Method for manufacturing high-strength steel pipes and high-strength steel pipes

Abstract

To provide a manufacturing method of a high strength steel pipe performing cylindrical bending molding of martensitic steel plate to bond butted end parts thereof through laser beam welding which hardly causes fracture of a HAZ part in a fluid pressure bulge processing.SOLUTION: A manufacturing method of a high strength steel pipe comprises: a first step where butted end parts of the high strength steel plate that is bended and molded in a cylindrical shape are bonded by laser beam welding; and a second step where a HAZ part in the vicinity of a weld bead part by laser beam welding of the first step is melted by laser heating with low heat input relative to the laser beam welding of the first step along the welding direction. The HAZ part softened by welding of the first step is melted, hardening is performed by rapidly cooling the HAZ part after being resolidified to generate a martensite phase to have high intensity. Heat input amount of the laser heating is lower than that of the laser beam welding at the first step so that reduction amount of rigidity of newly generated HAZ part is decreased.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention relates to a manufacturing technology (pipe manufacturing technology) of high-strength steel pipes using a steel plate having a matrix phase of martensite structure as a raw material. The high-strength steel pipes targeted by the present invention also include circular pipe test pieces for hydraulic bulge tests in circular pipes for investigating the material deformation behavior of steel plates having a matrix phase of martensite structure.

Background Art

[0002] In recent years, from the viewpoints of improving the collision safety and fuel efficiency of automobiles, high-strength steel plates have been widely used for steel plates for automobile parts, and accordingly, the application of martensitic steel plates having a matrix phase of martensite structure is expanding. By the way, for the purpose of obtaining a material model for forming simulation, the following tests are conducted. That is, a circular tubular test piece (circular pipe test piece) is created from a plate material, an internal pressure and a stress in the pipe axis direction are applied to this circular pipe test piece, the strain of the test piece is measured, and it is a test for measuring the biaxial stress of the plate material. A material testing machine therefor is disclosed in, for example, Patent Document 1, and a test method (biaxial bulge test method) is disclosed in, for example, Non-Patent Documents 1 and 2. Here, when subjecting a martensitic steel pipe to such a test, it is necessary to bend a martensitic steel plate (plate material) into a cylindrical shape and weld its plate edge (butt joint end) to manufacture a circular pipe test piece. In addition, the use of high-strength steel pipes manufactured using martensitic high-strength steel plates as raw materials is also expanding.

[0003] High-strength martensitic steel sheets are steel materials whose strength is adjusted to a predetermined level by controlling the martensitic phase through composition and quenching and tempering treatments during the manufacturing process. Therefore, when welded, the heat effect of welding can cause localized changes in strength that deviate from the original performance. Areas with reduced strength become more prone to breakage compared to the general area. Pipes, in particular, require welding during the pipe manufacturing process, and are often subjected to secondary processing such as bending, crushing, bulging, expansion, and reduction during use, so deformation concentrates in areas with reduced strength, making them prone to fracture. Furthermore, even without secondary processing, in applications where internal pressure from liquids, etc., is applied, there is a concern that rupture may originate from areas with reduced strength.

[0004] The mechanism by which high-strength martensitic steel achieves the required strength involves a combination of factors, including the martensitic phase, the dispersion of precipitates, and the refinement of crystal grains. In any case, changes in the state due to thermal effects can alter the strength (softening or hardening), so caution is necessary when using it in applications involving welding or exposure to heat. During welding, the weld bead, which is heated to a high temperature and temporarily melted, is re-hardened by rapid cooling after solidification, creating a martensite phase and becoming stronger than the base material. On the other hand, the HAZ (Heat Affected Zone, also called the heat-affected zone or HAZ softening zone) near the weld bead, where melting did not occur, is softened as the martensite phase of the base material is tempered by the heat. When a steel pipe is manufactured by bending a martensitic steel plate (sheet material) into a cylindrical shape and welding its edges (butt joint ends), the Vickers hardness distribution in the widthwise cross-section of the weld (cross-section perpendicular to the welding direction) is an indicator of strength, and can be seen, for example, in Figure 9.

[0005] When a martensitic high-strength steel pipe with a welded joint having the hardness distribution shown in Figure 9 undergoes hydraulic bulging by sealing both ends and applying internal pressure with liquid, cracking occurs in the softened HAZ (High-Area Zone). This cracking in the HAZ is a fracture mode of "Type A" HAZ cracking, schematically shown in Figure 4, which will be described later. In this case, the weakened portion of the HAZ in the welded joint preferentially cracks, and the pressure resistance performance expected from the base material strength is not achieved. The fundamental solution to avoid such problems is to prevent the softening of the HAZ (heat-affected zone), but eliminating the thermal effects of welding is difficult.

[0006] Conventionally, techniques (welding methods) for suppressing fracture in the HAZ (heat-affected zone) near the weld bead have been proposed for high-strength members, for example in Patent Documents 2 to 4. Patent Document 2 describes an overlap welding method that includes a martensitic structure and can suppress fracture in the HAZ (heat-affected zone) softened area of ​​steel plate members joined by spot welding. This welding method involves irradiating a laser beam to form a molten and solidified area that extends 3 mm or more outward from the edge of the spot weld nugget, crossing the edge of the nugget. This hardens the HAZ softened area of ​​the spot weld, thereby suppressing fracture in the HAZ.

