JPH11335731A - Production of high strength steel excellent in resistance to sulfide stress corrosion cracking - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain steel excellent in stress corrosion cracking by subjecting the steel, which consists of elements of a specified ratio and the balance Fe with inevitable impurities, contains the elements of a specified ratio in the impurities and is in specified relation between an Mo content and a V content, to heat treatment after hot working. SOLUTION: This steel contains, by mass, 0.2-0.35% C, 0.05-0.5% Si, 0.1-1% Mo, 0.1-1% Mn, 0.1-1.2% Cr, 0.005-0.5% V, 0.005-0.15 Ti+0.5 Zr with <=0.1% one kind or the kinds among Ti, Zr respectively and the balance Fe with inevitable impurities, further contains <=0.25% P, <=0.01% S, <=0.1% Ni, etc., in the impurities and further, satisfies formula for the contents of Mo, V. In the formula, A: 4930 e<(-0.0133> D<)> , B: 1.07×18<-8> e<(0.0187> D<)> , D: E-100 e<(-0.52> M<o-5.15> V<+0.4)> , C is respectively 150, 190, 270 for producing the steel of yield stress (ksi) of 110 to 125, >125 to <=140, >140 to <=155, E is 770, 755, 740, (e) is a base of a natural logarithm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is excellent in sulfide stress cracking resistance used in casings and tubing for oil and gas wells, drill pipes for drilling, line pipes for transportation, and piping for chemical plants. The present invention relates to a method for manufacturing a high-strength steel material.

[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, with the deepening of oil and gas wells, steel materials used in this field, particularly steel pipes, are required to have higher strength than ever before.

That is, a yield stress (hereinafter referred to as YS) of 80 ksi class (80 to 90 k
si) and 90 ksi class (more than 90, 100 ksi or less)
In recent years, 110 ksi class (110-125 ksi) and 125 k
High-strength steel pipes of si-class (greater than 125 and 140 ksi or less) having excellent sulfide stress crack resistance have come to be used. Further, there is an increasing demand for an ultra-high strength steel pipe having a YS of 140 ksi or more and excellent in sulfide stress cracking resistance.

[0004] In general, as the strength of a steel material increases, its resistance to sulfide stress cracking (hereinafter, “sulfide stress cracking” is referred to as “SSC”)
Is worse). Therefore, the biggest challenge for increasing the strength of steel materials used in an environment containing a large amount of hydrogen sulfide is improving the SSC resistance.

In order to improve the SSC resistance, the structure of the steel material is changed to a structure in which martensite accounts for about 80% or more, tempering is performed at a high temperature, and the steel is made highly clean.
Countermeasures have been taken so far, such as changing the structure of the steel material to a fine-grained structure.

[0006] Among these, the method of refining the structure is particularly noted because of the following advantages.
Development has been advanced.

That is, the first advantage is that when the strength of a steel material increases, its brittle cracks propagate in units of crystal grains or so-called “fracture units”. Therefore, when the structure is refined, the deterrent to cracks increases. is there. Secondly, the grain refinement itself also contributes to an increase in strength. Thirdly, grain refinement increases the grain boundary area per unit volume, so indirectly reduces the grain boundary segregation of impurity elements and reduces grain boundary brittleness. Is prevented.

[0008] As a general method of grain refinement of steel structure,
There are a method using transformation and working deformation, and a method for suppressing grain growth during recrystallization after working deformation. When the cast steel ingot is hot-formed into a steel material having a predetermined shape such as a steel pipe, it is inevitably deformed by processing, and is refined by repeating processing and recrystallization.

However, quenching in heat treatment for adjusting the strength after working is generally performed in the austenite region, that is, in the A region.
Since heating must be performed to a temperature of c3 or more, crystal grain growth is likely to occur, and it is desirable to lower the heating temperature during quenching in order to keep the crystals fine. However,
Being fine granules and lowering the quenching temperature
This is a factor that greatly reduces the hardenability, and it becomes difficult to secure a sufficient hardened structure in which 80% or more is martensite at the time of hardening by ordinary cooling means. Further, when the quenching temperature is lowered for grain refinement, the alloy element hardly dissolves in the matrix during quenching, and low-temperature tempering is required to increase the strength of the steel material. Tempering at a low temperature significantly lowers the SSC resistance as described below. On the other hand, if a large amount of alloying elements is added to ensure hardenability, the workability of the steel is deteriorated and the production cost is increased.

Japanese Unexamined Patent Publication (Kokai) No. 61-9519 discloses "a method for producing high-strength steel excellent in sulfide corrosion cracking resistance" in which quenching is carried out after rapid heating to reduce the grain size.

Japanese Unexamined Patent Publication (Kokai) No. 59-232220 discloses a "method of producing high-strength steel excellent in sulfide corrosion cracking resistance" in which steel is quenched twice and refined.

However, all of the methods described in the above-mentioned publications have a yield stress (YS) of 90 ksi class (over 90 and 100 ksi or less) or 100 ksi class (1).
(Exceeding 00 and 110 ksi or less). In these methods, when YS has a high strength of 110 ksi or more, the desired SSC resistance cannot always be obtained.

