Research Of Microstructural Characteristics And Properties Of Acicular Ferrite In High-strength Micro-alloy Steels

In this paper two different heat-treatment proces including static isothermal quenching with different temperature and dynamic thermo-mechanical process with different parameters are designed to study the acicular ferrite in a commerial X70 grade high-strength micro-alloy steels. Different forms of acicular ferrite were analyzed in the same place were investigated through the use of OM、SEM、TEM. The acicular ferrite were definited through the use of EBSD technical and also the contaction of the microstructure and performance of the acicular ferrite were analysied.The results shown that AF clusters which was marked out by Vickers hardness intentions exhibit different morphology in OM 、SEM and EBSD maps after different heat treatment process. The ferrite after isothermal quenching treatment all show a shape of block instead of acicular or long strip;The AF grains at as-rolled state are in a separate forms which has an uncompelete radial or chrysanthemum shape. While after thermal simulation process AF clusters exihibit a complete shape of radial or chrysanthemum in OM and SEM maps,meanwhile after EBSD analysis the adjacent grains has a small misorientation angles and thus show a similar color in Euler maps.The ?- 40 impact toughness of steels after isothermal quenching treatment has been measured by Charpy impact test,the result shows that the steels after three isothermal quenching treatment processes all get a good impact toughness,the impact toughness are all over 280 J. The EBSD and TEM technique was used to analysed the microstructure of the typical AF to illstrate the reason why it has good impact toughness.The microstructure of acicular ferrite was analysised by TEM technique,the acicular ferrite shows a shape of strip or acicular, with some apparent embosses on its coarse surface.Meanwhile large numbers of dislocations and sub-units appeared in the acicular ferrite. There are also a lot of precipitates in the matrix of ferrite. So the AF can improve its toughness through the influence of dislocation-strengthening and percipitation-strengthening.As for EBSD,the distribution of misorientation angles of AF was analysed,the results show that with the existence of typical radial AF clusters. Two peaks appeares at angles of 5-10° and 50-60° in the distribution histograms of misorientation angles,when the misorientation angles belong to 5-10°,the dislocation-strengthening is the mainly strengthen method and whenit comes to 50~60°,what was called HAGBs, these HAGBs can act as obstacles to prevent the propagation of crack, So the AF can improve its toughness through these methods.Meanwhile,the mean effective grain size of acicular ferrite can also be calculated by EBSD technique, the grain size of isothermal quenching treatment is smallest, shown 1.63μm,meanwhile the effective grain size of the three state are all blew 5μm and the fraction of effective grain size below 2 μm reaches to 80%.It means that the grain size of all the three states are very small,it can also explains the reason why the steels containing of acicular ferrite has a good impact properties.