JPH10204587A - High cr steel for line pipe, excellent in sulfide stress corrosion cracking resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high Cr steel for line pipe, excellent in corrosion resistance and stress corrosion cracking resistance in a carbon dioxide gas environment and a hydrogen sulfide environment as well as in weldability and toughness in a weld zone and reduced in alloying elements. SOLUTION: This steel has a composition which consists of, by weight, <=0.03% C, <=0.40% Si, <=0.35% Mn, <=0.020% P, <=0.005% S, <=5.0% Ni, 10-13% Cr, 1.0-4.0% Mo, <=0.03% To, 0.02-0.08% Al, <=0.02% N, 0.0005-0.003% Ca, and the balance Fe with inevitable impurities and in which the value of M determined by equality M=%Cr+%Mo×1.3-%Ni is regulated to <=11.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high Cr line pipe steel having excellent sulfide stress corrosion cracking resistance.

[0002]

2. Description of the Related Art In pipelines used for transporting gas or oil containing carbon dioxide or hydrogen sulfide,
It is necessary to take measures against stress corrosion cracking due to sulfide. Therefore, when a normal carbon steel pipe is used, a corrosion inhibitor (inhibitor) is used in combination, or a duplex stainless steel pipe or a clad steel pipe having high corrosion resistance is used. However, the use of a corrosion inhibitor has problems of environmental pollution as well as cost. Further, there is a problem that the material cost of the duplex stainless steel pipe and the clad steel pipe is high.

Against this background, AISI (American Iron and Steel Institute) 410 steel is a relatively inexpensive material. However, this steel has a problem in weldability and the like, and the problem of stress corrosion cracking in a hydrogen sulfide environment still remains.

A material having weldability and excellent stress corrosion cracking resistance in a hydrogen sulfide environment and a method for producing the same have been proposed.
Some have been proposed. For example, JP-A-55-215
No. 66 discloses that Mn is 1.0 to 3.5% and Cr is 10%.
Steel containing 13.5% (martensitic stainless steel,
The same applies hereinafter). Similarly, JP-A-4-99
No. 128 discloses that Cu is 1.2 to 4.5% and Cr is 11%.
-14% steel, disclosed in Japanese Patent Application Laid-Open No.
A steel containing 1.1 to 4.0% o and 11 to 14% Cr has been proposed.

Japanese Patent Application Laid-Open No. 5-156408 proposes a steel in which Cr is increased to 13 to 17%.
264192 also proposes a steel in which Cr is increased to 13 to 17%. On the other hand, Japanese Patent Application Laid-Open No. 6-306549 proposes a steel in which Cr is reduced to 10 to 13%. Further, Japanese Patent Application Laid-Open No. 8-3642 discloses Cu, N
A steel has been proposed in which i is 4.0% or less, Co is 2.0% or less, Mo and W are 3.0% or less, and the contents of these elements are defined by a certain relational expression. Furthermore, Japanese Patent Application Laid-Open No. 8-3642 proposes adding Nb, V, and Ti in a total amount of 0.1% or more.

[0006]

However, the technique described in Japanese Patent Application Laid-Open No. 4-99128, Japanese Patent Application Laid-Open No.
In the technique described in Japanese Patent No. 19, it is necessary to add Cu and Co. These elements have a problem that weldability and weld toughness are low.

JP-A-5-156408 and JP-A-6-156408
In the technique disclosed in Japanese Patent No. 264192, Cr needs to be added in a large amount of 13% or more. There is also a problem that the toughness of the heat affected zone is not good.

In the technique described in Japanese Patent Application Laid-Open No. 6-306549, the added elements are kept low overall, but the toughness and sulfide stress corrosion cracking resistance of the heat affected zone are not sufficient. In the technique described in Japanese Patent Application Laid-Open No. 8-3642, C
Although the upper limits are set for u, Ni, Co, Mo, and W, and the added elements are suppressed to some extent, the toughness and sulfide stress corrosion cracking resistance of the heat affected zone are not sufficient.

The present invention solves these problems of the prior art, is excellent in weldability and weld toughness, and is excellent in corrosion resistance and stress corrosion cracking resistance in a carbon dioxide gas environment and a hydrogen sulfide environment. To provide less high Cr line pipe steel.

[0010]

SUMMARY OF THE INVENTION The present invention provides, in weight percent,
C: 0.03% or less, Si: 0.40% or less, Mn:
0.35% or less, P: 0.020% or less, S: 0.00
5% or less, Ni: 5.0% or less, Cr: 10 to 13%,
Mo: 1.0 to 4.0%, Ti: 0.03% or less, A
l: 0.02 to 0.08%, N: 0.02% or less, C
a: 0.0005 to 0.003%, the balance being Fe and unavoidable impurities, and the value of M determined by the following formula is 11
This is a high Cr line pipe steel having the following excellent sulfide stress corrosion cracking resistance.

