Kohsaku Ushioda

Control of Structures and Properties of Cold-Rolled Sheet Steels

Precise microstructural control is essential to obtain the required properties of sheet steels. Since excellent formability is required for mild steels, the control of grain structure and texture are of utmost importance. First, the roles of initial grain boundaries prior to cold rolling, shear bands, and the interaction between C and Mn during recovery are critically reviewed as the essential factors controlling recrystallization texture. A model for the grain size of hot bands in interstitial free (IF) steels is presented in conjunction with texture control. Moreover, the grain growth during annealing, which leads to the development of ND//⟨111⟩texture, is discussed from the viewpoint of the pinning effect due to fine precipitates. The texture memory effect, typically found in Mn-IF steels, is also discussed. Concerning high strength steel sheets, the history of the development is firstly introduced. The discussion is focused on transformation induced plasticity (TRIP) steel by placing an emphasis on the relationship between the mechanical properties and the plastic stability of retained austenite. Furthermore, the prediction model for structures in TRIP steels during the continuous annealing process is presented based on the competitive reaction between bainite transformation and cementite precipitation together with a prediction model for the stress-strain curve. Finally, the factors to be overcome for further strengthening of sheet steels are described together with the future direction of research.

Effects of cementite morphology on short-fatigue-crack propagation in binary Fe–C steel

The effects of cementite morphology on short-fatigue-crack propagation were investigated by subjecting smooth steel specimens to rotating-bending fatigue tests. Intergranular and transgranular cementite precipitation steels free of other phases were prepared through heat treatments. In the intergranular cementite precipitation steel, short-fatigue-crack propagation was decelerated at the grain boundaries, and accelerated in the grain interiors. In contrast, the transgranular precipitation steel showed the deceleration of the crack propagation in the grain interiors but not in the vicinity of the grain boundaries.

Observation of co-segregation of titanium and boron at the interface between recrystallized and unrecrystallized grains in cold-rolled interstitial-free steel sheets.

It has been reported that the addition of ppm levels of B strongly retarded the growth of recrystallized grain into unrecrystallized grains in the process of cold-rolling and annealing of Ti-added interstitial-free (IF) ferritic steels. This phenomenon was explained by solute drag effect based on the assumption that, during annealing, B atoms segregate at the interface between recrystallized and unrecrystallized grains where they interact with Ti atoms. To verify this, atom probe tomography analysis of the interface was performed in Ti-added IF steels with and without B addition. Needle tips containing the interface identified from electron backscattering diffraction analysis, were produced by focused ion beam milling with the lift-out method. To increase the experiment reliability, the misorientation angle of the aimed interface was compared with that estimated by field ion microscopy analysis. Considerable amount of Ti segregation was observed at the interface in the steel without B addition, which increased with increasing amount of B segregation in the steel with B addition. The results suggest that the retardation of the interface migration was caused by solute drag effect based on the simultaneous co-segregation of Ti and B due to their attractive interaction.

The Effect of Cold Rolling Reduction on Shear Band and Texture Formation in Fe-3%Si Alloy

The effect of cold rolling reduction on shear band formation and crystal orientation within shear bands and annealing texture were investigated in Fe-3%Si {111}<112> single crystals. Several types of shear bands were observed with different angles to rolling direction, dependent on rolling reduction. As for shear band formation, those with smaller angles were formed earlier and those with larger angles were formed later. Regarding crystal orientation along shear bands after rolling reduction, orientation distribution from the initial became large in accordance with reduction and even exceeded Goss orientation when rolling reduction became larger than 40%. After annealing, however, recrystallized grains along shear bands were mainly Goss grains regardless of reduction. The speculated reason for the dominance of Goss after annealing is that Goss subgrains with less density of dislocations were surrounded by largely deformed areas.

Mechanism on Heterogeneous Nucleation of the Primary Al Phase on TiAl3 of a Hot-Dip Zn-11%Al-3%Mg-0.2%Si Coating on Steel Sheet

The solidification structure of a hot-dip Zn-11%Al-3%Mg-0.2%Si coating with a Ti addition on a steel substrate was investigated. Steel sheet was coated using a laboratory hot-dip galvanizing simulator. The coating was subsequently characterized via optical and high resolution scanning electron microscopy with EBSD and high temperature X-ray diffractometry. The hot-dip coating consisted of a combination of a Zn/Al/MgZn2 ternary eutectic structure, primary Al phase and MgZn2 phase. TiAl3 acts as a heterogeneous nucleation site for Al, which was shown to have perfect lattice coherency with TiAl3 as epitaxial Al growth from the TiAl3 was found. The growth direction of Al is along <110> and has a random texture, whereas Zn has a rather strong ND//<0001> fiber texture.

