Optimization of the chemical composition of supermartensitic stainless steel using thermodynamic calculations

Supermartensitic steels have a unique complex of chemical and mechanical properties, so pipes made of these steels have found their application in the oil and gas industry. However, these steels tend to form a banding structure. The banding structure is the cause of the products properties anisotropy and in addition can lead to the semi-finished products destruction at the production stage. In this work, the steel of the supermartensitic class 10Kh13N3MFB was investigated for its tendency to form a defect “banding” in the structure. For this, thermodynamic calculations and mathematical modeling were carried out using the Thermo-Calc-3.01 software. The following program functions were used: calculation of phase equilibria for the average chemical composition, as well as for compositions with a maximum of austenite-forming elements with a minimum of ferrite-forming elements and with a maximum of ferrite-forming ones with a minimum of austenite-forming elements. An constructed thermodynamic equilibria analysis showed that steel 10Kh13N3MFB tends to form a banded structure. To reduce the likelihood of banding, it is necessary to carry out crystallization and hot rolling of steel only in single-phase states (δ-ferrite and austenite, respectively). Therefore, further calculations in Thermo-Calc were aimed at determining the steel chemical composition, at which the alloy crystallizes in the single-phase δ-region, excluding the appearance of phase segregation, and establishing the temperature range of hot deformation. After analyzing the data obtained, the optimal chemical composition of the steel melting was proposed and the temperature ranges of hot rolling were determined for the average chemical composition with different contents of Ni - 2, 3, 4 wt. %.