JP2018162507A - High-strength oil well steel and oil well pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel material for a high-strength oil well, which has excellent SSC resistance, corrosion resistance comparable to that of low alloy steel from the viewpoint of total corrosion, and excellent toughness. SOLUTION: The chemical composition is mass%, C: 0.50 to 1.40%, Si: 0.05 to 1.00%, Mn: 3.0 to 24.0%, Ni: 5.0. ~ 19.0%, Al: 0.003 ~ 0.06%, P: 0.03% or less, S: 0.03% or less, N: 0.10% or less, Cr: 0 to 2.0%, Mo: 0 to 2.0%, Cu: 0 to 2.0%, Ti: 0 to 0.5%, V: 0 to 0.5%, Nb: 0 to 0.5%, Ca: 0 to 0 .01%, Mg: 0-0.01%, B: 0-0.005%, balance: Fe and impurities, and the metal structure is austenite single phase, high-strength steel for oil wells. [Selection diagram] None

Description

  The present invention relates to a high-strength steel material for oil wells and oil well pipes.

Oil wells and gas wells such as crude oil and natural gas containing hydrogen sulfide (H ₂ S) (hereinafter, oil wells and gas wells are simply referred to as “oil wells”) are steel sulfides in a wet hydrogen sulfide environment. Since stress cracking (hereinafter referred to as “SSC”) becomes a problem, an oil well pipe having excellent SSC resistance is required. In recent years, the strength of low-alloy sour well pipes has been increased for casing applications.

The SSC resistance decreases rapidly as the steel strength increases. Therefore, it is 110 ksi class (yield stress: 758 to 862 MPa) that can secure SSC resistance under the environment of NACE solution A (NACE TM0177-2005) containing 1 bar H ₂ S, which is a general evaluation condition. Up to steel. In many cases, in steel materials of higher strength of 125 ksi class (yield stress: 862 to 965 MPa) and 140 ksi class (yield stress: 965 to 1069 MPa), under a limited partial pressure of H ₂ S (for example, 0.1 bar or less) SSC resistance can only be ensured. Since demand for high-strength materials due to the deepening of oil wells is expected to increase further in the future, it is necessary to develop oil well pipes with higher corrosion resistance and higher strength and superior SSC resistance.

  SSC is a type of hydrogen embrittlement in which hydrogen generated on the surface of a steel material in a corrosive environment diffuses into the steel and breaks due to a synergistic effect with the stress applied to the steel material. In steel materials with high SSC sensitivity, cracks are easily generated with a lower load stress than the yield stress of steel materials.

  Many studies have been made so far on the relationship between the metal structure of low alloy steel and the SSC resistance. Generally, in order to improve the SSC resistance, it is said that it is most effective to make the metal structure a tempered martensite structure, and it is said that a fine-grain structure is desirable.

  However, tempered martensite has a body-centered cubic (hereinafter referred to as “BCC”) structure. The above tempered martensite and ferrite having a BCC structure are inherently highly susceptible to hydrogen embrittlement. Therefore, it is extremely difficult to completely prevent SSC in a steel mainly composed of tempered martensite or ferrite. In particular, since the SSC sensitivity increases as the strength increases as described above, it can be said that obtaining a steel material having high strength and excellent SSC resistance is a difficult task in low alloy steel.

  On the other hand, excellent SSC resistance can be easily obtained by using a steel having an austenite structure of a face-centered cubic crystal (hereinafter referred to as “FCC”) that is essentially low in hydrogen embrittlement sensitivity. For example, Patent Documents 1 to 3 disclose high-strength steel materials that are excellent in SSC resistance and contain a large amount of Mn, which is an austenite stabilizing element.

International Publication No. 2015/012357 International Publication No. 2016/052271 International Publication No. 2016/052397

  In the steel materials described in Patent Documents 1 to 3, excellent SSC resistance and high strength are compatible. However, Patent Documents 1 to 3 do not discuss the toughness of high-strength steel, and leave room for improvement.

  An object of the present invention is to provide a high-strength steel material for oil wells having excellent SSC resistance, having corrosion resistance comparable to that of low alloy steels from the viewpoint of overall corrosion, and an oil well pipe using the same. And

  This invention is made | formed in order to solve said subject, and makes a summary the following steel materials and oil well pipes for high strength oil wells.

