JPS5848611B2 - Steel cooling method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for cooling steel materials, and more specifically, the present invention relates to a method for cooling steel materials, and more specifically, cooling water is injected from a plurality of nozzles arranged in parallel and substantially perpendicular to the conveyance direction of the steel materials to cool the steel materials. In particular, the present invention relates to a method of cooling a steel material that can be effectively cooled with a minimum amount of water and that can obtain a steel material having a uniform structure and shape after cooling.

In recent years, large structures and large-diameter high-tensile steel pipes have been manufactured.
As these materials are now available on the market, demand for high-strength steel materials is increasing.

A common method for obtaining this high-strength steel material is to add alloying elements that contribute to increased strength, but the addition of alloying elements is limited by the upper limit of carbon equivalent from the viewpoint of weldability and formability. However, there is a limit to how much it can be used.

Furthermore, in recent years, it has been difficult to increase the amount of alloying elements added, both from the standpoint of resource conservation and from the standpoint of increasing the cost of steel materials.

Therefore, it is necessary to produce high-strength steel materials without increasing the amount of alloying elements added, and for this purpose, for example, quenching and tempering treatments are attracting attention.

Hardening of steel is generally performed by heating the steel to an Ar point or higher and then cooling with water to give the steel the required structure and properties.

In this case, the most important point is to obtain a martensitic structure by cooling the steel material at a high cooling rate from a high temperature of Ar3 point or higher.

Further, the cooling rate when cooling a steel material from a high temperature is determined by the heat transfer coefficient of the surface of the steel material, if the dimensions and material properties of the steel material are the same.

However, in the case of water cooling, the heat transfer coefficient generally varies depending on the surface temperature of the steel material, and when cooling from a temperature of Ar 3 or higher, as in the case of quenching, the surface temperature decreases to around 300°C, and As a result, the heat transfer coefficient increases.

Therefore, when increasing the cooling rate, it is preferable to rapidly lower the surface temperature of the steel material in the initial stage of cooling.

For this purpose, for example, as shown in FIGS. 1a and 1b, a header 2 is provided approximately at right angles to the conveyance direction A of the steel material 1, and a plurality of spray nozzles 3 are attached to this header 2. A water stream 4 is jetted to cool the surface of the steel material.

However, when cooling is performed by jetting cooling water from a nozzle in this way, it is necessary to make the distribution of water volume and density in the width direction of each spray uniform at the point where each spray of cooling water collides with the steel surface. is difficult.

Further, since the water jet ejected from each nozzle spreads radially, the water density decreases as the distance from each nozzle increases in the conveyance direction of the steel material.

That is, as shown in FIGS. 1a and 1b, when cooling with cooling water from a plurality of nozzles arranged in parallel in the width direction, parts of adjacent sprays overlap, resulting in the so-called interference part 4a. On the contrary, the water density increases, and as the spray moves away from the collision position, a difference in water density occurs between the center of the spray and the interference area.

To explain in more detail, when a water stream is injected from a single nozzle, the spray spreads radially, so as the steel material is conveyed in the longitudinal direction, the water density on the surface of the steel material decreases.

Furthermore, when a plurality of nozzles are arranged as shown in Figure 1 a and b, and a water stream is jetted from each of these nozzles, the situation is the same at the center of the spray as when using a single nozzle, and in the longitudinal direction As the steel progresses, the water density decreases, but at the interference part of the spray, water is supplied from the center of both adjacent sprays, so the water density increases as the steel material is transported in its longitudinal direction. do.

For example, Figure 2 shows the relationship between the longitudinal distance of the steel material and the water density ratio (interference area/center area) after spray collision, and in particular, the ○ mark indicates an overlap ratio of 0.5, the △ mark indicates an overlap ratio of 0.25, and the mouth mark indicates an overlapping ratio of 0.5. is overlapping O. It is O.

According to Figure 2, at the spray collision position, the water density is higher at the center than at the interference part, but when the distance from the collision position in the longitudinal direction is about 901n1ft or more, the water density is higher at the interference part. This results in a difference in water density between the center of the spray and the interference area.

By the way, as mentioned above, when water cooling is performed using nozzles, it is common to cool the steel material while conveying it along the direction in which the spray from each nozzle advances.

Therefore, when the conveyance speed of the steel material is low, there is no big problem because the steel is cooled while there is no difference in water flow density between the center of the spray and the interference area, but when the conveyance speed is high, Since the cooling effect is greater in areas where the difference in water density is large, a difference in cooling capacity occurs in the width direction of the steel material.

As a result, the structure of the steel material after cooling becomes non-uniform, and the difference in thermal stress and transformation stress causes non-uniform shape.

From this point of view, it is conceivable to reduce the interval between nozzles to increase the overlap of sprays, but this increases the amount of water and is not preferred from the standpoint of equipment operation.

