JP2002060893A - Oil-well steel having excellent sulfide stress corrosion cracking resistance, and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide oil-well steel having excellent sulfide stress corrosion cracking resistance and manufacturable by means of round CC(continuous casting) with high production efficiency and its manufacturing method and also to provide oil-well steel having excellent cracking resistance and its manufacturing method. SOLUTION: In the oil-well steel having excellent sulfide stress corrosion cracking resistance, a composition of Ca-type nonmetallic inclusions comprises >=50 mass%, in total, of CaS and CaO and also comprises <50 mass% compound oxide of Ca and Al, and the hardness of the steel ranges from HRC 21 to 30, and further, the total content X (mass%) of CaO and CaS satisfies the inequality, 100-X<=120-(10/3)×HRC. The method for manufacturing the steel is also provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for oil wells having excellent sulfide stress corrosion cracking resistance and suitable for applications such as casings for oil wells and gas wells, tubing and drill pipes for drilling.

[0002]

2. Description of the Related Art Seamless steel pipes, which provide higher reliability than welded pipes, are often used in harsh oil well environments and high-temperature environments, and provide higher strength, improved toughness, and resistance to sulfide stress cracking. (Hereinafter referred to as SSC resistance) is always required to be improved.

[0003] A seamless steel pipe is generally manufactured by an inclined rolling method. That is, the billet is pierced by a piercing mill to form a hollow shell, rolled by a plug mill or a mandrel mill, and after reheating, the outer diameter is reduced by a stretch reducer and the thickness is reduced slightly to a product size.
Tempered and tempered.

Recently, production cost reduction has been pursued.
In order to increase production efficiency, continuous casting (hereinafter, referred to as round CC) billet is used as a billet. However, when the round CC is applied, the nozzle for pouring molten steel from the tundish into the mold has a small diameter, so that nozzle clogging is likely to occur. To prevent this, C
Although a is added, it has been found that this Ca causes a decrease in SSC resistance. The mechanism by which the SSC resistance is reduced is that, during use in an oil well environment, Ca-based inclusions melt and pitting occurs.
Is presumed to accelerate the occurrence of

[0005] From the previous studies, considerable knowledge has been obtained on the chemical composition and heat treatment conditions of steel for improving SSC resistance. Based on these findings, steels having excellent SSC resistance have been developed. Was. For example, a steel having a martensite structure of 80 to 90% or more, a high-temperature tempered steel, a steel having a fine-grained structure, a high yield ratio steel, a steel containing no coarse carbide, and the like. However, when these steels are made into round billets by the round CC method and a seamless steel pipe is manufactured by the above method, it has been found that the SSC resistance is reduced due to Ca-based inclusions and the steel may not be usable. .

However, at present, there is no effective countermeasure against a decrease in SSC resistance that occurs when Ca is used during the round CC. If Ca is not used, application of round CC in continuous casting and further improvement in production efficiency such as an increase in the ratio of continuous CC cannot be achieved.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide an oil well steel having excellent sulfide stress corrosion cracking resistance, which can be produced by round CC with good production efficiency, and a method for producing the same.

[0008]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have studied various chemical compositions using round CC-seamless pipe-off-line or in-line heat treatment, 90-125 ksi (620-862 MPa). )
Of steel pipes for oil wells with high strength and their SSC resistance
The following findings were obtained by investigating the properties and examining the correlation between the SSC resistance and the Ca-based inclusion composition. In particular, C
An advanced fractional quantification method was applied to the composition analysis of the a-system inclusions as described later.

A) 5 ppm of Ca in steel by adding Ca
When the above remains, the Ca-based nonmetallic inclusion mainly contains C
It is composed of a composite oxide of a and Al (hereinafter referred to as Ca-Al-O), CaO and CaS.

B) Ca-based nonmetallic inclusions are all N
ACE TM0177 bath (NaCl + CH ₃ COOH solution) [Hereafter,
In an acid solution such as NACE bath], it dissolves into pits and becomes a source of stress concentration.

C) Ca—Al—O comprises CaO and CaS
Larger than. Its approximate size is Ca-Al
—O is several tens μm, and CaO and CaS are several μm.

D) The higher the relative ratio of Ca—Al—O, the more
Ca-based nonmetallic inclusions tend to be large.

