JP3709706B2 - Thick steel plate cooling method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for cooling a thick steel plate, and more particularly to an accelerated cooling method in which a thick steel plate immediately after rolling is uniformly and rapidly cooled and heat-treated.
[0002]
[Prior art]
In recent years, in the manufacturing process of thick steel plates, high-strength and high-toughness steel plates are obtained by performing controlled rolling to obtain predetermined characteristics by controlling the rolling amount and rolling temperature for each rolling pass and then water-cooling (accelerated cooling) the steel plates. The technology to obtain is widely performed. By combining controlled rolling and accelerated cooling, it has become possible not only to reduce additive elements and greatly reduce production costs, but also to produce thick steel plates with excellent weldability.
[0003]
In accelerated cooling, since cooling water is sprayed from a cooling nozzle onto the surface of a high-temperature steel plate at 200 to 900 ° C., boiling heat transfer occurs on the surface of the steel plate. Because of this phenomenon, a cooling rate several tens to several hundred times higher than that of air cooling or the like 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 can be manufactured as described above. it can.
[0004]
However, this boiling heat transfer phenomenon has a small heat transfer coefficient in the film boiling state and a large heat transfer coefficient in the nucleate boiling state. When the steel plate temperature is high, film boiling is the main component, but when the temperature is low, transition to nucleate boiling tends to increase the heat transfer coefficient. The temperature at which transition from film boiling to nucleate boiling varies depending on the condition of the momentum of the cooling water and the surface condition of the steel sheet, and is not constant, so when transitioning from film boiling to nucleate boiling in part of the steel sheet, the cooling rate increases locally. Heat transfer becomes very unstable. Therefore, it is difficult to uniformly control the cooling rate of the entire steel plate, and large temperature unevenness is likely to occur.
[0005]
The occurrence of such temperature irregularities not only causes variations in the mechanical properties of the product, but also causes deformations such as ear waves and medium elongation, and flattening and heat treatment must be performed again after the rolling process. Reduce efficiency.
[0006]
In recent years, it has become clear that unevenness in the surface properties of the steel sheet, such as surface roughness, is a major cause of temperature unevenness. Unevenness in scale thickness distribution on the steel sheet due to uneven heating in the heating furnace and temperature unevenness during rolling, and unevenness in the surface roughness of the steel sheet occurs when the degree of progress of surface wear of the rolling roll is partially different. When this is cooled, the mechanism of the transition from film boiling to nucleate boiling varies locally and the cooling rate varies greatly. In order to solve the above problems, for example, the following countermeasures have been tried.
[0007]
For example, Japanese Patent Publication No. 63-54046 discloses that when a metal tube is cooled 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 to 500 ( There is disclosed a technique for cooling by forming a uniform rough surface with a crest of 50 to 500 μm.
[0008]
In JP-A-1-284418, the surface roughness after rolling of the material to be rolled is adjusted to be not less than the vapor film thickness of the cooling medium (about 10 μm for water) and not more than 30 μm in terms of Rz, and after rolling. A technique for performing cooling after accelerating oxidation of a new surface is disclosed.
[0009]
Japanese Patent Laid-Open No. 2-70017 discloses a technique for cooling a heat transfer surface of a metal material by providing a uniform roughness pattern composed of a large number of linear grooves having a groove depth of 25 μm or less in terms of Rz. Is disclosed.
[0010]
[Problems to be solved by the invention]
The technique disclosed in the publication has the following problems.
(a) The technology disclosed in the three publications described above is a strong cooling type cooling device in which a film boiling state is relatively difficult to form, that is, a steel plate is restrained by a roller or a platen (a jig that pushes 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 body 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 in which the distance between the cooling nozzle and the steel plate is 1 m or more without restriction of a roller or the like, the film boiling region is still locally. And large temperature unevenness is likely to occur.
[0011]
(b) The technique disclosed in Japanese Patent Application Laid-Open No. 2-70017 provides regular irregularities on the steel surface in a state where there is no scale or is covered with a very thin scale. Since the irregularities are relatively conspicuous, the surface appearance of the product deteriorates and may not be applied depending on the product / use.
[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 examinations and obtained the following findings.
[0014]
(a) When a scale with a certain thickness is formed on the surface of a steel plate, the scale is cracked by water cooling, and the starting point of nucleate boiling is generated at the tip of the crack, thereby increasing the heat transfer coefficient.
