JPS61223168A - High strength steel having superior delayed fracture resistance - Google Patents

Abstract

PURPOSE:To improve the delayed fracture resistance and strength of a steel contg. prescribed percentages of C, Si, Mn, Cr, Mo, etc. by restricting the amounts of P, S and Ni as impurities in the steel and heat treating the steel at a prescribed temp. CONSTITUTION:A steel contg. 0.15-0.45% C, <=1.5% Si, 0.01-1.5% Mn, 0.5-2% Cr, 0.3-1.5% Mo+1/2W, 0.01-0.2% V, etc., is manufactured. The amounts of P, S and Ni as impurities in the steel are restricted to <=0.02% P, <=0.01% S and <=0.1% Ni. The steel is heated to the Ac3 point or below, quenched and tempered at 580 deg.C - the Ac point under conditions which satisfy >=16.8X10<3> PLM value represented by a formula PLM=T(20+logt) [where T is the tempering temp. ( deg.K) and (t) is the holding time (hr)]. The resulting steel has ASTM austenite grain size No. >=8.5, superior delayed fracture resistance and strength.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention provides a
The present invention relates to a high-strength steel that has a yield strength exceeding 0.2% yield strength) and excellent delayed fracture resistance, and is suitable for uses such as oil country tubular goods.

Conventional technology In recent years, in response to the growing need to secure energy from a long-term perspective, the development of new oil and gas fields has become active in various parts of the world. Energy extraction is becoming more common than ever, and development is now being focused on oil and natural gas in harsh environments such as those deep beneath the surface of the earth, which had previously been abandoned. Advanced technology is becoming necessary.

For example, in recent years, extremely deep areas (over 15,000 feet deep) and 0.5 psi per foot of depth have been
Wells for oil and natural gas extraction are increasingly being drilled into geological formations with higher ground pressure than the so-called "standard state", where pressure increases of more than 515 gf/mm" (OJ) are expected. To perform stable work in such an environment, a vehicle of V-150 class or higher [
S M Y S (Specified Mini II
Ium YieldStrength (standard minimum yield strength) is 150ksi (105,5kgf10+m”)
It is believed that oil country tubular goods having extremely high strength as described above are necessary, and the current situation is that the demand for a stable supply thereof is increasing.

However, when low-alloy steel, which has traditionally been used as oil country tubular goods, has a high strength of V-150 class or higher, the static temperature drops below the yield point due to embrittlement of the austenite grain boundaries. There was an inherent risk of ``delayed failure'' in which failure occurred even under heavy loads. Generally speaking, in oil fields, when a well becomes old and no longer produces self-gushing water, the pumping efficiency is improved by applying water or gas pressure or adding acid (acidizing), which is called secondary recovery. When adding acid or in oil fields under acidic environments, low alloy steels have conventionally had the problem of increasing the risk of delayed fracture due to the presence of hydrogen.

On the other hand, maraging steels such as 18Ni-5Mo-7,5Co and austenitic high alloys and high alloy steels
It is known to have better delayed fracture resistance than ordinary low alloy steel. However, maraging steel has %C
Since it contains O, there are problems such as high cost and poor low temperature toughness. On the other hand, austenitic high-alloys and high-alloy steels require a large amount of cold working to obtain strength, which is inefficient, and the high content of Ni and Cr makes them expensive. Because of these problems, none of them are used simply for high-strength oil country tubular goods, and are only put into practical use in some extremely limited environments, especially from the economic point of view. there were.

On the other hand, JP-A-58-61219 and JP-A-58-84
No. 960 discloses a high-strength steel with excellent delayed fracture resistance. However, the steel described in JP-A No. 58-61219 only pursues the effect of cleaning grain boundaries by reducing P and N, and furthermore, since the austenite grain size is large, the above-mentioned severe It cannot exhibit sufficient delayed fracture resistance in the environment.

On the other hand, the steel described in JP-A-58-84960 is also exclusively L.
The steel described in this publication is also unable to exhibit sufficient delayed fracture resistance in the above-mentioned harsh environment by only pursuing the effects of suppressing segregation of P by x and controlling sulfide morphology by Ca.

