JP6137082B2 - High strength stainless steel seamless steel pipe excellent in low temperature toughness and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength stainless steel seamless pipe and a method for producing the same, and more particularly to improvement of low-temperature toughness.

  In recent years, due to problems such as soaring energy prices such as crude oil and depletion of oil resources, oil fields with deeper depths that were not previously excluded, and oil fields under severe corrosive environments (so-called sour environments) including hydrogen sulfide, etc. Development of oil fields and gas fields under severe weather conditions such as those in the extreme north has been actively conducted.

  Steel materials used in such an environment are required to have high strength, excellent corrosion resistance (sour resistance), and excellent low temperature toughness. In addition to the conventional thin steel pipe, a thick steel pipe is also required in consideration of transport efficiency, pressure resistance, and the like.

Conventionally, 13% Cr martensitic stainless steel is often used as a steel material in oil fields and gas fields in a corrosive environment containing carbon dioxide CO ₂ , chlorine ions Cl ^{− and the} like. However, under the sour environment, 13% Cr martensitic stainless steel has insufficient corrosion resistance, and recently, the use of duplex stainless steel with reduced C content and increased Cr content and Ni content has been expanded. .

For example, Patent Document 1 describes a method for producing martensitic stainless steel having excellent corrosion resistance and sulfide stress corrosion cracking resistance. In the technique described in Patent Document 1, by weight%, C: 0.005-0.05%, Si: 0.05-0.5%, Mn: 0.1-1.0%, Cr: 10-15%, Ni: 4.0-9.0%, Cu : Steel containing 0.5 to 3%, Mo: 1.0 to 3%, Al: 0.005 to 0.2%, N: 0.005 to 0.1%, with Nieq adjusted to -10 or more, hot-worked to room temperature After natural cooling, heat treatment is performed at a temperature not lower than an Ac ₁ point and a temperature at which the austenite fraction is 80% or less, and further, heat treatment is performed at a temperature or less at which the austenite fraction is 60%. Thereby, it consists of an above-described composition, a tempered martensite phase, a martensite phase, and a retained austenite phase. The total fraction of the tempered martensite phase and the martensite phase is 60 to 90%, and the balance is the retained austenite phase. It is said that martensitic stainless steel (steel plate) having a structure and excellent in corrosion resistance and sulfide stress corrosion cracking resistance in a wet carbon dioxide environment and a wet hydrogen sulfide environment is obtained.

Patent Document 2 describes a method for producing a high-strength stainless steel seamless steel pipe for oil wells that has excellent corrosion resistance. In the technique described in Patent Document 2, in mass%, C: 0.005 to 0.05%, Si: 0.05 to 0.5%, Mn: 0.2 to 1.8%, Cr: 15.5 to 18%, Ni: 1.5 to 5%, Mo : 1 to 3.5%, V: 0.02 to 0.2%, N: 0.01 to 0.15%, O: 0.006% or less, Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 19.5 and Cr + Mo + 0.3Si-43.5C-0.4Mn -Ni-0.3Cu-9N≥11.5 steel pipe material is heated, piped by hot working, and then piped and then cooled to room temperature at a cooling rate equal to or higher than that of air cooling to seamlessly pass through the specified dimensions. Then, after reheating the seamless steel pipe to a temperature of 850 ° C. or higher, cooling it to 100 ° C. or lower at a cooling rate of air cooling or higher, and then performing a quenching and tempering treatment to heat it to a temperature of 700 ° C. or lower. It is said. As a result, it has a structure including a ferrite phase of 10 to 60% by volume and the balance being a martensite phase, yield strength: 654 MPa or more, Charpy impact test absorption energy at a test temperature: −40 ° C. is 50 J It is said that a high-strength stainless steel pipe for oil wells having the above-mentioned high toughness and sufficient corrosion resistance in a severe corrosive environment up to 230 ° C. containing CO ₂ and Cl ⁻ is obtained.

Japanese Patent Laid-Open No. 10-1755 Japanese Patent No. 5109222

  However, the techniques described in Patent Documents 1 and 2 target steel materials having a thickness of up to 12.7 mm, and there is no mention of thick steel materials exceeding a thickness of 15 mm. Recently, the depth of oil wells has increased, and steel materials used for oil wells are often used with thick steel materials. As the thickness increases, it is difficult to apply a desired processing strain to the center of the thickness in the normal hot working method, and the structure of the center of the thickness tends to become coarse. Therefore, there is a problem that the low-temperature toughness of the central portion of the wall tends to be lowered in the thick-walled steel material as compared with the thin-walled steel material. However, Patent Documents 1 and 2 do not mention any improvement in low-temperature toughness in thick steel materials.

An object of the present invention is to solve the problems of the prior art and to provide a high-strength seamless steel pipe for oil wells and a method for producing the same, which is excellent in low-temperature toughness of seamless steel pipes, in particular, low-temperature toughness in the central portion of the wall thickness. And “High strength” as used herein refers to the case where the yield strength is YS: 654 MPa or more, and “excellent in low temperature toughness” means that the absorbed energy vE ₋₄₀ of the Charpy test is 50 J at a test temperature of −40 ° C. This is the case. The wall thickness of the high strength seamless steel pipe for oil wells of the present invention is preferably 15 mm or more and less than 100 mm, which is relatively thick.

  In order to achieve the above-described object, the present inventors have intensively studied various factors affecting the low temperature toughness of a martensitic stainless steel seamless pipe. As a result, in order to improve the low-temperature toughness of the thick-walled stainless steel seamless pipe, particularly the low-temperature toughness at the center of the wall thickness, in addition to the martensite phase and the ferrite phase, an appropriate amount of stable austenite phase, It came to mind that it is effective to make the structure included.

