JP6315076B2 - Manufacturing method of high strength stainless steel seamless steel pipe for oil well - Google Patents

Description

The present invention relates to a seamless steel pipe for oil wells that contains carbon dioxide (CO ₂ ), chlorine ions (Cl ⁻ ), etc., and is suitable for use in oil wells and gas wells in extremely severe corrosive environments. The present invention relates to a method for producing a high-strength stainless steel seamless pipe for oil wells that combines excellent corrosion resistance and excellent low-temperature toughness.

  In recent years, from the viewpoint of soaring energy prices such as crude oil due to an increase in global energy consumption, and the depletion of petroleum resources, deep oil fields (deep oil fields), hydrogen sulfide, etc. that have not been previously excluded The development of oil fields and gas fields in severe corrosive environments under the so-called sour environment, and oil fields and gas fields in severe weather environments such as those in the extreme north, has been actively conducted. As a steel pipe used in such an environment, a steel pipe having high strength, excellent corrosion resistance (sour resistance), and excellent low temperature toughness is required. Also, the thickness of the steel pipe is required to vary from thin to thick.

Conventionally, 13% Cr martensitic stainless steel has been widely used as a steel material used for mining in oil fields and gas fields under the environment containing carbon dioxide CO ₂ , chlorine ion Cl ^{− and the} like. However, since 13% Cr martensitic stainless steel does not have sufficient corrosion resistance in a sour environment, there has been a demand for higher strength and improved corrosion resistance (sour resistance). In response to such a demand, for example, Patent Document 1 describes a high-strength stainless steel pipe for oil wells excellent in corrosion resistance and a method for producing the same. In the technique described in Patent Document 1, in mass%, C: 0.005 to 0.05%, Si: 0.05 to 0.5%, Mn: 0.2 to 1.8%, P: 0.03% or less, S: 0.005% or less, Cr: 15.5 -18%, Ni: 1.5-5%, Mo: 1-35%, V: 0.02-0.20%, N: 0.01-0.15%, O: 0.006% or less, and Cr + 0.65Ni + 0.6Mo + 0.55Cu- A steel tube material having a composition satisfying 20C ≧ 19.5 and Cr + Mo + 0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N ≧ 11.5 is heated and formed by hot working. Cool to room temperature at a cooling rate to obtain a seamless steel pipe of the specified dimensions, then reheat the seamless steel pipe to a temperature of 850 ° C or higher, then cool to 100 ° C or lower at a cooling rate of air cooling or higher, and then lower to 700 ° C or lower. It is characterized by performing a quenching-tempering treatment that is heated to a temperature.

This high-strength stainless steel seamless pipe has a structure containing a ferrite phase of 10-60% by volume and the balance being a martensite phase, yield strength: high strength of 654 MPa or more, CO ₂ and Cl ⁻ In addition, it has sufficient corrosion resistance even in a severe corrosive environment at a high temperature up to 230 ° C, and it has high toughness with absorbed energy at -40 ° C of Charpy impact test of 50J or more.

Japanese Patent No. 5109222 (Japanese Patent Laid-Open No. 2005-336595)

  In the technique described in Patent Document 1, most hot working is performed in a two-phase temperature range of a ferrite phase and an austenite phase. The fraction of the ferrite phase and the austenite phase varies depending on the heating and holding temperature and the heating time, but in general, the higher the temperature, the higher the fraction of the ferrite phase. Since ferrite has a fast grain growth, if it is kept at a high temperature, it tends to coarsen even if the initial grains are fine.

  Generally, the ferrite grains can be divided and refined by hot working. However, with thick materials, even if hot working is performed, it is difficult to impart sufficient strain to the center of the wall thickness, so ferrite grains cannot be separated at the center of the wall thickness. The coarsening of the ferrite grains may also occur by cooling after cold rolling. Coarse ferrite grains, particularly connected coarse ferrite grains, tend to be a propagation path for brittle cracks. For this reason, for example, in a steel pipe rolled at a high temperature with a high fraction of ferrite phase, in particular, a structure containing coarse ferrite grains is formed at the center of the thickness, and the toughness is often lowered. Further, when the ferrite grains become coarse, the strength is also affected, and in particular, the yield strength may be reduced.

  For this reason, in order to ensure the desired strength and toughness in the stainless steel seamless steel pipe, it is necessary to appropriately perform the temperature control in the hot rolling conditions and the subsequent heat treatment. However, Patent Document 1 does not mention temperature control during high-temperature heating or hot working of the material, and the temperature during high-temperature heating or hot working of the material affects the low-temperature toughness of the stainless steel seamless pipe. There is no mention of the impact. Moreover, the seamless steel pipe targeted by the technology described in Patent Document 1 is a steel pipe having a wall thickness of up to 12.7 mm at the most. Therefore, Patent Document 1 describes a thick steel pipe having a wall thickness exceeding 13 mm. There is no mention until.

  An object of the present invention is to provide a method for producing a high-strength stainless steel seamless pipe for oil wells that has high strength, excellent corrosion resistance, and excellent low-temperature toughness. .

