JP6066216B2 - Structure excellent in low temperature toughness and method for producing the same - Google Patents

Description

  The present invention relates to a structure using a steel for a line pipe, which is an API (American Petroleum Institute) 5L pipe grade, and having a joint excellent in low temperature toughness by friction stir welding at the joint, and a method for manufacturing the same.

  Steels for line pipes are API (American Petroleum Institute Regulations) pipes composed of C, Si, Mn as main components and addition of V, Ca, Al, Mo or alloy components such as Ti, B, etc. The specification 5L grade is often used. The steel is manufactured by predetermined quenching and tempering (tempering) or controlled rolling (TMCP) using a converter. In recent years, the specification of low temperature toughness of users has been strictly demanded. This is because each part of the pipe is exposed to a low temperature (about −40 ° C.) during an emergency stop of the pipeline, and the risk of brittle fracture is pointed out. On the other hand, a fusion welding technique such as submerged arc welding (SAW) is used for assembling pipes and structures, and a heat-affected zone (hereinafter referred to as HAZ) exposed to high temperatures (1250 to 1400 ° C.) by these fusion weldings. It is said that the toughness decreases. Therefore, even if a base material excellent in low temperature toughness is manufactured, it is difficult to satisfy the low temperature toughness of HAZ.

  As these improved technologies, Patent Document 1 adds Ti, N, Nb, V, and B, and precipitates fine TiN in the steel, thereby reducing the austenite grain size of the HAZ and improving the toughness of the HAZ. Something has been proposed. However, since TiN almost dissolves in the region where the maximum temperature reached 1400 ° C., the crystal grain size becomes coarse near the boundary with the weld metal. As a result, the toughness cannot be improved near the boundary.

  Further, Patent Document 2 proposes that austenite coarsening when exposed to a high temperature for a long time is suppressed by controlling the number of inclusion particles and the like, thereby improving HAZ toughness. However, the control of these fine inclusions is difficult to control in a general steelmaking process, and in particular, melting in an electric furnace leads to an increase in cost.

  On the other hand, an improvement technique using a friction stir welding (hereinafter referred to as FSW) technique is also being studied. FSW is not accompanied by melting because it is a solid-phase bonding utilizing the plastic flow of metal caused by frictional heat between the rotating tool and the bonding member. Thereby, it is known that not only the heating temperature at the time of joining is low, but also a fine austenite grain size can be obtained by strong processing of plastic flow.

  Patent Document 3 proposes a technique for producing a high-strength and high-toughness structure using the FSW technique. In this proposal, it is shown that a high-strength and high-toughness joint can be obtained by joining under a sufficient condition that does not cause defects when joining. However, application of FSW is only one pass on the surface, and in the case of FSW having a relatively shallow junction depth, there are cases in which joining from both sides is required when joining thick materials. In this case, the material properties, particularly toughness, may be deteriorated in the stirring section that is affected by the heat of the next pass.

  In patent document 4, the technique of joining the surface by FSW after constructing the welding route part of a back surface by fusion welding is proposed. With this method, it is possible to join thick materials and the HAZ area is reduced. However, since fusion welding is used, a portion with reduced toughness is formed.

Japanese Patent Publication No.55-26164 JP 2001-226739 A Special table 2012-509178 gazette Special table 2012-513306 gazette

  As a conventional problem, in the conventional tempered steel for line pipes, the manufacture of structures that include joints that satisfy the user's requirement to ensure low temperature toughness at the boundary between the base metal and the welded part and in the heat affected zone is necessary. Furthermore, since the joint portion needs to have higher strength than the base material (overmatch requirement), it is necessary to form a joint portion having high strength and high toughness.

  Therefore, the basic object of the present invention is to provide a structure having a high strength and high toughness joint using the FSW technology. First, the base material has higher toughness than conventional fusion welding and is a base material. The purpose of the present invention is to clarify the FSW bonding conditions in which a higher strength joint can be produced, and secondly, to improve the boundary between the base material and the joint and the low temperature toughness of the HAZ.

  With respect to this invention, based on API-X65 steel, the tensile properties of the joints and the Charpy impact properties with notches in the HAZ were studied under many FSW joining conditions. In addition, for application to a thick material, bonding was performed from both sides of the material, and the characteristics of the portion exposed to the thermal effect of the next pass were evaluated. Furthermore, it compared with the fusion welding currently performed conventionally, and examined the superiority of the FSW joint. As a result, by optimizing the FSW bonding conditions, a bonding method capable of obtaining a high-strength and high-toughness bonded portion and HAZ without a bonding defect even with a thick material was found, and the present invention was achieved. .

