JP2001271134A - Low alloy steel with excellent sulfide stress cracking resistance and toughness - Google Patents

Abstract

(57) [Summary] [Problem] Even if YS has a high strength of 155 ksi or more,
Provide a steel material having excellent toughness as well as SSC resistance. SOLUTION: In mass%, C: 0.2 to 0.35%, S
i: 0.05 to 0.5%, Mn: 0.1 to 1%, P:
0.025% or less, S: 0.01% or less, Cr: 0.1
-1.2%, Mo: 0.1-1%, B: 0.0001-
0.005%, Al: 0.005 to 0.1%, V: 0.
05 to 0.5%, Ni: 0.1% or less, N: 0.01%
Hereinafter, O (oxygen): 0.01% or less, with the balance being Fe
Mo and V contents satisfy the following formulas (1) and (2), and the yield stress is 1060MP.
a (155 ksi) or more, a low alloy steel excellent in sulfide stress cracking resistance and toughness. 0.03 ≦ Mo × V ≦ 0.3 (1) 0.5 × Mo−V + GS / 10 ≧ 1 (2) Here, GS indicates the ASTM particle number of the prior austenite particles.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a sulfide stress cracking resistance suitable for casings and tubing for oil and gas wells, drill pipes for drilling, line pipes for transportation, and pipes for petrochemical plants. The present invention relates to a steel material excellent in yield strength and toughness and having a yield stress of 1060 MPa (155 ksi) or more and a method for producing the same.

[0002]

2. Description of the Related Art With the recent tightening of the energy situation, crude oil and natural gas containing a large amount of hydrogen sulfide, which have been shunned so far, are being used.
Transport and storage have become necessary. Furthermore, in order to deepen oil and gas wells, improve transport efficiency, and reduce costs, steel materials used in this field, particularly steel pipes, are required to have higher strength than ever.

That is, 80 k, which has been widely used in the past,
Si class [Yield stress (YS) is 80 to 90 ksi (552)
621 MPa) or 90 ksi class [for example, YS is 90
To 100 ksi (621 to 686 MPa)], steel pipes with excellent resistance to sulfide stress cracking have recently been replaced with 110 ksi.
si class [YS is 110-125 ksi (758-862)
MPa)] or 125 ksi class [YS is 125 to 140 k
si (862 to 965 MPa)], a high-strength steel pipe excellent in sulfide stress cracking resistance is used, and YS is 140 ksi (965 MPa to 1068 MPa).
Demands for ultra-high-strength steel pipes having excellent sulfide stress cracking resistance are also increasing. Generally, as the strength of a steel material increases, the sulfide stress cracking property (hereinafter referred to as SSC) increases. Therefore, the biggest challenge for increasing the strength of steel used in an environment containing a large amount of hydrogen sulfide is SS.
This is an improvement in resistance to C (hereinafter referred to as SSC resistance).

With respect to the improvement of SSC resistance, (1) steel is made highly clean, (2) the structure of the steel material is a fine grain structure,
It is known that this can be achieved by (3) making the structure of the steel material a structure in which martensite is about 80% or more, and (4) performing high-temperature tempering.

[0005] It is said that SSC of high-strength steel is generated and propagates from the former austenite grain boundaries, and therefore, as described in (1) above, the impurity elements such as P and S are reduced to reduce the brittleness of the former austenite grain boundaries. It is effective to prevent SSC from improving SSC resistance.

Further, when the grain size is reduced, the deterrence against cracking is increased, and the grain boundary area per unit volume is increased, so that grain boundary segregation of impurity elements is reduced indirectly and grain boundary embrittlement is prevented. Therefore, the grain refinement of the structure as described in (2) is also effective for improving the SSC resistance. Increasing the martensite ratio to form a uniform structure as in (3) above and increasing the tempering temperature to reduce internal strain as in (4) above are also effective in improving SSC resistance. It is said.

For example, Japanese Patent Application Laid-Open No. 62-253720 discloses a method for improving SSC resistance by reducing impurity elements such as Mn and P. In addition, Japanese Unexamined Patent Publication No.
Japanese Patent Application Laid-Open No. 9-232220 discloses a method in which the structure is refined by twice quenching heat treatment to improve SSC resistance. Japanese Unexamined Patent Publication No. Hei 6-322478 discloses that the structure is refined by induction heating and has excellent SSC resistance.
A method for obtaining a 5 ksi grade steel material is disclosed. Also,
Japanese Patent Application Laid-Open No. H8-311551 discloses that a direct quenching method is used to increase the quenchability and the tempering temperature to improve the SS resistance.
A method for manufacturing a steel pipe having a C property and a strength of 110 ksi class to 140 ksi class is disclosed.

