| As the development of shipbuilding, the ship craft trends to be larger and lighter, so the common strength ship-boards couldn't meet the needs of modern ship, and the proportion of high strength ship-boards sharply increases in the field of shipbuilding. Beside high strength, the high strength ship-board requires good impact ductility at low temperature. Traditionally it adds Nb or Nb &Ti for fulfilling the grain refinement, so high strength and impact ductility will be obtained. However the steels with the addition of V could avoid transversal crack, otherwise, V is wide distributed, beside, China is one of there V production countries in the world. So the steel exhibit cost benefits as adding only V and Ti.Firstly, the thermal-mechanical simulation tests are carried out on THERMECMASTOR-Z thermal simulator and the static transformation temperature and dynamic CCT diagrams are then obtained. Secondly, the main chemical composition design of steel plate is low carbon-low Mn-VN, then according to the result of thermal-mechanical simulation tests, there different controlled rolling and controlled cooling experiments have been carried out on microalloyed steel with varied V and N. And the final VN composition is determined by means of comparing the mechanical properties. Then the microstructure and properties of steel plates at the state of controlled rolling & controlled cooling were tested. The paper's main conclusions are as follow: Static transformation temperatures Ar1 and Ar3 decreased with increasing of contents of Neff, however, Ac1 and Ac3 did not change; When cooling rate is down to 4℃/s, microstructures in these micro-alloyed steels are composed of ferrite and pearlite at room temperature. And cooling rate over 4℃/s, bainite began to appear. When cooled above 9℃/s, there are no pearlite in the microstructures which consists only of bainite and ferrite; The steels have optimum mechanical properties by the addition of content of V up to 0.08%, and N up to 0.008%,it is found that the tensile strength is above 390MPa, and the longitudinal notched bar impact strength is about 200J at -40℃in RCR+ACC process; The optimal technological parameter: the heating temperature is about 1180±20℃, the finishing temperature is 1050±10℃and the final cooling temperature is 550±20℃, the cooling rate is controlled between 10 and 15℃/s, under such controlled rolling and controlled cooling processing, the steels have the most precipitate. The yield strength and tensile strength are 457MPa, 573MPa individually, the elongation percentage and reduction of area are 27%, 74%. So plasticity satisfied the high strength ship-board. And the transverse impact toughness is also acceptable, it is about 45J at -60℃; Precipitations of second-phase are found on the grain boundary, sub-boundary and on the matrix of ferrite. The main precipitations of VC and TiN have been characterized using transmission electron microscopic (TEM). When it is at a low rolling temperature, the grains of precipitations will be finer. |
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