JP4964480B2 - High strength steel pipe excellent in toughness of welded portion and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength steel pipe excellent in toughness of a welded portion and a method for producing the same, and in particular, has a high strength of X80 to X100 according to the American Petroleum Institute (API) standard, and at a cryogenic temperature of −60 ° C. Further, the present invention relates to a high-strength steel pipe excellent in toughness of a welded portion (welded metal portion and weld heat affected zone: HAZ; Heat Affected Zone) and a method for producing the same.

  In recent years, line pipes used for pipelines for long-distance transportation of crude oil and natural gas realize (1) improved transport efficiency under high pressure, (2) improved local welding efficiency due to thinning, etc. For this reason, there is a tendency for higher tension. Up to now, line pipes up to X80 in the American Petroleum Institute (API) standard have been put into practical use. For the reasons described above, there is a need for pipelines having higher strength.

Conventionally, X100 high-strength linepipes have been studied based on the production method of X80 class linepipes (for example, see Non-Patent Documents 1 and 2), but such linepipes are low-temperature toughness, especially HAZ toughness. Therefore, an epoch-making high-strength steel pipe that solves such a problem is desired.
In recent years, with the depletion of resources, drilling and transportation projects of crude oil and natural gas in the Arctic region are being actively studied, and the HAZ toughness improvement of high strength steel pipes of X80 or higher at extremely low temperatures of -60 ° C is strong. It is desired.

The HAZ toughness of the low alloy steel is (1) crystal grain size, (2) MA constituent (mixture of martensite and austenite: hereinafter sometimes abbreviated as MA),
And (3) presence or absence of grain boundary embrittlement, (4) microsegregation of elements, and so on. Among them, the size of the HAZ crystal grains is known to have a great influence on the low-temperature toughness, and many techniques for refining the HAZ structure have been developed and put into practical use.

For example, a technique has been proposed in which TiN is finely dispersed to improve the HAZ toughness during high heat input welding of 490 MPa class high-strength steel (see, for example, Non-Patent Document 3).
However, in the steel described in Non-Patent Document 3, since the precipitates are exposed to a high temperature of 1400 ° C. or higher in the vicinity of the melting line, most of them are coarsened or melted, the HAZ structure is coarsened, and the HAZ toughness is reduced. There is a problem of deterioration.

  In order to solve the above-mentioned problems, Ti oxide is finely dispersed in steel, and intragranular acicular ferrite (hereinafter sometimes abbreviated as IGF) is generated in the HAZ at the time of welding. Techniques have been proposed for refining the HAZ structure and improving HAZ toughness.

In addition, as a measure for improving HAZ toughness, a large number of fine oxides composed of Mg and Al are dispersed in steel in order to suppress the growth of HAZ austenite (γ) grains near the melting line heated by heat exceeding 1400 ° C. A technology is proposed that combines the technology of compositely depositing 0.01 to 0.5 μm of TiN in the core and the technology of refining HAZ γ grains near the melting line by IGF generation. (For example, see Patent Document 3).
Japanese Patent Laid-Open No. 63-210235 Japanese Patent Laid-Open No. 1-15321 JP 2002-212670 A “NKK technique (No. 138)” Nippon Steel Pipe Co., Ltd., 1992, pp. 24-31 “The 7th offshore Mechanics Arc Engineering”, The American Society of Mechanical Engineers, 198 pp. 179-185 "Iron and Steel" Japan Steel Association, June 1979, Vol. 65, No. 8, p. 1232

  However, even with the above-described technique, the production of MA in HAZ cannot be completely suppressed at a high strength of X80 or higher, and HAZ toughness deteriorates. Therefore, there is a better technique as a measure for improving HAZ toughness. Was needed.

  The present invention has been made in view of the above circumstances, and in particular, it has a high strength of X80 to X100 according to the American Petroleum Institute (API) standard, and the toughness of the welded portion is excellent even at an extremely low temperature of −60 ° C. An object of the present invention is to provide a high-strength steel pipe and a manufacturing method thereof.

