JPH11285724A - Method for cooling thick steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the cooling of a high temp. thick steel plate without developing the unevenness and to facilitate the application to control cooling of the thick steel plate by developing a scale layer having a specific thickness on the steel plate after rolling and cooling with water after executing the rolling reduction with rolls having a specific height of ruggedness on the surface. SOLUTION: The steel plate 1 is rolled to a target plate thickness with a thick plate rolling mill 11 and carried on a roller table 12. After rolling, the steel plate is cooled in the atmosphere for 10-15 sec on the roller table 12 to develop the scale having 10-30 μm thickness. Successively, the ruggedness on the roll surface for light rolling reduction is made to 10-30 μm, desirably, 15-25 μm to execute the light rolling reduction to the steel plate 1 with an upper side roll 20 and a lower side roll 21 in the light rolling reduction device 19. The ruggedness pattern in the light rolling reduction roll may be used in randum size but this is desirable to be a fixed regularity. After executing the light rolling reduction to the steel plate 1, the steel plate is passed through the cooling device 13 to cool the upper and the lower surfaces of the steel plate 1 with the water.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for cooling a thick steel plate, and more particularly to an accelerated cooling method for uniformly and rapidly cooling and heat-treating a thick steel plate immediately after rolling.

[0002]

2. Description of the Related Art In recent years, in the manufacturing process of steel plates,
2. Description of the Related Art A technique for obtaining a high-strength, high-toughness steel sheet by performing water-cooling (accelerated cooling) on a steel sheet after controlling rolling to obtain predetermined characteristics by controlling a rolling reduction and a rolling temperature for each rolling pass is widely performed. . By combining controlled rolling and accelerated cooling, it has become possible to not only significantly reduce the production cost by reducing the added elements, but also to produce a thick steel plate excellent in weldability.

In accelerated cooling, cooling water is injected from a cooling nozzle onto the surface of a steel sheet having a high temperature of 200 to 900 ° C., so that boiling heat transfer occurs on the surface of the steel sheet. Due to this phenomenon, a cooling rate of several tens to several hundreds times higher than that of air cooling can be obtained, so that a steel sheet having a fine crystal structure can be obtained, and a steel sheet having high strength and high toughness as described above can be manufactured. it can.

However, in this boiling heat transfer phenomenon, the heat transfer coefficient is small in the state of film boiling and large in the state of nucleate boiling. When the temperature of the steel sheet is high, film boiling is mainly performed, but when the temperature is low, the state transitions to nucleate boiling, and the heat transfer coefficient tends to rapidly increase. The transition temperature from film boiling to nucleate boiling varies depending on the momentum of cooling water and the surface condition of the steel sheet, and is not constant, so the transition from film boiling to nucleate boiling in part of the steel sheet locally increases the cooling rate And the heat transfer becomes very unstable. Therefore, it is difficult to uniformly control the cooling rate of the entire steel sheet, and large temperature unevenness is likely to occur.

[0005] The occurrence of such temperature unevenness not only causes variations in the mechanical properties of the product, but also causes deformation such as ear waves and medium elongation, and requires flatness correction and heat treatment again in the process after rolling. Such as reducing production efficiency.

In recent years, it has been clarified that unevenness in the surface properties of a steel sheet, such as surface roughness, is a major cause of uneven temperature. Uneven heating in the heating furnace and uneven temperature caused during rolling causes uneven thickness distribution on the steel sheet.If the degree of progress of surface wear on the rolling rolls is partially different, uneven surface roughness of the steel sheet occurs However, it has been elucidated the mechanism by which the time when the film transitions from film boiling to nucleate boiling is locally varied when the material is cooled, causing a large variation in the cooling rate. In order to solve the above problems, for example, the following countermeasures have been attempted.

For example, Japanese Patent Publication No. 63-54046 discloses that when cooling a metal tube from a high temperature, after removing the mill scale before cooling, the surface has an average roughness of 2 μm or more and a PPI value of 50 or more. ~ 500 (interval between peaks is 50 ~ 500
A technique for cooling by forming a uniform rough surface (μm) is disclosed.

