JPH03281727A - Method for induction heating of metal tube - Google Patents

Abstract

PURPOSE:To enable the induction heating enabling to effectively eliminate uneven heat due to a thickness deviation by selecting frequency of electric source so that heat-penetrated depth becomes a specific ratio to the thickness at the thinnest part in the circumferential direction of a pipe at the time of induction-heating by inserting the pipe to be treated into an annular coil. CONSTITUTION:At the time of executing the induction heating by inserting a metal pipe into an annular coil, the electric source having a frequency so that the heat-penetrated depth delta becomes 50-100% of the thickness tmin at the thinnest part of the metal pipe, is supplied into the annular coil. By this method, in both parts of the thin part and the thick part, generated Joule heat becomes I<2>.Rdelta (Rdelta is resistance in the range of the penetrated depth delta) to uniformly transmit the Joule heat and the uneven heat is drastically reduced and the quality, etc., after executing the heat treatment is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for induction heating of metal pipes used in solution heat treatment of steel pipes and the like.

(Prior art) With the recent introduction of factory automation, the induction heating method, in which the pipe to be treated is inserted through a circular coil, has become popular for solution treatment of steel pipes.In induction heating, the power supply frequency f is high ( As you can see, the penetration depth δ becomes shallower, and the heating range is limited to the surface layer.For this reason, when heating a steel pipe, the entire thickness direction of the steel pipe can be heated efficiently (from the perspective of heating). The depth δ is generally set to be approximately the same as the wall thickness E of the steel pipe, or slightly thicker than this.

However, when there is a relatively large thickness deviation in the circumferential direction, such as in a seamless steel pipe, uneven heat is generated in the circumferential direction due to the thickness deviation, and the texture of the heat-treated steel pipe deteriorates. There is a risk of unevenness or bending of the steel pipe during heat treatment.

In order to prevent uneven heat from occurring even in uneven-walled pipes, for example, Japanese Patent Application Laid-Open No. 52-119549 discloses a penetration depth δ.
An induction heating method is disclosed in which a power supply frequency f is selected such that f is extremely larger than the wall thickness of the steel pipe. This method seems to be effective when the steel pipe is thick. However, for example, in the case of a thin-walled pipe with a wall thickness of 73 mm or less, uneven heat is hardly eliminated. The reason why increasing the penetration depth δ is not effective in eliminating uneven heat in thin-walled pipes is explained as follows.

[Problem to be solved by the invention]

The mechanism by which the temperature of metal pipes increases due to induction heating is that the resistance H of the pipe wall increases due to the generation of an induced lightning current in the pipe wall.
Joule heat BiR is generated due to this, and this heat is transferred in the wall thickness direction and the pipe circumferential direction. The thickness of the metal tube [
FIG. 3 shows a case in which a current with a penetration depth δ that is extremely larger than the average wall thickness is applied.

The induced current I is constant in the circumferential direction, and the penetration depth δ is larger than the thickest wall W L IIam, so the Joule heat JLH,h generated in the thinnest southern part and the Joule heat Jt generated in the thickest part , , becomes as above.

, J t□8-I2 ・Rt,,, I J Ls-m = 1” ・RLN-8 Here, R1
, , , R1- represent the respective resistances of the R thin wall portion and the thickest interior, which are inversely proportional to the wall thickness and are expressed by the following formula.

Rt,<io-K/tsty+RtN-,
= = K / t s*, (K' constant) Therefore, the dicole heat generated in the thinnest southern part J1. ++in is the heat generated inside the thickest part, which is larger than Jt, 4-, and generates uneven heat. This uneven heat is calculated by the uneven thickness rate (Ll, IMX
+, li*) /lζIt depends on this, so meat JtsuL
For thin-walled pipes with a small (average wall thickness), slight wall thickness deviation ([□
w LMin) will also produce large uneven heat.

An object of the present invention is to provide an induction heating method that can effectively eliminate uneven heat in a thin-walled tube.

(Means for Solving the Problems) In the induction heating force method of the present invention, when the annular coil ζ is inserted through the tube to be treated and heated by induction, the penetration depth is the thinnest southern portion of the tube in the circumferential direction. The present invention is characterized in that a power source having a frequency of 50 to 100% of 11 is supplied to the annular coil.

(Function) FIG. 1 is a diagram showing the relationship between the wall thickness of the metal tube and the penetration depth in the induction heating force method of the present invention.

The penetration depth δ is 50 to 1 of the top wall thickness Lllin of the metal pipe.
00% is selected. Then, the Joule heat generated in both the thin-walled portion and the thick-walled portion becomes 12 Rδ. Rδ
is the resistance in the region of penetration depth δ.

The inner surface from the area of penetration depth δ is heated by heat transfer, but during the heat transfer process, Joule heat moves both in the wall thickness direction and in the pipe circumferential direction, and the diffusion of Joule heat is equalized. Therefore, although uneven heat is not completely prevented, its degree is significantly reduced.

The penetration depth δ is the top wall thickness t. , 7 exceeds lOO%, a difference occurs in the Joule heat generated between the thin wall portion and the thick wall portion, making it impossible to sufficiently eliminate uneven heat.

If the immersion i3 depth δ is less than 50% of the J'X LNi++ in the thinnest part, the region where Joule heat is generated becomes narrow, the heat transfer region becomes wide, and the heating rate decreases.

In a thin-walled tube, the wall thickness is originally small, so even if the immersion depth δ is less than 100% of the wall thickness of the expanded part, the heat transfer area is small, and the penetration depth δ is 100 or less of the top wall thickness t. The decrease in heating rate due to this is small.

〔Example〕

The present invention will be explained in detail below.

SUS 304 Yolinal outer diameter 25-1 average meat r￥
Two steel pipes with various wall thickness deviations are passed through a circular coil arranged in four stages at a speed of 500 m/Hr.
It was induction heated to 1150'c for 4 degrees. TM, source frequency Ni1OkHz, 400kHz+7), and the penetration depth δ is 5peng for the former and lpeng for the latter. The results are shown in Table 1 and Figure 2. Also, set the power frequency 1 to 1oO
kHz (penetration depth δ is I, 7 am)
Shown in the table.

Table 1 As can be seen from Tables 1, 2, and Figure 2, even in the case of thin-walled pipes, power supply frequency 1 By increasing the frequency of The effect becomes more pronounced as the thickness deviation rate increases.

Although the above embodiment is an example of heating a stainless steel pipe, the present invention can be applied to steel pipes other than stainless steel made of carbon steel, alloy steel, etc., and can also be applied to metal pipes other than steel pipes. It is possible.

(Effects of the Invention) As explained above, the induction heating method of the present invention can effectively eliminate uneven heat caused by uneven thickness even in thin-walled pipes with a wall thickness of 3 or more, and improve the heat treatment quality. It is highly effective in improving the temperature and preventing pipe bending during heat treatment.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing the relationship between metal pipe wall thickness and penetration depth in the method of the present invention, Fig. 2 is a graph showing the effect of implementing the method of the present invention, and Fig. 3 is a diagram showing the metal pipe wall thickness in the conventional method. It is a schematic diagram showing the relationship between and the penetration depth. Figure Figure

Claims (1)

[Claims] (1) When inserting a tube to be treated through the annular coil and performing induction heating, a power source with a frequency such that the penetration depth is 50 to 100% of the thickness of the thinnest part of the tube in the circumferential direction is applied to the annular coil. A method for induction heating a metal tube, characterized in that: