JP6855962B2 - Welded steel pipe - Google Patents

Description

The present invention relates to a welded steel pipe, for example, when manufacturing a welded steel pipe such as a UOE steel pipe, the low temperature toughness of the weld heat affected portion is excellent due to low heat input, and high temperature cracking of the pipe end due to low heat input can be prevented. For example, the present invention relates to a welded steel pipe suitable for use in a pipeline laid in a cold region or a deep sea having permanent frozen soil.

UOE steel pipes are often used as steel pipes used in oil and natural gas pipelines. In recent years, UOE steel pipes are increasingly used in deep-sea wells and cold regions. Therefore, in order to improve the fracture safety of UOE steel pipes, high strength and thickening (thickness over 25 mm) are promoted, and the weld heat affected zone Heat Affected Zone (hereinafter referred to as "HAZ") is used. Excellent low temperature toughness is required.

However, in the production of thick-walled UOE steel pipes, the amount of heat input in welding inevitably increases. Therefore, the metal structure of HAZ becomes coarse, and the low temperature toughness of HAZ tends to decrease. So far, many inventions for improving the low temperature toughness of HAZ of UOE steel pipe have been proposed.

For example, Patent Document 1 discloses a high-strength UOE steel pipe having excellent seismic performance and low-temperature toughness of HAZ, which defines the chemical composition of the base material and the weld crack susceptibility index Pcm, and Patent Document 2 discloses the base material. And the chemical composition of the weld metal is specified, and after heating at 580 to 750 ° C for 10 minutes or more, stress relief annealing (SR treatment) is performed to cool at a cooling rate of 1 ° C / sec or less, and high strength with excellent SR characteristics. And a method for producing a high toughness UOE steel pipe is disclosed. The inventions disclosed in Patent Documents 1 and 2 basically improve the low temperature toughness of HAZ by defining the chemical composition of the base material.

On the other hand, Patent Documents 3 and 4 disclose an invention for improving the low temperature toughness of HAZ by devising a welding method of welding by using gas shielded arc welding and submerged arc welding in combination. In the method disclosed in Patent Documents 3 and 4, the amount of heat is applied deeply in the plate thickness direction of the steel plate by gas shielded arc welding, and then submerged arc welding is performed to refine the metal structure of HAZ. Improves low temperature toughness of HAZ. A method of reducing the heat input per pass by multi-layer welding of the outer surface welding is also known, but this method increases the manufacturing cost because the welding efficiency deteriorates more than twice as much as usual, and the mass production welding process. It is difficult to apply as.

Further, Patent Document 5 discloses a method for improving the low temperature toughness of HAZ by limiting the fusion line of the submerged arc welded portion to a specific shape.

Japanese Unexamined Patent Publication No. 2009-235460 Japanese Unexamined Patent Publication No. 8-269566 Japanese Patent No. 5521632 Japanese Patent No. 5515850 Japanese Unexamined Patent Publication No. 2016-150364

Welding when manufacturing UOE steel pipes is usually performed with one pass on the inner and outer surfaces from the viewpoint of production efficiency. At this time, when a thick base metal is welded (thick-walled welding) to produce a pipe, it is necessary to increase the penetration depth (increase the height of the weld bead). As the penetration depth increases due to the thickening, the amount of heat input to the weld increases, and as a result, the bead shape tends to increase. Therefore, in thick-wall welding, the metal structure of HAZ is coarsened due to the increase in heat input, and the low temperature toughness of HAZ is lowered.

In order to secure the penetration depth (weld bead height) while suppressing the amount of heat input low, it is conceivable to form the weld bead into a vertically long shape to narrow the width of the weld bead. However, when a weld bead is formed by this method, component segregation occurs in the weld metal due to the vertically elongated shape of the weld bead, and a weld crack called a high temperature crack (also referred to as “hot crack” in the present specification) occurs. .. In particular, high-temperature cracking is likely to occur on the inner surface welding side, and becomes a serious welding defect that can cause breakage of the line pipe. In addition, when high-temperature cracking occurs, the mechanical performance of the welded steel pipe naturally deteriorates significantly. Therefore, so far, the improvement of the low temperature toughness of HAZ by this method has not been actually performed.

