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The aim of the present study was to investigate the possibilities of reaching yield strengths beyond 600 MPa for low carbon bainitic hot strip steels by vanadium microalloying together with suitable base alloying. The processing conditions and levels of carbon and nitrogen chosen in this laboratory investigation correspond to those of a typical 8 mm hot strip steel containing 0.04 mass% carbon and 0.010 mass% nitrogen from electric arc furnace practice processed in conventional or compact strip mills. It was found that a base alloying corresponding to 1.4 mass% Mn, 1.0 mass% Cr and 0.25 mass% Mo is required to form a fully bainitic structure after coiling at 400°C. The decisive factors determining the strength of bainitic hot strip steels are firstly the bainite transformation temperature and secondly the extent to which recovery of the densely dislocated bainitic ferrite can be prevented. The results of this study demonstrate that vanadium microalloying effectively prevents the recovery of the bainitic ferrite and leads to retention of the strength of the virgin bainite after coiling. This is primarily due to retardation of recovery by fine vanadium carbonitrides precipitates on dislocations and only to a lesser extent to true precipitation strengthening. With 0.08 mass% V together with 0.010-0.020 mass% N the yield strength lies in the range of 750-790 MPa compared to 680 MPa for a similar reference steel without vanadium. By raising the chromium content to 2 %, yield strengths in the range of 840-880 MPa have been reached. This is attributed to a lowering of the bainite transformation temperature resulting from the higher base alloying.KEY WORDS: high strength steel; bainite; alloying; hot rolling; cooling; microstructure; mechanical properties.* 1 This paper is dedicated to Michael Korchynsky, who recently retired from Stratcor Inc. after 60 years service to steel and vanadium microalloying. The authors of this paper, as well as former colleagues in Stockholm, are indebted to him for his profound engagement in our research and for countless rewarding discussions over the years. * 2 All steel compositions in this paper are expressed in mass%. Alloy Design and Experimental SteelsOut of the normal alloying elements, manganese, molybdenum and chromium are the most effective for raising the hardenability of steel, defined as their ability to prevent formation of ferrite and pearlite during hardening. Their hardenabilities expressed as the Grossman coefficient 6) are all high but decline somewhat in the order Mn, Mo and Cr, viz. 4.10, 3.14 and 2.83. In the present case, the retardation of bainite formation is particularly significant. Here, the coefficient differs more between these three elements, Mn 4.10, Cr 1.16 and Mo 0. 7) Hence, to stimulate bainite formation and at the same time prevent formation of ferrite/pearlite, molybdenum is most effective, followed by Cr and then Mn. In accordance with this, 1.4% Mn, 1.0% Cr and 0.25% Mo were chosen as the standard composition to create an adequate base hardenabilit...

A model for interphase precipitation, with a predictive capacity, is presented. This article deals with its application to V-microalloyed steels. The model rests on an analysis of the growth of the Vdepleted zone ahead of a sheet of V(C,N) particles and the simultaneous advance of the ␥ /␣ interface in which it was nucleated. It is shown that volume diffusion of V cannot explain the observed intersheet spacings and that a faster diffusion process is required. It is postulated that the ␥ /␣ boundary will bow out some time after a sheet of V(C,N) particles has formed in it. Part of the V in the ␥ will then be fed to V(C,N) particles in the sheet by boundary diffusion as the ␥ transforms to ␣. The V content at the front will, thus, be lower than the initial content in the austenite. However, the reduction will be less the further the interface has moved away from the sheet of V(C,N) particles. At a sufficient distance, the V content is again high enough to allow new V(C,N) particles to nucleate, and a new sheet of particles will form. Between the two sheets, there will be a ledge (or superledge) that will advance along the first sheet. The height of the ledge will, thus, be determined by the distance in which V(C,N) particles can again be nucleated. The model exhibits reasonably good agreement with observed values of intersheet spacing, with its temperature dependence and transition from interphase to general precipitation, and with its dependence on C, V, and N content. It also provides physically sound explanations of these dependencies. I. BACKGROUNDsome high-temperature region where the chemical driving force for precipitation is low, nature chooses the sites where THE precipitation of V carbonitrides in V-microalloyed nucleation is energetically most favored, viz., the interface. steels can occur either randomly in ferrite in the wake of At lower temperatures where the driving force is large, we the migrating austenite-ferrite (␥ /␣) interface (general premight expect general nucleation in the ferrite matrix to occur. cipitation) or by interphase precipitation characterized by At high transformation temperatures, ϳ800 ЊC, for typical the development of sheets of particles parallel to the ␥ /␣ compositions of V-microalloyed structural steels, the interface formed repeatedly, with rather regular spacing.interphase precipitation consists of irregularly spaced and Many investigations have shown that, for compositions typioften curved sheets of V(C,N) particles. With decreasing cal of V-alloyed structural steels, the general precipitation temperatures, the occurrence of curved rows of precipitates takes place at lower temperatures, typically below 700 ЊC, diminishes and the dominant mode is regularly spaced, plaand the interphase precipitation takes place at higher nar sheets of particles (Figure 1). Below about 700 ЊC, temperatures.the interphase precipitation is commonly found to be less Figure 1 shows the typical morphology of interphase prefrequent, and random precipitation from supersaturated fercipitation of V...

The present work has concentrated on the roles of vanadium, nitrogen and carbon in controlling the microstructures and strength of steels designed for hot rolled long products. Effects of cooling rate and additional microailoying with titanium have also been included. MicrostructuresOptlcal micrographs of the final structures are presented in Fig. 2 for the steel D22whlch was essentlally typical of all the steels with the higher carbon level (-0.220/0 C), irrespective