JP5796370B2 - High-strength, thick-walled ERW steel pipe with excellent sour resistance in ERW welds - Google Patents

Description

The present invention relates to a high-strength thick-walled electric resistance welded steel pipe excellent in sour resistance of an electric resistance welded part, and more specifically, by defining the equivalent circle diameter of the oxide alone and the group of the seam part of the electric resistance welded steel pipe, The present invention relates to a high-strength thick-walled ERW steel pipe that suppresses hydrogen-induced cracking (HIC) starting from the seam and has excellent sour resistance in an ERW weld.
Here, “excellent sour resistance” means that a specimen including an ERW welded part is subjected to a HIC test that is immersed in a solution A solution specified by NACE at 25 ° C. for 96 hours. It means that the plate thickness direction crack ratio (CTR) measured by ultrasonic flaw detection perpendicular to the weld seam surface is less than 1.5%. “High strength” means strength of API (American Petroleum Institute) standard grade X65 or higher, and “thick” means that the thickness is 10 mm or more.

  Conventionally, ERW steel pipes for high-strength, thick-line pipes have been subjected to heat input adjustment and V-shape angle adjustment depending on experience from the viewpoint of improving welded part quality. Qualitatively, the weld quality has been improved qualitatively by high heat input and proper V-shape angle (approximately 2 to 3 degrees). However, the adjustment based on such experience does not always guarantee 100% sour resistance performance, and sometimes significant HIC (hydrogen induced cracking) occurs and cannot be suppressed. .

  On the other hand, the molten steel produced by melting in the converter is vacuum degassed and / or Mn, Si deoxidized so that the oxygen in the molten steel is 250 ppm or less, then the alloy composition is Ti: 10 to 70 wt%, the balance: Fe , Mn, Si, one or more types and alloys that are inevitable impurities are added to molten steel, and steel with a specific steel composition is continuously cast with high toughness for ERW steel pipes with excellent sour resistance Is known (see Patent Document 1). Thereby, it is said that only the oxide inclusions having a Ti oxide as a main component and a particle size of 200 μm or less are contained in the steel, thereby improving the sour resistance of the ERW weld.

Japanese Patent No. 3293024

The conventional ERW steel pipe manufacturing method based on the qualitative welding condition management described above has not yet achieved a product that exhibits stable and excellent sour resistance properties. Moreover, there is a concern that the method of Patent Document 1 is limited to Ti-containing steel and its application range is narrow. These points were problems.
The present invention solves the above-mentioned problems, and does not generate hydrogen-induced cracks in the ERW welds in the NACE stipulated HIC test, regardless of whether or not Ti is contained. The object is to provide a meat electric welded steel pipe.

  The inventors changed various heat input conditions and welded end face shape conditions of ERW welding for Ti-containing and non-Ti-containing materials of ERW steel pipes for high-strength thick line pipes of API standard X65 and X70 grades. An electric resistance welding experiment was conducted, and the relationship between the form of oxide remaining in the electric resistance welded part (by SEM observation) and the sour resistance of the electric resistance welded part (HIC test results stipulated by the NACE) was investigated in detail.

  As a result, regardless of the presence or absence of Ti, as shown in FIG. 1, the individual equivalent circular diameters of the oxide 1 are 20 μm or less, and a plurality of oxides have an adjacent spacing of 100 μm or less. The present invention was made by obtaining the knowledge that hydrogen-induced cracking does not occur only when the equivalent circular diameter of cluster 2 which is a cluster gathered together (however, it is over 0 μm) is 300 μm or less. Here, the equivalent circular diameter is a diameter of a circle having the same area as the target figure in the cross-sectional view. When the object is the cluster, the cluster figure is a figure surrounded by the outermost tangent line 3 of the group of oxides as shown in FIG.

