Advanced methods of studying the structure when developing technology for production of microalloyed pipe steel

The integrated application of methods of quantitative metallography and mathematical modeling makes it possible to study the patterns of structure formation when varying the technological parameters of rolled steel production. Using the example of niobium microalloyed steel, the application of results of quantitative metallography in mathematical modeling of the microsegregations formation during solidification and cooling of slabs in a continuous casting line is shown. Based on actual and calculated data, the influence of microsegregations on the microstructure formation of rolled products in the combined process of casting and hot rolling, a feature of which is the absence of cooling and heating stages of slabs, is shown. It has been established that during the δ→γ transformation in low-carbon microalloyed steel, microchemical heterogeneity of niobium is formed in the cast structure. It has been determined that one of the factors influencing the formation of heterogeneous prior austenite grains in rolled products is microsegregation. Calculations and experiments have confirmed that increasing the temperature of a continuously cast slab makes it possible to reduce the proportion of niobium carbonitrides precipitated in the slab. The results of the research, introduced into the technology, provided rolled products made of low-carbon microalloyed niobium steel with a thickness of 7–8 mm with impact strength indicators of more than 250 J/cm2 up to a test temperature of –60 °C on longitudinal samples. The shear area fraction is also at a stable and extremely high level of 100 %.