JPH09249935A - High strength steel material excellent in sulfide stress cracking resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel material (particularly a seamless steel tube) com posed of inexpensive low alloy steel and having high strength and excellent sulfide stress cracking resistance and its production. SOLUTION: The high strength steel material excellent in sulfide stress cracking resistance is composed of a low alloy steel of 0.20-0.60% C (carbon) content and has a metallic structure consisting of 1-10% volume fraction of retained austenite and the balance essentially martensite. Particularly, proof stress in this steel material is >=77.3kgf/mm<2> , and further, the cracking critical stress (σ th) in the NACE TM 0177 bath is regulated to a value not lower than actual yield stress. As to the method of manufacturing this steel material, a low alloy steel of 0.20-0.60% C (carbon) content is hot-worked, hardened, and then tempered at a temp. exceeding the Ac1 transformation point. In this method, it is preferable that hardening is carried out by means of direct hardening from hot working and tempering is performed at a temp. in the region between a temp. higher than the Ac1 transformation point and (Ac1 transformation point + 30 deg.C).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel having a high strength and excellent sulfide stress cracking resistance, particularly, a metallographic structure different from that of a conventional low alloy steel, and is excellent in sulfide stress cracking resistance. The present invention relates to a remarkably excellent steel material and a manufacturing method thereof. The steel material of the present invention is particularly suitable as a seamless steel pipe used for collecting and transporting crude oil, natural gas and the like under a corrosive environment containing sulfide.

[0002]

2. Description of the Related Art Heretofore, a martensitic structure has been considered as a metallographic structure of a low alloy high strength steel required to have resistance to sulfide stress cracking. For example, JP-A-52-66815
In the invention disclosed in the publication, the martensite structure ratio during quenching is specified to be 90% or more. In addition, JP-A-5-
The invention of the "method for producing oil country tubular goods" proposed in Japanese Patent No. 271772 is also characterized in that the martensite ratio is 90% or more by performing tempering treatment at an Ac ₁ transformation point or lower. In addition,
In such steels, the structure other than martensite is mostly bainite.

Further, in order to further improve the sulfide stress cracking resistance, means such as grain refinement, direct quenching, and component improvement have been studied. For example, in Japanese Patent Laid-Open No. 54-117311,
A method is disclosed in which the austenite grain size before quenching is made fine by defining the heating rate. Further, JP-A-62-30846 discloses the shape of austenite grains after hot rolling and direct quenching after hot rolling.
Further, JP-A-55-69246 discloses an invention in which Cu and W are added, and JP-A-62-253720 discloses an invention in which the contents of Mn, P and Mo are specified.

All of these steels are tempered below the Ac ₁ transformation point. That is, it is based on the premise that austenite is not formed during tempering and residual austenite is not generated after tempering. Conventionally, the formation of retained austenite has been avoided as much as possible because the following points have been a concern.

When retained austenite is formed, the strength of ordinary quench-tempered materials is significantly reduced, and a predetermined strength cannot be obtained.

[0006] The retained austenite itself corrodes and becomes a starting point of sulfide stress cracking, and the retained austenite acts as a hydrogen trap site to increase the stored hydrogen concentration.

On the other hand, Japanese Patent Application Laid-Open No. 58-161720 discloses a method of increasing the resistance to sulfide stress cracking by using bainite as a metal structure. Also, the Japan Institute of Metals, Bulletin, 21st
Vol. 16, No. 16 (1982), p. 441, it is reported that, depending on the type of bainite structure, it may be superior in sulfide stress cracking resistance to martensite structure.

With the above-developed technology, for example, in the case of a high strength low alloy oil country tubular good which requires sulfide stress crack resistance, C110 grade [YS ≧ 110 ksi (77.3 kgf
/ mm ² )]. Currently C125 grade [YS
≥125 ksi (87.9 kgf / mm ² )] or higher high strength and high corrosion resistance low alloy oil country tubular goods are desired, but conventional steel mainly composed of tempered martensite structure and high strength exceeding C110 grade, It is difficult to provide excellent stress corrosion cracking resistance.

