Steel pipe and method for manufacturing the same

Abstract

To provide: a steel pipe configured so as to suppress occurrence of deformation while suppressing the content of Mn; and a manufacturing method of the steel pipe.SOLUTION: A manufacturing method of a steel pipe according to the present invention comprises: heating a steel sheet having a component composition containing, by mass%, 0.001 to 0.06% of C, 0.10 to 0.30% of Si, 0.10 to 0.50% of Mn, 0.030% or less of P, and 0.015% or less of S, and the balance Fe with inevitable impurity and hot-forming the heated steel sheet into an open tube; joining both edge portions of the open tube; and draw-rolling the open tube at a draw-rolling finishing temperature of an Ar3 transformation point or lower.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a steel pipe and a method for manufacturing the same, and more specifically, to a technique capable of providing a thin-walled welded steel pipe that is less likely to deform at a low cost.

Background Art

[0002] Generally, welded steel pipes are used as steel pipes for pipes. A welded steel pipe is formed by shaping a steel plate into a tubular shape and joining both edge portions thereof, and is manufactured by various manufacturing methods from a small diameter to a large diameter. The main manufacturing methods include forge welding (forge-welded steel pipe) and electric resistance welding (electric welded steel pipe). The manufacturing method of a forge-welded steel pipe is mainly a manufacturing method adopted for small-diameter steel pipes. After continuously passing a steel plate through a heating furnace and heating it, it is formed into a tubular shape by a forming roll to form an open pipe. High-pressure air is blown onto the end faces of both edge portions of the open pipe to remove the scale of the edge portions and raise the temperature by an oxidation exothermic reaction. Both edge portions are butt-joined by a forge welding roll to form a solid-phase joint, and then, drawing rolling is performed hot to finish to a predetermined outer diameter and wall thickness. The feature of this manufacturing method is that it has high productivity by performing the entire process hot until finishing.

[0003] On the other hand, the manufacturing method of an electric welded steel pipe is mainly a manufacturing method adopted for small-diameter to medium-diameter steel pipes. A strip steel is continuously supplied, formed into a tubular shape by a forming roll to form an open pipe, and then, the end faces of both edge portions of the open pipe are heated to above the melting point of steel by high-frequency induction heating. Then, both edge portion end faces are butt-welded by a squeeze roll and finished to a predetermined outer diameter and wall thickness. Furthermore, in the case of an electric welded steel pipe, there is also a manufacturing method in which forming and drawing rolling are performed hot in the same manner as a forge-welded steel pipe, and it has high productivity like a forge-welded steel pipe.

[0004] These general-purpose steel pipes are standardized by Japanese Industrial Standards (JIS), such as JIS G3452 (commonly known as SGP). While the aforementioned standards specify outer diameter and wall thickness, carbon steel pipes with a certain outer diameter and wall thickness that meet the standards have a weight corresponding to their outer diameter, making them difficult for workers to carry during use. This necessitates multiple people to carry them, leading to problems such as reduced work efficiency. Due to these problems, there is a growing demand for lightweight, thin-walled welded steel pipes that can be easily carried by workers.

[0005] In the manufacturing process of these thin-walled welded steel pipes, the same manufacturing methods as for forge-welded steel pipes and electric resistance welded steel pipes are used as with conventional general-purpose piping steel pipes. However, one drawback of thin-walled welded steel pipes is that, due to their thin wall thickness, they are more susceptible to deformation compared to conventional general-purpose piping steel pipes. Specifically, thin-walled welded steel pipes, being thinner than general-purpose steel pipes, have a problem in that even if their strength characteristics such as yield strength and tensile strength are equivalent to those of general-purpose steel pipes, they have less resistance to deformation under external forces, making them prone to deformation such as dents.

[0006] Therefore, it is desirable to increase deformation resistance by increasing the strength of the steel pipe, and as a manufacturing method for increasing the strength of welded steel pipes, for example, Patent Document 1 is cited. Patent Document 1 proposes a manufacturing method that improves the strength of the joint by hot cutting both ends before joining. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 4-313471 [Overview of the project] [Problems that the invention aims to solve]

[0008] According to the method proposed in Patent Document 1, the strength of the joint in a thin-walled welded steel pipe can be increased. However, the strength increase obtained by this method is only at the joint, and it is not possible to increase the strength of the entire steel pipe, including the base material, and therefore it is not possible to suppress the occurrence of deformation.

