JPH09225504A - Production of hot rolled steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce the hot rolled steel plate having reduced scale flaw and excellent surface quality by easily improving secondary scale peelability with an existing continuous hot finish rolling mill. SOLUTION: When the hot rolled steel plate of specified thickness is produced so that a slab heated to a specified temp. is subjected to rough rolling and successively passing each stand of a continues hot finish rolling mill consisting of seven stands or more, a tension applied between stands is set to 2-5kgf/mm<2> for the steel plate passing the stand of at least a fifth stand and more to upstream side.

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 hot-rolled steel sheet having few scale surface defects.

[0002]

2. Description of the Related Art Generally, a hot-rolled steel sheet includes a heating step of heating a slab to a temperature of 1150 to 1300 ° C. in a heating furnace and a rough rolling step of rolling the heated slab to a thickness of about 30 mm in a rough rolling mill. , A finish rolling step of rolling a rough rolled slab to a predetermined plate thickness in a continuous 7-stand continuous finishing mill, and a runout winding in which a finish-rolled steel plate is water-cooled to a desired material and wound on a coiler. It is manufactured by the taking process.

When manufacturing a hot-rolled steel sheet by the above steps, the primary scale produced on the surface of the steel sheet in the heating step and the secondary scale produced in the rough rolling step after the primary scale is removed can be sufficiently removed. is necessary. If rolling is performed without sufficiently removing the primary scale and the secondary scale, the remaining scale bites into the steel sheet, causing scale flaws, which lowers the product value.

The primary scale produced in the heating step is thick in order to produce it at a high temperature for a long time, and, in addition, a reduction in the width direction by a VSB (vertical scale breaker) or the like which is usually carried out is used in combination. It can be easily removed by jetting high-pressure water from existing equipment.

The secondary scale produced in the rough rolling step is also thick because the surface temperature of the steel sheet is high, and therefore,
As disclosed in Japanese Patent Laid-Open No. 63-68214, it can be removed by jetting high-pressure water having a pressure of 25 gf / mm ² or more to the steel sheet at the entrance side of the finishing rolling mill train.

However, it is difficult to remove the secondary scale produced in the finish rolling process by the continuous hot finishing mill, which is usually composed of 7 stands, because the distance between the stands is short and the strip running time is short.

As a means for preventing such surface defects caused by secondary scales that occur in the finish rolling process, for example, Japanese Patent Laid-Open No. 7-48622 discloses that the surface of a steel sheet immediately after rolling is cooled with high pressure water. , A method of suppressing the generation of scale by lowering the surface temperature thereof (hereinafter referred to as prior art 1) is disclosed in JP-A-3-271381. A method (hereinafter referred to as prior art 2) for suppressing the generation of scale by applying a specific oily scale inhibitor is disclosed. Further, JP-A-53-43661 discloses a method of preventing scale formation and growth by covering the space between the final stand of the finishing rolling mill and the winding machine with a tunnel type compartment in a non-oxidizing gas atmosphere. (Hereinafter, referred to as Prior Art 3) is disclosed.

[0008]

However, according to the prior art 1, since the steel sheet is cooled with high-pressure water immediately after each reduction or each two reductions, the temperature of the steel sheet is inevitably lowered. Therefore, since the predetermined finishing temperature cannot be secured for the production of ultra-low carbon steel sheet and thin steel sheet with high rolling finishing temperature, which are in great demand in recent years, the prior art 1 is applied. I can't.

The method of Prior Art 2 is a method of rolling rolls and / or
Alternatively, a device for applying the scale inhibitor to the rolled steel sheet is required, and a specific chemical substance has to be added as the scale inhibitor, resulting in high cost.

When the method of Prior Art 3 is applied to the continuous hot finish rolling step, a large amount of equipment cost is required and
Since it is impossible to deal with a case where a rolling misroll or the like occurs in the compartment, it is difficult to carry out the operation.

Therefore, an object of the present invention is to solve the above-mentioned problems, to eliminate the need for expensive equipment investment, and to easily improve the peelability of the secondary scale by the existing continuous hot finish rolling equipment. The object of the present invention is to provide a method capable of producing a hot-rolled steel sheet having excellent surface quality with few scale defects.

[0012]

From the above-mentioned viewpoint, the present inventors can easily improve the peelability of the secondary scale by the existing continuous hot finish rolling facility, and the surface with few scale flaws. We have conducted intensive research to develop a method capable of producing high quality hot rolled steel sheet.

