JPH0377256B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for continuously heat-treating metal strips that have been previously cut into arbitrary dimensions.

It has long been well known that heat treatment of metal strips such as rails and steel pipes increases their strength, toughness, and quality, and methods for this include whole heating in a furnace or transfer heating using induction heating. However, in recent years, transfer heating using induction heating has become the mainstream for heat treatment of strip materials due to its energy saving properties, cleanliness, and high quality.

However, a heat treatment method in which the strip is moved at a constant speed in the longitudinal direction, heated by induction heating through a heat treatment means consisting of an inductor equipped with a cooling means, and cooled immediately after heating also has the disadvantage that the temperature at the end of the strip may vary. The only drawback is that it becomes mottled compared to the stationary part.

In order to eliminate this temperature unevenness, many techniques have been developed over the years, and each has shown its effectiveness.
Particularly effective among these is a heat treatment method using dual-frequency heating, in which heating is performed using two frequencies, one lower and one higher, based on an appropriate frequency that allows heating to a substantially uniform temperature over the entire length of the heated strip. It is.

In other words, when the heating frequency of the strip is lower than the appropriate frequency that allows heating substantially uniformly over the entire length of the strip including the tube end, the temperature at the end will decrease as shown in Figure 1. On the other hand, if the heating frequency is higher than the appropriate frequency, as shown in Figure 2,
By cleverly utilizing the so-called edge effect of induction heating, in which the temperature at the end becomes higher, when the temperatures obtained when heating at dual high and low frequencies are superimposed, a constant temperature is maintained over the entire length of the strip. This is a heat treatment method using two possible frequencies.

This method is extremely effective and has been used to great effect in large-scale equipment for mass production of small numbers of products with limited dimensions, such as relatively large diameter steel pipes. When used in small-scale equipment for small-scale production, two types of heating power sources are required, which not only has the disadvantage of increasing the economic burden, but also makes it impossible to maintain the same temperature at the beginning and end of the steel pipe. There is a technical drawback that there is no such thing, and it cannot be said to be completely satisfactory.

In other words, the heating spots that occur at the ends of strips are not only caused by the edge effect at the ends during induction heating described above, but are also caused by the unavoidable phenomenon of induced current flowing through the joints of steel pipes. be.

Next, the phenomenon will be explained using diagrams.

FIG. 3 shows a very general relationship between the steel pipe 1 and the heating inductor 2. The steel pipe 1 and the heating inductor 2 are connected by a connecting core 3 made of the same material as the steel pipes. The flow of current in the process of passing through the inductor 2 through the joint between the connecting core 3 and the steel pipe is shown in FIGS. 4 to 8 for any one winding of the inductor. In Figures 4 to 8, the inductor is drawn extremely diagonally and the current is shown as a single line, but this is for illustration purposes only; in reality, the inductor is not so diagonally. Of course, the current also flows with a certain width.

FIG. 4 shows a state in which the stationary portion of the steel pipe 1 is being heated, and the current i naturally flows in parallel to the inductor 2. However, as shown in Fig. 5, when the joint between the connecting core 3 and the end of the steel pipe 1 enters the inductor 2, the current i that had been flowing parallel to the inductor 2 flows through the joint with high electrical resistance. It flows along the end of the steel pipe 1, which has a lower impedance than the one that flows parallel to the inductor 2, as shown by the dotted line. This state continues until the joint has completely passed through the inductor 2 as shown in FIG. 8, so the temperature at the end of the steel pipe 1 becomes high and the temperature at the end of the core on the steel pipe 1a side becomes low.
This phenomenon naturally occurs on the starting end side of the steel pipe 1a, and in this case, the temperature is lower than on the terminal end side.

The above has described continuous heat treatment of steel pipes, but the same applies to continuous heat treatment of not only steel pipes but also other metal strips.

The inventor of the present invention has devised a method for reducing the temperature difference between the starting end and the terminal end of a steel pipe, which is caused by an unavoidable phenomenon caused by an induced current flowing in the joint between the connecting cores of the steel pipes 1 and 1a, as well as for induction heating. As a result of extensive research into methods for reducing temperature unevenness caused by edge effects at the ends of tubes, we have achieved the present invention, which can be applied not only to steel tubes but also to continuous heat treatment of other metal strips. is the rail,
While moving a metal strip such as a steel pipe in its longitudinal direction, the metal strip is heated by induction heating through a heat treatment means consisting of an inductor equipped with a cooling means, and the metal strip is successively cooled immediately after heating. In the method of connecting and continuously heat-treating the metal strips, a core is used when connecting the metal strips, and the material of the core is changed between the starting end core that is in contact with the starting end side of the metal strips and the terminal end side of the metal strips. The impedance of the starting core is always higher than that of the ending core, while the heating frequency is set to an appropriate frequency to uniformly heat the entire metal tube including the ends. If the impedance is relatively high, the impedance of the starting core should be the same or higher than the impedance of the metal strip, and the impedance of the terminating core should be lower, and the heating frequency should be set to the appropriate frequency. If the impedance is relatively low, the impedance of the starting core is higher than the impedance of the metal strip, and the impedance of the trailing core is the same or higher than the impedance of the metal strip.

