JPH06330139A - Method for smelting steel for line pipe having excellent sulfide stress crack resistance of weld zone - Google Patents

Abstract

PURPOSE:To execute ladle refining of steel having the excellent sulfide stress crack resistance in moist hydrogen sulfide atmosphere. CONSTITUTION:The steel consisting, by weight %, of 0.03 to 0.09 C, 0.1 to 0.4 Si, 0.5 to 1.0 Mn, <=0.015 P, (0.0010 S, 0.010 to 0.050 Nb, 0.005 to 0.03 Al, 0.001 to 0.003 Ca, 0.005 to 0.025 Ti, <=0.0050 O and (%Ca{1-98(%0)}/(%S)>1.7 and 0.07<(%Ca)/(%Al)<0.17 and 6<(%Si)/(%Al)<25 and the balance iron and inevitable impurities is subjected to ladle refining.

Description

Detailed Description of the Invention

[0001]

This invention relates to an AP having excellent weld sulfide stress cracking resistance in a wet hydrogen sulfide environment.
The present invention relates to ladle refining of I grade X42 to X56 line pipe steel.

[0002]

2. Description of the Related Art Since oil and natural gas produced in recent years contain hydrogen sulfide very often, the line pipes for transporting these oil and natural gas are hydrogen sulfide in the presence of water such as seawater. More likely to be exposed to the environment (sour environment). In the sour environment, hydrogen penetration into the steel due to corrosion of the steel surface is promoted by the catalytic action of hydrogen sulfide. On the other hand, a stress in the circumferential direction, that is, a hoop stress acts on the line pipe during operation due to the pressure of the fluid passing through the inside.

[0003] Sulfide stress cracking is known as a cause of line pipe failure that occurs under such environmental conditions. In particular, sulfide stress cracking that occurs in the high hardness portion of the weld has been frequently experienced. Therefore, as a method for preventing sulfide stress cracking, it is general to regulate the maximum hardness of the welded portion, and it is usually required to set the Vickers hardness to 248 or less. In order to meet this demand, steel pipe manufacturers reduce the carbon equivalent of the steel pipe material and manufacture the material by a so-called TMCP, which is a thermomechanical treatment method including controlled rolling and accelerated cooling, in order to compensate for the strength reduction. The introduction of TMCP technology has improved the resistance to sulfide stress cracking of the steel pipe base material by controlling the metallographic structure, as well as preventing the hardening of the weld.

However, in the TMCP steel, a softening region having a lower strength than the base metal is inevitably formed in the weld heat affected zone (HAZ) because of the low carbon equivalent. In particular, a UOE steel pipe having a high welding heat input has a high degree of softening. As a method for evaluating the sulfide stress crack resistance of line pipes, NACE standard TM0177 is used.
-90, but in this case, the setting criterion for the load stress is determined based on the strength of the base material, so TMC including the softened portion is included.
It is known that in P steel, cracking occurs preferentially in the softened portion where the stress condition becomes severe. In addition, recently, mainly in the United Kingdom, a test method for performing a sulfide stress cracking resistance evaluation test of a line pipe, a so-called CAPCIS test, is being standardized. In the CAPCIS test, the HAZ softened portion is included in the evaluation, and since ultrasonic flaw detection is adopted as an evaluation means, minute internal cracks generated in the softened portion can be regarded as a problem.

As a method of preventing the sulfide stress cracking of the softened portion, conventionally, (1) a method of setting the base material strength to a high value in anticipation of softening has been adopted. Further, (2) the following measures are disclosed as a method for preventing softening. A method of preventing the HAZ from being deteriorated in strength by precipitation hardening of a nitride or the like (for example, JP-A-62-284043). A method of heat-treating a welded portion after welding to improve softening (for example, JP-A-4-168220). These means are effective in terms of increasing the strength of the softened portion or preventing softening, but have the following drawbacks when actually applied to the production of line pipes.

[0006]

When the base metal strength is set high as described in (1) above, for example, when a steel pipe having a strength of X60 is supplied in response to a demand of X46, the cost of the alloy is unnecessarily increased. It will increase the manufacturing cost.
Also, depending on the customer, since the upper limit of strength is set,
Such a method does not work at all.

