JPH0331421A - Method for heating grain oriented silicon steel slab - Google Patents

Abstract

PURPOSE:To produce the grain oriented silicon steel sheet having excellent magnetic characteristics in a rolling direction by subjecting a silicon steel slab to a heat treatment under specific conditions, then to a hot rolling at the time of producing the grain oriented silicon steel sheet by subjecting the slab to a hot rolling and cold rolling then to a decarburization annealing and final finish annealing. CONSTITUTION:The slab of an ordinary silicon steel contg. 2.0 to 4.5wt.% Si is heated to the extent of not exceeding 1300 deg.C in a gas combustion type heating furnace. The slab is once taken out of the heating furnace and the scale formed on the surfaces of the slab by the heating is removed. The slab is then charged into the induction heating furnace and is heated to a 1350 to 1490 deg.C range in an inert gaseous atmosphere of <=1% O2 concn. and is then hot rolled. This hot rolled sheet is subjected to one pass or >=2 passes of the cold rolling includ ing the intermediate annealing and is thereby worked to the steel sheet of the final sheet thickness. The steel sheet is in succession subjected to the decarburization annealing in a gaseous H2 atmosphere and finally an annealing and separating agent essentially consisting of MgO is applied thereon and the steel sheet is subjected to the final finish annealing.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to the production of grain-oriented silicon steel sheets having excellent magnetic properties in the rolling direction, and relates to the production of silicon steel slabs as the steel sheet material. High temperature heating will be described below.

As is well known, unidirectional silicon steel plate has (110) on the plate surface.
It is composed of secondary recrystallized grains with <001> axes aligned in the plane and rolling direction, and it is important to selectively grow the crystal grains with the above orientation during final annealing.
, MnSe, AIN, and other so-called inhibitors are uniformly dispersed before final annealing (110) (00
1) It is necessary to suppress the growth of other crystal grains so that oriented secondary recrystallized grains can selectively grow.

It is already well known that such control of the dispersion form can be obtained by dissolving these precipitates into solid solution during slab heating prior to hot rolling, and then hot rolling with an appropriate cooling pattern. By the way.

Slab heating for this purpose is usually carried out at a high temperature of 1300° C. or higher, but the surface temperature is usually heated to 1350° C. or higher in order to sufficiently heat the core.

(Prior art) Regarding high-temperature heating of slabs, Japanese Patent Publication No. 47-14627
As described in JP-A No. 60-145318, after the first slab heating to a predetermined temperature (1250 to 1300°C), the slab is taken out of the heating furnace and immediately heated. A method has been proposed in which the material is placed in a separate heating furnace and heated to 1350° C. or higher.

(Problem to be solved by the invention) However, scale (F
If the second slab heating is performed without completely removing oxides such as eO and Sin, even if heating is performed in a non-oxidizing atmosphere, the oxidation of the remaining scale will be further accelerated, causing local damage. A layer close to pure iron without St is formed.

During the subsequent annealing process during the cold rolling process, a subscale layer several μm thick is formed on the surface of the steel sheet. nFeO3iot) is formed, whereas
An oxide layer is formed that is thinner than the surrounding area near FeO alone.

Such differences in subscale composition and thickness in the surface layer remain as a bald pattern at the product stage, resulting in defects in appearance.

To prevent partial overoxidation during the second slab heating,
The oxygen concentration during the second slab heating should be kept as low as possible (e.g. 0
．． 5% or less) or avoiding long-term heating, but these have not been implemented because they impair operability and equipment maintenance.

Therefore, an object of the present invention is to propose an advantageous heating method for a silicon steel slab that can prevent partial overoxidation during the second slab heating.

(Means for Solving the Problems) The inventors believe that in order to prevent the occurrence of bald pattern defects on the surface of steel sheets, the use of oxidation sources such as FeO that oxidizes the St in the base steel in the second slab heating stage is necessary. This invention was completed based on the knowledge that it is essential that there is no such thing.

That is, this invention hot-rolls a silicon-containing steel slab after heating, then cold-rolls it once or twice or more with intermediate annealing to achieve the final thickness, and then decarburizes it and then final finishes it. When producing a grain-oriented silicon steel plate by annealing, the silicon-containing steel slab is heated to a temperature not exceeding 1300"C, and then the scale generated on the surface of the steel plate is removed, and then the temperature range is 1350 to 1490C. In this method of heating grain-oriented silicon steel slabs, it is particularly advantageous to perform the second heating of the slab in an inert atmosphere with an oxygen concentration of 1% or less.

Materials to which this invention can be applied include Si: 2.0 to 4.
There is a slab for grain-oriented silicon steel sheet containing 5 nt% (hereinafter simply referred to as %), which is obtained by continuous casting or by blooming a steel ingot, and contains S and Se as inhibitors.

A total of 0 of 1 or 2 or more types selected from ^l
．． 01-0.06%, Mn: 0.02-0.10%
It is preferable that it be included.

