Study On Microstructure And Mechanical Properties Of EH47 Ship Plate Steel With Special Thickness

High strength and extra-thick ship plate steel is used for offshore platforms,shipbuilding industry,offshore facilities and equipment,especially for key supporting parts.Low alloy high strength EH47 steel plate is widely used in shipbuilding and offshore platform construction.It requires high strength,good plasticity,corrosion resistance,low temperature impact toughness and good welding performance.In this paper,we try to explore the influences of the microstructure and crystallographic orientation of EH47 marine steel plate on its strength,low temperature toughness and other comprehensive mechanical properties,analyze the influences of different heat treatment processes on its structure and properties,and optimize the alloy composition design and production process on this basis.The results show that the microstructure near surface of TMCP EH47 steel plate is acicular ferrite and a small amount of polygonal ferrite.The microstructure of 1/4 thickness of plate is mainly a large number of polygonal ferrite,a small amount of acicular ferrite and degraded pearlite.The size of degraded pearlite at 1/4 thickness is less than 5 um.The microstructure of 1/2 thickness of plate is coarse polygonal ferrite,a small amount of acicular ferrite and degraded pearlite,and the size of degraded pearlite at 1/2 thickness is 5-10 um.The volume fraction of grains with < 110 > orientation is higher and the volume fraction of grains with < 100 > orientation is lower in the grain structure of the sheet,which has better low temperature toughness.Under unidirectional tension,the degree of grain deformation is related to Schmid factor.The location of crack initiation is obviously selective.The location of crack initiation avoids in grains with < 110 > oriented and preferentially forms in the location of grains with smaller Schmid factor.After normalizing treatment,the microstructure of EH47 steel sheet is polygonal ferrite and granular bainite.The grain is slightly coarsened,and the tensile strength is slightly decreased.Large grain boundary misorientation can effectively deflect micro crack propagation.With the increase of the proportion of large grain boundary misorientation and increase of density of M-A islands,the low temperature impact toughness of sheets improves.The microstructure after quenching and tempering is lath bainite and polygonal ferrite.The grain is refined,the degraded pearlite mass disappears,the M-A islands and carbides disperse finely and precipitate continuously along the grain boundaries.After quenching at 930? and tempering at 600?,the yield strength is 584 MPa and the tensile strength is 657 MPa,which are 27.8% and 16.4% higher than that of TMCP state respectively.the impact energy at-60? is more than 250 J,which meets the requirements of low temperature toughness of F grade steel plates,has a large surplus,and the strength-toughness coordination is excellent.Based on the above results,the microstructure of TMCP steel sheet after improvement is similar to that of steel sheet after quenching at 930? and tempering at 600?.The product has excellent combination of strength and toughness.The yield strength is 655 MPa,and the tensile strength 690 MPa.Impact energy is more than 180 J at-80? with stable performance.