JPS63134633A - Cooling method for steel pipe - Google Patents

Abstract

PURPOSE:To assure desired mechanical properties and metal structure without generating quenching cracks by making combination use of annular outside surface cooling headers and bar-shaped inside surface cooling headers to cool a steel pipe so that the prescribed cooling rate pattern and the max. value of transient thermal stress are assured. CONSTITUTION:The steel pipe 2 emitted from a heating furnace 1 is moved by conveying rollers 4 and is passed between the annular outside surface cooling headers 51-58 and the bar-shaped inside surface cooling headers 15, by which the steel pipe is cooled. The inside surface cooling headers 15 are preliminarily shifted in an adequate moving direction from the outside surface cooling header 51 by a driving device 16. Force cooling nozzles are used for misting nozzles to maintain an air-water ratio at about 5-10. The respective cooling capacities are so set in this constitution as to assure the cooling rate pattern satisfying the temp. history during cooling intrinsic to the kind of the steel to be heated necessary for assuring the mechanical properties and metal structure required for the steel pipe 2 after the cooling over the entire wall thickness direction of the steel pipe and the max. value of the transient thermal stress near the inside and outside surfaces to prevent generation of the quenching cracks during the progression of martensite transformation near the inside and outside surfaces of the steel pipe 2 under cooling.

Description

[Detailed Description of the Invention] [Industrial Application Field] There are two methods for cooling steel pipes: one using a cooling header and the other using an immersion method. The present invention relates to a method for cooling a steel pipe using a single type cooling header, and more particularly, to a method for cooling a heated steel pipe in which the desired metal weave and mechanical properties can be imparted by strictly controlled cooling from the inner and outer surfaces of the steel pipe. In particular, the present invention relates to a cooling method suitable for thick-walled steel pipes having a wall thickness of 15 mm or more.

[Conventional technology]

Conventionally, cooling of thick-walled steel pipes during quenching, etc. has been mainly carried out by using a spray nozzle in combination with an external cooling header and an internal cooling header. Because the water volume range is narrow, it is difficult to adjust the cooling capacity according to the steel pipe dimensions and cooling specifications. In addition, the internal cooling start timing (corresponding to the fixed position of the internal cooling header) is a major item when performing internal and external cooling, but it has not been adjusted based on the material and dimensions of the steel pipe.

Therefore, when performing cooling control, it is common practice to keep the cooling capacity of the header constant, keep the position of the internal cooling header constant, and only adjust the conveyance speed according to the steel pipe dimensions, material, cooling specifications, etc. It was a target.

On the other hand, recently, in cooling hot-rolled steel sheets, the cooling rate pattern has been controlled in addition to the coiling temperature (cooling end point temperature), but in this case, the control is within a relatively small range of cooling capacity. In many cases, the adjustment range of the cooling capacity can be narrow, and it is possible to deal with the problem by making small improvements to existing means without using any new methods or equipment.

[Problem that the invention seeks to solve]

However, in the case of cooling steel pipes, high-speed cooling is often required, and in particular, in cases such as quenching, there are requirements such as a cooling rate of 50° C./S or more even in the portion where the cooling rate is slowest. In this way, when the absolute value level of the required cooling capacity is high and the cooling capacity needs to be controlled over a wide range, the above-mentioned improvements to the existing means cannot be used at all, and in the case of thick-walled steel pipes, It becomes even more difficult to respond.

The present invention is applicable not only to thin-walled steel pipes with a wall thickness of 15 mm and no drips, but also to
The object of the present invention is to provide a highly accurate cooling method for steel pipes that can strictly and stably provide the required metal structure and mechanical properties even to thick-walled steel pipes with a wall thickness of 15 mm or more.

[Means for solving problems]

By the way, the present inventors have previously developed a new steel pipe cooling method for the purpose of imparting desired Mi weave and properties to steel pipes.
The application was filed by the applicant (Japanese Patent Application No. 12333/1986)
No. 4 and No. 61-3340), these methods were developed from the following viewpoints.

