Study On The Microstructure Controlling And Mechanical Properties Of High-performance Offshore Engineering Steel

The high performance offshore engineering steel with high strength,high toughness,high plasticity and good weldability are developed to meet the requirements of lightweight equipment,high service safety and energy saving and emission reduction.This paper was focused on the modification of the macrostructure or microstructure segregation and heterogeneous phases which were formed during continuous casting,rolling,cooling and isothermal heat treatment’ in offshore steel plates via heat treatment,which controls the local chemical composition and microstructure,so as to obtain better mechanical properties.The thesis revealed the influence of alloying element partitioning on the stability of reversed austenite and decomposition of M/A constituents during intercritical heat treatment and presented the methods to improve the plasticity and toughness utilizing the principle.On the other hand,the effect of alloying elements enrichment in reversed austenite on the density of austenite nucleation during re-austenitization was investigated and the theory and mechanism of austenite grain refinement via optimizing the quenching and tempering process was studied.The fine and thermal-stable retained austenite was formed by multi-steps intercritical heat treatment and alloying elements partition,which was aimed to the zonal microstructure caused by the segregation of continuous casting billet in a C-Mn steel.The elongation and toughness of experimental steel was greatly enhanced by retained austenite so as to successfully manufacture the electrical resistance welding(ERW)pipeline with good expansion performance which meet the mechanical properties requirement of expansion casing pipe for deep sea oil and gas exploitation.The results showed that the micro-segregation(zonal)microstructure enriched with C and Mn was discontinuous distribution in the experimental steel.The micro-segregation(zonal)microstructure within weld zone was significantly formed in the ERW pipeline.The micro-segregation microstructure was transformed into thermal stable retained austenite by normalizing+intercritical annealing+intercritical tempering heat treatment.The uniform elongation of experimental steel was improved from 15%before heat treatment to 25%after multi-step intercritical heat treatment,which was attributed to the transformation induced plasticity(TRIP)of retained austenite.The weld performance of ERW expansion casing in C-Mn low alloy steel was clearly improved by the TRIP effect of retained austenite.For the 460 MPa grade and 60 mm thickness and multi-phases of ferrite and bainite steel plates processed by thermo-mechanical controlling process(TMCP)and relaxation process,since the low cooling rate in the center(1/2t)of heavy steel plate and the solute partitioning during the relaxation,large size martensite and austenite(M/A)constituent occurred at the center of the plate which deteriorated the low temperature toughness of center specimens.Some M/A constituents at 1/2t of multi-phases plate steel decomposed and some small size of M/A constituents transformed to austenite after one-step intercritical annealing.According to the partitioning theory of alloy elements from ferrite to reversed austenite,the alloying elements further enriched into austenite formed by the reversion of M/A constituent during intercritical annealing and 6%(volume fraction)stable retained austenite was formed.Therefore,the low temperature toughness of center specimen of intercritical annealed experimental steel was further improved.In addition,the super-high strength heavy gauge steel plate(690 MPa,100 mm)processed by TMCP process had lath bainite at the range of surface to quarter(1/4t)of the plate,while there was granular bainite and few lath bainite at the 1/2t,and huge of large size M/A constituent existed in granular bainite.The low temperature toughness was deteriorated by the large size M/A constituent,which needs heat treatment process to modify.The alloy elements enriched martensite and depleted ferrite obtained by intercritical annealing was firstly investigated in this paper,which increases the interface density of heterogeneous phases at 1/2t of plate steel.The austenite nucleation rate during complete re-austenitization was enhanced by the chemical interface with uneven compositions so as to refine austenite grain size,increase transformation driving force during quenching,reduce the volume fraction and size of M/A constituent and improve the toughness of 690MPa(100mm thickness)plate steel.In order to achieve the super-high strength,high toughness-plasticity in heavy gauge steel plate,the austenitization behavior during reheating and transformation behavior during cooling were paid more attention in this paper.Two morphorlogies of equiaxed reversed austenite(ERA)and lamellar reversed austenite(LRA)were observed after intercritical annealing.The LRA held the K-S orientation relationship(OR)((111)fcc/(011)bcc,[-101]fcc//[-1-11]bcc)with the prior austenite grain.The LRA was coarsened gradually and impinged with the increasing austenitization temperature and final formed an austenite grain which kept the almost identical orientation with the prior austenite grain,the process is referred as the construction of the prior austenite grain.While the ERA held the high angle misorientation with the adjacent austenite grain and had the near K-S OR with one adjacent austenite grain.There is no OR between new fine austenite grains.The ERA can refine the prior austenite grain effectively.The bainite/martensite microstructure with fine effective grain size was obtained by the intercritical annealing and austenitization quenching and intercritical tempering.The alloying elements enriched martensite(M)and depleted intercritical ferrite(IF)were formed after intercritical annealing.The transformation free energy of austenite in martensite was increased.So the ERA nucleating at the prior austenite grain boundary and intragranular was enhanced,which refined the prior austenite grain.The cementite lamellar and.retained austenite lamellar replaced the M/A constituent in hot rolled steel plate after heat treatment.And in combination with bainite/martensite microstructure with fine effective grain size,the experimental steel(690 MPa,100 mm)achieved super-high strength,high toughness,and high plasticity in the whole plate.