Study On Hot Rolling Deformation Mechanism Of TiB₂-reinforced Steel

As a new type of automobile steel,the TiB2-reinforced steel with high elastic modulus,low density and other more advantages,has a wide range of applications.The addition of TiB2 particles makes it more complicated than the traditional materials in the control of the structure.It is easy to cause cracks if the hot rolling process is inappropriate.In this context,this study was carried by MMS-200 thermal simulation testing machine,DIL805A/D thermal deformation instrument,0450×450 two-high reversing hot mill,scanning electron microscope and mechanical properties testing methods to complete some researches:?the dynamic recrystallization of austenite under different thermal simulation process was studied by using true stress-strain curves;?the effect of different thermal deformation parameters on the microstructure and properties of TiB2-reinforced steel;?the effect of different temperatures on the stress relaxation rate of TiB2-reinforced steel in the stress relaxation test;?the improved hot rolling process of TiB2-reinforced steel was proposed in order to inhibit the generation of cracks finally.The results and conclusions are as follows:(1)The effect of different thermal deformation parameters on the microstructure and properties of TiB2-reinforced steel.For hypereutectic composition(marked as TB-13),the debonding of large TiB2 particles is obviously reduced with the increase of the deformation temperature.For hypoeutectic composition(marked as TB-9),when the deformation temperature is changed,no particle breakage occurs in the structure,and the microstructure is very dense.The interface between the eutectic particles and the matrix is well bonded and regular.There is no obvious interface debonding.Whether the strain rate is 1/s-1 or 0.01/s-1 and whether the reduction is 30%or 60%,the effect of the strain rate and the reduction on the microstructure of both tested steels was not obvious,especially for the debonding of large particles.(2)Analysis of thermal simulation of high temperature cracking can be obtained:the reason caused the deformation cracks of TiB2-reinforced steel are the interface debonding between the particles and the matrix.The higher temperature makes the greater difference between the hardness of the matrix and the particles,which results in more serious stress concentration at the interface and the interface is more prone to be debonded.That results in cracking phenomenon finally.In fact,with the decrease of temperature,the debonding is more serious.With the increase temperature,stress relaxation caused by austenite dynamic recrystallization eliminates higher degree of dislocations before interface debonding.In addition,it is further verified that the cause of hot-rolled plate cracks of TiB2-steel is the interface debonding between the particles and the matrix by the hot-rolled plate cracks and tensile fracture microstructure.(3)For TB-13,the content of large particles is higher,as the aspect ratio of TiB2 particles is larger,the debonding of particles makes the stress level drop significantly.The curves show that the stress level is no longer decreased with the increase of temperature.For TB-9,all particles are small eutectic particles.All do not produce any particle debonding at all deformation temperatures and the interfacial adhesion is good.The flow stress-strain curve changes with the deformation temperature accorded with the normal austenite dynamic response recrystallization law,that is,the flow stress decreases with the increase of deformation temperature in the deformation process.(4)For both two tested steel,the stress relaxation rate increases with the deformation temperature increases,that is,the stress relaxation is more serious.This is because when the temperature is high,as the temperature increases,the degree of austenite dynamic recovery recrystallization is greater and the stress relaxation rate is higher in a short period after the deformation.Further,the dynamic recrystallization of austenite under high temperature greatly ameliorates the debonding phenomenon of the interface between the matrix and TiB2 particles caused by high dislocation.(5)Based on the research and analysis,an improved hot rolling process is proposed.Control the temperature of furnace insulation,use the recrystallization law of austenite and combine with repeated rolling process to achieve the purpose of resisting cracks.The idea put forward so that the hot cracking slab cracking problem can be effectively resolved.