Mechanical Properties of TRIP Steel Microalloyed with Ti
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INTRODUCTION
IN recent years, a considerable research effort has been made in the field of advanced highstrength multiphase steel for automotive applications. The primary aim is to reduce the car weight by an increased use of high-strength steel.[1] TRansformation-Induced Plasticity (TRIP) steel offers promising possibilities for improved passive safety in specific applications, e.g., for B-pillars, which require a superior crash worthiness. Steels containing typically 0.2 pct C-1.5 pct Mn-1.5 pct Si (mass pct) can be processed in such a manner that a triple-phase microstructure and a pronounced TRIP-aided mechanical behavior can be obtained.[2] Past studies have, however, revealed large difficulties in hot dip galvanizing of standard high-Si coldrolled CMnSi TRIP steel.[3] Si is also known to cause a low ductility level in the as-cast condition and an increase in the ductile-to-brittle transition temperature. These are the main reasons to keep the Si content of TRIP steels low. This can be achieved by the partial or full substitution of Si by elements with similar properties.[4] It is well known that Al and P, in addition to being strong ferrite stabilizers, inhibit the formation of cementite during austempering.[1,5–7] The CMnSiAlP TRIP steel concept[5] has, therefore, been proposed to achieve a combination of a high tensile strength close to 900 MPa and a total elongation of about 28 pct. For reasons of weldability, the C content has to be kept as low as possible (£0.25 pct). Microalloying with Ti, Nb, or V is one of the methods used to achieve a further increase of the strength of CMnAlSiP TRIP steel, without deteriorating its weldability by an increase of the C content.[8–13] By doing so, DANIEL KRIZAN, Senior Researcher, is with the Research and Development Center for Cold Rolled Strip, VoestAlpine Stahl GmbH, Linz, Austria. BRUNO C. DE COOMAN, Professor, is with the Graduate Institute of Ferrous Metallurgy, Pohang University of Science and Technology, Pohang, South Korea, and Director with the Materials Design Laboratory, Pohang University of Science and Technology, Pohang, South Korea. Contact e-mail: decooman@ postech.c.kr Manuscript submitted October 1, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
a tensile strength level above 980 MPa, combined with an elongation larger than 20 pct, can be achieved.[14] Nb in solid solution is well known to retard both the recrystallization during hot deformation and the austenite-to-ferrite transformation in CMnSi cold-rolled TRIP steels. In addition, Nb combines with C and N to form small precipitates. These retard recrystallization and grain growth, and result in precipitate strengthening.[9] In addition, Nb has been reported to exert a pronounced influence on the grain size development of both ferrite and austenite and to retard the bainite formation. It increases the C content of the retained austenite, and reduces the martensite start temperature Ms.[8,13] V can also be used to control the transformation behavior of TRIP steels, albeit during the later processing st
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