The Influence of Vanadium on Ferrite and Bainite Formation in a Medium Carbon Steel
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THE use of vanadium in engineering steels can be traced to the early 20th century, when its benefits for automotive application were rapidly recognized.[1,2] A century later, vanadium microalloyed ferrite-pearlite steels have become standard for a large variety of high-performance forged components. In the case of automotive crankshafts, for example, addition of 0.1 to 0.15 pct V to a 38MnSi5 (0.38 pct C-1.5 pct Mn-0.6 pct Si wt pct) steel grade raises the yield strength by over 100 MPa. This dramatic increase is due to the interphase precipitation phenomenon, which naturally occurs as ferrite forms from austenite upon cooling,[1] and through which a very fine dispersion of V(C,N) is obtained without the need for any complex heat-treatment. A detailed review of the relative stability of carbides, nitrides, and carbonitrides formed with vanadium, together with relevant thermodynamic data, can be found in Reference 1.
T. SOURMAIL is with Asco Industries Research (CREAS), BP 70045, Hagondange, France. Contact e-mail: [email protected] C. GARCIA-MATEO and F.G. CABALLERO are with the Department of Physical Metallurgy, National Center for Metallurgical Research (CENIM-CSIC), Madrid, Spain. S. CAZOTTES and T. EPICIER are with the Univ. Lyon, INSA-Lyon, MATEIS, CNRS UMR 5510, University Claude Bernard Lyon I, 69621 Villeurbanne, France. F. DANOIX is with Normandie Univ, UNIROUEN, INSA Rouen, CNRS, Groupe de Physique des Mate´riaux, 76000 Rouen, France. D. MILBOURN is with Vanitec Ltd., Suite 5, 90 Calvery Road, Tunbridge Wells, Kent TN1 2UN, UK. Manuscript submitted October 8, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
It is thus possible to achieve yield strength (YS) and ultimate tensile strength (UTS) of typically 620 and 900 MPa, respectively, in as-forged parts after natural cooling. In the case of smaller components such as connecting rods, higher percentages of vanadium (0.3 to 0.35 wt pct) have led to the development of one of the highest performance forging steel grades (36MnV4,[3]), with YS and UTS as high as 910 and 1180 MPa, respectively, while retaining a fully ferrite-pearlite microstructure. Nevertheless, mechanical properties usually remain below or at 1000 MPa in most cases, and the loss in ductility which comes with higher tensile strength is not acceptable for all applications. Quenching and tempering are often used to achieve tensile strength of 1000 MPa and above. Nonetheless, interest is growing both among steel-makers and end users for so-called bainitic steel grades.[4–11] These are designed to achieve a microstructure consisting mainly of bainite, without the need for specific heat-treatment. That is to say, the microstructure is obtained through natural cooling either after hot forging or hot rolling. These grades often exhibit mechanical properties on par with or in excess of those offered by quenched and tempered forging steels. Because interphase precipitation does not occur during formation of bainite, addition of vanadium to bainitic special steels is not as commonly use
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