JPS5919969B2 - Online cooling method for thick steel plates - Google Patents

Description

[Detailed Description of the Invention] This invention provides the following features: Immediately after final rolling in hot rolling of thick steel plates,
In other words, from a state where the thick steel plate is at a fairly high temperature, this steel plate is
The present invention relates to an online cooling method for a thick steel plate that minimizes cooling distortion of the steel plate during accelerated cooling at a cooling rate of 0°C/SeCg).

For the purpose of making steel tougher, quenching (quenching) of heated steel above its transformation point has traditionally been carried out, but most of this is offline, that is, it improves the properties of steel. In order to do this, the steel is heated to a constant temperature in a heating furnace and then rapidly cooled.On-line quenching, which rapidly cools the steel sheet while it is still at a high temperature immediately after hot rolling, has only recently been developed.

One thing to consider when using online cooling is that some steel plates that are rolled in a hot rolling mill and flow through a rolling line should be rapidly cooled, while others may be cooled by normal air cooling without rapid cooling. When the latter passes through the rolling line, a cooling method must be adopted in which the cooling header does not interfere with the running of the steel plate.

In particular, hot-rolled steel plates rolled by plate rolling mills are
As typified by so-called controlled trawling, some rolls pass through the rolling line in a greatly deformed shape, and for this reason, when cooling thick steel plates online, the method of cooling the top surface of the steel plate becomes an issue.

That is, it is difficult to lower the top cooling header to a predetermined height from the surface of the steel plate only when the steel plate to be cooled passes, and to move it to a position where it does not interfere with the running of the steel plate when other steel plates are passing. Not only is it extremely complicated, but the maintenance of the equipment is also difficult.

In addition, if the header for cooling the top surface of a steel plate is always located in a fixed position and is to be activated only when a steel plate to be cooled online passes, a sufficient distance should be maintained between the header for cooling the top surface of the steel plate and the steel plate. Due to this necessity, there have been major restrictions on the cooling method and cooling capacity for cooling the top surface of the steel plate.

For these reasons, it is virtually impossible to perform fine cooling capacity control online, and it has been impossible to perform cooling control that minimizes the cooling distortion of the steel plate or the variation in mechanical values within the steel plate.

However, online rapid cooling does not require reheating the steel plate, and has many advantages such as improved productivity and simplified processes, so it is desirable to develop a cooling method that can finely control cooling capacity online. It is rare.

Currently, hot strip mills use laminar flow cooling on a runout table, but most of the cooling is done under tension, the finishing temperature is high, and winding is required after cooling. Therefore, plate deformation due to cooling is hardly a problem.

In addition, cooling after rolling thick plates is currently performed using showers, and the cooling capacity is usually around 2 to 3"C/Sec at most, and it is used to shorten the cooling time rather than to improve material quality. The current situation is that

On-line quenching (quenching) can be achieved either by setting the height of the top cooling header sufficiently high, taking into account the steel plate with the largest amount of deformation among all the steel plates passing through the rolling line, or by rolling it. There are constraints such as the need to cool the plate by forcibly suppressing the plate deformation using, for example, pinch rolls.

In the rolling line, both heat-treated material that undergoes online cooling and ordinary material that does not undergo cooling pass through it, so the method of cooling the material by forcibly suppressing plate deformation due to rolling is to keep the height of the top cooling header sufficiently high. Compared to other methods, it is disadvantageous in terms of cost and operation, so when performing online rapid cooling, a top cooling header must be installed at a height that is higher than the amount of deformation of the steel plate passing through the rolling line. However, the amount of deformation of steel plates produced by conventional thick plate hot rolling mills is large, and the amount of deformation is 1000 to 1
Since the thickness reaches 500 mm, the two sides of the steel plate to be cooled are
Cooling by laminar flow is appropriate.

In this case, other cooling methods such as spraying or misting can be considered, but if the distance from the steel plate to be cooled is 1000 to 1500 mm, the cooling capacity will be extremely reduced, so it is not recommended to use the top surface cooling method of the steel plate to be cooled. is practically unusable.

