JPH0365410B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for cooling a steel strip using rolls whose interiors are cooled, by improving the degree of adhesion of the steel strip to the rolls to achieve uniform cooling in the width direction.

Methods for cooling high-temperature steel strips in continuous annealing furnaces include the gas jet method, which sprays gas, the mist cooling method, which sprays a mist of a gas-liquid mixture, the water spray method, the method of immersing the steel strip in water, and the method of cooling the steel strip on an internally cooled roll. There is a roll cooling method that cools by wrapping a band around it. Among these methods, the gas jet method has the disadvantage that the energy consumption of the blower is large. In addition, in methods where water and steel strip come into direct contact, there are problems such as the formation of an oxide film on the surface of the steel strip due to the oxygen in the water, resulting in discoloration. In this case, post-treatment equipment is required, which increases equipment costs. There are major drawbacks. In contrast to these methods, the roll cooling method is being adopted in continuous annealing furnaces and the like because it has the characteristics of low energy consumption and equipment costs, and the ability to maintain good surface properties of the steel strip.

However, in the roll cooling method, cooling tends to be uneven in the width direction, and therefore, there is a problem that flatness is likely to deteriorate due to uneven cooling, especially in thin objects. The inventors conducted various studies on this problem and found that the floating of the steel strip due to thermal expansion of the gas in the gap between the roll and the steel strip is one of the causes of uneven cooling, and the present invention was developed. It came to this. The contents will be explained below.

Looking microscopically at the contact surface between the steel strip and the cooling roll,
Since the contact pressure generated by the tension of the steel strip is very small, the contact occurs only at a small portion of the fine irregularities on the surface as shown in Figure 1. Here, the ratio between the true contact area S ₁ and the apparent contact area S ₀ can be approximately determined by the following equation.

S ₁ /S ₀ = P/H (1) where P: Contact pressure (Kg/mm ² ) H: Hardness of the softer material (Kg/mm ² ) A steel strip is wrapped around a roll (wrapping The contact pressure P when the angle is arbitrary) is determined by the following formula.

P=σt/R (Kg/mm ² ) (2) where σ: Tensile stress (Kg/mm ² ) t: Plate thickness (mm) R: Roll radius (mm) To give a standard calculation example, for example: σ=1Kg/
mm ² , t=0.4mm, R=400mm, P=0.001
Kg/mm ² , that is, approximately 0.1 atm. Assuming a high temperature and setting H = 20 Kg/mm ² , S ₁ /S ₀ =0.00005 (0.005%). That is, S ₁ /S ₀ is extremely small, and it can be seen that even if S ₁ /S ₀ changes due to changes in conditions, most of the apparent contact area remains non-contact. In this non-contact portion, there is atmospheric gas that is drawn in when the steel strip is wound around the row. Therefore, the temperature T _G of this gas is equal to the temperature T ₀ of the atmospheric gas immediately before it is sandwiched between the roll and the steel strip, and
Immediately after being sandwiched between the steel strip and the roll (for a very short period of time), the temperature is almost equal to that of the atmospheric gas. After being sandwiched between the steel strip and the rolls, it is heated by the steel strip and cooled by the rolls, and in a short time the average value of the steel strip temperature T _S and roll surface temperature T _R is
It reaches a value T ₁ of about T _S +T _R /2. (If we look at it in more detail, the closer to the steel strip in the gap the temperature is closer to T _S , and the closer to the roll the temperature is closer to T _R , but this temperature distribution is due to the phenomenon explained below. does not have much of an effect, so hereafter we will consider the average value of the gas temperature in the gap as _T
Due to the change from T ₀ to T ₁ , the pressure P _G and/or volume V _G of the gas changes. Considering the gas as a perfect gas, the change at this time follows the following equation.

P _G V _G / (T _G +273) = const. (3) However, the unit of T _G is [℃].

Here, the phenomenon in the case of the conventional roll cooling method is as follows.
This will be discussed using calculation examples. In the conventional roll cooling method, the ambient temperature T ₀ in the cooling chamber is considerably lower than the steel strip temperature T _S . Here, we will introduce one of the continuous annealing furnaces for cold-rolled steel sheets.
Considering the case of the secondary cooling zone, T _O = 150℃. Also, T _S = 700℃, T _R = 100℃, and the atmospheric pressure is 1
Let it be atmospheric pressure. The tension σ, the plate thickness t, and the roll radius R are the same as in the calculation example described above. Therefore,
The surface pressure P generated by the tension is 0.1 atm, and according to the above theory, T ₁ becomes T ₁ =T _S +T _R /2=400°C.

