JPH10272511A - Water cooling method for high temperature steel - Google Patents

Abstract

(57) [Problem] To provide a method for rapidly and uniformly cooling a high-temperature steel material using cooling water. A method for generating a red scale on a surface of a high-temperature steel material and cooling the same, and a method for producing a steel material having a thickness of 20-100 mm
A method of hot rolling with a μm scale attached thereto, rolling at 900 ° C. or lower, and then water cooling.

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 high-temperature steel material, and more particularly, to a method for cooling a high-temperature steel material quickly and uniformly.

[0002]

2. Description of the Related Art If a high-temperature steel material is subjected to quenching, controlled cooling, or the like, high strength and high toughness can be obtained even with a low alloy steel, so that a steel material having good weldability and high economic efficiency can be manufactured. When such a treatment is performed, it is important that a uniform cooling can be achieved while a high cooling rate is obtained. When rapidly cooling a high-temperature steel material using water, it is sometimes difficult to cool quickly and uniformly because of the boiling phenomenon of water.

[0003] When cooling water comes into contact with a high-temperature steel material, a vapor film is instantaneously generated on the surface of the steel material. When a vapor film is generated, the heat retained in the steel material moves to the cooling water side via the vapor layer, so that the cooling rate is low. This condition is called film boiling. When the temperature of the steel material decreases and a vapor film is hardly generated, the frequency of direct contact between the steel material and the cooling water gradually increases, and finally, the solid-liquid contact state is continuously generated. In the solid-liquid contact state, the retained heat of the steel material is efficiently transferred to the cooling water, so that the cooling rate is increased. This state is called transition boiling or nucleate boiling.

[0004] In the water cooling method, the cooling rate in a high temperature region at the initial stage of cooling is relatively slow because of the film boiling phenomenon.
However, if there is a part that cools quickly for some reason,
Since that part shifts to transition boiling, the cooling rate is further increased. Therefore, when rapidly cooling a high-temperature steel material, cooling is likely to occur when cooling in a state where both boiling regions are mixed. If cooling unevenness occurs, quality variation and product shape deteriorate, so that the application of rapid cooling is restricted. Cooling unevenness is particularly remarkable in a thick steel material that has a large amount of retained heat and maintains a high temperature for a long time.

[0005] Improvements for increasing the cooling rate of steel materials and improving the cooling accuracy (medium accuracy of cooling stop temperature) have been attempted for a long time. Scale present on the surface of hot steel also affects cooling. For this reason, in recent years, in addition to the study of the cooling medium (for example, a cooling device for uniformly applying high-pressure water to the surface of the steel sheet and a method of using the cooling device), a cooling control that takes into account the surface properties of the steel material, particularly the scale condition. A method has been proposed.

Japanese Patent Laid-Open Publication No. Hei 6-79324 predicts the thickness of a scale layer on the surface of a steel sheet. If the scale immediately before cooling is thick, cooling is performed in a weakening direction. A method for manufacturing a controlled cooling steel sheet in which the conditions are modified to improve the accuracy of the cooling stop temperature is disclosed.
However, the thickness of the scale varies not only in the length direction but also in the width direction of the steel sheet, and there is still a problem in achieving uniform cooling by this method.

Japanese Patent Application Laid-Open No. Hei 6-330155 discloses a method of controlling the cooling rate and cooling stop temperature of a thick steel plate by adjusting the pressure of high-pressure water for removing scale to control the thickness of the scale. A method is disclosed for controlling the heat transfer coefficient. However, it is difficult to accurately control the thickness of the scale by this method, and if the pressure of the high-pressure water for removing the scale is adjusted, conversely, the scale adhesion state becomes uneven, and as a result, the temperature becomes uneven. There is also a risk.

[0008]

An object of the present invention is to provide a method for rapidly and uniformly cooling a high-temperature steel material using cooling water.

[0009]

The gist of the present invention resides in a water cooling method for a high-temperature steel material described in the following (1) and (2).

(1) A water cooling method for a high-temperature steel material in which a red scale is uniformly generated on the surface of a high-temperature steel material and then water-cooled.

