Manufacturing method of butt welded steel pipe

Abstract

To provide a manufacturing method for a forge-welded steel pipe whose buckling is suppressed, and a forge-welded steel pipe.SOLUTION: In a manufacturing method for a forge-welded steel pipe, when a steel plate is heated to form the steel plate in a pipe shape and end parts in a width direction of the steel plate which are made face each other are butted to each other so as to forge-weld the parts to each other, the following formula (1) is satisfied: 110>(▵T×U) / C...(1), in which ▵T(°C)=|Te(°C)-Tb(°C)|, where Tb(°C): a temperature of a central part in a width direction of the steel plate after heating and before butting, Te(°C): a temperature of the end parts in the width direction of the steel plate after heating and before butting, C(%): a ratio of a plate thickness t (mm) with respect to a width L (mm) of the steel plate after heating and before butting, 100×t / L, U(%): an upset ratio at the time of butting, and 100×(L-X) / L, X(mm): a preset pipe circumferential length at a t / 2 position in a thickness direction from an outer surface in the forge-welded steel pipe after butting.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a butt-welded steel pipe having excellent outer surface properties suitable for piping, and to a butt-welded steel pipe. [Background technology]

[0002] Forged-welded steel pipes, which are inexpensive and easy to work with, have been used in piping facilities such as water pipes. One method for connecting such pipes is, for example, a grooving joint. In this grooving joint, the outer surface of the pipe end is machined into a concave shape, and a gasket and housing are attached to this section to join the pipes. At this time, if there is a step in the circumferential direction of the pipe on the outer surface of the pipe, as shown in Figure 1, there is a possibility that fluid will leak from this point. This step is caused by the steel strip buckling due to excessive force caused by upset when joining (colliding).

[0003] For example, Patent Document 1 discloses a technique for preventing buckling at the forge welded part by spraying cooling water on the forge welded part after forging to cool it down. Patent Document 2 introduces a technique for preventing flattening abnormalities by changing the roll gap of the rolls used for forging to change the amount of upset. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 3682682 [Patent Document 2] Japanese Patent Application Publication No. 2-182318 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the techniques proposed in Patent Documents 1 and 2 cannot be said to be sufficient as techniques for butt-welded steel pipes that improve buckling, and there has been a demand for the establishment of other techniques. In order to solve the above problems, the present invention aims to provide a manufacturing method for butt-welded steel pipe and a butt-welded steel pipe that suppresses buckling. [Means for solving the problem]

[0006] In order to solve the above problems, the present inventors have investigated methods for preventing buckling of steel plates such as steel strips (hereinafter sometimes simply referred to as steel strips) that occurs during joining (when abutting). As a result, it was found that buckling is affected by the upset rate (upset rate at abutment U (%)), which can be calculated from the width of the steel strip immediately before joining (abutment), the difference (△T) between the temperature (Tb) at the center of the steel strip (the center of the steel strip (steel plate) in the width direction) and the temperature (Te) at the end of the steel strip (the end of the steel strip (steel plate) in the width direction), and the ratio (t / L) of the steel strip width (L) to the plate thickness (t).

[0007] The higher the upset rate, the stronger the joining strength becomes, but on the other hand, excessive stress is generated at the joint (abutment part), making the end of the steel strip more likely to buckle. On the other hand, the greater the temperature difference between the center and ends of the steel strip, the more stress is concentrated at the ends (edges) of the steel strip, making the joining strength stronger but also making the ends more susceptible to buckling. In addition, the upset rate that does not cause buckling and the suitable range of the temperature difference between the center and ends of the steel strip also change depending on the ratio of the steel strip width to the plate thickness. In this way, we investigated the relationship between the upset rate, the difference in temperature between the center of the steel strip and the end of the steel strip, and the ratio of the steel strip width to the plate thickness, and found that buckling of the resulting forged-welded steel pipe can be suppressed by keeping the value obtained from this equation within a suitable range.