[0013]

The object of the present invention is to provide a YS
It is an object of the present invention to provide a method for producing a steel material suitable for oil wells and gas wells having excellent SSC resistance while having a high strength of 110 to 155 ksi (758 to 1068 MPa) and various facilities related to them and chemical plant facilities. .

The goal of improving SSC resistance is NACE.
(National Association of Corrosion Engineers) T
When a constant load test was performed in a bath specified in the M0177A method (0.5% acetic acid + 5% saline solution at 25 ° C. saturated with 1 atm of hydrogen sulfide at 25 ° C.), the critical stress (σth) for cracking was YS of steel. 80% or more. It is known that if the above conditions are satisfied, the steel material can sufficiently withstand the use in the recent severe corrosive environment.

[0015]

The gist of the present invention relating to a method for producing a high-strength steel material having excellent SSC resistance is as follows.

(1) In mass%, C: 0.2 to 0.35
%, Si: 0.05 to 0.5%, Mn: 0.1 to 1%,
Cr: 0.1 to 1.2%, Mo: 0.1 to 1%, Al:
0.005 to 0.1%, B: 0.0001 to 0.01
%, Nb: 0.005 to 0.5%, V: 0.005 to
0.5%, one or two of Ti and Zr are each 0.1% or less, and Ti + 0.5Zr: 0.005 to
0.15%, W: 0 to 1%, Ca: 0 to 0.01%, the balance being Fe and unavoidable impurities, P in the impurities: 0.025% or less, S: 0.01% Below, Ni:
0.1% or less, N: 0.01% or less, O (oxygen): 0.
A sulfide having a yield stress of 110 to 155 ksi, wherein a steel having a Mo and V content of not more than 01% and satisfying the following formula (1) is hot-worked and then subjected to heat treatment of quenching and tempering. A method for producing high-strength steel with excellent stress cracking properties.

2Mo + 5V ≦ B × C−A + 1.4 (1) where A: 4930e ^(−0.0133D) B: 1.07 × 10 ⁻⁸ e ^(0.0187D) D: E− 100e ^{(-0.52Mo-5.15V + 0.4)} C, E: When producing steel material with a yield stress of 110 or more and 125 ksi or less C: 150, E: 770 When producing a steel material with a yield stress exceeding 125 and 140 ksi or less C: 190, E: 755 When a steel material with a yield stress exceeding 140 and 155 ksi or less is produced. C: 270, E: 740 e: Base of natural logarithm Mo, V are contents (% by mass) (2) In mass% , C: 0.2-0.35%, Si: 0.
05-0.5%, Mn: 0.1-1%, Cr: 0.1-
1.2%, Mo: 0.1-1%, Al: 0.005-
0.1%, B: 0.0001 to 0.01%, Nb: 0.
005 to 0.5%, V: 0.005 to 0.5%, one or two of Ti and Zr are each 0.1% or less,
And Ti + 0.5Zr: 0.005 to 0.15%, W:
0 to 1%, Ca: 0 to 0.01%, the balance being Fe and unavoidable impurities, P in the impurities: 0.025
%, S: 0.01% or less, Ni: 0.1% or less,
N: 0.01% or less, O (oxygen): 0.01% or less, Mo, V, and Nb contents satisfy the following formula (2). 110-1 characterized by quenching and then tempering
A method for producing a high-strength steel material excellent in sulfide stress cracking resistance having a yield stress of 55 ksi.

2Mo + 4Nb + 5V ≦ B × C−A + 1.4 (2) where A: 4930e ^(−0.0133D) B: 1.07 × 10 ⁻⁸ e ^(0.0187D) D: E− 100e ^{(-0.52Mo-9.36Nb-5.15V + 0.4)} C, E: When producing a steel material with a yield stress of 110 or more and 125 ksi or less C: 150, E: 770 A steel material with a yield stress of more than 125 and 140 ksi or less When manufacturing: C: 190, E: 755 When yielding stress exceeds 140, and when manufacturing a steel material of 155 ksi or less C: 270, E: 740 e: Base of natural logarithm Mo, V and Nb are contents (mass%) The present inventors have studied the effect of grain refinement for improving SSC resistance.
Although it is effective for steel materials less than 0 ksi, the effect of improving SSC resistance becomes unstable for high-strength steel materials of 110 ksi or more. It was confirmed that no improvement effect was observed.

Therefore, in order to develop a method of manufacturing a steel material having a high strength of YS of 110 ksi or more and excellent in SSC resistance by a method other than the method of refining the structure, the chemical composition of the steel was examined. As a result of various experiments, the following findings were obtained.

A) The amount of diffusible hydrogen occluded in steel in a hydrogen sulfide environment (hereinafter referred to as occluded hydrogen concentration) differs depending on the composition of the steel.

B) Elements Mo and V greatly affect hydrogen storage in a hydrogen sulfide environment. Increasing the content of both elements increases the tempering softening resistance and enables high-temperature tempering to reduce internal strain. It has the function of increasing the stored hydrogen concentration. The reason is that diffusible hydrogen is trapped at the interface between Mo-based and V-based fine carbides that contribute to temper softening resistance, that is, precipitation strengthening.

