Preparation method of gradient high-silicon steel by molten salt electrolysis

Abstract

A preparation method of gradient high-silicon steel by molten salt electrolysis includes: weighing the inorganic fluoride salt and the inorganic silicon salt, mixing them uniformly and then drying; heating the electrolysis container over the melting point of the electrolyte, passing the inert gas through the electrolysis container, and connecting the electrode to the power supply to perform constant current electrolysis, after the electrolysis is finished, the cathode is taken out, washed and dried, placing the dried cathode in a constant temperature region of an annealing furnace; under a protective gas atmosphere, heating the cathode to the target temperature, and maintaining the temperature for a period of time; after the heat treatment, cooling the cathode to the room temperature, during which the cathode is always placed in the furnace.

Claims

1. A preparation method of gradient high-silicon steel by molten salt electrolysis, characterized in that it comprises the following steps: (1) electrolyte preparation: weighing the inorganic fluoride salt and the inorganic silicon salt, mixing them uniformly and then drying; wherein the molar ratio of the inorganic silicon salt to the inorganic fluoride salt is not more than 1:9, the inorganic silicon salt includes Na.sub.2SiF.sub.6, K.sub.2SiF.sub.6, Li.sub.2SiF.sub.6 or SiO.sub.2; (2) molten salt electrolysis: placing the electrolyte in an electrolysis container, then immersing a cathode and an anode into the electrolyte, heating the electrolysis container to 550° C.-950° C., passing the inert gas through the electrolysis container, and connecting the electrode to the power supply to perform constant current electrolysis, after the electrolysis is finished, the cathode is taken out, washed and dried, wherein the cathode is low silicon steel or pure iron, and the anode is single crystal silicon or polycrystalline silicon, the current density during the electrolysis is 1-20 mA/cm.sup.2, and the thickness of the cathode is 0.05 mm-1 mm; (3) high temperature annealing: placing the dried cathode in a constant temperature region of an annealing furnace; under a protective gas atmosphere, heating the cathode to the target temperature which is 1000-1200° C. at the rate of 5-10° C./min, and maintaining the temperature for a period of time; after the heat treatment, cooling the cathode to the room temperature at the rate of 5-10° C./min ,during which the cathode is always placed in the furnace, and then the gradient high-silicon steel with a uniform silicon concentration difference from the outside to the inside is obtained, wherein the expression of the temperature maintaining time t/min, the target temperature , T′/° C., the thickness of the cathode , x/mm, the difference of the target silicon content in the center of the cathode and the initial silicon content of the cathode, ΔSi/%, and the silicon content gradient k is as follows: $t=\frac{106\times 10^8\times e^{\frac{-T^{\prime }}{70}}\times x^{0.75}\times \Delta \mathrm{Si}}{k^{1.05}},$ the expression of the silicon concentration gradient k, the difference between the target silicon content on the surface of the cathode and the target silicon content in the center of the cathode, ΔSt%, and the thickness of the cathode, x/mm is: $k=\frac{△{\mathrm{Si}}^{\prime }}{x/2};$ wherein, the target silicon content on the surface of the cathode is not less than 6% and the target silicon content in the center of the cathode is not less than 3%.

2. The preparation method according to claim 1, characterized in that the inorganic fluoride salt in the step (1) comprises at least one of LiF, NaF and KF.

3. The preparation method according to claim 1, characterized in that both the inorganic fluoride salt and the inorganic silicon salt have a purity no less than 98%.

4. The preparation method according to claim 1, characterized in that the drying in the step (1) specifically comprises heating the mixture of the inorganic fluoride salt and the inorganic silicon salt to 200-300° C. in a vacuum furnace and maintaining the temperature for 12 h or more.

5. The preparation method according to claim 1, characterized in that the components of the low silicon steel in the step (2) comprises 0 wt %≤Si≤4.5 wt %, Fe≥95 wt %, and the balance being unavoidable residual elements.

6. The preparation method according to claim 1, characterized in that the purity of the anode in the step (2) is not less than 98%.