The gist of the present invention is as follows.
(1) By mass%, C: more than 0.03% and 0.10% or less, Si: 0.6% or less, Mn: 0.8% or more and 2.5% or less, P: 0.015% or less, S : 0.001% to 0.005%, Nb: 0.005% to 0.05%, Ti: 0.005% to 0.03%, Al: 0.005% or less, N: 0.00. 001% to 0.006%, O: 0.006% or less, Mg: 0.0001% to 0.005%, Ni: 0.1% to 1.0%, Cu: 0.1% to 1.0%, Cr: 0.1% to 1.0%, Mo: 0.1% to 1.0%, V: 0.01% to 0.1%, B : 0.0003% or more and 0.002% or less, Ca: 0.0005% or more and 0.005% or less, containing at least one or two or more, with the balance being iron And inevitable impurities, the following formula (1)
Pb = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + Mo + V (1)
A base material having a Pb value defined in the range of 2.3 to 3.5;
In mass%, C: 0.035% to 0.10%, Si: 0.6% or less, Mn: 1.5% to 2.2%, P: 0.015% or less, S: 0.00. 005% or less, Nb: 0.005% to 0.03%, Ti: 0.005% to 0.03%, B: 0.0003% to 0.002%, Al: 0.05% or less N: 0.001% to 0.01%; O: 0.015% to 0.050%; Mg: 0.0001% to 0.005%; Ni: 0.1 %: 2.5% or less, Cu: 0.1% or more, 1.0% or less, Cr: 0.1% or more, 1.5% or less, Mo: 0.1% or more, 1.5% or less, V: 0 0.01% or more and 0.03% or less, Ca: 0.001% or more and 0.005% or less, containing at least one or more of them, with the balance being iron It consists of fine inevitable impurities,
The following formula (2)
Pw = C + 0.11Si + 0.03Mn + 0.02Ni + 0.04Cr + 0.07Mo + 1.46Nb (2)
A Pw value defined in the range of 0.15 to 0.30, and a weld metal part in a range of Nb + V ≦ 0.03%,
The weld metal toughness of the weld zone characterized in that the volume fraction of the mixture of martensite and austenite (MA constituent) is less than 1% in the weld metal zone and the weld heat affected zone in the range from the melt line to 5 mm. Excellent high strength steel pipe.

(2) by forming a steel sheet having a composition as set forth in claim 1, followed after welding, a method of producing a high strength steel pipe expanding process is provided for performing tube expansion, the expanding process is the steel pipe After the inner and outer surfaces are welded, the weld metal affected zone and the weld heat affected zone in the range of 5 mm from the melt line are heated at a temperature in the range of 300 to 500 ° C. and held in this temperature range for 2 to 300 seconds. After that, a method for producing a high-strength steel pipe excellent in toughness of a welded portion, characterized in that it is air-cooled and then expanded .

  In order to ensure the strength of X80 to X100 when applying the low C—Nb—Ti component steel as the base material of the steel pipe, the present inventor makes the alloy element addition amount of the base material and the weld metal within an appropriate range. The amount of Nb + V of the weld metal part was limited to an appropriate range. In addition, after welding, the welded part is heated to a temperature range of 300 to 500 ° C., held in this temperature range for 2 seconds to 300 seconds, and then air-cooled, which is harmful to the toughness in the weld metal part and HAZ. It was found that the MA was reduced and the HAZ toughness at an extremely low temperature of −60 ° C. was improved, and the present invention was completed.

According to the high-strength steel pipe excellent in toughness of the welded portion and the manufacturing method thereof according to the present invention, the MA in the weld metal portion and the HAZ is reduced by the above configuration. Thereby, the high strength steel pipe of X80-X100 by API specification excellent in the toughness of a welded part can be obtained.
When the high-strength steel pipe excellent in toughness of the welded portion of the present invention is adopted in a pipeline in an extremely cold region, the safety of the pipeline is remarkably improved and the transportation efficiency can be drastically improved. Become.

Hereinafter, embodiments of a high-strength steel pipe excellent in toughness of a weld according to the present invention and a manufacturing method thereof will be described.
In addition, since this embodiment is described in detail for better understanding of the gist of the invention, the present invention is not limited unless otherwise specified.

  According to the earnest study by the present inventors, the weld heat-affected zone (HAZ) toughness is greatly increased in (1) the chemical composition of the steel and (2) the structure (the size of the crystal grains and the dispersion state of the hardened phase). Therefore, in order to improve the HAZ toughness, it is considered indispensable to optimize the steel components and refine the crystal grains, particularly to reduce the hardened phase such as MA.

  In order to reduce welding costs, a steel pipe manufactured by welding the inner and outer surfaces of a steel plate (for example, a UOE steel pipe or the like) is welded with one inner surface and one outer surface. In this case, the toughness of the region (coarse grains + Ac1 part) in which the vicinity of the melting line of the inner surface welding is heated to a temperature of 1400 ° C. or higher and is reheated immediately after the outer surface welding to the Ac1 temperature is the lowest. This is because carbon (C) is concentrated in a region transformed into austenite (γ) when reheated immediately above the Ac1 temperature, and a large amount of cured phase such as MA containing a large amount of C is contained in the subsequent cooling process. It is for generating to. Since M-A contains a large amount of C, it is hard and easily becomes a point of occurrence of brittle fracture. In order to increase the strength of steel, it is inevitably necessary to increase the amount of alloying element added. However, the amount of MA produced increases in coarse grains and Ac1 part, and the HAZ toughness greatly deteriorates.

Therefore, as a result of intensive studies on a method for suppressing the formation of MA in order to prevent deterioration of toughness in the coarse grains + Ac1 part, the present inventors have determined that after the welding on the outer surface side is finished, the weld metal part and the molten wire The heat-affected zone of 3 to 5 mm is heated and held in a temperature range of 300 to 500 ° C. for a time of 2 seconds to 300 seconds, and then air-cooled, whereby MA which is harmful to toughness is tempered and hardened. Has been found to be extremely effective in improving HAZ toughness.
Moreover, when the welding heat affected zone from the weld metal part and the melt line to 5 mm is heated in the temperature range of 300 to 500 ° C. for 2 seconds or more and 300 seconds or less after the outer surface welding is finished, The precipitation of Nb and V contained increases the hardness and lowers the low temperature toughness. Therefore, by limiting the amount of Nb + V added to the weld metal part, it is possible to prevent toughness deterioration of the weld metal part. I found it.