[0009] Another known method for improving strength properties is to add elements that contribute to increased strength to the material, but this has the problem of increasing the cost of the material. Specifically, elements that contribute to increased strength include C, Mn, Ti, V, and Nb. In particular, increasing the Mn content can increase strength, but from the standpoint of processability, weldability, and cost, it is desirable to suppress the Mn content, as well as Ti, V, and Nb. Thus, there was a strong need to establish a new technology for thin-walled welded steel pipes that suppressed deformation.

[0010] The present invention has been made in view of the above circumstances, and aims to provide a steel pipe and a method for manufacturing the same, which suppress deformation while suppressing the Mn content, and more particularly a thin-walled welded steel pipe and a method for manufacturing the thin-walled welded steel pipe. In this invention, suppressing deformation means that the ratio of the indentation depth (maximum indentation depth) d (mm) of the steel pipe to the outer diameter D (mm), which is d / D, is 0.010 or less. [Means for solving the problem]

[0011] The inventors, through diligent research to solve the above problems, have discovered a novel and important finding: in order to realize a thin-walled welded steel pipe that is less prone to deformation, by setting the end temperature of the drawing-rolling process to below the Ar3 transformation point of the steel pipe during the manufacturing of the welded steel pipe, the deformation resistance of the steel pipe increases, and the processing strain increases, thereby achieving increased strength without the need for alloy additions that contribute to increased strength. Furthermore, the inventors have discovered a novel and important finding: by setting the dimensional ratio of the outer diameter to the wall thickness before and after drawing-rolling to a specific range, the optimal processing strain is applied, resulting in a greater effect and improved formability. Based on this finding, the inventors conducted further studies and, as a result, completed the present invention.

[0012] The gist of this invention is as follows: [1] A steel sheet having a composition by mass% containing C: 0.001~0.06%, Si: 0.10~0.30%, Mn: 0.10~0.50%, P: 0.030% or less, S: 0.015% or less, with the remainder being Fe and unavoidable impurities, is heated and hot-formed to form an open tube. The two edges of the open tube are joined together. A method for manufacturing steel pipes, comprising performing drawing-rolling at a drawing-rolling termination temperature below the Ar3 transformation point. [2] A method for manufacturing steel pipes according to [1], wherein the starting temperature for drawing and rolling is below the Ar3 transformation point. [3] The ratio of the outer diameter D1 (mm) to the wall thickness t1 (mm) of the tube before drawing and rolling, t1 / D1, is 0.020 to 0.030. A method for manufacturing a steel pipe according to [1] or [2], wherein the ratio of the outer diameter D2 (mm) to the wall thickness t2 (mm) of the pipe after drawing and rolling, t2 / D2, is 0.030 to 0.050. [4] A steel pipe having a base material portion and a joint portion, the component composition being, in mass%, C: 0.001~0.06%, Si: 0.10~0.30%, Mn: 0.10~0.50%, P: 0.030% or less, S: 0.015% or less, with the remainder being Fe and unavoidable impurities. The yield strength is 200 to 300 MPa, and the tensile strength is 300 to 450 MPa. The ratio of the wall thickness t (mm) to the outer diameter D (mm), that is, t / D, is 0.030 to 0.050. A steel pipe in which the ratio of the depth d (mm) of the dent to the outer diameter D (mm), that is, d / D, is 0.010 or less.

Effect of the Invention

[0013] According to the present invention, it is possible to provide a steel pipe in which deformation is unlikely to occur while suppressing the Mn content, particularly a thin-walled welded steel pipe in which the occurrence of deformation is suppressed. The steel pipe manufactured by the method of the present invention is extremely effective in terms of being unlikely to deform and being manufactured at low cost.

Brief Description of the Drawings

[0014] [Figure 1] It is a drawing for explaining equipment for manufacturing the steel pipe of the present invention.

Embodiment for Carrying Out the Invention

[0015] Hereinafter, as an explanation of the steel pipe of the present invention and its manufacturing method, in this embodiment, the manufacturing method of the steel pipe and the steel pipe will be explained in this order.

[0016] <Manufacturing Method of Steel Pipe> The manufacturing method of the steel pipe of the present invention is to heat a steel sheet having a component composition containing, in mass%, C: 0.001 to 0.06%, Si: 0.10 to 0.30%, Mn: 0.10 to 0.50%, P: 0.030% or less, S: 0.015% or less, and the balance being Fe and unavoidable impurities, and then form it hot to obtain an open pipe, join both edge portions of the open pipe, and perform drawing rolling at a drawing rolling end temperature below the Ar3 transformation point.