[0008] The present invention was completed based on these findings and through further investigation, and employs the following means to solve the above problems. [1] Heat the steel plate, The steel plate is formed into a tubular shape; The widthwise ends of the facing steel plates are butted together to perform forge welding; A method for producing a forged welded steel pipe, wherein the following formula (1) is satisfied when the butting is performed. 110>(△T×U) / C ···(1) Here, in formula (1), △T(℃)=|Te(℃)-Tb(℃)|, Tb (℃): Temperature of the center of the steel plate in the width direction after heating and before abutment, Te (℃): Temperature of the width direction end of the steel plate after heating and before abutment, C (%): Ratio of plate thickness t (mm) to plate width L (mm) after heating and before abutment; 100×t / L, U(%): Upset rate at the time of abutment; 100×(LX) / L, X (mm): The pipe circumference at the position t / 2 in the wall thickness direction from the outer surface of the forged steel pipe after butting, which is set in advance. [2] A forged steel pipe in which a forge-welded abutment portion is formed in the pipe body in the pipe axial direction, A forge-welded steel pipe in which the difference in outer diameter between adjacent steel pipes measured at 1° intervals within a range of 15° on both sides of the pipe circumference, with the forge-welded abutment portion at the 0° position, is 1.0 mm or less. Effect of the Invention

[0009] According to the present invention, buckling at the abutment portion of a forge-welded steel pipe can be suppressed. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram for explaining a state in which a butt joint (forge welded butt joint) of a forge welded steel pipe is buckled. [Diagram 2] FIG. 2 is a schematic diagram showing a cross section in direction C of a butt-welded steel pipe of the present invention (a cross section perpendicular to the axial direction of the butt-welded steel pipe). [Diagram 3] FIG. 3 is a schematic diagram of a manufacturing facility for the butt-welded steel pipe of the present invention. [Figure 4] FIG. 4 is a cross-sectional view of a forge welding roll. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The manufacturing method of the butt-welded steel pipe and the butt-welded steel pipe of the present invention will be described below. The manufacturing method of the butt-welded steel pipe of the present invention comprises heating a steel plate, forming the steel plate into a tubular shape, and forge-welding the widthwise ends of the facing steel plates by butting them together, and when the butting is performed, the following formula (1) is satisfied. 110>(△T×U) / C ···(1) Here, in formula (1), △T(℃)=|Te(℃)-Tb(℃)|, Tb (℃): Temperature of the center of the steel plate in the width direction after heating and before abutment, Te (℃): Temperature of the width direction end of the steel plate after heating and before abutment, C (%): Ratio of plate thickness t (mm) to plate width L (mm) after heating and before abutment; 100×t / L, U(%): Upset rate at the time of abutment; 100×(LX) / L, X (mm): The pipe circumference at the position t / 2 in the wall thickness direction from the outer surface of the forged steel pipe after butting, which is set in advance.

[0012] Fig. 1 is a diagram for explaining the state in which the butt-welded abutment part of a butt-welded steel pipe is buckled as viewed in a cross section perpendicular to the axial direction. Fig. 2 is a schematic diagram showing a C-direction (width direction) cross section (cross section perpendicular to the rolling direction (L direction)) of the butt-welded steel pipe of the present invention. As shown in FIG. 1, in a conventional method for manufacturing a butt-welded steel pipe 100, a buckling portion 300 is formed at a butt-welded abutment portion 200 formed by abutting edge portions (widthwise ends) against each other. In this regard, in the butt-welded steel pipe 1 obtained by the manufacturing method of the butt-welded steel pipe of the present invention, as shown in FIG. Before explaining the details of the above formula (1) in the manufacturing method of the forged steel pipe of the present invention, the process of manufacturing a forged steel pipe from a steel strip (steel plate) will be explained below based on the manufacturing line equipment of the forged steel pipe, with reference to Figure 3.

[0013] The forge-welded steel pipe of the present invention is manufactured by using steel plates and butting and forge-welding the widthwise ends (hereinafter also referred to simply as edge portions) of the steel plates. The steel plate as the raw material is preferably a steel strip, but cut plates such as thin plates and thick plates can also be used. In the following, an example will be described in which the steel plate is a steel strip. The steel plate used may have a widthwise length≦rolling direction length, or a widthwise length>rolling direction length.

