JPS5848612B2 - Steel cooling method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for cooling steel materials, and more particularly to a cooling method that can effectively cool steel materials by appropriately arranging water cooling nozzles during quenching of steel materials.

In recent years, there has been an increasing demand for high-strength steel materials for manufacturing large structures and large-diameter high-tensile steel pipes.

In order to improve the strength of steel materials, the most common method is to add alloying elements that contribute to increased strength.

However, from the viewpoint of weldability and formability, there is an upper limit to the carbon equivalent, and there is also a limit to the amount of alloying elements added.

Furthermore, in recent years, it is not desirable to increase the amount of alloying elements added in order to save resources or to increase the cost of steel materials.

Therefore, it is necessary to manufacture high-strength steel materials without increasing the amount of alloying elements added, and one example of this is quenching and tempering.

This quenching is normally performed by heating the steel material to a temperature above the Arg point and then cooling it with water, but the most important part of this process is that the steel material is cooled at a high cooling rate from a high temperature above the Ar3 point to form a martensitic structure. It is to obtain.

This cooling rate is determined by the heat transfer coefficient of the steel surface when the dimensions and materials of the steel are the same, and in order to obtain high strength steel through quenching, it is necessary to maintain the heat transfer coefficient of the steel surface at an appropriate level. be.

In general, in the case of water cooling, the heat transfer coefficient 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 heat transfer coefficient decreases as the surface temperature decreases until around 300°C. growing.

Therefore, in order to increase the cooling rate, the surface temperature of the steel material must be rapidly lowered during the initial stage of cooling. For this reason, for example, cooling water with a high water density can be injected onto the steel material being transported to increase the cooling rate. It's getting bigger.

Also, the cooling water 3 is applied to the steel material 1 being conveyed in the direction of the arrow.
There are two methods: one is to spray from the hole-shaped nozzle of the nozzle 2 (see Figures 1 a and b), and the other is to spray from the slit-like nozzle of the nozzle. After the collision, the spray 3 will flow along the steel surface.

In other words, the water flow near the collision position with the steel material is in a turbulent state and has a high ability to remove heat from the surface of the steel material, but as it moves away from the collision location, the water flow approaches a laminar flow state and the heat removal ability decreases.

In addition, as shown in Fig. 1 a and b, the actual cooling curve inside the steel material 1 being transported and cooled is obtained, the surface temperature and heat transfer coefficient are calculated from this curve, and the surface temperature during spray cooling is further calculated. The water density is determined using a regression equation between temperature, heat transfer coefficient, and water density, and the relationship between this water volume density and the distance after spray collision is obtained, and the relationship shown in Figure 2 is obtained (in Figure 2, , A is when the water density at the spray collision position is 10,000 (l/min-m), when the water density at the mouth is 90,000 (l/min-m2), and C is 8,000 (l/min-@'). When , two is 6 0
0 0 (l/min ゜m")).

This water density is the water density that effectively contributes to the cooling of the steel material, and is hereinafter referred to as the effective water density.
After 100 mm, the effective water density decreases significantly.

If the conveyance speed of the steel material is high, the cooling time for the steel material is short in the region where the effective water flow density is large, so
A group of nozzles in a row cannot provide sufficient cooling.

Therefore, in order to prevent the cooling performance from decreasing due to such an increase in the conveying speed, cooling is usually performed by arranging a plurality of nozzle groups in the conveying direction, as shown in FIGS. 1a and 1b.

However, in the conventional example, these nozzle groups are not necessarily arranged in optimal conditions, and furthermore, the cooling capacity does not improve much despite the increase in the total amount of cooling water, and the water supply and drainage equipment is excessive. Improvement is desired.

In order to improve these drawbacks, the present inventor proposes a cooling method that was established through repeated experiments on the cooling performance of a plurality of nozzle groups, and that can effectively cool steel materials with a minimum amount of water.

In the method of the present invention, when cooling the steel material by jetting cooling water from the two-stage cooling nozzle group onto the surface of the steel material being transported, cooling water is sprayed onto the steel material surface from the first-stage cooling nozzle group. 30
At the same time, the cooling water from the second stage cooling nozzle is made to collide with the steel surface at an incident angle of 25 to 40', and further cooling from the second stage water cooling nozzle group. The feature is that cooling is performed by adjusting the amount of water to be 30 to 40% of the total amount of cooling water.

First, the inventors determined the change in the water volume density that effectively contributes to cooling, that is, the effective water volume density, in the water flow injected from the first-stage cooling nozzle group, and as shown in Figure 2 above. It was as shown in

From the situation shown in Figure 2, after the cooling water spray collision,
It can be seen that the effective water density decreases significantly after about 50 to 100 mm.

Therefore, the inventors of the present invention have noticed that effective cooling can be achieved by arranging the second stage nozzle group and injecting cooling water at a position where the effective water flow density begins to decrease.

In addition, a scale layer exists on the surface of the steel material, and since the scale layer is disadvantageous for cooling, we focused on increasing the impact force of the cooling water in order to peel off the scale layer.

On the other hand, in the second and subsequent stages, the injected water flow penetrates the water film on the steel surface formed by the jet water flow from the first stage nozzle group and is cooled by coming into direct contact with the steel surface. I paid attention.

Therefore, based on these findings, the inventors of the present invention
In addition, as shown in b, when the steel material 1 is cooled by two groups of two-stage cooling nozzles, there are various factors that affect the cooling performance, that is, the incident angle of the cooling water to the steel material (θ1, θ
2) We conducted numerous experiments regarding the distance (l) between the first stage cooling nozzle group and the second stage cooling nozzle group, the distribution ratio of the amount of cooling water to each stage cooling nozzle group, etc. However,
The results shown in Tables 1 to 3 were obtained.

That is, as is clear from Table 1, each incident angle θ1, θ2
There is a proper combination of cooling nozzles, in which cooling water is sprayed from the first stage nozzle group at an incident angle of 30 to 35 degrees, and from the second stage nozzle group at an incident angle of 25 degrees.
It can be seen that good cooling performance is obtained only when sprayed at ~40'.

Moreover, from Table 2, it was found that the distance (l) between each cooling nozzle group was also related to the cooling capacity, and it was found that this value preferably ranged from 50 to 90.

In addition, the amount of cooling water from the nozzle group at each stage is shown in Table 3.
The amount of water in the cooling nozzle group in the first stage is 30 to 4 times the amount of total cooling water.
It can be seen that good cooling performance can be obtained at 0%.

In short, in the method of the present invention, first, the angle of incidence (θ1) is
Cooling water is sprayed at an angle of 30 to 35 degrees, and the second stage cooling nozzle group sprays the steel plate at an incident angle (θ2) of 25 to 40 degrees.
At the same time as cooling water is sprayed with ', the cooling water from the second stage cooling nozzle is adjusted so that it accounts for 30 to 40% of the total amount of cooling water.

If these conditions are satisfied at the same time, the cooling capacity will be greatly improved, a large cooling rate can be obtained with a small amount of water, and the water supply and drainage equipment will become more compact, which will be advantageous in terms of collection.

In other words, when cooling with the method of the present invention, steel materials transported at a higher conveyance speed can be sufficiently cooled with the same amount of cooling water as when cooling with a single nozzle group, and the cooling equipment capacity is increased.

Furthermore, compared to the conventional cooling method using a plurality of nozzle groups, the total amount of cooling water can be significantly reduced, which is advantageous in terms of water supply and drainage equipment and operation.

Note that the method of the present invention is applicable not only to the above-mentioned quenching and tempering but also to cooling of steel materials in general.

[Brief explanation of drawings]

FIGS. 1a and 1b show that a plurality of cooling nozzle groups
A plan view and a side view when cooling the steel material, and FIG. 2 are graphs showing changes in the effective water density in the conveying direction of the steel material. Code, 1... Steel material, 2... Nozzle, 3
······Cooling water.

Claims (1)

[Claims] 1. When cooling the steel material by jetting cooling water from the two-stage cooling nozzle group onto the surface of the steel material being transported, the cooling water is sprayed from the first-stage cooling nozzle group at an angle of 30 to 35 degrees to the surface of the steel material. At the same time, the cooling water from the second stage cooling nozzle is made to collide with the steel surface at an incident angle of 25 to 40 degrees, and the amount of cooling water from the second stage water cooling nozzle group is A method for cooling steel materials, characterized by adjusting the amount of water to 30 to 40% of the total amount of water.