JPH1150148A - Manufacturing method of high strength and high corrosion resistant seamless steel pipe - Google Patents

Abstract

(57) [Summary] [Problem] A yield stress excellent in SSC resistance is 110 to 155.
A method for producing a high-strength, high-corrosion-resistant seamless steel pipe having a softened layer on the surface which can be used for oil wells and related facilities of ksi (758-1068 MPa) class. SOLUTION: In weight%, C: 0.2 to 0.35%, S
i: 0.05 to 0.5%, Mn: 0.1 to 1%, Cr:
0.3-1.2, Mo: 0.2-1%, sol. Al:
0.005 to 0.50%, Ti: 0.005 to 0.5
%, B: 0.0001 to 0.005%, and Nb:
Including 0.1% to 0.5%, V, W, Zr,
A billet containing one or more of Ca, the balance being Fe and steel of unavoidable impurities, is subjected to hot piercing and final finishing rolling at the time of rolling at 1000 to 1
After performing processing with a thickness reduction rate of 40% or more in a temperature range of 150 ° C., it is directly quenched and tempered from a thickness center temperature of 1000 ° C. or more and a surface temperature of less than 1000 ° C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention is excellent in strength and sulfide stress cracking resistance used in casings and tubing for oil wells, drill pipes for drilling, line pipes for transportation, and piping for chemical plants. And a method for manufacturing a seamless steel pipe.

[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. In addition, in order to deepen oil wells, improve transportation efficiency, and reduce costs, materials used in this field, especially steel pipes,
There is a demand for higher strength than ever. That is,
Conventionally widely used yield stress (YS) is 80 to 90
In recent years, 110 ksi (758 MPa) grade has been used in place of ksi (552-621 MPa) steel pipe, and demand for 125 ksi (862 MPa) grade or more is also increasing.

[0003] In general, steel materials are more susceptible to stress cracking as the strength increases. Therefore, the biggest challenge for increasing the strength of the material used in such a deteriorating environment is to improve the resistance to sulfide corrosion cracking (SSC resistance). Conventionally studied and generally known measures against this SSC resistance are to have a martensite structure of about 80% or more, to have high cleanliness, to have a high-temperature tempered fine grain structure, and to have a high yield ratio. Other means include low Mn (prevention of segregation), nitride formation, and Zr addition.

[0004] Quenching and tempering steel to the same strength level is less than tempering at low temperatures after incomplete quenching.
It is well known that tempering at a high temperature after sufficient quenching results in a steel material with much better toughness. The above indicates that the SSC resistance has the same tendency.

[0005] SSC is considered to be a type of hydrogen embrittlement, like delayed fracture, and increasing the toughness of the substrate is effective in preventing cracking. Further, it is better that the non-metallic inclusions serving as crack initiation points are as small as possible, and S and O causing the non-metallic inclusions are as small as possible to achieve high cleanliness.
With regard to the fine grain structure, the brittle cracks extend in units of crystal grains or grain boundaries when the strength is increased. Therefore, when the grain size is reduced, the deterrent to cracks increases. In addition, since fine graining itself contributes to an increase in strength, emphasis has been particularly placed on fine graining as a high-strength material having excellent SSC resistance.

[0006] Transformation, working deformation, suppression of grain growth during recrystallization after working deformation, and the like are generally used as a method of grain refinement. When the ingot after casting is formed into a steel pipe or the like while being heated, it is inevitably subjected to working deformation, and is refined by repeated processing and recrystallization.

However, since quenching requires heating to a temperature higher than the transformation point (Ac ₃ point), crystal grain growth is likely to occur. To keep the crystals fine, the heating temperature during quenching must be lowered. Is desirable.

However, fine grains and a low quenching temperature are also factors that greatly reduce the quenchability, and it is necessary to secure a structure in which 80% or more is martensite at the time of quenching by ordinary cooling means. Becomes difficult. Further, if a large amount of alloying element is added to ensure quenchability, the workability of the steel is deteriorated, which further increases the cost.

[0009] Therefore, a method is employed in which an element that forms fine carbonitrides such as Nb is added to suppress not only grain growth during recrystallization but also grain growth during a heating process during quenching. Many.

In the heat treatment of quenching and tempering, countermeasures for obtaining a fine grain structure, such as low-temperature quenching, twice quenching, or rapid heating quenching by induction heating, have been taken for some time. Recently, from the viewpoint of energy saving and cost reduction by simplifying the process, a direct quenching method in which quenching is carried out immediately from a high temperature at the end of hot rolling together with an additional element has been studied. However, in the direct quenching method, the crystal grain size of the obtained product tends to be larger than that of a normal method of cooling once and then reheating and quenching.

