Caratterizzazione microstrutturale e meccanica di un acciaio Q&P al Silicio e Molibdeno

The increasing demand by car producers for materials with high resistance and ductility is leading to the development of new advanced high strength steels (AHSS) in order to develop light cars, to reduce fuel consumption and to increase safety passenger with higher crash resistance. Recently a new family of AHSS is under study: quenching and partitioning (Q&P), which by an innovative heat treatment would lead to tensile strength above 1000-1200MPa and elongation higher than 10%. The Q&P process is made mainly in two step: quenching to a temperature between martensite start (Ms) and martensite finish (Mf) temperatures and partitioning to a relatively low temperature in order to promote carbon diffusion from carbon supersaturated martensite to retained austenite (in this way it's leading to stabilization of the austenite phase). In the composition of Q&P steel there are always elements provide enough hardenability (as Mn, Mo) and some that suppress the transformation of austenite in to ferrite and/or bainite during quenching. Another important requirement is to suppress completely the carbide precipitation during partitioning heat treatment, this is possible alloying elements such Si and/or Al (carbide formation retarding). In this study, low carbon steel alloyed with Mn, Si and Mo, was subjected to different thermal Q&P cycles by the thermomechanical simulator (changing annealing temperature always in intercritical field, quenching and partitioning temperatures and times) with the aim to evaluate the influence of process parameters on microstructure and consequently on mechanical properties. This work is part of a wider study about Q&P process which regards research on different compositions and thermal parameters. The critical point of steel was measured with "Differential Thermal Analysis" (DTA); the microstructure was investigated by optical and electronic microscopy (SEM); volume fraction of austenite retained at room temperature was measured by X rays diffraction; HV hardness measurements were performed. All samples studied have very fine microstructure, the mainly phases are: ferrite (intercritical and epitaxial), lath martensite, retained austenite and austenite with different transformation rate. In some samples phases unwanted were found as tempered martensite and bainite: their presence means that reactions competitive to partinioning were happened. Morphology and the amount of all phases depend by thermal parameters. Considering the numbers of variables of Q&P process, the multiplicity and the different morphologies of phases, the different amount and transformation rate of “retained” austenite, it's very difficult to establish a significant correlation at the present state of research among hardness, microstructure and mechanical properties. Transmission electron microscopy (TEM) examinations and identification of present phases by Electron back scattering diffraction (EBSD) are in progress and they could be useful to clarify. Anyway, about the first three cycles (they differ only for soaking temperature) it's possible to observe that: the third cycle has the highest tensile strength (1294 N/mm2), in agreement with hardness and the presence in the microstructure of martensite almost with rods morphology, lower amount of tempered martensite and ferrite (higher soaking temperature) than cycles 1 and 2. Unfortunately, the ratio Rp0.2/Rm is too high and elongation is not satisfying. The QP1 sample has yield strength and elongation modest (in agreement with copious amounts of tempered martensite and a few areas of retained austenite). Cycles 2 shows lower Rm than QP3 but yield strength and elongation are sufficient: all of these characteristics are in accordance with hardness, amount of retained austenite determined, martensite in forms of rods and plates and higher amount of ferrite and tempered martensite than QP3. The samples subjected to fourth and fifth cycles have satisfactory mechanical characteristics: in particular QP4 sample has high tensile strength, good Rp0.2/Rm ratio and elongation. About the last one, elongation is lower than QP5 sample, probably because of connection with the lower amount of retained austenite. There are some analogies between QP2 and QP5 experimental results, it follows that the different quenching temperature and the little difference between the two partitioning temperatures don't have influenced the mechanical behaviour. It's impossible to correlate the retained austenite fraction of all samples with mechanical behavior, but it was observed a correlation between mechanical characteristics and the presence of abundant "packets" (alternated platelets of lath martensite and austenite/ ferrite/bainite). In this case, austenite is close to martensite (supersaturated solution of C), so especially it's enriched of Carbon as it's useful for Q&P effect, but X-Ray diffraction can't detect only these areas because it measure the total amount of austenite. Overall, all mechanical properties of samples after 2, 4 and 5 cycles are acceptable (the only one analogies of treatment is intermediate soaking temperature). In particular, after cycle 4 it was obtained high tensile strength, similar to the value of QP3, without lacking in elongation and Rp0.2/Rm ratio.