Study On The Intra-granular Ferrite Induced By The Composite Inclusions With Low Frequency Pulse Magnetic Field

Oxide metallurgy technology is a new point of view in the field of materials and metallurgy,which use the inclusions in steel to induce the nucleation of ferrite,to achieve the purpose of refining the microstructure of steel and obtain excellent toughness.In order to study the effect of intra-granular ferrite on the refinement of microstructure in low carbon steel,inclusions were prepared in accordance with the condition of inducing intra-granular ferrite structure to study the distribution of inclusions and the mechanism of grain refinement under the effect of pulsed magnetic field,the effect of cooling rate on the formation of needle ferrite and the effect of isothermal treatment on the nucleation of ferrite cores were analyzed.Taking a low carbon steel containing artificial preparation of manganese sulfide inclusions as the research object,applying the pulse magnetic field whose cycle is 0.5s and voltage is 200 V to the process of liquid-solid and solid-solid transition.The results showed that mainly two types of inclusions which can induce intra-granular ferrite were observed,one was artificially added manganese sulfide inclusions,the other one were AlSi-Ti-O-S-Mn composite inclusions spheroidized by high melting point inclusions.The continuous cooling transition curves of the test steels at different cooling rates were determined and found that only at the medium cooling rate intra-granular ferrite can be obtained,and the best cooling rate was 5~7?/s.When the temperature was at 730~600?,with the decreasing of the temperature,the shape of intra-granular ferrite was changed from polygon to acicular,along with the increasing of the axial ratio and anisotropic increase,the nucleation incubation time of ferrite on inclusions is longer than that of grain boundary.The austenite grain size increased with the increase of austenitized temperature,and under the condition that the grain size was less than 100?m or the austenitized temperature was less than 1200? was not easy to form the intra-granular acicular ferrite.