Hot deformation behaviors of ferritic Fe-Si alloys

Abstract

Fe-Si alloys are very interesting steels showing excellent mechanical and magnetic properties. In case of magnetic properties, some Fe-Si alloys, called as electrical steels, have been widely used as very important steels in the electrical industry. The magnetic properties of Fe-Si alloys are mainly dependent on their crystallographic texture such as {110}<001> Goss texture and {100}<0vw> Cube fiber which are strongly developed by the hot deformation of ferritic temperature range. Furthermore, their magnetic properties are improved by alloying more silicon contents. The increase of silicon contents in Fe-Si alloys, however, is detrimental to mechanical properties, such as tensile and yield strengths. In the present work, the hot deformation behaviors of Fe-Si alloys containing various silicon contents were studied in order to comprehend the effect of silicon on the hot working and the crystallographic texture of Fe-Si alloys. The Fe-Si alloys containing up to 4wt% Si were selected in order to investigate the silicon effect on the mechanical properties at elevated temperatures by torsion and compression test. Constitutive parameters of hot working were determined and compared with various deformation modes. In addition, the hot deformed microstructure with strains were investigated by using EBSD and TEM in order to clarify whether the dynamic recovery or dynamic recrystallization takes place in ferritic steels during hot deformation. The torsion texture in Fe-Si alloys at elevated temperatures were measured by using EBSD and analyzed by means of ODFs. In the first part of this thesis, the silicon effect on the mechanical properties of Fe-Si alloys was measured by torsion and compression test. Fe-Si alloys took place dynamic recrystallization, showing a continuously decreasing flow stress after single stress peak at torsion test. The constitutive equations calculated from torsion and compression test indicated that the hot deformation behavior in Fe-Si alloys was well evaluated at both deformation modes. The activation energies of hot deformation were close to the that of lattice self diffusion in α-iron and n values were in the range of five power law which means the hot deformation of Fe-Si alloys occurs by a thermally-activated, diffusion-controlled dislocation climb mechanism. When Fe-Si alloys are deformed at temperatures below 850°C, the activation energy increased with increasing Si content due to enhanced solid solution hardening effect by Si atoms. On the other hand, the activation energy does not change so much with silicon contents in the high temperature above 850°C, indicating the effect of Si contents on hot deformation becomes negligible. In the second part of the present work, the dynamic softening of Fe-2wt%Si steel was discussed by using EBSD and TEM. The microstructure of ferritic Fe-2wt%Si steel deformed at the range of 550°C to 1050°C consisted of equiaxed crystallites surrounded by partly HABs and partly LABs. In case of the low deformation temperature of 550°C, the equiaxed crystallites were intensively found at shear bands and original grain boundaries which imply that RDRX by the progressive rotation of subgrains occurred. However, when the specimens were deformed at temperatures above 0.5Tm (more than 650°C), the HABs were homogeneously distributed in the hot deformed microstructure. In addition, a gradual increase of the number of boundaries having a misorientation in the range of 10° to 18° was observed in the specimen deformed at 850°C which can be explained by the gradual increase of the misorientation of sub-grains leading the CDRX. In the last part of the thesis, the torsion textures of Fe-Si alloys at elevated temperatures were evaluated by using EBSD. The torsion texture of Fe-Si alloys was similar with the results of J. Baczynski. The F {110}<001> texture and J {110}<112> texture developed at the early deformation stage were gradually consumed by the recrystallized D {112}<111> texture and E {110}<111> texture. D2 (1-1-2)[111]orientation become dominant with the relatively weak D1 (11-2)[111] and E {110}<111> orientations at very large strains. During hot deformation, the addition of Si in Fe-Si alloys is found to retard the grain boundary movement and the evolution of torsion textures.