Study On The Heat Treatment Process And Microstructue Evolution Of A Quenched And Tempered Steel With High Strength And Ductility

The quenched and tempered(QT)steel used in mooring chain plays a fixed role in ocean engineering and is an important part of mooring positioning system of marine engineering facilities.Presently,the most advanced QT steel certified by the classification societies has the lowest tensile strength of 1000 MPa and the impact energy of 58 J(-20?),which is not enough to satisfy the need of some deepwater engineerings.So developing a new high strength and ductility QT steel is very necessary and has a profound practical meaning and a huge economic value in the fields of deep water development and ocean transportation.Therefore,in view of the deepwater drilling platform for performance of the advanced mooring chain QT steel,this study conducted a research work for developing a new QT steel with the tensile strength of 1110 MPa and the impact energy of 110 J(-20?).Based on the optimization of Cr and Ni compositions of alloy elements in steel,this paper explored the heat treatment process of new mooring chain steel.A new QT steel with a tensile strength of 1162 MPa and an impact energy of 138 J(-20?)was developed successfully in the laboratory and the strength and ductility of its?90 mm bar manufactured under the conditions of industrialization were tested.The microstructural morphology and internal fine structure of martensite and temperd sorbite were studied by the OM,SEM and TEM;by the thermal simulation test,the behavior of transformation and its mechanism during the decomposition of supercooled austenite under continuous cooling conditions were summarized and the mechanisms and dynamics in transformations of bainite and martensite were also emphatically analyzed.The research results show that:(1)The best heat treatment process for the experimental steel is 920?×1 h+600?×1.5 h and the industrial trial steel treated by the above heat treatment process has a tensile strength 1130 MPa,low-temperature(-20?)impact 154 J and ratio of yield strength to tensile strength 0.904,so the research goal is realized in the study.(2)After tempering and tempering treatment,the carbides of bar alloy cementite,ellipsoid or spherical M₂₃C₆ type carbides are distributed on the grain boundary of the original austenite,the boundary of the original lath and the ?-Fe matrix.After tempering at a high temperature,the boundary of lath in microstructure is largely disappeared,but the ?-Fe is still in the form of strip.The main reasons for the high strength and toughness of the steel in this study are the disappearance of small angle grain boundary,the dispersion of large amount of nano-precipitations,the ?-Fe lath and the high density dislocation in the lath.(3)In the CCT curve of tested steel,there are mainly bainite and martensite phase region,but only very small ferrite and pearlite phase region.With the cooling rate of sample gradually increases from 0.1?/s to 20?/s,the transformation product of supercooled austenite changes from granular bainite to upper/lower bainite and then to martensite(lath martensite and acicular martensite),the critical quenching rate of steel is about 2?/s.(4)When the austenite is cooled slowly to room temperature at a cooling rate in 0.1?/s?1?/s,there are two different bainites,namely the granular bainite and lath bainitic ferrite.The polygonal ferrite matrix in granular bainite is formed and grown in bulk in the austenite carbon-poor zone,and the granular structure of granular bainite is irregularly distributed on ferrite matrix.When the cooling rate is above 0.7?/s,bainitic ferrite nucleates a lot at the grain boundary of original austenite and grows into the grain in the form of lath with a high density of dislocation.(5)When the cooling rate is greater than 2?/s,the supercooled austenite transforms completely into martensites containing lath martensite and acicular martensite.As the temperature of supercooled austenite is below M_s,martensite phase transition occurs rapidly,and the lath martensite with the substructure of high density dislocation is generated;with the increasing of the supercooling degree of austenite,the carbon-rich austenite transforms into the acicular martensite with high density dislocation,and some of which has a twin substructure.In addition,the acicular martensite occurs self-tempering to a certain extent,so nano-carbides are distributed in martensitic ferrite matrix.(6)The results of martensite phase transition dynamics analysis show that the relationship between the volume fraction of martensite f and the temperature T is f=5.331+1.400×10^-2T.