JPH027372B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a cooling method that does not cause deformation in water cooling during heat treatment of metal pipes, particularly various steel pipes. (Prior Art) In recent years, there has been an increase in the number of drilling opportunities for crude oil and gas, which have strong sour properties, and the line pipes used for transporting such crude oil and gas are increasingly required to have excellent sour resistance properties. In the past, this kind of sour resistance was mainly required for relatively high-strength materials of ×60 or higher, but the recent trend is that sour resistance is required even for line pipes with lower strength. The number of cases is increasing. In response to these changes,
Low-strength linepipes, which have been manufactured as hot-rolled or normalized, now need to be heat-treated by quenching and tempering to ensure sour resistance. Since low-strength steel pipes generally have low hot strength, they tend to bend easily during heat treatment, and in particular, large bends tend to occur during water cooling for quenching. There have already been many proposals regarding the cooling of metal pipes, especially steel pipes, all of which take into consideration the prevention of deformation caused by cooling and the improvement of productivity by increasing the quenching cooling capacity without interfering with uniformity. For example, Japanese Patent Publication No. 53-32097 proposes a device that uniformly cools a steel pipe by conveying it while rotating and uniformly spraying a large number of fluid jets in a tangential direction from the outer circumference of the pipe. . Also special public service in 1984
Publication No. 19370 discloses a method and device for uniformly cooling steel pipes by applying curtain-shaped jets from a number of nozzles arranged at equal intervals in the circumferential direction in a direction of 45° to 80° with respect to the traveling direction of the steel pipe. There is. Although these proposals have a certain effect on cooling ordinary steel pipes, they are not fully satisfactory when applied to steel pipes with low hot strength and thin walls, such as the low-strength line pulp mentioned above. It's not something you can get good results from. The reason why low-strength metal pipes tend to bend when cooling is that even though high-strength materials have uneven stress that can be sufficiently absorbed by their high rigidity, metal pipes with low hot strength are easily deformed. This is thought to be due to the fact that The cause of stress non-uniformity in different parts of the metal tube during cooling is that the cooling rate of the metal tube differs depending on the part of the steel pipe, and the variation in the cooling rate is due to the difference in the temperature of the metal tube surface (heat transfer surface). ) is mainly caused by unevenness such as roughness and mill scale adhesion. The reality is that no effective method has been proposed to date to prevent the deformation of metal tubes by suppressing variations in cooling rate caused by microscopic non-uniformity on the surface of metal tubes. . (Problems to be Solved by the Invention) As mentioned above, in a broad sense, the surface characteristics of metal tubes that are heat treated include variations in surface roughness, adhesion state of mill scale, surface condition of mill scale, etc. If the hot strength of the metal tube is low or if its rigidity is low due to a thin wall thickness, etc., this may cause large deformation when the metal tube is cooled with water. An object of the present invention is to prevent deformation of a metal tube that is easily bent as described above due to heat treatment and cooling. Further, the present invention can prevent deformation caused by cooling of metal tubes having low hot strength and low rigidity, and is also effective as a strain-free cooling method for ordinary metal tubes. (Means for Solving the Problems) The present invention advantageously solves the problems described above, and its gist is as follows: A heat treatment method for heating a metal tube to a predetermined quenching temperature and then cooling it. In the case of a metal tube with mill scale attached to its surface, the mill scale is removed so that the temperature of the tube surface (heat transfer surface) becomes 450 to 650 degrees Celsius from heating within 0.5 to 3 seconds after the start of cooling. In the case of a metal tube that has been cooled, slow cooling is performed so that the tube surface (heat transfer surface) temperature reaches 550 to 700℃ within 0.5 to 3 seconds from the start of cooling, and each part of the metal tube surface (heat transfer surface) is cooled. This is a method for heat treatment of metal tubes, which is characterized by reducing the temperature difference between 1 and 2, and then performing normal strong cooling from that temperature. The present invention will be specifically explained below based on the drawings. First, deformation after cooling due to surface conditions of the metal tube will be explained. When cooling a high-temperature metal, film boiling heat transfer occurs first, then transition boiling heat transfer, nucleate boiling heat transfer, and convection heat transfer cool the metal to room temperature. This is schematically shown in Figure 1, which is called a cooling curve. The heat transfer surface temperature called the quench point shown in FIG. 1 is related to the temperature at which the vapor film cannot exist stably and the vapor film collapses. That is, the cooling rate during quenching is not uniform, and even if water cooling is performed under the same conditions, cooling is relatively slow in the temperature range above the quench point, and cooling is rapidly accelerated beyond the quench point. Furthermore, the quench point varies depending on the surface conditions of the heat transfer surface, as described above. In general, a rough surface will increase the quench point.
