JPH10280037A - Production of high strength and high corrosion-resistant seamless seamless steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a high strength and high corrosion resistant seamless steel pipe excellent in SSC resistance, having yield stress of a 110 to 155 ksi (758 to 1068 MPa) class and usable for oil wells and various equipment connected thereto. SOLUTION: In the final stage of hot piercing for a steel pipe contg., by weight, 0.20 to 0.35% C, 0.05 to 0.5% Si, 0.1 to 1.0% Mn, 0.3 to 1.2% Cr, 0.2 to 1.0 Mo, 0.005 to 0.5% sol.Al, 0.005 to 0.5% Ti, 0.0001 to 0.005% B and 0.1 to 0.5% Nb and furthermore contg., at need, V, W, Zr, Ca or the like, it is subjected to working of >=40% at 1000 to 1150 deg.C, is quenched from >=1000 deg.C directly as it is and is tempered. Moreover, the steel pipe having the above compsn. is reheated at 1000 to 1150 deg.C and is subjected to quenching and tempering.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention 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 make oil wells deeper, improve transport efficiency, and reduce costs, materials used in this field, particularly steel pipes, are required to have higher strength than ever. That is, the yield stress conventionally used widely is 80 to 90 ksi (55 ksi).
Instead of steel pipe of 2-621MPa, recently 110ksi (758MPa)
a) grades are used and even 125ksi (862MP
a) There is a growing demand for higher grades.

[0003] In general, as the strength of steel increases, it becomes more susceptible to stress corrosion cracking. Therefore, the greatest challenge in increasing the strength of the material used in such a deteriorating environment is the resistance to sulfide stress corrosion cracking (resistance to SSC).
It is an improvement. To counter this SSC resistance, the following measures have been generally studied and are generally known: 1) A structure consisting of about 80% or more martensite 2) High cleanliness 3) High temperature tempering 4) Fine grain structure 5) A high yield ratio is required. Other means include 6) low Mn (prevention of segregation) 7) nitride formation 8) Zr addition.

[0004] When quenching and tempering steel to the same strength level, a steel material having much better toughness is required to be sufficiently quenched and tempered at a high temperature than to be tempered at a low temperature after incomplete quenching. Is well known. The above 1) and 3) show that the SSC resistance has the same tendency. SSC is considered to be a kind of hydrogen embrittlement like delayed fracture, and increasing the toughness of the base is effective in suppressing cracking. It is preferable that the non-metallic inclusions that are the starting points of the cracks are as small as possible, and the S and O that are the causes of the non-metallic inclusions are as small as possible to achieve the high cleanliness of 2). Regarding the fine-grained structure of 4), when the strength increases, the brittle crack propagates in crystal grain units or grain boundary units.
Finer grains increase the deterrent to cracking. In addition, since grain refinement itself also contributes to an increase in strength, emphasis has been placed on grain refinement as a high-strength material having excellent SSC resistance.

[0005] As a method of grain refinement, transformation, working deformation, suppression of grain growth during recrystallization after working deformation and the like are generally used. When the cast steel ingot is hot and processed into a steel pipe or the like, it is inevitably deformed by processing, and is refined by repeated processing and recrystallization.

However, since quenching requires heating to a temperature higher than the transformation point, crystal grain growth is likely to occur. To keep the crystals fine, it is desirable to lower the heating temperature during quenching. However, fine grains and lowering the quenching temperature are also factors that greatly reduce the quenchability, and it is difficult to secure a structure in which 80% or more is martensite during quenching with ordinary cooling means. It is becoming.
If a large amount of alloying elements is added to ensure hardenability, the workability of the steel is deteriorated, which further increases the cost. Therefore, a method is often adopted 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 quenching heating.

