Effect of Vanadium and Strain Rate on Hot Ductility of Low-Carbon Microalloyed Steels

Hot tensile tests were conducted in this study to investigate the effect of strain rate (10−3 and 10 s−1) and vanadium content (0.029 and 0.047 wt.%) on the hot ductility of low-carbon microalloyed steels. The results indicate that a hot ductility trough appears at a low strain rate (10−3 s−1) because of the sufficient time for ferrite transformation and the growth of second particles, but it disappears at a high strain rate (10 s−1). The hot ductility is improved with the increase in strain rate at 700 °C or higher temperatures. In addition, with the increase in vanadium content, the large amounts of precipitate and increased ferrite transformation result in poor hot ductility of steels fractured at a low temperature range (600~900 °C). However, when the steel is fractured at a high temperature range (1000~1200 °C), more vanadium in the solid solution in the austenite inhibits the growth of parental austenite grains and results in grain refinement strengthening, slightly improving the hot ductility.

Visualization of Prior Austenite Grain Boundaries in Low-Carbon Steels

Knowledge of the size of the prior austenite grain is of key importance. If abnormal grain growth occurs during austenitization, the resultant inhomogeneous microstructure may negatively affect the strength and toughness properties of the final product. The visualization of prior austenite grain boundaries with an etchant based on picric acid has been applied for years. Despite this long-time experience, it is often challenging to achieve sufficiently good visualization of prior austenite grain boundaries in many steel grades, especially low-carbon steels. This work will study the effect of the cooling rate from austenitizing temperature down to room temperature, of the subsequent tempering treatment and the etchant on the visualization of prior austenite grain boundaries in a low-carbon microalloyed steel. All these parameters have an impact on the etching result. A suitable etchant for the visualization of prior austenite grain boundaries in a low-carbon microalloyed steel could be found.

Features of the formation of structure and mechanical properties in rolled products of various thicknesses from low-carbon microalloyed steel produced by casting and rolling complex

The article considers results of the study of microstructure parameters effect on the impact strength in temperature range from 0 to –80 °C in 20 °C increments of Charpy samples with a sharp stress concentrator and Mesnager test pieces with a circular stress concentrator from rolled coils of low-carbon microalloyed steel with various thicknesses. The used roll products were produced in conditions of JSC “Vyksa Metallurgical Plant”. The tests were performed using optical and scanning electron microscopy. It is shown that with the same chemical composition and thermomechanical treatment modes, the metal of smaller thickness (6, 8 mm) is characterized by higher strength properties (on average, by 10 MPa for temporary resistance, by 30 MPa for yield strength) and a margin for viscous properties at negative temperatures at close values of grain score and average grain size corresponding to 10 – 11 numbers according to the State standard GOST 5639. The metal with a thickness of 12 mm has the lowest level of cold resistance, and the temperature of brittle transition is minus 50 °C. Structure of rolled products of various thicknesses has a variation in grain size. Rolled metal of smaller thicknesses have a smaller grains corresponding to number 14, rolled metal of larger thicknesses has a larger grains corresponding to number 8. By conducting electron microscopic studies using the backscattered electron method, it was found that a greater number of large-angle boundaries, which are barriers for brittle cracks propagation, are observed in the 6, 8 mm thick rolled products. The constructed orientation maps of the microstructure showed the presence of pronounced deformation texture corresponding to the orientations <110>||RD (rolling direction) and (<113>...<112>)||RD for rolled products with a thickness of 6 mm.