| Steel with 6.5wt%Si,which has high permeability,high electrical resistivity and low magnetostriction,is an ideal soft magnetic material.However,with the increasing of Si content,lots of ordered phases and Fe-Si covalent bondings can be formed in the matrix,which leads to the reduction of room temperature plasticity.Therefore,it is very hard to fabricate 6.5wt%Si steel thin sheets by cold rolling method.Strip casting can directly produce as-cast strips with the thickness close to final product,if the 6.5wt%Si steel is fabricated by strip casting,the heavy hot rolling reduction can be eliminated,leading to the cracks being avoided during hot rolling as possible.On the other hand,because of the sub-rapid solidification characteristic,the microstructure of the as-cast strip consists of fine equiaxed grains,the original ductility of 6.5wt%Si as-cast strip can be improved,making the yield of subsequent rolling process to be significantly improved.This paper develops the warm-cold rolling method based on the combination of strip casting and deformation induced disordering by the investigations on the aspects of ordered phases,warm rolling reduction ratio,warm rolling temperature and stress-relief annealing systematically,and according to the investigations on the microstructure,texture,surface qualities and rolling reduction ratio,the optimal process window has been determined.Based on the above results,the cold rolled 6.5wt%Si steel thin sheets with good magnetic properties have been successfully fabricated.The main conclusions of this paper are as follows:(1)The formation of ordered phases during cooling process can be very efficiently decreased by fast cooling.However,because the B2 ordered phase structure is relatively simple,it is very difficult to completely suppress the formation of B2 ordered phase structure,whereas,the DO3 ordered phase structure is relatively complex,making it easily to be suppressed.Therefore,fast cooling method cannot easily make the matrix to be completely disordered.With the increasing of cooling rate,the room temperature plasticity of 6.5wt%Si steel can be improved.When the B2 ordered domain size is about 1.2 nm,and DO3 ordered phase is completely suppressed after fast cooling,the matrix can obtain a certain amount of room temperature plasticity.Although the fast cooling method can improve the room temperature plasticity,it is not suit for relatively large size sheet due to the high internal stress during cooling process.(2)The ordered phase of the 6.5wt%Si steel can be destroyed by the deformation induced disordering phenomenon during warm rolling at relatively low temperature,leading to the reduction of order degree.Owing to the effect of lots of Fe-Si covalent bondings,the dislocations can make a lot of random cross slip during plastic deformation,moreover,because the length of the periodic atomic arrangement for the B2 ordered structure is relatively long,the ordered phase structure in 6.5wt%Si steel has low stability,therefore 15%warm rolling reduction ratio can significantly reduce the size of ordered phase.When the warm rolling reduction ratio exceeds 45%,very small B2 ordered phase can be observed in matrix,at the same time,the warm rolled sheet can be soften.When the warm rolling reduction ratio is 65%,the matrix is complete disordered.After room temperature three-point bending test,it can be found that the room temperature plasticity of 6.5wt%Si steel can also be improved with the increasing of warm rolling reduction ratio.When the warm rolling reduction ratio exceeds 45%,the warm rolled sheet can exhibit a certain amount of room temperature plasticity.Based on the strip casting method,and making the warm rolling reduction ratio to be about 55%,the 0.25 mm thick cold rolled 6.5wt%Si steel thin sheet with good surface quality has been successfully fabricated,and the magnetic induction B8 of cold rolled sheet is measured to be about 1.443 T after annealing,which is improved by about 13.6%with comparison to the CVD product(3)When the temperature exceeds 300?,the B2 ordered phase,which is formed during heat treatment,begins to grow,while the temperature for the growing of DO3 ordered phase is about 350?.In the temperature range from 300? to 400?,the B2 ordered domain size can be monotonically increased,however,when the temperature is 450?,the B2 ordered domain size is suddenly decreased.While for the DO3 ordered phase,in the temperature range from 200 ? to 450?,the size can be increased with the increasing of temperature.According to the precipitation behavior of ordered phases,the above abnormal phenomenon can be attributed to the fact that when the DO3 ordered phase nucleates and grows,the position exchanging between Si atoms and Fe atoms is random in the DO3 ordered phase structure,in which the exchanged Fe atom is the closest to the Si atom,moreover,the DO3 ordered phase is based on the B2 ordered phase,therefore,the original ordered atomic arrangement in the B2 ordered phase structure can be destroyed,leading to the decrease of the B2 ordered phase size.From the tensile test,it can be found that the ductile-brittle transition temperature of hot rolled 6.5wt%Si steel sheet is approximate 300?,when the temperature is higher than 300?,the overall plasticity shows an increased tendency,the corresponding fracture surface morphology is mainly about dimples.After analysis,temperature is the main factor in determining the plasticity of 6.5wt%Si steel,but not the ordered phases.Combining the results,the ideal warm rolling process,which can improve cold rolling property,has been determined,i.e.warm rolling reduction ratio is from 55%to 65%and warm rolling temperature is from 300? to 350?.In practical testing,when the warm rolled sheets with the width less than 80 mm,which is fabricated by the ideal warm rolling process,are performed to the room temperature cold rolling,the success rate is about 80%,and the cold rolled sheets have good magnetic properties.(4)When the disordered warm rolled 6.5wt%Si sheets are performed to the low temperature stress-relief annealing at 400?,the reordering phenomenon can be appeared,at this time,the B2 ordered phase begins to nucleate,while when the temperature is increased to 500?,the DO3 ordered phase begins to nucleate,and both the two kinds of ordered phases can be grown with the increasing of temperature.The mechanical properties of warm rolled 6.5wt%Si thin sheet can be significantly improved by controlling the stress-relief annealing temperature.When the temperature is 350?,the dislocation density in the warm rolled sheet is decreased,while the warm rolled sheet still maintains disordering,therefore,the macrohardness and plasticity of the warm rolled sheet can be decreased and increased,respectively.Although the 350? annealed specimen can obtain a certain amount of room temperature plasticity,the fracture mode is still brittle fracture,it can be concluded that the ordered phases should not be the main reason for the room temperature brittleness of 6.5wt%Si steel,while the Fe-Si covalent bonding,which has high bonding energy,should be the main reason.In practical testing,the edge crack of the cold rolled sheet,which has been taken the stress-relief annealing at 350? for 1 h before cold rolling,can be decreased.(5)In the cases of same rolling reduction ratio,compared to the warm rolled sheet,1)The cold rolled 6.5wt%Si steel thin sheet has better surface quality;2)Because the shear deformation amount of the sheet surface is smaller during room temperature cold rolling,the intensities of y texture in surface and center layers of cold rolled sheet are weaker,especially for the {111}<112>component at surface layer and {111}<110>component at center layer,leading to the intensities of y and ? textures being weaker and stronger after annealing,respectively;3)Because the stored energy for cold rolling process is higher,the recrystallized grain size of the cold rolled 6.5wt%Si steel sheet is smaller after annealing.Therefore,based on the factors above,the 6.5wt%Si steel thin sheet fabricated by cold rolling has better magnetic properties after annealing.Furthermore,the magnetic properties of the cold rolled 6.5wt%Si steel thin sheets can be significantly changed when the hot rolling and cold rolling reduction ratios are different.In the cases of the thickness being 0.3 mm and warm rolling reduction ratio being 55%,when the hot rolling and cold rolling reduction ratios are 47%and 50%,respectively,the cold rolled 6.5wt%Si steel thin sheet has the best magnetic properties.If the cold rolling reduction amount exceeds 0.45 mm,a certain amount of microcracks can be formed in the cold rolled sheet due to the room temperature brittleness and relatively large cold rolling reduction amount,leading to the magnetic properties of cold rolled 6.5wt%Si steel thin sheet being significantly decreased. |
PDF Full Text Request