As a result of further studies by the present inventors, the ratio between the Ni concentration in the austenite phase and the Ni concentration in the surrounding martensite phase, (C _Ni ) _γ / (C _Ni ) _M, is the stability of the austenite phase. By controlling the heat treatment and adjusting the ratio (C _Ni ) _γ / (C _Ni ) _M to the appropriate range (1.15 or more), a certain amount of stable austenite phase is secured. I found out that I can do it. The inventors have also found that it is necessary to make the ferrite phase finer in order to further improve the low temperature toughness of the thick stainless steel seamless steel pipe.

In other words, after the martensitic stainless steel material is heated to the Ac ₄ transformation point or higher, which becomes a ferrite single phase, accelerated cooling is performed and hot working is performed with the ferrite → austenite transformation suppressed, and the structure is adjusted. Furthermore, Ni is concentrated in the austenite phase by heating it to a ferrite + austenite two-phase temperature and then rapidly cooling and further tempering at least once to stabilize a predetermined amount of austenite phase. Furthermore, it was found that an appropriate amount of granular martensite phase was formed in the ferrite phase, and the ferrite phase was divided and finely divided to obtain a microstructure.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.050% or less, Si: 0.50% or less, Mn: 0.20 to 1.80%, Cr: 15.5 to 18.0%, Ni: 1.5 to 5.0%, Mo: 1.0 to 3.5%, V: 0.02 -0.20%, Al: 0.05% or less, N: 0.01-0.15%, O: 0.006% or less, the composition consisting of the balance Fe and unavoidable impurities, the martensite phase of 50% or more by volume, and 3 -15% of austenite phase and the balance is composed of ferrite phase, and Ni concentration (C _Ni ) _γ (mass%) in the austenite phase and Ni concentration (C _Ni ) _M in the martensite phase _M A high-strength stainless steel seamless pipe for oil wells excellent in low temperature toughness, characterized in that the ratio to (mass%), (C _Ni ) _γ / (C _Ni ) _M , is 1.15 or more.
(2) In (1), the structure is a structure in which in the ferrite phase, a part of the martensite phase is granular having a particle size of 10 μm or less and containing 3 or more per 100 μm ^{2 of} ferrite particles. High strength stainless steel seamless pipe for oil wells.
(3) In (1) or (2), in addition to the above-mentioned composition, the high-strength stainless steel seamless steel pipe for oil wells further comprising, by mass%, Cu: 3.5% or less.
(4) In any one of (1) to (3), in addition to the above composition, in mass%, Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, W: 3.0% or less, B: A high-strength stainless steel seamless pipe for oil wells having a composition containing one or more selected from 0.01% or less.
(5) In any one of (1) to (4), in addition to the above composition, the composition further includes one or two selected from Ca: 0.01% or less and REM: 0.01% or less by mass%. A high-strength stainless steel seamless steel pipe for oil wells characterized by having a composition.
(6) When the steel material is subjected to hot working including piercing and rolling to make a thick-walled seamless steel pipe, the steel material is mass%, C: 0.050% or less, Si: 0.50% or less, Mn: 0.20 ~ 1.80%, Cr: 15.5 ~ 18.0%, Ni: 1.5 ~ 5.0%, Mo: 1.0 ~ 3.5%, V: 0.02 ~ 0.20%, Al: 0.05% or less, N: 0.01 ~ 0.15%, O: 0.006% or less The steel material is composed of the balance Fe and unavoidable impurities, and the steel material is heated to a heating temperature of 1150 to 1350 ° C., and then the heated steel material is cooled from the cooling start temperature at the thickness center temperature. Accelerated cooling at an average cooling rate of 2.0 ° C./s or higher until a cooling stop temperature at which the temperature difference is at least 50 ° C. or higher and 800 ° C. or higher, and then subjected to hot working including the piercing and rolling, and then cooled The steel pipe is made into a steel pipe, and then the seamless steel pipe is heated to a temperature of 750 to 950 ° C. and rapidly cooled, and the heating temperature is a temperature of 550 to 680 ° C. Second stage treatment with the at least one superior process for producing a high strength stainless seamless steel pipe for oil well in low temperature toughness characterized by applying heat treatment the consisting of heating.
(7) In (6), after the steel material is heated to a heating temperature of 1150 to 1350 ° C., the piercing rolling is performed without performing the accelerated cooling before the hot working, and after the end of the piercing rolling, Starts cooling and accelerates the temperature range where the temperature difference from the cooling start temperature at the wall thickness center temperature is at least 50 ° C to 2.0 ° C / s or higher and to the cooling stop temperature of 800 ° C or higher. A method for producing a high-strength stainless steel seamless pipe for oil wells, which is cooled and then subjected to the hot working other than the piercing and rolling to form a seamless steel pipe, and then the heat treatment is performed on the seamless steel pipe.
(8) In (6) or (7), in addition to the said composition, it is set as the composition which contains further Cu: 3.5% or less by mass%, The manufacturing method of the high strength stainless steel seamless steel pipe for oil wells characterized by the above-mentioned.
(9) In any one of (6) to (8), in addition to the above composition, in addition to mass, Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, W: 3.0% or less, B: A method for producing a high-strength stainless steel seamless steel pipe for oil wells, wherein the composition contains one or more selected from 0.01% or less.
(10) In addition to the composition described above in any one of (6) to (9), the composition further comprises one or two selected from Ca: 0.01% or less and REM: 0.01% or less by mass%. A method for producing a high-strength stainless steel seamless steel pipe for oil wells.

