JPH07197125A - Production of high strength steel pipe having excellent sulfide stress corrosion crack resistance - Google Patents

Abstract

PURPOSE:To obtain a steel pipe having excellent SSCC resistance and strength by acceleration-cooling after hot pierce-rolling a specific composition of steel billet, successively, mandrel-rolling to make a seamless steel pipe and, thereafter, immediately quenching and tempering. CONSTITUTION:The composition of the steel is made to be, by wt.%, 0.15-0.4 C, 0.1-1 Si, 0.3-1 Mn, 0.1-1.5 Cr, 0.3-1 Mo, 0.0005-0.003 B, 0.01-0.1 Al, 0.003-0.01 N, <=0.015 P, <=0.005 S, and further, one or more kinds of 0.01-0.05 V, 0.01-0.05 Nb and 0.01-0.03% Ti and the balance Fe. This steel billet is heated to 1250-1350 deg.C and the Mannesmann type seamless rolling is applied to execute hot-piercing. This pipe is acceleration-cooled to the recrystallizing temp. +50 deg.C to the recrystallizing temp. +100 deg.C. Rolling reduction is executed to this pipe by the mandrel-rolling at <=30% and at the recrystallizing temp. to the recrystallizing temp. +30 deg.C, and finished at <=Ar3 transformation point to make the seamless steel pipe. This pipe is immediately water-hardened and, thereafter, the tempering is executed.

Description

Detailed Description of the Invention

The present invention has a high resistance to sulfide stress corrosion cracking (hereinafter abbreviated as SSCC) which occurs particularly in an environment in which wet hydrogen sulfide and stress are combined.
The present invention relates to a method for producing a seamless steel pipe for oil wells or gas wells having a high yield strength of 75 kgf / mm ² or more.

[0002]

2. Description of the Related Art With the recent urgency of energy circumstances, deep wells and hydrogen wells containing hydrogen sulfide, which have not been developed up to now, are also targets for development, and have high strength as oil well pipes to be used for them. The production of seamless steel pipes with high resistance to SSCC is highly desirable. Steel pipes used as oil well pipes are usually subjected to a single heat treatment of quenching and tempering, but from experience to date, to prevent SSCC, the hardness should be 20 to 20 on the Rockwell C scale.
It is considered effective to regulate it to 22 or less.

However, according to the regulation of the hardness, the yield strength must be 70 kgf / mm ² at most, and it is difficult to manufacture a steel pipe having higher strength and SSCC resistance. On the other hand, as a method for evaluating the SSCC resistance, a constant load tensile test in a 5% aqueous solution of hydrogen sulfide saturated NaCl-0.5% acetic acid defined in NACE standard TM-01-77 in the United States is well known. There is. Maximum stress (σt) that does not cause SSCC specified here
"Crack limit ratio (σth / YS)", which is a ratio of h) to the nominal yield stress (YS) of the material, expressed as a percentage, achieving both SSCC resistance of 85% or more and high strength of yield strength 75 kgf / mm ² or more. As a method, it is known that miniaturization of crystal grains is effective.

As a measure for realizing this, as disclosed in Japanese Patent Laid-Open No. 59-232220,
The manufactured steel pipe is subjected to quenching and tempering treatments twice or more, or as disclosed in JP-A-59-119324, a method of significantly increasing the temperature rise before quenching treatment is used. Proposed. On the other hand, seamless steel pipes (seamless steel pipes) used for oil wells or gas wells have been used after hot pipe rolling, as disclosed in JP-A-57-39129, from the viewpoint of energy saving and cost saving in recent years. Utilizing the temperature held therein, water quenching is performed from a temperature of A _c3 point or higher, followed by tempering. This method is called the so-called direct quenching method and has been widely used recently.

However, in comparison with the conventional pipe manufacturing method in which the pipe is cooled to room temperature after rolling, the temperature is raised to a predetermined quenching temperature by reheating, and then quenching is performed, the pipe manufactured by direct quenching is austenite Since the ferrite transformation and the transformation process in the opposite direction are omitted, the crystal grains tend to be coarsened, and thus the SSCC resistance is unavoidably deteriorated.

[0006]

As described above, the seamless steel pipe produced by direct quenching is advantageous in terms of energy saving or cost saving, but since the crystal grain size tends to become coarse, SSCC resistance is high. It has been considered difficult to apply it to steel pipes used in oil wells or gas wells that require high performance. Therefore, from the viewpoint of energy saving, the present invention provides a fine crack that achieves both a SSCC resistance of 85% or more and a high yield strength of 75 kgf / mm ² or more in “crack limit ratio (σth / YS)” by one-time quenching and tempering heat treatment. The purpose is to produce a high-strength seamless steel pipe having a grain structure. Specifically, controlled rolling is applied in the mandrel rolling in the seamless steel tube rolling process, and after the rolling is finished, the above-mentioned direct quenching is performed to obtain a fine grain structure, which has excellent SSCC resistance and yield point strength of 75 kgf /
The purpose is to achieve high strength of mm ² or more.

