JP5857491B2 - Low yield ratio resistant HIC welded steel pipe with excellent weld toughness after SR and method for producing the same - Google Patents

Description

  It relates to a welded steel pipe and its manufacturing method used for plant piping of petroleum refining equipment and the like under a wet hydrogen sulfide environment, and particularly 350 to 550 MPa that achieves both weld metal toughness, sour resistance and low yield ratio after SR and PWHT. The present invention relates to a class welded steel pipe and a method for manufacturing the same.

  The quality of crude oil is decreasing year by year, and the hydrogen sulfide concentration is increasing. Therefore, the resistance to wet hydrogen sulfide corrosion stress, that is, excellent resistance to hydrogen induced cracking (HIC) and resistance to sulfide stress corrosion cracking (SSC) (both of these combined) Called sex). In addition, since the plant piping uses steel pipes of various shapes attached by various joint manufacturing methods, if only some of the steel pipes have a high yield ratio, stress concentration will occur in the surrounding steel pipes and joints. If defects exist that could not be found by inspection at such sites, there is a possibility of destruction from those sites.

  Therefore, in plant piping, it is common to use a welded steel pipe having a yield ratio of 90% or less in order to reduce strength heterogeneity with surrounding members. In addition, a welded steel pipe used as a plant pipe is generally subjected to stress relief annealing (SR) in order to reduce residual stress generated by pipe making, and is also referred to as post-weld heat treatment (PWHT) after welding with the surrounding members. A local SR to the weld is performed. Therefore, it is important to ensure the characteristics after being subjected to such heat treatment, in particular, the yield ratio and the weld zone toughness.

  Many studies have been made to ensure sour-resistant performance mainly in the line pipe field. To improve HIC characteristics, the amount of second-phase structure is reduced by lowering C, and Mn-S is lower. Generally, techniques such as reduction of elongation MnS due to reduction, reduction of center segregation due to low Mn-low P, spheroidization of MnS by optimization of Ca addition amount, and suppression of Ca cluster formation are generally used. On the other hand, in order to improve SSC characteristics, it is effective to reduce the surface layer hardness of the base metal, HAZ, and weld metal, and the hardenability is reduced due to the reduction of alloy elements and the optimum cooling and quenching conditions are optimized. Generally, reduction of surface hardness by tempering and tempering is performed.

  On the other hand, various investigations have also been made in the field of thick steel plates and welded steel pipes for plant facilities that perform stress relief annealing, such as pressure vessels and fitting members, as well as plant piping.

  For example, in Patent Documents 1 and 2, a low CB-free system is selected as a component system that effectively reduces the HAZ hardness while reducing a decrease in the strength of the base material, and Nb is added to accelerate immediately after controlled rolling. A method is disclosed in which the lack of strength is compensated for by precipitation strengthening by performing cooling or direct quenching + tempering.

  Patent Documents 3 and 4 disclose a method for achieving both sour resistance performance and hot workability by performing normalization on a thick steel plate whose structure is uniformly refined by controlled rolling. Patent Document 5 discloses a method of improving the SSC resistance by reducing the surface hardness after quenching and tempering by optimizing the rolling heating temperature of the B-added steel.

  Patent Document 6 discloses a method for improving the HIC resistance and the SSC resistance by obtaining a uniform bainite structure by quenching immediately after bending for the manufacture of a fitting member that undergoes a hot forming process. ing. Further, regarding the welded portion characteristics of the steel plate for pressure vessels, Patent Document 7 and Patent Document 8 disclose a method for suppressing the increase in hardness and optimizing the SSC resistance of the welded portion by optimizing the chemical composition of the weld metal. ing.

Japanese Patent Laid-Open No. 2-8322 JP-A-2-263918 Japanese Patent Laid-Open No. 8-283839 JP-A-8-283840 JP 59-74219 JP 7-87225 A Japanese Patent Laid-Open No. 4-120240 Japanese Patent Laid-Open No. 5-200583

  However, the thick steel plate produced by direct quenching or TMCP disclosed in Patent Documents 1 and 2 undergoes hot transformation because the microstructure refined by rolling is retransformed when hot working is performed. Even if heat treatment is performed after processing, the originally obtained strength-toughness balance cannot be obtained, and the stretched structure formed by controlled rolling serves as a HIC crack propagation path, so that HIC characteristics can be secured. There is a problem that it is difficult.

  In Patent Documents 3 and 4, the HIC resistance performance and the hot working performance are compatible by making a fine uniform structure over the entire thickness by the normalization process, but the manufacturing method is difficult to ensure the strength. In order to ensure, a large amount of alloy is required, which not only increases the cost, but also increases the HAZ hardness, which causes a problem of deterioration in SSC resistance.

  In Patent Document 5, the surface layer hardness of the base material can be effectively reduced, but since the optimum amount of C is high and B addition is essential, it is difficult to reduce the HAZ hardness in this range. Yes, the generation of SSC starting from HAZ cannot be avoided.

  Patent Document 6 uses a quenching and tempering process to improve HIC resistance and SSC resistance, but does not disclose a method for achieving both a yield ratio and weld toughness. Further, Patent Documents 7 and 8 disclose methods for ensuring sour resistance performance including not only the base material but also the welded portion, and these also aim at achieving both the yield ratio and the toughness of the welded portion. A method is not disclosed.

  On the other hand, many studies have been disclosed for thick steel plates and welded steel pipes with low yield ratios mainly in the field of construction and line pipes, but in these studies, most means for achieving both sour resistance performance are disclosed. However, there has been no case in which the toughness of the weld metal part after the stress-relief annealing is ensured at the same time.

  As described above, according to the inventions so far, it has been difficult to produce a welded steel pipe having a low yield ratio without reducing the weld toughness and sour resistance performance after stress relief annealing.

