JP2002038219A - Method for producing martensitic stainless steel pipe - Google Patents

Abstract

(57) [Summary] [Problem] To provide a method for manufacturing a martensitic stainless steel pipe which does not crack and can be quenched for a short time. The method of the present invention is characterized in that the pipe is strongly cooled (first cooling) with water until the outer surface temperature of the pipe falls below (Ms point + 400 ° C.) to a temperature higher than the Ms point. Temperature is ΔaMs point + (1-a) Mf point; (However, a = 0.9-0.
5) After gas cooling (second cooling) until the temperature becomes｝, when the pipe is strongly cooled (third cooling) with water until the outer surface temperature of the pipe becomes room temperature, the second cooling
The switching from the cooling to the third cooling is performed based on the measured temperature of the outer surface of the tube.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for producing quench cracks in a martensitic stainless steel pipe, particularly a martensitic stainless steel pipe which is required to have excellent carbon dioxide gas corrosion resistance and sulfide stress cracking resistance. Related to the manufacturing method.

[0002]

2. Description of the Related Art In recent years, the demand for oil country tubular goods for oil and natural gas extraction has increased due to the energy situation, and a steel pipe manufacturing technique with high productivity has been desired. In a severe environment where oil and natural gas are collected, a steel pipe having high strength and corrosion resistance represented by martensitic stainless steel is used. Since this type of steel pipe is highly susceptible to quenching, a quenching method having a low cooling rate such as air quenching is generally employed.

[0003] However, in air quenching, even if quenching cracks can be prevented, productivity is poor and various properties such as carbon dioxide gas corrosion resistance and sulfide stress cracking resistance are degraded. there were.

To solve such a problem, Japanese Patent Application Laid-Open No. 6-100935 discloses Mo, C
The material is made of martensitic stainless steel with the positive addition of u and Ni and the reduction of C, P, S, and N, and the cooling start temperature from 750 ° C or higher to the cooling stop temperature from 550 to 350 ° C is 2 ° C / sec. A method has been proposed in which cooling is performed at the above cooling rate, and then cooling to room temperature at a cooling rate equal to or higher than air cooling.

In Japanese Patent Application Laid-Open No. 10-17934, the temperature of the outer surface of the tube is calculated from the quenching start temperature to (Ms point + 400).
℃) lower than the Ms point, and then strongly cooled (first cooling) until the temperature of the tube outer surface is less than the Ms point at an average heat transfer coefficient of 以下 or less at the end of the strong cooling. Then, after performing the weak cooling (second cooling) until the temperature becomes higher than (the intermediate temperature between the Ms point and the Mf point), the average cooling rate of the inner surface of the tube is set to 8 until the temperature of the outer surface of the tube becomes lower than the Mf point. ° C / s
A method of strongly cooling (third cooling) the outer surface of the tube so as to be equal to or more than ec has been proposed.

However, the method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 6-100935 requires an essential addition of an expensive alloying element in order to improve the properties of the martensitic stainless steel pipe, which increases the production cost of the product. There is a problem.

On the other hand, the method disclosed in Japanese Patent Application Laid-Open No. Hei 10-17934 can prevent the occurrence of quenching cracks without adding an expensive alloy element, and is ideal as a method for quenching martensitic stainless steel pipes. However, when the second cooling of the weak cooling in the method shown therein is replaced with water cooling by spray water or the like, not only is it likely to be supercooled, but also the switching from the second cooling to the third cooling is accurately performed. However, there is a problem that it is difficult to reliably prevent the occurrence of burning cracks.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to prevent the occurrence of sintering cracks without adding an expensive alloy element,
Moreover, it is an object of the present invention to provide a method for producing a martensitic stainless steel pipe having various favorable properties such as carbon dioxide gas corrosion resistance and sulfide stress cracking resistance.