(2) The thickness of the steel material is 20 to 100 μm.
A hot-rolling method for hot-rolling a high-temperature steel material, in which hot rolling is performed by attaching the scale described above, rolling is completed at 900 ° C. or lower to uniformly generate a red scale, and then cooled using water.

The present inventors investigated the effect of the scale properties of the steel sheet surface on the cooling performance in water cooling. As a result, when hot-rolling a steel sheet with the appropriate thickness scale attached, a red scale is formed on the entire surface of the steel sheet, which is usually a film boiling area, so the cooling rate is fast even in high temperature areas where the cooling rate is slow It has been found that a cooling rate can be obtained.

Here, the red scale is a red-colored scale formed on the surface of a hot-rolled steel material. The main composition of the red scale is Fe, the main component of hematite.
₂ O ₃ (hematite).

The following describes the experimental results that led to this finding.

C: 0.13% by weight, Si: 0.45% by weight
%, Mn: 1.02%, P: 0.013%, S: 0.0
12% thick, 145mm wide, 1 length containing 05%
A 45 mm steel plate was heated to 1200 ° C., and the holding time at 1200 ° C. was adjusted to generate a scale having a predetermined thickness on the steel plate surface. Remove the steel sheet from the heating furnace and 900 ℃
The steel sheet with the scale attached is rolled with a rolling reduction of 5% using a laboratory rolling mill.
The mixture was allowed to cool to 0 ° C., and then cooled with water using a spray nozzle to room temperature (steel plate A). For comparison, a steel sheet having the same chemical composition and dimensions as above was used in the same manner as above.
It was kept in a heating furnace heated to 0 ° C., and a scale having almost the same thickness was formed on its surface. The steel sheet was taken out of the heating furnace, allowed to cool to 800 ° C. without rolling, and then cooled to room temperature by water cooling using a spray nozzle under the same conditions as for the steel sheet A (steel sheet B). In each steel plate, a thermocouple was previously embedded at a depth of 3 mm from the surface, and the temperature history of the steel plate during water cooling was measured.

FIG. 3 shows the temperature history of each steel sheet after the start of cooling. In FIG. 3, the solid line shows the temperature history of the steel sheet A rolled and cooled with the scale attached, and the broken line shows the temperature history of the steel sheet B cooled without rolling. On the steel sheet surface after cooling, the entire surface of the steel sheet A is covered with a red-based scale (hereinafter simply referred to as “red scale”), and the gray or silver-white scale (hereinafter simply referred to as “simply referred to as“ red scale ”) that is normally recognized in the steel sheet B. Normal scale ")
Was covered with. As shown in FIG. 3, the cooling rate of the steel sheet A in the film boiling region of about 450 ° C. or higher is about twice that of the steel sheet B.

In general, the scale layer formed in the atmosphere at 560 ° C. or higher has wustite (Fe
O), magnetite (Fe ₃ O ₄ ) in the middle layer, and hematite (Fe ₂ O ₃ ) in the surface layer. Wustite is the thickest of each layer. For example, in a temperature range of 700 ° C. or more, FeO is 95% of the thickness of the entire scale layer.
%, Fe ₃ O ₄ accounts for ₄ to 5%, and Fe ₂ O ₃ accounts for 1 to ₂ %.
%. At 1300 ° C. or higher, the dissociation pressure of Fe ₂ O ₃ becomes 0.21 oxygen partial pressure in air.
Since it exceeds atm, Fe ₂ O ₃ disappears. Further, since the decomposition temperature of Fe ₂ O ₃ decreases when the oxygen partial pressure on the scale surface decreases, it is said that, for example, Fe ₂ O ₃ is not formed in steam.

