JPS6240086B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for cooling a hot thick steel plate that has been rolled and formed, and a cooling device for a hot thick steel plate that implements this method. The purpose of this is to prevent a decrease in strength due to insufficient cooling of the rear end portion of the steel plate during cooling, and to prevent improper deformation due to uneven cooling.

In recent years, many controlled cooling methods have been developed in which hot-rolled thick steel plates are water-cooled to a predetermined temperature at an arbitrary cooling rate to improve the mechanical properties of the steel plates, such as strength or toughness. Steel plates can now be produced efficiently.

The purpose of controlled cooling of thick steel plates is to produce high-quality heat-treated steel plates by jetting a large amount of cooling water onto both the top and bottom of a continuously running steel plate and cooling it to a predetermined temperature at a desired cooling rate. Masu.

In such a continuous heat treatment process, as the length of the steel plate to be cooled increases, it becomes difficult to make the cooling state uniform in the longitudinal direction.Especially in the unsteady parts at the front and rear ends of the steel plate, over-cooling or under-cooling occurs, resulting in poor quality and yield. The decline in the number of children has become a major problem.

The occurrence of excessive or insufficient cooling conditions at the leading and trailing ends of the steel plate is thought to be caused by the following two points.
That is, the first is the temperature distribution in the longitudinal direction of the steel sheet immediately after rolling, and the second is the uneven cooling that occurs during controlled cooling.

Conventional techniques targeting the first cause, the non-uniform temperature distribution in the longitudinal direction of the steel plate immediately after rolling, include Japanese Patent Publication No. 55-36051, Japanese Patent Application Laid-Open No. 86622-1982, and Japanese Patent Application Laid-Open No. 1986-86622. No. 29862, etc., and it is thought that it can exhibit a reasonably high level of practicality.

In this way, there are various conventional techniques to eliminate the first cause, the non-uniform temperature distribution in the longitudinal direction of the steel sheet immediately after rolling, but the second cause, non-uniform cooling that occurs during controlled cooling, can be eliminated. There is no prior art technology that does this.

For this reason, even if it is possible to obtain a steel plate with a uniform longitudinal temperature distribution by using the various techniques described above to eliminate the first cause, the controlled cooling performed afterwards will eliminate the second cause. Due to the "uneven cooling that occurs during controlled cooling," the rear end portion of this type of heat-treated steel sheet in the longitudinal direction (the direction in which the steel sheet travels during cooling) would be insufficiently cooled.

This "non-uniform cooling that occurs during controlled cooling" has become clear from an investigation of the strength distribution in the longitudinal direction of the steel plate, which shows that the strength of the rear end of the steel plate is lower than that of the center due to insufficient cooling. ing.

Although a clear explanation of the reason for the non-uniform cooling that occurs during controlled cooling is currently unknown, one theory is that cooling water is jetted onto both the upper and lower surfaces of a high-temperature steel plate running in one direction. If you do,
On the top surface of the steel plate, the cooling water jetted from the cooling nozzle stays on the steel plate surface and sequentially flows down from both sides of the steel plate to the bottom of the roller table, but most of the accumulated water stays in place due to its inertia, and it looks like As the steel plate travels, a flow of accumulated water is generated toward the rear end of the steel plate, and the depth of the accumulated water on the upper surface of the steel plate increases toward the rear end, which indicates that the cooling state of the rear end of the steel plate is It is thought that this will worsen, resulting in insufficient cooling and a decrease in mechanical strength. This phenomenon becomes particularly noticeable when the rear end of the steel plate has an upward curvature.

The reason why the depth of accumulated water reduces the cooling performance of the water ejected from the nozzle is that the accumulated water prevents the cooling water ejected from the nozzle from directly reaching the surface of the steel plate, making it impossible to form nucleate boiling, which has a high cooling capacity. It is conceivable that the steel plate could be cooled only in a film boiling state with low cooling capacity.

In this way, the insufficient cooling that occurs at the rear end of hot-worked thick steel sheets is a major cause of quality deterioration and yield reduction of heat-treated steel sheets as products, so effective countermeasures must be developed as soon as possible. That was what was hoped for.

The present invention has been devised to solve the above-mentioned problems in the conventional example and to satisfy the conventional demands.The present invention is designed to solve the problems in the conventional example described above and to satisfy the conventional demands. During controlled cooling, the surface temperature is lowered to such a level that it can be cooled to a nucleate boiling state even in an immersion cooling state using accumulated water.
Cooling is performed at the same cooling rate as other parts, thereby improving the quality and yield of controlled cooling steel sheets.