[0007]

The method for producing a seamless steel pipe according to the present invention is to provide a seamless steel pipe having fine crystal grains by direct quenching and tempering heat treatment and exhibiting excellent SSCC resistance. It is a feature,
As a result of studying the measures for realizing this feature, the inventors have invented the following manufacturing method.

(1) The invention of claim 1 is a method for producing a high-strength steel pipe (component composition is wt%) excellent in sulfide stress corrosion cracking resistance, which comprises the following steps. (A) As a main component, C: 0.15 to 0.4%, Si: 0.1 to 1% Mn: 0.3 to 1%, Cr: 0.1 to 1.5%, Mo: 0.1 to
1%, B: 0.0005 to 0.003%, Al: 0.01 to 0.1
%, N: 0.003 to 0.01% P: 0.015% or less, S: 0.005% or less, V: 0.01 to 0.05%, Nb: 0.01 to 0.05%, Ti:
Contains one or more of 0.01-0.03%,
A step of preparing a billet whose balance consists essentially of Fe; (b) a step of heating the billet between 1250 and 1350 ° C., followed by hot perforation by Mannesmann seamless rolling; and (c) the hot A step of accelerating and cooling the perforated billet to a temperature range of recrystallization temperature + 50 ° C or higher and recrystallization temperature + 100 ° C or lower, and (d) rolling the billet accelerated to the temperature range by mandrel rolling to reduce the rolling amount by 30% or more. And (e) immediately quenching the seamless steel pipe with water (e) immediately after the rolling to complete the rolling at a temperature not lower than the recrystallization temperature and not higher than the recrystallization temperature + 30 ° C. and not lower than the _{Ar 3} transformation point. f) A step of tempering the water-quenched seamless steel pipe at a temperature below the A _C1 transformation point.

(2) The invention of claim 2 is the composition of the billet according to claim 1, further comprising Ca: 0.000.
A method for producing a high-strength steel pipe excellent in sulfide stress corrosion cracking resistance, which comprises the same steps as those of claim 1, containing 5 to 0.01%.

[0010]

In view of the features of the method for producing a seamless steel pipe according to the present invention, that is, from the viewpoint of energy saving and cost saving, direct quenching and tempering are adopted, and in this case as well, it has fine crystal grains and is excellent. Resistance to SSCC
As a result of various studies on various measures for realizing the feature that a seamless steel pipe having excellent properties can be obtained, the following findings were obtained. That is, desired high strength and SS resistance
It was found that there is a correlation between the prior austenite grain size of the tempered martensite structure and the SSCC resistance, which are essential for obtaining CC properties. Figure 1 shows σth / YS
Shows the relationship between the grain size and the yield strength at which is 0.85 or more. In the conventional manufacturing method, after pipe rolling is finished, air cooling is carried out, followed by quenching and tempering heat treatment. In this case, the grain size number is at most ASTM No. In 8, σth
The yield strength at which / YS is 0.85 or more is 65 kgf / m
Stay at m ² .

According to FIG. 1, the critical yield strength is 75 kgf.
The grain size of not less than / mm ² is ASTM No. 10 is, for fine graining, immediately before direct quenching, that is,
It is important to make the grain size fine at the end of rolling.
Paying attention to this point, rolling was performed while varying the rolling temperature and the reduction ratio of the mandrel rolling, and then the prior austenite grain size of the tempered martensite structure obtained by performing direct quenching and tempering heat treatment was investigated. As a result, the structure of the finest grain is obtained is that cooling is rapidly performed after the end of billet drilling until the start of mandrel rolling, the growth of crystal grains during this period is suppressed, and the rolling is finished immediately above the recrystallization temperature. It turned out to be the case. The present invention has been completed based on these findings.

Regarding the chemical composition of the steel, the reasons why the chemical composition of the steel is limited as claimed in the present invention will be described below. C: 0.15 to 0.4%. C is an essential element for improving the hardenability and the tempering resistance in addition to the function of ensuring the strength of the low alloy steel, and its content is set to 0.15% or more. However, a large content exceeding 0.4% causes cracks during quenching and deterioration of toughness, so the C content is set to the above range.

