JPH02267241A - Steel for line pipe having excellent hydrogen induced cracking resistance and sulfide stress corrosion cracking resistance - Google Patents

Abstract

PURPOSE:To improve the hydrogen induced cracking resistance and sulfide stress corrosion cracking resistance of the steel in a low pH environment by regulating the contents of C, Si, Mn, P, S, Al, N, etc., in a steel and specifying its ACR value. CONSTITUTION:The compsn. of the steel for line pipes is constituted of, by weight, <0.05% C, <=0.5% Si, 0.5 to 1.7% Mn, <=0.0070% P, <=0.0010% S, 0.01 to 0.10% Al, <=0.0050% N, <0.0010% O, 0.0010 to 0.0040% Ca and the balance Fe with inevitable impurities. Furthermore, the value of ACR found by the formula is regulated so as to satisfy 0.5<=ACR<=2.8. It required, optimum amounts of one or more kinds among Cu, Ni, Cr, Mo, Nb, V, Ti and B are moreover incorporated thereto. The steel material is suitable to the transportion of petroleum, natural gas, etc.

Description

Detailed Description of the Invention <Industrial Application Field> The present invention provides hydrogen-induced cracking (hydrogen-induced cracking) suitable for transporting oil and natural gas that form a humid environment containing hydrogen sulfide (++xs) (hereinafter referred to as a sour environment). The present invention relates to a line pipe steel with excellent resistance to 111C) and sulfide stress corrosion cracking (SSC).

<Conventional Technology> Since the advent of oil mining, extraction of so-called sour oil and natural gas, including Has, has become more active, and in recent years major pipelines have been designed on the premise of sour environments.

In this design, old c, ss causing fatal destruction
The biggest challenge is to prevent the occurrence of cracks, which are unique to sour environments such as those found in sour environments such as C, and the solution to this problem has a great deal to do with the selection of materials to be used, as well as construction costs. Countermeasures include surface coating of pipes, pre-treatment of transported materials, and use of chemicals called inhibitors, but the excellent +11C and SSC resistance of the steel plate itself, which is the material of the pipe, is inexpensive and This is the basis for providing stable safety.

Both 111c and SSC are cracks caused by hydrogen (H) atoms generated by corrosion on the surface of 18S contained in the transported material entering the steel material.

1113 occurs when H that has entered the steel material accumulates in MnS in the steel material and oxide-based inclusions on clusters, and then becomes molecular, and the embrittlement effect of H itself and the pressure of molecularization cause crack openings. Furthermore, the segregation bands in steel facilitate the propagation of cracks that occur, leading to large and dangerous cracks.

111c prevention methods include: (1) Suppressing surface corrosion by adding Ni, Cu, and Cr.

(2) MnS is elongated by rolling to increase cracking susceptibility, and Ca, Rf! By adding M, it becomes a CaS or REMEI compound, which maintains a spheroidized state even after rolling and reduces cracking susceptibility.

(JP-A No. 53-14606, JP-A No. 54-38214
(3) Reduce the amount of Mn, P, and S, or perform soaking diffusion treatment before rolling to reduce the degree of segregation.

(JP-A No. 52-111815, JP-A No. 50-9751
No. 5) (4) Improving the micro&ll#s of the segregated part by heat treatment after rolling or recently by mechanical heat treatment technology (TMCP). (Unexamined Japanese Patent Application No. 112722/1983, Special Publication No. 112722/1983
-23056, Special Publication No. 62-35452) have been proposed.

On the other hand, the SSC problem has been experienced for a long time in oil country tubular goods that come in contact with crude oil, etc., which are not subjected to sour treatment, but the pipe materials used have a yield point or 0.2% proof stress that exceeds 60 to 70 kgf/Kakutake. It was made of high strength steel. Therefore, it mainly reduces the surface hardness (e.g. NAC
[! The standard Rc (Rockwell C hardness) ≦22) has been taken as a preventive measure, and has been implemented in the form of limiting the carbon equivalent of steel materials and limiting the conditions during welding work.

〈Problem to be solved by the invention〉 However, in order to transport sour oil and natural gas, API5L has the ability to withstand higher hydrogen concentration environments.
Increasingly, customers are looking for line pipes in the X46 to X65 class. Evaluation of resistance in this harsh environment is
Low pH NACE solution (5% NaCN + 0.5% Cl
100H+saturated 1IIS), but the methods for preventing llIc and SSC listed above are not necessarily effective, and no definitive method for preventing them has yet been found.