That is, among the methods for producing a structure excellent in low-temperature toughness according to the present invention, the first present invention is an API (American Petroleum Institute) 5L pipe grade in mass%, C ≦ 0.26%, Mn ≦ 1 .65%, P ≦ 0.03%, S ≦ 0.03%, (V + Nb + Ti) 1 or more ≦ 0.15%, the joint portion of steel for a line pipe that have a composition the balance being Fe and unavoidable impurities Friction stir welding is used , and in the friction stir welding, the welding conditions are 20 × 10 ^{3 to} 120 × 10 ³ r · kg / mm, the heat input index represented by the following formula (1), and the rotational speed of the friction tool. 80 to 200 rpm, the rotational pitch of the friction tool (the distance traveled while the tool rotates once) is 0.3 to 0.8 mm / r, and the joint is stronger than the base material and in the heat affected zone Measured in sub-size V-notch Charpy impact test Brittle fracture appearance transition temperature (FATT) is at -70 ° C. or less, and the absorbed energy at -70 ° C. and forming the joint is at least 40 J.
q (heat input index) = indentation load (kg) × rotational speed (rpm) × friction tool shoulder diameter (mm) / (joining speed (mm / min) × plate thickness (mm)) (1)

The method for producing a structure having excellent low temperature toughness according to the second aspect of the present invention is characterized in that, in the first aspect of the present invention , the friction stir welding is performed by a joining path on both surfaces of the steel for line pipes. .

The structure excellent in low temperature toughness according to the third aspect of the present invention is API (American Petroleum Institute) 5L pipe grade , mass%, C ≦ 0.26%, Mn ≦ 1.65%, P ≦ 0.03%. , S ≦ 0.03%, (V + Nb + Ti) 1 or more ≦ 0.15%, the friction stir joining portion to the joint of that line steel pipe having a composition the balance being Fe and unavoidable impurities is formed, wherein The maximum EBSD grain size when the boundary angle is 15 ° or more at the joint is 12.0 μm or less, the joint is stronger than the base material, and the sub-size V notch Charpy impact test is performed in the heat affected zone. The ductile brittle fracture surface transition temperature (FATT) measured in this manner is −70 ° C. or lower, and the absorbed energy at −70 ° C. is 40 J or higher.

According to the present invention, the joint has higher strength than the base material, and higher toughness can be obtained.
Further, as API (American Petroleum Institute) 5L pipe grade, product numbers X42 to X60, X65, and X70 are shown (2012 version), and the following composition (mass%) can be exemplified.
C ≦ 0.26%, Mn ≦ 1.65%, P ≦ 0.03%, S ≦ 0.03%, (V + Nb + Ti) 1 type or more ≦ 0.15%, balance Fe and inevitable impurities

  The sub-size V-notch Charpy impact test can be performed, for example, according to JIS Z2242: 2005.

Hereinafter, the limited range of conditions in the present invention will be described in detail.
Heat input index: 20 × 10 ^{3 to} 120 × 10 ³ r · kg / mm
The heat input index is given by the following equation, and is a value resulting from heat input during bonding.
q (heat input index) = indentation load (kg) × rotational speed (rpm) × tool shoulder diameter (mm) / (joining speed (mm / min) × plate thickness (mm)) (1)
The heat input index is an index necessary for examining an appropriate joining condition. If it is less than 20 × 10 ³ r · kg / mm, a joining defect occurs, a tool breaks, and 120 × 10 ³ is obtained. If it exceeds the above range, generation of burrs and deterioration of the properties of the joint and HAZ due to excessive joining heat input occur, so the above range is desirable. For the same reason, it is more desirable that the lower limit is 35 × 10 ³ r · kg / mm and the upper limit is 80 × 10 ³ r · kg / mm.

Friction tool rotation speed: 80-200 rpm
The rotational speed of the friction tool is a factor that affects the degree of plastic flow during joining. That is, when the rotational speed is low, defects occur in the joint due to insufficient plastic flow, so the lower limit is desirably 80 rpm. When the rotational speed is high, the plastic flow becomes excessive, and burrs are generated and defects and toughness are reduced accordingly. Further, the increase in the rotational speed also affects the reduction in the joint and HAZ toughness. For this reason, it is desirable that the upper limit of the rotational speed of the friction tool be 200 rpm. For the same reason, it is more desirable to set the lower limit to 100 rpm and the upper limit to 150 rpm.