Japanese Patent Application Laid-Open No. 1-284418 discloses that the surface roughness of a material to be rolled after rolling is adjusted to be not less than the vapor film thickness of the cooling medium (about 10 μm in the case of water) and not more than 30 μm in terms of Rz, and There is disclosed a technique of performing cooling after accelerated oxidation of a new surface after rolling.

Japanese Patent Laid-Open Publication No. Hei 2-70017 discloses that a heat transfer surface of a metal material is provided with a uniform roughness pattern composed of a large number of linear grooves having a depth of 25 μm or less in Rz, and cooling. A technique for performing the above is disclosed.

[0010]

The technique disclosed in the above publication has the following problems. (a) The technology disclosed in the above three publications discloses that a steel plate is restrained by a cooling device of a strong cooling type in which a film boiling state is relatively unlikely to be formed, that is, a roller or a platen (a jig for pressing the steel plate from above and below). However, in the case of an apparatus that performs cooling by bringing the cooling nozzle close to the object to be cooled, it is relatively effective. However, in the case of a weak cooling type cooling device generally used for controlled cooling of a thick plate, that is, a device having no constraint such as a roller and having a distance of 1 m or more between a cooling nozzle and a steel plate, a film boiling region is still locally localized. And large temperature unevenness is likely to occur.

(B) The technique disclosed in Japanese Patent Application Laid-Open No. Hei 2-70017 is to impart regular irregularities to the surface of a steel material without a scale or with a very thin scale. In addition, since the regular irregularities are relatively conspicuous, the surface appearance of the product is deteriorated, and it may not be applicable depending on the product or application.

[0012] In order to solve the above problems, an object of the present invention is to provide a method for uniformly accelerating and cooling a rolled steel sheet.

[0013]

Means for Solving the Problems In order to solve the above-mentioned problems, the inventors conducted various examination tests and obtained the following findings.

(A) When a scale of a certain thickness is formed on the surface of a steel sheet, a crack is generated in the scale by water cooling, a starting point of nucleate boiling is generated at a tip of the crack, and a heat transfer coefficient is increased.

[0015] Further, the fragments are separated from the steel plate from the cracked scale and dispersed in the cooling water, so that the vapor film between the cooling water and the steel plate is broken, and the heat transfer coefficient between the cooling water and the steel plate is increased. Has the effect of doing However, when all of the scale fragments are separated from the steel sheet, the heat transfer coefficient to the steel sheet is reduced again, or the flow of the cooling water causes the heat transfer coefficient to be locally uneven.

(B) If a part of the scale is bitten into the steel sheet under light pressure, it can be used as a starting point of nucleate boiling.
The heat transfer coefficient can be adjusted by adjusting the distribution density of the scale to be cut. Further, if the depth at which the scale bites is set to a certain limit or less, there is no problem of scale biting, and no noticeable pattern is generated on the steel sheet surface.

The present invention has been made based on the above findings, and the gist thereof is as described in (1) and (2) below. (1) In the cooling method of cooling the steel plate with water after rolling the steel plate,
A scale layer having a thickness of 10 to 30 μm is formed on the rolled steel sheet, and then rolled down with a roll having irregularities of a height of 10 to 30 μm, and then cooled with water. Method.

(2) The steel plate according to the above (1), wherein the interval between the apexes, ridges, or tops of the adjacent protrusions is 2 to 10 times the height of the protrusions. Cooling method.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is to form a scale layer of a predetermined thickness on the surface of a steel sheet before water cooling, and fix a part of the scale on the steel sheet surface to serve as a starting point of nucleate boiling. This eliminates the need to use a strong cooling type cooling device that closes the cooling nozzle and injects high-pressure cooling water while constraining the steel plate with a roller or platen, and reduces the distance between the cooling nozzle and the steel plate by unconstraining the steel plate. A relatively weak cooling device that secures 1 m or more can perform uniform and stable cooling. Hereinafter, the principle of the present invention will be described.