M =% Cr +% Mo × 1.3-% Ni First, the reasons for limiting the chemical components will be described.

C: When C exceeds 0.03%, sulfide stress corrosion cracking resistance and weldability deteriorate. Therefore, C
The amount is specified as 0.03% or less.

Si: If Si is added in excess of 0.40%, the workability is impaired, and the toughness and weldability of the heat affected zone are deteriorated. Therefore, the amount of Si is specified to be 0.40% or less.

Mn: If Mn is added in excess of 0.35%, the toughness of the heat affected zone and the resistance to sulfide stress corrosion cracking are deteriorated. Mn has a particularly remarkable tendency as compared with other elements such as C and Si, and the reduction of Mn is particularly important. Therefore, the Mn content is specified to be 0.35% or less.

P: If P exceeds 0.020%, the toughness and sulfide stress corrosion cracking resistance of the heat affected zone are deteriorated. Therefore, the P content is regulated to 0.020% or less.

S: If S exceeds 0.005%, the toughness and sulfide stress corrosion cracking resistance of the heat affected zone are deteriorated. Therefore, the amount of S is regulated to 0.005% or less.

Ni: Ni is added in order to obtain good corrosion resistance and base metal toughness. However, if the content exceeds 5%, the effect is saturated, and not only the cost is unnecessarily increased but also the weldability is deteriorated. Therefore, the amount of Ni is specified to be 5.0% or less.

Cr: To obtain good corrosion resistance, 1
It is necessary to add 0% or more of Cr. However, the addition exceeding 13% leads to deterioration of the toughness of the heat affected zone.
Therefore, the Cr content is defined in the range of 10 to 13%.

Mo: To obtain good sulfide stress corrosion cracking resistance, it is necessary to add 1.0% or more of Mo. However, if added in excess of 4.0%, the toughness and weldability of the heat affected zone deteriorate. Therefore, when the Mo amount is 1.0
It is specified in the range of ~ 4.0%.

Ti: Since the addition of a small amount of Ti improves the toughness and sulfide stress corrosion cracking resistance of the heat affected zone, it is an indispensable element for reducing other alloying elements. However, if it is added in excess of 0.03%, the toughness and sulfide stress corrosion cracking resistance of the heat affected zone are significantly deteriorated. Therefore, the amount of Ti is regulated to 0.03% or less.

N: If N exceeds 0.02%, the toughness of the base metal and the heat affected zone is impaired, and the weldability is degraded.
Therefore, the amount of N is regulated to 0.02% or less.

Ca: Addition of Ca improves sulfide stress corrosion cracking resistance. The lower limit is 0.0005%. However, if it exceeds 0.003%, the sulfide stress corrosion cracking resistance is rather deteriorated. Therefore,
The amount of Ca is defined in the range of 0.0005 to 0.003%.

The balance is Fe and inevitable impurities, but the inevitable impurities may include any element other than those described above as long as the object of the invention is not impaired.
As impurities, there are those mixed from scrap and those mixed in steelmaking work, but they may be included as impurities as long as they are mixed in normal work.

Next, the effect of elements that improve corrosion resistance or sulfide stress corrosion cracking resistance on the toughness of the weld heat affected zone was examined. These elements include, in particular, N
i, Cr, and Mo were examined. A Charpy impact test was performed on the weld heat affected zone of steels with various addition amounts of these elements.

FIG. 1 is a diagram showing the relationship between Ni, Cr, Mo and the toughness of the weld heat affected zone obtained by organizing the test results. The horizontal axis of the figure is% Cr +% Mo × 1.3, and the vertical axis is%.
Ni is shown. Here,% Cr and the like indicate the weight% of each element. In the figure, the mark ○ indicates -20 ° C in the Charpy impact test.
Indicates that the absorption energy (vE-20) is 50 J (joule) or more, and the symbol ● also indicates 50 J or less.

From FIG. 1, it can be seen that the region where the Charpy absorbed energy is 50 J or more (indicated by a circle) is a region above the diagonal straight line in the drawing (high Ni, low Cr / Mo side). The equation of this straight line is:% Cr +% Mo × 1.3-% Ni = 11, and when expressed by the aforementioned M, M = 11. here,
% Cr and the like indicate the weight% of each element. The region where the Charpy absorbed energy is 50 J or more (marked with ○) is M
Is equal to or less than 11, and an area in which M is equal to or less than 50 J (marked by ●) is an area in which M exceeds 11. Thus, in order to obtain good weld heat affected zone toughness, the value of M determined by Ni, Cr, and Mo needs to be 11 or less.