In other words, cooling is promoted. Conversely, a smooth surface will lower the quench point and retard cooling. If the quench point of the cooling curve differs depending on the portion of the metal tube surface (heat transfer surface) as described above, a large temperature difference will occur at a relatively early stage of cooling. Deformation occurs due to thermal stress based on this temperature difference or transformation stress related to the transformation point existing in this temperature range, and remains even after cooling. In other words, the deformation that occurs during cooling of the metal tube surface from the film boiling region is caused by the temperature difference in the high temperature region, especially the unevenness of the quench point. Therefore, in cooling a practical metal tube exhibiting such cooling characteristics, it is extremely important to realize a cooling method that aligns the cooling curves at each location on the surface of the metal tube. If sufficient cooling water is supplied at the beginning of the cooling of the metal tube, as shown in Figure 2, the quenching point is greatly accelerated, and the vapor film does not collapse on the surface (which has not yet reached the quenching point). The temperature difference with the heat transfer surface (heat transfer surface) becomes significantly large (A and B in the figure indicate the quench point). Therefore, for a certain period of time after the start of cooling, by avoiding strong cooling that may partially raise the quench point and using slow cooling, the temperature difference when the quench point is reached as shown in Figure 2 can be reduced. T can be decreased from ΔT ₁ to ΔT ₂ (BD=ΔT ₁ , BC=ΔT ₂ ). After that, by moving from slow cooling to strong cooling, each part can uniformly transition from transition boiling to nucleate boiling heat transfer, and the variation in cooling conditions of each part of the metal tube can be significantly reduced, resulting in a drastic reduction in deformation. . The slow cooling conditions here are cooling conditions that do not raise the quench point partially as described above, and if expressed in terms of the practical cooling water volume density,
This means an area not exceeding 4m ³ /m ²・min. Another method for aligning the quench points at various locations on the surface of the metal tube is to roughen the heat transfer surface. It is desirable that the protrusions on the rough surface be larger than the thickness of the steam film on the heat transfer surface, and when the protrusions break through the steam film and come into direct contact with cooling water, the collapse of the steam film is promoted using the protrusions as nuclei, resulting in quenching. points increase. For example, when mill scale is removed by stain blasting, etc., the surface is cleaned and protrusions are formed uniformly, so each surface (heat transfer surface)
It is even more effective if the quench point temperatures of the parts are the same and the method is combined with the above method. According to the research of the present invention, in the case of cooling water with a practical water temperature of 20 to 45 degrees Celsius, the quench point temperature of practical metals varies depending on the conditions of the heat transfer surface, but
It was within the range of ℃. Therefore, depending on the hot strength and rigidity of the metal to be cooled,
The slow cooling temperature range at the initial stage of cooling is different, but when mill scale is attached, the heat transfer surface temperature is 450℃ to 650℃.