In a high temperature range of 700 to 800 ° C. where cooling is started, the film is usually in a film boiling state. Since a steam film is formed between the steel sheet surface and the cooling water, and heat transfer is performed only by radiant heat transfer through the steam film, the heat transfer coefficient is small. As the cooling proceeds and the temperature decreases, the vapor film is broken at a certain temperature (hereinafter, referred to as a quench temperature). A transition from the quench temperature to the nucleate boiling state occurs, and the cooling water and the steel plate come into direct contact with each other to perform heat transfer. The destruction process of this vapor film is very unstable,
The unevenness in temperature is apt to occur because the steam film is broken and locally cooled rapidly due to the unevenness of the surface roughness of the steel sheet and the portion where the steam film is maintained and the cooling is delayed.

Even in a high temperature range of 700 to 800 ° C., in a strong cooling type cooling device that breaks a vapor film by colliding high-pressure jet water, the quench temperature is increased and the entire surface of the steel sheet is simultaneously quenched and uniformly cooled. Can be cooled. However, in a general weak-cooling type cooling device, the vapor film is hardly broken, and it is not possible to improve cooling unevenness.

In the present invention, first, the effect is obtained that the scale fragments dispersed in the cooling water in the high temperature region (film boiling region) destroy the vapor film in the film boiling state. Secondly, the so-called fin effect lowers the temperature at the tip of the fin portion to lower the temperature below the quench point to form the starting point of nucleate boiling. In addition, a scale having good wettability is used as the fin portion.

As means for realizing the above effect, a scale having a predetermined thickness is formed on the surface of the steel sheet 1 after rolling, and this scale is cracked by light pressure, and at the same time, a part of the scale is cut into the steel sheet. Next, inject cooling water,
The cracked scale is peeled off and dispersed in cooling water.

FIG. 1 is a schematic view showing the behavior near the boundary between the steel sheet and the cooling water. In the figure, a cooling water 2 is separated from a steel plate 1 by a vapor film 3. The fragments of the scale peeled from the steel sheet 1 are dispersed in the cooling water 2 as a floating scale 4. In the vicinity of the steel sheet 1, the floating scale 4 enters the vapor film 3, and the fin portions 6 of the floating scale 3 become the starting point of nucleate boiling. As shown in FIG. Destroyed, high temperature range (above quench point)
The heat transfer coefficient is improved.

When all of the scale is peeled off and dispersed in the cooling water, only the first effect can be expected. However, a part of the scale is fixed to the surface of the steel sheet and becomes a biting scale 5 as shown in FIG. Fin part 6 of scale 5
Is the starting point of nucleate boiling, and vapor bubbles 7 are generated. In this state, the steel sheet is directly cooled, and cooling is further promoted.

When the scale is cracked with the aim of the first effect, if the size of the scale fragments is set to be about the same as the thickness of the vapor film to several times the film thickness, the floating scale 4 will be formed. It becomes easy to enter the vapor film, and the effect of destruction of the vapor film can be increased. The uniform cracks of the above-mentioned size can be formed in the scale by imprinting a uniform pattern on the surface of the roll to be lightly reduced.

Even when a part of the scale is fixed to the surface of the steel sheet with the aim of the second effect, a uniform unevenness distribution is provided by providing a regular uneven pattern on the surface of the roll to be lightly reduced. Can be obtained. If the scale shards penetrate too deeply into the steel plate, they become scale flaws.
The thickness of the scale must be kept in an appropriate range, and the depth of the unevenness must be kept in an appropriate range.

Based on the above considerations, the reasons for limiting the conditions of the present invention will be described below. The thickness of the scale formed after rolling is 10 to 30 μm. If the thickness of the scale is less than 10 μm, even if a crack is applied to the scale under light pressure, it is difficult to peel when the cooling water is injected, and the dispersion density when the peeled scale is dispersed in the cooling water is This is because the first effect is reduced because the size is small and the size of the scale fragments is smaller than the thickness of the vapor film, and the effect of destruction of the vapor film is small. When the thickness of the scale is larger than 30 μm, the time required to generate a scale having a predetermined thickness after rolling becomes longer, and the rolling efficiency is reduced, or the temperature of the steel sheet is lowered and a predetermined cooling start temperature cannot be secured. This is because the scale is deeply cut into the steel sheet by light pressure reduction and scale flaws are caused. A more preferred scale thickness is 15 to 25 μm.