In FIG. 1, the upper limit (Ni: 5%) at which the effect of the addition of Ni is saturated is shown by a straight line parallel to the horizontal axis, which is necessary for obtaining good corrosion resistance or sulfide stress corrosion cracking resistance. Lower limit of Cr and Mo (10% and 1.0%, respectively)
% Cr +% Mo × 1.3, which is 11.3%) is shown by a straight line parallel to the vertical axis. From this, Ni, Cr,
As for the three elements of Mo, the region of the present invention corresponds to 3 in FIG.
It is indicated by a triangular area surrounded by straight lines of a book.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION The steel smelting of the present invention may be carried out by using a converter, an electric furnace, or any other production method capable of controlling chemical components within the scope of the invention. This steel is cast into a shape such as a slab for use as steel for line pipes. Then, if a steel plate is manufactured by hot rolling, a steel plate for a line pipe is obtained. The hot rolling may be manufactured by a thick plate mill, but when the width may be narrow (for small diameter pipes), the hot rolling may be manufactured by a hot strip mill.

[0029]

EXAMPLES Steel having the chemical composition shown in Table 1 was produced,
The sulfide stress corrosion cracking resistance, weldability, and toughness of the heat affected zone were examined. The corrosion resistance was examined by a corrosion test in artificial seawater saturated with carbon dioxide. The investigation of sulfide stress corrosion cracking resistance was performed by stress corrosion cracking test in artificial seawater saturated with a mixed gas of carbon dioxide and hydrogen sulfide. The weldability was investigated by a bending test. In order to investigate the toughness of the heat-affected zone of the weld, a V-notch Charpy test piece was cut out from the heat-affected zone of the weld and an impact test was performed.

[0030]

[Table 1]

The results of these tests are also shown in Table 1.
In this table, the M value is determined by the aforementioned Ni, Cr, and Mo.
, SSC indicates the stress corrosion cracking test, and toughness indicates the toughness of the heat-affected zone. Judgment of the test results was as follows: corrosion resistance was 0.5 mm / year or less of corrosion loss, SSC was not ruptured by immersion for 720 h in stress corrosion cracking test, and toughness was -2.
In Table 1, the absorbed energy at 0 ° C. was 50 J or more and the weldability was a bending radius of 2.0 t or less.

The developed steels (invented steels) A to L satisfying the chemical composition and the value of M according to the present invention have passed all the tests and have sufficient corrosion resistance and sulfide stress corrosion cracking resistance. It also has excellent weldability and toughness of the heat affected zone.

On the other hand, steel M has a high C content and a high Ni content, and the M value exceeds 11, and none of the sulfide stress corrosion cracking resistance, the toughness of the weld heat affected zone, and the weldability are sufficient. Steel N
Has a high Mn content and a low Cr content, and the corrosion resistance, sulfide stress corrosion cracking resistance, and toughness of the weld heat affected zone are not sufficient. Steel O
Has a high Ti content, and the sulfide stress corrosion cracking resistance and the toughness of the weld heat affected zone are not sufficient.

The steel P has a high Cr content, Mo content, and S content.
The value also exceeds 11, and none of the corrosion resistance, sulfide stress corrosion cracking resistance, toughness of the weld heat affected zone, and weldability are sufficient. Steel Q has a high P content and a low Mo content, has no added Ti, and has insufficient sulfide stress corrosion cracking resistance and toughness of the weld heat affected zone. Steel R has a high Si content and a high N content, and the toughness and weldability of the heat affected zone are not sufficient.

The steel S has an M value exceeding 11, and the sulfide stress corrosion cracking resistance and the toughness of the weld heat affected zone are not sufficient.
Steel T has a Ca content higher than the range of the invention and an M value exceeding 11, and the sulfide stress corrosion cracking resistance and the toughness of the weld heat affected zone are not sufficient. Steel U contains Ni, Cr, Mo, and other chemical elements within the scope of the invention, but has an M value exceeding 11, and the toughness of the weld heat affected zone is not sufficient.

[0036]

According to the present invention, by specifying the amounts of Ni, Cr and Mo and the relation between these elements, the amount of Ti and the amount of other main elements, sufficient corrosion resistance and sulfuration resistance can be obtained. A high Cr line pipe steel having excellent material stress corrosion cracking properties and excellent weldability and toughness of the heat-affected zone can be provided at low cost.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between Ni, Cr, Mo and the toughness of a heat affected zone by welding.

Claims (1)

[Claims] 1. The method according to claim 1, wherein C: 0.03% or less, Si:
0.40% or less, Mn: 0.35% or less, P: 0.02
0% or less, S: 0.005% or less, Ni: 5.0% or less, Cr: 10 to 13%, Mo: 1.0 to 4.0%, T
i: 0.03% or less, Al: 0.02 to 0.08%,
N: 0.02% or less, Ca: 0.0005 to 0.003
%, The balance of Fe and inevitable impurities, and the value of M determined by the following formula is 11 or less, and is a high Cr line pipe steel excellent in sulfide stress corrosion cracking resistance. M =% Cr +% Mo × 1.3-% Ni