That is, the present invention is as follows.
(1) By mass%, C: 0.01 to 0.15%, Si: 0.005 to 0.9%, Mn: 0.2 to 1.3%, P: 0.01% or less, S: It contains 0.01% or less, Al: 0.1% or less, the balance is composed of Fe and inevitable impurities, and the equivalent circular diameter of each oxide in the ERW weld is 20 μm or less. and a plurality of said oxide Ri der equality circular diameter 300μm or less clusters is more gathered population below nearest neighbor spacing 100 [mu] m, CTR value of the electric resistance welding portion is Ru der less than 1.5% A high-strength thick-walled electric-welded steel pipe having a plate thickness of 10 mm or more and excellent in sour resistance of an electric-welded welded portion having an API standard grade X65 or more .
(2 ) The electric resistance welding as described in ( 1 ) above, which contains 1 type or 2 types selected from Cu: 0.5% or less and Ni: 0.5% or less by mass% High-strength, thick-walled electric-welded steel pipe with excellent sour-resistance characteristics.
( 3 ) It is 1% or 2 types chosen from Cr: 3.0% or less and Mo: 2.0% or less by mass%, It is described in said ( 1 ) or ( 2 ) characterized by the above-mentioned. High-strength, thick-walled ERW steel pipe with excellent sour resistance for ERW welds.
( 4 ) The above, characterized by containing one or more selected from Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less in mass% A high-strength, thick-walled electric-welded steel pipe excellent in sour-resistance characteristics of the ERW welded portion according to any one of ( 1 ) to ( 3 ).
( 5 ) By mass%, it contains Ca: 0.005% or less, and it has excellent sour resistance in the ERW weld part according to any one of the above ( 1 ) to ( 4 ) High strength thick ERW steel pipe.

  The high-strength thick-walled ERW steel pipe of the present invention does not generate hydrogen-induced cracks in the ERW welds in the NACE stipulated HIC test. We can provide ERW steel pipes for meat line pipes.

It is explanatory drawing of the oxide form requirements of this invention.

[Oxide morphology requirements]
First, the oxide form requirements of the present invention will be described.
According to the knowledge of the inventors, the end face to be welded is oxidized and an oxide is generated at the time of ERW welding, but the generated oxide is discharged along the flow of molten steel pushed out by abutting pressure welding. At this time, it is difficult for all the oxides to be discharged, and some of the oxides remain in the ERW weld, but when the equivalent circular diameter of the remaining oxides exceeds 20 μm, It can be the starting point of HIC. Therefore, it was limited that each equivalent circular diameter of the oxide in the ERW welded portion was 20 μm or less.

  In addition, the oxide remaining in the electric resistance welded portion is dispersed in an unspecified range. A group in which a plurality of dispersed oxides gather together at a re-adjacent interval of 100 μm or less behaves as if it is one oxide, and may become a starting point of HIC. This group is defined as a cluster, and the conditions under which the cluster is the starting point of the HIC were determined by the above-mentioned ERW welding experiment. As shown in FIG. 1, the outermost tangent 3 of the group of oxides 1 forming the cluster 2 It was found that if the equivalent circular diameter of the enclosed region (that is, the equivalent circular diameter of the cluster) was 300 μm or less, the cluster would not be the starting point of HIC. Therefore, the equivalent circular diameter of the cluster is limited to 300 μm or less.

By the way, the reason why the thickness of 10 mm or more is defined as “thick” is that the influence of HIC is small when the thickness is less than 10 mm. However, if the thickness is less than 11.9 mm, the HIC sensitivity is relatively small and the HIC suppressing effect of the present invention is not noticeable. Therefore, in the present invention, the thickness is preferably 11.9 mm or more.
The reason why the API standard grade X65 or higher is defined as “high strength” is that the low grade of less than X65 has a low HIC cracking susceptibility, and a sour-resistant material can be produced without any practical problems.

[Manufacturing process]
The ERW steel pipe according to the present invention is basically formed by forming a tube material having an initial shape into a strip shape into a tubular shape and abutting edges (corresponding to band width end portions) of the formed V shapes. At the stage where the pipes are formed by sewing and welding, and the pipes are still formed, there are usually oxides having an equivalent circular diameter of more than 20 μm or clusters having an equivalent circular diameter of more than 300 μm at some part in the pipe length direction. The existence probability of these oxides and clusters can be reduced to a considerable extent by adjusting welding heat input conditions, upset conditions, welded edge groove shape conditions, etc. Is difficult.