[0007] Patent Document 3 describes an overlap welding method in which multiple steel plates having a martensitic structure are overlapped and a laser is irradiated to form a substantially circular laser weld. In this welding method, a laser is irradiated in a straight line through the outer edge of the substantially circular laser weld to form a hardened area that crosses the HAZ softened area around the substantially circular laser weld, thereby suppressing fracture in the HAZ softened area. Patent Document 4 describes a welding method for joining multiple high-tensile steel plates by laser screw welding. This welding method includes a tempering step after the nugget formation step, in which the laser irradiation spot diameter is larger and the power is lower than that of the laser used in the nugget formation step, thereby reducing the hardness of the irradiation point and its surroundings. As a result, the hardness of the parts of the heat-affected zone around the nugget that maintain high hardness is reduced, the area of ​​reduced hardness is expanded, and the condition of locally reduced hardness is eliminated. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Patent No. 5838165 [Patent Document 2] Japanese Patent Publication No. 2015-171731 [Patent Document 3] International Publication No. 2015 / 119159 [Patent Document 4] Japanese Patent Publication No. 2017-87263 [Non-patent literature]

[0009] [Non-Patent Document 1] Toshihiko Kuwahara, et al., "Measurement and Analysis of Plastic Deformation Characteristics of 5000 Series Aluminum Alloy Tubes Subjected to Axial Force and Internal Pressure," Plasticity and Processing, Japan Society for Technology of Plasticity, 2003, Vol. 44, No. 506, pp. 281-286. [Non-Patent Document 2] Fuminori Sugawara, et al., "Development of a biaxial bulge testing machine enabling continuous measurement of large-strain biaxial stress-strain curves of metal sheet materials," Plasticity and Processing, Japan Society for Technology of Plasticity, 2013, Vol. 54, No. 624, pp. 57-63. [Overview of the project] [Problems that the invention aims to solve]

[0010] However, Patent Documents 2 to 4 concern spot welds of structural members (overlap welded members) that make up an automobile body. Patent Documents 2 and 3 both describe a method of irradiating a portion of the HAZ softened area of ​​an automobile part with a laser to harden the HAZ softened area and suppress fracture in the HAZ softened area. In Patent Documents 2 and 3, it is sufficient to partially harden the HAZ softened area that is prone to fracture, corresponding to the direction of stress generated in the automobile part during a collision with the vehicle body. However, in the case of high-strength steel pipes used in hydraulic bulge processing, a HAZ softened area is formed along the entire length of the welded portion of the edge of the cylindrical bent steel plate (plate material), and stress perpendicular to the weld is generated along the entire length of the weld, so the methods of Patent Documents 2 and 3 cannot be directly applied. Furthermore, in Patent Document 4, since the area of ​​the HAZ softened is expanded by irradiating it with a laser, in the case of high-strength steel pipes used in hydraulic bulge processing, it actually becomes easier for them to break when internal pressure is applied.

[0011] Therefore, the object of the present invention is to provide a method for manufacturing high-strength steel pipes in which martensitic steel sheets are bent into a cylindrical shape and the butt ends are joined by laser welding, and in which fracture at the HAZ (High-Area Zone) is less likely to occur during secondary processing such as bulging. Another object of the present invention is to provide a high-strength steel pipe obtained by welding the butt ends of a tubularly bent martensitic steel plate together, which is less prone to fracture in the HAZ (High-Area Zone) during secondary processing such as bulging. [Means for solving the problem]

[0012] The inventors of this invention conducted extensive research to solve the above problems and found that the problems can be solved by laser heating the HAZ (heat-as-a-zone) area near the weld bead under specific conditions along the welding direction to form a molten area (molten bead area). Furthermore, they found that particularly high effectiveness can be obtained by optimizing the conditions for forming the molten area by laser heating. This invention is based on such findings and its gist is as follows. [1] In a method for manufacturing a high-strength steel pipe, in which a high-strength steel plate having a martensitic matrix structure is bent into a cylindrical shape and its butt ends are joined by laser welding, The first step is to join the butt ends by laser welding, A method for manufacturing a high-strength steel pipe, characterized by having a second step in which, at least the HAZ portion near the weld bead portion at both sides of the weld bead portion formed by laser welding in the first step, laser heating and melting the portion along the welding direction with a lower heat input than that of the laser welding in the first step.

[0013] [2] In the manufacturing method of [1] above, in the first step, the ratio Wu / Wo of the bead width Wu on the inner surface side of the pipe to the bead width Wo on the outer surface side of the pipe in the weld bead portion is set to 0.10 to 0.80, which is a manufacturing method of a high-strength steel pipe. [3] In the manufacturing method of [1] or [2] above, in the second step, the portion of the HAZ portion is laser-heated from the outer surface side of the pipe, and the melted portion due to the laser heating is prevented from reaching the inner surface side of the pipe, which is a manufacturing method of a high-strength steel pipe. [4] In the manufacturing method of [3] above, in the second step, the depth of the melted portion due to laser heating is set to 20 to 80% of the pipe thickness, which is a manufacturing method of a high-strength steel pipe. [5] In any of the manufacturing methods of [1] to [4] above, in the second step, the portion including the position of the HAZ portion where the hardness is minimized in the Vickers hardness distribution in the width direction of the welded portion by the laser welding in the first step is laser-heated and melted, which is a manufacturing method of a high-strength steel pipe. [6] In any of the manufacturing methods of [1] to [5] above, the high-strength steel pipe is a circular pipe test piece for a biaxial bulge test, which is a manufacturing method of a high-strength steel pipe.

[0014] [7] In a welded steel pipe having a cylindrical bent body of a high-strength steel plate with a martensite structure as the matrix phase and a welded portion joining the butted ends of the bent body, At both sides of the weld bead portion in the welded portion, at least in the portion of the HAZ portion near the weld bead portion, a molten bead portion is formed along the longitudinal direction of the welded portion, which is a high-strength steel pipe. [8] In the high-strength steel pipe of [7] above, the ratio Wu / Wo of the bead width Wu on the outer surface side of the pipe to the bead width Wo on the inner surface side of the pipe in the weld bead portion is 0.10 to 0.80, which is a high-strength steel pipe.

[0015] [9] In the high-strength steel pipe of [7] or [8] above, the molten bead portion is formed from the outer surface side of the pipe toward the inner surface side in the pipe thickness direction and is formed so as not to reach the inner surface side of the pipe, which is a high-strength steel pipe.

[10] In the high-strength steel pipe described in [9] above, the depth of the molten bead portion is 20 to 80% of the pipe wall thickness, and the high-strength steel pipe is characterized by this.

[11] In any of the high-strength steel pipes described in [7] to

[10] above, the molten bead portion is formed so as to include the position of the HAZ portion where the hardness is minimum in the Vickers hardness distribution in the width direction of the welded portion, and the high-strength steel pipe is characterized by this.

