JP2000017389A - Cr-Mo SERIES LOW ALLOY SEAMLESS STEEL PIPE EXCELLENT IN TOUGHNESS AND ITS Cr-Mo SERIES LOW ALLOY STEEL - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain good toughness in a steel pipe even in the case the crystal grains are coarse ones by forming the main structure of martensite, controlling the old austenite grain size to specified value and precipitating carbides mainly of an M3C type other than an M23C6 type into the old austenite grain boundaries. SOLUTION: This steel pipe contains, by weight, 0.05 to 1.5% Cr and 0.05 to 1.0% Mo. The main structure is formed of tempered martensite, the old austenite grain size is controlled to No.<=7 in the size number prescribed by JISG0551, and carbides mainly of an M3C type other than an M23C6 type are precipitated into the old austenite grain boundaries. This steel pipe is produced by using a billet of Cr-Mo series low alloy steel as the stock, subjecting the steel pipe subjected to hot finish forming by a Mannesman-mandrel pipe making method or the like to quenching treatment by heat treating equipment provided on the back-end of a finish rolling mill in a state in which it is held to the temp. more than the Ar3 transformation point and thereafter executing tempering treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a Cr alloy having excellent toughness.
-Mo-based low alloy steel seamless steel pipe and Cr for this seamless steel pipe
-Mo type low alloy steel. More specifically, after hot pipe production, so-called in-line heat treatment, in which the pipe is not cooled to a temperature lower than the Ar ₃ transformation point, but is quenched by effectively utilizing the heat retained during hot working and then tempered. A seamless steel pipe made of a Cr-Mo low alloy steel manufactured by the process, in which the main structure is tempered martensite and the former austenite crystal grain size is 7 or less of the grain size number specified in JIS G 0551. Regardless, a Cr-Mo low alloy steel seamless steel pipe with excellent toughness and a Cr-M for the same
o-based low alloy steel.

[0002]

2. Description of the Related Art Seamless steel pipes are more reliable than welded pipes and are therefore often used in harsh oil well environments and high-temperature environments, and are required to have higher strength, improved toughness and improved sour resistance. Always required. In order to satisfy these requirements, conventionally, a highly hardenable Cr-Mo steel represented by, for example, 1Cr-0.5Mo steel containing up to 1.5% of Cr and up to 1.0% of Mo is used, The size of the crystal grains has been reduced to 8 or more fine grains by a grain size number specified in JIS G 0551 by off-line tempering treatment.
However, from the viewpoint of production efficiency and energy saving,
In-line heat treatment is more advantageous.

[0003] Therefore, it has been studied to secure the toughness by utilizing the thermomechanical treatment to make the crystal grains finer and to omit the off-line heat treatment. For example,
Japanese Patent Application Laid-Open No. 61-238917 discloses a method of obtaining a steel material having good toughness by directly quenching after refining the structure by combining cooling and heating in-line.

[0004] However, since it requires processing at an appropriate temperature, cooling and reheating, the amount of energy saving is not so large. In addition, there is a problem that it is difficult to apply to all manufacturing sizes due to restrictions of the device. Therefore, there is a need for a means capable of securing toughness even with a relatively coarse crystal grain size obtained by in-line heat treatment.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to reduce the crystal grain size of relatively coarse grains, specifically, the coarse grain of prior austenite having a grain size of 7 or less of the grain size number prescribed in JIS G 0551. Nevertheless, Cr-
An object of the present invention is to provide a Mo-based low alloy steel seamless steel pipe and a Cr-Mo-based low alloy steel therefor.

[0006]

SUMMARY OF THE INVENTION The gist of the present invention is to provide the following (1) Cr-Mo based low alloy steel seamless steel pipe having excellent toughness and the following (2) and (3) seamless steel pipes. Cr-M
o-based low alloy steel.