In the heat treatment of quenching and tempering, measures for obtaining a fine grain structure have been taken for a long time, such as low-temperature quenching, double quenching or rapid heating quenching by induction heating. Recently, from the viewpoint of energy saving and cost reduction by simplification of the process, a direct quenching method in which quenching is performed 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 the usual method of cooling and then reheating and quenching. As a countermeasure, for example,
No. 49 discloses a method of forcibly cooling during rolling in order to obtain a fine grain structure, further reducing and then quenching as it is, and Japanese Unexamined Patent Publication No. 5-271772 discloses a steel to which Mo, Nb, Ti and B are added. The invention discloses a method of cooling directly to Ar ₃ or less during rolling after hot piercing, transforming, and then heating again to perform rolling and direct quenching. Also,
PCT-WO-96 / 36742 also discloses a method in which Nb and Ti are added in a complex manner, and after the pipe is made, the heat is supplemented and then directly quenched.

[0008]

In the above-described manufacturing method presented so far, the target strength level, that is, the yield stress level is mainly 90 ksi (621 MPa). However, when the pressure exceeds 110 ksi (758 MPa), it is difficult to say that these methods are always stable and provide sufficient high strength and SSC resistance. In addition, the direct quenching method for the purpose of simplifying the process also requires extra steps such as cooling, heating, and rolling to reach 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 high-strength, high-corrosion-resistant steel pipe which has excellent SSC resistance and a yield stress of 110 to 155 ksi (758 to 1068 MPa) class and which can be used in oil wells and related facilities. It is to provide a manufacturing method.

[0010] There is an API (American Petroleum Institute) standard for high-strength seamless steel pipe. No C110 or higher standard is set for this, but here C110 class {yield stress 110-125 ksi (758-862 MPa)}, C125 class {Yield stress of 125 to 140 ksi (862 to 965 MPa) and C140 grade {Yield stress of 140 to 155 ksi (965 to 1068 MPa)}}, and the present invention is directed to a method of manufacturing these high-strength steel pipes.

[0011]

SUMMARY OF THE INVENTION The present inventors have made various studies on a method for producing a high-strength seamless steel pipe having a yield stress exceeding 110 ksi and having excellent SSC resistance at a lower cost. Advanced.

[0012] As described above, the refinement of the crystal structure is achieved by using the anti-SS
It is indispensable for improving the C property, but the result of the study shows that the yield stress
When the material exceeds 110 ksi (758 MPa), it is clear that even if the material becomes coarser, quenching sufficiently and tempering at a high temperature have a greater effect of improving SSC resistance.

It has also been found that the tempering temperature is preferably 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 application for the time being. In order to secure sufficient strength even in such high-temperature tempering, it is effective to add a large amount of Cr or Mo that improves quenching properties and increases tempering softening resistance. 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 amount of occluded hydrogen increases, thereby deteriorating the SSC resistance. Regarding Mo, a large amount of Mo precipitates needle-like carbides of Mo, which may be a starting point of SSC, and there is a limit in increasing the amount of Mo. Therefore, as a result of further study of strengthening elements in place of these, it was found that N
It has been found that the content of b is effective.

It is known that the addition of Nb is effective for suppressing the growth of crystal grains, that is, for refining the crystal structure. N
In the grain refinement by the addition of b, a sufficient effect is usually exerted by adding a small amount of about 0.01%, and the effect is saturated even if a large amount is added. Therefore, the addition is usually up to 0.1%. . However, more than 0.1% of Nb was added, and
When quenching is performed at a temperature of 0 ° C. or higher, a high-strength steel pipe material that can maintain the strength after tempering even when tempered at a high temperature and has extremely excellent SSC resistance is obtained.

The effect obtained by adding Nb in a larger amount than usual is not necessarily clear, but may be due to the following several reasons.

SSC is a type of hydrogen embrittlement, and occurs when hydrogen atoms generated by corrosion in a hydrogen sulfide environment enter steel. Hydrogen involved in this hydrogen embrittlement is “diffusible” hydrogen that can diffuse in steel at a temperature of about room temperature, and this hydrogen diffuses into a stress concentration portion where there is a high risk of starting cracking. As the hydrogen concentration increases, the critical stress for crack generation decreases and the SSC sensitivity increases.

