Correlation of Microstructure and Mechanical Properties of Thermomechanically Processed Low-Carbon Steels Containing Bor
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OVER the past decades, structural steel plates have been steadily developed to have high strength and toughness with good weldability using different alloying elements and heat-treatment conditions, and applied to building, bridge, pressure vessel, pipeline, and offshore structures. The recent development of thermomechanical control process (TMCP) technology composed of controlled rolling and accelerated cooling has shown that it can efficiently produce high-strength steel plates with low carbon or ultralow carbon, which provides a good combination of toughness and weldability by lowering the volume fraction of carbide-containing microconstituents at the expense of strength and hardness.[1–3] Recently, with ever increasing environmental requirements, many studies of the utilization of scrap in BYOUNGCHUL HWANG and TAE-HO LEE, Senior Researchers, and CHANG GIL LEE, Principal Researcher, are with the Ferrous Alloys Group, Korea Institute of Materials Science, Changwon 641-010, Korea. Contact e-mail: [email protected] Manuscript submitted July 3, 2009. Article published online October 29, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
steelmaking process have been actively carried out to reduce the cost for refining tramp elements and to maximize the recyclability of steel scraps. The Cu, a representative tramp element, can contribute to solid solution strengthening as well as precipitation strengthening by Cu precipitation, because it has a relatively high solubility in austenite but is almost insoluble in ferrite.[4–6] It also improves hardenability and lowers the austenite-ferrite transformation temperature, leading to a finer ferrite microstructure. Meanwhile, it is noted that a small amount of B efficiently increases the hardenability of steels, because it retards the diffusional transformation of austenite to ferrite through its segregation to austenite grain boundaries.[7] Therefore, the application of TMCP to the low-carbon or ultralow-carbon steels containing B and Cu is very attractive for the development of high-strength and high-toughness steel plates from the viewpoint of recyclability and economics. The requirement for high-strength steel plates with high toughness and good weldability has led to intensive examination of continuously cooled microstructures of low-carbon or ultralow-carbon steels. The nonequiaxed ferritic microstructures usually formed during TMCP VOLUME 41A, JANUARY 2010—85
have been regarded as very complicated compared to the equiaxed or polygonal ferritic microstructures of conventional hot-rolled steels. There are many difficulties in identifying the phases and sometimes serious confusion in the terminology of the microstructures.[8–11] The temperature range for the formation of the nonequiaxed ferritic microstructures with different morphologies and properties is intermediate, between those at which austenite transforms to equiaxed ferrite and to martensite, and thus is similar to that in which bainite forms in medium-carbon steels. These nonequiaxed ferritic microstructures are to b
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