Rotational pitch of friction tool: 0.3 to 0.8 mm / r
The distance that the friction tool moves during one rotation is the rotation pitch. The rotation pitch is given by the following equation (2), and is a value applied in the case of moving joint as in the present invention.
Rotation pitch = joining speed / tool rotation speed (mm / r) (2)
The increase in the rotation pitch increases the load applied to the tool material in addition to the occurrence of defects in the joint due to insufficient plastic flow, and causes the tool to break. Therefore, the upper limit is preferably set to 0.8 mm / r. . When the rotational pitch is low, the plastic flow becomes excessive, and burrs are generated and the accompanying defects are generated. Therefore, it is desirable to set the lower limit to 0.3 mm / r. For the same reason, it is more desirable to set the lower limit to 0.4 mm / r and the upper limit to 0.6 mm / r.

EBSD particle size: 12.0 μm or less at maximum EBSD (electron beam backscatter diffraction method) is a method for measuring the orientation of each crystal. In general, in the case of steel, it has been reported that the crystal grain size (EBSD grain size) surrounded by a large angle boundary of 15 ° or more has a correlation with toughness. The finer the EBSD grain size, the better the low temperature toughness of the steel. According to the FSW bonding conditions of the present invention, a bonded portion having a maximum EBSD particle size of 12.0 μm or less is obtained, and good low temperature toughness is obtained. On the other hand, when the maximum EBSD particle size exceeds 12.0 μm, good low temperature toughness cannot be obtained.

Method of joining thick materials Normally, the joining depth of FSW is determined by the pin shape of the friction tool. The longer the pin, the thicker the material can be constructed. However, as the joining depth increases, the load required for joining increases and a large-scale device is required. Further, the load on the friction tool is increased, and the friction tool may be broken during the joining. In order to solve these problems, construction is performed on both sides of the groove so that thick materials can be joined even with a joining depth by a conventional rotary tool. Furthermore, by selecting an appropriate bonding condition, it is possible to bond without deteriorating characteristics even in a bonded portion that is affected by the heat of the next pass.

  As described above, according to the present invention, by friction stir welding, the strength is higher than that of the base material, and the low temperature toughness can be improved at the joint and HAZ as compared with the conventional fusion welding. That is, the ductile brittle fracture surface transition temperature (FATT) measured by the subsize V-notch Charpy impact test is −70 ° C. or lower, even if the joint is higher in strength than the base metal and is a heat affected zone, and A joint having excellent low-temperature toughness with an absorbed energy at −50 ° C. of 40 J or more can be obtained. For this reason, a thick material can be joined by carrying out double-sided construction.

It is the schematic which shows the process of the joining state in one Embodiment of this invention. Similarly, it is a figure which shows the friction tool used for embodiment. It is a figure which shows the extraction | collection point of the sample for EBSD measurement in the Example of this invention. Similarly, it is a figure which shows the sampling procedure of a subsize Charpy impact test piece. Similarly, it is a graph showing the correlation between the heat input index and the maximum EBSD particle size. Similarly, it is a figure which shows the correlation of a heat input index and the absorbed energy in test temperature -70 degreeC. Similarly, it is a figure which shows the sampling procedure of a joint tensile test piece. Similarly, it is a figure which shows the test piece cross-section observation result (rotation speed: 200 rpm) after a joint tensile test.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
The steel for line pipes used for this invention is prescribed | regulated by API (American Petroleum Institute) 5L pipe grade (2012), and can show the following components.
C ≦ 0.26%, Mn ≦ 1.65%, P ≦ 0.03%, S ≦ 0.03%, (V + Nb + Ti) 1 type or more ≦ 0.15%, balance Fe and inevitable impurities

The steel for line pipes can be provided through hot rolling, quenching, and tempering treatment. For example, after heating to about 1000 ° C., hot rolling can be performed, followed by quenching at 950 ° C. and tempering at 560 ° C.
In addition, the said manufacturing method of the steel for line pipes is an illustration, and the manufacturing method of the steel for line pipes used for this invention is not limited above.

