Application Of Simulation Methods On Research Of Bainite Steels

Simulation methods are applied widely for composition and process design during recent years. In the present work, combination of simulation methods including numerical simulation, artificial neural network and small-sample physical simulation were applied to guide composition and process design of bainite forging and casting with large size and 1500MPa grade super high strength steel, aiming at investigating the application of simulation methods on developing bainite steels with good strength and toughness. In addition, the strengthening and toughening mechanism of studied materials and ultra high cycle fatigue behaviors were studied based on the experimental results.The experimental results show that the structure and properties of studied bainite steels can be predicted correctly based on temperature field distribution during heat treatment calculated by NSHT heat treatment numerical simulation program, CCT diagram predicted by artificial neural network and heat treatment process simulation results obtained by small sample control cooling. Then the design of composition and heat treatment procedure can be guided adequately on the basis of above prediction results.The temperature field distribution of 400mm*800mm*800mm heavy section steel forging for reactor pressure vessel was numerical simulated by NSHT program. Experimental materials were originally designed by tungsten alloying on the adjusted composition of SA508-3 and their CCT diagrams were predicted by artificial neural network. Combing above results the structure and properties corresponding to arbitrary sites of the forging were predicted. Small sample control cooling was applied to simulate quenching of the typical sites of the forging to obtain information of structure and properties. Composition and heat treatment process were optimized based on above results. Experimental results show that satisfactory properties and property homogeneity with the forging section were obtained by tungsten alloying on SA508-3 after quenching and high temperature tempering, satisfying standards for RPV large size forging.The air cooling procedure after austenizing was numerical simulated for Mn-Si-Cr bainite steel plate cast with 60mm thickness. The structure of studied steels with different composition was predicted based on combination of numerical simulation results and CCT diagram, and the properties of typical site of the cast were predicted by small sample simulating heat treatment. Combining above results the hardenability of experimental materials for 60mm-thick cast were analyzed. According to prediction results the composition and heat treatment procedure were designed and good hardenability was obtained for studied steels. The prediction results are in consistence with measurement ones.For 1500MPa grade high strength steel with duplex phase of carbide free bainite/martensite(CFB/M) microstructure, the modified CCT diagram was established by small samples to simulate the air cooling of steel rods with different size. Thermal-mechanical treatments under different deformation temperature then control cooling were carried out by small samples. Continuous cooling transformation mechanism of experimental steel was studied, and the influences of cooling rate and deformation temperature on structure and properties were discussed. The results provide guidance for heat treatment design to obtain CFB/M structure with high strength and toughness. The CFB/M microstructure was obtained according to experimental results and the ultra high cycle fatigue behavior was investigated.