  ADVANTAGE OF THE INVENTION According to this invention, the high strength stainless steel seamless steel pipe for oil wells excellent in low temperature toughness can be manufactured easily, and there is a remarkable industrial effect. Further, according to the present invention, the structure of the seamless steel pipe can be refined to the thickness center portion with a relatively small processing amount, and in particular, the thickness seam where the processing amount at the thickness center portion cannot be increased. Even in a steelless pipe, there is an effect that the low temperature toughness can be improved.

It is explanatory drawing which shows typically an example of the manufacturing equipment row | line | column which can be used by this invention.

  FIG. 1 shows an example of a production equipment row suitable as equipment for producing the seamless steel pipe of the present invention. In addition, it cannot be overemphasized that in this invention, it is not limited to this equipment row | line | column.

  Examples of equipment for manufacturing seamless steel pipes include a heating device 1 and a hot working device 2 including a piercing and rolling device 21 and a rolling device 22. In the present invention, the cooling device 3 is used as the heating device 1. It is preferable to use a production equipment line arranged between the piercing and rolling device 21 or between the piercing and rolling device 21 and the rolling device 22.

  The heating device 1 used in the present invention may be any heating furnace capable of heating steel materials such as slabs and steel slabs to a predetermined temperature, and is not particularly limited. Any conventional heating furnace such as a heating furnace can be used. Moreover, there is no problem as an induction heating type heating furnace.

  Further, as the piercing and rolling apparatus 21 used in the present invention, any generally known piercing and rolling apparatus such as a Mannesmann tilting piercing machine or a hot extrusion piercing machine using a barrel roll, a cone roll or the like can be applied. Further, the rolling device 22 which is one of the hot rolling devices 2 may be a rolling device arranged in the order of an elongator 221, a plug mill 222, a reeler (not shown), a sizer 223, or a mandrel, depending on the purpose. Any generally known rolling device such as a rolling device (not shown) provided with a mill and a reducer can be applied.

  In addition, the cooling device 3 used in the present invention is a steel material having a stainless steel composition, and has a cooling capacity capable of obtaining an average cooling rate of at least 2.0 ° C./s or more at the surface of the material to be cooled and the center of the wall thickness. It is preferable to have an apparatus. If the cooling capacity is insufficient and only cooling slower than the above average cooling rate is possible, a desired phase distribution cannot be obtained, and a desired fine structure cannot be secured even if hot working is subsequently applied. The upper limit of the cooling rate is not particularly limited, but is preferably 50 ° C./s from the viewpoint of preventing cracking and bending. In addition, in FIG. 1, although the state currently cooled from the outer surface side is shown, it is not limited to this, and it cannot be overemphasized that you may cool from an outer surface side and an inner surface side.

  Note that a device row in which a heat retaining device (not shown) is disposed on the exit side of the rolling device 22 may be used. The heat retaining device is provided as necessary to slow down the cooling rate after hot working. In the case of stainless steel composition, if the cooling is too fast after processing, the nonequilibrium ferrite phase is cooled without causing the α → γ transformation, and the desired fine austenite grains cannot be produced, and the refinement of the steel pipe structure is achieved. become unable. Note that it is sufficient for the heat retaining device to have a heat retaining capacity that can be adjusted to a cooling rate of at least about 20 ° C./s at the surface temperature of the material to be cooled.

  Below, the manufacturing method of this invention high strength stainless steel seamless steel pipe is demonstrated.

  In the present invention, the steel material is subjected to hot working including piercing and rolling to obtain a seamless steel pipe. First, the reasons for limiting the composition of the steel material used will be described. Hereinafter, mass% in the composition is simply expressed as% unless otherwise specified.

  Steel materials used in the present invention are: C: 0.050% or less, Si: 0.50% or less, Mn: 0.20 to 1.80%, Cr: 15.5 to 18.0%, Ni: 1.5 to 5.0%, Mo: 1.0 to 3.5%, V : 0.02 to 0.20%, Al: 0.05% or less, N: 0.01 to 0.15%, O: 0.006% or less, and has a composition consisting of the balance Fe and inevitable impurities.

C: 0.050% or less
C is an important element related to the strength of martensitic stainless steel. In the present invention, C is preferably contained in an amount of 0.005% or more in order to ensure a desired strength. On the other hand, if the content exceeds 0.050%, sensitization during tempering due to Ni inclusion increases. For this reason, from the viewpoint of corrosion resistance, it is desirable that C is less. For these reasons, C is limited to 0.050% or less. In addition, Preferably it is 0.030 to 0.050%.

Si: 0.50% or less
Si is an element that acts as a deoxidizer, and it is desirable to contain 0.05% or more. If the content exceeds 0.50%, the corrosion resistance is lowered and the hot workability is also lowered. For this reason, Si was limited to 0.50% or less. In addition, Preferably it is 0.10 to 0.30%.

Mn: 0.20 to 1.80%
Mn is an element having an action of increasing the strength, and in order to obtain such an effect, it needs to be contained in an amount of 0.20% or more. On the other hand, if the content exceeds 1.80%, the toughness is adversely affected. For this reason, Mn was limited to 0.20 to 1.80%. In addition, Preferably it is 0.20 to 1.00%.

Cr: 15.5-18.0%
Cr is an element that has a function of forming a protective film and improving the corrosion resistance, and further increasing the strength of the steel by solid solution. In order to obtain such an effect, the content of 15.5% or more is required. On the other hand, if the content exceeds 18.0%, the hot workability is lowered and the strength is further lowered. For this reason, Cr was limited to 15.5-18.0%. In addition, Preferably it is 16.6 to 18.0%.