C) Nb which does not form a solid solution in the steel by ordinary quenching heat treatment is completely dissolved in the steel at the time of quenching only when directly quenched, and precipitates as fine carbide during the subsequent quenching to increase the temper softening resistance. . However, like Mo and V, it has a function of increasing stored hydrogen. Therefore, by containing these elements, internal strain due to high-temperature tempering can be reduced, but on the other hand, the fine carbide itself has an effect of increasing the occluded hydrogen concentration. However, when normal quenching and tempering are performed after direct quenching or direct quenching and tempering, the Nb carbide is coarsened and has no effect on temper softening resistance.

D) SSC resistance is remarkably improved by the synergistic effect of reduction of stored hydrogen and high-temperature tempering. That is, SS
When C is generated, the hydrogen concentration locally increases due to defects, dislocations, and the like, which results in cracking. However, in addition to reducing the occluded hydrogen concentration, internal strain can be further reduced by high-temperature tempering. Thus, the generation of SSC due to the local increase in the concentration of hydrogen is suppressed.

E) Mo, V and Nb in steel
(However, Nb is only in the case of direct quenching) The content is preferably set to an appropriate amount according to the strength level of the steel material.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The conditions specified in the manufacturing method of the present invention will be described below in detail. In addition, “%” of the content of the chemical component indicates “% by mass”.

(A) Chemical composition of steel C: 0.2 to 0.35% C is an element effective for improving hardenability and improving strength. However, if the content is less than 0.2%, the hardenability is reduced and the SSC resistance is sometimes reduced. On the other hand, if it exceeds 0.35%, the amount of carbides increases, the number of diffusible hydrogen trap sites increases, and the SSC resistance decreases. In addition, the susceptibility to cracking increases. Therefore, C
Was 0.2 to 0.35%. A preferred upper limit of the C content is 0.3%.

The term "trap site" as used herein means that hydrogen is not fixed so strongly that it cannot be diffused, but that hydrogen present as a solid solution in steel is more stable in that portion. This refers to a local portion where the concentration is relatively higher than the hydrogen concentration level of the base (base).

Si: 0.05-0.5% Si is an element effective in deoxidizing steel, and is also an element that increases tempering softening resistance and improves SSC resistance. For the purpose of deoxidation, the content needs to be 0.05% or more.
However, if the content exceeds 0.5%, the toughness is lowered and the grain boundary strength is lowered, so that the SSC resistance is also lowered. Therefore, the content of Si is 0.05
-0.5%. Note that a preferable upper limit of the Si content is 0.3%.

Mn: 0.1-1% Mn is an element effective for ensuring the hardenability of steel.
For this purpose, a content of 0.1% or more is required. However, if the content exceeds 1%, segregation at the grain boundaries causes SS resistance.
This leads to a decrease in C properties and toughness. Therefore, the content of Mn is set to 0.1 to 1%. Note that the upper limit of the Mn content is desirably 0.5%.

Cr: 0.1 to 1.2% Cr has the effect of increasing the hardenability and increasing the tempering softening resistance to enable high temperature tempering, thereby improving the SSC resistance. In order to ensure the above-mentioned effects, the Cr content needs to be 0.1% or more. However, if Cr is 1.
If the content exceeds 2%, Cr is actively dissolved in an acidic wet environment containing hydrogen sulfide to increase the corrosion rate, and on the contrary, the SSC resistance is lowered. Therefore, the content of Cr is set to 0.1 to 1.2%. Note that a preferable upper limit of the Cr content is 0.5%.

Mo: 0.1 to 1% Mo is an important element in the present invention together with V and Nb, and improves the hardenability, increases the tempering softening resistance, enables high-temperature tempering, and improves the SSC resistance. Has the effect of improving However, if the content is less than 0.1%, the above effects cannot be obtained. On the other hand, when the content exceeds 1%, needle-like Mo carbides precipitate by tempering, diffusible hydrogen is trapped, the concentration of occluded hydrogen is increased, and the SSC resistance is rather lowered due to the concentration of stress around it. Therefore, the content of Mo is set to 0.1 to 1%.

Al: 0.005 to 0.1% Al is an element necessary for deoxidizing steel. However, if the content is less than 0.005%, the effect cannot be sufficiently obtained. On the other hand, if the content exceeds 0.1%, coarse Al-based inclusions such as Al ₂ O ₃ increase, and the toughness and SSC resistance decrease. Therefore, the content of Al is 0.1.
005 to 0.1%. A desirable range of the Al content is 0.01 to 0.05%. Note that, in the present invention, Al
Is so-called “sol. Al (acid-soluble Al)”.

B: 0.0001 to 0.01% B has an effect of improving the hardenability of steel even in a trace amount.
However, if the content is less than 0.0001%, the effect is not sufficient, and if it exceeds 0.01%, the toughness and SSC resistance are reduced.
The content of B is 0.0001 to 0.0
1%. The desirable range of the B content is 0.0
002 to 0.002%.