(1) The chemical composition is mass%,
C: 0.50 to 1.40%
Si: 0.05-1.00%,
Mn: 3.0 to 24.0%
Ni: 5.0 to 19.0%,
Al: 0.003-0.06%,
P: 0.03% or less,
S: 0.03% or less,
N: 0.10% or less,
Cr: 0 to 2.0%,
Mo: 0 to 2.0%,
Cu: 0 to 2.0%,
Ti: 0 to 0.5%,
V: 0 to 0.5%
Nb: 0 to 0.5%,
Ca: 0 to 0.01%,
Mg: 0 to 0.01%,
B: 0 to 0.005%,
Balance: Fe and impurities,
The metal structure is an austenite single phase,
High strength steel for oil wells.

(2) The chemical composition is mass%,
Cr: 0.1 to 2.0%, and
Mo: 0.1 to 2.0%,
Containing one or two selected from
The steel material for high strength oil wells as described in said (1).

(3) The chemical composition is mass%,
Cu: 0.1 to 2.0%,
Containing
The steel material for high strength oil wells as described in said (1) or (2).

(4) The chemical composition is mass%,
Ti: 0.005 to 0.5%,
V: 0.005-0.5% and
Nb: 0.005 to 0.5%,
Containing one or more selected from
The steel material for high strength oil wells according to any one of (1) to (3) above.

(5) The chemical composition is mass%,
Ca: 0.0005 to 0.01%, and
Mg: 0.0005 to 0.01%,
Containing one or two selected from
The steel material for high strength oil wells according to any one of (1) to (4) above.

(6) The chemical composition is mass%,
B: 0.0005 to 0.005%,
Containing
The steel material for high strength oil wells according to any one of (1) to (5) above.

(7) The yield stress is 965 MPa or more.
The steel material for high strength oil wells according to any one of (1) to (6) above.

  (8) An oil well pipe made of the steel material for high well oil according to any one of (1) to (7) above.

  According to the present invention, it is possible to obtain a steel material having high strength, excellent SSC resistance, and excellent toughness.

  In high Mn austenitic steel, deformation occurs due to twin induced plasticity (TWIP). In the deformation by TWIP, since the increase of the flow stress accompanying the increase in strain rate hardly occurs, there is a characteristic that the deformation is concentrated in a part without being propagated to the adjacent part. For this reason, the absorbed energy of the high Mn austenitic steel in the Charpy impact test is low.

  One factor that determines whether deformation is caused by TWIP is Stacking Fault Energy (SFE). SFE is determined by the alloy composition, and alloying elements such as Mn and N tend to lower SFE, while Ni tends to increase SFE.

  The present inventor has found that, in a high Mn austenitic steel, by appropriately increasing the Ni content and increasing the SFE, deformation due to TWIP can be suppressed and deformation due to dislocation motion can be promoted. And it became possible to improve the toughness of the high Mn austenitic steel by increasing the deformation rate dependency of the flow stress and propagating the deformation to the adjacent part.

  The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

1. Chemical composition The reasons for limiting each element are as follows. In the following description, “%” for the content means “% by mass”.

C: 0.50 to 1.40%
Carbon (C) has an effect of stabilizing the austenite phase, and has an effect of promoting work hardening, and is an essential element for increasing the strength. Therefore, it is necessary to contain 0.50% or more of C. On the other hand, if the C content is excessive, the melting point of the material is remarkably lowered, and hot working at a low temperature is forced, and hot workability is lowered. Moreover, since cementite remains at the time of solution forming and impact performance is deteriorated, the C content is made 1.40% or less. In order to obtain a high-strength oil well steel with a better balance between strength and toughness, the C content is preferably 0.55% or more. Further, the C content is preferably 1.30% or less, and more preferably 1.20% or less.