As a countermeasure, Japanese Patent Laid-Open Publication No.
In Japanese Patent Application Laid-Open No. 5-271772, Mo, Nb, Ti, B, etc. are added. A method is disclosed in which steel is once cooled to an Ar ₃ point or less and transformed during rolling after hot piercing, and then heated again, rolled, and directly quenched. PCT-WO-96 / 36742 discloses Nb
There is also disclosed a method of adding titanium and Ti in a composite manner, supplementing the heat after pipe production, and then directly quenching.

The above-mentioned methods are all methods for increasing the SSC resistance of the entire steel base constituting the steel pipe or the steel sheet. In order to further increase the SSC resistance of the steel pipe, for example, the steel pipe is required to have the following requirements. It is effective to soften the surface in contact with the corrosive environment and the vicinity of the surface to such an extent that the required strength is not affected. This is because, as described above, the SSC susceptibility increases as the steel material increases in strength, so that only the surface where the hydrogen concentration is high in contact with the corrosive environment and the vicinity of the surface are reduced in strength to enhance the SSC resistance. Things.

The simplest method for softening the surface of a steel material having a martensitic structure is a method in which the temperature in the vicinity of the surface is reduced to an Ar ₃ point or less to partially transform the ferrite, followed by cooling. However, according to this method, not only the structure having a martensite ratio of 80% or more, which is desirable for SSC resistance, cannot be maintained, but also the C released during ferrite transformation is concentrated in the retained austenite, and the austenite becomes martensite. However, although it is softened, the SSC resistance may be degraded.

In this sense, in order to soften the surface and the vicinity of the surface, the vicinity of the surface is once transformed into ferrite and then reheated or covered to form an austenite single phase, and then quenched from that state to form a fine-grained martensitic single phase. Although a method of obtaining tissue is desirable, this method has difficulty in controlling the temperature. There is also a method of reducing the hardness of the martensite structure by decarburizing the surface layer, but this method requires atmosphere control at the time of heating and cannot be said to be a low-cost manufacturing method.

[0015]

In the above-described manufacturing method, the target strength level, that is, the yield stress level is 90 ksi (621 MPa).
Was the main. However, if the intensity level is 110 ksi (7
When the pressure exceeds 58 MPa), it is difficult to say that these methods always provide stable and sufficient high strength and SSC resistance. In addition, the direct quenching method for the purpose of omitting the steps also requires extra steps such as cooling, heating or rolling before reaching the point, and the simplification that significantly reduces the cost is still sufficiently achieved. It doesn't seem to be.

An object of the present invention is to provide a yield stress excellent in SSC resistance of 110 to 155 ksi (758 to 1068 MPS).
An object of the present invention is to provide a method capable of more efficiently producing a high-strength, high-corrosion-resistant seamless steel pipe which can be used in a) grade oil wells and related facilities.

The high-strength seamless steel pipe is made of an API.
There are (American Petroleum Institute) standards. No C110 class or higher standard is set for this, but here, as an extension of that standard, depending on the strength,
C110 class Yield stress 110-125ksi (758-
862 MPa)}, C125 class} Yield stress 125-14
0 ksi (862 to 985 MPa)} and C140 class {yield stress 140 to 155 ksi (985 to 1068M)
Pa), and the present invention is directed to a method for producing these high-strength seamless steel pipes.

[0018]

The present inventors have found that the yield stress exceeds 110 ksi (758 MPa) and that the SS
Various studies were made on a method for producing a high-strength seamless steel pipe having excellent C properties at a lower cost.

As described above, the refinement of the crystal structure can be achieved by using the anti-S
It is indispensable for improving the SC property, but the result of the study shows that when the yield stress exceeds 110 ksi (758 MPa), even if the material becomes somewhat coarse, it is sufficiently quenched and tempered at a high temperature. It became clear that the effect of improving SSC resistance was larger.

It has been found that the tempering temperature is desirably 650 ° C. or higher, and preferably 680 ° C. or higher, even for a high-strength material having a yield stress exceeding 125 ksi (862 MPa), which is the target of practical use for the time being. In order to secure sufficient strength even in such high-temperature tempering, it is necessary to improve the hardenability and further increase the temper softening resistance by using Cr or Mo.
Is effective in large amounts.

However, when a large amount of Cr is added, the corrosion rate in an acidic aqueous solution containing hydrogen sulfide (H ₂ S) increases, and the concentration of occluded hydrogen increases, thereby deteriorating the SSC resistance. As for Mo, a large amount of Mo is added as needle-like Mo.
There is a danger that this will cause the starting point of SSC, and there is a limit in increasing the addition amount.

Therefore, as a result of diligent studies on strengthening elements in place of these, it has been found that the content of Nb exceeding 0.1% is effective.

It is known that the addition of Nb is effective in suppressing the growth of crystal grains, that is, in refining the crystal structure. The refining by adding Nb at the time of reheating usually shows a sufficient effect by adding a small amount of about 0.01%, and the effect is saturated even if a large amount is added. Usually, it is added.