Si: 0.1 to 1% Si has a function as a deoxidizing agent for steel, as well as a function for improving the strength of the steel material, so it is necessary to add 0.1% or more, but if it exceeds 1%, toughness deteriorates. Since the grain boundary strength is also reduced, its upper limit is set to 1%.

Mn: 0.3 to 1% Mn acts as a deoxidizing agent for steel, improves the hardenability, and is an effective component for preventing red heat embrittlement, and is added in an amount of 0.3% or more. On the other hand, if the content exceeds 1%, the toughness deteriorates and the grain boundary strength also decreases, so the upper limit is made 1%.

Cr: 0.1 to 1.5% Cr is an element that has a significant effect on the improvement of hardenability, and has the effect of increasing the strength of steel, but its content is 0.1.
If it is less than%, the desired effect cannot be obtained in the above action,
Addition of a large amount obviously lowers the SSCC resistance, so the upper limit is made 1.5%.

Mo: 0.1 to 1%. Mo has the effect of increasing the tempering resistance of steel, but 0.1
%, The effect is small. On the other hand, if the content exceeds 1%, the steel becomes brittle and the toughness deteriorates. Therefore, the Mo content is in the above range.

B: 0.0005% to 0.003% B improves the hardenability of steel and can be contained in an amount of 0.005% or more. However, the addition of a large amount not only saturates the effect, but also causes cracking during hot working, so B is included. The amount is in the above range.

Al: 0.01 to 0.1%. Al is an element useful as a deoxidizing agent for steel, is an element that combines with Ti to form N by combining with N in the steel to form a nitride, and makes the effect of B manifest 0.01. % Or more. Further, an excessive content exceeding 0.1% is Al ₂ O.
₃ is increased and the SSCC resistance is lowered, so Al
Content of 0.01 to 0.1%.

N: 0.003 to 0.01%. N forms a nitride with Ti and Al, and particularly TiN has a function of suppressing grain growth of steel and refining it. Therefore, in order to precipitate the added Ti in the steel as TiN that effectively acts on the grain refinement, in consideration of stoichiometry,
It is made 0.003% or more. On the other hand, if the content is more than 0.01%, the effect of B addition is reduced and the hardenability is deteriorated. Therefore, the N content is set to 0.003 to 0.01%.

P: 0.015% or less. P is a harmful element that lowers the grain boundary strength, and particularly if its content exceeds 0.015%, the SSCC resistance deteriorates, so its content is made 0.015% or less.

S: 0.005% or less. S is an unavoidable impurity in steel, and if contained in a large amount, MnS
Is formed, and this becomes the starting point of cracking. Therefore, the S content is set to 0.005% or less.

Nb and V: Both are 0.01% to 0.05%. Since Nb and V have an effect of making recrystallized grains fine during rolling, they are elements to be added when finer crystal grains are desired, and this effect is not sufficient if less than 0.01%.
If the content exceeds 0.05%, not only the effect will be saturated, but also the toughness will be reduced.
It is set to 1% to 0.05%.

Ti: 0.01 to 0.03%. Ti fixes N as TiN similarly to Al to improve the hardenability of B and finely disperses and precipitates in the billet, so that it has an effect of suppressing coarsening of crystal grains during billet heating. However, if the content is less than 0.01%, the desired effect cannot be obtained. On the other hand, if the content exceeds 0.03%, there is no effect in suppressing the crystal grain growth due to coarsening of TiN by aggregation. Not only that, it causes deterioration of toughness, so the Ti content is made 0.01 to 0.03%.

Ca: 0.0005 to 0.01%. Ca precipitates S as spheroidized sulfide and is resistant to SSCC
It is an element added because it has the effect of further improving the property, and this effect is not sufficient if less than 0.0005%,
Even if it exceeds 0.0100%, not only the effect is saturated, but also the toughness is reduced, so the Ca content is 0.00
It was set to 05 to 0.01% (claim 2).

Regarding the manufacturing process of steel pipe, a billet having the above-described composition is manufactured by a continuous casting method or an ingot-agglomeration method at a temperature of 1250-13.
Heat to 50 ° C. In this heating, at a low temperature of less than 1250 ° C., the solid solution of the carbide present in the billet is not sufficient, the desired strength cannot be obtained by precipitation hardening during tempering, and the undissolved carbide reduces the toughness. Cause On the other hand, at temperatures above 1350 ° C., although carbides form a solid solution, the crystal grains are remarkably coarsened, and even if proper controlled rolling is performed by mandrel rolling, ASTM N
o. A fine grain structure of 10 or more does not occur, and the desired SSCC resistance
I can't get sex.

