JPH1157838A - Production of thick steel plate superior in laser cutting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To add an even scale thickness superior in adherence and to improve laser cutting by cooling the steel plate with an injection of a water stream like a curtain against a steel plate in its width direction while moving at a high temperature. SOLUTION: After a hot rolling, the steel plate 1 is reheated in an austenite threshold. The water stream like a curtain (a continuous water stream runs without cutting on the steel surface) is sprayed from a slit nozzle 4 arranged in a slit shape towards a width direction of the plate 1 against the steel plate 1. The slit nozzles 4 in a cooling zone are arranged 20 units each on the upper and the bottom face of the steel plate 1 and cool the plate from the both faces, the upper and bottom. It is desirable that all the cooling zones are of the same cooling system. A slit shape nozzle is desirable for obtaining a water stream like a curtain, but the nozzle shape is not specifically limited as long as a water stream like a curtain is obtainable on the steel plate 1 surface. A thickness of the steel plate 1 to be manufactured is desirous to be 6 mm or more wherein a generation of Fe3 O4 is easy.

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 thick steel plate having a uniform scale thickness and excellent laser cutability.

[0002]

2. Description of the Related Art Steel structures such as ships, buildings, and bridges use a large amount of thick steel plates. In the work of these steel structures, welding and cutting occupy much of the construction cost and man-hour, and automation of not only the welding method but also the cutting method has been considerably advanced in order to rationalize the work. As a method for cutting a thick steel plate, a thermal cutting method such as gas cutting, plasma cutting, or laser cutting has conventionally been used.

[0003] Gas cutting is the most commonly used cutting method because there is no upper limit on the plate thickness that can be cut and the apparatus is simple. However, fully automated technology for controlling and monitoring the combustion flame has not yet been completed. In addition, there are disadvantages such as a large cutting width, large thermal deformation, and a low cutting speed.

In plasma cutting, high-speed cutting is possible for a steel plate having a thickness of 50 mm or less, but it is difficult to secure cutting accuracy, and since the life of the torch is only several hours, productivity is reduced due to replacement. However, complete automation is not easy.

[0005] On the other hand, laser cutting has a very narrow cutting width, so that thermal deformation due to cutting heat input is small, common cutting can be performed in addition to cutout cutting, good cutting quality, and good cutting quality. Less fume,
In addition to the advantage that the working environment can be maintained well, there is a great advantage that the cutting operation can be easily fully automated.

Therefore, the laser cutting method is the most effective method for streamlining steel plate cutting. However, in the case of thick steel plates, the range of the appropriate laser cutting speed is limited, which hinders the expansion of the application. In other words, if the cutting speed is increased, the energy efficiency per unit cutting length decreases, and the entire sheet thickness cannot be melted, or even if it can be melted, the molten metal cannot be sufficiently removed. There is a problem that the molten metal adheres to the cut lower surface as dross.

On the other hand, when the cutting speed is reduced, the cutting width increases and the amount of molten metal also increases.
A defective cutting portion called a notch, in which the cutting width partially increases, occurs. For this reason, on both the high speed side and the low speed side of the cutting speed, there is a critical speed for obtaining good cutting quality, and this is also a reason for preventing an increase in the thickness of a thick steel plate that can be laser cut. Therefore, there is a demand for a thick steel plate that can adopt a wide range of appropriate laser cutting speed, that is, has stable laser cutting properties.

As a factor affecting such laser cutting properties, there is a property of scale generated on the surface of a steel sheet manufactured by hot rolling. In particular, it is considered that the adhesiveness of the scale has a close relationship with the laser cutting ability. This is because, when the adhesion of the scale is improved, the scale can be prevented from peeling off due to thermal shock during laser cutting, so that a stable cutting quality can be obtained and a wide range of cutting speed for obtaining good cutting quality can be adopted. This is because stable workability can be obtained.

For this reason, many attempts have been made to improve the properties of the scale of a steel sheet for the purpose of improving the laser cutting properties. These attempts include controlling the chemical composition of the steel material, or the hot rolling conditions of the steel sheet and the conditions after the rolling. Can be classified into three types: a production method combining water cooling and a combination of a chemical component and a production method. The following describes a conventional technique.

