Microstructure Evolution And Variation Law Of Impact Toughness Of Ultra-High Strength F690 Steel Use For Hull Structures

Ultra-high strength,highly ductile,heavy steel plates are required to construct large ships for efficient transportation and large drilling platforms to be constructed in deep sea areas.The F690 grade steel will be an important part of the hull structure and offshore platform in future.In present,domestic and foreign researchers mainly concentrated in the effects of chemical composition and heat treatment process on the microstructures and mechanical properties for the yield strength of 690 MPa steels with high strength and good weldability.The effects of different microstructures obtained under different austenitizing temperatures on the impact toughness,impact fracture behavior and cleavage fracture mechanism of the materials are rarely reported.However,it's crucial for the design of the heat treatment process and their applications in hull structure safety.A low carbon MnCrMoNiCu alloyed steel has been developed to produce highly ductile thick plates for offshore structure and shipbuilding.The present work was to reveal how austenitizing temperature affect the microstructure,impact toughness,impact fracture behavior and cleavage fracture mechanism of the steel.For that,a series of thermal treatments on the test steel were conducted employing a Gleeble 3800 thermomechanical simulator.These involved reheating samples at different temperatures(1 168?1623 K(895?1350?))generating different prior austenite sizes,respectively,followed by a continuous cooling transformation process.Charpy V notch(CVN)toughness of the samples were tested.The microstructural evolution on the austenite sizes was studied,impact fracture feature was characterized,the critical event of cleavage fracture was identified.The CVN toughness was maximized in the samples which were reheated at 1273 K(1000?)containing refined lathlike bainite(LB).Finally,the fundamental causes of the varied CVN treated under different austenitizing temperatures were elucidated.The results shows that,the rise of austenitizing temperature results in the average austenite grains coarsened being:15.8?m,17.9?m,22?m,59?m and 233.1?m,and correspondingly makes the average packets larger being:13.4?m(bainite packet),16.9?m(bainite packet),15.5?m(bainite packet),28.2?m(martensite packet)and 73.6?m(martensite packet)for the austenitizing temperature changes being 1168,1223,1273,1473 and 1623K(895,950,1000,1200 and 1350?).The resultant microstructures after the continuous cooling transformation(CCT)are evolved from mixture of granular bainite and lath bainite,lath bainite,to lath martensite as the isothermal for the five austenitizing temperatures.The rise of impact toughness for the austenitizing temperature changes being 1168,1223 and 1273K,and then decreases with the increase of austenitizing temperature.The reason of the difference in the toughness of the specimens under austenitizing at different temperatures is as follows:(1)The macroscopic results show that there are some differences in terms of the microstructure types,the size and number of martensite-austenite components,the grain size and the width of the lath;(2)Microscopically,the specimens austenitized at 1000? have the highest critical cleavage stress ?f,followed by austenitized samples at other temperatures.In the impact fracture process,grain boundary,bainite(or martensite)packet boundaries,internal lath of bainite(or martensite)packets with large angle grain orientation,can significantly hinder the crack propagation.The greater of the number,the higher of the impact absorbed energy.The critical event of cleavage fracture in the notch for the testes steel is the extension of the crack through the boundary of bainite(or martensite),and the size of the largest bainite(or martensite)packets characterizes the critical event size.