The present invention has been made in view of this problem of the prior art, and secures the penetration depth while suppressing the amount of heat input low by forming the weld bead into a vertically long shape and narrowing the bead width. Based on the method of improving the low temperature toughness of HAZ, it is an object of the present invention to provide a welded steel pipe having excellent low temperature toughness of HAZ, which can suppress component segregation in the weld metal and prevent the occurrence of high temperature cracks.

If the amount of heat input is suppressed and the weld bead is vertically elongated in the thickness direction of the base metal plate, the amount of heat input can be suppressed to prevent a decrease in absorbed energy given to HAZ, that is, a decrease in toughness, but the weld bead becomes vertically elongated. As a result, component segregation occurs inside the weld metal, and weld cracks called high-temperature cracks (hot cracks) are likely to occur particularly in the inner surface welded portion.

As shown in FIG. 1, which is an explanatory view showing an outline of a weld bead of a welded steel pipe according to the present invention, the present inventor determines the shape of the weld bead in the vicinity of a position where a high temperature crack (hot crack) occurs in the weld bead. By changing to a specific shape, specifically, by increasing the width near the position where high temperature cracks (hot cracks) occur in the weld bead, and changing from a vertically long bead to a barrel-shaped bead, the steel pipe We came up with the idea of increasing the amount of welding metal from the inner and outer surfaces of the steel, which is in contact with each other (metal touch part).

Then, by examining various inner surface welding conditions and groove conditions, changing the welding conditions of the first to third electrodes of submerged arc welding performed with several electrodes per inner surface pass, and further changing the groove conditions. The width of the weld bead near the position where the high temperature crack (hot crack) occurs in the thickness direction of the base metal plate can be expanded, whereby a welded steel pipe having excellent low temperature toughness of HAZ and no high temperature crack can be obtained. Knowing that, the present invention was completed.

The present invention is a welded steel pipe characterized in that conditions 1 to 3 described below are satisfied.

(Condition 1) A welded steel pipe having a groove portion at the end of the upper and lower surfaces of a steel plate having a plate thickness of more than 28 mm, and a welded joint portion in which a root face is brought close to each other and submerged arc welded from the inner and outer surfaces.

(Condition 2) When the thickness of the steel sheet is t (mm), the circle-equivalent particle size of the former austenite in the heat-affected zone near the welding line in the 1 / 4t portion of the steel sheet is 94 μm or less.

(Condition 3) When the height (length in the base plate thickness direction) of the weld bead formed by welding is T (mm), the bottom of the weld bead for inner surface welding is 1 in the base plate thickness direction. The width w (mm) of the weld bead at a position separated by / 8T (1 / 8T portion) satisfies the following equation (1), and is separated from the lowermost portion by 1 / 8T to 3 / 8T in the thickness direction of the base metal plate. The width x (mm) of the weld bead at the position (1 / 8T portion to 3 / 8T portion) satisfies the equations (2) and (3).

x ≧ 8.6 mm ・ ・ ・ ・ ・ (1)
x ≧ (1 / 0.2234) ln (y / 0.3525) ・ ・ ・ ・ ・ (2)
1 / 8T ≤ y ≤ 3 / 8T ... (3)
However,
x: Width of weld bead (mm)
y: Position (mm) away from the bottom in the base metal plate thickness direction
T: Height of weld bead not including pre-filling (mm)
Is.

According to the present invention, HAZ has excellent low temperature toughness and can prevent the occurrence of high temperature cracks that can cause serious welding defects. Therefore, it is used for line pipes in cold regions such as deep sea wells and permafrost. Suitable welded steel pipes such as UOE steel pipes can be provided.

