Study On Microstructure And Properties Of 9NiCrMo Steel Thick Plate

With the development of ocean engineering equipment continuously toward the direction of large-scale, the thickness of the steel plate was increasing, which brings some problems to high strength and high toughness of thick plate. Thickness made its organization uneven, which inevitably lead to non-uniformity performance of steel. 9NiCrMo steel had high alloy element content (especially the content of Ni element). After passed the modulation process, the steel can obtain very high strength and better low-temperature toughness. At the same time, weldability and corrosion resistance of steel were better. So this kind of 9NiCrMo steel was widely used in shipbuilding, ocean engineering and other fields. Accordingly, the further study of the development micro structure and properties of the high-strength and high-ductile 9NiCrMo steel, and clarify the main factors affecting the strength and toughness of 9NiCrMo,.in order to providing a theoretical basis and guidance for industrial production.Simulate different cooling methods in experiment the laboratory.9NiCrMo steel after dealing with the modulation, the organization was formed by secondary-martensite and reversed-austenite. This organization was different from the other low-Ni steel. The organization of water cooling, oil cooling, and air cooling and buried sand cooling had different bainite content. The organization after water-cooling was a single martensite and had high strength and low-temperature toughness. The organization after oil cooling contains a certain amount of bainite, which formed hybrid organization of the martensite and bainite. This kind of organization made steel have high strength and best low-temperature toughness in four kinds of cooling methods, was 190J. The structure after fast air cooling and buried sand cooling contented more bainitic, even appeared the MA organization, so that the strength and toughness steel was decreased. Analysis on causes of this phenomenon,, we can see that, when there is a certain amount of bainite in the structure, the martensite lath were refined, the length of packet boundary and block boundary increased, then increased of the length of large angle grain boundary to improve the low-temperature toughness of steel. At the same time, we think the smaller the size of block was the effective grain size of control the toughness. The content of reversed austenite in the steel also impacted toughness of steel. And the content of reversed austenite after oil cooling was the most. Under the function of these two factors, we can summarize that low temperature toughness of steel after oil cooling was the best.Analyzing the industrial manufacture of 80mm and 100mm 9NiCrMo thick plate, we can see that industrial production of thick plate had good hardenability. Metallurgical quality also had impact to thick steel plate. The surface, a quarter to the center of steel plate had different types of non-metallic inclusions, which affect the toughness of the steel; and the original austenite grain size of every section was bigger and bigger, which affect the strong and toughness. The surface got a single martensite structure, a quarter and center got martensite/bainite mixed organization. Bainite can refine martensit, and when the volume fraction of bainite was in a certain range, the organization got the best mixed proportion, which made the steel achieve the best toughness. The content of reversed austenite was higher, the low-temperature toughness of steel. In this experiment, the quarter of 80mm and 100mm thick plate location respectively had the best mixed proportion of martensite and bainite mixing proportion, and the content of reversed austenite in the quarter of the location was highest. In summary, under the impact of the metallurgy quality, martensite and bainite mixed organization and reverse austenite, the every section of the plate had a higher strength, and the low temperature toughness of a quarter of plate was best. Meanwhile, under the effect of metallurgy quality and the hardenability, the strong and toughness match of the every section of 80 mm thick plates was better than the corresponding section of 100 mm.