JP4077859B2 - Forged steel pipe excellent in workability, manufacturing method thereof, and manufacturing equipment line - Google Patents

Description

  The present invention relates to a forged steel pipe that is required for high workability and is used for flaring, grooving, thread rolling, and the like, and in particular, a forged steel pipe excellent in quality of a forged joining portion, a manufacturing method thereof, and a manufacturing method. Regarding the equipment line.

  In recent years, in the piping market, mechanical joints have become widespread from the viewpoint of strict and diversified required quality of joints, and forged steel pipes that can withstand strong processing such as flaring, grooving, or rolling screws. The demand is growing. In general, forged steel pipes are low in manufacturing cost, but when subjected to strong machining, cracks caused by the forging joints are generated, making them unsuitable for strong machining applications.

  FIG. 8 shows a general method for manufacturing a forged steel pipe. First, the edge part of the steel strip 1 is formed by the edge forming roll 2 and heated to 1100 to 1350 ° C. in the heating furnace 3. The heated steel strip is continuously formed into a tubular shape by the forming roll 4 and immediately before forging. Then, oxygen is blown to both edge portions of the tubular skelb by the oxygen blowing nozzle 6 and forged by the forging roll 5. In the forged joint of the forged steel pipe manufactured in this way, streak-like wrinkles (hereinafter referred to as streaks or outer streaks) are generated on the outer surface of the abutted portion of the steel pipe, and an oxide is formed inside the abutted portion. There are so-called contaminants in which etc. remain. These are major factors for cracking during strong processing.

It is considered that the outer surface streak is generated when the steel strip is slit to a predetermined width, and the edge portion is sagging and cannot be fully abutted. In addition, it is considered that the contaminants are generated when the scale generated during the heating or forming of the steel strip bites into the forging contact portion and remains.
And as a method of preventing the generation of these external streaks and impurities, the outer surface of the forging contact portion is cut and removed between the forging roll and the squeeze roll as in Patent Document 1 to prevent the outer streaking of the forging contact portion. For the purpose of reducing the amount of scale generated and reducing the fuel consumption rate, as in the method and Patent Document 2, both edges of the tubular skelb are heated by plasma arc immediately before forging, and then oxygen is blown to both edges. There has been proposed a method characterized by pipework by forging.

However, even with these methods, the workability that can withstand flaring, grooving, or rolling screws required as a mechanically welded steel pipe for a joint cannot be satisfied. According to the method of Patent Document 1, since cutting is performed hot, the cutting tool life is extremely short, the cutting surface is not smooth, and it is difficult to perform a healthy contact.
On the other hand, in the method of Patent Document 2, the equipment cost is high and the collision state is still unstable.

On the other hand, Patent Document 3 proposes a forged welded steel pipe excellent in workability in which the outer surface streak depth and contaminant occupancy of the abutting portion of the forged welded pipe are controlled within a specific range. That is, the outer streak depth of the forging contact portion of the forged steel pipe is 0.25 mm or less, and the foreign matter occupation ratio A value shown in the following formula << 1 >> is 3.0% or less. Forged steel pipe with excellent workability.
A = (L / t) × 100 (%)... << 1 >>
However, in the above formula, A: the occupancy ratio of impurities, L: the value of the total length of all impurities in the forged contact area (mm), t: the thickness of the forged contact area (mm) .

Japanese Patent Laid-Open No. 03-248715 JP-A-57-4319 JP 2000-317516 A

However, even the technique of the above-mentioned Patent Document 3 cannot sufficiently and withstand strong processing such as flare processing, grooving processing, or rolling screw.
In view of these circumstances, the present invention provides a welded steel pipe excellent in workability that can improve the bond strength of a welded contact portion and can stably withstand strong working, a manufacturing method thereof, and a manufacturing equipment line. It is the purpose.