On the other hand, regarding cooling of the lower surface of the steel plate, in the so-called laminar flow where cooling water flows, it is impossible to overcome the action of gravity and cause water to collide with the lower surface of the steel plate from the lower surface cooling header located below the steel plate. Therefore, as a method for cooling the lower surface of the steel plate, it is necessary to use a cooling method in which a considerable pressure is applied to the header and water in the form of water drops collides with the lower surface of the steel plate, that is, a spray or mist cooling method.

The present invention has been made in view of the above points, and includes an online cooling method for controllingly cooling a thick steel plate that is in a high temperature state following hot rolling, in which the upper surface of the thick steel plate is cooled by laminar flow, and the upper surface of the thick steel plate is cooled by laminar flow. The lower surface of the thick steel plate is cooled by spray or mist, and the water ratio of the cooling water used for cooling the lower surface of the thick steel plate and the upper surface of the thick steel plate is set to 1.5 to 4.0, and the header pressure of the laminar flow is set to 0.5. kg/i
G or less, and the cooling stop temperature is 400° C. or more.

This invention will be explained in more detail with reference to Examples.

Amount of deformation of the steel plate after cooling (Hereinafter, in this specification, the deformation of the steel plate after cooling will be quantitatively expressed as the amount of deformation per 1 m of length.

) is 5 mrn/m or less, distortion correction using a leveler is not necessary.

However, if the amount of deformation is larger than this, it will be necessary to correct it with a leveler, and if the amount of deformation increases further, the amount of correction with a leveler will increase, resulting in material deterioration due to work hardening and variations in mechanical properties in each part of the plate. Not only does this result in an increase, but it may also become impossible to correct.

Therefore, reducing the amount of deformation of the steel plate also reduces the variation in mechanical values in the plate thickness.

FIG. 1 shows the relationship between the amount of deformation of a 19 x 1500 mi x 3000 mm steel plate in an unstraightened state after online cooling and the variation in tensile strength among mechanical properties.

As is clear from Figure 1, reducing the amount of deformation of the steel plate does not require leveler correction;
However, the amount of correction can be made extremely small.

Further, there is an advantage that the variation in tensile strength of the plate thickness can be reduced, and a stable product can be obtained.

Deformation due to cooling of a steel plate is caused by the difference in cooling capacity between the upper and lower surfaces of the steel plate.

There are various factors that can be considered to control the cooling capacity of the upper and lower surfaces of the steel plate, but one that has a large influence is the water volume ratio of the upper and lower surfaces of the steel plate.

Figure 2 shows the water volume ratio of the upper and lower surfaces of the steel plate when rapidly cooling the steel plate to room temperature with a minimum thickness of 10 mm, which is thought to be able to be cooled to the extent that deformation of the steel plate does not become a problem during online cooling, that is, the lower surface of the steel plate. The relationship between the total amount of cooling water on the side/total amount of cooling water on the top side of the steel plate (hereinafter this ratio will be referred to as R) and the amount of plate deformation is shown.

As shown in the relationship between the cooling stop temperature and the amount of plate deformation in FIG. 5, the thicker the plate, the smaller the amount of deformation of the steel plate obtained when cooling under the same conditions.

As is clear from Figure 2, the amount of deformation that satisfies the range of 5 mm/m or less is 1.5≦R for plate thicknesses of 10 mm or more.
≦4.0, and if the amount of deformation is within this range, leveler correction is not necessary as described above.

Therefore, it is preferable that the water ratio between the upper and lower surfaces of the steel plate is 1.5 to 4.0.

It is known that increasing the amount of cooling water increases the cooling capacity, but as the amount of water increases, the cooling capacity becomes saturated.

Increasing the amount of cooling water in this way means increasing the header pressure, but the pressure in the laminar flow header was conventionally known as laminar flow, which is a suction flow, that is, a header pressure of 0.0 kg/air G. However, in recent years, people have begun to consider controlling the cooling capacity by controlling the amount of water in laminar flow, and for this purpose header structures that increase header pressure have been developed, making it possible to use the same header over a wide range of water amounts. It has become possible to maintain and control laminar flow.