The term “high strength” as used herein refers to the case where the yield strength YS: 654 MPa (95 ksi) or higher is maintained, and “excellent low temperature toughness” refers to the Charpy impact test in accordance with JIS Z 2242. Test temperature: The case where the absorbed energy vE- ₄₀ at −40 ° C. is 50 J or more.

The term “excellent corrosion resistance” as used herein refers to a test solution held in an autoclave: 20% by mass NaCl aqueous solution (liquid temperature: 230 ° C, 100 atm CO ₂ gas atmosphere). The corrosion rate when the immersion time is 7 days (168 hours) is less than 0.12 mm / y.

  In order to achieve the above-mentioned object, the present inventors diligently studied various factors affecting the low-temperature toughness of a high-strength stainless steel seamless pipe having a ferrite-martensite two-phase structure. As a result, in the stainless steel seamless steel pipe having the above-described two-phase structure, it has been thought that the refinement of the ferrite phase is the most effective means for improving the low temperature toughness.

  When manufacturing a high-strength stainless steel seamless steel pipe having a ferrite-martensite two-phase structure as described above, a steel pipe material adjusted to a predetermined composition is usually heated to a predetermined heating temperature range, such as piercing and rolling. Perform hot working. In a predetermined heating temperature range, the steel pipe material exhibits a structure composed of two phases of ferrite and austenite, but as the heating temperature rises, the fraction of the ferrite phase increases. Moreover, the ferrite grains are likely to grow rapidly and become coarse ferrite grains by being held at a high temperature for a long time. When coarse ferrite grains are formed, the low temperature toughness decreases.

  Therefore, the present inventors have further studied a measure for preventing the formation of the coarse ferrite grains. As a result, it has been found that when the austenite phase fraction is heated to a temperature range where the volume fraction is less than 10%, the growth rate of ferrite grains starts to increase rapidly. Based on this knowledge, the present inventors, in the heating process in the production of seamless steel pipes, if the austenite phase fraction is heated in a temperature range where the volume fraction is 10% or more, the grain growth of the ferrite grains during heating It was found that can be suppressed.

  If the above-mentioned heating is applied in all heating processes in the production of seamless steel pipes, and the growth of ferrite grains during heating can be suppressed, the ferrite grains can be divided and refined in the subsequent hot rolling process, and the structure becomes finer It was found that can be easily achieved.

The present invention has been completed on the basis of such findings and further studies. That is, the gist of the present invention is as follows.
(1) A steel pipe material processing step in which a steel material is heated and hot-worked to form a steel pipe material, and a hot-working step in which the steel pipe material is heated, piped, formed, and made into a seamless steel pipe, A heat treatment step for performing heat treatment on the seamless steel pipe, and a method for producing the seamless steel pipe,
The steel material is in mass%, C: 0.005-0.05%, Si: 0.05-0.5%, Mn: 0.2-1.8%, P: 0.03% or less, S: 0.005% or less, Cr: 15.5-18.0%, Ni : 1.5 to 5.0%, Mo: 1.0 to 3.5%, V: 0.02 to 0.2%, Al: 0.001 to 0.050%, N: 0.001 to 0.15%, O: 0.006% or less, the following formula (1) and the following (2 )formula
Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 19.5 (1)
Cr + Mo + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N ≧ 11.5 (2)
(Here, Cr, Ni, Mo, Cu, C, Si, Mn, N: content of each element (mass%))
And the balance is composed of Fe and inevitable impurities, and the heating in the steel pipe material processing step and the hot working step is performed at least in the volume fraction of the austenite phase at the time of heating. Performed at a temperature condition of 10% or more,
In the heat treatment in the heat treatment step, a method for producing a high-strength stainless steel seamless pipe for oil wells having a yield strength YS: 654 MPa or more, which is subjected to a tempering treatment to be heated to a temperature of 700 ° C. or less.
(2) In (1), in the heat treatment in the heat treatment step, before the tempering treatment, after reheating to a temperature of 850 ° C. or higher, a quenching treatment is performed to cool to 100 ° C. or lower at a cooling rate of air cooling or higher. Manufacturing method for high strength stainless steel seamless steel pipes for oil wells.
(3) In (1) or (2), in addition to the above-mentioned composition, the high strength stainless steel joint for oil wells further containing, by mass%, one group or two or more groups selected from the following groups A to C: Manufacturing method of steelless pipe.
Group A: Cu: 3.5% or less,
Group B: 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 selected from 0.01% or less,
Group C: one or two selected from Ca: 0.01% or less, REM: 0.01% or less

  According to the present invention, a high-strength stainless steel seamless pipe for oil wells that has high strength, excellent corrosion resistance, and excellent low-temperature toughness can be easily manufactured, and has a remarkable industrial effect. According to the present invention, even with a relatively small amount of processing, the ferrite grains can be made fine up to the thickness center of a high-strength stainless steel seamless steel pipe, and the thickness at which the processing amount at the center of the thickness cannot be increased. Even in the seamless steel 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 process of a seamless steel pipe.

