Non-oriented electrical steel sheet with fine magnetic performance, and calcium treatment method therefor

Abstract

A non-oriented electrical steel sheet with fine magnetic performance, and a calcium treatment method therefor, including an RH (Ruhrstahl-Heraeus) refinement step. The RH refinement step sequentially comprises a decarbonization step, an aluminum deoxidation step, and a step of adding calcium alloy. In the step of adding calcium alloy, time when the calcium alloy is added satisfies the following condition: time interval between Al and Ca/total time after Al=0.2-0.8. In this method, production cost is reduced, the production process is simple, a normal processing cycle of RH refinement is not affected, the device is convenient in operation and is controllable, and foreign substances are controllable in both shape and quantities. The non-oriented electrical steel sheet prepared according to the present invention has fine magnetic performance, and the method can be used for mass production of the non-oriented electrical steel sheet with fine magnetic performance.

Claims

1. A calcium treatment method for a non-oriented electrical steel comprising a RH refining process, the RH refining process comprising decarbonization step, aluminum deoxidation step, and calcium alloy addition step in sequence, wherein in the calcium alloy addition step, the time for adding calcium alloy satisfies the following conditions:
time interval between time for Al and time for Ca/Total time period after time for Al=0.2-0.8, wherein, the time interval between time for Al and time for Ca is the time interval between the time point for adding aluminum in said aluminum deoxidation step and the time point for adding calcium alloy in said calcium alloy addition step, and the total time period after time for Al is the time interval between the time point for adding aluminum in said aluminum deoxidation step and the end point of the RH refining process; wherein the method results in a non-oriented electrical steel having a chemical composition by weight percentage as follows: C0.005%, Si 0.2-3.4%, Mn 0.2-1.0%, P0.2%, S0.003%, Al 0.2-1.2%, N0.005%, O0.005%, Ca of 0.0005%-0.0017%, and balance being Fe and unavoidable impurities, and wherein further a magnetic induction of the non-oriented electrical steel is greater than or equal to 1.76 T and an iron loss of the non-oriented electrical steel is less than or equal to 5.7 W/kg.

2. The calcium treatment method of claim 1, wherein the addition amount of said calcium alloy ranges between 0.5 kg/t steel and 1.2 kg/t steel.

3. The calcium treatment method of claim 2, wherein said calcium alloy is added in two or more batches.

4. The calcium treatment method of claim 2, wherein said calcium alloy is added in three or more batches, and the addition amount for each batch of said calcium alloy does not exceed 40% of the total addition amount of said calcium alloy.

5. The calcium treatment method of claim 1, wherein said calcium alloy is subjected to a passivating treatment.

6. The calcium treatment method of claim 1, wherein said calcium alloy has the following chemical composition by weight percentages: Ca 18-27%, Mg 2-6%, Si 20-35%, Al 1-9%, Zr 1-5%, and balance being Fe and unavoidable impurities.

7. The calcium treatment method of claim 1, further comprising a step of silicon deoxidation before said aluminum deoxidation step.

8. The calcium treatment method of claim 1, wherein the content of sulfur in liquid steel is maintained to be 0.003% before said calcium alloy is added.

9. The calcium treatment method of claim 8, wherein the content of sulfur in liquid steel is maintained to be 0.003% by desulfurization of molten iron or molten steel.

10. A non-oriented electrical steel manufactured by a calcium treatment method comprising a RH refining process, the RH refining process comprising a decarbonization step, aluminum deoxidation step, and calcium alloy addition step in sequence, wherein in the calcium alloy addition step, the time for adding calcium alloy satisfies the following conditions: time interval between time for Al and time for Ca/Total time period after time for Al=0.2-0.8, wherein, the time interval between time for Al and time for Ca is the time interval between the time point for adding aluminum in the aluminum deoxidation step and the time point for adding calcium alloy in the calcium alloy addition step, and the total time period after time for Al is the time interval between the time point for adding aluminum in the aluminum deoxidation step and the end point of the RH refining process, wherein the non-oriented electrical steel has a chemical composition by weight percentage as below: C0.005%, Si 0.2-3.4%, Mn 0.2-1.0%, P0.2%, S0.003%, Al 0.2-1.2%, N0.005%, O<0.005%, Ca of 0.0005%-0.0017%, and balance being Fe and unavoidable impurities, and wherein further magnetic induction of the non-oriented electrical steel is greater than or equal to 1.76 T and an iron loss of the non-oriented electrical steel is less than or equal to approximately 5.7 W/kg.