[0009]

Means for Solving the Problems The present inventors have conducted various studies to achieve the above object, and as a result, have found that
The temperature of the outer surface of the tube at the end of the second cooling in the method disclosed in Japanese Patent No.
Point f: (where a = 0.9 to 0.5)}, and if the weak cooling is performed by gas cooling instead of water cooling, quenching cracks do not occur and good corrosion resistance to carbon dioxide gas It has been found that stable production of martensitic stainless steel pipes having heat resistance and sulfide stress cracking resistance is possible.

The gist of the present invention based on the above findings is the following method for producing a martensitic stainless steel pipe.

[0011] Following the first cooling, in which the outer surface of the tube is strongly cooled from the quenching start temperature until the outer surface temperature of the tube falls within a range from (Ms point + 400 ° C) or lower to a temperature higher than the Ms point, the outer surface temperature of the tube is increased. ｛AMs point + (1-a) Mf point; (however,
a = 0.9 to 0.5) a second cooling in which the outer surface of the tube is weakly cooled until the temperature reaches｝, and a second cooling in which the outer surface of the tube is strongly cooled until the outer surface temperature of the tube becomes room temperature after the second cooling. 3. A method for producing a martensitic stainless steel pipe performing 3 cooling, wherein the first cooling and the third cooling are performed with water and the second cooling is performed with gas, and the switching from the second cooling to the third cooling is performed by actually measuring the outer surface of the pipe. A method for producing a martensitic stainless steel pipe based on temperature.

In the first cooling in the method of the present invention, the water cooling conditions such as the required cooling time from the start of cooling to the end of cooling and the required amount of cooling water are determined by measuring a quenching start measured temperature on the outer surface of the tube, a cooling end target temperature, It is preferable to start the calculation under the conditions obtained in advance.

In the second cooling, the gas flow rates of the plurality of gas cooling means divided in the longitudinal direction of the pipe are adjusted so that the temperature in the longitudinal direction of the pipe at the end of the second cooling becomes uniform. It is preferable to change the temperature based on the actually measured temperature of the outer surface of the tube in the longitudinal direction of the tube at the end of one cooling.

Here, the gas cooling of the second cooling means cooling by leaving to cool or blowing an inert gas such as air, nitrogen gas or argon gas.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a method for producing a martensitic stainless steel pipe of the present invention will be described in detail.

First, the martensitic stainless steel as the material to be used in the present invention is a so-called martensitic stainless steel, which is a component designed for the purpose of providing good corrosion resistance. The chemical composition is not particularly limited. However, martensitic stainless steel pipes used as oil well pipes for oil and natural gas extraction generally have C: 0.3% or less in mass%.
Cr: 11-15% martensitic stainless steel is used. Therefore, also in the present invention, the material martensitic stainless steel has a C: 0.3% or less,
Cr: In addition to 11 to 15%, containing appropriate amounts of Si and Mn,
It is preferable to use those having the lowest possible contents of P and S as impurities. More preferably, in addition to the above components,
If necessary, an appropriate amount of Mo, Ni, Al, N, Nb, T
It is desirable to use steel containing one or more selected from i, V, Cu, Ca, Mg and B.

Next, the quenching method of the present invention will be described with reference to the schematic diagram shown in FIG.

As shown in FIG. 1, in the present invention, the temperature T ₁ (first cooling end temperature) in the range where the outer surface temperature of the tube from the start of quenching is lower than (Ms point + 400 ° C.) to higher than the Ms point. First cooling, in which the water is strongly cooled with water, is performed. The quenching start temperature during the first cooling is equal to or slightly higher than the heating temperature performed before quenching (100 ° C.).
Degree) low temperature, usually 750 to 1100 ° C.

In the first cooling, it is necessary to cause tensile plastic deformation on the outer surface of the tube in a high temperature range. For this purpose, the tube is strongly cooled with water until the outer surface temperature becomes (Ms point + 400 ° C.) or lower. There is a need. However, if the pipe is strongly cooled with water until the outer surface temperature becomes equal to or lower than the Ms point, the transformation start timing of the outer surface side and the inner surface side is greatly shifted, so that internal stress accompanying the transformation is generated, and sintering cracks occur. Therefore, the first cooling is performed when the outer surface temperature of the tube is equal to or lower than (Ms point + 400 ° C.).
It was decided to strongly cool with water until the temperature reached a range higher than the point. Usually, the Ms point of the steel targeted by the present invention is 20
Since the temperature is 0 to 300 ° C., the upper limit of the cooling end temperature T ₁ in the first cooling is approximately 700 to 600 ° C.

