CN115141972A - 125 ksi-grade sulfide stress cracking resistant low-alloy oil well pipe steel and preparation method thereof - Google Patents
125 ksi-grade sulfide stress cracking resistant low-alloy oil well pipe steel and preparation method thereof Download PDFInfo
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- 229910001566 austenite Inorganic materials 0.000 claims abstract description 19
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
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- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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Abstract
The invention belongs to the field of low-alloy high-strength steel, and particularly relates to 125 ksi-grade sulfide stress cracking resistant low-alloy oil well pipe steel and a preparation method thereof. The steel comprises the following chemical components in percentage by weight: 0.20 to 0.40 percent of C, 0.1 to 0.3 percent of Si, 0.5 to 0.6 percent of Mn, 0.5 to 0.7 percent of Ni, 0.5 to 0.7 percent of Cu, 0.5 to 0.6 percent of Cr, 0.8 to 0.9 percent of Mo, 0.15 to 0.30 percent of V, 0.015 to 0.035 percent of Nb, less than or equal to 0.010 percent of O, less than or equal to 0.005 percent of S, less than or equal to 0.010 percent of P, and the balance of Fe. Mixing the raw materials according to the requirements of chemical components, and smelting and pouring to obtain a steel ingot or a continuous casting billet; carrying out hot processing on the steel ingot or the continuous casting billet, and carrying out austenite homogenization on the steel ingot or the continuous casting billet before hot processing; and carrying out hot processing on the steel ingot or the continuous casting billet at the temperature of Ac3 or above, and carrying out heat treatment after the hot processing. The invention solves the double contradiction relationship between the high strength and the high toughness of the low alloy oil well pipe steel and the SSC resistance, and breaks through the challenging technical problem that the 125ksi grade low alloy oil well pipe steel does not break after 720 hours of SSC resistance.
Description
Technical Field
The invention belongs to the field of low-alloy high-strength steel, and particularly relates to 125 ksi-grade sulfide stress cracking resistant low-alloy oil well pipe steel and a preparation method thereof.
Background
From hydrogen sulphide (H) 2 S) induced Sulfide Stress Cracking (SSC) is a difficult problem in the oil well pipe production process. With the depletion of oil and gas resources in shallow wells with low corrosiveness, the development of high-pressure deep wells with high corrosiveness has been increasing. The deep drilling of oil and gas wells requires that oil well steel pipes have high strength and high toughness; on the other hand, corrosive H exists in oil and gas wells 2 The concentration of S gas is increased, and the requirement on the SSC resistance of the steel pipe is also improved. However, as the strength grade of steel increases, an increase in SSC sensitivity occurs.
At present, for containing H 2 The highest strength of the S acid environment low alloy oil and gas well steel pipe is limited within 110ksi level, namely yield strength of 758 MPa. To obtain oil well pipe steel with strength levels exceeding 110ksi (e.g., 125ksi, yield strength over 862 MPa), the dual contradictory relationships between high strength and SSC resistance, and high strength and toughness need to be solved. However, balancing the contradiction between high strength and high toughness, high strength and SSC resistance performance is extremely challenging. Therefore, the development of 125 ksi-grade low-alloy high-strength high-toughness SSC-resistant oil well pipe steel is a technical problem to be solved urgently by steel enterprises at home and abroad at present.
Disclosure of Invention
Aiming at the technical problems, the invention aims to provide 125 ksi-grade sulfide stress cracking resistant low-alloy oil well pipe steel and a preparation method thereof, which break through the dual technical bottlenecks of high strength, high toughness and high strength and SSC resistance and obtain the low-alloy oil well pipe steel which meets the 125 ksi-grade high strength, high toughness and excellent and stable SSC resistance.
The technical scheme of the invention is as follows:
the 125 ksi-grade sulfide stress cracking resistant low-alloy oil well pipe steel comprises the following chemical components in percentage by weight:
0.20 to 0.40 percent of C (preferably 0.25 to 0.35 percent), 0.1 to 0.3 percent of Si, 0.5 to 0.6 percent of Mn, 0.5 to 0.7 percent of Ni, 0.5 to 0.7 percent of Cu (preferably 0.5 to 0.6 percent), 0.5 to 0.6 percent of Cr, 0.8 to 0.9 percent of Mo, 0.15 to 0.30 percent of V, 0.015 to 0.035 percent of Nb0.010 percent of O, less than or equal to 0.005 percent of S, less than or equal to 0.010 percent of P, and the balance of Fe.
