Microstructure Evolution Mechanism And Regulation Of A Copper-Alloyed Crack Arresting Steel Through Thermo-mechanical Treatments

With the rapid development of the world economy,marine equipment such as ships and offshore platforms continue to develop in a large-scale direction,and the large-scale equipment has significantly deteriorated the stress state of the structure.In order to prevent fracture accidents and ensure structural safety,crack arrest steels with higher grades,larger thicknesses and the ability to inhibit the propagation of brittle cracks are required to be used in key parts where brittle fractures or catastrophic fractures may occur.The mechanical properties and welding properties of crack arrest steel depend on the composition design.The thickness direction microstructure and property uniformity of the large-thickness crack arrest steel are the key factors determining the properties of the steel plate.Understanding the transition mechanism of phase structure during thermo-mechanical treatmentis is the premise guarantee to guide the preparation process to obtain a uniform structure.Different microstructures,including phase composition and texture,are important factors affecting the crack arrest performance.Therefore,this paper studies from four aspects:alloy design,introduction of copper-rich nanophase strengthening instead of carbon strengthening,microstructure control mechanism,and the key factor affecting crack arrest toughness-the evolution law of texture to provide theoretical support for the production of large thickness crack arresting thick steel plate.In this paper,low carbon equivalent design is used to ensure good toughness and weldability of thick plate crack arrest steel,Ti-Nb microalloyed crack arrest steel is used,and TiN high temperature liquid phase precipitation is used to promote austenite nucleation and inhibit its grain growth.During the controlled rolling and controlled cooling process,Ti and Nb undergo strain-induced precipitation and interphase precipitation to refine the grain and strengthen the matrix.At the same time,the combination design of copper-rich nanophase alloy elements Cu,Ni,Al and Mn is used,and the matrix is strengthened by the dispersion and precipitation of copper-rich nanophase clusters to make up for the lack of strength caused by low-carbon equivalent design,and successfully develop a new type of crack arrest steel.The matrix structure of the crack arrest steel is composed of granular bainitic ferrite,acicular ferrite and lath bainitic ferrite,and the second phase precipitation particles include nano-scale(Ti,Nb)(C,N)and copper-rich nanophase which dispersed and distributed in the matrix to produce precipitation strengthening.The yield strength reaches 606～616MPa,the tensile strength reaches 758～783MPa,the elongation reaches 21.9～23.4%,and the impact energy at-80℃ reaches 246J.The law of austenite grain growth of crack arrest steel,the effect of solution and aging heat treatment on the microstructure and mechanical properties of crack arrest steel are studied.The study found that the carbonitride of Ti and Nb precipitated in the crack arrest steel can effectively pin the grain boundaries and inhibit the growth of austenite grains,and it will dissolve back into austenite above its full solution temperature,losing the effect of pinning grain boundaries,and accelerating the growth rate of austenite grains.The grain growth of crack arrest steel is not obvious at solution temperature of 875℃,900℃ and 925℃,but it is obvious at solution temperature of 950℃.The aging treatment has an effect on the matrix structure and the precipitation of the second phase of the crack arrest steel.With the prolongation of the aging time,the matrix structure will recover and recrystallize,and the second phase will precipitate.The increase of aging temperature will increase the diffusion rate of copper-rich nanophase forming elements,shorten the aging peak time.The copper atoms are precipitated in the form of copper-rich nano-phases,which will not damage the plasticity and toughness of the ferrite matrix while producing precipitation strengthening,and the strength and toughness-plasticity match well.The phase transformation law and the second phase precipitation law of static and dynamic continuous cooling of crack arrest steel are studied.The high temperature deformation enlarges the cooling rate range of polygonal ferrite,granular bainite ferrite and acicular ferrite.The high temperature deformation slightly reduces the transformation start temperature,and at the same time accelerates the transformation speed of supercooled austenite.The former is caused by the combined effect of high-temperature dynamic ferrite transformation,strain-induced precipitation,recovery and recrystallization,while the latter is caused by high density of dislocations and high deformation storage energy providing ferrite nucleation sites and growth driving forces,respectively.The polygonal ferrite formed by low cooling rate has coarse grain,large Angle grain boundary,low orientation gradient,dislocation density and residual stress.The bainite ferrite formed by high cooling rate has smaller grain size,higher proportion of low angle grain boundary,and correspondingly higher orientation gradient,dislocation density and residual stress.High proportion of low angle grain boundary,orientation gradient,dislocation density and residual stress can be obtained by high temperature deformation.In the dynamic ferrite region,the proportion of small Angle grain boundary,orientation gradient,dislocation density and residual stress are lower due to the release of residual stress and dislocation consumption.This study will contribute to the optimization of thermo mechanical control process and the precise control of microstructure and properties in high-performance thick steel production.The texture evolution law of thermomechanical treatment and cold rolling annealing and their effects on microstructure and mechanical properties of crack arrest steel are studied.During hot rolling,plastic deformation,recovery,recrystallization and strain-induced precipitation all affect the formation of texture.In the temperature range of 850℃ to 950℃,increasing rolling temperature and rolling deformation are beneficial to the formation of{114}～{113}<110>,{112}～{223}<110>on α orientation line and the formation of {001}<110>on the α orientation line and the {111}<112>-{332}<113>texture on the ε orientation line without recrystallization.After rolling at 950℃ with 50%deformation induced recrystallization,the {112}<111>texture with high strength and sharpness is formed on the εorientation line.This study can provide theoretical guidance for improving toughness and crack arrest performance of thick crack arrest steel by texture control.