Thus, in normal rolling, Fe ₂ O ₃
Is hardly generated and red scale is hardly generated. Exceptionally, when there is a de-scaling failure before the start of rolling, the red scale may be partially generated in a strip-like shape in the rolling direction. However, conventionally, in hot rolling (particularly, a finishing rolling mill for producing a hot-rolled steel sheet), the steel sheet is subjected to finish rolling with descaling in order to prevent surface defects from occurring. It is not covered with red scale. On the other hand, when hot rolling is performed with the scale attached as described above, a steel sheet in which a red scale is uniformly generated can be obtained. The reason for the occurrence of red scale is not clear, but the scale before rolling, which is mainly composed of FeO, is broken into powder by being rolled at a temperature of 900 ° C. or less, oxidation is promoted, and Fe ₂ O is promoted. than become ₃ is presumed that or not.

The reason why the cooling rate of the steel sheet on which the red scale is generated is high is not clear, but is presumed as follows. Since the red scale surface layer is broken into powder by rolling,
The surface roughness is coarser than the normal scale. Therefore, when the cooling water collides with the scale surface, the contact area between the cooling water and the scale surface increases, and cooling of the surface layer of the scale is promoted. Further, the inside of the red scale has much more voids than the normal scale. For this reason, the resistance to heat transfer in the scale layer increases, and the temperature of the outermost surface of the scale layer significantly decreases at the moment when the cooling water collides. As a result of these two phenomena, solid-liquid contact with cooling water is promoted at the surface portion of the red scale. For this reason, it is presumed that the cooling capacity is increased when the red scale is generated.

It is known that the cooling capacity increases as the thickness of the scale increases and the thermal resistance increases. However, according to the method of the present invention, the cooling can be effectively performed without increasing the thickness of the scale. Speed can be increased.

The present invention has been completed based on the new finding that a steel material having a red scale on the surface of a steel sheet can rapidly and uniformly cool as described above.

[0022]

Embodiments of the present invention will be described below.

In the present invention, in a method of cooling a high-temperature steel material using water, a red scale is generated on the entire surface of the steel material to be cooled, and the steel material is cooled.

The red scale to which the present invention is applied is a red-yellow-red scale which is observed by visually observing the surface of a steel material after hot rolling and cooling. In the present invention, the color of the red scale is defined as follows in the Munsell color chart system. That is, the hue H defined in the Munsell color chart system is 1.5R-1.
0R (red) and 0YR to 10YR (yellow-red), lightness V
Is in the range of 2.0 to 6.2, and the saturation C is in the range of 0.7 to 5.0. Here, H: 1.5R is a red color close to reddish purple, and H: 10YR is a yellow red color close to yellow. The lightness V is an ideal black value of 0 and a white value of 10, and the difference between them is quantified so that the difference in the sense of brightness is a uniform step. The chroma C is 0 for an achromatic color. Numerical values are expressed as the color increases.

When the area ratio occupied by the red scale is at least 80% of the surface of the steel material, the effect of increasing the cooling rate aimed at by the present invention can be obtained. This is because even if there is a portion where the red scale is partially left and there is a portion where the cooling rate is low, the ratio is 2
If it is less than 0%, the influence on the cooling rate of the entire steel material is small.

In order to generate red scale on the steel surface,
For example, a method of adjusting the chemical composition of the steel, such as increasing the Si content of the steel, is also possible. However, a normal scale of an appropriate thickness is attached to the surface of the steel material, hot rolling is performed without removing this scale, and rolling is completed at 900 ° C or less to generate a red scale. It is suitable as a method for stably obtaining. When the red scale is generated by this method, if the temperature at which the hot rolling is completed is too low, the deformation resistance increases and the rolling becomes difficult. Therefore, the hot rolling is preferably completed at 700 ° C. or more. The thickness of the normal scale on the steel sheet surface before rolling is preferably in the range of 20 μm to 100 μm. When the thickness is less than 20 μm, the scale is hardly crushed at the time of rolling and a red scale is hardly generated.
The normal scale thickness of the steel sheet surface before rolling is 10
If the thickness exceeds 0 μm, the scale peels off during rolling, making it difficult to uniformly generate red scale. Normal scale thickness of the steel sheet surface before rolling is 20 ~ 50μm
Is more preferable.

