Three-dimensional Morphology Analysis And Growth Kinetics Of Intragranular Ferrite

Intracrystalline acicular ferrite is the key link to improve the toughness of shipboard steel.Its nucleation morphology mechanism is an important factor affecting the mechanical properties of steel.In-depth study of the threedimensional morphology of ferrite can effectively predict the evolution of steel structure,configure the most reasonable components and effective heat treatment system.Firstly,the optimum precipitation temperature and cooling rate of acicular ferrite were measured by quenching phase change instrument.The heating system of 200?m austenite was determined for the test steel,and the heat treatment process diagram for in situ observation test was used as a reference.The austenite pinning effect was analyzed by calculating the total solid solution temperature of the second phase particles.The three-dimensional threephase particle(FIB)was used to construct the three-dimensional three-phase particle ferrite,grain boundary ferrite and interlocking structure.Morphology,the orientation relationship between grains was characterized by backscattered electron beam(EBSD).The process of ferrite transformation was analyzed by high temperature confocal microscopy in situ observation technique,and the actual growth rate of ferrite was calculated.The theoretical diffusion mode of ferrite internal elements under diffusion-type phase transition is calculated by Matlab.The research conclusions are as follows:1)The phase transition temperature for the precipitation of a large amount of acicular ferrite is 757°C~569°C,the cooling rate is controlled at 5°C/s ~15°C/s;the test steel is isothermal at 1200°C for 2 hours,you can get 200 ?m The austenite;NbC has a solid solution temperature range of 825°C~1051°C,which is consistent with the temperature corresponding to the abrupt change in austenite size in the test,indicating that the NbC pinning effect is obvious.2)After 750 °C isotherm for 1 min,the acicular ferrite growth rate is faster,the intergranular collision impact force is larger,and there are a lot of defects on the surface of the obtained acicular ferrite,which is not conducive to the performance of the steel;after 700 °C isothermal 1 min The growth rate of acicular ferrite is slower and the impact impact is small.The three-dimensional morphology of ferrite is smooth and octagonal cone,and there is no sub-grain,which is conducive to the formation of dense interlocking structure and improve the toughness of steel.3)The three-dimensional morphology of the ferrite interface is flat,and there are a large number of gullies on the surface.The three-dimensional shape of the grain boundary ferritic body is a triangular pyramid with a pointed top,and the three-dimensional shape of the ferrite grain of Wei's is "nail".The shape is composed of one piece of Wei's body and two pieces of Wei's body.These phenomena are not conducive to the performance of steel.4)The solute diffusion model of quasi-equilibrium model and local distribution model is established,which is consistent with the experimental ferrite growth rate,and explains the formation mechanism of acicular ferrite threedimensional morphology.This model can accurately calculate the composition of different test steels.The diffusion mode of solute elements at heat treatment temperature and accurate prediction of the three-dimensional morphology of ferrite have important significance for the selection of heat treatment system for steel materials.Figure 62;Table 5;Reference 67.