[0015]
In addition, the fragments from the cracked scale are peeled off from the steel sheet and dispersed in the cooling water, thereby destroying the vapor film between the cooling water and the steel sheet and increasing the heat transfer coefficient between the cooling water and the steel sheet. is there. However, when all the scale fragments are peeled off from the steel sheet, the heat transfer coefficient to the steel sheet is lowered again, or the heat transfer coefficient is locally uneven due to the flow of the cooling water.
[0016]
(b) If a part of the scale is bitten into the steel plate by light pressure, it can be the starting point of nucleate boiling. The heat transfer coefficient can be adjusted by adjusting the distribution density of the scale to be bitten. Further, if the depth at which the scale is bited is set to a certain limit or less, there is no problem of scale biting and no noticeable pattern is generated on the surface of the steel plate.
[0017]
The present invention has been made on the basis of the above findings, and the gist thereof is as follows (1) to (2).
(1) In a cooling method in which a thick steel plate is rolled and then cooled with water, a roll having a thickness of 10 to 30 μm on the rolled steel plate and then having a height of 10 to 30 μm on the surface is provided. A method of cooling a thick steel plate, characterized by cooling with water after reduction.
[0018]
(2) The method for cooling a thick steel plate according to (1), wherein the unevenness of the protrusions adjacent to each other is 2 to 10 times the height of the unevenness. .
[0019]
DETAILED DESCRIPTION OF THE INVENTION
It is an object of the present invention to form a scale layer having a predetermined thickness on the surface of a steel plate before water cooling, and fix a part of the scale to the surface of the steel plate as a starting point for nucleate boiling. As a result, there is no need to use a strong cooling type cooling device that constrains the steel plate with a roller or a platen and closes the cooling nozzle to inject high-pressure cooling water. Cooling can be performed uniformly and stably with a relatively weak cooling device that secures 1 m or more. The principle of the present invention will be described below.
[0020]
In a high temperature range of 700 to 800 ° C. where cooling is started, the film is normally in a film boiling state. A steam film is formed between the steel plate surface and the cooling water, and heat transfer is performed only by radiant heat transfer through the steam film, so the heat transfer coefficient is small. When the cooling proceeds and the temperature decreases, the vapor film is destroyed at a certain temperature (hereinafter referred to as a quench temperature). The transition from the quench temperature to the nucleate boiling state is such that the cooling water and the steel plate come into direct contact and heat transfer is performed, and the heat transfer coefficient is rapidly increased to cause rapid cooling. The destruction process of this vapor film is very unstable, and there are a part where the vapor film is destroyed and locally cooled due to variations in the surface roughness of the steel sheet, and a part where the vapor film is maintained and cooling is delayed. Therefore, temperature unevenness is likely to occur.
[0021]
Even in a high temperature range of 700 to 800 ° C., in the strong cooling type cooling device that breaks the vapor film by colliding with high-pressure jet water, the quench temperature is raised, and the entire surface of the steel sheet is started rapidly to cool uniformly. Can do. However, in a general weak-cooling type cooling device, the vapor film is not easily broken, and the cooling unevenness cannot be improved.
[0022]
In the present invention, first, the effect is 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. Second, the so-called fin effect lowers the temperature of the tip of the fin portion, and aims at the effect that the starting point of nucleate boiling is formed below the quench point. And a scale with good wettability is used as a fin part.
[0023]
As a means for realizing the above-described effect, a scale having a predetermined thickness is generated on the surface of the steel sheet 1 after rolling, and the scale is cracked by light reduction, and at the same time, a part of the scale is bitten into the steel sheet. Next, cooling water is sprayed, the scale having cracks is peeled off, and dispersed in the cooling water.
[0024]
FIG. 1 is a schematic diagram showing the behavior in the vicinity of the boundary between a steel plate and cooling water. In the figure, the cooling water 2 is separated from the steel plate 1 by a vapor film 3. The pieces of scale peeled from the steel plate 1 are dispersed in the cooling water 2 as the floating scale 4. In the vicinity of the steel plate 1, the floating scale 4 enters the vapor film 3, and the fin portion 6 of the floating scale 3 becomes the starting point of nucleate boiling, and the vapor film is caused by nucleate boiling in the vapor film 3 as shown in FIG. It is destroyed and the heat transfer coefficient in the high temperature range (above the quench point) is improved.