The relationship between the scale growth rate, temperature and time is as follows:

X ² = K _p · t K _p = K ₀ · exp (−Q / RT) where X: weight increase due to oxidation of steel, t: oxidation time,
K _P : parabolic rate constant, K ₀ : constant, T: absolute temperature of steel, R: gas constant If the coefficients of the formula are experimentally obtained for a steel having a typical chemical composition, the final descaling can be performed. By estimating the temperature and time until the rolling after the completion, the thickness of the scale immediately before the rolling can be estimated. The scale thickness is
The thickness can be adjusted to a predetermined value by the temperature (T) of the steel, the time (t) until rolling after descaling, and the like.

Although the rolling reduction in the final rolling is not particularly specified, it is preferable to reduce the rolling reduction by 5% or more in order to sufficiently crush the scale on the steel sheet surface. In order to generate a uniform red scale on the entire surface of the steel sheet, it is more preferable to perform rolling at a rolling reduction of 10% or more. The upper limit of the rolling reduction may be any high. When rolling is continuously performed by a plurality of rolling mills as in a hot strip mill, the total rolling reduction from the entry side to the exit side of the rolling mill group may exceed 90% in some cases.

After the final rolling, water cooling is performed. This cooling method is optional, and may be any of a commonly used laminar flow method, high-pressure spray method, and mist cooling method which is a mixture of high-pressure air and cooling water. The time from the end of the final rolling to the start of cooling is not particularly limited, and may be a condition usually applied.

The cooling method of the present invention can be applied to both ordinary steel and special steel. In particular, steel containing 0.2% or more of Si is preferable because a red scale is easily generated. Si
More preferably, the content is 0.5% or more.

Conventionally, red scale is often locally generated in the form of a band on the surface of a steel material, which impairs the appearance of the product and causes uneven cooling. In the present invention, the red scale is formed almost uniformly on the surface of the steel material and cooled. Therefore, the appearance quality is improved as compared with the conventional steel material in which the red scale is partially formed.

According to the method of the present invention, rapid cooling in a high temperature range is possible, and the entire steel material is relatively uniformly cooled.
Therefore, the present invention is preferably applied to hot-rolled steel sheets, thick plates, and other hot-rolled steel materials that are used as materials for welded structures and the like that require mechanical properties rather than appearance quality. The form of the steel may be any form such as a steel plate, a section steel, a section steel and the like. In particular, it is suitable for manufacturing a hot-rolled steel sheet in which red scale is easily generated uniformly. If necessary, after cooling, a treatment such as pickling may be applied to remove the red scale before use.

[0034]

【Example】

(Example 1) A hot-rolled steel sheet was rolled using a continuous hot rolling mill. The hot rolling mill used includes a rough rolling mill, a finishing rolling mill, a cooling zone, and a winding machine. In front of the rough rolling mill and the finishing rolling mill, there is a descaling device using high-pressure water.
The finishing mill consists of 7 stands. An infrared radiation thermometer for measuring the surface temperature of a substantially central portion in the width direction of the steel sheet is provided between the exit side of the finishing mill and the cooling zone.
Further, an infrared radiation thermometer capable of measuring a temperature distribution in a width direction of the steel plate is provided on an entrance side of the winding machine. Measure the surface temperature of the steel sheet with a thermometer installed in the middle of the cooling zone, and if the temperature of the steel sheet is outside the target intermediate temperature, set the temperature after the intermediate thermometer so that the target winding temperature can be obtained. Is adjusted.

[0035] The chemical composition of the steel used is C: 0.
07%, Si: 0.78%, Mn: 1.49%, P:
0.01%, S: 0.005%, residual Fe and unavoidable impurities. Two steel slabs of this chemical composition were descaled with high-pressure water according to a usual method and then rough-rolled. The roughly rolled steel was transported by a transport roll to a position before the finish rolling mill. One of the steels was rolled by a finish rolling mill without descaling before the finish rolling mill, with the scale generated during the transportation being attached to the surface (example of the present invention). Rolling dimensions are 2.9mm thick,
The width was 835 mm. The thickness of the scale on the surface of the steel at the side of the finish rolling mill was predicted to be 50 μm by the above-mentioned prediction formula which was obtained in advance.