FIG. 1 is an explanatory view showing an outline of a weld bead of a welded steel pipe according to the present invention. FIG. 2 is an explanatory view showing the position of the weld bead of the welded steel pipe according to the present invention in the thickness direction of the base metal plate. FIG. 3 is a graph showing the relationship between the bead width at the weld bead 1 / 8T portion of the inner surface welding and the occurrence rate of hot cracks at the pipe end. FIG. 4 is a graph showing the relationship between the bead width at the weld bead 2 / 8T portion of the inner surface welding and the occurrence rate of hot cracks at the pipe end. FIG. 5 is a graph showing the relationship between the bead width at the weld bead 3 / 8T portion of the inner surface welding and the occurrence rate of hot cracks at the pipe end. FIG. 6 is a graph showing the relationship between the bead width at the weld bead 4 / 8T portion of the inner surface welding and the occurrence rate of hot cracks at the pipe end. FIG. 7 is a graph showing the relationship between the bead width at the weld bead 5 / 8T portion of the inner surface welding and the occurrence rate of hot cracks at the pipe end. FIG. 8 is a graph showing the relationship between the bead width at the welding bead 6 / 8T portion of the inner surface welding and the occurrence rate of hot cracks at the pipe end. FIG. 9 is a graph showing the relationship between the bead width at the weld bead 7 / 8T portion of the inner surface welding and the occurrence rate of hot cracks at the pipe end. FIG. 10 is a graph showing the relationship between the bead width at the weld bead 8 / 8T portion of the inner surface welding and the occurrence rate of hot cracks at the pipe end.

The present invention will be described with reference to the accompanying drawings. In the following description, the case where the welded steel pipe is a UOE steel pipe will be taken as an example, but the application target of the present invention is limited to the UOE steel pipe in light of the action and effect of the present invention of avoiding defects in the welded joint. This does not apply equally to welded steel pipes obtained by submerged arc welding of steel sheets.

The UOE steel pipe according to the present invention satisfies the above-mentioned conditions 1 to 3.

(1) Condition 1
The UOE steel pipe according to the present invention has a welded joint. The welded joint is formed by submerged arc welding from the inner and outer surfaces with a groove provided at the end of the upper and lower surfaces of a steel plate having a thickness of more than 28 mm and the root faces approaching each other.

The groove portion is formed by cutting into a triangle when viewed from the side surface of the steel plate as usual.

When submerged arc welding is performed from the inner and outer surfaces of a steel plate in a tubular shape, the outer surface welding is usually performed after the inner surface welding. At this time, the weld bead (outer surface bead) is formed by the outer surface welding so as to cover the weld bead (inner surface bead) formed by the inner surface welding, and becomes a welded joint.

The reliability of the UOE steel pipe is improved by securing a certain amount of the parts of the inner bead that are overlapped by the outer surface welding (the overlapping part of the weld bead formed by welding from the inner and outer surfaces). Not only that, it is possible to partially eliminate the component segregation caused by the inner surface welding. It is preferable to secure the thickness of the overlapping portion of the inner surface bead and the outer surface bead by 2 to 4 mm in the plate thickness direction.

Here, the reason for using a steel plate having a plate thickness of more than 28 mm as a material will be described. That is, as the plate thickness increases, the heat input tends to increase as the penetration depth increases due to the thickening, and the shape of the weld bead also tends to increase. Therefore, in thick-wall welding, the crystal grains of HAZ, which is the base material, become coarse due to the increase in heat input, and the low temperature toughness of HAZ decreases. In particular, the low temperature toughness of HAZ is significantly reduced in a steel sheet having a thickness of more than 28 mm. Therefore, the thickness of the steel plate is set to more than 28 mm. The upper limit of the plate thickness is not particularly limited, but the plate thickness of the steel plate is 40 mm or less in consideration of the actually manufacturable range.

(2) Condition 2
When the thickness of the steel sheet is t (mm), the particle size equivalent to the circle of the old austenite in the heat-affected zone near the welding line in the 1 / 4t portion of the steel sheet is 94 μm or less.

Generally, the thicker the base metal is, the more heat is input to perform submerged arc welding. At this time, the larger the amount of heat input, the higher the temperature of HAZ, and the larger the particle size of the transformed austenite. After welding, the HAZ is cooled and undergoes a phase transformation, but the crystal grains of the HAZ structure largely depend on the grain size of the produced austenite (former austenite grain size).

In the present invention, welding is performed with a lower heat input amount than usual on the premise that the decrease in low temperature toughness of HAZ is prevented by suppressing the heat input amount. Although it depends on the chemical composition and thickness of the steel plate, the inner surface welding: 5.6 kJ / mm or less heat input (usually about 6.5 to 6.9 kJ / mm), the outer surface welding: 5.4 kJ / mm or less. According to the HAZ survey results when welding with the amount of heat input (usually about 6.3 to 6.7 kJ / mm), the 1 / 4t part (1 / from the plate thickness surface) that is considered to be most affected by the heat input. Since the circle-equivalent particle size of the former austenite (former austenite grains in contact with the welding line) in the HAZ near the welding line at a distance of 4 tons is 94 μm or less, the low temperature toughness of the HAZ does not decrease. Therefore, the circle-equivalent particle size of the former austenite in the HAZ near the weld line at the 1/4 t portion of the steel sheet is 94 μm or less.