The manufacturing method of the high strength stainless steel seamless steel pipe for oil wells of the present invention is a steel material, a steel pipe material processing step for heating and hot-working to make a steel pipe material, and the steel pipe material is heated, piped, molded, A method of manufacturing a seamless steel pipe, comprising a hot working process for making a seamless steel pipe and a heat treatment process for heat-treating the seamless steel pipe, wherein the steel material is in mass%, C: 0.005-0.05%, Si : 0.05-0.5%, Mn: 0.2-1.8%, P: 0.03% or less, S: 0.005% or less, Cr: 15.5-18.0%, Ni: 1.5-5.0%, Mo: 1.0-3.5%, V: 0.02- Contains 0.2%, Al: 0.001 to 0.050%, N: 0.001 to 0.15%, O: 0.006% or less so as to satisfy the formula (1) and the following formula (2), and the balance from Fe and inevitable impurities The heating in the steel pipe material processing step and the hot working step is performed at a temperature at which the austenite phase at the time of heating is at least 10% by volume fraction. Performed under the conditions, the heat treatment in the heat treatment step is subjected to tempering treatment of heating to a temperature of 700 ° C. or less. The high strength stainless steel seamless steel pipe for oil wells obtained by this production method has a yield strength of YS: 654 MPa or more.
Record
Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 19.5 (1)
Cr + Mo + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N ≧ 11.5 (2)
Here, Cr, Ni, Mo, Cu, C, Si, Mn, N: Content of each element (mass%)
An example of a manufacturing process of a seamless steel pipe that can be suitably used in the present invention is shown in FIG.

  In the present invention, first, in the steel pipe material processing step, the steel material is heated and hot worked to obtain a steel pipe material.

  In addition, the steel raw material used by this invention does not need to specifically limit the manufacturing method. It is preferable that molten steel having a desired composition is melted using a conventional melting furnace such as a converter or an electric furnace, and a slab produced by a conventional casting method such as a continuous casting method is used as a steel material. In addition, there is no problem even if the steel piece manufactured by the ingot-making and ingot rolling method is used as a steel material.

The steel material used in the present invention is in mass%, C: 0.005-0.05%, Si: 0.05-0.5%, Mn: 0.2-1.8%, P: 0.03% or less, S: 0.005% or less, Cr: 15.5- 18.0%, Ni: 1.5-5.0%, Mo: 1.0-3.5%, V: 0.02-0.2%, Al: 0.001-0.050%, N: 0.001-0.15%, O: 0.006% or less, the following formula (1) And the following equation (2)
Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 19.5 (1)
Cr + Mo + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N ≧ 11.5 (2)
(Here, Cr, Ni, Mo, Cu, C, Si, Mn, N: content of each element (mass%))
So that the balance is composed of Fe and inevitable impurities. Furthermore, in addition to the above composition, the steel material may contain one group or two or more groups selected from the following groups A to C.
Group A: Cu: 3.5% or less,
Group B: 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 selected from 0.01% or less,
Group C: one or two selected from Ca: 0.01% or less, REM: 0.01% or less

  Next, the reasons for limiting the composition of the steel material used will be described. Hereinafter, the mass% in the composition is simply expressed as%.

C: 0.005-0.05%
C needs to contain 0.005% or more in order to secure a desired strength. On the other hand, when C exceeds 0.05%, sensitization during tempering due to Ni inclusion increases. For this reason, the C content is limited to a range of 0.005 to 0.05%. In addition, Preferably, C content is 0.030 to 0.040%.

Si: 0.05-0.5%
Si is an element that acts as a deoxidizer, and in order to obtain such an effect, it is necessary to contain 0.05% or more of Si. On the other hand, the content of Si exceeding 0.5% decreases the corrosion resistance and hot workability. For this reason, Si content was limited to the range of 0.05 to 0.5%. In addition, Preferably, Si content is 0.10 to 0.30%.

Mn: 0.2-1.8%
Mn has the effect of increasing strength. In order to obtain such an effect, it is necessary to contain 0.2% or more of Mn. On the other hand, when Mn is contained exceeding 1.8%, toughness is reduced. For this reason, Mn content was limited to 0.2 to 1.8% of range. In addition, Preferably, Mn content is 0.20 to 1.00%. More preferably, the Mn content is 0.30 to 0.50%.

P: 0.03% or less
P is an element that segregates as an impurity at grain boundaries and has an adverse effect on corrosion resistance, low temperature toughness, and the like. In the present invention, it is desirable to reduce the P content as much as possible, but excessive reduction leads to an increase in refining costs. A P content of 0.03% or less is acceptable because it does not have a significant adverse effect on corrosion resistance, low temperature toughness, and the like. For this reason, the P content is limited to 0.03% or less. Preferably, the P content is 0.02% or less.

S: 0.005% or less
S is an element that significantly reduces hot workability, and it is desirable to reduce it as much as possible, but excessive reduction leads to an increase in refining costs. If the S content is 0.005% or less, normal hot working can be performed. For this reason, S content was limited to 0.005% or less. Preferably, the S content is 0.002% or less.