111c, (1) Ni+Cu+Cr addition method does not exhibit its effect unless pn is 5 or more in the case of Cu, and (2) Ca, R[1M addition method also requires a harsher sour environment. However, this is not a sufficient preventive measure because it is difficult to completely spheroidize MnS in the center of the steel where impurities tend to accumulate.If large amounts of Ca and REM are added for the purpose of completely spheroidizing MnS, it will cause the steel to become spherical. The soaking-diffusion treatment of segregated elements (3), which increases the amount of cluster-like nonmetallic inclusions (hereinafter referred to as inclusions) other than the central portion and even increases HIC susceptibility, is disadvantageous in terms of cost. Even if the reduction of P and S is assumed, there is a limit to the reduction of Mn from the viewpoint of ensuring the strength of the steel material, and it is difficult to cope with the severe sour environment discussed here. (4) Post-rolling heat treatment and the use of TMCP may be effective, but the former has a limit in terms of capacity, especially during mass production, and the latter naturally has a limit in control capacity.

By the way, SSC is different from the previous ones, ^PI5L-X4
6~-X65 class relatively low strength line pipe,
It has been found that this phenomenon occurs even in materials whose hardness does not exceed Rc-22, and conventional hardness restrictions have become meaningless, with the exception of locally hardened parts due to welding and the like.

The purpose of the present invention is thus to address NAC [! It is an object of the present invention to provide a steel for line pipes that makes it possible to prevent the generation of old C and SSC in a more severe sour environment such as I-liquid.

<! Means for Solving Iff> According to the research conducted by the present inventors, the former C and 5SCO problems mentioned above under the more severe sour environment are mainly caused by inclusions in the steel material being deeply involved. It has been found that it is possible to dramatically improve the load C resistance and SSC resistance by reducing the amount and optimally controlling the distribution state. This can be achieved by controlling the composition of steel.

That is, in the present invention, C: less than 0.05%, 31:0.5
% or less, Mn amount 0.5-1.7%, P: 0.00
70% or less, S: 0.0010% or less, ^J! (
Soj! ): 0.01-0.10%, N: 0
．． 0.0050% or less, On less than 0.0010%, Ca
+ 0.0010 to 0.0040%, and the following formula (
+), (2), and if necessary, Cu
: 0.5% or less, Nl F 0.5% or less, Cr:
1.0% or less, No: 0.5% or less, Nb: 0
．． 10% or less, v:o, to% or less, Tl! 0.0
Hydrogen-induced cracking resistance and sulfide stress corrosion resistance characterized by containing one or more of 1 to 0.10%, B: 0.0050% or less, and the remainder consisting of Fe and inevitable impurities. A linepipe steel with excellent crackability.

0.5≦^CR≦2.8
(1)・−・−・・−・−(2) If this method is followed, the heavy C resistance and resistance of steel materials can be improved even in harsh environments such as low pH NACE solutions, which conventional methods could not adequately handle. SSC properties can be dramatically improved.

Effect〉 The reason for limiting the component range of the present invention will be explained.

Although the present invention focuses on controlling inclusions in steel materials,
In order to cope with the harsh sour environment that conventional methods cannot reach, it is basically necessary to avoid the ferrite-pearlite structure. The basic ingredient range to achieve this is C:0
．． Less than 0.05%, Sf: 0.5% or less, Mn: 0.5~
1.7%, P: 0.0070% or less1, S:
0.010% or less, Aj! :0.01~0.10%, N
: Must be 0°ooso% or less.

Although it is disadvantageous for improving the strength of the base material, the amount of C is intentionally set to 0.05.
The reason for setting it below % is that if the C content is high, martensite will be generated when controlled cooling is performed by TMCP, which will have a negative effect on toughness and ductility, promote a local increase in hardness of the welded joint, and cause cold cracking. This is because sensitivity increases. Furthermore, too much C concentrates between the dendrite trees during solidification, changing the solidification phenomenon into a form that promotes the segregation of P, S, and even h, but the upper limit of C should be limited to less than 0.05%. This phenomenon can be suppressed.

Sl is an element that is inevitably included in the deoxidation process of steel,
In order to improve the toughness of the heat affected zone (IIAZ) of welded joints, it is necessary to limit it to 0.5% or less.

Mn is an extremely important element that improves strength and toughness at the same time, but the required strength (here ^P
In order to obtain a strength equivalent to X46 to X70 of the IA5L standard, 0.5% or more is required. However, the amount of Mn is 1.7
%, a large amount of martensite will be produced and the above-mentioned segregation will become extremely pronounced, promoting the propagation of utc and ssc, resulting in an opposite effect.

P is an element that promotes center segregation and is preferably low in content, but from the viewpoint of economy it is set to 0.0070% or less. If this amount is exceeded, even mild center segregation will cause locally abnormal structures and promote cracking.

Al is an element that is inevitably included for deoxidation, but 0.0
If it is less than 1%, deoxidation will be insufficient and the toughness of the base material will deteriorate, so the lower limit was set at 0.01%. Moreover, when Al exceeds 0°10%, it deteriorates the HAZ toughness and forms cluster-like oxide-based inclusions, increasing the 1lIc and ssc susceptibility. Therefore, the upper limit was set at 0.10%.