  Therefore, an object of the present invention is to provide a welded steel pipe having a low yield ratio and a method for manufacturing the same without reducing weld toughness and sour resistance performance after stress relief annealing.

  In order to solve the above-mentioned problems, the inventors diligently studied the chemical composition, manufacturing method, and structure of steel materials for thick steel plates that are subjected to normalization and quenching and tempering treatment, which are considered to have excellent hot workability. The following findings were obtained.

  First, in order to obtain excellent HIC resistance, it has been confirmed that it is effective to optimize the addition amount of low C-low Mn-low S-low P-Ca, as has been said. In particular, with respect to the optimum addition amount of Ca, by controlling the ACR defined by the formula (3) to 1.0 to 4.0, the central portion of the plate thickness due to the spheroidization of the elongated MnS (hereinafter referred to as 1 / 2t) It was found that the HIC cracking at a thickness of 1/4 part (hereinafter referred to as 1/4 t) due to the generation of HIC cracks and Ca clusters can be suppressed.

  In addition, even when the elongation MnS is almost lost, it has been found that at 1 / 2t, HIC cracking occurs starting from fine precipitates such as NbC and defects such as air bubbles when the hardness of the central segregation part is high. .

  In the present invention, as an index for rationally evaluating the influence of the component on the hardness of the center segregation part, the PHIC defined by the formula (2) is used, so that a 1/2 t HIC crack caused by the center segregation can be obtained. It was possible to evaluate the effect of alloying components. As PHIC increases, the hardness of the center segregation part increases and cracking is likely to occur. For example, when an X65 welded steel pipe for line pipes is produced with TMCP, an attempt was made to sufficiently suppress the occurrence of HIC cracking under NACE sour conditions. In this case, it was necessary to make PHIC 0.95 or less.

  On the other hand, by performing quenching and tempering treatment, it is possible to (1) suppress the propagation of HIC cracks by equiaxed organization and (2) reduce the hardness of the central segregation part by tempering treatment, and the upper limit of PHIC It was found that the value increased. Further, it was found that the quenching and tempering treatment can secure the base material strength with a lower component than the normalizing treatment, so that the HAZ hardness can be reduced and the SSC resistance is excellent. In addition, it is necessary to reduce the HAZ hardness in order to ensure the SSC resistance, but as an influence of the component on this, it has been confirmed again that low CB-free is desirable as is conventionally said.

  On the other hand, it is known that the yield ratio of quenched and tempered steel increases as C decreases, and it is difficult to achieve both sour resistance performance. Therefore, in the present invention, the relationship between the form of the structure obtained by quenching and tempering and the yield ratio was investigated. As a result, the main structure is polygonal ferrite or pseudo-polygonal ferrite, and the yield ratio is low in the case of a structure having a hard second phase between the grains. It was found that the yield ratio becomes smaller as the average particle size is larger and the fraction of the hard second phase is larger. Moreover, it was found that it is important to make both structures equiaxed in order to achieve both HIC resistance and to achieve both characteristics by making the average aspect ratio 2 or less.

  A method for ensuring the toughness of the welded portion after the stress relief annealing was studied under the constraints of the steel material components and thick steel plate manufacturing conditions as described above. As a result, it has been found that the toughness of the weld metal part is greatly influenced by the amount of O contained in the weld metal and elements Nb, V, and Ti that promote precipitation embrittlement. In particular, the influence of the amount of O is large, and it has been found that by applying a highly basic flux or a calcined flux, the amount of O is reduced to 0.035% or less, which greatly improves the toughness.

  Nb, V, and Ti have a large degree of toughness deterioration due to Nb, and their contribution to embrittlement can be organized as Nb = V / 2 = Ti / 2. Moreover, about Ti, it turned out that Ti except for the amount of Ti consumed as an oxide and nitride contributes to toughness deterioration not by the total amount of Ti contained in the weld metal but by precipitation as TiC. Therefore, reducing the amount of Nb and V contained in the base material, using a wire not containing Nb and V, and reducing the dilution of Nb and V from the base material, and further containing Al and Ti contained in the weld metal It has been found that by appropriately controlling N, O, it is possible to prevent the penetration of Nb, V or Ti involved in precipitation embrittlement into the weld metal, thereby greatly improving the toughness.

  The present invention is obtained by further examining the above-described knowledge, and the gist thereof is as follows.

  A first invention is a welded steel pipe manufactured by forming a base material made of a thick steel plate into a tubular shape and joining the seam portion thereof by submerged arc welding, wherein the base material of the welded steel pipe is in mass%. C: 0.03% or more and less than 0.08%, Si: 0.5% or less, Mn: 0.5 to 1.5%, P: 0.010% or less, S: 0.0030% or less, Al : 0.005 to 0.050%, Ti: 0.005 to 0.025%, B: 0.0003% or less, Ca: 0.0005 to 0.0050%, O: 0.0030% or less Further, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.025% or less, V: 0.050% or less 1 type or 2 types or more selected from among them, and Ceq specified by the formula (1) is 0.28 or more and specified by the formula (2) The PHIC is 1.00 or less, the ACR defined by the formula (3) is 1.0 to 4.0, the balance is Fe and unavoidable impurities. 10 to 60% by volume of polygonal ferrite and pseudo-polygonal ferrite having a diameter of 40 μm or less and an average aspect ratio of 2.0 or less, and a hard second phase that is bainite or martensite or a mixed structure thereof, The hardness difference between the pseudo-polygonal ferrite and the hard second phase is set to Hv 20 to 100, and the weld metal part of the welded steel pipe is mass%, C: 0.03 to 0.10%, Si: 0.00. 50% or less, Mn: 0.8 to 1.5%, P: 0.030% or less, S: 0.010% or less, Al: 0.050% or less, Ti: 0.01 to 0.04%, B: 0.0 005 to 0.0040%, Ca: 0.0040% or less, N: 0.0080% or less, O: 0.035% or less, further Cu: 1% or less, Ni: 1% or less, Cr: 1 % Or less, Mo: 1% or less, Nb: 0.025% or less, and V: 0.050% or less. The Pcm defined by the formula (4) is 0. It is a low yield ratio resistant HIC welded steel pipe excellent in weld toughness after SR, characterized in that PSR defined by formula (5) is 0.025 or less.

