Evolution of {100} texture induced by surface nucleation during γ → α phase transformation in electrical steel

Abstract

전기강판 재료는 크게 방향성 전기강판과 무방향성 전기강판으로 분류된다. 방향성 전기강판의 경우 일반적으로 변압기용 소재로 쓰이며 한 방향으로 배향돼 있는 {110}<001>, Goss 집합조직을 사용한다. Goss 집합조직은 한 방향으로 자기적 특성이 우수하다. 반면 무방향성 전기강판의 경우 대표적으로 전기자동차 모터 혹은 발전기용 소재에 사용되는데 이때 가장 이상적인 집합조직은 {100}<0vw>, cube on-face 집합조직이다. Cube-on-face 집합조직은 회전 방향으로 자기적 특성이 균일하면서 우수하다. Cube-on-face 집합조직은 ND 방향으로 <100>, 동시에 ND와 수직인 방향으로는 랜덤하게 배향돼 있다. 연구가 활발히 진행돼 있고 상용화된 Goss 집합조직과는 달리 cube-on-face 집합조직은 연구가 부족한 실정이다. Cube-on-face 집합조직은 꿈의 재료로 알려져 있다. 본 연구는 감마 → 알파 상변태 열처리시 시편에 응력을 가하게 되면 핵생성장벽이 낮은 {100} 핵이 표면에 유도되고 더불어 벌크 내부보다 표면에 핵생성이 우세하다면 {100}<0vw> 집합조직 형성에 대한 가설을 제시한다. 인장응력 혹은 압축응력을 가하여 cube on-face 집합조직 형성의 핵심인 표면핵생성 연구를 하였다.

Previously, it was shown that the {100}<0vw> texture could be evolved when the specimen is under the appropriate stress during γ → α phase transformation. Here the effect of the various process conditions on the evolution of the {100}<0vw> texture was examined. The texture evolution was affected by the magnitude of tensile stress, the furnace atmosphere, the heat treatment time (or grain size) and the cooling rate. In Fe–1wt%Si, the {100}<0vw> texture was 78.3% under the H2 atmosphere with a flow rate of 1000 standard cubic centimeter per minute (sccm) whereas it was 16.2% under the N2 atmosphere with a flow rate of 1000 sccm. The marked decrease in the percentage of the {100}<0vw> texture under the N2 atmosphere is believed to be related with the surface oxidation of the specimen. In Fe–2wt%Si–1wt%Ni, the {100}<0vw> texture was 8.9% when the specimen was heat treated at 1100 ℃ for 5 min whereas it increased to 66.0% when the specimen was heat treated for 6 h. This increase of the {100}<0vw> texture is believed to be related with the increase in the grain size. Under the optimum processing condition, the {100}<0vw> texture could be increased to as high as 92.6% in Fe–2wt%Si–1wt%Ni. In order to study the effect of the uniaxial compressive stress on the development of a cube-on-face texture, two sheet samples were heat treated with different positions to have a different self load, resulting in a different stress. One specimen was heat treated at 1120 ℃ for 60 min with the broad face positioned horizontally to the specimen holder and the other specimen was heat treated with the broad face positioned vertically to the specimen holder with the other condition being the same. The former produced a weak {100}<0vw> texture, being only 30.9% whereas the latter produced a strong {100}<0vw> texture, being 81.1%. We found out that such a texture difference is related to the difference in the nucleation barrier coming from the stress disparity. Based on this understanding, specimens with a different percentage of the {100}<0vw> texture could be manufactured. The measurements of magnetic properties showed that the specimen with 78.4% of {100}<0vw> and 3.2% of {111}<uvw> had B50 = 1.75 and W15/50 = 2.19, but the specimen with 0.7% of {100}<0vw> and 81.3% of {111}<uvw> had B50 = 1.61 and W15/50 = 2.50. The results show that the magnetic flux density of B50 was increased by 8.7% and the eddy current loss of W15/50 was decreased by 12.4% due to the evolution of the {100}<0vw> texture.