Cr: 15.5-18.0%
Cr has a function of forming a protective film and improving corrosion resistance. Furthermore, the strength of the steel is increased by solid solution. In order to obtain such an effect, it is necessary to contain 15.5% or more of Cr. On the other hand, when Cr is contained in a large amount exceeding 18.0%, the hot workability is lowered and the strength is also lowered. For this reason, the Cr content is limited to the range of 15.5 to 18.0%. In addition, Preferably, Cr content is 16.6 to 17.5%.

Ni: 1.5-5.0%
Ni has the effect of strengthening the protective film and enhancing the corrosion resistance. Furthermore, it dissolves to increase the strength of the steel and further improve the toughness. Such an effect is recognized when the Ni content is 1.5% or more. On the other hand, if Ni exceeds 5.0%, the retained austenite phase increases and the strength decreases. For this reason, Ni content was limited to 1.5 to 5.0%. In addition, Preferably, Ni content is 2.5 to 4.5%. More preferably, the Ni content is 3.5 to 4.0%.

Mo: 1.0-3.5%
Mo increases the resistance to pitting corrosion generated by chlorine ions (Cl ⁻ ) and improves the corrosion resistance. In order to obtain such an effect, it is necessary to contain 1.0% or more of Mo. On the other hand, when a large amount of Mo exceeds 3.5%, the strength decreases and the material cost increases. For this reason, Mo content was limited to 1.0 to 3.5% of range. In addition, Preferably, Mo content is 2.0 to 3.5%. More preferably, the Mo content is 2.0 to 3.0%.

V: 0.02-0.2%
V has the effect of increasing the strength and improving the corrosion resistance. In order to obtain such an effect, it is necessary to contain 0.02% or more of V. On the other hand, if V exceeds 0.2%, toughness decreases. For this reason, the V content is limited to the range of 0.02 to 0.2%. In addition, Preferably, V content is 0.02 to 0.08%. More preferably, the V content is 0.04 to 0.07%.

Al: 0.001 to 0.050%
Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it is necessary to contain 0.001% or more of Al. On the other hand, the content of Al exceeding 0.050% adversely affects toughness. For this reason, Al content is limited to 0.001 to 0.050%. More preferably, the Al content is 0.002 to 0.030%.

N: 0.001 to 0.15%
N is an element that significantly improves the pitting corrosion resistance. In order to obtain such an effect, the N content of 0.001% or more is required. On the other hand, when N exceeds 0.15%, various nitrides are formed and the toughness is lowered. For this reason, N content was limited to 0.001 to 0.15% of range. In addition, Preferably, N content is 0.002 to 0.008%.

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 the O content 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. Therefore, the O (oxygen) content is limited to 0.006% or less.

In the present invention, the above-described components are within the above-described range, and the following formulas (1) and (2)
Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 19.5 (1)
Cr + Mo + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N ≧ 11.5 (2)
The content is adjusted so as to satisfy. Here, Cr, Ni, Mo, Cu, C, Si, Mn, and N are content (mass%) of each element. In addition, when calculating the left-side value of the formulas (1) and (2), the elements not included in the elements described in the formulas (1) and (2) are calculated as 0%. To do.

By adjusting the Cr, Ni, Mo, Cu, and C contents so as to satisfy the formula (1), the corrosion resistance in a high-temperature corrosive environment containing CO ₂ and Cl ⁻ is remarkable at a high temperature up to 230 ° C. improves. From the viewpoint of corrosion resistance in a high temperature corrosive environment containing CO ₂ and Cl ⁻ , the value on the left side of the formula (1) is preferably 20.0 or more.

  Moreover, hot workability improves by adjusting Cr, Mo, Si, C, Mn, Ni, Cu, and N content so that Formula (2) may be satisfied. In the present invention, P, S, and O are remarkably reduced, but only reducing P, S, and O ensures the hot workability necessary and sufficient for forming martensitic stainless steel seamless pipes. In order to make martensitic stainless steel seamless pipes, Cr, Mo, Si, C, Mn should be satisfied so that P, S, and O are reduced and (2) is satisfied. It is important to adjust the contents of Ni, Cu and N. In addition, from the viewpoint of improving hot workability, it is preferable that the right side value of the formula (2) is 12.0 or more.

  The balance other than the above components is Fe and inevitable impurities. As an inevitable impurity, for example, Co: 0.1% or less is acceptable.

  In addition to the basic components described above, one or more groups selected from Group A to Group C can be further contained as selective elements.

Group A: Cu: 3.5% or less Group B: Cu strengthens the protective film, suppresses the penetration of hydrogen into the steel, and improves the resistance to sulfide stress corrosion cracking. In order to acquire such an effect, it is desirable to contain Cu 0.5% or more. On the other hand, if Cu content exceeds 3.5%, CuS grain boundary precipitation occurs, and hot workability decreases. For this reason, when Cu is contained, the Cu content is preferably limited to 3.5% or less. In addition, Cu content becomes like this. More preferably, it is 0.6 to 1.2%, More preferably, it is 0.8 to 1.14%.