The inventors of the present invention have found that a forged welded steel pipe excellent in workability can be obtained by controlling the state of the inner surface bead portion within a certain range with respect to the forged welded joint portion of the forged welded steel pipe.
The gist of the present invention is as follows.
( 1 ) The streak depth (do) of the forging contact portion on the outer surface of the forged steel pipe is 0.15 mm or less, and the bead height (hb) of the forging contact portion on the inner surface of the forged steel pipe is 0.10 mm or more. in streaks depth of the forge abutment (di) is not less than 0.25mm, further contaminants occupancy of forge welding abutments are defined by the following formula <1> (a) satisfies the following <2 > And <3> A forged steel pipe excellent in workability characterized by being in the range of the formula.
A = (L / t) × 100 ... <1>
However, A: Contamination ratio (%)
L: Total length in the thickness direction of the steel pipe in the forging contact area (mm)
t: Thickness of steel pipe (mm)
A ≦ −33 · do + 5.0 ... <2>
A ≦ −20 · di + 5.0 ... <3>
(2) the <1> forge welding abutments contaminants occupancy (A) is not more than 5.0%, which is defined by the formula, and, characterized in that it is a range of below <4> formula (1 Forged steel pipes with excellent workability as described in) .
A ≦ 33 · hb-3.3 ... <4>
( 3 ) The forged welded steel pipe having excellent workability according to (1) or (2) , wherein the base material of the forged welded steel pipe is Si killed steel or Al-Si killed steel.
( 4 ) In the manufacturing method of the forged steel pipe which edge-forms a steel strip, heats it, shape | molds in a tubular shape, and forge-contacts,
Cutting the edge of the steel strip, then forming the edge,
During the heating, the heating temperature is 1300 ° C. or less,
The method for producing a forged steel pipe excellent in workability according to any one of (1) to ( 3 ), wherein an upset rate is 3.0% or more during the forge welding.
(5) At least cutting forming equipment for cutting and forming the edge of the steel strip , equipment for forming the edge , a heating furnace for heating the steel strip at 1300 ° C. or less, equipment for forming the heated steel strip into a tubular shape, and molding was a steel tube equipment for forge welding in upset of 3.0% or more, in the production equipment string of forge welding steel pipe sequentially with the upstream side,
A cutting forming facility for cutting and forming the edge of the steel strip and a facility for forming the edge of the steel strip on the downstream side of the heating furnace for heating the steel strip are sequentially arranged (1) to (1) to ( 3 ) A production equipment row for producing a forged steel pipe excellent in workability according to any one of ( 3 ).

  By using the forged steel pipe of the present invention, even if flaring, grooving, thread rolling, etc., which are severely processed, can be performed without causing cracks at the abutting portion, the manufacturing cost is low. Using forged steel pipes, it can be applied to mechanical joints, etc., and inexpensive and short-term piping work can be performed.

Below, the forged steel pipe excellent in the workability of this invention and its manufacturing method are demonstrated in detail.
First, in the present invention, the outer streak depth (do) of the forging contact portion of the forged steel pipe is 0.15 mm or less, and the foreign matter occupancy ratio A shown in the following <1> is expressed by the following <2> It is characterized by being in the range of
A = (L / t) × 100 (%) ... <1>
However, A: Contamination ratio (%)
L: Total length in the thickness direction of the steel pipe in the forging contact area (mm)
t: Thickness of steel pipe (mm)
A ≦ −33 · do + 5.0 ... <2>

When manufacturing a steel pipe for a mechanical joint, the pipe end portion of the steel pipe is required to have a workability of 1.5 or more in terms of the expansion ratio by flaring.
Expansion rate = D '/ D
D: Outer diameter of the raw pipe D ': Outer diameter of the steel pipe after being expanded Under such strong processing, if the outer surface streaks remain, stress concentrates due to the notch effect, and cracking tends to occur. In addition, if there are impurities in the abutting portion, the adhesive strength of the abutting portion is reduced, and cracks are likely to occur in the abutting portion.