(1) Cr: 0.05 to 1.5% by weight
%, Mo: 0.05 to 1.0% Cr-Mo low alloy steel seamless steel pipe containing tempered martensite and a prior austenite grain size of JIS G05
51 or less 7 No. grain size number defined in types except M ₂₃ C ₆ in the old austenite grain boundaries, mainly M ₃
Cr-Mo low alloy steel seamless steel pipe with C type carbide precipitated.

(2) C: 0.15 to 0.35% by weight
%, Si: 0.1 to 1.5%, Mn: 0.1 to 2.5
%, P: 0.05% or less, S: 0.004% or less, so
l. Al: 0.001 to 0.1%, Cr: 0.05 to
1.5%, Mo: 0.05-1.0%, Nb: 0-0.
05%, Ti: 0 to 0.05%, V: 0 to 0.15%,
Ca: 0 to 0.0050%, Mg: 0 to 0.0050
%, REM: 0 to 0.0050%, N: 0.01% or less, B: 0 to 0.0030%, the balance being Fe and unavoidable impurities, the amount of effective B obtained by the following formula or the formula A Cr-Mo based low alloy steel for seamless steel pipes, wherein [B] is 0 to less than 0.0005% and the relationship between Cr and Mo satisfies the following formula.

When Ti <3.419 × N [B] = B + 0.2259 × Ti−0.7723 × N... When Ti ≧ 3.419 × N [B] = B ······· Mo × Cr ≦ 0.55 ···································································································· , The content (% by weight) of each element in the steel.

(3) In weight%, C: 0.15 to 0.35
%, Si: 0.1 to 1.5%, Mn: 0.1 to 2.5
%, P: 0.05% or less, S: 0.004% or less, so
l. Al: 0.001 to 0.1%, Cr: 0.05 to
1.5%, Mo: 0.05-1.0%, Nb: 0-0.
05%, Ti: 0 to 0.05%, V: 0 to 0.15%,
Ca: 0 to 0.0050%, Mg: 0 to 0.0050
%, REM: 0 to 0.0050%, N: 0.01% or less, B: 0 to 0.0030%, the balance being Fe and inevitable impurities, the amount of effective B obtained by the following formula or the formula [B] is 0.0005 to 0.0015%, and a Cr-Mo low alloy steel for a seamless steel pipe having a chemical composition in which the relationship between Cr and Mo satisfies the following formula.

When Ti <3.419 × N [B] = B + 0.2259 × Ti−0.7723 × N... When Ti ≧ 3.419 × N [B] = B... ······· Mo × Cr ≦ 0.16 ···················································· Each content (% by weight).

The alloy steel of the present invention described in the above (2) and (3) preferably has a P content of 0.015% or less.

The present invention has been completed based on the following findings. That is, the present inventors have earnestly studied a method for improving the toughness of a Cr-Mo based low alloy steel seamless steel pipe having a coarse crystal grain size manufactured by an in-line heat treatment process from the viewpoint of production efficiency and energy saving; The following has been found.

In a steel material manufactured by a so-called off-line heat treatment process in which a steel material once cooled to around room temperature after hot working is reheated to a temperature above the A _C3 transformation point to perform a quenching and tempering treatment, old austenite grains are used. M ₂₃ precipitated in the world
Toughness decreases when C ₆ type carbide is coarsened. Therefore, if appropriate the B content which promotes coarsening of carbides during quenching, M ₂₃ C ₆ type carbide becomes fine,
It is known that sufficient toughness can be ensured (iron and steel, vol. 72, No. 2, page 233 (1986)).

In the case of a steel material having a fine grain structure produced by an off-line heat treatment process and having a prior austenite grain size of 8 or more of the grain size number specified in JIS G 0551, the above method is certainly sufficient. . But,
In the case of a steel material manufactured by an in-line heat treatment process and having a coarse grain structure having a prior austenite grain size of 7 or less of the grain size number specified in JIS G 0551, controlling the B content alone does not improve the toughness at all.

