Study On Microstructure And Property Of 360MPa Grade Ship Plate Steel

High-strength ship plate steels have been widely used in shipbuilding industry, The mature process of 360MPa grade ship plate steel has been well explored by severel domestic steel factories and the major problem with this steel is to optimize further the process and designation of alloys.The Nb-microalloyed 360MPa grade ship plate steel was rolled experimentally by measuring the recrystallization curve and the continuous cooling transformation curve. The V-microalloyed 360MPa grade ship plate steel was rolled with different rolling process by substituting Nb with cheaperⅤ. Mechanical properties of these steels were measured by using tensile test, Charpy V-notch impact test, microhardness test and the microstructures were observed by OEM, SEM and TEM. The microstructures and itensities were studied which were influnced by different microalloying elements and rolling process. Conclusions by means of systerm study are as follows:(1) The rolling process was optimized according to the recrystallization curve, continuous cooling tranformation curve measured and the experience of manufacturing other grade of steel:the starting temperature of finish rolling is about 880℃,the final temperature is about 850℃,the total reduction of non-recrystallization zone rolling is 70%, controlled cooling technology should be applied after rolling with a cooling velocity of 5℃/s and a final cooling temperature of 450℃.The plates can be cooled naturely after controlled cooling.(2) The ferrite structures are influenced evidently by the shape of austenite grain after rough rolling and the latter is depend on the characteristic of microalloy element and ruduction in pass.When the steels are microalloyed by Nb, the sizes of ferrite grains are not uniform. Microalloyed by V, if the pass ruduction of rough rolling is not large enough the ferrite grains are coarse and the banded structure is dramatic, then if the pass ruduction is large enough the ferrite grains are small and uniform.(3) The precipitation temperature of TiN, V(C,N), Nb(C,N) varis from each other, TiN precipitates at high temperature when the continuous casting is cooled, Its main function is to suppress the coarsening of grains during austenitizing and intervals of rolling passes. V(C,N) precipitates with a dispersive distribution in ferrite grain and keeps a coherency with the matrix. It's main function is to strengthen the ferrite. Nb(C,N) precipitates at the end of rough rolling and during finish rolling, it will suppress the recrystallisation effectively.(4) Strength calculation results show that, in V-microalloyed steel, the intensity can gain an increment of 60MPa with fine ferrite grains caused by large pass reduction of rough rolling.The added 0.03～0.05% V can bring a precipitation strengthening of 50-70MPa. Therefore, relative large pass reduction should be applied when the V-microalloyed ship plate steels is produced, the steel can be cooled naturely after rolling.