JP2005272900A - High-strength uoe steel pipe having excellent low-temperature toughness of seam weld metal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a UOE steel pipe for a high-strength line pipe which has a weld zone having excellent low-temperature toughness and is manufactured by shaping a steel sheet having a tensile strength of 800 to 1,100 MPa to a cylindrical shape, then joining the butt parts of the steel sheet by welding. <P>SOLUTION: The UOE steel pipe is manufactured by shaping the steel sheet having the tensile strength of 800 to 1,100 MPa to the cylindrical shape, then fixing the butt parts of the steel sheet by tack welding or the like, and seam welding the butt parts by submerged arc welding. The high-strength UOE steel pipe having the seam weld metal excellent in the strength and the low-temperature toughness can be obtained by regulating the chemical components of the weld metal. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a high-strength line pipe excellent in low-temperature toughness having a tensile strength of 800 MPa or more, and UOE (or sometimes referred to as UO) for line pipes mainly used for transportation of natural gas and oil. It relates to steel pipes.

  In recent years, line pipes have become increasingly important as a long-distance transportation method for crude oil and natural gas. Currently, the American Petroleum Institute (API) standard X65 is the basic design for trunk line pipes for long-distance transportation. (1) Improvement of transportation efficiency due to high pressure and (2) Outer diameter of line pipe In order to improve local construction efficiency by reducing wall thickness and weight, higher strength line pipes are desired. So far, line pipes up to X80 have been put into practical use, but the need for high-strength line pipes has increased, and there is a demand for high-strength line pipes exceeding X100 (tensile strength of 760 MPa or more).

  The production of these line pipes requires (1) a steel material with a good balance between strength and low temperature toughness, and (2) a weld with excellent strength and low temperature toughness.

Steel pipes of relatively large size for line pipes are generally called UOE steel pipes or UO steel pipes. After a steel sheet is U-shaped press formed, it is further formed into a tubular shape by O-shaped press forming, and the butt end of the steel sheet is temporarily formed. It is manufactured by joining by an inner surface seam welding and an outer surface seam welding. At this time, for the tack welding, a gas shield arc welding method such as MAG welding or CO ₂ welding is mainly used, and recently, a laser welding method or the like is also used. Usually, tack welding is welded from the outer surface side of the steel plate butt portion formed into a tubular shape. In addition, seam welding mainly uses multi-electrode submerged arc welding (hereinafter also referred to as SAW welding method), which is superior in both welding efficiency and welding quality, as main welding after tack welding. Usually, in the seam welding, after the inner surface seam welding is performed from the inner surface side of the steel pipe seam portion that has been tack welded, the outer surface seam welding is performed from the outer surface side of the steel pipe seam portion.

  As a conventional UOE steel pipe manufacturing method, for example, as disclosed in Patent Document 1 and the like, the strengthening elements in the steel material are limited and the microstructure is mainly composed of a low temperature transformation structure such as martensite or bainite to ensure strength and toughness. doing. On the other hand, the welding material applied at the time of seam welding of this steel material is also intended to ensure the strength and toughness of the weld metal in the seam welded part by limiting the composition of the components in the same manner as the steel material. However, seam welds are different from those that are relatively easy to control strength and toughness depending on rolling conditions and components, such as steel, and the properties of weld metal as-welded are due to component segregation during welding and thermal cycles. There is a tendency for mechanical characteristics, particularly toughness variation, to increase.

  In particular, in a high-strength steel pipe having a tensile strength exceeding 850 MPa, it is not easy to stably secure low-temperature toughness while maintaining the tensile strength of the seam welded portion. For this reason, conventionally, although the average value satisfies the required value, a low value is often generated for each value and is not stable. Observing the soundness of the seam weld, it is natural that not only the average toughness value but also each value is important.

  Even in the seam welded part, for example, as disclosed in Patent Document 2, the toughness of the welded heat affected zone can be secured by limiting the amount of alloy added to the base metal, but high strength is ensured for the weld metal. For this reason, consideration is given to hot cracking due to the added amount of alloy, but toughness is not particularly considered.

  Similarly, in Patent Document 3, the hardness is adjusted to ensure the toughness of the weld heat affected zone, but the toughness of the weld metal is not particularly positively considered. Therefore, the toughness at −20 ° C. is only ensured.