Fine precipitates such as dislocations, carbides and nitrides in the steel 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. It is a local portion where the density is relatively higher than the density level. If the steel has the same composition, the progress of corrosion on the surface in the aqueous hydrogen sulfide environment is almost the same, and the amount of hydrogen generated thereby is also the same.
Some of the generated hydrogen enters the steel and others escape to the outside such as underwater. In that case, if the hydrogen concentration of the steel base is low, the ratio incorporated into the steel increases, and if it is high, the ratio decreases. In steel having many hydrogen trap sites such as dislocations and precipitates, hydrogen is unevenly distributed in that portion, and therefore, if the amount of invaded hydrogen is the same, the hydrogen concentration as a steel base becomes low. Therefore, in steels with many dislocations and fine precipitates, that is, high-strength steels that are strengthened by dislocations and fine precipitates,
If the amount of corrosion is the same, the amount of hydrogen uptake in the steel as a whole tends to increase as compared with low-strength steel. If the amount of hydrogen in the steel as a whole is large, the amount of hydrogen agglomerated in the stress concentration portion increases, and the hydrogen concentration in that portion tends to be higher, so that the SSC resistance decreases.

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 high temperature tempering improves the SSC resistance is presumed to be due to the reduction of dislocations, which are trap sites for diffusible hydrogen. Generally, an increase in the tempering temperature significantly lowers the strength, but when Nb is contained in a large amount, a decrease in the strength due to high-temperature tempering can be suppressed. It is believed that the reduction in strength is mainly due to the precipitation of fine carbides, and it is considered that the dispersion state and form of the precipitates have been changed by the addition of a large amount of Nb.

Although fine precipitates usually become trap sites for hydrogen as in the case of dislocations as described above, a study of materials to which Nb is added in an amount of more than 0.1% shows that hydrogen storage is smaller than that of other elements. The amount is small. That is, it was presumed that a change in the dispersion state or form of the precipitate due to the addition of a large amount of Nb also had an effect of reducing the action of the hydrogen as a trap site. As described above, the addition of 0.1% or more of Nb enables high-temperature tempering without significantly reducing the strength,
In addition, the formed precipitates have a low hydrogen storage capacity and reduce the absorption of hydrogen into the steel, so that they are extremely effective for obtaining high-strength steel pipes having excellent SSC resistance.

It has also been confirmed that the effect of the addition of Nb can be further improved by performing a process of reducing the cross-sectional reduction rate in the temperature range of 1000 to 1150 ° C. to 40% or more in the final step of hot rolling. Was. The final process of hot rolling is
When the amount of Nb is large as described above, it greatly affects the dispersion state of Nb precipitates, and also effectively acts to refine the crystal structure, resulting in good results.

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.

(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 then tempered to temper to a required strength. : 0.20 to 0.35%, Si: 0.05 to 0.5%, Mn: 0.1 to 1%, P: 0.025% or less, S: 0.01% or less, Cr: 0.3 to 1.2%, Mo: 0.2 to 1%, sol.Al: 0.005 to 0.5%, Ti: 0.005 to 0.5%, B: 0.0001 to 0.005%, Nb: 0.1 to 0.5%, V: 0.5% or less, W: 1% or less, Zr: 0.5% or less, Ca: 0.01% or less, Ni: 0.1% or less, N: 0.01% or less, O: 0.01% or less, with the balance being 1000% in the final finishing rolling step in hot piercing and rolling a steel billet composed of Fe and unavoidable impurities. After performing processing of 40% or more in the temperature range of ~ 1150 ° C, quenching directly from a temperature of 1000 ° C or more and then tempering Wherein, the method of producing a high strength and high corrosion resistance seamless steel pipe having a yield stress of 758～1068MPa.

(2) A method for producing a seamless steel pipe which is tempered to a required strength by quenching and tempering, wherein C: 0.20 to 0.35%, Si: 0.05 to 0.5%, Mn: 0.1 to 1% by weight. P: 0.025% or less, S: 0.01% or less, Cr: 0.3 to 1.2%, Mo: 0.2 to 1%, sol. Al: 0.005 to 0.5%, Ti: 0.005 to 0.5%, B: 0.0001 to 0.005%, Nb: 0.1-0.5%, V: 0.5% or less, W: 1% or less, Zr: 0.5% or less, Ca: 0.01% or less, Ni: 0.1% or less, N: 0.01% or less, O: 0.01% or less A steel pipe having a chemical composition consisting of Fe and unavoidable impurities is heated to 1000 to 1150 ° C., quenched, and then tempered, and has a high strength and high corrosion resistance with a yield stress of 758 to 1068 MPa. No steel pipe manufacturing method.

