JP5009514B2 - Non-oriented electrical steel sheet - Google Patents

Description

  The present invention relates to a non-oriented electrical steel sheet, and more particularly to a non-oriented electrical steel sheet that is substantially free of Al and has low iron loss after magnetic annealing.

  Non-oriented electrical steel sheets are classified into full-process materials and semi-process materials depending on the manufacturing method. Among these, the full process material is given a predetermined magnetic property by finish annealing on the steel manufacturer side, and is used as it is by customers. On the other hand, the semi-processed material may be subjected to finish annealing on the steel manufacturer side, but given magnetic properties are given by strain relief annealing after punching by the customer. At the time of strain relief annealing in the semi-process material, since the processing strain is removed and the crystal grains grow at the same time, the iron loss can be further reduced. For this reason, strain relief annealing is also called “magnetic annealing” (hereinafter referred to as magnetic annealing).

  In order to improve the grain growth at the time of magnetic annealing, it is effective to reduce the amount of inclusions and precipitates in the steel sheet. In particular, various methods for suppressing the precipitation of VN have been proposed because V mixed as an impurity precipitates as VN during magnetic annealing and inhibits grain growth. For example, a technique is disclosed in which 0.1 to 1% of Al is added to precipitate N as AlN to prevent VN precipitation (Patent Document 1).

  However, recently, from the viewpoint of motor recycling, there is a movement to melt the core of a used motor and reuse it as a casting in a motor frame or the like. If it tries to use, the problem that the viscosity of molten steel will increase at the time of casting and a shrinkage cavity will arise will arise. Therefore, when reusing and recycling the electromagnetic steel sheet as a casting, it is necessary to substantially add no Al.

As a substantially Al-free electrical steel sheet, the present inventors made Si: 1.0% or less, Al: 0.004% or less, and the relationship between V and N as impurities log (V × N) <− 5.44. A non-oriented electrical steel sheet excellent in low magnetic field characteristics has been proposed (Patent Document 2).
Japanese Patent Laid-Open No. 10-18006 Japanese Patent No. 2718410

  However, when the inventors subsequently investigated, as in the electromagnetic steel sheet described in Patent Document 2, even if Al was substantially not added, even if the relationship between V and N as impurities was controlled, after magnetic annealing It has become clear that a sufficiently low iron loss may not always be obtained.

  An object of the present invention is to provide a non-oriented electrical steel sheet that is substantially free from Al and has low iron loss after magnetic annealing.

The present inventors have examined the grain growth property and magnetic characteristics during magnetic annealing using a magnetic steel sheet substantially free of Al, and found the following.
(1) As impurities, not only V, but also the amount of Nb in mass% is significantly reduced from the conventional 0.002%, and the weighted total amount of Al, V, and Nb is controlled to a predetermined value or less. This is effective in promoting grain growth.
(2) To improve the magnetic flux density in a low magnetic field, it is effective to control the uniformity of crystal grains after magnetic annealing.

The present invention has been made based on such knowledge, in mass%, C: 0.005% or less, Si: 4% or less, P: 0.2% or less, Mn: 0.05 to 1.0%, S: 0.005% or less, N: 0.005% or less, Nb: 0.0008% or less, Al: 0.004% or less, V: 0.004% or less, the balance being Fe and inevitable impurities, and [Al] + [V] + 5 × [Nb] A non-oriented electrical steel sheet satisfying ≦ 0.002% is provided; provided that [M] represents the content (mass%) of the element M.

In addition, in the steel plate thickness cross section after magnetic annealing at 750 ° C. × 2 hr, when the total area of crystal grains exceeding 3 times the average crystal grain size is S1, and the total area of crystal grains of 3 times or less is S2, S1 / S2 is the higher the magnetic flux density B ₃ 0.3 or less is obtained.

  In the present invention, at least one element selected from Sb: 0.001 to 0.05%, Sn: 0.001 to 0.1%, Ni: 0.1 to 5%, and Co: 0.1 to 5% is further contained in mass%. be able to.

  According to the present invention, a non-oriented electrical steel sheet having substantially no Al added and low iron loss after magnetic annealing can be obtained.

Details of the present invention will be described below. (Note that “%” in relation to ingredients means mass% unless otherwise specified.)
1. Ingredients
C: 0.005% or less so that magnetic aging does not become a problem.

  Si: It is an element effective in increasing the specific resistance of the steel sheet and lowering the iron loss, but if it exceeds 4%, the magnetic flux density in the low magnetic field is reduced with the reduction of the saturation magnetic flux density, so 4% or less, Preferably it is 0.1% or more.

  P: An element effective for improving the punching workability of the steel sheet, but if it exceeds 0.2%, the steel sheet becomes brittle, so it is 0.2% or less, preferably 0.005% or more.

