| Offshore jack-up platform gear rack steel is the key material to the legs,which has the characteristics of high strength,high toughness,large thickness,strong corrosion resistance,high technical content and great difficulty in production.In this paper,a 177.8mm thickness,690MPa gear rack steel is used as experimental material.The mechanical properties and microstructure uniformity of gear rack at surface,1/4 thickness and 1/2 thickness were studied.The influence of the segregation at the surface,1/4 thickness and the 1/2 thickness on the cryogenic toughness and strength of the steel was analyzed.And the influence of off-line heat treatment on hardenability and low temperature toughness of the gear rack steel was investigated.The main work of this paper is as follows:(1)The CCT curve of 40%deformation and no deformation of tested steel during continuous cooling of austenite was measured on the MMS-300 simulation machine.The results showed that there is no ferrite and pearlite transformation and the bainite and martensitic transformation occur during the process of continuous cooling of austenite.When the cooling rate was between 0.5? and 2? per second,the bainite structure in the experimental steel gradually changed from the granular to the lath.When the cooling rate was between 2? and 5? per second,the microstructure of the steel was bainite and martensite.When the cooling rate was greater than or equal to 5? per second,the microstructure of the steel was full of martensite.After 40%deformation,the starting temperature of bainite transformation and martensite transformation decreased by about 60? and 40? respectively.The deformation had little effect on the Bf and Mf points of the experimental steel.And the hardness of steel increased by about 30(HV100)after 40%deformation.(2)The high temperature austenite transformation behavior was observed in situ by high temperature laser confocal microscopy.And the influence of temperature,holding time and cooling rate on austenite was studied.The results showed that the grain size increases exponentially with the increase of austenitizing temperature when the holding time is constant.And the austenite grain was abnormal coarsening when the steel was hold at 1250? for 15 minutes.When the austenitizing temperature was constant,the grain size of austenite increased inconspicuously with the increase of holding time.The time of bainite and martensitic transformation decreased with the increase of cooling rate.(3)The study on the microstructure and properties of the experimental steel showed that the surface of the steel is tempered martensite,and the 1/4 thickness is tempered sorbite and the 1/2 thickness is tempered bainite and tempered sorbite.The critical point of phase transformation at different positions was different because of the inhomogeneity of the composition in the thickness direction.The phase transformation point of the surface was about 30? to 50? higher than that of 1/4 thickness of tested steel,and the phase transformation point of 1/4 thickness was about 10? to 30? higher than that of core.The segregation of Ni,Cr,Nb,Mo in the experimental steel did increase the hardenability of the steel plate,however the segregation also led to a decrease in cryogenic toughness. |
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