7. The preparation method according to claim 1, characterized in that the inert gas in the step (2) is argon or nitrogen with a purity no less than 99%; and the protective gas in the step (3) is argon or nitrogen with a purity no less than 99%.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is the scanning electron micrograph of the electrolytic product of Example 1.

(2) FIG. 2 is the graph showing the element content distribution from the surface to the interior of the diffusion layer of the electrolytic product of Example 1.

(3) FIG. 3 is the scanning electron micrograph of the heat-treated product of Example 1.

(4) FIG. 4 is the graph showing the Si content distribution of the heat-treated product of Example 1.

DETAILED DESCRIPTION OF THE DISCLOSURE

Example 1

(5) 30 g LiF with a purity of 99.5%, 68 g KF eutectic salt with a purity of 99.9% and 5.0 g K.sub.2SiF.sub.6 with a purity of 99.9% were weighed and mixed at 300 r/min for 1 h by a planetary high-energy ball mill. The iron crucible which was dried by wiping with alcohol before use, filled with the mixture was placed in a horizontal furnace, the temperature of the furnace filled with high-purity Ar was maintained at 200° C. for 12 h.

(6) A single crystal Si rod with a diameter of 5 mm and a length of 50 mm was used as an anode, and a low-silicon steel sheet with a thickness of 0.3 mm was used as a cathode. The chemical composition of the low-silicon steel sheet, counting by mass percentage, were Si: 3.0%, C: 0.0022%, Als: 0.55%, Mn: 0.31%, O: 0.0060%, P: 0.011%, S: 0.0017%, N: 0.0019%, Ti: 0.0018%, V: 0.0014%, Nb: 0.0015%, B: 0.0049%, Sn: 0.105%, and the balance being Fe. The cathode silicon steel sheet was successively ground 2 min on 240-mesh, 500-mesh, 800-mesh and 1000-mesh sandpaper, and then polished on 1-3-mesh metallographic sandpaper. The dimensions of the silicon steel sheet were measured with a vernier caliper, and the surface area immersed in molten salt during electrolysis was calculated to be 2.08 cm.sup.2 (calculated as 2 cm immersed). The electrode was successively washed with deionized water and absolute ethanol on the surface and then dried in a 70° C. oven for use.

(7) The above-mentioned iron crucible filled with salt was horizontally placed in a constant temperature region of a vertical resistance furnace with crucible clamps. A low-silicon steel sheet cathode was fixed on one end of a stainless-steel rod with a 0.1 mm iron wire and was inserted into the middle of the furnace tube through a pipe orifice of an upper furnace cover of the resistance furnace. The Si rod anode was placed by the same process. After checking the resistance furnace installed with good air tightness, high-purity Ar gas and circulating water were introduced into the furnace, and then the furnace was heated to 600° C. at 5° C./min. After reaching the target temperature, the temperature was maintained for 15 min to completely melt the mixture.

(8) A multimeter was connected above the two electrodes, the position of molten salt and the depth of immersion of electrode into the molten salt was measured. The anode and cathode were both immersed into the molten salt for 2 cm. After fixing the electrode position, the electrode was connected to the power supply, with the constant current of 10.4 mA (5 mA/cm.sup.2) and the electrolysis was performed for 3 h. After electrolyzing, the electrode was slowly lifted 2 cm above the molten salt surface and cooled to room temperature at the rate of 5° C./min. The electrode sheet was taken out from the furnace, the residual molten salt on the surface of silicon steel sheet was wiped by cotton dipping deionized water. Then the deionized water on the surface was remove by alcohol. and then the silicon steel sheet was dried in a 70° C. oven. The scanning electron micrograph of the electrolytic product could be seen in FIG. 1, and the element content distribution diagram of the electrolytic product diffusion layer from the surface to the inside could be seen in FIG. 2.

(9) The specific process of the diffusion annealing heat treatment is as follows.