The 125 ksi-grade sulfide stress cracking resistant low alloy oil well pipe steel has the following phase transition temperature: ac1 is 760 +/-20 ℃; ac3 is 810 + -20 ℃.
The preparation method of the 125 ksi-grade sulfide stress cracking resistant low-alloy oil well pipe steel comprises the following steps of:
step one, mixing raw materials according to the requirements of chemical components, and smelting and pouring the raw materials to obtain a steel ingot or a continuous casting billet;
step two, hot processing is carried out on the steel ingot or the continuous casting billet, the austenite of the steel ingot or the continuous casting billet before hot processing is homogenized at the temperature of 1100-1200 ℃, and the homogenization accumulation time is not less than 2 hours;
step three, carrying out hot working on the steel ingot or the continuous casting billet at the temperature above Ac3, wherein the final hot working temperature is not lower than Ac3+50 ℃, and the accumulated deformation of the hot working is more than 80%;
and step four, performing heat treatment after hot processing.
The preparation method of the 125 ksi-grade sulfide stress cracking resistant low-alloy oil well pipe steel comprises the following heat treatment process in the fourth step:
(1) Preserving the heat of the hot processed steel ingot or continuous casting billet at the temperature of 30-50 ℃ above Ac3 for 0.5-1.5 hours, and cooling the oil to room temperature;
(2) Heating the cooled steel ingot or continuous casting billet in the step (1) to a temperature which is 20-40 ℃ above Ac1 and lower than Ac3, preserving the heat for 0.5-1.0 hour, and cooling the oil to room temperature;
(3) And (3) heating the steel ingot or continuous casting billet cooled in the step (2) to 50-80 ℃ below Ac1, preserving the heat for 0.5-1.5 hours, and cooling the steel ingot or continuous casting billet to room temperature in air.
The structure of the low alloy oil well pipe steel comprises tempered martensite and granular austenite, the volume of the austenite is 4-6% of the total volume of the structure, the shape is granular and uniformly distributed, and the size is not more than 2 microns.
According to the preparation method of the 125 ksi-grade sulfide stress cracking resistant low-alloy oil well pipe steel, the strength grade of the low-alloy oil well pipe steel meets the 125ksi requirement, namely the yield strength at room temperature is more than or equal to 862MPa, the tensile strength is more than or equal to 950MPa, and the impact power of a full-size V-shaped notch at room temperature is more than 160J.
The preparation method of the 125 ksi-grade sulfide stress cracking resistant low-alloy oil well pipe steel has excellent SSC resistance, and the low-alloy oil well pipe steel does not break after being maintained for at least 720 hours under the constant load of 85% yield strength by referring to NACE TM0177 standard method A solution.
The design idea of the invention is as follows:
the nature of SSC generation of low alloy oil well pipe steel is from H under stress conditions below yield strength 2 H atoms generated by S enter a matrix structure of the steel and then are locally aggregated, so that early hydrogen embrittlement fracture is caused. Therefore, the main measures for improving hydrogen embrittlement resistance are 'H prevention', 'H fixation' and 'H capacity'. The technical idea of 'H prevention' is that through component improvement or surface modification and tissue design, a 'barrier' structure for preventing H from entering a matrix is formed on the surface of a material, and H atoms are reduced to enter the material as far as possible; the technical idea of 'fixing H' is to make H atoms entering the material uniformly dispersed and distributed, avoid local enrichment and cannot easily move; the 'H capacity' is the critical H content for increasing the hydrogen embrittlement capacity of the materialUnder the same conditions, more H atoms enter the material without hydrogen embrittlement. For example, an austenite structure with stronger hydrogen embrittlement resistance is adopted to replace a martensite structure with weaker hydrogen embrittlement resistance.