In JP-A-7-155975 and JP-A-8-3692, the surface roughness (center line average roughness) of the scale is controlled to 3 μm or less, and Cu or Ni,
A laser cutting steel sheet containing Cr is disclosed. In addition, as a manufacturing method, it is particularly preferable that the rolling end temperature is 850 ° C. or higher. However, this technology
It cannot be applied to steels having different chemical components, and it is not easy to control the surface roughness of the scale within the above range, and it can be said that the versatility is poor.

Japanese Patent Application Laid-Open No. 5-112821 discloses a method in which the chemical composition of steel is limited and the rolling end temperature is set to 800 ° C. or higher to improve the laser cutting property.
When the rolling end temperature falls below this temperature, the surface roughness of the scale is degraded.However, according to the research of the present inventors, the laser cutting performance is actually improved because the scale adhesion is reduced by this method. Did not.

Japanese Patent Application Laid-Open No. 7-48622 discloses a method of manufacturing a scale-controlled steel sheet in which the rolling end temperature is 850 ° C. or less. However, according to the study by the inventors, when the rolling end temperature is 850 ° C. or less, the FeO is again rolled after rolling.
Since the scale mainly composed of (wustite) grows and the thickness of the scale increases, the adhesiveness of the scale is deteriorated and the laser cutting property is insufficient.

Japanese Patent Application Laid-Open No. 7-48623 discloses a method in which water is cooled to a temperature of 800 to 700 ° C. immediately after the end of rolling. FeO is formed again
Because of the growth, the scale becomes thick, and the laser cutting property is not sufficient.

In Japanese Patent Application Laid-Open No. 7-178414, 800 ° C.
After rolling at 900 ° C., a method of cooling to 800 ° C. or less at a cooling rate higher than natural cooling is disclosed in
In Japanese Patent No. 218119, rolling of 850 is performed after limiting the chemical composition of steel material and descaling conditions during rolling.
A method has been proposed in which the temperature is lowered to 700 to 600 ° C. after cooling at 700 to 720 ° C., followed by air cooling. According to these methods, the steel sheet surface is quenched by water cooling after rolling, so that the steel sheet passes through a temperature range in which FeO 2 grows in a short time. Therefore, the growth of FeO after rolling is suppressed, and a thin film having excellent adhesion is obtained. Since a scale can be obtained, laser cutting properties are also improved.

However, even if a scale having excellent adhesion is obtained by water cooling after rolling, this is due to a reduction in average scale thickness. “Smoothness” of the scale thickness (properties) of the steel sheet surface after cooling occurs, and in actual cutting work, there is a problem that the laser cutability is reduced.

As described above, a method for improving the scale thickness or scale adhesion of a steel sheet has not always been established from the prior art, and although water cooling after rolling applied to a steel sheet is a preferable direction, The above-mentioned problem of ensuring stable laser cutting properties of the entire steel sheet has not been solved yet.

[0017]

DISCLOSURE OF THE INVENTION The present invention has been made to solve these problems, and it is possible to optimize the method of manufacturing a steel sheet without limiting the chemical composition of the steel material, thereby improving the adhesion. Excellent uniform scale thickness,
It is an object of the present invention to provide a general-purpose method for producing a thick steel plate having excellent laser cutting properties.

[0018]

Means for Solving the Problems The inventors have found that unevenness in scale thickness on the surface of a steel sheet occurs during water cooling after rolling.
That is, as a result of intensive research on a cooling method of a steel sheet in order to improve laser cutting properties by preventing "scale unevenness", the following invention was reached.

That is, the first invention is characterized in that a thick steel plate excellent in laser cutting properties is characterized in that the steel plate is cooled by jetting in a width direction of a moving steel plate in a high temperature state by a curtain-shaped water flow. It is a manufacturing method of.

According to a second aspect of the present invention, the curtain-shaped water flow is
This is a method for producing a thick steel plate excellent in laser cutability, characterized by being generated by a slit-shaped nozzle.

[0021] A third invention is a method for producing a steel sheet having excellent laser cutting properties, wherein the steel sheet is a steel sheet which has been subjected to hot rolling. That is, in a manufacturing method of water-cooling the steel sheet after the completion of hot rolling, it is a method of cooling while spraying a curtain-shaped water flow in the width direction of the steel sheet.

A fourth invention is characterized in that the steel sheet is a steel sheet reheated to an austenite region after hot rolling. That is, in the heat treatment for quenching or normalizing a steel sheet, a method is used in which cooling water is injected by cooling water in the width direction of the steel sheet using a curtain-shaped water flow to cool the steel sheet.