Problems to be Solved by the Present Invention The present invention has been made in view of the problems of the prior art as described above.
ksi (105,5 kgf/mm"), its delayed fracture resistance is far superior to that of conventional low-alloy steels, and it The purpose of the present invention is to provide a high-strength steel that is much cheaper than alloy steel and is suitable for use as oil country tubular goods.

Means for Solving the Problems In order to achieve the above objectives, the inventors conducted detailed research on the chemical composition of steel materials, manufacturing conditions including heat treatment, and the relationship between the resulting structure and properties. As a result of repeated efforts, we have obtained the findings shown in (a) to (0) below. Namely, (a) Delayed fracture occurs when steel suddenly becomes brittle after a period of time when the steel is bent under a static load. This is a phenomenon in which a large amount of rupture occurs.
It is said to be a type of hydrogen embrittlement that occurs due to hydrogen that has entered the steel from the external environment or hydrogen that has entered the steel due to plating, etc., but the austenite grain size of the steel is
It has been found that the occurrence of delayed fracture can be suppressed by adjusting the TMN (L) to fine grains of 8.5 or more and quenching to obtain a martensite or low-temperature bainite structure and then tempering.

υ Carbides in steel serve as a place for hydrogen to accumulate. Therefore, if these carbides exhibit a notch defect shape such as needles or rods, or coarsely aggregate, this becomes the starting point and delayed fracture is likely to occur. However, it has been found that when Zr is contained in steel, the carbides are finely dispersed into spherical shapes and the delayed fracture resistance is significantly improved.

(C) PLM≧16.8X103 [however, PLM=
T (20+ log t), T: tempering temperature K
), t: holding time (hr)), it was found that the carbides were spheroidized and the occurrence of delayed fracture was suppressed.

” (d) C: 0.15-0.45%
, Si: 1.50% or less, Mn: 0.01 to 1.50%
Cr: 0.50 to 2 as an alloy component to steel containing
．． 00%, Mo + each W: OJO ~ 1.50%, V: 0.
01 to 0.20%, Nb: 0.005 to 0.20%, after quenching by heating above the Acs point,
At a temperature of 80°C or above and below Ac1 point, and the above P0 value is P
Even if the tempering treatment is performed under the conditions of 0≧16.8X1G', if the austenite grain size ASTMNα is 8.5 or more,
Yield strength: 150ksi (105.5kgf/mm)
It was found that a high strength exceeding that of

(e) Ni in steel promotes pitting (pitting corrosion), especially under low pH conditions, which becomes the starting point for delayed fracture, but if the Ni content is kept below 0.10%, It was found that the occurrence of pitting was suppressed and, therefore, delayed fracture resistance was improved.

(0) Refinement of austenite grains increases the yield ratio (yield strength/tensile strength), and therefore improves delayed fracture resistance because the tensile strength can be suppressed for the same yield strength. It was found to be effective.

The present invention was made based on the above findings, and includes: C: 0.15 to 0.45%, Si: 1.50% or less, Mn: 0.01 to 1.50%, Cr: 0.50. ~2.00%, Mo or W or both: Mo+1/2W
0.30~1.50%, V: 0.01~0.20%, Nb: 0.005~0.20%, Zr: 0.01~0.15%, Al: Q, Ql~0 Contains:
:0.0003~0.0050% 3rd category: Ca
: 0.001~0.030% 01 or more, the balance consists of Fe and unavoidable impurities, and Ps in the impurities
, Nt content is P: 0.020% or less, S: 0.010% or less, Ni: 0.10% or less, and is heat-quenched on a dot core above the Acs point, and then 58
At a temperature of 0°C or above and below the Ac+ point, and PLM≧16.8
X103 [However, P L, = T (20+ log
t), T: tempering temperature K), t: holding time (hr)
) is characterized in that it provides a high strength steel with an austenite grain size of 8.5 or more in ASTM Na and excellent delayed fracture resistance.

Function Next, in the present invention, the reason why the chemical composition, heat treatment, and grain size of the steel are limited as described above will be explained.