However, more than 0.1% of Nb was added, and then heating was performed at a high temperature (1200 ° C. or higher) required for a seamless pipe, and the pipe was completed in a state where Nb was substantially dissolved. Thereafter, direct quenching from a high temperature of 1000 ° C. or higher where NbC is not precipitated, followed by tempering to precipitate fine NbC, can maintain the strength after tempering even when tempered at a high temperature, and has extremely low SSC resistance. An excellent high-strength material for steel pipes was obtained.

As described above, the effect obtained by adding Nb in a larger amount than usual is not always clear, but there are several reasons as follows.

SSC is a type of hydrogen embrittlement, and is caused by intrusion of hydrogen atoms generated by corrosion in a hydrogen sulfide environment into steel. Hydrogen involved in this hydrogen embrittlement is
"Diffusible" hydrogen that can diffuse in steel at temperatures around room temperature, and when this hydrogen diffuses into the stress concentration area where there is a high risk of becoming the starting point of cracking and the hydrogen concentration increases, the critical stress for cracking And SSC sensitivity is increased.

Dislocations in the steel and fine precipitates such as carbides and nitrides act as trap sites for diffusible hydrogen. The term "trap site" used here does not mean that hydrogen is fixed so strongly that it cannot be diffused, but that hydrogen in solid solution in steel is more stable when present in that part, and the hydrogen concentration in the steel base It is a local part where the density is relatively higher than the level.

If the steel has the same composition, the progress of corrosion on the surface in a hydrogen sulfide environment is the same, and the amount of hydrogen generated thereby is the same, and the ratio of hydrogen entering the steel is included. Is the same. For this reason, if there are many trap sites, the hydrogen concentration in steel will increase and SSC resistance will fall.

When the tempering temperature is increased, a large amount of dislocations introduced by martensitic transformation during quenching gradually disappear. One of the reasons that the tempering from a high temperature improves the SSC resistance is presumed to be due to the decrease in the dislocations, which are trap sites for diffusible hydrogen. Because
The higher the strength, the higher the SSC susceptibility is because the dislocations and precipitates that contribute to strengthening also act as trap sites for diffusible hydrogen, increasing the diffusible hydrogen concentration in the steel.

In general, an increase in the tempering temperature significantly lowers the strength. However, when Nb is contained in a large amount, as described above, a decrease in the strength due to the high-temperature tempering can be suppressed. This reduction in strength is mainly attributed to the precipitation of fine carbides.
It is considered that this was changed by adding a large amount of.

The fine precipitates usually serve as hydrogen trap sites as in the case of dislocations as described above.
Examining steels that have been added more and that have been directly quenched from high temperatures, they have a lower hydrogen storage capacity than carbides of other elements. That is, it was presumed that the change in the dispersed state or form of the precipitate due to the addition of a large amount of Nb and the direct quenching and tempering also had the effect of reducing the action of the hydrogen as a trap site.

As described above, the addition of Nb exceeding 0.1% makes it possible to perform high-temperature tempering without significantly lowering the strength, and furthermore, the resulting precipitate has a small hydrogen storage capacity,
Since hydrogen absorption into steel is reduced, it is extremely effective to obtain a high-strength steel pipe having excellent SSC resistance.

The effect of adding a large amount of Nb is further improved by performing a process of reducing the thickness in the temperature range of 1000 to 1150 ° C. to 40% or more in the final step of hot rolling. It was also confirmed. As described above, when the Nb content is large, the Nb content greatly affects the dispersion state of Nb precipitates, and also effectively acts on the refinement of the crystal structure, resulting in good results.

By the way, Nb has a lower solid solubility than other carbide forming elements. Therefore, at the time of reheating, 0.0
Even if added at about 1%, undissolved carbides remain, which is effective in suppressing coarsening. As described above, usually, over 0.1% is not added because the effect is saturated.

However, at a high temperature of 1200 ° C. or more required for a seamless pipe, even if Nb is added in an amount of more than 0.1%, almost a solid solution is obtained.
In the final step of hot rolling, when processing with a wall thickness reduction rate of 40% or more is performed in a temperature range of 1000 to 1150 ° C, 1
Above 000 ° C, it remains in solid solution.

However, in the final step of hot rolling,
When rolling is performed using a mandrel mill, heat is removed by contact between the mandrel bar and the inner surface of the tube, and the inner surface temperature decreases. At this time, if the surface temperature is lower than 1000 ° C., NbC starts to precipitate rapidly in the vicinity of the surface and NbC precipitates before quenching, and no longer contributes to strengthening.

Therefore, when directly quenched from a state in which only the vicinity of the surface is lower than 1000 ° C. and the other part is higher than 1000 ° C., in a high Nb steel to which more than 0.1% of Nb is added,
A steel pipe having a softened layer on the surface, in other words, having a reduced strength layer, is obtained. The surface temperature at this time is, of course, Ar ₃
The softened layer has a martensite content of 80 or more.
The structure is about 90% or more, and there is no difference in structure from the center of the thickness. That is, the SSC resistance is good in terms of the structure, and the SSC resistance is better because the surface and the surface layer in the vicinity thereof are softened.
Thus, a steel pipe exhibiting the properties can be obtained.

