JPH10244312A - Rough rolling method of hot rolling steel sheet with excellent surface property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rough rolling method of a hot rolling steel sheet with the excellent surface property to remove a wedge type scale which is generated inside a heating furnace and whose partial is bitten in the base steel. SOLUTION: This is a rough rolling method of a hot rolling steel sheet where a slab extracted from a heating furnace 10 is sprayed with a high pressure water, executed with descaling, and rolled roughly, before rolling roughly, more than two times descaling are executed, and further, among descaling, more than one descaling is executed at the 950 deg.C or more surface temperature of the slag, and it is executed after the slab executed with the preceding descaling is oxidized in atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a surface texture capable of producing a product steel sheet having few scale flaws by removing a wedge-shaped scale that is difficult to remove by descaling by commonly used high-pressure water spray. The present invention relates to a method for rough rolling of a hot-rolled steel sheet having excellent heat resistance.

[0002]

2. Description of the Related Art Conventionally, in a rough rolling method of a hot-rolled steel sheet,
As a general method of removing scale (mainly FeO) formed on a slab or the like in a hot-rolling heating furnace in order to prevent the occurrence of scale flaws on the product steel sheet, a slab or the like extracted from the heating furnace is subjected to width reduction. A method in which high-pressure water is sprayed immediately before the coarse No. 1 rolling mill (R1), before and after the coarse No. 2 reverse rolling mill (R2), and immediately before the coarse No. 3 rolling mill (R3) after the width reduction by the device. It has been known. Also, Japanese Patent Laid-Open No. 58-16
As described in Japanese Patent No. 3250, a method of completely removing scale by using a brush roll during rolling has been proposed.

[0003]

However, the conventional method for rough rolling of a hot-rolled steel sheet has the following problems to be solved. That is, in the above rough rolling method, S
As shown in FIG. 8, the heating furnace generated scale generated when the steel material containing i and S is heated in the heating furnace is a concentration of Si and S as shown in FIG. Since the wedge-shaped scale remains without being removed by the above high-pressure water spray, it appears as a scale flaw on the surface of the steel sheet during cold rolling after roughing and finishing rolling. become. Also, Japanese Patent Laid-Open No. 58-16
In the method described in Japanese Patent No. 3250, there is a possibility that the ground iron may be ground together with the wedge-shaped scale by the brush roll, but there is a problem that the life of the brush roll is shortened.

[0004] The present invention has been made in view of such circumstances, and is intended to provide a hot rolled steel sheet having an excellent surface property capable of shortening or removing a wedge-shaped scale biting into a part of ground iron generated in a heating furnace. An object of the present invention is to provide a rough rolling method.

[0005]

According to the present invention, there is provided a semiconductor device comprising:
The rough rolling method for a hot-rolled steel sheet having excellent surface properties described above is a rough rolling method for a hot-rolled steel sheet in which high pressure water is sprayed onto a slab extracted from a heating furnace to perform descaling, and rough rolling is performed. Before the rough rolling,
Performing the descaling at least one time, and performing at least one of the descaling, the slab having a surface temperature of the slab of 950 ° C. or more and performing the descaling prior to the descaling. Performed after oxidizing. Here, the reason why the surface temperature of the slab to be descaled is set to 950 ° C. or higher is that the surface temperature is 95 ° C.
If the temperature is lower than 0 ° C., sufficient scale-off is not promoted by the secondary oxidation after the first descaling, and therefore, the effect of shortening the wedge-shaped scale that has penetrated the ground iron is small.

A rough rolling method for a hot-rolled steel sheet having excellent surface properties according to a second aspect of the present invention is the method for rough rolling a hot-rolled steel sheet having excellent surface properties according to the first aspect, wherein the atmospheric oxidation before the descaling is performed. Is set to 5 seconds or more. Here, the reason why the atmospheric oxidation before the descaling is set to 5 seconds or more is as follows.
This is because if the atmospheric oxidation is shorter than 5 seconds, the scale-off is not sufficiently promoted by the secondary oxidation, so that the effect of shortening the wedge-shaped scale is small. The rough rolling method of a hot-rolled steel sheet having excellent surface properties according to the third aspect of the present invention is described in the first aspect.
Or the rough rolling method of a hot-rolled steel sheet having excellent surface properties according to 2, wherein the interface temperature between the ground iron and the scale of the slab extracted from the heating furnace is maintained at or above the melting point of Fe ₂ SiO ₄ , Perform first descaling. Here, the reason why the first descaling is performed while maintaining the interface temperature between the ground iron of the slab extracted from the heating furnace and the scale equal to or higher than the melting point of Fe ₂ SiO ₄ is that the interface temperature is Fe
_{2 When} the melting point is lower than the melting point of SiO ₄ , the wedge-shaped scale is in a solidified state, and the removal performance of the scale formed in the heating furnace is reduced (the scale partially remains on the wedge-shaped scale cut into the ground iron). This is because the scale-off is not promoted by the secondary oxidation, and the effect of shortening the wedge-shaped scale is small.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the accompanying drawings to provide an understanding of the present invention. Here, FIG. 1 is a schematic configuration diagram of a rough rolling facility for hot-rolled steel sheets to which the method for rough-rolling hot-rolled steel sheets having excellent surface properties according to the embodiment of the present invention is applied.