(1) Reason for limiting the composition C: C is an effective component for increasing the hardenability and strength of steel, as well as refining the grains, but if it is less than 0.15%, it will cause a decrease in strength and hardening. Therefore, the desired strength cannot be tempered at a high temperature to make the carbide spheroidized, and it becomes difficult to obtain the desired fine-grained steel, resulting in increased delayed fracture susceptibility.

On the other hand, if C content exceeds 0.45%, susceptibility to quench cracking during quenching increases and also causes deterioration of toughness, so the C content is set at 0.15 to 0.45%.

Si: Si is an element necessary for deoxidizing steel and increasing its strength, and is also effective for raising the transformation point so that high temperature tempering can be performed stably.

However, if the Si content exceeds 1.50%, the toughness will deteriorate significantly and the pH will be low! At the boundary, the delayed fracture resistance may deteriorate, so the upper limit is set to 1.50.
%.

In addition, in order to further improve delayed fracture resistance by making the austenite grains as small as possible, the Si content should be set to 0.80.
% or less, and in order to further improve delayed fracture resistance in a low pH environment, (Si +
It is preferable that the value of Mn ) is 0.80% or less.

Mn: Mn is an effective element for deoxidizing, desulfurizing, and improving hardenability, but if it is contained in large amounts, it will deteriorate the workability and delayed fracture resistance of steel, so the upper limit has been set to 1. .5
It was set to 0%. In the case of low alloy steel, the value of (Si+Mn) should be set to 0.8 in order to reduce delayed fracture susceptibility in a low PH1 environment.
Although it is effective to reduce the In content to 0.01% or less, it is extremely difficult to reduce the In content to less than 0.01% in terms of manufacturing steel and increases costs.
01 to 1.50%. 0 to obtain stable fine grain steel
．． It is preferable to add 20% or more.

Cr: Cr has the effect of increasing the hardenability, strength, and temper softening resistance of steel, and is an effective element for obtaining high-strength steel by high-temperature tempering treatment, but its content is 0.5%. If the content is less than 0.50 to 2.00%, the desired effect cannot be obtained, while if the content exceeds 2.0θ%, the toughness will deteriorate and the susceptibility to quench cracking will increase. did.

Mo5W: Both Mo and W have the effect of increasing the hardenability, strength, toughness, corrosion resistance, and temper softening resistance of steel, enabling high-temperature tempering treatment, and improving delayed fracture resistance. It was decided to contain either one or both of the following.

The content of Mo and W is defined as (Mo1''/AW) because the atomic weight of W is about twice that of Mo, and is about half that of Mo in terms of the above-mentioned effects.

If the value of (Mo 10%W) is less than 0.30%, the desired effect cannot be obtained in the above actions, while if this value exceeds 1.50%, the effects of addition will be saturated, and even more A substantially unnecessary amount of M that cannot obtain the effect of increasing strength
The content of Mo and/or W is set to 0.30 to 1.50% in terms of (Mo + 'A W), since the content of Mo and W causes an increase in cost.

■: ■ is an element that is effective in increasing the strength of steel, imparting resistance to temper softening, and refining the grain, and is effective in enabling high-temperature tempering treatment and improving delayed fracture resistance, but 0.01%
If it is less than 0.2%, the above effect cannot be obtained, and on the other hand, if it is added in a large amount exceeding 0.20%, the toughness will be deteriorated, so it is set at 0.01 to 0.20%.

Nb: Nb is effective in improving the strength and toughness of steel, imparting temper softening resistance, and grain refinement, and is also effective in improving delayed fracture resistance, but if it is less than 0.005%, The effect was not sufficient, and on the other hand, if the content exceeded 0.20%, the effect would be saturated, and it would also cause deterioration of toughness, so it was set at 0.005 to 0.20%. .

Zr: Zr is an important element in the present invention and has the effect of dispersing carbides into fine spherical particles in the steel and significantly improving delayed fracture resistance, but if it is less than 0.01%, the effect is small; If it exceeds .15%, toughness will deteriorate, so it is set at 0.01 to 0.15%.

Al: Al is effective in stabilizing deoxidation of steel, homogenizing it, and refining the grains, but if it is less than 0.01%, the desired effect cannot be obtained; on the other hand, if it is less than 0.10% If the content exceeds 0.01, the effect will be saturated, and the increase in inclusions will cause cracks and deteriorate toughness.
It was set to 0.10%.

