JP2013104080A - Non-oriented magnetic steel sheet and method for manufacturing the same - Google Patents

Abstract

An unoriented electrical steel sheet having an unprecedented high magnetic flux density and low iron loss is provided at a low cost.
C: 0.005% or less, Si: 0.1-2.0%, Mn: 0.05-0.6%, P: 0.100% or less, N: 0.0030% or less, Al: 0.01 to 0.05%, B: N in a ratio of B: N = 0.9 to 1.2, nonmagnetic precipitate AlN having an average diameter of 10 to 200 nm, number density of 10 / [mu] m ³ contains the following, a non-oriented electrical steel sheet B50 average rolling direction and the direction perpendicular to the rolling direction is not less than 1.75 T, a slab heating temperature of 1050 ° C. to 1250 ° C., the coiling temperature of the coil 780~Ac1 With the transformation point, the annealing temperature in the final annealing process is set to 800 ° C. to Ac1 transformation point.
[Selection] Figure 1

Description

  The present invention relates to a non-oriented electrical steel sheet having an α-γ transformation (ferrite-austenite transformation) and excellent magnetic properties. Furthermore, even when hot-rolled sheet annealing is omitted, a high magnetic flux density is obtained. The present invention relates to a method for producing a non-oriented electrical steel sheet that can exhibit low iron loss.

  In recent years, there has been an increasing demand for higher efficiency of equipment even in the field where low-grade non-oriented electrical steel sheets have been used. The magnetic steel sheet used is required to have a high magnetic flux density and a low iron loss while suppressing costs. Here, the low grade non-oriented electrical steel sheet is often a component composition range generally having a low Si content and an α-γ transformation. In such a low-grade non-oriented electrical steel sheet, many methods for improving the magnetic properties by omitting hot-rolled sheet annealing have been proposed.

For example, Patent Document 1 proposes a method in which hot rolling is finished at an Ar3 transformation point or higher, and the temperature is slowly cooled from the Ar3 transformation point to an Ar1 transformation point at 5 ° C / sec or less. However, it is difficult to perform this cooling rate by hot rolling of the actual machine.
Patent Document 2 proposes a method of adding Sn to steel and controlling the finishing temperature of hot rolling in accordance with the Sn concentration to obtain a high magnetic flux density. However, this method is limited to a Si concentration of 0.4% or less, and is insufficient for obtaining a low iron loss.

Patent Document 3 proposes a steel plate that has a high magnetic flux density and excellent grain growth during strain relief annealing by limiting the heating temperature and finishing temperature during hot rolling. In this method, since there is no process such as self-annealing instead of hot-rolled sheet annealing, a high magnetic flux density cannot be obtained.
Patent Document 4 proposes that in hot rolling, a rough bar before finish rolling is heated online, the finishing temperature of hot rolling is Ar1 + 20 ° C. or higher, and the winding temperature is 640 to 750 ° C. This method aims at detoxification of precipitates, and a high magnetic flux density is not obtained.

JP-A-6-1927331 JP 2006-241554 A JP 2007-217744 A JP-A-11-61257

The invention relating to the omission of hot-rolled sheet annealing that has been proposed for the non-oriented electrical steel sheet having the α-γ transformation did not satisfy the actual machine manufacturability and the characteristics of the steel sheet at the same time as described above.
An object of the present invention is to provide a non-oriented electrical steel sheet having an unprecedented high magnetic flux density and low iron loss at low cost, and to optimize the component composition and manufacturing conditions.

  The present inventors paid attention to a self-annealing technique applied to a coil immediately after hot rolling as a technique for omitting hot-rolled sheet annealing. And in the case of steel having α-γ transformation, the greatest factor that hinders grain growth during self-annealing and finish annealing is found to be AlN precipitated in the cooling process of hot rolling, and a method for controlling the AlN This has led to the present invention.

