JP3375998B2 - Manufacturing method of non-oriented electrical steel sheet - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a non-oriented electrical steel sheet having both excellent magnetic properties and excellent steel sheet shape. 2. Description of the Related Art Non-oriented electrical steel sheets are classified into various grades according to their magnetic property levels, and include large-sized rotating machines, medium-sized rotating machines, general-purpose motors, motors for home appliances, transformers and stabilizers. Widely used as iron core material. In recent years, there has been a particularly strong demand for energy saving, improvement of characteristics of electric devices, miniaturization, and the like. Therefore, improvement of magnetic characteristics of iron core materials used for these electric devices has become increasingly important. [0003] In addition, such iron cores are often manufactured by punching a large number of electromagnetic steel sheets into a predetermined shape and then laminating them, so that the improvement of the shape of the steel sheet, especially the accuracy of the thickness, is the same as the magnetic properties. Is considered important. This is because, in the case of electromagnetic steel sheets having poor thickness accuracy, there are problems in that the height of the iron core varies when the steel sheets are laminated, and that the upper surface of the iron core is inclined due to thickness deviation. In particular, in recent years, with the automation of the core manufacturing line, further improvement in the thickness accuracy is desired. [0004] Since non-oriented electrical steel sheets are usually manufactured through a hot rolling step, a cold rolling step, and a finish annealing step, poor sheet thickness accuracy in a hot rolled sheet remains until after cold rolling. , Which is a cause of poor thickness of the product. [0005] There is no such defective shape of the hot rolled sheet.
As a hot rolling method suitable for obtaining a non-oriented electrical steel sheet having good magnetic properties, there are techniques disclosed in JP-A-51-74923 and JP-A-4-180522. JP-A-51-74923
The technique disclosed in Japanese Patent Application Laid-Open Publication No. H11-163,086 is a method in which hot rolling is completed in the α single phase region. In this method, by performing the γ → α transformation at the preceding stage of the finish rolling, the thickness unevenness caused by the instability of the rolling caused by the sudden decrease in the deformation resistance at the time of the γ → α transformation is reduced to an α single phase. This is intended to be corrected in the latter stage of the finish rolling. On the other hand, the technology disclosed in Japanese Patent Application Laid-Open No. 4-180522 discloses that the slab heating temperature is set to 1100 ° C. or less and the final stand outlet temperature is set to (820 + 80 × Si%) to (870 + 80 × Si
%). By controlling the temperature to ° C., the magnetism is improved, and a γ → α transformation is performed between stands by a cooling device. In this method, in order to prevent temperature unevenness in the width direction and local elongation caused by a sudden decrease in deformation resistance due to the γ → α transformation, a γ → α transformation is performed between stands by a cooling device, thereby obtaining a complete γ. Rolling in the area or the complete α area. [0006] However, Japanese Patent Application Laid-Open
In the method disclosed in JP-A-51-74923, although the thickness unevenness in the longitudinal direction of the hot-rolled sheet is improved, the width direction of the hot-rolled sheet caused by the instability of the rolling caused by the γ → α transformation in the final stage of the finish rolling. It is difficult to correct the shape defects such as the thickness deviation (crown) and the insufficient width of the sheet even in the stage after the finish rolling. Further, in the method disclosed in Japanese Patent Application Laid-Open No. 4-180522, it is extremely difficult to stably transform γ → α between stands because there are portions where the temperature varies, such as a skid portion, in the longitudinal direction of the hot-rolled sheet. It is difficult to say that it is practical. An object of the present invention is to propose a method for manufacturing a non-oriented electrical steel sheet which is excellent in both steel sheet shape and magnetic properties by advantageously overcoming the above problems and stabilizing hot rolling. Means for Solving the Problems In order to achieve the above object, the present inventors have conducted various studies on the hot deformation behavior of a non-oriented electrical steel sheet together with various conditions of a hot rolling process. The following findings were obtained. (1) As shown in JP-A-51-74923 and JP-A-4-180522, in non-oriented electrical steel sheets, the decrease in deformation resistance due to γ → α transformation is remarkable, and after γ → α transformation Is about 1/2 that before transformation. The reason that the rolling becomes unstable during the finish rolling is that, in addition to the non-uniform temperature in the longitudinal direction of the hot-rolled sheet (such as skid), the sudden decrease in deformation resistance due to the γ → α transformation described above causes This is because the deformation resistance greatly changes between the stands. FIG. 1 shows the result of examining the time change of the deformation resistance in the third stand when the non-oriented electrical steel sheet is hot-rolled by the seven-stand continuous finishing rolling mill. As shown in FIG.
