Non-oriented silicon steel and its manufacturing method

Abstract

An unoriented silicon steel having high magnetic conductivity and low iron loss at a working magnetic density of 1.0-1.5 T and method for manufacturing same. By proper deoxidation control in a RH refining and high-temperature treatment for a short time in a normalizing step, the method can reduce the amount of inclusions in the silicon steel and improve grain shape, so as to improve the magnetic conductivity and iron loss of the unoriented silicon steel at a magnetic density of 1.0-1.5 T.

Claims

1. A method for producing a non-oriented silicon steel, comprising the following steps in sequence: a) steel making, b) hot rolling, c) normalizing, d) cold rolling, and e) annealing, wherein, said steel making step a) is used for obtaining a casting slab having the following components by weight: C0.005%, 0.1%Si2.5%, Al1.5%, 0.10%Mn2.0%, P0.2%, S0.005%, N0.005%, Nb +V+Ti0.006%, and the balance being Fe and inevitable impurities; said steel making step a) includes RH refining, wherein decarbonization treatment and deoxidation treatment are proceeded in RH refining; the input amount of the deoxidizer Y satisfies the following formula: Y =Km([O]50), wherein [O] represents the content of free oxygen in unit of ppm upon the completion of decarbonization; K represents a coefficient indicating deoxidation capacity of the deoxidizer, and is in the range from 0.3510.sup.3 to 1.7510.sup.3; m represents the weight of molten steel contained in the steel ladle, in the unit of ton; in said normalizing step c), the hot-rolled steel strip after hot rolling is heated to a temperature of phase transformation point temperature Ac.sub.1 or above and 1,100 C. or below and is held for a time period t of 1090 s, and the steel strip after holding is cooled at a cooling speed of 15 C/s or less to a temperature of 650 C. and then is cooled naturally, and in said annealing step e), the cold-rolled steel strip after cold rolling is heated to a temperature of 7001,050 C. and is held for 1120 sec, and then is cooled naturally.

2. The method for producing a non-oriented silicon steel according to claim 1, wherein said casting slab further contains Sn and/or Sb, wherein the content of Sn is 0.1 wt % or less, and the content of Sb is 0.1 wt % or less.

3. The method for producing a non-oriented silicon steel according to claim 1, wherein said deoxidizer in said RH refining is aluminum, silicon iron, or calcium.

4. The method for producing a non-oriented silicon steel according to claim 3, wherein K is 0.8810.sup.3 when said deoxidizer in said RH refining is aluminum.

5. The method for producing a non-oriented silicon steel according to claim 3, wherein K is 1.2310.sup.3 when said deoxidizer in said RH refining is silicon iron.

6. The method for producing a non-oriented silicon steel according to claim 3, wherein K is 0.7010.sup.3 when said deoxidizer in said RH refining is calcium.

7. The method for producing a non-oriented silicon steel according to claim 1, wherein a final rolling temperature in said hot rolling step b) is 800900 C.

8. The method for producing a non-oriented silicon steel according to claim 1, wherein in said cold rolling step d), the rolling reduction is 45% or more.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows the relation between the grain size of non-oriented silicon steel and its magnetic permeability .sub.15 and iron loss P.sub.15/50.

(2) FIG. 2 shows the relation between the grain size of non-oriented silicon steel and its magnetic permeability .sub.15 and yield strength.

(3) FIG. 3 shows the relation between the magnetic permeability (.sub.10+.sub.15) and iron loss P.sub.15/50 of non-oriented silicon steel and its motor efficiency.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) Firstly, the reasons of limiting various ingredients contained in the casting slab for producing non-oriented silicon steel of the present invention are explained below.

(5) Si: being soluble in ferrite to form substitutional solid solution, improving resistivity of the substrate and significantly reducing the iron loss and increasing the yield strength, it is one of the most important alloying elements in non-oriented silicon steel. However, if silicon content is too high, it will deteriorate the magnetic permeability of silicon steel products and the processabilty is difficult. Therefore, in the present invention, Si content is limited to 0.1-2.5 wt %.