[12] In any of the high-strength steel pipes described in [7] to

[11] above, the high-strength steel pipe is characterized by being a circular pipe test piece for a biaxial bulge test. [Effect of the Invention]

[0016] According to the method of the present invention, in a method for manufacturing a high-strength steel pipe in which a martensitic steel plate is bent and formed into a cylindrical shape and the butted ends thereof are joined by laser welding, it is possible to manufacture a high-strength steel pipe in which breakage at the HAZ portion is unlikely to occur during secondary processing such as hydraulic bulge processing. In particular, in this manufacturing method, by optimizing the formation conditions of the molten portion by laser heating in the second step, it is possible to manufacture a high-strength steel pipe in which breakage at the HAZ portion is appropriately suppressed during secondary processing such as bulge processing. Further, the high-strength steel pipe of the present invention has an advantage that breakage at the HAZ portion is unlikely to occur during secondary processing such as bulge processing, and in particular, by optimizing the formation conditions of the molten bead portion, breakage at the HAZ portion is appropriately suppressed during secondary processing such as bulge processing. [Brief Description of the Drawings]

[0017] [Figure 1] An explanatory drawing schematically showing the situation (upper figure) in which the butted ends of a steel plate bent and formed into a cylindrical shape are joined by laser welding during the manufacture of a martensitic high-strength steel pipe, and the macrostructure of the cross section in the width direction of the welded portion by the laser welding (lower figure). [Figure 2] Regarding the case where the laser welding of the butted ends during pipe manufacture as shown in FIG. 1 is carried out with different heat inputs by changing the welding speed while keeping the welding conditions other than the welding speed (laser output, etc.) the same, drawings schematically showing the Vickers hardness distribution in the width direction of the welded portion (including the peripheral portion thereof) and the macrostructure of the cross section in the width direction of the welded portion. [Figure 3] Figure 2 summarizes the results and shows a graph illustrating the relationship between laser welding speed, bead width, HAZ width, and hardness reduction in the HAZ. [Figure 4] An explanatory diagram showing the status of hydraulic bulging on steel pipes and the type of cracking that occurs in steel pipes during hydraulic bulging. [Figure 5] An explanatory diagram (upper figure) schematically showing the macrostructure of the widthwise cross-section of the weld formed by the first step laser welding and the area to be laser heated in the second step in one embodiment of the present invention, and a diagram (lower figure) schematically showing the Vickers hardness distribution in the widthwise direction of the weld (including its peripheral area). [Figure 6] An explanatory diagram (upper figure) schematically shows the macrostructure of the widthwise cross-section of the welded portion formed by the first step laser welding and the molten bead portion (molten portion) formed in the HAZ portion thereof by the second step laser heating, in one embodiment of the present invention, and a diagram (lower figure) schematically shows the Vickers hardness distribution in the widthwise direction of the welded portion and the molten bead portion (including its peripheral portion). [Figure 7] Figure 7(a) shows a specific example of the Vickers hardness distribution in the width direction of the welded portion (including its peripheral portion) after the first step of the present invention, and Figure 7(b) shows a specific example of the Vickers hardness distribution in the width direction of the welded portion and molten bead portion (including its peripheral portion) after the second step of the present invention. [Figure 8]Figure 8(a) shows the Vickers hardness distribution in the width direction of the weld (including its surrounding area) when the second step of the present invention is not performed (i.e., the HAZ area is not melted by laser heating), and is a schematic explanatory diagram showing the macrostructure of the cross-section in the width direction of the weld. Figure 8(b) shows the Vickers hardness distribution in the width direction of the weld and molten bead (including its surrounding area) when the melting depth D of the molten area is 53% of the pipe thickness in the second step of the present invention, and is a schematic explanatory diagram showing the macrostructure of the cross-section in the width direction of the weld and molten bead. Figure 8(c) shows the Vickers hardness distribution in the width direction of the weld and molten bead (including its surrounding area) when the melting depth D of the molten area is 76% of the pipe thickness in the second step of the present invention, and is a schematic explanatory diagram showing the macrostructure of the cross-section in the width direction of the weld and molten bead. [Figure 9] This drawing shows an example of the Vickers hardness distribution in the width direction of the welded area (including its surrounding area) in a high-strength steel pipe manufactured by bending a martensitic steel sheet (plate material) into a cylindrical shape and welding its edges (butt joint ends). [Modes for carrying out the invention]

[0018] The present invention relates to a method for manufacturing (pipe making) high-strength steel pipes, which involves bending (processing) a high-strength steel sheet with a martensitic matrix into a cylindrical shape and joining its butt ends by laser welding (welding from the outer surface of the pipe). As mentioned above, the strength of martensitic high-strength steel is adjusted by controlling the martensitic phase through quenching and tempering treatments during the manufacturing process. As a result, the heat-induced changes in the state during welding during pipe making cause the heat-induced zone (HAZ) to soften, and this area becomes a preferred site for deformation concentration and cracking. Therefore, the present invention was developed by investigating a manufacturing (pipe making) method to obtain high-strength steel pipes that do not suffer from the above-mentioned problems.

[0019] In this invention, high-strength steel plate (high-strength steel pipe) refers to a steel plate with a tensile strength of 590 MPa or higher. The main strengthening mechanism of this high-strength steel plate utilizes a martensitic structure, and the strength is adjusted to a predetermined level by properly performing quenching and tempering treatments during the manufacturing process. The high-strength steel pipes manufactured according to this invention include those in their original pipe shape, as well as those that have undergone secondary processing such as bending, crushing, bulging, expanding, and reducing in diameter. In addition to general steel pipes used for fluid transport or as structural components, the invention also includes circular pipe test pieces for hydraulic bulge testing of circular pipes.

[0020] First, we will explain the results of our investigation into appropriate welding conditions and welds when manufacturing high-strength martensitic steel pipes using laser welding. Figure 1 is a schematic diagram illustrating the process of joining the butt ends of steel plates bent into a cylindrical shape by laser welding during the manufacturing of martensitic high-strength steel pipes (upper figure), and the macrostructure of the widthwise cross-section of the welded joint (lower figure). Methods for bending steel sheets into a cylindrical shape typically include press forming and roll forming.

