Steel pipe and manufacturing method thereof

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 manufacturing method thereof, and more specifically, to a technique capable of providing a thin-walled welded steel pipe which is less susceptible to deformation at a low cost. [Background technology]

[0002] Generally, welded steel pipes are used as steel pipes for piping. Welded steel pipes are made by forming a steel plate into a tubular shape and welding both edges of the tubular shape. They are manufactured by various manufacturing methods from small diameter to large diameter, and the main manufacturing methods include forge welding (forge welded steel pipes) and electric resistance welding (electric resistance welded steel pipes). The manufacturing method of butt welded steel pipe is mainly adopted for small diameter steel pipe, in which steel plate is heated by continuously passing it through a heating furnace, then formed into a tube by forming rolls to make an open pipe, high pressure air is blown onto both edge faces of the open pipe to remove scale from the edges and raise the temperature by an exothermic oxidation reaction, both edges are butted together by butt welding rolls to form a solid state weld, and then hot reduction rolling is performed to finish to the specified outer diameter and wall thickness. A feature of this manufacturing method is that it has high productivity because it is done hot up to the finishing stage.

[0003] On the other hand, the manufacturing method of electric resistance welded steel pipe is a manufacturing method mainly adopted for small to medium diameter steel pipes, in which a steel strip is continuously supplied, formed into a tubular shape by forming rolls to form an open pipe, and then both edge end faces of the open pipe are heated to above the melting point of the steel by high frequency induction heating, and then both edge end faces are butt welded by squeeze rolls to finish to a specified outer diameter and wall thickness. Furthermore, electric resistance welded steel pipe can also be manufactured by hot forming and rolling, just like forged steel pipe, and has high productivity just like forged steel pipe.

[0004] Such general piping steel pipes are standardized by Japanese Industrial Standards, such as JIS G3452 (commonly known as SGP). The above standards stipulate the outer diameter and wall thickness, but carbon steel pipes having a wall thickness stipulated by the above standards and having an outer diameter of a certain level or more have a weight corresponding to the outer diameter, making them difficult for workers to carry when in use, and requiring multiple people to carry them, resulting in problems such as reduced work efficiency. Due to these problems, there is an increasing demand for lightweight, thin-walled welded steel pipes that can be easily carried by workers.

[0005] In manufacturing this thin-walled welded steel pipe, the same manufacturing methods as for conventional steel pipes for general piping are used as for forge-welded steel pipes and electric-resistance welded steel pipes. However, since thin-walled welded steel pipes have a thinner wall thickness, they are more prone to deformation than conventional steel pipes for general piping. Specifically, thin-walled welded steel pipes are thinner than general-purpose steel pipes, and therefore have the problem that even if their strength properties, such as yield strength and tensile strength, are equivalent to those of general-purpose steel pipes, they have lower deformation resistance to external forces and are therefore more susceptible to deformations such as dents.

[0006] Therefore, it is desirable to increase the deformation resistance by increasing the strength of the steel pipe, and an example of a manufacturing method for increasing the strength of a welded steel pipe is Patent Document 1. Patent Document 1 proposes a manufacturing method for improving 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 Summary of the Invention [Problem to be solved by the invention]

[0008] According to the method proposed in Patent Document 1, it is possible to increase the strength of the joint in a thin-walled welded steel pipe. However, this method increases the strength only at the joint, and does not increase the strength of the entire steel pipe including the base material, so it is not possible to prevent the occurrence of deformation.

[0009] In addition, as another method for improving strength characteristics, it is generally known that elements that contribute to increasing strength are added to the material, but this has the problem of increasing the cost of the material.Specifically, elements that contribute to increasing strength include C, Mn, Ti, V, and Nb, and in particular, increasing the content of Mn can increase strength, but from the standpoint of workability, weldability, and cost, it is desirable to suppress the content of Mn as well as Ti, V, and Nb. Thus, there has been a demand for the establishment of a new technology for thin-walled welded steel pipes that suppresses the occurrence of deformation.

[0010] The present invention has been made in consideration of the above circumstances, and has an object to provide a steel pipe in which the occurrence of deformation is suppressed while the Mn content is suppressed, and a manufacturing method thereof, in particular a thin-walled welded steel pipe and a method for manufacturing the thin-walled welded steel pipe. In the present invention, suppressing the occurrence of deformation means that d / D, which is the ratio of the recess depth (maximum recess depth) d (mm) of the steel pipe to the outside diameter D (mm), is 0.010 or less. [Means for solving the problem]

[0011] As a result of intensive research by the inventors to solve the above problems, in order to realize a thin-walled welded steel pipe that is less likely to deform, when reducing is performed in the manufacture of welded steel pipe, the end temperature of the reducing roll is set to the Ar3 transformation point of the steel pipe or less, thereby increasing the deformation resistance of the steel pipe and increasing the processing strain, thereby achieving an increase in strength without adding an alloy that contributes to increasing strength. Furthermore, the inventors have obtained the novel and important finding that by setting the dimensional ratio of the outer diameter and wall thickness before and after reducing roll to a specific range, an optimal processing strain is applied, thereby making the effect even greater and obtaining good formability. The present inventors conducted further studies based on this finding and as a result completed the present invention.