(6) Al: being soluble in ferrite to improve resistivity of the substrate, coarsing grains and reducing eddy current loss, and hardly deteriorating the magnetic permeability of silicon steel products. In addition, Al also has the effect of deoxidation and nitrogen fixation. However, if Al content is too high, smelting and casting will be difficult, and thus subsequent processability is difficulty. In the present invention, Al content is limited to 1.5 wt % or less.

(7) Mn: being similar to Si and Al, it also can improve resistivity of steel and reduce iron loss; in addition, Mn can enlarge phase zone, slow down the phase transformation speed from to , and thus effectively improve hot rolling plasticity and hot-rolled sheet structure. Meanwhile, Mn can bond with the impurity element S to form stable MnS and eliminate the harm of S for magnetic property. If Mn content is too low, the above beneficial effects are not obvious; if Mn content is too high, it will deteriorate the beneficial texture. In the present invention, Mn content is limited to 0.1-2.0 wt %.

(8) P: adding a certain amount of phosphorus into steel can improve the processability of steel strip, however, if P content is too high, it will deteriorate the cold rolling processability of steel strip. In the present invention, P content is limited to 0.2% or less.

(9) C: being harmful for magnetic property, it is an element which intensively hinders the growth of grains while expanding the phase zone; an excessive amount of C will increase the transformation amounts of both phase zones and in normalizing treatment, significantly reduce the phase transformation point temperature Ac.sub.1, cause the abnormal refinement of crystal structure and thus increase iron loss. In addition, if the content of C as an interstitial element is too high, it will be disadvantage for the improvement of the fatigue property of silicon steel. In the present invention, C content is limited to 0.005 wt % or less.

(10) S: being harmful for both processability and magnetic property, it is easy to form fine MnS particles together with Mn, hinder the growth of annealed grains of the finished products and severely deteriorate magnetic property. In addition, it is easy for S to form low-melting-point FeS and FeS.sub.2 or eutectic together with Fe and cause the problem of hot processing brittleness. In the present invention, S content is limited to 0.005 wt % or less.

(11) N: it is easy for N as an interstitial element to form fine dispersed nitrides with Ti, Al, Nb or V, and it also intensively hinders the growth of grains and deteriorates iron loss. If N content is too high, the amount of nitride precipitates increases, which intensively hinders the growth of grains and deteriorates iron loss. In the present invention, N content is limited to 0.005 wt % or less.

(12) Nb, V, Ti: all of they are elements unfavorable for magnetic property. In the present invention, the total content of Nb, V and Ti is limited to 0.006 wt % or less.

(13) Sn, Sb: as segregation elements, they have the effect of surface oxidation resistance and surface nitridation resistance. Adding an appropriate amount of Sn and/or Sb contributes to increase aluminum content in silicon steel and prevent the formation of a nitride layer in the surface layer of silicon steel. In the present invention, Sn content is set to 0.1 wt % or less, and Sb content is set to 0.1 wt % or less.

(14) Next, the present inventor investigates the effect of the grain size of non-oriented silicon steel (silicon content: 0.852.5 wt %; thickness of silicon steel: 0.5 mm) on the magnetic permeability .sub.15, iron loss P.sub.15/50 and yield strength .sub.s. The results are shown in FIG. 1 and FIG. 2.

(15) FIG. 1 shows the relation between the grain size of non-oriented silicon steel and its magnetic permeability .sub.15 and iron loss P.sub.15/50. It can be seen from FIG. 1 that, when the grain size of non-oriented silicon steel is between 60 m and 105 m, non-oriented silicon steel with both high magnetic permeability and low iron loss can be obtained.

(16) FIG. 2 shows the relation between the grain size of non-oriented silicon steel and its magnetic permeability .sub.15 and yield strength .sub.s. It can be seen from FIG. 2 that, when the grain size of non-oriented silicon steel is between 60 m and 105 m, non-oriented silicon steel with both high magnetic permeability and yield strength can be obtained.