E) A steel having as little as less than 50% Ca-Al-O has good SSC resistance.

F) In particular, a steel satisfying the following equation in relation to hardness is NACE TM0177 METHO.
In DA (uniaxial tensile test), no fracture occurs in NACE bath with an applied stress of 85% of the specified minimum yield stress.

100−X ≦ 120− (10/3) × HR
C However, 21 ≦ HRC ≦ 30 g) If deoxidation becomes excessive, Al becomes excessive and Ca-A
1-O is easily generated. Further, CaO is hardly generated.

H) On the other hand, if the deoxidation is insufficient, MnS is generated and the SSC resistance is reduced.

I) If desulfurization becomes excessive, Ca becomes excessive and Ca-Al-O is easily generated.

J) If the desulfurization is insufficient, MnS is generated, and a large amount of Ca is required, and the amount of Ca-based inclusions increases, so that the SSC resistance also decreases.

K) The proper inclusion composition is Ca, Al,
It can be obtained by setting O and S to the following ranges.

-0.005 ≦ (Ca / 40-S / 32) × sol.Al × TotalO
× 1000000 ≦ 0.0042 The present invention has been made based on such findings, and the gist thereof is as follows.

(1) In mass%, C: 0.15 to 0.35
%, Si: 0.1 to 1.5%, Mn: 0.1 to 2.5
%, P: 0.025% or less, S: 0.004% or less, sol.
Al: 0.001-0.1%, Ca: 0.0005-
0.005%, the composition of Ca-based nonmetallic inclusions is
The sum of CaS and CaO is 50% by mass or more;
When the composite oxide of Al and Al is less than 50% by mass and the hardness of the steel is in the range of 21 to 30 in terms of HRC, the hardness of the steel and the total amount X (% by mass) of CaO and CaS are expressed by the following formula ( Oil well steel excellent in sulfide stress corrosion cracking resistance that satisfies 1).

100−X ≦ 120− (10/3) × HRC (1) (2) Ca or a Ca-containing substance is added to the molten steel after the start of the converter and before casting. To 0.0005
Not less than 0.005% by mass, and S, Al, Ca and Total O (oxygen) in steel are represented by the following formula (2)
A method for producing oil well steel having excellent sulfide stress corrosion cracking resistance, which is controlled to satisfy the following.

-0.005 ≦ (Ca / 40−S / 32) × sol.Al × TotalO × 1000000 ≦ 0.0042 (2) Here, the element symbol in the formula indicates the content (% by mass).

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for defining the chemical composition of the steel for oil wells according to the present invention will be described below. Hereinafter, all percentages are expressed as mass%.

C: C is an element necessary for securing the strength of the steel pipe. If it is less than 0.15%, the hardenability is insufficient, the tempering temperature is lowered, and the required performance can be secured. difficult. On the other hand, if the content exceeds 0.35%, quenching cracks occur and the toughness is further deteriorated.
And Desirably, it is 0.2 to 0.3%.

Si: Si is added for the purpose of deoxidation. Further, it strengthens the tempering softening resistance and contributes to an increase in strength. For the purpose of deoxidation, it is necessary to contain 0.1% or more. Further, if the content exceeds 1.5%, the hot workability becomes extremely poor, so the upper limit was made 1.5%. Preferably,
0.1-0.5%.

Mn: Mn is an element that increases the hardenability of steel and is effective in ensuring the strength of a steel pipe. If it is less than 0.1%, both strength and toughness decrease due to insufficient hardenability. Meanwhile, 2.
When the content exceeds 5%, the segregation increases and the toughness decreases, so the upper limit is set to 2.5%. Preferably, 0.
2-1%.

P: P is inevitably present in steel as an impurity. If it exceeds 0.025%, it segregates at the grain boundaries and lowers the toughness. The lower the better.

S: S combines with Mn or Ca to form inclusions and is stretched by hot rolling. If the content is too large, the toughness is reduced. The lower the better.

Sol. Al: Al is an element necessary for deoxidation, and sol. If the content of Al is 0.001% or less, the steel quality is deteriorated due to insufficient deoxidation, and the toughness is reduced. However, when the content exceeds 0.1%, the toughness is rather lowered, which is not preferable. Therefore 0.001
To 0.1%. However, excess Al makes Ca—Al—O easy to generate in view of the balance with Ca, S, and other elements. Desirably, it is 0.01 to 0.05%.