[0012] The gist and configuration of the present invention are as follows. [1] A steel sheet having a composition containing, by 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 balance being Fe and unavoidable impurities, is heated and hot-formed into an open tube; Joining both edges of the open tube; A manufacturing method for steel pipes in which reducing rolling is performed at a finishing temperature below the Ar3 transformation point. [2] The method for producing a steel pipe according to [1], wherein the reduction rolling start temperature is set to the Ar3 transformation point or lower. [3] The ratio of the outer diameter D1 (mm) to the wall thickness t1 (mm) of the tube before the rolling is t1 / D1, which is 0.020 to 0.030; and a ratio of the outside diameter D2 (mm) to the wall thickness t2 (mm) of the tube after the reducing rolling, t2 / D2, is 0.030 to 0.050. [4] A steel pipe having a base metal portion and a joint portion, the composition of which is, by 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 balance 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 outer diameter D (mm) to the wall thickness t (mm), t / D, is 0.030 to 0.050; A steel pipe having a ratio of d / D, which is the ratio of the recess depth d (mm) to the outer diameter D (mm), of 0.010 or less. Effect of the Invention

[0013] According to the present invention, it is possible to provide a steel pipe which is less likely to deform while suppressing the Mn content, in particular a thin-walled welded steel pipe which is less likely to deform. The steel pipe produced by the method of the present invention is extremely effective in that it is less likely to deform and can be produced at low cost. [Brief description of the drawings]

[0014] [Figure 1] FIG. 2 is a diagram for explaining an equipment for manufacturing a steel pipe according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] As a description of the steel pipe and its manufacturing method of the present invention, in this embodiment, the manufacturing method of the steel pipe and the steel pipe will be described in that order.

[0016] <Steel pipe manufacturing method> The method for producing a steel pipe of the present invention comprises heating and hot forming a steel plate having a composition containing, by mass%, 0.001-0.06% C, 0.10-0.30% Si, 0.10-0.50% Mn, 0.030% or less P, 0.015% or less S, with the balance being Fe and unavoidable impurities, and then joining both edges of the open pipe and performing reducing rolling at a reducing rolling finish temperature not exceeding the Ar3 transformation point.

[0017] The chemical composition of the steel sheet will be described below. Unless otherwise specified, "%" representing the content of each component represents "% by mass."

[0018] C: 0.001 to 0.06% C increases the strength of steel when contained at 0.001% or more, so the C content is set to 0.001% or more. On the other hand, if the C content exceeds 0.06%, it affects the weldability and plating cracking susceptibility, so the C content is set to 0.06% or less, preferably 0.05% or less.

[0019] Silicon: 0.10 to 0.30% Silicon is an element that has the effect of improving the platability of steel pipes. In order to obtain this effect, the silicon content is set to 0.10% or more, and preferably 0.15% or more. On the other hand, if the Si content exceeds 0.30%, the formability decreases, so the Si content is set to 0.30% or less, preferably 0.25% or less.

[0020] Mn: 0.10~0.50% Mn is an important component added to increase strength, and a Mn content of 0.10% or more increases the strength of steel. Therefore, the Mn content is set to 0.10% or more. On the other hand, the higher the Mn content, the higher the manufacturing cost. Also, if 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 costs, the Mn content is set to 0.50% or less, and preferably 0.40% or less.

[0021] P:0.030% or less P is an unavoidable impurity, and the lower the content, the better. However, an excessive reduction in P content leads to an increase in manufacturing costs, so the P content is set to 0.030% or less, and preferably 0.025% or less.

[0022] S: 0.015% or less S is an unavoidable impurity, and the lower the content, the better. However, since excessive reduction of S leads to an increase in manufacturing costs, the S content is set to 0.015% or less, and preferably 0.010% or less.

[0023] The above is the composition of the steel plate used in manufacturing the steel pipe of the present invention, with the balance being Fe and unavoidable impurities. From the viewpoint of cost, it is preferable that the above composition contains, as other elements, Ti, Nb, and V in an amount of 0.03% or less.