Moreover, although the composition of a weld metal is prescribed | regulated in patent document 4, since a structure | tissue is mainly an acicular ferrite, intensity | strength is 800 MPa range at the maximum, and it is inadequate with respect to a further high strength steel. Further, the toughness is -20 ° C as an index, and difficulty arises when further low temperature toughness is required.
With regard to the weld metal, for example, in Patent Document 5, the components of the weld metal are specified to ensure strength and toughness. However, after welding, the steel pipe including the weld metal is heated and cooled to a high temperature to perform heat treatment. However, it cannot be applied to UOE steel pipes used as welded.

Japanese Patent Application No. 11-002042 Japanese Patent Laid-Open No. 2000-109592 Japanese Patent Laid-Open No. 10-306347 JP-A-9-824203 JP-A-11-172375

  The present invention is a UOE steel pipe for high-strength line pipes, which is manufactured by forming a steel sheet having a tensile strength of 800 to 1100 MPa into a cylindrical shape with a weld having excellent low-temperature toughness and then joining the butted parts of the steel sheet by welding. Is to provide.

  As a result of intensive investigation and analysis of the chemical composition and mechanical properties of the weld metal in the UOE steel pipe having a tensile strength of 800 to 1100 MPa, the present inventors have a strength that satisfies the seam weld metal part of the high-strength UOE steel pipe. The inventors have invented a weld metal having stable low temperature toughness.

  The gist of the present invention is as follows.

(1) In a UOE steel pipe composed of a base material having a tensile strength of 800 to 1100 MPa and a seam weld metal portion having a tensile strength of 850 to 1100 MPa,
The weld metal is in mass%,
C: 0.03 to 0.09%,
Si: 0.08 to 0.5%,
Mn: 1.4 to 2.3%
P: 0.01% or less,
S: 0.01% or less,
Ni: 1.3-3.3%,
Cr: 0.4 to 1.3%,
Mo: 0.5 to 2.5%,
Total amount of Cr and Mo: 1 to 2.5%,
Ti: 0.004 to 0.02%,
O: 0.016-0.035%
Containing
Nb: limited to less than 0.01%,
The balance is composed of Fe and inevitable impurities, and the Pcw value defined by the following formula (1) satisfies 0.45 to 0.8, and the welded structure is a bainite structure. High strength UOE steel pipe with excellent low temperature toughness of seam weld metal.
Pcw = C + 0.09 × Si + 0.08 × Mn + 0.06 × Ni + 0.11 × Cr + 0.014 × Mo (1)
Here, C, Si, Mn, Ni, Cr, and Mo are mass% of each element.

  (2) The high strength UOE steel pipe excellent in the low temperature toughness of the seam weld metal according to the above (1), characterized in that Nb is limited to 0.02% or less in the base material.

(3) In mass%,
C: 0.04 to 0.1%,
Ni: 4-6%,
Cr: 1.8-3%,
Mo: 2 to 3.5%,
Ti: 0.02 to 0.04%
Containing
Nb: limited to 0.03% or less,
The weld metal is formed by submerged arc welding using a welding wire whose balance is Fe and inevitable impurities, and the seam weld metal according to (1) or (2) is excellent in low-temperature toughness Strength UOE steel pipe.

  Hereinafter, embodiments of the present invention will be described in detail.

  In general, high-strength UOE steel is U-pressed on a high-strength steel plate with a tensile strength of 800 Mpa or more, and after O-pressed butt ends are tack welded, and then seam welded (hereinafter referred to as the pipe inner surface and the outer surface). The inner / outer surface seam welding is performed) to be joined. In the seam welding, one-layer welding is performed on each of the tube inner surface side and the tube outer surface side by using multi-electrode submerged arc welding which is superior in terms of normal welding efficiency and quality. When the outer surface seam welding is performed after the inner surface seam weld metal is formed, the inner surface seam weld metal is partially melted by welding heat input, and a heat affected zone is formed around the inner surface seam weld metal.

  The present inventors conducted an experiment in which the base steel plate, welding material, welding conditions, etc. were changed for the cause of reducing the toughness of the seam weld metal of high-strength UOE steel, and particularly the relationship between the composition of the weld metal and the toughness. As a result of investigation, it was found that the toughness is lowered particularly in the case of a weld metal having a high Nb content.

