Mechanical Properties And Ductility Improvement Mechanisms Of Rare Earth Microalloyed High Silicon Electrical Steel

The high silicon electrical steel(Fe-6.5wt.%Si alloy)possesses excellent soft magnetic properties,such as high permeability,low iron loss and near-zero magnetostriction,etc.,making it an ideal soft magnetic material in high frequency electrical equipment,energy-saving transformers and electric vehicles,etc.However,the severe brittleness and poor cold forming ability of the alloy makes it difficult to fabricate the thin sheet on industrial scale via common casting-rolling method,which seriously restricts a wide application of the alloy.It can be found in many studies on rare earth micro-alloying in various metallic materials that the rare earth addition can cause significant changes on structures,phase transformations,mechanical,physical and chemical properties,etc.By focusing on the brittleness essence of high silicon electrical steel,this paper presented to improve plastic deformability and thus to decrease cold rolled thin sheet fabrication difficulty of high silicon electrical steel via rare earth micro-alloying method,and mainly investigated bonding properties and alloying effects of rare earth elements in high silicon electrical steel,studied the influences of rare earth on the structures and mechanical properties,and the corresponding physical nature and the condition to improve the deformability of the alloy by rare earth addition were understood.Effect and its mechanisms of rare earth on reducing thin sheet rolling fabrication difficulty of the alloy were also studied.The results via first-principles calculation based on density functional theory(DFT)indicates that the bonding energy levels and orbitals of rare earth atoms in high silicon electrical steel are similar,and there exists significant electron transfer between rare earth atom and other atoms in the alloy hence induced significant change on bonding properties of the alloy,and the rare earth Y atom caused partial Fe atoms to transfer more electrons to Y and other atoms,whereas the rare earth La and Ce can transfer electrons to partial Fe and Si atoms significantly.With an increasing rare earth content,the tensile deformability at 400℃ of as-cast specimens increased and then decreased,and the deformability improvement effect is better when the content of Y,La,Ce and Gd in the alloy is about 0.01wt.%,0.01wt.%,0.02wt.%and 0.03wt.%,respectively,and the tensile elongation to failure at 400℃ can be increased from about 7%to about 22～23%maximally by rare earth addition.Investigation results on microstructures and crystal structures of ingots with a various Ce content shows that the grain size of the alloy decreased significantly when Ce content increased over 0.021wt.%,and the enrichment near the grain boundary of oversaturated rare earth element is the main cause for the grain size refinement.Ordered degree presented a decreasing tendency with an increasing Ce content,and the electron transfer effect of rare earth element makes the Fe and Si atoms tend to be dragged by rare earth atom during the ordering process of the alloy,which will hinder the disorder-order transformation and the formation of B2 ordered structure.Whereas the ordered degree will stop decreasing when Ce content increases over 0.021wt.%.Solid solution rare earth atoms improved tensile deformability at 400℃ of the alloy via the effects of ordered degree reduction and covalent interaction decrease of partial Fe-Fe bonds and Si-Fe bonds.Hence,taking full advantage of deformability improvement effect of rare earth by increasing rare earth solid solution content,and preventing inter-granular brittleness caused by enriched rare earth near grain boundary via preventing rare earth content from exceeding the solid solubility is a basic condition to improve the deformability of the alloy via utilizing rare earth.The uniform tensile elongation to failure of hot-rolled sheet specimens was improved significantly at 300～500℃ due to that the 0.02wt.%Ce addition reduced the content and size of B2 ordered structure,which will contribute to decrease the warm-rolled thin sheet fabrication difficulty.Although the microstructures and structures of non-rare earth addition alloy and Ce addition alloy were of no obvious difference after heavy deformation by hot forging,hot rolling and warm rolling,the tensile yield strength σ0.2 was decreased from 1618.5MPa to 1515.3MPa,the ultimate tensile strength was decreased from 1652.9MPa to 1552.5MPa and the tensile elongation to failure was increased from 0.67%to 0.80%of warm rolled specimens by 0.02wt.%Ce addition at room temperature respectively.The deformation resistance reduction and deformability improvement at room temperature was mainly caused by covalent interaction decrease of partial Fe-Fe bonds and Si-Fe bonds induced by rare earth.Adding rare earth Ce whose content below its solid solubility can decrease the cold-rolled thin sheet fabrication difficulty,whereas caused no obvious damage to magnetic properties of cold-rolled thin sheet after annealed at 1200℃.