  That is, the feature of the present invention is that when a low C—Nb—Ti component steel material is applied as a base material of a steel pipe, the alloy element addition amount to the base material is expressed by a Pb value in order to ensure the strength of X80 to X100. It is limited to the proper range defined, the alloy addition amount of the weld metal part is limited to the proper range defined by the Pw value, and the Nb + V amount is limited to the proper range. M- which is harmful to toughness in weld metal parts and HAZ by heating and holding in the temperature range of 300 to 500 ° C. for 2 seconds to 300 seconds in the temperature range from the metal part and the melting line to 5 mm, and then air-cooling. A is to temper A and to improve the HAZ toughness at an extremely low temperature of −60 ° C.

  The present inventors have found that the toughness is significantly improved when the volume ratio of MA is less than 1% in the HAZ from the weld metal part and the melt line to 5 mm. Thus, in order to improve the toughness by reducing the amount of MA generated in the weld metal part and HAZ, the weld part is heated in the temperature range of 300 to 500 ° C. for 2 seconds or more and 300 seconds or less after welding. After that, it is necessary to air-cool. Heating at a temperature lower than 300 ° C. has little effect of tempering MA, and when it exceeds 500 ° C., precipitation strengthening elements such as Nb and V are precipitated, and carbides are coarsened to increase the toughness of the weld metal part. Since it has an adverse effect, the heating temperature was limited to a range of 300 to 500 ° C. Furthermore, if the holding time is less than 2 seconds, the effect of tempering MA is small, and if it exceeds 300 seconds, not only the productivity is lowered, but also the carbide is coarsened and adversely affects the toughness of the weld metal part. The holding time during heating was limited to the range of 2 to 300 seconds.

  The reason for air-cooling the weld after heating is to prevent cracking of the weld. The reason why the range from the weld metal and the melt line to 5 mm is heated is that the generation of MA is most remarkable in the range from the melt line to 5 mm. In addition, although a welding part can be heated by a high frequency, width radiation heating, etc., the heating method is not specifically limited, It can employ | adopt suitably.

[Composition composition of high-strength steel pipe with excellent weld toughness]
The high-strength steel pipe excellent in the toughness of the welded portion of the present invention (hereinafter sometimes abbreviated as high-strength steel pipe) is mass%, C: more than 0.03% and 0.10% or less, Si: 0.6 % Or less, Mn: 0.8% or more and 2.5% or less, P: 0.015% or less, S: 0.001% or more and 0.005% or less, Nb: 0.005% or more and 0.05% or less, Ti: 0.005% or more and 0.03% or less, Al: 0.005% or less, N: 0.001% or more and 0.006% or less, O: 0.006% or less, and Mg: 0 0.001% to 0.005%, Ni: 0.1% to 1.0%, Cu: 0.1% to 1.0%, Cr: 0.1% to 1.0%, Mo : 0.1% to 1.0%, V: 0.01% to 0.1%, B: 0.0003% to 0.002%, Ca: 0.000 % 0.005% or more of the following, contain more than at least one or two, the balance being iron and unavoidable impurities,
The following formula (1)
Pb = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + Mo + V (1)
A base material having a Pb value defined in the range of 2.3 to 3.5;
In mass%, C: 0.035% to 0.10%, Si: 0.6% or less, Mn: 1.5% to 2.2%, P: 0.015% or less, S: 0.00. 005% or less, Nb: 0.005% to 0.03%, Ti: 0.005% to 0.03%, B: 0.0003% to 0.002%, Al: 0.05% or less N: 0.001% to 0.01%; O: 0.015% to 0.050%; Mg: 0.0001% to 0.005%; Ni: 0.1 %: 2.5% or less, Cu: 0.1% or more, 1.0% or less, Cr: 0.1% or more, 1.5% or less, Mo: 0.1% or more, 1.5% or less, V: 0 0.01% or more and 0.03% or less, Ca: 0.001% or more and 0.005% or less, containing at least one or more of them, with the balance being iron It consists of fine inevitable impurities,
The following formula (2)
Pw = C + 0.11Si + 0.03Mn + 0.02Ni + 0.04Cr + 0.07Mo + 1.46Nb (2)
Pw value defined in the range of 0.15 to 0.30 and Nb + V ≦ 0.03% of the weld metal part, 5 mm from the weld metal part and the melting line In the welding heat-affected zone in the range up to, the volume ratio of the mixture of martensite and austenite (MA constituent) is less than 1% and is roughly configured.

“Component composition of the base material”
Hereinafter, the reasons for limiting the component composition of the base material of the high-strength steel pipe excellent in toughness of the weld according to the present invention will be described.