  In general, Nb in the base steel plate is precipitated as Nb carbonitride during the production of the steel plate and refines the structure and has the effect of improving precipitation hardening and hardenability, so that the tensile strength is 800 MPa or more. It is often used for steel plates. Since the submerged arc welding used for normal seam welding has a large amount of base metal steel sheet, the amount of Nb in a weld metal formed by seam welding a steel sheet having a high Nb content tends to increase.

On the other hand, Nb in weld metal formed by melting and solidification by seam welding is not precipitated as carbonitride due to the high cooling rate after solidification, and exists as solid solution Nb, improving the strength of the weld metal. It is known to contribute to For this reason, when seam welding a high-strength steel sheet of 800 MPa or more, the Nb content in the welding material used at the time of welding is increased in order to improve the weld metal to the same level as the base metal, and the strength is improved by the solid solution Nb of the weld metal. Has been done.
As a result of detailed observation of the weld metal fracture surface after the impact test of the seam weld with a scanning electron microscope, the present inventors found that there are many causes of the reduction in the toughness of the seam weld metal in the heat affected zone of the inner seam weld metal. It has been found that Nb carbide to be used as a starting point of fracture reduces toughness. This is because during the outer surface seam welding, a large amount of dissolved Nb was precipitated as Nb carbide in the heating and cooling process in the heat-affected zone of the inner surface seam weld metal heated and cooled to a high temperature by the heat input. I think.

  The present invention has been made on the basis of the above knowledge. When producing a high strength UOE steel pipe having a tensile strength of 800 MPa or more, Nb, which has been widely used as a strengthening element for base steel and weld metal, is used as much as possible. Reduced toughness due to Nb carbides in seam weld metal while preventing tensile strength equal to or higher than that of the base metal by optimizing the composition and structure of the seam weld metal excluding Nb. The technical idea is to improve low temperature toughness.

  In the present invention, it is assumed that the base steel plate of the UOE steel pipe has a tensile strength of 800 to 1100 MPa. The reason why the steel sheet tensile strength is 800 MPa or more is that the toughness reduction due to the Nb carbide of the UOE steel pipe seam weld metal becomes remarkable with this steel sheet strength. In addition, when the steel plate tensile strength exceeds 1100 MPa, the weld metal has a very large reduction in toughness in order to ensure the tensile strength of the weld metal equal to or higher than that of the steel plate. At this tensile strength level, even according to the present invention, the toughness of the weld metal cannot be sufficiently improved, so the upper limit of the steel plate tensile strength is set to 1100 MPa as a preferable condition.

  In the present invention, in order to improve the toughness of a seam weld metal of a UOE steel pipe whose base steel plate has a tensile strength of 800 to 1100 MPa, it is necessary to properly define the composition, structure and tensile strength of the seam weld metal.

  The component composition of the seam weld metal specified in the present invention will be described below. Note that “%” shown below means “% by mass” unless otherwise specified.

  The component composition of the seam weld metal of the present invention regulates the Nb content as much as possible and maintains the strength / toughness balance of the weld metal with the steel plate strength of 800 to 1100 MPa in tensile strength of 850 to It is necessary to specify in order to maintain a weld metal characteristic of toughness 130J or higher at 1100 MPa and −30 ° C.

  C is an element effective for generating a bainite structure in the weld metal structure and improving the strength of the bainite structure. In order to sufficiently improve the tensile strength of the weld metal, its content is set to 0.03% or more. However, if C is added excessively, the hardness of the weld metal becomes too high, and sufficient toughness cannot be obtained, and the risk of cold cracking also increases. Therefore, the upper limit of the content is set to 0.09%.

  Si has an action of suppressing the generation of blowholes during seam welding, and the content thereof is set to 0.08% or more in order to sufficiently exhibit this action and sufficiently prevent blowholes. However, when Si is added excessively, the toughness of the seam weld metal is lowered, so the upper limit of its content was made 0.5%.

  Mn is an important element in securing the tensile strength of the seam weld metal. In order to sufficiently improve the tensile strength of the weld metal, its content is set to 1.4% or more. However, excessive addition causes a reduction in the toughness of the weld metal, so the upper limit of its content was made 2.3%.