On the other hand, when precipitation hardening is used as in the above (2), the degree of softening is certainly reduced, but the softened portion is not completely eliminated. Further, since the nitride of V is used as the precipitation element, the fracture toughness of the welded portion, which is one of the important characteristics in the line pipe, is reduced. Further, although the method (2) is certainly a means for preventing softening, a large heat treatment means is required for application to the production of UOE steel pipes, and productivity is impaired, resulting in an increase in cost. Therefore, it is not a realistic method. As described above, as a method for obtaining the sulfide stress cracking resistance of the welded portion, there are few means for improving softening, and even if it can be adopted, it is not an industrially advantageous means.

Therefore, assuming the presence of HAZ softening,
Securing the sulfide stress cracking resistance of the softened part is an issue in manufacturing a line pipe having excellent sulfide stress cracking resistance of the welded part.

[0009]

The present invention advantageously solves the above-mentioned problems by adding Ca, which is necessary for controlling the morphology of sulfides in ladle refining, and then adding Ca, Al and Si. By adjusting the amount, the morphology of the oxide, which is the starting point of sulfide stress cracking, is controlled to obtain sulfide stress cracking resistance even in the softened portion.

That is, the gist of the present invention is that
% By weight, C: 0.03 to 0.09%, Si: 0.1
To 0.4%, Mn: 0.5 to 1.0%, P: 0.01
5% or less, S: 0.0010% or less, Nb: 0.01
0-0.050%, Al: 0.005-0.03%, C
a: 0.001-0.003%, Ti: 0.005-
0.025%, O: 0.0050% or less is contained, and if necessary, V: 0.01 to 0.1%, Ni:
0.05-0.5%, Cu: 0.05-0.5%, C
In the ladle refining of steel containing one or two or more of r: 0.05 to 0.5% and Mo: 0.05 to 0.5%, and the balance consisting of iron and unavoidable impurities, the following ( 1),
Ca, Al, Si so as to satisfy the expressions (2) and (3).
The method for producing a steel for line pipes having excellent resistance to sulfide stress cracking of welded portions is characterized by adjusting the addition amount of.

[0011]

[Equation 3]

0.07 <(% Ca) / (% Al) <0.17 (2) 6 <(% Si) / (% Al) <25 (3)

[0013]

[Function] The inventors of the present invention have manufactured X42 to X5 manufactured on an actual machine.
Experiments were performed to reproduce sulfide stress cracking in the weld softened portion having high crack susceptibility for two grades of various UOE line pipes, and the starting point of cracking was observed by observing the fracture surface of the initial crack generated in the softened portion. As a result, when the sulfide morphology control by Ca is not sufficiently performed, relatively large cracks occur, and the cracks start from elongated MnS,
When the sulfide morphology control by Ca is sufficiently performed, relatively small cracks occur, and the origins of the cracks are CaO.Al ₂ O ₃ based oxides or Al ₂ O ₃
It was found to be a SiO ₂ type oxide. Therefore, in order to prevent the sulfide stress cracking of the softened portion, the present inventor first
It was considered necessary to sufficiently control the morphology of sulfides. As a result of investigating various UOE line pipes of X42 to X56 grades manufactured on an actual machine, it is necessary to satisfy the following formula (1) with respect to the amount of Ca necessary for sufficiently controlling the form of sulfide in ladle refining. It was found that

[0014]

[Equation 4]

After satisfying these conditions, it is assumed that the existence state of oxides varies depending on the addition amounts of Ca and Al and the addition amounts of Si and Al, and various Ca / Al and Si / Al UOEs are changed. Regarding line pipe, NACE standard TM0177-90 from the seam welded portion in the direction perpendicular to the weld line.
A test piece according to method A was prepared, a constant load test was carried out according to the same standard, and a stress σth at a limit at which the test piece did not break was determined after 720 hours. σth is normalized by the standard minimum yield stress SMYS of the steel pipe, and σth / SMYS ≧ 0.
8, it was judged that it had excellent sulfide stress cracking resistance, and the occurrence of sulfide stress cracking in the weld was examined. As shown in FIG. 1, it has been found that excellent sulfide stress cracking resistance can be obtained even in the softened portion if the following expressions (2) and (3) are satisfied in ladle refining.