If the circle-equivalent particle size of either the left or right austenite with the inner / outer surface of the UOE steel pipe and the weld bead (inner surface bead and outer surface bead) is greater than 94 μm, the low temperature toughness of HAZ is lowered at that portion. Therefore, the circle-equivalent particle size of the former austenite in HAZ near the welding line is 94 μm or less on the front and back surfaces (inner and outer surfaces of the steel pipe) and on the left and right sides of the welding bead. Further, the smaller the circle-equivalent particle size of the former austenite, the higher the low-temperature toughness of HAZ. Therefore, the lower limit of the circle-equivalent particle size of the former austenite is not particularly specified, but is usually 40 μm or more.

(3) Condition 3
When the height of the weld bead formed by welding (length in the base plate thickness direction) is T (mm), it is 1/8 T away from the bottom of the weld bead for inner surface welding in the base plate thickness direction. The width w (mm) of the weld bead at the position satisfies the following equation (1), and the width of the weld bead at the position y (mm) separated from the lowermost portion in the base plate thickness direction by 1/8 T to 3/8 T. x (mm) satisfies Eqs. (2) and (3).

w ≧ 8.6 mm ・ ・ ・ ・ ・ (1)
x ≧ (1 / 0.2234) ln (y / 0.3525) ・ ・ ・ ・ ・ (2)
1 / 8T ≤ y ≤ 3 / 8T ... (3)
However, x is the width of the weld bead (mm), y is the position away from the bottom in the base metal plate thickness direction (mm), and T is the height of the weld bead not including the prefill (mm).

Hot cracks can occur in both inner and outer surface welds, but more likely to occur in inner surface welds. Therefore, in order to suppress component segregation on the inner bead and prevent high temperature cracking, attention is paid to the bead width of the inner bead. If at least the profile of the weld line of the outer surface weld metal becomes the same as the profile of the weld line of the inner surface weld metal, it is considered that high temperature cracking can be prevented for the entire weld line welded from the inner and outer surfaces.

FIG. 2 is an explanatory view showing a penetration position in the base metal plate thickness direction of the weld bead of the UOE steel pipe according to the present invention.

Welding conditions (plate thickness, groove shape, heat input, welding speed) were changed in various ways, and the profile of the welding line (width of the weld bead) of the inner bead was investigated. More specifically, for the collected sample, the width of the weld bead and the pipe end at eight positions (1 / 8T part to 8 / 8T part) separated from the bottom of the inner bead in the base metal plate thickness direction. The correlation with the incidence of hot cracks was investigated.

FIGS. 3 to 10 are graphs showing the results of investigation of the relationship between the width of the weld bead at the 1 / 8T to 8 / 8T portion of the inner bead and the occurrence rate of hot cracks at the pipe end, respectively.

As shown in the graphs of FIGS. 3 to 5, as the width of the weld bead at the 1 / 8T to 3 / 8T portion increases, the occurrence rate of hot cracks at the pipe end tends to decrease. On the other hand, as shown in the graphs of FIGS. 6 to 10, the increase in the width of the weld bead at the 4 / 8T to 8 / 8T portion (the surface layer side of the weld bead) and the occurrence rate of hot cracks at the pipe end. No relevance is found in.

From the results shown in the graphs of FIGS. 3 to 10, there is a correlation between the width of the weld bead in the 1 / 8T portion to the 3 / 8T portion and the hot crack, and the thicker the width of the weld bead, the more hot crack occurs. It tends to be suppressed. When the conditions of the welding bead width w ≧ 8.6 mm at the 1 / 8T part and the welding bead width ≧ 11.2 mm at the 2 / 8T part and the welding bead width ≧ 12.6 mm at the 3 / 8T part are satisfied. No high temperature cracking occurs. On the other hand, as shown in the graphs of FIGS. 6 to 10, no correlation is observed between the width of the weld bead and the hot crack in the 4 / 8T portion to the 8 / 8T portion.