(C: more than 0.03% and 0.10% or less)
C needs to be added in an amount exceeding 0.03% in order to ensure the strength and toughness of the base material and the HAZ. However, if it exceeds 0.10%, the toughness of the base metal and the HAZ is lowered and the weldability is deteriorated, so 0.10% was made the upper limit.

(Si: 0.6% or less)
Si is an element added for deoxidation and strength improvement of the base material, but if added in a large amount, the field weldability and the HAZ toughness deteriorate, so the upper limit was made 0.6%. For the deoxidation of steel, only Ti described later is sufficient, and Si does not necessarily have to be added.

(Mn: 0.8% to 2.5%)
Mn is an element indispensable for securing the strength of the base material and the low temperature toughness, and its lower limit is 0.8%. However, if the amount of Mn is too large, not only the hardenability of the steel will increase and the field weldability and HAZ toughness will deteriorate, but also the center segregation of the continuously cast steel piece will be promoted, and the low temperature toughness will also deteriorate. The upper limit was 2.5%.

(P: 0.015% or less)
P is an unavoidable impurity in the present invention, and its content is 0.015% or less. The reason why the content of P is described above is to further improve the low temperature toughness of the base material and the HAZ. Reduction of the amount of P reduces the center segregation of a continuous casting slab, prevents a grain boundary fracture, and improves low temperature toughness.

(S: 0.001% or more and 0.005% or less)
S is an important element in the high-strength steel pipe of the present invention. In order to precipitate a sulfide on the oxide as an IGF transformation nucleus, it must contain 0.001% or more of S. However, if the S content exceeds 0.005%, the toughness of the base material and the HAZ deteriorates, so the upper limit was made 0.005%.

(Nb: 0.005% to 0.05%)
Nb not only suppresses recrystallization of ν during controlled rolling and refines crystal grains, but also contributes to precipitation hardening and hardenability, and has the effect of strengthening steel. It is an essential element. In order to obtain such an effect, 0.005% or more of Nb needs to be added. However, if the amount of Nb is too large, the HAZ toughness deteriorates, so the upper limit was made 0.05%.

(Ti: 0.005% to 0.03%)
Ti is an indispensable element in the present invention that forms fine TiN, refines the microstructure by suppressing coarsening of ν slabs during slab reheating and HAZ, and improves the low temperature toughness of the base material and HAZ. is there. In order to exhibit this effect, an addition amount of 0.005% or more is necessary. Further, if the amount of Ti is too large, TiN becomes too large or precipitation hardening due to TiC occurs, so that the low temperature toughness is deteriorated, so the upper limit was made 0.03%.

(Al: 0.005% or less)
Al usually has an effect as a deoxidizing element. 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: 0.001% to 0.006%)
N forms TiN and suppresses coarsening of the ν grains of HAZ during reheating of the slab and improves the low temperature toughness of the base material and HAZ. The minimum amount necessary to obtain such an effect is 0.001%. However, if the amount of N is too large, it will cause deterioration of the HAZ toughness due to slab surface flaws or solute N, so the upper limit must be limited to 0.006%.

(O: 0.006% or less)
Since O is an unavoidable impurity that deteriorates the toughness of the steel pipe, it is preferably as small as possible. In particular, if the amount of O exceeds 0.006%, a large amount of coarse oxide is generated in the steel, and the toughness deteriorates, so the upper limit was made 0.006%.

  Next, the reason for adding at least one or more of Mg, Ni, Cu, Cr, Mo, V, B, and Ca to the base material of the high strength steel pipe excellent in toughness of the welded portion of the present invention Will be described. The main purpose of adding these elements to the basic components of the base material is to improve the properties such as strength and low temperature toughness without impairing the characteristics of the steel of the present invention. Therefore, the amount added is a property that should be limited by itself.

(Mg: 0.0001% to 0.005%)
Mg forms a fine oxide of Al and Mg, and fine TiN is produced using this oxide as a production nucleus. Since this TiN is chemically stable even at a high temperature of 1400 ° C. or higher, it exhibits the effect of suppressing the coarsening of γ grains and improves the HAZ toughness. When the amount of Mg is 0.0001% or less, the above-described effect is small. Further, if the amount of Mg added is too large, the HAZ toughness is deteriorated, so the upper limit was made 0.005%.

(Ni: 0.1% or more and 1.0% or less)
Ni improves the strength and low-temperature toughness of the base material without adversely affecting weldability and HAZ toughness, but the effect is small at an addition amount of 0.1% or less, and addition of 1.0% or more Since it is not preferable for weldability, the upper limit was made 1.0%.

(Cu: 0.1% to 1.0%)
Cu has substantially the same effect as Ni, and is also effective in corrosion resistance, resistance to hydrogen-induced cracking, etc., and needs to be added in an amount of 0.1% or more. However, if Cu is added excessively, the toughness of the base material and HAZ deteriorates due to precipitation hardening, and Cu-cracks are generated during hot rolling, so the upper limit was made 1.0%.

(Cr: 0.1% to 1.0%)
Cr has the effect of increasing the strength of the base material and the welded portion, and it is necessary to add 0.1% or more. However, when there is too much addition amount of Cr, field weldability and HAZ toughness will deteriorate remarkably. For this reason, the upper limit of the Cr content was set to 1.0%.