In the second invention, a steel slab having the component composition of the base material of the welded steel pipe described in the first invention is heated and hot-rolled, and then the temperature of (Ac ₁ + Ac ₃ ) / 2 to Ac ₃ points from room temperature. to the heating, after holding, the thickness and water cooling the temperature range of 800 to 500 ° C. at a cooling rate 15 to 80 ° C. / s, was prepared cooling after re-heating to a temperature of 550 ° C. to Ac ₁ point from room temperature, holding A steel plate is formed into a tubular shape, and its seam portion is joined by submerged arc welding to form a welded steel pipe having a weld metal portion having the component composition described in the first invention. This is a method for producing a low yield ratio resistant HIC welded steel pipe excellent in weld zone toughness.

  In a third invention, the submerged arc welding is performed by using a wire containing Nb: 0.005% by mass or less, V: 0.005% by mass or less and the balance Fe and unavoidable impurities, a highly basic molten flux or The method for producing a low yield ratio resistant HIC welded steel pipe excellent in welded portion toughness after SR according to the second aspect of the invention, characterized in that the firing type flux is used.

  A fourth invention is characterized in that the WSR defined by the formula (6) from the Nb and / or V content of the base material and the base material dilution ratio A is 0.025 or less. It is a manufacturing method of the low yield ratio resistance HIC welded steel pipe excellent in the weld part toughness after SR described in the third invention.

  The present invention makes it possible to produce 350-550 MPa class welded steel pipes that have both sour resistance, low yield ratio, and weld toughness after stress relief annealing. Applicable to piping.

  The reasons for limiting the respective constituent requirements of the present invention will be described below.

  1. The base material component composition, structure form, and manufacturing method of the low yield ratio welded steel pipe excellent in sour resistance performance according to the present invention will be described.

1.1 Component Composition of Base Material In the following description, all component% means mass%.

C: 0.03% or more and less than 0.08% C is the most effective element for increasing the hardenability during the quenching process and increasing the strength of the base material. If C is less than 0.03%, sufficient strength cannot be ensured, and if it is 0.08% or more, the fraction and hardness of the second phase structure increase and the HIC performance deteriorates. Further, since the HAZ hardness also increases, the C content is set to a range of 0.03% or more and less than 0.08%. Preferably, it is 0.03% or more and less than 0.05% of range.

Si: 0.5% or less Si is added for deoxidation, but if Si is added in excess of 0.5%, toughness and weldability deteriorate, so the Si amount is 0.5% or less. Preferably it is 0.3% or less.

Mn: 0.5 to 1.5%
Mn is added to improve the strength and toughness of the base metal, but if it is less than 0.5%, the effect is not sufficient, and if added over 1.5%, the hardness of the central segregation part increases and MnS is generated. Since the HIC performance is deteriorated due to the above, the Mn content is set in the range of 0.5 to 1.5%. Preferably, it is 1.0 to 1.4% in range.

P: 0.010% or less P is an unavoidable impurity and remarkably increases the hardness of the central segregation part, and as a result, deteriorates the HIC performance. Since this tendency becomes remarkable when it exceeds 0.010%, the amount of P is made 0.010% or less. Preferably, it is 0.008% or less.

S: 0.0030% or less S is generally an MnS inclusion in steel, but the form is controlled from MnS to a spherical Ca (O, S) inclusion by addition of Ca. However, if the amount of S is large, the total amount of Ca (O, S) inclusions increases and becomes the starting point of HIC cracking, so the amount of S is made 0.0030% or less. Preferably, it is 0.0010% or less.

Al: 0.005 to 0.050%
Al is added as a deoxidizer and needs to be added in an amount of 0.005% or more to fix the oxide. However, if it exceeds 0.050%, the cleanliness is lowered and the ductility is lowered. Is in the range of 0.005 to 0.050%.

Ti: 0.005-0.025%
Ti is an essential element for forming TiN and suppressing coarsening of γ grains during heating and holding before quenching to ensure the toughness of the base material. Further, since TiN is stable even at high temperatures, CGHAZ formed when welding is refined to improve toughness and reduce HAZ hardness. In order to obtain these effects, addition of 0.005% or more is necessary. However, addition of more than 0.025% causes TiN to become coarse and the pinning force is saturated. In order to precipitate as TiC in the inside and deteriorate toughness, the amount of Ti is made 0.005 to 0.025% of range. Preferably, it is 0.005 to 0.015% of range.

B: 0.0003% or less B is an element harmful to the SSC resistance. In the present invention, in order to suppress the mixing of B as much as possible, a steelmaking raw material is examined and the content is made 0.0003% or less.

Ca: 0.0005 to 0.0050%
Ca is an element effective in improving the ductility and improving the HIC resistance by controlling the form of sulfide inclusions, but the effect is small if it is less than 0.0005%, and in the case of addition exceeding 0.0050%. Since the generation of Ca clusters serves as the starting point of HIC cracking and the starting point of ductile cracks during deformation, the Ca content is in the range of 0.0005 to 0.0050%.

O: 0.0030% or less O forms an oxide with Al, Ca, etc., and exists as an inevitable inclusion in the steel. When an oxide with an O content exceeding 0.0030% is generated, it becomes the starting point of HIC cracking and the starting point of ductile cracks, so the amount of O is 0.0030% or less.