(17) Furthermore, the present inventor investigates the effect of the magnetic permeability (.sub.10+.sub.15) and iron loss P.sub.15/50 of non-oriented silicon steel (0.5 mm thickness) on its motor efficiency. FIG. 3 shows the relation between the magnetic permeability (.sub.10+.sub.15) and iron loss P.sub.15/50 of non-oriented silicon steel and its motor efficiency, and the motor used is a 11 kw6 grade motor. The inventor finds from FIG. 3 that, when the magnetic permeability (.sub.10+.sub.15) and iron loss P.sub.15/50 of non-oriented silicon steel satisfy the following formula, a high motor efficiency can be obtained.
.sub.10+.sub.158,000(1);
.sub.15865.7+379.4P.sub.15/50(2)
.sub.10+.sub.1510,081352.1P.sub.15/50(3)

(18) Next, the present invention will be further described in conjunction with examples, but the protection scope of the present invention is not limited to these examples.

EXAMPLE 1

(19) Firstly, a casting slab containing the following ingredients as calculated by weight percentage is obtained by steel making: C 0.0035%, Si 0.85%, Al 0.34%, Mn 0.31%, P 0.023%, S 0.0027% and N 0.0025%, Fe and other unavoidable impurities as the remains; RH refining is used in the steel making, wherein Al as the deoxidizer is used for deoxidation treatment in RH refining. In Example 1, the weight of molten steel in the steel ladle is 285 ton, the content of free oxygen upon completion of decarbonization is 550 ppm, and the input amount of Al is 125 kg.

(20) Next, the casting slab is subject to hot roll to form hot-rolled steel strip, wherein the final rolling temperature is 800 C. or more, and the thickness of hot-rolled steel strip after hot rolling is 2.6 mm.

(21) Then, the hot-rolled steel strip is subject to the normalizing high-temperature treatment for short-time, i.e., the hot-rolled steel strip is heated to 980 C. and hold for 20 s, and then is cooled to 650 C. at a cooling speed of about 15 C./s, and is naturally cooled.

(22) Next, the hot-rolled steel strip after normalizing treatment is subject to cold roll to form the cold-rolled steel strip, which has a thickness of 0.5 mm after cold rolling.

(23) Finally, at an atmosphere of nitrogen and hydrogen, it is subject to anneal at 800 C. for 18 s, and thus non-oriented silicon steel in Example 1 is obtained.

EXAMPLE 2

(24) Non-oriented silicon steel in Example 2 is produced in the same method as that used in Example 1, except the content of free oxygen upon completion of decarbonization and the input amount of Al are respectively changed to 400 ppm and 87.5 kg.

EXAMPLE 3

(25) Non-oriented silicon steel in example 3 is produced in the same method as that used in Example 1, except the content of free oxygen upon completion of decarbonization and the input amount of Al are respectively changed to 300 ppm and 62.5 kg.

EXAMPLE 4

(26) Non-oriented silicon steel in Example 3 is produced in the same method as that used in Example 1, except the content of free oxygen upon completion of decarbonization and the input amount of Al are respectively changed to 280 ppm and 57.5 kg.

COMPARATIVE EXAMPLE 1

(27) Non-oriented silicon steel is produced in the same method as that used in Example 1 except the input amount of Al is changed to 115 kg.

COMPARATIVE EXAMPLE 2

(28) Non-oriented silicon steel is produced in the same method as that used in Example 1 except the input amount of Al is changed to 135 kg.

COMPARATIVE EXAMPLE 3

(29) Non-oriented silicon steel is produced in the same method as that used in Example 1, except there is no deoxidation treatment in RH refining.

(30) The inclusions of non-oriented silicon steel (0.5 mm thickness) in the above examples and comparative examples are evaluate in grade by GB10561-2005 method, and their magnetic permeability (.sub.10+.sub.15), iron loss P.sub.10/50 and P.sub.15/50 and motor efficiency (11 kw6 grade motor) are measured. The results are shown in Table 1.

