CN114395696A

CN114395696A - Steel for oil well pipe, preparation method of steel and oil well pipe

Info

Publication number: CN114395696A
Application number: CN202210191344.XA
Authority: CN
Inventors: 邹喜洋; 严伟; 吴光辉; 史显波; 周正平; 单以银; 朱林; 曾天翼; 郑秋艳
Original assignee: Institute of Metal Research of CAS; Hengyang Valin Steel Tube Co Ltd
Current assignee: Institute of Metal Research of CAS; Hengyang Valin Steel Tube Co Ltd
Priority date: 2022-02-28
Filing date: 2022-02-28
Publication date: 2022-04-26

Abstract

The invention provides steel for an oil well pipe, a preparation method thereof and an oil well pipe. The chemical components of the steel for the oil well pipe comprise the following components in percentage by weight: 0.4 to 0.6 percent of C, less than or equal to 0.3 percent of Si, less than or equal to 0.5 percent of Mn, 0.8 to 1.2 percent of Cr, 0.6 to 0.9 percent of Mo, 0.1 to 0.3 percent of V, less than or equal to 0.05 percent of Nb, less than or equal to 0.05 percent of Al, less than or equal to 0.03 percent of N, less than or equal to 0.005 percent of S, less than or equal to 0.01 percent of P, 0.7 to 1.5 percent of Cu, the weight percentage of Ni and Cu is more than or equal to 0.5 and less than or equal to 1, and the balance of Fe and inevitable impurities. By balancing the chemical components of the steel for the oil well pipe, the double contradiction relationship between high strength and high toughness and between high strength and SSC resistance is solved.

Description

Steel for oil well pipe, preparation method of steel and oil well pipe

Technical Field

The invention relates to the technical field of pipeline steel for petroleum and natural gas exploitation, in particular to steel for an oil well pipe, a preparation method of the steel and the oil well pipe.

Background

In the field of oil and gas production, development of highly corrosive high-pressure deep wells has been increasing with depletion of oil and gas resources in shallow wells having low corrosivity. The deep drilling of oil and gas wells requires that oil well steel pipes have high strength and high toughness; on the other hand, many oil and gas wells contain corrosive hydrogen sulfide (H)₂S), the corrosion resistance of the steel pipe is also required to be improved.

Low alloy steel pipe exposed to H-containing atmosphere₂In the acidic environment of S, hydrogen embrittlement cracking (SSC) occurs due to sulfide stress cracking, and SSC is more likely to occur in high-strength steel.

Heretofore, for containing H₂Highest strength of low alloy oil and gas well steel pipe in acid environment of SLimited to a 110ksi rating, i.e., yield strength of 758 MPa. For obtaining oil well pipe steel with strength level over 110ksi (such as 125ksi level, yield strength over 862MPa), the dual contradictory relationships of high strength and high toughness, and high strength and SSC resistance need to be solved. In recent years, 125ksi high strength oil well pipe steel which is developed at the beginning obtains high strength (yield strength is higher than 862MPa), but the SSC resistance is unstable, and the requirement that the steel can not be broken after being loaded with 85% of minimum yield strength under the condition of Method A in NACE TM0177 standard and kept for 720 hours is rarely met.

Disclosure of Invention

The invention mainly aims to provide steel for an oil well pipe, a preparation method thereof and an oil well pipe, and aims to solve the problem that the steel for the oil well pipe in the prior art cannot simultaneously meet the requirements of high strength, high toughness, high strength and SSC resistance.

In order to achieve the above object, according to one aspect of the present invention, there is provided a steel for oil well pipes, comprising the following chemical components in percentage by weight: 0.4 to 0.6 percent of C, less than or equal to 0.3 percent of Si, less than or equal to 0.5 percent of Mn, 0.8 to 1.2 percent of Cr, 0.6 to 0.9 percent of Mo, 0.1 to 0.3 percent of V, less than or equal to 0.05 percent of Nb, less than or equal to 0.05 percent of Al, less than or equal to 0.03 percent of N, less than or equal to 0.005 percent of S, less than or equal to 0.01 percent of P, 0.7 to 1.5 percent of Cu, the weight percentage of Ni and Cu is more than or equal to 0.5 and less than or equal to 1, and the balance of Fe and inevitable impurities.

Further, the yield strength of the steel for the oil well pipe at room temperature is more than 862MPa, and the tensile strength is more than 950 MPa; the impact energy of the full-size V-shaped notch is more than 130J at the temperature of 0 ℃.