  Although the above is the basic component of the present invention, in order to obtain desired strength and toughness, one or more selected from Cu, Ni, Cr, Mo, Nb, and V shown below may be contained. Good.

Cu: 0.5% or less Cu is an effective element for improving toughness and increasing strength. However, when Cu is added in excess of 0.5%, weldability deteriorates. The amount of Cu is preferably 0.5% or less.

Ni: 0.5% or less Ni is an effective element for improving toughness and increasing strength. However, when Ni is added in excess of 0.5%, weldability deteriorates. The amount of Ni is preferably 0.5% or less.

Cr: 0.5% or less Cr increases the hardenability and improves the temper softening resistance to reduce the decrease in strength after tempering, so as to ensure the strength of the quenched and tempered steel. However, when Cr is added, the Cr content is preferably 0.5% or less.

Mo: 0.5% or less Mo has both effects of improving hardenability and improving resistance to temper softening and reducing strength reduction after tempering, and the effect is greater than Cr. Although it is the most effective element for securing the strength of the quenched and tempered steel, the weldability deteriorates due to the addition exceeding 0.5%. Therefore, when Mo is added, the Mo amount is 0.5%. The following is preferable.

Nb: a 0.025% or less _{(Ac 1 + Ac 3) /} 2~Ac 3 points of both precipitation of NbC during effect and tempered enhance the hardenability effect, is effective in increasing strength, additives As the amount increases, the amount of dilution to the weld metal increases and the toughness after stress relief annealing deteriorates. If the addition amount exceeds 0.025%, even if other precipitation elements such as V are not included, excellent weld toughness after stress relief annealing cannot be obtained. Therefore, when Nb is added, the Nb amount is It is preferable to make it 0.025% or less. More preferably, it is less than 0.010%.

V: 0.050% or less V is effective in increasing the strength due to both the effect of enhancing hardenability and the precipitation of VC during tempering treatment. However, as Nb is added, the amount added to the weld metal increases. The dilution amount increases and the toughness after stress relief annealing deteriorates. If the addition amount exceeds 0.050%, even if other precipitation elements such as Nb are not included, excellent weld toughness after stress relief annealing cannot be obtained. It is preferable to set it as 0.050% or less.

  In the present invention, the ranges of Ceq, PHIC and ACR defined in the formulas (1) to (3) are further defined.

Ceq: 0.28 or higher The higher the value of Ceq, the higher the hardenability and the higher the strength. Ceq is set to 0.28 or more in order to obtain the strength of 350 to 550 MPa class targeted in the present invention.

PHIC: 1.00 or less PHIC is an equation devised to estimate the material of the central segregation part from the content of each alloy element. The higher the PHIC, the higher the concentration of the central segregation part, and the central segregation part hardness becomes higher. To rise. In the present invention, the hardness of the center segregation part is reduced by performing the tempering treatment. However, if the PHIC exceeds 1.00, HIC cracking due to the hardening of the center segregation part occurs. 00 or less.

ACR: 1.0-4.0
Ca has a high affinity with O. First, CaO is generated, and the remaining Ca binds to S to form CaS. ACR is an index representing the existence form of O, S and Ca in these steels. When ACR is less than 1.0, O and S exist excessively with respect to Ca, so S becomes MnS. Promotes 1 / 2t HIC cracking. On the other hand, if it exceeds 4.0, excessively added Ca becomes a cluster and promotes 1 / 4t HIC cracking. Therefore, the ACR is in the range of 1.0 to 4.0. Preferably, it is the range of 1.5-3.5.

1.2 About the metal structure of the base metal In the present invention, the fraction, the particle diameter, and the aspect ratio of the metal structure at the center portion (1/2 t) of the weld steel pipe base material are defined.

Volume fraction of polygonal ferrite and pseudo-polygonal ferrite: 10-60% by volume
If the volume fraction of polygonal ferrite and pseudo-polygonal ferrite is less than 10% by volume, the yield ratio increases greatly, so the lower limit is made 10% by volume. The yield ratio is lowest when the fraction is about 40% by volume. However, when the volume fraction exceeds 60% by volume, the decrease in strength is significant. Therefore, the volume fraction of polygonal ferrite and pseudopolygonal ferrite is 10-60. The volume is in the range.
Preferably, it is the range of 20-50 volume%.

Average particle diameter of polygonal ferrite and pseudo-polygonal ferrite: 40 μm or less Because the toughness deteriorates when the average particle diameter of polygonal ferrite and pseudo-polygonal ferrite, which is the main structure of the welded steel pipe base material of the present invention, exceeds 40 μm, The average particle size of polygonal ferrite and pseudo-polygonal ferrite is 40 μm or less. Preferably, it is 25 μm or less. On the other hand, the finer the soft ferrite, the better the toughness. However, since the increase in the yield ratio is not so large, the lower limit of the average grain size is not particularly specified. Polygonal ferrite is a massive ferrite and is a white structure observed with an optical microscope when nital etching is performed. On the other hand, pseudo-polygonal ferrite has a slightly square shape compared to polygonal ferrite, and refers to a slightly baked structure than polygonal ferrite. In addition, it is mentioned as a discrimination | determination reference | standard of a structure | tissue that neither structure | tissue contains lath-like carbide | carbonized_material in a grain.

Polygonal ferrite and pseudo-polygonal ferrite average aspect ratio: 2.0 or less Polygonal ferrite and pseudo-polygonal ferrite are preferably equiaxed because propagation resistance to HIC cracking increases, but the aspect ratio is 2.0. If it exceeds, the propagation resistance becomes weak and the HIC characteristics deteriorate, so the average aspect ratio of polygonal ferrite, pseudo-polygonal ferrite and hard second phase is 2.0 or less.