[0021] Figure 2 schematically shows the Vickers hardness distribution in the width direction of the weld (including its surrounding area) and the macrostructure of the cross-section of the weld in the width direction of the weld, when laser welding of the butt joint end during pipe manufacturing, as shown in Figure 1, is performed with different heat inputs by changing the welding speed while keeping all welding conditions other than the welding speed (laser power, etc.) the same. In the laser welding shown in Figure 2, all welding conditions other than the welding speed (laser power, etc.) the same, and the heat input of the laser welding was decreased in the order of (A) → (B) → (C) by increasing the welding speed in the order of (A) → (B) → (C). Here, Vickers hardness is measured according to JIS Z2244-1:2020 "Vickers hardness test - Part 1: Test method". The Vickers hardness distribution in the width direction of the weld (including its surrounding area) is determined by measuring the center of the plate thickness in a cross section perpendicular to the weld bead under conditions of a test force of 100 gf and an indentation spacing of 0.1 mm.

[0022] As shown in Figure 2, the weld bead is harder and stronger than the base material, and a softer, less hard HAZ (HAZ softening) area exists around the weld bead. The width of the weld bead (the region with higher hardness than the base material) and the width of the HAZ (the region with lower hardness than the base material), as well as the amount of hardness reduction, change depending on the heat input of the laser welding. In other words, the faster the laser welding speed, that is, the lower the heat input of the laser welding, the smaller the bead width and HAZ width become, and the amount of hardness reduction in the HAZ (the reduction in the minimum hardness of the HAZ relative to the base material hardness) is also reduced. Figure 3 summarizes the results of Figure 2 and shows the relationship between the laser welding speed and the bead width, HAZ width, and the amount of hardness reduction in the HAZ.

[0023] For high-strength steel pipes manufactured under the conditions shown in (a), (b), and (c) of Figure 2, hydraulic bulging was performed by applying hydraulic pressure to the inside of each pipe until it fractured, and the locations where cracks (fractures) occurred were observed. Figure 4 shows the implementation status of this hydraulic bulging and the morphology of cracks (fractures) in the steel pipes during hydraulic bulging. As shown in Figure 4, the following types of cracks can occur in steel pipes during hydraulic bulging. • HAZ cracking (Type A) where the fracture occurs in the HAZ (Hazard Axle) area (HAZ softening area) • Base metal cracking (Type B): No cracks occur in the weld bead or HAZ area, but the base metal fractures. • Butt joint cracking (Type C) where the weld bead fractures before the HAZ (Heat-Absorbing Zone).

[0024] In Figure 2, high-strength steel pipes manufactured under conditions (a), (b), and (c) did not exhibit base metal cracking (Type B). However, in conditions (a) and (b), where the welding speed was relatively slow and the heat input from laser welding was large, HAZ cracking (Type A) occurred. On the other hand, in condition (c), where the welding speed was faster than in (a) and (b) and the heat input from laser welding was small, butt joint cracking (Type C) occurred. In the example where butt joint cracking (Type C) occurred, it is believed that the heat input from the laser welding was insufficient, resulting in the failure to form a proper weld bead and thus a poor joint. In contrast, in the example where HAZ cracking (Type A) occurred, the heat input from the laser welding was sufficient, and a proper weld bead was formed, but the HAZ area is thought to have been the starting point for the crack.

[0025] Therefore, we investigated the appropriate laser welding conditions during pipe manufacturing (butt welding) that can suppress butt joint cracking (Type C) during hydraulic bulge processing. In laser welding of butt joints during pipe manufacturing (welding from the outer surface side of the pipe) as shown in Figure 1, we investigated the appropriate conditions for the ratio Wu / Wo, which is the bead width Wo on the outer surface side (front side) of the pipe irradiated with the laser, to the bead width Wu on the inner surface side (back side) of the pipe. Hereinafter, the bead width Wo on the outer surface of the pipe will be referred to as "front bead width Wo," and the bead width Wu on the inner surface of the pipe will be referred to as "back bead width Wu." In laser welding of butt joints, the heat input was adjusted, and the Wu / Wo ratio was varied within the range of 0.00 to 0.99 during pipe fabrication. The resulting steel pipes had a thickness of 0.8 mm, a diameter of φ55 mm, and a tensile strength of 780 MPa. Generally, the heat input in laser welding can be adjusted by adjusting one or more of the following: laser power, welding speed, shielding gas flow rate, reflectivity, focusing lens, laser spot diameter, beam mode, etc. In this pipe fabrication, the heat input of the laser welding was also adjusted by adjusting the laser power, welding speed, etc.

[0026] The obtained steel pipes were subjected to hydraulic bulging as shown in Figure 4, and the locations where fractures occurred were observed. The results (crack morphology during hydraulic bulging) are shown in Table 1, along with Wu / Wo, etc. Table 1 also shows the expansion ratio measured at the point where the diameter of the cracked sample (circular pipe) was largest. Here, the expansion ratio is defined by the following formula. (Pipe expansion rate)=(D1-D0) / D0×100(%) However, D1: Maximum outer diameter of the circular pipe after hydraulic bulge processing where cracking occurred. D0: Outer diameter of the circular pipe before hydraulic bulging. This expansion ratio represents the circumferential elongation of the sample (circular pipe) and is one of the indicators of processability in hydraulic bulging. In hydraulic bulging, if deformation is concentrated in a localized area in the circumferential direction of the circular pipe and fracture occurs, the expansion ratio will be small. On the other hand, in hydraulic bulging, if deformation is dispersed over a wide area in the circumferential direction of the circular pipe and expands, the expansion ratio will be large. For this reason, in butt joint cracks (Type C), where the weld bead fractures before the HAZ (Heat-Area Zone), and in HAZ cracks (Type A), where fracture occurs in the HAZ, the expansion ratio will be small. Conversely, in base material cracks (Type B), where no cracks occur in the weld bead or HAZ, and the base material deforms and fractures over a wide area in the circumferential direction of the circular pipe, the expansion ratio will be large.