The deformation resistance varies greatly even within the same coil. (2) However, the amount of change in the deformation resistance due to the γ → α transformation strongly depends on the draft and the strain rate,
By using a low draft and a high strain rate, the amount of change in deformation resistance can be significantly reduced. Therefore, the γ → α transformation is completed during the finish rolling by controlling the rolling speed, the roll gap and the cooling conditions based on the above experimental results, and a low rolling reduction and a high strain rate are applied during this period. After continuous rolling,
It has been found that not only the rolling is stabilized, but also the magnetic properties are improved. The present invention is based on the above findings. [0010] That is, the present invention relates to a Si + Al: 1.8 wt%
An electromagnetic steel slab containing (hereinafter simply indicated by%) containing:
After hot rolling and then cold rolling once or twice including intermediate annealing, and then producing a non-oriented electrical steel sheet by performing finish annealing, in the continuous finishing rolling step of the above hot rolling When the sheet temperature is in the range of (γ → α transformation start temperature + 20 ° C.) to (γ → α transformation end temperature−20 ° C.), rolling is performed under the conditions of a rolling reduction: 40% or less and an equivalent strain rate: 50 s ⁻¹ or more. And a method for producing a non-oriented electrical steel sheet. Hereinafter, the present invention will be specifically described based on the experimental results that led to the present invention. 2 and 3, C: 0.003%, Si: 0.25%, Mn: 0.20%, P;
The measurement result of the true stress-true strain curve at the time of hot compression processing of the steel containing 07%, S: 0.0030%, and Al: 0.25% is shown. FIG. 2 shows the case where the strain rate is 5 s ^-1 (constant),
FIG. 3 shows a case where the strain rate is 50 s ^-1 (constant). The temperature during processing is 920 ° C just above the γ → α transformation point (γ single phase)
890 ° C (α single phase) immediately below. As is clear from FIG. 2, when the strain rate is low (5 s ^-1 ), A
→ As shown in B, the stress difference between the γ phase and the α phase is very large. On the other hand, when the strain rate is high (50
s ^-1 ), when the true strain is large, the stress difference between the γ phase and the α phase is large as shown by C → D in FIG. 3, but when the true strain is small, E → As shown by F, it was found that the stress difference between the γ phase and the α phase was significantly reduced. Then, next, in order to investigate the effects of the rolling reduction and strain rate during hot rolling on the deformation resistance,
C: 0.0025%, Si: 0.50%, Mn: 0.15%, P: 0.02%,
A 4 mm-thick rough rolled sheet for non-oriented electrical steel sheets containing 0.0030% of S and 0.001% of Al was converted to a high-speed hot rolling mill (roll diameter: 30
0 mm), one-pass rolling was performed at 920 ° C just above the γ → α transformation temperature and 890 ° C just below the γ → α transformation temperature. γ
→ The degree of change in deformation resistance during α transformation is Kα / Kγ
(Where Kα indicates the deformation resistance at 890 ° C. and Kγ indicates the deformation resistance at 920 ° C.). FIG. 4 shows the obtained results. As apparent from FIG. 1, a high Kα / Kγ
In order to obtain a low rolling reduction and a high strain rate, it is important to reduce the rolling rate to 40% or less and an equivalent strain rate of 50 s ^-1 or more to obtain an excellent value of Kα / Kγ ≧ 0.80. Obtained. The rolling reduction (r) and the equivalent strain rate Is represented by the following equation. (Equation 2) Reduction ratio: 40% or less, equivalent strain rate: 50 s
Under the condition of ⁻¹ or more, it was found that the change in deformation resistance due to the γ → α transformation was small. Therefore, the non-oriented electrical steel sheet was actually hot-rolled by a continuous finishing rolling mill including seven stands. In Table 1, C: 0.003%, Si: 0.25%, Mn: 0.20
%, P: 0.07%, S: 0.0030%, and Al: 0.25% The rolling conditions and rolling results when the non-oriented electrical steel sheet is continuously finish-rolled are shown. In the table, stands surrounded by squares are those whose sheet temperature is in the range of (γ → α transformation start temperature + 20 ° C.) to (γ → α transformation end temperature−20 ° C.). FIG.
The deformation resistance at each stand is shown below. [Table 1] As is apparent from FIG. 1, according to the present invention, the sheet temperature ranges from (γ → α transformation start temperature + 20 ° C.) to (γ → α transformation end temperature−20 ° C.), and the rolling reduction is 40% or less. When the finish rolling was performed under the conditions of an equivalent strain rate of 50 s ⁻¹ or more, the rolling stability was extremely good, and as a result, a hot-rolled sheet having excellent thickness accuracy could be obtained. Next, the reason why the component composition of the raw material in the present invention is limited to the above range will be described. Si + Al: 1.8% or less Both Si and Al effectively contribute to the reduction of iron loss by increasing the specific resistance. Si and Al also have the effect of increasing the γ → α transformation temperature. According to the present invention, γ →
This action is extremely useful because it requires alpha transformation. However, if the content is too large, the magnetic flux density will decrease and the cost will increase.
%. In the present invention, the composition of the components is not particularly limited except for the above-mentioned Si and Al, but the preferred composition range of the other components is as follows. C: 0.0050% or less C significantly deteriorates magnetic properties due to aging precipitation. Mn: 0.50 to 1.5% Mn reacts with S to form MnS.
MnS is finely dispersed and hinders grain growth, resulting in deterioration of iron loss. On the other hand, if the content exceeds 1.5%, the cost increases, so the Mn content is desirably set to 0.50 to 1.5%. S: not more than 0.0050% As described above, S forms precipitates MnS that inhibit grain growth, so that it is desirable to minimize the incorporation of Sn, and preferably not more than 0.0050%. P: 0.20% or less P increases the hardness and effectively contributes to the improvement of punching performance.