Hard second phase structure If the second phase structure is hard, strain concentration occurs due to a hardness difference from soft ferrite, and the yield ratio decreases. At that time, in order to make a hardness difference with the soft ferrite, the hardness difference is small and the yield ratio cannot be reduced unless the second phase structure is hard bainite or martensite or a mixed structure thereof. On the other hand, as-quenched bainite or as-quenched martensite deteriorates the toughness of the base material, the hardness difference from soft ferrite becomes excessive, and it becomes a propagation path of HIC cracks. Therefore, the second phase structure is tempered bainite or tempered martensite. It is good to use a site or a mixed organization.

  In addition, it is preferable that there are few structures other than the said polygonal ferrite, pseudo-polygonal ferrite, and a hard 2nd phase, However, It is permissible to 10%.

Hardness difference between polygonal ferrite and pseudo-polygonal ferrite and hard second phase: Hv20-100
The yield ratio decreases as the hardness difference between the polygonal ferrite and pseudo-polygonal ferrite and the hard second phase increases. However, if the hardness difference is too large, the toughness of the base metal deteriorates and the HIC crack propagation path becomes an HIC. Characteristics deteriorate. When the hardness difference exceeds Hv100, the effect is significant, so the upper limit is set to Hv100. On the other hand, since the yield ratio does not decrease when the hardness difference is less than Hv20, the lower limit of the hardness difference is Hv20. Preferably it is Hv20-60.

1.3 Base Material Manufacturing Method Continuous Casting The ACR specified in the present invention is the optimum range for continuous casting, and the ingot forming method cannot appropriately suppress the formation of MnS and Ca clusters. We will use it.

Quenching _{temperature: (Ac 1 + Ac 3)} / 2~Ac 3 points quenching temperature without residual polygonal ferrite and quasi-polygonal ferrite of untransformed and becomes greater than ₃ points Ac, it is impossible to lower the yield ratio. Further, when the quenching temperature is less than (Ac ₁ + Ac ₃ ) / 2, the concentration of C into the reverse transformed austenite becomes remarkable, and extremely hard martensite and island martensite (MA) are generated during quenching. The quenching temperature is in the range of (Ac ₁ + Ac ₃ ) / 2 to Ac ₃ points since it becomes a starting point of HIC cracking.
Although it is desirable to obtain Ac ₃ points and Ac ₁ point by a four-master test or the like, it may be obtained by the equations (7) and (8).

Quenching cooling rate: 15-80 ° C / s
The cooling rate from 800 ° C. to 500 ° C. during quenching is 15 ° C./s or more. The faster the quenching cooling rate, the harder the structure produced when the reverse transformed austenite transforms again, and the strength can be increased, and the yield ratio can be lowered. When the quenching cooling rate is less than 15 ° C./s, a second phase having sufficient hardness cannot be obtained, so that the desired strength and yield ratio cannot be obtained. Is generated in a large amount and becomes a starting point of HIC cracking, and the toughness is also deteriorated. Therefore, the quenching cooling rate is set in the range of 15 to 80 ° C./s.

Tempering temperature: 550 ° C. to Ac ₁ point Tempering temperature is 550 ° C. to Ac ₁ point. By performing the tempering process, the hardness of the central segregation part is lowered and the HIC performance is improved. Also, the surface layer hardness is reduced, and the SSC characteristics are improved. This effect cannot be obtained at temperatures lower than 550 ° C., and if it exceeds Ac ₁ point, reverse transformation occurs, and the high C reverse transformation structure deteriorates toughness. Therefore, the tempering temperature is in the range of 550 ° C. to Ac ₁ point. To do.

  2. The composition of the weld metal obtained by submerged welding of a welded steel pipe having excellent weld toughness after stress relief annealing according to the present invention, the welding material, and the weld cross-sectional shape are defined.

2.1 Component composition of weld metal In the following explanation, all component% is mass%.

C: 0.03-0.10%
C is a component that greatly enhances hardenability. However, if it is less than 0.03%, the strength is insufficient. On the other hand, if it exceeds 0.10%, carbide and martensite are likely to be generated, and the toughness is lowered and the weld metal hardness is reduced. Increases, and the SSC resistance deteriorates, so the C content is in the range of 0.03 to 0.10%. Preferably, it is 0.03 to 0.08% of range.

Si: 0.50% or less Si is added as a deoxidizer, but since it is also a component that enhances hardenability, when added excessively, a coarse structure called upper bainite is generated, and toughness is reduced. Si amount is 0.5% or less. Preferably it is 0.1 to 0.5% of range.

Mn: 0.8 to 1.5%
Mn is necessary as a deoxidizer and a component that enhances hardenability. However, if it is less than 0.8%, its effect is poor. On the other hand, if it exceeds 1.5%, upper bainite (UB) tends to be formed, and toughness is increased. Therefore, the Mn content is in the range of 0.8 to 1.5%.

P: 0.030% or less
P is an impurity element contained in a trace amount in the base material and the welding material, and deteriorates the welded portion characteristics by causing grain boundary embrittlement after stress relief annealing, so it is desirable to reduce it as much as possible. Since the tendency of deterioration becomes remarkable when it exceeds 0.030%, the amount of P is made 0.030% or less.

S: 0.010% or less S is an impurity element contained in a trace amount in the base material and the welding material, and causes a decrease in ductility of the welded part by generating MnS, and therefore it is desirable to reduce it as much as possible. The amount of S is 0.010% or less.

Al: 0.050% or less Al is added as a deoxidizer. However, if excessively added, Al ₂ O ₃ increases and the effect of refining the structure due to the Ti-based oxide cannot be used. .050% or less.

Ti: 0.01-0.04%
Ti improves the toughness by forming fine ferrite, but if less than 0.01%, this effect is poor, whereas if over 0.04%, the solid solution Ti as it is welded increases, and after stress relief annealing In order to precipitate as TiC and deteriorate toughness, the amount of Ti is set in the range of 0.01 to 0.04%.