■ In order to obtain the desired metallographic structure and mechanical properties of steel pipes, it is essential not only to cool them thoroughly but also to control the temperature history during cooling.

■ In order to perform this steel pipe cooling, a new cooling header and cooling method that can strictly control the cooling capacity are required.

■ As a cooling header, misting jet nozzles, which were mainly used for slow cooling, have been replaced with air-water ratios (
A cooling nozzle with a small ratio of supplied air to water and a large amount of water is used.

In other words, misting jet nozzles were conventionally used for slow cooling, such as cooling control in CC (continuous casting), and were used at an air/water ratio of 30 or more, but with this method, they are used for strong cooling of steel pipes. It is used at an air-water ratio of about 5 to 10. In other words, if the air-water ratio is less than 5, the energy of collision of the mist particles with the material to be cooled decreases, resulting in a decrease in cooling ability, and if it exceeds 10, the mist particle size becomes small and sufficient cooling ability cannot be obtained.

In addition, the amount of water used is in the range of 5 to 2017 minutes per nozzle, whereas conventionally it was about 5 to 717 minutes per nozzle. In other words, 5I! If it is less than 201/min/nozzle, it is difficult to obtain a stable cooling capacity for each nozzle, and if it exceeds 201/min/nozzle, the effect on the cooling capacity when changing the amount of water decreases, and there is no point in adjusting the amount of water.

■ As a cooling means, a plurality of the above-mentioned headers (annular headers) are installed in series on a steel pipe conveyor line whose speed can be adjusted. Set independently.

■ In particular, regarding patent application No. 3340/1986, conditions were set so that the temperature difference in the wall thickness direction during the cooling process was within a range that would not cause quench cracking near the cooling surface of the steel pipe during martensitic transformation, and countermeasures against quench cracking were taken. shall be.

However, according to subsequent research by the present inventors, it was found that these methods were insufficient in handling mainly thick-walled steel pipes.

The present invention adds measures against thick-walled steel pipes and further improved measures against quench cracking to the method of the prior application described above, and the gist thereof is to provide a misting jet nozzle in which the amount of water and the air-water ratio can be adjusted. An annular external cooling header with variable cooling capacity and a plurality of annular external cooling headers installed in a row in the direction of steel pipe movement in a variable speed roller conveyor, and a misting jet that can also adjust water volume and air/water ratio < li The cooling capacity is variable using a nozzle, and ') a rod-shaped internal cooling header that is movable in the direction of movement of the steel tube inside the 8 tubes is used, and the steel tube heated to a predetermined temperature is heated to a predetermined temperature between the annular enclosure and the rod-shaped internal cooling header. Cooling is performed by inserting and moving between the
The moving speed of the steel pipes, the cooling capacity of each outer cooling header, the fixing position of the inner cooling header, and the cooling capacity of the inner cooling header should be adjusted to ensure the following cooling rate pattern and to ensure the maximum transient thermal stress value shown below. The gist of this article is a cooling method for steel pipes that is characterized by the following settings.

Cooling rate pattern: A cooling rate pattern that satisfies the temperature history during cooling that is specific to the type of steel to be heat treated and is necessary to obtain the mechanical properties and metallographic structure required for the steel pipe after cooling over the entire thickness direction of the steel pipe.

Maximum value of transient thermal stress: The maximum value of transient thermal stress near the inner and outer surfaces of a steel pipe during cooling without causing quench cracking during martensitic transformation.

[For production]

The effectiveness of the present invention will be described below.

○ Effectiveness for the mechanical properties and metallographic structure required of the steel pipe after cooling When viewing quenching as cooling of the steel pipe, the temperature history during cooling that should be set should be determined based on the required structure and metallographic structure of the steel pipe after cooling. It can be determined from hardness as a mechanical property (Vickers hardness) and CCT line times known for each steel pipe material.