  Mn: 0.05% or more is necessary to prevent red hot brittleness during hot rolling, but if it exceeds 1.0%, the magnetic flux density decreases, so 0.05 to 1.0%.

  S: Precipitated as MnS and hinders grain growth, so 0.005% or less.

  N: Since a very small amount of Al, V, Nb, which will be described later, and fine nitrides are formed and grain growth is inhibited during magnetic annealing, it is set to 0.005% or less.

Nb: To investigate the effect of Nb on iron loss after magnetic annealing, C: 0.0030%, Si: 0.50%, Mn: 0.30%, P: 0.100%, Al: tr, S: 0.002%, V: tr ., N: 0.0025% constant, steel with varying Nb content in the range of 0.0001-0.0070% was melted in the laboratory, hot-rolled, pickled, and cold-rolled to a thickness of 0.5 mm Finish annealing at 750 ° C. × 30 s and magnetic annealing at 750 ° C. × 2 hr were further performed. Then, magnetic properties were measured using a 25 cm Epstein test piece. Nb was analyzed by ICP (inductively coupled plasma) mass spectrometry. FIG. 1 shows the relationship between the Nb amount and the iron loss (W _15/50 ). It can be seen that when the Nb amount is 0.0008% or less, a low W _15/50 of 4.4 W / kg or less can be obtained. Further, when the Nb content is 0.0004% or less, a lower W _15/50 of 4.0 W / kg or less is obtained. On the other hand, when the Nb content is 0.0009% or more, the iron loss is remarkably high. In order to investigate this cause, when the structure of the sample after magnetic annealing was observed with a transmission electron microscope (TEM), many very fine Nb-based precipitates of about 80 nm were observed at the grain boundaries. It was found that the precipitates inhibited the grain growth and markedly increased the iron loss. From this, it has been clarified that, in a magnetic steel sheet substantially free of Al, even if the amount of Nb is extremely small, the grain growth property is remarkably inhibited.

  Al: If present in a trace amount, fine AlN is formed and magnetic properties are inhibited, so 0.004% or less.

  V: For example, carbide other than nitride is formed to inhibit grain growth, so 0.004% or less.

[Al] + [V] + 5 × [Nb]: After controlling the amounts of Nb, Al, and V as described above, [Al] + [V] + 5 × [Nb] will be described next. Need to be controlled. C: 0.0025%, Si: 0.50%, Mn: 0.30%, P: 0.09%, S: 0.002%, N: 0.0020%, V is 0.001 to 0.006%, Nb is 0.0001 to 0.0030%, Al is 0.001 The steel changed to ~ 0.005% was melted in the laboratory, hot-rolled, pickled, cold-rolled to a thickness of 0.5 mm, subjected to finish annealing at 720 ° C x 1 min, and further 750 ° C x 2 hr Magnetic annealing was performed. Then, iron loss (W _15/50 ) was measured by the above method. As shown in FIG. 2, it is understood that the iron loss is remarkably reduced when [Al] + [V] + 5 × [Nb] ≦ 0.002% . In order to investigate this cause, the structure of the sample after magnetic annealing was observed with TEM. As a result, very fine V-Nb and V-Nb-Al precipitates of about 100 nm were observed in materials with high iron loss. A large number were observed at the grain boundaries, and it was found that the fine precipitates hindered the grain growth and significantly increased the iron loss. From this, when Al, V, and Nb coexist, they form a composite precipitate, so it is necessary to limit the amount of Nb as an impurity compared to the case where Nb exists alone. It became clear. Here, the correlation between [Al] + [V] + 5 × [Nb] and iron loss (W _15/50 ) is good. Nb is Al, V is 5 times the grain growth suppression effect during magnetic annealing This is thought to be due to the strength.

  The balance other than the above component elements is Fe and inevitable impurities, and is further selected from Sb: 0.001 to 0.05%, Sn: 0.001 to 0.1%, Ni: 0.1 to 5%, Co: 0.1 to 5% At least one element can be added to improve magnetic properties.

