DICTRA Simulation of Holding Time Dependence of NbC Size and Experimental Study of Effect of NbC on Austenite Grain Grow
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TRODUCTION
WITH the development of technology, the demands for offshore structures including the offshore structure steels grow to explore resources in the deep ocean.[1] Since the offshore structure immersed in the sea suffers from seawater corrosion and atmospheric corrosion as well as the sea wave pounding, the harsh service conditions require higher properties of steels, such as high strength, low-temperature toughness,[2] weldability,[2] corrosion[3] resistance. To reduce weight and production cost, high strength steels are developed, such as steel EQ70 with the yield strength and tensile strength higher than 690 and 770–940 MPa, respectively.
SU-FEN TAO and GUI-LIN SUN, Ph.D. Students, FU-MING WANG, Professor, and ZHAN-BING YANG, Associate Professor, are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China, and also with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing. Contact e-mail: [email protected] CHANG-RONG LI, Professor, is with the School of Materials Science and Engineering, University of Science and Technology Beijing. Manuscript submitted July 20, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
Steel EQ70, a quenched and tempered steel for offshore structure, is recently developed as a kind of high strength and low alloy steel plate with a thickness ranging from 70 to 220 mm. The steel mainly achieves high strength and toughness through the adjustment of carbon and other alloying elements coupled with an appropriate heat treatment process.[4] The role of microalloying elements is to improve the strength and toughness of steels by grain refinement and precipitation strengthening.[5] However, grain refinement is in conflict with precipitation strengthening with regard to soaking temperature. Thus, soaking temperature should be chosen carefully for the highest effect of alloying element on the strengthening. Appropriate soaking temperature must be the temperature at which the austenite grains evenly distribute and the contents of the alloying elements in matrix are high enough so as to result in precipitation at lower temperatures. Therefore, the study on grain growth behavior is essential for determining the austenitizing temperature. Austenite grains in carbon steels grow gradually as temperature increases.[6] Sellars et al.[7] have proposed a mathematical model for describing the austenite grain growth of C-Mn steel. However, as for the steel alloyed with aluminum, niobium, vanadium or titanium, a
sudden grain growth may occur[8,9] when the soaking temperature is higher than the grain coarsening temperature, at which the equilibrium existing between the driving forces for grain growth and grain pinning is broken. Then the subsequent transformation product will distribute unevenly or be abnormally large, and degrade the steel performance. Steel EQ70 is alloyed with grain refining elements Al, Nb, and Ti, all of which can form into small sparingly soluble precipitates,
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