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Improving and Characterizing Ferrous Materials Properties by Texture Analysis

Title
Improving and Characterizing Ferrous Materials Properties by Texture Analysis
Authors
오규환
Date Issued
2012
Publisher
포항공과대학교
Abstract
Texture, or preferred orientation, is a fundamental phenomenon resulting from the microstructure evolution that takes place during various process including casting, thin film fabrication, and thermomechanical processing of materials. These textures are largely responsible for the directionality of materials properties. So, to know better about the materials, it is necessary a deeper understanding of their textures. The deep drawability, e.g., the limiting drawing ratio or the plastic strain ratio, of steel sheets is well known to increase with increasing density of γ fiber (<111>//ND with ND denoting the sheet normal direction) component in their textures. Therefore, steel sheets for deep drawing are desired to have as high density of γ fiber as possible. A two-step rolling-annealing process has been developed to increase the γ fiber component in the recrystallization texture of a copper-bearing bake hardening steel. The two step process comprises the first rolling by a low reduction in thickness and subsequent annealing at 780 °C, followed by the second rolling by a high reduction and subsequent annealing at 780 °C. The first rolling process aims at seeding the γ fiber oriented grains, so that they can grow at the expense of differently oriented grains developed in the second rolling process. In this way the density of γ fiber component in the recrystallization texture of the bake hardening steel much increases compared with that in the conventional one-step rolling-annealing process. When the first rolling reduction was 8 or 10%, abnormal grain growth occurred during annealing. This phenomenon is known as the strain annealing process which has been proposed to increase the grain size in low carbon steel, IF steel, and silicon steel. It uses the grain growth by applying a rolling reduction low enough not to cause recrystallization, followed by annealing. In this case, the driving force for the grain growth is the difference in strain energy between neighboring grains. The effects of the strain annealing increased with increasing temper rolling reduction and decreasing impurity content. A study has been made of the evolution of the microstructures and textures in three kinds of low-carbon steel sheets (MAFE, BH, and IF) having well developed <111>//ND texture that were rolled by low reductions and annealed at 780 °C in Ar atmosphere. The steel sheets developed different microstructures and textures, even though their initial textures and thermomechanical treatments were similar. MAFE steel showed an unusual behavior that grains with high Taylor factors survived and grew very rapidly. This unusual behavior and the differences in microstructure and texture have been discussed. The effect of stacking fault energy on the formation of deformation twin in Fe-18% Mn-x Al-0.6% C steel with x being 0-3% subjected to a true tensile-strain of 0.4 has been investigated. The tensile deformation textures are approximated by major <1 1 1> and minor <1 0 0> fibres along the tensile axis parallel to the rolling direction (RD). Twinning is dependent on the grain orientation and takes place well on the <111> // RD fibers, and this tendency increases with increasing stacking fault energy (SFE). The orientation dependence of twinning is well described by Schmid’s law. We also measured the twin thickness and lamellae spacing by electron backscatter diffraction. The twin thickness and lamellae spacing increase with increasing SFE. The difference of twin thickness according to SFE was analyzed by means of nucleation and growth kinetics of twins during plastic deformation. Using the Hall-Petch relationship, we found the contribution of twinning to the tensile strength. Little study has been made of the texture of high Mn austenite steel rolled over 60 %, possibly due to its high hardening rate. The rolling and recrystallization textures of 60 to 80% cold-rolled Fe-18% Mn-x Al-0.6% C steel with x being 0-3% have been investigated. The α fiber (<110>//ND) connecting Goss {110}<001> and brass {110}<112> orientations developed in the cold-rolled sheets, whose orientation density tended to increase with increasing rolling reduction and Al content. The α fiber (<110>//ND) rolling texture transformed into the recrystallization texture consisting of the main Goss component and minor brass component after annealing at 800 °C for 10 min, implying that the brass orientation transform into the Goss orientation during recrystallization. The orientation transformation was discussed based on the strain-energy-release-maximization model for recrystallization texture.
URI
http://postech.dcollection.net/jsp/common/DcLoOrgPer.jsp?sItemId=000001391208
http://oasis.postech.ac.kr/handle/2014.oak/1666
Article Type
Thesis
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