening of δ phase was negligible compared with that of γ phase. A simple constitutive equation, which takes into account temperature, strain and strain rate, could successfully describe the measured flow stress of carbon steel by hot tensile test. The calculated flow stress was in good agreement with experimentally measured data.

Using a 2-dimensional coupled thermo-elasto-plastic finite element model, the thermo-mechanical behaviours of the strand and mold were analyzed. The calculated geometry and temperature distributions of the solidifying shell and mold were compared with experimental observations. The calculated geometry around corner region was in good agreement with experimental observations. The mold wear was analyzed by a new dimensionless parameter of "Apparent Wear Parameter" which is inversely proportional to yield stress of the mold at service temperature and directly proportional to the interfacial pressure between the strand and mold. The effects of narrow face taper and carbon concentration of cast steel on mold wear were analyzed using the apparent wear parameter. With increasing narrow face taper, the possibility of mold wear increased due to increasing interfacial pressure. With increasing carbon concentration, the width of worn region of 35 mm at 0.05 and 0.1 wt% C steels decreased to 15 mm of 0.2 wt% C steel due to uniform thermal contraction of 0.2 wt% C steel during solidification. The calculated behaviours of mold wear were compared with used mold in industrial operation. The calculated worn region of mold based on the apparent wear parameter were in good agreement with industrial observations.

A new unit layer model for the equiva lent t hermal co nductivity of layered steel strips has been proposed. The equivale nt thermal conductivity is a functio n of strip thickness, surface characteristics and compressive stress. The modeled eq ui valent thermal conduct ivity corresponds well to the ex perimental data. Finite element analyses (FE M) fo r cooli ng of hot rol led coil have been carried out under various cooling condi tions using the equivale nt ther mal conduct ivity as the thermal conductivity in radial di rection of hot rol led coi l. A new calculation procedure using ABAQUS has been developed, where th e radi al compress ive ther mal stres s is taken into account for ca lculatio n of the eq ui va lent th ermal conduct i vi tι or th e orthotropic stress dependent ther ma l co nductivity . The calcul ated cool ing curv es using the orthotrop ic stress dependent thermal conductivity have been compared with data calculated usi ng isot ropic and orthotropic stress independent conductiv ity and with experimenta l data. The cooling curves ca lcul ated usi ng eq uivalent thermal conductivity as rad ial thermal conductivi ty are in better ag reement with ex per imental data.

During martensite transformation, parent austenite usually has an orientation relationship with newly transformed martensite and thereby a crystallographic texture of the austenite has a great influence on a texture development in the inherited martensite. For a given orientation relationship, there are several equivalent orientations of the inherited phase, which is called variant. Table 1 shows the 24 variants for Kurdjumov–Sachs (K–S) orientation relationship which is usually observed in carbon steels. In idealized case, all variants can appear in an austenite grain with an equal probability during the transformation. However, it has been reported that some variants preferentially appeared during the transformation. In other words, some variants possibly have greater relative probability to be selected. This phenomenon, which is called variant selection, is known to have a significant effect on the texture development in the inherited phase. The slip system of parent phase, the grain boundary orientation and the existence of stress have been known to affect the variant selection during the transformation and the development of transformation texture. As for the slip activity, the criterion of variant selection was imposed in terms of slip distribution and was applied to diffusion-controlled transformation. Ray et al. reviewed the various kinds of transformation texture that is normally encountered in austenite-to-ferrite transformation in steel. As for the bainitic transformation in steel, the variant selection had been investigated by an experimental observation. Recently, present authors suggested that the probability for a nucleation site to really act during displacive transformation could be derived for each variant as a function of the mechanical interaction energy between externally applied stress and lattice deformation based on the Kurdjumov–Sachs (K–S) relationship. In present study, the variant selection in mechanically induced martensitic transformation of metastable austenite is investigated with respect to the interaction between external stress and lattice deformation of the transformation. The orientations of parent austenite and newly transformed martensite are measured for tensile and compressive deformation using electron back-scattered diffraction (EBSD). For an individual austenite grain, the orientation of 24 K–S variants are evaluated and compared with measured orientation of martensite. The interaction energy between externally applied stress and lattice deformation is calculated for each 24 K–S variant and the probability of variant selection is assessed. The assessed probability is compared with the experimental results.

Effects of carbon and sulfur on the longitudinal surface cracks have been investigated by calculating the non-equilibrium pseudo binary Fe-C phase diagram and introducing the strain in brittle temperature range for continuous casting of steels. The cracking tendency as a function of carbon content was well described by the strain in brittle temperature range. The strain in brittle temperature range was influenced by the other solute elements as well as carbon. The carbon content at which longitudinal surface cracking is maximized decreased with increasing sulfur content. At a given carbon content, the possibility of surface cracking increased with increasing sulfur content.