Evolution of Heterogeneous Deformation Structure and Recrystallization Texture of Steel

Heterogeneous deformation during rolling is a crucial issue for elucidating recrystallization behavior. The progress thus far in our understanding of heterogeneity has been reviewed focusing on grain boundary and shear band. A statistical study on heterogeneous deformation structure using EBSD revealed that heterogeneity along the grain boundary can be classified into three types: 1) relatively flat boundary, 2) irregularly serrated boundary, and 3) boundary associated with fine grains. The fine grains in type 3 seem to be dynamically recovered as a cold-rolled state. Shear band formation is considered to be caused by plastic instability that is accelerated, for instance, by dynamic strain aging. A shear band is revealed to have a feature of recovered fine cells with Goss orientation already embedded in the shear band. The application of the phase-field method is exploited to predict recrystallization behavior and texture evolution during annealing based on the subgrain growth model. In simulation, a bulging mechanism seems to be dominant. Thus, a more rigorous description of the heterogeneous deformation structure is needed in the future

Two-Dimensional Grain Growth Simulation by Local Curvature Multi-Vertex Model

A local curvature multi-vertex model was developed. This model is the straightforward two-dimensional topological network model based on the physical principles which are the curvatures of grain boundaries and the grain boundary tensions at triple junctions. The model was applied to the artificial random microstructure under some conditions of grain boundary characters. The misorientation distribution was changed very little under constant grain boundary energy and mobility, but it was change much under grain boundary character dependent on misorientation. Therefore, in order to discuss actual textures, it is important to take grain boundary characters into account.

Change in the microstructure and mechanical properties of drawn pearlitic steel with low-temperature aging

Hydrogen embrittlement is a serious problem in high-strength steels. Drawn pearlitic steel shows excellent resistance to hydrogen embrittlement despite its high strength, and aging treatment at a low temperature can simultaneously improve its strength and hydrogen-embrittlement resistance. To clarify the mechanism for this we have used thermal desorption analysis (TDA) and the newly developed precession electron diffraction analysis method in the transmission electron microscope. After aging at 100 °C for 10 min, the amount of hydrogen seen amount on the TDA curve reduced at around 100 °C. In contrast, when aging was performed at 300 °C, the hydrogen amount further reduced at around 100 °C and the unevenly deformed lamellar ferrite zone was locally recovered. For the samples that were aged at the low temperature, we confirmed that their yield strength and relaxation stress ratios increased simultaneously with improvement in the hydrogen-embrittlement property. We infer that segregation of carbon or formation of very fine carbide in dislocations during aging is the cause of these behaviors.

Fundamentals for Controlling the Microstructure and Properties of Cold Rolled and Continuously Annealed Sheet Steels

The strict control of microstructure is indispensable for meeting the demands for sheet steels with superior properties. This paper reviews the state-of-the-art related to continuous annealing process from the viewpoint of physical metallurgy. For mild steel, control of texture is one of the most important issues ensuring deep drawability. Therefore, the fundamental guiding principles on texture control are reviewed. Heterogeneity in cold rolled structures is discussed, with emphasis placed on grain boundary, shear band and cementite in terms of preferential nucleation of recrystallized grains. Furthermore, C-X interactions in the recovery stage are discussed in conjunction with scavenging effects. The ND//<111> texture usually evolves according to the grain growth, which is enhanced by reducing the pinning effect. The ductility in low-C steels is significantly influenced by the state of C, which implies the importance of C control in the continuous annealing process. As for high strength sheet steels, the basic guiding principles for dual phase and transformation induced plasticity (TRIP) steels are reviewed in terms of phase transformation and TRIP effect. Because the strength of steels currently in practical use is considered to be only a part of their potential, steels are materials with many advantages and technical problems that remain to be solved.

Precipitation Behaviour of Mg and Si at RT in Pre-Aged Al-Mg-Si Alloys

The precipitation behavior of Mg and Si during storage at RT in Al-Mg-Si alloys pre-aged at 90°C was studied using a tensile test and differential scanning calorimetry (DSC) measurement. Specimens were solutionized at 530°C, water-quenched and then pre-aged for 2, 6 and 12 hours at 90°C during which small precipitates were formed. In the pre-aged alloy, the strengthening rate at RT has two stages. In the initial stage, the yield strength increases slowly with the aging time and in the final stage, it increases rapidly. In the initial stage, the strength in the pre-aged alloy is smaller than that in the non pre-aged alloy, while in the final stage, the strength in the pre-aged alloy is larger than that in the non pre-aged alloy. Furthermore, the period of the initial stage is dependent on the pre-aging period at 90°C. The DSC curves of alloys in the initial stage do not show the presence of clusters, while those in the final stage do. It seems that in the initial stage Mg and Si atoms accumulate around the small precipitates that have been formed in pre-aging at 90°C while in the final stage, the clusters of these atoms are formed.