In consideration of the following, the cooling end temperature T ₁ in the first cooling is (Ms point + 20 ° C.) to (M
(s point + 100 ° C.). That is, there is a possibility that temperature unevenness generated during heating before quenching may remain after the first cooling. Eliminate temperature unevenness just above the Ms point in the second cooling described below. That too high a cooling end temperature T ₁ of the first cooling productivity required time is significantly longer for the next second cooling is reduced.

The method of the first cooling using the strong cooling water is not particularly limited. However, it is preferable that the cooling device is designed so as to suppress the occurrence of remarkable temperature unevenness. From this point of view, it is preferable that the cooling device be designed so that the steel pipe is always moving rather than in a stationary state and that the water of the refrigerant does not concentrate on only a part of the outer surface of the steel pipe. Specifically, for example, a cooling device that supplies laminating water to a pipe outer surface that is rotated in a circumferential direction around a pipe axis as a rotation axis, A cooling device configured to supply water from a nozzle can be used.

The first cooling is performed with a heat transfer coefficient of several thousand to 1,000.
This is a strong cooling of 0 W / m ² K, and the cooling is completed in several seconds to several tens of seconds. Therefore, the start and end of water cooling
It is necessary to have good responsiveness, and from this point of view, it is preferable to use a cooling device provided with an openable and closable shutter that instantaneously shuts off the supply of water to the steel pipe between the water supply means and the steel pipe to be cooled. Good.

Next, the second cooling will be described. As shown in FIG. 1, in the present invention, following the above-mentioned first cooling, the outer surface temperature of the tube is changed to ΔaMs point + (1-a) Mf;
Until (a = 0.9 to 0.5)｝, the system is allowed to cool or weakly cool with a so-called gas by blowing an inert gas such as air, nitrogen gas or argon gas.

Here, the second cooling is made to be weak cooling by gas, and the cooling end temperature T _{2 is set} to a value of ｛aMs point + (1-a)
Mf; (where a = 0.9 to 0.5)}
For the following reasons.

(1) In the case where the gas is weakly cooled from a temperature higher than the Ms point, the uniformity of temperature unevenness generated by the first cooling is promoted mainly by conduction heat transfer in the steel pipe. As a result, the outer surface starts martensitic transformation almost at the same time, and the generation of stress in the steel pipe due to the unbalanced transformation is suppressed, and sintering cracks do not occur.

(2) Using Water as Refrigerant The outer surface of the pipe immediately after the first cooling is in a so-called "wet" state with water attached. Even if we try to weakly cool the outer surface of the tube in this "wet" state with mist that is spray water or gas-liquid mixed water without leaving a gap, the desired weak cooling is not stably realized, and it tends to overcool, Occurs. This is because in the so-called water cooling using spray water or mist, when the surface to be cooled is cooled from a high temperature where the surface to be dried is dry, a heat transfer pattern similar to film boiling is obtained, and a relatively weak cooling ability is obtained. This is because, when the surface to be cooled in the "" state is cooled, a heat transfer form similar to transition boiling or nucleate boiling is exhibited, and high cooling ability is exhibited.

Accordingly, in the present invention, the second cooling is so-called gas cooling by allowing the cooling to be performed or blowing an inert gas such as air, nitrogen gas or argon gas. Therefore, when the gas cooling is allowed to cool, the water constituting the "wet" state generated by the first cooling evaporates and disappears, and the inert gas such as air, nitrogen gas or argon gas is blown (forced). In the case of air cooling, etc., the gas is removed by the blow pressure, so that the spread of uneven temperature due to "wetting" is prevented.