[0014] FIG. 3 is a schematic diagram of a manufacturing facility for the butt-welded steel pipe 1 of the present invention. First, as shown in Fig. 3, a steel strip 5 unwound from a coil 4 is passed through a looper 6, and the widthwise end portion (edge ​​portion) of the steel strip 5 is formed by an edge forming machine 7. Although not particularly limited in the present invention, conceivable methods for forming the edge portion include a method of rolling the steel strip end portion with a roll to form the end portion, and a method of cutting the end portion by cutting. Then, the entire width of the steel strip 5 is heated in a heating furnace 8, the width of the steel strip is measured by a steel strip width gauge 9, and the thickness of the steel strip is measured by a steel strip thickness gauge 10. Then, the steel strip is continuously formed into a tubular shape by a forming forge welding roll group 11 equipped with forge welding rolls, while the width direction end portion (edge ​​portion) is butted. Just before forge welding, for example, it is possible to blow oxygen-mixed air from a nozzle 12 to the width direction end portion (edge ​​portion), and then further heat the edge portion and forge weld it to join it. In addition to the method shown in FIG. 3, a method using induction heating, a method using flame radiation, and a method using laser radiation are also considered. Furthermore, the steel strip is reduced to the desired outer diameter by a diameter reducing roll 13, and a forge welded steel pipe 1 is manufactured. Here, oxygen-mixed air refers to a gas in which oxygen has been mixed into air until a desired oxygen concentration (volume %) is reached. If the oxygen concentration in the oxygen-mixed air is less than 21 volume %, the amount of heat removed by the blown air will be greater than the heat generated by oxidation, and the temperature of the steel strip will drop below the temperature after heating. For this reason, it is preferable that the oxygen concentration in the oxygen-mixed air is 21 volume % or more. Also, the oxygen concentration is preferably 22% by volume or more. If the oxygen concentration is too high, the temperature of the forge welded joint may exceed the melting point and melt, so the oxygen concentration is preferably 45% by volume or less, and more preferably 40% by volume or less. The oxygen concentration O2 (vol %) in the oxygen-mixed air can be measured by using an oxygen concentration meter attached in the nozzle. The obtained forge-welded steel pipe 1 has a forge-welded abutment portion 2 (see FIG. 2 again). In this equipment line, the widthwise ends (edges) of the steel strip are forge welded to join them, and then the forge-welded steel pipe is finished by reducing rolling, but the forge-welded steel pipe may also be finished without reducing rolling.

[0015] 110>(△T×U) / C...Equation (1) Next, the formula (1) which prescribes the butting time in the manufacturing method of the butt-welded steel pipe of the present invention will be described. When a butt-welded steel pipe is manufactured using the equipment shown in FIG. 3, the formula (1) is satisfied during the collision in the forming forge welding machine 11 (particularly the forge welding rolls provided in the forming forge welding machine 11). Here, in formula (1), △T(℃)=|Te(℃)-Tb(℃)|, Tb (℃): Temperature of the center of the steel plate in the width direction after heating and before abutment, Te (℃): Temperature of the width direction end of the steel plate after heating and before abutment, C (%): Ratio of plate thickness t (mm) to plate width L (mm) after heating and before abutment; 100×t / L, U(%): Upset rate at the time of abutment; 100×(LX) / L, X (mm): Pre-set pipe circumference at t / 2 position in the wall thickness direction from the outer surface of the butted steel pipe after butting, It is.

[0016] Buckling is more likely to occur as the temperature difference ΔT between the widthwise center of the steel plate (center of the steel strip) and the widthwise end of the steel plate (end of the steel strip) and the upset rate U become larger, but the degree of influence varies depending on the plate thickness t and the width L of the steel plate. The greater the temperature difference ΔT and the upset rate U are relative to the plate thickness t, the greater this relationship becomes, making buckling more likely to occur. As a result of investigating the relationship between this relationship and buckling at the product joint, it was found that buckling can be suppressed when (ΔT×U) / C is less than 110. Therefore, in the manufacturing method for butt-welded steel pipes of the present invention, formula (1) is set to satisfy 110>(ΔT×U) / C. Formula (1) is preferably 100>(ΔT×U) / C. In addition, although there is no particular lower limit for (ΔT×U) / C in formula (1), if C is too large and / or (ΔT×U) is too small, the formability may deteriorate, or the upset force may become too small, resulting in a decrease in the joining force. Therefore, (ΔT×U) / C is preferably 50 or more, and more preferably 55 or more.