  The above formula <1> defines the abundance of contaminants in the forge joint since the forged steel pipe has a different wall thickness for each pipe size. The measurement of the contaminants is performed by observing the contaminants along the thickness direction of the cross section of the forging contact portion using an optical microscope and measuring the length of each contaminant. A value obtained by adding all the measured lengths of each contaminant is defined as L (see FIG. 1), and L is divided by the thickness t of the steel pipe and is referred to as a contaminant occupation rate.

FIG. 2 is a schematic view of a forged contact portion of a forged steel pipe.
A broken line part is a forging contact part. The outer surface streak depth is measured by measuring the depth of the streak generated on the outer surface side of the forging contact portion as the outer surface streak depth. The inner surface streak depth is measured as the inner surface streak depth from the highest part of the bead part to the lowest part of the forging contact part. The height (hb) of the inner surface bead portion was (tB)-(t). That is, it is the difference between the thickness (tB) of the highest part of the bead part and the thickness (t) of the steel pipe.

  Figure 3 shows whether the horizontal axis is the outer streak depth and the vertical axis is the foreign matter occupancy ratio. Samples are manufactured at each level, and flare processing is performed at an expansion ratio of 1.3. Is determined. As a result, when no cracks were generated, cracks began to occur when the outer surface streak depth exceeded 0.15 mm, and in the sample without outer streaks, the cracks were observed when the contaminant occupancy ratio exceeded 5.0%. Is starting to occur. Moreover, when an outer surface streak and a foreign substance exist in a forging contact part, the crack has generate | occur | produced except the area | region which satisfy | fills said <2> Formula. From these results, in the present invention, the outer surface streak depth of the forged steel pipe is set to 0.15 mm or less and to the range of the above formula <2>.

However, even if the outer surface streak depth and the foreign matter occupancy ratio are defined, it has been impossible to stably process the expansion ratio of 1.5 or more without cracking.
Accordingly, the inventors focused on the inner surface streaks of the forged contact portion of the forged steel pipe and examined the presence or absence of cracks during processing.

Fig. 4 shows whether the horizontal axis is the depth of the inner streak and the vertical axis is the occupancy rate of impurities. Samples are manufactured at each level, and flare processing is performed at an expansion ratio of 1.5. Is determined. As a result, when there were no impurities, cracks began to occur when the inner surface streaks exceeded 0.25 mm, and cracks began to occur when the sample with no inner surface streaks exceeded 5.0%. . Further, when the inner surface streaks and impurities are present in the forging contact portion, cracks are generated outside the region satisfying the following <3>. From these results, in the present invention, the inner surface stripe depth of the forged steel pipe is 0.25 mm or less and the range of the following <3> formula is defined.
A ≦ −20 · di + 5.0 ... <3>

Accordingly, the present inventor has various pressing forces of the forging contact portion for the purpose of setting the outer surface streak depth and the inner surface streak depth of the forging contact portion of the forged steel pipe to 0.15 mm and 0.25 mm or less, respectively. As a result of examining the outer surface streak depth and the inner surface streak depth and the inner surface bead shape of the forging contact portion, the following was found.
FIG. 5 is a conceptual diagram showing the state of the inner surface bead portion of the forging contact portion at various times with various pressing forces during forging contact.
A broken line part is a forging contact part, and when a pressing force is large, a bead part can swell on the inner surface side of the steel pipe. The height (hb) of the bead portion was (tB)-(t). That is, it is the difference between the thickness (tB) of the highest part of the bead part and the thickness (t) of the steel pipe. The depth from the highest part of the bead part to the lowest part of the abutting part is measured as the inner surface stripe depth.
In the order of (a) to (c) in FIG.
(A) is a case where the pressing force is the smallest, and streaks are generated in the outer and inner forging contact portions of the forged steel pipe.
In (b), the pressing force is slightly larger than in (a), and the outer surface streak depth of the forged steel pipe is reduced. Beads are generated on the inner surface of the forged steel pipe, and the inner surface stripe depth at the center of the bead is reduced.
(C) is a case in which the pressing force is further increased, and the outer surface stripe depth of the forged steel pipe is further reduced. Although the height of the bead is further increased on the inner surface of the forged steel pipe, the inner surface stripe depth at the center of the bead is further reduced as compared with (b).
By increasing the pressing force at the time of forging contact, the inner bead height increases, but the outer streak depth and inner streak depth can be reduced.