According to the literatures and the like so far, it is presumed that the scale flaws on the surface of the steel sheet during hot rolling are caused by the mechanism described below. 9 on the scale
FeO, which accounts for 0% or more, exhibits ductility at high temperatures.
Therefore, as shown in the schematic view of the steel plate cross section in FIG.
When the scale 2 formed on the surface of the sheet is rolled in a thin state, the scale 2 is also stretched at a rate equal to the rolling reduction of the steel sheet 1 as the base material without causing any flaw.

On the other hand, when the scale 2 formed on the surface of the steel plate 1 is rolled in a thick state as shown in the schematic view of the steel plate cross section in FIG. However, many cracks 3 are generated in the thickness direction of the scale 2. Such a crack 3
It is considered that the occurrence of is caused by the residual stress caused by the difference in the physical property values between the steel plate 1 and the scale 2, or the deformation resistance difference between the steel plate 1 and the scale 2, which occurs with the increase in the thickness of the scale 2. When rolling is performed again with such cracks 3 present, the base metal bites into the gaps between the cracks 3 and causes scale flaws.

The inventors of the present invention have paid attention to the cracks in the scale thickness direction that occur after the rolling described above, and as a result of research, they are caused by the secondary scale produced in the finish rolling process consisting of the continuous hot finish rolling mill. In order to reduce scale flaws, if high tension is applied to the steel plate in the former stand where the temperature of the steel plate is high and the scale is thick due to the long threading time, the scale cracks caused by rolling develop and scale Therefore, it was found that the scale can be easily removed by injecting cooling water to the steel sheet in this state.

The present invention was made on the basis of the above findings, in which a slab heated to a predetermined temperature is roughly rolled, and then each stand of a continuous hot finish rolling mill composed of 7 or more stands. In the method for producing a hot-rolled steel sheet in which a hot-rolled steel sheet having a predetermined thickness is rolled by sequentially passing the hot-rolled steel sheet while applying a predetermined tension to the hot-rolled steel sheet, the continuous hot finish rolling mill is provided at least upstream of the fifth stand. It is characterized in that the tension between the stands applied to the steel sheet passing through the stand is set to ² Kgf / mm ² or more and 5 Kgf / mm ² or less.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a continuous hot finish rolling mill composed of seven or more stands passes through at least a stand upstream of a fifth stand,
It is necessary to set the inter-stand tension applied to a steel plate having a high temperature and a long running time to ² Kgf / mm ² or more. By applying such a high tension to the steel sheet, cracks develop in the scale produced by rolling, and the scale easily peels off.

Therefore, in this state, the scale can be easily removed by spraying the cooling water onto the steel sheet from the nozzles provided between the stands. As a result, the scale is not brought into the next stand, scale defects are prevented from occurring, and the scale thickness in the finish rolling step is reduced, so that pickling in the next step is simplified.

The inter-stand tension applied to the steel sheet passing through at least the stand upstream of the fifth stand is 2 kg.
If it is less than f / mm ² , the above effect cannot be obtained. On the other hand, when the tension between the stands exceeds 5 Kgf / mm ² , the tension becomes excessive, and the plate width defect is likely to occur. Therefore, the inter-stand tension applied to at least the steel plate passing through the stand on the upstream side of the fifth stand is ² Kgf / mm ² or more and 5 Kg.
It should be f / mm ² or less.

The pressure of the cooling water sprayed on the steel plate by the cooling device provided between the stands is the same as the current pressure of 10 kg.
It may be about f / cm ² , but it is more effective if the injection position of cooling water is provided on the entrance side of the next stand. The tension between the stands on the steel plate may be applied by any method, but when the tension is applied by using the existing looper, bending strain is applied to the steel plate, so that the peelability of the scale is further improved.

[0021]

Next, the method of the present invention will be further described with reference to examples. Example 1 Using a hot rolling mill having a constant tension load mechanism as shown in the schematic diagram of FIG. 3, the effect of tension on the peelability of the surface scale produced on the rolled steel sheet was investigated. In FIG. 3, 4 is a hot rolling mill, 5 is a tension load device, 6 is a non-oxidizing atmosphere heating furnace, and a predetermined tension is applied to the steel plate 1 rolled by the rolling mill 4 by the tension load device 5. It is supposed to be done.