Since elements other than Nb have a high solid solubility, low-Nb steels containing 0.1% or less of Nb can be hardened with a temperature difference from the Ar ₃ point or more as described above. No intensity difference occurs. Therefore, in order to soften the surface of the low Nb steel, it is necessary to temporarily transform the surface to an Ar ₃ point or lower as described above, which is costly. In contrast,
In the case of high Nb steel containing more than 0.1% of Nb, the surface temperature naturally falls below 1000 ° C. by setting the finishing temperature in the mandrel mill to about 1050 ° C., and a special operation is not required, thereby increasing the cost. A softened steel pipe can be easily obtained without inviting.

Based on the above findings, the present invention has been completed by clarifying the limits of conditions under which the effect can be sufficiently exerted. The gist of the present invention is as follows.

A method for producing a seamless steel pipe which is hot pierced and rolled, formed into a steel pipe shape, directly quenched as it is, and tempered to temper to a required strength.
0.2-0.35%, Si: 0.05-0.5%, M
n: 0.1-1%, Cr: 0.3-1.2, Mo: 0.
2-1%, sol. Al: 0.005 to 0.50%, T
i: 0.005 to 0.5%, B: 0.0001 to 0.0
05%, Nb: more than 0.1% and 0.5% or less, V: 0 to 0%
0.5%, W: 0 to 1%, Zr: 0 to 0.5%, Ca:
0 to 0.01%, with the balance being Fe and unavoidable impurities, with P, S, Ni, N and O in the impurities.
(Oxygen) is P: 0.025% or less, and S: 0.
In the final finish rolling step when hot-piercing and rolling a steel billet of not more than 01%, not more than 0.1% of Ni, not more than 0.01% of N, not more than 0.01% of O,
40% reduction in wall thickness in the temperature range of 1000-1150 ° C
After the above processing, the center temperature of the wall thickness
758 to 1068, characterized by direct quenching from 0 ° C. or more and a surface temperature of less than 1000 ° C., followed by tempering.
A method for producing a high-strength, high-corrosion-resistant seamless steel pipe having a yield stress of MPa and a softened layer on the surface.

[0041]

BEST MODE FOR CARRYING OUT THE INVENTION

 1. Chemical composition "%" in the following chemical composition is all% by weight.

C: 0.2 to 0.35% C is an element necessary for enhancing hardenability and improving strength. However, if its content is less than 0.2%,
The quenching hardness is insufficient, and the required high strength after tempering cannot be obtained. Conversely, if its content exceeds 0.35%,
Carbides increase and SSC resistance decreases. Therefore,
C content was set to 0.2 to 0.35%. A desirable upper limit of the C content is 0.3%.

Si: 0.05-0.5% Si is an element necessary for deoxidation of steel, and is an element which increases temper softening resistance and improves SSC resistance. Embrittle. For the purpose of deoxidation and improvement of SSC resistance, a content of 0.05% or more is necessary. However, if the content exceeds 0.5%, not only the toughness is reduced, but also the grain boundary strength is reduced. , SSC resistance is reduced. Therefore,
The Si content was 0.05 to 0.5%. Desirable S
The upper limit of the i content is 0.30%.

Mn: 0.1-1% Mn is an element necessary for deoxidizing steel. For the purpose of deoxidation, a content of 0.1% or more is necessary. However, if the content exceeds 1%, not only the toughness is reduced, but also the grain boundary strength is weakened, and the SSC resistance is reduced. Therefore, the Mn content is
0.1-1%. The upper limit of the desirable Mn content is
0.5%.

Cr: 0.3 to 1.2% Cr secures hardenability, increases strength, and improves SSC resistance. However, in order to obtain a steel capable of ensuring a yield stress of 110 ksi (758 MPa) or more, which is the target of the present invention, if the content is less than 0.3%, the effect of improving hardenability is insufficient. Conversely, if the content exceeds 1.2%, the corrosion rate increases in an environment containing hydrogen sulfide, which causes an increase in the occluded hydrogen concentration and deteriorates the SSC resistance. Therefore, the Cr content is 0.1.
3 to 1.2%. A desirable range is 0.5 to 0.8
%.

Mo: 0.2% to 1% Mo, like Cr, improves quenching properties to increase strength, increases tempering softening resistance, enables high-temperature tempering, and improves SSC resistance. However, in order to obtain a steel capable of ensuring a yield stress of 110 ksi (758 MPa) or more, which is the target of the present invention, the content is 0.2%.
If less than the above, the above effect is not sufficient. Conversely, if the content exceeds 1%, a needle-like Mo carbide serving as a starting point of SSC is precipitated due to a high stress concentration coefficient, and the SSC resistance is deteriorated. Therefore, the Mo content is set to 0.2 to 1%. A desirable range is 0.3 to 0.8%.