11. The non-oriented electrical steel of claim 10, wherein the addition amount of said calcium alloy ranges between 0.5 kg/t steel and 1.2 kg/t steel.

12. The non-oriented electrical steel of claim 10, wherein said calcium alloy is added in two or more batches.

13. The non-oriented electrical steel of claim 10, wherein said calcium alloy is added in three or more batches, and the addition amount for each batch of said calcium alloy does not exceed 40% of the total addition amount of said calcium alloy.

14. The non-oriented electrical steel of claim 10, wherein said calcium alloy is subjected to a passivating treatment.

15. The non-oriented electrical steel of claim 10, wherein said calcium alloy has the following chemical composition by weight percentages: Ca 18-27%, Mg 2-6%, Si 20-35%, Al 1-9%, Zr 1-5%, and balance being Fe and unavoidable impurities.

16. The non-oriented electrical steel of claim 10, said method further comprising a step of silicon deoxidation before said aluminum deoxidation step.

17. The non-oriented electrical steel of claim 10, wherein the content of sulfur in liquid steel is maintained to be 0.003% before said calcium alloy is added.

18. The non-oriented electrical steel of claim 10, wherein the content of sulfur in liquid steel is maintained to be 0.003% by desulfurization of molten iron or molten steel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 provides the diagram of inclusion control effect of the finished steel products in the ordinary furnace number (without being added with calcium alloy) and in the calcium treatment furnace number of the present invention (added with calcium alloy).

(2) FIG. 2 shows the effects of the addition amount of calcium alloy on the iron loss and magnetic induction of finished steel products.

(3) FIG. 3 shows the effects of the sulfur content of liquid steel on the iron loss of finished steel products in the ordinary furnace number and in the calcium treatment furnace number of the present invention.

(4) FIG. 4 shows the effects of various addition modes of calcium alloy on calcium content in the wire feeding furnace number, in the calcium treatment furnace number of the present invention and in the ordinary furnace number.

DETAILED DESCRIPTION OF THE INVENTION

(5) Next, the method of the present invention will be further described in conjunction with the attached figures and examples, but the present invention is not limited to these examples herein.

(6) The steel making process of the non-oriented electrical steel comprises converter blowing, RH refining and continuous casting process.

(7) The RH refining process of the present invention comprises decarbonization step, aluminum deoxidation step and calcium alloy addition step in sequence. As shown in FIG. 1, calcium alloy is added in a specific period of RH refining in the furnace number of the present invention, and the inclusions contained in the finished steel products thus manufactured are large in size and low in amount, so the steel thus manufactured has a high purity and the finished steel products thus manufactured have excellent electromagnetic performance. In the ordinary furnace number (without being added with calcium alloy), the inclusions contained in the finished steel products thus manufactured are small in size and high in amount, so the steel thus manufactured has a low purity and the finished steel products thus manufactured can not be guaranteed of excellent electromagnetic performance.

(8) In the present invention, the RH refining process comprises decarbonization step, aluminum deoxidation step and calcium alloy addition step in sequence, where in the calcium alloy addition step, the time for adding calcium alloy satisfies the following conditions:
Time interval between time for Al and time for Ca/Total time period after time for Al=0.20.8,

(9) wherein, the time interval between time for Al and time for Ca is the time interval between the time point for adding aluminum in said aluminum deoxidation step and the time point for adding calcium alloy in said calcium alloy addition step, and the total time period after time for Al is the time interval between the time point for adding aluminum in said aluminum deoxidation step and the end point of the RH refining process.