Therefore, T _G will rise from 150℃ to 400℃, and from equation (3), the value of R _G V _G is 400 + 273 / 150 + 273
= 1.6 times. Therefore, if the volume of the gas does not change, the pressure will increase 1.6 times to 1.6 atmospheres. However, the force trying to press the steel strip against the roll is 1.1 atmospheres, which is the sum of the outside atmospheric pressure of 1 atmosphere and the surface pressure of 0.1 atmospheres due to the above-mentioned tension, which is less than 1.6 atmospheres and is not balanced.
Therefore, in reality, the volume of the gas increases by about 1.45 times, so that P _G V _G becomes 1.6 times, and P _G becomes 1.1 atmospheres. At this time, the volume of the gas increases by 1.45 times, so the steel strip separates from the roll and becomes floating.

From the above calculations, it is considered that in the conventional method, the steel strip often lifts due to thermal expansion of the gas. Furthermore, it is thought that the degree of floating varies in the width direction because the expanded gas gradually escapes from the surrounding gaps.

As analyzed above, in the conventional technology, there was a problem that the adhesion between the steel strip and the cooling roll was insufficient, resulting in a decrease in cooling efficiency and uneven cooling in the width direction of the steel strip. .

An object of the present invention is to solve the problems of the prior art and to provide a method for cooling a steel strip with good cooling efficiency and uniformity in the width direction.

According to the present invention, in a method of cooling a steel strip by continuously bringing the steel strip into contact with the surface of a cooling roll,
The method for cooling a steel strip as described above, characterized in that the temperature of the atmospheric gas immediately before being drawn into the gap between the cooling roll and the steel strip is made higher than the average value of the temperature of the steel strip before cooling and the surface temperature of the cooling roll. is provided.

The method of the present invention can be achieved by increasing the temperature of the entire atmospheric gas in the cooling chamber housing the cooling roll so that the temperature of the steel strip before cooling is higher than the average value of the surface temperature of the cooling roll. Also, by blowing gas into the wedge-shaped space between the steel strip and the roll at a position just before the strip is wound, and making the temperature of the gas higher than the average value of the steel strip temperature before cooling and the surface temperature of the cooling roll. It can be achieved.

The present invention will now be described in more detail with reference to the accompanying drawings.

FIG. 2 is a schematic illustration of a roll cooling device, entirely arranged within a cooling chamber, for carrying out the method of the invention.

The steel strip 1 has a group of rolls 2a, 2 which are internally cooled.
It runs wrapped around b, 2c, 2d, and 2e. The entire cooling device is placed in a cooling chamber 3, and the atmospheric gas is adjusted.

According to the present invention, the temperature of the entire atmosphere in the cooling chamber 3 may be maintained higher than the average temperature of the temperature of the steel strip 1 before cooling and the surface temperature of the cooling roll 2a, or as shown in FIG. and cooling rolls 2a, 2
flat nozzles 4a, 4b, 4c, 4 arranged in wedge-shaped spaces between each of b, 2c, 2d, 2e;
High temperature gas may be injected in d and 4e.

According to the method of the invention, the ambient temperature in the cooling chamber T ₀
The phenomenon when the temperature is set to 400°C or 700°C will be theoretically analyzed in the same way as above.

When T ₀ is 400℃, there is no change in gas temperature, so the gas in the gap is at 1 atm, and the pressure from the outside is
The gas in the gap is responsible for 1 atm out of 1.1 atm and 0.1
The roll will take charge of the air pressure at the point of direct contact. Furthermore, if T ₀ is made higher than 400℃, for example
If the same temperature as T _S is 700℃, the value of P _G V _G in the gap is 400 + 273 / 700 + 273 = 0.7 using the same calculation as above.
Double. Therefore, assuming that V _G does not change,
P _G becomes 0.7 times that is, 0.7 atm, and the external pressure
Of the 1.1 atm, 0.7 atm is taken care of by the gas in the gap, and the remaining 0.4 atm is taken care of by the roll in the direct contact area. In other words, the steel strip is pressed against the roll with a pressure four times greater than the surface pressure due to tension. (Actually, in such a case, the steel strip is attracted to the rolls, so the volume of the gap, that is, the volume of gas, V _G , decreases somewhat, and P _G becomes a value somewhat larger than 0.7 atmospheres. ) It is difficult to check whether the above phenomena are occurring as calculated, but as explained below, the atmospheric gas temperature or the injection gas temperature
The effects of the present invention were confirmed by experiments of the present invention in which T ₀ was increased, and the following estimations were confirmed to be correct, at least qualitatively.