The present invention thus made is as follows.
(1) By mass%, C: 0.005% or less, Si: 0.1-2.0%, Mn: 0.05-0.6%, P: 0.100% or less, N: 0.0030 %: Al: 0.01-0.05%, B: N in a ratio of B: N = 0.9-1.2, with the balance being Fe and inevitable impurities, with an average diameter of 10-200 nm Non-magnetic precipitate AlN, a number density of 10 pieces / μm ³ or less and a structure composed of ferrite grains not containing an unrecrystallized structure, and the average grain diameter of the ferrite grains is 30 to 200 μm, A non-oriented electrical steel sheet having an average magnetic flux density B50 in the rolling direction and a direction perpendicular to the rolling direction (flux density when excited at a frequency of 50 Hz and a magnetizing force of 5000 A / m) of 1.75 T or more.
(2) Further, at least one of Sn and Sb is contained in an amount of 0.05 to 0.2% by mass, and the average magnetic flux density B50 in the rolling direction and the direction perpendicular to the rolling is 1.77 T or more. The non-oriented electrical steel sheet according to (1).

(3) Non-directionality in which slab having the steel composition described in (1) or (2) is hot-rolled, cold-rolled without being subjected to hot-rolled sheet annealing, and subjected to finish annealing on the cold-rolled steel sheet. In the hot rolling process, the slab heating temperature is set to 1050 ° C to 1250 ° C, the coil winding temperature is set to 780 ° C to Ac1 transformation point, and the annealing temperature in the finish annealing step is set to 800 ° C to Ac1. A method for producing a non-oriented electrical steel sheet, characterized by having a transformation point.

  According to the present invention, a steel sheet with low iron loss and high magnetic flux density can be obtained at low cost. It can contribute to high efficiency of various devices such as motors.

It is a figure which shows the relationship between finish rolling finish temperature FT and magnetic flux density B50. It is a figure which shows the relationship between finish rolling finish temperature FT and iron loss W15 / 50.

First, the experimental results that led to the present invention will be described.
Steel ingots composed of steels X and Y having the composition shown in Table 1 were melted in the laboratory. Both steels were confirmed by a Formaster test that the Ar1 transformation point was 960 ° C, the Ar3 transformation point was 1020 ° C, and the Ac1 transformation point was 1055 ° C.

  These steel ingots were heated at a temperature of 1150 ° C. for 1 hour and subjected to hot rolling. At that time, the finish rolling finish temperature FT was changed in the range of 880 ° C to 1080 ° C. The finished thickness is 2.5 mm. In order to simulate the self-annealing in the actual coil, the hot rolled material immediately after the finish rolling was inserted into a furnace having a furnace temperature of 850 ° C., held for 15 minutes, and then taken out and air-cooled. These hot-rolled materials were pickled, cold-rolled to a thickness of 0.5 mm, and then subjected to finish annealing at 900 ° C. for 30 seconds.

A 55 mm × 55 mm test piece was cut out from the obtained cold-rolled annealed plate, and magnetic measurement was performed in the rolling direction (L direction) and the direction perpendicular to the rolling direction (C direction) by the excitation current method defined in JIS C2556. FIG. 1 shows the relationship between the finish rolling finish temperature FT and the average magnetic flux density B50 in the L / C direction (the magnetic flux density at a frequency of 50 Hz and a magnetizing force of 5000 A / m) for the two steels.
In the steel X with no B added, the magnetic flux density B50 decreases as the FT increases. On the other hand, in the steel Y with B addition, the decrease in B50 is hardly observed. The structure after simulated self-annealing becomes finer with FT in Steel X, but hardly changes in Steel Y.

  For steels X and Y, when the microstructure of the hot-rolled sheet having the highest FT of about 1060 ° C. is observed by SEM, in steel X, AlN fine precipitates are observed at the grain boundaries. No precipitate was observed. FIG. 2 shows the relationship between FT and iron loss W15 / 50 (iron loss at a frequency of 50 Hz and a maximum magnetic flux density of 1.5 T). Even if FT is high temperature, the iron loss is reduced by the steel Y to which B is added.