Although N may generate coarse nitride-based inclusions and increase sensitivity to old C and SSC, it must be kept at 0.0050% or less in order to maintain the toughness of the welded joint, especially the toughness of the weld metal. When Nl exceeds o, ooso%, solid solution N in the weld joint metal increases due to N dissolving into the weld metal from dilution of the base metal, and additive elements such as B, which have a large effect on improving toughness in small amounts, cannot be effectively utilized. .

Now, next, the most important compositions in the present invention, S, O
, Ca component range will be described. As has been conventionally said, S and Ca have a great relationship in controlling the spheroidization form of elongated MnS by rolling, which promotes the propagation of llIc and SSC. In order to create a spherical shape with the lowest SSC sensitivity and to prevent fine particles from gathering and clustering, it is necessary to change the CR value to the range defined by equation (2) to the range of equation (1), ACR4! S
The amount of 0 is also taken into account, rather than the amount of Ca. This is due to the spheroidal morphology caused by conversion of MnS to CaS depending on the amount of 0.
This is because <8 abilities are affected.

0.5≦ACR≦2.8
(1) When ACR is smaller than 0.5, the amount of Ca is small or the amount of S is large, so that the spheroidization of HnS due to CaS formation is insufficient, and elongated MnS remains. On the other hand, 8CR is 2
．． If it exceeds 8, spheroidization is promoted, but the generated CaS is large and clustered, resulting in 111
c. Increases SSC sensitivity.

Even if these conditions are met, it is still difficult to cope with today's harsh sour environments, as the amount of oxide-based inclusions is still large. Oxide inclusions are 0.0010%
It is necessary to reduce it to below. 0 amount is 0.001
If it is 0% or more, minute spherical oxide inclusions (Ca
b, ^280. or a complex compound thereof) serves as the origin of llIC.

SSC occurs.

In the present invention, in addition to the above components, Cu: 0.5% or less, N1j 0.5% or less, Cr:
1.0% or less, Mo: 0.5% or less, Nb: 0.1
0% or less, V: 0.10% or less, Tl: 0.0
1 to 0.10%, B: 0°0050% or less, or 211 or more.

The main purpose of including these elements is to improve the strength and toughness and to expand the applicable plate thickness without impairing the characteristics of the steel of the present invention, and the amount of addition should be limited by oneself. .

Ni is an effective element for improving corrosion resistance and heavy carbon resistance.
Moreover, it improves the strength and toughness of the base material. However, it is also a hardening element, and if it exceeds 0.5%, the SSC resistance deteriorates in terms of hardness, which has been a problem in the past, so the upper limit was set at 0.5%.

As mentioned above, Cu is effective in preventing hydrogen intrusion in a relatively high pn sour environment, but if it exceeds 0.5%, M
Even if 1 is added, cranks occur during rolling, making manufacturing difficult. At the same time, like Xi, Cu is also a hardening element, and the lower the content, the better, and the upper limit was set at 0.5%.

Cr is an element that increases the strength of both the base metal and the welded part.
Although it can be used to expand the applicable plate thickness range, it is a hardening element and must be limited from the viewpoint of weldability, and the upper limit is 1.0%.
It is.

Mo has a great addition effect because it suppresses the formation of pearlite in the microstructure of steel and promotes bainite formation, which is good for old C and SSC.However, it also makes it easier to generate martensite, making it easier for sea bream to harden. , welded joint H
Considering the hardening of AZ, a limit is necessary and the upper limit is 0.5
%.

Nb is included to refine the rolling structure, improve hardenability, and precipitation harden. It is an important element that improves both strength and toughness. Although its effect is large up to 0.10%, , even if it is added in excess of 10%, the effect on the material quality is small, and on the contrary it deteriorates the toughness of the welded joint +1AZ. Therefore, the upper limit was set at 0.10%.

V has almost the same effect as Nb, but the upper limit is 0.01%
It is acceptable up to

Ti has an extremely strong affinity with N. The formation of TiH fixes free nitrogen, which has a negative effect on toughness, and has the effect of improving the toughness of the base metal and welded joint +1AZ, but this effect is not obtained when the amount is less than 0.01%. It is impossible to do so, and if it is more than 0 or 10%, it is hard! 87 H deteriorates toughness.

B segregates at austenite grain boundaries during rolling, improving hardenability and making it difficult to form a pearlite structure;
If oso% is exceeded, BN or B-constituent
The toughness of the base metal and the welded joint 11AZ deteriorates. Therefore, the upper limit was set to o, ooso%.

<Example> Examples of the present invention will be described.