For example, there is a nozzle in which an aperture is inserted in the middle, which was proposed by the same applicant in Japanese Patent Application No. 109586/1986.

Inserting a restriction in the middle of the nozzle in this way makes it possible to maintain laminar flow over a wide range, increase header pressure, and spray water evenly across the length of the header. .

FIG. 3 shows the relationship between the header pressure and the cooling rate when single-sided cooling is performed by changing the header pressure in the range of nozzle diameter/aperture diameter -1.5 to 3.0.

As is clear from Figure 3, the header pressure is 0.4k
At y/iG or more, the cooling rate is almost saturated, and even if the pressure is increased by 0.5 kg/critG or more, the effect does not change substantially even if some safety is considered.

Therefore, header pressure P in case of laminar flow cooling
It is appropriate that cooling is performed in the range of PI7≦0.5 (kg/Cr?tG), preferably 0.4 or less.

Experiments have shown that when rapid cooling is performed using laminar flow for the purpose of improving material quality, large local distortions may occur when the plate thickness is less than 10 mm, and the variation in the mechanical values of the plate begins to increase rapidly. Confirmed.

FIG. 4 shows the relationship between plate thickness and variation in plate wall tensile strength at a cooling rate of 5 to 6° C./SeC.

As can be seen from FIG. 4, when the plate thickness is 10 mm or more, the variation in plate tensile strength is extremely small and becomes stable.

On the other hand, when the cooling stop temperature is increased, the amount of deformation of the steel plate becomes smaller.

Further, as a result of experiments in the range of 1.5≦R≦4.0, it was found that the amount of deformation of the steel plate can be well approximated by the following equation.

As shown in Figure 5, when the cooling stop temperature becomes 400°C or lower, the amount of deformation of the steel plate suddenly increases and the plate thickness
Since the amount of deformation is approximately 5 mm/m when the cooling stop temperature is 400°C at 0 mm, t≧10 (tomo) and T≧
400 CC) is preferable.

As explained above, according to the present invention, the upper surface of the steel plate is cooled by laminar flow, the lower surface of the steel plate is cooled by spray or mist, and the ratio of the amount of cooling water used for cooling the lower surface and the upper surface of the steel plate, that is, the water amount ratio of the upper and lower surfaces. is set to 1.5 to 4.0, and the header pressure of the laminar flow is set to 0.5 kg/
i G or less, and by setting the cooling stop temperature to 400°C or higher, not only can an accelerated cooling steel sheet with excellent mechanical properties with less deformation be obtained without restraining the steel sheet during cooling, but also Since there is little deformation of the thick steel plate, fluctuations in the cooling capacity due to the deformation of the steel plate can be minimized, and as a result, there is extremely little variation in the characteristic values that indicate the mechanical properties of each part of the obtained pre-conditioned plate thickness. Industrially useful effects such as

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between the amount of plate deformation and the variation in plate tensile strength, Figure 2 is a diagram showing the relationship between the water and sewage flow ratio and the amount of plate deformation, and Figure 3 is a diagram showing the relationship between the amount of plate deformation and the variation in plate thickness tensile strength. Figure 4 is a diagram showing the relationship between header pressure and cooling rate, Figure 4 is a diagram showing the relationship between plate thickness and plate wall tensile strength, and Figure 5 is a diagram showing the relationship between cooling stop temperature and plate deformation amount. It is.

Claims (1)

[Claims] 1. In an online cooling method for controlled cooling of a thick steel plate in a high temperature state following hot rolling, the upper surface of the thick steel plate is cooled by laminar flow, the lower surface of the thick steel plate is cooled by spray or mist, and the lower surface of the thick steel plate is cooled. The water volume ratio of the cooling water used for cooling the upper surface of the thick steel plate is set to 1.5 to 4.0, and the header pressure of the laminar flow is set to 0.5 kg.
/criY G or less, and the cooling stop temperature is 40
．． An online cooling method for thick steel plates characterized by cooling the steel plate to 0°C or higher.