JP2008223134A - High strength electric resistance welded line pipe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin high strength electric resistance welded pipe in which t/D is reduced at a low cost. <P>SOLUTION: Disclosed is a high strength electric resistance welded line pipe produced through the process of cold roll molding, electric resistance welding, seam heat treatment and sizing from a hot coil, and in which the outside diameter is 200 to 610 mm and the ratio of thickness/outside diameter (t/D) is ≤2%. The pipe has an acicular-ferrite metallic structure with the average crystal grain size of ≤5 μm, and in which the tensile strength in the circumferential direction after flattening is ≥700 N/mm<SP>2</SP>, a 0.5% proof stress is ≥550 N/mm<SP>2</SP>, and the occupancy area ratio of the oxide in the electric resistance welding abutted part is ≤0.1%. The line pipe comprises, as desirable components, >0.04 to 0.08% C, 0.04 to 0.08% Nb, 0.05 to 0.1% V, 0.1 to 0.5% Ni, 0.1 to 0.5% Cu, 0.05 to 0.20% Mo and 0.01 to 0.03% Ti, and in which the contents of Ni, Cu and Mo satisfy 1.0>3Mo+Ni+Cu>0.8. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a thin electric seam line pipe of API X80 grade or higher used for transportation of oil or natural gas.

  Pipelines for transporting oil and natural gas are being strengthened to improve transport efficiency through high-pressure operations, to reduce the weight of steel materials through thinning, and to improve welding efficiency. Large-diameter UO steel pipes and spiral steel pipes are mainly used for the trunk lines of pipelines. However, highly efficient and inexpensive ERW steel pipes may be used for branch lines and small-scale pipeline trunk lines. Many. Since an electric resistance steel pipe is manufactured by cold forming a hot coil, a high-strength electric resistance line pipe requires a high-strength hot coil.

  Patent Document 1 discloses a hot coil that satisfies the strength of API X70 grade and has high toughness, and a manufacturing method thereof.

  Patent Document 2 discloses a hot coil having strength and HIC resistance equal to or higher than API X65 grade and a manufacturing method thereof.

  Patent Document 3 discloses a hot coil having API X80 grade strength and a manufacturing method thereof.

  Patent Document 4 discloses a method for producing a thick steel pipe having API X100 grade strength.

  Patent Document 5 discloses a method for manufacturing an electric resistance welded steel pipe having API X65 to 80 grade strength.

Japanese Patent No. 3846729 JP 2006-274338 A JP 2005-281838 A JP-A-8-104922 JP-A-8-176669

  In order to increase the strength of an electric resistance welded pipe, there is a high need for thinning rather than a high pressure operation, and a high strength electric resistance welded pipe with a small t / D is required. However, in the manufacturing process of the ERW steel pipe, the compressive processing is applied in the circumferential direction by the sizer after the ERW welding and seam heat treatment processes, so the yield strength in the circumferential direction is greatly reduced by the Bauschinger effect. In the case of a steel pipe having a large t / D, the decrease in yield strength is suppressed by the influence of work hardening at the time of forming the steel pipe, but the decrease in the yield strength in the circumferential direction increases as t / D decreases. This effect is extremely serious because the standard strength of line pipes is mainly defined by the yield strength in the circumferential direction.

  In Patent Documents 1 to 3, the strength of the hot coil, which is a steel pipe material, is only increased, and no significant decrease in yield strength after pipe forming when t / D is low is considered. Specifically, it is not possible to produce a thin high-strength ERW steel pipe of API X80 grade or higher.

  Patent Document 4 discloses an API X80 grade electric resistance welded steel pipe, but since it is a thick steel pipe with a wall thickness of 15 mm, it is easy to manufacture because the margin of decrease in yield strength by the sizer is small, and expensive Ni, Mo addition is high and economical.

  Patent Document 5 discloses an API X80 grade thin-walled high-strength ERW steel pipe with t / D = 1.6%. However, the amount of expensive Mo added is high and the economy is poor.

  In view of the above problems, an object of the present invention is to provide a thin high-strength ERW line pipe having a strength of API X80 grade and having a t / D of 2% or less at a low price.