Therefore, in order to achieve the above object, a factor governing the toughness of a Cr-Mo based low alloy steel material having a coarse grain structure with a prior austenite crystal grain size of 7 or less of the grain size number specified in JIS G 0551 is considered. A search was conducted to find out.
As a result, the carbide precipitated at the former austenite grain boundary is reduced to M
_{When the} type is changed from the ₂₃ C ₆ type to the M ₃ C type, the toughness is drastically improved in spite of the coarse grain structure. Further, if the content of B, Cr and Mo is controlled as follows, the M ₃ C type carbide is surely precipitated at the grain boundary instead of the M ₂₃ C ₆ type carbide.

First, the content of B contained in steel is limited to 0.0030% or less. In addition, free B that is not bonded to N before quenching (this is referred to as effective B in the present specification), and the effective B amount [B] obtained by the above equation or the equation is 0 to 0.0005.
% And the content of Cr and Mo is set to a value satisfying the above equation. Alternatively, the effective B amount [B] is set to 0.0
005-0.0015%, and the contents of Cr and Mo are set to values satisfying the above formula.

FIGS. 1 and 2 are examples of diagrams showing the relationship between the effect of the Cr and Mo contents and the effective B content [B] on M ₃ C type carbide precipitation and toughness. 1 indicates the effective B amount [B] is 0 to less than 0.0005%, and FIG.
The amount [B] is 0.0005 to 0.0015%, and the contents of Ti and N are 0 to 0.03% and 0.005%, respectively.
It is the case of 30 to 0.0070%.

The symbol "○" in the figure indicates that M ₂₃ C ₆
No type carbides precipitated at all indicates that toughness is good only in M ₃ C type carbide, "●" mark, M ₂₃ C ₆ type in addition to the M ₃ C type carbide in grain boundaries This indicates that carbides were precipitated and the toughness was not good.

Here, M in the above-mentioned M ₂₃ C ₆ or M ₃ C is mainly Cr, and in addition, it is Fe, Mo, Mn or the like.

When the content of P in the impurities is set to 0.015% or less under the above conditions, the toughness is further improved.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the toughness of the present invention, C
The most preferred Cr-Mo low alloy steel according to the present invention, which is used for obtaining an r-Mo low alloy steel seamless steel pipe, will be described. In the following, “%” means “% by weight”.

<< Chemical Composition of Steel >> C: C is added for the purpose of securing strength. However, if the content is less than 0.15%, hardenability is insufficient,
Even if the tempering temperature is lowered, it is difficult to secure the required strength. On the other hand, when the content exceeds 0.35%, C
Even if the contents of r and Mo are suppressed as described later, M ₂₃ C
Precipitation of the _six types of carbides at the grain boundaries cannot be suppressed, and the toughness decreases. In addition, burning cracks easily occur, which makes mass production difficult. Therefore, the C content is 0.15 to
0.35%. The preferred range is 0.20 to 0.28
%, And a more preferable range is 0.22 to 0.25%.

Si: Si is an element that is usually added for the purpose of deoxidizing steel and that increases tempering softening resistance and contributes to an increase in strength. The deoxidizing effect is obtained at a content of 0.1% or more. However, when the content exceeds 1.5%, the hot workability becomes extremely poor. For this reason, the Si content is 0.1.
1 to 1.5%. The preferred range is 0.3-0.8%
It is.

Mn: Mn is an element effective for increasing the hardenability of steel and ensuring the strength of a seamless steel pipe. But,
If its content is less than 0.1%, hardenability will be insufficient, and both strength and toughness will decrease. On the other hand, when the content exceeds 2.5%, segregation increases, and on the contrary, the toughness decreases.
Therefore, the Mn content is set to 0.1 to 2.5%.

Although Mn is smaller than Cr and Mo described below, Mn has an effect of precipitating M ₂₃ C ₆ type carbide. Therefore, the content is preferably as small as possible from the viewpoint of securing high toughness, and at most up to about 0.8%, more preferably 0.5%.
% Is desirable.