B: 0.0005 to 0.0040%
B is a component that greatly enhances the hardenability and forms fine acicular ferrite by synergistic effect with Ti to improve toughness. However, if less than 0.0005%, this effect is poor, while 0.0040% is reduced. If exceeding, the hardness of the welded portion is remarkably increased and the SSC resistance is deteriorated, so the B amount is made 0.005 to 0.0040%.

Ca: 0.0040% or less Ca is a component mixed from the base material, and does not affect the welded portion characteristics in a small amount. However, if it exceeds 0.0040%, the weld ductility decreases due to an increase in CaO-based inclusions, so the Ca content is set to 0.0040% or less.

N: 0.0080% or less N is a component that is inevitably contained in the weld metal, but when it exceeds 0.0080%, inclusions increase and further bond with B to pro-eutectoid ferrite at grain boundaries. The amount of N is made 0.0080% or less because it promotes the formation of N and reduces toughness.

O: 0.035% or less O is a component inevitably contained in the weld metal, but when it exceeds 0.035%, inclusions increase and further bond with B to pro-eutectoid ferrite at grain boundaries. The amount of O is made 0.035% or less because it promotes the formation of and reduces toughness.

  The above is the basic component of the present invention. However, in order to obtain a desired weld strength, one or more selected from Cu, Ni, Cr, Mo, Nb, and V shown below may be added. Good.

Cu: 1% or less Cu is a component that enhances hardenability, and is a component mixed from the base metal and wire plating. However, if it exceeds 1%, the hardenability becomes excessive and the toughness is reduced. When Cu is added, the amount of Cu is preferably 1% or less.

Ni: 1% or less Ni is a component that enhances hardenability, is contained by mixing from the base material and being supplied from the wire, and increases the strength without degrading toughness by adding a large amount. However, when the content exceeds 1%, the SSC characteristics deteriorate. Therefore, when Ni is added, the amount of Ni is preferably 1% or less.

Cr: 1% or less Cr is a component that enhances hardenability and is contained by mixing from the base material and being supplied from the wire, but if it exceeds 1%, hardenability becomes excessive and lowers toughness. For this reason, when Cr is added, the Cr content is preferably 1% or less.

Mo: 1% or less Mo is a component that enhances hardenability and refines the weld metal structure to improve toughness. However, if it exceeds 1%, the weld metal reheated portion is embrittled, so Mo is added. The amount of Mo is preferably 1% or less.

Nb: 0.025% or less Nb is a component that enhances hardenability and is contained by mixing from the base material, but significantly deteriorates toughness after stress relief annealing. Therefore, it should not be added as much as possible. However, when Nb is added, the Nb content is preferably 0.025% or less from the viewpoint of compatibility with the base material characteristics.

V: 0.050% or less V is a component that enhances hardenability and is contained by mixing from the base material, but significantly deteriorates the toughness after stress-relief annealing, but not as much as Nb. Therefore, it should not be added as much as possible, but when V is added, the V amount is preferably 0.050% or less from the viewpoint of coexistence with the base material characteristics.

  In the present invention, PCM and PSR defined by the equations (4) and (5) are further defined.

Pcm: 0.12 or more Pcm is originally an index representing the risk of welding cold cracking, but it is known that there is also a good correlation with the strength of the weld. In order to obtain a desired joint strength, it is necessary to set it to 0.12 or more, so the lower limit was set to 0.12. On the other hand, the higher the Pcm, the higher the hardness of the welded portion. Therefore, it is preferably in the range of 0.12 to 0.18.

PSR: 0.025 or less PSR is an index indicating toughness deterioration due to precipitation embrittlement after stress relief annealing as described above. If the PSR exceeds 0.025, the toughness deterioration due to stress-relief annealing increases, and the desired toughness cannot be obtained even if the oxygen is reduced or the structure is refined. Therefore, the PSR is set to 0.025 or less.

  Ti is preferentially precipitated as oxide and nitride during welding, and the remaining Ti is precipitated as carbide during SR and adversely affects toughness. Therefore, Tieff is defined and becomes an oxide and nitride. The influence of Ti is excluded.

2.2 Welding materials Wire Nb, V
The component of the weld metal is composed of a component of the welding material and a component diluted from the base material. In order to ensure the toughness of the welded portion after stress relief annealing, it is desirable to reduce the amounts of Nb and V. On the other hand, Nb and V work effectively to ensure the strength and toughness of the base material. Therefore, in order to increase the amount of Nb and V that can be added to the base material, positive addition of Nb and V to the wire is not preferable. However, you may contain Nb and V in the range accept | permitted as an unavoidable impurity. The ranges are Nb ≦ 0.005% and V ≦ 0.005%.

Flux In order to reduce the amount of O in the weld metal, it is necessary to use a highly basic molten flux or a fired flux. Regarding the melt type flux, it is desirable that the basicity BL defined by the formula (9) is 1.0 or more.

2.3 Base material dilution rate In the present invention, the upper limit of the WSR defined by the equation (6) is calculated from the Nb and V amounts of the base material and the dilution rate A of the base material measured from the circumferential cross section of seam welding. Is specified. WSR is an index indicating the mixing of Nb and V from the base metal to the weld metal. By using this index, the weld metal toughness after stress relief annealing when the groove shape, weld heat input, and the number of weld stacks are different. Changes can be evaluated. Therefore, the upper limit is set to 0.025%, which is the same as PSR of weld metal. More preferably, considering the influence of Ti, it is made 0.022%. The base material dilution rate is calculated by equation (10).