(10) The calcium treatment method of the present invention adds calcium alloy in a specific period of RH refining so as to get the form and amount of inclusions under control, and in the present method, the production cost of calcium alloy is low, the production process of calcium alloy is simple, and the addition modes of calcium alloy do not influence the normal treatment cycle of RH refining, and the equipment are convenient for operation and controllable.

(11) On the other hand, the effective calcium concentration of liquid steel is an important factor determining the sufficient modification of inclusions. In order to ensure a better calcium treatment effect, the present invention further puts forward its requirements on the addition amount of calcium alloy. FIG. 2 shows the effects of the addition amount of calcium alloy on the iron loss and magnetic induction of the finished steel products. Iron loss refers to the electric energy loss of the silicon steel material under a specific magnetic field intensity and current intensity and at a certain frequency. Magnetic induction refers to the magnetic flux density, which, usually represented by the symbol B, is a fundamental physical quantity employed to describe the intensity and direction of a magnetic field. In physics, the intensity of a magnetic field is represented by magnetic induction intensity (also called magnetic flux density), i.e., a high magnetic induction intensity denotes a strong magnetic induction while a low magnetic induction intensity denotes a weak magnetic induction. The unit of magnetic flux density is Tesla, i.e., T for short. As shown in FIG. 2, when the addition amount of calcium alloy ranges between 0.5 kg/t steel and 1.2 kg/t steel, the finished steel products have a relatively low iron loss and high magnetic induction, and thus have an excellent magnetic property. Thus, in order to ensure the electromagnetic performance of the finished steel products, the addition amount of calcium alloy is set between 0.5 kg/t steel and 1.2 kg/t steel. The calcium alloy is added in two or more batches. Preferably the calcium alloy is added in three or more batches, and the addition amount for each batch of said calcium alloy does not exceed 40% of the total addition amount of said calcium alloy.

(12) In order to increase the retention time of calcium in liquid steel, facilitate the sufficient reaction between calcium and liquid steel and achieve a satisfactory effect of inclusion improvement, the calcium alloy is subjected to a passivating treatment, which means to appropriately increase the surface oxide layer of calcium alloy to reduce its reaction rate.

(13) Besides, the chemical ingredients of calcium alloy are limited. The differences from previous tests lie in that in the test calcium alloy is used to significantly reduce aluminum content and silicon content is appropriately increased so as to increase the melting point of calcium alloy; calcium content is adjusted to control the degree of intense reaction between calcium and liquid steel, and Mg, Zr and other elements are appropriately added to increase the solubility of calcium in liquid steel and increase its yield. In the present invention, the calcium alloy has the following chemical composition by weight percentages: Ca 1827%, Mg 26%, Si 2035%, Al 19%, Zr 15%, and balance being Fe and unavoidable impurities.

(14) As found by the present inventor after test, if aluminum deoxidation is directly employed, small inclusions will be generated. The viscosity of liquid steel will increase even if silicon alloy is added after that, so it will be difficult for aluminum oxide inclusions to float upward and to be eliminated, and the calcium treatment has a poor effect on silicon oxide modification. If silicon deoxidation is adopted before aluminum deoxidation, i.e., adopting the two-step deoxidation method (silicon deoxidation and aluminum deoxidation in succession), it will be relatively easier for aluminum oxide inclusions to float upward and to be eliminated. Aluminum has the strong deoxidizing effect, and thus the aluminum oxide inclusions generated by the subsequent deoxidation will be able to be further eliminated by the calcium treatment to generate the calcium aluminate having a low melting point, and the dispersed tiny granular inclusions are inhibited. Thus, in order to better control the form and amount of inclusions, based on the prevent invention, silicon deoxidation is employed before the aluminum deoxidation step, i.e., adopting the two-step deoxidation method (silicon deoxidation and aluminum deoxidation in succession).