The inventors of the present invention have made extensive studies in order to solve the above problems, and by optimizing the tensile strength, structure and chemical composition of the steel, an ERW steel pipe having a high yield strength can be obtained even when t / D is 2% or less. It was. The gist is as follows.
(1) Outer diameter 200 mm or more and 610 mm or less, thickness / outer diameter ratio manufactured from hot coil with thickness of 4 mm or more and 12 mm or less through cold roll forming, electro-sealing welding, seam heat treatment, sizer process (T / D) is an electric resistance welded steel pipe of 2% or less, the metal structure is an acicular ferrite structure having an average crystal grain size of 5 μm or less, and the tensile strength in the circumferential direction after flattening is 700 N / mm ² or more, 0 A high-strength electric resistance line pipe characterized by having a 5% proof stress of 550 N / mm ² or more and an oxide occupying area ratio of an electric resistance welding contact portion of 0.1% or less.
(2) By mass% C: more than 0.04 to 0.08%
Si: 0.1 to 0.3%
Mn: more than 1.6 to 2.0%
P: 0.02% or less S: 0.003% or less Nb: 0.04 to 0.08%
V: 0.05-0.1%
Ni: 0.1 to 0.5%
Cu: 0.1 to 0.5%
Mo: 0.05-0.20%
Ti: 0.01-0.03%
Al: 0.05% or less, N: 0.005% or less, and the content of Ni, Cu, Mo satisfies the following <1> formula, and the balance is Fe and inevitable impurities. High strength ERW line pipe as described.
1.0> 3Mo + Ni + Cu> 0.8 <1>
(3) In mass%,
S: 0.0020% or less Ca: 0.001 to 0.003%
The high-strength ERW linepipe according to (2), characterized in that:

  The high-strength ERW line pipe of the present invention has low tensile strength and proof stress despite its low t / D and thin wall thickness. Therefore, the existing line pipe can be made thinner, Use weight can be greatly reduced. Excellent toughness due to its fine metal structure. Since the area ratio of the oxide in the ERW weld abutting portion is low and seam heat treatment is performed, the ERW weld portion has the same level of soundness as the base material. In addition, it is a highly productive ERW pipe with a chemical component that suppresses the use of expensive alloying elements as much as possible. Therefore, the manufacturing cost is significantly lower than UO steel pipe, spiral steel pipe, and seamless steel pipe of the same strength level. .

The inventors have repeatedly studied to obtain a thin-walled high-strength steel pipe having a strength equal to or higher than API X80 grade and having a t / D of 2% or less. When t / D is 2% or less, the yield strength in the circumferential direction is significantly reduced due to the Bauschinger effect caused by the compressive stress in the sizer. As a result of investigation of a huge production record, the relationship between t / D and YR (ratio between yield strength and tensile strength) is shown by a data band as shown in FIG. 1, and when t / D becomes 2% or less, YR is 0. It became clear that it fell to .79-0.73. Therefore, it was found that the tensile strength of the steel pipe must be 700 N / mm ² or more in order to secure 550 N / mm ² which is the lower limit of the proof stress of API X80 grade when t / D is 2% or less. More preferably, it is 750 N / mm ² or more.

  In order to obtain such a hot coil for high-strength ERW steel pipe, the chemical component range is strictly defined as described later, and normal conditions used for obtaining a high-strength steel sheet are applied as hot rolling conditions. Can be obtained.

  In this case, it is important to make the metal structure an acicular ferrite having an average crystal grain size of 5 μm or less. This is a necessary condition for ensuring the necessary minimum toughness while obtaining high tensile strength. Acicular ferrite here is a structure that does not have a layered structure like pearlite, and in which carbides are uniformly dispersed, and unlike bainite, does not leave traces of prior austenite grain boundaries. Such acicular ferrite has the property that the ferrite transformation occurs at a lower temperature than pearlite and the dislocation density is high. In the present invention, if the acicular ferrite in the metal structure occupies 90% or more in area ratio, it is regarded as an acicular ferrite structure. Here, the minimum toughness is API X80 grade, the absorbed energy at −20 ° C. is 100 J or more in the JIS Z 2202 V notch test piece (50 J or more in the 1/2 subsize test piece), and vTrs is − A condition of 50 ° C. or lower is a preferable condition. Moreover, in order to make the metal structure an acicular ferrite having an average crystal grain size of 5 μm or less, it can be realized by adopting a normal rolling method for obtaining a fine grain structure.

