Non-oriented silicon steel and manufacturing process thereof

Abstract

The present invention provides a non-oriented silicon steel with excellent magnetic properties and a manufacturing process therefor. During the manufacturing process of the present invention, the temperature T of the molten steel of steel tapped from a converter during steelmaking and the carbon content [C] and the free oxygen content [O] comply with the following formula: 7.2710.sup.3[O][C]e.sup.(5000/T)2.9910.sup.4, and the final annealing step uses tension annealing at a low temperature for a short time. A non-oriented silicon steel with a low iron loss, and excellent anisotropy of iron loss can be obtained by means of the manufacturing process of the present invention.

Claims

1. A method for producing non-oriented silicon steel, the method comprising the following steps in sequence: a) steel making, b) hot rolling, c) normalizing, d) cold rolling, and e) annealing, wherein, a casting slab containing the following composition by weight percentage is obtained by said steel making step a): C 0.0010.004%, Si 2.54.0%, Al 0.51.5%, Mn 0.101.50%, P0.02%, S0.002%, N0.003%, B0.005%, where Mn/S300, Al/N300, and the balance being Fe and unavoidable impurities, wherein, said steel making step a) includes converter steel making, in which the temperature T of molten steel during tapping on converter, the carbon content [C] and the free oxygen content [O] satisfy the following formula:
7.2710.sup.3[O][C]e.sup.(5,000/T)2.9910.sup.4, wherein said steel making step a) further includes a step of RH refining, and in said RH refining, a deoxidation is implemented at the end of decarbonization first by using FeSi alloy and then by using FeAl alloy, in sequence; in said annealing step e), the cold-rolled steel strip is heated to 9001,050 C., and then is subject to thermal insulation under a tension of 0.51.5 MPa for a period of time t of 860 sec; wherein said casting slab contains Sn and Sb, the content of Sb+2Sn ranging between 0.0010.05 wt %; and wherein further, the non-oriented silicon steel so produced comprises a grain diameter between 100 and 200 m, a grain equivalent axial coefficient L between 1.05 and 1.35, an iron loss P15/502.40 W/kg, and an anisotropy of iron loss 10%.

2. The method for producing non-oriented silicon steel according to claim 1, wherein in said annealing step e), the temperature is 9201,000 C. and the tension is 11.3 MPa.

3. The method for producing non-oriented silicon steel according to claim 1, wherein the casting slab obtained in said steel making step a) satisfies: 350(Mn/S)600, 350(Al/N)600.

4. The method for producing non-oriented silicon steel according to claim 1, wherein said cold rolling step d) has a reduction rate of 7088%.

5. The method for producing non-oriented silicon steel according to claim 1, wherein a batch furnace of normalization is used in said normalizing step c) where the steel strip is subject to a thermal insulation at 780880 C. for 26 hours under a protection atmosphere of nitrogen and hydrogen.

6. The method for producing non-oriented silicon steel according to claim 1, wherein a continuous annealing of normalization is used in said normalizing step c) where the hot-rolled steel strip is firstly heated to 850950 C. at a heating rate of 515 C./sec, and is subject to a thermal insulation under a protection atmosphere of nitrogen for a period of time t of 1090 sec, then is cooled to 650 C. at a cooling rate of 10 C./sec or less, and is finally left for natural cooling.

7. The method for producing non-oriented silicon steel according to claim 6, wherein, in said normalizing step c), the hot-rolled steel strip is heated to 850930 C.

8. The method for producing non-oriented silicon steel according to claim 1, wherein said hot rolling step b) has a deformation of 80% or more at 950 C. or more.

9. The method for producing non-oriented silicon steel according to claim 8, wherein in said hot rolling step b), the maximum temperature difference between various positions of the hot-rolled steel strip is 20 C. or less.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows the relationship between the Mn/S ratio of the casting slab for manufacturing the non-oriented silicon steel and the iron loss P.sub.15/50 of the non-oriented silicon steel.

(2) FIG. 2 shows the relationship between the S content of the casting slab for manufacturing the non-oriented silicon steel and the iron loss P15/50 of the non-oriented silicon steel.