Aiming at the technical idea of improving the hydrogen embrittlement resistance of the steel material, the invention integrates the three measures, and adopts a proper amount of Cu alloying on the design of the alloy components of the 125 ksi-grade low-alloy oil well pipe steel to ensure that a Cu-rich film barrier is formed on the surface of the material to reduce H atoms from entering the material and prevent H; the nanometer V carbide and the nanometer Cu-rich phase irreversible hydrogen trap are utilized to ensure that H atoms entering the material are uniformly dispersed and distributed and cannot be easily moved to 'fix H'; in the aspect of tissue regulation, a special heat treatment process is adopted to form an austenite tissue with stronger H capacity, so that hydrogen embrittlement does not occur even more H atoms enter the austenite tissue. The composition design is combined with the structure regulation, three measures for improving the hydrogen embrittlement resistance of the material are realized simultaneously, and finally the high-strength and high-toughness 125 ksi-grade SSC-resistant low-alloy oil well pipe steel is obtained.
The functions and contents of the main elements in the invention are explained as follows:
carbon (C): c has a triple effect in the steel for oil well pipes of the present invention: firstly, solid solution strengthening; secondly, forming nano carbide to play a role in precipitation strengthening; thirdly, the nano carbide plays a role in hydrogen trapping and H fixation. In order to obtain this effect, the C content needs to be ensured to be 0.2wt% or more; however, when the C content exceeds 0.4wt%, coarse carbides are easily formed. Therefore, the upper limit of the C content is 0.4wt%.
Chromium (Cr): cr is an element for improving hardenability and general corrosion resistance in the steel for oil country tubular goods of the present invention. In order to obtain this effect, it is necessary to contain at least 0.5wt% of Cr; however, the Cr content is not more than 0.6wt%, otherwise coarse Cr-containing compounds are easily formed with C in the system steel, so that the Cr content is controlled to 0.5 to 0.6wt%.
Molybdenum (Mo): mo plays two roles in the oil well pipe steel of the present invention: firstly, solid solution strengthening effect; secondly, mo carbide is formed to improve the tempering resistance. In order to achieve the double effects, the content of Mo is controlled between 0.8 and 0.9 weight percent.
Vanadium (V): v is an important element which plays a role in precipitation strengthening, tempering resistance improvement and nano hydrogen trap in the steel for the oil well pipe. V and C are combined to form nano VC in the heat treatment process, so that H atoms entering the material are uniformly dispersed and distributed, and hydrogen embrittlement fracture caused by local hydrogen enrichment is avoided. According to the composition range of C in the steel of the invention, the proportion of V in the steel of the invention is controlled to be 0.15-0.30 wt% optimally.
Copper (Cu): cu is a key alloy element in the oil well pipe steel of the invention, and has multiple functions in the steel of the invention: firstly, the SSC resistance of the steel is improved, and the performance is as follows: (1) "H-proof": cu and H 2 S forms Cu sulfide which is attached to the surface of the material, and hydrogen can be effectively prevented from entering the material; (2) "solid H": a nano-sized Cu-rich phase is formed in the tempered tissue and serves as an irreversible hydrogen trap to capture hydrogen entering the material, so that the hydrogen is uniformly dispersed and distributed in a material matrix, and hydrogen embrittlement fracture caused by local hydrogen enrichment is avoided. Secondly, the strength is improved, which is shown as follows: and a Cu-rich phase with a nano size is precipitated in the structure after tempering, and the effect of precipitation strengthening is achieved. Thirdly, cu is an austenite forming element and is beneficial to forming austenite after being regulated and controlled in a critical zone (Ac 3-Ac 1 temperature interval). The multiple effects of Cu in the steel of the present invention are beneficial to the strength, toughness and SSC resistance of the steel. However, the minimum content of Cu should be higher than 0.5wt%, and excessive Cu content tends to cause hot shortness, so that the maximum content of Cu should not exceed 0.7wt%, while adding Ni element in close proximity to the content to compensate for the deficiency of Cu in hot working of the steel.