Si: 0.05-1.00%
Silicon (Si) is an element necessary for deoxidation of steel, and if its content is less than 0.05%, deoxidation is insufficient and a lot of non-metallic inclusions remain, and the desired resistance. SSC property cannot be obtained. On the other hand, when the content exceeds 1.00%, the grain boundary strength is weakened, and the SSC resistance is lowered. Therefore, the Si content is set to 0.05 to 1.00%. The Si content is preferably 0.10% or more, and more preferably 0.20% or more. Moreover, it is preferable that Si content is 0.80% or less, and it is more preferable that it is 0.60% or less.

Mn: 3.0 to 24.0%
Manganese (Mn) is an element that can stabilize the austenite phase at low cost. Mn is an element that has the effect of reducing SFE and promoting deformation by TWIP. In the present invention, it is necessary to contain 3.0% or more of Mn in order to sufficiently exhibit the effect. On the other hand, Mn is preferentially dissolved in a wet hydrogen sulfide environment, and a stable corrosion product is not formed on the material surface. As a result, the overall corrosion resistance decreases as the Mn content increases. If Mn is contained in an amount exceeding 24.0%, the standard corrosion rate of the low alloy oil country tubular goods may be exceeded, so the Mn content needs to be 24.0% or less. The Mn content is preferably 5.0% or more, and more preferably 7.0% or more. The Mn content is preferably 22.0% or less.

In the present invention, the above-mentioned “standard corrosion rate of low alloy oil country tubular goods” refers to solution A (5% NaCl + 0.5% CH ₃ COOH aqueous solution, 1 bar H ₂ S saturation specified in NACE TM0177-2005). ) Is a corrosion rate converted from the amount of corrosion when immersed in 336h, and it is 1.5 g / (m ² · h).

Ni: 5.0 to 19.0%
Nickel (Ni) is an element that can stabilize the austenite phase like C and Mn, but unlike Mn, in addition to the generation of deformation twins during plastic deformation, it promotes deformation by dislocation. , An element that has the effect of increasing the absorbed energy in impact tests. Therefore, it is necessary to contain 5.0% or more of Ni. On the other hand, if it is contained excessively, the residual carbide during solutionization is promoted and the toughness is deteriorated. Therefore, the Ni content needs to be 19.0% or less. The Ni content is preferably 5.5% or more, and more preferably 6.0% or more. Moreover, it is preferable that Ni content is 18.0% or less, and it is more preferable that it is 17.0% or less.

  In order to stabilize the austenite phase, the total content of Mn and Ni is preferably 10.0% or more, and more preferably 12.0% or more. On the other hand, if the total content is too high, the melting point of the steel material is unnecessarily lowered, so the hot working temperature must be lowered, and productivity may be lowered. Therefore, the total content of Mn and Ni is preferably 40.0% or less, and more preferably 35.0% or less.

Al: 0.003 to 0.06%
Since aluminum (Al) is an element necessary for deoxidation of steel, it is necessary to contain 0.003% or more. However, if the Al content exceeds 0.06%, coarse oxides are likely to be mixed, which may adversely affect toughness and corrosion resistance. Therefore, the Al content is set to 0.003 to 0.06%. The Al content is preferably 0.008% or more, and more preferably 0.012% or more. Further, the Al content is preferably 0.05% or less, and more preferably 0.04% or less. In the present invention, Al means acid-soluble Al (sol. Al).

P: 0.03% or less Phosphorus (P) is an element unavoidably present in steel as an impurity. However, when the content exceeds 0.03%, segregation to the grain boundary becomes remarkable, and the SSC resistance is deteriorated. Therefore, the P content needs to be 0.03% or less. The P content is desirably as low as possible, preferably 0.02% or less, and more preferably 0.015% or less. However, excessive reduction significantly increases the manufacturing cost of the steel material, so the lower limit is preferably 0.001%, more preferably 0.005%.

S: 0.03% or less Sulfur (S) is inevitably present in the steel as an impurity as in the case of P, but when it exceeds 0.03%, segregation to the particle size becomes remarkable and the sulfide system The SSC resistance is lowered by generating inclusions. Therefore, the S content needs to be 0.03% or less. Note that the S content is desirably as low as possible, preferably 0.015% or less, and more preferably 0.010% or less. However, excessive reduction significantly increases the manufacturing cost of the steel material, so the lower limit is preferably 0.0005%, and more preferably 0.001%.