The international standard for oil country tubular goods is API (American Petroleum Institute) 5CT, but the C110 class is not standardized therein. In the standard, "C" means corrosion resistant grade, but C90 of YS ≧ 90ksi (63.3kgf / mm ² ) or more
It is only standardized up to this point, and C110 class and above are handled as the standard of each oil country tubular manufacturer. However, here, for convenience of explanation, C110, C125, C140 and the like are used to indicate the strength level of the steel pipe.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to provide an inexpensive low alloy steel material having high strength and excellent resistance to sulfide stress cracking, and for example, a joint for oil country tubular goods. An object of the present invention is to provide a method for manufacturing the above-mentioned steel material, which is also suitable for manufacturing a steel pipe without a steel pipe.

More specifically, for example, yield strength (YS)
Is 77.3 kgf / mm ² (110 ksi) or higher, that is,
Above C110 grade, NACE TM 0177 bath (0.5% acetic acid +
To provide a high-strength steel material having a sulfide stress cracking resistance of which cracking limit stress (σth) in 5% sodium chloride, 1 atm hydrogen sulfide saturation, 25 ° C) is 80% or more of proof stress, and an inexpensive manufacturing method thereof. It is in.

[0012]

The gist of the present invention is the following steel material (1) and its manufacturing method (2).

(1) A low alloy steel having a C (carbon) content of 0.20 to 0.60% and a residual austenite volume fraction of 1
High strength steel material with excellent sulfide stress cracking resistance characterized by having a metallographic structure consisting mainly of martensite at ~ 10%, especially with a proof stress of 77.3 kgf / mm ² or more, and
NACE TM 0177 High-strength steel with excellent sulfide stress cracking resistance with a critical crack initiation stress (σth) in the bath of 80% or more of the yield strength.

(2) A low alloy steel having a C (carbon) content of 0.20 to 0.60% is hot worked, quenched, and then tempered at a temperature exceeding the Ac ₁ transformation point. 1) A method for producing a high-strength steel material having excellent sulfide stress cracking resistance.

In the above method (2), it is desirable that the quenching is performed by a direct quenching method from hot working, and the tempering is performed in a temperature range exceeding the Ac ₁ transformation point to "Ac ₁ transformation point + 30 ° C". Further, a step of supplementing heat at the Ac ₃ transformation point or higher before direct quenching and / or a step of “reheating-quenching” between direct quenching and tempering may be added.

The low alloy steel to which the present invention is applied can contain not only the C content of 0.20 to 0.60% but also various alloy elements in combination. Preferred low alloy steels include those having the following chemical composition:

% By weight, C: 0.20 to 0.60%, Si: 0.05 to 3.0%, Mn: 0.05 to 1.0%, P: 0.025% or less, S: 0.01% or less, sol.Al: 0.005 to 0.50%, Cr : 0.1 to 1.5%, Mo: 0.1 to 1.5%, Nb: 0.005 to 0.5%, Ti: 0.005 to 0.5%, V: 0 to 0.5%, B: 0 to 0.01%, W: 0 to 1.0%, Zr: 0-0.5%, Ca: 0-0.01%, Ni: 0.1% or less, N: 0.01% or less, O: 0.01% or less, Fe and unavoidable impurities: balance

[0018]

DETAILED DESCRIPTION OF THE INVENTION As described above, in low alloy steel materials, the presence of retained austenite has conventionally been considered to cause a reduction in strength and a reduction in sulfide stress cracking resistance. But,
Austenite itself has extremely low susceptibility to sulfide stress cracking. For example, it is well known that a steel material having an austenite single-phase structure has low susceptibility to hydrogen embrittlement. Therefore, even in the case of a low alloy steel mainly composed of martensitic structure, if a proper amount of retained austenite is present, it may be useful for improving the sulfide stress crack resistance.

Based on such an estimation, the present inventor conducted a detailed study from the viewpoint of the metallographic structure and confirmed that the presence of an appropriate amount of retained austenite rather improved the sulfide stress cracking resistance. . Moreover, it was found that the retained austenite is preferentially generated in the old austenite grain boundary, and as a result, the grain boundary strength is increased and the strength of the steel material is improved. The present invention is based on such a new finding.

Sulfide stress cracking of high-strength steel of the quenching-tempering type originates from the former austenite grain boundary crack (obviously, the "old" of the former austenite is to indicate that it is after the martensitic transformation). Things). The conventional methods that have been adopted to prevent the former austenite grain boundary cracking are grain refining by induction heating and addition of grain refining elements such as Ni, and reduction of grain boundary segregation elements such as P and S. As a result, these are understood to be methods for increasing the grain boundary strength.

