Effects Of Rolling Process On Microstructure And Mechanical Property Of Heavy Plate With High Strength And High Toughness

In order to achieve the breakthrough of key technology of high-end equipment manufacturing,development of high strength heavy plate is imperative.However,the heavy plate produced by conventional process of casting,rolling and heat treatment containes many typical heavy plate problems(such as internal defects and performance inhomogeneity along thickness direction),which restrict the application of heavy plate and high-end equipment.Therefore,it would be significant to explore new process to eliminate internal defectsand improve the performance of heavy plate.In this paper,the low alloy high strength heavy plate was studied,and effect of rolling process and heat treatment on microstructure via finite element simulation,SEM,TEM,EBSD,and the influence of process parameter was revealed by comparing mechanical properties produced by different process.Also,the rolling process was studied in industry condition.The results are as follows:Considering dynamic recovery,dynamic recrystallization,static recovery and static recrystallization,a model was built to predict the change of prior austenite grain and energy distribution during rolling based on the results of ABAQUS model.The model shows that,comparing to plate produced by uniform temperature rolling,the larger deformation in the center of plate produced by gradient temperature rolling remarkablely decreases the incubation period of static recrystallization,and promotes static recrystallization.Gradient temperature rolling can introduce large deformation and high stress in center to promote the crack closure(crack,air holes,loosen,for instance),and also can decrease lamellar microstructure induced by segregation through full recrystallization,hence can improve qualification rate during flaw detection.As to plate with thickness of more than 120 mm,conventional rolling process can only produce third grade plate,but gradient temperature rolling can produce first and second plate;in terms of 60?80 mm plate,the first grade ratio was increased from 40%of conventional rolling process to 85.7%by gradient temperature rolling.In term of strength,the strength distribution along thickness direction in GTR plate was more uniform than UTR plate;strength difference from plate surface to center is 33?37 MPa in GTR plate,but is about 77 MPa in UTR plate.As to strain of necking and fracture,UTR plate is similar along thickness direction,which leads the Z direction tensile sample to fracture in center;however,GTR plate has a better ductility in center area than in quarter thickness,which results in the fracture of quarter direction.Besides,GTR plate represents a higher lamellar tearing resistance than UTR plate.As to precipitate particles,from surface to center,the number of precipitations gradually decreases in UTR plate and increases in GTR plate.The large deformation in center and quarter thickness of GTR plate can induce titanium and niobium carbides at high temperature and effectively promote the nucleation sites of particles.Besides,unlike distribution and size of particles produced by other process,the carbides in center and quarter thickness of GTR plate is nano-scale and distributed in the form of cluster,which is related with the specific deformation condition of gradient temperature rolling and required further studied.Based on simulation results of Z direction tensile,under ordinary conditions(strain gradient:-0.00217?0.00433/mm,strength gradient:-8.67?1.3 MPa/mm),increasing the ductility and strength of central area can effectively suspend strain concentration,improve the reduction of area and lamellar tearing resistance,and also makes fracture location gradually move toward surface.Thus,gradient temperature rolling can improve the strength gradient and ductility gradient along thickness direction,and thereby increase Z direction performance.From plate surface to center,the average prior austenite grain size exhibits a notable increase from 39.3 to 78.1?m in as-rolled UTR plate,but represents a remarkable decrease from 69.0 to 31.1 ?m in as-rolled GTR plate,which verifies the contributions of GTR process on effective refinement of prior austenite grain in half and quarter thickness.It shold be noting that,some intragranular acicular ferrite,fine polygonal ferrites and qusi-polygonal ferrites were observed in surface of GTR plate.From plate center to surface,the percent of intercritical ferrite increases from 14.0%to 26.0%in UTR plate,but decreases from 27.0%to 4.8%in GTR plate.The change rule of high angle grain boundary frequency in thickness direction was similar to intercritical ferrite percent,which implies acicular intercritical ferrite forming during intercritical quenching can effectively promote the formation of high angle grain boundary and the refinement of grain size with respect to cleavage fracture.Microstructure is composed of M/A islands,coarse tempered martensite and qusi-polygonal ferrite in tempered specimen.The appearances of large sized M/A islands in the center and quarter of UTR plate is related to the relatively low cooling speed,few precipitates and small percent of acicular austenite during intercritical quenching.The strength,strength uniformity and impact toughness of GTR plate processed by TMCP&tempering are much better than those of UTR plate.The GTR plate with tempering temperature of 650? exhibits optimal mechanical properties and accords to the standard of Q890E grade heavy plate.