JPH05117751A - Method for hot-rolling continuously cast slab for grain-oriented electrical steel sheet - Google Patents

Abstract

PURPOSE:To eliminate an internal defect and to improve the magnetic characteristic by changing the heating temp. of a slab with Mn and S contents in the slab for grain-oriented electrical steel sheet obtd. with a continuous casting, treating and successively applying rougher-rolling and finish-rolling. CONSTITUTION:The slab for grain-oriented electrical steel sheet containing 0.02-0.85% C and 2-4.5% Si is continuously cast. This slab is heated at 1300-1450 deg.C with an electric type heating furnace in non-oxidizing atmosphere after preheating at 900-1250 deg.C with a gas combustion type heating furnace. Then, by changing the heating temp. in the electric-type heating furnace according to the component values of Mn and S contained in the slab, high-temp. solution treatment is executed. Successively, the rougher-rolling and finish-rolling are quickly executed. By this method, the finish product having excellent surface characteristic and magnetic characteristic can stably be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for hot rolling a slab for unidirectional electrical steel produced by continuous casting in a process for producing a unidirectional electrical steel sheet, and in particular, it has good surface defects and magnetic properties. The present invention relates to a hot rolling method for obtaining a grain-oriented electrical steel sheet having excellent characteristics.

[0002]

2. Description of the Related Art As is well known, grain-oriented electrical steel sheets are widely used as iron core materials for transformers and the like due to their excellent magnetic characteristics such as high magnetic flux density and low iron loss. As a characteristic of the manufacturing method, in order to obtain secondary recrystallized grains in which the so-called Goss orientation of [110] <001> is highly integrated, Mn
Inhibitor elements such as S and AlN that control the grain orientation are used. In order for this inhibitor element to appropriately bring about the intended action, after sufficiently dissociating and forming a solid solution (hereinafter referred to as solution heat treatment) during slab heating prior to hot rolling, hot rolling is performed under appropriate conditions. It is very important to precipitate finely and uniformly dispersed by cooling,
For solution treatment of such an inhibitor element, for example, 1300
The slab is heated to a high temperature above ℃.

When a gas combustion type heating furnace is used as a method for performing high temperature heating of the slab, a large amount of molten scale is generated in the heating furnace and not only the yield is deteriorated, but also
Not only does this impair the stable operability of the heating furnace, but it also causes defects in the surface of the product and so-called edge cracking defects at the ends in the width direction.
In addition, since it takes an extremely long time to reach the target temperature in this type of heating furnace, coarsening of the slab structure occurs in the meantime, and a huge stretched grain is expressed in the subsequent rolling, As a result, it also causes instability in the magnetic properties of the product.

With respect to the above problems, Japanese Patent Laid-Open No. 62-10
In Japanese Patent No. 3322 and Japanese Patent Laid-Open No. 62-10214, a slab is preheated to about 1230 ° C. in a gas combustion type heating furnace, and subsequent high temperature heating is performed in an induction heating furnace controlled to an inert atmosphere for a short time. It is suggested to do so. In particular, the former adjusts the frequency in the induction heating so that the central part temperature (this temperature is described to be determined by the amounts of MnS, Se, and Al) and the surface part temperature are made uniform. In the latter, an induction heating method is disclosed in which the temperature difference between the slab surface and the central portion is kept within a certain range. However, according to these methods, induction heating is performed at 1250 ° C.
Alternatively, it may be heated to any temperature in the range of 1300 ° C. or higher to around 1400 ° C., and the (MnS, Se, Al
There is no specific specified temperature in the description). When using such a heating method, the slab may be overheated, so internal slab cracks in the slab, deterioration of magnetic properties due to coarsening of crystal grains, increase in energy consumption per unit cost, or insufficient heating. There is a problem that the magnetic properties are deteriorated by the above.

[0005]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems of the prior art in the process of manufacturing a grain-oriented electrical steel sheet, and is an atmosphere-controlled internal heating type electric heating. The slab is heated at an appropriate temperature that is determined from the component values of the inhibitor element by this method, and there are no defects such as internal melting of the slab, and continuous casting for unidirectional electrical steel sheets with excellent magnetic properties and productivity. It is an object to provide a hot rolling method for slabs.

[0006]

In order to achieve the above object, the present invention has the following constitution. That is, C: 0.
A slab for unidirectional electrical steel sheets produced by continuous casting containing 02 to 0.085% and Si: 2 to 4.5% is preheated to 900 to 1250 ° C. in a gas combustion type heating furnace, and then nonoxidized. In the method of inserting into an electric heating furnace in a characteristic gas atmosphere and heating to a temperature range of 1300 to 1450 ° C., the slab heating temperature in the electric heating furnace is determined by the component values of Mn and S contained in the slab. It is a hot rolling method for a continuously cast slab for unidirectional electrical steel sheets, which comprises varying and treating, and then rough rolling and finish rolling.

In the present invention described above, it is preferable to perform wide reduction rolling after preheating the slab, or to perform preliminary rolling at a reduction rate of less than 40% after wide reduction rolling.

