JPS5844131B2 - Heat treatment method for steel pipes - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method for heat treatment of steel pipes, in which the steel pipes are heated and quenched while being transferred.

In general, in large diameter steel pipes, the ratio tw/dw of the diameter dw and wall thickness tw is small, and in particular, when heat treatment is performed to heat to AC 3 points or more such as quenching or standard, the cross section may be flattened or bent in the length direction. It is a known fact that deformation is likely to occur.

These deformations are classified by temperature range as shown in Figure 1a.

As shown in b, there are a region A of deformation due to its own weight, a region B of deformation due to non-uniform heating, and a region C of deformation due to non-uniform cooling.

Region A is the so-called deformation region due to high-temperature softening of the material, and it is known that metallic materials rapidly soften when a predetermined temperature determined by the material is exceeded due to rapid growth of crystal grains due to secondary recrystallization. ing.

In region B, if there is a temperature difference, so-called temperature unevenness, in the heating temperature on the circumference of the steel pipe, deformation occurs due to a difference in thermal expansion.

In region C, the metal first contracts due to rapid cooling after heating, and then expands due to martensitic transformation at around 300-400℃, but if cooling becomes uneven and temperature unevenness occurs, the metal will shrink. Each point on the circumference contracts and expands differently, causing deformation.

The following measures can be considered for each of these areas.

The area B is heated as uniformly as possible.

That is, the eccentricity of the heating coil 2 for induction heating and the steel pipe 3,
If, for example, the steel pipe 3 is to be transferred without rotating in order to prevent so-called misalignment, a mechanism that detects the eccentricity between the heating coil 2 and the steel pipe 3 and automatically aligns the central axis is required.

However, this can be considerably improved by transferring the steel pipe 3 while rotating.

Furthermore, if the steel pipe 3 has uneven thickness, etc., it is possible to appropriately select the frequency of the heating power source.

For region C, it differs depending on whether the steel pipe 3 is rotating or not.

When there is rotation, (a) uneven heating due to eccentricity with respect to the heating coil 2 is eliminated.

(b) It is preferable in various respects, such as less deformation due to its own weight.

On the other hand, in the case of no rotation, improvements to the cooling device 15 may be made, such as a self-aligning mechanism to eliminate misalignment between the steel pipe 3 and the annular cooling device 15, and precision machining of the cooling water injection nozzle to align the cooling start points. Is required.

For region A, it depends on the length lo of the region heated to the Ac3 point or higher.

Of course, to be exact, there is a zone that is somewhat longer than this, and even after cooling begins, it still enters the deformation region.

Therefore, it is desirable that this length lo be as short as possible, but this length lo is limited by the following conditions.

That is, (a) in manufacturing the heating coil 2, the heating coil 2
There is a limit to the power that can be input to 0 unit size.

Therefore, once the required processing tonnage, that is, the processing speed is determined, the minimum required coil length and number of coils will be determined.

(b) When the wall thickness of the steel pipe 3 is thick, it is necessary to take a relatively long heating time in order to minimize the temperature difference between the inside and outside of the pipe.

Condition (a) is unavoidable because it is related to the manufacturing technology of the heating coil 2.

The conventional solution to the condition (b) has been to select the frequency of the heating power source.

For this reason, since the rate of temperature increase at the A c 3 transformation point is slow, the crystal grains of the metal become enlarged, making it difficult to improve quality.

This invention was made to eliminate the above drawbacks.
After heating at a predetermined power density to a first temperature at which the strength of the steel material rapidly decreases, it is heated to a second temperature exceeding the Ac3 transformation point at a power density higher than the power density when heated to the first temperature. It is an object of the present invention to provide a method for heat treatment of steel pipes, which can refine metal crystal grains and improve quality.

Hereinafter, an embodiment of the present invention will be described while comparing it with a conventional method.

FIG. 2 is a diagram illustrating a conventional quenching process by induction heating for large diameter pipes as a coil configuration diagram.

First, coil 1 heats from room temperature to 750°C, and then coil 2 heats to the final heating temperature of 950°C.

Note that the frequency of the heating power source is generally 50 or 60 Hz.

In this case, the size of the steel pipe 3 is 1016mmφX12.7
mm thickness, root canal: C: 020%, Mn: 0.75%
If the carbon steel pipe is made of carbon steel pipe of about

The power required for heating is the sum of the power required for heating the steel pipe 3 and the power equivalent to the loss due to radiation, excluding the efficiency of the heating coils 1 and 2, Pwl and Pw2
(KW ), the transfer speed of the steel pipe 3 is 6 mm/se
When c, the power of coil 1 and coil 2 are each 1092
W to 853, W to 853, and coil 2 corresponding to 3 points of Ac.
The average heating rate is 2.2°C/see.

Note that the average heating rate UH of the coil 1, which is responsible for heating up to the first temperature at which the strength of the steel material rapidly decreases, is 2.00/
sec.

However, as shown in the figure, the length of the coil is 800 mm, and the gap between the coils is 1200 mm.

Note that 4 is a roller that supports the steel pipe 3.