0.07 <(% Ca) / (% Al) <0.17 (2) 6 <(% Si) / (% Al) <25 (3) Based on the above facts, the reason to be described later If the above conditions are satisfied in ladle refining after limiting the chemical components with, the conclusion is that it is possible to manufacture API grade X46 to X52 line pipes with excellent resistance to sulfide stress cracking in welds. Got

Next, the reasons for limiting the components in the present invention will be described. Since C is a strengthening element, it is made 0.03% or more to obtain a desired strength. On the other hand, if added in a large amount, the hardness of the base material of the line pipe and the welded portion will increase, the toughness will decrease, and sulfide stress cracking will easily occur in a hydrogen sulfide environment, so 0.09% or less. To do.

Si is a deoxidizing element. If it is less than 0.1%, sufficient deoxidizing power cannot be obtained, and if it exceeds 0.4%, the steel is embrittled, so that 0.1 to 0. 4%.
Mn forms MnS, which is a starting point of sulfide stress cracking, and co-segregates with P that promotes embrittlement of steel to promote crack propagation and progress. Therefore, the amount of Mn added is as low as possible. Is preferable. However, since Mn is an essential element for obtaining strength and toughness, it is set to 0.5 to 1.0% in order to obtain the strength of the line pipe from X42 to X52.

P is an element that facilitates the propagation of sulfide stress cracking due to segregation, and the lower the better, 0.015
% Is the upper limit. Since S is combined with Mn to form MnS, which is the starting point of sulfide stress cracking, S is preferably as low as possible. From the viewpoint of obtaining stable crack resistance,
The upper limit is 0.0010%.

Nb is added in an amount of 0.010% or more in order to make the rolling structure finer grain, improve hardenability and precipitation hardening.
Even if added over 0%, the effect of adding a large amount is small, and rather coarse carbides are formed and hydrogen-induced cracking resistance is lowered, so 0.050% is made the upper limit. Al
Is important as a deoxidizing element, but if added in a large amount, it contaminates the steel and reduces toughness, so 0.005 to 0.
It is set to 03%.

[0021] Ca is added in an amount of 0.001% or more in order to control the shape of sulfide inclusions such as MnS. However, if added in a large amount, the steel is contaminated, so Ca is made 0.003% or less. The lower limit of 0.005% of the amount of Ti added is the minimum amount at which fine TiN is formed and micronization of the microstructure is expected, and the upper limit is 0.025% from the condition that the toughness does not decrease due to TiC. .

O remains in the steel to form an oxide, but O
When the amount is large, and as a result, the amount of oxide is large, it becomes a starting point of hydrogen-induced cracking and sulfide stress cracking and also lowers toughness, so 0.0050% is made the upper limit. In the present invention, in addition to the above elements, V, Ni, C may be further added if necessary.
One, two or more of u, Cr and Mo may be added.

V is added as a strengthening element in an amount of 0.01% or more, and if added excessively, the toughness is lowered, so V is made 0.1% or less. Ni, Cu, Cr, and Mo are all added when it is necessary to increase the hardenability and strength of steel, but excessive addition forms a low temperature transformation product, resulting in toughness and hydrogen-induced cracking resistance. Is impaired, so 0.5
% Is the upper limit. Further, if less than 0.05%, the effect of addition is small, so 0.05% is the lower limit.

The present invention provides excellent welding by adjusting the amounts of Ca, Al and Si added in accordance with the above formulas (1), (2) and (3) in ladle refining of line pipe steel having the above-mentioned components. Imparts sulfide stress cracking resistance to the part. Equation (1) shows that elongated MnS causes relatively large sulfide stress cracking.
This is a condition necessary to sufficiently control the morphology of sulfides in order to prevent the formation of sulfide. The above formula (2) is CaO · A existing in a group that causes relatively small sulfide stress cracking.
Conditions for preventing formation of l ₂ O ₃ -based oxide, the above formula (3)
Is a condition for preventing the formation of Al ₂ O ₃ .SiO ₂ -based oxides existing in groups that cause relatively small sulfide stress cracking. These conditions are also sufficient for obtaining the sulfide stress cracking resistance of the base material of the steel pipe.

