박판주조법으로 제조한 마그네슘 합금 판재의 가공 열처리에 따른 미세조직과 집합조직 변화

Abstract

Mg alloys are the lightest commercial alloys developed so far and have great potential for high performance automotive applications. For the successful application of Mg alloy sheet products, thermo-mechanical treatment (TMT) such as hot/warm rolling is needed to modify the microstructure so that an optimum combination of mechanical properties can be obtained. Basal texture, however, becomes stronger during TMT, which has an adverse effect on the formability at low temperatures. Also such textured alloys usually exhibit strong tension-compression strength asymmetry. This is especially true for conventional ingot cast Mg alloys since a fairly large amount of rolling reduction is required to produce thin sheet products. It has been recently shown that the application of twin-roll casting (TRC) technology can alleviate some of the problems encountered in conventional Mg alloy sheet products. One of the important characteristics of TRC process is that it can produce thin Mg sheet with thickness ranging from 2 to 5 mm directly from the melt, providing a fast solidification rate and the concomitant refinement of microstructural features. Also the total amount of rolling reduction required to produce the final gauge is much smaller for TRC Mg alloys than for conventional ingot cast alloys. Therefore, TRC Mg alloys can have much reduced basal texture than ingot cast Mg alloys after TMT. However, TRC Mg alloys still show rather pronounced basal texture, which needs to be further randomized to improve their formability. Therefore it is important to modify the texture of Mg alloys by various TMTs. However, the detailed mechanism of texture evolution during thermo-mechanical treatment is not yet clear. In the present study, the effect of TMT on the evolution of texture by recrystallization has been investigated. To understand the details of texture evolution during rolling and reheating, EBSD analyses were conducted on the sheets given each rolling passes and/or reheating. One unique feature of this experimental set-up is the observation of the same area before and after the reheating so that the exact microstructural and texture evolution can be analyzed. It shows that basal texture becomes stronger with an increase in the rolling reduction as expected. However, re-heating between rolling passes results in the reduction of basal texture. The deformation bands consisting of twins act as nucleation sites for static recrystallization. Static recrystallization induced by re-heating between rolling passes results in a weakening of basal texture, which becomes more evident with an increase in re-heating time. During reheating, {0001}<11-20> texture changes to {0001}<10-10> texture. The change of direction of grains by reheating will lead to the development of random texture during further processing. It would also have a beneficial effect on reducing the anisotropy of the sheets.It also shows that the rolling temperature and reduction ratio can affect the texture evolution. More importantly, recrystallization behavior of Mg alloys is quite different depending on the alloy system. This is mainly due to the difference in nature of deformation twins formed during rolling. In the case of Mg alloys containing rare-earth (RE) elements, deformation twins are most tension twins, while other alloy systems contain either compression or double twins. There is a splitting of basal poles towards the RD when the deformation bands consist of double twins, while a broadening of the angular distribution of basal poles towards the TD occurs when the deformation bands consist of tension twins. Tension twins present in RE containing alloys promote the formation of recrystallized grains with random texture, resulting in improved formability of Mg-RE alloys.