Impurity elements oxygen (O), sulfur (S), phosphorus (P): o, S and P are main impurity elements in the steel of the present invention, and they easily form non-metallic inclusions in the steel, resulting in a decrease in toughness of the steel. Therefore, the content of the steel is reduced as much as possible to obtain higher toughness as the smelting cost allows, and the preferable O, S and P of the steel are respectively as follows: o is less than or equal to 0.010wt%, S is less than or equal to 0.005wt%, and P is less than or equal to 0.010wt%.
The invention has the advantages and beneficial effects that:
1. the invention solves the double contradiction relationship between the high strength and the high toughness of the low alloy oil well pipe steel and the performance of the high strength and the SSC resistance, and realizes the SSC resistance 125ksi grade low alloy oil well pipe steel meeting the strict standard requirement.
2. The low-alloy oil well pipe steel has the advantages of easily controlled alloy components, simple preparation and heat treatment process and easy realization of industrial large-scale production.
3. The strength grade of the low-alloy oil well pipe steel reaches 125ksi grade, namely the yield strength is higher than 862MPa, and the low-alloy oil well pipe steel has excellent sulfide stress cracking resistance, and breaks through the very challenging technical problem that the 125ksi grade low-alloy oil well pipe steel does not break after 720 hours of SSC resistance.
Drawings
FIG. 1 is an optical topographical map of the steel of example 3.
FIG. 2 is an EBSD topography of the steel of example 3.
FIG. 3 is a parallel 3 samples of the steel of example 1 that did not break after SSC test loading 720.
FIG. 4 is a texture map of the surface of an unbroken sample of the steel of example 1.
FIG. 5 is an energy spectrum of the surface of an unbroken sample of the steel of example 1.
FIG. 6 is a morphology of the nano-precipitates formed in the steel structure of example 2.
Detailed Description
In the concrete implementation, the main preparation methods of some typical examples of the steel of the present invention are as follows, and the preparation methods different from the above are described at the corresponding examples.
The raw materials are mixed according to the chemical components of the invention, and the steel ingot is obtained by 50kg of vacuum induction smelting and pouring.
The ingot was austenitized at 1170 ℃ for 4 hours, followed by hot forging at an initial forging temperature of about 1120 ℃ and an end forging temperature of about 900 ℃, and finally forged into a round bar having a diameter of 30mm with a cumulative forging deformation of about 86%. Followed by different heat treatments of different embodiments.
Cutting a mechanical property sample from the material subjected to heat treatment, wherein the specification of a tensile sample is 5mm in diameter and 25mm in gauge length, and the test temperature is room temperature; the impact specimen size was 10mm × 10mm × 55mm, V-notch, and the test temperature was room temperature.
SSC resistance test A constant load sample was cut and processed according to NACE TM0177 standard, and the time to break was recorded using a standard Methoda, solution A with a loading stress of 85% (862 MPa 85% =733 MPa) of the minimum yield strength of 125ksi grade steel.
The invention will now be described by way of comparison of various examples, which are provided for illustrative purposes only and to which the invention is not limited.
Example 1
The low-alloy oil well pipe steel comprises the following chemical components in percentage by weight: 0.24% of C, 0.21% of Si, 0.56% of Mn, 0.57% of Ni, 0.66% of Cu, 0.56% of Cr, 0.83% of Mo, 0.20% of V, 0.021% of Nb, 0.008% of O, 0.002% of S, 0.005% of P and the balance of Fe. Wherein Ac1 of the steel is 773 ℃ and Ac3 of the steel is 825 ℃.
The heat treatment process comprises the following steps:
(1) Firstly, transferring the steel material after forging to a heating furnace at 860 ℃ (Ac 3+35 ℃) for heat preservation for 1 hour, and then cooling the steel material to room temperature;
(2) Then heating the steel material to 800 ℃ (Ac 1+27 ℃) and preserving the heat for 0.5 hour, and then cooling the steel material to room temperature by oil;
(3) Finally, the steel material is heated to 700 ℃ (Ac 1 and 73 ℃) and is kept for 0.7 hour, and then the steel material is cooled to the room temperature by air.