P and S: Yield strength is particularly 150 ksi (105,
In order to improve the toughness of the steel and to improve its delayed fracture resistance, it is important to reduce the amount of impurities P and S as much as possible in low-alloy high-strength steels exceeding 5 kgf/m+s. It is desirable to set the upper limits of P and S contents to 0.020% and 0.0%, respectively, in consideration of the balance with the manufacturing cost of steel.
It was set at 10%.

Ni: Ni promotes the occurrence of pitting especially in a low pH environment, which becomes the starting point for delayed fracture, so in order to improve delayed fracture resistance, Ni as an impurity was added to 0.1
It was set as 0% or less.

Ti: Ti is an effective element for increasing the strength and refining steel, but if it is less than 0.01%, the above effects cannot be obtained;
Addition of more than 10% leads to deterioration of toughness, so the content is set in the range of 0.01 to 0.10%.

B: B is effective in improving hardenability, and thereby improving strength, toughness, and delayed fracture resistance. However, if the amount of B is less than 0.0003%, it is difficult to obtain the effect of addition, and even if it is added in excess of 0.0050%, the effect of addition is saturated and no further property improvement effect can be expected; B
The content was set to o,oooa~0,0050%.

Ca: Ca is effective in spheroidizing inclusions in steel and improving the toughness in the direction perpendicular to the rolling direction, especially in high-strength steel, but if it is less than 0.001%, this effect cannot be obtained; If it exceeds 0.030%, the effect not only becomes saturated, but also non-metallic inclusions such as oxides of the steel increase, reducing the cleanliness of the steel and increasing the susceptibility to delayed fracture. Therefore, the Ca content range is 0.001 to 0.
．． 030%.

(2) Reason for limiting heat treatment and particle size Conventionally, the yield strength was 150 ksi (105,5 kgf/
For high-strength oil country tubular goods made of low-alloy steel exceeding mm2), hot-rolled steel pipes are reheated to Ac3 point or higher and then quenched, or hot-rolled steel pipes are made and then directly quenched at a temperature of Ac3 point or higher. , and then tempered at a temperature below its own point. However, directly quenched steel pipes have coarse austenite grains (ASTMN 17 or less) and are extremely susceptible to delayed fracture. On the other hand, research by the present inventors has revealed that in the case of reheated and quenched steel, the delayed fracture characteristics vary greatly depending on the austenite grain size and tempering temperature.

That is, the present inventors have discovered that C, Si, Mn, CrSMo
,W.

■ Using various steels whose contents of Nb, Zr, AI, PlS, and Ni are within the range of the present invention, the austenite grain size is improved by using various means such as heat treatment, processing heat treatment, and a combination of cold working and heat treatment. Change this to 450 to 650
It was tempered at ℃ for 30 minutes. Parallel section 8 from each steel plate.
5. , φ round bar tensile test specimens were collected and subjected to a tensile test, and the delayed fracture characteristics were investigated only for those that were confirmed to have a yield strength (0.2% proof stress) near 170 ksi (119.5 kgf/ll 1 m2). did.

The delayed fracture characteristics are shown in Fig. 1(a), a perspective view of the whole, and Fig. 1(a).
Figure (c) shows the details of the U-notch part. Five specimens were cut out from one tempered steel plate, a wedge was inserted into the U-notch part, and then immersed in hot water at 80°C for 5,000 hours. Investigate whether or not an outbreak has occurred, and use the results as a second
Shown in the figure. In FIG. 2, ◯ indicates that no cracking was observed in any of the five test pieces, and × indicates that cracking was observed in any or all of the five test pieces.

As shown in Figure 2, the austenite grain size is A37MN.
If α is less than 8.5, cracks will occur even if the tempering temperature is increased; on the other hand, if the tempering temperature is less than 580°C, cracks will occur even if the austenite grain size is as fine as 8.5 or more by ASTM NLIL. It turns out. Therefore, in the present invention, the austenite grain size is determined according to ASTM No. It is limited to steels that are quenched at a temperature of 8.5 or higher and tempered at 580°C or higher.