In other words, St is added to increase the resistivity of the steel sheet and reduce iron loss, and if it is added at 2%, the crystal orientation will be damaged in the final high-temperature annealing due to α-T transformation, while the upper limit of 4.5% will affect the cold rollability. It can be determined from points.

In addition, the lower limit of the total amount of S, Ss, and AI was set at 0.005% because it was the minimum amount for these MnS and MnSe + AIN to be finely dispersed in the steel and function as an inhibitor. The reason why it is set at 0.060% is disadvantageous because if it increases beyond this, high temperature heating exceeding the scope of this invention will be required to dissolve them into solid solution during slab heating and use them as an inhibitor, which will further promote oxidation. be.

Regarding the amount of Mn, it was also set at 0.00 to ensure the amount of inhibitor.
The lower limit is 0.2%, and the upper limit of 0.1% is required in that the slab heating temperature for dissociation and solid solution of MnS and MnSe is not high.

In addition, other inhibitors include Sb, Sn, and A.
s.

It is known that grain boundary segregation type elements such as Pb + Bi + Cu and Mo, as well as other nitride forming elements such as BN and VN, can be included at the same time. This does not impair the quality improvement effect of the addition, and the addition of these is also included within the scope of the present invention.

(Function) A slab having the above components is first heated in a conventional gas-fired slab heating furnace, but at this time the slab temperature is 13.
After heating to a temperature not exceeding 00°C, the steel plate is extracted from a furnace to remove scale from the surface of the steel plate, and then charged into a slab induction heating furnace until the slab temperature reaches 1350°C or higher and 1490°C.
Heat to the following range. The slab temperature referred to here refers to the temperature at the lowest temperature part in the thickness direction of the slab (calculated by heat transfer calculation based on the measured value of the surface temperature).

Generally, the temperature at the center of the slab is difficult to rise, and it is important to regulate the temperature of the coldest point within the slab within the above range for stabilizing the magnetic properties of the final product. Whether or not the slab center temperature was within each of the above temperature ranges was confirmed by measuring the temperature by inserting a thermocouple into the center of the thickness of the slab.

The reason why the slab temperature in the gas-fired slab heating furnace for preheating was set to be 1300° C. or lower is because if the temperature was higher than this, the generation of slag in the gas-fired furnace would rapidly begin. Note that the lower limit is that it becomes difficult to obtain uniform heating during the second heating.11
The temperature is preferably 00°C.

After this first slab heating, scale generated on the surface of the steel plate is mechanically removed (for example, by brushing, etc.) after the surface is rapidly cooled using, for example, high-pressure spray water.

Then, the slab was charged into an induction heating furnace and the temperature was raised to 1350℃~1.
Heat to a range of 490°C. The reason why the second slab heating was limited to this range is as follows.

As mentioned at the beginning, the reason why slab heating is required is the solid solution of the inhibitor, MnS contained in the steel,
Depending on the amount of MnSe and AIN, the lower limit is determined as the dissociation solid solution condition for making the required amount function as an inhibitor. On the other hand, if the slab heating temperature becomes too high, the slab crystal grains will coarsen, resulting in the generation of band-like fine grains in the product, which will deteriorate the magnetism and further increase the energy cost, so the upper limit is regulated.

Figure 1 shows the appropriate temperature at the center of the slab in an induction heating furnace, with Se: 0.022% as an inhibitor,
A 3.45% silicon steel slab containing 0.08% Mn and 0.025% Sb was heated by combined heating using a gas-fired slab heating furnace and a slab induction heating furnace according to the method of the present invention. After that, the magnetic properties of the final product when it was made into a hot coil of 2.5 mm thickness and finished into a product with a thickness of 0.30 mm using the known cold rolling process were determined by the magnetic properties of the final product when heated in a slab induction heating furnace and during soaking. It is shown for the center temperature of the slab, and each slab has a temperature of 1 at each soaking temperature.
0 to 5 m1n was maintained.

When the heating temperature was in the range of 150°C to 1350°C, the magnetic flux density B1° value of the product changed to a stable value of 1.897.

Figure 2 shows how the oxidation loss changes depending on the oxygen content of the atmosphere during induction heating and the surface temperature during slab heating. After heating the slab in a furnace until the center temperature reaches 1150℃, it is heated in a slab induction heating furnace for 20 to 40 m1n, and after the surface temperature reaches the temperature indicated in the figure, it is held for 10 m1n, and then extracted to reduce the oxidation loss. This is what we investigated.

It is clear that by suppressing the amount of 02 in the heating atmosphere to 1.0% or less when the slab surface temperature is 1350° C. or higher, the oxidation loss can be significantly reduced.