The contents of the present invention will be described in detail below. At the center of the continuously cast slab for unidirectional electrical steel,
A dense segregation zone of impurities such as S and P occurs. This is an unavoidable phenomenon that occurs because the heat transfer coefficient of the slab for grain-oriented electrical steel sheets is small.

In order to improve the magnetic properties of the grain-oriented electrical steel sheet, a high temperature solution treatment is performed at the slab stage. The slab having such a thick segregation zone is heated at a high temperature of 1300 to 1450 ° C. In this case, since the S-enriched portion has a low melting point, grain boundary melting occurs, which becomes an internal hole in the subsequent processing stage, which causes blistering defects in the steel sheet.

In particular, when the slab width is integrated to improve the productivity and the large width reduction rolling is performed, this internal opening is promoted. This is because the force of pushing and spreading acts more on the center segregation due to the large horizontal reduction. Therefore, if one pass is subjected to a light reduction and the number of passes is reduced, the stress can be dispersed and blistering defects can be suppressed. Therefore, the reduction amount of one pass is preferably 50 mm or less.

It is preferable that the slab that has been subjected to the large-width reduction rolling be subjected to rapid pre-rolling. That is, if hot deformation is applied in the plate thickness direction by this rolling, blistering defects do not occur even if heated at a high temperature thereafter. It can be presumed that this is because the addition of strain promoted the recrystallization of the central segregation zone and increased the grain boundary strength. A reduction ratio of less than 40% is sufficient for that purpose.
Also, the slab rolled by wide reduction has a so-called dogbone shape, but the dogbone of this pre-rolling is eliminated and the shape is made to have good heating efficiency, so that it can be easily charged into the induction heating furnace in the subsequent process. It also plays a role of stabilizing.

In the present invention, the continuous cast slab is subjected to 900-1250 in a gas combustion type heating furnace before performing wide reduction rolling.
Preheating is performed at a relatively low temperature of ℃.

The preheated slab is immediately taken out to the rolling line and, if necessary, after the above-described wide reduction rolling and pre-rolling, an electric heating furnace such as induction heating in a non-oxidizing atmosphere such as N ₂ or Ar gas. Then, the solution is subjected to high temperature solution treatment.

This high-temperature solution treatment in the present invention aims to form a solid solution of MnS, AlN, etc. contained in the slab,
In order to impart excellent magnetic properties to the final product that has undergone cold rolling, finish annealing, etc., it is heated to a temperature range of 1300 to 1450 ° C. In this temperature range, it is contained in the slab as described below. A feature is that the heating temperature is changed according to the component values of Mn and S as the inhibitor forming element.

The reason for changing the slab heating temperature in the electric heating furnace depending on the component values of Mn and S is as follows.
First of all, heating at an unnecessarily high temperature increases the occurrence rate of blistering surface defects due to internal melting in the low melting point region of the concentrated segregation zone such as S and P. Ear cracking is aggravated, which leads to deterioration of intermediate quality and product quality such as a decrease in yield and deterioration of sheet-passability in subsequent steps.

Secondly, heating at an unnecessarily high temperature causes an increase in energy consumption rate, that is, an increase in cost.

Further, Mn which is an inhibitor forming element,
The reason why the heating temperature is changed depending on the component values of Mn and S among S, Al and N is as follows.

That is, the solution temperature of MnS and AlN is calculated from the thermal equilibrium equation in steel as shown in FIGS. As is clear from the figure, the calculated solution temperature of MnS is 1305 to 1335 ° C in the composition range of the inhibitor forming element of the grain-oriented electrical steel sheet. On the other hand, Al
The calculated solution temperature of N is 1165 to 1295 ° C, and M
The temperature is lower than that of nS. Therefore, Mn, S, A
This is because the solution formation of the inhibitor forming element, which is the most important item in the slab heating of the grain-oriented electrical steel sheet containing l and N as the inhibitor components, is rate-controlled by MnS.

FIG. 3 shows the characteristics of the product manufactured by the following method, which is obtained by the relationship between the [Mn] × [S] value contained in the slab and the soaking temperature in the electric heating furnace (induction heating furnace). It is shown. That is, in% by weight, C: 0.0
8%, Si: 3.25%, Al: 0.027%, N:
0.0090%, Sn: 0.10%, Cu: 0.05%
And Mn: 0.065 to 0.090% and S:
0.020 to 0.030% is combined and Mn x S is set to 0.
The range is 0013 to 0.0027 with the balance being substantially Fe.
Each sample of the continuous casting slab consisting of the above is heated to an average temperature of 1200 ° C. in a gas heating furnace, and then heated to 15 in a vertical rough rolling mill.
The width of the slab is reduced to 1300 to 1 using an induction heating furnace controlled to a N ₂ atmosphere after performing 0 mm width reduction rolling, further straightening the dogbone and reducing it by 20% using a rough rolling machine.
After soaking for 20 minutes at a multi-level temperature of 350 ° C, the slab heated in this way is extracted from the heating furnace and immediately rough-rolled and finish-rolled and wound as a 2.3 mm thick hot coil. It was These hot coils with different hot rolling conditions were pickled, pre-cold rolled to a thickness of 1.5 mm, hot coil annealed, and finally cold rolled to 0.22 mm.
A thick steel strip was used. This cold-rolled steel strip was further subjected to decarburization annealing at 835 ° C., an annealing separator was applied, then finish annealing was performed at 1200 ° C., and tension coating was applied to produce a high magnetic flux density grain-oriented electrical steel sheet. is there. The magnetic properties (iron loss, W17 / 50), surface defects (blisters), and ear pit depth of the hot coil were measured for each product obtained in this way, and these measurement results are shown in the legend attached to the figure. It is evaluated and divided for each mark, as shown in FIG.