[0027] In Table 1, the heat input from laser welding increases in the order from No. 1 to No. 10, and as the heat input increases, the front bead width Wo and the back bead width Wu also increase. Furthermore, the larger the heat input, the larger the Wu / Wo ratio becomes. Under conditions where the heat input of laser welding was large and the back bead width was large (Wu / Wo > 0.80) (No. 8 to No. 10), perforation due to burn-through of the molten portion occurred frequently, making hydraulic bulging impossible. Also, under conditions where the heat input of laser welding was small and the back bead width was small (Wu / Wo < 0.10) (No. 1, No. 2), the joint across the entire cross-section in the thickness direction was insufficient, resulting in butt joint cracking (Type C) during hydraulic bulging, and the expansion ratio was 0.0 to 1.2%.

[0028] On the other hand, under the condition 0.10 ≤ Wu / Wo ≤ 0.80 (No.3 to No.7), no holes were created due to melt-through in the molten portion, and the cracking pattern during hydraulic bulging was HAZ cracking (Type A) rather than butt joint cracking (Type C). However, the expansion rate increased compared to the case of butt joint cracking (Type C), but remained at 2.2-4.7%. Therefore, in order to suppress butt joint cracking (Type C) during hydraulic bulge processing, it is preferable to adjust the heat input by laser welding so that the ratio of the front bead width Wo to the back bead width Wu / Wo is in the range of 0.10 to 0.80.

[0029] [Table 1]

[0030] Based on the above results, the inventors investigated a method for manufacturing high-strength steel pipes that is less prone to HAZ cracking (Type A) during hydraulic bulge processing, preferably one that suppresses HAZ cracking, and devised the following novel manufacturing method. In other words, this manufacturing method is characterized by having a first step of joining the butt ends of high-strength steel plates bent into a cylindrical shape by laser welding, and a second step of heating and melting the HAZ portion near the weld bead of the first step's laser welding with a lower heat input than the laser welding in the first step, along the welding direction. This makes it possible to obtain a high-strength steel pipe that is less prone to cracking in the HAZ portion during hydraulic bulge processing.

[0031] The manufacturing method will be described below with reference to Figures 5 and 6. Figure 5 is an explanatory diagram (upper figure) schematically showing the macrostructure of the widthwise cross-section of the weld formed by the laser welding in the first step and the area to be laser heated in the second step in one embodiment of the present invention, and a diagram (lower figure) schematically showing the Vickers hardness distribution in the widthwise direction of the weld (including its peripheral area). Point P in Figure 5 indicates the position (widthwise position) of the HAZ area where the hardness is minimum in the Vickers hardness distribution in the widthwise direction of the weld. Figure 6 is an explanatory diagram (upper figure) schematically showing the macrostructure of the widthwise cross-section of the weld formed by the laser welding in the first step and the molten bead (molten area) formed in the HAZ area by laser heating in the second step in one embodiment of the present invention, and a diagram (lower figure) schematically showing the Vickers hardness distribution in the widthwise direction of the weld and the molten bead (including its peripheral area). In Figure 6, the Vickers hardness distribution in the width direction of the welded area (including its surrounding area) in Figure 5 is shown by a dashed line.

[0032] In the first step of the present invention, the butt ends of high-strength steel plates bent into a cylindrical shape are joined by laser welding, as shown in Figure 5, to form a high-strength steel pipe. Preferably, the heat input is adjusted by adjusting the laser welding conditions, such as laser output and welding speed, so that the weld bead portion 1 reaches from the outer surface side to the inner surface side of the pipe and Wu / Wo = 0.10 to 0.80. In the second step, as shown in Figures 5 and 6, the steel pipe is laser-heated and melted at least the portion of the HAZ (i.e., the portion of the HAZ near the weld bead 1) on both sides of the weld bead 1, along the welding direction, with a lower heat input than in the first step, to form a molten portion (molten bead 2). This laser heating to form the molten portion (molten bead 2) is performed from the outer surface of the pipe. Typically, in the second step, as shown in Figure 5, laser heating is performed first on one side of the HAZ (first time), and then on the opposite side (second time). The method for adjusting the heat input in laser heating is the same as the method for adjusting the heat input in laser welding described above, and is generally adjustable by adjusting one or more of the following: laser power, heating speed (movement speed of the laser irradiation unit; the same applies hereinafter), shielding gas flow rate, reflectivity, focusing lens, laser spot diameter, beam mode, etc. Therefore, the amount of heat input can be adjusted by, for example, adjusting the laser output, heating rate, etc.

[0033] If the amount of heat input from laser heating in the second step is even slightly lower than the amount of heat input from laser welding in the first step, then by laser heating the HAZ area to form a molten area, cracking in the HAZ area becomes less likely, and a certain effect can be obtained. More preferable conditions, as will be described later, are that in the laser welding of the first step, an amount of heat input is required for the weld bead 1 to reach from the outer surface to the inner surface of the pipe, whereas in the laser heating of the second step, it is preferable that the amount of heat input is such that the molten area (molten bead 2) does not reach from the outer surface to the inner surface of the pipe. The preferred conditions for the melting depth D of the molten area formed by laser heating in the second step will also be described later.

[0034] In the first step of laser welding, the HAZ (Heat-Absorbing Zone) areas that form on both sides of the weld bead 1 are softened due to the tempering of the martensite phase of the base material by the heat effect of welding, as shown in Figure 5. In the second step, as shown in Figure 6, these softened HAZ areas are melted by laser heating to form a molten area (molten bead 2), and then rapidly cooled after resolidification to re-quench the area and generate the martensite phase, making it harder (higher strength) than the base material. At this time, the heat input for laser heating is lower than the heat input for laser welding in the first step. As a result, the range of the newly generated HAZ area (HAZ width) around the molten area (molten bead 2) due to laser heating in the second step is narrower than the HAZ width due to welding in the first step. Consequently, the amount of hardness reduction in the newly generated HAZ area is also reduced, thereby suppressing the occurrence of HAZ cracking during hydraulic bulge processing.