However, there has been a large number of studies on steel materials having a strength of 110 ksi class or less, and it has been difficult to improve the SSC resistance of steel materials of higher strength, for example, 125 to 140 ksi class. There are few examples. Furthermore, 155 ksi class YS is 155 ksi (1
There is no study on steel materials having a strength of｝ 060 MPa) or more.

On the other hand, it has recently been found that, in addition to the decrease in SSC resistance due to the increase in strength, the decrease in toughness is also a serious problem, particularly for low alloy steels having a YS of 140 ksi (965 MPa) or more.

[0010]

The object of the present invention is to provide a YS
Is to provide a steel material having excellent SSC resistance and toughness even if it has a high strength of 1060 MPa (155 ksi) or more.

The specific goal of SSC resistance is NACE
(National Association of Corrosion Engineers) T
The crack initiation limit stress (σth) in a constant load test in a bath (0.5% acetic acid at 25 ° C saturated with hydrogen sulfide + 5% saline) specified in the M0177-96A method was 85% of YS of steel. % Or more. In addition, the target of toughness is a fracture surface transition temperature (a temperature at which the area ratio of ductile / brittle fracture surface becomes 1: 1) is −10 ° C. or less in consideration of use environment and transport environment.

[0012]

Means for Solving the Problems The present inventors have conducted experiments and studies in order to solve the above-mentioned problems, and as a result, have obtained the following knowledge.

A) The SSC resistance and toughness of a 155 ksi grade steel material are largely controlled by the form of carbide.

B) To improve SSC resistance, tempering is performed by adding an alloy element of Mo, V and Nb which is an element forming a fine MC type carbide having a tetragonal structure (hereinafter simply referred to as MC). It is effective to raise the temperature.

C) Further, the above alloy element has a hardenability.
Has the effect of increasing the martensite ratio in the tissue.
This also has the effect of improving SSC resistance. d) To improve toughness, avoid precipitation strengthening by Nb and V as much as possible.
Use Mo to get M _ThreeC-type carbide (hereinafter simply referred to as M_ThreeC and
It is most effective to slow the growth of

E) However, V precipitates as fine carbides, though not as much as Nb, while maintaining consistency with the base material, and reduces toughness. However, V carbide sufficiently dissolves in steel at around the normal quenching temperature of 900 ° C. and contributes to precipitation strengthening at the time of subsequent tempering, so that there is no problem of toughness reduction due to coarsening. On the other hand, Mo is a coarse M ₃ C mainly composed of Fe.
And the M ₃ C increases the tempering temperature.
Does not reduce toughness as much. From such a viewpoint, by utilizing Mo and making it contained in a well-balanced manner with V, SSC resistance and toughness can be simultaneously improved.

The present invention has been made based on the above findings, and the gist is as follows.

(1) In mass%, C: 0.2 to 0.35
%, Si: 0.05 to 0.5%, Mn: 0.1 to 1%,
P: 0.025% or less, S: 0.01% or less, Cr:
0.1 to 1.2%, Mo: 0.1 to 1%, B: 0.00
01-0.005%, Al: 0.005-0.1%,
N: 0.01% or less, V: 0.05 to 0.5%, Ni:
0.1% or less, W: 1.0% or less, O (oxygen): 0.0
1% or less, the balance being Fe and impurities,
A low alloy steel material having an o and V content satisfying the following formulas (1) and (2) and having a yield stress of 1060 MPa (155 ksi) or more and excellent in sulfide stress cracking resistance and toughness.

0.03.ltoreq.Mo.times.V.ltoreq.0.3 (1) 0.5.times.Mo-V + GS / 10.ltoreq.1 (2) Here, the symbol of the element is the content (mass) of each element. %), GS
Indicates the ASTM grain size number of the prior austenite grains. (2) Instead of a part of Fe, 0.005 to 0.1% of N
The low alloy steel material according to the above (1) containing b.

(3) Instead of part of Fe, Ti: 0.0
The low-alloy steel material according to any one of (1) and (2) above, which contains one or two of 0.05 to 0.03% and Zr: 0.005 to 0.06%. (4) The low alloy steel material according to any one of (1) to (3), which contains 0.3 to 1% of W instead of part of Fe. (5) Instead of part of Fe, 0.0001 to 0.01%
The low alloy steel material according to any one of the above (1) to (4), containing Ca.