A slab with the chemical composition shown in Table 1 manufactured in the converter series casting process is
After reheating to 100°C to 1200°C, apply a reduction of 65% or more in the non-recrystallized temperature range of 850°C or lower, just above the Ar3 temperature (within +10°C) determined by the composition of each slab.
Controlled cooling was performed from The cooling rate was 10 to b/see, and the cooling stop temperature was less than 550°C and 350°C or more. This rolling process produced steel plates of 10 to 32 inches. 1st
The sea bream 8111-N11il in the table is the steel of the present invention,
2 to 21 are comparative steels. Table 2 shows the mechanical properties, heavy C resistance, and SSC resistance of the invention steel and comparative steel.

In the 111C test, specimens with a thickness of 20 fins and a length of 100 fins were cut from the front and back surfaces of the steel plate, each 1 m thick, with the rolling direction aligned with the longitudinal direction of the specimen. In addition, for the SSC test, a round bar tensile test piece with a parallel part of 25 cm and a diameter of 6 m was taken from a place as close as possible to the plate surface.

As for the test conditions, the old C test was conducted without applying external stress.
The SSC test was conducted by applying a tensile stress of 80% of the yield strength of the base material using a constant load type tensile tester.

The dipping solution used was 5% NaCl-0.5% CH3CO0II at 25°C, generally called NACE solution, saturated with Il and S (911ζ3), and the dipping time was I
The time was 96 hours for IIG and 720 hours for SSC.

N[L12 to Na16 are steel plates with too large an amount of O. o
If l is too large, even if 8CR is satisfied (Na1
2゜13.16), does not satisfy both the weight resistance C and SSC characteristics, No, 12+13, the ACR value is appropriate.
Satisfied with Ic characteristics but poor SSC resistance, magnetic】4 is C
a. The O amount exceeds the appropriate value, even if the sl is small (
However, both the 111C resistance and SSC characteristics are poor, while the N
1115 has an appropriate amount of Ca, but a large amount of S and 0 AC
R is too small, and sufficient control of the shape of inclusions is not achieved, resulting in a resistance of 1110. SSC characteristics are poor. Na16 has too much P, so HIC resistance is poor even if the ACR value is appropriate. Rules 17 to 21, Of is appropriate. However, 8cL1
In 7, the S amount is a little too high, but above all the N amount is 80ρps.
As shown in Table 2, WI4Fj is as high as +.

Low-temperature toughness is poor. NcL18 has too high Ca and Sl. In 81119, the Ca content is slightly too high and the ACR value is beyond the appropriate value. Sou 20 is appropriate except for C and Nil.

However, the toughness was extremely poor due to the excessive amount of N, and center segregation was promoted due to the high amount of C, and although the 111c resistance was good, the SSC characteristics were deteriorated. Sou 21 is suitable for non-mass sales of C, but the Mnl is higher than Nct20 at +4, which promotes center segregation above +41120, so the weight resistance of C is also deteriorated. In other words, among the comparative samples that do not meet the scope of the present invention, there is no one that simultaneously satisfies all of the mechanical properties, weight resistance c, and SSC properties.

<Effects of the invention> As mentioned above, low pi NAC [! Even in harsh environments such as solutions, the present invention has been able to dramatically improve the heavy C resistance and SSC resistance of steel materials.

Claims (1)

[Claims] (1) C: less than 0.05%, Si: 0.5% or less, Mn
: 0.5-1.7%, P: 0.0070% or less, S: 0
．． 0010% or less, Al: 0.01-0.10%, N:
0.0050% or less, O: less than 0.0010%, Ca:
Contains 0.0010 to 0.0040%, and the following formula (1)
, (2), and the remainder consists of Fe and unavoidable impurities, and the steel has excellent hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance. 0.5≦ACR≦2.8・・・・・・・・・(1)A
CR=(Ca(%)-O(%)×{0.18+0.01
3×Ca(%)})/(1.25×S(%))・・・・
......(2) (2) C: less than 0.05%, Si: 0.5% or less, Mn
: 0.5-1.7%, P: 0.0070% or less, S: 0
．． 0010% or less, Al: 0.01-0.10%, N:
0.0050% or less, O: less than 0.0010%, Ca:
Contains 0.0010 to 0.0040%, and the following formula (1)
, (2), Cu: 0.5% or less, Ni:
0.5% or less, Cr: 1.0% or less, Mo: 0.5% or less, Nb: 0.10% or less, V: 0.10% or less, Ti
: 0.01 to 0.10%, B: 0.0050% or less 1
A steel for line pipes having excellent resistance to hydrogen-induced cracking and resistance to sulfide stress corrosion cracking, characterized in that the steel contains one or more of the following: Fe and unavoidable impurities. 0.5≦ACR≦2.8・・・・・・・・・(1)A
CR=(Ca(%)-O(%)×{0.18+0.01
3×Ca(%)})/(1.25×S(%))・・・・
・・・・・・(2)