P: P is inevitably present in steel as an impurity. If the content exceeds 0.05%, it segregates at the grain boundaries and lowers the toughness.
From the viewpoint of further improving the toughness, it is preferable to reduce the content. When the content is 0.015% or less, good toughness is obtained, and when the content is 0.008% or less, the toughness is further improved. Become.

S: S is inevitably present as an impurity in the steel, like P described above. The content is 0.004
%, It binds with the above-mentioned Mn or Ca or REM when added later to form inclusions and lowers the toughness. From the viewpoint of further improving the toughness, it is preferable to reduce the content. If the content is 0.0015% or less, good toughness is obtained, and if it is 0.0008% or less, the toughness is further improved. Become.

Sol. Al: Al is an element necessary for deoxidizing steel. However, the content of sol. Al
If it is less than 0.001%, the steel quality is deteriorated due to insufficient deoxidation, and the toughness is reduced. On the other hand, sol. 0.1% in Al
If the content exceeds 3, the toughness is rather lowered, which is not preferable. Therefore, sol. Al content is 0.0
01% to 0.1%. The preferred range is 0.01 to
0.04%.

Cr, Mo: Cr is an element useful for improving hardenability. However, if the content is less than 0.05%, the hardenability is poor, and the required strength cannot be obtained. On the other hand, if the content exceeds 1.5%, the decrease in toughness is large. Therefore, the Cr content is 0.05 to 1.5%.
And

Mo is added to enhance hardenability and temper softening resistance. Also, there is an effect of improving sour resistance. However, if the content is less than 0.05%, the above effects cannot be obtained. On the other hand, if the content exceeds 1.0%, the toughness deteriorates. For this reason, the Mo content is 0.1.
05 to 1.0%.

However, the balance between the contents of Cr and Mo is important, which suppresses the precipitation of M ₂₃ C ₆ type carbide at the grain boundaries, and adversely affects the toughness in addition to the M ₃ C type. In some cases, carbides such as MC type, M ₆ C type, and M ₇ C ₃ type may be slightly contained. However, in order to mainly precipitate M ₃ C type carbide at grain boundaries, an effective B Depending on the quantity [B], the quantity must satisfy the following formula or the formula. That is, when the effective B amount [B] is 0 to less than 0.0005%, the amount satisfies the formula, and the effective B amount [B] is 0.0005 to 0.0015%.
In the case of, the amount must satisfy the formula.

Mo × Cr ≦ 0.55... Mo × Cr ≦ 0.16... Here, the element symbols are the contents (% by weight) of the respective elements in the steel.

Nb: Nb has the effect of increasing the variation in the strength of the seamless steel pipe manufactured by the in-line heat treatment process, so it is better not to add Nb from the viewpoint of suppressing the variation in the strength. However, Nb is an element which greatly strengthens steel by secondary precipitation and greatly contributes to improvement in strength. Therefore, when it is desired to obtain the effect, Nb can be added. However, if the content is less than 0.005%, the above effects cannot be obtained. On the other hand, when the content exceeds 0.05%, the variation in strength and the decrease in toughness increase. For this reason, the Nb content when added is 0.1.
005-0.05% is desirable, and 0.005-0.02% is more desirable.

Ti: Ti need not be added. If added, it has a strong bonding force with N, so that Ti
It has the effect of forming N and fixing N, preventing B in the steel from becoming BN during quenching, and making it free B effective for hardenability. It also has the effect of strengthening the steel by the secondary precipitation effect and improving the strength. Therefore, in order to obtain the effect, Ti can be added. But,
If the content is less than 0.005%, the above effects cannot be obtained. On the other hand, when the content exceeds 0.05%, a large amount of TiN or TiC precipitates and the toughness is reduced. For this reason, the Ti content when added is desirably 0.005 to 0.05%, more preferably 0.005 to 0.03%.
% Is desirable.