  In an ERW steel pipe, it is required that the ERW welded portion has the same strength and toughness as the base metal. However, in the present invention, the oxide occupation area ratio of the ERW abutting portion is preferably 0.1% or less. Is specified to be 0.05% or less, and it is essential to perform seam heat treatment after ERW welding, so that the same strength and toughness as the base metal can be secured. Here, the oxide occupation area ratio is measured by performing a Charpy test at 160 ° C. with a Charpy test piece having a notch in the electro-welding joint, and observing the fracture surface of the test piece after the test with an optical microscope. In addition, seam heat treatment refers to heat-treating only the vicinity of the ERW welded part by, for example, induction heating after ERW welding. After heating and quenching, heating is performed to Ac1 point or less. The latter method is more desirable. In order to set the oxide occupation area ratio of the electric stitching contact portion to 0.1% or less, for example, the relationship between the welding speed and the welding input is the second type in which the oxide occupation area ratio is 0.1% or less. This can be realized by setting the welding phenomenon region and welding (see, for example, “Iron and Steel, 70 (1984), S1162 (FIG. 1)”). Alternatively, the area occupied by the oxide in the electro-welding abutting portion can be reduced to 0.1% or less by surrounding the welded portion with a shield box and welding in an argon gas atmosphere.

  The reason for limiting to hot coils with a plate thickness of 4 mm or more and 12 mm or less is that API X80 grade thick ERW steel pipes with a wall thickness of 15 mm or more are known, and if the plate thickness is less than 4 mm, cold forming becomes difficult. It is. A more desirable wall thickness is 6 to 10 mm.

  The reason why the outer diameter is specified to be 200 mm or more and 610 mm or less is that the outer diameter range in which t / D ≦ 2%, which did not exist in the past, can be realized within the above-described limit range of the plate thickness. More desirably, the thickness is 350 to 550 mm.

  The chemical components of the high-strength ERW line pipe of the present invention will be described below.

  The lower limit of C exceeds 0.04%, which is the minimum amount for ensuring the strength of the base metal and the welded portion, low temperature toughness, precipitation strengthening by adding Nb and V, suppression of recrystallization, and effect of grain refinement. is there. However, if the amount of C is too much, low temperature toughness and on-site weldability are deteriorated, so the upper limit was made 0.08%.

  Si is an element added for deoxidation and strength improvement. In order to obtain the effect, it is necessary to contain 0.1% or more. On the other hand, if added in a large amount, the toughness and weldability deteriorate, so the content was limited to 0.1 to 0.3%.

  Mn is an element essential for obtaining an acicular ferrite structure and ensuring strength and toughness, and its lower limit is over 1.6%. However, too much Mn not only increases the hardenability of the steel and deteriorates the local weldability, but also promotes the center segregation of continuously cast steel pieces and deteriorates the toughness, so the upper limit is 2.0%. did.

  Since P and S are elements that adversely affect toughness, P is 0.02% or less, S is 0.003% or less, and more preferably, S is limited to 0.002% or less.

  Nb contributes to refinement of crystal grains and precipitation strengthening, and has an effect of strengthening steel. In order to obtain the strength defined by the present invention, it is necessary to contain 0.04% or more. However, even if Nb is added in excess of 0.08%, not only a further strength improvement effect can be obtained, but also the weldability and toughness are adversely affected. Therefore, the content is specified to be 0.04 to 0.08%.

  V precipitates as a carbide during the slow cooling after hot coil winding, and contributes to strengthening. In order to obtain the strength defined by the present invention, it is necessary to contain 0.05% or more. However, even if V is added in excess of 0.10%, not only a further strength improvement effect is not obtained, but also the weldability and toughness are adversely affected, so the content was specified to be 0.05 to 0.10%.

