On the transformation and growth behavior of intragranular acicular ferrite during semi-solid alloy steel cooling process

Uncommon high temperature intragranular acicular ferrite (IAF) transformation was found and discussed during semi-solid M2 steel cooling processing. The nucleation and growth behavior of IAF was clarified by combining in situ microscopy and other microstructural characterization methods. The effect of IAF on mechanical properties was investigated. The results showed that the fraction, morphology of IAF matrix and carbides was affected by the semi-solid cooling conditions. The high alloying elements and semi-solid controlled cooling contributed to the precipitation of granular and nano vanadium carbide (VC). The precipitation of nano VC carbide during semisolid cooling could play an effective nucleation substrate for IAF. The orientation relationship between the VC carbide and IAF followed (011)alpha//(200)VC, [111]alpha//[011]VC orientation relationship and the lattice misfit was about 4.57%. Semi-solid processing could provide a new method for obtaining IAF. The growth rate of IAF during semi-solid processing is a relatively slow range of 0.7-7 mu m/s. The IAF transformation during semi-solid cooling happened at extremely high temperature but much lower growth rate. The growth behavior might follow the diffusional transformation mode. The strength and hardness for semi-solid specimen were higher as compared with conventional forming procedure specimen. The compressive strength and hardness for semi-solid M2 steel could reach 4474 MPa and 65.6 HRC, respectively. The formation of IAF and fine carbide precipitation during semi-solid cooling process was the key factor for the mechanical response. The co-existence of IAF and fine carbide could be a new microstructural construction strategy to tailor the mechanical properties of semi-solid alloy steel.