In addition to these methods, the method of grain boundary strengthening newly investigated by the present inventors is utilization of retained austenite. That is, it is a method of preventing sulfide stress cracking in the austenite grain boundaries by precipitating austenite in the austenite grain boundaries by utilizing the low hydrogen embrittlement susceptibility of austenite. More specifically, the idea is to use austenite, which has a high resistance to sulfide stress cracking, as a barrier to prevent the progress of sulfide stress cracking in the former austenite grain boundaries.

As described above, since the sulfide stress cracking of such high strength steel originates from the grain boundary cracking, the prevention of the grain boundary cracking leads to the improvement of the sulfide stress cracking resistance.

One of the methods for precipitating and retaining austenite at the former austenite grain boundaries will be described later.
It is tempering at a temperature above the Ac ₁ transformation point. The reason why austenite is preferentially precipitated in the former austenite grain boundaries by this tempering is that the diffusion of elements in the grain boundaries is fast. Fortunately, during precipitation of austenite, grain boundary segregation elements such as Mn are incorporated into the austenite phase, so the former austenite grain boundaries that remain as a tempered martensite structure are cleaned, and this also contributes to sulfide stress stress cracking. Sex is improved. The impurity element taken into the retained austenite does not substantially pose a problem because the susceptibility of austenite itself to sulfide stress cracking is low.

Thus, contrary to the conventional wisdom that retained austenite is not desirable for sulfide stress cracking resistance, in high strength steel, retained austenite is precipitated at the former austenite grain boundary to clean the grain boundary itself. , It has become possible to dramatically improve sulfide stress cracking resistance by preventing the development of sulfide stress cracking at the former austenite grain boundaries.

Although it has been reported that retained austenite can be used as a trap for hydrogen, the present inventor cannot agree with such a view. This is because, in a wet hydrogen sulfide environment, hydrogen that has penetrated into the steel by corrosion is diffusible hydrogen that causes hydrogen embrittlement at the time of penetration. That is, sulfide stress cracking, which is hydrogen embrittlement, is caused before trapped by retained austenite as non-diffusible hydrogen that does not contribute to hydrogen embrittlement. In particular, in the wet hydrogen sulfide environment, the maximum level of hydrogen invades compared with other corrosive environments, so it can be said that the effect of residual austenite as a hydrogen trap is not sufficient at all.

The excellent resistance to sulfide stress cracking of the steel material of the present invention is due to the action of residual austenite to clean the grain boundaries themselves and prevent the development of sulfide stress cracks at the former austenite grain boundaries, and the retained austenite precipitates. It is to eliminate the internal strain of the martensite structure due to moderate high temperature tempering.

The amount of retained austenite in the steel material of the present invention is
It is necessary that the volume fraction is 1% or more. If it is less than this, the above effect is not exhibited. It should be noted that even if the volume fraction is 1%, it preferentially precipitates at the grain boundaries during tempering, so the volume occupancy rate of the retained austenite at the former austenite grain boundaries becomes extremely high. On the other hand, if the retained austenite exceeds 10% in volume fraction, the strength is significantly reduced.

The structure of the steel material of the present invention other than the retained austenite is mainly martensite (tempered martensite) in order to secure the strength. A structure such as bainite or ferrite may be mixed up to a volume fraction of about 20%.

There are various chemical compositions of the raw material steel for imparting the metal structure as described above and making the steel material excellent in strength and sulfide stress cracking resistance. Examples of the composition include various alloy components. Hereinafter, the action of the alloying element and the desired content will be described. % With respect to the content of alloy components means% by weight.

I. Desirable Chemical Composition C: C is an essential element for enhancing hardenability and strength. If it is less than 0.20%, the hardenability is insufficient and high strength cannot be obtained. On the other hand, if it exceeds 0.60%, the susceptibility to quench cracking increases. Therefore, the proper range of C content is 0.20-
Although it is 0.60%, the desirable upper limit is 0.40% or less.

Si: Si is an element useful for deoxidizing steel,
It is also an element that enhances temper softening resistance and improves sulfide stress cracking resistance. It also has the effect of stabilizing the austenite that precipitates during tempering. For the purpose of deoxidation, 0.05
% Or more content is desirable. Furthermore, from the viewpoint of stabilizing retained austenite, 1.5% or more is desirable. However, if it exceeds 3.0%, the toughness of the steel material decreases.