Example 2
The low-alloy oil well pipe steel comprises the following chemical components in percentage by weight: 0.30% of C, 0.25% of Si, 0.54% of Mn, 0.63% of Ni, 0.56% of Cu, 0.55% of Cr, 0.82% of Mo, 0.18% of V, 0.022% of Nb, 0.008% of O, 0.003% of S, 0.005% of P and the balance of Fe. Wherein Ac1 of the steel is 760 ℃ and Ac3 is 817 ℃.
The heat treatment process comprises the following steps:
(1) Firstly, transferring the steel material after forging to a heating furnace with the temperature of 850 ℃ (Ac 3+33 ℃) for heat preservation for 1 hour, and then carrying out oil cooling to the room temperature;
(2) Then heating the steel material to 790 ℃ (Ac 1+30 ℃) and preserving the heat for 0.5 hour, and then cooling the steel material to room temperature;
(3) Finally, the steel material is heated to 690 ℃ (Ac 1 and 70 ℃) and is kept warm for 1 hour, and then the steel material is cooled to room temperature by air.
Example 3
The low-alloy oil well pipe steel comprises the following chemical components in percentage by weight: 0.35% of C, 0.20% of Si, 0.56% of Mn, 0.55% of Ni, 0.60% of Cu, 0.58% of Cr, 0.87% of Mo, 0.17% of V, 0.019% of Nb, 0.007% of O, 0.004% of S, 0.005% of P and the balance of Fe. Wherein Ac1 of the steel is 755 ℃, and Ac3 of the steel is 810 ℃.
The heat treatment process comprises the following steps:
(1) Firstly, transferring the steel material after forging to a 840 ℃ (Ac 3+30 ℃) heating furnace for heat preservation for 1 hour, and then carrying out oil cooling to room temperature;
(2) Then heating the steel material to 780 ℃ (Ac 1+25 ℃) and preserving the heat for 0.5 hour, and then cooling the steel material to room temperature by oil;
(3) Finally, the steel material is heated to 705 ℃ (Ac 1 and below 50 ℃) and is kept for 1 hour, and then is cooled to room temperature by air.
Example 4
Example 4 the steel had the same composition as in example 3.
The heat treatment process comprises the following steps:
(1) Firstly, transferring the steel material after forging to a 840 ℃ (Ac 3+30 ℃) heating furnace for heat preservation for 1 hour, and then carrying out oil cooling to room temperature;
(2) The steel stock was then heated to 705 ℃ (Ac 1 below 50 ℃) for 1 hour and subsequently air cooled to room temperature.
Example 5
The composition of the steel of example 5 is the same as that of example 3.
The heat treatment process comprises the following steps:
(1) Firstly, transferring the steel material after forging to a 840 ℃ (Ac 3+30 ℃) heating furnace for heat preservation for 1 hour, and then carrying out oil cooling to room temperature;
(2) Then heating the steel material to 780 ℃ (Ac 1+25 ℃) and preserving the heat for 0.5 hour, and then cooling the steel material to room temperature by oil;
example 6
Example 6 the steel had the same composition as in example 3.
The heat treatment process comprises the following steps:
(1) Firstly, transferring the steel material after forging to a 840 ℃ (Ac 3+30 ℃) heating furnace for heat preservation for 1 hour, and then carrying out oil cooling to room temperature;
(2) Then heating the steel material to 780 ℃ (Ac 1+25 ℃) and preserving the heat for 1.5 hours, and then cooling the steel material to room temperature by oil;
(3) Finally, the steel material is heated to 705 ℃ (Ac 1 and below 50 ℃) and is kept warm for 1 hour, and then air cooling is carried out to the room temperature.
Comparative example 1
The steel comprises the following chemical components in percentage by weight: 0.23% of C, 0.22% of Si, 0.55% of Mn, 0.56% of Ni, 0.02% of Cu, 0.56% of Cr, 0.84% of Mo, 0.20% of V, 0.023% of Nb, 0.007% of O, 0.002% of S, 0.005% of P and the balance of Fe. Wherein Ac1 of the steel is 780 ℃ and Ac3 is 830 ℃.