(3) Further, when the second cooling is gas cooling, it is possible to measure the pipe outer surface temperature during the second cooling, which cannot be measured by water cooling. since the cooling end temperature T ₂ can be accurately managed, in addition to it it is possible to prevent the occurrence of quenching cracks more reliably, it is possible to achieve also the elimination of temperature variations in the longitudinal direction of the tube as described below.

In the second cooling, gas cooling by blowing an inert gas such as air, nitrogen gas or argon gas is employed while leaving the cooling as little as possible in order to prevent the required cooling time from becoming extremely long. It is desirable. In addition, since inert gas such as nitrogen gas and argon gas is expensive, it is most preferable to use forced air cooling with air.

Further, the gas cooling device is required to be provided in parallel with the cooling device for performing the first cooling, and to be designed so that switching between the two can be performed instantaneously.

Further, in the second cooling, the cooling end temperature T ₂ is raised to the point of ｛aMs point + (1-a) Mf point;
0.9-0.5)｝, for the following reason. That is, in the second cooling, it is important to promote a certain degree of martensitic transformation, and in particular, to ensure that there is no untransformed part of martensitic transformation in the entire outer surface of the tube. However, if the cooling end temperature T ₂ is too close to the Ms point, there is a high possibility that there is a portion where the martensitic transformation has not started due to slight temperature unevenness on the outer surface of the tube. In particular, when the pipe outer surface temperature is not measured at a plurality of locations, the danger becomes higher. Therefore, it is necessary to set the cooling end temperature T _{2 to} a temperature lower than the Ms point so as to sufficiently absorb the influence of temperature unevenness on the outer surface of the tube. However, if the cooling end temperature T ₂ is too low, residual amount of austenite martensite transformation proceeds excessively in the cooling rate smaller the second cooling is increased, so lowering the corrosion resistance. Therefore, in the present invention, {AMS point cooling end temperature T ₂ of the second cooling in consideration of the influence of the two + (1-a)
Mf point; (however, a = 0.9 to 0.5)}.

The above-mentioned cooling end temperature T ₂ in the second cooling is calculated as follows: ｛aMs point + (1-a) Mf point;
a = 0.7-0.5)}. this is,
If the coefficient a exceeds 0.7, the internal stress generated due to the martensitic transformation is large, which may cause cracks between cooling and tempering in some cases.

Next, the third cooling will be described. As shown in FIG. 1, in the present invention, following the second cooling described above, a third cooling is carried out in which the outer surface temperature of the pipe is strongly cooled with water until the outer surface temperature becomes room temperature. The third cooling may employ the same cooling method as the first cooling described above, or may employ another method. However, from the viewpoint of simplifying the equipment,
It is preferable to employ the same cooling method as the cooling.

It should be noted that the third cooling is to perform aggressive cooling from the start of the third cooling without performing a halfway cooling, for the following reason.
That is, the outer surface of the pipe is dry at the end of the second cooling, and if halfway cooling is performed here, "wet"
This is because there is a large temperature unevenness between the two parts, which are divided into portions where no cracks are generated and portions where no cracks are generated, which may cause sintering cracks. Therefore, in the third cooling, it is necessary to actively perform strong cooling from the start of the cooling to instantaneously generate “wetting” over the entire outer surface, thereby suppressing the occurrence of temperature unevenness.

According to the above-described method of performing the steps from the first cooling to the third cooling, the martensitic stainless steel pipe can be cooled (quenched) without causing quenching. However, some residual stress is generated in the quenched martensitic stainless steel pipe, and if left as it is for a long time, cracks may occur due to the residual stress. For this reason, the martensitic stainless steel pipe after quenching should be tempered as early as possible, specifically within 24 hours, preferably within 12 hours after quenching.

The tempering is performed by A_C1Perform below the transformation point
The tempering temperature is not particularly limited.
L80 gray defined by I (American Petroleum Institute) standard
593 ℃ or more_C1Beyond the transformation point
Below, desirably 650 ° C or higher A _C1Temper below the transformation point
Is good.

