| High-carbon tool steel SK85 is often used as hand saw blades,scrapers and fitter assembly tools.However,the hot rolled steel is easy to break when uncoiling and the effect of spheroidizing annealing is not good.Low carbon pressure vessel steel SA537CL1 is supplied in a normalized state,but there are also some problems such as insufficient strength after normalizing and post-weld heat treatment,especially the low qualified rate of impact property.The morphology and distribution of carbides play a key role in controlling the properties of steel.In allusion to the SK85 and SA537CL1 hot-rolled steels provided by a steel mill,this paper focuses on the analysis of the evolution of carbide morphology and distribution from hot-rolled state to heat-treated state and its effect on mechanical properties,aiming at providing theoretical guidance for optimization of production processes and improvement of comprehensive properties.The research results of this paper are as follows:·Study on uncoiling fracture of SK85 hot-rolled sheet and spheroidizing effect（1）The head and tail parts of the hot-rolled steel have a faster cooling rate while the middle part has a slower cooling rate,which leads to the different morphology and uneven distribution of cementite and uncoiling fracture of hot-rolled steel may occur at various positions.The key to solve the problem of the material fracture is to apply reasonable cooling intensity to ensure the uniform distribution of microstructure during hot-rolling.（2）The holding time of spheroidizing annealing can be shortened by refining the original structure of the hot-rolled steel and increasing the cold-rolling deformation rate.The finer the original structure is,the finer and more uniform the carbide after spheroidizing annealing is,and the higher the degree of spheroidization is.With the extension of holding time,the carbide goes through the stages of lamellar dissolution,spheroidization,coarsening and growth.If the holding time is too long or too short,the lamellar cementite will be formed.（3）The coarsening of carbide particles formed with in grain of the cold rolled samples satisfies the cubic growth law while the coarsening of carbide particles at the grain boundary satisfies the quartic growth law,which conforms to LSW coarsening theory.（4）With the increase of average carbide particle size,the tensile strength of the test steel is slightly reduced and the plasticity is improved.The fine and uniform spherical carbide structure can significantly improve the strength and toughness of the test steel while the existence of lamellar cementite is not conducive to the improvement of the comprehensive properties of the test steel.（5）The spheroidizing effect has little effect on the microstructure and properties after quenching and tempering.The reason is that the quenching temperature is much higher than the Accmline and the carbide particles re-dissolved during quenching,so the genetic behavior of microstructure is not significant.·Carbide evolution and its effect on mechanical properties of SA537CL1 hot rolled steel（80mm）during heat treatment（1）The structure of the hot-rolled thick steel is unevenly distributed from the surface layer to the core.It seems to be the normal structure of ferrite and pearlite from the surface layer to 1/4 depth,where the carbide form is lamellar and the granular bainite microstructure appears at the core because of the segregation caused by Mn,Ni and Si.（2）The ferrite grains are refined,the sharp corners of cementite are rounded and the fine carbonitrides are dispersed in the the normal structure after normalizing,resulting in the decrease of strength and the increase of low temperature toughness.After post weld heat treatment,the cementite spheroidizes,coarse carbides appear at grain boundarie and a few compound carbonitrides coarsen,resulting in the decrease of strength and low temperature impact toughness.However,the overall strength of the normal structure is good and the low-temperature impact toughness is excellent.（3）After normalizing and post-welding heat treatment of the abnormal structure at the core,the morphology of the carbide gradually changes from discontinuous block and island to ferrite matrix with granular and acicular carbides.Due to the existence of abnormal structure in each stage of heat treatment,the strength is insufficient and the low-temperature impact toughness is seriously low.The key to solve the problem of unqualified properties at the core is to reduce or eliminate the segregation of Mn by optimizing alloy composition and improving continuous casting,rolling and normalizing processes. |