V: V may not be added. If added, it has the effect of strengthening the steel by the secondary precipitation effect and improving the strength, like Nb and Ti described above. Also, V
Has a large solubility in VC during hot working, so that it is completely dissolved in quenching in-line, and does not cause a variation in strength. Therefore, when the effect is desired, V can be added. However, if the content is less than 0.01%, the above effects cannot be obtained. On the other hand, 0.3
If the content exceeds 0%, the toughness is greatly deteriorated. Therefore, the V content when added is 0.01 to 0.30%.
And more preferably 0.05 to 0.2.
It is desirable to set it to 5%.

Ca, Mg, REM These elements need not be added. When added, these elements react with S in the steel to form sulfates in the 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. So if you want to get that effect,
One or more selected from Ca, Mg, and REM (Ce, La, Y, etc.) can be added.

However, if the content of each element is less than 0.0005%, the above effects cannot be obtained. On the other hand, if any element is contained in excess of 0.0050%, the amount of inclusions in the steel increases, the cleanliness decreases, and various performances decrease. Therefore, the content of these elements when added is 0.0005 to 0.00
The content is desirably set to 50%, more preferably 0.000%.
It is desirably set to 0.05 to 0.0025%.

N: N is inevitably present in steel as an impurity, similar to S and P described above, and Al, Ti and Nb
And form a nitride. In particular, the content is 0.1.
If it exceeds 01%, a large amount of AlN or TiN precipitates and adversely affects toughness, SSC resistance and HIC resistance. Therefore, the N content is set to 0.01% or less. A preferred upper limit is 0.007%.

B: B may not be added. If added, the hardenability is improved, and it is particularly effective when manufacturing a thick steel pipe. Therefore, to obtain this effect, B
Can be added. However, B contributing to the improvement of hardenability is free B not combined with N (effective B)
It is. However, the effective B amount [B] is greatly influenced by N and Ti when added, and the relationship between the two is “Ti <
In the case of “3.419 × N”, the following expression, “Ti ≧ 3.
In the case of “419 × N”, the value is obtained by the following equation.

[B] = B + 0.2259 × Ti−0.7723 × N [B] = B The relationship between N and Ti when added is “Ti <
In the case of “3.419 × N”, not all N is fixed as TiN by the added Ti, and excess N bonds with B to generate BN. For this reason, the effective B amount [B] is smaller than the added B amount, and becomes a value obtained by the above equation. When the value obtained by the formula is “negative”, it means that all of the added B amount is combined with N and is fixed as BN, so that the effective B amount [B] is 0.

On the other hand, when the relationship between N and Ti when added is “Ti ≧ 3.419 × N”, the added T
i fixes all N as TiN and does not generate BN. For this reason, the effective B amount [B] in this case is equal to the added B amount, and is a value represented by the equation.

The effect of improving hardenability is obtained when the effective B amount [B] is 0.0001% or more. However, excessive effective B
Promotes grain boundary precipitation of M ₂₃ C ₆ type carbide which reduces toughness, and the prior austenite grain size is JIS G
The toughness of a Cr-Mo based low alloy steel material having a coarse grain structure having a grain size number of 7 or less specified by 0551 is reduced.

However, the effective B is not present at all, or even if it is present, the effective B amount [B] is 0.0005.
% And the contents of Cr and Mo are adjusted so as to satisfy the above-described formula, carbides of the M ₃ C type mainly precipitate at the grain boundaries instead of the carbides of the M ₂₃ C ₆ type, and the toughness is reduced. Improve dramatically.

In the region where the effective B content [B] is 0.0005% or more, the upper limit is limited to 0.0015%, and the contents of Cr and Mo are adjusted so as to satisfy the above-mentioned relationship. Then, as in the case described above, M ₃ C type carbide mainly precipitates at the grain boundaries instead of M ₂₃ C ₆ type carbide, and the toughness is dramatically improved.