Nb: 0.005 to 0.5% Nb is an element which is effective for grain refinement by a pinning effect as an undissolved carbide in ordinary quenching and tempering heat treatment.
Further, if the solid solution is completely dissolved at the time of quenching by the direct quenching method, it can be used for tempering softening resistance and SSC resistance can be improved. To obtain this effect, Nb should be 0.005
% Or more. On the other hand, if the content exceeds 0.5%, coarse Nb carbides become trap sites for diffusible hydrogen and increase the amount of hydrogen occlusion, so that the SSC resistance decreases. Therefore, the content of Nb is 0.005 to
0.5%.

V: 0.005 to 0.5% V precipitates as fine carbides at the time of tempering to increase tempering softening resistance and enables high-temperature tempering, thereby improving S resistance.
It has the effect of improving SC properties. To ensure this effect, the content of V is set to 0.005% or more. On the other hand, V
If the content exceeds 0.5%, V carbides are coarsened and do not contribute to strengthening. In addition, the coarse carbides serve as trap sites for diffusible hydrogen and increase the amount of hydrogen occlusion, thereby deteriorating the SSC resistance. I do. Therefore, the content of V is set to 0.005 to 0.5%.

The contents of Mo, V and Nb must satisfy the following formulas (1) and (2) according to the heat treatment method and the target range of YS.

Ti and Zr: One or two kinds are each 0.1% or less, and Ti + 0.5 Zr: 0.005 to
0.15% Ti and Zr respectively convert N, which is an impurity in steel, into TiN
Or for the purpose of fixing as ZrN. Further, Ti or Zr in excess of the amount required for fixing N becomes carbide and is finely precipitated, and has an effect of increasing tempering softening resistance. The reason why N needs to be fixed is to prevent B added to improve quenchability from becoming BN, and to maintain B in a solid solution state to secure sufficient quenchability.

Such an effect can be surely obtained when the value of Ti + 0.5Zr is 0.005% or more with respect to the contents of Ti and Zr. However, at the value of Ti + 0.5Zr, the value of 0.1.
Even if more than 15% of Ti and Zr are added, the above-mentioned effect is saturated, so that the cost increases, and further, coarse carbides increase, and toughness and SSC resistance may decrease.

The value of Ti + 0.5Zr is 0.005
Since it only needs to be 0.15%, Ti
And Zr need not be compounded. When Zr is not added, that is, when Ti is added alone, if its content exceeds 0.1%, carbides increase, resulting in toughness and SSC resistance.
Is reduced. Conversely, when Ti is not added, that is, when Zr is added alone, if Zr is contained in an amount exceeding 0.1%, carbides increase and toughness and SSC resistance decrease.

W: 0 to 1% W is contained if necessary. If W is contained, it has the effect of improving the hardenability, increasing the tempering softening resistance, being advantageous for high temperature tempering, and improving the SSC resistance. .
In order to ensure the above-mentioned effect, the content of W should be set to 0.1.
It is preferable to set it to 3% or more. However, when the content exceeds 1%, the precipitated carbides are coarsened, and the above-mentioned effect is saturated or reduced. In addition, the coarsened carbides serve as trap sites for diffusible hydrogen, and the SSC resistance is lowered. I do. Therefore, when W is contained, the upper limit is set to 1%. Note that a preferable upper limit of the W content is 0.7%.
It is.

Ca: 0 to 0.01% Ca is contained as necessary, and combines with S in steel to form a sulfide, thereby improving the shape of inclusions and improving SSC resistance. Therefore, when it is desired to secure the above effects, Ca may be contained. In order to surely obtain the above-mentioned effects, the content is preferably 0.0001% or more. However, when the content exceeds 0.01%, not only does the SSC resistance decrease, but also the toughness decreases, and defects such as ground flaws easily occur on the steel material surface.
Therefore, when Ca is contained, the upper limit is 0.01%.
And

P: not more than 0.025% P is inevitably present in steel as an impurity, but segregates at grain boundaries and degrades SSC resistance. In particular, if the content exceeds 0.025%, the deterioration of SSC resistance becomes remarkable. Therefore, its content needs to be 0.025% or less. It is desirable that the P content be as low as possible in order to increase the SSC resistance.

S: 0.01% or less S is inevitably present in steel as an impurity like P, but it is resistant to segregation at grain boundaries and generation of a large amount of sulfide-based inclusions. SSC property will be reduced.
In particular, if the content exceeds 0.01%, the decrease in SSC resistance becomes significant. Therefore, its content is 0.01
% Or less. It is desirable that the content of S be as low as possible in order to increase the SSC resistance.

Ni: 0.1% or less Ni is present as an impurity in steel, and deteriorates SSC resistance in steel having a chemical composition range specified in the present invention. In particular, when the Ni content exceeds 0.1%, the SSC resistance is significantly reduced. Therefore, the content of Ni is 0.1%
It was as follows. In addition, when Ni is inevitably contained in the Cr raw material, it is industrially extremely difficult to reduce the Ni content to 0 (zero) when Cr is contained,
It is desirable to minimize it.