  P is desirably reduced as much as possible in order to reduce mechanical properties such as toughness in the seam weld metal. Part of the P in the weld metal is caused by the inclusion of P contained in the base material during seam welding. In the present invention, considering the carry-in from the base material, the content is limited to 0.01% or less from the viewpoint of suppressing the decrease in toughness of the weld metal.

  S, like P, lowers the mechanical properties such as toughness of the weld metal. Therefore, considering the mixing from the base metal, the content is made 0.01% or less from the viewpoint of suppressing the decrease in the toughness of the weld metal. Restricted.

  Ni is an effective element for improving the hardenability of the seam weld metal, making the weld metal a bainite structure, improving strength, and improving toughness. In order to sufficiently improve the strength and toughness of the weld metal, its content is set to 1.3% or more. However, if Ni is added excessively, hot cracking is likely to occur in the weld metal, so the upper limit of its content was made 3.3%.

  Cr is an effective element for improving the hardenability and making the weld metal a bainite structure and improving the strength. In order to sufficiently improve the strength of the weld metal, its content is set to 0.4% or more. However, excessive addition of Cr causes a decrease in toughness, so the upper limit of its content was made 1.3%.

  Mo, like Cr, is an element necessary for improving the hardenability, making the weld metal a bainite structure and improving the strength, and in order to sufficiently obtain this effect, its content is 0.5% or more. did. However, excessive addition of Mo does not increase the weld metal strength more than necessary and lowers the toughness, so the upper limit of its content is 2.5%, preferably 2.0%.

  Since the above Cr and Mo produce similar effects, it is necessary to define the total amount of these in order to produce bainite having an appropriate strength. In order to control the strength improvement due to the hardenability effect and the toughness reduction due to the excessive strength improvement, the total amount of Cr and Mo was specified in the range of 1 to 2.5%.

  Ti is added from the welding wire at the time of seam welding, and a part thereof is oxidized and exists in the weld metal as Ti oxide, and the other is present in the weld metal as solute Ti. Ti oxide contributes to refinement of the weld metal structure and has an effect of improving toughness, and solute Ti contributes to improvement of the strength of the weld metal even in a small amount. In the present invention, the content is made 0.004% or more in order to sufficiently obtain the strength improvement effect by solute Ti. However, if its content exceeds 0.02%, the solid solution Ti becomes excessive, and conversely, the toughness decreases. The upper limit of the Ti content was set to 0.02% in order to combine with oxygen in the weld metal to generate Ti oxide without any excess and to refine the weld metal structure.

  O is mixed into the weld metal from the welding wire and the atmosphere during seam welding, but the amount is defined by the deoxidation action of Ti. In the case where the O content in the weld metal is less than 0.016%, the Ti oxide present in the weld metal is reduced, the excess Ti is present as solute Ti, so the strength is increased, and the toughness is reduced. The content was set to 0.016% or more. However, if its content exceeds 0.035%, on the contrary, the amount of Ti oxide in the weld metal increases, and it becomes easy to form coarse oxides that do not contribute to the refinement of the structure, and toughness decreases.

  As described above, Nb precipitates as Nb carbide in the heat-affected zone of the inner surface seam weld metal during outer surface seam welding, and serves as a starting point of fracture to reduce toughness. Therefore, the content of Nb is reduced as much as possible in the present invention. There is a need. In the present invention, the Nb content in the weld metal is limited to less than 0.01% in order to sufficiently improve the toughness by suppressing the precipitation of Nb carbide in the heat-affected zone of the inner surface weld metal. If the content is higher than this, the toughness of the seam weld metal cannot be stably improved, and in particular, the toughness of the inner surface seam weld metal part is lowered.

In addition, in the present invention, in consideration of the balance between the tensile strength of the weld metal and the base metal strength, in order to ensure the tensile strength of 850 to 1100 MPa and improve the toughness, Therefore, it is necessary to define the component composition so that the Pcw value defined by the following formula (1) satisfies 0.45 to 0.8. As a result, the weld metal structure can be made into a bainite structure main structure excellent in strength and toughness.
Pcw = C + 0.09 × Si + 0.08 × Mn + 0.06 × Ni + 0.11 × Cr + 0.014 × Mo (1)
Here, C, Si, Mn, Ni, Cr, and Mo are mass% of each element.