Group B: 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 selected from Group B: Nb, Ti , Zr, W, and B are all elements contributing to an increase in strength, and can be selected and contained as necessary. In order to obtain such an effect, it is desirable to contain Nb: 0.03% or more, Ti: 0.03% or more, Zr: 0.03% or more, W: 0.2% or more, B: 0.0002% 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.

Group C: Ca: 0.01% or less, REM: One or two types selected from 0.01% or less Group C: Ca, REM has the effect of spheroidizing the shape of sulfide inclusions, and intervenes It is an element that has the effect of reducing the lattice strain of the matrix around the object and reducing the hydrogen trapping ability of inclusions, and can contain one or two kinds as necessary. In order to obtain such an effect, it is desirable to contain Ca: 0.0005% or more and REM: 0.001% or more. However, if it contains more than Ca: 0.01% and REM: 0.01%, the corrosion resistance decreases. For this reason, when it contains, it is preferable to limit to Ca: 0.01% or less and REM: 0.01% or less.

  In the steel pipe material processing step of the present invention, the steel material having the above composition is heated to a predetermined temperature by the heating device 1 shown in FIG. 1 and hot-worked by the hot processing device 2 to obtain a steel pipe material. A round billet of a predetermined shape is used. As the hot working apparatus 2, a hot working apparatus such as a steel slab rolling mill (rough and finish rolling mill) to be used is usually applicable.

  Next, the obtained steel pipe material is charged into a steel pipe material heating device 3 and heated to a predetermined temperature, and then the heated steel pipe material is hot-worked by a hot working device 4 to produce a product pipe (seam A hot-working process is carried out. In the middle of hot working, for example, reheating may be performed by the reheating device 5 and hot working may be continued.

  The hot working device 4 used in the hot working process is usually a hot working device used when the steel pipe material is a seamless steel pipe having a predetermined size, for example, Mannesmann-plug mill method, Mannesman-mandrel mill method. Any hot working apparatus such as can be applied. As the hot working apparatus 4, there is a piercing and rolling apparatus 41 that performs piercing and rolling on a heated steel pipe material to form a hollow material. As the piercing and rolling device 41, for example, a Mannesmann inclined piercing machine using a barrel-type roll, a cone-type roll, or the like can be applied. In addition to this, any conventionally known piercing and rolling apparatus such as a hot extrusion piercing machine can be applied.

  Further, as the hot working device 4, there is a rolling device 42 that performs hot working on the hollow material obtained by the piercing and rolling device 41 to obtain a seamless steel pipe having a predetermined shape. As the rolling device 42, any conventionally known rolling device that performs reduction rolling, straightening rolling, or the like can be applied. For example, as the rolling device 42, there are an elongator 421 for expanding the hollow element tube, a plug mill 422 for extending the expanded hollow element thinly and long, a reeler 423 for smoothing the inner and outer surfaces of the hollow element tube, and a hollow element tube. It is preferable that the sizing mill 424 that adjusts to a predetermined size is arranged in that order.

  In addition, the reheating apparatus 5 for compensating for the drop in the steel pipe temperature can be installed in the intermediate process of hot rolling. In FIG. 1, the example provided in the entrance side of the sizing mill 424 is shown. Although not shown, depending on the purpose, as the rolling device 42, a mandrel mill in which the hollow hollow tube is a hollow steel tube of a predetermined size, a reducer that performs a slight reduction to adjust the outer diameter and thickness. May be sequentially arranged.

  In the present invention, the heating of the steel material in the steel pipe material processing step described above, and the heating or reheating of the steel pipe material in the hot processing step described above are performed under a temperature condition where the austenite phase is at least 10% by volume fraction. Do.

  The heating device 1 used for heating the steel material in the steel tube material processing step may be any ordinary heating furnace that can heat the steel material to a predetermined temperature, and is not particularly limited. For example, the walking beam heating furnace Can be illustrated. Note that an induction heating furnace may be used. Moreover, the steel pipe raw material heating apparatus 3 used for heating the steel pipe raw material in the hot working process described above may be any ordinary heating furnace capable of heating the steel pipe raw material to a predetermined temperature, and is not particularly limited. A rotary hearth type heating furnace can be exemplified. Note that an induction heating furnace may be used.

  In the present invention, the heating in the steel tube material processing step, the heating in the hot processing step, or the reheating causes the ferrite grains to grow under a temperature condition in which the austenite phase fraction is less than 10% in volume fraction. The structure of the product pipe becomes coarse, and it becomes impossible to secure desired characteristics, particularly desired low temperature toughness. For this reason, heating (including reheating) in the steel pipe material processing step and the hot processing step was limited to temperature conditions where the austenite phase fraction was 10% or more in terms of volume fraction. If the austenite phase fraction during heating is less than 10% in terms of volume fraction, the ferrite grains grow remarkably, and it becomes impossible to secure desired fine ferrite grains, and the low temperature toughness of the product pipe is lowered.