Therefore, in the inspection process after ERW welding, ultrasonic flaw detection is performed using the “ultrasonic flaw detection apparatus and ultrasonic flaw detection method” described in Japanese Patent No. 4544240 to oxidize the equivalent circular diameter of more than 20 μm. A portion in the tube length direction where an object or a cluster having an equivalent circular diameter of more than 300 μm is present is detected, and the detected portion is cut and removed, and the rest is used as a product tube. Thereby, the electric resistance welded steel pipe which satisfies the said oxide form requirements over the full length of a product pipe is obtained.
[Chemical composition]
In the ERW steel pipe according to the present invention, the chemical composition (abbreviated composition) considers the total cost reduction when laying the line pipe, especially considering the demands of customers who place importance on lowering the transportation cost of the steel pipe. And it was designed on the basis of a composition that can achieve high strength (API standard X65 grade or higher). The reasons for limiting the preferred ranges for the individual components will be described below. Each component content unit of the composition is mass% and is abbreviated as%. The composition of the base material part is adjusted at the melting stage of the tube material. The composition of the welded portion is almost the same as the composition of the base metal part because the electro-welding process does not involve the addition of alloying elements.
(C: 0.01 to 0.15%) C is an element that contributes to precipitation strengthening as a carbide, but if it is less than 0.01%, sufficient strength cannot be secured, while if it exceeds 0.15%, pearlite. In addition, the second-phase structure fraction such as martensite increases, making it difficult to ensure the required sour resistance. For this reason, C: 0.01 to 0.15%. In addition, More preferably, it is C: 0.02-0.07%.
(Si: 0.005 to 0.9%) Si is added for deoxidation, but if it is less than 0.005%, the deoxidation effect is not sufficient. Si: 0.005 to 0.9% is used for significant deterioration.
(Mn: 0.2 to 1.3%) Mn is added to ensure strength and toughness, but if it is less than 0.2%, the effect is not sufficient. Since the fraction increases and it is difficult to ensure excellent sour resistance, Mn: 0.2 to 1.3%.
(P: 0.01% or less) P is an element that deteriorates the electric resistance weldability, but if it is 0.01% or less, there is no problem, so P: 0.01% or less.
(S: 0.01% or less) In general, S is a MnS inclusion in steel, and it is better as it is smaller because it becomes the starting point of HIC. If it is 0.01% or less, there is no problem, so S: 0.01% or less.
(Al: 0.1% or less) Al is added as a deoxidizer, but if it exceeds 0.1%, the cleanliness of the steel decreases and the toughness deteriorates, so Al: 0.1% or less .

For the purpose of further improving the strength, sour resistance, etc. of the electric resistance welded steel pipe, in addition to the above components, one or two selected from Cu: 0.5% or less and Ni: 0.5% or less Species, Cr: 3.0% or less, Mo: One or two selected from 2.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% One or more selected from the following, Ca: 0.005% or less can be selected and contained.
(Cu: 0.5% or less) Cu is effective in improving the sour resistance by generating a surface layer coating, but if it exceeds 0.5%, a problem arises in the manufacturability of the tube material, so Cu: 0.5% The following.
(Ni: 0.5% or less) Ni is an element effective for improving toughness and increasing strength, but if it exceeds 0.5%, a hardened second phase is likely to be formed, leading to deterioration of HIC characteristics. Ni: 0.5% or less.
(Cr: 3.0% or less) Similar to Mn, Cr is an effective element for obtaining sufficient strength even at low C. However, if it exceeds 3.0%, the second phase is likely to be formed, and HIC characteristics Therefore, Cr: 3.0% or less.
(Mo: 2.0% or less) Mo is an element effective for obtaining sufficient strength even at low C as in Mn and Cr. However, if it exceeds 2.0%, the second phase is likely to be formed. In order to deteriorate the HIC characteristics, Mo: 2.0% or less.
(Nb: 0.1% or less) Nb improves strength and toughness by fine precipitation of carbonitride and fine graining of the structure, but when it exceeds 0.1%, the cured second phase tends to increase, In order to significantly deteriorate the HIC characteristics, Nb: 0.1% or less.
(V: 0.1% or less) V also contributes to an increase in strength by fine precipitation of carbonitrides in the same manner as Nb. However, if it exceeds 0.1%, the cured second phase fraction increases in the same manner as Nb. V: 0.1% or less because the HIC characteristics deteriorate significantly.
(Ti: 0.1% or less) Ti, like Nb and V, contributes to strength increase by fine precipitation of carbonitride, but if it exceeds 0.1%, the second phase fraction cured in the same manner as Nb Since it increases and the HIC characteristics are remarkably deteriorated, Ti: 0.1% or less.
(Ca: 0.005% or less) Ca is an element necessary for controlling the morphology of elongated MnS, which tends to be the starting point of HIC, but when added over 0.005%, excessive Ca oxide and sulfide are generated. And Ca: 0.005% or less because it leads to toughness deterioration.