The heat treatment process comprises the following steps:
(1) Firstly, transferring the steel material after forging to a heating furnace with 870 ℃ (Ac 3+40 ℃) for heat preservation for 1 hour, and then carrying out oil cooling to room temperature;
(2) Then heating the steel material to 810 ℃ (Ac 1+30 ℃) and preserving the heat for 0.5 hour, and then cooling the steel material to room temperature by oil;
(3) Finally, the steel material is heated to 700 ℃ (Ac 1 and 80 ℃) and is kept warm for 0.5 hour, and then air cooling is carried out to the room temperature.
Comparative example 2
The steel comprises the following chemical components in percentage by weight: 0.25% of C, 0.21% of Si, 0.53% of Mn, 0.67% of Ni, 0.64% of Cu, 0.54% of Cr, 0.83% of Mo, 0.01% of V, 0.03% of Nb, 0.008% of O, 0.002% of S, 0.005% of P and the balance of Fe. Wherein Ac1 of the steel is 770 ℃ and Ac3 is 830 ℃.
The heat treatment process comprises the following steps:
(1) Firstly, transferring the steel material after forging to a heating furnace at 860 ℃ (Ac 3+30 ℃) for heat preservation for 1 hour, and then carrying out oil cooling to room temperature;
(2) Then heating the steel material to 800 ℃ (Ac 1+30 ℃) and preserving the heat for 0.5 hour, and then cooling the steel material to room temperature by oil;
(3) Finally, the steel material is heated to 700 ℃ (Ac 1 and below 70 ℃) and is kept warm for 0.7 hour, and then air cooling is carried out to the room temperature.
Table 1 shows the mechanical properties and SSC fracture time of the steels of different examples and comparative examples
TABLE 1
| Numbering | Yield strength (MPa) | Tensile strength (MPa) | Elongation (%) | Impact work (J) | SSC fragmentation time (hours) |
| Example 1 | 878 | 955 | 21.0 | 183 | >720 |
| Example 2 | 893 | 964 | 20.5 | 177 | >720 |
| Example 3 | 909 | 984 | 20.0 | 165 | >720 |
| Example 4 | 955 | 1024 | 14.5 | 95 | 120 |
| Example 5 | 1005 | 1108 | 10.5 | 47 | 22 |
| Example 6 | 833 | 900 | 20.0 | 126 | 372 |
| Comparative example 1 | 815 | 881 | 22.0 | 186 | 48 |
| Comparative example 2 | 787 | 877 | 20.5 | 155 | 8 |
The results show that the steel according to the invention obtains a microstructure of tempered martensite and granular austenite with a volume fraction of austenite of about 5% and an average austenite size of about 1.2 μm, see figures 1 and 2, respectively.
The yield strength of the oil well pipe steel obtained by the components and the preparation process (such as examples 1-3) is not lower than 870MPa, the tensile strength is not lower than 950MPa, the impact energy is higher than 160J, and the 125ksi strength level is completely met.
Importantly, SSC performance can be maintained under severe conditions without breaking for at least 720 hours (fig. 3).
When no Cu (comparative example 1) or V (comparative example 2) was added to the composition, not only did the strength of the oil well pipe steel not meet the 125ksi strength grade, but also the SSC resistance was poor, and at worst, cracking occurred in less than 10 hours (comparative example 2).
The invention is premised on component design, and the heat treatment process is guaranteed. For example, examples 4 and 5, although having the same chemical composition as example 3, do not satisfy the target requirements of the present invention without any step of the heat treatment process of the present invention. This is mainly attributed to the fact that the small amount and small size austenite structures with stronger H-proof ' Cu-containing ' membrane structure ' and ' solid H ' nanometer precipitated phase and ' H-tolerant ' capacity as shown in figures 4-6 and 2 are obtained through component design and tissue regulation, namely, heat treatment process in the invention. Moreover, even if the heat treatment links are not lacked, if one link parameter is not properly selected, as in example 6, the heat treatment process in the second step has too long heat preservation time (1.5 hours), the subsequently formed austenite has more content and larger size, the strength and toughness do not meet the requirements, and the SSC resistance does not meet the standard requirements.
