Cooling Rate Dependence of Boron Distribution in Low Carbon Steel

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THE addition of a small amount of boron (B) remarkably increases the hardenability of low carbon, low alloy steels.[1–5] This beneficial effect on hardenability occurs because segregation of B atoms to austenite grain boundaries reduces the grain boundary energy and thus retards transformation of austenite to ferrite. B atoms are easily segregated to the austenite grain boundaries during general heat treatment and have a strong tendency to interact with lattice imperfections.[6] In general, the grain boundary segregation of B in steel occurs by two mechanisms: equilibrium and nonequilibrium segregation.[7] Equilibrium grain boundary segregation (EGS) occurs by the movement of solute atoms from inside the grain matrix to loosely packed regions such as grain boundaries, thus reducing the grain boundary free energy. Nonequilibrium grain boundary segregation (NGS) occurs during cooling from high temperatures by the diffusion of vacancy-solute complexes toward grain boundaries. The NGS of B is generally accepted to be the usual dominating process in steel.[8–10] DONG JUN MUN, Postdoctoral Candidate, JAE SANG LEE, Associate Professor, and YANG MO KOO, Professor, are with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea. Contact e-mail: [email protected] EUN JOO SHIN, Senior Researcher, is with the Neutron Science Division, Korea Atomic Energy Research Institute, Daejeon 305-353, Republic of Korea. KYUNG CHUL CHO, Senior Researcher, is with the Steel Making Research Group, Technical Research Laboratories, POSCO, Gwangyang 545-090, Republic of Korea. Manuscript submitted July 20, 2011. Article published online November 17, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A

According to the mechanism of the NGS, solute B atoms are transported to the grain boundaries after forming complexes with vacancies. A large number of supersaturated vacancies can be produced in a material during quenching, irradiation, or deformation, and thus, NGS of B can be observed through these kinds of processes. In quenching-induced segregation,[11–13] supersaturated vacancies formed during quenching from a higher temperature to a lower temperature are annihilated at grain boundaries during isothermal holding; then vacancy-boron complexes diffuse from the grain interior to the grain boundaries along the vacancy gradients with an increasing holding time (HT) at a holding temperature TH, and the maximum grain boundary segregation of B occurs at a certain time that is generally known as the critical time.[12] NGS caused by diffusion of vacancy-boron complexes gradually disappears if sufficient time is allowed for the system to reach full equilibrium. This is a very important concept in understanding NGS of B in steel. The purpose of this study is to investigate the CR dependence of B segregation to grain boundaries in low carbon steel. Although the grain boundary segregation of B during the continuous cooling[9] and isothermal holding[13] process has been studied by some resea