Deformation Softening Mechanism Of Warm Rolled Fe-6.5%Si Steel

As the ultra-high voltage transmission,high frequency information technology and national defense industry rapidly develop in recent years,the market demand for Fe-6.5%Si steel has been increasing in order to further reduce the noise and iron loss,as well as improve energy efficiency.Fe-6.5%Si steel exhibits excellent soft magnetic properties,such as high permeability,near-zero magnetostriction,low coercive force and iron loss,thus it has wide application prospects in high-frequency fields.Fe-6.5%Si steel experiences a significant decline in the plastic deformation capacity due to the effect of brittleness at room temperature.It is almost impossible that the well-performance Fe-6.5%Si sheets are manufactured using the conventional rolling process,which has severely restricted its manufacture and application in the industrial fields.In this paper,the manufacturing procedure of hot rolling+annealing+warm rolling is adopted,and the deformation softening mechanism of warm rolled Fe-6.5%Si steel is investigated to provide the theoretical basis and technological support for industrial production.The main work and experimental results of this paper are as follows:(1)Not only are the microstructures of Fe-6.5%Si steel investigated during hot rolling and annealing,but also the effects of cooling methods after hot rolling and annealing on the order-disorder transformation are elaborated.The recrystallization degree of hot-rolled sheet is high because of the higher finish rolling temperature,in this case,the {111}<110>and {111}<112>orientation grains with high Taylor factor exist in {100}<001>shear zone,which provides a lot of nucleation positions for the subsequent recrystallization.The ferrite grains after hot rolling refine gradually with increasing rolling reduction ratios.After Fe-6.5%Si steel is annealed at 950? for 1 h,the equiaxed recrystallization grains reach more even,which reduces the tendency to the brittleness of materials.The restraining of ordered phase transition can be considered as a process accompanied with the refinement of grains.When adopting the water-cooling process the higher cooling rate,the restrained nucleation and growth of B2 ordered phase induce a decline in the phase size.After the temperature quickly decreases to B2-DO3 phase transition temperature,it is detrimental to DO3 ordered phase nucleation in B2 phase.For this reason,in contrast to A2-B2 phase transition,the degree of inhibition on B2-DO3 phase transition by the water-cooling process is more prominent.The B2 phase content significantly decreases,meanwhile the generation of DO3 phase is completely suppressed by adopting the water-cooling process after hot rolling and annealing,which reduces the anti-phase boundary energy and improves the plastic deformation ability of Fe-6.5%Si steel.(2)The order-disorder transition temperature is determined by differential scanning calorimeter(DSC)method,and the influence rules of B2 and DO3 ordered phases on the hardness of Fe-6.5%Si steel are systematically studied,as well as the reordering phenomenon within holding time before warm rolling is analyzed.DSC test results show that the A2-B2 and B2-DO3 phase transition temperatures are about 760? and 640? respectively,and formation temperature of DO3 ordered phase is 400?.B2 ordered phase generated in steel is attributable to ordering of Fe and Si atoms in the nearest-neighbor position,while based on B2 ordered phase,DO3 ordered phase forms depending on Fe and Si atoms in the next-nearest-neighbor position.After the unstable DO3 ordered phase grows up to some extent,the action of spinodal decomposition makes its stability enhanced.The nucleation and growth process of B2 ordered phase is very similar to the growth process of recrystallization grains,and the sizes of B2 phase gradually increase with the increase of holding time.While DO3 ordered phase is more similar to secondary phase precipitates in Fe-6.5%Si steel,and the growth of DO3 ordered phase nucleated on B2 phase matrix is a process of ordered structures swallowing up each other.In B2 phase zone,the ordering time does not much affect the hardness of Fe-6.5%Si steel,while the values of microhardness in B2+DO3 phase zone show a rapid upward trend with the extended holding time,which indicates the higher hardness value mainly comes from DO3 ordered phase.The reordering degree depends on the ordering degree of annealed sheets,and the residual B2 and DO3 ordered phases in the annealed sheets no longer need the reordering again,and go directly into the matrix,leading to an increase in the reordering degree.The lower the order degree,the lower the reordering degree of annealed sheet,and a large number of untransformed A2 disordered phase will be conducive to warm rolling process smoothly.