JPH06256894A - High strength line pipe excellent in hydrogen induced cracking resistance - Google Patents

Abstract

PURPOSE:To prevent the generation of hydrogen induced cracking on a line pipe even in an NACE environment by specifying the compsn. and furthermore limiting the size and P concn. of the spot segregated parts of Mn and the effective Ca content required for the control of the form of sulfides. CONSTITUTION:This pipe line contains, by weight, 0.03 to 0.09% C, 0.1 to 0.6% Si, 1.1 to 1.5% Mn, <=0.015% P, <=0.0010% S, 0.010 to 0.050% Nb, 0.005 to 0.05% Al and 0.002 to 0.004% Ca and contains one or more kinds among 0.005 to 0.025% Ti, 0.01 to 0.1% V, respectively 1.0% of Ni, Cu and Cr and <=0.5% Mo, and the balance iron. Then, the size of Mn segregated spots, the areas in which the concn. of Mn is >=1.32 is regulated to <500mum, the concn. of P in the segregated party is regulated to <0.035 and the effective Ca ratio by the formula: effective Ca ratio [(%Ca) (1-98(%O))}/(%S) is regulated to >=1.7.

Description

Detailed Description of the Invention

[0001]

This invention relates to API grade X60, which has hydrogen-induced cracking resistance in a moist hydrogen sulfide environment.
To X70 line pipe.

[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, and so-called hydrogen-induced cracking may occur even when there is no additional stress from the outside. Therefore, hydrogen-induced cracking resistance is required for line pipes that may be exposed to sour environments.

Regarding the mechanism of occurrence of this hydrogen-induced cracking,
Various studies have been conducted, and there is a mechanism that cracks occur due to gas pressure when infiltrating hydrogen diffuses and accumulates at the interface between the nonmetallic inclusions stretched by hot rolling and the base iron, and becomes molecular hydrogen. Widely recognized. A representative of this stretched inclusion is MnS. Further, in the slab produced by continuous casting, since center segregation is generally present, MnS is likely to be formed due to segregation of Mn, and hardness that promotes crack propagation due to segregation of Mn and P. Area of high.

Based on the above-mentioned research on the crack generation mechanism, the following hydrogen-induced crack prevention measures have heretofore been adopted and put into practical use in the production of line pipes, and the effects have been improved. (1) Highly purified S is reduced as much as possible in the steelmaking stage to reduce the amount of MnS. Further, P is reduced as much as possible to reduce the hardness of the segregated portion. (2) Reduction of macro center segregation At the solidification end of continuous casting, measures such as preventing bulging of the slab are taken to reduce macro segregation.

(3) Morphology control of sulfides The sulfide morphology is changed to Mn by Ca treatment in the secondary refining.
From S to CaS that is difficult to be stretched during hot rolling. (4) Controlled Rolling, Microstructure Control by Accelerated Cooling At the rolling stage of the steel pipe original plate, controlled rolling and accelerated cooling are applied to make the metallographic structure as uniform as possible to increase the crack resistance (for example, JP-A-58-133348). Issue).

[0006]

As a test method for evaluating hydrogen-induced cracking resistance, TM02 standardized by NACE is used.
84 is widely used. In this method, a strip-shaped test piece cut out from a line pipe is subjected to an immersion test in a sour environment, and the cross section of the test piece is observed to determine the rate of hydrogen-induced cracking. The test environment of the standard has a PH of about 5, but with the recent shift to the sour environment of oil wells, the PH specified by NACE standard TM0177-90 Method A
It has become common to evaluate in about 3 environments (hereinafter referred to as NACE environment).

Further, regarding the determination of the occurrence of cracks, rather than observing the cross section, the test piece is ultrasonically flaw-tested to determine the crack more strictly, that is, more strict quality assurance is required. There is. However, only by applying the above-mentioned conventional technique, it is not possible to completely eliminate the cracks detected by ultrasonic waves in the immersion test in the NACE environment. From this viewpoint, NAC detected by ultrasonic flaw detection
Setting a condition for preventing hydrogen-induced cracking that occurs in the E environment is an issue in manufacturing a high-strength line pipe having excellent hydrogen-induced cracking resistance.