(6) The steel having the chemical composition described in any of the above (1) to (5) is hot-worked, and then 880 to 95
A quenching treatment is carried out after holding at a temperature within a temperature range of 0 ° C. for 10 minutes or more, and then a tempering treatment is carried out. Yield strength excellent in sulfide stress cracking resistance and toughness is as low as 1060 MPa (155 kis) or more. Manufacturing method of alloy steel.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the chemical composition and production conditions of the steel material of the present invention will be described in detail. In addition, "%" of the content of the chemical component indicates "% by mass".

Chemical composition of steel: C: 0.2 to 0.35% C is an element effective for improving hardenability and improving strength. If the content is less than 0.2%, the hardenability is reduced and sufficient SSC resistance and toughness cannot be obtained in many cases. On the other hand, if it exceeds 0.35%, the amount of carbides increases and becomes hydrogen trap sites, so that the SSC resistance decreases and the susceptibility to sintering cracks also increases. Therefore, the content of C is set to 0.2 to 0.35%. Preferably 0.25-0.
30%.

Si: 0.05-0.5% Si is an element effective for deoxidizing steel, and also has the effect of increasing temper softening resistance. For the purpose of deoxidation, the content needs to be 0.05% or more. However, when its content is 0.1.
If it exceeds 5%, precipitation of the softened ferrite phase is promoted, and the SSC resistance is significantly reduced. Therefore, the content of Si is set to 0.05 to 0.5%. The upper limit of the preferred Si content is 0.3%.

Mn: 0.1-1% Mn is an element effective for securing the hardenability of steel. For this purpose, a content of 0.1% or more is required. However, if the content exceeds 1%, it segregates at the grain boundaries and lowers SSC resistance and toughness. Therefore, Mn
Was set to 0.1 to 1%. Note that the upper limit of the Mn content is desirably 0.5%.

P: not more than 0.025% P is inevitably present in steel as an impurity, but segregates at grain boundaries and degrades SSC resistance and toughness. In particular, when the content exceeds 0.025%, the deterioration of SSC resistance and toughness becomes remarkable. For this reason, even if it is mixed as an impurity, its content needs to be 0.025% or less. In addition, in order to increase SSC resistance and toughness, it is desirable that the P content be as low as possible.

S: 0.01% or less S is inevitably present in steel as an impurity like P, but it is segregated at grain boundaries and forms a large amount of sulfide-based inclusions. Decreases SSC resistance and toughness. In particular, if the content exceeds 0.01%, the SSC resistance
The deterioration of the properties and toughness becomes remarkable. Therefore, even if it is mixed as an impurity, its content needs to be 0.01% or less. It is desirable that the content of S be as low as possible in order to increase the SSC resistance.

Cr: 0.1 to 1.2% Cr has the effect of improving the hardenability. In order to ensure this effect, the Cr content needs to be 0.1% or more. However, when Cr is contained in excess of 1.2%, in an acidic moist environment containing hydrogen sulfide, Cr is actively dissolved to increase the corrosion rate and lower the SSC resistance.
Therefore, the content of Cr is set to 0.1 to 1.2%.
Preferably it is 0.4-1%.

Mo: 0.1 to 1% Mo is an important element in the present invention which improves quenching properties and is concentrated in M ₃ C to delay its growth and increase temper softening resistance. When Nb and V are added at the same time, fine MC is formed and high-temperature tempering is enabled.
Improves C properties. If the content is less than 0.1%, the above effects cannot be obtained. On the other hand, if the content exceeds 1%, in addition to the above effects being saturated, needle-like Mo carbides precipitate during tempering, trapping hydrogen, increasing the amount of occluded hydrogen, and increasing the concentration of stress in the vicinity thereof. The SSC resistance may be reduced on the contrary. Therefore, the content of Mo is set to 0.1 to 1%. Preferably it is 0.3-0.7%.

B: 0.0001 to 0.005% B has a function of improving the hardenability of steel in a small amount.
However, if the content is less than 0.0001%, the effect is not sufficient, and if the content exceeds 0.01%, Cr
₂₃ (C, B) ₆ is precipitated, and the SSC resistance and toughness are reduced, so that the content of B is 0.0001 to 0.005%.
And The desirable range of the B content is 0.000.
2 to 0.002%.