[0035] Figure 7(a) shows a specific example of the Vickers hardness distribution in the width direction of the welded area (including its surrounding area) after the first step of the present invention, and Figure 7(b) shows a specific example of the Vickers hardness distribution in the width direction of the welded area and molten bead area (including its surrounding area) after the second step of the present invention. Furthermore, from the above-mentioned viewpoint, in the second step, when the HAZ portion generated by the laser welding in the first step is laser-heated and melted, it is preferable to laser-heat and melt the portion including the position of the HAZ portion where the hardness is minimum in the Vickers hardness distribution in the width direction of the weld (point P in Figure 5) to form the molten portion (weld bead portion 2).

[0036] Furthermore, the inventors investigated preferred conditions for the depth of the molten portion (molten bead portion 2) formed by laser heating in the second step. For high-strength steel pipes laser-welded under the same laser welding conditions as in Figure 2(a), the HAZ (heat-as-avoided zone) was laser-heated to form a molten area (molten bead 2), as shown in Figure 6. The amount of heat input due to laser heating was adjusted by controlling the laser output and heating rate. This allowed the depth of the molten area (molten bead 2) (hereinafter referred to as "molten depth D") to be varied from 0% (without laser heating) to 100% of the pipe thickness of the high-strength steel pipe. Hydraulic bulging, as shown in Figure 4, was then performed on these high-strength steel pipes, and the locations where fractures occurred were observed to determine the crack morphology (Types A to C in Figure 4) during hydraulic bulging.

[0037] As a result, when the melting depth D was set to 20-80% of the pipe thickness, HAZ cracking (Type A) was suppressed, and in all cases, base material cracking (Type B) occurred. In contrast, when the second step of the present invention was not performed (the HAZ was not melted by laser heating) (melting depth D = 0%), HAZ cracking (Type A) occurred. Furthermore, as shown in Example 2 described later, HAZ cracking (Type A) occurred in both cases where the melting depth D was less than 20% of the pipe thickness and when it was more than 80%. This is because when the melting depth D is less than 20% of the pipe thickness, the hardening and strengthening by laser heating is limited to the vicinity of the surface, while when the melting depth D exceeds 80% of the pipe thickness, the amount of hardness reduction (degree of softening) in the new HAZ area that occurs around the molten area due to laser heating becomes large, and in both cases, the softened HAZ area becomes the starting point for cracking.

[0038] Figure 8(a) is a diagram showing the Vickers hardness distribution in the width direction of the weld (including its surrounding area) when the second step of the present invention is not performed (i.e., the HAZ area is not melted by laser heating), and a schematic explanatory diagram showing the macrostructure of the cross-section in the width direction of the weld. Figure 8(b) is a diagram showing the Vickers hardness distribution in the width direction of the weld and molten bead (including its surrounding area) when the melting depth D of the molten area is 53% of the pipe thickness in the second step of the present invention, and a schematic explanatory diagram showing the macrostructure of the cross-section in the width direction of the weld and molten bead. Furthermore, Figure 8(c) is a diagram showing the Vickers hardness distribution in the width direction of the weld and molten bead (including its surrounding area) when the melting depth D of the molten area is 76% of the pipe thickness in the second step of the present invention, and a schematic explanatory diagram showing the macrostructure of the cross-section in the width direction of the weld and molten bead.

[0039] In Figures 8(i) and 8(iii), the softened HAZ area in Figure 8(a) is hardened (increased in strength) by quenching due to laser heating (formation of the molten area) in the second step. Furthermore, by lowering the heat input for laser heating compared to the laser welding in the first step, the decrease in hardness in the newly formed HAZ area is also reduced, thereby suppressing the occurrence of cracks in the HAZ area during hydraulic bulge processing. From the results described above, it is preferable that the molten portion (molten bead portion 2) formed by laser heating in the second step does not reach the inner surface of the pipe, and further, it is preferable that the molten depth D be 20 to 80% of the pipe thickness. However, as shown in the examples described later, even if the molten depth D is outside the range of 20 to 80% of the pipe thickness, laser heating of the HAZ portion to form a molten portion makes it less likely for cracks to occur in the HAZ portion, and a certain effect (effect of the invention) can be obtained.

[0040] The high-strength steel pipe manufactured by the manufacturing method of the present invention described above is a welded steel pipe having a cylindrical bent form (formed material) of a high-strength steel plate having a martensitic matrix phase and a welded joint that joins the butt ends of the bent form, wherein a molten bead 2 (a bead generated by the base material being laser-heated and melted, and then re-solidified) is formed along the longitudinal direction of the weld at least in the portion of the HAZ (i.e., the portion of the HAZ near the weld bead 1) on both sides (outer surface side) of the weld bead 1 in the weld joint. For the reasons mentioned above, this high-strength steel pipe preferably has the following configuration. (i) The ratio Wu / Wo of the bead width Wo on the outer surface of the pipe to the bead width Wu on the inner surface of the pipe in the weld bead section 1 is 0.10 to 0.80. (ii) The molten bead portion 2 is formed in the direction of the pipe thickness from the outer surface side to the inner surface side of the pipe, and not reaching the inner surface side of the pipe. (iii) The depth of the molten bead 2 (molten depth D) is 20-80% of the pipe thickness. (iv) The molten bead portion 2 is formed to include the position of the HAZ portion where the hardness is minimized in the Vickers hardness distribution in the width direction of the weld. [Examples]

[0041] In the following description, a pipe formed by joining the butt ends of high-strength steel plates bent into a cylindrical shape using laser welding (first step) is referred to as a "step 1 welded pipe." Furthermore, a "step 2 processed pipe" is formed by creating a molten area (molten bead area) on both sides of the weld bead portion of this "step 1 welded pipe" by laser heating the HAZ portion near the weld bead portion from the outer surface of the pipe along the welding direction with a lower heat input than the laser welding in the first step (second step). This "step 2 processed pipe" is the high-strength steel pipe (product steel pipe) manufactured according to the present invention. In this embodiment (and in other experiments conducted to complete the present invention), a CO2 laser welding machine with a wavelength of 10.6 μm was used for the first step of laser welding and the second step of laser heating. The output of this welding machine can be arbitrarily set in the range of 0.5 to 15 kW, and the welding speed can be arbitrarily set in the range of 500 to 20000 mm / min using NC control. Nitrogen was used as a shielding gas to prevent oxidation of the welded area.