A steel plate 1 having a thickness of 20 mm, a width of 100 mm, and a length of 250 mm is placed in a non-oxidizing atmosphere heating furnace 6 for 100
After heating at 0 ° C. for 1 hour and holding, it was left in the atmosphere for 5 seconds, then rolled by a hot rolling mill 4 at a reduction rate of 40%, and immediately cooled in an non-oxidizing atmosphere. The tension was applied to the steel sheet 1 rolled by the rolling mill 4 by the tension loading device 5 while varying the tension in the range of 0 to 6 Kgf / mm ² .

Steel plate 1 rolled and cooled in this way
After ultrasonic cleaning and removing the peeled surface scale,
The thickness of the scale in the section in the plate thickness direction was examined. FIG. 4 is a graph showing the relationship between the scale thickness after rolling and the load tension during rolling. As is clear from FIG. 4, the scale thickness begins to decrease when a tension of ² Kgf / mm ² is applied to the steel plate 1 during rolling, and when the load tension exceeds ² Kgf / mm ² to 3 Kgf / mm ² , the scale thickness Fell sharply. Then, when the load tension exceeds 3 Kgf / mm ² , the decrease in scale thickness slows down.

Example 2 A sheet bar 8 having a thickness of 30 mm was finish-rolled to a hot-rolled steel sheet having a thickness of 4 mm by a continuous hot finishing rolling mill 7 consisting of 7 stands 7a to 7g shown in FIG. In FIG. 5, 9 is a looper provided between the stands 8a to 8g, and 10 is a cooling device. The steel plate temperature condition including the runout cooling and the rolling condition were the same, and only the tension between the stands by the looper 9 is shown in Table 1,
Four conditions of B, C and D were set.

Condition A shown in Table 1 is a comparative example, and conditions B, C and D are inventive examples. The pressure of the cooling water by the cooling device 10 provided between the stands is 10 Kgf / cm ²
It was fixed at. The scale thickness of the coil tail end after the finish rolling under the conditions A, B, C and D was measured and is also shown in Table 1.

[0026]

[Table 1]

As is clear from Table 1, the conditions B, C and D are such that the inter-stand tension applied to the steel sheet passing through the stand on the upstream side of the fifth stand is ² Kgf / mm ² or more.
In the case of the present invention, the scale thickness was 5.6 μm or less, which was extremely small, and as a result of the visual inspection in the subsequent step, almost no scale flaw was generated.

On the other hand, in the case of the comparative example of the condition A in which the tension between the stands applied to the steel strip passing through the stand on the upstream side of the fifth stand is less than ² kgf / mm ² , the scale thickness is As thick as 11.3 μm, visual inspection in the post-process confirmed the occurrence of scale flaws.

[0029]

As described above, according to the method of the present invention, it is possible to easily improve the peelability of the secondary scale by the existing continuous hot finish rolling equipment without requiring expensive equipment investment. This can produce a hot-rolled steel sheet with less scale flaws and excellent surface quality, which is industrially useful.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a cross section of a steel sheet when rolled in a state where the scale thickness before rolling is thin.

FIG. 2 is a schematic diagram showing a cross section of a steel sheet when rolled in a state where the scale thickness before rolling is thick.

FIG. 3 is a schematic explanatory view of a hot rolling mill having a constant tension load mechanism.

FIG. 4 is a graph showing the relationship between the scale thickness after rolling and the load tension during rolling.

FIG. 5 is an explanatory diagram showing a continuous hot finish rolling mill train including seven stands.

[Explanation of symbols]

 1 Steel plate 2 Scale 3 Crack 4 Hot rolling mill 5 Tension load device 6 Non-oxidizing atmosphere heating furnace 7 Continuous hot finish rolling mill 8 Sheet bar 9 Looper 10 Cooling device

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Sadakazu Masuda, Marunouchi 1-2-2, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Hiroshi Sekine 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Inside the Steel Pipe Corporation (72) Inventor Takahiko Ogura 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Steel Pipe Corporation

Claims (1)

[Claims] 1. A slab heated to a predetermined temperature is roughly rolled, and then successively passed through each stand of a continuous hot finishing mill composed of seven or more stands while applying a predetermined tension. In the method for manufacturing a hot-rolled steel sheet that is rolled into a hot-rolled steel sheet having a predetermined thickness, an inter-stand tension applied to a steel sheet that passes through at least a stand upstream of a fifth stand of the continuous hot finish rolling mill, A method for producing a hot-rolled steel sheet, characterized in that it is ² Kgf / mm ² or more and 5 Kgf / mm ² or less.