Then, the present inventors further examined the presence or absence of cracks during processing by paying attention to the inner surface bead height of the forged contact portion of the forged steel pipe.
Figure 6 shows the internal bead height (hb) on the horizontal axis and the occupancy rate on the vertical axis. Samples were prepared at each level, and flare processing was performed at an expansion ratio of 1.5. It is determined whether the quality is good or not. As a result, when there are no impurities, cracks start to occur when the inner bead height is less than 0.10 mm. Further, when impurities are present, cracks are generated outside the region satisfying the following <4> formula. However, if the foreign matter occupancy ratio exceeds 5.0%, even if the inner bead height is further increased to eliminate the outer surface streak depth and the inner surface streak depth, cracks occur as shown in FIGS. Will occur. From these results, in the present invention, the inner surface bead height of the forging contact portion is 0.10 mm or more, the foreign matter occupation ratio is 5.0% or less, and is defined in the range of the following formula <4>.
A ≦ 33 · hb-3.3 ... <4>

Based on the above findings, the present inventors examined the conditions during pipe making and whether or not cracking occurred when flaring the forged steel pipe.
If the outer streak depth and the inner streak depth of the forging contact portion are large, cracking occurs due to the notch effect when flare processing is performed. In addition, if there are impurities in the forging contact area, the adhesive strength of the forging contact area is reduced, and cracks are likely to occur in the forging contact area.

  In order to reduce the inner surface streak depth of the forging contact portion of the forged steel pipe, it is effective to remove the inner surface streaks together with the beads after forging. However, in this case, cutting online will result in hot cutting, which will be technically difficult as described above, such as the tool life and roughened cutting surface, and cutting after cooling after pipe forming. Then, the production efficiency is significantly reduced, and the cost is also increased.

One way of thinking for the above problem is to reduce the inner surface stripe depth even with the inner surface bead left.
As before, if there is burrs or sag on the skelb edge of the steel strip, if a large pressing force is applied during forging, the inner bead height will increase, but the groove between the remaining burrs and sag is deep. Cracks occur due to the notch effect. Therefore, in order to leave the inner bead after the forging, it is necessary to clean the edges of the edges beforehand, and the pressing force at the time of the collision needs to be increased. .

As shown in FIG. 5 described above, when the pressing force at the time of forging is increased and the inner bead is increased, the inner line depth and the outer surface line depth are decreased accordingly, and the notch effect when flare processing is performed is decreased. Therefore, the inner surface bead height may be 0.10 mm or more so that the inner surface stripe depth and the outer surface stripe depth become 0.25 mm or less and 0.15 mm or less , respectively.
For this purpose, the edge portion of the steel strip is cut and formed before the steel strip is heated, and further roll-formed and then heated and forged.

Next, an example of the manufacturing method for obtaining the forged steel pipe of this invention is demonstrated.
FIG. 7 is a diagram showing an example of the manufacturing process of the forged steel pipe of the present invention. FIG. 8 is a diagram showing an example of a manufacturing process of a conventional forged steel pipe. What is different from the conventional manufacturing process is that in the present invention, the quality of the forging contact portion is a point, and for that purpose, cutting forming and then roll forming are performed in edge forming of the steel strip before heating.
First, edge cutting is performed by the steel strip edge cutting equipment 7 before heating the steel strip. The edge foreign matter is completely removed by performing edge cutting. Thereafter, edge forming is performed with the roll 2 to adjust the shape of the edge. By carrying out these, impurities remaining in the abutting portion are reduced, and the edge surface is smoothed to prevent the occurrence of streaks on the outer and inner surfaces of the forged steel pipe as much as possible. Conventionally, as disclosed in Japanese Patent Application Laid-Open No. 04-31471, after forming with an edge roll as edge forming, preheating was performed to perform edge cutting. However, in hot cutting, steel is soft. A normal edge shape cannot be obtained, and the life of the cutting tool is extremely short, which is not practical.