N: 0.01% or less N is present in steel as an impurity and segregates at grain boundaries to lower toughness and SSC resistance. However, when its content is 0.1.
The content of N is set to 0.01% or less because the content of N is acceptable if it is 01% or less. It should be noted that N is dissolved in the steel from the atmosphere and the like during smelting. Therefore, it is extremely difficult industrially to make the content 0 (zero), but it is desirable to reduce the content as much as possible.

O (oxygen): 0.01% or less O is present as an impurity in steel and segregates at grain boundaries to lower toughness and SSC resistance. However, when its content is 0.1.
Since the content of O is acceptable if it is 01% or less, the content of O is set to 0.01% or less. Since O is dissolved in the steel from the atmosphere during the smelting, it is extremely difficult industrially to make the content 0 (zero), but it is desirable to reduce the content as much as possible.

Next, with respect to Mo, Nb and V, it is necessary to satisfy the following equations (1) and (2) according to the heat treatment method and the target range of YS as described above.

In the case of ordinary quenching and tempering heat treatment, 2Mo + 5V ≦ B × C−A + 1.4 (1) where A: 4930e ^(−0.0133D) B: 1.07 × 10 ⁻⁸ e ^{( 0.0187D)} D: E-100e ^{(-0.52Mo-5.15V + 0.4)} C, E: When producing steel having a yield stress of 110 or more and 125 ksi or less C: 150, E: 770 Yield stress exceeds 125 and 140 ksi When producing the following steel materials: C: 190, E: 755 When the yield stress exceeds 140 and when producing steel materials of 155 ksi or less C: 270, E: 740 e: The base of the natural logarithm Direct quenching 2Mo + 4Nb + 5V ≦ B × C −A + 1.4 (2) where A: 4930e ^(−0.0133D) B: 1.07 × 10 ⁻⁸ e ^(0.0187D) D: E−100e ^{(−0.52Mo-9.36Nb) -5.15V + 0.4)} C, E: yield stress of 110 or more, 125 ksi When producing the lower steel material: C: 150, E: 770 When producing a steel material with a yield stress exceeding 125 and 140 ksi or less C: 190, E: 755 When producing a steel material with a yield stress exceeding 140 and 155 ksi or less C : 270, E: 740 e: base of natural logarithm The SSC resistance performance was improved by the synergistic effect of the reduction of the stored hydrogen concentration and the high-temperature tempering as described above, and the index value of the following equation obtained by repeating various tests ( Hereinafter, this is referred to as an SSC resistance index).

SSC resistance index = (hydrogen storage concentration) /1.07×
10 ^-8 e ^(0.0187D) D indicates ° C. at the annealing temperature. Further, the stored hydrogen concentration can be measured by various measuring methods, and the unit is ppm.

The value of e ^(0.0187D) decreases as the tempering temperature D decreases. That is, as the temperature becomes higher, the dislocations disappear due to the diffusion movement, the internal strain is eliminated, and the diffusible hydrogen hardly collects at such dislocations.

FIG. 1 shows that YS is 140 ksi class (140 to 140 ksi class).
It is a figure which shows the relationship between SSC resistance index and SSC resistance of 155 ksi) steel.

The horizontal axis is the value (SSC resistance index) obtained by dividing the measured value of the stored hydrogen concentration in various steel types by 1.07 × 10 ⁻⁸ e ^(0.0187D) obtained by the above equation. On the other hand, the vertical axis is the ratio of the breaking limit stress to the actually measured YS by the constant load test according to the NACE TM0177A method described later. It can be seen that the SSC resistance can be improved by reducing the SSC resistance index. That is, the SSC resistance index is (storage hydrogen concentration) /
Since it is 1.07 × 10 ⁻⁸ e ^(0.0187D), it is preferable to use a steel of a component system in which the occluded hydrogen concentration becomes small.

The stored hydrogen concentration in the hydrogen sulfide environment is Mo,
Depending on the contents of V and Nb, they can be expressed by the following formulas (4) and (5) depending on the case of ordinary quenching and tempering heat treatment and the case of direct quenching and tempering.

In the case of normal quenching and tempering heat treatment, the absorbed hydrogen concentration = A + 2Mo + 5V-1.4 (4) In the case of direct quenching The absorbed hydrogen concentration = A + 2Mo + 4Nb + 5V-1.4 (5) A: 4930e ^{( -0.0133D)} D: Tempering temperature (° C.) A in the first term corresponds to the concentration of hydrogen trapped by internal strain, and is reduced with high-temperature tempering. On the other hand, the second and subsequent terms correspond to temper softening resistance, that is, the concentration of hydrogen trapped at the interface of fine carbides that contribute to precipitation strengthening. In the case of the direct quenching process, Nb contributes to precipitation strengthening.
It is necessary to consider the hydrogen storage effect of Nb.

The tempering temperature is affected by each alloy element for each strength, and can be expressed by the following equation obtained by regression calculation.