High temperature by satisfying the condition of 1 / 8T portion welding bead width w ≧ 8.6mm, 2 / 8T portion welding bead width ≧ 11.2mm, and 3 / 8T portion welding bead width ≧ 12.6mm. The reason why cracks do not occur is that by increasing the width of the weld bead at the bottom surface of the weld metal where component segregation is likely to occur, the segregation in the weld metal is dispersed and alleviated, and the segregation of the inner bead is alleviated. It is presumed that this suppresses the occurrence of high temperature cracking. Further, by securing the width of the inner bead itself widely from the vicinity of the bottom surface portion of the weld metal, the fastening force of the weld metal itself can be increased.

Therefore, attention is paid to the width of the weld bead at a position (1 / 8T to 3 / 8T portion) separated from the lowermost portion of the welding bead for inner surface welding in the base metal plate thickness direction by 1/8 T to 3/8 T.

The height T of the weld bead, that is, the wall thickness of the weld bead depends on the thickness of the steel plate used for the UOE steel pipe, the welding conditions, and the like. For this reason, UOE steel pipes with various plate thicknesses were manufactured, and the width of the weld bead in the 1 / 8T to 3 / 8T part of the weld bead where the occurrence of high temperature cracking was 5% or less was investigated to determine the position of the weld bead. The relationship between weld bead widths was organized and generalized.

As a result, the width x of the weld bead in the 1 / 8T portion to the 3 / 8T portion (1 / 8T ≦ y ≦ 3 / 8T) is x ≧ (1 / 0.2234) ln (y / 0.3525). It has been found that the incidence of high temperature cracking is reduced when the bead shape is satisfied. However, y is a position separated from the lowermost portion in the thickness direction of the base metal plate (the depth position of the weld bead).

Further, even if this equation is satisfied, it is necessary to make the shape of the weld bead wider from the bottom surface of the weld bead. When a steel plate having a thickness of more than 28 mm is used from the above-mentioned investigation using a steel plate having a thickness of 28.5 mm, the width w of the weld bead at the penetration position 1/8 T portion is 8.6 mm or more.

Since the component segregation can be reduced as the widths w and x of the weld beads are larger, the upper limits of the widths w and x of the weld beads are not specified. However, considering the welding efficiency and the like, the width x of the welding bead in the 1 / 8T to 3 / 8T portion is usually 20 mm or less. In particular, the width w of the weld bead at the 1 / 8T portion is usually 15 mm or less.

Prepare a thick steel plate (plate thickness 28.5-40 mm) of 0.06 mass% C with a carbon equivalent Ceq of 0.41 mass%, and apply it to the upper and lower surfaces of the thick steel plate assuming the production of UOE steel pipes. The groove was cut and submerged arc welding was performed from the upper and lower surfaces.

For the welding wire, a solid wire (manufactured by Nippon Steel & Sumitomo Metal Welding Industry Co., Ltd.) was used for each electrode, and a molten flux (manufactured by the same company) was used for the flux. Table 1 shows the combinations of welding wires.

Further, a welded joint was manufactured by variously changing the shape (welding bead shape) of the weld metal on the groove and the inner surface welding side (the side welded first) according to the welding conditions shown in Table 2. Regarding the outer surface welding side (the side welded later), the amount of heat input was substantially fixed at 5.2 to 5.4 kJ / mm, and welding was performed.

A macro sample is taken for the bead shape of the joint after welding, the sample is etched, the height T of the weld bead (inner bead) on the inner surface welding side is measured, and it is divided into 8 stages in the base metal plate thickness direction. Then, the width of the inner bead at each position (1 / 8T part, 2 / 8T part, 3 / 8T part) separated from the bottom by 1 / 8T, 2 / 8T, and 3 / 8T was measured (see also FIG. 2). ..

In addition, the presence or absence of high-temperature cracks was confirmed by performing an ultrasonic flaw detection test and X-ray photography on the welded portion. Here, the presence or absence of high-temperature cracking was determined based on the absence of high-temperature cracking in both the ultrasonic flaw detection test and the X-ray imaging.