(3) FIG. 3 shows the relationship between the Al/N ratio of the casting slab for manufacturing the non-oriented silicon steel and the iron loss P15/50 of the non-oriented silicon steel.

(4) FIG. 4 shows the relationship between the total content of nitrogen and oxygen at a depth of 30 m from the surface of the non-oriented silicon steel and the iron loss P15/50 of the non-oriented silicon steel.

(5) FIG. 5 shows the relationship between the equivalent axial coefficient of the grains of the non-oriented silicon steel and anisotropy of iron loss of the non-oriented silicon steel.

DETAILED DESCRIPTION OF THE INVENTION

(6) Firstly, the reasons for limiting various ingredients contained in the casting slab for manufacturing the non-oriented silicon steel in the present invention are explained as follows.

(7) 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. If Si content is too low, its effect of reducing iron loss will become insignificant; if Si content is too high, not only its effect of reducing iron loss obviously decreases, but also it will cause processing difficulty. In the present invention, Si content is limited to 2.54.0 wt %.

(8) Al: being soluble in ferrite to improve resistivity of the substrate, coarsen grains, reduce iron loss and improve yield strength while deoxidating and fixing nitrogen, but easily causing oxidation inside the surface of finished steel sheet products. If Al content is too low, its effect of reducing iron loss, deoxidating and fixing nitrogen will become insignificant; if Al content is too high, it is difficult to smelt and cast, magnetic induction decreases and process is difficult. In the present invention, Al content is limited to 0.51.5 wt %.

(9) Mn: being similar to Si and Al, it also can improve the resistivity of steel and reduce iron loss, bond with the impurity element S to form stable MnS and eliminate the harm of S for magnetic property. In addition to preventing hot shortness, it's also soluble in ferrite to form substitutional solid solution, has a function of strengthening solid solution, and improves the yield strength of the matrix. If Mn content is too low, the above effects will become insignificant; if Mn content is too high, both the phase transformation point temperature Acl and recrystallization temperature of the silicon steel will reduce, and there will be - phase transformation when heat treatment, thus deteriorate the favorable crystal texture. In the present invention, Mn content is limited to 0.101.50 wt %.

(10) Furthermore, the present inventor has investigated the relationship between the Mn/S ratio and the iron loss P.sub.15/50 of the non-oriented silicon steel. FIG. 1 shows the relationship between the Mn/S ratio of the casting slab for manufacturing the non-oriented silicon steel and the iron loss P.sub.15/50 of the non-oriented silicon steel. As shown in FIG. 1, a good effect of reducing iron loss (P.sub.15/50) is observed when the Mn/S ratio is 300 or more, and that the effect of reducing iron loss (P.sub.15/50) becomes basically saturation when the Mn/S ratio reaches 600. In the present invention, the Mn/S ratio is limited 300 or more, and preferably between 350 and 600.

(11) S: being harmful for both processability and magnetic property, it is easy to form fine MnS particles together with Mn, hinders the growth of annealed grains of the finished products and severely deteriorates the magnetic property. In addition, it is easy for S to form low-melting-point FeS and FeS.sub.2 or eutectic crystal together with Fe and causes the problem of hot processing brittleness. The present inventor has investigated the influence of S content on the iron loss P.sub.15/50 of the non-oriented silicon steel. FIG. 2 shows the relationship between the S content of the casting slab for manufacturing the non-oriented silicon steel and the iron loss P.sub.15/50 of the non-oriented silicon steel. As shown in FIG. 2, the iron loss P.sub.15/50 of the non-oriented silicon steel is deteriorated when S content exceeds 0.002 wt %. In the present invention, S content is limited to be 0.002 wt % or less.

(12) P: adding a certain amount of phosphorus into steel can improve the processability of the 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 be 0.02% or less.

(13) C: being harmful for the 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 Acl, 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 disadvantageous for the improvement of the fatigue property of silicon steel. If C content is too high, it will cause magnetic failure; if C content is too low, it will significantly reduce the yield strength. In the present invention, C content is limited to 0.0010.004 wt %.

