JPS62124226A - Heat treatment of metal tube - Google Patents

Abstract

PURPOSE:To prevent deformation of easily bendable metal tube due to heat treatment cooling, by reducing temp. difference at each part of metal pipe at starting point of rapid cooling, while performing slow cooling for a prescribed time from the start of cooling, then carrying out conventional rapid cooling, in cooling metal tube after quenching. CONSTITUTION:Metal pipe such as steel tube having, especially thin wall thickness is heated to quenching temp., then slowly cooled for several sec to reduce temp. difference at each part of metal tube at starting point of rapid cooling. Condition of the slow cooling is <=4m<3>/m<2>.sec by exhibition of water quantity density of actual cooling water. Next, the metal tube subjected to slow cooling is conventionally and rapidly cooled and heat treatment of metal tube is finished. In this way, nonstress cooling of low strength - low rigidity metal tube becomes possible. Consequently, heat treatment cooling of steel tube, etc., which was conventionally not possible to be quenched and tempered is now made possible, and characteristic such as sour resistance of tube can be improved.

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.

(Conventional technology) In recent years, opportunities to drill for sour crude oil and gas have increased.
Line pipes used for transporting these materials are increasingly required to have excellent sour resistance properties. conventionally,
This kind of sour resistance was mainly required for relatively high-strength materials of X60 or higher, but the recent trend is that sour resistance is also required for line pipes with lower strength. The number of cases is increasing. In response to these changes, it has become necessary for low-strength linepipes, which had previously been manufactured as hot-rolled or simply manufactured, to be heat-treated by quenching and tempering in order to guarantee sour resistance properties. 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.

Regarding the cooling of metal pipes, especially steel pipes, many proposals have already been made, 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 an apparatus that uniformly cools a steel pipe by transporting it while rotating and uniformly spraying a large number of fluid jet streams tangentially to the steel pipe from the outer circumferential direction. Furthermore, in Japanese Patent Publication No. 56-19370, a large number of nozzles arranged at equal intervals in the circumferential direction are
A method and apparatus for uniform cooling by applying a violent jet in a direction of 80° to 80° is disclosed. 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, especially thin walls, such as the low-strength line pipes mentioned earlier. It's not something you can get good results from.

The reason why bends tend to occur when cooling low-strength metal pipes 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 putting it away. The cause of uneven stress in different parts of the metal tube during cooling is that the cooling rate of the metal pipe differs depending on the part of the steel pipe.
The fluctuation in the cooling rate is mainly caused by the unevenness of the metal tube surface (heat transfer surface), such as the roughness and the state of 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.

The object of the present invention is to prevent the deformation of such easily curved metal tubes 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: (1) heating a metal tube to a predetermined quenching temperature and cooling it; In the heat treatment method of the metal tube, the cooling is performed for several seconds after the start of cooling to reduce the temperature difference between the various parts of the metal tube at the starting point of strong cooling, and then normal strong cooling is performed. Heat treatment method.

(2) 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 narrow surface, the heat transfer surface temperature increases within 0.5 to 3 seconds from the start of cooling. The temperature should be 450°C to 650°C, and in the case of metal pipes with mill scale removed, the start of cooling should be 0.5 to 650°C.
The heat transfer surface temperature changes from the heating temperature to 550℃ to 70℃ in 3 seconds.
This is a heat treatment method for a metal tube, which is characterized by performing slow cooling to reduce the temperature difference between each part of the metal tube at the starting point of strong cooling to 0°C, 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.

Generally, the rougher the surface, the higher 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 part of the metal tube, a large temperature difference will occur at a relatively early stage of cooling. Thermal stress based on this temperature difference or transformation stress related to the transformation point existing in this temperature range causes deformation and remains even after cooling.

In other words, the deformation that occurs when the metal tube is cooled from the film boiling region is caused by temperature differences in the high temperature region, particularly by uneven quench points.

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 part of the metal tube.

If sufficient cooling water is supplied at the beginning of the cooling of the metal tube, the cooling after the quench point will be greatly accelerated as shown in Figure 2, and the vapor film will not collapse and the quench point will not be reached yet. The temperature difference with the area where there is no
, B indicates 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 performing slow cooling, the temperature difference ΔT at the time of reaching the quench point shown in Figure 2 is achieved. can be reduced from ΔT1 to ΔT2 (BD=ΔT1゜B
C-ΔTz). 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 mentioned above, and if expressed in terms of practical cooling water density, it is 4 m"/m" min.
means a range not exceeding

Another way to align the quench points of each part 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 vapor film is promoted using the protrusions as nuclei. , the quench point increases. For example, if mill scale is removed by shot blasting or the like, the surface will be cleaned and protrusions will be formed uniformly, so the quench point temperature of each part will be the same, and it will be more effective to combine it with the above-mentioned 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 is within the range of 500 degrees Celsius to 850 degrees Celsius. there were.

Therefore, the slow cooling temperature range at the initial stage of cooling differs depending on the hot strength and rigidity of the metal to be cooled, but when mill scale is attached, the heat transfer surface temperature can be set to 450C to 650"C, and when mill scale is removed, 550'C ~ in roughened state
It was confirmed that it is good to lower the temperature to 700u and then strongly cool it.

