Studies On Technology And Microstructure Performance Of The Nb Micro-alloyed Q345R Plate

The Q345R series steel is the most widely used, and the largest amount of pressurevessel steel, and requires very high comprehensive mechanical properties, especially thelow-temperature impact toughness. For the performance requirements, the solution is to addthe right mnount of alloying elements, using Vacuum treatment to improve the purity of theliquid steel, and the use of controlled rolling and cooling process.Micro-alloying element Niobium has a good strengthening effect on high strength lowalloy steel. The vacuum treatment can reduce the content of hydrogen, nitrogen, oxygen,sulfur, nonmetallic inclusions and other elements in the steel, and control the form ofnonmetallic inclusions, which can effectively improve the cleanliness of the steel. Controlledrolling and controlled cooling technology can significantly improve the ultimate performanceof products. The effect of above three factors to improve the performance of the Q345Rpressure vessel steel is analyzed in this paper. According to the results of the analysis, theactual production experience and the consideration of lower cost，the suitable controlledrolling and controlled cooling process system of Niobium micro-alloyed Q345R plate hasbeen found out.The Q345R of pressure vessel steel were piloted in accordance with the process systemidentified above in a factory. Finally, through tensile test and impact test, the phase diagramsof microstructure of the specimen and test data are derived and a comprehensive analysis ofits structures and performance are carried out. The results showed that, the tempered sorbitewhich consists of ferrite and pearlite formed in Q345R steel after the quenching andtempering treatment. The ferrite precipitated along the austenite grain boundary and thepearlite in spherical shape occurred atfer elevated temperature tempering treatment. Themicrostructures along crossing and rolling direction were uniform. The amount ofnonmetallic inclusions was less and the distribution was homogeneous in morphology. TheNiobium micro-alloyed Q345R steel was produced by using RH vacuum treatment andreasonable control rolling and controlled cooling process. Compared with the standards andsimilar products of other manufacturers, Q345R steel possessed higher strength, goodductility and low temperature impact properties, for1) its yield strength was more than360MPa,2) the elongation rate was over30%,3) the minimum impact energy in0°C was142J,4) lateral impact energy in-60°C was beyond45J.