Ni: 1.5-5.0%
Ni is an element that has an action of strengthening the protective film and improving the corrosion resistance, and further increasing the strength of the steel by solid solution and further improving the toughness. Such an effect is recognized when the content is 1.5% or more. On the other hand, if the content exceeds 5.0%, the stability of the martensite phase decreases and the strength decreases. For this reason, Ni was limited to 1.5 to 5.0%. In addition, Preferably it is 2.5 to 4.5%.

Mo: 1.0-3.5%
Mo is an element that increases resistance to pitting corrosion caused by Cl ⁻ . In order to acquire such an effect, it is necessary to contain 1.0% or more. On the other hand, if the content exceeds 3.5%, the strength decreases and the material cost increases. For this reason, Mo was limited to 1.0 to 3.5%. In addition, Preferably it is 2.0 to 3.5%.

V: 0.02 to 0.20%
V is an element that increases strength and improves corrosion resistance. In order to obtain such an effect, a content of 0.02% or more is required. On the other hand, if the content exceeds 0.20%, toughness decreases. For this reason, V was limited to 0.02 to 0.20%. In addition, Preferably it is 0.02 to 0.08%.

Al: 0.05% or less
Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it is desirable to contain 0.002% or more. On the other hand, if it exceeds 0.05%, the toughness is adversely affected. For this reason, Al was limited to 0.05% or less. When Al is not added, an inevitable impurity of about 0.002% is allowed.

N: 0.01-0.15%
N is an element that remarkably improves the pitting corrosion resistance. In order to obtain such an effect, N is required to be contained in an amount of 0.01% or more. On the other hand, if the content exceeds 0.15%, various nitrides are formed and the toughness is lowered. For this reason, N was limited to the range of 0.01 to 0.15. In addition, Preferably it is 0.02 to 0.08%.

O: 0.006% or less
O (oxygen) exists as an oxide in steel and adversely affects various properties. For this reason, it is desirable to reduce as much as possible. In particular, when O is contained in a large amount exceeding 0.006%, the hot workability, toughness and corrosion resistance are remarkably deteriorated. For this reason, O was limited to 0.006% or less.

  The above components are basic components. In the present invention, in addition to this basic composition, Cu: 3.5% or less and / or Nb: 0.2% or less, Ti: 0.3% or less, Zr: 1 or more selected from 0.2% or less, W: 3.0% or less, B: 0.01% or less, and / or Ca: 0.01% or less, REM: 1 selected from 0.01% or less A seed | species or 2 types can be contained as needed.

Cu: 3.5% or less
Cu strengthens the protective film and suppresses the penetration of hydrogen into the steel, improving the resistance to sulfide stress corrosion cracking. Such an effect becomes remarkable when the content is 0.5% or more. On the other hand, if the content exceeds 3.5%, CuS grain boundary precipitation occurs, and hot workability decreases. For this reason, when it contains, it is preferable to limit Cu to 3.5% or less. In addition, More preferably, it is 0.8 to 1.2%.

One or more selected from Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, W: 3.0% or less, B: 0.01% or less
Nb, Ti, Zr, W, and B are all elements that increase the strength, and can be selected and contained as necessary. Such effects are recognized when Nb: 0.03% or more, Ti: 0.03% or more, Zr: 0.03% or more, W: 0.2% or more, B: 0.01% or more. On the other hand, inclusions exceeding Nb: 0.2%, Ti: 0.3%, Zr: 0.2%, W: 3.0%, and B: 0.01% respectively reduce toughness. For this reason, when it contains, it is preferable to limit to Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, W: 3.0% or less, B: 0.01% or less, respectively.

One or two selected from Ca: 0.01% or less, REM: 0.01% or less
Both Ca and REM have the effect of spheroidizing the shape of sulfide inclusions, reducing the lattice distortion of the matrix surrounding the inclusions, and reducing the hydrogen trapping ability of inclusions It is an element and can contain 1 type or 2 types as needed. Such an effect becomes remarkable when Ca: 0.0005% or more and REM: 0.001% or more. On the other hand, when it contains exceeding Ca: 0.01% and REM: 0.01%, toughness will fall. For this reason, when it contains, it is preferable to limit to Ca: 0.01% or less and REM: 0.01% or less.

  The balance other than the components described above consists of Fe and inevitable impurities. Inevitable impurities are P: 0.03% or less and S: 0.005% or less.

  The method for producing a steel material having the above composition need not be particularly limited. Using a conventional smelting furnace such as a converter or electric furnace, the molten steel having the composition described above is melted, and a slab (round slab) is obtained by a conventional casting method such as a continuous casting method. It is preferable to use a raw material. In addition, it is good also as a steel raw material as a steel slab of a predetermined dimension by hot-rolling a slab. Moreover, it is set as a steel slab by the ingot-making-slab rolling method, and there is no problem as a steel raw material.

  First, a steel material having the above-described composition is charged into the heating device 1 and heated to a heating temperature of 1150 to 1350 ° C.

Heating temperature: 1150-1350 ° C
If the heating temperature is less than 1150 ° C., a ferrite single-phase structure cannot be obtained, and refinement of the structure using transformation from the single phase cannot be achieved. Further, the deformation resistance becomes too high, and subsequent hot working becomes difficult. On the other hand, at 1350 ° C. or higher, deformation due to its own weight occurs, or accumulation of strain due to molding (processing) becomes difficult. For this reason, the heating temperature of the steel material was limited to a temperature in the range of 1150 to 1350 ° C. In addition, it is preferably 1200 to 1300 ° C. from the viewpoint of low deformation resistance and easy processing, and a large temperature difference during cooling.