If the content increases, the magnetic properties deteriorate. Therefore, the content is preferably set to 0.20% or less. Further, if necessary, Cu: 0.01 to 1.0%,
Sn: 0.02 to 0.2%, Sb: 0.010 to 0.30%, B: 3 to 50 p
One or more selected from pm and the like can be contained. These elements have the effect of improving the texture and increasing the magnetic flux density. However, if they are added in large amounts, it is disadvantageous in terms of cost. Therefore, it is desirable that these elements be contained in the above range. Next, the hot rolling conditions according to the present invention will be described. First, the heating temperature is desirably about 1200 ° C. or less. This is because at temperatures exceeding this, MnS forms a solid solution and precipitates finely during hot rolling, thereby deteriorating iron loss. As for the finish rolling, as described above,
Γ → α transformation during continuous finish rolling and the sheet temperature becomes (γ
→ α transformation start temperature + 20 ° C) ~ (γ → α transformation end temperature -20)
℃) range, controlling the rolling temperature, roll gap, cooling conditions, etc., rolling reduction ≤ 40%, strain rate ≥ 50 s
It is important to perform continuous rolling under the condition of ^-1 . This is because the rolling conditions in the above temperature range are such that the rolling reduction is> 40.
%, Or when the strain rate is less than 50 s ⁻¹ , the change in transformation resistance due to transformation becomes large, and rolling becomes unstable. The reason why the sheet temperature range is given a margin of ± 20 ° C. with respect to the γ → α transformation temperature range is that the sheet thickness and the temperature distribution in the sheet width direction are considered. Further, although the magnetic properties can be further improved by annealing the hot-rolled sheet, such annealing treatment involves an increase in cost, and may be performed as necessary. After the above hot rolling, cold rolling is performed once or twice including intermediate annealing, and then finish annealing is performed. After finish annealing, cold rolling of 15% or less can be performed (so-called semi-process non-oriented electrical steel sheet). EXAMPLES C: 0.0030%, Si: 0.35%, Mn: 0.25%,
P: 0.03%, S: 0.0028%, solAl: 0.001%, the remainder is molten steel substantially having the composition of Fe, and then slab is formed into a 230mm thick slab by continuous casting. Hot rolling was performed under the indicated rolling conditions to obtain a hot-rolled sheet having a thickness of 2.5 mm. Thereafter, the hot-rolled sheet was subjected to pickling and descaling, and then cold-rolled to form a 0.5-mm-thick cold-rolled sheet, followed by continuous annealing at 750 ° C. for 30 seconds to obtain a product. From the non-oriented electrical steel sheet thus obtained, eighty 30 × 80 mm Epstein test pieces were sampled from the rolling direction and eight from the direction perpendicular to the rolling direction, for a total of 16 pieces, and the magnetic properties were measured. Table 3 also shows the obtained results. [Table 2] [Table 3] As is evident from Table 3, when hot rolling was performed under the conditions according to the present invention, the rolling was stabilized, and as a result, particularly good magnetic properties and steel sheet shapes were obtained. Thus, according to the present invention, in the hot rolling step of a non-oriented electrical steel sheet, the sheet temperature is (γ → α transformation start temperature + 20 ° C.) to (γ → α transformation end temperature−20 ° C.) Is continuously rolled under the conditions of a draft of ≦ 40% and a strain rate of ≧ 50 s ⁻¹ , it is possible to obtain a non-oriented electrical steel sheet excellent in both steel sheet shape and magnetic properties.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing a time change of deformation resistance in a third stand when a non-oriented electrical steel sheet is hot-rolled by a seven-stand continuous finishing rolling mill. FIG. 2 is a graph showing a true stress-true strain curve when hot compression processing is performed at a strain rate of 5 s ⁻¹ . FIG. 3 is a graph showing a true stress-true strain curve when hot compression processing is performed at a strain rate of 50 s ⁻¹ . FIG. 4 shows that the rolling reduction and the strain rate are determined by the deformation resistance (Kα / K
It is a figure which shows the influence which affects (gamma). FIG. 5 is a graph showing deformation resistance at each stand in hot rolling.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-264619 (JP, A) JP-A-4-107215 (JP, A) JP-A-63-53214 (JP, A) JP-A-4- 235223 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/12 C21D 9/46 501 B21B 3/02 C22C 38/00-38/60

Claims (1)

(57) [Claims 1] An electromagnetic steel slab containing 1.8 wt% or less of Si + Al was hot-rolled, and then cold-rolled once or twice including intermediate annealing. Thereafter, in producing a non-oriented electrical steel sheet by performing finish annealing, in the continuous finishing rolling step of the hot rolling, the sheet temperature is (γ → α transformation start temperature + 20 ° C.) to (γ → α transformation end temperature). (-20 [deg.] C.), a method for producing a non-oriented electrical steel sheet, wherein rolling is performed under the conditions of a draft of 40% or less and an equivalent strain rate of 50 s- ¹ or more.