Effect of Niobium on the Ferrite Continuous-Cooling-Transformation (CCT) Curve of Ultrahigh-Thickness Cr-Mo Steel
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INTRODUCTION
PRESSURE-RETAINING vessels made of Cr-Mo bainite steel, which are widely used in petrochemical and power plants, must be thick (400 mm) and have high tensile strength (600 MPa). However, large-scale vessels of thickness over 400 mm can be subjected to mass effect, which leads to microstructural transformation in the midsection of the vessel through ferrite transformation, and this, in turn, leads to a reduction in the tensile strength.[1,2] Therefore, alloys with improved hardenability and reduced mass effect are being designed by varying the chemical composition of the alloy to prevent the reduction in tensile strength.[3–9] The hardenability of existing Cr-Mo steel, which is used for commercial pressure vessels with thickness less than 300 mm, has been improved by the addition of B,[10] Ni,[11] Mn,[12] and Cr.[13] To improve the hardenability of Cr-Mo steels with thickness over 400 mm, a higher quantity of alloying elements is required. However, it is difficult to increase the amount of alloying elements beyond a certain limit imposed by the material specifications of Cr-Mo steel. Furthermore, the addition SANGHOON LEE, Senior Researcher, is with the Industrial Technology Support Division, Korea Institute of Materials Science, Changwon 641-831, Korea. HYESUNG NA, Doctoral Candidate, and CHUNGYUN KANG, Professor, are with the Department of Materials Science and Engineering, Pusan National University, Geumjeong-gu, Busan 609-735, Korea. Contact e-mail: kangcy@ pusan.ac.kr BYUNGHOON KIM, Senior Researcher, and DONGJIN KIM, Principal Researcher, are with the Corporate R&D Institute, Doosan Heavy Industries & Construction, Changwon, Gyeongnam 641-792, Korea. Manuscript submitted December 16, 2011. Article published online February 5, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
of an excessive amount of Mn, Ni, and B can reduce the impact properties [like ductile-brittle transition temperature (DBTT)] by segregation in the austenite grain boundary. The addition of Cr and Mo leads to the formation of metal carbide (MC) precipitates, which increases the tensile strength of the alloy but reduces the impact properties.[14–18] Therefore, further investigation into alternative alloying elements that prevent deterioration of mechanical properties and improve the hardenability is required. Research on Cr-Mo steel has resulted in the development of a new method, i.e., the superhardenability treatment, in which Ti (~0 pct to 0.05 pct) or Al (~0 pct to 0.03 pct) is added[19,20]; however, this method has not yet been applied commercially. A large number of studies on Nb has focused on its strength-hardening effect in thicker Cr-Mo steels. However, its effect on the ferrite continuous-coolingtransformation (CCT) curve has not been completely investigated. On the other hand, the hardenability of microalloyed Nb steel has been extensively studied using a thin plate high-strength low-alloy (HSLA) steel.[21–29] According to the research on HSLA steels, when Nb is dissolved in the steel until the solubility limit without any
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