Another slab of steel having the same chemical composition was descaled and coarse-rolled in accordance with a usual method, subjected to descaling with high-pressure water on the entrance side of a finishing mill, and finished and rolled to the same size (comparative example). . The exit speed of the finish rolling is 570 mpm and the exit temperature is 830 ° C. for all the steel sheets. Water cooling by the laminar method is started 1.5 seconds after leaving the finish rolling mill, and the target temperature is cooled to 450 ° C. It was wound into a coil.

A blackish red-brown scale was formed on the entire surface of the steel sheet of the present invention. The reddish-brown scale is generated between the start of finish rolling and the entrance of the cooling device. The color is Munsell color chart system, hue H: 7.2 to 10R and 0 to 3.1YR, lightness V:
3.5-5.5, chroma C: 0.8-2.3. The scale of the steel sheet surface of the comparative example was gray or silver-white, and no red scale was generated.

The temperature in the middle of the cooling zone when rolling and cooling according to the conditions of the present invention was 590 ° C., and the temperature in the middle of the cooling zone of the steel sheet of the comparative example was 630 ° C. The comparative example was 40 ° C. higher than the method of the present invention. Both are cooled under the same conditions up to the position of the intermediate thermometer in the cooling zone.
Therefore, according to the method of the present invention, it was found that the cooling rate during this period was 20% faster than that of the comparative example. Also, 450
In order to cool down to ° C., in the comparative example, rapid cooling had to be performed in which the amount of cooling water in the cooling zone after the intermediate thermometer was increased by 15% compared to the present invention.

1 and 2 show the temperature distribution in the width direction of the steel sheet measured immediately before the winder. FIG. 1 shows the case of the present invention, and FIG. 2 shows the case of the comparative example. As shown in FIG. 1, in the example of the present invention, cooling is performed to approximately 450 ° C. uniformly in the width direction. As shown in FIG. 2, in the comparative example, portions where the temperature is abnormally lowered are observed at several places in the plate width direction. As described above, in the comparative example,
In order to cool to 450 ° C., the amount of cooling water in the latter half of the cooling zone had to be increased more rapidly than in the present invention. For this reason, in the comparative example, it is considered that the film partially shifted from the film boiling region to the transition boiling or nucleate boiling region, and that portion was supercooled. In the example of the present invention, since the cooling in the first half of the cooling zone was sufficient, the cooling in the second half could be weakly cooled. Thus, it is considered that the state of film boiling was maintained to the end and cooled uniformly.

In order to improve the mechanical properties of steel, a cooling pattern of rapidly cooling a high temperature region and weakly cooling a low temperature region is often preferred. This is because the crystal structure becomes finer by rapidly cooling the high-temperature region, and the effect of adjusting the properties by removing the internal strain by weakly cooling the latter half is obtained. According to the method of the present invention, rapid cooling is possible in the first half of the cooling zone, so that the latter half can be weakly cooled. For this reason,
This is a preferred method for obtaining excellent mechanical properties.

[0041]

According to the method of the present invention, a high-temperature steel material can be cooled at a high cooling rate without cooling unevenness. For this reason, quenching of steel materials and application of controlled cooling become easy. In addition, no special equipment is required, and the method is economical.

[Brief description of the drawings]

FIG. 1 is a diagram showing a temperature distribution in a sheet width direction of a steel sheet which is hot-rolled and cooled by applying the method of the present invention.

FIG. 2 is a diagram showing a temperature distribution in a sheet width direction of a steel sheet cooled by hot rolling by a conventional method.

FIG. 3 is a diagram showing the results of an experiment comparing the cooling state of a steel sheet with and without a red scale.

Claims (2)

[Claims] 1. A water cooling method for a high-temperature steel material in which red scale is uniformly generated on the surface of a high-temperature steel material and then water-cooled. 2. A high-temperature method in which a scale having a thickness of 20 to 100 μm is adhered to the surface of a steel material, hot-rolled, the rolling is completed at 900 ° C. or less to uniformly generate a red scale, and then cooled using water. Water cooling method for steel.