(Mo: 0.1% to 1.0%)
Mo is an element that increases the strength of the base metal and the welded portion. However, when it exceeds 1.0%, the toughness and weldability of the base material and the HAZ are deteriorated similarly to Cr. Further, when the addition amount is 0.1% or less, the above-described effect is small.

(V: 0.01% to 0.1%)
V has substantially the same effect as Nb, but the effect is much weaker than Nb. In order to exhibit the effect, addition of 0.01% or more is necessary. Further, the upper limit is acceptable up to 0.1% from the viewpoint of on-site weldability and HAZ toughness.

(B: 0.0003% or more and 0.0050% or less)
B is a very small amount, which dramatically enhances the hardenability of the steel and provides good strength and toughness. In order to exhibit this effect, addition of 0.0003% or more is necessary. Moreover, since HAZ toughness will deteriorate when there is too much addition amount of B, the upper limit was made into 0.002%.

(Ca: 0.0005% or more and 0.005% or less)
Ca controls the form of sulfide (MnS) and improves low-temperature toughness (increased absorbed energy in the Charpy test, etc.), and also exhibits a remarkable effect in improving sour resistance. When the addition amount of Ca is 0.0005% or less, the above-described effect is small. When the addition amount exceeds 0.005%, a large amount of CaO—CaS is formed to form clusters and large inclusions, and the cleanliness of steel. Not only harms, but also adversely affects on-site weldability. Therefore, the Ca addition amount is limited to 0.0005% or more and 0.005% or less.

(Pb value: 2.3 to 3.5)
In order to satisfy the strength of the steel pipe as the target strength of X80 to X100, it is necessary to optimize the addition amount of the alloy element. That is, the Pb value defined by the following formula {Pb = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + Mo + V} needs to be in the range of 2.3 to 3.5.
If the Pb value is less than 2.3, the target strength of X80 or higher cannot be secured. On the other hand, when the Pb value exceeds 3.5, the formation of MA becomes remarkable and the HAZ toughness deteriorates. For this reason, the range of Pb value was limited to 2.3-3.5.

“Composition composition of weld metal”
Hereinafter, the reason for limiting the component composition of the weld metal part of the high strength steel pipe excellent in the toughness of the weld part according to the present invention will be described.

(C: more than 0.035% and 0.10% or less)
In order to prevent hot cracking of the weld metal part, C needs to be added in an amount of 0.035% or more. This is because if the amount of C is less than 0.035%, δ solidification occurs in the process of solidification after welding and hot cracking occurs. However, if the C content exceeds 0.10%, the low temperature toughness of the weld metal part deteriorates, so the upper limit was made 0.10%.

(Si: 0.6% or less)
S is an element added for deoxidation and strength improvement of the weld metal part, but if added in a large amount, low temperature toughness and on-site weldability deteriorate, so the upper limit was made 0.6%.

(Mn: 1.5% to 2.2%)
Mn is an element indispensable for ensuring the strength and low temperature toughness of the weld metal part, and its lower limit is 1.5%. However, if the amount of Mn added is too large, the hardenability of the steel increases and the low temperature toughness and on-site weldability deteriorate, so the upper limit was made 2.2%.

(Nb: 0.005% to 0.03%)
Nb has the effect | action which strengthens steel, and the addition amount of 0.005% or more is required. However, if Nb is added in excess of 0.03%, it will adversely affect on-site weldability and toughness, so the upper limit was made 0.03%.

(Ti: 0.005% to 0.03%)
Ti forms fine TiN and improves the low temperature toughness of the weld metal part. In order to develop such an effect of TiN, a Ti addition amount of at least 0.005% is necessary. However, if the amount of Ti is too large, TiN coarsening or precipitation hardening due to TiC occurs and the low temperature toughness deteriorates, so the upper limit must be limited to 0.03%.

(B: 0.0003% or more and 0.002% or less)
B is an element that dramatically increases the hardenability of steel by adding a very small amount. In order to obtain such an effect, B must be added in an amount of at least 0.0003%. On the other hand, when B is added excessively, not only the low temperature toughness is deteriorated, but also the effect of improving the hardenability of B may be lost, so the upper limit was made 0.002%.

(Al: 0.05% or less)
Al usually has an effect as a deoxidizing element. However, if the Al content exceeds 0.05%, Al non-metallic inclusions increase to impair the cleanliness of the steel, so the upper limit was made 0.05%.

(N: 0.001% to 0.01%)
N forms TiN and improves the low temperature toughness of the weld metal part. The minimum amount of N required for this is 0.001%. However, if the amount of N is too large, the low temperature toughness is deteriorated, so the upper limit must be suppressed to 0.01%.

(O: 0.015% to 0.050%)
O forms an oxide in the weld metal part, acts as a nucleus of intragranular transformed ferrite, and is effective in refining the structure. However, if the amount of O is too large, the low temperature toughness of the weld metal deteriorates and welding defects such as slag entrainment occur. For this reason, the lower limit of the amount of O is set to 0.015%, and the upper limit is set to 0.050%.