N: 0.10% or less Nitrogen (N) is usually treated as an impurity element in steel materials and is reduced by denitrification. However, since N is an element that stabilizes the austenite phase, a large amount of N may be contained in order to stabilize the austenite. On the other hand, if N is contained excessively, the high-temperature strength is increased, the processing stress during hot working is increased, and hot workability is lowered. Therefore, the N content needs to be 0.10% or less. From the viewpoint of refining costs, it is not necessary to denitrify unnecessarily, and the lower limit of the N content is preferably 0.001%.

Cr: 0 to 2.0%
Chromium (Cr) is an element that improves the overall corrosion resistance, and may be contained as necessary. However, if its content exceeds 2.0%, it often remains as a carbide during solution treatment, and there is a risk of reducing both SSC resistance and absorbed energy, so the Cr content is 2.0. % Or less. The Cr content is preferably 1.5% or less, and more preferably 1.0% or less. In addition, when it is desired to obtain the effect of improving the overall corrosion resistance, the Cr content is preferably 0.1% or more, more preferably 0.2% or more, and 0.5% or more. More preferably.

Mo: 0 to 2.0%
Molybdenum (Mo) is an element that stabilizes corrosion products in a wet hydrogen sulfide environment and improves overall corrosion resistance, and may be contained as necessary. However, if the Mo content exceeds 2.0%, it often remains as a carbide during solutionization, which may cause a decrease in SSC resistance and absorbed energy. Moreover, since Mo is an extremely expensive element, the Mo content is set to 2.0% or less. The Mo content is preferably 1.5% or less, and more preferably 1.0% or less. In order to obtain the above effect, the Mo content is preferably 0.1% or more, more preferably 0.2% or more, and further preferably 0.5% or more. .

Cu: 0 to 2.0%
Copper (Cu) is an element that stabilizes the austenite phase, and may be contained as needed as long as the amount is small. However, a large amount of Cu causes Cu brittleness during hot working, and remarkably lowers the ductility of steel at high temperatures. Therefore, the Cu content is set to 2.0% or less. The Cu content is preferably 1.5% or less, and more preferably 1.0% or less. In addition, when obtaining the effect of stabilizing austenite, the Cu content is preferably 0.1% or more, and more preferably 0.2% or more.

Ti: 0 to 0.5%
V: 0 to 0.5%
Nb: 0 to 0.5%
Titanium (Ti), vanadium (V), niobium (Nb) is an element that contributes to strengthening steel by forming fine carbides or carbonitrides by combining with C or N, and contained as necessary You may let them. However, even if these elements are contained in a large amount, the effect is saturated and the impact absorption energy may be lowered. Therefore, the content is made 0.5% or less. In order to acquire said effect, it is preferable to contain 0.005% or more of 1 or more types selected from these elements, and it is more preferable to contain 0.01% or more.

Ca: 0 to 0.01%
Mg: 0 to 0.01%
Calcium (Ca) and magnesium (Mg) have the effect of improving the hot workability of steel by being detoxified in combination with S, which degrades the hot workability. It also has the effect of changing the form of inclusions and improving toughness and corrosion resistance. However, even if these elements are contained in a large amount, not only the effect is saturated, but also the amount and size of inclusions are increased, and on the contrary, toughness and corrosion resistance are lowered. Therefore, the content of each element is set to 0.01% or less. The content of each element is preferably 0.006% or less. Moreover, when both Ca and Mg are contained, the total content is preferably 0.01% or less. In order to acquire the said effect, it is preferable to contain 0.0005% or more of 1 type or 2 types of Ca or Mg, and it is more preferable to contain 0.0010% or more.

B: 0 to 0.005%
Boron (B) has the effect of improving the hot workability of steel. However, when B is contained in a large amount, the precipitation amount of carbon boride is increased and the toughness of the steel is lowered. If the B content exceeds 0.005%, the toughness is greatly deteriorated, so the B content is set to 0.005% or less. The B content is preferably 0.004% or less, and more preferably 0.002% or less. In order to obtain the above effect, the B content is desirably 0.0005% or more.