  In addition, since the austenite phase fraction during heating depends on the composition and the heating temperature, in the present invention, the austenite phase fraction and the heating temperature are determined in advance according to the composition of the target steel pipe by an equilibrium calculation or a heating experiment. Know the relationship in detail. Thereby, the lower limit temperature at which the volume fraction of the austenite phase is less than 10% can be determined. The “heating temperature” here is the maximum heating temperature of the heating from the viewpoint of the austenite phase fraction during heating. Note that the holding time at the maximum heating temperature is not particularly limited. The influence of the holding time at the maximum heating temperature is small compared to the influence of the heating temperature, and it is desirable to hold it for a short time from the viewpoint of productivity, etc., but in the range where the heated material is uniformly heated to the predetermined temperature. Needless to say, it is limited.

  In the present invention, the lower limit temperature at which the austenite phase fraction is less than 10% in terms of volume fraction is obtained from the relationship between the austenite phase fraction during heating and the heating temperature ascertained in advance. The heating described above is carried out by adjusting the temperature to be lower than the temperature. And it is preferable to set it as 1000 degreeC or more from a viewpoint of keeping hot workability favorable. In the case of performing piercing and rolling as hot working, it is more preferable that the temperature range is 1100 ° C. or higher.

  The steel pipe material heated under the above-described heating conditions is subjected to a hot working process to obtain a product pipe (seamless steel pipe). The hot working process is not particularly limited as long as it can be a seamless steel pipe having a predetermined dimension and shape, and a normal hot working apparatus is used so that the predetermined dimension and shape can be secured. Hot working such as piercing rolling, stretching rolling, reduced diameter rolling, straightening rolling and the like can be performed. After these hot workings, the steel can be cooled to room temperature at a cooling rate equal to or higher than that of air cooling to obtain a seamless steel pipe (heat treated raw pipe) having a predetermined size and shape. When cooled to room temperature at an appropriate cooling rate equal to or higher than air cooling, the martensite phase becomes the main structure, and a desired quenched structure can be obtained. Further, the “cooling rate higher than air cooling” referred to herein means 0.01 ° C./s or higher.

  The obtained heat-treated raw pipe (seamless steel pipe) is then heat-treated in a heat treatment step.

  In the heat treatment in the heat treatment step, a tempering treatment is performed by heating to a temperature of 700 ° C. or lower.

  By heating to a tempering temperature of 700 ° C. or lower, preferably 400 ° C. or higher, and tempering, the structure is mainly composed of a tempered martensite phase and has a structure consisting of a fine ferrite phase and a retained austenite phase. And a high-strength stainless steel seamless steel pipe (product pipe) having excellent low-temperature toughness.

  Moreover, you may perform a quenching process before the above-mentioned tempering process. The quenching treatment is preferably a treatment of reheating to a quenching temperature: 850 ° C. or higher and then cooling to 100 ° C. or lower, preferably 50 ° C. or lower at a cooling rate of air cooling or higher. If the quenching temperature is less than 850 ° C., quenching may be insufficient and desired strength may not be ensured. For this reason, it is preferable to limit the quenching temperature to a temperature of 850 ° C. or higher. Further, if the quenching temperature is too high, the amount of tempered martensite may be reduced and desired quenching characteristics may not be obtained. Therefore, the temperature is preferably 1050 ° C. or lower. Further, the “cooling rate higher than air cooling” referred to herein means 0.01 ° C./s or higher.

  By these heat treatments, a structure mainly composed of a tempered martensite phase and having a fine ferrite phase and a retained austenite phase can be obtained.

  The “subject” here refers to the phase with the largest area ratio. The “structure mainly composed of a tempered martensite phase” refers to a case where the phase occupies 50% or more by volume ratio. The residual austenite phase is 20% or less by volume. Further, the ferrite phase is 10 to 40% by volume, preferably 20 to 35%. If the ferrite phase is less than 10%, desired corrosion resistance cannot be obtained. On the other hand, if the content exceeds 40%, the strength decreases.

  Further, in the present invention, mainly a tempered martensite phase, as a measurement method of the structure having a fine ferrite phase and a retained austenite phase, first, from a seamless steel pipe, to collect a specimen for structure observation, Polishing so that the cross section perpendicular to the tube axis direction (C cross section) becomes the observation surface and corroding (corrosion of bilera liquid (mixed with picric acid, hydrochloric acid and ethanol in proportions of 2 g, 10 ml and 100 ml, respectively)) The tissue is observed and imaged using an optical microscope (magnification: 100 times). Using the obtained tissue photograph, image analysis is performed to obtain a tissue fraction. For ferrite grains, measure by the EBSD method (Electron Backscatter Diffraction Method) using a scanning electron microscope, and calculate the area of the average grain with the area where the orientation of adjacent grains differ by 5 ° or more as different grains. .

The retained austenite phase fraction is measured using an X-ray diffraction method. In X-ray diffraction, the diffraction X-ray integral intensity of the (220) plane of austenite γ and the (211) plane of α is measured, and the following formula γ (volume ratio) = 100 / {1+ (IαRγ / IγRα)}
(Here, Iα: α integral strength, Iγ: γ integral strength, Rα: α crystallographic theoretical calculation value, Rγ: γ crystallographic theoretical calculation value)
Calculate using. The fraction of the tempered martensite phase is calculated as the remainder other than these phases.