(10) The electrolytic product was placed in a constant temperature region of an annealing furnace, with carbon blocks placed at the mouth of the annealing furnace and the cover tightly closed. After checking that the gas tightness was good, the annealing furnace was cleaned with high-purity Ar gas for three times to remove residual oxygen in the furnace chamber, and finally high-purity Ar was introduced. The furnace was heated to 1150° C. at the rate of 5° C./min and the temperature maintaining time was set to 18 min according to the Si concentration gradient k of 16.0%.Math.mm.sup.−1. After heating treatment, the sample is cooled to room temperature in the furnace at a rate of 5° C./min so as to obtain a high-silicon steel with a uniformly decreasing of Si content gradient. The scanning electron micrograph of the heat-treated product was shown in FIG. 3, and the Si content distribution of the heat-treated product was shown in FIG. 4. After determination, the Si content on the surface of the gradient high-silicon steel obtained in this example was 6.5%, the inside Si content was 4.1%, and the Si content decreased uniformly in a gradient from the surface to the inside of high-silicon steel. The concentration gradient was 16.0%.Math.mm.sup.−1, with the difference from the presetting gradient value being within 1%, and satisfying the expected requirements. Iron loss P.sub.10/50=0.65 W.Math.kg.sup.−1, P.sub.10/400=8.7 W.Math.kg.sup.−1, tensile strength was 680 MPa, elongation was 7.5%. The surface of the sample was smooth, without dendrites. No defects such as pores were observed. The density of the product was high.

Example 2

(11) This example was prepared by the same process of Example 1, except that the electrolysis current was set to 6.28 mA (3 mA/cm.sup.2) and the electrolysis was performed for 4 h. The temperature maintaining time of the heat treatment was set to 8 min according to the Si concentration gradient k of 16.0%.Math.mm.sup.−1.

(12) After determination, the Si content on the surface of the gradient high-silicon steel obtained in this example was 6.0%, the inside Si content was 3.5%, and the Si content decreased uniformly in a gradient from the surface to the inside of high-silicon steel. The concentration gradient was 16.6%.Math.mm.sup.−1, with the difference from the presetting gradient value being within 1%,and satisfying the expected requirements. Iron loss P.sub.10/50=0.77 W.Math.kg.sup.−1, P.sub.10/400=9.1 W.Math.kg.sup.−1, tensile strength was 630 MPa, elongation was 8.5%. The surface of the sample was smooth, without dendrites. No defects such as pores were observed. The density of the product was high.

Example 3

(13) This example was prepared by the same process of Example 1, except that the chemical composition of low-silicon steel sheet ,counting by mass percentage, are Si: 2.5%, C: 0.0022%, Als: 0.55%, Mn: 0.31%, O: 0.0060%, P: 0.011%, S: 0.0017%, N: 0.0019%, Ti: 0.0018%, V: 0.0014%, Nb: 0.0015%, B: 0.0049%, Sn: 0.105%, and the balance being Fe. The electrolysis current was set to 4.16mA (2mA/cm.sup.2) and the electrolysis was performed for 6 h. The temperature maintaining time of the heat treatment was set to 20 min according to the Si concentration gradient k of 13.0%.Math.mm.sup.−1 t.

(14) After determination, the Si content on the surface of the gradient high-silicon steel obtained in this example was 5.5%, the inside Si content was 3.5%, and the Si content decreased uniformly in a gradient from the surface to the inside of high-silicon steel. The concentration gradient was 13.4% mm.sup.−1, with the difference from the presetting gradient value being within 1%, and satisfying the expected requirements. Iron loss P.sub.10/50=0.81 W.Math.kg.sup.−1, P.sub.10/400=9.9 W.Math.kg.sup.−1, tensile strength was 610 MPa, elongation was 10.1%. The surface of the sample was smooth, without dendrites. No defects such as pores were observed. The density of the product was high.

Example 4

(15) This example was prepared by the same process of Example 1, except that the electrolysis current was set to 20.8 mA (10 mA/cm.sup.2) and the electrolysis was performed for 2 h. The temperature maintaining time of the heat treatment was set to 16 min according to the Si concentration gradient k of 17.0%.Math.mm.sup.−1.