[0025]
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 steel plate surface and becomes the bite scale 5 as shown in FIG. The fin part 6 becomes a starting point of nucleate boiling, and the vapor bubble 7 produces | generates. In this state, the steel sheet is directly cooled, and cooling is further promoted.
[0026]
Aiming at the first effect, when the scale is cracked, if the size of the scale fragment is made equal to the thickness of the vapor film to several times the thickness of the vapor film, the floating scale 4 becomes a vapor film. It becomes easy to enter and the effect of vapor film destruction can be increased. In order to make a uniform crack of the above-mentioned size in the scale, it can be realized by imprinting a uniform pattern on the surface of the roll that is lightly pressed.
[0027]
Aiming at the second effect, even when a part of the scale is fixed to the surface of the steel sheet, a uniform unevenness distribution is obtained by providing a regular uneven pattern on the surface of the roll that is lightly pressed. Can do. If scale debris penetrates too deep into the steel sheet, it will become scale wrinkles, so the thickness of the scale must be kept within an appropriate range, and the depth of irregularities must be kept within an appropriate range.
[0028]
Based on the above consideration, the reasons for limiting the conditions of the present invention will be described below.
The thickness of the scale generated after rolling is 10 to 30 μm. If the thickness of the scale is less than 10 μm, even if the scale is cracked by light pressure, it is difficult to peel off when cooling water is injected, and the dispersion density when the peeled scale is dispersed in cooling water is This is because the first effect is reduced due to the small size and the size of the scale fragments being smaller than the thickness of the vapor film, and the vapor film destruction effect is small. If the thickness of the scale is larger than 30 μm, it takes a long time to generate a scale having a predetermined thickness after rolling, the rolling efficiency is lowered, or the temperature of the steel sheet is lowered, so that a predetermined cooling start temperature cannot be secured. And, due to light pressure, the scale deeply digs into the steel sheet and becomes scale flaws. A more preferable scale thickness is 15 to 25 μm.
[0029]
In order to generate the scale, by cooling in the air for 10 to 15 seconds after rolling, the predetermined scale thickness is ensured without affecting the rolling efficiency while ensuring the predetermined cooling start temperature. be able to.
[0030]
In order to secure the thickness of the scale, when the cooling time is long and there is a problem in rolling efficiency or there is a problem of a decrease in the steel sheet temperature, accelerated oxidation may be performed after rolling. As a method of accelerated oxidation, there is a method of spraying an oxygen-enriched gas on the surface of a steel plate or heating with a direct fire burner. Further, when the cooling start temperature, the scale thickness, and the like are significantly different between the front end portion and the rear end portion of the steel plate, the rear end portion of the steel plate may be heated in a reducing atmosphere.
[0031]
In the normal cooling method, the steel sheet after finish rolling is transported to the cooling device in a time of less than 10 seconds. Therefore, the scale generation thickness is not sufficient, and the cooling method of the present invention actively cools down after rolling. Need to be adjusted. Therefore, the cooling method of the present invention is difficult to apply in a continuous process such as a hot-rolled steel sheet, and is limited to application in a process that can be processed in a single unit such as a thick steel sheet.
[0032]
The unevenness of the roll surface that is lightly pressed is 10 to 30 μm. If the height of the unevenness is less than 10 μm, it is impossible to effectively crack the scale having the thickness of 10 to 30 μm, and the scale corresponding to the convex portions of the roll unevenness is effectively cut into the steel plate. It is because it cannot be made. If the scale is pushed down to force the scale into the steel sheet, the scale will bite over the entire surface of the steel sheet, resulting in an inappropriate fin distribution and the possibility of 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 will increase, and the scale will dig deeply into a scale wrinkle. A more preferable height of the unevenness is 15 to 25 μm.
[0033]
The uneven pattern of the lightly pressed roll may be random, but preferably has a certain regularity. That is, according to the first object, since the scale fragments that peel according to the cracks under light pressure are larger than a certain size and are aligned, the destruction effect of the film boiling is enhanced. This is because a uniform heat removal effect can be obtained when the fin portions are substantially uniformly distributed on the steel plate surface.