  FIG. 1 is a diagram showing the relationship between the Pcw value of seam weld metal, tensile strength, and toughness.

  When the Pcw value of the seam weld metal is less than 0.45, the amount of ferrite produced in the weld metal structure increases, so that the required tensile strength of 850 MPa or more cannot be ensured from the balance with the base metal strength. On the other hand, when the Pcw value of the seam weld metal exceeds 0.80, the tensile strength of the seam weld metal is too high, and the weld metal has a sufficient toughness (preferably a toughness of 130 J or more even at −30 ° C.). I can't get it. Therefore, in the present invention, the weld metal structure is mainly bainite, the tensile strength thereof is in the range of 850 to 1100 MPa, and the Pcw defined by (1) above is ensured in order to ensure sufficient tensile strength and toughness of the seam weld metal. The component composition of the weld metal was defined so that the value was in the range of 0.45 to 0.8.

  Further, the seam weld metal structure of the present invention needs to be mainly composed of a bainite structure excellent in strength and toughness. When the weld metal structure is mainly composed of a ferrite structure, the strength of the weld metal is lowered, and a tensile strength of 850 MPa or more cannot be secured. Further, when the weld metal structure is mainly composed of a martensite structure, the strength of the weld metal becomes too high and the toughness is lowered, and the preferable toughness of the weld metal (the toughness is 130 J or more even at −30 ° C.) cannot be obtained.

In order to make the seam weld metal a bainite main structure, it is possible to adjust the component composition of the seam weld metal so as to be within the specified range.
As described above, by adjusting the other component composition while regulating the Nb content in the seam weld metal as much as possible to obtain a bainite main structure, the tensile strength is 850 to 1100 MPa and the toughness at −30 ° C. It becomes possible to obtain a weld metal excellent in strength and toughness of 130 J or more.

  The method for reducing the Nb content during seam welding as much as possible is performed by limiting the Nb content in the base material steel plate or the welding material used during seam welding.

  When the Nb content in the welding material used during seam welding is an unavoidable amount of impurities, the Nb content in the weld metal is affected by the Nb content in the base steel and the welding heat input. It depends on the amount of penetration of the components. For this reason, it is preferable to reduce the amount of Nb mixed from the base material by reducing the heat input amount during seam welding or by welding under a condition in which the base steel plate melts shallowly.

  In the present invention, in order to reduce the Nb content in the weld metal as much as possible without limiting the component composition and welding conditions of the welding material at the time of seam welding more than necessary, in the base steel plate having a tensile strength of 800 to 1100 MPa. It is preferable to limit the Nb content to 0.02% or less. Nb contained in the base steel plate is mixed into the weld metal when the steel plate partially melts during seam welding, but the Nb is consumed by being partially oxidized and discharged out of the weld metal as slag. The In the present invention, considering the Nb yield in the weld metal affected by the base metal penetration ratio and the oxidation ratio during seam welding, the upper limit of the preferable Nb content is set to 0.02%.

  When submerged arc welding that is normally used as seam welding is performed, a welding wire and a flux are used as the welding material. Among these, the component composition of the weld metal is determined by the component composition of the welding wire.

  The component composition of the welding wire at this time depends on the component composition of the base steel plate, the welding heat input during seam welding, and the penetration amount conditions so that the component composition and structure of the weld metal specified in the present invention can be obtained. This is possible by adjusting the wire component, and is not particularly limited. In addition, since normal seam welding uses multi-electrode submerged welding using a multi-electrode wire excellent in welding efficiency and quality, it is preferable to adjust the composition of the welding wire in consideration of the current ratio and welding conditions of each electrode. .

  However, in order to stably ensure the mechanical properties of the strength and toughness of the above-described weld metal of the present invention, the component yield in the weld metal due to penetration and oxidation of the base steel plate component during seam welding is considered. And it is more preferable to prescribe | regulate the following components among the component compositions in a welding wire.

  The C content in the welding wire is a dilution by the penetration of C in the base steel plate during seam welding in order to stably obtain the C content range of the weld metal described above: 0.03 to 0.09%. Therefore, it is preferable to limit the content to 0.04 to 0.1%.