(16) After determination, the Si content on the surface of the gradient high-silicon steel obtained in this example was 6.5%, the inside Si content was 4.0%, and the Si content decreased uniformly in a gradient from the surface to the inside of high-silicon steel. The concentration gradient was 16.6%.Math.mm.sup.−1, with the difference from the presetting gradient value being within 1%, and satisfying the expected requirements. Iron loss P.sub.10/50=0.66 W.Math.kg.sup.−1, P.sub.10/400=8.7 W.Math.kg.sup.−1, tensile strength was 670 MPa, elongation was 7.5%. The surface of the sample was smooth, without dendrites. No defects such as pores were observed. The density of the product was high.

Example 5

(17) This example was prepared by the same process of Example 1, except that the mass weight and the purity of K.sub.2SiF.sub.6 were 2.6 g (0.5 mol %), and 99.9% respectively, and the electrolysis was performed for 6 h. The temperature maintaining time of the heat treatment was set to 17 min according to the Si concentration gradient k of 16.0%.Math.mm.sup.−1.

(18) After determination, the Si content on the surface of the gradient high-silicon steel obtained in this example was 6.3%, the inside Si content was 4.0%, and the Si content decreased uniformly in a gradient from the surface to the inside of high-silicon steel. The concentration gradient was 15.3%.Math.mm.sup.−1, with the difference from the presetting gradient value being within 1%, and satisfying the expected requirements. Iron loss P.sub.10-50=0.65 W.Math.kg.sup.−1, P.sub.10-400=8.9 W.Math.kg.sup.−1, tensile strength was 680 MPa, elongation was 7.7%. The surface of the sample was smooth, without dendrites. No defects such as pores were observed. The density of the product was high.

Example 6

(19) This example was prepared by the same process of Example 1, except that the cathode was a pure iron sheet with a thickness of 0.3 mm, and the electrolysis was performed for 6 h. The temperature maintaining time of the heat treatment was set to 56 min according to the Si concentration gradient k of 17.0%.Math.mm.sup.−1.

(20) After determination, the Si content on the surface of the gradient high-silicon steel obtained in this example was 6.0%, the inside Si content was 3.6%, and the Si content decreased uniformly in a gradient from the surface to the inside of high-silicon steel. The concentration gradient was 16.1%.Math.mm.sup.−1, with the difference from the presetting gradient value being within 1%, and satisfying the expected requirements. Iron loss P.sub.10/50=0.79 W.Math.kg.sup.−1, P.sub.10/400=9.2 W.Math.kg.sup.−1, tensile strength was 620 MPa, elongation was 8.6%. The surface of the sample was smooth, without dendrites. No defects such as pores were observed. The density of the product was high.

Example 7

(21) This example was prepared by the same process of Example 1, except that the electrolysis temperature was set to 700° C. and current to 6.28 mA (3 mA/cm.sup.2). The temperature maintaining time of the heat treatment was set to 17 min according to the Si concentration gradient k of 16.0%.Math.mm.sup.−1.

(22) After determination, the Si content on the surface of the gradient high-silicon steel obtained in this example was 6.4%, the inside Si content was 4.0%, and the Si content decreased uniformly in a gradient from the surface to the inside of high-silicon steel. The concentration gradient was 16.0%.Math.mm.sup.−1, with the difference from the presetting gradient value being within 1%, and satisfying the expected requirements. Iron loss P.sub.10/500=0.68 W.Math.kg.sup.−1, P.sub.10/400=8.8 W.Math.kg.sup.−1, tensile strength was 660 MPa, elongation was 7.9%. The surface of the sample was smooth, without dendrites. No defects such as pores were observed. The density of the product was high.

Example 8

(23) This example was prepared by the same process of Example 1, except that during the diffusion annealing heat treatment, the furnace was heated to 1000° C. and the temperature maintaining time was set to 35 min according to the Si concentration gradient k of 16.0%.Math.mm.sup.−1.

(24) After determination, the Si content on the surface of the gradient high-silicon steel obtained in this example was 6.5%, the inside Si content was 4.0%, and the Si content decreased uniformly in a gradient from the surface to the inside of high-silicon steel. The concentration gradient was 16.7%.Math.mm.sup.−1, with the difference from the presetting gradient value being within 1%, and satisfying the expected requirements. Iron loss P.sub.10/500=0.68 W.Math.kg.sup.−1, P.sub.10/400=8.8 W.Math.kg.sup.−1, tensile strength was 680 MPa, elongation was 7.6%. The surface of the sample was smooth, without dendrites. No defects such as pores were observed. The density of the product was high.