Embodiments of the present invention will be described below with reference to the drawings.

The method for cooling a hot thick steel plate according to the present invention includes pre-cooling a rear end portion over a predetermined length range of a hot thick steel plate 6 discharged and conveyed from a rolling mill 1, and then pre-cooling this rear end portion. The hot thick steel plate 6 is uniformly cooled over its entire length.

That is, before uniformly cooling the entire hot thick steel plate 6 discharged and conveyed from the rolling mill 1,
By cooling only the rear end portion of the hot thick steel plate 6 over a predetermined length range where insufficient cooling occurs, when the entire hot thick steel plate 6 is uniformly cooled,
This is to prevent insufficient cooling from occurring in the rear end portion of the hot thick steel plate 6 over a predetermined length range.

Based on many actual measurement results, the rear end portion of the hot thick steel plate 6 that needs to be pre-cooled over a predetermined length range is approximately 250 to 600 mm from the rear end of the hot thick steel plate 6.
In addition, the degree of preliminary cooling for this part is approximately 100°C (for example, when cooling the hot thick steel plate 6 from 800°C to 400°C, the above-mentioned hot thick steel plate 6 Good results were obtained by pre-cooling the rear end portion to 700°C.

Next, a configuration example of a cooling device that is considered to be most suitable for carrying out the method for cooling hot thick steel plates according to the present invention will be described with reference to the drawings.

This cooling device includes a conveying device 2 that conveys the hot thick steel plate 6 rolled and discharged from the rolling mill 1, and a rear end over a predetermined length range of the hot thick steel plate 6 discharged and conveyed from the rolling mill 1. It consists of a pre-cooling device 4 that pre-cools the section, and a main cooling device 3 that cools the hot thick steel plate 6 uniformly over its entire length. Both cooling devices 3 and 4 are provided with cooling water flow opening/closing valves 31 and 41 and cooling water amount adjustment valves 32 and 42 which are individually controlled to open and close.

The conveyance device 2 sets a conveyance line for conveying the hot thick steel plate 6 rolled and formed by the rolling mill 1 to a destination, and in the illustrated embodiment, it is constituted by a large number of conveyance rollers.

The main cooling device 3 has a sufficiently long cooling zone, and by injecting cooling water from above and below to the supplied hot thick steel plate 6, it cools the hot thick steel plate 6 over its entire length. Although it is designed to uniformly cool the steel plate 6, in the illustrated embodiment, the hot thick steel plate 6
It is constructed by a combination of a pair of cooling function parts having the same configuration, which are disposed above and below a conveying device 2 that conveys.

This pair of cooling function parts includes a large number of nozzles 30 whose nozzle openings are opened toward the hot thick steel plate 6 transported by the transport device 2, and control for cutting off the supply of cooling water injected from the nozzles 30. A combination of a cooling water supply valve 31 and a flow rate control valve 32 to supply cooling water from the main cooling water supply pipe 5 to each nozzle 3.
0, and a cooling water supply pipe 33 that supplies the water to the cooling water.

In the illustrated embodiment, a plurality of combinations of cooling water supply valves 31 and flow rate regulating valves 32 are connected in parallel, and each cooling water supply valve 31 is provided with a plurality of combinations connected in parallel.
The combination of the flow control valve 32 and the flow control valve 32 includes 2 to 3
Nozzles 30 are connected, which allows for more fine-grained cooling power settings, and
This is to enable the length of the cooling zone of the main cooling device 3 to be freely set.

The preliminary cooling device 4 has almost the same configuration as one of the cooling function parts of the main cooling device 3 described above, but is smaller in scale.

That is, a combination of a nozzle 40, a cooling water supply valve 41 and a flow rate adjustment valve 42 that control the supply cutoff of cooling water to the nozzle 40, and a combination of a cooling water supply valve 41 and a flow rate adjustment valve 42 that control the supply of cooling water to the nozzle 40, and supply cooling water from the main cooling water supply pipe 5 to each nozzle 40. It is composed of a cooling water supply pipe 43 that supplies cooling water.

As described above, this preliminary cooling device 4 only needs to cool the rear end portion of the hot thick steel plate 6, so its cooling zone is limited to the length of the rear end portion of the hot thick steel plate 6. This is a good thing, but depending on the cooling state of the hot thick steel plate 6 by the preliminary cooling device 4,
It goes without saying that the cooling zone of the preliminary cooling device 4 can be made shorter.

