JPH06192740A - Method for heat-treating thick walled cast steel pipe - Google Patents

Abstract

PURPOSE:To eliminate unevenness in the heat treatment of a thick walled cast steel pipe by uniformizing the temp. on each part of a thick walled cast steel pipe air-cooled to a two phase region hardening temp. thereafter leaving it to stand for a fixed time and executing hardening. CONSTITUTION:A thick walled cast steel pipe A is charged to the inside of a heating furnace 1 and is held at a temp. at which its structure is formed into a one of an austanic single phase for a suitable time to austenitize the structure of the thick walled cast steel pipe A. This thick walled cast steel pipe A is discharged from the heating furnace 1 and is air-cooled for suitable time to a two phase region hardening temp. at which its structure is formed into two phases of austenite and ferrite. While the air-cooled thick walled cast steel pipe A is rotated, a misty cooling fluid is fed to the inside from the jetting port 3a of a nozzle 3 and the inside of the thick walled cast steel pipe A is subjected to forced cooling to uniformize the temp. at each part. The thick walled cast steel pipe A is which the temp. has been uniformized is left to stand for a certain time to stabilize the temp. distribution, and after that, it is charged into a water tank 2 and is hardened to obtain the thick walled cast steel pipe A free from unevenness in heat treating quality.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment method for a thick cast steel pipe, and more specifically, it is a thick cast steel pipe heated to have an austenite single phase structure, and the structure has two phases of austenite and ferrite. The present invention relates to a heat treatment method for a thick-walled cast steel pipe, which comprises performing air-cooling in a two-phase region quenching temperature to a target temperature region and then performing quenching from that temperature region.

[0002]

2. Description of the Related Art In a thick cast steel pipe used as a material for building structural materials, pipelines, etc., in recent years, a yield ratio has been lowered as one means for improving its performance. This is because if the yield ratio of the thick cast steel pipe is lowered, its deformability and the like are improved, and the earthquake resistance and safety of building structural materials and pipelines are improved. In order to lower the yield ratio of the thick cast steel pipe (for example, 0.8 or less), FIG. 5 (heat treatment conditions shown with time on the horizontal axis and temperature on the vertical axis)
As shown in Fig. 1, after the ordinary quenching (that is, quenching from the austenite single-phase region) is performed on the thick cast steel pipe, the thick cast steel pipe at a low temperature is treated with the two-phase structure of austenite and ferrite. After being heated to the two-phase region quenching temperature, the temperature is maintained at that temperature for an appropriate time, and then quenching from that temperature (hereinafter referred to as the two-phase region quenching) is performed. After the quenching in the two-phase region, tempering is performed in the same manner as in ordinary quenching. When a series of heat treatments including such quenching in the two-phase region are performed, a low yield ratio which cannot be obtained only by performing ordinary quenching and tempering is obtained. By the way, when performing a series of heat treatments as described above, a step of heating the thick cast steel pipe so that the structure becomes an austenite single phase,
Two heating steps, a step of heating the thick cast steel pipe so that the structure has two phases of austenite and ferrite, are required, and a large amount of energy is consumed for heating. Furthermore, since two quenching steps are required in addition to the two heating steps, it takes a long time to complete the entire process. Therefore, in order to reduce the above-mentioned energy during heating and the above-mentioned processing time, FIG. 4 (an explanatory view schematically showing the heat treatment method) and FIG. 6 (horizontal axis shows time, vertical axis shows temperature). Therefore, a heat treatment method as shown in the illustrated heat treatment conditions) is considered. That is, after the thick cast steel pipe A heated in the heating furnace 1 so that the structure becomes an austenite single phase is air-cooled to a temperature region where the target is the two-phase region quenching temperature where the structure becomes two phases of austenite and ferrite. A heat treatment method has been developed in which the thick cast steel pipe A is put into the water tank 2 to perform quenching from the temperature range (hereinafter referred to as direct two-phase quenching). Even after the direct two-phase region quenching, tempering is performed and tempering is carried out in the same manner as in the ordinary quenching and the two-phase region quenching described above.

