Phase Diagram And As-cast Structure Characteristics Of A High Nitrogen Crmn Austenitic Steel Containing V And Nb

High nitrogen CrMn austenitic steel is a new class of high-performance structuralmaterials.18Mn18Cr0.5N steel used in retaining rings of generators, is a typicalrepresentative of it. However, surface cracking and coarse grains usually occur when theretaining ring blanks are subjected to hot processing, which seriously restricts themanufacture and development of retaining rings. Retaining rings of large generatorsmanufactured by this steel has been mainly dependent on imports so far. Microalloyinghas been widely used in improving the microstructure and properties of high strength lowalloy steel and non-quenched and tempered steel. But there is not much research on itsapplication to high alloy steel (especially high nitrogen CrMn austenitic steel). The effectlaw of microalloying on its as-cast structure and properties is not yet very clear.The pseudo-binary phase diagram, solubility products of carbonitrides in austeniteand as-cast structure characteristics for the18Mn18Cr0.5N steel containing V and Nbwere studied by thermodynamic calculation, differential scanning calorimetry (DSC),optical metallurgy (OM), X-ray diffraction (XRD), scanning electron microscope (SEM),energy diffraction spectrum (EDS), electrolytic extraction, etc. And the steel compositionwas optimized. The main results are as follow:For the tested steel, the liquidus and solidus temperature is1362℃and1320℃,respectively; MX-type microalloyed carbonitride starts to precipitate from liquid, δferrite and austenite at1350℃,1320℃å'Œ1160℃, respectively; ε, M23C6and σ start toprecipitate at957℃,940℃and815℃, respectively; the actual melting peaktemperature is1420℃; MX dissolution peak temperature is1350℃.The as-cast structure consists of γ matrix and precipitates. Most precipitates areirregularly blocky with sharp edges, whose size is mainly5~10μm, and precipitates areusually located in the grain boundary or trigeminal grain boundary. The precipitates canbe described as (Nb,V)4N3.38, which come from liquid and whose volume fraction isabout0.75%.The theoretically calculated and experimentally determined solubility productformulae of precipitate in austenite in the temperature range of1000~1200℃are lg Ksp7593/T0.19and lg Ksp3918/T2.48(T is absolute temperature, K),respectively. At the same temperature, the experimentally determined solubility productis higher than the theoretically calculated one.The atomic ratio of Nb to V in precipitates is about5:1, which means that Nbcontributes more to the formation of precipitates than V. And the content of C is verysmall, indicating that the effect of Nb and V on stabilizing C is slight. The element Nbmainly exists in the form of precipitates, so Nb content should be controlled in designinghigh nitrogen CrMn austenitic steel containing Nb. Only rely on heat treatment can noteasily modify the precipitates size. Severe multi-pass hot deformation can be used toimprove the size and distribution of precipitates.Taking these aspects (the effect of microalloying elements on the δ ferrite existingtemperature range, precipitate characteristics and the solubility product of precipitate in γ)into consideration, the recommended addition of Nb and V is0.1~0.2wt%and0.2~0.3wt%, respectively.