As shown in FIG. 1, a rough-rolling equipment A for a hot-rolled steel sheet to which a rough-rolling method for a hot-rolled steel sheet having excellent surface properties according to an embodiment of the present invention is applied is provided near a heating furnace 10 on the extraction side. Two heating furnace outlet descaling devices 11 and 12
And a first roughing mill (R1) 13 installed downstream of the heating furnace outlet descaling devices 11 and 12, and a descaling device 14 provided immediately before the first rough rolling mill (R1) 13.
A coarse No. 2 reverse rolling mill (R2) 15 installed downstream of the coarse No. 1 rolling mill 13;
Descaling devices 16 and 17 provided before and after
It comprises a coarse No. 3 rolling mill (R3) 18 installed downstream of the No. 3 reverse rolling mill 15 and a descaling device 19 provided immediately before the coarse No. 3 rolling mill 18.

Each of the above descaling devices 11, 12,
14, 16, 17 and 19 consist of conventional high pressure water spray devices. As an example, twelve nozzles are arranged in the width direction, and the ejection pressure of high-pressure water is 140 kgf / cm ² ,
The ejection flow rate is 100 liters / min. / Book. Heating furnace 10
From, for example, thickness 200mm ~ 300mm, width 900
mm to 1200 mm, length 5000 mm to 10000 m
m is extracted at 1100 ° C. to 1250 ° C., and the thickness of the scale generated in the heating furnace 10 is 1.0 mm to 2.0 mm.
mm. In the coarse No. 1 rolling mill (R1) 13, the coarse No. 2 reverse rolling mill (R2) 15, and the coarse No. 3 rolling mill (R3) 18, the slab is reduced by a predetermined thickness by a reduction device.
In the coarse No. 2 reverse rolling mill (R2) 15, the slab is passed a plurality of times (five times in this embodiment) to reduce the slab by a predetermined thickness.

Next, a rough rolling method for a hot-rolled steel sheet having excellent surface properties according to an embodiment of the present invention will be described. 2 and 3 show the first embodiment (level A),
By comparing the second embodiment (level B) and the conventional example (level C, level D), the use (o), non-use (x), and heating of each descaling device shown in FIG. An example of the relationship between the elapsed time from furnace extraction and the slab surface temperature is shown. The slab extracted from the heating furnace 10 at a surface temperature of 1200 ° C. in the first embodiment, 1230 ° C. in the second embodiment, and a level C of 1200 ° C. and a level D of 1230 ° C. in the conventional example is shown in FIG. As shown in FIGS. 1 and 2, in the first embodiment, only the heating furnace outlet descaling device 12 immediately after the heating furnace 10 is used, and in the second embodiment,
High pressure water is jetted only at the heating furnace outlet descaling device 11 at an ejection pressure of 140 kgf / cm ² and at an ejection flow rate of 100 liters / minute. Thereafter, as in the conventional example, the descaling device 14 and the coarse No. 1 rolling mill are used. (R1) 13, descaling device 16, coarse No. 2 reverse rolling mill (R2) 1
5, descaling device 17, coarse No. 2 reverse rolling mill (R2) 15, descaling device 16, coarse No. 2 reverse rolling mill (R2) 15, descaling device 17, coarse 2
No. reverse rolling mill (R2) 15, descaling device 1
6, the coarse strip No. 2 reverse rolling mill (R2) 15, the descaling device 19, and the coarse No. 3 rolling mill (R3) 18 are descaled and reduced on the metal strip in this order.