Example: A gas blowing nozzle was installed on the inlet side of the cooling roll installed in the primary cooling zone of a continuous annealing furnace for cold rolled steel sheets, and gas at various temperatures was sprayed between the roll and the steel strip, A cooling experiment was conducted on a steel strip with a thickness of 0.4 mm, a width of 1200 mm, an entrance temperature of 650°C, and a tensile stress of 1 Kg/mm ² . In this experiment, the roll surface temperature was about 120°C. As a result of the experiment, the blown gas temperature T ₀
The lower the value, the more uneven the cooling in the width direction became, and there was a tendency for the cooling near the edges to become insufficient, and at the same time, the deterioration in flatness became more noticeable. On the other hand, T ₀
However, at temperatures above 600°C, almost no uneven cooling in the width direction or deterioration of flatness was observed. FIG. 3 shows the relationship between the injection gas temperature T ₀ and the cooling rate of the steel strip. The cooling rate is the value at the center of the width. From Figure 3
Although it seems contrary to common sense that a high T ₀ actually increases the cooling rate, it can be seen that the phenomenon predicted from the above theory is occurring. Note that when T ₀ is small, a phenomenon occurs in which the edges are difficult to cool, so if the cooling rate of the steel strip is considered as an average over the entire width, this tendency becomes even stronger.

The results of the above experiments are interpreted as follows.
Thermal crown (convex) of the cooling roll due to heat from the steel strip and plate shape on the entry side (elongation in the middle, wave, etc. However, the steel strip used in the experiment was almost flat)
The degree of adhesion between the steel strip and the roll varies in the width direction due to the influence of That is, areas where the surface pressure is weak, and in extreme cases, floating areas, etc. occur. Thermal expansion of the gas in the gap when T ₀ is low will promote such floating, and as mentioned above, it is assumed that the escape of gas from the gap will also differ in the width direction, so the floating in the width direction will increase. As the number of parts increases, the degree of floating also varies in the width direction. For this reason, the cooling rate differs in the width direction, and the degree of shrinkage due to the temperature drop differs in the width direction, resulting in deterioration of flatness. On the other hand, when T ₀ is high, the steel strip is attracted to the roll, so the steel strip and the roll are in close contact over the entire width. At this time as well, the contact pressure through the direct contact part is thought to vary in the width direction due to the influence of the thermal crown, plate shape, etc., but if the gas pressure in the gap becomes low, it will be as shown in Figure 1. It is considered that the degree to which the steel strip and roll in this state come closer than usual and the volume of the gap becomes smaller is quite small. Because of this, and because in the roll cooling method, heat conduction through the interstitial gas accounts for a larger proportion than through the direct contact area, which has a very small area, S ₁ /S ₀ changes as the pressure changes. It is estimated that the way heat is transferred does not change much even if For this reason, it is thought that when T ₀ is high, cooling will be uniform in the width direction. The fact that the slope of the graph in Figure 3 is small when T ₀ is large and large when T ₀ is small also agrees with the above reasoning.

As described above, the present invention has made it possible to uniformly cool the steel strip. The gap between the roll and the steel strip is about the same size as the surface roughness, so the amount of gas in the gap is very small, so a nozzle is used to inject high-temperature gas between the roll and the steel strip. When spraying, the amount can be considerably reduced.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of the state of contact between the cooling roll surface and the steel strip. FIG. 2 is a schematic diagram of an apparatus for carrying out one embodiment of the method of the invention. FIG. 3 is a graph showing the relationship between the temperature of the injection gas and the cooling rate of the steel strip. (Main reference numbers) 1: steel strip, 2a-2e: cooling roll, 3: cooling chamber, 4a-4e: gas injection nozzle.

Claims (1)

[Claims] 1. In a method of cooling a steel strip by continuously bringing the steel strip into contact with the surface of a cooling roll, the temperature of atmospheric gas just before it is drawn into the gap between the cooling roll and the steel strip. A method for cooling a steel strip, characterized in that the temperature of the steel strip is made larger than the average value of the temperature of the steel strip before cooling and the surface temperature of the cooling roll. 2. In a method of cooling a steel strip by continuously bringing the steel strip into contact with the surface of the cooling roll in a cooling chamber that accommodates a cooling roll, the temperature of the atmospheric gas in the cooling chamber is equal to the temperature of the steel strip before cooling. A method for cooling a steel strip, characterized in that the surface temperature of the cooling roll is made higher than the average value. 3. In a method of cooling a steel strip by continuously bringing the steel strip into contact with the surface of a cooling roll, gas is introduced into the wedge-shaped space between the steel strip and the roll at a position immediately before the steel strip is wound around the cooling roll. A method for cooling a steel strip, comprising blowing the gas and making the temperature of the gas higher than the average value of the temperature of the steel strip before cooling and the surface temperature of a cooling roll.