Changes in precipitates in such B-added steel and B-free steel are considered as follows.
Since the solubility of AlN is smaller in the α phase than in the γ phase, a large amount of AlN precipitates when the parent phase is transformed from γ to α. On the other hand, when the γ grains are processed, the structure of the γ phase before transformation includes an unrecrystallized structure in some cases, and even if recrystallized, the grain size is smaller than the γ grain size before reduction. When the parent phase is transformed, α nuclei are generated with the old γ grain boundary as a precipitation site, and a fine α phase structure is formed. Since AlN easily precipitates simultaneously with the transformation, the grain boundaries of α grains become precipitation sites, and AlN precipitates in a fine and large amount. When B is added thereto, BN is preferentially formed in the γ region, and precipitation of AlN during transformation can be suppressed. This improves grain growth during self-annealing and finish annealing after cold rolling, and improves magnetic flux density and iron loss.

From the above experiments, it was confirmed that by adding B to steel having a component composition having α-γ transformation, excellent magnetic properties can be obtained even when the finish hot rolling finish temperature is high.
The present invention has been made on the basis of such examination results, and the requirements of the non-oriented electrical steel sheet and the manufacturing method thereof defined in the present invention will be sequentially described in detail below.

First, the reasons for limiting the component composition of steel used in the non-oriented electrical steel sheet of the present invention will be described. Below,% of content means the mass%.
<C: 0.005% or less>
C is a harmful element that degrades iron loss and also causes magnetic aging, so 0.005% or less. Preferably it is 0.003% or less.
<Si: 0.1 to 2.0%>
Si is an element that increases the specific resistance of steel and decreases iron loss, and the lower limit is 0.1%. Excessive addition reduces the magnetic flux density. Therefore, the upper limit of Si is 2.0%. Preferably it is 0.1 to 1.6%.

<Mn: 0.05 to 0.6%>
Mn increases the specific resistance of the steel and coarsens the sulfide to render it harmless. However, excessive addition leads to steel embrittlement and cost increase. Therefore, it is made 0.05 to 0.6%. Preferably it is 0.1 to 0.5%.
<P: 0.001 to 0.1%>
P is added to ensure the hardness of the steel sheet after recrystallization. Excessive addition causes embrittlement of the steel. Therefore, it is 0.001 to 0.1%. Preferably it is 0.001 to 0.08%.

<N: 0.0030% or less>
N produces BN, AlN, and the like, and when it is finely precipitated, the grain growth is degraded during self-annealing or finish annealing after cold rolling. In order to prevent this, the content is made 0.0030% or less. Preferably it is 0.0020% or less.
<Al: 0.01 to 0.05%>
Since Al is useful as a deoxidizer, the lower limit is set to 0.01%. If it becomes excessive, AlN is preferentially precipitated over B, so 0.05% or less. Preferably it is 0.02 to 0.04%.

<B: Ratio of N to B / N = 0.9 to 1.2>
If B is contained in such a range that B / N = 0.9 to 1.2 in terms of the ratio with N under the Al content being limited as described above, N is given priority over Al. Fix and produce coarse BN. Therefore, fine precipitation of AlN at the time of transformation from γ to α is suppressed, and crystal grain growth is improved during self-annealing or finish annealing after cold rolling. If B / N is less than 0.9, the production of fine AlN has priority over the production of coarse BN. If B / N is greater than 1.2, B dissolves, degrades the texture after cold rolling recrystallization, and lowers the magnetic flux density B50. Preferably, B / N = 0.9 to 1.1.

<At least one of Sn and Sb: 0.05 to 0.2%>
Sn and Sb are added as necessary to improve the texture after cold rolling recrystallization and increase the magnetic flux density. However, excessive addition causes the steel to become brittle. For this reason, when adding, it is good to set it as 0.05 to 0.2%. Preferably it is 0.05 to 0.15%.

The non-oriented electrical steel sheet of the present invention has the steel composition of the α-γ transformation system as described above, and the balance of the composition is Fe and inevitable impurities.
Next, other features of the non-oriented electrical steel sheet of the present invention will be described.