Sol. Al (acid soluble Al): 0.005
０．５0.5% Al is an element necessary for deoxidation of steel. As a result of adding to the molten metal to sufficiently ensure the soundness of the slab, sol. Al
Contained in steel as 0.005%
If it is less than 1, a sufficiently sound slab cannot be obtained. Conversely, if the content exceeds 0.5%, inclusions increase and toughness decreases. In addition, seamless steel pipes for oil country tubular goods are often cut with connection threads at the ends of the pipes. However, if there is a large amount of Al, defects tend to occur in the threaded portions. Therefore, sol. Al
The content was 0.005 to 0.5%. A desirable range is 0.01 to 0.1%.

Ti: 0.005 to 0.5% Ti is added for the purpose of fixing N, which is an impurity in steel, as TiN. Further, excess Ti, which is necessary for fixing N, becomes carbide and is finely precipitated, and has an effect of increasing the tempering softening resistance. The fixation of N is for suppressing the below-mentioned B added for improving hardenability from becoming BN, and for maintaining B in a solid solution state to secure sufficient hardenability. However, if the content is less than 0.005%, the above effects cannot be obtained, and if the content exceeds 0.5%, the toughness decreases. Therefore, the Ti content is 0.005
-0.5%. A desirable range is 0.01 to 0.1.
%.

B: 0.0001 to 0.005% B improves the hardenability by a trace amount, and particularly, the SS resistance of thick materials
Improves C properties. However, its content is 0.0001%
If it is less than the above, the above effects cannot be obtained. Conversely, if the content exceeds 0.005%, toughness and SSC resistance decrease. Therefore, the B content is 0.0001 to 0.00.
5%. Desirable range is 0.0002 to 0.00
2%.

Nb: more than 0.1% and 0.5% or less Nb is an element that plays the most important role in the present invention. That is, quenching from a high temperature is enabled by suppressing grain growth during heating, the tempering softening resistance after quenching is significantly increased, and sufficient strength is maintained even at high temperature tempering, and the SSC resistance is reduced due to the increase in strength. That is to prevent it. To obtain such an effect, the Nb content must be at least 0.1% or more, and if the content is 0.1% or less, after high-temperature tempering (650 ° C. or more) desirable for improving SSC resistance. It is difficult to ensure high strength.

However, if the content exceeds 0.5%, in addition to the decrease in toughness, when high-temperature tempering is performed to adjust the strength, it exceeds the Ac ₁ point, and austenite is formed. Since a part of austenite becomes fresh martensite after cooling, SSC resistance is lowered. At a desirable tempering temperature (650 ° C. or higher), the strength becomes too high and the SSC resistance deteriorates. Therefore, N
The b content was set to be more than 0.1% and 0.5% or less. A desirable range is 0.2-0.4%.

P: 0.025% or less P is inevitably present in steel as an impurity. If the content exceeds 0.025%, segregation occurs at the grain boundaries, and in particular, the SSC resistance of high-strength steel is reduced. Therefore,
The P content was 0.025% or less. The lower the P content, the better.

S: 0.01% or less S is inevitably present as an impurity in steel, like P described above. If the content exceeds 0.01%, segregation occurs at the grain boundaries, while sulfide-based inclusions are formed, and particularly the SSC resistance of high-strength steel is reduced. Therefore, the P content is set to 0.01% or less. The content of S is preferably as low as possible, and the desirable upper limit is 0.00.
1%.

Ni: 0.1% or less Ni is present in steel as an impurity, and has high toughness and SSC resistance.
Reduce the nature. However, since there is no problem if the content is 0.1% or less, the upper limit is set to 0.1%. Note that Ni is inevitably contained in the Cr raw material, and it is industrially impossible to reduce its content to 0 (zero), but it is desirable that the content be as small as possible.

N: 0.01% or less N is present as an impurity in steel and, like the above-mentioned Ni,
Decreases toughness and SSC resistance. However, since there is no problem if the content is 0.01% or less, the upper limit is set to 0.01%. It should be noted that N enters the steel from the atmosphere or the like, and its content is set to 0 (zero).
Like Ni, it is not industrially possible, but it is desirable to have as little as possible.

O: 0.01% or less O (oxygen) is present in the steel as an impurity and decreases toughness and SSC resistance, similarly to Ni and N described above.
However, since there is no problem if the content is 0.01% or less, the upper limit is set to 0.01%. Note that O
Enters the steel from the air and the like, like N,
It is industrially impossible to reduce the content to 0 (zero), but it is desirable that the content be as small as possible.

In the present invention, one or more selected from the following V, W, Zr and Ca can be added to the steel having the above-mentioned chemical composition, if necessary.

