JPS63125622A - Method for continuous cooling of steel strip in heat treatment stage - Google Patents

Abstract

PURPOSE:To uniformly cool a heated steel strip and to obtain a steel sheet having good quality by adjusting the supply rate of cooling water in a mist mixture and executing slow cooling while the temp. of the steel strip is in a high-temp. region and in a region below the transition film boiling region. CONSTITUTION:The steel strip heated in a heat treatment furnace such as annealing furnace is continuously cooled from the high temp. region down to an ordinary temp. by injecting the mist mixture composed of the cooling water and cooling gas thereto. The supply rate of the cooling water is so adjusted as to attain the slow cooling while the temp. of the above-mentioned steel strip is in the high-temp. region of about 1,200-700 deg.C and is in the region below the transition film boiling region of the cooling water in the mist mixture of about 400-300 deg.C. The degree of the cooling in the above-mentioned region where the abrupt cooling of the above-mentioned steel strip is liable to arise is thereby adjusted and the uniform and gentle cooling over the entire region is attained. The product having good material quality and shape is thus obtd.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to cooling technology when continuously heat treating steel strip in a continuous annealing furnace, a continuous normalizing furnace, etc. We propose a method of air-water cooling in which the heated steel strip is injected with water to continuously cool the steel strip.

(Prior art) Conventionally, when a steel strip is heat treated in a heat treatment furnace (hereinafter referred to as an example of a "continuous annealing furnace"), the high temperature steel strip S that has undergone heating and soaking treatment is placed in the cooling zone of the furnace. While the plate is passed from the direction indicated by the arrow in FIG. 2, the air-water coolant sprayed onto the plate surface cools it to a predetermined temperature. The reference numeral 1 in the figure is a cooling water header, and 2 is a plenum chamber for introducing gas.

When the cooling water W and the cooling gas G supplied to these are ejected from the header 1 and plenum chamber 2, respectively, they become a mixed mist at the nozzle 2a provided in the chamber 2, and are directly directed toward the sheet material S to be cooled. Sprayed.

FIG. 2 also shows a cooling curve when the steel strip is cooled by a conventional cooling method.

This cooling curve is a diagram showing the result of cooling a high-temperature steel strip S with air and water to room temperature at a uniform air-water mixing ratio and a uniform spraying speed over the entire length of the cooling zone. That is,
In the high temperature region Δ where the temperature of the steel strip S is high, rapid cooling is performed, and thereafter, gradual cooling is performed for a while. However, at a point not indicated by α in the figure, the surface of the steel strip S begins to be wetted by the cooling water W in the atomized mixed mist, so it gradually becomes strongly cooled, and after passing through region B, it is rapidly cooled again and returns to room temperature. Become.

(Problems to be Solved by the Invention) The rapid cooling in the high temperature region A described above is caused by the fact that the steel strip S has a relatively large amount of heat dissipation since it is at an extremely high temperature.

Also, from relatively low temperature (450 to 350℃) to low temperature (20℃).
The rapid cooling in region B toward 0 to 150° C.) occurs in the transition boiling region Z where the cooling water W in the mist having boiling characteristics as shown in FIG. 3 moves from the film boiling region X to the nucleate boiling region Y. This results from the increase in cooling capacity from the minimum heat flux (1) to the maximum heat flux (old) when moving from the minimum heat flux (1) to the maximum heat flux (old).

If the above-mentioned rapid cooling occurs during the cooling process of the steel strip S, the steel strip S will be cooled unevenly, resulting in uneven material quality in the width direction of the steel strip S, wrinkles and elongation due to thermal stress. There was a problem in that the quality deteriorated due to shape defects such as.

In short, an object of the present invention is to provide a cooling method that can realize uniform cooling without sudden changes in temperature gradient.

(Means for solving the problem) The purpose of the above is to use a method whose gist is the configuration described below, that is, to mix a steel strip heated in a heat treatment furnace such as an annealing furnace with cooling water and cooling gas. When the temperature of the steel strip is in the high temperature range and below the transition boiling range of the cooling water in the mixed mist when continuously cooling from a high temperature range to room temperature by spraying mist This can be reliably achieved by employing a continuous cooling method during steel strip heat treatment, which is characterized by adjusting the supply amount of cooling water to provide gradual cooling.

(Function) The basic concept of the method of the present invention is that the temperature gradient during cooling of the steel strip is large, that is, the cooling effect of the high temperature region and the mixed mist changes rapidly from film boiling to nucleate boiling. Regarding the transition boiling region accompanied by a heat removal effect, the cooling capacity is reduced by suppressing the amount of cooling water supplied, and cooling is performed so that the overall cooling rate is uniform.

As a result, shape defects due to thermal distortion and the like can be eliminated.

For this reason, in the present invention, as shown in FIG. 1, the primary cooling zone 10 to the tertiary cooling zone 30 each independently have
It is necessary to use a furnace constructed so that the flow of the cooling gas G and the flow of the cooling gas G can be controlled.