(P: 0.015% or less, S: 0.005% or less)
In the high-strength steel pipe of the present invention, the amounts of impurity elements P and S contained in the weld metal part are set to 0.015% or less and 0.005% or less, respectively.
The main reason why the contents of P and S are in the above range is to further improve the low temperature toughness. Reduction of the P content has the effect of preventing grain boundary fracture and improving low temperature toughness. Moreover, reduction of the amount of S has the effect of reducing MnS and improving toughness.

  Next, the reason why at least one or two or more of Mg, Ni, Cu, Cr, Mo, V, and Ca are added to the weld metal portion of the high-strength steel pipe excellent in toughness of the weld portion of the present invention. explain. The main purpose of adding these elements to the basic components of the weld metal part as required is to improve the properties such as strength and low-temperature toughness without impairing the characteristics of the steel of the present invention. Therefore, the amount added is a property that should be limited by itself.

(Mg: 0.0001% to 0.005%)
Mg has the effect of improving the toughness by refining the solidification structure of the weld metal part, but there is no effect when the amount of Mg added is 0.0001% or less. Moreover, since the toughness of a weld metal part will deteriorate when there is too much addition amount of Mg, the upper limit was made into 0.005%.

(Ni: 0.1% to 2.5%)
Ni is added for the purpose of increasing the strength without deteriorating the low temperature toughness and on-site weldability of the weld metal part. However, if there is too much Ni addition amount, not only economical efficiency but low temperature toughness etc. will be deteriorated, so the upper limit was made 2.5% and the lower limit was made 0.1%.

(Cu: 0.1% to 1.0%)
Cu, like Ni, increases the strength of the weld metal part without degrading the low-temperature tough field weldability. However, if Cu is added excessively, the low temperature toughness deteriorates, so the upper limit was made 1.0%. The numerical value of 0.1% defined as the lower limit of Cu is the minimum value at which the effect on the material due to the addition of Cu becomes remarkable.

(Cr: 0.1% to 1.5%)
Cr increases the strength of the weld metal part. However, if the amount of Cr added is too large, the low temperature toughness and on-site weldability are significantly deteriorated. For this reason, the upper limit of the Cr amount is 1.5%, and the lower limit is 0.1%.

(Mo: 0.1% to 1.5%)
Mo is an element added to improve the hardenability of the steel. To obtain this effect, an addition amount of at least 0.1% is necessary. However, excessive Mo addition deteriorates low temperature toughness and on-site weldability, so the upper limit was made 1.5%.

(V: 0.01% to 0.03%)
V has almost the same effect as Nb, but the effect is weaker than Nb. V has the effect of increasing the strength by strain-induced precipitation. In order to obtain the sufficient effect, the lower limit of the addition amount needs to be 0.01%. Further, the upper limit of the amount of V is allowable up to 0.03% from the viewpoint of on-site weldability and low temperature toughness.

(Ca: 0.001% to 0.005%)
Ca controls the form of sulfide (MnS) and improves low temperature toughness (increased absorbed energy in the Charpy test, etc.). However, when the Ca content is 0.001% or less, there is no practical effect. When the Ca content exceeds 0.005%, a large amount of CaO—CaS is generated and a weld defect is generated. For this reason, Ca addition amount was limited to 0.001% or more and 0.005% or less.

(Pw value: 0.15 to 0.30)
In order to satisfy the target strength of X80 to X100 in the weld metal part, it is necessary to optimize the amount of alloying element added. That is, the Pw value defined by the following formula {Pw = C + 0.11Si + 0.03Mn + 0.02Ni + 0.04Cr + 0.07Mo + 1.46Nb} needs to be in the range of 0.15 to 0.30.
If the Pw value is less than 0.15, the target weld strength of X80 or higher cannot be secured. Moreover, when Pw value exceeds 0.30, the production | generation of MA becomes remarkable, while toughness deteriorates, a low temperature crack generate | occur | produces. For this reason, the range of the Pw value was limited to 0.15 to 0.30.

(Nb + V ≦ 0.03%)
In the present invention, it is necessary that the weld metal part has the above composition, and the relationship between the addition amounts of Nb and V is in the range of Nb + V ≦ 0.03%.
When Nb and V contained in the weld metal part are precipitated, the hardness is increased and the low temperature toughness is lowered. However, by adjusting the amount of Nb + V added to the weld metal part within the above range, Toughness is improved.

“Volume ratio of mixture of martensite and austenite (MA): less than 1%”
In the high strength steel pipe excellent in toughness of the welded portion of the present invention, the base metal and the weld metal portion have the above-mentioned composition, and in the weld metal affected zone and the weld heat affected zone in the range from the melt line to 5 mm, The volume ratio of MA, which is a mixture of sites and austenite, is less than 1%. By reducing the harmful MA to the improvement of the toughness, the HAZ toughness is improved, and high strength steel pipes of X80 to X100 according to API standards can be obtained. In particular, by improving the low temperature toughness of HAZ at an extremely low temperature of −60 ° C., when the high-strength steel pipe of the present invention is used in a pipeline in a cold region, the safety of the pipeline is remarkably improved and transportation is performed. The efficiency can be dramatically improved.