Mn: Mn is also an element useful for deoxidizing steel. For the purpose of deoxidation, 0.05% or more is desirable, and like Si, it also has a function of stabilizing austenite precipitated during tempering. On the other hand, if Mn exceeds 1.0%, the toughness decreases, so the desirable content of Mn is 0.05 to 1.0%. The upper limit of Mn is more preferably 0.5%.

P: P is inevitably present in the steel as an impurity, but if it exceeds 0.025%, it segregates at the grain boundaries and particularly reduces the sulfide stress cracking resistance of high strength steel, so it is mixed as an impurity. However, it should be kept below 0.025%. Of course, the lower the content, the more desirable.

S: S, like P, is inevitably present in the steel as an impurity. If S exceeds 0.01%, segregation occurs at grain boundaries, and particularly in high strength steel, sulfide stress cracking resistance decreases. Further, sulfide-based inclusions are generated to reduce sulfide stress cracking resistance. Therefore, even if it is mixed as an impurity, it is preferably 0.01% or less and kept as low as possible.

Cr: Cr enhances the hardenability of steel to increase the strength and also the resistance to sulfide stress cracking.
Assuming C125-140 grade, if it is less than 0.3%, the required effect of improving hardenability cannot be obtained. On the other hand, if Cr exceeds 1.5%, in a hydrogen sulfide-containing environment, the corrosion rate increases and the stored hydrogen concentration increases, which deteriorates the sulfide stress cracking resistance. Therefore, the proper content of Cr is 0.1
˜1.5%, and more preferably 0.3 to 1.2%.

Mo: Mo, like Cr, improves the hardenability of steel and contributes to higher strength, and also enhances temper softening resistance to improve sulfide stress cracking resistance. For C140 grade, less than 0.2% is insufficient. On the other hand, when Mo exceeds 1.5%, needle-like Mo carbide, which has a high stress concentration factor and is a starting point of stress corrosion cracking, is deposited, so that the sulfide stress cracking resistance is deteriorated. Therefore, the proper Mo content is 0.1-1.5%.
And preferably 0.2 to 1.0%.

V: V need not be added, but if added, it increases the temper softening resistance of the steel and contributes to grain refinement to improve sulfide stress cracking resistance. The Mo-added material has sufficient resistance to sulfide stress cracking even if V is not added, but addition of V further improves resistance to sulfide stress cracking. In particular, it is effective for further exerting the effect of improving the tempering softening resistance by direct quenching. When the direct quenching-tempering process is applied to a steel containing V, a steel having a high strength of 98.4 kgf / mm ² (140 ksi) or more and having an extremely high temper softening resistance can be obtained even after high temperature tempering.

On the other hand, if the V content exceeds 0.5%, not only the effect is saturated, but also segregation deteriorates the sulfide stress cracking resistance. Therefore, even when V is used, the upper limit of the content should be 0.5%. 0.1-0.3 is preferred
%.

Sol.Al: Al (aluminum) is an element effective for deoxidizing steel. The content as sol.Al is 0.005%
If less than, the effect is small. However, if it exceeds 0.50%, the inclusions in the steel increase and the toughness decreases. It should be noted that a seamless steel pipe for oil well pipes is often provided with a connecting screw at its pipe end, but if there is a large amount of Al, defects are likely to occur in the threaded portion. For the above reasons, the proper content of sol.Al is 0.005.
~ 0.50%.

Ti: Ti fixes N, which is an impurity in steel, as TiN. If Ti is contained in the steel, N is fixed as TiN, so that B (boron) described later does not precipitate as BN during direct quenching. Therefore, B exists in a state effective for hardenability and improves hardenability during direct quenching. Ti above N is fixed as TiN
It has the effect of expanding the non-recrystallization temperature range to high temperatures and partially accumulating processing strain at high temperatures. When performing "heat supplement" described later, if the supplement heat temperature is set to a low temperature and it takes a certain time,
Fine recrystallization can be obtained. As will be described later, the fixed time is 10 seconds to 30 minutes when the supplementary heating temperature is 850 to 1100 ° C. Further, solid solution Ti finely precipitates during tempering after direct quenching and improves temper softening resistance, so that it is possible to temper at high temperature together with Mo and V. 0.
If it is less than 005%, its effect is small, and if it exceeds 0.50%, the toughness deteriorates.
It should be 5 to 0.50%.