Next, the perforated billet obtained by hot perforation by Mannesmann seamless rolling is recrystallized at +
Accelerated cooling to a temperature range of 50 ° C or higher and recrystallization temperature + 100 ° C or lower. This step is intended to suppress grain growth that occurs during the start of mandrel rolling after hot drilling. At this time, in the air cooling that has been performed conventionally, even if proper control rolling is performed by mandrel rolling due to the crystal grain growth that occurs in the cooling process, the fine grain structure of No. 10 or more does not occur, and the desired resistance is not obtained. SSCC property cannot be obtained. This step is performed by, for example, water cooling or mist cooling, and the cooling rate at the time of accelerated cooling is usually 1 to 20 ° C. /
It is desirable to set to sec.

Then, mandrel rolling is started in the above cooling end temperature range. The reason why rolling is started in this temperature range is to refine the austenite by recrystallization in the rolling process and to finish rolling at a temperature not lower than the recrystallization temperature. Next, the reason why the rolling reduction in mandrel rolling is limited to 30% or more is that the reduction of grain size by recrystallization is not sufficient when the rolling reduction is less than 30%.
o. This is because a fine grain structure of 10 or more cannot be obtained. A particularly important point in this mandrel rolling process is to set the rolling end temperature to a recrystallization temperature or higher and a recrystallization temperature + 30 ° C. or lower and an _{Ar 3} transformation point or higher.

That is, since the structure at the end of rolling is brought by the direct quenching following the mandrel rolling, when rolling is completed below the recrystallization temperature, extended austenite is formed and the SSCC resistance deteriorates. . Also, if the rolling is finished at a temperature higher than the recrystallization temperature + 30 ° C., grain growth occurs due to recovery after rolling, resulting in ASTMN
o. A fine grain structure of 10 or more does not occur, and the desired SSCC resistance
I can't get sex. Further, when rolling is completed at the _{Ar 3} transformation point or lower, a two-phase structure of austenite and ferrite is formed, hardenability deteriorates, and desired strength and SSCC resistance cannot be obtained.

Here, the recrystallization temperature is the composition of the target steel,
Since this is a physical property value that changes depending on the combination of the grain size and the rolling reduction before the start of rolling, in order to determine this, for example, by performing a high-temperature high-speed compression test, the rolling of an actual machine is reproduced, and the recrystallization temperature is It is necessary to ask. The seamless steel pipe after completion of the mandrel rolling is immediately water-quenched from the end temperature of rolling, and then tempered at a temperature not higher than the _AC1 transformation point. The tempering temperature is an A _C1 transformation point temperature as an upper limit so as not to precipitate austeite, but the tempering temperature may be adjusted depending on the strength of the target steel pipe.
That is, when lowering the strength, this temperature is increased, and when increasing the strength, tempering is performed at a lower temperature.

[0030]

EXAMPLES Examples of the present invention will be described. The properties of the seamless steel pipes produced using the steels A to I having the composition shown in Table 1 are shown in Table 2 together with the production method. The present invention steel, a seamless pipe is produced by using a billet produced by a continuous casting method or an ingot making method as a raw material,
The rolling method of the seamless steel pipe was a combination of the Mannesmann piercing method and the mandrel rolling method. Billet diameter is about 170
~ 300 mm. The outer diameter of the seamless steel pipe is approximately 114
It is ~ 244 mm and the wall thickness is about 10-15 mm.

The recrystallization temperature of each steel shown in Table 2 is the most important factor in determining the rolling temperature and the rolling reduction of the controlled rolling carried out by the manufacturing method according to the present invention, and as described above. The high temperature and high speed compression test was performed to reproduce the rolling in the actual machine and the recrystallization temperature was obtained for each. According to the present invention, since accelerated cooling such as water cooling is performed in the process before Mandrel rolling after Mannesmann-type perforation, grain growth that occurs when air cooling is performed in this conventional process is suppressed, and direct quenching and crystal after tempering are performed. The particle size is A
STM No. A fine grain structure of 10 or more was obtained.

In particular, B steel, C steel and F containing Nb
For steel, ASTM No. 12 or more fine crystal grains were obtained. Further, the invention steel shows a yield strength of 75 kgf / mm ² or more when tempered at the temperature shown in Table 2,
In addition, the cracking limit ratio (σth / YS) was 0.85 or more, indicating excellent SSCC resistance. On the other hand, since the amounts of Mo added in the G steel, the amounts of Cr and Mo added in the H steel, and the amounts of Ti added in the I steel are not appropriate, the grain size is ASTM No. Although 10 or more was achieved, σth / YS could not satisfy 0.85 or more.