The surface temperature of the metal strip subjected to the descaling and reduction under the above conditions is as shown in FIG. FIG. 3 shows the relationship between the elapsed time from the heating furnace extraction and the surface temperature of the slab. For the level A (△), the first descaling is performed after the slab is extracted from the heating furnace, and then the atmospheric oxidation is performed. This is a case where de-scaling is performed with time, and the level C (○) is a conventional example used for comparison.
Level B (□) is the case where the temperature of the slab extracted from the heating furnace is raised, the interface scale is melted, the first descaling is performed, and then atmospheric oxidation is performed for a predetermined time to perform descaling. D (●) shows the case of descaling only in front of the first roughing mill (R1) 13 under the same extraction conditions as the level B.

1 and 3, the levels A, B, C, D
Level A is the case where the slab is extracted from the heating furnace at about 1200 ° C., then the first descaling is performed, and then the atmospheric oxidation is performed for a predetermined time (5 seconds or more) to perform the descaling. As shown in (△), the surface temperature is lower than that of the conventional level C (○) in which descaling is performed only immediately before the first roughing mill (R1) 13. In this case, the state of the scale immediately before the roughing mill 1 (R1) 13 from the heating furnace extraction is as shown in FIG. A thick scale 22 and a wedge-shaped scale 21a in a molten state are formed on the surface. The thick scale 22 on the surface of the slab ground iron 20 is removed by the first descaling performed after extraction from the furnace. Next, the surface of the ground iron 20 of the slab is oxidized in the atmosphere, and the surface of the wedge-shaped scale 21 is simultaneously descaled by the next descaling so that the wedge-shaped scale 21 disappears or becomes shallow. The level C (）) is a conventional example, and similarly to the level A, the interface between the surface scale 22 and the ground iron 20 at the time of descaling is at or below the solidification temperature. For level B (□), the temperature of the slab extracted from the heating furnace is increased to 1230 ° C., the first descaling is performed with the interface scale in a molten state, and then the atmospheric oxidation is performed for a predetermined time (5 sec or more). When the scale is applied, the level D (●) is for the coarse No. 1 rolling mill (R1).
This is the case where the de-scaling is performed only immediately before 13.

FIG. 4 compares the length indexes of the wedge-shaped scales remaining after the completion of the rolling at the levels A, B, C and D. Comparing the level A and the level C at the same extraction temperature (1200 ° C.), the length of the level A is shorter than that of the level C after the end of rolling. Also, comparing level B and level D at the same extraction temperature (1230 ° C),
Level B has a shorter wedge-shaped scale remaining after rolling than Level D. Note that the level D has a longer length of the wedge-shaped scale remaining after the completion of rolling than the level C. This is because the extraction temperature of the level D is higher than that of the level C, and thus the length of the wedge-shaped scale generated in the heating furnace is longer. That's why.

The present inventor estimated the mechanism of the occurrence of scale flaws in the product steel sheet according to the conventional method as shown in FIG. First, in the heating furnace 10, S
A wedge-shaped scale 21 partially encroaching on the ground iron 20 and a scale 22 made of FeO on the surface of the ground iron 20 due to the concentration of i and S. Next, the scale 2 made of FeO on the surface of the ground iron 20 by descaling with high-pressure water
2 is removed, but the wedge-shaped scale 21 is not removed as it remains in the ground iron 20. Furthermore, the wedge-shaped scale 21 finally appears as a scale flaw of the product steel sheet at the time of cold rolling by the rough and finish rolling.

Therefore, a method of removing the wedge-shaped scale 21 that has penetrated the ground iron 20 has been intensively studied. As a result, the following could be found. After removing the scale formed in the furnace, the wedge trapped in the base iron by promoting (secondary oxidation + descaling of the base iron surface), that is, scale off by secondary oxidation. The length of the mold scale can be reduced. In order to improve the removability of the scale (FeO) generated in the heating furnace, it is effective to descaling the wedge-shaped scale confined in the ground iron in a molten state. Hereinafter, these will be described in detail.