Furthermore, a sample for microstructure observation is taken from the location of the base material 1 / 4t of the joint, the cross section in the thickness direction is mirror-polished and etched, and then a photograph is taken with an optical microscope. The circle-equivalent particle size of the old austenite in the HAZ in the vicinity of was calculated. Here, the circle-equivalent particle size of the old austenite is about 10 old austenite grains (40 in total) in the vicinity of the welding line at the base material 1 / 4t portion of the left and right welding lines with the front and back surfaces of the base material and the welding bead sandwiched. The measurement was performed, and the average value was taken as the circle-equivalent particle size of the old austenite.

Table 3 shows the height of the inner bead, the width of the inner bead, the particle size equivalent to the old austenite circle in HAZ, and the occurrence rate of high-temperature cracking, which does not include the surplus of the manufactured welded joint. Since there were variations in the penetration depth of the inner surface weld bead and the width of the weld bead even in the welded joint manufactured under the same conditions, Table 3 shows the measured average values of the welded joint manufactured under the same conditions, and the temperature is particularly high. Regarding cracks (high temperature crack occurrence rate in Table 3), the ratio of the number of high temperature cracks to the total number of UOE steel pipes manufactured under the same conditions is shown. The underline in Table 3 indicates that it is out of the scope of the present invention, the low temperature toughness of HAZ is not good, or high temperature cracking has occurred.

The same measurement was performed for the outer bead, the bead height and the bead width, but the samples of Examples (No. 1 to 14) having the same base metal plate thickness were substantially fixed and the outer surface was welded. Due to the formation of exterior beads with similar profiles, only representative samples were measured. Further, no particular measurement was performed on the samples of Examples (No. 15 and 16) having a base material plate thickness of 35 mm or more. The height T of the outer surface bead for the sample of Examples (No. 1 to 14) in which the measurement was performed was 19.5 to 19.8 mm, and the width of the outer surface bead of the 1 / 8T portion was 10.78 to 10. It was 85 mm, the width of the outer bead of the 2 / 8T portion was 13.09 to 13.18 mm, and the width of the outer bead of the 3 / 8T portion was 14.32 to 14.45 mm.

As shown in Table 3, in the examples of the present invention (No. 11, No. 15 and No. 16) satisfying all the provisions of the present invention, the high temperature crack occurrence rate was 0%.

On the other hand, in the comparative examples (Nos. 1 to 10, 12 to 13) that did not satisfy the provisions of the present invention, the occurrence rate of high temperature cracking was high. In particular, No. In No. 14, although the high temperature crack occurrence rate was zero, the particle size equivalent to the old austenite circle in HAZ exceeded 100 μm, and there was a concern that the low temperature toughness of HAZ would decrease. For several samples including 14, the absorbed energy at −20 ° C. at the welding line position of 1 / 4t portion of the base metal on the inner surface welding side was measured. As a result, No. In No. 14, the absorbed energy value was only 83J (3-point average), and the low temperature toughness of HAZ was insufficient.

On the other hand, in the examples of the present invention (No. 11, No. 15 and No. 16), the absorbed energy values were all 200 J or more (3-point average of each sample), and the low temperature toughness of HAZ was also good.

Claims (1)

A welded steel pipe having a welded joint in which a groove is provided at the end of the upper and lower surfaces of a steel plate having a thickness of more than 28 mm and submerged arc welding is performed from the inner and outer surfaces with the root face close to each other.
When the thickness of the steel plate is t (mm) and the height of the weld bead formed by welding is T (mm),
The circle-equivalent particle size of the former austenite in the heat-affected zone near the welding line at the 1 / 4t portion, which is 1 / 4t away from the thickness surface of the steel sheet, is 94 μm or less.
The width w (mm) of the weld bead at a position 1 / 8T away from the bottom of the welding bead for inner surface welding in the base plate thickness direction satisfies the following equation (1), and the base plate thickness direction from the bottom. A welded steel pipe in which the width x (mm) of the weld bead at a position y (mm) separated from 1 / 8T to 3 / 8T satisfies the equations (2) and (3).
w ≧ 8.6 mm ・ ・ ・ ・ ・ (1)
x ≧ (1 / 0.2234) ln (y / 0.3525) ・ ・ ・ ・ ・ (2)
1 / 8T ≤ y ≤ 3 / 8T ... (3)
However,
x: Width of weld bead (mm)
y: Position (mm) away from the bottom in the base metal plate thickness direction
T: Height of weld bead not including pre-filling (mm)

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