  Steels (A to L) having chemical components shown in Table 1 are made into slabs by a continuous casting method, re-heated and hot-rolled at a high temperature (more than 950 ° C.) so as to have a plate thickness of 15 to 120 mm. Air-cooled until. Subsequently, after removing the surface scale by shot blasting, quenching and tempering treatment was performed under the conditions shown in Table 2 to produce thick steel plates (Nos. 1 to 17). The quenching and tempering holding temperature was the furnace holding temperature, and the holding time was the time from when the furnace reached the target temperature of −20 ° C. until the thick steel plate was taken out of the furnace. The cooling rate at the time of quenching was as follows. When producing the thick steel plate (No. 1), a sample was taken from the as-quenched (before tempering) steel plate, and the hardness was 1 / 2t and the CCT line was maintained at 930 ° C for 10 min. The cooling rate for each thickness was estimated by comparing the hardness in the figure. The tempering was cooled by air cooling. As a comparison, thick steel plate No. No. 15, after reheating the continuously cast slab to 1150 ° C., reducing the temperature by 950 ° C. or less to 50% and finishing the rolling at 760 ° C., making the thickness 30 mm, then from 720 ° C. to 400 ° C. at 10 ° C./s It was prepared by water cooling and air cooling to room temperature (referred to as TMCP).

The steel plate manufactured by the above-mentioned method is cut into 3000 to 12000 mm in the longitudinal direction, made into a steel pipe shape by cold working (UOE method or bending roll method), and the seam portion is welded by submerged arc welding. Manufactured). Table 3 shows the thick steel plate used for the steel pipe material of the welded steel pipe, the pipe making method, and the welding conditions. Table 4 shows the brand or chemical composition of the welding wire used for welding, and Table 5 shows the brand or chemical composition of the welding flux. Welding wire and welding flux whose components are described are those newly produced in this study, or components analyzed, and those with brands are commercially available products that are generally available This time, component analysis was not performed. Also, pipes made by UOE method are grooved into X shape, then C-U-O press is performed, carbon dioxide gas welding is performed from the outer surface side, tack welding is performed, inner surface one-layer welding is performed, and then One-layer outer surface welding was performed. On the other hand, pipes made by the bending roll method are grooved into a Y shape, then tubed with multiple bending presses, the inner surface is multilayered, and the outer surface is grooved by gas gouging. Processed and multilayer welded.

  These welded steel pipes were subjected to characteristic evaluation tests and microstructure quantification by the following methods. Tensile tests were conducted for samples with a tube thickness of less than 40 mm by collecting full thickness tensile test pieces defined by API from coupons flattened from the circumferential direction of the steel tube with a press. For pipes with a thickness of 40 mm or more, a round bar tensile test piece with a diameter of 12.7 mm in accordance with ASTM A370-07a was taken from the 1 / 2t position of the coupon taken from the circumferential direction of the steel pipe without flattening. I went.

  At this time, those having a tensile strength exceeding 415 MPa (corresponding to the lower limit of A516-Gr.60) and those having a yield ratio (0.5% yield stress / tensile strength × 100) of 85% or less were regarded as acceptable.

  In the Charpy test, the base metal was sampled from the direction perpendicular to the rolling at the 1 / 2t position, and the welded portion was the outer surface side welded portion at the center of the welded portion in the circumferential direction (plate thickness 10 mm only) Three 2 mmV notch test pieces taken from the size Charpy test piece from the center of the welded portion (1/2 t position) were tested at -50 ° C., and the average value was used.

The HIC characteristics were determined based on NACE Standard TM0284-2003 by taking 3 samples each and placing 96 specimens in 5% NaCl + 0.5% CH ₃ COOH aqueous solution saturated with hydrogen sulfide having a pH of about 3. After immersion for a period of time, the presence or absence of cracks on the entire surface of the test piece was investigated by ultrasonic flaw detection, and evaluated by a crack area ratio (CAR). Here, the maximum value of each steel plate was defined as the CAR of the steel plate, and CAR ≦ 6% was regarded as acceptable.

Based on NACE Standard TM0177-2005, the SSC characteristics were obtained by taking two round bar tensile samples for the base metal and the weld, applying a load stress of 80% of the base metal, and having a pH of about 3 It was evaluated whether or not the test piece was immersed in a 5% NaCl + 0.5% CH ₃ COOH aqueous solution saturated with hydrogen for 720 hours for fracture. At this time, when both were not broken, it was evaluated as No crack, and when even one was broken, it was evaluated as Crack.

  The microstructure fraction and aspect ratio are 400 times with an optical microscope after 3% nital etching after mirror-polishing the L-surface observation sample taken from the 1 / 2t position of each thick steel plate (No. 1-17) The three photos were taken and analyzed by image analysis. At this time, the average grain diameter of polygonal ferrite and pseudo-polygonal ferrite was the average value of the equivalent circle diameter of each crystal grain, and the aspect ratio was the average value of the long side / short side of each crystal grain. For the fraction of the second phase, the fraction of the portion other than the polygonal ferrite and the pseudo-polygonal ferrite was adopted, and for the aspect ratio, the average value of the long side / short side of each structure and texture group was adopted.

  AW and AY, which are the shape factors of the welded part, are taken from the prepared welded joint so that the circumferential surface becomes the observation surface, a macro sample is taken out and the structure is revealed by nital etching, and a photograph is taken from there. It calculated | required by measuring an area by analysis. The groove cross-sectional area AK was determined by extracting a shape by pressing a bundle of wires aligned with the groove cross-section just before seam welding. The WSR value was obtained from the AW, AY, AK and the base material Nb, V amount obtained by the above measurement method.

  The chemical composition of the weld metal was carried out by cant back analysis taken from a surface that coincided with the Charpy test evaluation position of the weld metal as much as possible (O, N, and B were wet analysis using chips).

  Table 6 shows the chemical analysis values of the weld metal part.

  Table 7 shows the test results of the tensile test, Charpy test, HIC test, and SSC test.