As the friction tool, a composite material of PCBN (cubic boron nitride sintered body) and a W-Re alloy can be used. However, in the present invention, the shape and material of the tool material are not limited to specific ones, and any tool that can be used for steel materials may be used. Any tool material capable of joining steel materials is possible, including, for example, ceramics, metals, composite materials, and other derived materials.
FIG. 2 shows an example of the friction tool 10. The friction tool 10 includes a columnar main body 11 and a columnar shoulder 12 that is provided on the distal end side of the main body 11 and has a larger diameter than the main body 11. The pin 13 has a truncated cone shape with a small diameter on the tip side. For example, the diameter of the main body portion is 25 mm, the length of the main body portion is about 60 mm, the diameter of the shoulder 12 is about 37 mm, the outer peripheral inclination with respect to the axial direction of the pin 13 is 30 degrees, and the depth of the pin 13 is 4 mm.

The friction tool 10 is disposed in a state where the pin 13 is in contact with the joint portion 2 of the line pipe steel 1 and applies a predetermined load to the line pipe steel 1 side while maintaining a predetermined rotational speed. The friction tool 10 is relatively moved in the joining direction by a predetermined rotation pitch. The rotational speed of the friction tool is desirably 80 to 200 rpm, and the rotational pitch is desirably 0.3 to 0.8 mm / r. Note that the line pipe steel 1 side may be moved with respect to the friction tool 10.
Further, the friction stir welding is performed under the joining conditions in which the heat input index represented by the following formula (1) is 20 × 10 ^{3 to} 120 × 10 ³ r · kg / mm.
q (heat input index) = indentation load (kg) × rotational speed (rpm) × friction tool shoulder diameter (mm) / (joining speed (mm / min) × plate thickness (mm)) (1)
A joint 3 is formed at the joint 2 by friction stir welding based on the above operation.

The joint 3 has higher strength than the base pipe steel 1 as a base material, and has a ductile brittle fracture surface transition temperature (FATT) of −70 ° C. or less as measured in a subsize V-notch Charpy impact test. And the absorbed energy in -70 degreeC is 40J or more.
The subsize V-notch Charpy impact test can be performed under the conditions specified in JIS Z2242: 2005.
Specifically, a test piece having a test piece width of 5 mm, a height of 10 mm, and a length of 55 mm was prepared, and the notch shape had a notch angle of 45 °, a notch depth of 2 mm, and a notch bottom radius of 0.25 mm. For the test, a refrigerant such as alcohol and liquid nitrogen was used, and the test was carried out after being kept constant for at least 10 minutes in a liquid bath maintained at ± 2 ° C. with respect to a predetermined test temperature.

Further, in the joint portion 3, the maximum EBSD (Electron Back Scatter Diffraction) particle diameter when the boundary angle is 15 ° or more is 12.0 μm or less.
The EBSD particle size can be determined by crystal analysis using a scanning electron microscope (SEM).

In the study of friction stir welding, API-X65 grade steel for line pipes was used.
For steel for line pipes, in mass%, C: 0.05%, Mn: 1.43%, P: 0.009%, S: 0.004%, V + Nb + Ti: 0.07%, balance Fe and inevitable Impurity material was used. After melting, the steel was heated to about 1000 ° C. and then hot-rolled, then quenched at 950 ° C. and tempered at 560 ° C.
As for the structural material subjected to the friction stir welding shown in FIG. 1, a sample 4 is taken as a test material from the center of the joint 3 where the single-sided one-pass joining is performed as shown in FIG. 3, and EBSD (OIM manufactured by TSL) is obtained. ) Measurement was carried out.
In the EBSD measurement, the measurement pitch was set to 0.05 μm, and the particle size measurement was performed based on the measurement results in the 100 μm × 100 μm range two visual fields. Table 1 shows the welding conditions for friction stir welding and the EBSD particle size measurement results. At the rotational speed of 50 rpm, defects occurred in the joints, so that good joints could not be obtained under these conditions. If the rotational speed is 200 rpm or less, even the maximum EBSD particle size is 12.0 μm or less.

In order to evaluate the impact characteristics of the joint and HAZ, friction stir welding was performed under the conditions shown in Example 1, and subsize Charpy impact test pieces were collected in the manner shown in FIG. Moreover, in this example, in order to consider the thermal effect of the next pass, the joint portion 3 was produced by double-sided pass construction. The joining conditions in the double-sided pass were the same.
In the specimen collection, the notch position was sampled at the center position in the joint width direction of the joint-side test piece 20. Moreover, in the HAZ side test piece 22, the notch position was sampled at a position adjacent to the joint width direction.
Further, for comparison, a comparative example in which the fusion welding is SAW using a welding material used for joining API-X65 steel is further facilitated.
The subsize Charpy impact test was performed under the conditions specified in JIS Z2242: 2005 in the same manner as in Example 1.
Table 2 shows the results of the subsize Charpy impact test.

