JPS6147886B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for uniformly heating rolled steel pipes (hereinafter simply referred to as rolled steel pipes), such as seamless steel pipes, by electric induction. In general, steel pipes manufactured by rolling or the like are more or less uneven-walled pipes in which the wall thickness varies in the circumferential direction or the axial direction. Such uneven thickness is characterized by the rolling conditions, especially the characteristics of the rolling mill. When a rolled steel pipe with uneven wall thickness is moved by a conveying device such as a pinch roller or a guide roller and is heated by induction through electric inductor coils arranged in series in two or more stages, Variations in wall thickness manifest as variations in temperature, making it extremely difficult to achieve uniform heating. However, various methods have been developed to uniformly heat rolled steel pipes with some variation in thickness by induction heating.
No. 52-122942, etc. The uniform induction heating devices described in these publications all attempt to achieve uniform heating by controlling the amount of power supplied to the inductor coil. Furthermore, in this type of uniform heating, it is generally attempted to keep the average temperature of each part of the steel pipe within a certain range. The present inventors completed the present invention after conducting various studies on a soaking method during induction heating in order to improve accuracy and reduce the range of temperature fluctuation in electric induction heating of rolled tubes. When we measure the variation in the thickness of the pipe wall for a single rolled steel pipe, as shown in Figure 1, it varies, although this is within the range allowed by the standard (in the case of Figure 1, the degree of thickness deviation is 8.5% on average). ). The solid line and the dotted line respectively show the tube wall thickness at opposing positions centering on the tube axis, compressed in the tube axis direction and expanded somewhat in the thickness direction for easy viewing. When a steel pipe with such wall thickness variations is induction heated, the temperature varies in accordance with the thickness variation shown in FIG. 1, as illustrated in FIG. Then, a temperature fluctuation range (Δθ) corresponding to the thickness fluctuation is generated. The inventors of the present invention have repeatedly conducted various searches in order to reduce the temperature fluctuation range (Δθ) caused by the above-mentioned variation in the thickness of the tube wall.
We have found a new control target necessary to make the temperature fluctuation range (Δθ') within the target range. That is, for example, in order to make the temperature fluctuation width (Δθ), which is used as a standard during heating, to the desired temperature fluctuation width (Δθ′), depending on the transition speed of the heated material and the number of stages of inductor coils for uniform heating, which are arbitrarily selected. Rather than controlling the output power to the inductor coil, one or both of the transition speed of the heated material and the number of inductor coil stages for uniform heating can be adjusted to the above-mentioned reference temperature fluctuation range (Δθ) and the desired value. The purpose can be sufficiently achieved by controlling the temperature fluctuation range in relation to the ratio to the temperature fluctuation range (Δθ'), that is, the temperature fluctuation range reduction rate (Δθ'/Δθ=α). The present invention will be described in detail below. In the present invention, when the transfer rate of the material to be heated is arbitrarily selected in advance for each steel pipe size, for example, the standard transfer rate in standard work, and the number of soaking coil stages in the soaking area is selected arbitrarily, for example, one stage. standard temperature range Δθ (for example, the difference between the respective temperatures (θ ₁ ) and (θ ₂ ) at position B in FIG. 2 Δθ=θ ₁
-θ ₂ ) to the temperature fluctuation width (Δθ′) and the standard transition speed (v ₀ ) when the standard temperature fluctuation width (Δθ) is obtained. , the so-called uniformity obtained by the ratio of the number of uniformly heated induction coils (n ₀ ) at that time, the transfer velocity v' of the heated material operated to obtain various Δθ', and the number n' of uniformly heated induction coils. The relationship with the thermal time magnification (β=n'/n ₀ /v'/v ₀ ) is determined by experiment. FIG. 3 shows the relationship between the temperature fluctuation range reduction rate (α) and the soaking time magnification (β) in the present invention. However, the relationship shown in Figure 3 was obtained under the following conditions. Standard transition speed v ₀ = 2.6 m/min Initial temperature T ₀ = 650℃ (Final temperature 680℃) Number of coils n ₀ = 3 (soaking coil) 4 (heating coil) Current I = Stage 750A Stage 750A Stage 900A Frequency f = 360−360−60Hz Thus, the temperature fluctuation range reduction rate (α) and soaking time multiplier (β) shown in Figure 3 are determined for each steel pipe size.
Using this relationship, select one or both of the transfer rate of the heated material and the number of uniform heating inductor coils for uniform heating, set this as the heating condition for uniform heating, and perform heating. That is, in induction heating of steel pipes to be heated, the target temperature fluctuation range Δθ' of the steel pipe is determined, and the standard temperature fluctuation range (Δθ) determined experimentally in advance for each group to which this steel pipe corresponds is determined. The ratio, that is, the temperature fluctuation width reduction rate (α) is determined. Next, β is determined from the relationship diagram between the heating temperature width reduction rate (α) and the soaking time magnification (β) determined for each steel pipe size group, and further, the transition speed v of the heated material at this β is calculated. ′ or the number of uniformly heated inductor coils (n′), or both are set so that β is obtained earlier. Heating for uniform heating is performed under these set conditions. In the above description of the present invention, we have described a uniform heating method in which the temperature fluctuation width is directly measured and the soaking conditions are set based on the relationship with the soaking time multiplier. It is also possible to measure the pipe thickness variation state in relation to the temperature variation width, estimate the corresponding temperature variation width Δθ, and estimate the soaking conditions by the method described above. Since the present invention is as described above, it is possible to perform uniform heating with a reduced range of fluctuation in heating temperature despite variations in the thickness of the tube wall. Example A rolled steel pipe was heated at a certain standard transition speed by one-stage heating of a soaking inductor coil in a soaking region to obtain a temperature distribution (Δθ) as shown in FIG. However, FIG. 2 shows an example under the following conditions. Standard transition speed v ₀ = 5.2m/min Initial temperature T ₀ = 670℃ (final temperature 680℃) Number of coils n ₀ = 1 (heating coil), 6 (heating coil) Current I = 1250A Frequency f = 60Hz and transition Similar heating was carried out with the heating speed set at 1/2 of the above speed and three stages of soaking inductor coils to obtain a temperature distribution (Δθ') as shown in FIG. In this way, the ratio of temperature fluctuation range, that is, α=Δθ′/Δθ
was calculated. On the other hand, the standard transition speed (v ₀ ) when the standard temperature fluctuation width (Δθ) is obtained, the number of soaked induction coils (n ₀ ) at that time, and the temperature fluctuation range (Δθ′) operated to obtain the The soaking time multiplier β, that is, (n′/n ₀ )/(v′/v ₀ ), is calculated from the transfer speed of the heating material (v′) and the number of soaking induction coils (n′). When a plurality of experimental data values of α and β obtained in this way are plotted in terms of the α/β relationship, the correlation shown in FIG. 3 is obtained. The target temperature fluctuation range Δ when induction heating a rolled steel pipe of the same size group as above
θ' is planned, and the temperature fluctuation range reduction rate α at that time is
If it is set as 0.62, then the soaking time multiplier β can be obtained as 6 from Fig. 3. That is,