[0034]
Fig. 2 is a conceptual diagram showing the distribution of irregularities on the roll surface. Fig. 2 (a) shows the case where dot-like projections are randomly arranged, and Fig. 2 (b) shows a point-like projection with a constant shape. In the case where the portions are arranged in a lattice shape, FIG. 10C shows the case where the parallel line-shaped convex portions having a fixed shape are arranged. In Fig. (A), the shape of the convex portion is illustrated as a cone, in Fig. (B) as a quadrangular pyramid, and in Fig. (C) as a triangular ridge, the regularity of the shape of one convex portion is not so important. Also, the shape of the recess is not so important. However, the distribution density of the convex portions is important as described below.
[0035]
It is desirable that the distance P between the vertices of adjacent convex portions is 2 to 10 times the size of the concave and convex portions. For example, in the case of (a) or (b) in the figure, the distance P is represented by the distance between the vertices of a certain convex part and the nearest neighboring convex part. As shown in FIG. In the case of a part, it is represented by the distance between ridge lines.
[0036]
If the interval P between the protrusions is less than twice the height H of the protrusions, the cracks generated in the scale generated on the surface of the steel sheet will be fine, and the floating scale aiming at the first effect will be too fine. This is because the distribution density of the bite scale fixed on the steel plate becomes too large and is rapidly cooled locally. If the distance P between the protrusions is larger than 10 times the height H of the protrusions, when the scale cracked by light pressure becomes a floating scale, the size becomes too large, and the bite scale fixed on the steel plate This is because the distribution density is too small to effectively cool, and the surface pressure of the convex portion increases, the bite scale deeply penetrates, and there is a risk of scale wrinkles and indentations on the steel sheet. A more preferable range of the protrusion interval P to the protrusion height H is 3 to 8 times.
[0037]
In actuality, it is easy to process the roll in a lattice shape as shown in FIG. 2B or a linear shape as shown in FIG. In particular, if the ridgeline direction is set to the roll circumferential direction as shown in FIG. The cross-sectional shape of the protrusion may be a quadrangle, trapezoid, triangle or the like, but if the shape of the apex is sharp, the wear is fast, so the radius of curvature of the apex is about 3 to 10 μm, or the top side is 3 to 10 μm in the case of a trapezoid It is desirable to be front and back.
[0038]
The light reduction may be performed by a thick plate rolling mill having predetermined irregularities on the work roll after generating a scale having a predetermined thickness, or may be performed by a rolling mill dedicated to light reduction separately from the thick plate rolling mill. Good. However, even if the plate rolling mill has the specified irregularities, the irregularities are worn quickly during rolling of each pass, and the irregularities are crushed by the surface pressure between the backup roll and work roll. It is desirable to provide a machine.
[0039]
Although slight unevenness is generated on the surface of the steel plate by light pressure, it can be easily smoothed by light pressure by hot leveling usually performed after water cooling, and the appearance of the final product is not impaired.
[0040]
FIG. 3 is a schematic view showing a thick plate controlled cooling facility to which the cooling method of the present invention is applied. The steel plate 1 rolled to the target plate thickness by the thick plate mill 11 reaches the cooling device 13 after a predetermined cooling time has elapsed to generate a predetermined scale thickness while being conveyed on the roller table 12. Next, the steel plate 1 is water-cooled on the upper and lower surfaces while passing through the cooling device 13. The steel plate temperatures before and after cooling are measured by upper and lower thermometers 16a and 16b provided on the inlet side of the cooling device 13 and upper and lower thermometers 17a and 17b provided on the outlet side, respectively. In the case of controlled cooling, these thermometers control the conveyance speed during water cooling or the amount of cooling water to be used so that the temperatures before cooling and after cooling fall within a predetermined range. A width direction thermometer 18 is installed on the exit side of the cooling device 13.
[0041]
In the water cooling device, for example, 10 upper surface water cooling nozzles 14 and lower surface water cooling nozzles 15 are arranged side by side in the transport direction. The upper surface water-cooling nozzle 14 is a laminar flow type, in which film-like laminar flow cooling water is jetted onto the surface of the steel sheet to cool the water, and has a high 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 control cooling devices for thick plates and hot-rolled steel plates, and can transport steel plates at high speed, which is advantageous in terms of equipment cost, cost, and efficiency.
