On the causes of instability of mechanical properties of rolled products made of 15Kh13N2 niobium-microalloyed martensitic steel

It was established that during solidification of 15Kh13N2 pre-peritectic martensitic steel, microalloyed to 0.04 % wt. Nb, carbide phases rich in niobium (from 54.2 to 79.4 % wt. Nb) with an average size from 0.2 to 4.2 μm are formed. It is shown that chromium, nickel, vanadium, titanium, silicon and manganese are directly liquated at the crystallization stage, enriching the interdendritic space and forming a solid solution with an increased content of carbide-forming elements. For cast metal after quenching with high tempering, it was determined that the average value of microhardness in the interdendritic space is 226 HV, and in the dendritic crystal zone - 146 HV. The revealed features of crystallization and the individual nature of the distribution of elements between the microliquation zones “dendritic crystal - interdendritic space” in the first approximation allowed for three-dimensional measurement to present the primary structure of 15Kh13N2 martensitic steel as a material in which more plastic dendritic crystals are separated from each other by a hard and brittle matrix. As a result, the banded structure of deformation (metallurgical heredity) recorded by metallographic analysis, consisting of zones that are practically incapable of homogenization, can be reflected in the instability of mechanical property values in the assessment of the quality of rolled products on a macro scale. At the same time, NbC-based carbide phases, detected in the in the eutectic zones last to harden, increase the likelihood of instability of the mechanical property values of rolled products. The selection of the optimal heat treatment mode was performed in laboratory conditions on a 25 mm diameter billet, which was tested in workshop conditions on a 190 mm diameter rolled product with quenching from a temperature of 990 °C and tempering at 710 °C of a 10 t charge. The resulting microstructure provided the required level of strength and resistance to brittle fracture at negative temperatures in rolled products due to the formation of a finely dispersed ferrite-carbide mixture with a certain directionality relative to the “former” martensite needles in each of the “dendritic crystal - interdendritic space” microliquation zones.