(15) It has also been found by the present inventor in the industrialized test that, in the calcium treatment, the relatively high content of sulfur in liquid steel will lead to the generation of CaS inclusions in large amount, make it difficult for aluminum oxide inclusions to be fully modified, influence the improvement effect of inclusions contained in the steel and unfavorable to the increase of the electromagnetic performance of the finished steel products. As shown in FIG. 3, when the content of sulfur in liquid steel is >30 ppm (i.e. >0.003%), iron loss is rapidly increased in both the furnace number of the present invention and in the ordinary furnace number, which is unfavorable to the increase of the electromagnetic performance of the finished steel products. Thus, in order to ensure the electromagnetic performance of the finished steel products, the content of sulfur in liquid steel is maintained to be 0.003% before the calcium alloy is added; preferably the content of sulfur in liquid steel is maintained to be 0.003% by desulfurization of molten iron or molten steel.

(16) The non-oriented electrical steel manufactured by the method of the present invention usually has a chemical composition by weight percentages as below: C0.005%, Si 0.23.4%, Mn 0.21.0%, P0.2%, S0.003%, Al 0.2%1.2%, N0.005%, O0.005%, and balance being Fe and unavoidable impurities. The non-oriented electrical steel further comprises Ca of 0.0005%.

(17) As shown in FIG. 4, the calcium content of the ordinary furnace number is <0.0005%. The calcium content of the wire feeding furnace number is 0.0005%, however, when the wire feeding method is employed for calcium treatment, it will cause significant environmental pollution, influence the circulation of liquid steel in vacuum, make it difficult to either ensure the actual treatment effect of liquid steel or put the circulation mode under control, which as a result influence the normal treatment cycle of RH refining; and impose relatively high requirements on the conditions of wire feeding equipment. In the furnace number of the present invention, calcium alloy is added in a specific period of RH refining so that the calcium content of the finished steel products thus manufactured is 0.0005%, and in the present method, the addition modes of calcium alloy do not influence the normal treatment cycle of RH refining, and the equipment are convenient for operation and controllable.

(18) In the following section, there are descriptions for the effects of the chemical ingredients of the non-oriented electrical steel of the present invention and the instructions on limiting their contents:

(19) C: Below 0.005%. C is an element which strongly inhibits the growth of grains of the finished products, and may easily deteriorate the magnetic property of the finished strip steel products and lead to severe magnetic aging. Thus, C content must be maintained below 0.005%.

(20) Si: 0.23.4%. Si is an element which can effectively increase the resistance of the finished strip steel products. When Si content is lower than 0.2%, it can not effectively reduce the iron loss; when Si content is higher than 3.4%, the magnetic flux density will significantly decline, accompanied by increased hardness and deteriorated processability.

(21) Mn: 0.21.0%. Like Si and Al, Mn can also increase the resistance of steel and improve the surface condition of electrical steel. Thus, it's necessary that Mn content is maintained to be above 0.2%. Meanwhile, when Mn content is higher than 1.0%, it will significantly increase the manufacturing cost and reduce the magnetic induction of the finished products.

(22) Al: 0.21.2%. Al is an element which can effectively increase the resistance of the finished strip steel products. When Al content is lower than 0.2%, it can not effectively reduce the iron loss, and the magnetic property of the finished products tends to be unstable; when Al content is higher than 1.2%, it will significantly increase the manufacturing cost and reduce the magnetic induction of the finished products.

(23) P: Below 0.2%. Adding a certain amount of P in steel can improve the processability of the steel sheet, however, when P content exceeds 0.2%, the cold-rolling processability of the steel sheet will be deteriorated.

(24) S: Below 0.003%. When S content exceeds 0.003%, it will significantly increase the amount of MnS and other S compounds precipitated, strongly inhibit the growth of grains, deteriorate the condition of iron loss and influence the modification effect of inclusions through calcium treatment.

(25) N: Below 0.005%. When N content exceeds 0.005%, it will significantly increase the amount of AIN and other N compounds precipitated, strongly inhibit the growth of grains and deteriorate the condition of iron loss.

(26) O: Below 0.005%. When O content exceeds 0.005%, it will significantly increase the amount of oxide inclusions, strongly inhibit the growth of grains and deteriorate the condition of iron loss.

EXAMPLES

(27) The following examples are illustrated to explain the implementation of the present invention, and can not be understood to constitute any limitation on the present invention.