Al: 0.005 to 0.1% Al is an element necessary for deoxidizing steel. However, if the content is less than 0.005%, a sufficient effect cannot be obtained.
On the other hand, when the content exceeds 0.1%, coarse Al-based inclusions increase and SSC resistance and toughness decrease. Therefore, the content of Al is set to 0.005 to 0.1%. Desirable range of Al content is 0.01 to 0.05%
It is. In the present invention, Al is a so-called “sol.A”.
1 (acid-soluble Al) ".

V: 0.05 to 0.5% V is an important element in the present invention which has a function of improving the SSC resistance by precipitating as fine carbide at the time of tempering and enabling high temperature tempering. To ensure this effect, the content needs to be 0.05% or more. On the other hand, if the V content exceeds 0.5%, the effect saturates and does not contribute to strengthening, and in addition, the toughness is reduced and the SSC resistance is reduced due to VC becoming a hydrogen trap site. Therefore, the content of V is set to 0.05 to 0.5%. Desirably, it is more than 0.1% and 0.2% or less.

Ni: 0.1% or less Ni is present in the steel as an impurity and lowers the SSC resistance in the steel having the chemical composition defined in the present invention. In particular, when the Ni content exceeds 0.1%, the SSC resistance is reduced.
The property is significantly reduced. Therefore, the content of Ni is set to 0.1% or less. Ni is inevitably contained in the Cr raw material, and when Cr is contained, it is industrially extremely difficult to reduce the Ni content to 0 (zero), but it is desirable to reduce the Ni content as much as possible.

N: 0.01% or less N is present in steel as an impurity and segregates at grain boundaries to lower toughness and SSC resistance. When adding Ti or Zr, TiN or ZrN is formed. If the N content exceeds 0.01%, N that cannot be fixed with Ti or Zr precipitates as BN, so that B also cannot sufficiently obtain the effect of improving the hardenability, and the SSC resistance and the toughness decrease. Also,
Excessive precipitation of TiN and ZrN greatly reduces toughness. Therefore, the content of N is set to 0.01% or less.
Note that N enters the steel from the atmosphere or the like, and it is extremely difficult industrially to make the content 0 (zero), but it is desirable to reduce the content as much as possible.

O (oxygen): 0.01% or less O is present as an impurity in steel and segregates at grain boundaries to prevent S
Reduces SC properties and toughness. However, the content is acceptable if the content is 0.01% or less, so the O content is set to 0.01% or less. O enters the steel from the atmosphere or the like, and it is extremely difficult industrially to make the content 0 (zero), but it is desirable to reduce the content as much as possible.

Nb: 0.1% or less Nb is an element to be contained as required. If it is contained, it is present as an undissolved carbide in ordinary quenching and tempering heat treatment, and is effective for fine graining by a pinning effect. . Also,
If the solid solution is completely dissolved at the time of quenching and the tempering temperature is raised by the direct quenching method, the tempering temperature can be increased and the SSC resistance can be improved. In order to obtain this effect, it is necessary to contain Nb in an amount of 0.005% or more. On the other hand, if the content exceeds 0.1%, Nb carbide significantly lowers toughness. Therefore, the content of Nb is set to 0.005 to 0.1%. A desirable upper limit of Nb is 0.05%.

Ti: 0.005 to 0.03%, Zr:
One or more of 0.005 to 0.06% Ti and Zr are elements to be contained as necessary, but have an effect of fixing N, which is an impurity in steel, as TiN or ZrN. Excess Ti than required for N fixation becomes carbide and finely precipitates, and has an effect of increasing tempering softening resistance. The fixation of N is necessary in order to prevent B added to improve quenchability from becoming BN, and to maintain B in a solid solution state to secure sufficient quenchability. In order to obtain such an effect, it is preferable that both Ti and Zr be 0.005% or more and the stoichiometric amount necessary for forming a nitride. Ti content is 0.0
If the content of Zr exceeds 3% and the content of Zr exceeds 0.06%, fine Ti carbides and Zr carbides excessively precipitate and greatly reduce toughness.
% And Zr were set to 0.06%. Desirable upper limits of Ti are 0.02% and Zr is 0.04%.

W: 0.3-1% W is contained as necessary. When it is contained, it has the effect of increasing the quenchability, increasing the tempering softening resistance and improving the SSC resistance. In order to ensure the above-mentioned effects,
Is preferably 0.3% or more. However, when the content exceeds 1%, the above effect is saturated or deteriorated, and in addition, excess W carbide becomes a trap site of hydrogen, and the SSC resistance is rather deteriorated. Therefore, the content of W is set to 0.3 to 1%. The upper limit of the W content is preferably set to 0.7%.

