JPH08311560A - Method for finish-annealing grain oriented electrical steel sheet - Google Patents

Abstract

PURPOSE: To provide a method for uniformizing magnetism in the longitudinal direction of the coil of a grain oriented electrical steel sheet. CONSTITUTION: In a low temp. side within 50 deg.C from the temp. range desired to the uniformization of a temp. raising velocity, a holding treatment is once executed, and the temp. difference ΔT between the hottest point and the coldest point in the coil is made to a value obtd. from a target temp. raising velocity V, thermal conductivity of the coil λ, thermal capacity C, density ρ, height of the coil H and thickness of the coil W. That is, at the time of becoming ΔT=a×(H/2×0.7+W/2×0.3)<2> and (a)=V×C×ρ/λ, the raising of temp. is restarted in the target velocity to execute an annealing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a grain-oriented electrical steel sheet in which magnetism is uniformly obtained in the longitudinal direction of the coil.

[0002]

2. Description of the Related Art Grain-oriented electrical steel sheets are mainly used as iron core materials for electrical equipment such as transformers and generators, and are required to have excellent magnetic properties. Usually, grain-oriented electrical steel sheet is S
A silicon steel slab containing 2 to 4% of i is hot-rolled, hot-rolled sheet annealed, and then cold-rolled to a desired sheet thickness, followed by decarburization annealing to perform decarburization and primary recrystallization. Inhibitors are created by nitriding, and finally, secondary annealing is performed by finish annealing to create magnetism.

The secondary recrystallization is an operation for selectively growing only the so-called Goss orientation so that the target easy axis of magnetization is parallel to the rolling surface, and the crystal is grown up to a temperature range in which the Goss orientation preferentially grows. This is achieved by suppressing grain growth with an inhibitor and gradually growing the grain by thermal decomposition of the inhibitor in a target temperature range. Therefore, the rate of temperature rise of the coil during secondary recrystallization, which regulates the decomposition rate of the inhibitor and the growth rate of crystal grains, is an important factor that affects the magnetic superiority and inferiority as a result of affecting the selectivity of the crystal orientation. Attempts have been made to further optimize the rate of temperature rise itself to raise the level of magnetism.

For example, Japanese Laid-Open Patent Publication No. 54-40227 discloses that the magnetic flux density can be improved by gradually heating between 900 and 1050 ° C. at which secondary recrystallization proceeds. However, at present, since the finish annealing is performed in a coil shape, the temperature rise rate in the coil at the time of secondary recrystallization is manifested, even though the annealing conditions greatly differ inside and outside the coil such as temperature deviation inside the coil. The annealing cycle has been determined without sufficient consideration for the homogenization. As a result, there is a magnetic gap in the longitudinal direction of the coils, which causes a problem in production such that the coils must be separated according to the magnetic grade before shipping.

[0005]

DISCLOSURE OF THE INVENTION The present invention relates to the rate of temperature rise during secondary recrystallization of a coil, which affects the orientation selectivity during secondary recrystallization, while taking into consideration the size of the coil and other factors in the coil. The aim is to establish a heat cycle setting method that can be the same in terms.

[0006]

Means for Solving the Problems The gist of the present invention is, in a method for manufacturing a grain-oriented electrical steel sheet, when performing finish annealing, temporarily hold at a lower limit temperature in a temperature region where the temperature rising rate at all points in the coil is desired to be uniform. After the treatment, the temperature difference ΔT between the hottest point and the coldest point in the coil is the target heating rate V, the thermal conductivity λ of the coil, the heat capacity C, the density ρ, the coil height H,
The annealing method is characterized in that when the value obtained from the coil wall thickness W is reached, reheating of the furnace is started at a target speed. ΔT is preferably ΔT = a × (H / 2 × 0.7 +
W / 2 × 0.3) ² and a = V × C × ρ / λ.

The present invention will be described in detail below. First, the one-dimensional consideration was given to the distribution state of the temperature inside the coil when the temperature rise inside the coil was made uniform. When the minute portion in the coil is rising at the temperature rising rate V = dT / dθ (T: temperature, θ: time), the following heat balance is established. V × C × ρ × dx = λ × (dT / dx | _{x = 0−} dT / dx | _{x = dx} ) = λ × d ² T / dx ² × dx where C: specific heat of coil, ρ: coil Density, x: distance, λ: thermal conductivity of coil. Therefore, if the heating rate can be made constant at all points in the coil, then d ² T / dx ² = a (a: constant), and the rate of positional change of the temperature gradient in the coil becomes constant. I understand.

As a result, the temperature distribution in the coil when the heating rate is uniform becomes the following parabolic distribution. T = a × X ² + b (b: constant) Therefore, letting the coil representative dimension be L, the outermost circumference of the coil,
The coldest spot temperature in the coil is b with respect to the hottest spot temperature a × L ² + b, and the difference ΔT between the hottest spot and the coldest spot temperature in the coil is ΔT = It can be seen that it becomes a × L ² . Furthermore, a is defined as the following equation from the above heat balance equation. a = V × C × ρ / λ

From the above consideration, the secondary recrystallization temperature range 100
As a method of equalizing the rate of temperature rise in the coil at 0 to 1100 ° C., the temperature difference between the hottest point and the coldest point in the coil can be calculated from the above-mentioned formula group before entering this temperature range. Calculated value Δ
It was considered most efficient to perform the holding process once so that T = a × L ² and then restart the reheating at the target heating rate V. However, in the actual coil, two-dimensional heat transfer occurs, and between the coil plates, Mg with a small thermal conductivity is used.
O is applied, and there is a difference from theoretical studies such as different heat transfer characteristics in the longitudinal and lateral directions of the coil. Therefore, a two-dimensional in-coil heat transfer model that can simulate actual in-coil heat transfer is used. It was created and the method of applying the above idea of equalizing the heating rate to the actual coil was examined.