[0008]

Means for Solving the Problems The present invention advantageously solves the above-mentioned problems, and is the size of the spot segregated portion of Mn, the P concentration of the segregated portion, and the sulfide content which are the starting points of hydrogen-induced cracking. NACE by limiting the amount of effective Ca required for morphology control
It is to prevent hydrogen-induced cracking in the environment.

That is, the gist of the present invention is that
% By weight, C: 0.03 to 0.09%, Si: 0.1
0.6%, Mn: 1.1 to 1.5%, P: 0.015%
Hereinafter, S: 0.0010% or less, Nb: 0.010
0.050%, Al: 0.005-0.05%, Ca:
Contains 0.002-0.004%, Ti: 0.005
.About.0.025%, V: 0.01 to 0.1%, Ni: 1.
0% or less, Cu: 1.0% or less, Cr: 1.0% or less,
Mo: contains 0.5% or less of one or more kinds, the balance is iron and inevitable impurities, and the concentration of Mn is an average M
The size of the Mn segregation spot, which is a region of 1.32 or more of the n concentration, is less than 500 μm, and the concentration of P in the segregation portion is 0.
A high-strength line pipe excellent in hydrogen-induced cracking resistance, characterized in that it is less than 035% and the effective Ca ratio calculated by the formula (1) is 1.7 or more. Effective Ca ratio = {(% Ca) (1-98 (% O))} / (% S) ……… (1)

[0010]

The present inventors have taken measures to prevent hydrogen-induced cracking (1) high purity, (2) reduction of macro center segregation, (3) morphology control of sulfide, (4) controlled rolling, microstructure by accelerated cooling. The fracture surface of hydrogen-induced cracking, which is still generated even with the control, was observed, and the cause was considered. As a result, even after the macro-center segregation is removed, hydrogen-induced cracking starts from the group-like MnS, and the region where the group-like MnS exists corresponds to the spot-like segregation part of Mn. And in that P
It was found that the Ca treatment did not act effectively in addition to the segregation of

Based on this result, the X60 manufactured by an actual machine
To various XOE grade UOE line pipes, the relationship between the size of the Mn spot segregation part, the P concentration of the segregation part and the occurrence of hydrogen-induced cracking in the NACE environment was investigated.
As shown in FIG. 1, the Mn concentration was 1.3 times the average Mn concentration.
When two or more regions are defined as Mn spot segregation parts, M
n spot segregation portion size is less than 500 μm, and
It was found that hydrogen-induced cracking does not occur if the following conditions regarding effective Ca are satisfied when the P concentration in the segregated portion is less than 0.035%. Figure 1 shows an effective Ca ratio of 1.7.
In the above case, the size of the Mn spot segregation part is 500μ.
If it is less than m and the P concentration in the segregated portion is less than 0.035%, it indicates that hydrogen-induced cracking does not occur.

In addition to the above-mentioned segregation-related conditions, the present inventors have examined the conditions for the amount of Ca necessary for sufficiently controlling the sulfide morphology. As a result, as shown in FIG. 2, when the effective Ca ratio represented by the equation (1) is 1.7 or more, it was found that hydrogen-induced cracking does not occur if the above-mentioned conditions for segregation are satisfied. . FIG. 2 shows that the size of the Mn spot segregated portion is less than 500 μm and the P of the segregated portion is P.
If the effective Ca ratio is 1.7 or more when the concentration is less than 0.035%, hydrogen-induced cracking does not occur.