The hot thick steel plate cooling device shown in FIG. 1 has the above-mentioned structure. Next, an example of the cooling operation of the hot thick steel plate 6 by this cooling device will be explained step by step.

The hot thick steel plate 6 rolled and discharged from the rolling mill 1 is conveyed by the conveying device 2, and as shown in FIG.
This hot thick steel plate 6 is located in the cooling zone of
Cooling water is ejected from the nozzle 40 of the pre-cooling device 4 to the rear end portion to pre-cool the rear end portion.

During this preliminary cooling, a part of the hot thick steel plate 6 may enter the cooling zone of the main cooling device 3 as shown in FIG. 3 must be kept in a stopped state.

Also, during this pre-cooling, in some cases,
The operation of conveying the hot thick steel plate 6 by the conveying device 2 may be stopped.

When the preliminary cooling is completed and the hot thick steel plate 6 transported by the transport device 2 enters the cooling zone of the main cooling device 3 as shown in FIG. 3, the cooling water supply valve 31 is opened. Then, a predetermined flow rate of cooling water is injected onto both the front and back surfaces of the hot thick steel plate 6 to cool the hot thick steel plate 6 to a predetermined temperature.

While the main cooling device is cooling the hot thick steel plate 6, the preliminary cooling device 4 may be operated, but in order to reduce energy costs, it is recommended that the preliminary cooling device 4 be stopped. good.

In this way, the rear end portion of the hot thick steel plate 6, which is insufficiently cooled, is cooled in advance by the amount of insufficient cooling, and then the whole is uniformly cooled to a predetermined temperature, so that the hot thick steel plate 6 Since the rear end part does not become insufficiently cooled and can be uniformly cooled over the entire length of the hot thick steel plate 6, it is possible to use a heat treated steel plate having uniform mechanical properties over the entire hot thick steel plate 6. You can get it.

Further, deformation of the rear end portion of the hot thick steel plate 6 due to uneven cooling can also be prevented.

Examples of experimental measurements showing the effects of the present invention are shown in FIGS. 4 and 5.

FIG. 4 is a diagram showing the tensile strength distribution along the longitudinal direction of a hot thick steel plate after cooling according to a conventional example, and the tensile strength of a portion 6a, which is the rear end portion of the hot thick steel plate, is
It is clearly shown that it is significantly lower than other parts.

On the other hand, FIG. 5 is a diagram showing the tensile strength distribution along the longitudinal direction of the hot thick steel plate 6 after cooling according to the present invention. It can be seen that the strength is almost the same as that of other parts, and that the entire cooling-treated hot thick steel plate 6 is formed to exhibit uniform tensile strength.

As is clear from the above description, the present invention can cool the entire hot thick steel plate uniformly over its entire length, so it can effectively eliminate uneven cooling of the hot thick steel plate to be heat treated. This also makes it possible to obtain a uniform mechanical strength distribution over the entire length of the hot thick steel plate, which is of great help in improving the quality of the heat-treated steel plate, as well as increasing the strength of the rear end of the steel plate. Since it is possible to prevent a decrease in yield due to cutting due to shortage, it exhibits many excellent effects such as greatly helping to reduce the price of high-quality heat-treated steel sheets.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an example of the basic configuration of a cooling device that implements the method of the present invention. FIG. 2 is an explanatory diagram showing a preliminary cooling operation state. Figure 3 shows
FIG. 3 is an explanatory diagram showing a main cooling operation performed after the preliminary cooling shown in FIG. 2; FIG. 4 is a strength distribution diagram showing changes in tensile strength over the entire length of a hot thick steel plate cooled by a conventional cooling method. FIG. 5 is a strength distribution diagram showing changes in tensile strength over the entire length of a hot thick steel plate cool-formed by the method of the present invention. Explanation of symbols, 1; Rolling machine, 2; Conveying device, 3;
Main cooling device, 4; Pre-cooling device, 6; Hot thick steel plate.

Claims (1)

[Claims] 1. After pre-cooling the rear end of a hot thick steel plate over a predetermined length range discharged and conveyed from a rolling mill, the hot thick steel plate with the rear end precooled is rolled over the entire length of the hot thick steel plate. A cooling method for hot thick steel plates.