(14) N: it is easy for N as an interstitial element to form fine dispersed nitrides with Ti, Al, Nb or V, which 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 be 0.003 wt % or less.

(15) Usually, Al content is increased to form coarsened AlN and reduce the influence of N element and other fine nitride. The Al/N ratio will directly influence the form and size of AlN. If Al content is too low, fine needle-like AlN will be formed, which seriously influences the magnetic domain motion and thus deteriorates iron loss. The present inventor has investigated the relationship between the Al/N ratio and the iron loss P.sub.15/50 of the non-oriented silicon steel. FIG. 3 shows the relationship between the Al/N ratio of the casting slab for manufacturing the non-oriented silicon steel and the iron loss P.sub.15/50 of the non-oriented silicon steel. As shown in FIG. 3, the iron loss is low when the Al/N ratio is 300 or more and preferably between 350 and 600, and that the effect of reducing iron loss becomes basically saturation when the Al/N ratio reaches 600. In the present invention, the Al/N ratio is limited to be 300 or more, and preferably between 350 and 600.

(16) O: it is harmful for the magnetic property and is able to form oxide inclusions during the steel making process, its amount and form significantly influence the magnetic property. Thus, in addition to reducing the final oxygen content in the steel making process as far as possible, it is also needed to reduce the amount of oxides and control their form through steel making techniques.

(17) B: When B is added in the steel with low Si content, it can reduce Al content and lower the steel making cost; when B is added in the steel with high Si content and Al content, it is in the solid solution state, and in this state, it can improve the crystal structure by its segregation along grain boundary while preventing embrittlement caused by P segregation and preventing the formation of internal oxide layer and internal nitride layer, thus promoting the growth of grains. However, as an interstitial atom, excessive B content will hinder the magnetic domain motion and reduce the magnetic property. Therefore, in the present invention, B content is limited to be 0.005 wt % or less.

(18) Next, the present inventor has investigated the influence of both the total amount of nitrogen and oxygen in the surface layer and the equivalent axial coefficient of the grains of the non-oriented silicon steel on the iron loss and/or the anisotropy of iron loss of the non-oriented silicon steel.

(19) The total content of nitrogen and oxygen in the surface layer of the non-oriented silicon steel represents the degree of surface nitridation and internal oxidation and the total amount level of oxides, which directly influences the iron loss level of the non-oriented silicon steel. FIG. 4 shows the relationship between the total content of nitrogen and oxygen at a depth of 30 m from the surface of the non-oriented silicon steel and the iron loss P.sub.15/50 of the non-oriented silicon steel. As shown in FIG. 4, the iron loss of the non-oriented silicon steel increases with the increase of the total content of nitrogen and oxygen, and the non-oriented silicon steel presents a low iron loss when the total content of nitrogen and oxygen is 300 ppm or less. Therefore, in order to obtain the non-oriented silicon steel having a low iron loss, the total content of nitrogen and oxygen in the surface layer of the non-oriented silicon steel should be reduced as far as possible.

(20) Said equivalent axial coefficient of the grains in the present invention is defined as follows: selecting samples in parallel to the sheet surface, rubbing off the surface layer to make the metallographic samples, observing the grain structure under a microscope, and respectively measuring the average diameter D.sub.L of the grain structure parallel to the rolling direction and the average diameter D.sub.C of the grain structure perpendicular to the rolling direction (i.e., cross direction). The ratio of the average diameter D.sub.L to the average diameter D.sub.C is defined as equivalent axial coefficient L of the grains, i.e., L=D.sub.L/D.sub.C.

(21) L is employed to characterize the shape features of the grains in the rolling direction and cross direction. When the L value is more approximate to 1, it means that the grains are more approximate to equivalent axial grains; when the L value is more deviated from 1, it means that the grains are more deviated from the equivalent axial form; the higher the L value is, the longer the grains in the rolling direction are, and the shorter the grains in the cross direction are. FIG. 5 shows the relationship between the equivalent axial coefficient of the grains of the non-oriented silicon steel and anisotropy of iron loss of the non-oriented silicon steel. As shown in FIG. 5, the non-oriented silicon steel has a low anisotropy of iron loss when the L value falls between 1.05 and 1.35. Therefore, in order to obtain the non-oriented silicon steel having an excellent magnetic property, preferably the equivalent axial coefficient L of the grains is set between 1.05 and 1.35.