  In the steel material having the composition range described above, a ferrite single phase is formed in a temperature range of 1200 ° C. or higher. In the temperature range of 1150 ° C. or more and less than 1200 ° C., a two-phase structure of ferrite and austenite is exhibited. However, since the ferrite is mostly contained in this temperature range, the structure can be sufficiently refined by subsequent processing.

  In the present invention, the heated steel material is then subjected to accelerated cooling by the cooling device 3.

  In accelerated cooling, the temperature of the steel material after heating is set as the cooling start temperature, and the temperature difference from the cooling start temperature is at least 50 ° C., and the cooling stop temperature at which the temperature is 800 ° C. or higher is 2.0 ° C. in terms of thickness center. The cooling process is accelerated cooling at an average cooling rate of at least / s. Hereinafter, unless otherwise specified, the temperature is the thickness center temperature.

  Here, the cooling start temperature is the temperature of the steel material before the start of cooling, and is preferably 950 ° C. or higher. If it is less than 950 degreeC, the temperature after cooling will become low too much, deformation resistance will become high, and subsequent process shaping | molding will become difficult. In the present invention, it is intended to make the microstructure state as much as possible with the ferrite phase and then subject to hot working to refine the structure, so accelerated cooling is started so that the microstructure state has as much ferrite phase as possible. Higher temperatures are preferred.

Accelerated cooling temperature range: 50 ° C or higher The temperature range of accelerated cooling, that is, the temperature difference between the cooling start temperature and the cooling stop temperature is at least 50 ° C or higher. When the temperature range of accelerated cooling is less than 50 ° C., a remarkable non-equilibrium phase fraction cannot be secured, and the effect of refining the structure by subsequent processing cannot be expected. For this reason, the temperature range of accelerated cooling was limited to 50 ° C. or higher. The larger the temperature range of accelerated cooling, the easier it is to secure a non-equilibrium phase fraction. In addition, Preferably it is 100 degreeC or more.

Cooling stop temperature for accelerated cooling: 800 ° C or higher The cooling stop temperature shall be 800 ° C or higher. When the cooling stop temperature is less than 800 ° C., the diffusion of elements is delayed, the phase transformation (α → γ transformation) by the subsequent processing is delayed, and the effect of refining the structure by the subsequent processing cannot be expected. For this reason, the cooling stop temperature of accelerated cooling is limited to 800 ° C. or higher. In addition, Preferably it is 1000-800 degreeC.

Average cooling rate of accelerated cooling: 2.0 ° C./s or more The average cooling rate of accelerated cooling is 2.0 ° C./s or more on the surface. When the average cooling rate is less than 2.0 ° C./s, the phase fraction in the non-equilibrium state cannot be secured, and the effect of refining the structure by the subsequent processing cannot be expected. For this reason, the average cooling rate of accelerated cooling was limited to 2.0 ° C./s or more. In addition, Preferably it is 5-20 degrees C / s. The upper limit of the average cooling rate is determined by the capacity of the cooling device and is not particularly limited, but is preferably 50 ° C./s or less from the viewpoint of preventing cracking and bending.

  The accelerated and cooled steel material is then subjected to piercing and rolling by a piercing and rolling device 221 to form a hollow material, and further subjected to hot working (hot rolling other than piercing and rolling) by, for example, a hot rolling device 222 or the like. , A seamless steel pipe having a predetermined dimension.

  In addition, without subjecting the steel material after heating to the above-described accelerated cooling, immediately after the heating, the steel material is subjected to hot working (piercing rolling) with a piercing and rolling device 221 to form a hollow material, and then accelerated cooling similar to the above-described conditions. May be applied. In this case, the temperature of the hollow material after piercing and rolling is about 1050 to 1200 ° C., which is lower than that immediately after heating, but the accelerated cooling under the above conditions can be sufficiently performed, and the effect of the accelerated cooling is almost changed. Make sure there is no. In this case, the cooling stop temperature for accelerated cooling is preferably 800 ° C. or higher.

  Hot working performed on steel materials (including hollow materials) after accelerated cooling is preferably processed in a state of structure with a lot of ferrite phase, corresponding to the cooling stop temperature of accelerated cooling, 800 ° C or higher, Preferably it is 1000-800 degreeC. Further, the hot working conditions only need to be a seamless steel pipe having a predetermined size, and ordinary working conditions can be applied, and there is no particular limitation. In the present invention, a desired microstructure can be refined even with a relatively low machining amount (rolling rate). However, from the viewpoint of further miniaturization of the microstructure, it is preferable that the cumulative machining amount is 50% or more.

  In addition, the cooling rate after being finished into a seamless steel pipe having a desired dimension by hot working is not particularly limited and may be allowed to cool, but it is 20 ° C on average from the end of hot working to 500 ° C. / S or less is preferable. After the hot working, when the cooling becomes faster at an average cooling rate exceeding 20 ° C./s, the nonequilibrium ferrite phase is cooled without causing the α → γ transformation, and the desired fine austenite grains cannot be produced, While refinement of the steel pipe structure cannot be achieved, Ni concentration in the generated austenite phase does not proceed, and the stability of the austenite phase cannot be improved.

  In the present invention, the seamless steel pipe obtained under the above-described conditions is further heated to a temperature in the range of 550 to 680 ° C. and a first stage treatment in which the heating temperature is reheated to 750 to 950 ° C. and rapidly cooled. The heat treatment consisting of the second stage treatment is performed at least once.

  The heat treatment consisting of the first stage treatment and the second stage treatment is a treatment performed for concentrating Ni in the austenite phase and increasing the content of the stable austenite phase.

  The first stage treatment is a heating temperature: reheating to 750 to 950 ° C. and quenching.