  Ni and Cu are elements necessary for improving the hardenability and securing an acicular ferrite structure. In order to obtain the effect, addition of 0.1% or more is necessary. However, since it is an expensive element and a large amount of addition deteriorates weldability, the content is 0.1 to 0. It was defined as 5%.

  Mo is an element necessary for improving the hardenability and securing an acicular ferrite structure, and precipitates as a carbide during slow cooling after winding of the hot coil and contributes to strengthening. In order to obtain the effect, addition of 0.05% or more is necessary. However, since it is an expensive element and a large amount of addition deteriorates weldability, the content is 0.05 to 0.00%. It was defined as 20%.

  Ti forms fine TiN, suppresses coarsening of austenite grains in the slab reheating and welded HAZ, refines the microstructure, and improves the toughness of the base material and the HAZ. In order to exhibit such an effect of TiN, it is necessary to add at least 0.01% Ti. However, if the amount of Ti is too large, TiN coarsening and precipitation hardening due to TiC occur and the toughness deteriorates, so the content was specified to be 0.01 to 0.03%.

  Al is an element usually contained in steel as a deoxidizer, and has an effect on the refinement of the structure. However, if the Al content exceeds 0.05%, Al-based non-metallic inclusions increase to impair the cleanliness of the steel, so the upper limit was made 0.05%.

  N forms TiN and suppresses the coarsening of austenite grains during slab reheating and welding HAZ, thereby improving the low temperature toughness of the base material and HAZ. However, if the amount of N is too large, it will cause deterioration of the HAZ toughness due to slab surface defects or solute N, so the upper limit was made 0.005%.

  Ca has the effect of fixing Ca in steel by forming CaS in steel and improving toughness and hot workability. In order to obtain the effect, addition of 0.001% or more is necessary, but if added too much, CaO is formed and the toughness is lowered, so the content is specified to be 0.001 to 0.003%. did. In addition, when not adding Ca positively, Ca content in steel is about 0.0001-0.0002 mass%.

In the present invention, in addition to the above, addition of Ni, Cu, and Mo so as to satisfy the <1> formula is defined as a preferable condition. This is to ensure an acicular ferrite structure, and when it is 0.8 or less, ferrite + pearlite is mainly, and when it is 1.0 or more, bainite is the main component.
1.0> 3Mo + Ni + Cu> 0.8 <1>

  In the present invention, the hot coil rolling conditions can be arbitrarily selected such that the metal structure and strength satisfy the present invention.

  The electric resistance welded steel pipe in the present invention includes both an electric heating method and an induction heating method.

(Example 1)
Thickness of the continuous cast slab with chemical symbol 1 shown in Table 1 is varied at the extraction temperature of 1230 ° C, the finishing total rolling reduction, the finishing rolling finishing temperature, the cooling rate until winding, and the winding temperature. After rolling into a 7 mm hot coil, it was formed into a tube with an outer diameter of 406.4 mm. Table 2 shows the strength in the plate width direction of the hot coil (plate), the strength in the circumferential direction of the flat tube, the metal structure, the crystal grain size, and the Charpy test result with a ½ subsize test piece. . In all cases, the oxide occupation area ratio of the ERW welding abutting portion was 0.1% or less.

  Tube symbols a to c are examples of the present invention in which the tensile strength and proof stress of the tube satisfy the API X80 grade, the absorbed energy at −20 ° C. is 50 J or more, the vTrs is −50 ° C. or less and the toughness is also good.

  On the other hand, because the coil symbol d was low in coiling temperature, the precipitation strengthening of Nb, V, and Mo was not sufficiently exhibited, and the strength of the hot coil was similar to the API X80 grade. This is an example in which the API X80 grade could not be satisfied due to the decrease.

  The tube symbol e is an example in which the average crystal grain size is large, the absorbed energy at −20 ° C. is 50 J or less, and the vTrs is −50 ° C. or more, resulting in poor toughness.

  The tube symbol f is a bainite structure in which the old γ grain boundaries flattened by non-recrystallization zone rolling remain clearly, and the strength was sufficient, but the minor axis of the old γ grains greatly exceeded 5 μm of the present invention. This is an example in which vTrs was high.