Formula (1) is Ca excluding Ca consumed as oxides, that is, Ca and S acting on the formation of sulfides.
Theoretically, it should be possible to control the morphology of MnS at 1 or more, but in reality, the MnS spot segregation portion is formed, so the ratio must be 1.7 or more. There is. Effective Ca ratio = {(% Ca) (1-98 (% O))} / (% S) ……… (1)

Based on the above facts, the chemical composition is limited for the reasons described below, and the size of the Mn spot segregation portion is
It was concluded that it is possible to manufacture API grade X60 to X70 line pipes with excellent hydrogen-induced cracking resistance in a NACE environment by limiting the P concentration in the segregation part and the amount of effective Ca required for morphology control of sulfide. 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, and if it is less than 0.1%, sufficient deoxidizing power cannot be obtained, and if it exceeds 0.6%, the steel becomes brittle, so that 0.1 to 0.1%. 6%.
Mn forms MnS, which is the starting point of hydrogen-induced cracking, and co-segregates with P, which promotes embrittlement of steel, to promote the propagation and progress of hydrogen-induced cracking. The lower the better. However, since Mn is an essential element for obtaining strength and toughness, X60
To obtain the strength of the X70 line pipe from 1.1 to
1.5%.

P is an element that facilitates the propagation of hydrogen-induced 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 hydrogen-induced cracking, S is preferably as low as possible. From the viewpoint of preventing hydrogen-induced cracking of the line pipe in the NACE environment, the upper limit is 0.0010%. N
b is added in an amount of 0.010% or more for the purpose of refining the rolling structure, improving the hardenability and precipitation hardening, but if added in excess of 0.050%, the effect of adding a large amount is small, but rather coarse. Form carbides and reduce hydrogen-induced cracking resistance.
The upper limit is 0.05%.

Al is important as a deoxidizing element, but if added in a large amount, it contaminates the steel and lowers toughness.
It is set to 0.005% to 0.05%. Ca is 0.002% in order to control the shape of sulfide inclusions such as MnS.
The above is added, but if added in a large amount, the steel is contaminated, so the content is made 0.004% or less. In the present invention, one or more of Ti, V, Ni, Cu, Cr and Mo are added to the above elements.

The lower limit of the Ti addition amount of 0.005% is the minimum amount at which fine TiN is formed and the microstructure is expected to be finely grained, and the upper limit is 0.025 from the condition that the toughness is not deteriorated by TiC. %. V is 0.01 as a strengthening element
% Or more, and if added excessively, the toughness decreases, so 0.1% or less. Ni, Cu, Cr, and Mo are added when it is necessary to increase the hardenability of steel and increase the strength, but excessive addition forms a low temperature transformation product, resulting in toughness and hydrogen-induced cracking resistance. Is lost, so N
The upper limit of i, Cu, and Cr is 1.0%, and the upper limit of Mo is 0.5%.

The present invention relates to a line pipe having the above-mentioned components, by limiting the size of the Mn segregation portion and the P concentration in the segregation portion, and by adjusting the effective Ca addition amount, excellent hydrogen-induced cracking resistance can be obtained. Imparts sex.

Even after the central segregation is reduced and the macroscopic central segregation is removed, if there is a spot-like Mn segregation portion, MnS is grouped in the spot segregation portion even if Ca treatment is performed. It is formed and acts as a starting point of hydrogen-induced cracking. Also, P tends to segregate in the Mn segregation portion, which promotes the progress of hydrogen-induced cracking.

It is quite difficult to eliminate the spot segregation portion at present, but by reducing the size of the spot segregation portion, it does not act as a starting point of hydrogen-induced cracking in the NACE environment. As a premise in this case, morphology control of sulfide by Ca is essential, and it is necessary to secure an effective amount of Ca that is combined with S to make the Mn segregated portion substantially harmless with respect to occurrence of hydrogen-induced cracking. .