  In the first stage treatment, heating is performed in a temperature range of 750 to 950 ° C. By reheating to the above temperature range, an austenite phase is formed by transformation from the martensite phase, and an austenite phase (granular) is also formed from the ferrite phase supercooled during accelerated cooling, and then rapidly cooled. Part of the formed austenite remains, and the others are transformed into martensite. Note that the austenite phase (granular) formed by transformation from the ferrite phase is transformed into a martensite phase (granular) and dispersed in the ferrite phase in a granular form.

  When the heating temperature in the first stage treatment is less than 750 ° C., formation of the austenite phase from the martensite phase is not recognized. On the other hand, if the temperature exceeds 950 ° C., the amount of austenite phase to be formed increases, the stability is lowered, and the desired amount of austenite phase cannot be retained. Therefore, the heating temperature of the first stage treatment was limited to 750 to 950 ° C.

  The holding (residence) time at the heating temperature is preferably about 60 min or less from the viewpoint of stabilization of austenite. The rapid cooling after heating is preferably water cooling.

  Subsequent to the first stage process described above, the second stage process is performed. The second stage treatment is a treatment in which the heating temperature is heated to a temperature in the range of 550 to 680 ° C. and allowed to cool. In the second stage treatment that follows the first stage treatment, the enrichment of Ni in the austenite phase formed in the first stage treatment is promoted, and the austenite phase is stabilized. When the heating temperature is less than 550 ° C., the temperature is low, the diffusion of Ni is delayed, and the concentration of Ni in the austenite phase is delayed. On the other hand, when the heating temperature is higher than 680 ° C., the austenite phase increases, the amount of Ni becomes dilute, and austenite is not stabilized. The cooling after heating is preferably slower from the viewpoint of the concentration of Ni in the austenite phase, and is allowed to cool.

  The temperature difference between the heating temperature of the first stage treatment and the heating temperature of the second stage treatment is preferably 150 ° C. or more. If this temperature difference is less than 150 ° C, the amount of austenite in the second stage treatment is too large, Ni concentration in the austenite phase is small, and stable austenite cannot be secured, so significant low temperature toughness cannot be expected. . For this reason, it is preferable to limit the temperature difference between the heating temperature of the first stage treatment and the heating temperature of the second stage treatment to 150 ° C. or more.

  In addition, it is preferable to repeat the heat processing which consists of a 1st step process and a 2nd step process at least once, Preferably several times. By repeating the above heat treatment a plurality of times, the stability of the austenite phase becomes higher, the amount of stable austenite at the time of deformation increases, and the low temperature toughness is improved.

The seamless steel pipe obtained by the above-described manufacturing method has the above composition and at least a thickness of 1/4 to 3/4, which is the thickness central portion, and a martensite phase with an area ratio of 50% or more, 3 ~ 15% austenite phase and the remainder is composed of ferrite phase, or part of the martensite phase is dispersed in the ferrite phase in the form of particles having a particle size of 10 μm or less and 3 or more per 100 μm ^{2 of} ferrite particles The ratio between the Ni concentration (C _Ni ) _γ (mass%) in the austenite phase and the Ni concentration (C _Ni ) _M (mass%) in the martensite phase, (C _Ni ) _γ / (C _Ni ) High strength stainless steel seamless pipe with excellent low temperature toughness, with _M , 1.15 or higher, yield strength YS: 654 MPa or higher, test temperature: −40 ° C., Charpy absorbed energy vE ₋₄₀ is 50 J or higher It is.

  Hereinafter, the reason for limiting the structure of the high-strength stainless steel seamless steel pipe of the present invention will be described.

Martensite phase: 50% or more in area ratio In the seamless steel pipe of the present invention, a martensite phase having a volume ratio of 50% or more is a main phase. The martensite phase is an important phase for securing a desired high strength. If the volume ratio is less than 50%, the strength is lowered and the desired high strength cannot be secured. For this reason, the martensite phase is a phase occupying 50% or more by volume ratio. A part of the martensite phase is dispersed as granular martensite grains in the ferrite grains.

Austenitic phase: 3-15% by volume
The austenite phase is rich in toughness and is dispersed to ensure excellent low temperature toughness. In order to obtain such an effect, it is necessary to contain 3% or more by volume. On the other hand, if the content exceeds 15%, the strength is lowered and the desired high strength cannot be ensured. For this reason, the austenite phase was limited to a range of 3 to 15% by volume ratio.

(C _Ni ) _γ / (C _Ni ) _M : 1.15 or more Ni concentration in the austenite phase is important for stabilizing the austenite phase. In particular, in the austenite phase formed in the martensite phase, Ni is concentrated as compared with the surrounding martensite phase, so that the stability is increased and the low temperature toughness is improved. Therefore, the ratio of the Ni concentration (C _Ni ) _γ (mass%) in the austenite phase and the Ni concentration (C _Ni ) _M (mass%) in the martensite phase, (C _Ni ) _γ / (C _Ni ) _M , Was limited to 1.15 or more.

  The Ni concentration of the austenite and martensite phases is determined by the energy dispersive X-ray spectrometer (EDS) and wavelength dispersive X-ray spectrometer attached to the scanning electron microscope (SEM) and transmission electron microscope (TEM). (WDS) can be used for measurement. When the austenite phase (grains) is 1 μm or less, it is preferable to measure by TEM-EDS from the viewpoint of measurement time and spatial resolution.