(31) TABLE-US-00001 TABLE 1 Deoxidation in RH refining Difference between the temperature of Content of free original molten Content in oxygen in molten Input Grade of Magnetic property steel and melting original steel upon completion amount type C .sub.10 + Motor point of steel molten steel of decarbonization of Al inclusions .sub.15 P.sub.10/50 P.sub.15/50 efficiency ( C.) (%) (ppm) (kg) (kg) (G/Oe) (w/kg) (w/kg) (%) Example 1 61 0.021 550 125 Grade 1.0 8,605 2.24 4.73 91.1 Example 2 81 0.034 400 87.5 Grade 1.0 8,629 2.17 4.62 91.5 Example 3 124 0.043 300 62.5 Grade 1.0 8,687 2.11 4.58 91.8 Example 4 147 0.06 280 57.5 Grade 1.5 8,578 2.32 4.89 90.6 Comparative 61 0.021 550 115 Grade 2.0 8,416 2.49 5.3 89.4 example 1 Comparative 61 0.021 550 135 Grade 2.0 8,449 2.45 5.1 89.9 example 2 Comparative No deoxidation in RH refining Grade 2.0 8,347 2.59 5.5 88.9 example 3

(32) It can be seen from Table 1 that, compared with Comparative Example 3 which does not adopt deoxidation process in RH refining, non-oriented silicon steel in the examples which use deoxidation process in RH refining significantly decreases the amount of inclusions. The magnetic permeability at 1.0 T and 1.5 T of non-oriented silicon steel in examples increases at least 100 G/Oe, and both iron loss and motor efficiency thereof are significantly improved.

(33) Furthermore, compared with Comparative Example 1 having an excessively low input amount of Al and comparative Example 2 having an excessively high input amount of Al, non-oriented silicon steel in examples has better magnetic permeability, iron loss and motor efficiency. Therefore, when the input amount of Al as the deoxidizer Y and the content of free oxygen upon the completion of decarbonization [O] satisfy the following formula: Y=Km([O]50) (wherein, K is 0.8810.sup.3), a more optimal improving effect can be obtained with respect to the magnetic permeability, iron loss and motor efficiency of non-oriented silicon steel.

EXAMPLE 5

(34) Firstly, a casting slab containing the following ingredients as calculated by weight percentage is obtained by steel making: C 0.001%, Si 2.15%, Al 0.35%, Mn 0.24%, P 0.018%, S 0.003% and N 0.0012%, Fe and other unavoidable impurities as the remains; RH refining is used in the steel making, wherein silicon iron or calcium as the deoxidizer is used for deoxidation treatment in RH refining. The input amount of deoxidizer Y and the content of free oxygen upon the completion of decarbonization [O] satisfy the following formula: Y=Km([O]50).

(35) Next, the casting slab is subject to hot roll to form hot-rolled steel strip, wherein the final rolling temperature is 800 C. or more, and the thickness of hot-rolled steel strip after hot rolling is 2.3 mm.

(36) Then, the hot-rolled steel strip is subject to the normalizing high-temperature treatment for short-time, i.e., the hot-rolled steel strip is heated to 980 C. and hold for 1090 s, and is cooled to 650 C. at a cooling speed of about 5/s, and then is naturally cooled.

(37) Next, the hot-rolled steel strip after normalizing treatment is subject to cold roll to form the cold-rolled steel strip, which has a thickness of 0.5 mm after cold rolling.

(38) Finally, at an atmosphere of nitrogen and hydrogen, it is subject to anneal at 800 C. for 20 s, and thus non-oriented silicon steel in Example 5 is obtained.

EXAMPLE 6

(39) Non-oriented silicon steel is produced in the same method as that used in Example 5, except the holding temperature in the normalizing step is changed to 1,030 C.

EXAMPLE 7

(40) Non-oriented silicon steel is produced in the same method as that used in Example 5, except the holding temperature in the normalizing step is changed to 1,050 C.