Ca: Ca is contained to prevent nozzle clogging in continuous casting. In order to obtain the effect, it is necessary to contain 0.0005% or more. In addition, Ca
If the amount is less than this, SSC caused by Ca-based nonmetallic inclusions
Therefore, the lower limit of Ca is 0.0005%. On the other hand, if the content exceeds 0.005%, the cleanliness of individual pieces is reduced, and the toughness is deteriorated. Therefore, the Ca content is set to 0.0005 to 0.005%. Desirably, it is 0.008 to 0.004%. In addition, Ca prevents the generation of easily stretchable MnS, which serves as a starting point of cracking, has the effect of making S difficult to elongate by rolling and forming fine CaS.

The steel for oil wells according to the present invention may contain at least the above-mentioned elements, and may further contain the following elements as necessary.

Cr: Cr is an element useful for improving the hardenability, but may not be contained if other elements ensure the minimum hardenability. However, it is advantageous to include it when producing a thicker steel pipe. When the Cr content is 0.1% or more, hardenability,
And it has the effect of increasing tempering softening resistance. Also, 1.5
%, The toughness deteriorates. Therefore, when Cr is contained, the content is set to 0.1 to 1.5%. Desirably, it is 0.2 to 1.2%.

Mo: Mo is preferably contained for the purpose of increasing hardenability and tempering softening resistance when producing a thick steel pipe. Also, there is an effect of improving sour resistance. When it is contained, the effect is not exhibited if it is less than 0.1%, so it is desirable to contain it at 0.1% or more. If it exceeds 1%, the toughness deteriorates, so it is preferable to set the content to 1% or less. Desirably, it is 0.2 to 0.8%.

Nb: In the off-line heat treatment process, Nb is effective for suppressing the growth of crystal grains during reheating by a pinning effect and for reducing austenite grains. In the case where it is contained, since the effect does not appear if it is less than 0.01%,
Desirably, the content is 0.01% or more. Desirably, it is 0.015 to 0.05%.

On the other hand, in the in-line heat treatment process in which quenching is performed while maintaining the temperature at the Ar 3 point or higher after the final rolling, such an effect is not only obtained, but if contained, most of NbC is hardened during quenching. Since it does not precipitate and precipitates during tempering, it has a precipitation behavior completely different from that of the off-line heat treatment, and thus greatly affects the strength. In particular, in a seamless steel pipe, a temperature difference always occurs between the surface layer in contact with the tool and the center of the wall thickness, and the solid solution amount of Nb changes depending on the position in the wall thickness direction due to the temperature difference. Variations occur. Therefore, since it is desired that the solid solution Nb is reduced to substantially zero, the content is preferably set to less than 0.005%, and more preferably 0.003% or less.

V: V is an element effective for improving SSC resistance. When V is contained, it has an effect of increasing the strength by secondary precipitation strengthening, and when obtaining a predetermined strength, can be tempered at a higher temperature, which contributes to an improvement in SSC resistance. In addition, since VC has a high solubility in the austenite region, all are in solid solution during in-line quenching, and do not cause a variation in strength. Content is 0.
If the content is less than 01%, the effect is not obtained. If the content exceeds 0.3%, the toughness is significantly deteriorated. Therefore, when it is contained, the content is set to 0.01 to 0.3%. Desirably, 0.02-
0.2%.

Ti: When Ti is contained, it has a strong bonding force with N, forms TiN stable at high temperatures, and has the effect of fixing N. If less than 0.005%, the effect is not obtained. On the other hand, if it is contained in a large amount exceeding 0.05%, TiC starts to precipitate in the final rolling temperature range, which causes a variation in strength as in the case of Nb. Therefore, in the case where it is contained, 0.
005 to 0.05%. Desirably, 0.01 to
0.03%.

W: Like W, W has the effect of improving hardenability and obtaining high strength, and has the effect of increasing temper softening resistance and improving SSC resistance, and is contained as necessary. If the content exceeds 1%, the effect is saturated, so the upper limit of the content is preferably set to 1%.