Granular martensite grains having a particle size of 10 μm or less: 3 or more per 100 μm ^{2 of} ferrite grains A part of the martensite phase is dispersed as granular martensite grains in the ferrite phase. By dispersing the granular martensite grains in the ferrite grains, the ferrite grains are divided, the apparent miniaturization is achieved, and the low temperature toughness is improved. From the viewpoint of making the ferrite grains fine among the granular martensite grains, three or more granular martensite grains having a particle diameter of 10 μm or less are dispersed per 100 μm ² of ferrite grains. In the present invention, the particulate martensitic grains having a particle size of more than 10 [mu] m, and does not contribute to the grain refining of a ferrite grain of shed too large, the particle size is limited to granular martensite grains 10 [mu] m, the ferrite grains 100 [mu] m ² Three or more per one were dispersed. When the dispersion of granular martensite grains having a particle size of 10 μm or less is less than 3 per 100 μm ^{2 of} ferrite grains, it does not contribute to the desired subdivision of the ferrite phase. In addition, granular martensite grains often exhibit a substantially elliptical shape, and the “particle diameter” herein refers to the major axis.

  In the present invention, an appropriate amount of granular martensite grains can be dispersed in the ferrite phase by the above-described two-stage heat treatment, so that it is difficult to add strain by hot working, especially in the wall of thick-walled seamless steel pipes. The low temperature toughness of the thickness center can be remarkably improved.

  Hereinafter, based on an Example, this invention is demonstrated further.

  Molten steel having the composition shown in Table 1 is melted in a converter and made into a slab (slab: thickness 260 mm) by a continuous casting method, and then the slab is pierced and round steel slab having a diameter of 350 mm. And steel material.

  The obtained steel material was charged into a heating device and heated to a heating temperature of 1250 ° C. and held for a certain time (45 min). The heated steel material is a cooling device using water spray, and is accelerated and cooled from the cooling start temperature to the cooling stop temperature shown in Table 2 at the average cooling rate shown in Table 2, and then in the piercing and rolling device. After being pierced and rolled into a hollow material, hot working (piercing and hot rolling) was performed with a rolling device.

  After hot working, it was cooled to a seamless steel pipe (outer diameter 273 mφ × inner diameter 209 mmφ). In some cases, the heated steel material is subjected to piercing and rolling by a piercing and rolling device to be a hollow material, and then subjected to accelerated cooling after piercing and rolling shown in Table 2, and thereafter a hot rolling device. And then rolled into a seamless steel pipe of the same dimensions. In addition, the cooling rate measured the surface temperature with the radiation thermometer, calculated | required the temperature of the thickness center by heat transfer calculation from the surface temperature, and calculated the average cooling rate.

  The obtained seamless steel pipe was subjected to heat treatment consisting of a first stage treatment and a second stage treatment under the conditions shown in Table 2. The first stage treatment was heated to the heating temperature shown in Table 2 and then cooled with water, and the second stage treatment was heated to the heating temperature shown in Table 2 and then allowed to cool. In some cases, the first stage treatment and the second stage treatment were repeated twice in that order. In some cases, the first-stage treatment and the second-stage treatment were performed, and then the second-stage treatment was repeated.

Test pieces were sampled from the obtained seamless steel pipe and subjected to a structure observation, a tensile test, and an impact test. The test method was as follows.
(1) Microstructure observation From the obtained seamless steel pipe, a specimen for microstructural observation was collected so that the cross section perpendicular to the rolling direction (C cross section) became the observation surface, and mechanical polishing and electrolytic polishing were performed and immediately scanned. The tissue was observed using a scanning electron microscope (SEM) (magnification: 500 to 5000 times). The structure was observed and imaged at the center of the thickness, and the structure fraction of the ferrite phase and the number of granular martensites in ferrite 100 μm ² were measured by image analysis (image processing) using the obtained structure photograph. . In addition, in the observation by SEM (magnification: 500 times), the structure fraction of the ferrite phase is observed, and in the observation by SEM (magnification: 2000 times, 5000 times), the granular martensite whose major axis in the ferrite is 10 μm or less. The number was measured and converted to the number in 100 μm ^{2 of} ferrite. In addition, when 80% or more of the interfaces of granular martensite are interfaces with ferrite, it was determined that granular martensite is present in the ferrite grains.

In addition, a transmission electron microscope (TEM) test piece (for a thin film) was collected from the obtained seamless steel pipe so that the central portion of the wall became the observation surface, and subjected to mechanical polishing and electrolytic polishing. The specimen was observed with a scanning electron microscope (TEM), and the structure was observed with TEM. Electron diffraction is used to identify austenite grains, and by using an energy dispersive X-ray spectrometer (EDS) attached to the TEM, the Ni concentration in the austenite (C _Ni ) _γ and the Ni concentration in the surrounding martensite (C _Ni ) _M was measured, and (C _Ni ) _γ / (C _Ni ) _M was calculated.

Further, from the obtained seamless steel pipe, an X-ray diffraction test piece was collected so that the central portion of the wall thickness would be the measurement surface, mechanically polished and electrolytically polished, and subjected to X-ray diffraction, and the volume of the austenite phase. The fraction _Vγ was calculated. In the X-ray diffraction, the X-ray integrated intensity of diffraction of the (220) plane of the austenite phase (γ) and the (211) plane of the ferrite phase (α) is measured, and the following formula V _γ (%) = 100 / {1+ _{(I α R γ / I γ} R α)}
Where I _α : Integral intensity of α,
I _γ : Integral intensity of γ,
R _α : crystallographically calculated value of α,
R _γ : Conversion was performed using a crystallographic theoretical calculation value of γ.
The fraction of the martensite phase was the remainder other than these phases.