  In order to avoid excessively increasing the Ni content in the steel sheet, the Ni content in the welding wire takes into account dilution due to penetration of C in the base steel sheet during seam welding, and the lower limit of the content. Is preferably 4.0%. On the other hand, if the Ni content in the welding wire exceeds 6%, the amount of Ni in the weld metal becomes excessive and causes defects such as hot cracks, so the upper limit is preferably made 6%.

  Similarly to Ni, Cr in the welding wire is preferably added to the weld metal mainly from the welding wire so that the Cr content in the weld metal is within the above range. Therefore, it is preferable that the lower limit of the Cr content in the welding wire is 1.8%. On the other hand, if the Cr content in the welding wire exceeds 3%, the upper limit of the Cr content in the weld metal is exceeded and the toughness decreases, so the upper limit of the content is 3%. preferable.

Similarly to Ni and Cr, Mo in the welding wire is preferably added to the welding metal mainly from the welding wire to make the range of the Mo content in the welding metal described above. For this reason, the lower limit of the Mo content in the welding wire is preferably 2%. On the other hand, if the Cr content in the welding wire exceeds 3.5%, the upper limit of the Mo content in the weld metal described above exceeds the upper limit of the toughness, so the upper limit of the content is 3.5%. Is preferable.
Considering the Ti yield in the weld metal due to oxidation consumption during welding, the Ti in the welding wire has a content of 0.02 to 0 in order to stably obtain the Ti content in the weld metal. 0.04% is preferable.

  Nb in the welding wire is preferably reduced as much as possible in order to obtain the Nb content in the above-described weld metal. In consideration of the fact that Nb in the base steel sheet may be added to some extent for microstructure control, and considering the Nb yield in the weld metal due to oxidation consumption during welding, the upper limit of the Nb content in the welding wire is set to 0. 0.03% or less is preferable. Thereby, Nb content in the above-mentioned weld metal can be made less than 0.01% without excessively limiting the Nb content in the base steel.

  Next, the effects of the present invention will be described with reference to the following examples.

  A steel sheet having the tensile strength and component composition shown in Table 2 is UO-pressed into a tubular shape and tack welded by MAG welding, and then seam welding is performed using three-electrode submerged arc welding. By combining welding wires having component compositions as shown in Table 3 and welding under heat input conditions of 2.0 to 3.2 kJ / mm shown in Table 3, seam weld metals having the component compositions shown in Table 3 were obtained. Formed. Table 4 shows the characteristics of the weld metal.

  A melt type flux was used as the submerged welding flux.

  After seam welding, a non-destructive inspection by X-ray was performed on a seam weld having a total length of 12 mm to determine the presence or absence of defects.

  In addition, as a mechanical test of the seam welded portion, a tensile test and an impact test of the outer surface weld metal were performed.

  As shown in FIG. 2 (a), the impact test piece has a notch 4 at the meeting portion of the inner surface seam weld metal 2 and the outer surface seam weld metal 3 that is ½ of the plate thickness in the plate thickness direction from the outer surface of the steel pipe. The impact test piece 1 (hereinafter referred to as “thickness ½ meeting portion”) and a center portion of the inner surface seam weld metal 2 (hereinafter referred to as inner surface weld metal) as shown in FIG. Two types of impact test pieces 1 with 4 were prepared. A 10 mm full-size impact test piece was used for gathering the meeting part, and a 5 mm sub-size impact test piece was used for the inner weld metal. In the impact test, the test temperature was set to −30 ° C., 6 impact test pieces collected were repeated, and an average value was obtained.

  Steel plates E and F in Table 2 are low Nb steel plates in which the Nb addition amount in the steel plate satisfies the more preferable range of 0.02% or less of the present invention, and the steel plates A to D deviate from the more preferable range of the present invention. Steel plate. Moreover, the welding wires eg in Table 1 are welding wires having a more preferable component range of the present invention, and the welding wires a to d are welding wires deviating from the above.

  Table 3 shows the chemical composition and tensile strength of the seam weld metal, and Table 4 shows the evaluation results of the tensile strength, toughness and weldability of the seam weld.

