Formation of Ultrafine Cellular Microstructure Around Alumina Particles in a Low-Carbon Steel
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RODUCTION
IN spite of many achievements for clean steels in recent years, it is inevitable that nonmetallic inclusions still exist in steel products.[1] Their cracking or decohesion from the matrix are the major sources of the formation of cracks or voids that lead to material failure.[2–7] Therefore, many studies have examined the effect of their size, shape, and distribution on the steel properties.[8–14] On the other hand, inclusions can also play the role of inoculants for a phase transformation. A few of them effectively promote the formation of intragranular ferrite idiomorphs or acicular ferrites, which results in strengthening and improved toughness of the alloys by grain refinement.[15–20] Although there have been many studies on inclusions in steels, it is difficult to find a detailed microscopic examination of the shallow region surrounding the inclusions. This is largely due to the experimental difficulties in retaining the region during metallographic sample preparation. Owing to the incoherency of the interface between the inclusion and the matrix, this region is normally susceptible to severe attack by chemical etching. In the current study, a very fine and peculiar microstructure surrounding an alumina (Al2O3) particle was discovered in a low-carbon steel plate by a modern ion microscopy. The mechanism of its formation is discussed in terms of the hydrostatic stress field obtained JUN-YUN KANG, Senior Researcher, is with the Korea Institute of Materials Science, Changwon, Republic of Korea. SANG HOON LEE, Team Leader, is with the FEI, Hillsboro, OR. YI-GIL CHO, Post Doctoral Fellow, SEUL CHAM KIM and HOON-HWE CHO, PhD Candidates, HEUNG NAM HAN and KYU HWAN OH, Professors, are with the Department of Materials Science and Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted December 4, 2012. Article published online April 26, 2013 4098—VOLUME 44A, SEPTEMBER 2013
by a thermo-elasto-plastic finite element (FE) analysis and the phase transformation kinetics in the matrix surrounding the alumina particle. To confirm the origin of the development of fine cellular microstructure, which might be the delayed phase transformation due to the large hydrostatic pressure, a microstructure produced with the aid of high-pressure high-temperature (HPHT) technology in diamond synthesis is presented and compared with the fine cellular one around the particle.
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EXPERIMENTAL
A steel plate, which satisfies the requirement of N80 grade steel as defined by the American Petroleum Institute (API), was used. The chemical composition of the steel was 0.3C-0.3Si-1.5Mn-0.015Nb-0.01Ti0.3Cr-0.15V (wt pct). Specimens were cut from the plate and heated to 1373 K (1100 °C) at a rate of 0.9 K/s, held at that temperature for 8 minutes, cooled to 673 K (400 °C) at 0.5 K/s, and then cooled to room temperature in the air. Other specimens were subjected to two cycles of austenitization. In each cycle, the sample was heated to 1273 K (1000 °C) at a rate
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