(3) In the final finishing rolling step in hot piercing and rolling, the temperature range of 1000 to 1150 ° C.
(2) above, characterized by using a steel pipe which has been worked at a rate of at least
A method for producing a high-strength, high-corrosion-resistant seamless steel pipe having a yield stress of 58 to 1068 MPa.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION

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

C: 0.20 to 0.35%, C is an element necessary for securing strength by quenching. Its content is 0.20
%, The quenching hardness is insufficient, and the high strength required after tempering cannot be obtained. If it exceeds 0.35%, the amount of carbides increases and the SSC resistance decreases, so the content is set to 0.20 to 0.35%. The upper limit of C is desirably up to 0.30%.

Si: 0.05-0.5%, Si is an element necessary for deoxidation of steel, and is an element which enhances temper softening resistance and improves SSC resistance. However, excessive Si content makes the steel brittle. For the purpose of deoxidation and improvement of SSC resistance, the content of 0.05% or more is necessary. However, if it exceeds 0.5%, the toughness is reduced and the SSC resistance is reduced.
%. The upper limit is desirably 0.3%.

Mn: 0.1-1%, Mn is an element necessary for suppressing hot brittleness due to S, and has an effect of improving hardenability. For these purposes, the content of 0.1% or more is necessary, but if it exceeds 1.0%, the toughness is reduced, and particularly the SSC resistance is reduced. M
The upper limit of n is desirably 0.3%.

Cr: 0.3 to 1.2%, Cr secures hardenability, increases strength and improves SSC resistance. In order to obtain a steel having a yield stress of 110 ksi (758 MPa) or more, which is the target of the present invention, if less than 0.3%, the effect of improving hardenability is insufficient, and if it exceeds 1.2%, the corrosion rate increases in an environment containing hydrogen sulfide. In addition, the concentration of occluded hydrogen is increased, thereby deteriorating the SSC resistance. Desirably, it is 0.5 to 0.8%.

Mo: 0.2 to 1%, Mo has the effect of improving the hardenability and the tempering softening resistance, like Cr. If the tempering temperature can be increased, the SSC resistance improves. If the content of Mo is less than 0.2%, the effect is not sufficient. However, if it exceeds 1%, acicular Mo carbides precipitate, and this has a high stress concentration coefficient and becomes a starting point of SSC, thereby deteriorating SSC resistance. Desirably, it is 0.3 to 0.8%.

Sol. Al (acid-soluble Al): 0.005 to 0.5% Al is an element necessary for deoxidizing steel. As a result of addition to the molten metal to ensure sufficient soundness of the slab, the slab is contained in the steel as sol.Al, but if less than 0.005%, defects remain in the slab. However, if it exceeds 0.5%, the inclusions increase and the toughness decreases. Further, in a seamless steel pipe for an oil country tubular good, a thread for connection is often cut at the pipe end. However, if there is a large amount of Al, a defect easily occurs in a threaded portion. Therefore, sol.Al
Is 0.005 to 0.5%.

Ti: 0.005 to 0.5% Ti is added for the purpose of fixing N, which is an impurity in steel, as TiN. Also, the excess T required for fixing N
i has the effect of increasing the temper softening resistance by becoming finely precipitated as carbides. Fixing N suppresses the formation of BN,
This is for maintaining B in a solid solution state and sufficiently improving hardenability. If it is less than 0.005%, the effect cannot be sufficiently obtained, and if it exceeds 0.5%, a bad effect of deteriorating toughness appears. Therefore, the content of Ti is set to 0.005 to 0.5%.

B: 0.0001 to 0.005% B improves the hardenability in a trace amount, and particularly improves the SSC resistance of thick materials. If it is less than 0.0001%, the effect of improving the hardenability is not obtained, and if it exceeds 0.005%, the toughness and SSC resistance are greatly reduced, so the B content is made 0.0001 to 0.005%.