Comparative Example 1

(25) This comparative example was prepared by the same process of Example 1, except that the electrolysis was performed for 6 h and the temperature maintaining time of the heat treatment was set to 30 min.

(26) After determination, the Si content on the surface and in the inside of the high-silicon steel obtained in this example were both 6.5%, with the uniform distribution of the Si in the substrate. Iron loss P.sub.10/50=0.65 W.Math.kg.sup.−1, P.sub.10/400=8.6 W.Math.kg.sup.−1, tensile strength was 720 MPa, elongation was 1.6%. The surface of the sample was smooth, without dendrites. No defects such as pores were observed. The density of the product was high. The brittleness was significantly higher and the ductility was poorer compared with the examples.

Comparative Example 2

(27) This comparative example was prepared by the same process of Example 1, except that the electrolysis current was set to 52.0 mA (25 mA/cm.sup.2) and the electrolysis was performed for 2 h. The temperature maintaining time of the heat treatment was set to 14 min according to the Si concentration gradient k of 30.0%.Math.mm.sup.−1.

(28) After determination, the Si content on the surface of the gradient high-silicon steel obtained in this example was 9.0%, the inside Si content was 4.5%, and the Si content decreased uniformly in a gradient from the surface to the inside of high-silicon steel. The concentration gradient was 30.0%.Math.mm.sup.−1, with the difference from the presetting gradient value being within 1% and satisfying the expected requirements. Iron loss P.sub.10/50=1.09 W.Math.kg.sup.−1, P.sub.10/400=13.1 W.Math.kg.sup.−1, tensile strength was 750 MPa, elongation was 0.8%. The surface of the sample was smooth, without dendrites. No defects such as pores were observed. The density of the product was high. the deposition rate of silicon was much higher than the diffusion rate of silicon into the substrate, due to the much high electrolysis current density, and the crystallization tended to grow along the electric field to the inside of the electrolyte, which caused the surface of the deposition layer to nodulation and the formation of dendritic crystals. The sample was brittle and poor in ductility. Dendritic formation was observed on the surface of the sample, and tens of micron-sized pores were observed, resulting in poor density.

Comparative Example 3

(29) This comparative example was prepared by the same process of Example 1, except that the weight of the K.sub.2SiF.sub.6 with a purity of 99.9% was 0 g(0 mol).The temperature maintaining time of the heat treatment was set to 5 min according to the Si concentration gradient k of 15.0%.Math.mm.sup.−1.

(30) After determination, the Si content on the surface of the gradient high-silicon steel obtained in this example was 5.5%, the inside Si content was 3.3%, and the Si content decreased in a gradient from the surface to the inside of high-silicon steel. The concentration gradient was 14.6%.Math.mm.sup.−1, with the difference from the presetting gradient value being within 1% and satisfying the expected requirements. Iron loss P.sub.10/50=1.25 W.Math.kg.sup.−1, P.sub.10/400=17.8 W.Math.kg.sup.−1, tensile strength was 690 MPa, elongation was 2.7%. The absence of Si ion in molten salt before electrolyzing resulted in non-silicon deposition at the early stage of electrolysis, which resulted in the deposition of impurities in the molten salt onto the surface of the cathode. Thus, the dendritic formation and tens of micron-sized pores which leaded to low density were observed on the surface of the sample with high brittleness and low ductility.

Comparative Example 4

(31) This comparative example was prepared by the same process of Example 1, except that the electrolysis temperature was set to 520° C. The temperature maintaining time of the heat treatment was set to 17 min according to the Si concentration gradient k of 11.0%mm.sup.−1.