[0017] Plate width L and plate thickness t after heating and before abutment Regarding the measurement of the plate width L and plate thickness t after heating and before abutment, when manufacturing a forge-welded steel pipe using the equipment shown in Fig. 3, the plate width L and plate thickness t are measured by a steel strip width gauge 9 and a steel strip thickness gauge 10 at the outlet side of the heating furnace 8, i.e., immediately before forge welding, more specifically, before forming the steel plate into a tubular shape. The measurement method may include, for example, a method in which visible light (wavelength 380 to 780 nm) is projected from the gauge onto the entire plate width and plate edge portion, and the plate width L and plate thickness t are measured from the amount of reflected light, but is not particularly limited thereto. The plate width L and plate thickness t are measured at regular intervals, such as every 1 m, starting from the tip of the steel plate (steel strip). The plate thickness t is the value measured by measuring the plate thickness at the end position in the width direction of the steel plate.

[0018] Upset rate U The upset rate (upset rate at abutment) U (%) refers to the reduction rate of the pipe circumference X (mm) after abutment relative to the plate width L (mm) before abutment. In other words, U = 100 × (LX) / L (%). Here, the pipe circumference X (mm) is a preset pipe circumference at the center of the wall thickness of the forge-welded steel pipe after butting (pipe circumference at a position t / 2 in the wall thickness direction from the outer surface of the forge-welded steel pipe). When manufacturing a forge-welded steel pipe using the equipment shown in Fig. 3, as described above, the plate width L and the plate thickness t of the steel strip are measured at the outlet of the heating furnace 8. In addition, as shown in Fig. 4, when the forge-welding rolls (two-roll method) provided in the forming forge-welding roll group 11 abut the width-direction ends (edges) of the steel strip formed into a tubular shape, the upset rate U (%) can be calculated using formula (2) defined based on the roll bottom radius R (see reference numeral 18) and the roll gap Rg (see reference numeral 19) of the rolls 16 and 17 that perform the abutment (joining). The upset rate is changed by the roll gap or roll bottom radius based on the measured plate thickness and plate width. U=100×(L-(2π(Rt / 2)+2Rg)) / L...(2) U: Upset rate at collision (%) L: Width of the steel plate (steel strip) after heating and before abutment (mm) t: thickness of steel plate (steel strip) after heating and before abutment (mm) R: Roll bottom radius (mm) Rg: Roll gap (mm)

[0019] Temperature Tb (℃) at the center of the width of the steel plate (center of the steel strip) and temperature Te (℃) at the edge of the width of the steel plate (edge ​​of the steel strip) Measurement is performed on the surface of the steel plate (steel strip) after heating and before abutment (just before forge welding), more specifically after the steel plate is formed into a tubular shape. Measurement methods include, but are not limited to, a method of measuring the infrared intensity of the object surface when the emissivity of the steel plate surface is set to 0.3. Measurement frequency is, for example, at regular intervals such as every 1 m, and measurements are performed at the same positions as those for measuring the plate width and thickness. In addition, the steel strip end temperature Te is determined by measuring the temperatures on the left and right sides of the steel strip end, and the higher temperature is used.