Welded steel pipe No. Since all of B, C, E, S, and Z satisfy the component range, structure form range, and production method range of the present invention, desired strength, toughness, HIC resistance and SSC resistance are obtained. On the other hand, a welded steel pipe No. which is a comparative example. Since the chemical components of the weld metal are all outside the scope of the present invention, the toughness of the weld metal part after SR is low. Welded steel pipe No. In F, since Nb exceeds the upper limit, Nb carbonitride is generated by heating with SR, and the toughness deteriorates due to precipitation embrittlement, and the toughness of the weld metal part after SR deteriorates.
Welded steel pipe No. H has a high yield retention ratio because it has a high quenching retention temperature, so that soft ferrite cannot be obtained. Welded steel pipe No. Since the quenching temperature is low, the ferrite fraction is too high, so that the hard second phase is dispersed in the ferrite and the HIC characteristics are deteriorated. Welded steel pipe No. Nu has a low quenching cooling rate and the hardness of the hard second phase is low, so the desired strength is not obtained. Welded steel pipe No. Since the steel has not been tempered, there is a large difference in hardness from the second phase, and cracks have occurred in the HIC. Welded steel pipe No. As for M, since Mn and PHIC exceed the upper limit, many cracks are observed in HIC. Welded steel pipe No. Wa has many cracks in HIC because of its low ACR. Welded steel pipe No. As for mosquitoes, since C exceeds the upper limit, many cracks are observed in HIC, and cracks are also generated in SSC. Welded steel pipe No. Since Y is not added with Ti, ferrite grains are coarsened and toughness is deteriorated. Welded steel pipe No. Since B is added, cracks occur in SSC.
Welded steel pipe No. Since RE is manufactured by TMCP and has a large aspect ratio of soft ferrite and second phase, many cracks are observed in HIC.

Claims (4)

A welded steel pipe having a base material made of a thick steel plate and a weld metal part , wherein the base material of the welded steel pipe is in mass%, C: 0.03% or more and less than 0.08%, Si: 0.5% or less Mn: 0.5 to 1.5%, P: 0.010% or less, S: 0.0030% or less, Al: 0.005 to 0.050%, Ti: 0.005 to 0.025%, B: 0.0003% or less, Ca: 0.0005 to 0.0050%, O: 0.0030% or less, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0 0.5% or less, Mo: 0.5% or less, Nb: 0.025% or less, and V: 0.050% or less, containing one or more selected from the formula (1) Ceq is 0.28 or more, PHIC defined by the formula (2) is 1.00 or less, and ACR defined by the formula (3) is 1.0 to 4 The structure of the central portion of the tube thickness, which is composed of the balance Fe and unavoidable impurities and is 1 / 2t of the base material, is polygonal ferrite and pseudopolygon having an average particle size of 40 μm or less and an average aspect ratio of 2.0 or less. It contains 10-60% by volume of null ferrite and a hard second phase that is bainite or martensite or a mixed structure thereof, and the structure other than the polygonal ferrite, pseudo-polygonal ferrite, and hard second phase is 10% by volume or less. There, the hardness difference between the polygonal ferrite and quasi-polygonal ferrite and the hard second phase and Hv20～100, further, the weld metal of the welded steel pipe, in mass%, C: from 0.03 to 0. 10%, Si: 0.50% or less, Mn: 0.8 to 1.5%, P: 0.030% or less, S: 0.010% or less, Al: 0.0 50% or less, Ti: 0.01 to 0.04%, B: 0.0005 to 0.0040%, Ca: 0.0040% or less, N: 0.0080% or less, O: 0.035% or less Further, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 1% or less, Nb: 0.025% or less, V: 0.050% or less Or a welded part after SR, characterized in that it contains two or more, Pcm defined by formula (4) is 0.12 or more, and PSR defined by formula (5) is 0.025 or less. Low yield ratio resistant HIC welded steel pipe with excellent toughness.

After heating and hot-rolling the steel slab having the component composition of the base material of the welded steel pipe according to claim 1 , after heating and holding from room temperature to a temperature of (Ac ₁ + Ac ₃ ) / 2 to Ac ₃ points, the temperature range of 800 to 500 ° C. and cooled at a cooling rate of 15 to 80 ° C. / s, again heated to a temperature of 550 ° C. to Ac ₁ point from room temperature, the steel plate manufactured by air cooling after holding, formed into a tubular By joining the seam parts by submerged arc welding, the structure of the central part of the pipe thickness, which is the 1 / 2t position of the base material, has polygonal ferrite having an average grain size of 40 μm or less and an average aspect ratio of 2.0 or less. pseudo polygonal ferrite of 10 to 60% by volume and comprises bainite or martensite or hard second phase which is a mixed structure, the polygonal ferrite, pseudo polygonal ferrite and hard Tissues other than the second phase is 10 vol% or less, wherein the polygonal ferrite and quasi-polygonal ferrite hardness difference between the hard second phase and Hv20～100, weld metal composed of composition of claim 1, wherein A method for producing a low yield ratio resistant HIC welded steel pipe excellent in weld toughness after SR, characterized by being a welded steel pipe having a portion. In the submerged arc welding, Nb and V are not included (however, Nb: 0.005% by mass or less and V: 0.005% by mass or less are allowed as unavoidable impurities) and the following formula (9): The low yield ratio resistant HIC welded steel pipe excellent in weld toughness after SR according to claim 2, characterized in that it is carried out using a high basic melt type flux having a defined basicity BL of 1.0 or more. Manufacturing method.

4. The SR after SR according to claim 2, wherein the WSR defined by the formula (6) is 0.025 or less from the Nb and / or V content of the base material and the base material dilution ratio A. 5. A method for producing a low yield ratio resistant HIC welded steel pipe excellent in weld toughness.