The present invention aims to solve the above-mentioned drawbacks, and specifically proposes a cooling method for steel that can uniformize the cooling capacity, improve the non-uniformity of the structure and shape, and furthermore keep the amount of water to a minimum. .

That is, in the method of the present invention, when cooling the steel material by injecting cooling water spray radially from a plurality of nozzles arranged perpendicularly to the conveyance direction of the steel material, each cooling water spray is applied at the steel material conveyance speed (V ) of the following formula, m≧0.4681ogV-1.032 (100≦V<7
00) m≧0.2371ogV-0.375 (700
≦V<3000) m≧0.4 5 (
3 0 0 o≦V) However, when m is the spray width, the overlap width is mXl! The value of this m is shown as .

■ indicates the conveyance speed of steel material (m7W/min).

It is characterized by being sprayed so that it overlaps the street.

The method of the present invention will be explained in detail below.

First, three nozzles were arranged in the width direction of a steel plate, a rectangular water receiving port was provided at a distance from each nozzle, and the density distribution of the amount of water received at each water receiving port was measured.

At this time, the water inlet is divided into 20 pieces at equal intervals, and the water density between the center of the spray sprayed from each nozzle and the interference area is measured. In particular, the distance in the longitudinal direction of the steel plate is Adjust by shifting the position of the water inlet, and adjust the overlap ratio (
The value of m) is the width of the overlapped part (l1) and the width of each spray (l
), then l1=mxl.

As a result, the results shown in Figure 3 were obtained as described above, and the second
It can be seen from the figure that when the overlap ratio (m) is 0.5 (indicated by a circle), the water density ratio is 1, indicating a uniform distribution, and uniform cooling can be achieved regardless of the conveyance speed of the steel material.

On the other hand, when the overlap ratio (m) becomes smaller (△ mark m=
0.25, mark m=0) The water density at the center of the spray decreases as it advances from the spray collision position, and as a result, the cooling capacity becomes non-uniform.

For this reason, when the conveyance speed of the steel material is low, the steel material is cooled at a short distance from the spray impact position, so it is uniformly cooled, but as the transport speed increases, the steel material is cooled considerably away from the spray impact position. It is also cooled non-uniformly.

Therefore, the present inventors determined the relationship between the conveyance speed (V) of the steel material and the overlap ratio (m), and obtained the results shown in FIG. 3.

In addition, the cooling rate has a large effect on the quality of the material after cooling, and in particular, if the cooling rate at the center of the spray and the center of each plate thickness at the interference part varies, the material and shape will become uneven, making it difficult to achieve the desired strength. I can't do it.

Therefore, the cooling rate and tensile strength of the central part of the spray were taken as Vc and Tsc, and the cooling rate and tensile strength of the interference part were taken as Vw and Tsw, and the same results were obtained for the cooling rate.

As a result, it is generally accepted that variation in tensile strength is within ±3%, and if we calculate the variation in cooling rate within this range from Figure 4, it is found that this value is within 0.1%. is necessary.

In addition, in Fig. 3, if the variation in cooling rate satisfies the condition of 0.1% or less, it is marked ○, and if the variation in cooling rate satisfies the condition of 0.1% or more, it is marked ●. If it is within %, it is necessary to satisfy the following formula for cooling.

m≧0.4681ogV-1.032 (100≦V<7
00) m≧0.2371ogV-0.375 (700≦
V<3000) m≧0.45 (30
00≦■) In short, if cooling water is injected and cooled under conditions where the above three equations hold true, the material and shape of the steel material after cooling can be maintained uniformly within the allowable range, and furthermore, the amount of cooling water can be kept at a minimum. It can be kept cool and cooled, which is advantageous in terms of equipment and operation.

[Brief explanation of the drawing]

Figures 1a and b are a plan view and a side view of the case where steel material being transported is cooled by multiple nozzles arranged in parallel, and Figure 2 shows the relationship between the longitudinal distance from the spray impact point and the water volume density ratio. FIG. 3 is a graph showing the relationship between steel material conveyance speed and spray overlap ratio, and FIG. 4 is a graph showing the relationship between variation in cooling rate and variation in material strength. Code, 1... Steel material, 2... Header, 3
...Spray nozzle, 4...Water flow, 4
a... Interference part.

Claims (1)

[Scope of Claims] 1. When cooling the steel material by injecting cooling water spray radially from a plurality of nozzles arranged perpendicularly to the conveyance direction of the steel material, each cooling water spray is applied at the steel material conveyance speed (
m≧0.4 6 8 1ogV-
1.0 3 2 (100≦V<700) m≧0.2
3 7 1ogV - 0.3 7 5 (700≦V
<3000) m≧0.45 (3000
≦V) However, m indicates the value of m where the overlap width is indicated by mxl, where the spray width is 1. ■ indicates the conveyance speed (ynx/min) of the steel material. A method of cooling steel material that is characterized by spraying the jets so that they overlap each other.