[0003]

When performing the above-mentioned heat treatment including direct two-phase region quenching, a thick cast steel pipe that is air-cooled to a temperature region in which the two-phase region quenching temperature is targeted is uniformly targeted as a whole. It cannot be said that the temperature (that is, the quenching temperature in the two-phase region) has been reached, and the temperature range after air cooling is quite large, and after air cooling, there is considerable temperature unevenness in the wall thickness direction / pipe axis direction (particularly the wall thickness direction). Has occurred. For example, outer diameter: 800m
m, inner diameter: 620 mm, length: 4 m, for a thick cast steel pipe, set the target temperature (target temperature after air cooling) at the center of wall thickness of the pipe center to 780 ° C. In the conducted state, the surface temperature of the pipe end portion was 730 ° C, the surface temperature of the pipe central portion was 745 ° C, and the internal temperature of the pipe central portion was 8 ° C.
It becomes 50 ° C. Therefore, there is a problem in that the heat treatment quality of the thick-walled cast steel pipe that has been quenched from the temperature range in which such temperature unevenness exists also has unevenness. The present invention has been made paying attention to such an actual situation, and can eliminate the unevenness of the heat treatment quality of the thick cast steel pipe that occurs when a series of heat treatments including the direct two-phase region quenching described above are performed. It is intended to provide a way.

[0004]

The heat treatment method for a thick cast steel pipe according to the present invention is characterized in that a thick cast steel pipe heated so that its structure becomes an austenite single phase is obtained. After air cooling to a target temperature range of the two-phase region quenching temperature is a phase, a heat treatment method of a thick cast steel pipe to quench from the temperature region, the thick cast steel pipe air-cooled to the temperature region By supplying a mist-like cooling fluid to the inside to forcibly cool the inside of the thick cast steel pipe, the temperature of each part of the thick cast steel pipe is made uniform, and the thickness is made uniform. The point is that after the thick cast steel pipe is left for a certain period of time, the quenching is performed.

When the cooling fluid is supplied to the inside of the thick cast steel pipe, it is preferable to supply the cooling fluid alternately from both ends of the thick cast steel pipe.

[0006]

[Operation] For thick-walled cast steel pipes that are air-cooled to the target temperature range of the two-phase region quenching temperature,
Although a considerable temperature unevenness inevitably occurs in the thick wall thickness direction), according to the method of the present invention, the thick cast steel pipe is supplied with a mist-like cooling fluid inside the thick cast steel pipe that is air-cooled to the temperature range. Is forcibly cooled, the temperature of each part of the thick cast steel pipe (in particular, the external temperature and the internal temperature of the thick cast steel pipe) is made uniform by the forced cooling, and the temperature is made uniform by the forced cooling. By allowing the thick cast steel pipe to stand for a certain period of time, the peculiar temperature distribution generated by the forced cooling is smoothed and stabilized. Therefore, the temperature unevenness of the thick cast steel pipe after air cooling is reliably eliminated, and the temperature of each part of the thick cast steel pipe is made uniform to the target two-phase region quenching temperature (or a temperature very close to that temperature). become.
Moreover, when the mist-like cooling fluid is supplied to the inside of the thick cast steel pipe, the supply condition can be easily changed, and the leaving time of the thick cast steel pipe can be easily changed. Therefore, the forced cooling rate of the thick cast steel pipe can be easily changed based on the change of the supply condition, and the temperature distribution of the thick cast steel pipe can be easily leveled based on the change of the standing time. Can be changed to.

[0007]

According to the method of the present invention, as described above,
Since the uneven temperature of the thick cast steel pipe after air cooling is eliminated and the temperature of each part of the cast steel pipe is made uniform to the target quenching temperature in the two-phase region (or a temperature very close to that temperature), direct two-phase The unevenness of the heat treatment quality of the thick-walled cast steel pipe that has been subjected to area quenching is prevented from occurring conventionally due to the temperature unevenness, so that the object of the present invention is achieved. Become. Moreover, as described above, since the forced cooling rate and the temperature distribution of the thick cast steel pipe can be easily changed, the heat treatment conditions can be finely adjusted and further homogenized.

When the cooling fluid is supplied to the inside of the thick cast steel pipe, if it is supplied from only one end of the thick cast steel pipe, the one end side portion of the thick cast steel pipe is the other end side. It is more likely to be forcibly cooled than the part, and temperature unevenness tends to occur easily in the pipe axis direction, but if the cooling fluid is supplied alternately from both ends of the thick cast steel pipe, as described above, The tendency of temperature unevenness to occur in all directions is suppressed, and more uniform heat treatment can be performed.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. In the drawings, the portions denoted by the same reference numerals as the conventional example indicate the same or corresponding portions.