In a practical heat-treated metal tube with mill scale attached, the lower limit of the quench point temperature is approximately soo℃, so there is no need to slowly cool the heat transfer surface temperature to 450C or less, and it is not necessary to cool the heat transfer surface temperature from 650℃ or higher. In the case of metal pipes with low hot strength and low rigidity when rapidly cooled, the bending will be 25;
1. When mill scale is removed and the surface is roughened, the lower limit of the quench point temperature is approximately 60.
Since the temperature is 0°C, there is no need to slowly cool the heat transfer surface temperature to 550°C or less, and if the heat transfer surface temperature is rapidly cooled from 700°C or higher, metal tubes with low hot strength and low rigidity will be bent greatly and the effect will be small. was also confirmed.

As for the actual degree of slow cooling, as a result of various experiments,
0.5s from the start of cooling when mill scale is attached
The heat transfer surface temperature decreases from the heating temperature to 450°C within ec to 3sec.
Cooling conditions that lower the temperature from ℃ to 650℃ are good, and when mill scale is removed and the surface is roughened, the cooling time is 0.5 sec to 3 sec.
The heat transfer surface temperature is between 550℃ and 70℃ from the heating temperature.
Cooling conditions that lower the temperature to 0°C are preferred. By doing this,
Slow cooling at the initial stage of cooling is performed without actually measuring the temperature of the heat transfer surface.
It is time-manageable and practical. Slow cooling time is 0
．． If the time is 5 seconds or less, the temperature of each part will be uneven, and if it is rapidly cooled thereafter, a large bend may occur.

When the slow cooling time is 3110 degrees or more, the temperature inside the wall thickness also decreases, and especially in heat treatment cooling such as quenching, the thick surface layer may not become a completely quenched structure, which is unsuitable.

(effect) This will be explained specifically 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 perform quenching and tempering heat treatment on low-strength line pipes and the like used for transporting crude oil and gas. These steel pipes have conventionally been manufactured as hot-rolled or simply manufactured, but from the viewpoint of sour resistance, quenching and tempering heat treatment is now required.

In cases where the hot strength is low, the wall thickness is thin, and the rigidity of the steel pipe is very low compared to normal steel pipes, the conventional quenching method of strong cooling from the initial stage of cooling will cause the bending due to cooling to is large and cannot be charged into existing tempering furnaces. Alternatively, transportation within the tempering furnace is not successful, causing work troubles, resulting in very low productivity of quenching and tempering heat-treated low-strength line pipes, and significantly increasing costs. In addition, in conventional hardening operations, it is recommended from a metallurgical point of view to cool as intensely 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 almost A nearly straight steel pipe can be obtained.

By the cooling method of the present invention, the problem of quenching deformation has been solved, and the quenching-tempering heat treatment of low-strength, low-rigidity steel pipes can now be performed stably.

(Example) As shown in Fig. 3, the following low-strength seamless line pipe material was The results are shown in Table 1. In FIG. 1, 2 indicates the conveying device for the pipe 1, and the arrow 3 indicates the direction of movement of the pipe 1.

Test material (1) Standard API -5LB (2) 4? l, l” 140.3φX4.5tX1
1.80OLAlthough the strong cooling start temperature in this example was measured by embedding a thermocouple in the surface of the pipe, the strong cooling start temperature is usually controlled by time control.

According to this, when the slow cooling time in the first stage of cooling is 0.5 sae or less, the strong cooling start temperature is high and the subsequent cooling curves for each part are different, so the curve is large, but the slow cooling time is about 0.
．． 5s@c or more, and when the heat transfer surface temperature is reduced to about 650℃ or less when mill scale is attached, and when the heat transfer surface temperature is reduced to about 700℃ or less when the mill scale is removed and the surface is roughened, the bending decreases rapidly. However, if the slow cooling time is about 3 seconds or more, the temperature of the heat transfer surface will be lower than 450℃ when mill scale is attached, and the temperature of the heat transfer surface will be lower than 550℃ when the mill scale is removed and the surface is roughened.
Moreover, the temperature of the considerably thicker portion of the optical layer decreased, and a martensitic structure could not be obtained. For the above reasons, the slow cooling time in the first stage of cooling is 0.5 sec to 3 a@
During step e, it is recommended to select conditions such that the strong cooling start temperature is 450°C to 650°C when mill scale is attached, and 550°C to 700°C when the mill scale is removed and the surface is roughened. This was confirmed.

〔Effect of the invention〕

As explained above, according to the present invention, a distortion-free cooling method for low-strength and low-rigidity metal tubes has been established. Heat treatment cooling is now possible, sour resistance etc.! Great for improving Eve's characteristics.

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. It is a block diagram of the quenching device. 1: /4' Eve, 2: Pipe conveyance device, 3
:No9ide direction of travel, Q:Water cooling device. Figure 1 12 Death p time; 5 ρ time

Claims (2)

[Claims] (1) In a heat treatment method in which a metal tube is heated to a predetermined quenching temperature and then cooled, the cooling is performed slowly for several seconds after the start of cooling to reduce the temperature difference between each part of the metal tube at the starting point of strong cooling, and then A method for heat treatment of metal tubes, characterized by performing intense cooling. (2) 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 heat transfer surface temperature changes to the heating temperature within 0.5 to 3 seconds from the start of cooling. to 450°C to 650°C, and in the case of metal pipes with mill scale removed, start cooling from 0.5°C to 650°C.
The heat transfer surface temperature changes from the heating temperature to 550℃ to 70℃ in 3 seconds.
A method for heat treatment of a metal tube, characterized by performing slow cooling to reduce the temperature difference between the various parts of the metal tube at the starting point of strong cooling to 0° C., and then performing normal strong cooling from that temperature.