Studies On Theory And Technology Of Microalloying Of0Cr(13～16)Ni(4,5)Mo(1,2)Super Martensitic Stainless Steel

0Cr(13-16)Ni(4,5)Mo(1,2) series of super low carbon martensitic stainless steels (super martensitic stainless steels) with higher strength, toughness, good cavitation and water erosion resistance and weldability are increasingly applied in construction of nuclear power engineering, large turbine, high pressure feed water pump and oil country tubular goods. The duplex microstrucure of tempered lath martensite with reversed austenite distributed along martensitic lath boundaries and within laths is obtained after normalizing and subsequent tempering above Acl temperature. Although the presence of reversed austenite and decerase in C content impart this series of stainless steel many good performance, its application is still limited by its relative low strength, insufficient corrosion resistance and excessive volune fraction of δ ferrite network. The present study attempts to improve the microstructure, mechanical and corrosion properties of super martensitic stainless steel through microalloying.Based on the chemical compositions of0Cr(13-16)Ni(4,5)Mo(1,2) martensitic stainless steel, the present study melted the steels with different content of N, the steels with high V and high N, the steels microalloyed by Nb and the steels microalloyed with Nb, V and N. The solubility of nitrogen in martensitic stainless steel0Crl6Ni5Mo when smelted by vacuum induction furnace under protective atmosphere of near normal pressure was investigated. The effects of the protective gas type and the nitrided ferrochromium addition level on nitrogen content in the high nitrogen martensitic stainless steel were discussed. Through tempering the tested steels in a wider temperature range, the effects of different methods of microalloying on phase composition and evolution of tempered microstructure were investigated by charactering the tested steel samples using laser scanning confocal microscope (LSCM), scanning and transmission electron microscope (SEM and TEM) and X-ray diffraction (XRD) and combining the calculation of phase diagram. The influence of different methods of microalloying on mechanical properties and corrosion resistance of super martensitic stainless steel was examined by conducting tensile, Chary impact and electron chemical polarization tests, and the effect mechanism was further discussed. Finally, one method of microalloying which can synthetically improve the mechanical properties and corrosion resistance of super martensitic stainless steel was found. The Conclusion was obtained as follows:1. The solubility of nitrogen in martensitic stainless steel0Crl6Ni5Mo when smelted by vacuum induction furnace under nitrogen protective atmosphere of near normal pressure is about0.18%(in mass, and it is in mass below when not specified), which is close to the value calculated theoretically, and does not change with the increasing amount of nitrided ferrochromium addition. The solubility of nitrogen in martensitic stainless steel0Crl6Ni5Mo when smelted by vacuum induction furnace under argon protective atmosphere of near normal pressure is lower than the theoretical level, and increases with the amount of nitrided ferrochromium addition.2. N as strong austenite stabilizing element, adding0.16%can completely suppress the occurrence of8ferrite, while addition of0.1%can decrease the amount of8ferrite to a level lower than1%in area fraction in0Crl5Ni5Mo martensitic stainless steel. Meanwhile, addition of N increases the volume fraction of retained austenite in martensitic stainless steel after normalizing. Tempering the martensitic stainless steels with and without N at lower temperature range of350℃～500℃results in secondary hardening behavior. The secondary hardening effect in steels with N is stronger that in the steels without N, while they both peaks at450℃. M23C6type of carbide occurs at prior austenite grain boundaries, inter-lath boundaries and within laths in0Cr(13-16)Ni(4-5)Mo martensitic stainless steels without N, while Cr2N starts to form at inter-lath boundaries and within laths in steels with N after tempering above500℃. The coherent orientation relationship of Cr2N with martensite matrix was revealed as [111]M//[001]Cr2N,(011)M//(100)Cr2N. Cr2N grows up gradually with increasing tempering temperature and loses coherent relationship with matrix, weakening the precipitate strengthening effect.3. Tempering the0Cr(13-16)Ni(4,5)Mo(1,2) series of martensitic stainless steel above Acl temperature (about525℃) promotes the formation of reversed austenite along prior austenite grain boundaries, inter-lath boundaries and within laths by nucleation-diffusion mechanism. The reversed austenite can be partially retained upon oiling cooling to room temperature after tempering due to its higher stability, which relates to its chemical composition and tempering temperature. The presence of reversed austenite and the increase in its volume fraction causes the decrease in strength of the martensitic stainless steel at room temperature. With increasing tempering temperature, the volume fraction of reversed austenite increases and reversed austenite partially grows up to big block. The volme fraction of reversed austenite decreases with further incease in tempering temperature due to the secondary retransformation of reversed austenite upon cooling after tempering caused by the decrease in stability of reversed austenite. N retards the formation of reversed austenite during tempering.4. Tempering above Acl temperature promotes simultaneously the precipitation of large amount of rod-like Cr2N at martensitic inter-lath boundaries and within laths in etiher the0Crl6Ni5Mo steel with N content of0.1%and0.12%, or the0Crl3Ni4Mo steel with only0.04%N. The coarsing of Cr2N with increasing tempering temperature not only weakens the precipitate strengthening effect, but also impairs the Charpy impact toughness of the martensitic stainless steel. More importantly, the corrosion resistance of the martensitic stainless steel is decreased by Cr2N precipitation, which causes depletion of Cr in vicinity of precipitates below the level required to form protective oxide layer.5. Although the addition of0.12%V into0Crl6Ni5Mo steel with0.1%N further retards the formation of reversed austenite, the toughness of the steel is effectively increased due to the variation of type of precipitates(Cr2Nâ†'VN). However, VN is prone to coarsen, causing its strengthening effect weak. Further addition of0.04%Nb can apparently increase the strength, but the coarsing of NbN impairs the toughness of the steel seriously. Moreover, because the N content in martensitic stainless is too high, addition of micro alloying element of Nb and V is not adequate to stabilize the N. The excess N combine with Cr to form Cr2N during tempering. Temperig above550℃promotes not only the precipitation of globular Nb and V carbo-nitride, but also the occurrence of Cr2N still due to the excessive N content in the steel microalloyed with0.025%Nb,0.09%V and0.06%N. Large amount of these precipitates nucleate and coarse at martensitic inter-lath boundaries,6. Decreasing the N content in0Crl3Ni5Mo(1,2) martensitic stainless steel to0.01%can effectively decrease the volume fraction of Cr rich precipitates formed during tempering above550℃. Further adding0.1%Nb can completely inhibits the occurrence of precipitation of M23C6and M7C3in0Crl3Ni5Mo(1-2) martensitic stainless steel during tempering above 550℃. The dynamical potential for precipitation of NbN is greatly reduced by decreasing the N content. Austenite grain refinement, which is beneficial for increasing the toughness of the steel, can be achieved through proper control of rolling and solution treatment temperature. Nano-scale Nb(C, N) precipitates occur at inter-lath boundaries and within martensite matrix in0Crl3Ni5Mo(1-2) martensitic stainless steel with Nb and low N, not only effectively increasing the strength, but also not causing much decrease in toughness. Most importantly, the depletion of Cr from matrix by precipitation of Cr rich precipitates is inhibited by suppression of occurrence of Cr rich precipitates by Nb. Thus, the corrosion resistance of the0Crl3Ni5Mo(1-2) martensitic stainless steel with Nb and low N is effectively improved to some extent. Among the tempering temperatures varied from550℃to700℃,0Crl3Ni5Mo(1-2) martensitic stainless steel with Nb and low N tempered at600℃exhibits excellent combined mechanical and corrosion properties, with yield strength of930-960MPa, elongation of19-20%and Charpy impact toughness of160J.