Figure 3 shows C: 0.25%, Si: 0.29%, Mn: 0
．． This is a CCT diagram for steel containing 42%, p:o, ots%, showing the relationship between the cooling temperature and Ac, the elapsed time after the temperature is turned off, and the hardness obtained.

According to this CCT chart, for example, if cooling is performed along the cooling curve (2), a mixed structure of bainite structure and martensite will be formed, and the Vickers hardness will be 402. Also cooling curve■
If cooling is performed along the lines shown in FIG.

Therefore, if the hardness and material of the steel pipe required for the steel pipe after quenching are given, the temperature history required during cooling during quenching will be determined.

Although Vickers hardness is taken up here as a mechanical property, since toughness, tensile strength, etc. are also controlled by the temperature history during cooling, the term "mechanical properties" in the present invention refers to all of these performances. It is.

Furthermore, the temperature history or cooling rate pattern during cooling in the cooling method of the present invention includes the cooling end point temperature.

Effectiveness against O quench cracking When steel pipes are externally cooled and quenched using a cooling header,
A deviation occurs in the temperature distribution in the direction of the steel pipe, and the temperature history differs depending on the position in the wall thickness direction.

Now, suppose that 100% multi-tinted & lI weave is required as the structure after quenching.
So that the part with the slowest cooling rate in the wall thickness direction passes only within the M (martensite) range that passes through the B (bainite) range, that is, below the cooling curve shown by the ■ curve. It is necessary to set the temperature history.

In this case, the above-mentioned first-to-file method (Japanese Patent Application No. 6l-33405)
Then, the cooling curve satisfies this condition at the point where the cooling rate is slowest (the inner surface of the steel pipe in the case of external cooling, the center of the wall thickness of the steel pipe in the case of inner and outer cooling), and the temperature difference in the wall thickness direction is A cooling control method has been considered in which water cooling heat is dissipated from the outer surface so that the vicinity of the cooling surface of the steel pipe is kept within a range where quenching cracks do not occur during martensitic transformation.

However, with this method alone, there is no problem with thin-walled steel pipes up to about 15m5, but when it comes to thick-walled materials such as 15 to 3Qs, the mechanical properties required of the steel pipe after cooling are insufficient due to the lack of cooling capacity. Not only is it impossible to obtain a good metallographic structure, but even if sufficient cooling capacity were achieved, quench cracking would occur due to the rapid temperature gradient near the outer surface of the steel pipe and thermal stress caused by expansion due to transformation.

The present invention provides a solution to these problems, in that only an external cooling header is required to ensure that the entire steel tube has the temperature fat layer necessary to obtain the required mechanical properties and metallographic composition of the steel tube after cooling. In addition, from the viewpoint of suppressing the sudden temperature gradient near the cooling surface of the steel pipe and the thermal stress caused by expansion due to transformation, prevention of quenching cracking is based on thermal stress analysis using the finite element method. This is a measure that has been introduced.

In other words, this idea of preventing quench cracking attempts to directly understand the thermal stress, which is the original cause of the occurrence of quench cracking, by the maximum value of transient thermal stress during the progress of martensitic transformation near the cooling surface of the steel pipe, and goes beyond the above-mentioned method. How to apply (Special application 6l-
33405), that is, replacing the thermal stress, which is the original cause of quench cracking, with the temperature difference in the thickness direction of the steel pipe, is taken one step further.

The thermal stress analysis during quenching will be described in detail later, but the literature includes Zhigang Wang and Tatsuo Inoue (Kyoto University) ``Analysis of temperature, structure, and stress during quenching of steel considering the stress dependence of phase transformation.'' ” Masahiro Kadokawa, Shigeru Nagagi, and Tatsuo Inoue (Kyoto University) ``Analysis of structural changes and stress during quenching and low-temperature tempering of steel'' is a useful reference.