  In addition, the structure of the present invention can be controlled by setting a specific component composition, performing a tempering process at a specific temperature, performing a quenching process at a specific temperature, and the like.

  In the quenching process, the quenching temperature is heated to a temperature of 850 ° C. or higher. Even in this case, the heating temperature is adjusted so as to satisfy the temperature condition that the austenite phase is 10% or more by volume. There is a need to. By heating while satisfying such heating conditions, grain growth can be suppressed and desired excellent mechanical properties can be stably obtained.

  In addition, the temperature which performs said heat processing shall be the temperature of the steel pipe outer surface.

  A steel pipe having such a structure has a high strength of yield strength: 654 MPa or more, and an excellent low temperature toughness with an absorption energy of 50 J or more at −40 ° C. in a Charpy impact test at the center of the thickness. It has a seamless steel pipe. The steel pipe manufactured by the manufacturing method of the present invention can be a thick steel pipe having a wall thickness exceeding 13 mm.

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

  Molten steel having the composition shown in Table 1 was melted in a converter and cast into a slab (slab: wall thickness 260 mm) by a continuous casting method.

  First, a test piece (20 mm × 20 mm × 10 mm) was sampled from the obtained cast piece and subjected to a heat treatment experiment. The test piece was heated to various temperatures in an experimental heat treatment furnace (small heat treatment furnace), held for 30 minutes, then immersed in water and rapidly cooled. From the obtained specimens, specimens for tissue observation were collected, polished and corroded (corrosion solution: Viera solution), and the tissues were observed with an optical microscope (magnification: 100 times) and imaged in each of 9 or more fields of view. . Image analysis of the obtained structure | tissue photograph was performed, and the area ratio of the martensite phase was computed. The area ratio of the obtained martensite phase was assumed to be equivalent to the volume fraction of the austenite phase at the heating temperature. And from the volume fraction of the obtained austenite phase, the relationship between the austenite phase fraction, the heating temperature and the holding time is obtained for each steel, and the lower limit temperature T (the volume fraction of the austenite phase is less than 10%) ° C). The obtained results are shown together in Table 1.

  From Table 1, it can be seen that the lower limit temperature T (° C.) at which the austenite phase is less than 10% is different for each steel having a different composition.

  Next, the obtained slab was charged into a heating device, heated under the conditions shown in Table 2 (billet rolling: maximum heating temperature, extraction temperature), subjected to hot working (billet rolling), and rounded. Billet (outer diameter: 260mmφ) was used as the steel pipe material. In addition, it stood to cool after hot processing.

  Next, the obtained steel pipe material is charged into a steel pipe material heating device, heated under the conditions shown in Table 2 (piercing and rolling: maximum heating temperature, extraction temperature), and heat in which a piercer, an elongator, a plug mill, and a reeler are sequentially installed. It was hot-worked with a hot-working machine, and further charged into a re-heating machine and hot-worked (rectified rolling) with a sizing mill to produce a seamless steel pipe (outer diameter 244.5 mmφ x wall thickness 13.84 mm). In addition, it air-cooled after hot processing.

  The heating conditions were five types of patterns, pattern a to pattern e.

  Then, the obtained seamless steel pipe was subjected to quenching treatment and tempering treatment under the conditions shown in Table 3 as a heat treatment step. In some cases, the quenching process was not performed and only the tempering process was performed. In addition, the hardening process was set as the process immersed in a water tank, after charging a steel pipe in the hardening heating furnace and heating to the hardening heating temperature shown in Table 3. A tempering treatment was performed under the conditions shown in Table 3 after the quenching treatment or without the quenching treatment. After tempering, air cooling was performed.

  Test pieces were sampled from the obtained heat-treated seamless steel pipe and subjected to structure observation, tensile test, impact test and corrosion resistance test. The test method was as follows.

(1) Microstructure observation A specimen for microstructural observation is collected from the obtained seamless steel pipe, and is polished and corroded so that a cross section (C cross section) perpendicular to the tube axis direction becomes an observation surface. Then, the tissue was observed and imaged using an optical microscope (magnification: 100 times). Using the obtained tissue photograph, image analysis was performed to obtain a tissue fraction. The ferrite grains were measured by an EBSD method using a scanning electron microscope, and the area of the average crystal grains was calculated with different crystal grains in regions where the orientations of adjacent grains differed by 5 ° or more.

The retained austenite phase fraction was measured using an X-ray diffraction method. In X-ray diffraction, the diffraction X-ray integral intensity of the (220) plane of austenite γ and the (211) plane of α is measured, and the following formula γ (volume ratio) = 100 / {1+ (IαRγ / IγRα)}
Here, the calculation was performed using the integral intensity of Iα: α, the integral intensity of Iγ: γ, the crystallographic theoretical calculation value of Rα: α, and the crystallographic theoretical calculation value of Rγ: γ. The fraction of the tempered martensite phase was calculated as 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 wall thickness of the obtained seamless steel pipe so that the pipe axis direction is the tensile direction, and JIS Z 2241 specified. The tensile test was carried out according to the above. The yield strength was the strength at elongation: 0.2%. Yield strength YS of 654MPa (95ksi) or higher was accepted.