Nb: 0.1-0.5%, Nb is an element that plays the most important role in the present invention. That is, quenching from a high temperature is made possible by suppressing grain growth during heating, the tempering softening resistance after quenching is significantly increased, and sufficient strength is maintained even at a high tempering temperature. To achieve this effect, at least
0.1% or more must be contained, and if less than 0.1%, SS resistance
It is difficult to ensure high strength after high-temperature tempering (650 ° C. or higher), which is desirable for C properties.

On the other hand, if it exceeds 0.5%, the toughness is reduced, and the tempering temperature for adjusting to the required strength is A ₁
It may exceed the transformation point. Therefore, the content range is limited to 0.1 to 0.5%. Desirable is 0.2-0.4%.

V: 0.5% or less, V may not be added, but is added if necessary because it precipitates as fine carbides during tempering and increases tempering softening resistance. Especially Nb
When added together, SSC resistance is improved. In order to exhibit the effect by addition, the content is preferably at least 0.005% or more. However, if added too much, the toughness is reduced, so the content should be at most 0.5%.

W: 1.0% or less, W may not be added, but has the effect of improving the hardenability and increasing the temper softening resistance as in the case of Mo. In order to exhibit the effect by addition, the content is preferably at least 0.005% or more. However, if the content exceeds 1.0%, not only the effect is saturated, but also segregation may deteriorate the SSC resistance, so the content is limited to 1.0%.

Zr: 0.5% or less, Zr is an expensive element and does not need to be added. However, when Zr is contained, the yield stress increases, and as a result, SSC resistance improves. This is interpreted to be due to the effect of reducing work hardening at the time of local yielding. At least 0.005% for a clear effect
Is desirable, but if it is too large, inclusions increase and the toughness deteriorates, so when adding, the upper limit of the content is 0.5%.

Ca: 0.01% or less, Ca may not be added, but may be added if necessary because it may improve SSC resistance. This is considered to be due to the effect of improving the shape of the inclusion by reacting with S in the steel to form a sulfide, thereby preventing the inclusion from becoming a stress concentration source at the starting point of cracking. When it is contained, the content is preferably at least 0.0001% or more in order to obtain the effect, but if it is too much, defects such as ground flaws may occur on the steel surface,
The maximum is 0.01%.

Inevitable impurities: The inclusion of inevitable impurities other than Fe and the above-described alloy components deteriorates the performance of steel, so the smaller the better, the better. Particularly, as an element for which the upper limit needs to be regulated, S forms sulfide-based inclusions, thereby deteriorating corrosion resistance and deteriorating SSC resistance.
P should be set to 0.01% or less, and P should be set to 0.025% or less, because the SSC resistance is greatly deteriorated when the strength is increased. Ni is restricted to 0.1% or less because steel having the composition range of the present invention deteriorates SSC resistance. In addition, since both N and O decrease toughness and SSC resistance,
It shall be 0.01% or less.

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

In the final stage of hot rolling, 1000 to 1150
The cross-sectional compression ratio in the temperature range of ° C is 40% or more. This is because the degree of processing up to 1150 ° C is less effective in refining the crystal structure because recrystallization proceeds immediately after processing, and a large amount of Nb is added at processing below 1000 ° C. In the case of the steel of the present invention, and the variation in hardness after quenching,
This is because there is a possibility that the deformation of the steel pipe after cooling may be increased. On the other hand, when the cross-sectional compression ratio in this temperature range is less than 40%, fine grained structure cannot be obtained and the SSC resistance becomes insufficient.

After finishing the final stage of rolling, it is quenched immediately. In this case, the quenching temperature is 1000 ° C. or higher. The upper limit temperature of quenching is not particularly defined, but processing at 1150 ° C or lower requires 40% or more, so there is a limit naturally. 1000 ℃
Quenching at a temperature lower than the above is difficult to obtain a material having good SSC resistance. This is because the dispersion state of precipitates such as NbC is not preferable, and a sufficient quenched state cannot be obtained. The quenching temperature is desirably 1050 ° C. or higher.