Next, the reason why the heating temperature for quenching is set to Ac3 or higher is to quench from a uniform austenite structure. The upper limit of the heating temperature for quenching is preferably below the austenite grain coarsening starting temperature, and as mentioned above, fine austenite grains with AsTMlk of 8.5 or more can be obtained in the final quenching process. There is a need to.

Next, the present inventors discovered that 0.30%C-0.30%Si
-0,43%Mn -0,64%Cr -0,46%Mo
-0,05%V-0,041%Nb-0,040%Zr
-0,039%^1-0.008%P -0,002
Using a steel having a composition of %S-0.03%Ni (self point near 55°C, Ac5 point = 850°C), the austenite grain size was adjusted to ASTM No. The temperature was adjusted to 10.5 and quenched, then heated to 600°C and held for 5 minutes and 1 hour, respectively.
Tempering was performed for 0 minutes, 15 minutes, and 30 minutes, and the same delayed fracture test as above was conducted on the steel pieces after tempering.

From the results of this experiment, cracks were observed at a rate of 215.115 for those that were tempered for 5 minutes and 10 minutes, respectively. However, no cracks were observed in the samples tempered for 15 and 30 minutes.

For tempering at 600°C for 10 minutes, P LM =
16.78 x 10', and for tempering at 600°C for 15 minutes, P LM = 16.938103.

Therefore, in the present invention, the condition that PLMI≧16.8 XlG3 is set. Note that this condition requires a tempering temperature of 580°C.
This indicates that tempering for 30 minutes or more is required. That is, as mentioned above, the temperature is 580°C or higher and the PL
Above 1 shows that when M≧16,8
This was confirmed in the experiment described above.

On the other hand, the above steel was tempered at 550°C for 3 hours (
When the above-mentioned delayed fracture test was conducted on PLM1=16.9×103), cracking occurred at a rate of 115. From this, for tempering, 580℃
It is clear that lack of any one of the above conditions (PLM≧16.8 XIO') is not favorable for improving delayed fracture resistance.

Therefore, in the method of the present invention, the temperature is 580°C or higher and P
LM≧16.8×10′ is defined as the tempering condition.

Further, in this case, if the tempering temperature exceeds the self-point, the strength of the steel material will not only change significantly, but also the delayed fracture susceptibility will increase, so the tempering temperature was set to be below the Ac1 point.

Next, the present invention will be explained using examples and comparing with comparative examples. It should be noted that these Examples are merely illustrative of the effects of the present invention, and of course do not limit the technical scope of the present invention.

Example 1 First, steels 1 to 17 having the chemical compositions shown in Table 1 were melted. These steels were then heated and rolled, quenched and tempered under the conditions shown in Table 2.

Austenite grain size (ASTM
No. ), and tensile tests and delayed fracture tests were conducted on the tempered samples.

The tensile test was conducted using a round bar test piece with a parallel portion of 8.5 mmφ, and the delayed fracture test was conducted under the following conditions. That is,
Five test pieces shown in FIG. 1 were cut out from each type of steel material. FIG. 1(a) shows the overall shape of the U-notched test piece, and FIG. 1(c) shows details of the U-notched test piece. After statically inserting a wedge into this U-notch, it was immersed in warm water at 80° C. for 5,000 hours to check for cracks.

The test results obtained are also shown in Table 2.

From the results shown in Table 2, the steel with the chemical composition range of the present invention is 5
Even if tempered under the conditions of 80℃ or higher and PLMI≧16.8
A yield strength exceeding 0.2% yield strength (0.2% yield strength) exceeding 0.2% yield strength (0.2% proof stress) is obtained, and there is no occurrence of delayed fracture, and either one or both of strength and delayed fracture resistance is superior to comparative steels. It is clear that the balance between this and delayed fracture resistance is extremely good.

Example 2 Steels 18 to 20 having the chemical composition shown in Table 3 were melted.

Next, these steels were heated and rolled, quenched under the conditions shown in Table 4, and then tempered.

Austenite grain size (ASTM
No. ), and a tensile test and a delayed fracture test were conducted on the tempered product under the same conditions as in Example 1.

The test results thus obtained are also shown in Table 4.