[0024] In the present invention, a steel pipe is produced from a steel plate having the above-mentioned composition. To obtain a steel pipe, first, a steel having the above-mentioned predetermined composition is melted and continuously cast to obtain a steel material (slab), or alternatively, the steel material is melted, cast, and rolled by splitting to obtain a steel material (slab, etc.). The steel material is then heated to a predetermined heating temperature. The heating temperature is not particularly limited, but is preferably 1100°C or higher in consideration of the increased load during rolling due to the increased deformation resistance, and is preferably 1300°C or lower in consideration of the increased amount of scale generated that may deteriorate the surface properties during rolling.

[0025] Thereafter, hot rolling is performed under a predetermined temperature condition to obtain a hot-rolled steel sheet. The hot rolling includes rough rolling and finish rolling. The conditions of the rough rolling do not need to be particularly limited as long as a sheet bar having a predetermined shape can be obtained. However, in the finish rolling, it is preferable to adjust the finish rolling end temperature to 750°C or higher. The coiling temperature may be 550 to 700°C. If the temperature is within the above range, it is possible to suppress instability of the shape due to meandering of the hot-rolled steel sheet after coiling and reduce the load on the coiler. The hot-rolled steel sheet is used to manufacture a steel pipe by the method shown below.

[0026] FIG. 1 is a diagram illustrating an example of equipment for producing a steel pipe according to the present invention. As shown in Fig. 1, first, a steel plate 2 loaded into a loading machine 1 is heated in a heating furnace 3. The steel plate 2 is then heated and hot-formed into an open pipe in a heating device and forming machine 4, and both edges are joined to form a raw steel pipe. Next, the raw steel pipe is reduced in a reducing mill 5 and cut to the desired pipe length by a cutting machine 6, and a steel pipe (welded steel pipe) 7 is obtained.

[0027] The manufacturing process of steel pipes will be described below, in particular the forming step in which a steel plate having the above-mentioned composition is heated and hot-formed to form an open pipe, the joining step in which both edges of the open pipe are joined to form a base steel pipe, and the reducing rolling step (hot reducing rolling step) in which the base steel pipe is subjected to reducing rolling will be described in order.

[0028] [Molding process] In the forming step, the steel sheet is heated and then hot-formed into an open tube. The method for forming the steel sheet is not particularly limited and may be a conventional method. The forming may be performed by roll forming.

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

[0030] [Reduction rolling process (hot reduction rolling process)] (Reducing rolling end temperature: below Ar3 transformation point) After the joining step, in the hot reducing rolling step (hot reducing rolling step), the base steel pipe is subjected to hot reducing rolling so as to have a predetermined outer diameter and wall thickness. In the hot reducing rolling, the end temperature of the hot reducing rolling is set to the Ar3 transformation point or lower in order to impart processing strain by increasing the deformation resistance of the base steel pipe during the reducing rolling and thereby realize an increase in strength. If the finishing temperature of the reducing rolling exceeds the Ar3 transformation point, a sufficient increase in strength cannot be realized and deformation cannot be sufficiently suppressed. The finishing temperature of the reducing roll is preferably (Ar3 transformation point - 70°C) or less. Although there is no particular lower limit, the finishing temperature of the reducing roll is preferably (Ar3 transformation point - 100°C) or more. Moreover, from the viewpoint of formability during the reducing rolling, the reducing rolling start temperature is more preferably set to the Ar3 transformation point or lower. The start temperature of the reducing rolling is preferably (Ar3 transformation point - 10°C) or lower. Although there is no particular lower limit, the start temperature of the reducing rolling is preferably (Ar3 transformation point - 30°C) or higher. The Ar3 transformation point can be obtained by a transformation point measurement test such as a Formaster test. The reducing rolling end temperature and the reducing rolling start temperature may be the temperature of the outer surface of the steel pipe.

[0031] t1 / D1: 0.020~0.030 Furthermore, the value of t1 / D1, which is the ratio of the outer diameter D1 (mm) and the wall thickness t1 (mm) of the pipe after the joining step and before the reducing rolling, is preferably 0.020 to 0.030. If the value of t1 / D1 is less than 0.020, deformation resistance is small, and forming may become unstable during reducing rolling. On the other hand, if the value of t1 / D1 exceeds 0.030, it may not be possible to form the steel sheet into a desired thickness after rolling, and therefore the value of t1 / D1 is preferably set to 0.020 to 0.030. The value of t1 / D1 can be adjusted by the molding conditions in the molding step (the dimensions of the molding rolls, the number of rotations of the rolls).