For quenching from 1000 ° C. or more, direct quenching immediately after the above-mentioned rolling is preferable, but once cooled (below 650 ° C. or about room temperature), the steel pipe is cooled to 1000 to 11 ° C.
Almost the same effect can be obtained by reheating to 50 ° C. and quenching. Here, the reheating temperature for quenching is set to 1150 ° C. if the temperature is too high, the crystal grains become coarse. In this case, a steel pipe manufactured using a billet having a chemical composition defined in the present invention may be used, but in the final stage of hot rolling, the cross-sectional compression ratio in a temperature range of 1000 to 1150 ° C. is 40%. After that, cool down to 400 ° C at 10 ° C / s
With the above cooling rate, the SSC resistance can be more effectively improved. This is because the form of the precipitate such as NbC can be made more preferable. Even if the cooling rate after rolling becomes too fast and the quenched state does not change, the effect does not change, but since the steel pipe may be deformed due to uneven cooling, it is preferable to keep the cooling rate to a degree that quenching does not occur. . The preferred cooling rate is 20 ° C
/ S or more.

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-described conditions, the tempering condition is adjusted to the required strength and the excellent SSC resistance is obtained. Requires tempering at 650 ° C or higher. However, it is more preferable that the required strength be obtained at a tempering temperature of 680 ° C. or higher.

[0046]

【Example】

Example 1 Steel having the chemical composition shown in Table 1 was melted using a 150 kg vacuum melting furnace. Steel symbols A to D, E to H, I and J, K and L, and M to O are melted at the same melting chance, respectively, and the composition is adjusted by adding a specific alloy element immediately before casting. doing. The obtained ingot was forged to form a billet for rolling having 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 steel pipe processing process or the finish rolling process,
After the rough rolling of 50%, finish rolling was performed in a temperature range lower than 1150 ° C, and quenching was performed immediately after the rolling. here,
For each steel sample, the temperature at the end of rolling, that is, the quenching temperature,
The temperature was set at 1020 to 1050 ° C. After quenching, tempering was performed by changing the temperature according to the components and the required strength. The test conditions for each of these steel samples are summarized in Table 2.

[0047]

[Table 1]

[0048]

[Table 2]

A JIS-14B test piece was sampled from each of the obtained steel sheet samples in parallel with the rolling direction, and the tensile strength was measured. The SSC resistance was evaluated by NACE TM-0117 Metod A
The method was performed in accordance with That is, 6.35 mm in diameter and 25.4 mm in length, parallel to the rolling direction from the center of the sample thickness.
Is a constant load test in an aqueous solution of 5% acetic acid + 5% salt saturated with hydrogen sulfide at 1 atm. Load stress is 80% of actual yield stress, temperature is 25
At ℃, continue the test with the load applied for 720 hours,
When the sample did not break during that time, it was determined that the SSC resistance was good.

The test results are shown in Table 2. here,
Test Nos. 1 to 4 or Test Nos. 5 to 8 compare the effect of the Nb content, but in Test No. 1 or Test No. 5, the Nb content is lower than the range defined by the present invention, The tempering temperature cannot be raised sufficiently to obtain the required strength,
C property is not good. Test No. 4 or Test No. 8 is the case where the Nb content is too large than the range of the present invention. However, in order to obtain the required strength, the tempering temperature exceeds the Ac ₁ transformation point, which is also the SSC resistance. It's causing it to get worse. Test number 10,
12, 13, and 15, the amount of C, the amount of Cr, or the amount of Mo, etc., deviates from the range specified by the present invention, and the manufacturing conditions are 40% or more processing in the finishing process of hot rolling and 1000 ° C or more. Even if quenching is performed at a high temperature and the required strength is obtained after tempering, the SSC resistance is insufficient. Test Nos. 17 to 20 and Test Nos. 21 to 24 show the effect of changing the rolling reduction in a temperature range lower than 1150 ° C. in finish rolling. Although the steel composition falls within the scope of the present invention, any of Test Nos. 17 and 21 in which the rolling degree is less than 40% results in poor SSC resistance. As can be seen from the comparison of these results, when manufactured by the method defined by the present invention, all of them show excellent SSC resistance.