Nb: Nb has an effect of expanding the temperature range in which the working strain of the steel material is accumulated to the high temperature range, and is an element effective for holding at a low supplementary heating temperature to obtain fine crystal grains. Its effect is stronger than Ti. Further, Nb in the steel material directly quenched as a solid solution is finely precipitated during the subsequent tempering. Therefore, it increases the temper softening resistance of the steel together with Mo, W and Ti, and improves the sulfide stress cracking resistance. If the content is less than 0.005%, the effect is not remarkable, while on the other hand,
If it exceeds 5%, the toughness decreases. Therefore, the proper Nb content is 0.005 to 0.5%.

W: W is an element which, like Mo, improves the hardenability of steel and enables higher strength, and also increases the resistance to temper softening and improves the resistance to sulfide stress cracking.
Mo-containing steel has a sufficient temper softening resistance without adding W, but by adding W together with Mo, the sulfide stress cracking resistance can be further improved. However, if the W content exceeds 1.0%, not only will the effect be saturated, but
The segregation deteriorates the stress corrosion cracking resistance.

Zr: Zr-containing steel has an increased yield point elongation during a tensile test, resulting in improved sulfide stress cracking resistance. Zr is also an expensive element and has sufficient resistance to sulfide stress cracking even if it is not added. However, Zr further improves resistance to sulfide stress cracking. On the other hand, the Zr content is 0.5
If it exceeds%, the inclusions increase and the toughness decreases.

B (Boron): B is an element that improves the hardenability of steel in a small amount, and is an element that is particularly effective in improving the hardenability of thick-walled materials and enhancing the resistance to sulfide stress cracking. However, in a thin material, sufficient sulfide stress cracking resistance can be obtained without addition. When added, the content is preferably 0.0001% or more where the effect becomes remarkable. However, if it exceeds 0.010%, the toughness and resistance to sulfide stress cracking of steel deteriorate, so the upper limit of the content should be 0.010%.

Ca: Ca reacts with S in steel to form sulfides, thereby improving the shape of inclusions and improving sulfide stress cracking resistance. The degree of the effect varies depending on the content of S, and if the deoxidation is not sufficient, the resistance to sulfide stress cracking may decrease. Therefore,
It is an element that can be appropriately selected whether to add. When added, its content is preferably 0.001 to 0.01%. This is because if it is less than 0.001%, the effect is not clearly exhibited, and if it is contained excessively, the toughness and sulfide stress cracking resistance are deteriorated and the surface defects of the steel material are caused.

Ni: From the viewpoint of retained austenite formation, a steel containing Ni is often studied. However, in an environment containing hydrogen sulfide, in a low alloy steel containing Ni, sulfide-resistant stress cracking due to pitting corrosion is caused. Is significantly degraded. Therefore, even if it is contained as an impurity, it should be kept to 0.1% or less.

N (Nitrogen): N is present as an impurity in steel and reduces toughness and sulfide stress cracking resistance.
It was set to 0.01% or less. Due to refining technology, N cannot be set to 0, but it is better to reduce N as much as possible.

O (oxygen): O is also unavoidably present in steel as an impurity, and reduces toughness and sulfide stress cracking resistance.
It is desirable to keep the upper limit of 0.01% and keep it as low as possible.

II. Method for Manufacturing Steel of the Present Invention In the steel of the present invention, retained austenite has a volume fraction of 1 to 10
The main feature is to have a% organization. One of the methods of producing a high-strength steel material having such a structure is
The method of (2) above. Hereinafter, the method will be described in the order of steps.

1. Melting, casting and hot working Melting and casting may be carried out by an ordinary method for producing a low alloy steel material. However, refining is performed to minimize impurities such as P, S, Ni, N, and O described above. Casting may be ingot casting or continuous casting.

2. Hot working (forging, piercing, rolling, etc.) After casting, hot working such as forging and rolling is performed. The slab obtained by continuous casting may be rolled into a plate as it is, and in the production of a seamless steel pipe, the billet obtained by continuous casting may be punched as it is.

In the case of a seamless steel pipe, rolling is performed using a mandrel mill or a plug mill after the above boring step. In the case of a plate material, the slab is roughly rolled and finish-rolled. Hereinafter, desirable processing conditions will be described by taking a method for manufacturing a seamless steel pipe as an example.