  In the high strength oil well steel of the present invention, the balance is Fe and impurities. Here, “impurities” are components that are mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when steel is industrially manufactured, and are allowed within a range that does not adversely affect the present invention. Means something.

2. Metal Structure As described above, when α ′ martensite and ferrite having a BCC structure are mixed in the metal structure, the SSC resistance is lowered. Therefore, in the present invention, the metal structure is an austenite single phase having an FCC (face centered cubic) structure. In addition, the said metal structure should just be an austenite single phase substantially.

  In particular, when the total volume fraction of α ′ martensite and ferrite is 0.1% or more, the SSC resistance is significantly lowered. For this reason, the total volume fraction of α ′ martensite and ferrite is preferably less than 0.1%.

  Further, in the present invention, the structure mainly composed of the FCC structure is allowed to contain ε martensite of the HCP structure in addition to the FCC structure as a steel matrix. The volume fraction of ε martensite is preferably 10% or less.

  The total volume fraction of α ′ martensite and ferrite can be measured by using a ferrite meter when the amount is too small to be detected by X-ray diffraction or X-ray diffraction. The volume fraction of ε martensite can be measured by an X-ray diffraction method.

3. Mechanical properties Although the SSC resistance rapidly decreases as the strength of the steel increases, the steel according to the present invention can achieve both high yield stress and excellent SSC resistance. The steel material for high strength oil well according to the present invention preferably has a yield stress of 965 MPa or more.

4). Manufacturing method Although there will be no restriction | limiting in particular if it is a manufacturing method which can provide said intensity | strength about the manufacturing method of the steel material which concerns on this invention, For example, the following method can be used.

<Melting and casting>
For melting and casting, a method performed by a general method for producing austenitic steel materials can be used, and the casting may be ingot casting or continuous casting. When producing a seamless steel pipe, it may be cast into the shape of a round billet for pipe making by round CC.

<Hot processing (forging, drilling, rolling)>
After casting, hot working such as forging, drilling and rolling is performed. In the manufacture of seamless steel pipes, when a round billet is cast by the above-described round CC, processes such as forging and split rolling for forming the round billet are not necessary. When the steel material is a seamless steel pipe, rolling is performed using a mandrel mill or a plug mill after the drilling step. Further, when the steel material is a plate material, the slab is roughly rolled and then finish-rolled. Desirable conditions for hot working such as piercing and rolling are as follows.

  The billet may be heated to such an extent that hot piercing with a piercing and rolling mill is possible, but a desirable temperature range is 1000 to 1200 ° C. There are no particular restrictions on piercing rolling and rolling by other rolling mills such as mandrel mills, plug mills, etc. However, in terms of hot workability, in order to prevent surface flaws, the finishing temperature should be 900 ° C or higher. Is desirable. Although there is no restriction | limiting in particular also in the upper limit of finishing temperature, it is good to keep it to 1100 degreeC.

  When manufacturing a steel plate, it is sufficient that the heating temperature of the slab or the like is a temperature range in which hot rolling is possible, for example, 1000 to 1200 ° C. The hot rolling pass schedule is arbitrary, but it is desirable to set the finishing temperature to 900 ° C. or higher in consideration of hot workability for reducing the occurrence of surface flaws, ear cracks and the like of the product. The finishing temperature is preferably up to 1100 ° C. like the seamless steel pipe.

<Solution heat treatment>
The steel material after hot working is rapidly cooled after being heated to a temperature sufficient to completely dissolve carbides and the like. In this case, after holding for 10 minutes or more in the temperature range of 900-1200 degreeC, it cools rapidly. When the heating temperature is less than 900 ° C., when carbides, particularly Cr and Mo, are contained, Cr—Mo based carbides cannot be completely dissolved, and Cr and Mo around the Cr—Mo based carbides cannot be dissolved. A deficient layer is formed, causing stress corrosion cracking due to the occurrence of pitting corrosion, and the desired SSC resistance may not be obtained. On the other hand, when the heating temperature exceeds 1200 ° C., grain boundary melting occurs, and both mechanical performance and corrosion resistance may not be obtained. In addition, if the holding time is less than 10 min, the effect of solid solution is insufficient and the carbide cannot be completely dissolved, and therefore, for the same reason as when the heating temperature is less than 900 ° C., the desired SSC resistance may not be obtained.