  After forming the edge, the steel strip is heated to a predetermined temperature, formed into a tubular shape, and forged. What is important is to increase the pressing force of the skelb during forging. The reason is that, as described above, when the pressing force is small, streaks can be formed on the inner and outer surfaces of the forged joint portion of the forged steel pipe, which causes cracks due to the notch effect. Further, as another measure, the inner bead height may be used as a measure of the pressing force as long as the inner bead height is 0.10 mm or more. In any case, it is sufficient that the outer surface stripe depth is 0.15 mm or less and the inner surface stripe depth is 0.25 mm or less. For this purpose, the inner surface bead height is required to be 0.10 mm or more, and the skelb edge is pressed to that extent. Increase power. In addition, increasing the pressing force is advantageous in that the impurities in the abutting portion due to the scale generated during the forging contact are discharged. In order to increase the pressing force, the upset amount may be simply increased to increase the pressing force of the skelb, but the heating temperature of the skelb itself may be lowered to increase the pressing force of the abutting portion. In any case, if the pressing force of the forged joint is such that the forging condition is such that the outer surface streak depth, inner surface streak depth, or inner surface bead height of the forged steel pipe falls within the scope of the present invention, Can be any method.

  Furthermore, such a joint is often applied to an application that requires corrosion resistance such as a water pipe, and is often processed in a state where a steel pipe is plated. Therefore, in such a case, a plating coating that can withstand strong processing is required. When the steel used as a base material contains Si, the plating peel resistance is improved. Therefore, Si-killed steel containing Si as the base material of the steel pipe (Al ≦ 0.010%, Si ≧ 0.010%) and Al—Si killed steel (Al> 0.010%, Si ≧ 0.010%) It is desirable to adopt.

  Hereinafter, the present invention will be described more specifically with reference to examples.

Example 1
A forged steel pipe was manufactured according to the manufacturing process shown in FIG. That is, the steel strip 1 is edge-cut with the steel strip edge cutting equipment 7, subsequently subjected to edge forming with the steel strip edge forming roll 2, then heated with the heating furnace 3, and formed into a tubular shape with the forming roll 4, A forged steel pipe was manufactured on the basis that oxygen was blown between the forming roll 4 and the forged welding roll 5 with oxygen or an air blowing nozzle 6 and then abutting forged welding was performed.
Under the conditions of the above production, and the occupancy ratio of the forged welded steel pipe manufactured as a result, the outer surface streak depth, the inner surface streak depth, the inner surface bead height, and the steel pipe under the condition of the expansion ratio of 1.5. Table 1 lists the occurrence rate of cracks depending on the presence or absence of cracks when flaring. In addition, about the incidence rate of a crack, the sample was extract | collected 30 pieces for every same manufacturing conditions at the time of forge-welded steel pipe manufacture, the flare process was performed, and the result was described.

No. Nos. 1 to 9 are forged steel pipes in the range defined in the present invention, and the occurrence rates of cracks are all 0%. No. Nos. 11 and 12 have a small upset rate, an insufficient pressing force at the time of forging, a large outer surface streak depth, a large inner surface streak depth, and cracks. No. 13 and 14, the upset ratio is sufficient, but the heating temperature of the skelb is too high, and as a result, the pressing force during forging is small, especially the inner bead height is low, the inner surface stripe depth is large, and cracking occurs is doing. No. Since 15 and 16 do not cut the steel strip edge, the occupancy rate of impurities in the forging contact portion is particularly high and cracks are generated.

(Example 2)
The base material of the forged steel pipe is Si killed as shown in Table 2, Al-Si killed, Al killed steel, and the forged welded pipe is manufactured under conditions such that the state after forging is within the scope of the present invention. Hot dip galvanizing was performed, and flaring was performed under the same conditions as in Example 1. The results were evaluated based on whether or not the plating was peeled off. The results are shown in Table 3.