Billet Heating Temperature The heating temperature may be any temperature at which hot punching can be performed by a punching machine. The optimum temperature depends on the material, and is determined by taking hot ductility and high temperature strength into consideration. Usually, heating is performed between 1100 ° C and 1300 ° C. The heating method may be either a gas heating furnace or induction heating. However, it is preferable to use a non-oxidizing atmosphere because thick oxide scale causes surface scratches.
In order to realize highly efficient billet heating, it is preferable that the billet length be as long as possible, and a cutting machine may be installed at the exit of the heating furnace and then introduced into the punching machine.

Punching condition Punching is a process for manufacturing a basic shape of a steel pipe by hot forming a through hole in a solid billet. There are no restrictions on the piercing method such as the inclined rolling method and the press piercing method. Note that when the surface temperature decreases, scratches are likely to occur at the time of punching, so an auxiliary heating device, such as an induction heating device, may be installed immediately before the punching machine.

Final Rolling Conditions Final rolling or final finishing rolling as referred to herein means a combination of processing by a mandrel mill and processing by a sizer. The finish rolling of the steel pipe is performed by a final rolling mill arranged at the subsequent stage of the punching machine. Rolling at this stage is a work in a low temperature range, and is therefore important in thermomechanical treatment. That is, in the case where direct quenching or "compensation heat + direct quenching" is carried out after the final finish rolling, a workability of 40% or more in terms of cross-section compression rate is desirable. By this processing, a grain refining effect due to recrystallization can be obtained. Further, from the viewpoint of making the particles fine, the finishing temperature is important in addition to the workability. 1050 ℃
Refinishing grains become coarse at finishing temperatures above 1050 ° C.
The following is desirable. On the other hand, finer recrystallized grains can be obtained by finishing at a lower temperature, but the production efficiency decreases, so 800 ° C or higher is desirable.

3. Heat treatment 3-1. Quenching As the quenching method, the steel material once cooled after hot working is heated to the quenching temperature and rapidly cooled, the "direct quenching method" in which it is rapidly cooled after hot working, and supplementary heat is applied before direct quenching. Method of adding process, method of performing reheating-quenching once or more between direct quenching and tempering, supplementary heat before direct quenching, and reheating-quenching once between direct quenching and tempering There is a method of performing the above, and any of them can be adopted.

When the steel material once cooled after hot rolling by the method of (1) is reheated and quenched, the heating temperature is set to the Ac ₃ transformation point or higher. Further, in the case of directly quenching immediately after the hot working, the finish temperature of the hot working is set to the Ar ₃ transformation point or higher, and the austenite state is rapidly cooled. In any case, the martensite structure is obtained by cooling with a suitable cooling medium (water, oil, gas, etc.). The direct quenching method not only contributes to energy saving and process rationalization such as process shortening, but also helps to improve the quality of steel materials such as grain refinement.

[0058] In the method of, of one or more times directly after quenching - heating temperature and Ac ₃ transformation point or more in the case of performing a "re-heating quenching". From the viewpoint of preventing coarsening during quenching, it is desirable to control the heating temperature to an Ac ₃ transformation point of + 200 ° C or less in reheating-quenching and to 1100 ° C or less in direct quenching.

In particular, when the steel having the above-mentioned chemical composition is directly quenched, the resistance to temper softening increases, so that even if residual austenite is produced by high temperature tempering, the target C12
There is an advantage that high strength of 5th grade or higher can be maintained.
Further, due to the high temperature tempering, the strain of the martensitic structure is eliminated, so that the sulfide stress cracking resistance of the tempered martensitic structure is significantly improved.

The "supplementary heat" in the method (1) means online heating for promoting recrystallization after hot working and before direct quenching. When this supplementary heat is applied, recrystallization occurs during that time, the austenite crystal grains become extremely fine, and as a result, the resistance to sulfide stress cracking is significantly improved. The temperature of supplementary heat is preferably 850 to 1100 ° C, and the time is preferably 10 seconds to 30 minutes.

By performing the above-mentioned reheating-quenching once or more (usually, up to two times is sufficient), it is possible to further reduce the grain size as compared with direct quenching, and the sulfide stress corrosion cracking resistance is improved. Will be even better. In particular, when the re-quenching is added after the direct quenching that produces the fine grain structure as described above, the crystal grains become finer, and the achievement level of the effect becomes higher.