Ca: 0.0001 to 0.01% Ca is an element to be contained as necessary. If Ca is contained, it combines with S in steel to form a sulfide and improves the shape of inclusions to improve the resistance. Improve SSC property. Therefore, when it is desired to secure the above-mentioned effects, Ca is preferably contained. In order to reliably obtain the above-mentioned effect, Ca is 0.0001.
% Or more is required. However, its content is 0.01%
If the ratio exceeds 2, the SSC resistance and toughness are rather lowered, and defects such as ground flaws easily occur on the surface of the steel material. Therefore, the content of Ca is set to 0.0001 to 0.01%.

Expressions (1) and (2) 0.03 ≦ Mo × V ≦ 0.3 (1) 0.5 × Mo−V + GS / 10 ≧ 1 (2) The present invention In a low alloy steel material having a yield stress of 155 ksi or more, Mo and V need to satisfy the following formula in order to achieve both SSC resistance and toughness.

From the viewpoint of improving SSC resistance, Mo × V is contained so as to be 0.03% or more as shown in the following formula.
It is necessary to increase the tempering temperature by utilizing precipitation strengthening.

0.03 ≦ Mo × V On the other hand, the precipitation strengthening due to excess Mo and V causes the MC to be a trap site for hydrogen, which in turn lowers the SSC resistance. For this reason, it is necessary to make Mo × V 0.3% or less as in the following equation (1).

Mo × V ≦ 0.3 (1) From the viewpoint of toughness, it is better to use the effect of Mo to suppress the growth of M ₃ C than to strengthen the precipitation by V. The reason for this is that M ₃ C, in which non-coherent precipitation occurs, is less sensitive to toughness than VC, in which coherent strain is fixed to the periphery. Mo is concentrated in the M ₃ C to delay its growth and has a function of increasing the temper softening resistance. Further, since the toughness is also affected by the prior austenite grain size, it is desirable to have a finer grain structure. From these viewpoints, it is necessary to contain Mo with respect to V and particle size so as to satisfy the following expression (2).

0.5 × Mo−V + GS / 10 ≧ 1 (2) GS indicates the ASTM particle number of the prior austenite particles.

Manufacturing method: The steel material of the present invention is prepared by subjecting a steel having the above-mentioned chemical composition to hot working into a sheet material or a seamless steel pipe, and then performing a quenching treatment to recrystallize the structure and to reduce the crystal grains. It is obtained by miniaturization. At this time, if the quenching temperature is less than 880 ° C., sufficient quenching cannot be performed, and a softening phase is locally formed, thereby deteriorating the SSC resistance. On the other hand, when the quenching temperature exceeds 950 ° C., the crystal grains become coarse and the toughness is reduced. The tempering temperature is not particularly limited.
The range of 00-700 degreeC is preferable. By tempering at a high temperature, internal strain is reduced or M ₃ C is uniformly dispersed to further improve SSC resistance. After the direct quenching or several quenching and tempering, the quenching treatment may be finally performed in the above temperature range. However, it is preferable to perform the quenching and tempering treatment only once from the viewpoint of reduction in manufacturing cost.

[0046]

EXAMPLES 20 low alloy steels having the chemical compositions shown in Table 1 were melted in a 150 kg vacuum melting furnace and a 150 ton converter to form steel ingots, and then hot worked to produce steel plates and steel pipes. did.

[0047]

[Table 1] Steel symbols (A), (B), (F), (G), (K) in the table
(M) is a converter melting material, and the others are vacuum melting materials.

Each vacuum-melted 150 kg steel ingot is
After heating to 0 ° C, hot forging, thickness 40mm, width 80
mm and a length of 250 mm. This steel slab was hot-rolled into a steel sheet having a thickness of 15 mm, quenched at the temperatures shown in Table 2, and subjected to tempering heat treatment to adjust the strength.

The ingot of the converter was made into a round billet, and a seamless steel pipe having an outer diameter of 250 mm and a wall thickness of 16 mm was manufactured by a usual Mannesmann / mandrel mill method.

[0050]

[Table 2] From the steel sheet and the seamless steel pipe, a round bar tensile test piece having a diameter of 6 mm and a length of 40 mm was sampled from the above-mentioned steel plate and the seamless steel tube so that the parallel portion was in the rolling direction, and a tensile test was performed at room temperature to obtain a yield stress (YS). ) Was measured.