In the present invention, the number density of nonmagnetic precipitates AlN having an average diameter of 10 to 200 nm in the steel sheet is suppressed to 10 pieces / μm ³ or less.
As a result of the observation as described above, in the component system of the present invention, the average diameter of AlN that has the greatest influence on the grain growth during self-annealing and finish annealing was 10 to 200 nm. Therefore, the number density of AlN of this size is specified. If the number density exceeds 10 / μm ³ , recrystallization and grain growth of the hot-rolled sheet are not sufficient during self-annealing, leading to a decrease in magnetic flux density. Further, it adversely affects recrystallization and grain growth during finish annealing after cold rolling. Preferably, it is 5 / μm ³ or less.

  Moreover, the structure of the steel sheet of the present invention is a structure composed of ferrite grains not containing an unrecrystallized structure, and the average grain diameter of the ferrite grains is 30 to 200 μm. When the average particle size is less than 30 μm, the hysteresis loss increases and the total iron loss increases. Preferably it is 40 micrometers or more, More preferably, it is 60 micrometers or more. If it exceeds 200 μm, the eddy current loss increases and the total iron loss increases. Preferably it is 150 micrometers or less.

  Further, in the steel sheet of the present invention, the average B50 in the rolling direction and the direction perpendicular to the rolling direction is 1.75 T or more, and 1.77 T or more when the steel sheet contains at least one of Sn and Sb. As described above, Sn and Sb have the effect of improving the texture after cold rolling recrystallization and improving the magnetic flux density B50.

Next, the manufacturing method for obtaining the electromagnetic steel sheet of this invention is demonstrated.
The manufacturing method of the present invention is a process in which hot rolling is performed on a slab having the steel composition described above, self-annealing by the heat of hot rolling is performed, cold rolling is performed, and then finish annealing is performed. Become.

The slab heating temperature in the hot rolling process is set to 1250 ° C. or less in order to prevent re-solution-fine precipitation of impurities such as sulfides and not to deteriorate iron loss. However, if the temperature is too low, the hot rolling ability is lowered, so the temperature is set to 1050 ° C. or higher. Preferably it is 1100 degreeC-1200 degreeC.
Subsequent rough rolling and descaling may be performed by an ordinary method, and the conditions are not particularly limited.
The finish rolling finish temperature FT of the hot rolling is not particularly limited because it has a small influence on magnetism as shown in the previous experiment, but it is preferably set to Ar1 transformation point + 20 ° C. or lower. This is because both iron loss and magnetic flux density are improved.

  The coil winding temperature after hot rolling is set to 780 ° C. to Ac1 transformation point. When the coil is water-cooled, the time until the water-cooling starts is 10 minutes or more. As a result, the hot-rolled sheet is self-annealed. As a result, the hot rolled structure becomes coarse and the magnetic flux density is improved. In addition, the precipitates are also coarsened to improve grain growth during finish annealing after cold rolling. Preferably it is 800 degreeC or more, More preferably, it is 850 degreeC or more.

  If the coiling temperature exceeds the Ac1 transformation point, it is transformed again in the cooling process, the structure before cold rolling becomes fine, and the magnetic flux density after cold rolling / recrystallization is lowered, which is not preferable. The coiling temperature can also be ensured by heating the hot-rolled sheet immediately before winding and raising the temperature. The method is not particularly limited, but induction heating or the like can be used. In this case, too, heating to the Ac1 transformation point or higher is not preferable because the magnetic flux density decreases.

  Next, the hot-rolled material is pickled without being subjected to hot-rolled sheet annealing, cold-rolled, and finish-annealed. In the final annealing step, the annealed structure is a ferrite phase that does not contain an unrecrystallized structure, and the average grain size of the ferrite grains is 30 to 200 μm. In order to make the average grain size of the ferrite grains 30 μm or more, the annealing temperature is set to 800 ° C. or more. However, when the transformation point Ac1 is exceeded, the structure becomes finer, and therefore, the transformation point Ac1 or less is set. Preferably it is 850 degreeC-Ac1 transformation point.