Next, a specific method for performing the above-described first to third cooling with high accuracy and a cooling device suitable for use in the method will be described.

The most significant feature of the above-described quenching method is that gas cooling is used as the second cooling. Thus, when the cooling medium is water, the cooling becomes impossible due to water obstruction. It is now possible to measure the outer surface temperature of the pipe on the way.

[0039] That is, when passing through the third cooling step from the first cooling above, the quenching crack does not occur are as described above, among others cooling end temperature T ₂ in the second cooling of quenching cracks had the greatest effect on the occurrence, to know the cooling end temperature T ₂ in the second cooling precisely is extremely important.

However, if a water cooling method such as spray water is adopted as a weak cooling method for the second cooling, the most important second cooling method is adopted.
Because the actual cooling end temperature T ₂ in the cooling can not be measured,
The cooling end timing of the second cooling must be managed by the time required from the start of the first cooling to the end of the second cooling, the amount of cooling water, and the like, based on the results of previous experiments and simulations. As a result, in practice, sometimes cooling end temperature T ₂ in the second cooling third cooling is performed before the a predetermined temperature, quench cracking is generated in this case, as described above, quench cracking A stable martensitic stainless steel pipe cannot be produced.

On the other hand, when the gas cooling method is used as the second cooling method as in the present invention, the second cooling method is used.
Since the cooling end temperature T ₂ of the cooling can be measured,
For example, if a temperature measuring means such as a non-contact radiation thermometer is provided in the cooling device to monitor the pipe outer surface temperature during the second cooling, when the temperature reaches the predetermined cooling end temperature T ₂ , It is possible to accurately shift to the third cooling, and it is possible to stably produce a martensitic stainless steel pipe free from sintering cracks.

When the cooling means rotates a pipe on a turning roller, a plurality of temperature measuring means are provided in the longitudinal direction of the cooling device, and the second cooling means is provided in the longitudinal direction of the cooling device. When the gas flow rate to the plurality of divided second cooling means is adjusted according to the temperature unevenness in the longitudinal direction of the pipe, the cooling end temperature in the longitudinal direction of the pipe in the second cooling is made uniform. Therefore, it is possible to more stably produce a martensitic stainless steel pipe free from burning cracks.

The method using the turning roller is as follows.
This is particularly effective for suppressing the temperature drop of the pipe portion that comes into contact with the turning rollers 2 and 2 and for achieving uniform temperature in the pipe longitudinal direction, especially in the longitudinal direction of a long pipe.

Further, the first temperature is measured by using the above-mentioned temperature measuring means.
When the tube outer surface temperature (quenching start temperature) immediately before the start of cooling is actually measured, the actual quenching start temperature can be accurately known. Water cooling conditions such as the amount of cooling water from the start of cooling to the end of the first cooling and the required cooling time can be obtained in advance. Therefore, when the first cooling is performed under the water cooling condition obtained in advance, the transition from the first cooling to the second cooling can be accurately performed for each pipe, and the martensitic system free from burning cracks. It is possible to manufacture a stainless steel pipe more stably.

In the above case, the cooling end temperature T ₁ in the first cooling is controlled by fine cooling control for each pipe.
Can be controlled to a lower temperature closer to the Ms point. As a result, it is possible to realize an optimum cooling pattern in which the required time for the second cooling is kept to a minimum, and the productivity is further improved. In addition, in the first cooling, an appropriate tensile plastic deformation is applied to the outer surface of the tube. Thus, the generation of stress due to the subsequent martensitic transformation is suppressed, and the effect of suppressing quench cracking is also obtained.

The water cooling conditions such as the required cooling time and the required cooling water amount are determined based on the results of previous experiments and simulations.
c) can be obtained by the following equation.

T = A (X / Q) ln ｛(T _1S -T _W ) /
(T _1E −T _W )｝ where A: a coefficient determined by cooling conditions and a positive value (l · se
c / mm · min), X: wall thickness of the pipe to be cooled (mm), Q: cooling water amount (l / min), T _1S : quenching start measured temperature (° C.), T _1E : cooling end target temperature (° C.) ), T _W : cooling water temperature (° C.).