To produce the scale, after rolling, 10 to
By leaving to cool in the atmosphere for 15 seconds, while maintaining the predetermined cooling start temperature, without affecting the rolling efficiency,
A predetermined scale thickness can be ensured.

In order to secure the thickness of the scale, the cooling time is prolonged, and if there is a problem with the rolling efficiency or a problem of a drop in the steel sheet temperature, accelerated oxidation may be performed after the rolling. As a method of the accelerated oxidation, there are a method of blowing an oxygen-enriched gas onto the surface of the steel sheet and a method of heating with a direct-fired burner.
When there is a significant difference in the cooling start temperature, the scale thickness, and the like between the front end and the rear end of the steel sheet, the rear end of the steel sheet may be heated in a reducing atmosphere.

In the ordinary cooling method, the steel sheet after finish rolling is transported to the cooling device in less than 10 seconds, so that the scale formation thickness is not sufficient. It is necessary to adjust the cooling time. Accordingly, it is difficult to apply the cooling method of the present invention in a continuous process such as a hot-rolled steel sheet, and is limited to an application such as a thick steel sheet that can be processed in a single unit.

The surface of the roll surface to be lightly reduced is 10 to 30.
μm. If the height of the unevenness is less than 10 μm,
This is because cracks cannot be effectively formed on a scale having a thickness of ３０30 μm, and the scale corresponding to the convex portion of the roll unevenness cannot be effectively cut into the steel sheet. If a heavy pressure is applied to force the scale to bite into the steel sheet, the scale will cut into the entire surface of the steel sheet, making it impossible to obtain an appropriate fin distribution and possibly causing scale flaws. If the height of the unevenness is larger than 30 μm, the surface pressure at the portion corresponding to the convex portion of the unevenness increases, and the scale penetrates deeply to form a scale flaw. A more preferable height of the unevenness is 15 to 25 μm.

The concavo-convex pattern of the light pressure roll may be random, but preferably has a certain regularity. That is, according to the first object, the size of the scale fragments separated according to the crack under light pressure is equal to or larger than a certain size, and the more uniform, the better the destruction effect of film boiling is,
This is because, when the fin portions of nucleate boiling are substantially uniformly distributed on the steel sheet surface, a uniform heat removal effect can be obtained.

FIGS. 2A and 2B are conceptual diagrams showing the distribution of unevenness on the roll surface. FIG. 2A shows a case where dot-like projections are randomly arranged, and FIG. FIG. 4C shows a case where parallel-shaped convex portions having a predetermined shape are arranged when the convex portions having a rectangular shape are arranged in a lattice shape. The shape of the protrusion is illustrated as a cone in FIG. 5A, a quadrangular pyramid in FIG. 5B, and a triangular ridge in FIG. 5C, but the regularity of the shape of one protrusion is not so important. Also, the shape of the recess is not so important. However, the distribution density of the protrusions is important as described below.

It is desirable that the distance P between the vertices of the adjacent convex portions is 2 to 10 times the size of the unevenness. The distance P is, for example, expressed as the distance between a certain convex portion and the vertex of the nearest convex portion in the case of FIG. 3A or FIG. 3B, and as shown in FIG. In the case of a part, it is represented by the distance between the ridge lines.

If the interval P between the projections is less than twice the height H of the projections, the cracks generated on the scale formed on the surface of the steel sheet become finer, and the floating scale aimed at the first effect becomes finer. This is because the distribution density of the biting scale fixed on the steel plate becomes too large and is rapidly cooled locally. The interval P between the projections is 1 of the height H of the projections.
If it is larger than 0 times, when the scale with cracks becomes a floating scale by light pressure reduction, the size will be too large, the distribution density of the bite scale fixed on the steel plate is too small, it can not be cooled effectively, In addition, the surface pressure of the projections is increased, and the bite scale is deeply penetrated, which may cause scale flaws and indentations on the steel plate. A more preferable range of the interval P between the projections and the height H of the projections is 3 to 8 times.