V: 0 to 0.5% V is an element which precipitates as fine carbides during tempering, has an effect of increasing tempering softening resistance after quenching, and improving SSC resistance. Therefore, when it is desired to obtain the effect, V can be added and contained. However, if the content is less than 0.005%, the above effects cannot be obtained,
If the content exceeds 0.5%, the toughness decreases. Therefore, the V content when added is 0.005 to 0.5%
It is necessary to

Zr: 0 to 0.5% Zr is an element having an effect of improving the yield point elongation during a tensile test and, as a result, further improving the SSC resistance. Therefore, in order to obtain the effect, Zr can be added and contained. However, its content is 0.0
If the content is less than 05%, the above effects cannot be obtained. If the content exceeds 0.5%, inclusions increase and toughness decreases. Therefore, the Zr content when added is 0.005 to 0.5
%. In addition, the above effect by Zr is as follows.
It is presumed that work hardening during local yielding was small.

W: 0 to 1% W is an element which, like Mo, has the effect of increasing the quenchability and improving the strength, and also increasing the temper softening resistance and improving the SSC resistance. For this reason, when it is desired to obtain the effect, W can be added and contained. However, if the content is less than 0.05%, the above effect cannot be obtained. If the content exceeds 1%, not only the effect is saturated, but also the SSC resistance is reduced due to segregation. Therefore, when added, the W content needs to be 0.05 to 1%.

Ca: 0 to 0.01% Ca is an element having the effect of improving the shape of inclusions by reacting with S in steel to form sulfides and improving SSC resistance. Therefore, when it is desired to obtain the effect, Ca can be added and contained. But,
If the content is less than 0.0001%, the above effects cannot be obtained, and if the content exceeds 0.01%, the toughness and the SS resistance are reduced.
Not only is the C property reduced, but also defects tend to occur on the surface of the steel pipe. Therefore, when Ca is added, the content of Ca is 0.1.
Must be 0001 to 0.01%. If Ca is added when the deoxidation is not sufficient, the SSC resistance is rather reduced.
Attention should be paid to this point, since the property is reduced.

2. Manufacturing Conditions In the hot rolling, that is, the rolling from piercing to subsequent rolling, the heating temperature of the billet is usually 1100 to 1300 ° C.
However, in the case of the method of the present invention, a higher one is preferable from the viewpoint of controlling the dispersion state of the precipitated NbC. However, since raising the temperature is limited in terms of heating equipment and processing equipment, a desirable temperature is 1150 to 1250 ° C.

The billet heated to the above temperature range is
It is extremely coarse. Therefore, in the present invention, it is not necessary to have an extremely fine grain structure. However, if the grain size is too large, the quenchability is good, but the toughness and SSC resistance are reduced. There is a need.

Therefore, in the present invention, in the final stage of the hot rolling, the thickness reduction rate in the temperature range of 1000 to 1150 ° C. is set to 40% or more. This is because the degree of processing up to 1150 ° C. depends on recrystallization immediately after processing,
There is no effect on grain refinement, and processing at a temperature lower than 1000 ° C., in the case of the steel of the present invention containing a large amount of Nb, causes a variation in hardness after quenching and deformation of the steel pipe after cooling. This is because there is a risk of increasing the size. Further, in the processing in which the wall thickness reduction rate is less than 40% in this temperature range, the structure is too coarse and the toughness and SSC resistance become insufficient.

Immediately after the final stage of rolling, quenching begins. In this case, the quenching temperature is less than 1000 ° C. on the surface and 1000 ° C. or more at the thickness center. this is,
This is because a softened layer is formed on the surface by depositing NbC on the surface and in the vicinity of the surface, quenching from a state where NbC is not deposited as much as possible in the entire thickness and recrystallization does not proceed.

If the temperature at the center of the thickness is less than 1000 ° C., NbC is remarkably precipitated before quenching, so that not only the desired strength cannot be ensured, but also the SSC resistance decreases. A desirable temperature at the center of the thickness is 1050 ° C. or more. The upper limit temperature at the center of the wall thickness at the time of quenching is not particularly defined, but since processing at 1150 ° C. or lower requires 40% or more, there is a limit naturally.

On the other hand, when the surface temperature is 1000 ° C. or higher,
Due to insufficient NbC precipitation before quenching, a necessary softened layer is not formed on the surface. Although the lower limit temperature of the surface is not particularly defined, if the temperature is lower than 950 ° C., the strength of the entire tube is reduced.

When the tube surface temperature and the center temperature of the wall thickness before quenching are rolled using a mandrel mill at a reduction rate of 40% or more, the finishing temperature, that is, the mill outlet side (the sizer is located downstream of the mandrel mill). In the case where fixed rolling mills such as the above are arranged in a row, the temperature can be easily obtained by setting the outer tube surface temperature at the outlet side of the fixed rolling mill to about 1050 ° C.

