JP2722970B2 - Heat input control method for wrought steel pipe - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat input control method for heating a tubular skeleton just before forging to a temperature optimum for forging in a production line of forged steel pipes.

[0002]

2. Description of the Related Art A forged steel pipe is formed by forming a band-shaped skeleton heated in a heating furnace into a tube with a forming roll, and heating the edge of the tubular skeleton by blowing oxygen from a welding horn. After heating using a forging roll, the edge is forged by a forging roll.

Conventionally, in such a forging process, the temperature of the forged portion is measured by measuring the temperature distribution immediately after the edge heater installed on the exit side of the heating furnace, and controlling the output of the edge heater so that the temperature distribution is optimized. Although the method of controlling was adopted, since the distance from the edge heater to the forge welding roll was large, and the heating was performed again with a welding horn or high frequency heating coil immediately before the forge welding roll, the true temperature just before forging was determined. There was a problem that control could not be performed and a defect occurred in the quality of the forged portion.

[0004] To solve such a problem,
No. 16193 proposes the following temperature control method. As shown in FIG. 7, the temperature of both edges of the tubular skeleton is measured separately immediately before the forging roll 5, and the heating device 8 just before the forging roll 5 is adjusted so that the peak temperature difference between the two edges is within the target range. The above-mentioned problem can be solved because the edge temperature immediately before forging is kept substantially constant by controlling the heating output of the '.

[0005]

However, although the method disclosed in Japanese Patent Publication No. 2-16193 can provide a forged steel pipe of good quality, it is not always possible to obtain a good quality forged steel pipe, and the yield is not always high. There was a problem of bad. That is, FIG. 1 is a cross-sectional structure diagram of the forged portion and the periphery thereof, but the strength and toughness of the forged portion cannot be improved unless the structure X obtained by heat denaturation is stabilized. And
The heat metamorphic structure X also differs depending on the manufacturing conditions such as the type, thickness, and line speed of the steel material. Therefore, without considering the thermal metamorphic structure of the forged part, simply controlling the difference between the peak temperatures of the left and right edges to the same,
The structure by the heating of the forged portion is not always subjected to the thermal denaturation that is optimal for forging, and the above-mentioned method cannot always provide a stable quality.

The present invention has been made in view of the above-mentioned problems, and has a stable quality by controlling the temperature of a tubular skeleton just before forging while taking into account the thermal metamorphic structure of a forged portion. An object of the present invention is to provide a method for controlling the heat input of a forged steel pipe that can be obtained more.

[0007]

Means for Solving the Problems For this reason, the present inventors have:
In order to find a factor that has a predetermined relationship with the thermal metamorphic structure of the stable forged part, as a result of extensive investigation and research under the conditions set so that the peak temperature difference between both edges is within the target range, a stable heat Since the width of the metamorphic structure coincides with the edge width heated to a predetermined temperature or higher, attention was first paid to the predetermined edge width. Here, the width of the thermal metamorphic structure is the range of X shown in FIG. Based on such a result, the present inventors focused on the edge width heated above the predetermined temperature,
As a result of repeating the test while varying the line speed and the type and thickness of the steel material, it was found that the width heated at a predetermined temperature or more, the line speed, and the type and thickness of the steel material depended.

That is, FIG. 2 shows that a flat test was performed on a high-strength steel and a general material at an arbitrary heating amount while varying the line speed, and the difference between the accepted material and the rejected material was heated to a predetermined temperature or higher. It is the graph plotted by the relationship between width and line speed. Here, as the reference temperature, a numerical value obtained from a test result in which the width of the stable thermally metamorphic structure coincides with the edge width heated to a predetermined temperature or more is used. As is clear from this graph, a stable thermal metamorphic structure can be obtained if the width of the edge portion at or above the predetermined temperature is within the predetermined range,
It can be seen that the appropriate edge width varies depending on the line speed and the type and thickness of the steel material.

Therefore, an optimum edge width that is equal to or more than a predetermined temperature that can form a stable structure is previously determined for each type, thickness, and line speed of the steel material, and then the temperature of the tubular skeleton edge portion immediately before forging on the line is determined. And measuring the actual edge width equal to or higher than a predetermined temperature, and performing feedback control so that the actual measured value approaches the value of the optimal heating range, using the optimum width as a parameter. Things.

The heat input control method for a forged steel pipe according to the present invention has been devised based on the above findings. When both edges of a tubular skeleton are heated by a heating device immediately before forging, the two edges are heated. It is characterized in that the heating temperature is controlled such that the width of the both edges becomes equal to or higher than a predetermined temperature within the target range while controlling the peak temperature to be equal to or higher than the target temperature.

Next, a specific control method according to the present invention will be described with reference to an apparatus configuration shown in FIG.

In this example of the apparatus configuration, a line is formed in the order of a heating furnace 1, an induction heating apparatus 2, an edge blower 3, a support roll 4, a welding horn 5, and a forging roll 6, as shown in FIG. In addition, linear ray thermometers 7, 8 are arranged immediately before the forging roll 6. The heating output of the induction heating device 2 and the air flow rate of the welding horn 5 are controlled by control devices 9 and 10 based on the measured values of the linear ray thermometers 7 and 8, respectively.