Here, the curtain-shaped water flow means a water flow in which the cooling water from the cooling nozzle is continuous without any break on the surface of the steel sheet, and is hydrodynamically laminar or turbulent. You may. While a conventional water stream injected from a pipe nozzle described later collides with a steel plate in a point-like manner, a curtain-shaped water stream collides in a linear manner.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The basic concept of the present invention will be described below by taking as an example the application to a steel sheet after hot rolling.
In general, it is considered that the adhesiveness of the scale generated on the steel sheet has a close relationship with the laser cutting property of the steel sheet. That is, when the adhesion of the scale is improved, scale peeling due to thermal shock at the time of laser cutting can be prevented, so that a good cutting quality can be obtained and a wide range of an appropriate laser cutting speed can be adopted.

On the other hand, in order to obtain a scale having excellent adhesiveness, it is necessary to reduce the thickness of the scale and make the scale composition Fe ₃ O ₄ (magnetite) rather than FeO (wustite). This is because a thick scale is likely to peel off due to stress at the time of processing or heating, and since FeO has a brittle structure including many pores, a denser Fe ₃ O ₄ has better adhesion. It depends.

Looking at the scale formation process on the steel sheet surface,
In a high temperature state in which a steel sheet is hot-rolled, FeO is generated as a scale. The generated FeO elongates by rolling and peels off by descaling, but the final thickness is determined by the scale generated during the rolling and the generation and growth of the scale in the cooling process after the rolling. Therefore, in order to reduce the scale thickness, it is necessary to suppress the formation of scale during rolling and to suppress the formation and growth of scale in the cooling process after rolling.

On the other hand, when the temperature of the steel sheet becomes lower than 570 ° C., the generated FeO starts to transform into Fe ₃ O ₄ . As described above, since Fe ₃ O ₄ is denser than FeO, transformation to Fe ₃ O ₄ is effective for improving the adhesion of the scale.

From the above, the method of cooling the hot-rolled steel sheet with water and then air-cooling is effective as a method of reducing the scale thickness and improving the adhesion of the scale. This is because the surface temperature of the steel sheet decreases rapidly due to water cooling,
This is because the growth of FeO after rolling is suppressed, and
Choose an appropriate cooling stop temperature, then air-cool,
This is because the isothermal transformation from FeO to Fe ₃ O ₄ can be promoted.

However, generally, when a wide steel sheet is water-cooled, the steel sheet after cooling has a non-uniform scale thickness. This uneven thickness of the scale, that is, "scale unevenness"
Occurs, the absorptance of the laser light differs in the "uneven" portion, so that the laser cutting conditions fluctuate, and there is a problem that stable laser cutting cannot be performed.

The present inventors have examined in detail the effects of cooling conditions on the scale properties, and found that instantaneous non-uniformity of cooling, which is injected into a high-temperature steel sheet, affects the scale properties, It has been found that the unevenness of the cooling water injected first to the hot steel sheet has the greatest effect on the "scale unevenness".

That is, F generated during and after rolling
The scale mainly composed of eO is rapidly cooled and contracted when cooling water is injected at a high temperature of 700 ° C. or higher. At this time, if the injection of the cooling water is non-uniform, nucleate boiling cooling occurs in the strongly cooled part and the steel sheet surface decreases to about 200 ° C., but film boiling cooling occurs in the weakly cooled part and the steel sheet surface temperature hardly decreases. For this reason, the contracted state of the scale is different, and this causes the occurrence of “scale unevenness”.

FIG. 1 is a diagram schematically showing the distribution of cooling capacity when a steel plate at 800 ° C. is cooled by the laminar water flow from the pipe nozzle shown in FIGS. 2 (a) and 2 (b). It is. In the vicinity of the laminar collision point just below the pipe nozzle,
While the cooling capacity is significantly increased, portions other than the collision point indicate that the cooling capacity is low. That is, at the point of collision,
The vapor film is broken by the laminar, and the cooling water is in a nucleate boiling state where it comes into direct contact with the steel sheet, whereas in other areas,
This is because there is a film boiling state in which a vapor film exists between the steel sheet and the cooling water.

Usually, in a cooling device using a pipe nozzle, since the pipe nozzles are arranged in a staggered shape as shown in FIG. 2C, the rolled steel sheet moves in the cooling device as shown in FIG. The distribution of the cooling capacity as shown is cancelled, and the entire cooling system is designed so as to obtain an average cooling capacity.