When the rolling with a wall thickness reduction rate of 40% or more is performed using a tube rolling machine other than a mandrel mill, specifically, a plug mill, the heat removal by the plug is extremely small. Forcibly cool by injecting cooling water into
Needless to say, the above-mentioned temperature difference may be generated and then quenched.

The tempering conditions are not particularly defined for the purpose of adjusting the strength to a predetermined value. However, when the quenching is completed under the above conditions, the tempering conditions are adjusted to the required strength and the S
To obtain the SC property, tempering at 650 ° C. or more is required. However, it is more preferable that the required strength be obtained at a tempering temperature of 680 ° C. or higher.

[0071]

【Example】

Example 1 Steel having the chemical components shown in Tables 1 and 2
Sixteen steels of Nos. A to P were melted using a 150 kg vacuum melting furnace. However, among steel Nos.
D, E to H, I and J, K and L, and M to O were respectively melted at the same melting chance, and the components were adjusted by adding a specific alloy element immediately before casting.

Then, the obtained ingot was forged to a thickness of 50%.
mm, a width of 80 mm, and a length of 250 mm. These billets are heated to 1250 ° C. in accordance with the conditions of the wall thickness reduction rate of the steel pipe processing step or finish rolling, and after rough rolling with a wall thickness reduction rate of 50%, finishing in a temperature range below 1150 ° C. After rolling, quenching was performed after rolling, and then tempering was performed. At this time, the thickness reduction rate in finish rolling, the surface temperature before quenching, and the temperature at the center of the thickness were variously changed. The tempering temperature is determined by the required strength / the yield stress of 110 to 15 depending on the chemical composition of the steel and the quenching conditions.
Various changes were made so as to obtain 5 ksi (758-1068 MPa)｝.

The surface temperature before quenching and the temperature at the center of the wall thickness were adjusted so that they were allowed to cool after finish rolling so that they reached a predetermined temperature. The surface temperature before quenching was measured with a radiation thermometer, and the temperature at the center of the wall thickness was measured with a thermocouple embedded in a billet before rolling. Table 3 summarizes the test conditions for each steel No.

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[Table 1]

[0075]

[Table 2]

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[Table 3]

From each of the obtained steel sheets, J was set in parallel with the rolling direction.
A No. 14B test piece specified in IS Z 2201 is sampled and tensile strength (yield stress “YS” and tensile strength “TS”)
Was measured.

The Vickers hardness (Hv) at the center of the thickness and at a position 0.5 mm below the surface was measured according to the method specified in JIS Z 2244.

Further, in order to confirm the effect of the softened layer formed on the surface, the evaluation of SSC resistance was carried out in parallel with the rolling direction from each sheet material in a width of 10 mm and a length of 75 mm including a surface with a thickness of 3 mm including the surface. , A 4-point bending test piece from which the mill scale on the surface was removed was collected, and NACETM-0177 Metho
dA was performed in accordance with a method. That is, it is a constant strain test in 0.5% acetic acid + 5% saline at 25 ° C. saturated with 1 atm of hydrogen sulfide. In addition, the amount of distortion (the amount of deflection) of the four-point bending test piece was controlled so that the applied stress became two conditions of 80% and 90% of the actual yield stress obtained in the above tensile test. The test time was set to 720 hours. In this test, those which did not break at a load stress of 90% of the actual yield stress were judged to have good SSC resistance.

The results are shown in Table 3. Here, in Test Nos. 1 to 16, the finish rolling temperature was 1050 ° C., and after the rolling, it was allowed to cool for a while, and the surface temperature was 970 ° C. less than 1000 ° C.
When it reached 20 ° C., it was quenched. In addition, trial number 17-
No. 32 is the same condition as that of test numbers 1 to 16 up to the finish rolling temperature, and the surface temperature of the finish rolling is 1010
And quenched at 1030 ° C. where the temperature at the center of the wall thickness is 1000 ° C. or higher. Furthermore, the test numbers 33 to 36 were obtained by investigating the effect of the thickness reduction rate in finish rolling, and the conditions other than the thickness reduction rate and tempering temperature in finish rolling were the same as those in test numbers 1 to 16. .

Hereinafter, representative test results of SSC resistance will be described.

For example, in test numbers 1 to 4, test number 1 is N
Due to the low b content, tempering must be performed at a low temperature in order to obtain C125-grade strength, and the SSC resistance is poor. On the other hand, in Test No. 4, since the Nb content was too large, the tempering temperature was too high and exceeded the Ac ₁ point, and it is considered that the SSC resistance was lowered. For trial numbers 5 to 8, trial number 1
And it can be considered in the same way as the test number 4.

The test numbers 10, 12, 13 and 15
Although the tempering temperature is below the Ac1 point and the tempering temperature is appropriate, it is considered that the SSC resistance was poor due to the inappropriate chemical composition of the steel.

Further, in the test numbers 33 to 36, the test number 3
Sample No. 3 had a small thickness reduction rate of 30% in finish rolling, and was coarse and had poor SSC resistance. These trials show that even with a load stress of 80% actual yield stress,
The C property was unsatisfactory.

