Research On Fabrication Of Low Carbon Ti-Nb Microalloyed Steel Plate With Gradient Structure And The Corresponding Strength And Toughness Property

Gradient structure metal materials can break the properties of materials that are originally coupled together,allowing one or more of the performance improvements to provide a new orientation for structural design in both strength and toughness of the material.In this dissertation,low carbon Ti-Nb microalloyed steel is studied.By introducing water cooling simultaneously during rolling,the interaction between deformation,precipitation and phase transformation is fully utilized to prepare gradient structure low carbon microalloyed steel plate with high strength and toughness.The effects of the continuous cooling phase transformation behavior of the experimental steel and the final cooling temperature,coupling with deformation and cyclic insertion cooling and deformation on the microstructure refinement of the experimental steel during the simultaneous insertion of water cooling during the rolling process were studied.On this basis,the gradient structure steel plate was fabricated by controlling deformation,phase transformation and precipitation,at the same time,the strength and toughness of the gradient structure steel plate were analyzed.The main research contents and conclusions of the article are as follows:(1)The continuous cooling transformation behavior of austenite at different cooling rates was studied,and the dynamic and static CCT curves were compared.It is found that as the cooling rate increased,the temperature of austenite transformation decreased.Compared on undeformed austenite at non-recrystallization region,the temperature of austenite transformation is higher than deformation in the austenite nonrecrystallization region,(2)The effects of cooling,deforming and reheating behavior on microstructure refinement were studied.The ferrite grain size at the time of intercooling between the two passes rolling is finer than that of the ferrite grains without cooling,and the lower the cooling temperature,the finer the ferrite grain size is.When deformed at 650?,due to dynamic recrystallization during deformation,an apparently fine ferritic grain with size of 1?3 ?m appeared in the microstructure.During the reheating temperature after deformation,the microstructure obtained when reheating to 900? is the finest.(3)The dynamic cyclic phase transformation process can refine the austenite grains and further refine the ferrite grains.With the increase of the times of dynamic cyclic transformations,the ferrite grains can be refined from 7.1 ?m to 3.3 ?m.The volume fraction of bainite decreased from the 71%to 0%.The proportion of high angle grain boundaries increased from 32.9%to 64.2%.The volume fraction of precipitated particles increased gradually from 0.7%to 2.1%.At the same time,the diameter of precipitated particles increased from 3.6 nm to 7.1 nm.The contribution of various strengthening methods to yield strength has also changed.When the times of cycles is increased from one time to four times,the grain boundary strengthening increased from 225 MPa to 330 MPa;the precipitation strengthening increased from 184 MPa to 361 MPa;the dislocation strengthening also increased.(4)After cooling to the ferrite region after the rough rolling stage,and then reheating to the two-phase region rolling,a gradient structural steel plate can be obtained.The ultra-fine grain layer on the surface of the steel plate has a thickness of about 1.5 mm and the grain size of about 3.1 ?m.The microstructure is elongated ferrite and equiaxed ferrite.The distribution size around the ferrite grain boundary is about 100?200 nm.The fine cementite with the diameter at the level of nanometer as a pinning effect on the ferrite grain boundary and hinders the growth of ferrite grains.The gradient layer has a thickness of about 2.5 mm,and the gradient layer is organized into equiaxed ferrite and pearlite of non-uniform size.The central coarse grained region is composed of ferrite and pearlite with an average grain size of 7.2 ?m.The reason for the formation of the gradient structure is mainly the temperature gradient in the thickness direction of the steel plate during the water-cooling process after the rough rolling stage,so that the surface is thermally deformed while the mid-thickness is still in a state of thermal deformation,and the surface layer is dynamically recrystallized to refine(5)The yield strength of the surface layer of the gradient structure steel plate is 31 MPa and 29 MPa higher than the gradient layer position and the mid-thickness position,respectively.The elongation of the surface position,the gradient layer position and the mid-thickness position were 17.1%,22.2%,and 20.5%,respectively.The gradient layer had excellent plasticity,and the elongation increased by 29.8%and 8.3%compared with the surface layer and the mid-thickness,respectively.Due to the special structure of the gradient,geometrically necessary dislocations are generated at the junction of the coarse and fine grains,which makes the deformation more compatible and improves the plasticity.The impact toughness at the surface position from-60? to 20? is higher than that at other locations.The impact toughness at the position of the gradient layer is lower than the surface position of the experimental steel plate,but slightly higher than the mid-thickness position of the experimental steel plate and the impact toughness and the compared steel plate.The impact fractures of the experimental steel plate and the compared steel plate are both ductile fractures,and there are obvious dimples.The improvement in toughness is mainly caused by the refinement of the grains.The ratio of the high angle grain boundary of the surface layer structure is 17.8%higher than the position of the mid-thickness,which is also the reason for the increase in toughness.