JPH10180338A - Scale flaw preventing method in hot finishing rolling - Google Patents

Abstract

PROBLEM TO BE SOLVED: To strike a cooling water against the wide area of a steel plate and to surely restrain the generation of scales in the part by distributing the cooling water within a prescribed quantity range per unit area toward the whole steel plate within a prescribed percentage range of the steel plate surface area in one or more sections among respective stands of a hot tandem finishing mill train and cooling the steel plate surface. SOLUTION: The finishing mill outlet side temperature FDT is operated from the condition of the measured temperature value by a finishing mill inlet side thermometer 4, rolling speeds of respective stands and the plate thickness or the like and compared with the measured value of FDT thermometer 5, the number using strip coolants 3 is increased when the measured value is larger than a prescribed FDT reference value, and the number using the same is reduced when it is smaller than the reference value. Signals controlling whether coolants 3 are used or not are transmitted from a FDT calculating device 11 to a strip coolant controller 12, the cooling water of ejecting quantity of >=100cc/cm<2> /min to <=300cc/cm<2> /min per unit area is distributed toward the whole steel plate 1 within the range of >=50% to <=70% of the surface area of the steel plate 1 in one or more sections among respective stands, whereby the surface of the stool plate is cooled.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preventing scale flaws in hot finish rolling, and more particularly to a method for producing a hot-rolled steel sheet having a good surface by preventing scale-induced surface defects.

[0002]

2. Description of the Related Art Usually, in hot rolling, a rolled material is 1000
After heating at a temperature of at least ℃ and rolling in a rough rolling mill, rolling is performed in a finishing rolling mill. Finish rolling is usually performed in a tandem rolling mill of 7 stands, and a predetermined thickness is obtained after rolling, and the temperature (FDT) on the exit side of the finishing mill is equal to or higher than a predetermined temperature (800 to 1).
000 ° C.). In order to keep the FDT at a predetermined temperature, the finishing mill is provided with a device for injecting coolant (strip coolant) such as cooling water. Usually, the rolling speed, the reduction ratio distribution, and the on / off state of the strip coolant of each stand are controlled to secure the FDT. By the way, when the temperature of the steel sheet is high or the amount of scale generated on the steel sheet surface is large, there is a problem that the scale bites into the steel sheet surface or the roll surface is roughened, and the roll roughness is transferred to the steel sheet surface, resulting in scale flaws. is there. The main measures for preventing scale flaws include removal of scale by a scale breaker on the entrance side of the finishing mill and suppression of scale generation by supply of strip coolant. In order to prevent scale flaws, it is sufficient to use a scale breaker and a strip coolant to the maximum extent, but there is a limit in securing the FDT.

[0003]

Conventionally, as a method for preventing scale flaws, use of a scale breaker on the entry side of a rolling mill, use of strip coolant, higher precision of FDT control, temperature on the entry side of a finishing mill (FET) ) Restrictions were taken as countermeasures. Japanese Patent Application Laid-Open No. 1-205581 discloses that a rolled material is charged into a finish rolling mill with the surface temperature of the upper and lower surfaces of the rolled material after rough rolling and descaling being 900 ° C. or less, and a surface temperature difference of 0 to 50 ° C. Techniques for generating scale and preventing scale flaws are disclosed. In an embodiment of this technique, a scale breaker is placed between the first stand (hereinafter referred to as F1) entrance and the first stand (F1) and the second stand (F2). Thermometers installed on the top and bottom, each temperature is 900 ℃
The scale breaker is controlled as follows.

However, when the above technique is to be practiced, since the steel sheet temperature is set to 900 ° C. or less before finishing, it is difficult to secure the FDT for a material having a high FDT. Occurs in a short time, and it may be difficult to completely prevent scale flaws by the above technique. SUMMARY OF THE INVENTION An object of the present invention is to provide a steel sheet having a good surface without generation of scale flaws by reliably suppressing the generation of scale between stands in consideration of such conventional problems.

[0005]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above problems and to obtain a steel sheet having a good surface without scale flaws.
Cooling water is injected at a rate of 100 cc / cm ² per unit area over the entire width of the steel sheet in the range of 50% to 70% of the steel sheet surface area in one or more sections between the stands of the row of hot tandem finishing mills for producing thin steel sheets. / Min or more and 300 cc / cm ² /
The present invention proposes a method for preventing scale flaws in hot finish rolling, characterized in that the surface of the steel sheet is cooled by distributing the steel sheet to a thickness of at most min. In the present invention, the cooling water injection area refers to an area where the injected cooling water directly hits the steel plate.