[Method for producing high-strength steel pipe with excellent weld toughness]
The manufacturing method of the high strength steel pipe excellent in the toughness of the welded portion of the present invention is a manufacturing method of a high strength steel pipe provided with a pipe expanding step of forming a steel sheet having the above-mentioned component composition and then welding and then expanding the pipe. In the pipe expanding step, after the inner and outer surfaces of the steel pipe are welded, the weld heat affected zone in the range from the weld metal part and the melt line to 5 mm is heated at a temperature in the range of 300 to 500 ° C., and this temperature range The method is schematically configured as a method in which the air is cooled for 2 seconds to 300 seconds and then air-cooled and then expanded .
Hereinafter, each limited item of the manufacturing method of the high strength steel pipe of this invention is demonstrated.

"Heating temperature: 300-500 ° C"
In the present invention, after the inner and outer surfaces of the steel pipe are welded, the welded heat affected zone in the range from the weld metal portion and the melt line to 5 mm is heated at a temperature in the range of 300 to 500 ° C.
As described above, in order to improve the toughness by reducing the amount of MA generated in the weld metal part and HAZ, it is necessary to heat the weld part in a temperature range of 300 to 500 ° C. after welding. Heating at a temperature lower than 300 ° C. has little effect of tempering MA, and when it exceeds 500 ° C., precipitation strengthening elements such as Nb and V are precipitated, and carbides are coarsened to increase the toughness of the weld metal part. Adversely affect.
In the production method of the present invention, the amount of MA produced can be reduced in the range where the production of MA is most prominent by heating the weld metal part and the range from the melt line to 5 mm.
In addition, as a heating method of a welding heat affected zone, although it can carry out by a high frequency, width radiation heat, etc., a heating method is not specifically limited, It can employ | adopt suitably.

"Holding time during heating: 2 to 300 seconds"
In the present invention, the holding time when heating the weld heat affected zone in the above temperature range is set to 2 seconds to 300 seconds. When the holding time at the time of heating is less than 2 seconds, the effect of tempering MA is small, and when it exceeds 300 seconds, not only the productivity is lowered, but also the carbide is coarsened and adversely affects the toughness of the weld metal part. Therefore, the holding time is set in the above range.

"Air cooling after heating"
In the present invention, after the welding heat-affected zone is heated within the above temperature range and holding time, the welding heat-affected zone is air-cooled. By cooling the weld heat affected zone with air after heating, the welded portion can be prevented from cracking.

  As described above, the high-strength steel pipe obtained by the production method of the present invention improves the HAZ toughness by reducing MA, and the strength of X80 to X100 is obtained according to API standards. It can be obtained as a steel pipe excellent in low temperature toughness of HAZ at an extremely low temperature.

  Hereinafter, examples of the high-strength steel pipe excellent in toughness of the welded portion according to the present invention will be given and the present invention will be described more specifically. However, the present invention is not limited to the following examples, It is also possible to carry out the present invention with appropriate modifications within a range that can be adapted to the gist described below, and these are all included in the technical scope of the present invention.

[Sample preparation]
Table 1 and Table 2 below have the chemical composition, and steel plates manufactured from steel slabs of each component by a converter-coupled casting method, and the plate thickness is 12 to 38 mm depending on the manufacturing conditions shown in Table 3 below. Steel pipes of the present invention steel (steel codes 1 to 12) and comparative steel (steel codes 13 to 22), which are made in the range of After heating and holding at this temperature for a time in the range of 3 to 320 seconds, air cooling was performed, and then tube expansion was performed, and various properties were investigated in an evaluation test described below.

[Evaluation test]
First, the mechanical properties of the base material are as follows: a round bar tensile test piece having a diameter of 12.7 mm and a gauge length of 50.8 mm is taken from the circumferential direction of the steel pipe and a tensile test is performed. A Charpy test was performed using test pieces collected from the 2t site, and the results are shown in Table 3 below.
In addition, after one layer of SAW (submerged arc welding) was performed on the inner and outer surfaces of the steel pipe, a Charpy test was performed using test pieces collected from the 1/2 t portion of the thickness of each steel. The notch positions were the center of the weld metal part and HAZ (position of 1 mm from the point where the weld metal of inner surface welding and outer surface welding intersects), and the results are shown in Table 3 below.
Also, the MA fraction of the weld metal part is determined by multiplying the position of the center of the weld metal part 1/4 t by 1000 times after the sample is polished and corroded with a corrosive solution that reveals MA. After taking a picture, it was obtained by an image analyzer and shown in Table 3 below.
In addition, HAZ MA is polished at a position 0.5 mm, 1 mm, 3 mm, and 5 mm from the 1/4 t melt line after the sample is polished and corroded with a corrosive solution that reveals MA. Was photographed at a magnification of 1000 times and then obtained by an image analyzer, and the maximum value of these values is shown in Table 3 below.