B: Since B significantly improves hardenability when B is contained, the required strength can be ensured by adding B when producing a thick steel pipe. If you want to include
If the content is 0.0030% or more, carbonitride tends to precipitate at the grain boundary, which causes deterioration of toughness.
It is good to make it 03%.

Mg: When Mg is contained, it reacts with S in steel to form sulfate in molten steel. This sulfate is spherical even after rolling, and does not extend in the rolling direction.
For this reason, it also has the effect of improving the impact properties in the direction perpendicular to the rolling and suppressing hydrogen-induced cracking. Content 0.0
When the content exceeds 05%, the amount of inclusions in the steel increases, the cleanliness decreases, and various performances decrease. Therefore, the content is preferably 0.005% or less.

N: N is inevitably present in steel. N
Combines with Al, Ti and Nb to form a nitride. In particular, when a large amount of AlN or TiN precipitates, toughness and SS
0.008% because it has an adverse effect on C properties and HIC resistance
It is better to do the following.

Zr: When Zr is contained, like Zr,
Since N, which is an impurity in steel, is fixed as nitride, B
Hardenability can be sufficiently secured. Further, since it is difficult to form carbides, it does not cause a variation in strength. On the other hand, if the content exceeds 0.1%, inclusions increase and toughness decreases, so the upper limit of the content is preferably 0.1%.

O: O is inevitably present in steel. O
Combines with Al and Ti to form an oxide. In particular, A
If a large amount of l ₂ O ₃ is precipitated, the toughness is adversely affected. Therefore, the content is preferably set to 0.008% or less. Also, C
a, S, Al, and other elements, excessive O
Is more preferable because it makes Ca-Al-O easier to produce.

Hereinafter, the non-metallic inclusion will be described.

Among Ca-based nonmetallic inclusions, CaO and CaS
Is 50% or more: Ca-based nonmetallic inclusions are composed of relatively large Ca—Al—O, relatively small CaO and CaS, and the higher the relative ratio of the sum of CaO and CaS,
As a whole, Ca-based nonmetallic inclusions tend to be small. C
The sum of aO and CaS is 50% or more, in other words,
When the balance of Ca-Al-O is less than 50%, the SS resistance is low.
C property is improved.

When a round billet is cast by continuous casting, Ca
Is added to molten steel, and the composition of inclusions is Al ₂ O ₃
Therefore, it becomes harmful large Ca-Al-O, and when Ca is further added, simple CaO is separated from the complex inclusion of Ca-Al-O, and the CaO immediately reacts with S. To CaS. That is, if the content of CaS is small, harmful MnS may remain and deteriorate sulfide stress cracking resistance. In addition, large C that is harmful if CaS is small
This means that the reaction from a-Al-O to harmless CaO or CaS has not progressed, resulting in poor sulfide stress cracking resistance. Therefore, CaS is preferably used in an amount exceeding 10%.

Hardness of Steel and CaO + CaS Content (X): Hardness of steel is set to 21 ≦ HRC ≦ 30, and hardness of steel (HRC)
And the amount X of CaO + CaS must satisfy the following expression.

100−X ≦ 120− (10/3) × HR
C Here, X is the amount of CaO + CaS (% by mass).

In the case of a high-strength oil country tubular good exceeding HRC30, the above equation cannot be used. This is considered to be because the SSC sensitivity becomes extremely high, so that the alloy element composition, the structure, the heat treatment conditions, and the like are closely related to the Ca-based nonmetallic inclusion composition. HRC is 2
If it is less than 1, the hardness required for oil well steel (oil well pipe) is not reached, so the hardness needs to be 21 or more.

Next, the manufacturing method will be described.

Method for adding Ca to molten steel: The refining step and method for adding the Ca-containing substance are not limited. However,
The Ca-containing substance is preferably 0.01 to 1.1 kg / ton of molten steel in terms of pure Ca. If it is less than 0.01 kg / ton of molten steel, it is difficult to increase the Ca concentration in the molten steel and the CaO concentration in the inclusions sufficiently. When the pressure exceeds 1.1 kg / ton of molten steel, the composition of Ca-based nonmetallic inclusions becomes Ca
-Cannot be less than 50% as Al-O;
-Al-O concentration tends to be excessive.