[0042] [Example 1] In the first step, Step 1 welded pipes No. 1 to 11 were fabricated by varying the heat input of the laser welding. For the weld bead sections of these Step 1 welded pipes, the front bead width Wo and back bead width Wu were measured, and the Wu / Wo ratio was calculated. The results are shown in Table 2. Furthermore, the HAZ (heat-induced zone) portion near the weld bead of these Step 1 welded pipes was laser-heated along the welding direction with a lower heat input than the laser welding process to form a molten area (molten bead), thereby obtaining the Step 2 processed pipe, which is the finished steel pipe. In this process, the melting depth D of the HAZ portion was set to 0.5 mm (50% of the pipe thickness). This finished steel pipe has a pipe thickness of 1.0 mm, a pipe diameter of φ80 mm, and a tensile strength of 980 MPa class.

[0043] The heat input for laser welding in the first step and the heat input for laser heating in the second step were adjusted according to the welding speed for laser welding and the heating speed for laser heating, respectively, as shown in Table 2. Furthermore, hydraulic bulging was performed on each of these Step 1 welded pipes and Step 2 processed pipes by sealing both ends and applying liquid pressure to the inside. The crack morphology in hydraulic bulge processing was classified into three types based on the positional relationship between the fracture site and the weld bead: HAZ cracking (Type A), base metal cracking (Type B), and butt joint cracking (Type C), as schematically shown in Figure 4. The expansion ratio was also determined using the same method as in Table 1. These results are shown together in Table 2.

[0044] According to these results, when Wu / Wo ≤ 0.09, the crack morphology in hydraulic bulge processing was determined to be butt joint cracking (Type C) for both Step 1 welded pipes and Step 2 processed pipes. Furthermore, the expansion ratio for both Step 1 welded pipes and Step 2 processed pipes was 1% or less. When 0.10 ≤ Wu / Wo ≤ 0.80, Step 1 welded pipes were judged to have HAZ cracks (Type A), and Step 2 treated pipes were judged to have base metal cracks (Type B). Furthermore, while the expansion ratio of Step 1 welded pipes with HAZ cracks was 1.6-5.0% (No. 3-8), the expansion ratio of Step 2 treated pipes with base metal cracks was 13.9-15.5% (No. 3-8), which was 2.8-9.1 times that of Step 1 welded pipes (No. 3-8, average 5.8 times). When Wu / Wo was 0.93 or less, the hydraulic bulge test could not be performed due to holes in the weld bead.

[0045] When the heat input of the laser is small, there is less melting during laser welding, resulting in a narrow bead width. Since the melting during laser welding does not easily reach the inner surface (back side) of the pipe, Wu / Wo is small, and sometimes Wu / Wo = 0. When Wu / Wo ≤ 0.09, the heat input is insufficient and the joint is inadequate, which is likely why both the Step 1 welded pipe and the Step 2 processed pipe developed butt joint cracks (Type C). When the heat input of laser welding is increased to 0.10 ≤ Wu / Wo ≤ 0.80, the inner surface (back side) of the pipe melts, the butt joint is sufficiently joined, and laser welding is performed properly. However, because the HAZ (heat-induced zone) softens, step 1 welded pipes develop HAZ cracks (type A), and although the expansion rate by hydraulic bulging is better than that of butt joint cracks (type C), it remains below 5%. In contrast, in step 2 processed pipes where the HAZ is laser heated and melted, the hardness of the HAZ recovers, and it turns into base metal cracking (type B). As a result, the expansion rate by hydraulic bulging can be increased to approximately 15% (average 14.5%).

[0046] In other words, when manufacturing a high-strength steel pipe by bending a high-strength steel plate with a martensitic matrix into a cylindrical shape and joining its butt ends by laser welding, in the first step, the butt ends are joined by laser welding to obtain a proper weld (Wu / Wo = 0.10 to 0.80), and in the second step, the HAZ area near the weld bead is laser heated along the welding direction with a lower heat input than the laser welding in the first step to melt it and form a molten area (molten bead), thereby obtaining a high-strength steel pipe that is less prone to cracking in the HAZ area during hydraulic bulge processing.

[0047] [Table 2]

[0048] [Example 2] In this second example, Step 1 welded pipes No. 4, No. 6, and No. 8, whose crack morphology during hydraulic bulge processing in Step 2 processed pipes in Example 1 was determined to be base material cracking (Type B), were used. For these Step 1 welded pipes, the HAZ area near the weld bead was heated by laser heating with a reduced heat input compared to the welding process, and a molten area (molten bead) was formed to create Step 2 processed pipes, No. 4-1 to 5, No. 6-1 to 5, and No. 8-1 to 5, which are the finished steel pipes. In these Step 2 processed pipes, the depth of the molten area (molten bead area), i.e., the melting depth D (mm), was varied by adjusting the heat input by laser heating. These finished steel pipes have a pipe thickness of 1.0 mm, a pipe diameter of φ80 mm, and a tensile strength of 980 MPa class. For these Step 2 processed tubes, hydraulic bulging was performed in the same manner as in Example 1, and the crack morphology was determined in the same manner as in Example 1, along with the expansion ratio. These results, along with Wu / Wo and melting depth D, are shown in Table 3. The heat input for laser welding in the first step and the heat input for laser heating in the second step were adjusted according to the welding speed for laser welding and the heating speed for laser heating, respectively, as shown in Table 3.