The results shown in Table 4 also show that steel within the chemical composition range of the present invention has a strength of 150 ksi (105.5 kgf/m) even when tempered at 580°C or higher and P0≧16.8xlO'.
A large yield strength (0.2% yield strength) exceeding 0.2% yield strength is obtained, and there is no occurrence of delayed fracture, and both strength and delayed fracture resistance are superior to comparative steels. It is clear that the balance is extremely good.

Example 3 Steel 19, which is a target steel having the chemical composition range defined by the present invention in Table 3 above, was heated and rolled, quenched under the conditions shown in Table 5, and then tempered.

Austenite grain size (ASTM
After tempering, a tensile test and a delayed fracture test were conducted under the same conditions as in Example 1. The test results are also shown in Table 5.

From the results in Table 5, it can be seen that the delayed fracture resistance of target steels within the chemical composition range defined by the present invention becomes good only when the processing conditions within the range of the present invention are satisfied.

Example 4 Steel 18, which is a target steel having the chemical composition range defined by the present invention in Table 3 above, was heated and rolled, then quenched under the conditions shown in Table 6, and then tempered.

Austenite grain size (ASTM
N(L) was measured, and the tensile test and delayed fracture test were conducted under the same conditions as in Example 1 for the tempered product. The test results are also shown in Table 6.

From Table 6, it can be seen that in the present invention, regardless of the method of refining austenite grains, the austenite grain size is
．． After adjusting the temperature to 8.5 or higher and quenching it, heat it to 580℃.
From the above, it can be seen that high-strength steel with excellent delayed fracture resistance can be obtained as long as it is tempered under the conditions of PLN≧16.8X10'.

Example 5 Of the steels shown in Table 1 above, steel 1.2.3.7, which is a steel subject to the chemical composition range specified by the present invention, and comparative steel 14, which is outside the scope of the present invention, were heated and rolled, Hardening was performed under the conditions shown in Table 7, and then tempering was performed. Measure the austenite grain size (ASTM No.) of the material before tempering,
After tempering, a tensile test was conducted under the same conditions as in Example 1, and according to Example 1, the specimens were immersed for 2000 hours in 5% saline (at room temperature) with HCI't'pH adjusted to 3.5. A delayed fracture test was conducted.The test liquid was 48
Replaced every hour.

The test results thus obtained are also shown in Table 7.

From the results shown in Table 7, it is clear that among the target steels within the chemical composition range specified by the present invention, steel 1.2 with (Si+Mn) of 0.80% or less and low Ni is particularly resistant to lag even in a low pH environment. It can be seen that the balance between breakability and strength is extremely good.

Effects of the Invention As mentioned above, the steel of the present invention has a strength of 150 ksi (10
It has become possible to manufacture inexpensive ultra-high strength oil country tubular goods that have high strength exceeding 5.5 kgf/mm') and excellent delayed fracture resistance, and the industrial effects thereof are extremely large. .

The steel of the present invention can be widely applied not only to ultra-high-strength oil country tubular goods but also to high-strength bolts having the same strength level as described above.

Note that in this specification, the percentages used to indicate the chemical components of steel are percentages by weight.

[Brief explanation of the drawing]

FIG. 1 shows the shape of a delayed fracture test piece, FIG. 1(a) is a perspective view of the entire test piece, and FIG. 1(6) shows details of the U-notch. Figure 2 shows 170 ksi (119,5 kgf/mm)
FIG. 2 is a diagram showing the influence of tempering temperature, degree (in the case of 30 minutes of holding), and austenite grain size on the delayed fracture resistance of the steel subject to the present invention having a yield strength near . Patent applicant Sumitomo Metal Industries Co., Ltd. Agent Patent attorney Masahiko Arai Figure 1 (5i) (b) Austenite degree (ASTM No.) Procedural amendment (spontaneous) 1. Indication of case Patent application No. 065132 of 1985 2. Name of the invention High strength steel with excellent delayed fracture resistance 3.
Relationship with the case of the person making the amendment Patent applicant address 5-15 Kitahama, Higashi-ku, Osaka Name
(211) Sumitomo Metal Industries, Ltd. 4, Agent 6, Number of inventions increased by amendment (none) 7, Subject of amendment Detailed explanation of the invention in the specification column 8, Contents of the amendment, page 26 of the specification Table 1 is as shown in the attached sheet (corrected).