[Example 2] In the same manner as in Example 1, a steel pipe having the composition shown in Table 1 was used to form a steel pipe in a processing step or finish rolling using a 50 mm thick, 80 mm wide, and 250 mm long billet for rolling. After heating at 1250 ° C and rough rolling at 50% according to the conditions of workability, 70% or less at the temperature range below 1150 ° C
Finish rolling was performed at a rolling reduction of 50%, and after the rolling was completed at a temperature of 1020 ° C. or more, it was allowed to cool to room temperature. The average cooling rate up to 400 ° C. was about 20 ° C./s, except for a part. The obtained steel sheet was reheated and quenched and tempered. Table 3 shows the temperature conditions for these heat treatments.

[0052]

[Table 3]

From the heat-treated plate specimen, the tensile strength was measured in the same manner as in Example 1, and the SSC resistance was evaluated. The results are also shown in Table 3. Test numbers 25 to 40 are the test numbers of Example 1.
Whereas 1 to 16 are prototypes made by direct quenching,
The same material is reheated and quenched. As can be seen, even when reheated, those having the chemical composition range defined by the present invention can obtain excellent SSC resistance by quenching from a high temperature and tempering at a high temperature. Test Nos. 41 to 44 or Test Nos. 45 to 48 are quenched and tempered by changing the heating temperature with steel having a chemical composition falling within the range of the present invention. If it is too high or too low to deviate from the scope of the invention, it is clear that the SSC resistance is not good. In the test numbers 49 and 50, the cooling after rolling was performed while keeping the temperature low, and the result was slightly unstable.

[0054]

According to the present invention, a high-strength, high-corrosion-resistant seamless steel pipe having excellent SSC resistance and a yield stress of 110 to 155 ksi (758 to 1068 MPa), which can be used in oil wells and related facilities, is provided. By a simple means of adding a large amount of Nb and quenching from a high temperature, it can be easily manufactured by direct quenching immediately after rolling or reheating quenching, and these steel pipes can be provided at high productivity and at low cost.

Claims (3)

[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.
C: 0.20 to 0.35%, Si: 0.05 to 0.5%, Mn: 0.1 to 1%, P: 0.025% or less, S: 0.01% or less, Cr: 0.3 to 1.2%, Mo: 0.2 to 1%, sol .Al: 0.005 to 0.5%, Ti: 0.005 to 0.5%, B: 0.0001 to 0.005%, Nb: 0.1 to 0.5%, V: 0.5% or less, W: 1.0% or less, Zr: 0.5% or less, Ca: 0.01 %, Ni: 0.1% or less, N: 0.01% or less, O: 0.01% or less, with the balance being the final finishing rolling step when hot-piercing and rolling a steel billet composed of Fe and inevitable impurities. , Which is characterized by having a yield stress of 758 to 1068MPa, characterized in that it is directly quenched from a temperature of 1000 ° C or higher after being subjected to processing of 40% or more in a temperature range of 1000 to 1150 ° C and then tempered. Manufacturing method of high strength and high corrosion resistance seamless steel pipe. 2. A method for producing a seamless steel pipe which is tempered to a required strength by quenching and tempering, wherein C: 0.20 to 0.35%, Si: 0.05 to 0.5%, Mn: 0.1 to 1%, P : 0.025% or less, S: 0.01% or less, Cr: 0.3 to 1.2%, Mo: 0.2 to 1%, sol. Al: 0.005 to 0.5%, Ti: 0.005 to 0.5%, B: 0.0001 to 0.005%, Nb: 0.1 to 0.5%, V: 0.5% or less, W: 1.0% or less, Zr: 0.5% or less, Ca: 0.01% or less, Ni: 0.1% or less, N: 0.01% or less, O: 0.01% or less, and the balance Is a high-strength, high-corrosion-resistant seamless steel pipe having a yield stress of 758-1068 MPa, characterized in that a steel pipe having a chemical composition comprising Fe and unavoidable impurities is heated to 1000-1150 ° C., quenched, and then tempered. Manufacturing method. 3. In the final finishing rolling step in hot piercing and rolling, after performing processing of 40% or more in a temperature range of 1000 to 1150 ° C., cooling to 400 ° C. or less at 10 ° C./s or more. 758. The 758 according to claim 2, wherein a steel pipe is used.
A method for producing a high-strength, high-corrosion-resistant seamless steel pipe having a yield stress of up to 1068 MPa.