That is, in a steel in which the relationship between Cr and Mo satisfies the above-described formula, if the effective B amount [B] is 0.0005% or more, M ₂₃ C ₆ type carbide precipitates at the grain boundaries,
The toughness is significantly reduced. In the case of steel in which the relationship between Cr and Mo satisfies the above-described formula, the effective B amount [B] is 0.001.
If it exceeds 5%, carbides of the M ₂₃ C ₆ type precipitate at the grain boundaries, and the toughness is significantly reduced.

Therefore, in the present invention, the effective B amount [B] obtained by the above equation or the equation
o is 0 to 0.
Less than 0005%, 0.000 for steel satisfying the formula
It was determined as 5 to 0.0015%.

If the B content in the steel exceeds 0.0030%, the precipitation of carbohydrates increases and the sulfide stress cracking resistance decreases. Further, depending on the cooling rate, the quenchability may be rather reduced. Therefore, the upper limit of the B content is set to 0.0030%.

<< About the Cr-Mo Low Alloy Steel Seamless Steel Pipe of the Present Invention >> The Cr-Mo low alloy steel seamless steel pipe of the present invention is, in terms of% by weight, Cr: 0.05-1.5%, Mo: : 0.
A Cr-Mo based low alloy steel seamless steel pipe containing 0.05 to 1.0%, in which the main structure is tempered martensite and the prior austenite grain size is 7 or less of the grain size number specified in JISG 0551. , M on old austenite grain boundary
In type except for the ₂₃ C _6, it is a seamless steel pipe mainly M ₃ C type carbides are precipitated. This seamless steel pipe
Using a billet of a Cr-Mo low alloy steel having the above chemical composition as a raw material, a steel pipe formed by hot finishing by a conventional method, for example, a Mannesmann-mandrel mill pipe forming method, was subjected to _Ar3
In a state where the temperature is maintained at a temperature equal to or higher than the transformation point, it is supplied to a heat treatment facility provided at a stage subsequent to the finish rolling mill to perform a quenching treatment, and then to perform a tempering treatment at, for example, 600 to 750 ° C.

[0050]

EXAMPLES 26 types of steels having the chemical compositions shown in Tables 1 and 2 were prepared. In the table, Nos. 1 to 20 are steels of the present invention, and Nos. 21 to 25 are steels of comparative examples.

[0051]

[Table 1]

[0052]

[Table 2]

A billet having an outer diameter of 276 mm made of each of the above steels was prepared, heated to 1250 ° C., and then subjected to a Mannesmann-mandrel mill tube manufacturing method to obtain an outer diameter of 244.5 mm.
It was formed into a seamless steel pipe having a thickness of 13.8 mm. While the temperature of the seamless steel pipe after forming is _{equal to} or higher than the _Ar3 transformation point, the seamless steel pipe is charged into an auxiliary heating furnace having a furnace temperature of 950 ° C., which constitutes a heat treatment facility provided at a subsequent stage of the finish rolling mill. After being left in the furnace for 5 minutes and uniformly supplementarily heated, water quenching was performed. Next, the seamless steel pipe after the water quenching was charged into a tempering furnace constituting the above heat treatment equipment, and was finished into a product pipe by an in-line heat treatment process of tempering under the conditions shown in Table 3.

From the longitudinal direction of each product tube obtained, API
(American Petroleum Institute) An arc-shaped tensile test piece specified by 5CT and a full-size No. 4 test piece specified by JIS Z 2202 are sampled, subjected to a tensile test and a Charpy impact test, and yield strength (MPa) is obtained. And fracture surface transition temperature (℃)
Was examined. Similarly, a grain size measurement specimen and a micro observation specimen are collected, and the size of the grain size of the prior austenite (grain number specified in JIS G 0551) and the type of carbide precipitated at the prior austenite grain boundary are extracted. The examination was performed using a replica method. The results of these investigations are also shown in Table 3.