  The present invention is characterized by a non-oriented electrical steel sheet having a high magnetic flux density and low iron loss as described above and a production method capable of producing the electrical steel sheet at a low cost by omitting hot-rolled sheet annealing. Hereinafter, the feasibility and effects of the present invention will be further described using examples.

<Example 1>
The molten steel melted in the converter is vacuum degassed, adjusted to the composition shown in Table 2, and continuously cast into a slab. The slab is hot-rolled and hot rolled with a thickness of 2.5 mm. A board was used. At that time, the slab heating temperature was 1200 ° C., the finish rolling end temperature was 1020 ° C., the winding temperature was 850 ° C., and the wound coil was water cooled after being held for 15 minutes after winding. This hot-rolled sheet was pickled and cold-rolled to 0.5 mm and subjected to continuous finish annealing at 850 ° C. for about 30 seconds.
The magnetic properties of the obtained material were evaluated by the Epstein method (JIS C 2556), the particle size was measured (JIS G 0552), and the precipitate was observed. The results are also shown in Table 1. The magnetic characteristics are shown as an average value in the L direction and the C direction.
A non-oriented electrical steel sheet having a composition within the range of the present invention can provide excellent magnetic properties.

<Example 2>
In mass%, C: 0.0011%, Si: 0.5%, Mn: 0.17%, P: 0.073%, N: 0.0019%, Al: 0.03%, B: 0.0. A slab having a component composition of 0018%, Sn: 0.095%, balance Fe and inevitable impurities was melted in a converter. This steel had an Ar1 transformation point of 955 ° C, an Ar3 transformation point of 985 ° C, and an Ac1 transformation point of 1018 ° C.
This slab was heated at a temperature of 1030 ° C. to 1300 ° C. and subjected to hot rolling. The finishing temperature of finish rolling was changed in the range of 880 ° C to 1080 ° C. Thereafter, the coil was wound at a coiling temperature of 750 ° C. to 1020 ° C., held in a coil for 15 minutes, and then cooled with water. In addition, the example whose winding temperature is 650 degreeC is water-injected and wound up immediately after completion | finish of hot rolling for a comparison. Thereafter, the hot-rolled sheet was pickled, cold-rolled to 0.5 mm, and subjected to finish annealing at 750 ° C. to 1050 ° C. for about 30 seconds.
As with Example 1, the obtained material was subjected to magnetic measurement, particle size measurement, and precipitation observation. Production conditions and measurement results are shown in Table 3.
The non-oriented electrical steel sheet manufactured by the manufacturing method within the scope of the present invention has excellent magnetic properties.

Claims (3)

% By mass
C: 0.005% or less,
Si: 0.1 to 2.0%,
Mn: 0.05 to 0.6%,
P: 0.100% or less,
N: 0.0030% or less,
Al: 0.01 to 0.05%,
B: Ratio of N to B / N = 0.9 to 1.2
And the balance consists of Fe and inevitable impurities,
A nonmagnetic precipitate AlN having an average diameter of 10 to 200 nm, a number density of 10 particles / μm ³ or less, and a structure composed of ferrite grains not containing an unrecrystallized structure, and the average grain diameter of the ferrite grains is 30 ~ 200 μm,
A non-oriented electrical steel sheet having an average magnetic flux density B50 in a rolling direction and a direction perpendicular to the rolling of 1.75 T or more.   Furthermore, 0.05% to 0.2% of Sn and Sb is contained by mass%, and the average magnetic flux density B50 in the rolling direction and the direction perpendicular to the rolling is 1.77 T or more. The non-oriented electrical steel sheet according to 1.   A method for producing a non-oriented electrical steel sheet, wherein the slab having the steel composition according to claim 1 or 2 is hot-rolled, cold-rolled without performing hot-rolled sheet annealing, and subjected to finish annealing on the cold-rolled steel sheet. In the hot rolling process, the slab heating temperature is 1050 ° C. to 1250 ° C., the coil winding temperature is 780 ° C. to Ac1 transformation point, and the annealing temperature in the finish annealing step is 800 ° C. to Ac1 transformation point. The manufacturing method of the non-oriented electrical steel sheet characterized by these.

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