As is clear from the figure, when the soaking temperature is low, the magnetic properties are deteriorated, and conversely, when the soaking temperature is high, surface defects or deterioration of the ear depth are observed. Then, there are good regions in which both the magnetic properties and the surface defects exceed the target values. That is, lines A and B with double circles
If the value is low, it changes to the low temperature side, and if it is high, it changes to the high temperature side.

As described above, the present invention has found out an appropriate solution soaking temperature according to the values of [Mn] and [S] in order to sufficiently dissolve MnS in the slab. Since heating is performed in a non-oxidizing atmosphere for a short time, no slag is generated, and further, deterioration of the characteristics and surface state due to the above-mentioned excess and deficiency heating can be eliminated, and productivity in heating operation can be improved. it can.

In the present invention, as a heating means in the solution heat treatment, for example, a short-time heating furnace (electric heating furnace) in which a slab is vertically inserted in an induction coil and the atmosphere can be made non-oxidizing is provided. It is preferable to install on-line or in the vicinity of the on-line, and also in the vicinity of a rough rolling machine having a preliminary rolling function.

The slab heated to a high temperature is immediately bitten into a rough rolling mill and thereafter rough-finish-rolled according to a conventional method and wound into a coil to obtain a hot-rolled steel sheet for unidirectional magnetic steel sheet. Then, this hot rolled steel sheet is treated by a usual method for producing a unidirectional magnetic steel sheet such as cold rolling or annealing, and a final product excellent in surface properties and magnetic properties can be stably obtained.

[0024]

Example: C: 0.08% by weight, Si: 3.20
%, Al: 0.026%, N: 0.0080%, Sn:
0.12%, Cu: 0.05%, Mn: 0.08%,
Each of the samples of the continuously cast slab containing S: 0.025% and the balance substantially consisting of Fe was subjected to an average temperature of 12 in a gas heating furnace.
After heating to 00 ° C., width reduction rolling of 150 mm is performed by a vertical rough rolling machine, and an induction heating furnace controlled to a non-oxidizing atmosphere is used to cool the inside slab 1300, 1310, 13
Soaking and holding were carried out for 20 minutes at 6 levels of temperature of 20, 1330, 1340, and 1350 ° C., and the slab heated in this way was extracted from the induction heating furnace and immediately rough-rolled and finish-rolled to a 2.3 mm thickness. I got a hot coil.

These hot coils under different rolling conditions were pickled, pre-cold rolled to a thickness of 1.55 mm, hot coil annealed, and finally cold rolled to 0.22 mm.
A thick steel strip was used. This cold-rolled steel strip was further subjected to deoxidation annealing at 835 ° C., an annealing separator was applied, finish annealing was performed at 1200 ° C., and tension coating was applied to obtain a high magnetic flux density grain-oriented electrical steel sheet.

Table 1 shows the magnetic properties, surface defects and hot coil edge crack depth of these final products. Those heated in the temperature range of the present invention are good in all of the magnetic properties, product surface defects, and hot coil edge cracking depth.

[0027]

[Table 1]

[0028]

As described above, according to the method of the present invention, slab heating in an electric heating furnace whose atmosphere can be controlled is obtained at an appropriate temperature according to the component values of Mn and S which are inhibitor forming elements. By doing so, it is possible to obtain a grain-oriented electrical steel sheet having excellent surface properties, magnetic properties and productivity.

[Brief description of drawings]

FIG. 1 is a diagram showing characteristics obtained by a relationship between a [Mn] × [S] value contained in a slab and a soaking temperature in an electric heating furnace (induction heating furnace).

FIG. 2 is a diagram showing the relationship between [Mn] × [S] and calculated solution treatment.

FIG. 3 is a diagram showing a relationship between [Al] × [N] and a calculated solution temperature.

Claims (1)

[Claims] 1. C: 0.02-0.085%, Si: 2
The slab for unidirectional electrical steel sheet produced by continuous casting containing ˜4.5% is heated to 900 to 1250 in a gas combustion type heating furnace.
Mn contained in the slab in a method of preheating to ℃, then inserting into an electric heating furnace in a non-oxidizing gas atmosphere and heating to a temperature range of 1300 to 1450 ℃,
A hot rolling method for a continuously cast slab for a grain-oriented electrical steel sheet, which comprises performing treatment by changing the slab heating temperature in the electric heating furnace according to the component value of S, and then performing rough rolling and finish rolling.