Influence of Cr and Cr+Nb on the interphase precipitation and the mechanical properties of microalloyed steels for the automotive industry

Steels for the car industry are engineered to enhance the strength (yield and ultimate), total elongation, and weldability to lighten cars and minimize fuel consumption. This can be achieved by combining ferritic matrixes toughen with nanoprecipitates formed during the austenite-to-ferrite transformation. Samples of two alloys were initially tested, V-Mo and Cr-V-Mo, which were claimed to have exceptional mechanical properties but with no complete information on the processing. At the Department of Materials Science and Engineering of the University of Sheffield, microalloyed steels V-Mo-0.4 and Cr-V-Mo-0.4 were produced to evaluate a large hot-working strain per pass before the isothermal transformation and to observe the prior-austenite grain behaviour. Results showed superior mechanical properties with a microstructure composed of a majority of ferrite. Values of σ_Y≈1GPa, σ_Ult≈1,3 GPa with total elongation of about ~14% were found for steels with no Cr additions, and σ_Y≈1GPa, σ_Ult≈1,3 GPa with total elongation of ~17% for steels with Cr addition. Another three alloys were manufactured at the University of Sheffield to evaluate lower magnitudes of strain per pass on the same compositions and add a new one (Cr-Nb-V-0.2) because of the well-known effect of Nb on the grain refinement of the final microstructure plus the addition of Cr which showed a higher percentage of ferrite after the isothermal transformation, and better mechanical properties than steel free of Cr. The change in the strain per pass during the hot-rolling produces a drop in the final mechanical properties of about 200 MPa for the yield stress, 300 MPa for the ultimate stress and about 3% in the total elongation. Also, less ferrite volume fraction was found in these last alloys. To clarify that, identification and quantification of the interphase precipitation within the ferrite were performed followed by the calculation of the strength contributions. It was established that steels with higher strain per pass during the hot-working produce more volume fraction of interphase precipitation, of dislocation density and maximize the solid solution strengthening contribution. Also, the addition of Cr showed higher precipitation of vanadium carbides.