[0006] Temperature control during operation is performed at a temperature of FET: 1010 ° C.
Hereinafter, FDT is specified in the range of 850 to 900 ° C. The upper limit regulation of the FET is for preventing scale flaws such as a burning scale and a roll surface roughness in the preceding stage of the finish rolling mill, and the FDT regulation is for securing the material of the rolled material. The setting of turning on / off of the strip coolant for preventing the roll surface from being roughened and burning scale is performed by calculating the finishing outlet temperature for all the cooling conditions between the stands and adopting the cooling condition that can obtain a predetermined temperature. The calculation of the exit temperature (FDT) of the finishing mill is determined as follows.

T _s -T _f = T _d + b ₀ + (b ₁ / h _F ) + (b ₂ / υ _F ) + ΔT _d (1) The above equation (1) indicates the temperature drop in the finishing mill. This is an empirical formula. Here, T _s : FET T _f : FDT h _F : finished plate thickness ： _F : rolling speed T _d : temperature drop under the descaler ΔT _d : adaptive correction term b _{0 to} b ₃ : coefficient.

The rolling temperature T ₁ of the F1 stand is determined by the FET:
It is obtained as shown in equation (2) from T _s and the temperature drop T _d under the descaler. T ₁ = T _s −T _d (2) From the measured value of the FET and the target value of the FDT, F1 is obtained by the equation (2).
The rolling temperature of each stand is obtained, and the rolling temperature of each stand is calculated by equation (3). n represents the number of stands.

T _i = T _i−1 + (T ₁ −T _f ) / n (3) Equation (1) is a form in which the amount of temperature drop from F1 to the last stand is equally distributed to each stand. I have. This is because the temperature drop of the rolled material is proportional to the transport time between stands (that is, inversely proportional to the plate speed) and inversely proportional to the thickness, so that it can be regarded as an isothermal drop between stands according to the constant law of mass flow. The rolling speed for obtaining the target FDT is given by the following equation obtained by modifying equation (1) to equation (3).

_ΥF0 = b ₂ / ｛T _s -T _f- (T _d + b ₀ + (b ₁ / h _F ) + b ₃ T _s + ΔT _{d に}よ_る (4) Cooling by the strip coolant is given by the following equation (2). The rolling speed is given to each stand in the form of, or calculated using the formula (4) without using the strip coolant, and manually used when the actual measured value of FDT is high. If the speed is too high, the FDT will be too high and scale flaws will easily occur, and if the speed is too low, a predetermined FDT cannot be secured at the rear end of the coil.

With a conventional strip coolant, about 10
The cooling water is injected at a rate of 100 to 2000 cc / cm ² / min per unit area of the cooling water, and the cooling water is 3
-20%, and the effect of reducing the average temperature of the steel sheet as well as the surface temperature of the steel sheet was great. Therefore, it was difficult to prevent the occurrence of scale flaws without reducing the average temperature of the steel sheet.

Therefore, the present inventors have developed a coolant supply method in which cooling water is applied to a wide area of the steel sheet surface.
This operates differently from the conventional strip coolant supply in the following points. Conventionally, the supply of strip coolant has focused on ensuring the steel sheet temperature (average temperature) within a predetermined temperature range, and a high cooling capacity has been required. However, in the supply of the wide area cooling strip coolant of the present invention, the area is kept large at 50% or more and 70% or less (about 5 m between stands) between the stands that hit the steel plate of the injected water. In the present invention, the steel sheet is distributed with a smaller cooling water injection amount per unit area than before, the cooling water with a small amount of water hits, and the steel sheet surface temperature is slightly lower than the scale generation temperature to reliably generate the scale. Curb, while
The feature is that the reduction of the average temperature of the steel sheet can be suppressed. With the conventional spray, the cooling area is small and the steel sheet is cooled with a high cooling capacity, so the plate surface temperature in the narrow part where the spray is hit is much lower than the scale generation temperature, but the surface temperature quickly Since the temperature was recovered and became higher than the scale generation temperature in a wide range, scale flaws were generated.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION For temperature control, setting calculation is performed for each steel type and standard before starting rolling, and the use, non-use and supply amount of strip coolant are determined so as to secure FDT. The determination of the use, non-use, and supply amount of the strip coolant after the start of rolling is calculated by a mechanism as shown in FIG. That is, the FDT is calculated from the temperature measured by the FET thermometer 4, the rolling speed of each stand, the sheet thickness, and other conditions, and is compared with the actually measured value of the FDT thermometer 5. When the measured value (or calculated value) of the FDT thermometer is larger than a predetermined FDT reference value, the number of strip coolants used is increased, and when it is smaller than the reference value, the number of strip coolants used is reduced. A control signal indicating whether to use or not use the strip coolant is sent from the FDT calculator to the strip coolant controller. The steel sheet 1 in the figure is rolled by a work roll 2 and cooled by a strip coolant 3. An FET thermometer 4 and an FDT thermometer 5 are mounted.