3-2. Tempering In order to make the metal structure of the final product steel material (steel pipe, steel plate, etc.) into retained austenite having a volume fraction of 1 to 10%, the tempering temperature is set to the Ac ₁ transformation point. You have to exceed it. This is because retained austenite does not form at grain boundaries in tempering at a temperature below the Ac ₁ transformation point. Further, in order to reduce the amount of retained austenite to 10% or less,
It is desirable that the tempering temperature does not exceed the Ac ₁ transformation point + 30 ° C.

Since tempering is an important process that determines the performance of the product, sufficient soaking property is required. The variation is preferably ± 10 ° C, more preferably ± 5 ° C. As a result, fluctuations in yield stress (YS) and tensile strength (TS) are ± 5 kgf /
It can be suppressed to mm ² or less. The tempering time may be about 30 seconds to 1 hour.

The structure containing retained austenite can also be obtained by the isothermal transformation treatment. This is the steel material after quenching
c After heating to a transformation point of ₃ or higher to austenite, it is rapidly cooled to around 300 to 600 ℃ in a salt bath, where it undergoes bainite transformation and is then cooled (water or oil cooled) to leave austenite. It is a thing. Compared with normal water quenching or oil quenching, it requires a salt bath facility and has the drawback of decreasing productivity, but it is a desirable heat treatment method from the viewpoint of the stability of retained austenite. This isothermal transformation treatment can also be used to manufacture the steel material of the present invention.

The main purpose of the present invention is to provide a high-strength and high-corrosion-resistant steel material having a C110 grade or higher and a method for producing the same, but it is naturally possible to apply the present invention to a low-strength oil country tubular good below C110 grade. Generally, the higher the strength is, the more the sulfide stress cracking resistance deteriorates. Therefore, it is relatively easy to manufacture a steel material having low strength and excellent sulfide stress cracking resistance. Also in the practice of the present invention, it is possible to freely manufacture steel materials in the respective target strength ranges by increasing or decreasing or omitting addition of strengthening elements such as C, Cr, Mo, and V.

[0066]

[Example] Steel having the chemical composition shown in Table 1 was melted and cast in a 150 kg vacuum furnace to form a 120 mm square × 1 m ingot, which was forged and a block 20 mm thick × 80 mm wide × 250 mm long I chose
Using these as test materials, various thick plate rolling conditions from rolling to final tempering were changed and examined. This thick plate rolling condition also corresponds to the working condition of the seamless steel pipe from perforation to finish rolling. The cross-section compressibility is almost the same for steel pipe rolling and steel plate rolling. Then, as shown in Table 2, the tempering temperature was changed according to the steel type and the quenching conditions.

[Quenching-Tempering Process] Trials Nos. 1 to 4 and 20 ... Reheat Quenching + Tempering Trials No. 5 to 8 ... Direct Quenching + Tempering Trials Nos. 9 to 12 and 17・ ・ ・ Supplementary heat + direct quenching + tempering Trial Nos. 13 and 14 ・ ・ Supplementary heat + direct quenching + reheating quenching + tempering Trial Nos. 15 and 16 ・ Supplementary heat + direct quenching + re- Heat quenching + reheat quenching + tempering trial number 18 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ Direct quenching + reheat quenching + tempering.

Each heating time was 5 minutes, the heating time during reheating and quenching was 10 minutes, and the tempering time was 20 minutes. The cooling medium during quenching was water. Sample No. 19 was subjected to an isothermal transformation treatment of "heating at 900 ° C for 45 minutes → cooling with salt bath (retention at 375 ° C for 1 hr) → oil cooling".

The heating temperature before hot rolling was 1250 ° C., the degree of rough rolling at 1100 ° C. or higher was 50%, and the finishing temperature was 100%.
The degree of finish rolling was 70% at 0 ° C. These are common conditions for all.

[0070]

[Table 1]

A test piece having the same size as the stress corrosion cracking resistance test piece described below was cut out from the test material after the heat treatment, and a tensile test was carried out, and the following sulfide stress cracking resistance test was carried out. Further, the amount of retained austenite was measured by the X-ray method.

The resistance to sulfide stress cracking was measured by a round bar type tensile test piece (parallel part 6.35φ × 2) taken from the L direction (rolling direction).
5.4 mm). The load stress is 8 of the actual yield stress of the base metal.
0%. The test solution is NACE TM 0177-90 (Test Meth
od by National Association of Corrosion Engineers)
NACE TM 0177 bath (0.5% acetic acid + 5% saline solution,
1 atmosphere of hydrogen sulfide saturated, 25 ° C). Those that did not break during the test period of 720 hr were regarded as good sulfide stress cracking resistance. Table 3 shows the test results.