Obtain [formula]. Therefore, since n ₀ = 1 and v ₀ = 1, β = 6n'/v' Here, if n' is set to 3, for example, v' becomes 1/2, and the number of equalizing inductor coils is reduced to 3 stages. If so, the speed should be reduced to half of the standard. In the same way as above, if the number of soaking inductor coils is set to one stage, the transition speed will be
Just set the deceleration to 1/6. FIG. 4 shows the results of heating with the soaking conditions set as described above, and the range of heating temperature fluctuation was greatly reduced.

[Brief explanation of the drawing]

Figure 1 is a graph showing the fluctuation state of the tube wall thickness in the tube axis direction of a rolled steel pipe. Figure 2 is a graph showing the temperature fluctuation in the tube axis direction after the soaking period in induction heating.
The figure is a graph showing the relationship between the temperature fluctuation reduction rate and the soaking time magnification, and FIG. 4 is a graph showing the temperature fluctuation in the tube axis direction when soaking is carried out by the method of the present invention.

Claims (1)

[Scope of Claims] 1. When soaking a rolled steel pipe with an induction heating coil, the temperature fluctuation range is determined from the temperature fluctuation range of each part of the steel pipe with respect to predetermined reference heating conditions or the thickness unevenness rate of the steel pipe measured in advance, and Temperature fluctuation width reduction rate α from temperature fluctuation width and desired set temperature fluctuation width
Then, using the correlation between the temperature fluctuation width reduction rate and the soaking time magnification determined in advance, the soaking time magnification β is calculated from the temperature fluctuation width reduction rate, and the soaking time magnification β and the predetermined standard heating are calculated. Electric induction equalization of rolled steel pipes is characterized by selecting one or both of the transfer velocity of the heated material v' and the number n' of induction equalizing coils to be activated that satisfy the following relationship based on the conditions (v ₀ , _n 0 ). Heating method: Soaking time multiplier β = (n′/n ₀ )/(v′/v ₀ ) However, v ₀ and n ₀ are the standard transition speed and the number of soaking induction coils at that time, respectively, and v′, n′ is the transfer rate of the heated material and the number of soaking induction coils.