It was confirmed that it is best to lower the temperature to 550°C to 700°C in a state where mill scale has been removed and the surface has been roughened, and then strongly cool it. In a practical heat-treated metal tube, the lower limit of the quench point temperature is approximately 500℃ when mill scale is attached, so there is no need to slowly cool the metal tube surface (heat transfer surface) temperature to 450℃ or less, and the heat transfer It was confirmed that when metal tubes are rapidly cooled from a surface temperature of 650℃ or higher, the effect is small due to large bends in the case of metal tubes with low hot strength and low rigidity. In addition, when the mill scale is removed and the surface is roughened, the lower limit of the quench point temperature is approximately 600℃, so there is no need to slowly cool the metal tube surface (heat transfer surface) temperature to 550℃ or less, and the metal tube surface ( It was also confirmed that metal tubes with low hot strength and low rigidity, when rapidly cooled from temperatures above 700℃ (heat transfer surface), have a large bend and are less effective. As for the actual degree of slow cooling, as a result of various experiments, it has been found that when mill scale is attached, the metal tube surface (heat transfer surface)
Cooling conditions where the temperature drops from the heating temperature to 450℃ to 650℃ are good, and when mill scale is removed and the surface is roughened, the metal tube surface (heat transfer surface) temperature will drop from the heating temperature within 0.5 seconds to 3 seconds. Cooling conditions that lower the temperature from 550℃ to 700℃ are preferable. By doing this, slow cooling in the initial stage of cooling can be managed based on time without actually measuring the temperature of the transmission surface, which is practical. If the slow cooling time is 0.5 seconds or less, the temperature of each part will be uneven, and if it is then rapidly cooled, large bends may occur. When the slow cooling time is 3 seconds or more, the temperature inside the wall thickness also decreases, and the thick surface layer may not become a completely quenched structure, which is not appropriate, especially in heat treatment cooling such as quenching. (Function) The following will be specifically described using quenching of seamless steel pipes as an example. Recently, drilling for crude oil and gas with strong sour properties has progressed, and it has become necessary to subject low-strength line pipes and the like used for transporting crude oil and gas to quenching and tempering heat treatment. These steel pipes have conventionally been manufactured as hot-rolled or normalized, but from the viewpoint of sour resistance, quenching-tempering heat treatment is now required. When the hot strength of these pipes is low, the wall thickness is thin, and the rigidity of the steel pipe is very low compared to normal steel pipes, the conventional method of strong cooling from the initial stage of cooling causes large bends due to cooling, cannot be charged into a tempering furnace. Alternatively, transportation within the tempering furnace is not successful, resulting in work troubles, resulting in very poor productivity of the quenching-tempering heat treated low-strength line pipe, and significantly increasing costs. In addition, in conventional hardening operations, it is recommended from a metallurgical point of view to cool as much as possible between 800°C and 500°C. In contrast, under the conditions of the present invention, by performing slow cooling in the first stage to equalize the temperature of the heat transfer surface of the steel pipe, and rapidly cooling from near the lowest quench point temperature on the cooling surface, quenching deformation is drastically reduced and the pipe becomes almost straight. A similar steel pipe can be obtained. By the cooling method of the present invention, the problem of quenching deformation has been solved, and it has become possible to stably perform quenching-tempering heat treatment of low-strength, low-rigidity steel pipes. Example As shown in Fig. 3, the following low-strength seamless line pipe material was used in a steel pipe external cooling device (Japanese Patent Publication No. 19370/1983) consisting of a large number of blocks Q ₁ to _{Q 10} in the direction of pipe 1 movement. The results are shown in Table 1. In FIG. 1, reference numeral 2 indicates a conveying device for the pipe 1, and arrow 3 indicates the direction in which the pipe 1 moves. Test material (1) Standard API−5LB (2) Size 140.3φ×4.5 ^t ×11.800 ^L

〔Effect of the invention〕

As explained above, according to the present invention, low strength -
Strain-free cooling methods for low-rigidity metal tubes have been established, such as
As mentioned above, it has become possible to heat-treat and cool steel pipes, which were previously impossible to undergo quenching-tempering heat treatment.
It is very useful for improving pipe characteristics such as sour resistance. In addition, a strain-free cooling method for heat treatment cooling of ordinary metal tubes has been realized, and has great industrial effects.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram showing the cooling process of high-temperature metal to explain the present invention, Fig. 2 is a diagram showing the cooling process of a metal tube to explain the present invention, and Fig. 3 is a schematic diagram showing the cooling process of a metal tube to explain the present invention. This is a block diaphragm in a quenching device. 1: Pipe, 2: Pipe conveyance device, 3: Pipe traveling direction, Q: Water cooling device.

Claims (1)

[Claims] 1. In a heat treatment method in which a metal tube is heated to a predetermined quenching temperature and then cooled, in the case of a metal tube with mill scale attached to its surface, the temperature of the tube surface (heat transfer surface) increases within 0.5 to 3 seconds from the start of cooling. From heating to 450-650℃
In the case of metal tubes with mill scale removed, slow cooling is performed so that the tube surface (heat transfer surface) temperature reaches 550 to 700℃ within 0.5 to 3 seconds from the start of cooling. A method for heat treatment of a metal tube, which comprises reducing the temperature difference between various parts of the metal tube surface (heat transfer surface), and then performing normal strong cooling from that temperature.