The Hot Workability of Cromanite<sup>TM</sup>, a High Nitrogen Austenitic Stainless Steel

Abstract

Columbus Joint Venture produces a High Nitrogen Austenitic Stainless Steel, called CROMANITE (EN 10 088 type 1 3820), containing a nominal 19% chromium, 10% manganese, 0.5% nitrogen and less than 1% nickel. At hot working temperatures, High Nitrogen Austenitic Stainless Steels are known to have a lower ductility than conventional austenitic stainless steels The factors which affect the hot workability of CROMANITE under production conditions have been studied and the hot working temperature range for this steel has been optimised From the literature it is known that high nitrogen steels are prone to edge cracking, if the δ-ferrite content is too high The effect of composition and slab reheat temperature on δ-ferrite content was determined and it was found that the δ-ferrite content varies with the slab reheat temperature. It was found that in some alloys, the amount of δ-ferrite increased substantially if the slab was heated above 1200°C In one case, the δ-ferrite increased from 3% at 1100°C to 30% at 1300°C. In addition to δ-ferrite content, it was found that chromium had a detrimental effect on hot ductility, while intermediate amounts of boron were beneficial With the production of CROMANITE, the nitrogen solubility, which has a strong affect on the δ-ferrite content, is determined principally by the Cr and Mn levels, the liquid metal temperature in the ladle and the nitrogen partial pressure at the slag-metal interface. From an understanding of these effects, the composition is selected to ensure that at least the minimum specified nitrogen content is obtained and that the hot workability of the steel is maximised Optimisation of the chemistry also takes into account alloying costs and steelmaking constraints The above has resulted in successfully hot rolling CROMANITE in gauges ranging from 50mm down to 3mm on a production scale.