Furthermore, by performing the second slab heating in an inert atmosphere with an oxygen concentration of 1% or less, it is possible to extremely reduce the occurrence of bald spots, as shown in FIG. Note that it is advantageous to adjust the atmosphere by, for example, reducing the opening of the second heating furnace as much as possible, and in addition, sealing in an inert gas such as N2 gas to maintain the furnace internal pressure higher than atmospheric pressure.

The steps after hot rolling for the slab heated under the conditions of this invention are not particularly different from usual, and depending on the amount and type of inhibitor, cold rolling including intermediate annealing and decarburization annealing and decarburization annealing are performed once or twice. This is followed by high-temperature box annealing.
A unidirectional silicon steel plate with a thickness of ~0.50 mm can be produced.

(Example) C: 0.03%, Si melted by continuous casting on a working plate
: 3.05%, Mn: 0.071%, Am;
0.001%, P: 0.004%, S: 0.0
When hot-rolling a silicon-containing steel slab with a thickness of 215 mm and a weight of approximately 8 mm containing 18% silicon, a thermocouple was inserted into the center of the thickness of the slab to measure the temperature. After heating the slab in a beam-type slab heating furnace for 3 hours until the central temperature reaches 1200°C, the slab is temporarily extracted outside the furnace, and after spraying high-pressure water on the front and back surfaces of the slab, it is heated with pinch rolls to After reducing the pressure for 1 ms, the surface was descaled by grinding with a wire brush, and then heated in a slab induction heating furnace to a temperature at the center of the slab of 1450°C.
Raise the temperature by about 30m1n until it reaches ℃, and at this temperature 15a+
After holding the steel sheet in the same position, it was hot rolled and finished into a 2.5 m hot rolled steel strip.

Next, the surface oxidation layer of the hot-rolled steel strip was removed by pickling, and then cold-rolled to an intermediate thickness of 0.65 mm, and rolled at 950°C for 3 seconds.
After performing intermediate annealing in a hydrogen/nitrogen mixed gas, the product was finished by cold rolling to a product thickness of 0.30 ma+.

After that, decarburization annealing was performed in wet hydrogen at 850°C + 3a+in, MgO was applied as an annealing separation material, and final annealing was performed in hydrogen at 1200°C for 10 hours.

The magnetic properties of the final product thus obtained were measured at 5 locations in the longitudinal direction of the coil, and found to be WB7s. :l, 20±
0.01 W/kg, aton 1.88±0.0
As a grain-oriented silicon steel similar to 05T, it was excellent with little variation in magnetism.

Furthermore, the results of surface observation of the product are shown in FIG. 4 in comparison with the case where no descaling was performed between the first and second slab heating. From the same figure, it can be seen that the product obtained according to the present invention has extremely less occurrence of bald spots compared to the product without descaling.

When the same material as in Example 1 was treated in the same manner, the slab induction heating furnace was sealed with Nt gas and the oxygen concentration in the atmosphere was set to 0.1 to 0.3%. A product with no texture was obtained. Note that the magnetic properties were the same as in Example 1.

(Effects of the Invention) According to the present invention, high-quality grain-oriented silicon steel can be obtained while avoiding surface defects in products resulting from high-temperature heating of silicon steel slabs.

[Brief explanation of drawings]

Figure 1 shows the slab center temperature (°C) in the soaking furnace when the slab was heated in the induction heating furnace and the magnetic flux density B of the final product. value(
Figure 2 shows the relationship between the slab surface temperature (°C) and the 02 content (%) in the furnace atmosphere during heating when the slab is heated in an induction heating furnace. Figure 3 is a graph showing the effect on weight loss (%); Figure 3 is a graph showing the relationship between the rate of occurrence of bald spots and oxygen concentration in the furnace; Figure 4 is a graph showing the rate of occurrence of bald spots. Figure 1 Slab ζ ptji: J1 degree (゛go) Figure 2 Figure 3 2nd power O @ time, inside the furnace [%02] Procedural amendment August 1, 1989

Claims (1)

[Claims] 1. A silicon-containing steel slab is heated and then hot rolled, and then cold rolled once or twice or more with intermediate annealing to reach the final thickness, and then decarburized and annealed. In producing a unidirectional silicon steel plate through the subsequent final finish annealing, the silicon-containing steel slab is heated to a temperature not exceeding 1300°C, scale formed on the surface of the steel plate is removed, and then 13
A method for heating a grain-oriented silicon steel slab, the method comprising heating a grain-oriented silicon steel slab to a temperature range of 50 to 1490°C. 2. Hot-roll the silicon-containing steel slab after heating, then cold-roll once or twice or more with intermediate annealing to reach the final thickness, and then decarburize it and then final finish annealing. In producing grain-oriented silicon steel sheets, the silicon-containing steel slabs are heated to a temperature not exceeding 1300°C, scale formed on the surface of the steel sheets is removed, and then the slabs are heated in an inert atmosphere with an oxygen concentration of 1% or less. 1350-1490
A method for heating grain-oriented silicon steel slabs characterized by heating to a temperature range of °C.