In the method of the present invention, highly accurate cooling control can be achieved, and this is because misting jet nozzles are adopted for the inner and outer cooling headers, and the cooling capacities of the outer cooling head and the inner cooling head are set independently. Needless to say, this is based on the fact that the conveying speed of the steel pipe and the fixing position of the inner cooling groove are continuously variable.

[Mode of implementation]

Hereinafter, the present invention will be explained in more detail with reference to the drawings.

FIG. 1 shows an example of an apparatus for carrying out the method of the present invention, in which (a) is a plan view and (b) is a side view. According to the figure, the high-temperature steel pipe 2 coming out of the heating furnace 1 is moved by the conveying roller 4 in the direction of the arrow 3, and the outer cooling header 51.5□...
.--.passes between 5@ and the internal cooling header 15. A plurality of misting jet nozzles are provided centripetally toward the pass line of the steel pipe 2 on the inner surface of each outer cooling hemlock.

In addition, in order to delay the internal cooling start timing from the external cooling start timing by a required period of time, the internal cooling header 15 is moved in the steel pipe conveying direction by the drive device 16 to a fixed position 7 with respect to the external cooling header. io can be adjusted freely.

The outer surface of this inner cooling header 15 has an outer cooling head.
The misting jet nozzles are arranged radially over the entire area of length lc' corresponding to the effective cooling length fc, and all the nozzles are adjusted under the same conditions (water amount, air-water ratio).

As mentioned above, misting jet nozzles have conventionally been used for relatively slow cooling applications, but in the present invention, misting jet nozzles are used in applications where the air-water ratio is The nozzle, which used to be large (more than 3O), has been made smaller and has a larger amount of water, and is used as a strong cooling nozzle to control the cooling rate of steel pipes. Specifically, the air/water ratio is desirably set within a range of about 5 to 10, which provides a high cooling capacity and a uniform misting jet flow.

However, since the appropriate value of the air-water ratio depends on the characteristics of the nozzle, the value of 5 to 10 is not necessarily fixed.

Figure 2 (A) shows the cooling capacity adjustment device for the external cooling header (installed for each head), and (2) shows the same device for the internal cooling header.

First, for the external cooling header, cooling water is supplied from the cooling water supply source 6 to the cooling header 5115□... via the cooling water flow rate control devices 7□, 7□..., and air is supplied from the air supply source 5115□... 8, the air is supplied to the cooling headers 5+, 52, . . . via an air flow rate control device 919z, . The cooling water and air are mixed by misting jet nozzles in each cooling header 51.5t... to form a misting jet stream, which is sprayed onto the outer surface of the steel pipe 2 passing through the header. That is, each external cooling header can independently adjust its cooling capacity to control the cooling rate.

In addition, the more external cooling headers are installed, the better the overall cooling capacity will be, and the steel pipes can be transferred at a faster speed, which will improve productivity, but due to equipment cost and equipment space constraints, in this example, eight stages are used. .

On the other hand, for the internal cooling header, since the header has only one head, the cooling water is supplied from the cooling water supply source 17.
Air is supplied from the air supply source 19 to the cooling header 15 via the air flow rate control device 20. Similar to the above, this cooling water and air are mixed by the misting jet nozzle of the cooling header 15 to form a misting jet stream, which is sprayed onto the inner surface of the steel pipe 2 passing outside the header.

Further, in the above-mentioned conveyance device, the conveyance speed can be arbitrarily set using the conveyance roller 4.

That is, for the cooling device as a whole, the conveyance speed and the cooling capacity of each cooling header can be freely selected depending on the dimensions, material, and cooling specifications of the steel pipes 2.

By using the above-mentioned device, it is possible to give the steel pipe a desired temperature history while preventing quench cracking. The method is described below.

As the cooling device, the example shown in FIG. 1 consisting of eight outer cooling headers and eight inner cooling headers is used.