Next, test numbers 18, 19, 22, 23, 2
In Nos. 5, 27, 30, and 32, the chemical composition of the steel was within the range of the invention, and even when the surface temperature during quenching was 1000 ° C. or higher, the SSC resistance was good at a load stress of 80% actual yield stress. However, they did not have the effect of forming the surface softened layer, and the SSC resistance was poor at a load stress of an actual yield stress of 90%.

Example 2 Steel No. Q steel having the chemical components shown in Tables 1 and 2 was melted in a 150-ton converter. Then, the obtained CC slab is subjected to bulk-rolling into a round billet having an outer diameter of 225 mm, and the round billet is heated to 1250 ° C. and pierced and rolled with a piercer to obtain an outer diameter of 250 mm and a wall thickness of 47 mm.
, And passed through a mandrel mill to elongate and roll to an outer diameter of 245 mm and a thickness of 15 mm (thickness reduction rate of 7).
0%) and subsequently formed into a seamless steel pipe having an outer diameter of 244.5 mm and a wall thickness of 13.84 mm using a sizer connected to a mandrel mill.

At this time, the outer surface temperature of the tube on the side of the sizer was set to 10
The temperature was adjusted to 50 ° C., and directly quenched when the surface temperature of the pipe became 970 ° C. and the temperature at the center of the wall thickness became 1020 ° C. immediately before the quenching device arranged continuously after the sizer,
Next, the tempering temperature is variously changed to obtain the required strength {yield stress 110 to 155 ksi (758 to 1068 MPa)}.
Was adjusted.

From each of the obtained steel pipes, J was set in parallel with the rolling direction.
A No. 14B test piece specified in IS Z 2201 is sampled and tensile strength (yield stress “YS” and tensile strength “TS”)
Was measured. Further, Vickers hardness (Hv) was measured at the center of the thickness and at a position of 0.5 mm below the surface according to the method specified in JIS Z 2244.

Further, in order to evaluate the SSC resistance, in order to confirm the effect of the softened layer formed on the surface, each steel pipe was formed into an inverted C-ring type so that a tensile stress acts on the inner side of the pipe, while The C-ring test piece from which the mill scale was removed was sampled, and the test was performed by a method based on NACE TM-0177 Method C. That is, a C-ring test in 0.5% acetic acid + 5% saline at 25 ° C. saturated with 1 atm of hydrogen sulfide. The load stress was set to two conditions of 80% and 90% of the actual yield stress obtained in the above tensile test. The test time was set to 720 hours. During this test, those which did not break at a load stress of 90% of the actual yield stress were tested for SS resistance.
The C property was determined to be good. These results are also shown in Table 3.

As shown in Table 3, these steel pipes (trial number 3
Nos. 7 to 39) did not break even at a load stress of 90% of the actual yield stress, and the SSC resistance was good.

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According to the present invention, the yield stress having excellent SSC resistance is 110 to 155 ksi (758 to 11068M).
Pa) grade, high-strength, high-corrosion-resistant seamless steel pipe having a softened layer on the surface that can be used for oil wells and related facilities, after hot working with a high Nb addition and a wall thickness reduction rate of 40% or more, Surface temperature is less than 1000 ° C, thickness center temperature is 1
It can be manufactured and provided with high productivity by a simple means of directly quenching and tempering from 000 ° C. or higher.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/00 301 C22C 38/00 301Z 38/54 38/54

Claims (1)

[Claims] 1. A method for producing a seamless steel pipe which is hot pierced and rolled, formed into a steel pipe shape, directly quenched as it is, and tempered to temper to a required strength, wherein C: 0% by weight. 0.2 to 0.35%, Si: 0.05 to 0.5%, Mn: 0.1 to 1%, Cr: 0.3 to 1.2, Mo: 0.2 to 1%, sol. Al: 0.005 to 0.50%, Ti: 0.005 to 0.5%, B: 0.0001 to 0.005%, Nb: more than 0.1% and 0.5% or less, V: 0 -0.5%, W: 0-1%, Zr: 0-0.5%, Ca: 0-0.01%, the balance being Fe and unavoidable impurities,
P, S, Ni, N and O (oxygen) in the impurities are respectively P: 0.025% or less, S: 0.01% or less, Ni: 0.1% or less, N: 0.01% or less, O: 0.01% or less The steel billet was subjected to a hot rolling process and a final finishing rolling stage in rolling at a temperature range of 1000 to 1150 ° C. and a thickness reduction rate of 40% or more. rear,
As it is, the center temperature of the thickness is 1000 ° C or more, and the surface temperature is 1
A method for producing a high-strength, high-corrosion-resistant seamless steel pipe having a yield stress of 758 to 1068 MPa and having a softened layer on the surface, which is directly quenched at a temperature of less than 000 ° C. and then tempered.