| With the rapid development of the economy and technology,domestic infrastructure and industrial construction continue to accelerate,and society's demand and performance for steels have also continued to increase.Low alloy steel is the main steel grade for engineering structural steel.Low alloy steel is widely used in construction,bridges,offshore platforms,etc.due to its excellent mechanical properties and low production cost.With the development of microstructure control,the mechanical properties of low alloy steel have been greatly improved.However,during the development of low alloy steel,the abnormal structure and mechanical properties caused by centerline segregation and the influence on welding performance have not been well resolved.The segregation in low alloy steel is mainly due to the centerline segregation of continuous casting billet resulting in.Therefore,to solve the effect of center segregation on the abnormal structure and mechanical properties and welding performance,it is necessary to start with the control of centerline segregation of continuous casting slabs.At present,there are quality defects such as segregation,delamination and cracks in steel products,of which centerline segregation is the main problem,and the mechanical properties of the steel plate are deteriorated,which is easy to cause large fluctuations in performance and seriously affect the welding stability.It is a scientific and engineering issue that requires in-depth research.This dissertation starts with the problem of centerline segregation of X70 pipeline steel,and studies the effect of centerline segregation of different grades of continuous casting slab on the microstructure and mechanical properties of rolled plates.The results show that the distribution of centerline segregation on the cross-section of the continuous casting slab is uneven,and it is completely inherited to the core of the rolled plate after rolling,and cannot be avoided or eliminated.The centerline segregation is mainly caused by the enrichment of carbon and manganese elements,and mainly by the enrichment of manganese elements.When the centerline segregation of the casting slab is Manesmman class 2 and below,the core microstructure of the rolled plate is composed of ferrite and a small amount of MA;while when the center segregation of the billet reaches class 3 or even higher,the core microstructure of the rolled plate is lath bainite/martensite,and the abnormal microstructure ratio can reach?63%.With the increase of the degree of segregation in the centerline of the slab,the tensile strength and yield strength of the corresponding rolled plate increase;the elongation decreases,and the strength and toughness fluctuations increase significantly.When the centerline segregation degree is higher than class 2,the fluctuation range of tensile strength will be expanded from?13MPa to?110MPa,and the fluctuation range of yield strength will be expanded from?15MPa to?30MPa;at the same time,the fluctuation of impact toughness is more significant.The ductile-brittle transition temperature increased from-90? to-65?.Aiming at the problem of centerline segregation in X70 pipeline steel welding,the effect of center segregation on the low temperature toughness of the heat affected zone of the welded joint was studied.The results show that when the segregation at the center of the slab reaches class 3 or higher,the impact fracture at the heat-affected zone of the rolled plate exhibits delamination and the impact energy fluctuation increases,especially as the test temperature decreases,the impact energy fluctuation increases significantly.Pipeline steel welding heat fracture zone impact fracture layering and impact energy fluctuations are due to the enrichment of carbon and manganese elements in the segregation zone,the phase transformation temperature of the segregation zone is reduced,and the faster cooling rate after welding in the heat affected zone promotes the formation of grain size in the segregation zone.The smaller martensite results in the concentration of stress distribution in the segregation zone,which forms the crack source and crack propagation path.In order to improve the centerline segregation to improve the microstructure homogeneity and mechanical properties of low-alloy steel,the effect of continuous casting process on the solidification structure and macro-segregation of the slab and the relationship between the microstructure uniformity and mechanical properties of the corresponding rolled material were studied.The study suggested that in low-alloy steels,the continuous casting process was adjusted to increase the cooling rate and temperature gradient during the solidification of the slab,promote the growth of columnar crystals,inhibit the growth of equiaxed crystals,and significantly increase the proportion of equiaxed crystals,thereby suppressing the segregation spots formed in equiaxed crystal region.The abnormal microstructure formed by the segregation inherited from the centerline segregation of the continuous casting billet is bainite/martensite in the pipeline steel,and is a relatively high proportion of degenerated pearlite in the section steel.Guided by the microstructure and mechanical properties of the rolled steel,the optimization of the continuous casting process can significantly improve the microstructure uniformity,increase the elongation and improve the low temperature toughness.In order to solve the problem of diffusion and enrichment of solute elements in the solidification front during the solidification process,the relationship between the cooling rate and the secondary dendrite arm spacing(SDAS),microsegregation and centerline segregation was studied by software Dictra.The research shown that the cooling rate and the SDAS has no obvious effect on the segregation of carbon between dendrites;and the cooling rate and the distance between dendrites have a particularly significant effect on the manganese segregation between dendrites.The higher the cooling rate,the smaller the SDAS,the lower the enrichment content of the manganese between the dendrites,although the manganese element also reversed to some extent after the molten steel was completely solidified,the enrichment content of the manganese between the dendrites was significantly higher than dendrit.Therefore,when the cooling rate is higher,the secondary dendrite spacing becomes smaller,and the enrichment content of manganese between dendrites decreases;the columnar crystals are developed,and the proportion of equiaxed crystals is significantly reduced,which significantly inhibits the formation of segregation in the equiaxed crystal region.The high cooling rate and the increase of the ratio of columnar crystals can significantly inhibit the centerline segregation of the continuous casting slab.Mainly through the cooling rate to inhibit centerline segregation,affecting the redistribution of solute elements.In the low alloy casting billet,the solidification structure with high columnar crystal ratio is more conducive to the control centerline segregation.In addition,by studying the effect of centerline segregation of HSLA bainite steel on the core microstructure and mechanical properties,and comparing the core microstructure of two thick plates with different degrees of centerline segregation,the study suggested that with the increase of the severity of segregation,the microstructure transform from bainite to martensite.The increase in hardenability is related to the decrease in toughness.A detailed analysis of the variation distribution of these two regions with different degrees of segregation shows that the microstructure with severe segregation,results in the larger size of the packet and Bain group.Due to the large grain size and the strong variant selection between different Bain groups,the adaptive adjustment of phase transformation strain is very limited,so that the deflection reduces the effective grain boundary to crack propagation. |
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