As for the critical temperature history (the part with the slowest cooling rate is the temperature history of the inner surface in the case of outer surface cooling), consider the cooling curve ■ (Figure 3).The cooling end point temperature is 180℃ (T□: martensite) (transformation end temperature) or lower.

In order to satisfy these conditions, we first calculate cooling in the thickness direction of the steel pipe using a temperature calculation model using the heat transfer coefficients of the inner and outer surfaces of the steel pipe, the timing of starting inner cooling, etc. required for cooling by each outer cooling header and inner cooling header. Determine the cooling rate pattern and use this cooling rate pattern to determine whether the maximum value of transient thermal stress near the inner and outer surfaces of the steel pipe is within a range that does not cause quench cracking near the end of martensitic transformation using a thermal stress analysis model. This sets the inner and outer cooling conditions.

These can be done by off-line simulation, and the calculation flowchart is shown in FIG. 4, and will be explained in detail below (see FIGS. 5 to 1O).

■, Loading calculation conditions 1, Cooling equipment specifications 2, CCT diagram various conditions Martensitic transformation start temperature 7.3 Termination degree "rxt" etc. 3, Steel pipe dimensions (outer diameter and wall thickness) 4, Required cooling rate 5, Cooling start temperature (uniform in thickness direction) T2C, cooling end temperature T.

etc.

n, *Determination of feeding speed 1' It is assumed that all cooling headers of hl-fh8 are used for cooling, and the feeding speed υ is determined by the following formula.

υTomi2. /1°However, 1. ;Total length of cooling header tf: Cooling time Note that this cooling time t is determined by determining the steel pipe cooling rate required to obtain the required mechanical properties and metallographic structure of the steel pipe after cooling from the CCT diagram. Defined by.

■ Assumption of external cooling conditions First, l1kL1 external cooling header (1 control zone)
Based on the heat transfer coefficient, calculate the temperature when passing through the external cooling tube. And then, the next
The above calculation is performed sequentially from the outer surface cooling to the hood, and the cooling rate pattern in the thickness direction of the steel pipe is determined by cooling only 7 gou to the outer surface cooling.

Although the mechanism by which quench cracking occurs has not been completely elucidated, the cooling surface of steel pipes (the outer surface for external cooling and the inner surface for internal cooling) is affected by rapid temperature gradients during cooling and thermal stress caused by expansion due to transformation. It is known to be prone to cracking.

The present invention is also basically based on this idea,
In order to reduce the temperature difference in the thickness direction of the steel pipe during martensitic transformation, strong cooling is performed on the outer surface immediately after the start of cooling to obtain the specified cooling rate, and after martensitic transformation begins, warm cooling is performed. External cooling control is performed to reduce the temperature difference in the thickness direction of the steel pipe.

Points to note in the above-mentioned outer surface cooling control method will be explained with reference to FIGS. 5(a) and 5(b).

In (a), when heating is performed after strong cooling immediately after the start of external cooling,
This is an example where the difference in cooling capacity between adjacent control zones is too large, causing recuperation on the cooling surface (outer surface). This is an example of cooling.

This reheating has an adverse effect on mechanical properties after quenching, such as variations in hardness in the thickness direction, and must be avoided for quality control reasons.

Note that the temperature calculation model used here is a one-dimensional heat transfer model in the thickness direction, and is a well-known technique, so its explanation will be omitted.

(2) Assumption of internal cooling conditions First, give the internal cooling start timing. The idea is that in the case of cooling the inner and outer surfaces, the cooling rate is lowest near the center of the steel pipe wall thickness, so the timing when the center of the steel pipe wall thickness reaches AC3 (austenite transformation start temperature during heating) during outer surface cooling is determined by the inner surface cooling rate. By providing this as the initial value for the cooling start timing, the aim is to increase the cooling rate at the center of the wall thickness of the steel pipe. This is based on the idea that the cooling rate during quenching is generally defined by the cooling rate between AC3 and Tss (martensite transformation start temperature).