[0049] According to these results, the crack morphology during hydraulic bulge processing of Step 2 processed pipes was determined to be HAZ cracking (Type A) for Step 2 processed pipes (No. 4-1, No. 8-1) where the melting depth D was less than 20% of the pipe thickness t. Similarly, for Step 2 processed pipes (No. 4-2~4-5, No. 6-1~6-4, No. 8-2~8-4) where the melting depth was between 20% and 80%, it was determined to be base material cracking (Type B). Similarly, for Step 2 processed pipes (No. 6-5, No. 8-5) where the melting depth was greater than 80%, it was determined to be HAZ cracking (Type A). If the heat input during laser heating to form the molten area is small, the molten depth D becomes small, and if the molten depth D is less than 20% of the pipe thickness (No. 4-1, No. 8-1), the hardness of the softened area in the HAZ (heat-affected zone) does not recover sufficiently due to insufficient heat input from the laser heating. For this reason, the crack morphology in the hydraulic bulge processing is considered to be HAZ cracking (Type A). However, in all of the Step 2 processed pipes that exhibited HAZ cracking (Type A), the expansion ratio was improved compared to the Step 1 welded pipes with HAZ cracking (No. 3-8, 1.6-5.0% in Example 1) (No. 4-1: 8.7%, No. 8-1: 7.0%) because the molten area (molten bead) was formed with laser heating using a reduced heat input compared to the welding in Step 1.

[0050] When the heat input for laser heating to form the molten zone is sufficiently large, and the molten depth D is 20-80% of the pipe thickness (No. 4-2-4-5, No. 6-1-6-4, No. 8-2-8-4), the hardness of the softened area in the HAZ zone is sufficiently restored. As a result, the base material deforms without the HAZ zone cracking, and the expansion rate until fracture during hydraulic bulging is significantly improved (14.1-15.8%) compared to step 1 welded pipes with HAZ cracking (No. 3-8 in Example 1, 1.6-5.0%). Consequently, it is considered that the crack morphology during hydraulic bulging shifted to base material cracking (type B) in step 2 processed pipes.

[0051] When the heat input during laser heating is even greater and the melting depth D exceeds 80% of the pipe thickness (No. 6-5, No. 8-5), the degree of softening of the newly formed HAZ (heat-induced zone) by laser heating increases. For this reason, the crack morphology in the hydraulic bulge processing of the Step 2 processed pipes is considered to be HAZ cracking (Type A). However, in all of the Step 2 processed pipes that exhibited HAZ cracking (Type A), the expansion ratio was improved compared to the Step 1 welded pipes with HAZ cracking (No. 3-8, 1.6-5.0% in Example 1) (No. 6-5: 7.2%, No. 8-5: 6.8%) because the molten area (molten bead) was formed by laser heating with a reduced heat input compared to the welding in the first step. In other words, in the manufacturing method of the present invention, by setting the melting depth D in the second step of laser heating to 20-80% of the pipe thickness, a high-strength steel pipe can be obtained in which cracking in the HAZ (Heat-Absorbing Zone) is appropriately suppressed during hydraulic bulge processing, and the effects of the present invention are particularly enhanced. Furthermore, for Step 2 processed pipes where the melting depth D is less than 20% or more than 80% of the pipe thickness, the expansion rate until fracture due to hydraulic bulging was increased compared to Step 1 welded pipes. This indicates that although the base metal did not crack, the formation of a molten zone by laser heating the HAZ (Heat-Absorbing Zone) made the HAZ less prone to cracking.

[0052] [Table 3] [Explanation of Symbols]

[0053] 1. Weld bead 2. Molten bead section

Claims

1. In a method for manufacturing high-strength steel pipes, in which a high-strength steel sheet having a martensitic matrix structure is bent into a cylindrical shape and its butt ends are joined by laser welding, The first step is to join the butt ends by laser welding, The process includes a second step in which, at least the HAZ portion near the weld bead at both sides of the weld bead formed by the first step of laser welding, the HAZ portion is laser heated and melted along the welding direction with a lower heat input than that of the first step of laser welding, The method for manufacturing a high-strength steel pipe, characterized in that, in the first step, the ratio Wu / Wo of the bead width Wo on the outer surface of the pipe and the bead width Wu on the inner surface of the pipe in the weld bead portion is set to 0.10 to 0.

80.

2. The method for manufacturing a high-strength steel pipe according to claim 1, characterized in that, in the second step, the HAZ portion is laser-heated from the outer surface side of the pipe, and the molten portion due to the laser heating does not reach the inner surface side of the pipe.

3. The method for manufacturing a high-strength steel pipe according to claim 2, characterized in that the depth of the molten portion by laser heating is 20 to 80% of the pipe thickness in the second step.

4. The method for manufacturing a high-strength steel pipe according to claim 1, characterized in that the second step involves laser heating and melting a portion of the Vickers hardness distribution in the width direction of the welded portion by laser welding in the first step, including the position of the HAZ portion where the hardness is minimized.

5. A method for manufacturing a high-strength steel pipe according to any one of claims 1 to 4, characterized in that the high-strength steel pipe is a circular pipe test specimen for biaxial bulge testing.

6. A welded steel pipe having a cylindrical bent form made of a high-strength steel plate having a martensitic matrix, and a welded joint that joins the butt ends of the bent form, At both sides of the weld bead in the aforementioned welded portion, a molten bead is formed along the longitudinal direction of the weld, at least in the HAZ portion near the weld bead. A high-strength steel pipe characterized in that the ratio Wu / Wo of the bead width Wo on the outer surface of the pipe to the bead width Wu on the inner surface of the pipe in the weld bead portion is 0.10 to 0.

80.

7. The high-strength steel pipe according to claim 6, characterized in that the molten bead portion is formed in the pipe thickness direction from the outer surface side to the inner surface side of the pipe, and does not reach the inner surface side of the pipe.

8. The high-strength steel pipe according to claim 7, characterized in that the depth of the molten bead portion is 20 to 80% of the pipe thickness.

9. The high-strength steel pipe according to claim 6, characterized in that the molten bead portion is formed to include the position of the HAZ portion where the hardness is minimized in the Vickers hardness distribution in the width direction of the weld.

10. A high-strength steel pipe according to any one of 6 to 9, characterized in that it is a circular pipe test specimen for biaxial bulge testing.

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