  In Invention Examples 1 to 6, each component of the seam weld metal and the Pcw value are within the range specified in the present invention, so that the tensile strength of the weld metal is maintained at a high strength of 880 MPa or more, and the thickness is 1/2 associated. It was possible to obtain excellent low temperature toughness at 30 ° C. on the part at an average of 136 J or more, and internal weld metal toughness at 30 ° C. on the average of 53 J or more. Moreover, even if it looked at the seam toughness value, the test piece extract | collected 6 by the weld line of the seam weld part had little variation, and was stable, and high toughness was obtained.

  Among these inventive examples, the inventive compositions 5 and 6 are such that the component composition of the base steel sheet is limited to the more preferable Nb content of the present invention, so that the amount of penetration of the Nb amount from the base metal during seam welding is small, The toughness value of the weld metal could be improved more stably.

  On the other hand, Comparative Examples 1 to 10 are examples in which each component of the seam weld metal and the Pcw value are out of the range defined in the present invention. As a result, any of the tensile strength of the weld metal, the toughness at 30 ° C. at the plate thickness ½ meeting portion, and the toughness at 30 ° C. of the inner surface weld metal was a result lower than the example of the present invention.

  In Comparative Example 1, since the Nb content of the weld metal deviates from the specified range of the present invention, the average value of the toughness at 30 ° C. is low for both the plate thickness ½ meeting part and the inner surface weld metal. A very low toughness value was measured with a large variation in each value of the six test pieces.

  In Comparative Example 2, the contents of Si, P, S, and Nb of the weld metal, the sum of the contents of Cr and Mo, and the Pcw value are far from the ranges specified in the present invention. As a result, the tensile strength of the seam weld metal becomes too high, and the average value of the toughness at 30 ° C. is low for both the plate thickness ½ meeting part and the inner surface weld metal. A very low toughness value with large variation was measured.

  In Comparative Example 3, the contents of Mn, Ni, Mo, and Nb, the total content of Cr and Mo, and the Pcw value are far outside the ranges defined in the present invention. As a result, the tensile strength of the seam weld metal becomes too high, and the average value of the toughness at 30 ° C. is low for both the plate thickness ½ meeting part and the inner surface weld metal. A very low toughness value with large variation was measured. Moreover, since the amount of Ni in the weld metal was large, hot cracking occurred.

  In Comparative Example 4, the C, P, S, Ni, Mo, and Nb contents of the seam weld metal, the sum of the Cr and Mo contents, and the Pcw value deviate from the range specified in the present invention, respectively, and Ti and Each content of O is out of the range defined in the present invention. As a result, the tensile strength of the seam weld metal becomes too high, and the average value of the toughness at 30 ° C. is low for both the plate thickness ½ meeting part and the inner surface weld metal. A very low toughness value with large variation was measured. Moreover, since the amount of Ni in the weld metal was large, hot cracking occurred.

  In Comparative Example 5, each of the Cr and Nb contents of the seam weld metal, the sum of the Cr and Mo contents, and the Pcw value are far from the ranges defined in the present invention. As a result, the tensile strength of the seam weld metal becomes too high, and the average value of the toughness at 30 ° C. is low for both the plate thickness ½ meeting part and the inner surface weld metal. A very low toughness value with large variation was measured.

  In Comparative Example 6, each of the Nb and Ti contents of the seam weld metal deviates from the range defined by the present invention, and the Ni content and the Pcw value deviate from the ranges defined by the present invention. As a result, the tensile strength of the seam weld metal is low, the average value of the toughness at 30 ° C. is low for both the thickness ½ meeting part and the inner surface weld metal, and the values of the six test pieces vary. Large and very low toughness values were measured.

  In Comparative Example 7, the C, Ni, and Cr contents of the seam weld metal and the Pcw value deviate from the range defined by the present invention, and the Nb content deviates from the range defined by the present invention. As a result, the tensile strength of the seam weld metal is low, the average value of the toughness at 30 ° C. is low for both the plate thickness ½ meeting part and the inner surface weld metal, and the values of the six test pieces vary. A large and low toughness value was measured.

  In Comparative Example 8, the Si, Mn, Mo, and O contents of the seam weld metal and the Pcw value are out of the range defined in the present invention, and the Nb content is out of the range defined in the present invention. Yes. As a result, the seam weld metal has a low tensile strength, and because the tensile strength is low, the average value of the toughness at 30 ° C. was high for both the plate thickness ½ meeting part and the inner surface weld metal. Because of the high Nb content, the values of the six test pieces varied greatly and very low toughness values were measured. In addition, blowholes were observed in the X-ray nondestructive inspection of the seam weld due to the low amount of Si.