In practice, it is easy to form the roll into a lattice shape as shown in FIG. 2B or a linear shape as shown in FIG. 2C. In particular, if the ridge direction is set to the circumferential direction of the roll as shown in FIG. The cross-sectional shape of the convex portion may be square, trapezoidal, triangular, or the like. If the shape of the vertex is sharp, the radius of curvature of the vertex is about 3 to 10 μm because the abrasion is rapid, or the vertex of the vertex is 3 to 10 μm. It is desirable to be before and after.

The above-mentioned light reduction may be carried out by a plate rolling mill having a predetermined unevenness on a work roll after a scale having a predetermined thickness is formed, or by a rolling mill dedicated to light reduction separately from the plate rolling mill. May go. However, even if the plate rolling mill is provided with predetermined irregularities, the irregularities are rapidly worn during rolling in each pass, and the irregularities are crushed by the surface pressure between the backup roll and the work roll. It is desirable to provide a machine.

Although slight irregularities are generated on the surface of the steel sheet by light reduction, it can be easily smoothed under light pressure by hot leveling usually performed after water cooling, and does not impair the appearance of the final product.

FIG. 3 is a schematic diagram showing a thick plate control cooling equipment to which the cooling method of the present invention is applied. The steel plate 1 rolled to the target plate thickness by the plate rolling mill 11 is
After a predetermined cooling time has elapsed in order to generate a predetermined scale thickness while being transported above, the cooling device 13 is reached. Next, the steel sheet 1 is water-cooled on the upper and lower surfaces while passing through the cooling device 13. The temperature of the steel sheet before and after the cooling is measured by the upper and lower thermometers 16 provided on the inlet side of the cooling device 13, respectively.
a, 16b and upper and lower thermometers 17a provided on the outlet side,
The temperature is measured at 17b. In the case of controlled cooling, the transport speed during water cooling or the amount of cooling water to be used is controlled by these thermometers so that the temperatures before and after cooling fall within a predetermined range. A width direction thermometer 18 is provided on the output side of the cooling device 13.
Is installed.

In the water-cooling device, an upper surface water-cooling nozzle 14 and a lower surface water-cooling nozzle 15 are arranged side by side in the transport direction, for example, ten at a time. The upper surface water-cooling nozzle 14 is of a laminar flow type, in which laminar cooling water in the form of a film is sprayed onto the surface of the steel plate to be water-cooled, and has a large heat transfer coefficient. Since the upper surface water-cooling nozzle 14 is arranged at a height of about 1 m from the steel plate surface so as not to collide even if the plate is deformed during cooling, a roller or a platen for restraining the steel plate is unnecessary. This type of cooling device is generally used for the control cooling device for thick plates and hot-rolled steel sheets.
It is advantageous in terms of cost and efficiency.

On the inlet side of the cooling device 13, a light reduction device 19 for reducing the steel plate lightly is installed. The light roll-down device 19 has a pair of upper and lower rolls 20 and 21, and the upper roll 20 is provided with an elevating device (not shown) so as to be able to retract upward when not in use. Roll 20, 2
1 has an uneven pattern. The light reduction device 19 may be a two-stage roll as shown in the figure, or may be a four-stage roll type or a leveler type composed of several roll rows.

[0043]

EXAMPLE A test was conducted to compare the cooling method of the present invention with a conventional cooling method. The test was performed on the rolling line shown in FIG. The rolls 20 and 21 of the light reduction device 19 in the same figure were provided with linear irregularities shown in FIG. Roll 20,
The roll diameter of No. 21 is 500 mm, and the height of the surface irregularities is 1
The distance between adjacent ridge vertices was set to 100 μm in both the circumferential direction and the axial direction of the roll.

In the example of the present invention, the roll was allowed to cool for about 15 seconds after rolling so as to produce a surface scale of about 15 μm, then lightly reduced by the light reduction device 19, and further cooled by the cooling device 13.