However, since the amount of shrinkage of the scale on the surface of the steel sheet due to cooling is determined by the state of the steel sheet still at a high temperature, that is, the condition of the first cooling zone of the cooling device, "scale unevenness" occurs in the steel sheet after cooling. It is extremely difficult to avoid.

On the other hand, when the steel sheet is cooled by a slit-shaped nozzle installed in the width direction of the steel sheet as shown in FIG. 3, a uniform curtain-shaped water stream collides with the steel sheet, so that the cooling water strikes at the collision point. As a result, a uniform cooling capacity distribution can be obtained in the plate width direction. Therefore, the shrinkage behavior of the scale on the surface of the steel sheet becomes uniform in the width direction of the steel sheet. As a result, the thickness of the scale after cooling becomes uniform.

FIG. 4 is a diagram schematically showing the shape of the scale on the surface of the steel sheet after being cooled by the cooling device shown in FIGS. FIG. 4A shows a case where cooling is performed by the pipe nozzle method of FIG. 2, but at the same interval as the pipe nozzle interval,
There is a thin part of the scale due to strong cooling,
"Scale unevenness" occurs in the plate width direction. On the other hand, FIG.
(b) shows a case where cooling was performed by a curtain-like water flow using the slit nozzle in FIG. 3, and a substantially uniform scale thickness was obtained in the plate width direction.

As described above, in the method for producing a steel sheet by hot rolling and water cooling, the cooling water is jetted from the slit-shaped nozzles in the width direction of the steel sheet while passing through the cooling zone, whereby the surface of the steel sheet is sprayed. The occurrence of "scale unevenness" can be prevented.

As described above, according to the present invention, a steel sheet is cooled by a curtain-shaped water flow to produce a scale having a small thickness and excellent adhesion without unevenness on the steel sheet surface. It is to let. Thereby, a stable steel cutting can be performed in a wide range of cutting speed, and a steel sheet having good cutting quality can be obtained.

This concept can be applied to a steel sheet manufactured by heat treatment in addition to the above-described method of manufacturing a steel sheet by rolling and water cooling. That is,
The present invention is also applicable to a quenching treatment or a normalizing treatment performed after re-heating a hot-rolled steel sheet to an austenite region.

Although the slit-shaped nozzle is a desirable nozzle for obtaining a curtain-shaped water flow, the shape of the nozzle is not particularly limited as long as a curtain-shaped water flow can be obtained on the steel sheet surface. For example, as shown in FIG. 5, it is also possible to obtain a curtain-shaped water flow on a steel plate surface using a flat spray nozzle. The hydrodynamics of the water flow may be a laminar flow state or a turbulent flow state.

The jet angle of the water stream from the nozzle is not particularly limited. That is, in addition to the method of dropping cooling water vertically downward from the nozzle toward the steel plate,
A slit-shaped nozzle may be installed near the steel plate, and a curtain-shaped water flow may flow along the steel plate in the traveling direction of the steel plate.

It is preferable that all the cooling zones are of the same cooling system. However, as long as the effect of the present system is maintained, only the cooling zone on the entrance side is cooled by a slit-shaped nozzle in the plate width direction. In the cooling zone thereafter, cooling may be performed by another method. Also, it is preferable to cool both the upper and lower surfaces of the steel plate with a slit-shaped nozzle, but laser cutting of the steel plate is usually performed by irradiating a laser beam from one side of the steel plate, so that it is cooled by a curtain-shaped water flow. It may be only one side of the steel plate.

In the present invention, the cooling water injection method is limited, and other rolling conditions and cooling conditions are not particularly limited. That is, during rolling, it is of course possible to use mill descaling with high-pressure water or shower cooling for controlling the rolling temperature in combination. However, the rolling end temperature is 900 to suppress the growth of the scale generated during rolling and the scale generated after rolling.
C. to 700.degree. C. is preferred.

The temperature at which the cooling is stopped is set at 600 ° C. to prevent the formation of FeO again after the cooling and to promote the isothermal transformation of FeO to Fe ₃ O ₄ during the air cooling process after the cooling. About 450 ° C. is preferable. Further, when the steel sheet is held in a temperature range of about 600 ° C. to 300 ° C. after the cooling, the transformation of the scale composition from FeO to Fe ₃ O ₄ is promoted, and the scale adhesion is further improved.