In the case of ordinary quenching and tempering heat treatment, tempering temperature, D (° C.) = E−100e
^{(-1.5Mo-5.15V + 0.4) In} case of direct quenching Tempering temperature, D (℃) = E-100e
^{(-1.5Mo-9.36Nb-5.15V + 0.4)} where E is 110 to 125 ksi when producing steel material E = 770 When 125 or more and 140 ksi or less steel material is produced E = 755 140 or more and 155 ksi When the following steel material is manufactured: E = 740 Accordingly, the SSC resistance index can be expressed by the following equation according to the range of YS, and is affected by the contents of Mo, V, and Nb.

(1) In the case of ordinary quenching and tempering heat treatment SSC resistance index = (A + 2Mo + 5V-1.4) / B A = 4930e ^(−0.0133D) B = 1.07 × 10 ⁻⁸ e ^(0.0187D) D = E-100e ^{(-0.52Mo-5.15V + 0.4)} (tempering temperature, ° C) E is as described above.

(2) In case of direct quenching SSC resistance index = (A + 2Mo + 4Nb + 5V-1.4)
/ B A = 4930e ^(−0.0133D) B = 1.07 × 10 ⁻⁸ e ^(0.0187D) D = E−100e ^{(−0.52Mo-9.36Nb-5.15V + 0.4)} E is as described above. .

If a component system capable of reducing the stored hydrogen concentration is selected, the numerator of the above formula can be reduced. On the other hand, if a component system capable of realizing high-temperature tempering is selected, the denominator is increased and the internal strain is reduced. And the SSC resistance can be improved by either method. However, the stored hydrogen concentration and the tempering temperature are not independent but closely correlated, and an optimum component balance actually exists.

FIG. 2 shows that YS is obtained by ordinary quenching and tempering heat treatment.
FIG. 6 is a diagram showing a change in the SSC resistance index when the content of Mo and V is changed in a case where is more than 140 ksi.

It can be seen that there is an optimum combination range of Mo and V for improving the SSC resistance, and a component system that makes the SSC resistance index equal to or less than a certain value should be selected to satisfy the desired SSC resistance. As a result of various tests, this value is 150 when YS is 110 class (110 to 125 ksi) and 1 when YS is 125 class (125 to 140 ksi).
It was 270 for 90 and 140 grades (140 to 155 ksi). Furthermore, excessive addition of Mo and V increased the effect of increasing the storage of hydrogen concentration, toward the direction of reducing rather the SSC resistance.

Accordingly, if each of the target SSC resistance indices is C, the component system should satisfy the following equation.

(A + 2Mo + 5V-1.4) /B.ltoreq.C, ie, 2Mo + 5V.ltoreq.B.times.C-A + 1.4, in the case of normal quenching and tempering heat treatment, (A + 2Mo + 4Nb + 5V-1.4) / B ≦ C That is, 2Mo + 4Nb + 5V ≦ B × C−A + 1.4Nb is the most effective element for increasing the SSC resistance without increasing the stored hydrogen concentration. However, in order to effectively utilize the tempering softening resistance, it is essential that the solid solution be made once in quenching by direct quenching, and it cannot be applied to ordinary quenching and tempering treatment.

If the chemical components are optimized according to the above formula, the refinement of the crystal grains is not always necessary for improving the SSC resistance. In the direct quenching method, coarsening of crystal grains is generally a problem, but if the optimum component range is satisfied, desired SSC resistance is satisfied even with coarse grains. The reason is SS
This is because the fracture mode of C is an intragranular fracture type unlike the conventional low-temperature tempered steel. Therefore, in the present invention, the SSC resistance is determined mainly by only the component system without being affected by the heat treatment and the working degree at the time of production. Of course, even if an attempt is made to reduce the grain size by means of, for example, performing heat treatment several times, the SSC resistance is not reduced and there is no problem. For example, if a certain degree of toughness is to be ensured, it is desirable to avoid excessive coarsening.

(B) Heat treatment conditions (B-1) Quenching conditions After the steel having the above chemical composition is melted by a usual method, for example, hot forging, piercing, rolling, etc. by a normal method. After being formed into a steel material of a predetermined shape such as a steel pipe or a steel plate by hot working, the steel material is quenched and tempered to adjust to a desired strength.

Hot working such as forging, piercing and rolling may be performed under ordinary conditions. For example, a steel ingot, billet or slab is heated to a temperature in a temperature range of 1000 to 1250 ° C. 900 from the point of prevention
It is desirable to finish a steel material of a predetermined shape such as a steel pipe or a steel plate at a temperature of １1100 ° C.

The quenching temperature and the holding time are not particularly limited. However, when it is desired to secure toughness, it is preferable to perform quenching heat treatment at a temperature of 850 ° C. to 1000 ° C. from the viewpoint of avoiding coarsening.

The quenching method need not be particularly limited. An ordinary quenching method such as oil quenching or water quenching may be appropriately used according to the chemical composition of the steel material. That is, for example, the quenching method may be determined so as to have a sufficient quenched structure (for example, a structure in which martensite is about 80% or more) based on a result of a preliminary investigation according to the chemical composition of the steel material in advance. .