Claims (4)

[Claims] (1) In weight%, C: 0.15-0.45%, Si: 1.50% or less, Mn: 0.01-1.50%, Cr: 0.50-2.00%, Mo or Either or both of W: Mo+1/2W 0.30-1.50%, V: 0.01-0.20%, Nb: 0.005-0.20%, Zr: 0.01-0 .15%, Al: 0.01~0.10%, the balance consists of Fe and unavoidable impurities, and the content of P, S, and Ni in the impurities is P: 0.0.
20% or less, S: 0.010% or less, Ni: 0.10% or less, and is quenched after heating to Ac_3 points or more, and then 5
P_L_M≧ at a temperature of 80℃ or higher and a temperature of Ac_1 point or lower
The austenite grain size, which was tempered under conditions satisfying 16.8×10^3, is ASTM No. High strength steel with excellent delayed fracture resistance of 8.5 or higher. However, P_L_M=T(20+logt) T: Tempering temperature (°k), t: Holding time (hr), (2) In weight%, C: 0.15-0.45%, Si: 1.50% or less, Mn: 0.01-1.50%, Cr: 0.50-2.00%, Mo or Either or both of W: Mo+1/2W 0.30-1.50%, V: 0.01-0.20%, Nb: 0.005-0.20%, Zr: 0.01-0 .15%, Al: 0.01 to 0.10%, and further contains either or both of Ti: 0.01 to 0.10% and B: 0.0003 to 0.0050%. , the balance is Fe and unavoidable impurities, and the contents of P, S, and Ni in the impurities are P:
0.020% or less, S: 0.010% or less, Ni: 0.10% or less, and is quenched after heating to Ac_3 points or more, and then 5
P_L_M≧ at a temperature of 80℃ or higher and a temperature of Ac_1 point or lower
The austenite grain size, which was tempered under conditions satisfying 16.8×10^3, is ASTM No. High strength steel with excellent delayed fracture resistance of 8.5 or higher. However, P_L_M=T(20+logt) T: Tempering temperature (°K), t: Holding time (hr), (3) In weight%, C: 0.15-0.45%, Si: 1.50% or less, Mn: 0.01-1.50%, Cr: 0.50-2.00%, Mo or Either or both of W: Mo+1/2W 0.30-1.50%, V: 0.01-0.20%, Nb: 0.005-0.20%, Zr: 0.01-0 .15%, Al: 0.01 to 0.10%, Ca: 0.001 to 0.030%, the balance consists of Fe and inevitable impurities, and the content of P, S, and Ni in the impurities are respectively, P: 0.0
20% or less, S: 0.010% or less, Ni: 0.10% or less, and is quenched after heating to Ac_3 points or more, and then 5
P_L_M≧ at a temperature of 80℃ or higher and a temperature of Ac_1 point or lower
The austenite grain size, which was tempered under conditions satisfying 16.8×10^3, is ASTM No. High strength steel with excellent delayed fracture resistance of 8.5 or higher. However, P_L_M=T(20+logt) T: Tempering temperature (°K), t: Holding time (hr), (4) In weight%, C: 0.15-0.45%, Si: 1.50% or less, Mn: 0.01-1.50%, Cr: 0.50-2.00%, Mo or Either or both of W: Mo+1/2W 0.30-1.50%, V: 0.01-0.20%, Nb: 0.005-0.20%, Zr: 0.01-0 .15%, Al: 0.01-0.10%, Ca: 0.001-0.030%, and further contains Ti: 0.01-0.10% and B: 0.0003-0. P:
0.020% or less, S: 0.010% or less, Ni: 0.10% or less, and is quenched after heating to Ac_3 points or more, and then 5
P_L_M≧ at a temperature of 80℃ or higher and a temperature of Ac_1 point or lower
The austenite grain size, which was tempered under conditions satisfying 16.8×10^3, is ASTM No. High strength steel with excellent delayed fracture resistance of 8.5 or higher. However, P_L_M=T(20+logt) T: Tempering temperature (°K), t: Holding time (hr),