In the present invention, the chemical composition of the steel is not particularly limited. Considering that laser cutting is frequently used in the manufacture of steel structures, steels of general chemical composition, i.e., in weight% (wt%), C: 0.01-0.25.
%, Si: 0.01-0.50%, Mn: 0.2-2.
0%, P: 0.03% or less, S: 0.03% or less, A
l: Steel containing 0.005 to 0.06%, the balance being Fe and unavoidable impurities.

In addition, if necessary, Ti:
0.03% or less, Nb: 0.05% or less, V: 0.1%
Hereinafter, Cu: 1.0% or less, Ni: 3.5% or less, C
Steel containing at least one of r: 3.0% or less and Mo: 0.6% or less may be used.

The thickness of the steel sheet produced according to the present invention is not particularly limited. However, when the sheet thickness is small, the cooling rate of the steel sheet after hot rolling increases, and Fe ₃ O ₄ is hardly generated. Therefore, the plate thickness is preferably 6 mm or more.

[0048]

Embodiments of the present invention will be described below. Example 1 C: 0.15%, Si: 0.20% by weight (wt%)
%, Mn: 0.8%, P: 0.018%, S: 0.01
A steel containing 0%, Al: 0.028%, and the balance consisting of iron and inevitable impurities was heated at 1150 ° C. by slab heating.
Hot rolling was performed to produce a steel plate having a thickness of 25 mm. After the hot rolling, the steel sheet was cooled from 750 ° C. to 450 ° C. using the cooling device shown in FIG. 6, and then cooled to room temperature by air cooling.

This cooling device has a total of 20 cooling zones.
There are zones, and the cooling method is the same for all zones. On the upper surface of the steel plate, a slit-shaped nozzle is disposed above the steel plate in the width direction, and cooling water falls from the slit-shaped nozzle onto the upper surface of the steel plate, thereby cooling the steel plate. The lower surface of the steel plate is cooled by the slit-shaped nozzle similarly to the upper surface.

After a bending test piece was taken from a steel plate and subjected to a 90 ° bending test, a tape peeling test was performed from the bent portion.
The scale peeling amount was evaluated. The amount of scale peeling was 31 mg / cm ² at the maximum in Comparative Examples described later,
It showed 9 mg / cm ² and was judged to have high adhesion. Further, there was no "scale unevenness", and the uniformity of the scale was good.

The steel sheet was subjected to laser cutting at various cutting speeds with a laser output of 2.5 kW using a 6 kW carbon dioxide laser processing machine. The upper limit cutting speed at which the entire plate thickness can be melted and dross does not adhere to the lower surface of the cut is 1250 mm / min, and the lower limit cutting speed at which a defective cut called a notch does not occur on the cut surface is 440.
mm / min, 960,
Compared to 610 mm / min, the appropriate laser cutting speed range was sufficiently wide, and the laser cutting property was excellent. The notch refers to a portion where the cutting width is increased by 50% or more.

Example 2 A steel sheet having the same composition as in Example 1 was hot-rolled under the same conditions to produce a steel sheet having a thickness of 25 mm, and the cooling apparatus shown in FIG. Cooled down. This cooling device has a total of 20 cooling zones, and the cooling system is the same for all zones.

On the upper surface of the steel plate, a slit-shaped nozzle having the shape shown in FIG. 7B is arranged near the steel plate above in the width direction of the steel plate. Is discharged, and the steel sheet is cooled.
The lower surface of the steel plate is cooled by a spray nozzle method.

In the same manner as in Example 1, the amount of scale peeling in the tape peeling test performed at the bent portion of the bending test was 7 mg.
/ Cm ² , which indicates that the adhesiveness is high. Also,
There was no "scale unevenness" and the uniformity of the scale was good. In the laser cutting test, the upper limit cutting speed: 1300
mm / min, lower limit cutting speed: 400 mm / min
And the appropriate cutting speed range is sufficiently wide, and the laser cutting property is excellent.

Example 3 A steel having the same composition as in Example 1 was hot-rolled under the same conditions to produce a steel plate having a thickness of 25 mm, and was cooled from 900 ° C. to 200 ° C. using the cooling device shown in FIG. . This cooling device has a total of 18 cooling zones, and only the first cooling zone on the entrance side has the slit-shaped nozzles shown in FIG. 7B arranged on the upper and lower surfaces in the width direction of the steel plate. A slit-like water flow is discharged in the direction of travel to cool.