In addition, in the above explanation, the high temperature region is about 1200~
The transition boiling region refers to a region of about 400 to about 300° C., which varies depending on the size and material of the steel strip S, the type of cooling water or cooling gas, etc.

In addition, the method of the present invention can achieve uniform and gradual cooling over the entire region by adjusting the degree of cooling in the region B below the transition boiling region to the high temperature region where rapid cooling of the steel strip S is likely to occur. The purpose is to do so. In this sense, in the high temperature region Δ, cooling is performed only by cooling gas G without supplying any cooling water W, or cooling is performed only by heat radiation without supplying cooling water W and cooling gas G at all. Alternatively, in region B below the transition boiling region, cooling may be performed by supplying a small amount of cooling water together with the cooling gas.

(Example) An example in which the present invention is applied to a cooling zone of a continuous steel annealing furnace will be described below with reference to the drawings.

First, a cooling zone for implementing the present invention will be explained based on FIG. The cooling zones through which the steel strip S heated to a high temperature of 1000°C or higher in the upstream heating and soaking process is passed are, from the upstream side, primary cooling zone 10, secondary cooling zone 20, and tertiary cooling zone 30. It consists of three cooling zones. The primary cooling zone 1
0 has a total length of 3.5 m in the furnace of this embodiment, and includes a plenum chamber 12 to which cooling gas (air) G is supplied via a fan 11, and a header to which cooling water W is supplied via a pump 13] 4. In this primary cooling zone 10, when cooling gas G is jetted out from the plenum chamber 12 at a rate of 30 m/sec and cooling water W is jetted out from the header 14 at a rate of 80 cc/min, the nozzle 2a
becomes a mixed mist. This mixed mist was passed through a plate with a thickness of 0.8 mm and a width of 1.3 mm at a line speed of 60 m/m.
The spraying is carried out toward the steel strip S of m to slowly cool it to about 700°C.

The secondary cooling zone 20 following the primary cooling zone 10 has a total length of 8 m, and includes a plenum chamber 22 to which cooling gas (air) 6 is supplied via a fan 21 and cooling water (water) W via a pump 23. A header 24 is provided. In the secondary cooling zone 20, a mixed mist formed by jetting cooling gas G from the plenum chamber 22 at a rate of 50+n/sec and jetting cooling water W at a rate of 200 cc/min from the header 24 is blown onto the steel strip S. Approximately 4
Cool down to 00°C. The terminal end of the secondary cooling zone 20 is a transition boiling region where the boiling of the cooling water W on the surface of the steel strip S changes from film boiling to nucleate boiling, and the surface of the steel strip S is a steam-water mixed phase region. The condition deteriorates and the separated cooling water W starts to get wet.

The tertiary cooling zone 30 following this secondary cooling zone 20 has a total length of 5 m.
It is equipped with a plenum chamber 32 to which cooling gas (air) G is supplied via a fan 31.

In the tertiary cooling zone 30, cooling gas G is ejected from the plenum chamber 32 at a rate of 1 m/sec to the steel strip S to cool the steel strip S to about 200°C.

When such cooling is performed, the steel strip S is cooled with a cooling curve as shown in FIG. In other words, the steel strip S is in the high temperature region △ and is easily cooled down rapidly due to its own heat dissipation.
In the secondary cooling zone 10, the supply amount of cooling water W and cooling gas G is reduced to achieve gradual cooling, and in the secondary cooling zone 20 located in the region from the high temperature region Δ to the transition boiling region B, the required amount Cooling is performed using cooling water ~1 and cooling gas G,
In the tertiary cooling zone 30, which is in the region below the transition boiling region B and where the cooling capacity of the cooling water W increases, gradual cooling is achieved only by the cooling gas G. As a result, the steel strip S was cooled slowly as a whole without uneven cooling, so that a product of good quality was obtained without causing shape defects.

(Effects of the Invention) As explained above, according to the continuous cooling method during steel strip heat treatment according to the present invention, sudden cooling of the steel strip is eliminated.
11- It is possible to prevent non-uniformity of abrasive quality and defective shapes, etc., and to manufacture steel plates of good quality.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of a cooling method according to the present invention; Fig. 2 is an explanatory diagram of a cooling method according to a conventional method; and Fig. 3 is a graph showing the relationship between temperature and heat flow east, which shows the boiling characteristics of cooling water. It is. 10... Primary cooling zone 20... Secondary cooling zone 30
...Third cooling zone 11.21, 31...Fans 14, 24...Cooling header

Claims (1)

[Claims] 1. When a steel strip heated in a heat treatment furnace such as an annealing furnace is continuously cooled from a high temperature range to room temperature by injecting a mixed mist of cooling water and cooling gas, the temperature of the steel strip is high. Continuous cooling during steel strip heat treatment characterized by adjusting the supply amount of cooling water so as to provide slow cooling when the cooling water in the mixed mist is in the region below the transition boiling region and when the cooling water in the mixed mist is in the region below the transition boiling region. Method.