Incidentally, the motor core material is required to have a high magnetic flux density, in particular, B ₃ from the viewpoint of torque in addition to the low iron loss. This operation magnetic flux density at the compressor motor or the like is a region of about 1.5T, the magnetic flux density when excited by B ₃ That magnetizing force 300A / m is substantially 1.5T about, magnetic flux density at magnetization power Is important. Therefore, steel containing C: 0.0025%, Si: 0.60%, Mn: 0.25%, P: 0.09%, S: 0.002%, N: 0.0020%, Nb: 0.0002%, Al: 0.0005%, V: 0.0010% In a laboratory, hot-rolled, pickled, cold-rolled to a thickness of 0.5 mm, subjected to finish annealing at 750 to 850 ° C x 1 min, and further magnetic annealing at 750 ° C x 2 hr. , it was measured B _3. As a result, it has been found that the magnetic flux density B ₃ may vary even if the amounts of Al, V, and Nb are within the range of the present invention. In order to investigate the cause of the variation in the magnetic flux density, the structure of the steel plate thickness cross section was investigated with an optical microscope. As a result, a material with a low magnetic flux density showed a marked nonuniformity in crystal grain size. In order to quantify the relationship between the uniformity of the crystal grain size and the magnetic properties, the total area of crystal grains exceeding three times the average grain size measured by the line segment method of JIS G 0552 is defined as S1, and it is 3 times or less. the total area of crystal grains and S2, shown in FIG. 3 the results of investigating the relationship between S1 / S2 and the magnetic flux density B _3. Here, the grain size of each crystal grain was obtained by calculating the area of the crystal grain by image processing and converting it into a circle as a circle diameter (equivalent circle diameter). FIG. 3 shows that S1 / S2 needs to be 0.3 or less in order to obtain B ₃ as high as 1.5T. When S1 / S2 exceeds 0.3, the crystal grain size is remarkably non-uniform, and magnetization non-uniformity is likely to occur. Therefore, it is considered that the magnetic flux density is lowered. When the cause of such a non-uniform crystal grain size was investigated, it was found that this was due to the high temperature of finish annealing. This is because part of the precipitate is re-dissolved when the temperature of the finish annealing is high, and a part of the precipitate is further dissolved during the subsequent magnetic annealing. This is thought to be due to weakening and abnormal grain growth. Therefore, in order to make S1 / S2 0.3 or less, it is desirable that the temperature of finish annealing be 800 ° C. or less.

  In the non-oriented electrical steel sheet of the present invention, the object is achieved if the components are within a predetermined range, and a normal method can be applied to the manufacturing method. That is, the molten steel blown in the converter is degassed and adjusted to a predetermined component, cast, hot rolled, then hot-rolled sheet annealed as necessary, one cold rolling, Alternatively, finish annealing is performed after a predetermined thickness is obtained by cold rolling at least twice with intermediate annealing. As described above, the finish annealing temperature is desirably 800 ° C. or lower.

  After the steel 1-30 adjusted to the components shown in Table 1 by degassing after blowing in a converter, the steel 1-30 was hot-rolled to produce a hot-rolled sheet having a thickness of 2.3 mm. Next, this hot-rolled sheet was pickled, cold-rolled to a thickness of 0.5 mm, subjected to finish annealing at the temperature shown in Table 1 for 30 seconds, and further subjected to magnetic annealing at 750 ° C. × 2 hours. Steel 28 seems to have a large amount of P, but cracking occurred during cold rolling, and subsequent annealing was not performed. And the magnetic characteristic and the crystal grain were measured by the above methods.

The results are shown in Table 1. In Steels 2 to 15 and 22 to 24, which are inventive examples in which the components are controlled within the scope of the present invention, W _15/50 after magnetic annealing is 4.5 W / kg or less, and very low iron loss can be obtained. Recognize. Further, in steels 2 to 15, 22, 29, and 30 whose components are within the scope of the present invention and whose finishing temperature is 800 ° C. or less, S1 / S2 is 0.3 or less, and B _{3 of} 1.50 or more is obtained. I understand that.

FIG. 3 is a diagram showing the relationship between [Nb] and iron loss after magnetic annealing (W _15/50 ). It is a figure which shows the relationship between [Al] + [V] + _5x [Nb] and the iron loss ( _{W15 / 50} ) after magnetic annealing. Is a diagram showing the relation between S1 / S2 and the magnetic flux density B ₃ after magnetic annealing

Claims (3)

In mass%, C: 0.005% or less, Si: 4% or less, P: 0.2% or less, Mn: 0.05 to 1.0%, S: 0.005% or less, N: 0.005% or less, Nb: 0.0008% or less, Al: 0.004 %, V: 0.004% or less, non-oriented electrical steel sheet consisting of remaining Fe and inevitable impurities and satisfying [Al] + [V] + 5 × [Nb] ≦ 0.002% ; ] Represents the content (mass%) of the element M.   In the steel plate thickness section after magnetic annealing at 750 ° C. × 2 hr, the total area of crystal grains exceeding 3 times the average crystal grain size is S1, and when the total area of crystal grains of 3 times or less is S2, S1 / 2. The non-oriented electrical steel sheet according to claim 1, wherein S2 is 0.3 or less.   Furthermore, the mass% contains at least one element selected from Sb: 0.001 to 0.05%, Sn: 0.001 to 0.1%, Ni: 0.1 to 5%, and Co: 0.1 to 5%. The non-oriented electrical steel sheet according to claim 2.

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