In this control example, the air flow rate f of the welding horn 5 immediately before the forging roll 6 is adjusted in order to obtain a stable forged portion having a thermally metamorphosed structure. The width W is used. Therefore, first, the relationship between the air flow rate f and the edge width W at which the temperature is equal to or higher than the predetermined temperature is determined in advance by a test for each type, thickness, and line speed of the steel material. This result becomes as in the graph shown in FIG. 4 (However, in this case assumes the right edge width W _R ≒ left edge width W _L, the edge width W _L of the left or right side, the relationship between the W _R This is set in the control device 10. Here, the reference temperature is a numerical value obtained from a test result in which the width of the stable thermally metamorphic structure matches the width of the edge heated to a predetermined temperature or higher, and is set to about 1350 ° C. here. The optimum of the edge width W _L, for also W _R, determined on the basis of the flat test shown in supra Figure 2, steel type and thickness, the test for each line speed. At this time, the edge width W _L of the forge welding pipes to pass the flat test As shown in the figure, the W _R because of the tolerance range, and eventually obtains the allowable range is set to the controller 10. Therefore, in the graph shown in FIG. 4, when the edge width having a target of W ₁ is an air flow f _U ~f _L to W ₂ to _W-3 it is set as the allowable band.

Further, the peak temperature T of the left and right edges is used as a parameter for adjusting the air flow rate f. This is because heating at a predetermined temperature or higher is required to achieve good forgeability. For this reason, the relationship between the air flow rate f and the edge peak temperature T is determined by a test for each type, thickness, and line speed of a steel material as described above. This result becomes as in the graph shown in FIG. 5 (although this is the peak temperature T _L of the left or right side, shows the relationship between T _R, assumes T _L ≒ T _R), which is also This is set in the control device 10. In addition, the lower limit temperature to be a reference is set uniformly and fixed (for example, 13
90 ° C).

If the plate center temperature T _C is also set as a parameter so as to be equal to or lower than a predetermined temperature, abnormal adjustment of the nozzle position of the welding horn 5 can be found. Therefore, in this control example, the upper limit temperature is set. And, when it is less than that, an alarm is output.

In the setting of the control device 10 as described above,
The temperature distribution of the tubular skeleton in front of the forging roll 6 is determined by a linear ray thermometer 8. This value is output to the control device 10 via the signal processing device. This output value is shown as a graph in FIG. In the figure, A is a left edge portion, B is a right edge portion, C is a plate center portion, D is a portion of the left edge portion having a peak temperature T _L ′, E is a right edge portion having a peak temperature T _R ′. W _R ′ indicates the width of the right edge at a predetermined temperature, that is, 1350 ° C. or more, and W _L ′ indicates the width of the left edge at 1350 ° C. or more. The measured value is, the edge width W _L which is set in the control device 10, W _R, the peak temperature T _L, T _R, the plate central portion temperature T
It is compared with a target value and _C, air flow f of the Welding horn is to be changed control be within the range of the set value according to the deviation.

A calculation method for changing and controlling the air flow rate f based on the deviation between the actually measured value and the target value is performed using, for example, the following equation.

[0018]

(Equation 1)

Here, first, the actually measured edge width value W _R ',
It is determined whether or not _WL ′ falls within the set allowable band, and if not, for example, W _{L in} FIG.
When the ₂ outside the range of W _3, the deviation [Delta] W ₁ between the target edge width _{W 1 (= W 1 -W R} '(OR W L')) in response to, g ₁ from the function table of the equation (1) = Δf ₁ = Δf ₁ (ΔW _R '
Seek _(OR W _L ')). If it is in the allowable band, the control is not changed.

Next, the measured value T of the peak temperature of the left and right edges
_L, T _R is, determines whether has reached above a set temperature, using the g ₁ if satisfied. If not, g ₂ = Δf ₂ from the function table of the above equation ( ₁ ) so as to reach the target lower limit (for example, T _{1 in} FIG. 5).
= Δf ₂ (ΔT _R '( _OR T _L ')) and use g ₂ . However, a limiter is applied so that the flow rate setting by this control does not exceed the allowable upper limit edge width (W _{3 in} FIG. 4) in the relation of W _R ′ ( _OR W _L ′) −f. Specifically, in the case of the following equation, f
and _{i +1} = f _U.

[0021]

(Equation 2)

Next, a test example in which a forged steel pipe is formed by applying the above control method will be described. For comparison, a tube was formed by a conventional method in which only the temperature difference between the left and right edges was considered. FIG.
As a result of operating the three-month line in the configuration shown in FIG. 1, it was found that the method of the present invention reduced the loss due to poor forging of the forged portion and improved the product yield compared to the conventional method.

[0023]

As described above, in the method for controlling heat input of a forged steel pipe according to the present invention, the temperature of the tubular skeleton is controlled immediately before forging while considering the thermal metamorphic structure of the forged portion. As a result, more stable products can be obtained, and a remarkable effect of improving the product yield can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional structure diagram of a forged portion of a forged steel pipe and its periphery.

[FIG. 2] A flat test is performed on a high-strength steel and a general material at various heating speeds with an arbitrary heating amount, and a difference between a pass material and a reject material is determined by a width and a line speed heated to a predetermined temperature or more. It is the graph plotted by the relationship of.

FIG. 3 is an explanatory diagram showing an apparatus configuration of a forged steel pipe production line to which a specific control method of the present invention is applied.

FIG. 4 is a graph showing a relationship between an air flow rate f of a welding horn and an edge width W at or above a predetermined temperature.

FIG. 5 is a graph showing a relationship between an air flow rate f of a welding horn and an edge peak temperature T.

FIG. 6 is a graph showing a temperature distribution at an edge of a tubular skeleton just before forging.

FIG. 7 is an explanatory diagram showing an apparatus configuration to which the temperature control method of Japanese Patent Publication No. 2-16193 is applied.

Claims (1)

(57) [Claims] When heating both edges of a tubular skeleton with a heating device immediately before forging, while controlling the peak temperature of both edges to be equal to or higher than a target temperature, the width of both edges that becomes equal to or higher than a predetermined temperature is set to a target temperature. A heat input control method for a forged steel pipe, wherein a heating temperature is controlled to fall within a range.