[0073]

[Table 2]

[0074]

[Table 3]

As shown in Table 3, even in trial Nos. 1 to 4 using the same steel A, trial No. 1 has a residual austenite amount of 0% because the tempering temperature is the Ac ₁ transformation point or lower, and thus the sulfurization resistance is low. The stress cracking property is poor. On the other hand, in trial No. 4, the tempering temperature was too high, and as a result, the amount of retained austenite was 10%.
Since, the resistance to sulfide stress cracking is deteriorated. Steel B
The same can be said for trial numbers 5 to 8 and trial numbers 9 to 12 using steel C.

Regardless of tempering temperature or quenching conditions,
If the retained austenite amount is in the range of 1 to 10%,
All yield strength (YS) is 77.3 kgf / mm ² (110 ks
Higher strength than i). Moreover, 80
It exhibits excellent resistance to sulfide stress cracking, which does not cause cracking even under a stress load of%.

Since the C content of the steel K of trial No. 21 was as small as 0.19%, the residual austenite content was only 0.5% and the strength did not reach the target value (YS ≧ 110 ksi). Also, the sulfide stress cracking resistance is not good. In addition, trial number 20 is steel J of the test material.
Since the C content was too high and quench cracking occurred during quenching, the tensile test and the sulfide stress cracking resistance test were stopped.

[0078]

As described above, the steel material of the present invention having a structure containing a retained austenite structure in a volume fraction of 1 to 10% and the remainder mainly composed of a tempered martensite structure, is resistant to sulfide stress cracking. Excellent in strength and high strength. This steel material can be manufactured easily and at low cost by the manufacturing method of the present invention, that is, by reheating quenching or by direct quenching and then tempering at a temperature exceeding the Ac ₁ transformation point. According to the present invention, a steel material extremely suitable as an oil country tubular good or the like used in a severe corrosive environment containing sulfide can be provided at low cost and with high productivity.

Claims (7)

[Claims] 1. A low alloy steel having a C (carbon) content of 0.20 to 0.60% by weight and having a volume fraction of retained austenite of 1
A high-strength steel material excellent in sulfide stress cracking resistance, characterized by having a metal structure in which the balance is up to 10% and the balance is mainly martensite. 2. The yield strength is 77.3 kgf / mm ² or more, and NACE
TM 0177 The crack initiation critical stress (σth) in the bath is 80
% Or more, the high-strength steel material excellent in sulfide stress cracking resistance according to claim 1. 3. The method according to claim 1 or 2 having the following chemical composition.
High-strength steel material described in. % By weight, C: 0.20 to 0.60%, Si: 0.05 to 3.0%, Mn: 0.05 to 1.0%, P: 0.025% or less, S: 0.01% or less, sol.Al: 0.005 to 0.50%, Cr: 0.0. 1 to 1.5%, Mo: 0.1 to 1.5%, Nb: 0.005 to 0.5%, Ti: 0.005 to 0.5%, V: 0 to 0.5%, B: 0 to 0.01%, W: 0 to 1.0%, Zr: 0 ~ 0.5%, Ca: 0-0.01%, Ni: 0.1% or less, N: 0.01% or less, O: 0.01% or less, Fe and unavoidable impurities: balance 4. A low alloy steel having a C (carbon) content of 0.20 to 0.60% is hot worked, quenched, and then tempered at a temperature exceeding the Ac ₁ transformation point. 2. The method for producing a high-strength steel material having excellent sulfide stress cracking resistance according to 2 or 3. 5. Quenching is carried out by a direct quenching method starting from hot working, and tempering is performed at a temperature exceeding the Ac ₁ transformation point and the “Ac ₁ transformation point + 30.
The method for producing a high-strength steel material having excellent resistance to sulfide stress cracking according to claim 4, which is carried out in a temperature range up to "° C". 6. The method for producing a high-strength steel material excellent in sulfide stress cracking resistance according to claim 5, further comprising a step of supplementary heating of the steel material at a temperature of Ac ₃ transformation point or higher before direct quenching. 7. The method for producing a high-strength steel material having excellent resistance to sulfide stress cracking according to claim 5 or 6, wherein reheating-quenching is performed once or more between direct quenching and tempering.