(22) In one preferable embodiment in the method of the present invention, in RH refining, deoxidation is implemented first by using FeSi alloy and then by using FeAl alloy. Firstly using FeSi alloy for deoxidation can effectively eliminate most of the free oxygen contained in the silicon steel, and the resulted deoxidized product SiO.sub.2 has large size and is easy to come up and be eliminated; then using FeAl alloy having a deoxidizing capacity better than FeSi alloy can easily eliminate the residual free oxygen in the silicon steel, significantly reduce the amount of oxide inclusions of the silicon steel, control the amount of oxide inclusions having a size of 500 nm or less contained in the final silicon steel products to be 40% or less, and thus weaken the pining effect of grain boundary and the pining effect of magnetic domain and improve the magnetic property of the silicon steel. The influence of FeSi alloy deoxidation and FeAl alloy deoxidation on the inclusions of the silicon steel is shown in Table 1.

(23) TABLE-US-00001 TABLE 1 <0.5 m 0.5~1 m 1~1.5 m 1.5~5 m 5~10 m FeSi alloy A large amount AlN and MnS AlN and MnS AlN and MnS A small amount deoxidation of MnS, Cu.sub.2S complex, some complex, some complex, a small of FeO and SiO.sub.2 and AlN MnS Cu.sub.2S amount of CaO, complex Al.sub.2O.sub.3, FeO and other complex FeAl alloy A large amount Mainly Mainly AlN AlN, Al.sub.2O.sub.3 and A small amount deoxidation of MnS and Cu.sub.2S MgO + MnS/Cu.sub.2S and Al.sub.2O.sub.3 SiO.sub.2 or Cu.sub.2S of FeO and Al.sub.2O.sub.3 complex complex

(24) In another preferable embodiment in the method of the present invention, in said hot rolling step b), a deformation at 950 C. or more is 80% or more. The influence of the high-temperature deformation in hot rolling (deformation at 950 C. or more) on steel strip structure is shown in Table 2. As shown in table 2, increasing the high-temperature deformation in hot rolling can reduce the fine precipitates in the steel strip and improve the recrystallization of grains. Therefore, in order to obtain the non-oriented silicon steel having an excellent magnetic property, in the method of the present invention, preferably in said hot rolling step b), a deformation at 950 C. or more is 80% or more.

(25) TABLE-US-00002 TABLE 2 Deformation at 950 C. Fine or more precipitates Recrystallization 1 30% Obviously visible Fiber structure at the core 2 50% Obviously visible Fiber structure at the core 3 60% Visible A small amount of Fiber structure at the core 4 80% Extremely few Fully recrystallized 5 85% Extremely few Fully recrystallized
In another preferable embodiment in the method of the present invention, the maximum temperature difference between various positions of the hot-rolled steel strip in the hot rolling step is preferably 20 C. or less, and further preferably 10 C. or less. The relationship between the maximum center-edge temperature difference of the steel strip and the maximum degree of convexity and edge crack is shown in Table 3. As shown in table 3, both the degree of convexity and the edge crack reached an excellent level when the temperature difference is 20 C. or less, and edge crack can be mostly avoided when the temperature difference is 10 C. or less. Therefore, in view of obtaining excellent sheet shape and preventing edge crack, the maximum temperature difference between various positions of the hot-rolled steel strip is preferably 20 C. or less, and further preferably 10 C. or less.