In Comparative Example 1, high pressure water of 150 kgf / cm ² was sprayed on the steel sheet after rolling by the descaling device 22 to remove the scale. Subsequently, the surface of the steel sheet was provided with irregularities having a height of 15 μm by light pressure reduction, and then cooled by the cooling device 13.

In Comparative Example 2, the roll was allowed to cool for about 15 seconds after rolling so as to form a surface scale of about 15 μm, and then cooled by the cooling device 13 without reducing the pressure.

The thickness of the steel sheet at the end of the finish rolling is 20.5 m
m, the target cooling start temperature is 750 ° C., and the target cooling stop temperature is 450 ° C. Further, the amount of reduction in the thickness of the plate due to the reduction by the light reduction device was 0.5 mm.

FIG. 4 is a graph showing the temperature distribution in the width direction after the cooling test. As shown in the figure, in the example of the present invention, the temperature deviation after water cooling was significantly reduced as compared with Comparative Examples 1 and 2. In addition, there was almost no rolling pattern on the steel sheet surface due to light rolling.

The difference between the present invention example and the comparative example 1 is the difference in the presence or absence of the surface scale when a light reduction is performed. Comparative Example 1 showed a slight improvement in cooling uniformity due to roughness of the steel sheet surface as compared with Comparative Example 2, but was inferior in cooling uniformity as compared to the present invention. In addition, a linear pattern remains on the surface of the steel sheet, and there was a problem in quality.

The difference between the present invention and Comparative Example 2 was the presence or absence of light reduction, and the conditions for the thickness of the surface scale were the same. Comparing Comparative Example 2 with the present invention example, it was found that mere generation of scale on the steel sheet surface did not contribute to uniform cooling.

As described above, it has been clarified that the method of the present invention can significantly reduce the temperature unevenness caused by controlled cooling and can stably produce the same.

The same effect can be obtained when the cooling method according to the present invention is applied not only to the cooling process of a thick steel plate but also to another steel material controlled cooling process such as a steel pipe or a shaped steel.

[0053]

According to the cooling method of the present invention, a high cooling rate can be obtained without evenly cooling a high-temperature thick steel plate. For this reason, it becomes easy to apply the controlled cooling of the thick steel plate. Moreover, there is no reduction in rolling efficiency and no increase in cost.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a behavior near a boundary between a steel plate and cooling water.

FIG. 2 is a conceptual diagram showing a distribution state of irregularities on a roll surface. FIG. 2A shows a case where point-like protrusions are randomly arranged, and FIG. FIG. 4C shows a case where the convex portions are arranged in a lattice shape, and FIG.

FIG. 3 is a schematic diagram showing a thick plate control cooling facility to which the cooling method of the present invention is applied.

FIG. 4 is a graph showing a temperature distribution in a width direction after a cooling test.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Steel plate 2 Cooling water 3 Steam film 4 Floating scale 5 Biting scale 6 Fin part 7 Steam bubble 8 Convex part 11 Plate rolling mill 12 Roller table 13 Cooling device 14 Upper surface water cooling nozzle 15 Lower surface water cooling nozzle 16a, 16b Thermometer 17a, 17b Thermometer 18 Width thermometer 19 Light pressure reduction device 20 Upper roll 21 Lower roll 22 Descaling device H Convex height P Convex interval

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Michiharu Hariki 4-5-33 Kitahama, Chuo-ku, Osaka City Sumitomo Metal Industries Co., Ltd.

Claims (2)

[Claims] In a cooling method of rolling a thick steel plate and cooling it with water, a scale layer having a thickness of 10 to 30 μm is formed on the rolled steel plate, and the surface has irregularities with a height of 10 to 30 μm. A method for cooling a thick steel plate, wherein the steel plate is rolled down and then cooled with water. 2. The method for cooling a thick steel plate according to claim 1, wherein the distance between the apexes, ridges, or tops of the protrusions adjacent to each other is 2 to 10 times the height of the protrusions. .