Low-Temperature Toughening Mechanism in Thermomechanically Processed High-Strength Low-Alloy Steels
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HIGH-STRENGTH low-alloy (HSLA) steels widely used for building, bridge, pressure vessel, offshore, and pipeline structures have increasingly required higher strength, enhanced low-temperature toughness, and improved weldability simultaneously. In recent years, special attention has been paid to sufficient deformability such as low yield to tensile strength ratio, high uniform elongation, and high work hardening exponent, since many steel structures are subjected to progressive or abrupt displacement arising from structure loading itself, ground movement under their service environment, and earthquake.[1,2] As the increase of strength is often accompanied by the deterioration of toughness or deformability, a precise metallurgical design comprising chemical composition and thermomechanical processing is indispensible for achieving an excellent balance of the mechanical properties of HSLA steels. Presently, dualphase microstructure is known to be very suitable for so-called strain-based design with higher deformability, because it offers superior strain capacity due to the initial yielding in soft ferrite phase and the subsequent strain hardening during load transfer to hard phase.[3–5] BYOUNGCHUL HWANG, Senior Researcher, and CHANG GIL LEE, Principal Researcher, Ferrous Alloys Group, and SUNG-JOON KIM, Principal Researcher, are with the Korea Institute of Materials Science, Changwon 641-010, Korea. Contact e-mail: entropy0@kims. re.kr Manuscript submitted April 24, 2010. Article published online November 10, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
In general, controlled rolling and cooling in the intercritical-phase region, i.e., (austenite + ferrite) twophase region, allow dispersion of stronger ferrite uniformly in the austenite, and subsequent accelerated cooling or quenching transforms the austenite to various low-temperature transformation phases depending on the cooling rate and start and finish cooling temperatures.[5–8] In dual-phase steels consisting of ferrite and low-temperature transformation phases, ferrite provides a beneficial effect for cleavage fracture resistance as well as good deformability, and the strength usually increases with the volume fraction of harder low-temperature transformation phases at the expense of toughness. However, recent progress in thermomechanical processing technology has shown that HSLA steels having higher strength and deformability can be efficiently fabricated without loss of toughness and weldability.[5,8] Because the morphologies of continuously cooled microstructures produced in low carbon or ultra-lowcarbon HSLA steels have been regarded as very complicated,[9–11] on the other hand, systematic studies are seriously needed to elucidate the low-temperature toughening mechanism in terms of microstructure and the associated grain boundary characteristics. In the present study, advanced thermomechanical processing conditions were employed to develop different dual-phase microstructures characterized by grain refinement and formation of various low-temperature transfo
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