[0020] The forged welded steel pipe 1 of the present invention obtained by the above-mentioned method is a forged welded steel pipe in which a forged welded abutment portion 2 is formed in the pipe body in the pipe axial direction, and the difference in outer diameter between adjacent steel pipes measured at 1° intervals within a circumferential range of 15° on both sides of the pipe outer periphery, with the forged welded abutment portion 2 at the 0° position, is 1.0 mm or less. The wall thickness of the forged steel pipe 1 of the present invention is not particularly limited, but in order to ensure a good balance between weight reduction and strength, it is preferably 1.00 mm or more and 4.00 mm or less, and more preferably 1.50 mm or more and 3.00 mm or less. The outer diameter of the forged-welded steel pipe 1 of the present invention is preferably from 48.6 mm to 114.3 mm. In addition, the forge-welded steel pipe of the present invention preferably has a tensile strength TS of 290 to 500 MPa, although this is not particularly limited. The tensile strength TS can be measured by a tensile test (JIS Z2241 (2020) test piece No. 12).

[0021] steel pipe wall thickness As shown in FIG. 2, the wall thickness of the steel pipe is measured in the C-direction cross section (cross section perpendicular to the L direction (pipe axis direction)) of the steel pipe at three circumferential points at 3 o'clock, 6 o'clock, and 9 o'clock when the position of the forge welded abutment part 2 is set to 0 o'clock, using a single ball micrometer or the like. In other words, if the outer circumference of the cross section of the forge welded steel pipe 1 is a circle, the circumferential length of the outer circumference of the forge welded steel pipe is measured starting from the forge welded abutment part 2, and the measurement positions are 1 / 4, 1 / 2, and 3 / 4 of that length, and positions 15 at 90°, 180°, and 270° when the position of the forge welded abutment part 2 is set to 0°. The wall thickness at each position 15 is then measured using a single ball micrometer or the like. The average value of the wall thicknesses at these three points is the wall thickness of the steel pipe.

[0022] Steel pipe outer diameter The outer diameter of the steel pipe is measured at 1° intervals within a range of 15° on both sides of the pipe circumference, with the forge welded joint 2 as the 0° position. At this time, as shown in FIG. 2, the outside diameter at the forge welded abutment portion 2 is measured with a point micrometer with a needle-shaped contact point, and this value is taken as the outside diameter of the steel pipe at the 0° position (see reference numeral 14). The circumference of each steel pipe is measured and divided (equally) into 360 parts, with the abutment part being the 0° position. Then, at each division point (1° per division), the outer diameter of the steel pipe is measured using a point micrometer within a range of 15° on both sides of the pipe circumference from the abutment part, and the difference in the outer diameters of adjacent steel pipes measured at 1° intervals can be evaluated.

[0023] Although not particularly limited, the forge-welded steel pipe 1 of the present invention preferably has a composition, by mass%, of C: 0.01-0.12%, Si: 0.1-0.5%, Mn: 0.2-1.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.01-0.5%, N: 0.1% or less, Mo: 0.1-0.5%, with the remainder being Fe and unavoidable impurities. EXAMPLES

[0024] The manufacturing equipment shown in Fig. 3 was used to manufacture the forge-welded steel pipes having the chemical composition shown in Table 1. That is, the steel strip discharged from the coil was passed through a looper to form the edge, and then the entire width of the steel strip was heated in a heating furnace and continuously formed into a tube by a forming forge welding machine, while the edge was butted and forged. Just before the forge welding, oxygen-mixed air (oxygen concentration: 21-40% by volume) was sprayed from a nozzle to raise the temperature, and the pipes were joined by forge welding. The pipes were then reduced to the desired outer diameter by a diameter reducing roll to manufacture the forge-welded steel pipes. At this time, the temperature difference △T (℃) between the temperature Tb (℃) at the center of the steel strip and the temperature Te (℃) at the end of the steel strip, the upset rate U (%) at the time of butting, and the ratio C of the width L of the steel strip to the thickness t after heating were within the ranges of the following formula (1). 110>(△T×U) / C ···(1) Here, in formula (1), △T(℃)=|Te(℃)-Tb(℃)|, Tb (℃): Temperature of the center of the width of the steel plate (steel strip) after heating and before abutment, Te (℃): Temperature of the width direction end of the steel plate (steel strip) after heating and before abutment, C (%): Ratio of plate thickness t (mm) to plate width L (mm) after heating and before abutment; 100×t / L, U(%): Upset rate at the time of abutment; 100×(LX) / L, X (mm): The pipe circumference at the position t / 2 in the wall thickness direction from the outer surface of the forged steel pipe after butting, which is set in advance. The width and thickness of the steel strip were measured by shining light from a measuring device onto the entire width and edge of the strip, and measuring the amount of reflected light. The temperature Tb at the center of the steel strip and the temperature Te at the end of the steel strip were measured from the intensity of infrared light on the surface of the steel strip, and the temperature Te at the end of the steel strip was measured on the left and right sides of the end of the steel strip, and the higher temperature was used. These measurements were taken at regular intervals from the beginning of the steel strip, and the values ​​were used to calculate equation (1).