[0017] Hereinafter, the component composition of the steel sheet will be explained. Unless otherwise specified, “%” representing the content of each component represents “mass%”.

[0018] C: 0.001 to 0.06% C increases the strength of steel by containing 0.001% or more. Therefore, the C content should be 0.001% or more. On the other hand, when the C content exceeds 0.06%, it affects the weldability and the susceptibility to plating cracks. Therefore, the C content should be 0.06% or less, preferably 0.05% or less.

[0019] Si: 0.10 - 0.30% Si is a component that has the effect of improving the plating property of the steel pipe. To obtain the above effect, the Si content should be 0.10% or more, preferably 0.15% or more. On the other hand, when the Si content exceeds 0.30%, the formability decreases. Therefore, the Si content should be 0.30% or less, preferably 0.25% or less.

[0020] Mn: 0.10 - 0.50% Mn is an important component added to obtain an increase in strength, and it increases the strength of steel by containing 0.10% or more. Therefore, the Mn content should be 0.10% or more. On the other hand, the higher the Mn content, the worse the manufacturing cost. Also, when the Mn content exceeds 0.50%, the workability is reduced due to an increase in strength and a decrease in elongation. Therefore, from the viewpoints of workability and manufacturing cost, the Mn content should be 0.50% or less, preferably 0.40% or less.

[0021] P: 0.030% or less P is an inevitable impurity, and the lower the content, the better. However, excessive reduction of P leads to an increase in manufacturing cost. Therefore, the P content should be 0.030% or less, preferably 0.025% or less.

[0022] S: 0.015% or less S is an inevitable impurity, and the lower the content, the better. However, excessive reduction of S leads to an increase in manufacturing cost. Therefore, the S content should be 0.015% or less, preferably 0.010% or less.

[0023] The above describes the component composition of the steel plate used in manufacturing the steel pipe of the present invention, with the remainder being Fe and unavoidable impurities. From a cost standpoint, it is preferable that the other elements in the above component composition be 0.03% or less each of Ti, Nb, and V.

[0024] In this invention, a steel pipe is manufactured from a steel plate having the above-described component composition. To obtain a steel pipe, first, steel having the predetermined component composition is melted and continuously cast to form a steel material (slab), or melted, cast, and then decomposed and rolled to form a steel material (slab, etc.). Next, the steel material is heated to a predetermined heating temperature. The heating temperature is not particularly limited, but considering that the load during rolling increases due to the increase in deformation resistance, it is preferable to set it to 1100°C or higher, and also, since the surface properties may deteriorate during rolling due to the increase in scale formation, it is preferable to set it to 1300°C or lower.

[0025] Subsequently, hot rolling is performed under predetermined temperature conditions to produce hot-rolled steel sheets. Hot rolling consists of rough rolling and finish rolling. The conditions for rough rolling do not need to be particularly limited, as long as a sheet bar of a predetermined shape can be produced. However, in finish rolling, it is preferable to adjust the finish rolling completion temperature to 750°C or higher. The winding temperature can be 550 to 700°C. Within this temperature range, it is possible to suppress the instability of the shape of the hot-rolled steel sheet due to meandering after winding and to reduce the load on the winding machine. Using these hot-rolled steel sheets, steel pipes are manufactured by the method shown below.

[0026] Figure 1 is a diagram illustrating an example of equipment for manufacturing the steel pipe of the present invention. As shown in Figure 1, first, the steel plate 2 that was loaded into the charging machine 1 is heated in the heating furnace 3. Then, the steel plate 2 is heated in the heating device and forming machine 4, and formed at a hot temperature to become an open pipe, and both edges are joined to become a raw steel pipe. Next, the raw steel pipe is drawn and rolled in the drawing mill 5 and cut to the desired pipe length in the cutting machine 6 to obtain a steel pipe (welded steel pipe) 7.

[0027] The manufacturing process for steel pipes will be described below. In particular, the forming process, in which a steel sheet having the above-mentioned component composition is heated and then hot-formed to form an open pipe; the joining process, in which the two edges of the open pipe are joined to form a base steel pipe; and the drawing-rolling process (hot drawing-rolling process), in which the base steel pipe is subjected to drawing-rolling, will be described in order.

[0028] [Molding process] In the forming process, the steel sheet is heated and then formed into an open tube while hot. The method of forming is not particularly limited and may be carried out according to conventional methods. The forming may be carried out by roll forming.