The conventional cooling device of FIG. 3 and FIG. 1 of the present invention are shown in FIG.
In both cases, use or non-use of the strip coolant is determined by the above method. However, in the conventional apparatus, the supply of the coolant between the stands is performed by a single row of nozzles having a small injection angle, and the cooling water injection amount per unit area of about 1000 to 2000 cc / cm ² / min is 3 times the surface area of the steel sheet between the stands.
-20%, and the effect of lowering the average temperature of the steel sheet together with the surface temperature of the steel sheet was great. In the prior art, it was difficult to prevent the occurrence of scale flaws without lowering the average temperature of the steel sheet.

Therefore, the inventors have developed a coolant supply technique in which cooling water is applied to a wide area of the steel sheet surface.
This operates differently from the conventional strip coolant supply in the following points. Conventionally, the supply of strip coolant has focused on ensuring the steel sheet temperature (average temperature) within a predetermined temperature range, and a high cooling capacity has been required. However, in the supply of the wide area cooling strip coolant according to the present invention, the area of the supply section of the strip coolant is larger than before, and the area where the injected water hits the steel plate is kept large. That is, the area of the sprayed cooling water is 50% or more of the area between the stands. For example, in a rolling mill having a stand-to-stand distance of about 5 m, the width is set to be equal to or more than the sheet width × 2.5 m in the rolling direction. In the present invention, as shown in FIG. 2, a number of rows of strip coolant 3 supply nozzles are provided between the stands (work rolls 2 and 2), and a steel plate 1 is supplied with cooling water having a volume substantially equal to the conventional total water volume. The amount of cooling water injected per unit is reduced, so that it is widely distributed and evenly applied to the surface of the steel sheet, the surface temperature of the steel sheet is slightly lower than the temperature at which the scale is generated, and the generation of scale is reliably suppressed, and the average temperature of the steel sheet Is characterized by the fact that the reduction of the amount can be suppressed. For example, the injection angle of the nozzle is set to 45 degrees or more, and
Cool ~ 70%. In addition, the nozzles are arranged in the width direction of the steel sheet so that the rolled material (steel sheet) can be cooled to the maximum width or more. The cooling range can be changed by turning ON and OFF for each strip coolant supply line.

If the area to be cooled is widened, the time during which the steel sheet is in contact with the cooling water is long, and the time during which the steel sheet is high after reheating can be shortened, and the generation of scale until the plate is caught in the next stand is suppressed. effective. An experiment was conducted to compare the cooling effect of the strip coolant for wide area cooling of the present invention with the conventional strip coolant and the effect of preventing scale flaws on the plate surface. In each of the experiments, the finish rolling speed was calculated by the above equation (1) under the condition that the strip coolant was not used. Inlet thickness 35.1mm, Outlet thickness 2.3m
FET using 10% low carbon steel with m and C content of 0.05% by weight
The temperature was set to 00 ° C. The rolling speed was determined by the above method, and the F7 exit rolling speed was set to 600 mpm. In the experiment, a seven-stand hot tandem rolling mill partially shown in FIG. 1 was used. The distance between each stand is 5 m.