FIG. 1 shows a thick-walled cast steel pipe A (that is, outer diameter: 800 mm, used for a building structure) cast by centrifugal casting.
A heat treatment method for achieving a low yield ratio of a thick cast steel pipe having an inner diameter of 620 mm and a length of 4 m (hereinafter, simply referred to as the thick cast steel pipe A) is schematically shown as an example of the method of the present invention. Has been done.

The procedure of the method of the present invention will be described in detail with reference to FIG. First, the thick cast steel pipe A is charged into the heating furnace 1 set at an appropriate temperature (930 ° C. in this example) to heat the structure so that the structure becomes an austenite single phase, and held for an appropriate time, After the structure of the thick cast steel pipe A is austenitized, the thick cast steel pipe A is taken out from the heating furnace 1, and the two-phase region quenching temperature where the structure becomes two phases of austenite and ferrite (this example Then 7
Appropriate time (for example, 80 ° C) to the target temperature range (for example,
Cool for 15-20 minutes). If it is left to be air-cooled, the actual temperature range is quite large, and there is considerable temperature unevenness in each part of the thick cast steel pipe A (especially in the thickness direction). Therefore, while rotating the thick cast steel pipe A in the state where the air cooling is performed and the temperature unevenness exists (or the state where the air cooling is performed and the temperature unevenness exists), the thick cast steel during the rotation. By supplying a mist-like cooling fluid (specifically, water mist) to the inside of the pipe A to forcibly cool the inside of the thick cast steel pipe A, the temperature of each part of the thick cast steel pipe A (particularly,
The thickness direction) should be made uniform. The thick cast steel pipe A is supplied with a mist-like cooling fluid inside the thick cast steel pipe A.
It is not always essential to rotate the thick cast steel pipe A when forcibly cooling the inside of the, but it is more effective to rotate the thick cast steel pipe A for temperature uniformity. Then, after making the temperature (particularly in the wall thickness direction) of each part of the thick cast steel pipe A uniform, the thick cast steel pipe A is left for a certain period of time and then put into the water tank 2 for quenching. The supply means for supplying the mist-like cooling fluid into the thick cast steel pipe A is, for example, arranged near the outer side of the thick cast steel pipe A in a state of extending in the pipe axis direction of the thick cast steel pipe A. And spout 3
The nozzle 3 in which a is directed to the inside of the thick cast steel pipe A is used. A blower (not shown) is arranged behind the nozzle 3, and the blowout port 3 of the nozzle 3 is used for blowing air by the blower.
The cooling fluid ejected in a mist form from a is placed on the mist-like cooling fluid and supplied to the inside of the thick cast steel pipe A.

In the case of performing the heat treatment in such a procedure, the thick cast steel pipe A which is air-cooled to the target temperature range of the quenching temperature in the two-phase region is left in the air-cooled state in the thickness direction / pipe axis direction (particularly, Although a considerable temperature unevenness occurs in the wall thickness direction), according to the method of the present invention, a mist-like cooling fluid is supplied from the nozzle 3 to the inside of the thick cast steel pipe A that is air-cooled to the above temperature range, Since the inside of the cast steel pipe A is forcibly cooled, the temperature of each part of the thick cast steel pipe A (in particular, the temperature inside and outside the thick cast steel pipe) is made uniform by the forced cooling. In addition, the thick cast steel pipe A after the temperature is made uniform by the forced cooling is left for a certain period of time, so that the forced cooling causes a peculiar temperature distribution in the thick cast steel pipe A. Even so, the peculiar temperature distribution is leveled and stabilized by the above-mentioned leaving. Therefore, the temperature unevenness of the thick cast steel pipe A is reliably eliminated, and thus the thick cast steel pipe A is
The temperature of each part of (1) is made uniform to the target two-phase region quenching temperature (or a temperature very close to that temperature). Moreover, when the mist-like cooling fluid is supplied to the inside of the thick cast steel pipe A, the supply amount thereof can be easily changed, and the leaving time of the thick cast steel pipe A can be easily changed. it can. Therefore, the forced cooling rate of the thick cast steel pipe A can be easily changed based on the change of the supply condition, and the thick cast steel pipe A can be changed based on the change of the standing time.
The method of smoothing the temperature distribution of can be easily changed.

