Research On TMCP Process,Microstructure And Properties Of Extra-High Strength Steel For Arctic Ships

Polar ships are navigating in the harsh polar seas,and the hulls are subject to repeated impacts of sea ice.Therefore,it is necessary to ensure that the polar marine steels have excellent low temperature properties,including strength,toughness,corrosion resistance and weldability.This paper relies on the national key research and development plan "Ultra-cold and ultra-low temperature environment marine steel and application(2016YFB0300700)" project,research on controlled rolling and controlled cooling(TMCP)process and microstructure performance control of polar marine steel.The composition design idea of low carbon and high manganese,composite addition of Ni,Mo,Nb and other alloy elements was adopted,the effects of TMCP process and Ni and Mo on the microstructure and properties under laboratory conditions,the influence law of Mo and Ni content was investigated.The chemical composition of the marine steel and the TMCP process route with excellent low temperature toughness are obtained.The main work of this paper is as follows:(1)The effect of austenite grain growth and the original austenite grain size on the microstructure after transformation was studied by laser high-temperature confocal microscopy.The results show that 1200? is the coarsening temperature of the austenite grains of the experimental steel,and the original austenite grain size has a significant influence on the microstructure size after the phase transformation,the heating temperature of the slab is set to 1150?.(2)Study on the continuous cooling transition of high temperature austenite using thermal simulation tester.The results show that ferrite and a small amount of pearlite are formed at low cooling rate.With the increase of cooling rate,the austenite gradually transforms into granular bainite and lath bainite,and the finer the structure,the M/A island also more subtle and diffuse.(3)By plotting the CCT curve of the experimental steel,it is found that the dynamic phase transition temperature of the experimental steel containing Mo decreases from 620? to 570?as the cooling rate increases from 0.5?/s to 30?/s.The end temperature is reduced from 545 0C to 450 0C.The effect of cooling rate on phase transformation of steel is similar,indicating that the increase of cooling rate will delay the phase transition;the dynamic phase transition temperature of Mo-containing experimental steel is slower than static phase transition.The starting temperature can be up to 28? higher,and the dynamic phase transition starting temperature of the Mo-free steel can be up to 58? higher than the static phase transition starting temperature.The deformation causes the CCT curve to move to the upper left corner,which expands the ferrite transition zone.(4)The effect of Mo element on the transformation law of austenite was studied by a thermal simulator.The results show that when 0.2%Mo is added to the steel,the initial and end temperatures of the phase transition can be reduced by about 20?,the ferrite and pearlite transformation are inhibited,and the influence on the bainite transformation temperature is small.(5)Under the different TMCP process conditions,the influence of Mo element on the microstructure and properties of polar marine steel was studied.It was found that when the red return temperature was 400? and 350?,the addition of Mo increased the content of granular bainite in the experimental steel,and the size and content of the M/A island increased,and the shape is changed from a spherical shape to a pointed shape or a strip shape,large angle grain boundary ratio decreases,low temperature toughness deteriorates;and when the red return temperature is 350?,the content of granular and lath bainite in the experimental steel containing Mo increased simultaneo usly.Compared with the non-Mo experimental steel,the yield strength increased from 544 MPa to 580 MPa,and the tensile strength increased from 713 to 744 MPa.The average impact energy at-80? is reduced from 270J to 52J.(6)Comparing the difference of microstructure and mechanical properties of experime ntal steel with different Ni content,it is found that with the increase of Ni content,the maximum size of M/A islands in experimental steels decreases from 4.17?m to 3.57 ?m,the content decreases from 0.39%to 0.21%,the distribution becomes more diffuse,the shape of M/A islands changes from sharp angle to spherical shape,and the proportion of large angle grain boundary increases from 37%to 57%,the lath bainite of 0.9Ni experimental steel is finer than that of 0.6Ni experimental steel,the lath spacing decreases from 0.6?m to 0.3?m,and the lath bainite spacing becomes finer.With the increase of Ni content,the low temperature toughness of experimental steel is improved.