The component composition of each steel pipe of the present invention steel (steel codes 1 to 12) and comparative steel (steel codes 13 to 22) is shown in Tables 1 to 2, and the production conditions and the results of each evaluation test are shown in Table 3. .
Tables 1 and 2 show chemical components of the steel pipe base material and the weld metal, and Table 3 shows the heating temperature, holding time, and cooling method of the welded part, as well as the mechanical properties of the steel pipe base material and the steel pipe welded part. The mechanical properties of

  As is apparent from the results shown in Tables 1 and 3, the high-strength steel pipes of the present invention (present invention steels 1 to 12) are excellent in the strength (YS, TS) of the base metal and the low temperature toughness, and are welded. It turns out that it is excellent in the toughness of a metal part and HAZ.

In contrast, the comparative steels 13 to 22 (conventional steel pipes) are inferior in characteristics because the chemical components and production conditions are not appropriate.
The comparative steel 13 does not satisfy the target strength because the Pb value of the base material is too low.
Moreover, since the Pb value of the base material is too high, the comparative steel 14 is inferior in HAZ toughness.
Moreover, since the comparative steel 15 has too low Pw value of a weld metal part, the intensity | strength of a weld part is low.
Moreover, since the comparative steel 16 has too high Pw value of a weld metal part, the toughness of this weld metal part is inferior.

Since the comparative steel 17 has too much Nb + V in the weld metal part, the toughness of this weld metal is inferior.
Moreover, the comparative steel 18 is an example in which the heating temperature is too low and the MA fraction is high and good low-temperature toughness cannot be obtained.
Moreover, since the comparative steel 19 has too high heating temperature, the toughness of a weld metal part is inferior.

The comparative steel 20 is an example in which the retention time after heating is too short, the MA fraction is high, and good low temperature toughness was not obtained.
Moreover, since the comparison steel 21 has a too long holding time after heating, the toughness of the weld metal part is inferior.
Moreover, since the comparative steel 22 was water-cooled after heating, a crack occurred in the weld.

From the above results, it is clear that the high-strength steel pipe excellent in toughness of the welded portion of the present invention is excellent in mechanical properties. In addition, when the high-strength steel pipe of the present invention (API standard: X80 to X100) is adopted in a pipeline in a very cold region, it is clear that the safety of the pipeline is remarkably improved and the transportation efficiency can be dramatically improved. It is.

Claims (2)

In mass%, C: more than 0.03% and 0.10% or less, Si: 0.6% or less, Mn: 0.8% or more and 2.5% or less, P: 0.015% or less, S: 0.0. 001% or more and 0.005% or less, Nb: 0.005% or more and 0.05% or less, Ti: 0.005% or more and 0.03% or less, Al: 0.005% or less, N: 0.001% or more 0.006% or less, O: 0.006% or less,
Further, Mg: 0.0001% to 0.005%, Ni: 0.1% to 1.0%, Cu: 0.1% to 1.0%, Cr: 0.1% to 1.0% 0% or less, Mo: 0.1% to 1.0%, V: 0.01% to 0.1%, B: 0.0003% to 0.002%, Ca: 0.0005% to 0 0.005% or less, containing at least one or more, and the balance consisting of iron and inevitable impurities,
The following formula (1)
Pb = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + Mo + V (1)
A base material having a Pb value defined in the range of 2.3 to 3.5;
In mass%, C: 0.035% to 0.10%, Si: 0.6% or less, Mn: 1.5% to 2.2%, P: 0.015% or less, S: 0.00. 005% or less, Nb: 0.005% to 0.03%, Ti: 0.005% to 0.03%, B: 0.0003% to 0.002%, Al: 0.05% or less N: 0.001% to 0.01%, O: 0.015% to 0.050%,
Furthermore, Mg: 0.0001% to 0.005%, Ni: 0.1% to 2.5%, Cu: 0.1% to 1.0%, Cr: 0.1% to 1. 5% or less, Mo: 0.1% to 1.5%, V: 0.01% to 0.03%, Ca: 0.001% to 0.005%, at least one or two Contains more than seeds, the balance consists of iron and inevitable impurities,
The following formula (2)
Pw = C + 0.11Si + 0.03Mn + 0.02Ni + 0.04Cr + 0.07Mo + 1.46Nb (2)
A Pw value defined in the range of 0.15 to 0.30, and a weld metal part in a range of Nb + V ≦ 0.03%,
The weld metal toughness of the weld zone characterized in that the volume fraction of the mixture of martensite and austenite (MA constituent) is less than 1% in the weld metal zone and the weld heat affected zone in the range from the melt line to 5 mm. Excellent high strength steel pipe. Molding a steel sheet having a composition as set forth in claim 1, followed after welding, a method of producing a high strength steel pipe expanding process is provided for performing tube expansion,
In the pipe expanding step, after welding the inner and outer surfaces of the steel pipe, the weld metal affected zone and the weld heat affected zone in the range from the melt line to 5 mm are heated at a temperature in the range of 300 to 500 ° C., and this temperature range is 2 seconds. A method for producing a high-strength steel pipe excellent in toughness of a welded portion, characterized in that after being held for 300 seconds or less, air-cooled and then expanded .