(26) TABLE-US-00003 TABLE 3 Maximum center-edge Maximum temperature degree of difference ( C.) convexity Edge crack 1 10 30 m No edge crack 2 15 30 m Occasional edge crack 3 20 35 m Slight edge crack 4 30 50 m Edge crack 5 >35 60 m Obvious edge crack

(27) 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

(28) In the first steel making step, a casting slab containing the following ingredients as calculated by weight percentage is obtained through RH refining and continuous casting: C 0.002%, Si 3.2%, Al 0.7%, Mn 0.50%, P 0.014%, S 0.001%, N 0.002%, B 0.002%, Nb 0.001%, V 0.002%, Ti 0.0015%, Zr 0.001%, Sn 0.008%, and the balance being Fe and unavoidable impurities; in the steel making step, the temperature T of molten steel during tapping on converter, the carbon content [C] and the free oxygen content [O] satisfy the following formula: 7.2710.sup.3[O][C]e.sup.(5,000/T)2.9910.sup.4, and, in RH refining, deoxidation is implemented first by using FeSi alloy and then by using FeAl alloy.

(29) In the following hot rolling step, the casting slab is heated to 1,100 C. and is rolled after thermal insulation, and the temperature at the end of hot rolling is 850 C. or more, wherein the deformation at 950 C. or more is 80% or more and the hot-rolled steel strip has a thickness of 1.5-3.0 mm.

(30) Then, the continuous annealing of normalization or batch furnace of normalization is adopted. When the continuous annealing of normalization is adopted, the normalization process is maintained for 1090 s at 850950 C., the heating rate of normalization is 515 C./s, and the cooling rate is 520 C./s; when the batch furnace of normalization is adopted, the normalization process is maintained for 26 h at 780880 C. under the protection atmosphere of hydrogen.

(31) Next, the hot-rolled steel strip after normalizing treatment is subject to cold rolling to form the cold-rolled steel strip, and the cold-rolled steel strip has a thickness of 0.270 5 mm after cold rolling, and the reduction rate of cold rolling is 7088%.

(32) Finally, the cold-rolled steel strip is subject to annealing. In the continuous annealing furnace, it is heated to 900 C. at the heating rate of 2545 C./s, and at such temperature, the annealing process is maintained for 860 s under the protection atmosphere of nitrogen and hydrogen and under the tension of 0.5 MPa, thus to obtain the non-oriented silicon steel in Example 1.

Example 2

(33) Non-oriented silicon steel in Example 2 is produced with the same method as that used in Example 1, except that the annealing temperature in the final annealing step is changed to 920 C.

Example 3

(34) Non-oriented silicon steel in Example 3 is produced with the same method as that used in Example 1, except that the annealing temperature in the final annealing step is changed to 1,020 C.

Example 4

(35) Non-oriented silicon steel in Example 4 is produced with the same method as that used in Example 1, except that the annealing temperature in the final annealing step is changed to 1,050 C.

Example 5

(36) Non-oriented silicon steel in Example 5 is produced in the same method as that used in Example 1, except that the tension in the final annealing step is changed to 1 MPa.

Example 6

(37) Non-oriented silicon steel in Example 6 is produced with the same method as that used in Example 1, except that the tension in the final annealing step is changed to 1.3 MPa.

Example 7

(38) Non-oriented silicon steel in Example 7 is produced with the same method as that used in Example 1, except that the tension in the final annealing step is changed to 1.5 MPa.

Comparative Example 1

(39) Non-oriented silicon steel in Comparative Example 1 is produced with the same method as that used in Example 1, except that the annealing temperature in the final annealing step is changed to 850 C.

Comparative Example 2

(40) Non-oriented silicon steel in Comparative Example 2 is produced with the same method as that used in Example 1, except that the annealing temperature in the final annealing step is changed to 1,100 C.

Comparative Example 3

(41) Non-oriented silicon steel in Comparative Example 3 is produced with the same method as that used in Example 1, except that the tension in the final annealing step is changed to 0.3 MPa.

Comparative Example 4

(42) Non-oriented silicon steel in Comparative Example 4 is produced with the same method as that used in Example 1, except that the tension in the final annealing step is changed to 2 MPa.

Comparative Example 5

(43) Non-oriented silicon steel in Comparative Example 5 is produced with the same method as that used in Example 1, except that the annealing time in the final annealing step is changed to 5 s.