  The upper limit of the holding time depends on the size and shape of the steel material and cannot be determined unconditionally. In any case, a time for soaking the entire steel material is required, but from the viewpoint of suppressing the manufacturing cost, an excessively long time is not desirable, and it is appropriate that the time is usually within 1 h. The cooling is preferably performed at a cooling rate equal to or higher than that of oil cooling in order to prevent precipitation of carbides (mainly Cr—Mo based carbides) and other intermetallic compounds during cooling.

  The lower limit of the holding time is a holding time when the steel material after hot working is once cooled to a temperature of less than 900 ° C. and then reheated to the temperature range of 900 to 1200 ° C. However, when the hot working finish temperature (finishing temperature) is in the range of 900 to 1200 ° C., the same effect as the solution heat treatment under the above conditions can be obtained if supplementary heating is performed for about 5 minutes or more at that temperature. It can be obtained and rapidly cooled without reheating. Therefore, the lower limit value of the holding time in the present invention includes the case where the end temperature (finished temperature) of hot working is in the range of 900 to 1200 ° C. and the temperature is supplemented for about 5 minutes or more.

<Cold processing>
The steel after the solution heat treatment is subjected to cold working in order to achieve a target yield stress, at least a strength of 965 MPa. In this case, it is preferable to perform cold working with a working degree (cross-sectional reduction rate) of 30% or more. Since the steel material according to the present invention maintains high ductility even after strong processing, even if the degree of processing is increased to 40%, cold working can be performed without causing fine cracks on the surface.

  The cold working method is not particularly limited as long as it is a method capable of uniformly processing a steel material. However, when the steel material is a steel pipe, it is industrially advantageous to use a so-called cold drawing machine using a perforated die and a plug or a cold rolling machine called a cold pilger mill. Further, when the steel material is a plate material, it is industrially advantageous to use a rolling mill that is used for manufacturing a normal cold-rolled steel sheet.

<Aging heat treatment>
Rather than using strengthening by cold working, an aging heat treatment may be performed on the material as it is in solution for the purpose of precipitation strengthening mainly by precipitation of carbides and carbonitrides. The aging heat treatment may be performed after the cold working process. The aging heat treatment is particularly effective when containing one or more of Ti, V and Nb. However, excessive aging heat treatment leads to the formation of excessive carbides, reducing the C concentration in the parent phase and causing destabilization of austenite. As heat treatment conditions, it is preferable to heat in the temperature range of 600 to 800 ° C. for several tens of minutes to several hours.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  17 types of steels A to Q having chemical components shown in Table 1 were melted in a vacuum high-frequency melting furnace and cast into a 50 kg ingot. Each ingot was heated at 1150 ° C. for 3 hours, and then finished into a square bar having a cross section of 50 mm in width and 50 mm in height by hot forging. After cutting to an appropriate length, the plate was heated at 1150 ° C. for 15 minutes and then finished into a plate material having a thickness of 16 mm by hot rolling. Further, a solution heat treatment was performed by holding at 1100 ° C. for 15 minutes and then cooling with water. Finally, the material after solution heat treatment was cold-rolled to prepare various test materials having a thickness of 8 mm to 12.8 mm.

  First, the total volume fraction of ferrite, α ′ martensite, and ε martensite was measured using X-ray diffraction and a ferrite meter (manufactured by Helmut Fischer, model number: FE8e3). In any of the test materials, a structure having a BCC structure was not detected, and was an austenite single phase.

Mechanical properties, SSC resistance, corrosion rate and toughness were investigated using the above test materials. A round bar tensile test piece having a parallel portion with an outer diameter of 4 mm and a length of 34 mm was taken from the test material in parallel with the plate length direction, and subjected to a tensile test at a strain rate of 4.9 × 10 ⁻⁴ / s at room temperature. went. In the present invention, it was judged that sufficient strength was obtained for those having a yield stress of 965 MPa or more.