In addition, Fig. 3 shows that the size, material, and transfer speed of the steel pipe 3 are
Similar to the one shown in the figure, this is a hardening process in which a coil was constructed based on the idea of the present invention.

In this configuration, coil 1 operates from room temperature to 450'C.
Coil 5 has a heating range of up to 750°C, and coil 2 has a heating range of 95°C, which is the final heating temperature, including 3 points A.
He will be in charge of heating the temperature down to 0℃.

In this case, the role of coil 2 is similar to that of coil 2 in FIG.

Therefore, the heating rate through the A c B point is 2.
2℃/see, which is also the same as in Figure 2, but in the first stage, the heating rate UH up to the first temperature at which the strength of the steel material rapidly decreases is 2.0℃/see in Figure 2. On the other hand, it is 1.0°C/sec, which is 1/2.

Therefore, it is necessary to consider the temperature distribution inside and outside the steel pipe 3, as well as the heat equalization effect in the heat dissipation zone between the heating coils 1 and 2°5, but roughly speaking, the improvement is approximately 1/2. That will happen.

Furthermore, FIG. 4 shows a quenching process consisting of three heating coils L2 and 5 in an embodiment of the present invention. This is a case where the length of the coil 2 responsible for the temperature up to the second temperature is 4001nrIl.

In this case, the heating rate UH passing through the three points A c is 4
．． The temperature rises as quickly as 3℃/see.

Therefore, the crystal grain size of the material after quenching is finer than in the case shown in FIG. 2, and the mechanical properties are improved.

Note that in FIGS. 2 to 4, an arrow 6 indicates the direction in which the steel pipe 3 is transferred.

Table 1 shows the average heating rate UH1 power density Pd of the heating coil, the temperature difference inside and outside the tube, etc. with the case of FIG. 2 set as 100 for the above three examples (FIGS. 2 to 4).

However, Table 1 shows that the transfer speed of the steel pipe is 6 ynm/se.
The steel pipe size is 1016φ x 12.7t, and the length of the heating coil is 800mm (coil 2 in Fig. 4 is 400mm).
mm), the distance between each coil is 12001 m, the frequency of the heating power source is 60 to 180 H2, and the material is equivalent to Al511020.

As can be seen from Table 1, in the conventional process shown in Figure 2, the power density of the heating coil 2, which is in charge of heating from the first temperature to the second final heating temperature, is relatively low; The characteristic of FIGS. 3 and 4, which show embodiments of the invention, is that the values are relatively high.

FIG. 5 shows the longitudinal position of the steel pipe 3 and the temperature T of the steel pipe 3 at each point at this position.

7 is a conventional method shown in FIG. 2, in which the coil 1 heats up to a first temperature point 8, and the coil 2 heats up to a second temperature point 9.

10 is the method shown in FIG. 8, in which coils 1 and 5 heat to a first temperature point 8, and coil 2 heats to a second temperature point 9.

11 is another embodiment of the present invention, which is the method shown in FIG. , the heating region up to the first temperature point 8 is made relatively long to reduce the temperature difference between the inside and outside of the steel pipe 3.

Furthermore, the second temperature point 12 is heated by the coil 2 at a power density greater than the power density when heating to the first temperature point 8.
Heat until.

13 is the deformation area of the steel pipe 3 due to its own weight when using the method shown in FIGS. 2 and 3, and 14 is the deformation area due to its own weight when using the method shown in FIG. The area is narrow.

According to this invention, after heating the steel at a predetermined power density to a first temperature at which the strength of the steel material rapidly decreases, the second temperature exceeds the AC temperature at a power density higher than the power density at the time of heating to the first temperature. By heating to the temperature of the first
From the first temperature to the second temperature, heat at a low power density for a relatively long time to reduce the temperature difference between the inside and outside of the steel pipe, and from the first temperature to the second temperature, heat at a high power density for a short time to achieve the A, c3 transformation. By increasing the heating rate beyond the point, it is possible to make crystal grains finer.

Therefore, the quality of steel pipes after induction heating can be greatly improved.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a heat treatment apparatus for quenching steel pipes using yFIJ induction heating, FIG. 2 is a block diagram of a conventional heat treatment apparatus, and FIG. 3 is a block diagram of an embodiment of the present invention. Fourth
The figure is a configuration diagram showing another embodiment of this invention, and FIG.
FIG. 4 is a temperature rise characteristic diagram showing the relationship between the longitudinal position of the steel pipe and the temperature t at each position of the steel pipe. In the figure, 3 is a steel pipe, 8 is a first temperature point, and 9 and 12 are second temperature points. Note that the same reference numerals in each figure indicate gaps or corresponding parts, respectively.

Claims (1)

[Claims] 1. In a heat treatment method for steel pipes in which the steel pipes are sequentially heated to a predetermined temperature by induction heating while being transported in the longitudinal direction and quenched, the temperature at which the strength of the steel material rapidly decreases is between 600°C and 8°C.
After heating to 00℃ at a predetermined power density, the temperature exceeds the AC3 transformation point at a power density higher than the above power density at 860℃~
A method for heat treatment of steel pipes, characterized by heating up to 870°C.