As a result, it was confirmed that the above idea can be universally applied to coils having different sizes by determining the typical sizes of the coils as follows. That is, L = H / 2 × 0.7 + W / 2 × 0.3 where H: coil height and W: coil wall thickness.

This relational expression shows that the temperature rise inside the coil is
It is shown that the heat supply by the heat conduction in the steel plate in the coil height direction is more dominant than the heat conduction in the coil thickness direction with the heat transfer resistance layer by MgO. Therefore, in the case of the actual coil, the temperature difference ΔT, which is a standard for starting the reheating, is ΔT = a × (H / 2 × 0.7 + W / 2 × 0.3) ² .

Regarding the temperature level to be temporarily retained, even if the temperature level is held at the lower limit of the temperature level for which the uniform heating rate is desired, the effect of equalizing the heating rate at all points in the coil when reheating is started is not achieved. , Also confirmed that it is sufficient. Regarding the retention time, if the heat cycle up to the retention temperature is specified, this can be quantified in relation to the coil size, but at the present time, given conditions such as the temperature level until retention and the rate of temperature rise after that. Depending on the above, the holding time is determined by simulating the behavior of the coldest point and the hottest point temperature from the above heat transfer model.

[0013]

EXAMPLES C: 0.055%, Si: 3.22%, M
n: 0.016%, S: 0.007%, Al: 0.02
A slab consisting of 69%, N: 0.008%, Sn: 0.017%, other iron and unavoidable impurities at 1220 ° C.
Coil heated to 2.3 mm, hot rolled to 2.3 mm, annealed to a hot rolled plate, cold rolled to 0.3 mm, decarburized and nitrided, and coated with MgO (height 1000 mm, wall thickness 400 mm ) 2
I prepared it. In performing these finish annealing, one set was processed by the conventional finish annealing cycle shown in FIG.

In the other set, the heat cycle was determined as shown in FIG. 2 for the purpose of equalizing the temperature rising rate in the coil in the range of 1000 to 1100 ° C. That is, first, the target ΔT of the temperature difference between the hottest point and the coldest point is set to the coil heating rate target V = 10 ° C.
/ Hr, thermal conductivity λ = 20 kcal / m / Hr / ° C, heat capacity C =
Calculation was performed as follows using 0.15 kcal / kg / ° C., density ρ = 7650 kg / m ³ , coil height H = 1 m, and coil wall thickness W = 0.4 m.

A = V × C × ρ / λ = 574 ° C./m ² ΔT = a × (H / 2 × 0.7 + W / 2 × 0.3) ² = 96.5 ° C.

Next, based on the heat cycle up to 1000 ° C., the temperature behavior between the hottest point and the coldest point was simulated by a heat transfer model, and a holding time of about 17 hours for obtaining the temperature difference of about 100 ° C. was obtained. It was Therefore, at 1000 ℃ 17
After the Hr retention, an annealing treatment was performed to raise the temperature at a target temperature rising rate of 10 ° C./Hr. 1000 to 1100 of the hottest and coldest points of each coil when annealed by these two heat cycles
Table 1 shows the actually measured values of the temperature rising rate in the ° C region and the actually measured changes in the magnetic field in the longitudinal direction of the coil after annealing.

[0017]

[Table 1]

In the case of annealing in the current cycle, there is a difference in the temperature rising rate between the hottest point and the coldest point, and as a result, it is considered that the magnetism in the longitudinal direction of the coil fluctuates.
When the heat cycle according to the present invention is used, the heating rates of the hottest point and the coldest point in the coil are almost equal,
It became possible to make the magnetism uniform.

[0019]

According to the present invention, it becomes possible to manufacture a grain-oriented electrical steel sheet having uniform magnetism in the longitudinal direction of the coil.

[Brief description of drawings]

FIG. 1 is a graph showing a current heat cycle of annealing in an example of the present invention.

FIG. 2 is a graph showing a heat cycle of annealing determined according to the present invention in Examples of the present invention.

Claims (2)

[Claims] 1. When performing finish annealing of a grain-oriented electrical steel sheet, a temperature holding process is performed once at a lower limit temperature of a temperature region where the temperature rising rate of all points in the coil is desired to be uniform, and the hottest point and the highest point in the coil are set. When the temperature difference between the cold spots reaches a value that was previously obtained based on the target heating rate, coil thermal conductivity, heat capacity, density, coil height, and coil wall thickness, the furnace is reheated at the target heating rate. A finish annealing method for a grain-oriented electrical steel sheet, which comprises starting temperature. 2. When performing finish annealing of a grain-oriented electrical steel sheet, a temperature holding treatment is performed once at a lower limit temperature of a temperature region where the temperature rising rate of all points in the coil is desired to be uniform, and the hottest point and the coldest point in the coil are cooled. The temperature difference between the points is a value obtained from the target heating rate V, the thermal conductivity λ of the coil, the heat capacity C, the density ρ, the coil height H, and the coil wall thickness W. ΔT = a × (H / 2 × 0.7 + W / 2 × 0.3) ² However, when a = V × C × ρ / λ, the reheating of the furnace should be started at the target heating rate. A method for finish annealing a grain-oriented electrical steel sheet.