Comparative Example 6

(44) Non-oriented silicon steel in Comparative Example 6 is produced with the same method as that used in Example 1, except that the temperature T of molten steel during tapping on converter in steel making, the carbon content [C] and the free oxygen content [O] fail to satisfy the following formula: 7.2710.sup.3[O][C]e.sup.(5,000/T)2.9910.sup.4.

(45) The iron loss P.sub.15/50 and the anisotropy of iron loss of non-oriented silicon steel (0.5 mm in thickness) in the above examples and comparative examples are measured, and the results are shown in Table 4.

(46) TABLE-US-00004 TABLE 4 Whether satisfying the following formula: Tension in 7.27 10.sup.3 [O] Temperature in the Annealing time [C]e.sup.(5,000/T) 2.99 the final final annealing in the final Iron loss P.sub.15/50 Anisotropy of 10.sup.4 annealing ( C.) (MPa) annealing (s) (W/kg) iron loss (%) Example 1 Yes 900 0.5 8~60 2.37 7.5 Example 2 Yes 920 0.5 8~60 2.35 8.0 Example 3 Yes 1,020 0.5 8~60 2.30 9.0 Example 4 Yes 1,050 0.5 8~60 2.26 9.6 Example 5 Yes 900 1 8~60 2.34 8.6 Example 6 Yes 900 1.3 8~60 2.28 9.1 Example 7 Yes 900 1.5 8~60 2.30 9.5 Comparative Yes 850 0.5 8~60 2.45 7.0 Example 1 Comparative Yes 1,100 0.5 8~60 2.26 11.1 Example 2 Comparative Yes 900 0.3 8~60 2.53 7.0 Example 3 Comparative Yes 900 2 8~60 2.46 10.1 Example 4 Comparative Yes 900 0.5 5 2.65 7.2 Example 5 Comparative No 900 0.5 8~60 2.48 8.0 Example 6

(47) It can be known from the above table, compared with the comparative examples, the non-oriented silicon steel in the above examples has a low iron loss and low anisotropy of iron loss. The non-oriented silicon steel has an iron loss P.sub.15/50 of 2.40 W/kg or less and anisotropy of iron loss of 10% or less at 0.5 mm thickness, wherein P.sub.15/50 represents the iron loss of the non-oriented silicon steel under a magnetic induction of 1.5 T at 50 Hz.

(48) In addition, the present inventor has measured the surface property and grain property of the non-oriented silicon steel in the above examples. The results show that the non-oriented silicon steel in the above examples has a grain diameter between 100 and 200 nm, and the grain equivalent axial coefficient L between 1.05 and 1.35. Furthermore, the total content of nitrogen and oxygen at a depth of 30 nm from the surface of the non-oriented silicon steel in the above examples is 300 ppm or less, and the amount of inclusions having a size of 500 nm or less contained in the non-oriented silicon steel is 40% or less.

(49) The experimental results of the present invention demonstrate that, in the present invention, by strictly controlling the relationship among the temperature T of molten steel during tapping on converter and the carbon content [C] and free oxygen content [O] and regulating the content of various ingredients in the casting slab, both the total content of nitrogen and oxygen and the amount of inclusions in the non-oriented silicon steel can be reduced, thus improve the structure and magnetic properties of the non-oriented silicon steel. Furthermore, by making low-temperature short-term annealing at the temperature of 9001,050 C. and under the tension of 0.51.5 MPa, the grains can rapidly grow and obtain a suitable grain equivalent axial coefficient, and thus reduce both the iron loss and the anisotropy of iron loss and increase the magnetic property of the non-oriented silicon steel.

BENEFICIAL EFFECTS OF THE PRESENT INVENTION

(50) By means of regulating the content of various ingredients in the casting slab through steel making, strictly controlling the relationship among the temperature T of molten steel during tapping on converter and the carbon content [C] and free oxygen content [O] to reduce the amount of inclusions and control their form, making low-temperature short-term annealing at a tension to control the form of grains, the present invention can obtain non-oriented silicon steel having excellent iron loss and anisotropy of iron loss. The non-oriented silicon steel in the present invention can satisfy the miniaturization and energy conservation requirements of electronic equipment, thus have a broad application prospect.