[0029] [Joining process] Next, in the joining process, the widthwise ends (both edges) of the open pipe obtained in the forming process are joined to obtain a raw steel pipe. The joining method is not particularly limited and can be carried out according to conventional methods. Examples of joining methods include forge welding and electric resistance welding.

[0030] [Drawing and Rolling Process (Hot Drawing and Rolling Process)] (End of drawing-rolling temperature: Below the Ar3 transformation point) Following the joining process described above, in the drawing-rolling process (hot drawing-rolling process), the material steel pipe is subjected to hot drawing-rolling to achieve a predetermined outer diameter and wall thickness. In the hot drawing-rolling process, the temperature at the end of the drawing-rolling is kept below the Ar3 transformation point in order to increase the deformation resistance of the material steel pipe during drawing-rolling, thereby applying processing strain and achieving increased strength. If the temperature at the end of the drawing-rolling process exceeds the Ar3 transformation point, a sufficient increase in strength cannot be achieved, and deformation cannot be adequately suppressed. The final temperature of the drawing-rolling process is preferably (Ar3 transformation point - 70°C) or lower. The lower limit is not particularly limited, but the final temperature of the drawing-rolling process is preferably (Ar3 transformation point - 100°C) or higher. Furthermore, the starting temperature for the drawing-rolling process is more preferably below the Ar3 transformation point, from the viewpoint of formability during the drawing-rolling process. The starting temperature for drawing-rolling is preferably (Ar3 transformation point - 10°C) or lower. The lower limit is not particularly limited, but the starting temperature for drawing-rolling is preferably (Ar3 transformation point - 30°C) or higher. The Ar3 transformation point is obtained by transformation point measurement tests such as the Formaster test. The end temperature of the drawing-rolling process and the start temperature of the drawing-rolling process may be the temperature of the outer surface of the steel pipe.

[0031] t1 / D1: 0.020~0.030 Furthermore, it is preferable that the value of t1 / D1, which is the ratio of the outer diameter D1 (mm) to the wall thickness t1 (mm) of the tube after the joining process and before the drawing and rolling process, be 0.020 to 0.030. If the value of t1 / D1 is less than 0.020, the deformation resistance is low, which may lead to unstable forming during drawing and rolling. On the other hand, if the value of t1 / D1 exceeds 0.030, it may not be possible to form the desired wall thickness after drawing and rolling. Therefore, it is preferable to set the value of t1 / D1 to 0.020 to 0.030. The value of t1 / D1 can be adjusted by the molding conditions in the molding process (dimensions of the molding rolls, rotation speed of the rolls).

[0032] t2 / D2: 0.030~0.050 Furthermore, the value of t2 / D2, which is the ratio of the outer diameter D2 (mm) to the wall thickness t2 (mm) of the tube after drawing and rolling, is preferably 0.030 to 0.050. If the t2 / D2 value is less than 0.030, shape defects such as dents may be more likely to occur. On the other hand, if the value of t2 / D2 exceeds 0.050, and the outer diameter remains constant, the wall thickness increases, leading to an increase in mass. This can make it difficult for workers to carry the object or increase the workload. Therefore, it is preferable to set the value of t2 / D2 to between 0.030 and 0.050. The value of t2 / D2 can be adjusted by the drawing-rolling conditions in the drawing-rolling process (dimensions of the drawing-rolling rolls, rotation speed of the rolls).

[0033] By manufacturing steel pipes using the above process, it is possible to produce steel pipes that are less prone to deformation (thin-walled welded steel pipes).

[0034] <Steel pipes (thin-walled welded steel pipes)> The steel pipe of the present invention has the above-described component composition and is obtained by the above-described method for manufacturing the steel pipe. Specifically, the steel pipe of the present invention is a steel pipe having a base material portion and a joint portion, and its component composition is, in mass%, C: 0.001~0.06%, Si: 0.10~0.30%, Mn: 0.10~0.50%, P: 0.030% or less, S: 0.015% or less, with the remainder being Fe and unavoidable impurities, with a yield strength of 200~300 MPa, a tensile strength of 300~450 MPa, a ratio of outer diameter D (mm) to wall thickness t (mm), t / D being 0.030~0.050, and a ratio of recess depth d (mm) to outer diameter D (mm), d / D being 0.010 or less.