In conclusion, the SSC-resistant high-strength high-toughness 125 ksi-grade low-alloy oil well pipe steel meeting the strict standard requirements can be obtained strictly according to the principle of component design and structure regulation, the yield strength at room temperature is more than or equal to 862MPa, the tensile strength is more than or equal to 950MPa, and the impact energy of the full-size V-shaped notch at room temperature is more than 160J. The low alloy steel for oil well pipes has excellent SSC resistance, and does not break after being maintained for at least 720 hours under a constant load of 85% yield strength by referring to NACE TM0177 standard method A solution.
The above embodiments are merely illustrative of the technical ideas and features of the present invention, and the purpose thereof is to enable those skilled in the art to understand the contents of the present invention and implement the present invention, and not to limit the protection scope of the present invention. All equivalent changes and modifications made according to the spirit of the present invention should be covered within the protection scope of the present invention.
Claims (7)
1. The 125 ksi-grade sulfide stress cracking resistant low-alloy oil well pipe steel is characterized by comprising the following chemical components in percentage by weight:
0.20 to 0.40 percent of C, 0.1 to 0.3 percent of Si, 0.5 to 0.6 percent of Mn, 0.5 to 0.7 percent of Ni, 0.5 to 0.7 percent of Cu, 0.5 to 0.6 percent of Cr, 0.8 to 0.9 percent of Mo, 0.15 to 0.30 percent of V, 0.015 to 0.035 percent of Nb, less than or equal to 0.010 percent of O, less than or equal to 0.005 percent of S, less than or equal to 0.010 percent of P, and the balance of Fe.
2. The 125ksi grade sulfide stress cracking resistant low alloy oil well pipe steel of claim 1, wherein the transformation temperature of the low alloy oil well pipe steel is: ac1 is 760 +/-20 ℃; ac3 is 810 + -20 ℃.
3. A method of producing a grade 125ksi sulfide stress cracking resistant low alloy oil well pipe steel of claim 1 or 2, comprising the steps of:
step one, mixing raw materials according to the requirements of chemical components, and smelting and pouring the raw materials to obtain a steel ingot or a continuous casting billet;
step two, carrying out hot processing on the steel ingot or the continuous casting slab, wherein the austenite of the steel ingot or the continuous casting slab before the hot processing is homogenized at the temperature of 1100-1200 ℃, and the homogenization accumulated time is not less than 2 hours;
step three, carrying out hot working on the steel ingot or the continuous casting billet at the temperature above Ac3, wherein the final hot working temperature is not lower than Ac3+50 ℃, and the accumulated deformation of the hot working is more than 80%;
and step four, performing heat treatment after hot processing.
4. The method for preparing 125ksi grade sulfide stress cracking resistant low alloy oil well pipe steel according to claim 3, wherein the heat treatment process in the fourth step is as follows:
(1) Preserving the heat of the hot processed steel ingot or continuous casting billet at the temperature of 30-50 ℃ above Ac3 for 0.5-1.5 hours, and cooling the oil to room temperature;
(2) Heating the cooled steel ingot or continuous casting billet in the step (1) to a temperature which is 20-40 ℃ above Ac1 and lower than Ac3, keeping the temperature for 0.5-1.0 hour, and cooling the steel ingot or continuous casting billet to room temperature;
(3) And (3) heating the steel ingot or continuous casting billet cooled in the step (2) to 50-80 ℃ below Ac1, preserving the heat for 0.5-1.5 hours, and cooling the steel ingot or continuous casting billet to room temperature in air.
5. The method of claim 4, wherein the structure of the low alloy steel for oil well pipes is selected from tempered martensite and granular austenite, the volume of the austenite is 4-6% of the total volume of the structure, the austenite is granular and uniformly distributed, and the size of the austenite is not more than 2 μm.
6. The method of claim 4, wherein the strength grade of the 125 ksi-grade sulfide stress cracking resistant low alloy oil well pipe steel meets 125ksi requirements, namely the room temperature yield strength is larger than or equal to 862MPa, the tensile strength is larger than or equal to 950MPa, and the room temperature full-size V-notch impact energy is larger than 160J.
7. The method of claim 4, wherein the low alloy steel for oil well pipe having excellent SSC resistance is not fractured under a constant load of 85% yield strength for at least 720 hours in accordance with NACE TM0177 standard method dA, solution A.
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