  The pipe symbol g is an example of a ferrite + pearlite structure, in which the strength was low and the toughness was poor because the average particle size was large.

(Example 2)
When the steel pipe of the pipe symbol a in Table 2 is electro-welded, the heat input condition is changed to intentionally create a portion with a high oxide occupation area ratio in the electro-resist welding joint, and quenching and tempering Seam heat treatment was performed. Table 3 shows the Charpy test results at −20 ° C. using a ½ sub-size test piece in which the weld is located at the notch position. Tube symbols a-1 and a-2 are examples of the present invention having absorbed energy equivalent to or higher than that of the base material.

  Tube symbols a-3 and a-4 are examples in which the oxide occupation area ratio is too high and the absorbed energy is reduced to 50 J or less.

(Example 3)
Continuous casting slabs having the chemical components shown in Table 1 were extracted at a temperature of 1230 ° C., a final rolling reduction of 80%, a finish rolling finish temperature of 860 ° C., a cooling rate of 10 ° C./s, and a winding temperature of 570 ° C. Rolled to width and piped. Table 4 shows the strength in the circumferential direction of the flattened tube, the metal structure, the crystal grain size, and the Charpy test results with a ½ subsize test piece.

  Pipe symbols A to H and P are examples of the present invention in which the tensile strength and proof stress of the pipe satisfy API X80 grade, the absorbed energy at −20 ° C. is 50 J or more, the vTrs is −50 ° C. or less and the toughness is also good.

  On the other hand, the pipe symbol I was an example in which the t / D was too large and the yield reduction due to the pipe making was small, but the wall thickness was too thick and the crystal grains were not sufficiently refined and the toughness was low. It is.

  The pipe symbol J is an example in which the amount of C was too high, so that the strength exceeded the API X80 grade and the toughness was low.

  The tube symbol K is an example in which sufficient strength cannot be obtained because the amount of C is too small.

  The pipe symbol L is an example in which the amount of Mn is too large, resulting in a mixed structure of bainite and acicular ferrite and low toughness.

  The tube symbol M is an example in which the amount of Mn is too small, so that a mixed structure of ferrite + pearlite and acicular ferrite is formed, and the strength and toughness are low.

  The pipe symbol N is an example in which since the <1> formula is 0.8 or less, a mixed structure of ferrite + pearlite and acicular ferrite is formed, and the strength and toughness are low.

  The pipe symbol O is an example in which the toughness is low because the <1> formula has a mixed structure of bainite and acicular ferrite because the formula is 1.0 or more.

It is a figure which shows the relationship between t / D of an ERW steel pipe, and YR of the circumferential direction.

Claims (3)

A hot coil having a thickness of 4 mm or more and 12 mm or less, manufactured by cold roll forming, electric seam welding, seam heat treatment, sizer process, outer diameter 200 mm or more and 610 mm or less, thickness / outer diameter ratio (t / D) ERW steel pipe with 2% or less, metal structure is an acicular ferrite structure with an average crystal grain size of 5 μm or less, and circumferential tensile strength after flattening is 700 N / mm ² or more, 0.5% A high-strength ERW line pipe characterized by having a proof stress of 550 N / mm ² or more and an oxide occupation area ratio of the ERW welding contact portion of 0.1% or less. In mass% C: more than 0.04 to 0.08%
Si: 0.1 to 0.3%
Mn: more than 1.6 to 2.0%
P: 0.02% or less S: 0.003% or less Nb: 0.04 to 0.08%
V: 0.05-0.1%
Ni: 0.1 to 0.5%
Cu: 0.1 to 0.5%
Mo: 0.05-0.20%
Ti: 0.01-0.03%
Al: 0.05% or less N: 0.005% or less, and the content of Ni, Cu, and Mo satisfies the following <1> formula, and the balance is made of Fe and inevitable impurities. High strength ERW line pipe as described.
1.0> 3Mo + Ni + Cu> 0.8 <1> In mass%,
S: 0.0020% or less Ca: 0.001 to 0.003%
The high-strength ERW line pipe according to claim 2, wherein:

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