[0035] The steel pipe of the present invention has a base material portion and a joint portion, and has a yield strength of 200 to 300 MPa and a tensile strength of 300 to 450 MPa. As a result, the steel pipe of the present invention can suppress deformation. Furthermore, the ratio of the outer diameter D (mm) to the wall thickness t (mm), t / D, is 0.030 to 0.050, and the indentation depth (maximum indentation depth) d (mm) satisfies d / D ≤ 0.010. As a result, the present invention makes it possible to obtain a steel pipe that sufficiently suppresses deformation, has a good appearance, and does not have any practical defects.

[0036] In the steel pipe of the present invention, the effect of suppressing deformation is greater when the wall is thin, so the wall thickness t is preferably 3.0 to 4.0 mm. [Examples]

[0037] Next, the present invention will be described in more detail based on the following examples. This is merely one preferred example, and the present invention is not limited in any way by this embodiment. Welded steel pipes were manufactured using the following procedure, and the properties of the obtained welded steel pipes were evaluated. First, welded steel pipes were manufactured using steel plates having the component composition shown in Table 1. During this process, the pipes were formed into open pipes by roll forming using a forming and forging machine, and both edges were joined. After joining, drawing and rolling were performed to obtain welded steel pipes (forging-welded steel pipes). A tensile test specimen was cut from the base material of the obtained welded steel pipe in the longitudinal direction. Using the aforementioned tensile test specimens, tensile tests were conducted according to the method compliant with JIS Z2241(2022), and the yield strength (YS), tensile strength (TS), and elongation (El) were evaluated. Furthermore, as an external evaluation of the steel pipes after manufacturing, the maximum indentation depth (indentation depth d) of the steel pipes was measured. The depth was measured using a depth gauge, and the largest value obtained by contacting the measuring probe with the indented portion was taken as the maximum indentation depth. The drawing-rolling conditions and the test and inspection results are shown in Table 2.

[0038] [Table 1]

[0039] [Table 2]

[0040] As can be seen from the results of the above examples, in the present invention, it is possible to increase the yield strength and tensile strength without excessively adding elements that contribute to strength increase, and while suppressing the Mn content, and to manufacture thin-walled welded steel pipes that suppress deformation. Furthermore, in the steel pipe of the present invention, the elongation was made 65% or more by suppressing the Mn content. In contrast, thin-walled welded steel pipes obtained under the conditions of comparative examples that do not satisfy the conditions of the present invention do not show an increase in yield strength and tensile strength, or they result in a deterioration of material costs and are not suitable for inexpensive manufacturing methods. [Explanation of Symbols]

[0041] 1 Charging machine 2 steel plate 3 Heating furnace 4. Heating device and molding machine 5. Drawing and rolling mill 6 cutting machine 7. Steel pipes (welded steel pipes)

Claims

1. A steel sheet having a composition in mass percent of C: 0.001-0.06%, Si: 0.10-0.30%, Mn: 0.10-0.50%, P: 0.030% or less, S: 0.015% or less, with the remainder being Fe and unavoidable impurities, is heated and then hot-formed to form an open tube. The two edges of the open tube are joined together. Ar ３ The drawing-out rolling is performed at a drawing-out rolling end temperature below the transformation point. A method for manufacturing a steel pipe, wherein the ratio of the outer diameter D2 (mm) to the wall thickness t2 (mm) of the pipe after drawing and rolling, t2 / D2, is 0.030 to 0.

050.

2. The starting temperature for drawing and rolling is Ar ３ A method for manufacturing a steel pipe according to claim 1, wherein the temperature is below the transformation point.

3. The outer diameter D of the tube before the aforementioned drawing and rolling process. １ (mm) and wall thickness t １ The ratio of (mm) is t １ / D １ A method for manufacturing a steel pipe according to claim 1 or 2, wherein the coefficient is 0.020 to 0.

030.

4. A steel pipe having a base material portion and a joint portion, the component composition being, in mass%, C: 0.001 to 0.06%, Si: 0.10 to 0.30%, Mn: 0.10 to 0.50%, P: 0.030% or less, S: 0.015% or less, with the remainder being Fe and unavoidable impurities. The yield strength is 200-300 MPa, and the tensile strength is 300-450 MPa. The ratio of the outer diameter D (mm) to the wall thickness t (mm), t / D, is 0.030 to 0.

050. A steel pipe in which the ratio d / D, which is the ratio of the recess depth d (mm) to the outer diameter D (mm), is 0.010 or less.