FIG. 5A shows a mechanism (corresponding to level A) for shortening (or removing) the length of a wedge-shaped scale by promoting scale-off by secondary oxidation, and FIG. 5B shows a conventional example. 4 shows the mechanism of descaling. In the mechanism shown in FIG. 5A for shortening (or removing) the length of the wedge-shaped scale by accelerating the scale-off by the secondary oxidation, the scale (FeO) 22 generated in the heating furnace and the ground iron 20 are cut off. The melted wedge-shaped scale 21a is connected to the heating furnace outlet descaling device 1 provided immediately after the heating furnace 10.
2, the scale 22 is almost entirely removed, but a part of the scale (FeO) 22a is formed near the surface of the solidified wedge-shaped scale 21 in the high temperature state (the surface temperature of the slab is about 950 ° C. to 1150 ° C.). Is attached. However, since the surface temperature of the slab is high, the scale-off is promoted by the secondary oxidation near the wedge-shaped scale 21 in the base iron 20, and the secondary oxidation part is removed by the descaling of the descaling device 14. The scale 22a is removed together with the wedge-shaped scale 21 that has penetrated the ground iron 20. Further, by the coarse No. 1 rolling mill (R1) 13, the coarse No. 2 reverse rolling mill (R2) 15, the coarse No. 3 rolling mill (R3) 18, and the descaling devices 16, 17, and 19, the high temperature is increased in the same manner as described above. By promoting the scale-off by the secondary oxidation in the state, the wedge-shaped scale 21 biting into the base iron 20 is gradually shortened (or removed).

The thick dotted line in FIG. 6 indicates the level B in FIG. 3, and the descaling in the molten state improves the removal performance of the scale generated in the heating furnace, and
In the mechanism for shortening the length of the wedge-shaped scale by promoting the scale-off by the secondary oxidation, the heating furnace 10
Since the wedge-shaped scale is in a molten state, the scale 22 generated therein is completely removed by the heating furnace outlet descaling device 11 provided immediately after the heating furnace 10, that is, the peelability of the scale 22 is improved. I do. At the same time, since the surface temperature of the slab is high, the scale-off is promoted by the secondary oxidation near the wedge-shaped scale 21 in the base iron 20, and the secondary oxidation part is removed by the descaling of the descaling device 14. . By promoting the scale-off by the secondary oxidation in the high temperature state, the wedge-shaped scale 21 biting into the base iron 20 is gradually shortened (or removed), so that it becomes even shorter than in the case of the level A.

Compared with the rough rolling method of the hot-rolled steel sheet according to the present embodiment, in the conventional method of level D, in the descaling of the descaling device 14, the elapsed time from the heating furnace extraction is longer. The wedge scale solidifies and some of the scale remains on the wedge scale. Thereafter, the coarse No. 1 rolling mill (R1) 13 and the coarse No. 2 reverse rolling mill (R2) 1
5. By the coarse No. 3 rolling mill (R3) 18 and the respective descaling devices 16, 17, 19, the scale-off is promoted by the secondary oxidation in the same manner as described above, so that the wedge-shaped scale 21 biting into the base iron 20 is gradually shortened. However, some will remain.

FIG. 6 is a graph showing the relationship between the elapsed time from the heating furnace extraction and the interface temperature of the slab (the interface temperature between the ground iron and the scale). In addition, the level B has shown the result at the time of simulation when the extraction temperature was 1230 degreeC, the slab thickness was 250 mm, and the scale thickness was 1.9 mm. Here, a melting point of 1177 ° C. of Fe ₂ SiO ₄ , which is estimated to generate a wedge-shaped scale in a molten state.
As described above, in order to maintain the interface temperature, it is understood that the elapsed time from the heating furnace extraction must be within 50 seconds. That is, by performing the first descaling within 50 seconds of the elapsed time from the heating furnace extraction, it becomes possible to descaling the wedge-shaped scale in a molten state. Therefore, the heating furnace outlet descaling device 11 provided immediately after the heating furnace 10 makes this possible.

Next, the conditions and results of the experiment for confirming the effect of the melting descaling will be described with reference to FIG.
It is shown in (B). Experiment No. shown in FIG. B1 indicates that the interface temperature between the scale and the iron base is the melting point of Fe ₂ SiO ₄ (11
77 ° C) or lower descaling is to confirm that there is much scale residue generated in the heating furnace,
Experiment No. shown in FIG. B2 is for confirming the effect of melting descaling. As the conditions for the experiment, the extraction temperature was set to 1230 ° C. and the interface temperature before descaling was set to 1107 ° C. for B1 and 121 ° C. for B2.
0 ° C.

FIGS. 7A and 7B schematically show a scale removal situation at the time of descaling based on a micrograph of the interface as a result of the experiment. The following results became clear from the above results. Comparing B1 and B2, it is possible to significantly reduce the scale generated in the heating furnace remaining at the interface by performing the melting descaling under the condition that the interface temperature exceeds 1177 ° C. Therefore, the scale that inhibits the secondary oxidation of the ground iron near the wedge-shaped scale can be removed by the melting descaling, and the effect of shortening the wedge-shaped scale increases by promoting the scale-off by the secondary oxidation.