EXAMPLE 8

(41) Non-oriented silicon steel is produced in the same method as that used in Example 5, except the holding temperature in the normalizing step is changed to 1,100 C.

COMPARATIVE EXAMPLE 4

(42) Non-oriented silicon steel is produced in the same method as that used in Example 5, except the holding temperature in the normalizing step is changed to 920 C.

(43) The grain size of the steel strip after normalizing treatment in the above examples and comparative examples are measured, and the magnetic permeability (.sub.10+.sub.15), iron loss P.sub.10/50 and P.sub.15/50 and motor efficiency (11 kw6 grade motor) of the final silicon steel products (0.5 mm thickness) are measured. The results are shown in Table 2.

(44) TABLE-US-00002 TABLE 2 Normalizing process parameter Grain size of Holding temperature Cooling speed steel strip after Magnetic property Motor in Normalizing before 650 C. normalizing .sub.10 + .sub.15 P.sub.10/50 P.sub.15/50 efficiency ( C.) ( C./s) (m) (G/Oe) (w/kg) (w/kg) (%) Example 5 980 5 133 9,068 1.49 3.25 90.6 Example 6 1,030 5 141 9,105 1.41 3.13 91.1 Example 7 1,050 5 148 9,189 1.37 3.01 91.3 Example 8 1,100 5 157 9,226 1.29 2.87 92.1 Comparative 920 5 114 8,965 1.58 3.41 87.4 example 4

(45) It can be seen from Table 2 that, compared with Comparative Example 4 which adopts low-temperature normalizing, the examples which adopt the normalizing high-temperature treatment for short-time significantly increase the grain size of steel strip after normalizing. The magnetic permeability at 1.0 T and 1.5 T of non-oriented silicon steel in examples increases at least 100 G/Oe, and both iron loss and the motor efficiency thereof are significantly improved.

(46) In addition, it can be seen from Tables 1 and 2 that, the iron loss P.sub.10/50 and P.sub.15/50 of non-oriented silicon steel in examples of the present invention are respectively 3.0 w/kg or less and 5.5 w/kg or less, and using non-oriented silicon steel in examples can obtain a motor efficiency of 90% or more.

(47) Furthermore, the present inventor measured the grain diameter, surface layer property, sulphur content and yield strength .sub.s of non-oriented silicon steel in examples 18. The results show that, non-oriented silicon steel in examples has a grain diameter of between 60 m and 105 m, S content of 15 ppm or less, the total nitride concentration in the surface layer of 020 m of 250 ppm or less, and the total nitride concentration of not more than 5.85C.sub.N. In addition, the yield strength .sub.s of non-oriented silicon steel in examples is no less than 220 MPa.

(48) Furthermore, the present inventor investigates the relation between the magnetic permeability and iron loss of non-oriented silicon steel at 1.0 T and 1.5 T in examples 18, and the results indicate that, the magnetic permeability of non-oriented silicon steel in examples satisfies the following formula:
.sub.10+.sub.158,000(1);
.sub.15865.7+379.4P.sub.15/50(2)
.sub.10+.sub.1510,081352.1P.sub.15/50(3)

(49) The experimental results of the present invention indicate that, by proper deoxidation control in RH refining and high-temperature treatment for short-time in the normalizing step, the present invention can reduce the amount of inclusions in the non-oriented silicon steel, improve grain shapes, and thus improve the magnetic permeability and iron loss of non-oriented silicon steel at 1.01.5 T and obtain a high motor efficiency.

BENEFICIAL EFFECTS OF THE PRESENT INVENTION

(50) By proper deoxidation control in RH refining and high-temperature treatment for short-time in the normalizing step, the present invention can provide the non-oriented silicon steel with high magnetic permeability and low iron loss. The non-oriented silicon steel in the present invention can obtain a motor efficiency of 90% or more when used as iron core in electronic devices, and satisfy miniaturization and energy conservation requirements of electronic devices such as rotary machines and static machines, thus has a broad application prospect.