Next, give the pufferfish's heat transfer coefficient to the internal cooling.

Generally, in the case of cooling the inner and outer surfaces of a steel pipe, the inner surface has a greater cooling effect than the outer surface, so approximately 50% of the outer surface cooling capacity is given as an initial value.

Then, the cooling rate pattern in the wall thickness direction of the steel pipe is determined using the external cooling heat transfer coefficient given in ■, the internal cooling start timing, and the internal cooling heat transfer coefficient described above (using the same temperature calculation model as in ■).

FIG. 6 shows the cooling rate pattern in the wall thickness direction of the steel pipe during cooling of the inner and outer surfaces at this time.

Regarding internal cooling, as in the case of external cooling described in (2) above, strong cooling is performed immediately after the start of internal cooling to increase the cooling rate, and warm cooling is performed after the start of martensitic transformation to reduce the temperature difference in the thickness direction of the steel pipe. It is desirable to try to reduce the
In order to achieve this, it is necessary to change the cooling capacity in the axial direction of the inner cooling tube, which makes the device configuration complicated.

Therefore, in the present invention, we have adopted an apparatus configuration in which the cooling capacity in the axial direction of the inner cooling header tube is constant (however, the absolute value of the cooling capacity is variable by adjusting the amount of water and the air/water ratio).

Next, the quality checks for the mechanical properties and metallographic structure required of the steel pipe after cooling are to check whether the prescribed cooling rate has been achieved in all parts of the steel pipe in the thickness direction, and to check the cooling end point temperature. This is done based on two points: whether or not the constraint conditions for are satisfied. If the predetermined cooling rate is obtained, proceed to the next step; if not, return to the step of assuming the inner cooling condition or outer surface cooling condition and repeat the above steps until the predetermined cooling rate is obtained. Repeat the calculation.

■Measures to prevent quench cracking In the present invention, from the viewpoint that ``quench cracking is likely to occur near the cooling surface of a steel pipe due to the thermal stress caused by the extreme temperature gradient during cooling and the expansion caused by transformation,'' The idea was to suppress the maximum value of transient thermal stress as much as possible to prevent quench cracking near the point at which the inner and outer surfaces of the steel pipe had completed G4'' martensitic transformation. The upper limit of the maximum transient thermal stress at this time is determined by the steel pipe material, dimensions, etc.
It is given as a constant value for each type of steel pipe.The change in thermal stress over time is determined by a thermal stress analysis model using the cooling rate pattern in the thickness direction of the steel pipe during cooling of the inner and outer surfaces in (2). This thermal stress analysis model performs bulletproof thermal analysis that takes into account the strain and stress that occurs during quenching of steel pipes, volumetric expansion associated with phase transformation, and structural changes during the deformation process in the steel pipe thickness direction. The simulation is performed using the finite element method. Since this analysis method is also a well-known technique, detailed explanation will be omitted.

Next, check for quench cracking by checking whether the maximum transient thermal stress value obtained from the above analysis is smaller than the upper limit value above.
If it is small, go to the next step, and if it is large, go back to the step assuming the inner cooling condition or outer cooling condition. j Repeat the above calculation until the maximum thermal stress value becomes smaller than the predetermined upper limit.

Figure 7 shows an example of the results of thermal stress numerical analysis. Since quenching cracks often occur in the axial direction of steel pipes, thermal stress was checked using circumferential thermal stress, and the heating conditions were 950 tx20 minutes. Details are given in the examples.

■, Determination of internal and external cooling conditions Based on the track transfer coefficient for each external cooling header obtained through the above steps, the heat transfer coefficient of the internal cooling header, and the internal cooling start timing, the water volume and air-water ratio for each external cooling header header, internal cooling Determine the amount of water in the heso goo, the air and water pressure, and the fixing position of the inner cooling heso goo.