(3) Impact test V-notch test specimens were collected from the thickness center of the obtained seamless steel pipe so that the direction perpendicular to the rolling direction (C direction) was the specimen longitudinal direction, and specified in JIS Z 2242 The Charpy impact test was conducted according to the above. The test temperature was −40 ° C., the absorbed energy was measured, and the toughness was evaluated. The number of test pieces was three each, and the average value thereof was the absorbed energy (J) of the seamless steel pipe. Test temperature: The case where the absorbed energy vE- ₄₀ at −40 ° C. was 50 J or more was evaluated as “◯” (pass) as “excellent in low temperature toughness”. Even if the average absorbed energy value is 50 J or more, if there is an individual value less than 50 J, it is evaluated as “△”, and if the average absorbed energy value is less than 50 J, “×”. And both were rejected.

(4) Corrosion resistance test A corrosion test piece (size: thickness 3 mm x width 30 mm x length 40 mm) was sampled from the obtained seamless steel pipe and subjected to a corrosion test. In addition, the corrosion resistance test was not implemented about the steel pipe outside this invention range.

The corrosion test was conducted by immersing the corrosion test piece in a test solution retained in an autoclave: 20% by mass NaCl aqueous solution (liquid temperature: 230 ° C., 100 atmospheres CO ₂ gas atmosphere), and immersion period: 7 days (168 Time). The test piece after the corrosion test was weighed, and the corrosion rate calculated from the weight loss before and after the corrosion test was obtained. The case where the corrosion rate was less than 0.12 mm / y was regarded as acceptable.

  The obtained results are shown in Table 3.

  In all of the examples of the present invention, the microstructure can be refined, the desired corrosion resistance is maintained, and the yield strength is 654 MPa or higher, but the absorbed energy at a test temperature of −40 ° C. is 50 J or more. The toughness is remarkably improved. On the other hand, in the comparative example outside the scope of the present invention, the maximum heating temperature in the heating in the steel pipe material processing step and the heating in the hot processing step is equal to or higher than the lower limit temperature at which the austenite phase is less than 10% in volume fraction. The structure cannot be refined and the desired high toughness cannot be ensured.

DESCRIPTION OF SYMBOLS 1 Heating apparatus 2 Hot processing apparatus 3 Steel pipe material heating apparatus 4 Hot processing apparatus 5 Reheating apparatus
41 Drilling and rolling equipment
42 Rolling equipment
421 Elongator
422 Plug mill
423 Leela
424 Sizing Mill

Claims (3)

Steel pipe material processing process that heats and hot-processes steel material to make steel pipe material,
The steel pipe material is heated, pipe-formed, molded, and a hot-working process to be a seamless steel pipe,
A heat treatment step of performing heat treatment on the seamless steel pipe, and a method for producing the seamless steel pipe,
The steel material is mass%,
C: 0.005-0.05%, Si: 0.05-0.5%,
Mn: 0.2 to 1.8%, P: 0.03% or less,
S: 0.005% or less, Cr: 15.5-18.0%,
Ni: 1.5-5.0%, Mo: 1.0-3.5%,
V: 0.02 to 0.2%, Al: 0.001 to 0.050%,
N: 0.001 to 0.15%,
O: 0.006% or less is contained so as to satisfy the following formula (1) and the following formula (2), with the balance being composed of Fe and inevitable impurities,
The heating in the steel pipe material processing step and the hot working step is performed under a temperature condition in which the austenite phase during the heating is at least 10% in volume fraction,
In the heat treatment in the heat treatment step, a high strength stainless steel for oil wells having a yield strength YS: 654 MPa or higher , an absorbed energy vE- ₄₀ at a test temperature of −40 ° C. of 50 J or higher, which is tempered to a temperature of 700 ° C. or lower. A method for producing seamless steel pipes.
Record
Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 19.5 (1)
Cr + Mo + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N ≧ 11.5 (2)
Here, Cr, Ni, Mo, Cu, C, Si, Mn, N: Content of each element (mass%)   2. The oil well according to claim 1, wherein in the heat treatment in the heat treatment step, before the tempering treatment, after reheating to a temperature of 850 ° C. or higher, a quenching treatment is performed to cool to 100 ° C. or lower at a cooling rate of air cooling or higher. Manufacturing method of high-strength stainless steel seamless pipe. The production of high strength stainless steel seamless steel pipes for oil wells according to claim 1 or 2, further comprising one group or two or more groups selected from the following groups A to C in mass% in addition to the composition. Method.
Group A: Cu: 3.5% or less,
Group B: 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 selected from 0.01% or less,
Group C: one or two selected from Ca: 0.01% or less, REM: 0.01% or less

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