Further, a round bar tensile test piece having a diameter of 6.35 mm and a length of 25.4 mm of the parallel portion was sampled so that the parallel portion was in the rolling direction, and subjected to SSC-resistant method according to the NACETM0177-96A method. A sex evaluation test was performed. This test consists of 0.5% acetic acid at 25 ° C. saturated with hydrogen sulfide +
In a constant load test in a 5% saline solution, the partial pressure of hydrogen sulfide was 0.003 atm assuming the actual environment, and the stress was 85% of YS.

In order to evaluate the toughness, a Charpy test specimen (V notch) of 10 mm × 10 mm × 55 mm was sampled in parallel with the direction perpendicular to the rolling direction. The direction in which the test piece was sampled was set to the direction perpendicular to the rolling direction because the evaluation in the direction perpendicular to the rolling direction was more severe than the evaluation in the longitudinal direction. Using this test piece, an impact test was performed at various temperatures, and the temperature at which the area ratio between the ductile fracture surface and the brittle fracture surface in the fracture surface became 1: 1 was determined as the fracture surface transition temperature vTs.
And

Table 2 shows the evaluation results of the measured YS, SSC resistance and toughness. In the evaluation of SSC resistance, when the sample did not break during the test time of 720 hours, the SSC resistance was determined to be good, and was evaluated as ○, and when broken, it was evaluated as ×.

The particle size of the prior austenite phase shown in Table 2 was determined by taking a microscopic test piece from the tempered material, embedding and polishing with a resin, and corroding the surface with a nital solution, followed by JISG
The particle size number is measured by the method according to No. 0551. As is evident from Table 2, it is understood that the steel having the chemical composition and the particle size number specified in the present invention has both good SSC resistance and toughness. On the other hand, in the comparative example, since the chemical composition and the particle size do not satisfy the requirements of the present invention, it is recognized that the SSC resistance or the toughness is not sufficient.

[0055]

According to the present invention, YS is 1060 MPa.
Good SS resistance even with high strength (155 ksi) or more
A steel material with both C properties and toughness is obtained, and it has excellent effects when used in casings and tubing for oil and gas wells, drill pipes for drilling, line pipes for transportation, and piping for petrochemical plants, etc. It is extremely effective in industry.

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 4K032 AA01 AA02 AA05 AA08 AA11 AA12 AA16 AA19 AA21 AA22 AA23 AA26 AA27 AA29 AA31 AA35 AA36 AA37 AA39 BA01 BA03 CA03 CF03

Claims (6)

[Claims] (1) C: 0.2 to 0.35% by mass, S
i: 0.05 to 0.5%, Mn: 0.1 to 1%, P:
0.025% or less, S: 0.01% or less, Cr: 0.1
-1.2%, Mo: 0.1-1%, B: 0.0001-
0.005%, Al: 0.005 to 0.1%, V: 0.
05 to 0.5%, Ni: 0.1% or less, N: 0.01%
Hereinafter, O (oxygen): 0.01% or less, with the balance being Fe
Mo and V contents satisfy the following formulas (1) and (2), and the yield stress is 1060MP.
a (155 ksi) or more, a low alloy steel excellent in sulfide stress cracking resistance and toughness. 0.03 ≦ Mo × V ≦ 0.3 (1) 0.5 × Mo−V + GS / 10 ≧ 1 (2) Here, the element symbol indicates the content (% by mass) of each element. , GS
Indicates the ASTM grain size number of the prior austenite grains (2) 0.005 to 0.1 instead of part of Fe
The low-alloy steel material according to claim 1, wherein the low-alloy steel material contains 0.1% Nb. 3. The method according to claim 1, wherein Ti: 0.005 to 0.005
0.03%, Zr: 1 of 0.005 to 0.06%
2. The composition according to claim 1, wherein the composition contains at least one species.
Or a low alloy steel material having the chemical composition according to any of 2. 4. The low-alloy steel material according to claim 1, wherein W contains 0.3 to 1% of W instead of part of Fe. 5. The method according to claim 1, wherein 0.0001 to 0.
2. The composition according to claim 1, further comprising 0.01% of Ca.
5. The low-alloy steel material according to any one of items 1 to 4. 6. A steel having the chemical composition according to any one of claims 1 to 5, which is hot-worked, then maintained at a temperature within a temperature range of 880 to 950 ° C. for 10 minutes or more, and then quenched. The yield stress which is excellent in sulfide stress cracking resistance and toughness characterized by tempering is 1060MP.
a (155 ksi) or more low alloy steel production method.