From this point of view, as a condition for preventing hydrogen-induced cracking in the NACE environment, when a region where the Mn concentration is 1.32 or more of the average Mn concentration is defined as the Mn spot segregation portion, the size of the Mn spot segregation portion is (Length in the direction perpendicular to the rolling direction, that is, the width of the Mn spot segregation portion) is 500
It is less than μm, the P concentration in the segregated portion is less than 0.035%, and the effective Ca ratio represented by the formula (1) is 1.7 or more. Effective Ca ratio = {(% Ca) (1-98 (% O))} / (% S) ……… (1)

[0024]

Example A steel having the chemical composition shown in Table 1 was melted, a slab was manufactured by continuous casting, and after rolling a thick plate, a UOE steel pipe was formed. The outer diameter of the steel pipe is about 30 inches, and the pipe thickness is about 20 mm. API specifications X
Satisfies 60 to X70. 180 ° from seam weld
Immersion test pieces were prepared from the base material of the separated steel pipe according to NACE standard TM0284, and NACE standard TM0177-9 was prepared.
The immersion test was performed according to the procedure of TM0284 under the environmental conditions of 0 MethodA.

There were 5 test pieces from the same steel pipe. After the completion of the immersion test, the test piece was scanned with an ultrasonic flaw detector with a frequency of 25 MHz, and the percentage of the width x length of 2000 mm ²
The value detected whether hydrogen-induced cracking occurs at a ratio of C, C
AR (%) was determined, and CAR = 0 was defined as having hydrogen-induced cracking resistance. When hydrogen-induced cracking occurred, the average value of the CAR of the five test pieces was taken as the CAR of the steel pipe.

The cross section of the crack was measured by EPMA, and M
The n-spot segregation size and the P concentration in the segregation part were measured. On the other hand, when hydrogen-induced cracking did not occur, hydrogen was introduced into the test piece by the electrolytic charging method, and the Mn spot segregation size and the P concentration in the segregation portion were measured in the cross section of the hydrogen cracking that occurred. As shown in Table 1, under the conditions according to the present invention,
In each case, CAR = 0%, and excellent hydrogen-induced cracking resistance was obtained.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

However, the Mn spot segregation size in Comparative Examples 1 and 2, the Mn amount and the Mn spot segregation size in Comparative Example 3, the P concentration in the segregation portion in Comparative Example 4, the Mn spot segregation size in Comparative Example 5, The P concentration in the segregated portion, the P amount in Comparative Example 6 and the P concentration in the segregated portion, the effective Ca in Comparative Example 7
In the comparative example 8, the Ca amount and the effective Ca ratio in Comparative Example 8 and the S amount and the effective Ca ratio in Comparative Example 9 are outside the scope of the present invention.
Hydrogen-induced cracking occurs respectively.

[0031]

According to the present invention, API grade X60 having hydrogen-induced cracking resistance in a humid hydrogen sulfide environment.
X70 line pipe is obtained from the above, the industrial effect is remarkably large.

[Brief description of drawings]

FIG. 1 is a diagram showing a P concentration in a segregated portion and a Mn spot segregated size.

FIG. 2 is a chart of hydrogen-induced cracking rate and effective Ca ratio.

Claims (1)

[Claims] 1. By weight%, C: 0.03 to 0.09%, Si: 0.1 to 0.6%, Mn: 1.1 to 1.5%, P: 0.015% or less, S: 0.0010% or less, Nb: 0.010 to 0.050%, Al: 0.005 to 0.05%, Ca: 0.002 to 0.004%, Ti: 0.005 to 0.005% 0.025%, V: 0.01 to 0.1%, Ni: 1.0% or less, Cu: 1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, or The size of the Mn segregation spot, which is a region containing two or more kinds, the balance being iron and unavoidable impurities, and having a Mn concentration of 1.32 or more of the average Mn concentration, is 500 μm.
Less, the P concentration in the segregated portion is less than 0.035%, and the effective Ca ratio calculated by the formula (1) is 1.7 or more. High strength excellent in hydrogen-induced cracking resistance. Line pipe. Effective Ca ratio = {(% Ca) (1-98 (% O))} / (% S) ……… (1)