(28) Molten iron and scrap steel are proportionally mixed, subjected to 300 ton converter smelting, RH refining for decarbonization and deoxidation, addition of calcium alloy for calcium treatment, and then continuous casting to finally obtain the continuous casting slab #A with 170250 mm in thickness and 8001,450 mm in width. See the related process parameters and magnetic property data and chemical ingredients of steel respectively in Table 1 and Table 2.

(29) The lower the iron loss is, the higher the magnetic induction is, and the better the magnetic property of the finished steel products is.

(30) The iron loss and magnetic induction are measured according to the standard JIS-C-2550.

(31) For the continuous casting slab #A, if the magnetic induction is 1.76 T and the iron loss is 5.7 W/kg, it suggests that the finished steel products have an excellent magnetic property; if the magnetic induction is <1.76 T and the iron loss is >5.7 W/kg, it suggests that the finished steel products have a poor magnetic property.

(32) TABLE-US-00001 TABLE 1 Iron Addition Adding Deoxidation Magnetic loss No. amount time mode induction (T) (W/kg) Example 1 0.53 0.24 Si, Al 1.764 5.43 Example 2 1.02 0.55 Si, Al 1.768 5.65 Example 3 1.13 0.73 Si, Al 1.762 5.50 Comparative 0.47 0.36 Si, Al 1.752 5.87 Example 1 Comparative 1.67 0.62 Si, Al 1.754 5.79 Example 2 Comparative 1.02 0.91 Si, Al 1.746 5.96 Example 3 Comparative 0.54 0.16 Si, Al 1.756 5.68 Example 4 Comparative 0.83 0.69 Al, Si 1.757 5.72 Example 5

(33) TABLE-US-00002 TABLE 2 No. C Si Mn P S Ca Al O N Example 1 0.0008 0.22 0.27 0.09 0.0022 0.0005 0.24 0.0015 0.0013 Example 2 0.0029 0.26 0.26 0.08 0.0024 0.0007 0.26 0.0028 0.0015 Example 3 0.0037 0.22 0.22 0.10 0.0021 0.0006 0.25 0.0009 0.0010 Comparative 0.0031 0.21 0.22 0.09 0.0045 0.0003 0.23 0.0021 0.0009 Example 1 Comparative 0.0033 0.24 0.24 0.09 0.0038 0.0008 0.27 0.0017 0.0009 Example 2 Comparative 0.0014 0.31 0.22 0.09 0.0041 0.0017 0.23 0.0014 0.0031 Example 3 Comparative 0.0042 0.27 0.22 0.09 0.0029 0.0002 0.24 0.0012 0.0012 Example 4 Comparative 0.0027 0.25 0.23 0.09 0.0038 0.0006 0.26 0.0007 0.0018 Example 5

(34) The addition amount refers to the amount of calcium alloy added in the calcium alloy addition step of RH refining.

(35) The adding time refers to the time for adding the calcium alloy in the calcium alloy addition step of RH refining, i.e., time interval between time for Al and time for Ca/total time period after time for Al.

(36) In the examples 13, the addition amount of calcium alloy ranges between 0.5 kg/t steel and 1.2 kg/t steel, and the adding time of calcium alloy ranges between 0.2 and 0.8; the two-step deoxidation method (Si deoxidation and Al deoxidation in succession) is adopted in all cases, with S content 0.003%; the finished steel products corresponding to the examples 13 have a magnetic induction 1.76 T and an iron loss 5.7 W/kg, which suggest that they have an excellent magnetic property, with Ca content 0.0005%.

(37) In the comparative example 1, the addition amount of calcium alloy is less than 0.5 kg/t steel; in the comparative example 2, the addition amount of calcium alloy is greater than 1.2 kg/t steel; in the comparative example 3, the adding time of calcium alloy is greater than 0.8; in the comparative example 4, the adding time of calcium alloy is less than 0.2; in the comparative example 5, a two-step deoxidation method (Al deoxidation and Si deoxidation in succession) is adopted; in the comparative cases 1, 2, 3 and 5, S content is greater than 0.003%. Thus, the finished steel products corresponding to the comparative examples 15 have a magnetic induction <1.76 T and an iron loss >5.7 W/kg, which suggest that they have a poor magnetic property.