[0032] t2 / D2: 0.030~0.050 Furthermore, the value of t2 / D2, which is the ratio of the outside diameter D2 (mm) and the wall thickness t2 (mm) of the tube after reducing rolling, is preferably 0.030 to 0.050. If the value of t2 / D2 is less than 0.030, shape defects such as dents may easily occur. On the other hand, if the value of t2 / D2 exceeds 0.050, when the outer diameter is constant, the wall thickness becomes thicker and the mass increases, which may make it difficult for the worker to carry the product or increase the workload. Therefore, the value of t2 / D2 is preferably set to 0.030 to 0.050. The value of t2 / D2 can be adjusted by the reducing rolling conditions (the dimensions of the reducing rolls and the number of rotations of the rolls) in the reducing rolling step.

[0033] By manufacturing steel pipes through the above process, it is possible to manufacture steel pipes that are less susceptible 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-mentioned composition and is obtained by the above-mentioned method for producing a steel pipe. Specifically, the steel pipe of the present invention is a steel pipe having a base material portion and a welded portion, and its chemical composition, in mass%, contains 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 balance being Fe and unavoidable impurities, and has 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) of 0.030-0.050, and a ratio of recess depth d (mm) to outer diameter D (mm) of 0.010 or less.

[0035] The steel pipe of the present invention has a base material portion and a welded portion, and has a yield strength of 200 to 300 MPa and a tensile strength of 300 to 450 MPa, which makes it possible to suppress deformation of the steel pipe of the present invention. Furthermore, t / D, which is the ratio of the outer diameter D (mm) to the wall thickness t (mm), is 0.030 to 0.050, and the recess depth (maximum recess depth) d (mm) satisfies d / D≦0.010. As a result, the present invention can provide a steel pipe that sufficiently suppresses the occurrence of deformation, has a good appearance, and is free from defects in practical use.

[0036] In the steel pipe of the present invention, the effect of suppressing deformation is large when the wall is thin, so that 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 examples. This is merely a preferred example, and the present invention is not limited to this example in any way. Welded steel pipes were produced according to the following procedure, and the properties of the resulting welded steel pipes were evaluated. First, welded steel pipes were manufactured using steel plates having the chemical compositions shown in Table 1. At that time, the pipes were formed into open pipes by roll forming using a forming forge welding machine, and both edges were joined. After the joining, reduction rolling was performed to obtain welded steel pipes (forge welded steel pipes). Tensile test pieces were cut out in the longitudinal direction from the base metal portion of the obtained welded steel pipe. The tensile test specimen was directly used to carry out a tensile test in accordance with JIS Z2241 (2022), and the yield strength (YS), tensile strength (TS), and elongation (El) were evaluated. In addition, the maximum dent depth (dent depth d) of the steel pipe was measured as an appearance evaluation of the steel pipe after production. The depth was measured using a depth gauge, and the largest value measured by contacting a gauge tip with the dent portion was taken as the maximum dent depth. The reducing 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 adding an excessive amount of elements that contribute to increasing strength, while suppressing the Mn content, and to manufacture a thin-walled welded steel pipe in which the occurrence of deformation is suppressed. Furthermore, in the steel pipe of the present invention, the elongation could be increased to 65% or more by suppressing the Mn content. In contrast, thin-walled welded steel pipes obtained under comparative example conditions that do not satisfy the conditions of the present invention did not show any increase in yield strength and tensile strength, or the material costs increased, making them unsuitable for an inexpensive manufacturing method. [Explanation of symbols]

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

Claims

1. a steel sheet having a 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, with the balance being Fe and unavoidable impurities, is heated and hot-formed into an open tube; Joining both edges of the open tube; Ar ３ A method of manufacturing steel pipes in which reducing rolling is performed at a reducing rolling finish temperature below the transformation point.

2. The reduction rolling start temperature is Ar ３ The method for producing a steel pipe according to claim 1, wherein the temperature is set to a transformation point or lower.

3. The outer diameter D of the tube before the reduction rolling １ (mm) and wall thickness t １ (mm) ratio t １ / D １ is 0.020 to 0.030, And the outer diameter D of the tube after the reduction rolling ２ (mm) and wall thickness t ２ (mm) ratio t ２ / D ２ The method for producing a steel pipe according to claim 1 or 2, wherein the σ is 0.030 to 0.

050.

4. A steel pipe having a base metal portion and a joint portion, the composition of which is, by 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 balance 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 outer diameter D (mm) to the wall thickness t (mm), t / D, is 0.030 to 0.050; A steel pipe having a ratio d / D of a recess depth d (mm) to an outer diameter D (mm) of 0.010 or less.

Images