That is, the present invention uses a core when connecting metal strips, and the material of the core is changed to a starting end core that contacts the starting end side of the metal strip and a terminal end core that contacts the terminal end side of the metal strip. The impedance of the starting core is always higher than that of the ending core, while the heating frequency is higher than the frequency appropriate for uniform heating of the entire metal strip including the ends. In this case, the impedance of the starting core is the same or higher than the impedance of the metal strip, and the impedance of the terminating core is lower, and the heating frequency is lower than the appropriate frequency. In this case, the impedance of the starting end core is higher than the impedance of the metal strip, and the impedance of the ending core is the same or higher, thereby reducing temperature unevenness at the end of the metal strip. This makes it possible to uniformly heat-treat the entire length of the metal strip.

Next, as an example of implementing the present invention, an example in which a steel pipe is continuously heat treated will be explained with reference to the drawings.

What is shown in FIG. 9 is a connecting core used in an embodiment in which steel pipes are continuously heat-treated, and the materials thereof are a starting end core 31 in contact with the starting end of the steel pipe and a terminal end core 31 in contact with the terminating end of the steel pipe. 32, and protrusions 33 and 34 are formed on the outside of each to fit into the ends of the steel pipe.

Therefore, if the heating frequency of the steel pipe is higher than the appropriate frequency for uniformly heating the entire steel pipe including the pipe ends, the terminal side core 32 is made of copper,
When the starting end core 31 is made of the same material as the steel pipe and used as a connecting core for the steel pipes, and the steel pipe is heat treated, the temperature rise at the terminal end of the steel pipe is suppressed, and the temperature rise at the starting end of the steel pipe is suppressed. There is almost no temperature drop,
It was possible to perform uniform heat treatment on the whole.

This is because if the material of the terminal core 32 is the same as that of the steel pipe, the current flows along the tube end with low impedance at the terminal end of the steel pipe, but Since the material of the core 32 is copper, the impedance is extremely low, and even if the current passes through a joint with high electrical resistance, the impedance is lower than that flowing along the tube end, and the current flows toward the terminal core 32. As a result, this works to offset the edge effect in induction heating, suppressing the temperature rise at the end of the steel pipe.On the other hand, on the starting end side of the steel pipe, the material of the starting end side core 31 is the same as that of the steel pipe. This is because even if the temperature on the starting end side tries to drop, it is canceled out by the edge effect and the temperature drop is suppressed.

In addition, if the heating frequency is lower than the above-mentioned appropriate frequency, the end-end core is made of the same material as the steel pipe, and the start-end core 31 is made of a magnetic material. When used as a core and subjected to continuous heat treatment to the steel pipe, the temperature rise at the end of the steel pipe was suppressed, and there was almost no temperature change at the start end, making it possible to uniformly heat treat the entire pipe. done.

This is because if the material of the end-side core 32 is the same as that of the steel pipe, a current will flow at the end of the steel pipe as shown in Figure 5, and the temperature at the end will tend to rise, but due to the edge effect. As a result, the temperature becomes approximately constant.On the other hand, on the starting end side of the steel pipe, the material of the starting end side core 31 is a magnetic material with higher impedance than the steel pipe, so the relationship between the copper core and the steel pipe described above is different. Similarly, the current flows toward the starting end of the steel pipe, where the impedance is low, and acts in a direction that cancels out the edge effect, so the temperature remains approximately constant.

In addition, when continuously heat-treating a steel pipe, it is desirable to prevent cooling water from entering the inside of the pipe. This will prevent cooling water from entering the inside of the steel pipe.

Further, in the above embodiments, continuous heat treatment of steel pipes was described, but the present invention is not limited to steel pipes, but can also be applied to continuous heat treatment of other metal strips. In this case, it goes without saying that the shape of the core should be suitable for the strip material to be heat treated.

Since the present invention is as described above, it is extremely useful as a continuous heat treatment method for metal strips.

[Brief explanation of drawings]

Figure 1 is a diagram showing the edge effect when a steel pipe is heated at a heating frequency lower than the appropriate frequency.
Figure 2 is a diagram showing the edge effect when a steel pipe is heated at a higher heating frequency than the appropriate frequency, Figure 3 is a diagram showing the very general relationship between a steel pipe and a heating inductor, and Figure 4 8 to 8 are diagrams for explaining the inevitable phenomenon that occurs when an induced current flows in a joint of steel pipes, and FIG. 9 is a front view of a core for connecting steel pipes. DESCRIPTION OF SYMBOLS 1, 1a...Steel pipe, 2...Inductor, 3...Coupling core, 31...Starting end side core, 32...Terminal side core, 33, 34...Protrusion.

Claims (1)

[Claims] 1 Metal strips such as rails and steel pipes are heated by induction heating through a heat treatment means consisting of an inductor equipped with a cooling means while being moved in their longitudinal direction, and the metal strips are cooled immediately after heating. In the method of sequentially connecting metal strips and continuously heat-treating them, a core is used when connecting the metal strips, and the material of the core is changed between the starting end core that is in contact with the starting end of the metal strip and the metal strip. The impedance of the start-side core is always higher than that of the end-side core by dividing it into the end-side core that is in contact with the end-side core, and the heating frequency is set so that the entire metal strip including the ends is heated uniformly. If the frequency is higher than the appropriate frequency, the impedance of the starting core is the same or higher than the impedance of the metal strip, and the impedance of the terminating core is lower, and the heating frequency is When the frequency is lower than the appropriate frequency, the impedance of the starting core is higher than the impedance of the metal strip, and the impedance of the terminating core is the same or higher. Continuous heat treatment method for metal strips.