Next, another embodiment will be described. When supplying the cooling fluid to the inside of the thick cast steel pipe A, if the supply is performed only from one end of the thick cast steel pipe A (the above-described embodiment is an example performed from only one end), One end side portion of the thick cast steel pipe A is more likely to be forcibly cooled than the other end side portion, and temperature unevenness tends to occur along the pipe axis direction, but a nozzle (not shown) having the same configuration as the nozzle 3 described above is thick. The thick cast steel pipe A is provided on both sides of the thick cast steel pipe A, and the mist-like cooling fluid is supplied by using both nozzles.
If it is performed alternately from both ends, the tendency that temperature unevenness occurs in the tube axis direction can be suppressed as described above.

In each of the above embodiments, mist-like water is supplied from the nozzle 3 into the thick cast steel pipe A (water is selected, for example,
Depending on the circumstances, a mist-like cooling fluid other than water may be supplied, although post-treatment is easy). Further, depending on the shape of the thick cast steel pipe A, the heat treatment conditions thereof, and the like, it may be possible to supply the cooling fluid in a state not called mist. In particular, when heat-treating a relatively thin cast steel pipe, it may be sufficient to simply supply air from the nozzle 3 as a cooling fluid.

As a nozzle 3 for supplying a mist-like cooling fluid into the thick cast steel pipe A, as shown in FIG.
An embodiment in which the jet port 3b is located at the center of the inside of the thick cast steel pipe A is also conceivable. In this case, a suction device (not shown) for sucking the mist-like cooling fluid jetted and supplied from the jet port 3b of the nozzle 3 is provided outside both ends of the thick cast steel pipe A, respectively. This method is effective for eliminating temperature unevenness in the pipe axis direction when the length of the thick cast steel pipe A is long.

As a device for supplying a mist-like cooling fluid into the thick cast steel pipe A, a nozzle 3 having a double supply path is used as shown in FIG. One of the jet outlets 3d and 3e of the path is positioned at one end of the thick cast steel pipe A, and the other jet port 3e of the jet ports is set to the thick cast steel pipe A. It is also conceivable that the embodiment is located at the center of the inside. According to such a nozzle 3, there is a tendency that the cooling ability on the downstream side in the supply direction of the cooling fluid tends to be relatively decreased due to the cooling fluid from the ejection port 3e located in the center of the inside of the thick cast steel pipe A. It can be solved by cooling.

In the above-mentioned embodiment, the method of the present invention is carried out in order to reduce the yield ratio of a thick cast steel pipe for a building structure. However, another thick cast steel pipe, for example, a pipe. The method of the present invention can be carried out even when the yield ratio of a thick cast steel pipe for a line is reduced.

It should be noted that reference numerals are given in the claims for convenience of comparison with the drawings, but the present invention is not limited to the configurations of the accompanying drawings by the entry.

[Brief description of drawings]

FIG. 1 is an explanatory view schematically showing a heat treatment method according to the present invention.

FIG. 2 is an explanatory view showing another embodiment of the method of the present invention.

FIG. 3 is an explanatory view showing another embodiment of the method of the present invention.

FIG. 4 is an explanatory view schematically showing a conventional heat treatment method.

FIG. 5 is a graph showing quenching conditions in the two-phase region.

FIG. 6 is a graph showing direct two-phase region quenching conditions.

[Explanation of symbols]

 A thick cast steel pipe

Claims (2)

[Claims] 1. A thick-walled cast steel pipe (A) heated to have an austenite single phase structure, is air-cooled to a temperature range targeting a two-phase region quenching temperature where the structure has two phases of austenite and ferrite. A heat treatment method for a thick cast steel pipe for quenching from that temperature range, comprising supplying a mist-like cooling fluid to the inside of the thick cast steel pipe (A) that is air-cooled to the temperature range. By forcibly cooling the inside of the cast steel pipe (A), the thick cast steel pipe (A)
In the method for heat treating a thick cast steel pipe, the temperature of each part is homogenized, the thick cast steel pipe (A) thus homogenized is allowed to stand for a certain period of time, and then the quenching is performed. 2. The heat treatment method for a thick cast steel pipe according to claim 1, wherein the cooling fluid is supplied into the thick cast steel pipe (A) alternately from both ends of the thick cast steel pipe (A). .