FIG. 2 is a front view schematically showing a quenching apparatus suitable for use in the method of the present invention. A tube 1 to be processed is rotated around its axis by turning rollers 2 and 2. Laminating water 3a, 3a arranged in two rows so as to sandwich the axis of the pipe 1 is placed above the turning rollers 2, 2.
The slit nozzles 3, 3 for supplying toward the outer surface are arranged. Further, between the turning rollers 2 and 2 and the slit nozzles 3 and 3, a shutter 4 that is openable and closable that instantaneously shields lamina water 3a and 3a supplied toward the outer surface of the pipe 1 is disposed. Further, the turning roller 2,
Gas supply nozzles 5, 5 and a thermometer 6 are arranged on the side of 2.

The method of the present invention using the quenching apparatus configured as described above is performed according to the following procedure.

First cooling: The shutter 4 is opened (solid line in the figure), the gas supply nozzles 5 and 5 are closed, the turning rollers 2 and 2 are driven to rotate the pipe 1 around its axis, and the slit is slit. The lamina water 3a, 3a is supplied from the nozzles 3, 3 toward the outer surface of the pipe 1.

Second cooling: While the rotation of the pipe 1 by the turning rollers 2 and 2 is continued, the shutter 4 is closed (the broken line in the figure), the gas supply nozzles 5 and 5 are opened, and the gas supply nozzles 5 and 5 are opened. To supply the gas to the outer surface of the tube 1. At this time, the external surface temperature of the tube 1 is measured by the thermometer 6, the temperature is shifted at the same time following the third cooling becomes a cooling end temperature T _2.

Third cooling: While the rotation of the pipe 1 by the turning rollers 2 and 2 is continued, the shutter 4 is opened (in a solid line state in the figure), the gas supply nozzles 5 and 5 are closed, and the slit nozzles 3 and 3 are closed. Lamina water 3a, 3a is supplied toward the outer surface of the pipe 1.

The above-mentioned slit nozzle may be a slit nozzle that supplies a single piece of laminating water immediately above the axis of the pipe 1. In addition, gas supply nozzles 5, 5
Is preferably divided into a plurality of pieces in the longitudinal direction of the tube 1 and arranged continuously. In this case, the thermometers 6 are arranged in the longitudinal direction of the tube 1 by the number of divisions of the gas supply nozzles 5, 5. Is preferably as described above.

[0054]

EXAMPLE An outer diameter of 151 m having the chemical composition shown in Table 1
A seamless test tube having a thickness of 5.5 m, a thickness of 5.5 mm, and a length of 1 m and a quenching apparatus shown in FIG. 2 were prepared, and a test was conducted to harden 10 test tubes under the respective conditions shown in Table 2.

At this time, the test tube was 980 in each case.
After heating to ° C., it was subjected to a test, and was rotated around its axis at a rotation speed of 60 rpm. Further, laminating water and spraying water used were those having a water temperature of 30 ° C., and air at room temperature was used for gas cooling (forced air cooling). Further, in the method of Test No. 2 of the present invention, the cooling end temperature of the second cooling was measured using a non-contact radiation thermometer, and the second cooling was switched to the third cooling based on the measurement result. .

[0056]

[Table 1]

[Table 2] In Table 2, Test No. 1 is a comparative method in which the cooling end temperature of the second cooling is out of the range specified in the present invention, Test No. 2 is the method of the present invention, and Test No. 3 is spray water cooling for the second cooling. The comparative method used, trial number 4 was a comparative method in which lamination was performed only from the start to the end of quenching, trial number 5 was a conventional method in which cooling was performed from the start to the end of quenching, and trial number 6 was a method from the start to the end of quenching. This is a conventional method that uses only forced air cooling (gas cooling).

