| During the last decade,oil exploration is being extended to abyssal regions,leading to the increased size of offshore platform.To this end,heavy gauge steel plates with yield strength exceeding 500 MPa are being increasingly needed to satisfy the requirement of safety and reliability.Combination of high strength and high toughness is a key technical problem needed to be solved during the development of offshore platform steel.Microstructural evolution,M-A constituents and precipitation of carbonitride during rolling,cooling and heat treatment are always greatly concerned.In the present paper,on the basis of TMCP and tempering,the key issues,i.e.,microstructral evolution and recrystallization of austenite,bainite transformation,decomposition of M-A constituents during tempering,microstructural evolution of heat affected zone and corrosion behavior were further studied.Chief original work is as follows:(1)Deformation temperature and strain rate are main factors affecting the high temperature deformation behavior of austenite.The high deformation and the smaller strain rate,the easier for occurring of dynamic recrystallization.With the increase of deformation temperature and interval time,static recrystallization softening rate increases.When deformation temperature is higher than that of strain induced precipitation temperature,the static behavior follows Avrami equation.When the deformation temperature is lower than that temperature,there has a platform of softening curve.Due to the strain rate(1-2s-1,)and effective strain(0.2)of each pass during the rolling of industrial produce,it is difficult to refine austenite grain size via dynamic recrystallization.In the temperature range of 1000-1050? static recrystallization can occur quickly,i.e.it can occur nearly 90%with the interval time 20s at 1050?.Therefore,the static recrystallization is the main softening mechanism in controlled rolling process of steel T1(Nb-V-Ti steel)and T3(Nb-Ti steel).(2)Comparing of different cooling processes after the same condition of hot rolling,different microstructures can be observed.Polygonal ferrite and M-A constituents can be obtained according to AC+UFC(Air cooling+Ultra fast cooling),and it has the lowest yield ratio and highest elongation.For UFC+AC+UFC(Ultra fast cooling+Air cooling+Ultra fast cooling),the microstructure consists of acicular ferrite,granular bainite and lath bainite/martensite,and it has the best impact toughness.Acicular ferrite and lath bainite/martensite can be noticed via DQ(Direct Quenching)technology with the highest yield and tensile strength.(3)The yield strength is more than 620MPa,the tensile strength is more than 720MPa,elongation is greater than 16%,and impact absorbed energy is greater than 100J for the temerping of steel T2(Nb-V-Ti steel),and it can satisfy the requirement of FH550 offshore platform steel.It is mixed microstructure of lower bainite and granular bainite for as-rolled sample,while it is tempered bainite after tempering.Carbides located at grain boundaries lead to the fluctuation of impact toughness.Both of necking and shear crack propagation can be observed in the impact fracture process.For as-tempered sample,the high angle grain boundaries proportion is 79.3%,and the average grain size is 7.61 ?m.Refined grain size and high angle grain boundaries lead to the excellent low temperature toughness of experimental steel.(4)The primary physical metallurgical processes occurred during tempering of GB include recovery of bainitic ferrite,precipitation of micro-alloyed carbides and decomposition of M-A constituents which further involves the cementite precipitation and recovery and recrystallization of highly dislocated ferrite matrix.Precipitation of cementite particles,which was initiated by the segregation of supersaturated carbon atoms to defects or cdusters in martensite laths,is one of the characteristic processes occurred during tempering.At the same time,martensite laths with low angle boundaries merged and recrystallized into polygonal ferrite grains under the driving force of stored energy in dislocation.Decomposition of M-A constituents ended in the formation of dispersed cementite particles and polygonal ferrite.Decomposition of M-A constituents and recrystallization of bainitic ferrite matrix resulted in the decrease of tensile strength,while this can be significantly compensated by the precipitation of micro-alloyed carbides at tempering temperature above 5000?.For tempered sample,the decomposition of M-A constituents has produced more potential nucleation sites for microvoids,i.e.ferrite/cementite interfaces.Then,with increasing strain during impact,microvoids continued to grow and coalesce accompanied by the deformation of ferrite matrix until fracture.(5)In general,the sample with heat input energy of 15 kJ/cm exhibited quasi-cleavage,which has a cleavage facet and,hence,a short unit crack path.Many ductile fracture bands(tear ridges)with small dimples were observed.The connection of cleavage facets by tear ridges is considered as the major energy-absorbing mechanism during cleavage crack propagation.The fracture surface showed several deflections during the propagation of primary cracks.For the sample with heat input energy of 80 kJ/cm,the fractograph showed complete cleavage fracture with large cleavage facets.These large cleavage facets propagate across the prior austenite grains in a linear manner and exhibit a river pattern.Tracing the river pattern,the probable cleavage initiation site can be located.The matching fracture surfaces indicated voids at locations where particles were present and pulled out.A cleavage crack can also initiate from the area between two nearby M-A constituents where residual phase transformation stress and stress concentration due to debonding of the M-A constituent overlap.The primary crack propagated in a straight manner,and the M-A constituent showed no sign of deformation.(6)The corrosion process can be divided into the initial and later stage in simulated atmospheric environment.At the initial stage,corrosion products present light brown and loose structure,and the corrosion rate increases continuously.At the later stage,the rust layer is deep brown at macro level.Dense structure of the inner rust layer leads to the protection of substrate,and results in the decrease of corrosion rate.After being corroded for 360h,the rust layer of Cr steel,Cr-Ni steel and Cr-Ni-Cu steel all consist of ?-FeOOH,?-FeOOH,?-FeOOH and large number of amorphous oxides.After being corroded for 876h,Fe3O4 was present in the rust layer for Cr and Cr-Ni steel,while the composition of Cr steel rust did not change.With the increase of corrosion time,corrosion products accumulate on the electrode surface gradually,and the anodic dissolution of substrate and the electrochemical reaction rate are suppressed.When the corrosion time increased from 540h to 1020h,the corrosion rate of Cr-Ni steel decreased from 1.25mm/a to 0.86mm/a,and the rust resistance increased from 231.2 ?·cm2 to 487.8 ?·cm2. |
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