[0055]

[Table 3]

As is clear from the results shown in Table 3, among the steels of the present invention, the effective B amount [B] ("* 2" in Tables 1 and 2)
0 to less than 0.0005%, and the formula “Mo
× Cr ≦ 0.55 ”, the seamless steel pipes (No. 1 to 10) manufactured from the steels of No. 1 to 10 of the present invention all have a grain size number of 7 or less, and M ₂₃ C ₆ type carbide was not precipitated, and mainly M ₃ C type carbide was precipitated. Therefore, the yield strength is 821 MPa
Despite the above high strength, the fracture surface transition temperature is -21 ° C
Below, it had good toughness. Above all, the toughness of the seamless steel pipes (Nos. 6 to 10) manufactured from the steels of the present invention of Nos. 6 to 10 in which the content of P is reduced indicates that the fracture surface transition temperature is -44.
It was much better below ℃.

When the content of B is 0.0005 to 0.0
030% and satisfies the formula "Mo × Cr≤0.16"
No. 11 to 20 seamless steel pipes (N
o. 11 to 20) all had a grain size number of 7 or less, and no M ₂₃ C ₆ type carbide was precipitated at the grain boundaries, but mainly M ₃ C type carbide.
For this reason, despite the high yield strength of 823 MPa or more, the fracture surface transition temperature was −20 ° C. or less, indicating good toughness. Above all, No. 16 with reduced P content
To 20 seamless steel pipes made of the steel of the present invention (Nos. 16 to 20).
The toughness of 20) was even better when the fracture surface transition temperature was -45 ° C or lower.

On the other hand, among the comparative steels, the content of each element is within the range specified by the present invention, but the relationship between Cr and Mo is defined by the above formula “Mo × Cr ≦ 0. 5
No. 5 "or" Mo × Cr ≦ 0.16 "
Seamless steel pipes made of comparative steels Nos. 21 to 24 (No. 21)
24) is in all of the following grain size number is No. 7, yet its the grain boundary was precipitated only M ₂₃ C ₆ type carbides. Therefore, although the yield strength was as high as 828 MPa or more, the fracture surface transition temperature was 20 ° C. or more and the toughness was poor.

The content of each element is within the range specified in the present invention, and the relationship between Cr and Mo also satisfies the above-mentioned formula “Mo × Cr ≦ 0.16” specified in the present invention. No. 25 in which the amount [B] does not satisfy the value specified in the present invention.
In the seamless steel pipe (No. 25) manufactured from the comparative steel No. 5, the grain size number was 5.5, and only the M ₂₃ C ₆ type carbide was precipitated at the grain boundaries. Therefore, although the yield strength is as high as 828 MPa, the fracture surface transition temperature is 4 MPa.
At 1 ° C., the toughness was remarkably inferior.

Further, the relationship between Cr and Mo, N, Ti and B
Although the relationship of satisfies the condition specified in the present invention, the seamless steel pipe (No. 26) made of the comparative steel of No. 26 having a high P content of 0.061% has a grain size number of 7 In the following, M ₂₃ C ₆ type carbide was not precipitated at the grain boundary, but mainly M ₃ C type carbide was precipitated.
However, since the P content is too high, the yield strength is 83%.
Although the strength was as high as 1 MPa, the fracture surface transition temperature was 19 ° C. and the toughness was poor.

[0061]

According to the present invention, the Cr-Mo low alloy steel seamless steel pipe has a prior austenite grain size of JIS G 055.
Despite being coarse grains having a grain size number of 7 or less as defined in No. 1, it has excellent toughness with a fracture surface transition temperature of -20 ° C or less. For this reason, reliability is high when used in a severe oil well environment or high temperature environment.

Further, the Cr-Mo-based low alloy steel for seamless steel pipes of the present invention is produced by a so-called in-line heat treatment process in which a quenching treatment and a tempering treatment are performed by effectively utilizing the retained heat during hot working. Except for M ₂₃ C ₆ type carbides at grain boundaries with grain size of 7 or less, mainly M ₃ C
A type of carbide precipitates. For this reason, the Cr-Mo low alloy steel seamless steel pipe excellent in toughness of the present invention can be manufactured at low cost.

