Carbide precipitation and high-temperature strength of hot-rolled high-strength, low-alloy steels containing Nb and Mo
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HIGH-STRENGTH, low-alloy (HSLA) steels are very important metallic materials. Their outstanding properties are high strength, excellent ductility, and good weldability.[1] After alloying elements were introduced, which improve these properties, the HSLA steels experienced considerable development. The general alloying elements used to improve the properties of steels are Nb, Ti, V, and B. These HSLA steels have been used in a variety of applications, including pipelines, cars, pressure vessels, ships, and offshore platforms. The strength of the conventional HSLA steel used in building frames is generally reduced by heat during a fire, thus impairing its durability. Therefore, fire protection is strictly specified under building regulations. Protection work, however, adds to the cost of construction substantially, increases construction time, and reduces usable interior space. The fire-resistant HSLA steels, are guaranteed, at 600 ⬚C, to be in the range of two-thirds the yield-strength value specified at room temperature.[2–6] These were developed for the fire safety regulations that govern the design of such buildings, in order to take into consideration the WON-BEOM LEE, Researcher, is with the Technical Research Laboratories, POSCO, Gyeongbuk, 790-785, Korea. Contact e-mail: wbeom70@ posco.co.kr SEUNG-GAB HONG, Student, and CHAN-GYUNG PARK, Professor, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology (POSTECH), Gyungbuk 790-784, Korea. SUNG-HO PARK, Senior Researcher, is with the Automotive Steel Research Center, Technical Research Laboratories, POSCO, Cheonnam, 545-090, Korea. Manuscript submitted March 12, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
temperature at which the steel loses a significant proportion of its strength or load-carrying capacity in the event of a fire. Among the common alloy elements added to these HSLA steels, Cr/Mo plays a unique and important role both by having a marked decreasing effect on the austenite transformation temperature and by forming various carbides with complex constitutions in the steels containing Cr/Mo. Kuo[7] has shown that Cr introduces two carbides (Cr23C6 and Cr7C3) and Mo introduces five carbides (MoC, Mo2C, M23C6, M6C, and Fe2MoC) into steels. It has been indicated[8–11] that the HSLA steels containing both Nb and Mo exhibit an acicular ferrite/bainite microstructure and, thus, excellent strength-retention characteristics at high temperature (up to 600 ⬚C to 700 ⬚C), in comparison to conventional HSLA steels containing Nb and V. It has also been reported[15,16] that Mo decreases the diffusivity of the carbide-forming species (i.e., Nb and C) and, thus, delays the precipitation of MC carbides. These findings strongly suggest that the coexistence of Nb and Mo play an important role in significantly enhancing the strength of the HSLA steels at both room temperature and high temperatures up to 600 ⬚C to 700 ⬚C. However, it is not evident why the coexistence of Nb and Mo in the steel results in higher yield str
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