  In Comparative Example 9, the Ni and O contents of the seam weld metal deviate from the range defined by the present invention, and the Ti content deviates from the range defined by the present invention. As a result, the tensile strength of the seam weld metal becomes too high, and the average value of the toughness at 30 ° C. is low at both the plate thickness ½ meeting part and the inner surface weld metal due to the large amount of oxide in the weld metal. The values of the test pieces of the book varied greatly and very low toughness values were measured.

  In Comparative Example 10, the total value of the Cr and Mo contents and the Pcw value of the seam weld metal are far outside the ranges defined in the present invention. As a result, the tensile strength of the seam weld metal becomes too high, and the average value of the toughness at 30 ° C. is low at both the plate thickness ½ meeting part and the inner surface weld metal due to the large amount of oxide in the weld metal. The values of the test pieces of the book varied greatly and very low toughness values were measured.

  2 to 5, the invention examples 1 to 6 and comparative examples 1 to 10 shown in Table 4 were subjected to the impact test of 6 test pieces at the plate thickness 1/2 meeting part and the inner surface weld metal at 30 ° C. The measured value of toughness is shown. Even in comparison with these inventive examples 1 to 6 and comparative examples 1 to 9, the present invention example has a high average value of toughness at 30 ° C. for both the thickness ½ meeting part and the inner surface weld metal, and It is apparent that a seam weld metal having small variations in the values of the six test pieces and having excellent low temperature toughness can be obtained.

  According to the present invention, it is possible to easily manufacture a high-strength UO steel pipe having a seam weld that can stably obtain low-temperature toughness, which greatly contributes industrially.

It is a figure which shows the relationship between the tensile strength and toughness of Pcw and a weld metal. It is a figure which shows the notch position of a welding part impact test. It is a figure which shows the toughness of the board thickness 1/2 gathering part of this invention Example. It is a figure which shows the toughness of the inner surface weld metal of an Example of this invention. It is a figure which shows the toughness of the board thickness 1/2 meeting part of a comparative example. It is a figure which shows the toughness of the internal weld metal of a comparative example.

Explanation of symbols

1 Impact test piece 2 Inside seam weld metal 3 Outside seam weld metal 4 Notch

Claims (3)

In a UOE steel pipe composed of a base material having a tensile strength of 800 to 1100 MPa and a seam weld metal portion having a tensile strength of 850 to 1100 MPa,
The weld metal is in mass%,
C: 0.03 to 0.09%,
Si: 0.08 to 0.5%,
Mn: 1.4 to 2.3%
P: 0.01% or less,
S: 0.01% or less,
Ni: 1.3-3.3%,
Cr: 0.4 to 1.3%,
Mo: 0.5 to 2.5%,
Total amount of Cr and Mo: 1 to 2.5%,
Ti: 0.004 to 0.02%,
O: 0.016-0.035%
Containing
Nb: limited to less than 0.01%,
The balance is composed of Fe and inevitable impurities, and the Pcw value defined by the following formula (1) satisfies 0.45 to 0.8, and the welded structure is a bainite structure. High strength UOE steel pipe with excellent low temperature toughness of seam weld metal.
Pcw = C + 0.09 × Si + 0.08 × Mn + 0.06 × Ni + 0.11 × Cr + 0.014 × Mo (1)
Here, C, Si, Mn, Ni, Cr, and Mo are mass% of each element. The high strength UOE steel pipe excellent in low temperature toughness of the seam weld metal according to claim 1, wherein Nb in the base metal is limited to 0.02% or less. % By mass
C: 0.04 to 0.1%,
Ni: 4-6%,
Cr: 1.8-3%,
Mo: 2 to 3.5%,
Ti: 0.02 to 0.04%
Containing
Nb: limited to 0.03% or less,
The high-strength UOE steel pipe excellent in low-temperature toughness of seam weld metal according to claim 1 or 2, wherein the weld metal is formed by submerged arc welding using a welding wire whose balance is Fe and inevitable impurities. .

Images