[Brief description of the drawings]

FIG. 1: Cr and Mo content and effective B content [B] are M
₃ and effects on C type carbide precipitation, in an example of a diagram showing the relationship between the toughness, effective B amount (B) is from 0 to 0.0005
It is a figure in the case of less than%.

FIG. 2 shows that the content of Cr and Mo and the effective B amount [B] are M
₃ and effects on C type carbide precipitation, in an example of a diagram showing the relationship between the toughness, effective B amount (B) is 0.0005
It is a figure in the case of 0.0015%.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Takahiro Kushida 4-33, Kitahama, Chuo-ku, Osaka-shi, Osaka Prefecture Within Sumitomo Metal Industries, Ltd. (72) Inventor Takeshi Ichinose 4-chome, Kitahama, Chuo-ku, Osaka-shi, Osaka 5-33 No. Sumitomo Metal Industries, Ltd.

Claims (4)

[Claims] (1) Cr: 0.05 to 1.5% by weight, M
o: A Cr-Mo low alloy steel seamless steel pipe containing 0.05 to 1.0%, in which the main structure is tempered martensite and the prior austenite grain size is JIS G 0551.
Or less 7 No. grain size number defined, in types except M ₂₃ C ₆ in the old austenite grain boundaries was largely superior toughness, characterized in that M ₃ C type carbide is precipitated Cr- Mo-based low alloy steel seamless steel pipe. 2. C: 0.15 to 0.35% by weight, S
i: 0.1 to 1.5%, Mn: 0.1 to 2.5%, P:
0.05% or less, S: 0.004% or less, sol. A
l: 0.001 to 0.1%, Cr: 0.05 to 1.5
%, Mo: 0.05 to 1.0%, Nb: 0 to 0.05
%, Ti: 0 to 0.05%, V: 0 to 0.15%, C
a: 0 to 0.0050%, Mg: 0 to 0.0050%,
REM: 0 to 0.0050%, N: 0.01% or less,
B: 0 to 0.0030%, the balance being Fe and inevitable impurities, the amount of effective B [B] determined by the following formula or the formula is 0 to less than 0.0005%, and C
A Cr-Mo based low alloy steel for seamless steel pipes, characterized in that the relationship between r and Mo has a chemical composition satisfying the following formula. When Ti <3.419 × N [B] = B + 0.2259 × Ti−0.7723 × N ... When Ti ≧ 3.419 × N [B] = B ... ···· Mo × Cr ≦ 0.55 ············································································ Is the content (% by weight) of each element. 3. C .: 0.15 to 0.35% by weight, S
i: 0.1 to 1.5%, Mn: 0.1 to 2.5%, P:
0.05% or less, S: 0.004% or less, sol. A
l: 0.001 to 0.1%, Cr: 0.05 to 1.5
%, Mo: 0.05 to 1.0%, Nb: 0 to 0.05
%, Ti: 0 to 0.05%, V: 0 to 0.15%, C
a: 0 to 0.0050%, Mg: 0 to 0.0050%,
REM: 0 to 0.0050%, N: 0.01% or less,
B: 0 to 0.0030%, the balance being Fe and unavoidable impurities, the amount [B] of effective B determined by the following formula or the formula is 0.0005 to 0.0015%,
A Cr-Mo based low alloy steel for seamless steel pipes, characterized in that the relationship between Cr and Mo has a chemical composition satisfying the following formula. When Ti <3.419 × N [B] = B + 0.2259 × Ti−0.7723 × N When Ti ≧ 3.419 × N [B] = B・ ・ ・ ・ ・ Mo × Cr ≦ 0.16 Is the content (% by weight) of each element. 4. The Cr-Mo low alloy steel for a seamless steel pipe according to claim 2, wherein the content of P is 0.015% by weight or less.