The relationship between the cooling water supply amount fi, the heat transfer coefficient hi, and the air-water ratio r in each cooling header is as shown in Figure 10. Therefore, once the required heat transfer coefficient is determined, the required cooling water supply amount,
The air/water ratio is determined.

Further, the internal cooling start timing can be controlled by controlling the fixed position 1io of the internal cooling hesogo, and is determined by the following equation.

j! 1o=c+xtis However, tis: Cooling start time difference between inner and outer surfaces [Example] For a steel pipe with an outer diameter of 196 φ, a wall thickness of 16 m*t, and a composition shown in FIG. 3, the composition shown in FIG. Assuming the use of a cooling device, the target Vickers strength is 512 and the conveyance speed is 0.
When the above calculation was performed assuming a speed of 16 m/sec, the cooling conditions shown in Table 1 were obtained for each outer cooling heel and inner cooling header, and the fixed position WlliOO value of the inner cooling heel was approximately 5001.

The equipment used in this example has a cooling water flow rate of 5 to 25 bar/hr, an air supply rate of 50"l OONm/hr, and a conveyance speed of 0.0 to each outer cooling groove and inner cooling groove.
Each can be adjusted continuously in the range of 5 to 0.4 m/s,
In addition, the effective cooling length per stage of external cooling header is 600m.
In order to prevent bending of the steel pipe, a conveyor roller 4 is provided for every two stages of the outer cooling header, and for the inner cooling header,
It is set to 500 Q wm to cover approximately the same range as the effective cooling length of the external cooling header.

When cooling was actually performed based on the above calculation results, the Vickers hardness distribution in the thickness direction was within a predetermined range of 514 to 524, and no embrittlement occurred.

For comparison, we used a conventional device (spray nozzle cooling for both the inner and outer surfaces) on the same type of steel pipe, set the target temperature and conveyance speed to be the same, cooled the inner and outer surfaces, and fixed the goo at a distance of 150 m.
Cooling was performed at f (constant regardless of the material dimensions of the steel pipe, etc.).

As a result, the Vickers hardness distribution in the wall thickness direction was 516 to 5.
Although it fell within the prescribed range of No. 28, quench cracking occurred at a rate of about 50%.

Table 1 FIG. 7 shows the thermal stress analysis results of this example.

The cooling conditions for each outer and inner cooling header are as shown in Table 1. The slowest cooling rate at the center of the wall thickness is 46'C/sce.
This satisfies the required cooling rate (46° C./sce or higher), and the maximum value of the inner surface co-directional thermal stress is 4 kg/Tsuru 2, which is a sufficient level to prevent quench cracking.

FIG. 8 shows an example of the control effect when the internal cooling start timing is changed. In other words, the inner cooling header fixing position j! io (a) Om, (b) 500m,
(c) The maximum value of circumferential thermal stress and the slowest cooling rate are compared for each set at 1100 mm.

At the time of 500m mentioned above, the slowest cooling rate was 46
℃/sce, which satisfies the required cooling rate, and the maximum circumferential thermal stress value is 4kt/1m'', which is within the range of not causing quench cracking. Maximum directional thermal stress is also 13kg/m”
There is a high risk of quench cracking, and in the case of (c) 1100Ii, there are problems with both the required cooling rate and the possibility of quench cracking.

Figure 9 shows an example of the control effect when only the internal cooling capacity is changed with the internal cooling header fixed position 1ioQm. By changing the maximum value of circumferential thermal stress and the slowest cooling rate,
This is a comparison of degrees. The slowest cooling rate is 4 in (a)
3℃/sec C% (47℃/sec at OL,
, l/'11 and 49°C/sec. Also, regarding the maximum value of circumferential thermal stress, due to the difference in internal cooling capacity, the inner surface is larger than the outer surface in (b) and (c), and there is a difference in the absolute value, but (a)
The inner surface cooling capacity is small, and the outer surface is larger than the inner surface.