  As a result of flaring, plating peeling after flare processing was not observed in Si killed and Al-Si killed steel, but plating peeling was observed in Al killing.

  The forged steel pipe manufactured by the method of the present invention has an outer surface streak depth of 0.15 mm or less at the forging contact portion, an inner streak depth of the forging contact portion of 0.25 mm or less, and an inner bead height of 0.10 mm. As described above, the occupation ratio of impurities is 5.0% or less, and the processability is excellent. In the future, there will be an increasing demand for forged steel pipes that can withstand severe processing as steel pipes for mechanical joints. Therefore, the effect of the forged steel pipe manufactured according to the present invention is extremely large.

The figure which shows the state of the foreign material in the forge contact part of a forge welded steel pipe. The figure which shows the outer surface stripe, the inner surface stripe, and the inner surface bead in the forge welding abutting part of a forge welded steel pipe. The figure which shows the relationship between the outer surface streak depth of a forged steel pipe, the foreign material occupation rate, and the presence or absence of a process crack. The figure which shows the relationship between the inner surface line depth of a forge welded steel pipe, a foreign material occupation rate, and the presence or absence of a process crack. It is a figure which shows the state of pressing force and bead at the time of forging contact, (a) is small pressing force, (b) is during pressing force, (c) shows the state of large pressing force. The figure which shows the relationship between the inner surface bead height of a forged steel pipe, a foreign material occupation rate, and the presence or absence of a process crack. The figure which shows an example of the manufacturing process for manufacturing this invention steel pipe. The figure which shows the manufacturing process of the conventional forged steel pipe.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Steel strip 2 Steel strip edge forming roll 3 Heating furnace 4 Forming roll 5 Forge welding roll 6 Oxygen or air spray nozzle 7 Steel strip edge cutting equipment

Claims (5)

The streak depth (do) of the forging contact portion on the outer surface of the forged steel pipe is 0.15 mm or less, and the bead height (hb) of the forging contact portion on the inner surface of the forged steel pipe is 0.10 mm or more, The streak depth (di) of the forging contact portion is 0.25 mm or less, and the foreign matter occupancy (A) of the forging contact portion defined by the following <1> formula is <2> and <3> A forged steel pipe excellent in workability characterized by being in the range of the formula.
A = (L / t) × 100 ... <1>
However, A: Contamination ratio (%)
L: Total length in the thickness direction of the steel pipe in the forging contact area (mm)
t: Thickness of steel pipe (mm)
A ≦ −33 · do + 5.0 ... <2>
A ≦ −20 · di + 5.0 ... <3> The <1> contaminants occupancy forge abutment (A) is not more than 5.0%, which is defined by the formula, and, according to claim 1, characterized in that in the range of below <4> formula excellent forge steel pipe in the workability.
A ≦ 33 · hb-3.3 ... <4> The forged welded steel pipe having excellent workability according to claim 1 or 2 , wherein the base material of the forged welded steel pipe is Si killed steel or Al-Si killed steel. In the method of manufacturing a forged steel pipe, which is formed by edge forming a steel strip, heating, forming into a tubular shape, and forge-contacting,
Cutting the edge of the steel strip, then forming the edge,
During the heating, the heating temperature is 1300 ° C. or less,
The method for manufacturing a forged steel pipe excellent in workability according to any one of claims 1 to 3 , wherein an upset rate is set to 3.0% or more during the forge welding. At least a cutting and forming equipment for cutting and forming an edge of the steel strip , an equipment for forming the edge , a heating furnace for heating the steel strip at 1300 ° C. or less, an equipment for forming the heated steel strip into a tubular shape, and a formed steel pipe and equipment for forge welding in upset of 3.0% or more, in the production equipment string of forge welding steel pipe sequentially with the upstream side,
The front stage of the heating furnace for heating the steel strip, the cutting forming equipment for cutting and forming the edge of the steel strip, and the equipment for forming the edge of the steel strip on the downstream side thereof are sequentially arranged. manufacturing equipment column for producing excellent forge steel workability according to any one of 3.

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