As a result of the test, in the inventive example, FATT was shifted to a lower temperature side than melt welding (SAW) in both the joint and HAZ, and excellent low temperature toughness was exhibited. In the inventive example, the absorbed energy (vE ₋₇₀ ) at −70 ° C. was 40 J or more. Further, the joint exhibited substantially the same impact characteristics as the base material at a rotational speed of 100 rpm. On the other hand, in Comparative Example 2, good low temperature toughness was not obtained.
The correlation between the heat input index, the EBSD grain size, and the toughness is shown in FIGS. As the heat input index increased, the EBSD particle size increased. When the heat input index was 120 × 10 ³ r · kg / mm or more, it became 12.0 μm or more, and good low temperature toughness of the joint was not obtained.

  In order to evaluate the strength of the joint portion, friction stir welding was performed under the conditions shown in Example 1, and a joint tensile test piece 5 including the joint portion 3 and the line pipe steel 1 as shown in FIG. It collected as a test material. The joint tensile test piece 5 was a micro test piece (GL 5 mm, thickness 0.34 mm), and a joint tensile test was performed under conditions where the test temperature was room temperature and the tensile speed was 0.5 mm / min. The test results are shown in Table 3.

  Further, FIG. 8 shows a structure observation result at the fracture position after the joint tensile test. The fracture position was the base material. From this, it was shown that the joint part obtained in the present invention example had higher strength than the base material, and a joint part having high strength and good low-temperature toughness was obtained by the present invention.

  INDUSTRIAL APPLICABILITY The present invention can be applied as friction stir welding (hereinafter referred to as FSW) of steel materials used for natural gas pipelines and the like, and has excellent low temperatures in joints and heat-affected zones particularly in environments where low temperature toughness is required It can be used as a joining method specialized in toughness. However, the scope of application of the present invention is not limited to these.

  Although the present invention has been described based on the above embodiment, the present invention is not limited to the above description unless departing from the scope of the present invention, and appropriate modifications can be made.

1 Steel for Line Pipe 2 Joint 3 Joint 10 Friction Tool 12 Shoulder 13 Pin

Claims (3)

API (American Petroleum Institute) 5L pipe grade , mass%, C ≦ 0.26%, Mn ≦ 1.65%, P ≦ 0.03%, S ≦ 0.03%, (V + Nb + Ti) 1 or more types ≦ 0.15%, using the friction stir welding to the joint portion for the line pipe steel that have a composition the balance being Fe and unavoidable impurities, in the friction stir welding, the bonding condition, represented by the following formula (1) The heat input index is 20 × 10 ^{3 to} 120 × 10 ³ r · kg / mm, the rotation speed of the friction tool is 80 to 200 rpm, and the rotation pitch of the friction tool (the distance moved during one rotation of the tool) is 0. The ductile brittle fracture surface transition temperature (FATT) measured by the subsize V-notch Charpy impact test in the heat-affected zone is −70 ° C. at 3 to 0.8 mm / r. Absorption energy at −70 ° C. below Method for producing a good structure low temperature toughness characterized in that over to form a joint at least 40 J.
q (heat input index) = indentation load (kg) × rotational speed (rpm) × friction tool shoulder diameter (mm) / (joining speed (mm / min) × plate thickness (mm)) (1) The method for producing a structure excellent in low-temperature toughness according to claim 1, wherein the friction stir welding is performed by a joining pass on both surfaces of the steel for a line pipe. API (American Petroleum Institute) 5L pipe grade , mass%, C ≦ 0.26%, Mn ≦ 1.65%, P ≦ 0.03%, S ≦ 0.03%, (V + Nb + Ti) 1 or more types ≦ 0.15%, the friction stir joining portion to the joint of that line steel pipe having a composition the balance being Fe and unavoidable impurities is formed at the junction, the maximum EBSD when used as a boundary angle 15 ° or more The particle size is 12.0 μm or less, the joint is stronger than the base metal, and the ductile brittle fracture surface transition temperature (FATT) measured by the subsize V-notch Charpy impact test in the heat-affected zone is − A structure excellent in low temperature toughness, characterized in that the absorbed energy at 70 ° C. or lower and −70 ° C. is 40 J or higher.

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