In the above embodiment, the number of times of descaling after the heating furnace extraction and before the descaling device 14 is one. However, if the surface temperature of the slab can be maintained at 950 ° C. or higher, the number is increased ( (2 or more times), the amount of secondary oxidation is increased, and the effect of shortening the wedge scale can be improved.

[0023]

According to the method for rough rolling of a hot-rolled steel sheet having excellent surface properties according to claims 1 to 3, the slab is subjected to descaling at least twice before performing the rough rolling. At least one descaling is performed after the surface temperature of the slab is 950 ° C. or more and the slab subjected to the descaling before the descaling is oxidized to the atmosphere. Descaling can be carried out at a high interface temperature with the furnace, so that the scale generated in the heating furnace can be removed, and the secondary oxidation at a high temperature thereafter significantly reduces the wedge-shaped scale compared to the conventional low-temperature descaling ( Or shortening), and thus the occurrence of scale flaws in the product steel sheet can be reduced.

In particular, in the method for rough rolling of a hot-rolled steel sheet having excellent surface properties according to the second aspect, since the atmospheric oxidation before descaling is set to 5 seconds or more, sufficient scale-off is promoted by secondary oxidation. Thus, the effect of shortening the wedge-shaped scale is further improved. In the method for rough rolling of a hot-rolled steel sheet having excellent surface properties according to the third aspect, the interface temperature between the ground iron and the scale of the slab extracted from the heating furnace is maintained at or above the melting point of Fe ₂ SiO _4. Since the first descaling is performed, the scale partially remaining on the wedge-shaped scale that has cut into the ground iron is removed, and the effect of shortening the wedge-shaped scale is further improved.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a hot-rolled steel sheet rough rolling facility to which a hot-rolled steel sheet rough rolling method having excellent surface properties according to an embodiment of the present invention is applied.

FIG. 2 is an explanatory diagram showing use and non-use conditions of a descaling device.

FIG. 3 is a graph showing a relationship between an elapsed time from a heating furnace extraction and a slab surface temperature.

FIG. 4 is a graph illustrating a shortening effect of a wedge-shaped scale.

FIG. 5 (A) is an explanatory diagram showing a mechanism for shortening the length of a wedge-shaped scale by promoting scale-off by secondary oxidation, and FIG. 5 (B) is an explanatory diagram showing a conventional mechanism for descaling.

FIG. 6 is a graph showing the relationship between the elapsed time from the heating furnace extraction and the interface temperature between the ground iron and the scale.

FIG. 7 is an explanatory diagram showing experimental conditions and results in a confirmation experiment on melting descaling.

FIG. 8 is a schematic diagram for explaining that wedge-shaped scales that have penetrated the ground iron are not removed but appear as scale flaws on the surface of the steel sheet during cold rolling.

[Explanation of symbols]

A Rough rolling equipment for hot rolled steel sheet 10 Heating furnace 11 Heating furnace outlet descaling device 12 Heating furnace outlet descaling device 13 Coarse No. 1 rolling machine 14 Descaling device 15 Coarse No. 2 reverse rolling machine 16 Descaling device 17 Descaling device Reference Signs List 18 Rough No. 3 rolling mill 19 Descaling device 20 Ground iron 21 Wedge scale 21a Wedge scale in molten state 22 Scale (FeO) 22a Scale (FeO)

Continuation of the front page (72) Inventor Shoichi Sakaguchi 1-1, Tobata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka New Nippon Steel Corporation Yawata Works (72) Inventor Masaharu Kameda 1 Tobata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka No. 1 New Nippon Steel Corporation Yawata Works

Claims (3)

[Claims] 1. A method for rough rolling of a hot-rolled steel sheet in which descaling is performed by spraying high-pressure water onto a slab extracted from a heating furnace to perform rough rolling, wherein the slab is subjected to rough rolling before the rough rolling is performed. The descaling is performed two or more times, and one or more of the descaling is performed, and the surface temperature of the slab is 950 ° C.
A rough rolling method for a hot-rolled steel sheet having excellent surface properties, wherein the slab subjected to the descaling before the descaling is oxidized in the atmosphere. 2. The method of claim 1, wherein the atmospheric oxidation before the descaling is performed for 5 seconds or more. 3. The first descaling is performed while maintaining the interface temperature between the ground iron and the scale of the slab extracted from the heating furnace at or above the melting point of Fe ₂ SiO _4. Item 3. The method for rough rolling of a hot-rolled steel sheet having excellent surface properties according to item 1 or 2.