In order to prevent clogging of the nozzle, Ca may be added during the period from tapping the converter to the casting. C
a addition is a method of adding the Ca-containing substance to the molten steel in the ladle,
There are a method of adding a Ca-containing substance to molten steel in a tundish during continuous casting, and a method of placing a Ca-containing substance in a tundish.

-0.005 ≦ (Ca / 40-S / 32) × sol.Al × TotalO
× 1000000 ≦ 0.0042: S, sol.Al and TotalO in steel
However, when the value exceeds 0.0042 in the above formula, excess Ca remains, and Al and O are large, Ca-Al-O increases remarkably and SSC resistance decreases. On the other hand, if the value is less than -0.005 in the above equation, the SSC resistance is reduced by MnS. Desirably, it is 0 to 0.0036.

The method of quantifying Ca-based inclusions by form is described in detail in Iron and Steel, vol. 82 (1996), No. 12, p.

If the water countermeasure of working in a glove box in an Ar atmosphere using anhydrous methanol is taken,
Ca-Al-O is quantitatively extracted by the bromine-methanol method. In addition, Ca-Al-O and CaS are quantitatively extracted by an AA (acetylacetone) -based electrolytic method.
That is, Ca-Al-O is obtained by converting the metal component in the bromine-methanol extraction residue into oxide, and
As for aS, the difference between Ca analysis values of the AA-based electrolytic extraction residue and the bromine-methanol extraction residue can be determined as Ca as CaS.

Although CaO cannot be extracted, oxygen in steel exists as an oxide, and the oxides are Ca—Al—O and CaO. The difference between the oxide equivalent oxygen amount of
It can be calculated as CaO.

The specific calculation formula is as follows. [Ca-Al-O] = [Ca (1)] × 56.06 / 40.08 + [Al] × 101.96 / 53.96 +
[Mg] × 40.31 / 24.31 + [Zr] × 123.22 / 91.22 [CaS] = ([Ca (2)]-[Ca (1)]) × 72.14 / 40.08 [CaO] = [TotalO]-([Ca ( 1)] × 16.00 / 40.08 + [Al] × 48.00 /
53.96+ [Mg] × 16.00 / 24.31 + [Zr] × 32.00 / 91.22} × 56.08 /
16.00) Here, [Ca (1)]: the amount of Ca in the bromine-anhydrous methanol extraction residue in the glove box [Ca (2)]: the amount of Ca in the AA-based electrolytic extraction residue [TotalO]: the amount of oxygen in the steel

The continuously cast steel is made into a seamless steel pipe by the ordinary Mannesmann method described above, and used after being quenched and tempered.

The heat treatment will be described below.

In the case of direct quenching and tempering in-line, if the final rolling temperature is lower than 950 ° C., an extended grain structure is formed, and anisotropy is generated in toughness and SSC resistance, and the anisotropy is not eliminated by subsequent heating. At a final rolling temperature exceeding 1100 ° C., the crystal grain size is remarkably coarsened, and fine grains are not formed even by supplementary heat. Therefore, the final rolling temperature is preferably 950 ° C. to 1100 ° C.

The uniformity of the temperature of the steel pipe is eliminated by eliminating the temperature unevenness, and at the same time, the dispersion of the strength is eliminated and the SS
When performing supplementary heat treatment after rolling to improve C property,
If the supplementary heat temperature exceeds 950 ° C., the toughness and SSC resistance are not good because the grains remain coarse. If the Ar value is less than 3 points, proeutectoid ferrite precipitates and a complete martensitic structure cannot be obtained. Therefore, the supplementary heat temperature is 3 points or more of Ar 95
The temperature is preferably set to 0 ° C. or lower. It is desirably 920 ° C. or lower.

The tempering temperature is preferably higher from the viewpoint of SSC resistance. Desirably, it is 680 ° C. or higher. However, if the temperature exceeds the Ac1 point, the material softens significantly.

Next, in the case of quenching and tempering off-line, if the heating temperature exceeds 980 ° C., coarse particles are formed and the toughness and SSC resistance are reduced. In addition, when the Ac point is less than 3 points, since austenite is not completely formed, a uniform structure cannot be obtained when quenched. So the heating temperature is Ac3
The temperature is preferably not less than the point and less than 980 ° C. Preferably 950
It is below ° C.