From the above, by adjusting the cooling capacity of each external cooling header and internal cooling header and the internal cooling start timing, it is possible to guarantee the metal structure and mechanical properties required for the product after cooling, and prevent quench cracking. You will understand.

〔Effect of the invention〕

As is clear from the above explanation, the method of the present invention not only can ensure desired mechanical properties and metal structure without causing quench cracking through strict cooling control, but also has the following effects. .

■ Conventionally, when it was necessary to perform quenching to generate a completely martensitic structure in order to obtain the desired product quality, expensive alloying elements were added or oil was used to avoid the risk of quench cracking. In some cases, quenching using an immersion method is used, which increases the manufacturing cost of high-grade steel pipes. However, according to the method of the present invention, these measures against quench cracking are not required at all, making it possible to significantly reduce manufacturing costs.

■ In addition, when using a continuous line for the quenching process and tempering process, the temperature at which the quenching end point is set to the minimum necessary temperature (just below the end temperature of martensitic transformation) increases the charging temperature for the tempering process. (This makes it possible to save energy.

(2) Furthermore, in the method of the present invention, minimum cooling rate control can be performed based on the CC7g diagram, and product quality can be stabilized, that is, mechanical properties can be stabilized. For example, regarding tensile strength, conventionally the strength deviation σ is 1.2 kg/
Overall, the strength deviation can be reduced to 1/2 to 2/3, such as from 0.8 kg/dragon to 0.8 kg/dragon.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a device suitable for carrying out the method of the present invention, Fig. 2 is a schematic diagram showing a cooling capacity adjustment device for each cooling header, and Fig. 3 is a schematic diagram showing the cooling capacity adjustment device of each cooling header. CCT diagram, Figure 4 is a calculation flowchart of the cooling control method of the present invention, Figure 5
Figure 6 is a graph showing the concept of external cooling control, and Figure 6 is a graph showing the cooling rate pattern in the thickness direction of the copper pipe during internal and external cooling.
Fig. 7 is a graph showing an example of thermal stress analysis results, Fig. 8 is a graph showing the effect of internal cooling start timing control, Fig. 9 is a graph showing the effect of internal cooling capacity control, and Fig. 10 is a graph showing the cooling water supply amount. It is a graph which shows the relationship between and a heat transfer coefficient. l: heating furnace, 2: steel pipe, 4: conveyance roller, 5. ~5, :
External cooling header, 6.17: Cooling water supply amount, 'y, t
s: cooling water flow rate control device, 8.19: air supply source, 9.
20 Air flow rate control device, 15: Internal cooling belly button, 16
: Drive device. Figure 5 Figure 6 Figure □: Outer surface cooling start timing

Claims (1)

[Claims] (1) An annular external cooling header with variable cooling capacity that uses misting jet nozzles that can adjust the water volume and air-water ratio, and that has multiple annular external cooling headers installed in a row in the steel pipe movement direction on a variable-speed roller conveyor. In addition, a rod-shaped internal cooling header with variable cooling capacity and movable in the direction of movement of the steel pipe inside the steel pipe is also used, which is configured using a misting jet nozzle that can also adjust the water volume and air-water ratio, to maintain a predetermined temperature. A heated steel pipe is inserted and moved between the annular outer cooling header and the rod-shaped inner cooling header to cool it. At that time, the moving speed of the steel pipe, the cooling capacity of each outer cooling header, the fixing position of the inner cooling header, and the inner surface are A method for cooling steel pipes, characterized in that the cooling capacity of the cooling header is set so as to ensure the following cooling rate pattern and the following maximum transient thermal stress value. Cooling rate pattern: A cooling rate pattern that satisfies the temperature history during cooling specific to the type of steel to be heat-treated, which is necessary to ensure the mechanical properties and metallographic structure required for the steel pipe after cooling throughout the thickness direction of the steel pipe. Maximum value of transient thermal stress: The maximum value of transient thermal stress near the inner and outer surfaces of a steel pipe during cooling without causing quench cracking during martensitic transformation.