[0025] Buckling was evaluated as not occurring when the difference in outer diameter of adjacent steel pipes measured at 1° intervals within a range of 15° on both sides of the pipe circumference from the forge weld abutment position was 1.0 mm or less. On the other hand, buckling was evaluated as occurring when the difference in outer diameter of adjacent steel pipes measured at 1° intervals within a range of 15° on both sides of the pipe circumference from the forge weld abutment position was more than 1.0 mm. More specifically, among the differences in the outer diameters of the steel pipes measured for each steel pipe, if the maximum outer diameter difference, which is the maximum value of the differences, was 1.0 mm or less, it was evaluated as no buckling, and if the maximum outer diameter difference was more than 1.0 mm, it was evaluated as having buckling.

[0026] Table 1 shows the chemical composition of the steel pipes, and Table 2 shows the manufacturing conditions of the steel pipes, the dimensions of the obtained steel pipes, and the evaluation results. The value on the right side of formula (1) in Table 2 is the maximum value when that variety was produced for the invention examples, and the minimum value when that variety was produced for the comparative examples. For each variety, a 1-m length of steel pipe including the portion with the maximum and minimum values ​​on the right side of formula (1) was taken, and the end of the steel pipe was measured using the method described above to evaluate the buckling of the steel pipes.

[0027] [Table 1] [Table 2]

[0028] Nos. 1 to 8 satisfied the range of formula (1), no buckling occurred near the manufactured forge welded joints, and no gaps occurred even when grooving joints were applied.

[0029] On the other hand, Nos. 9 to 12 were outside the range of formula (1), and buckling occurred in the manufactured forge welded joint, resulting in a gap between the joint and the pipe when the grooving joint was applied. [Explanation of symbols]

[0030] 1,100 forge welded steel pipe 2, 200 Forge weld abutment part 300 Buckling part 4 Coils 5 Steel strip 6. Looper 7 Edge forming machine 8 Furnace 9. Steel strip width measuring device 10. Steel strip thickness gauge 11 Form-welded rolls (forged rolls) 12 Nozzles 13. Diameter reduction roll 14 Steel pipe outer diameter 15 Steel pipe wall thickness measurement position 16 rolls 17 Rolls 18 Roll bottom radius 19 Roll gap

Claims

[Claim 1] A method for manufacturing a forged steel pipe having a composition containing, by mass%, C: 0.01-0.12%, Si: 0.1-0.5%, Mn: 0.2-1.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.01-0.5%, N: 0.1% or less, Mo: 0.1-0.5%, with the remainder being Fe and unavoidable impurities; The steel plate is heated, The steel plate is formed into a tubular shape; The widthwise ends of the facing steel plates are butted together to perform forge welding; A method for manufacturing a butt-welded steel pipe, wherein the following formula (1) is satisfied when the butt is performed. 110>(△T×U) / C...(1) Here, in formula (1), △T(℃)=|Te(℃)−Tb(℃)|, Tb (°C): temperature of the center of the width direction of the steel plate after heating and before abutment; Te (° C.): Temperature of the width direction end of the steel plate after heating and before abutment, C (%): ratio of plate thickness t (mm) to width L (mm) of the steel plate after heating and before abutment; 100 × t / L, U (%): upset rate at the time of collision; 100×(L−X) / L, X (mm): Pre-set pipe circumference at a position t / 2 in the wall thickness direction from the outer surface of the forged steel pipe after butting.

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