Subsequently, the SSC resistance was evaluated by a constant load test as follows. A plate-like smooth test piece was collected, and a stress corresponding to 90% of the yield stress was applied to one surface by a four-point bending method. Then, as a test solution, immerse in solution A (5% NaCl + 0.5% CH ₃ COOH aqueous solution, 1 atm. H ₂ S saturated) specified in NACE TM0177-2005, hold at 336 h at 24 ° C., and break? Judged whether or not. As a result, no fracture occurred in all the test materials.

Further, in order to evaluate the overall corrosion resistance, the corrosion rate was determined by the following method. The above test material was immersed in the above solution A for 336 h at room temperature, the corrosion weight loss was determined, and converted to an average corrosion rate. In the present invention, the case where the corrosion rate is 1.5 gm ⁻² h ⁻¹ or less is considered to be excellent in overall corrosivity.

And the subsize Charpy impact test piece of thickness 5mm was extract | collected in the board width direction, and the V notch was provided in the board length direction. Then, a Charpy impact test was performed at −10 ° C., and an impact value (Impact Value) which is an absorbed energy per unit area was calculated from the obtained absorbed energy. In the present invention, those having an impact value of 30 J / cm ² or more are considered to have sufficient toughness.

  These results are summarized in Table 2.

From Table 2, Test Nos. 1 to 12, which are examples of the present invention, have a yield stress of 965 MPa or more and are excellent in SSC resistance, and the corrosion rate can be suppressed to 1.5 gm ⁻² h ⁻¹ or less, which is a target value. It is. Furthermore, the impact value by the Charpy impact test is 30 J / cm ² or more, and it is understood that the toughness is also excellent.

  On the other hand, Test Nos. 13 and 14 in which the C content and the Mn content do not satisfy the lower limits specified in the present invention, respectively, did not obtain the target strength. Test No. 15 resulted in poor overall corrosion resistance because the Mn content was too high. In Test No. 16, since the Ni content was too low, a sufficient impact value could not be obtained. And since test number 17 had too high Ni content, the carbide | carbonized_material remained in the structure | tissue, and the impact value fell on the contrary.

  According to the present invention, it is possible to obtain a steel material having high strength, excellent SSC resistance, and excellent toughness. Therefore, the high-strength oil well steel according to the present invention can be suitably used for oil well pipes in a wet hydrogen sulfide environment.

Claims (8)

Chemical composition is mass%,
C: 0.50 to 1.40%
Si: 0.05-1.00%,
Mn: 3.0 to 24.0%
Ni: 5.0 to 19.0%,
Al: 0.003-0.06%,
P: 0.03% or less,
S: 0.03% or less,
N: 0.10% or less,
Cr: 0 to 2.0%,
Mo: 0 to 2.0%,
Cu: 0 to 2.0%,
Ti: 0 to 0.5%,
V: 0 to 0.5%
Nb: 0 to 0.5%,
Ca: 0 to 0.01%,
Mg: 0 to 0.01%,
B: 0 to 0.005%,
Balance: Fe and impurities,
The metal structure is an austenite single phase,
High strength steel for oil wells. The chemical composition is mass%,
Cr: 0.1 to 2.0%, and
Mo: 0.1 to 2.0%,
Containing one or two selected from
The steel material for high strength oil wells according to claim 1. The chemical composition is mass%,
Cu: 0.1 to 2.0%,
Containing
The steel material for high strength oil wells according to claim 1 or 2. The chemical composition is mass%,
Ti: 0.005 to 0.5%,
V: 0.005-0.5% and
Nb: 0.005 to 0.5%,
Containing one or more selected from
The steel material for high strength oil wells according to any one of claims 1 to 3. The chemical composition is mass%,
Ca: 0.0005 to 0.01%, and
Mg: 0.0005 to 0.01%,
Containing one or two selected from
The steel material for high strength oil wells according to any one of claims 1 to 4. The chemical composition is mass%,
B: 0.0005 to 0.005%,
Containing
The steel material for high strength oil wells according to any one of claims 1 to 5. The yield stress is 965 MPa or more,
The steel material for high strength oil wells according to any one of claims 1 to 6.   An oil well pipe made of the steel material for high strength oil well according to any one of claims 1 to 7.