(3)The influences of warm rolling temperature on the ordered structures and dislocation configuration of Fe-6.5%Si warm-rolled sheets are investigated,meanwhile a suitable warm rolling temperature is confirmed,thereby decreasing the content of ordered phases and soften Fe-6.5%Si steel matrix.In 450?550? temperature range,the ordered degree of warm-rolled sheets increases significantly with decreasing the warm rolling temperatures,and the dislocation glide softening is the main softening factor of Fe-6.5%Si warm-rolled sheet.While the change in the content of ordered phases is not obvious in 600?650? temperature range,and the dynamic recovery plays a vital role in the soften effect.When the warm rolling is held at 450?,the larger value of kernel average misorientation(KAM)exists in shear band due to the low extent of recovery.While the KAM value of 650? warm-rolled sheet along grain boundaries is high,which shows that the stress concentrates on the grain boundaries,meanwhile there is a significant increase in the total amount of CSL boundaries with low energy and strong bonding force that inhibit the stress concentration and intergranular cracking,thereby reducing the boundary brittleness of warm-rolled sheets.With the increasing of rolling temperatures,the surface indentation morphology transforms from the sink-in type into the pile-up type,and the value of microhardness decreases from 415 HV to 369 HV.The warm-rolled sheets have fractured at the elastic stage in 450?500?temperature range,while 600? warm-rolled sheet exhibits obvious plastic deformation,therefore,to ensure the smooth development of warm rolling process,the warm rolling temperature should be set above 600?.(4)The influences of warm rolling reduction ratio on the ordered structures and dislocation configuration of Fe-6.5%Si warm-rolled sheets are investigated,and the relationship between ?-orientation texture and coincidence site lattice(CSL)boundaries is explored,thereby improving the working embrittlement of Fe-6.5%Si steel.When the pass reduction ratio is improved from 8%to 14%at 600? during warm rolling,the microhardness value of warm-rolled sheet rises from 362 HV to 417 HV.The higher superdislocation density exists in warm-rolled sheets with the lower pass reduction ratio,however,in contrast to the softening effect of dislocation glide and dynamic recovery,the softening effect is much more notable than the work hardening.When the warm rolling reduction ratio increases from 50%to 85%,the superdislocations in pairs are changed into the single dislocation prone to cross-slip.In this case,the dislocation mobility is enhanced and the dislocation glide softening effect is pronounced.Then the corresponding nano-hardness value is reduced from 6.4 GPa to 3.7 GPa,while the fracture deflection value increases from 6.2 mm to 18.2 mm.Adopting the higher warm rolling reduction ratio is beneficial to form the strong ?-fiber textures,the {111}<110>and {111}<112>orientations with strong intensity means that the frequency of CSL boundaries is high.At the same time,the increased total CSL boundaries enhance the intergranular bonding strength,effectively prevent the expansion of cracks along the grain boundaries,in this case,the mixed fracture mode of inter-and trans-granular cleavage is transformed into cleavage+dimple mixed fracture,which significantly improves the working embrittlement of warm-rolled sheets.(5)The thermodynamic and dynamic aspects of order-disorder transformation are studied,furthermore,regarded as the main work softening mechanism of Fe-6.5%Si steel during warm rolling,the ordered phase transformation and dynamic recovery,as well as the dislocation glide are revealed.In Fe-6.5%Si steel,Fe atom with the attribute of the strongest diffusivity can cause high-concentration thermal vacancies in B2 and DO3 ordered structures.Based on the vacancy diffusion mechanism,a large number of vacancies can increase the anti-phase boundary energy in steel due to the increasing atom movability,and the dislocation glide has an obvious effect in promoting the vacancy diffusion in the crystal during warm rolling,thereby completing the transition from B2 ordered structure to A2 disordered structure.Fe and Si atoms of DO3 ordered structure do not slip along a dislocation line,but a layer and then a layer of atoms slip in turn,exhibiting a disordered state,which results from the superdislocation glide during plastic deformation.B2 and DO3 ordered structures are broken and cut through superdislocation movement during the deformation process of warm rolling,which hinders the growth and amalgamation of ordered phases and reduces the ordered degree of the matrix,and then the anti-phase boundary energy is reduced accordingly.Even though the dislocation density in warm-rolled sheets will increase the risk of work hardening,but the destruction of ordered phases caused by the deformation decreases the difference of energy state between the antiphase domain boundary and the matrix,and reduces the increasing range of energy caused by the independent slip of partial dislocations.Therefore,the softening effect caused by dislocation glide is much more notable than the work hardening effect due to the increase of dislocation density,which can contribute to the plastic deformation ability of Fe-6.5%Si steel.In this paper,aiming at the manufacturing procedure of hot rolling+annealing+warm rolling,the softening mechanisms of rolling deformation of Fe-6.5%Si steel are revealed,and the improvement of plastic deformation ability are realized,which provides a theoretical basis for production development of Fe-6.5%Si steel.