[0025]

[Example] Steels having chemical compositions shown in Tables 1 and 2 (following Table 1) were melted in a converter, and the components were adjusted by ladle refining.
Slab is manufactured by continuous casting, and after rolling thick plate, UOE
It was made into a steel pipe. The size of the steel pipe has an outer diameter of about 30 inches and a pipe thickness of about 20 mm, and each steel pipe satisfies API standards X42 to X52, although it depends on the composition. NACE standard TM0177-9 from the seam weld to the direction perpendicular to the weld line
A test piece according to 0 method A was sampled and produced,
A similar test piece was sampled from the base material of the steel pipe 180 ° away from the seam weld. A constant load test was performed according to the same standard, and after 720 hours, a limit stress σth at which the test piece did not break was determined. σth is the standard minimum yield stress S of the steel pipe
It was judged to have excellent sulfide stress cracking resistance by standardizing with MYS and having σth / SMYS ≧ 0.8.

As shown in Table 2, under the conditions according to the present invention, in both cases, σth / SMYS for both the weld and the base metal.
≧ 0.8, and excellent sulfide stress cracking resistance was obtained. However, in Comparative Example 1, the value of formula (1), in Comparative example 2 the amount of Ca and the value of formula (1), in Comparative example 3 the amount of S and the value of formula (1), and in Comparative examples 4 and 5, The values of the formula (2), the values of the formula (3) in the comparative examples 6 and 7, and the values of the formula (2) and the formula (3) in the comparative examples 8 and 9 are outside the scope of the present invention. In addition, the sulfide stress cracking resistance is deteriorated.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

INDUSTRIAL APPLICABILITY According to the present invention, AP having excellent resistance to sulfide stress cracking of welds in a humid hydrogen sulfide environment
I grades X42 to X52 steels for line pipes are obtained, so that the industrial effect is remarkably large.

[Brief description of drawings]

[Fig. 1] Ca / Al ratio of 0.07 to 0.
17 is a diagram showing that the sulfide stress cracking resistance of the welded portion is high when the Si / Al ratio in 17 is in the range of 6 to 25. FIG.

Claims (2)

[Claims] 1. By weight%, C: 0.03 to 0.09%, Si: 0.1 to 0.4%, Mn: 0.5 to 1.0%, P: 0.015% or less, S: 0.0010% or less, Nb: 0.010 to 0.050%, Al: 0.005 to 0.03%, Ca: 0.001 to 0.003%, Ti: 0.005 to 0.025. %, O: 0.0050% or less, and the balance of Ca and A to satisfy the following formulas (1), (2) and (3) in the ladle refining of steel containing the balance of iron and inevitable impurities.
1. A method of melting steel for line pipes, which is excellent in sulfide stress cracking resistance of a welded portion, characterized by adjusting an addition amount of 1 and Si. [Equation 1] 0.07 <(% Ca) / (% Al) <0.17 (2) 6 <(% Si) / (% Al) <25 (3) 2. By weight%, C: 0.03 to 0.09%, Si: 0.1 to 0.4%, Mn: 0.5 to 1.0%, P: 0.015% or less, S: 0.0010% or less, Nb: 0.010 to 0.050%, Al: 0.005 to 0.03%, Ca: 0.001 to 0.003%, Ti: 0.005 to 0.025. %, O: 0.0050% or less, V: 0.01 to 0.1%, Ni: 0.05 to 0.5%, Cu: 0.05 to 0.5%, Cr: 0. In the ladle refining of steel containing one or two or more of 05 to 0.5% and Mo: 0.05 to 0.5%, and the balance being iron and inevitable impurities, the following (1), ( A line pie excellent in sulfide stress cracking resistance of welds, characterized in that the addition amounts of Ca, Al and Si are adjusted so as to satisfy the expressions (2) and (3). The method of melting of use steel. [Equation 2] 0.07 <(% Ca) / (% Al) <0.17 (2) 6 <(% Si) / (% Al) <25 (3)