(38) Molten iron and scrap steel are proportionally mixed, subjected to 300 ton converter smelting, RH refining for decarbonization and deoxidation, addition of calcium alloy for calcium treatment, and then continuous casting to finally obtain the continuous casting slab #B with 170250 mm in thickness and 8001,450 mm in width. See the chemical ingredients and related process parameters and magnetic property data of steel respectively in Table 3 and Table 4.

(39) For the continuous casting slab #B, if the magnetic induction is 1.69 T; the iron loss is 3.8 W/kg, it suggests that the finished steel products have an excellent magnetic property; if the magnetic induction is <1.69 T; the iron loss is >3.8 W/kg, it suggests that the finished steel products have a poor magnetic property.

(40) TABLE-US-00003 TABLE 3 Iron Addition Adding Deoxidation Magnetic loss No. amount time mode induction (T) (W/kg) Example 4 1.17 0.41 Si, Al 1.702 3.78 Example 5 1.17 0.80 Si, Al 1.694 3.65 Example 6 0.83 0.60 Si, Al 1.696 3.41 Comparative 0.83 0.72 Si, Al 1.684 3.92 Example 6 Comparative 0.33 0.18 Al, Si 1.686 3.75 Example 7

(41) TABLE-US-00004 TABLE 4 No. C Si Mn P S Ca Al O N Example 4 0.0028 1.25 0.69 0.002 0.0018 0.0009 0.25 0.0010 0.0032 Example 5 0.0019 1.38 0.57 0.002 0.0027 0.0008 0.26 0.0014 0.0026 Example 6 0.0027 1.41 0.87 0.001 0.0022 0.0008 0.26 0.0009 0.0009 Comparative 0.0043 1.39 0.83 0.02 0.0042 0.0002 0.37 0.0017 0.0026 Example 6 Comparative 0.0036 1.41 0.59 0.02 0.0025 0.0003 0.41 0.0014 0.0017 Example 7

(42) The addition amount refers to the amount of calcium alloy added in the calcium alloy addition step of RH refining.

(43) The adding time refers to the time for adding calcium alloy in the calcium alloy addition step of RH refining, i.e., time interval between time for Al and time for Ca/total time period after time for Al.

(44) In the examples 46, the addition amount of calcium alloy ranges between 0.5 kg/t steel and 1.2 kg/t steel, and the adding time of calcium alloy ranges between 0.2 and 0.8; the two-step deoxidation method (Si deoxidation and Al deoxidation in succession) is adopted in all cases, with S content 0.003%; the finished steel products corresponding to the examples 46 have a magnetic induction 1.69 T and an iron loss 3.8 W/kg, which suggest that they have an excellent magnetic property, with Ca content0.0005%.

(45) In the comparative example 6, S content is greater than 0.003%; in the comparative example 7, the addition amount of calcium alloy is lower than 0.5 kg/t steel, and the adding time of calcium alloy is less than 0.2; a two-step deoxidation method (Al deoxidation and Si deoxidation in succession) is adopted. Thus, the finished steel products corresponding to the comparative examples 67 have a magnetic induction<1.69 T or an iron loss>3.8 W/kg, which suggest that they have a poor magnetic property.

(46) Table 14 indicate that, by controlling the adding time for calcium alloy within the range of 0.20.8, controlling the addition amount of calcium alloy within the range of 0.5 kg/t steel1.2 kg/t steel, adopting the two-step deoxidation method (Si deoxidation and Al deoxidation in succession), and limiting S content to be 0.003%, the effect of inclusion control can be stably improved to produce the finished steel products with excellent magnetic property and effectively increase the Ca content of steel.

INDUSTRIAL APPLICABILITY

(47) The method of the present invention has the following advantages: reduced production cost, simplified production process, convenient control of equipment and controllable form and amount of inclusions without influencing the normal treatment cycle of RH refining. The non-oriented electrical steel manufactured by the method of the present invention has an excellent magnetic property, and the present method can be employed for the large-scale production of the non-oriented electrical steel with excellent magnetic property.