FIG. 2 shows an outline of a plan view of the steel plate cooling apparatus under the conditions of the present invention shown in FIG. FIG. 3 shows an outline of a conventional apparatus. In the equipment under the conventional conditions, the temperature of the steel sheet at each stand is sequentially calculated based on the temperature obtained from the thermometer on the entrance side of the finishing mill, the rolling position (sheet thickness) obtained from each rolling stand, and the rolling load data. The use, non-use, and supply amount of the strip coolant are determined so that the outlet temperature becomes a predetermined temperature. In the apparatus of the present invention, the method of calculating the FDT, the use of the strip coolant,
The method of determining the non-use and the supply amount is the same as that of the conventional method, but the supply of the strip coolant 3 is provided in a large number of rows as shown in FIG. The flow rates of the conventional and the present invention are 1000 cc / cm ² / min and 100 to 30, respectively.
The area of the coolant injection section occupying between the stands (between the work rolls) at 0 cc / cm ² / min is as follows:
4%, 50-70%.

(Example 1) First, an experiment was conducted to compare the conventional cooling conditions with the conditions of the present invention. The results are shown in Table 1. Table 1 shows the difference in effect between the conventional strip coolant and the strip coolant of the present invention. FET
Is 1010 ° C or less, and the target value of FDT is 850 to
900 ° C. The cooling length indicates the length of a side perpendicular to the width of the steel plate in the region where the strip coolant is injected. The ratio S / C * in the table indicates the ratio of the area (area) where the strip coolant is injected to the surface area of the steel plate between the stands. Spray is number 1 among numbers 1-7
Nos. 6 and 7 were not used. The FDT was 860 ° C. for both conditions. The flow rate of the coolant per unit area is 1000 cc / cm ² / m under the conditions of the comparative example.
in, 300 cc / cm ² / min under the conditions of the present invention. The range where the cooling water is applied is the sheet width x
It was 200 mm (4% of the surface area of the steel plate between the stands), and in the present invention, the width was 2500 mm (50% of the surface area of the steel plate between the stands). Cooling water was supplied by spray. As a result of the experiment, scale flaws were generated on the surface of the steel sheet under the conditions of the comparative example. However, under the conditions of the present invention, no scale flaw was generated and the steel sheet surface was good. Further, when the scale thickness of the steel sheet after rolling was measured, it was 9 μm under the conditions of the comparative example and 8 μm under the conditions of the present invention. It can be seen that the present invention can reduce the scale on the steel sheet surface and can prevent scale flaws. .

[0019]

[Table 1]

(Embodiment 2) The FET, F
An experiment was performed to compare the conditions with the changed DT and the conditions of the present invention. The same rolled material was used as described above. Table 2 shows the experimental results
Shown in Table 2 is a table showing the effects of the temperature of the rolled material and the use conditions of the strip coolant on the FDT and scale flaws on the steel sheet surface. In Table 2, x indicates that a scale flaw was generated, and Δ indicates that the steel sheet surface was good.

When the amount of cooling water was 400 cc / cm ² / min, the FDT could not satisfy a predetermined temperature. In Comparative Example 1, only spray number 1 was used so that FDT could be secured. As a result, the FDT reached 880 ° C., which could satisfy the predetermined temperature, but scale flaws occurred. In Comparative Example 2, spray numbers 2 and 3 were used by increasing the number of sprays used so as not to generate scale flaws as in Comparative Example 1. As a result, generation of scale flaws could be prevented, but it was impossible to secure a predetermined temperature of the FDT.

Therefore, a spray was used as Comparative Example 3 and the FET temperature was set to 20 ° C. so that FDT could be secured.
An experiment was conducted to investigate the state of FDT and scale flaws. Strip coolant conditions were the same as in Comparative Example 2, and only the FET was changed. As a result, FDT was secured, but scale flaws could not be prevented.
In the example of the present invention, the FET was set to 1000 ° C., and the strip coolant was set to the same as that of Comparative Example 2. As a result, the scale thickness was 8 μm and the FDT was 860 ° C., and no scale flaw was observed on the steel sheet surface.

From the above, it has been clarified that by applying the cooling according to the present invention, scale flaws can be prevented and FDT can be reliably ensured.

[0024]

[Table 2]

(Example 3) In addition, an experiment was conducted in which the flow rate of the strip coolant was changed in the examples of the present invention to investigate whether or not FDT and scale flaws were generated. Table 3 shows the experimental conditions and results. Table 3 shows that the water amount of the strip coolant is FDT.
3 is a table showing the effect on scale flaws. × indicates that scale flaws have occurred, and Δ indicates that the steel sheet surface is good.