Then, the pipe surface immediately after quenching was visually observed, and the number of pipes in which the occurrence of quenching cracking was observed even in a part was examined. In addition, the time required from the start of quenching to the end of quenching and the microstructure of the quenched structure were micro-observed to determine the martensite ratio (volume%) in the structure. The results of these investigations are also shown in Table 2.

As shown in Table 2, in the method of the present invention No. 2, no cracking occurred, a good structure having a martensite ratio of 99% was obtained, and the required time was as short as 51 seconds. It is.

On the other hand, in the comparison method of Test Nos. 1 and 3, the structure was good with martensite ratios of 99% and 100%, and the required time was as short as 32 seconds and 13 seconds. Among them, four and six cracked.

In the comparative method in which the quenching from the start to the end of Test No. 4 was carried out only with lamina water, the required time was as short as 12 seconds and the structure was as good as 100% martensite. Burn cracking occurred in seven of them.

Further, according to the conventional method of cooling the sample No. 5 from the start to the end of quenching, quenching cracks did not occur.
With a martensite ratio of 90%, the structure was somewhat unsatisfactory, and the required time was extremely long at 4500 seconds.

Further, in the conventional method in which the quenching from the start to the end of test number 6 is performed only by forced air cooling (gas cooling),
No cracking occurred, but the martensite ratio was 93%
The organization was somewhat unsatisfactory, and the required time was long at 467 seconds.

In the comparison method, the time required for test number 3 was 13
The extremely short second is that the second cooling subsequent to the first cooling is spray water cooling to the surface to be cooled under the "wet" state, so that the second cooling is supercooled, and is weak like the second cooling by the gas of the present invention. This is because cooling was not realized.

[0064]

According to the method of the present invention, it is possible to prevent the occurrence of quenching cracks even without adding an expensive alloy element, so that a martensitic stainless steel pipe having good corrosion resistance can be manufactured at low cost and with high yield. can do.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a cooling pattern according to the present invention.

FIG. 2 is a schematic front view showing an example of a quenching apparatus suitable for use in the method of the present invention.

[Explanation of symbols]

 1: steel pipe, 2: turning roller, 3: slit nozzle, 3a: lamina water, 4: shutter, 5: gas supply nozzle, 6: thermometer.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 3H111 AA01 BA03 DA08 DA26 DB08 DB27 EA12 4K042 AA06 BA06 BA14 CA16 DA01 DD02 DD05 DE01 DE06 EA02

Claims (3)

[Claims] 1. An external surface of a tube following the first cooling for strongly cooling the external surface of the tube from a quenching start temperature to a temperature within a range from (Ms point + 400 ° C.) to a temperature higher than the Ms point. Temperature is ΔaMs point + (1-a) Mf point; (however,
a = 0.9 to 0.5) a second cooling in which the outer surface of the tube is weakly cooled until the temperature reaches｝, and a second cooling in which the outer surface of the tube is strongly cooled until the outer surface temperature of the tube becomes room temperature after the second cooling. 3. A method for producing a martensitic stainless steel pipe performing 3 cooling, wherein the first cooling and the third cooling are performed with water and the second cooling is performed with gas, and the switching from the second cooling to the third cooling is performed by actually measuring the outer surface of the pipe. A method for producing a martensitic stainless steel pipe based on temperature. 2. The water cooling condition from the start of the first cooling to the end of the first cooling is determined in advance based on the actually measured temperature of the start of quenching of the outer surface of the tube and the target temperature of the end of cooling by the first cooling. The method for producing a martensitic stainless steel pipe according to claim 1, wherein the first cooling is started under cooling conditions. 3. A second cooling gas cooling means for cooling with gas is arranged in the longitudinal direction of the pipe, and the gas flow rate of each gas cooling means is adjusted in the longitudinal direction of the pipe at the end of the second cooling. The method for producing a martensitic stainless steel pipe according to claim 1 or 2, wherein the temperature is changed based on the actually measured temperature of the pipe outer surface in the pipe longitudinal direction at the end of the first cooling so that the temperature becomes uniform.