After the second cooling zone, the upper and lower surfaces are cooled by spray nozzles. As in Example 1, the amount of scale peeling at the bent portion in the bending test was 18 mg.
/ Cm ^2, which is slightly lower than in Examples 1 and 2, but the adhesion was judged to be good.

There was no "scale unevenness". Upper limit cutting speed in laser cutting test: 1100 mm / mi
n, lower limit cutting speed: 500 mm / min. Also in this case, the appropriate cutting speed range is slightly narrowed as compared with Examples 1 and 2, but since there is no "scale unevenness", laser cutting can be stably performed within the range, and the laser cutting property is excellent. Was determined to be.

Comparative Example A steel having the same composition as in Example 1 was hot-rolled under the same conditions to produce a steel sheet having a thickness of 25 mm, and was cooled from 770 ° C. to 520 ° C. using the cooling device of the comparative example shown in FIG. Cool. This cooling device has a total of 20 cooling zones, and the cooling system is the same for all zones. The upper surface of the steel plate is cooled by a pipe laminar, and the lower surface of the steel plate is cooled by a spray nozzle.

The amount of scale peeling performed in the same manner greatly varies depending on the sampling position of the test piece, and is 31 mg / max.
cm ² , and the adhesion was insufficient. In addition, obvious "scale unevenness" occurred. When the same laser cutting test was performed, the appropriate laser cutting conditions changed in this "scale unevenness" part, so stable laser cutting could not be performed under certain conditions, and the upper limit of cutting where good cutting quality was obtained The speed is 960 mm / min, and the lower limit of the cutting speed at which no cutting failure occurs is 610 mm / min for the same reason.
The appropriate cutting speed range was extremely narrow, and the laser cutting properties were insufficient.

[0060]

As described above, according to the present invention,
During cooling of the hot-rolled steel sheet after rolling, generation of “scale unevenness” can be completely prevented, and thus a scale having a uniform thickness and high adhesion can be obtained. Therefore, at the time of laser cutting, it is possible to prevent peeling of the scale due to thermal shock, and it is not necessary to consider fluctuations in laser cutting conditions due to "scale unevenness", so that the cutting quality is stable and can be achieved. A wide range of appropriate laser cutting speed can be adopted, and good workability can be obtained. As a result, a steel sheet excellent in laser cutting property can be stably manufactured in mass production, and this is an invention which spreads a laser cutting method having various advantages.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a cooling capacity distribution of a pipe laminar.

FIG. 2A is a cross-sectional view illustrating a pipe laminar. (B) It is a perspective view explaining a pipe laminar. (C) It is a figure which shows the distribution of the collision point of a pipe laminar.

FIG. 3 is a perspective view illustrating a slit nozzle.

FIG. 4A is a cross-sectional view illustrating a scale shape of a steel sheet after being cooled by a pipe nozzle. (B) It is sectional drawing which shows the shape of the scale of the steel plate after cooling by a slit nozzle.

FIG. 5 is a diagram illustrating a cooling device using a flat spray nozzle.

FIG. 6 is a sectional view illustrating a cooling device according to the first embodiment.

FIG. 7A is a cross-sectional view illustrating a cooling device according to a second embodiment. (b) It is a perspective view explaining the cooling device of Example 2.

FIG. 8 is a cross-sectional view illustrating a cooling device according to a third embodiment.

FIG. 9 is a cross-sectional view illustrating a cooling device of a comparative example.

[Explanation of symbols]

 1: Steel plate 2: Table roll 3: Drain roll 4: Slit nozzle 5: Cooling water header 6: Flow control valve 7: Spray nozzle 8: Pipe nozzle 9: Flat spray nozzle X: Transport direction of steel plate

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Nobuyuki Ishikawa 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Kokan Co., Ltd. (72) Inventor Makoto Kabazawa 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd.

Claims (4)

[Claims] 1. A method for producing a thick steel plate excellent in laser cutability, comprising injecting a curtain-shaped water flow in a width direction of a moving steel plate in a high temperature state and cooling the steel plate. 2. The method according to claim 1, wherein the curtain-shaped water flow is generated by a slit-shaped nozzle. 3. The method according to claim 1, wherein the steel sheet has been hot-rolled. 4. The method according to claim 1, wherein the steel sheet is a steel sheet reheated to an austenite region after hot rolling.