(32) After determination, the Si content on the surface of the gradient high-silicon steel obtained in this example was 5.1%, the inside Si content was 3.6%, and the Si content decreased in a gradient from the surface to the inside of high-silicon steel. The concentration gradient was 10%.Math.mm.sup.−1, with the difference from the presetting gradient value being within 1% and satisfying the expected requirements. Iron loss P.sub.10/50=1.15 W.Math.kg.sup.−1, P.sub.10/400=18.6 W.Math.kg.sup.−1, tensile strength was 660 MPa, elongation was 3.3%. The low electrolysis current resulted in the high brittleness and low ductility of the sample, the dendritic formation and tens of micron-sized pores which leaded to low density were observed on the surface of the sample.

Comparative Example 5

(33) This comparative example was prepared by the same process of Example 1, except that the furnace was heated to 900° C. during the diffusion annealing heat treatment and the temperature maintaining time of the heat treatment was set to 60 min.

(34) After determination, the Si content on the surface of the gradient high-silicon steel obtained in this example was 7.8%, the inside Si content was 3.5%, and the Si content decreased in a gradient from the surface to the inside of high-silicon steel. The concentration gradient was 28.67%.Math.mm.sup.−1. Iron loss P.sub.10/50=0.88 W.Math.kg.sup.−1, P.sub.10/400=9.8 W.Math.kg.sup.−1, tensile strength was 720 MPa, elongation was 3.0%. The sample had high brittleness and poor ductility. The silicon concentration difference between the surface and inside of the high-silicon steel was more than 4% and the concentration gradient was large. The application of gradient silicon steel at this concentration difference was not reported at present.

Comparative Example 6

(35) This comparative example was prepared by the same process of Example 1, except that the temperature maintaining time of the heat treatment was set to 30 min.

(36) After determination, the Si content on the surface of the gradient high-silicon steel obtained in this example was 5.7%, the inside Si content was 4.7%, and the Si content decreased uniformly in a gradient from the surface to the inside of high-silicon steel. The concentration gradient was only 7.2%.Math.mm.sup.−1. Iron loss P.sub.10/50=1.13 W.Math.kg.sup.−1, P.sub.10/400=12.8 W.Math.kg.sup.−1, tensile strength was 640 MPa, elongation was 1.9%. The lower concentration gradient of Si, the more brittle of the sample and the higher loss of the iron.

Comparative Example 7

(37) This comparative example was prepared by the same process of Example 1, except that the temperature maintaining time of the heat treatment was 10 min.

(38) After determination, the Si content on the surface of the gradient high-silicon steel obtained in this example was 7.1%, the inside Si content was 3.5%, and the Si content decreased uniformly in a gradient from the surface to the inside of high-silicon steel. The concentration gradient was 24%.Math.mm.sup.−1, which was much high level. Iron loss P.sub.10/50=1.06 W.Math.kg.sup.−1, P.sub.10/400=12.9 W.Math.kg.sup.−1, tensile strength was 670 MPa, elongation was 2.6%.

Comparative Example 8

(39) This comparative example was prepared by the same process of Example 1, except that K.sub.2SiCl.sub.6 was replaced with K.sub.2SiF.sub.6. The temperature maintaining time of the heat treatment was set to 5 min according to the Si concentration gradient k of 15.0%.Math.mm.sup.−1.

(40) After determination, the Si content on the surface of the gradient high-silicon steel obtained in this example was 5.5%, the inside Si content was 3.3%, and the Si content decreased in a gradient from the surface to the inside of high-silicon steel. The concentration gradient was 14.6%.Math.mm.sup.−1, with the difference from the presetting gradient value being within 1% and satisfying the expected requirements. Iron loss P.sub.10/50=1.25 W.Math.kg.sup.−1, P.sub.10/400=22.8 W.Math.kg.sup.−1, tensile strength was 690 MPa, elongation was 2.5%. Compare with Example 3, Stable Si ion was not formed in the molten salt before electrolyzing resulted in non-silicon deposition at the early stage of electrolysis, which resulted in the deposition of impurities in the molten salt onto the surface of the cathode. Thus, the dendritic formation and tens of micron-sized pores which leaded to low density were observed on the surface of the sample with high brittleness and low ductility.

(41) The embodiments described above are only preferred embodiments of the present invention and are not intended to limit the present invention. It is within the scope of the present invention for those skilled in the art to make equivalent substitutions and alterations on the basis of the present invention.