A direct quenching method of quenching immediately after hot rolling may be applied. The heating temperature and the working ratio at this time are not particularly limited.
Heated to a temperature of 000 ° C to 1250 ° C;
It is desirable to finish at a temperature of 0 ° C. There is no particular limitation on the quenching method. Further, normal quenching and tempering may be performed after direct quenching or direct quenching and tempering. In this case, since the precipitation softening resistance of Nb is not effective, it becomes the same as the case of the normal quenching and tempering treatment.

(B-2) Tempering Conditions The tempering method is not particularly defined. Although the tempering time is not particularly limited, it is generally preferable to hold the tempering for 10 minutes or more from the viewpoint of performing a uniform heat treatment on the entire steel material. The cooling method after tempering is not particularly limited, and may be performed by a normal cooling method such as standing cooling, air cooling, mist water cooling, or water cooling.

The method for measuring the stored hydrogen concentration is not particularly specified, and NACE TM0177A described later is not specified.
Any method can be used as long as the concentration of hydrogen absorbed in the test solution of the method can be accurately compared between steel materials. For example, a method is generally used in which a steel material is immersed in a test solution for 24 hours or more, taken out, immersed in a glycerin solution at 45 ° C. for 72 hours or more, and the concentration of released hydrogen is measured.

In addition, a recently used simple method of elevated temperature hydrogen analysis may be used. In this method, a steel material is immersed in a test liquid for 24 hours or more, taken out, raised at a constant speed from room temperature, and analyzed for the amount of hydrogen released. In this case, it is said that all of the diffusible hydrogen is released at 400 ° C. or lower, and the amount of hydrogen may be measured and compared.

[0073]

EXAMPLES A 150 kg steel ingot having the chemical composition shown in Tables 1 to 3 was melted by a conventional method using a vacuum melting furnace.

[0074]

[Table 1]

[0075]

[Table 2]

[0076]

[Table 3]

These ingots were heated to 1250 ° C. and then hot forged to form slabs having a thickness of 40 mm × 80 mm width × 250 mm length. This steel slab was quenched after hot rolling by the following methods (1) and (2), and heat treated for tempering to adjust the strength.

(1) The slab was heated to 1250 ° C. for 1 hour, hot rolled to a thickness of 14 mm at a rolling finish temperature of 950 ° C., and further ground to finish to a thickness of 12 mm.

The steel sheet having a thickness of 12 mm is heated at 850 ° C. to 95
After quenching by heating to a temperature range of 0 ° C.,
The tempering treatment was performed at various temperatures described in No. 7. The quenching was water quenching. "QT" described in the column of "Heat treatment method" in Tables 4 to 7 indicates this quenching and tempering treatment.

(2) The steel sheet was heated to a temperature of 1250 ° C., subjected to hot rolling, finished to a thickness of 14 mm, directly quenched, and then tempered at various temperatures listed in Tables 4 to 7. The quenching was water quenching. "DQT" in the column of "Heat treatment method" in Tables 4 to 7 indicates this quenching and tempering treatment.

[0081]

[Table 4]

[0082]

[Table 5]

[0083]

[Table 6]

[0084]

[Table 7]

A tensile test specimen was taken in parallel with the rolling direction from a 12 mm thick steel sheet after the hot rolling and heat treatment of (1) and (2) above, and a tensile test was conducted at room temperature (room temperature) to measure YS. did.

The diameter of the parallel portion of the steel sheet having a thickness of 12 mm after tempering is 6.
A round bar tensile test specimen having a length of 35 mm and a length of 25.4 mm was sampled and subjected to a SS-resistant method according to the NACETM0177A method.
The C property was evaluated.

That is, 2 saturated with 1 atm of hydrogen sulfide
A constant load test was performed in 0.5% acetic acid + 5% saline at 5 ° C. to evaluate SSC resistance. Change the applied stress to 72
80% maximum stress that did not break during the 0 hour test time
And those of the actual YS or higher were determined to have good SSC resistance.

After immersing for 24 hours in a 0.5% acetic acid + 5% saline solution at 25 ° C. saturated with hydrogen sulfide as described above, the sample was taken out, and the hydrogen concentration released at 400 ° C. or less was measured by a temperature-raising hydrogen analysis method. It was measured as a concentration.

The results of the various tests are shown in Tables 4 to 7.

As is clear from Tables 4 and 5, any steel having a chemical composition within the range specified by the present invention after adjusting the strength to the range of each YS has a ratio of 80% or more of the actually measured YS in the constant load test. It does not break even under applied stress, and has good SSC resistance.

On the other hand, when the value is out of the range specified in the present invention, the breaking limit stress in the constant load test is 8%.
It is less than 0% and is inferior in SSC resistance.