[0042]
On the entry side of the cooling device 13, a light reduction device 19 for lightly reducing the steel plate is installed. The light reduction 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 retracted upward when not in use. The rolls 20 and 21 are provided with an uneven pattern. In addition to the two-stage roll as shown in the figure, the light reduction device 19 may be a four-stage roll type or a leveler type constituted by several roll rows.
[0043]
【Example】
A test for comparing the cooling method of the present invention with the cooling method according to the conventional method was conducted. The test was conducted 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 as shown in FIG. The roll diameters of the rolls 20 and 21 were 500 mm, the height of the irregularities on the surface was 15 μm, and the interval between adjacent convex vertices was 100 μm in both the circumferential direction and the axial direction of the roll.
[0044]
In the example of the present invention, the steel sheet was allowed to cool for about 15 seconds after rolling so as to generate a surface scale of about 15 μm, then lightly reduced by the light reduction device 19 and further cooled by the cooling device 13.
[0045]
In Comparative Example 1, 150 kgf / cm ² of high-pressure water was sprayed onto the rolled steel plate 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, and then cooled by the cooling device 13.
[0046]
In Comparative Example 2, the steel sheet was allowed to cool for about 15 seconds after rolling so as to generate a surface scale of about 15 μm, and then cooled by the cooling device 13 without light reduction.
[0047]
The steel plate thickness at the end of finish rolling is 20.5 mm, the target cooling start temperature is 750 ° C., and the target cooling stop temperature is 450 ° C. In addition, the reduction in plate thickness due to the reduction by the light reduction device was set to 0.5 mm.
[0048]
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. Also, there was almost no reduction pattern on the surface of the steel sheet due to light reduction.
[0049]
The difference between the inventive example and Comparative Example 1 is the difference in the presence or absence of a surface scale when light reduction is applied. Compared to Comparative Example 2, Comparative Example 1 showed a slight improvement in cooling uniformity due to the roughening of the surface of the steel sheet, but the cooling uniformity was inferior compared to the inventive example. Moreover, the linear pattern remained on the steel plate surface, and there was a problem in quality.
[0050]
The difference between the inventive example and Comparative Example 2 was the presence or absence of light pressure, and the condition of the surface scale thickness was the same. Comparing Comparative Example 2 with the inventive example, it was found that simply generating a scale on the steel sheet surface does not contribute to uniform cooling.
[0051]
As described above, it has been clarified that the method according to the present invention can greatly improve the temperature unevenness caused by controlled cooling and can be stably manufactured.
[0052]
The cooling method according to the present invention is not limited to the cooling process of thick steel plates, and the same effect can be obtained when applied to other steel material controlled cooling processes such as steel pipes and shaped steels.
[0053]
【The invention's effect】
According to the cooling method of the present invention, a high cooling rate can be obtained without unevenly cooling a high-temperature thick steel plate. For this reason, application of the controlled cooling of the thick steel plate becomes easy. Moreover, there is no reduction in rolling efficiency or increase in cost.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing the behavior in the vicinity of the boundary between a steel plate and cooling water.
FIG. 2 is a conceptual diagram showing the distribution of unevenness on the roll surface. FIG. 2 (a) shows a case where dot-like convex portions are randomly arranged, and FIG. In the case where the convex portions are arranged in a lattice shape, FIG. 10C shows the case where the parallel-shaped convex portions having a fixed shape are arranged.
FIG. 3 is a schematic diagram showing a thick plate controlled cooling facility to which the cooling method of the present invention is applied.
FIG. 4 is a graph showing the temperature distribution in the width direction after the 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 Thick plate 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 direction thermometer 19 Light reduction device 20 Upper roll 21 Lower roll 22 Descaling device H Convex height P Convex spacing

Claims (2)

In a cooling method in which a thick steel plate is rolled and then cooled with water, a scale layer having a thickness of 10 to 30 μm is formed on the rolled steel plate, and then rolled down with a roll having a height of 10 to 30 μm on the surface. A method for cooling a thick steel plate, characterized by cooling with water. The unevenness | corrugation of Claim 1 is a cooling method of the thick steel plate characterized by the space | interval of the vertex of an adjacent convex part, a ridgeline, or a top part being 2 to 10 times the height of an unevenness | corrugation.

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