  The balance excluding the above components is Fe and inevitable impurities. It is preferable that inevitable impurities (O and others) be as small as possible.

Table 1 uses steel strips (A to I) showing composition, plate thickness, YS, and TS as pipe materials, and performs various types of welding heat input conditions, welded edge groove conditions, etc. ERW steel pipes having outer diameters and strength grades shown in Table 2 were produced. Each of the steel strips was manufactured by a method in which a steel slab was rolled to a predetermined plate thickness by hot rolling and then wound into a hot coil.
Evaluate the sour resistance of the ERW welded part of the manufactured ERW steel pipe by the thickness direction crack rate (CTR) measured by the following HIC test. If the CTR of the welded part is less than 1.5%, sour resistance The characteristic was determined to be acceptable (◯), otherwise it was rejected (x).
[HIC test]
(Test piece) A full-thickness test piece having a width of 20 mm and a length (pipe length direction) of 100 mm, with the center position of the ERW welded portion (position of the weld seam surface) at the center of the test piece width direction.
(Test method) A solution A solution defined by NACE was used and immersed for 96 hours at 25 ° C, and then ultrasonic flaw detection was performed perpendicularly to the weld seam surface to measure CTR.

Moreover, the oxide form in the ERW weld was investigated by the following method.
[Oxide morphology survey method]
Using the welding seam surface as the surface to be observed, SEM (scanning electron microscope) is used to observe magnifications of 100 to 1000 times and the number of fields of view of 10 or more. Was measured.

These measurement results are shown in Table 2. The value of the equivalent circle diameter of the oxide and cluster for each steel pipe in Table 2 is the maximum value of the measured data.
From Table 2, all of the examples of the present invention show excellent sour-resistant characteristics and pass, whereas the comparative examples show that the equivalent circular diameters of oxides and / or clusters deviate from the scope of the present invention and fail. is there.

1 Oxide 2 Cluster 3 Outermost tangent

Claims (5)

In mass%, C: 0.01 to 0.15%, Si: 0.005 to 0.9%, Mn: 0.2 to 1.3%, P: 0.01% or less, S: 0.01 % Or less, Al: 0.1% or less, with the balance being composed of Fe and inevitable impurities , the individual equivalent circular diameter of the oxide in the ERW weld is 20 μm or less, and wherein a plurality of oxide equality diameter of clusters is more gathered population below nearest neighbor spacing 100μm is Ri der below 300 [mu] m, the CTR value der Rukoto less than 1.5% of the electric-resistance welded portion A high-strength thick-walled ERW steel pipe with excellent sour resistance for ERW welds with a plate thickness of 10 mm or more and API standard grade X65 or more . 2. The sour resistance of an electric resistance welded portion according to claim 1, comprising one or two selected from Cu: 0.5% or less and Ni: 0.5% or less by mass%. High-strength thick-walled ERW steel pipe with excellent characteristics. It contains 1 type or 2 types chosen from Cr: 3.0% or less and Mo: 2.0% or less by mass%, The electric resistance welded part of Claim 1 or 2 characterized by the above-mentioned. A high-strength, thick-walled ERW steel pipe with excellent sour resistance. By mass%, Nb: 0.1% or less, V: 0.1% or less, Ti: claim 1, characterized in that it contains one or two or more selected from 0.1% or less to 4. A high-strength thick-walled ERW steel pipe excellent in sour resistance characteristics of the ERW welded portion according to any one of 3 above. The high-strength thick-walled electric-welded steel pipe excellent in sour resistance of the ERW weld according to any one of claims 1 to 4 , characterized by containing Ca: 0.005% or less in mass%. .

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