When the steel A was subjected to Mannesmann perforation and then air-cooled as a conventional method, the crystal grains were ASTM No. It coarsened to 9, and σth / YS remained at 0.65. When the billet heating temperature of A steel is 1380 ° C,
Coarsening of crystal grains occurs during heating, and the grain size after pipe manufacturing is ASTM No. 7.5, and σth / YS is 0.5
Stayed at 5. When the reduction rate of mandrel rolling in steel B is 20%, recrystallization during rolling is not sufficient and the grain size is ASTM No. 9, and σth / YS is 0.
Stayed at 65. Further, when the end temperature of the mandrel rolling in the F steel was set to 900 ° C., which is lower than the recrystallization temperature, the crystal grains after pipe production were expanded grains in which austenite was elongated, and σth / YS remained at 0.55.

[Table 1]

[Table 2]

[0034]

As described above, according to the method for producing a seamless steel pipe of the present invention, a seamless steel pipe having a high level of high strength and sulfide stress corrosion cracking resistance at the same time can be obtained by a single quenching and tempering heat treatment. Moreover, since no additional equipment is required, cost saving and energy saving are achieved. Such a manufacturing method maximizes the effect of energy saving by direct quenching, and in the cooling step prior to controlled rolling, the water spraying device already provided for scale removal can be used, resulting in a production cost. It has an advantage over the conventional manufacturing method in that it is not significantly required.

[0035]

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the grain size and the yield strength at which σth / YS is 0.85 or more.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Noriaki Otomari No. 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (2)

[Claims] 1. A method for producing a high-strength steel pipe excellent in sulfide stress corrosion cracking resistance, which comprises the following steps (the composition of components is wt%:
Is). (A) As a main component, C: 0.15 to 0.4%, Si: 0.1 to 1% Mn: 0.3 to 1%, Cr: 0.1 to 1.5%, Mo: 0.1 to 1%, B: 0.0005 to 0.003%, Al: 0.01 to 0.1%, N: 0.003 to 0.01% P: 0.015% or less, S: 0.005% or less, V: 0.01 to 0.05%, Nb: 0.01 to 0.05%, Ti: 0.01 to 0.03% A step of preparing a billet containing one kind or two or more kinds, and the balance being substantially Fe, and (b) heating the billet between 1250 and 1350 ° C., and then hot-punching by Mannesmann seamless rolling And (c) accelerating and cooling the hot-drilled billet to a temperature range of recrystallization temperature + 50 ° C. or higher and recrystallization temperature + 100 ° C. or lower, and (d) billet accelerated and cooled to the temperature range. Is subjected to mandrel rolling to reduce the temperature by 30% or more, and at a temperature not lower than the recrystallization temperature and not higher than the recrystallization temperature + 30 ° C. and not lower than the _{Ar 3} transformation point. A step of finishing rolling to obtain a seamless steel tube at a certain temperature, (e) a step of immediately water quenching the seamless steel tube, and (f) a step of tempering the water-quenched seamless steel tube at a temperature not higher than the A _C1 transformation point. 2. A method for producing a high-strength steel pipe excellent in sulfide stress corrosion cracking resistance, which comprises the following steps (component composition is wt%:
Is). (A) C: 0.15 to 0.4%, Si: 0.1 to 1% Mn: 0.3 to 1%, Cr: 0.1 to 1.5%, Mo: 0.1 to 1%, B: 0.0005 to 0.003%, Al: 0.01 to 0.1% , N: 0.003 to 0.01% P: 0.015% or less, S: 0.005% or less, and one or two of V: 0.01 to 0.05%, Nb: 0.01 to 0.05%, Ti: 0.01 to 0.03%. A billet containing at least one species and further containing Ca: 0.0005 to 0.01% and the balance substantially consisting of Fe; and (b) heating the billet between 1250 and 1350 ° C., Hot-drilling by Mannesmann seamless rolling; (c) accelerating and cooling the hot-drilled billet to a temperature range of recrystallization temperature + 50 ° C. or higher and recrystallization temperature + 100 ° C. or lower; The billet that has been accelerated and cooled to the temperature range is subjected to mandrel rolling to reduce it by 30% or more at a recrystallization temperature or higher and a recrystallization temperature + 30 ° C or lower. And a step of the seamless steel pipe to exit the rolling at A _r3 transformation point or higher temperatures, (e) a step of water immediately quenching the seamless steel, (f) water quenching the said seamless steel pipe A _C1 transformation point Step of tempering at the following temperature. [0001]