Nitrogen-containing steels and methods of their production

Abstract

The systems of alloying Fe – Cr – N, Fe – Cr – Mn – N, Fe – Cr – Ni – Mn – N, Fe – Cr – Ni – N are considered and attention is paid to the compositions of developed or already used steels. Mechanical, operational and other properties of a number of modern nitrogenalloyed steels with an equilibrium and super-equilibrium concentration of nitrogen are considered. The optimal intervals of their doping with nitrogen are given and the contribution of nitrogen to formation of the structural-phase state and the complex of their properties is estimated. For example, in the Fe – Cr – N system of practical interest are the austenitic steels Fe – (21 – 22) Cr – (1.1 – 1.3) N, solid solution hardened, technologically plastic, with a yield strength of 800 MPa and high corrosion resistance. Corrosion-resistant high-strength austenitic steels are in demand of the Fe – Cr – Mn – N system, such as Fe – (18 – 21) Mn – (14 – 22) Cr – (0.4 – >0.6) N, in which nickel as austenite-forming element is completely or partially replaced by manganese and nitrogen. Examples of steels of the Fe – Cr – Mn – Ni – N system with high service properties are given. Since alloying steels with nitrogen requires an assessment of the maximum possible level of its content (solubility) in the metal and the creation of conditions for the introduction of nitrogen into the liquid metal and its preservation in the solid metal, attention is paid to: calculations of nitrogen solubility, taking into account the effect on it of the chemical composition of steel, temperature and pressure at which alloying occurs; the concept of compositionally stable nitrogen content and the coefficient of compositional stability. The main methods of production of nitrided steels are considered. The quality of metal in open smelting and after refining electro-slag remelting (ESR) is compared. The latter makes it possible to preserve nitrogen during the remelting of nitrided steels, to ensure its uniform distribution along the height and cross-section of the ingot, to obtain ingots with a good surface and a dense structure with a radialaxial orientation and without shrinkage defects. The advantages of the method of electroslag remelting under pressure (PESR) are noted – the ability to obtain high-quality metal with a nitrogen content above its equilibrium concentration (under standard conditions) and to provide an almost ideal ecology of production.