For example, when the strength of steel D in Table 1 was adjusted to C110 grade by ordinary quenching and tempering,
As shown in Test No. 4 in the table, the desired SSC resistance was satisfied, but Test No. 20 in Table 4 adjusted to C125 grade,
In Test No. 36 in Table 5 adjusted to C140 grade,
The desired SSC resistance is not satisfied because the balance between Mo and V is inappropriate. This D steel can be N if the direct quenching method is applied.
The precipitation strengthening of b can be effectively used, and good SSC resistance is exhibited even in C125 grade (test No. 28 in Table 5) and C140 grade (test No. 44 in Table 6).

The steels I and J in Table 1 have respective components within the scope of the present invention. However, as shown in Test Nos. 49 to 60 in Table 6, the SSC resistance index in any range of YS was as follows. The desired SSC resistance is not satisfied because the value does not satisfy the requirements of the present invention.

As shown in Test Nos. 61 to 87 in Tables 6 and 7, K steels to 11 steels in Tables 2 and 3 have unsatisfactory SSC resistance because each component is out of the range of the present invention. . Test Nos. 61 to 87 only describe examples in which YS was adjusted to 110 ksi class by direct quenching.
Needless to say, the sensitivity to SSC is further increased at 5 ksi class and 140 ksi class, and the SSC resistance becomes poor. In addition, the effect of tempering softening resistance due to Nb cannot be expected with normal QT, and it goes without saying that SSC resistance is poor.

[0095]

According to the manufacturing method of the present invention, YS is 11
Steel with excellent SSC resistance can be easily obtained even with a high strength of 0 to 155 ksi, and oil and gas well casings and tubing containing large amounts of hydrogen sulfide, drill pipes for drilling, drill pipes for transportation, It can be used for line pipes, and also piping for chemical plants, and has an extremely large industrial effect.

[Brief description of the drawings]

FIG. 1 is a diagram showing a correlation between an SSC resistance index and a critical breaking stress by a constant load test.

FIG. 2 is a diagram showing the correlation between the Mo and V content balance and the SSC resistance index.

Claims (2)

[Claims] (1) C: 0.2 to 0.35% by mass, S
i: 0.05 to 0.5%, Mn: 0.1 to 1%, Cr:
0.1 to 1.2%, Mo: 0.1 to 1%, Al: 0.0
05-0.1%, B: 0.0001-0.01%, N
b: 0.005 to 0.5%, V: 0.005 to 0.5
%, One or two of Ti and Zr are each 0.1%
% Or less, and Ti + 0.5Zr: 0.005 to 0.1
5%, W: 0-1%, Ca: 0-0.01%, the balance being Fe and unavoidable impurities.
0.025% or less, S: 0.01% or less, Ni: 0.1
%, N: 0.01% or less, O (oxygen): 0.01%
A steel having a Mo and V content satisfying the following formula (1) is hot-worked, and then subjected to heat treatment of quenching and tempering, and has a yield stress of 110 to 155 ksi. For producing high-strength steel with excellent heat resistance. 2Mo + 5V ≦ B × C−A + 1.4 (1) where A: 4930e ^(−0.0133D) B: 1.07 × 10 ⁻⁸ e ^(0.0187D) D: E−100e ^{(− 0.52Mo-5.15V + 0.4)} C, E: When producing a steel material with a yield stress of 110 or more and 125 ksi or less C: 150, E: 770 When producing a steel material with a yield stress exceeding 125 and 140 ksi or less C: 190 , E: 755 In the case of producing a steel material having a yield stress exceeding 140 and 155 ksi or less C: 270, E: 740 e: Base of natural logarithm Mo, V are contents (% by mass) 2. In mass%, C: 0.2 to 0.35%, S
i: 0.05 to 0.5%, Mn: 0.1 to 1%, Cr:
0.1 to 1.2%, Mo: 0.1 to 1%, Al: 0.0
05-0.1%, B: 0.0001-0.01%, N
b: 0.005 to 0.5%, V: 0.005 to 0.5
%, One or two of Ti and Zr are each 0.1%
% Or less, and Ti + 0.5Zr: 0.005 to 0.1
5%, W: 0-1%, Ca: 0-0.01%, the balance being Fe and unavoidable impurities.
0.025% or less, S: 0.01% or less, Ni: 0.1
%, N: 0.01% or less, O (oxygen): 0.01%
And the content of Mo, V and Nb is the following formula (2)
Is characterized by hot-working steel satisfying the following conditions, directly quenching after hot-working, and then tempering.
A method for producing a high-strength steel material having a yield stress of 0 to 155 ksi and excellent in sulfide stress cracking resistance. 2Mo + 4Nb + 5V ≦ B × C−A + 1.4 (2) where A: 4930e ^(−0.0133D) B: 1.07 × 10 ⁻⁸ e ^(0.0187D) D: E−100e ^{(− 0.52Mo-9.36Nb-5.15V + 0.4)} C, E: when producing steel materials with a yield stress of 110 or more and 125 ksi or less C: 150, E: 770 when producing steel materials with a yield stress exceeding 125 and 140 ksi or less C: 190, E: 755 When a steel material with a yield stress exceeding 140 and 155 ksi or less is manufactured. C: 270, E: 740 e: Base of natural logarithm Mo, V and Nb are contents (% by mass).