(2) Tensile test A round bar tensile test piece (parallel part 6mmφ x GL20mm) was taken from the center of the thickness of the obtained seamless steel pipe so that the rolling direction was the tensile direction, and stipulated in JIS Z 2241 A tensile test was carried out in accordance with the tensile properties (yield strength YS, tensile strength TS, elongation EL). The yield strength YS was the strength at 0.2% elongation.
(3) Impact test V-notch test specimens were collected from the center of thickness of the obtained seamless steel pipe so that the direction perpendicular to the rolling direction (C direction) was the specimen longitudinal direction, and JIS Z 2242 A Charpy impact test was conducted in accordance with the regulations. The test temperature was −40 ° C., and the absorbed energy vE ₋₄₀ (J) was determined. The number of test pieces was three each, and the average value thereof was taken as the absorbed energy of the steel pipe.

  The obtained results are shown in Table 3.

In all of the examples of the present invention, even in the central portion of the wall thickness exceeding 15 mm, YS: 654 MPa or more and vE ₋₄₀ : 50 J or more, a thick-walled seamless steel pipe showing high strength and high toughness. Yes. On the other hand, in the comparative examples that are outside the scope of the present invention, the desired high strength is not obtained, the desired high toughness is not obtained, or both are not obtained.

DESCRIPTION OF SYMBOLS 1 Heating apparatus 2 Hot working apparatus 21 Punching and rolling apparatus 22 Rolling apparatus 221 Elongator 222 Plug mill 223 Sizer 3 Cooling apparatus

Claims (9)

% By mass
C: 0.050% or less, Si: 0.50% or less,
Mn: 0.20-1.80%, Cr: 15.5-18.0%,
Ni: 1.5-5.0%, Mo: 1.0-3.5%,
V: 0.02 to 0.20%, Al: 0.05% or less,
N: 0.01 to 0.15%, O: 0.006% or less, the composition comprising the balance Fe and inevitable impurities,
In the range of thickness 1 / 4-3 / 4 thick central portion, by volume, more than 50% of martensite phase, and 3% to 15% of austenite phase, Ri Do balance being ferrite phase, and The ferrite phase has a structure in which a part of the martensite phase is granular with a particle size of 10 μm or less and includes 3 or more per 100 μm ^{2 of} ferrite particles , and the Ni concentration (C _Ni ) _γ ( %) And the ratio of Ni concentration (C _Ni ) _M (% by mass) in the martensite phase, (C _Ni ) _γ / (C _Ni ) _M , is 1.15 or more, characterized by low temperature toughness High strength stainless steel seamless steel pipe for oil wells. The high-strength stainless steel seamless pipe for oil wells according to claim 1, wherein the composition further comprises Cu: 3.5% or less in terms of mass% in addition to the composition. In addition to the above composition, in addition to mass, Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, W: 3.0% or less, B: 0.01% or less The high-strength stainless steel seamless steel pipe for oil wells according to claim 1 or 2 , wherein the composition contains a seed or more. In addition to the above composition, by mass%, Ca: 0.01% or less, REM: claims 1, characterized in that a composition comprising one or two species selected from among 0.01% or less any 3 High-strength stainless steel seamless steel pipe for oil wells. A method for producing a high-strength stainless steel seamless steel pipe for oil wells with excellent low-temperature toughness according to claim 1, wherein the steel material is subjected to hot working including piercing and rolling to form a seamless steel pipe.
The steel material in mass%,
C: 0.050% or less, Si: 0.50% or less,
Mn: 0.20-1.80%, Cr: 15.5-18.0%,
Ni: 1.5-5.0%, Mo: 1.0-3.5%,
V: 0.02 to 0.20%, Al: 0.05% or less,
N: 0.01 to 0.15%, O: 0.006% or less, the steel material of the composition consisting of the balance Fe and inevitable impurities,
After the steel material is heated to a heating temperature of 1150 to 1350 ° C., the heated steel material is cooled so that the temperature difference from the cooling start temperature at the thickness center temperature is at least 50 ° C. or more and 800 ° C. or more. After accelerated cooling at an average cooling rate of 2.0 ° C./s or higher up to the stop temperature, it is subjected to hot working including the piercing and rolling to be a seamless steel pipe, and then to the seamless steel pipe,
A low temperature characterized by performing at least one heat treatment consisting of a first stage treatment that is reheated to 750 to 950 ° C. and rapidly cooled, and a second stage treatment that is heated to a temperature of 550 to 680 ° C. A method for producing high-strength stainless steel seamless steel pipes with excellent toughness for oil wells. The steel material is heated to a heating temperature of 1150 to 1350 ° C. and then subjected to the piercing and rolling without performing the accelerated cooling before the hot working, and cooled at the thickness center temperature after the piercing and rolling is finished. The temperature difference from the start temperature is at least 50 ° C. or higher, and the cooling is accelerated to the cooling stop temperature in the temperature range of 800 ° C. or higher at a cooling rate of 2.0 ° C./s to 20 ° C. 6. The method for producing a high-strength stainless steel seamless pipe for oil wells according to claim 5 , wherein hot-working other than rolling is performed to obtain a seamless steel pipe, and then the heat treatment is performed on the seamless steel pipe. The method for producing a high-strength stainless steel seamless steel pipe for oil wells according to claim 5 or 6 , wherein the composition further comprises Cu: 3.5% or less by mass% in addition to the composition. In addition to the above composition, in addition to mass, Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, W: 3.0% or less, B: 0.01% or less The method for producing a high-strength stainless steel seamless steel pipe for oil wells according to any one of claims 5 to 7 , wherein the composition contains a seed or more. In addition to the above composition, by mass%, Ca: 0.01% or less, REM: any of claims 5-8, characterized in that a composition comprising one or two species selected from among 0.01% or less A method for producing a high-strength stainless steel seamless pipe for oil wells according to claim 1.

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