The FET was used at 1000 ° C., the spray was used only for No. 2, and the others were not used. The flow rate of spray No. 2 was changed from 0 to 400 cc / cm ² / min as shown in the table. As a result, when the flow rate of the strip coolant is 100 to 300 cc / cm ² / min, scale flaws can be prevented and the finishing outlet temperature can be satisfied, but when the flow rate is small as in Comparative Examples 1 and 2, Has a scale flaw and the flow rate is 400 cc / cm ² / mi
It was found that when the number was too large, the FDT could not be secured.

[0027]

[Table 3]

(Embodiment 4) In the present invention, the area occupied by the area where the strip coolant is sprayed out of the area divided between the finishing stands was changed, and the influence on the scale flaws on the steel sheet surface and the FDT were changed. An experiment was conducted to investigate the effect on the environment. Under any conditions, the FET temperature is 1000 ° C,
Strip coolant flow rate is 300cc / cm ² / m
The area where the strip coolant was sprayed in the area partitioned between the finishing stands was changed to 40 to 80%. The spray area of spray number 2 was changed.

The results are shown in Table 4. As a result, the condition 1 (the ratio of S / C * =
In (40), the FDT was able to satisfy the predetermined temperature, but scale flaws were generated and the condition 5 (S / C ratio * = 8)
In 0), no scale flaw was generated, but FDT was not satisfactory. Under conditions 2 to 4 where the cooling length was 2500 to 3500 mm, no scale flaw was observed and FDT could be satisfied.

[0030]

[Table 4]

With the above technique, when the finished plate thickness and the rolling speed are different, the cooling capacity can be appropriately determined for each stand by the same means as described above. Predetermined FDT
If a large cooling capacity is required to ensure the cooling, measures such as switching the coolant pressure to two or more levels can be used to satisfy both the effects of preventing scale formation and reducing the temperature of the steel sheet at the same time. You can make requirements.

Scale flaws are most likely to occur at the front stage (F1 to F3) of a finishing mill where a large amount of scale is generated, and it is important to suppress the growth of scale in this range.
The above technique can obtain the effect only in the stage preceding the finishing stand. Normally, the distance between the stands of the hot finishing mill is 5 to 6 m.
What is necessary is just to arrange about 3 rows of m spray headers. Since this method aims to keep the steel plate surface over a large area and keep cooling for a long time, the cooling water can be supplied by any of a spray method, a laminar method and a slit laminar method.

[0033]

In the conventional method, the strip coolant is used to cool the steel sheet, and when injected, the temperature of the steel sheet greatly drops, and the temperature of the steel sheet may be too low. There were limitations to using sprays to prevent them. The use of the strip coolant supply method for wide area cooling according to the present invention makes it possible to surely suppress the generation of scale in the area of the steel sheet because the cooling water is applied to the wide area of the steel sheet. Since the temperature drop can be determined according to the FDT, the temperature drop of the steel sheet can be reduced, and the FD can be easily determined.
T can be secured.

[Brief description of the drawings]

FIG. 1 is a system diagram showing a steel plate cooling device according to an embodiment.

FIG. 2 is a plan view showing the steel plate cooling device of the embodiment.

FIG. 3 is a system diagram showing a conventional steel plate cooling device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Steel plate 2 Work roll 3 Strip coolant 4 Finishing mill entrance thermometer 5 Finishing rolling mill exit thermometer 6, 7 Temperature signal 8, 9, 10 Rolling speed plate thickness signal 11 FDT calculation device 12 Strip coolant control device 13 Rolling direction

Continuing on the front page (72) Kunio Isobe, 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Pref. Kawasaki Steel Engineering Co., Ltd. (72) Kenji Tominaga 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. Kawasaki Steel Works, Mizushima Works

Claims (1)

[Claims] 1. A steel sheet having a surface area of at least one section between stands of a series of hot tandem finishing mills for producing thin steel sheets.
% Of cooling water per unit area in the range of 100 cc / cm ² / min to 300 c
A method for preventing scale flaws in hot finish rolling, wherein the surface of the steel sheet is cooled at a distribution of c / cm ² / min or less.