The tempering temperature is preferably higher from the viewpoint of SSC resistance. Desirably, it is 680 ° C. or higher. However, if the temperature exceeds the Ac1 point, the material softens remarkably.

[0066]

EXAMPLES Steels having the chemical compositions shown in Table 1 and having symbols A1 to F were melted and continuously cast into round billets. For each steel, Ca and other components were adjusted sequentially in a tundish during continuous casting.

[0067]

[Table 1] Thereafter, an outer diameter of 244.5 mm and a wall thickness of 13.15 mm were obtained from the actual billet by the Mannesmann mandrel mill method.
An 8 mm seamless steel pipe was manufactured, and quenched and tempered at the temperatures shown in Table 2.

[0068]

[Table 2] Next, from the seamless steel pipe after the heat treatment, the parallel portion was 6.35φ.
× 25.4 mm tensile test pieces were collected from each, and NAC
The SSC test was performed according to E TM0177 Method A. That is, a uniaxial tensile test was performed in a 0.5% acetic acid + 5% saline solution at 25 ° C. saturated with hydrogen sulfide at 1 atm with a stress of 85% of the specified minimum yield stress (SYS). Those that did not break within the test period of 720 h were considered to have good SSC resistance (those that broke even one were bad).

All the test pieces were subjected to HRC hardness measurement after the SSC test. Hardness is an average value of three-point measurements.

Table 2 shows the results. Representative S
The test results of the SC property will be described below.

Among the symbols A1 to A3, the symbol A3 has a high S and poor SSC resistance. Since MnS was observed on the fractured surface, it is considered to be SSC originating from MnS.

In the symbols B1 to B4, the symbol B2 is
aSSC was generated from the pitting corrosion of the inclusions that had melted down.
The relationship between the Ca inclusion composition and the hardness is out of the claims. The symbol B4 indicates that Ca-Al-
O-rich Ca-based nonmetallic inclusion composition was removed. Although the tempering temperature was increased and the hardness was controlled to be lower, SSC still occurred.

In the symbols C1 to C3, the trial number C3 is
aSSC was generated from the pitting corrosion of the inclusions that had melted down.
At the time of RCC, a large amount of Ca-Al-O was generated due to excessive Al.

In the symbols D1 to D3, the symbol D3 is
At the time of CC, a large amount of Ca-Al-O was generated due to excessive O. SSC resistance was also poor.

[0075]

According to the present invention, a steel for oil wells having excellent sulfide stress corrosion cracking resistance can be obtained, and excellent effects can be obtained when used for casings, tubing, drill pipes and the like of oil wells and gas wells. Demonstrate. Furthermore, the round CC can be applied in the manufacturing method, and the ratio can be continuously increased to reduce the manufacturing cost.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Yoshihiko Higuchi 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Prefecture Inside Sumitomo Metal Industries, Ltd. (72) Hirofumi Kurasuya 4-chome Kitahama, Chuo-ku, Osaka-shi, Osaka No. 5 33 Sumitomo Metal Industries, Ltd. F term (reference) 4K013 AA09 BA14 EA03 EA12 EA25 FA02

Claims (2)

[Claims] C. 0.15 to 0.35% by mass%, S:
i: 0.1 to 1.5%, Mn: 0.1 to 2.5%, P:
0.025 or less, S: 0.004% or less, sol.Al:
0.001-0.1%, Ca: 0.0005-0.00
5%, the composition of Ca-based nonmetallic inclusions is such that the sum of CaS and CaO is 50% by mass or more, the composite oxide of Ca and Al is less than 50% by mass, and the hardness of steel is Is H
Within the range of 21 to 30 in RC, the hardness of the steel and the total amount X (mass%) of CaO and CaS satisfy the following formula (1). Excellent oil well steel. 100−X ≦ 120− (10/3) × HRC (1) 2. After the start of the converter and before casting, the molten steel
a or Ca-containing substance is added, and Ca
005 to 0.005% by mass or more,
A method for producing oil well steel excellent in sulfide stress corrosion cracking resistance, characterized in that S, Al, Ca and Total O (oxygen) in the steel are controlled so as to satisfy the following formula (2). -0.005 ≦ (Ca / 40-S / 32) × sol.Al × TotalO × 1000000 ≦ 0.0042 (2) Here, the element symbol in the formula indicates the content (% by mass).