Further, the metallographic structure of the steel for the oil well pipe comprises tempered sorbite and austenite, the volume of the austenite is 3-5% of the total volume of the metallographic structure, and the volume of the tempered sorbite is 95-97% of the total volume of the metallographic structure.

Further, the steel for oil well pipes is saturated in H₂The steel can not be broken after being maintained for at least 720 hours under the stress of 733MPa loaded in the S environment.

In order to achieve the above object, according to one aspect of the present invention, there is provided a method of manufacturing the above steel for oil well pipes, the method comprising: step S1, mixing the chemical components of the steel for the oil well pipe to obtain raw materials, and smelting and pouring the raw materials to obtain a steel ingot; step S2, preserving heat of the steel ingot, forging the steel ingot in an austenite single-phase region, and cooling the steel ingot to room temperature to obtain a cooled steel ingot; and step S3, carrying out heat treatment on the cooled steel ingot to obtain the steel for the oil well pipe.

Further, in the step S2, the heat preservation temperature is 1150-1200 ℃, the heat preservation time is not less than 4h, and the preferable heat preservation time is 4-12 h.

Further, in step S2, the forging includes initial forging and final forging, wherein the temperature of the initial forging is 1100-1150 ℃, and the time of the initial forging is 5-60 min; the final forging temperature is 950-1000 ℃, and the final forging time is 5-60 min; preferably, the forging ratio of forging is more than 8, and the forging ratio of forging is 8-20; the cooling is preferably air-cooled, and the cooling rate is preferably 0.5 ℃/s to 5 ℃/s.

Further, in step S3, the heat treatment includes: step S31, heating the cooled steel ingot to a first preset temperature, and carrying out first heat preservation at the first preset temperature to obtain a first steel ingot; step S32, heating the heat-insulated steel ingot to a second preset temperature, carrying out second heat insulation at the second preset temperature to obtain a second steel ingot, and cooling the second steel ingot to room temperature to obtain a third steel ingot; step S33, tempering the third steel ingot to obtain a fourth steel ingot; and step S34, cooling the fourth steel ingot to room temperature to obtain the steel for the oil well pipe.

Further, the first preset temperature is 600-700 ℃, and the time coefficient of the first heat preservation is preferably 2.0-4.0; the second preset temperature is 810-860 ℃, and the second heat preservation time is preferably 60-90 minutes; preferably, the cooling in step S32 is oil cooling at a rate of 5 ℃/S to 20 ℃/S.

Further, the tempering temperature of the tempering treatment is 680-730 ℃, and the tempering time coefficient of the tempering treatment is 3.0-9.0.

According to another aspect of the present invention, there is provided an oil country tubular good which is manufactured using the above oil country tubular good steel.

By applying the technical scheme of the invention, the strength of the steel for the oil well pipe can be improved by increasing the carbon content; the carbon content is increased, so that the strength can be improved by a solid solution strengthening effect, and by increasing the types (carbide-Cr carbide, vanadium carbide, molybdenum carbide, nitride-vanadium nitride and Cu-rich phase) and the number of precipitated phases, on one hand, the precipitated phases play a role in precipitation strengthening, on the other hand, the precipitated phases can also serve as hydrogen traps to increase the density of the hydrogen traps, so that hydrogen entering the material is captured, the hydrogen is uniformly dispersed and distributed in a material matrix, the hydrogen diffusion and aggregation are inhibited, and the SSC resistance is improved; meanwhile, the toughness of the steel for the oil well pipe is effectively improved by increasing the molybdenum content. By combining the effects, the SSC resistance of the steel for the oil well pipe is improved. By balancing the chemical components of the steel for the oil well pipe, the double contradiction relationship between high strength and high toughness and between high strength and SSC resistance is solved.

Drawings

The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the invention and, together with the description, serve to explain the invention and not to limit the invention. In the drawings:

FIG. 1 shows an SEM image of a metallographic structure of example 1 of the present invention;

FIG. 2 shows an XRD pattern of a metallographic structure of example 1 of the present invention;

FIG. 3 is a view showing an OM diagram of a bainite structure in example 22 of the present invention;

FIG. 4 shows a TEM image of a nanosized precipitate phase of example 22 of the present invention;

FIG. 5 is an SEM photograph showing Cu sulfide formation on the surface of the material in example 22 of the present invention;

FIG. 6 shows an EDS diagram of the surface formation of Cu sulfide on the material of example 22 of the present invention.

Detailed Description

It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict. The present invention will be described in detail below with reference to the embodiments with reference to the attached drawings.

As analyzed by the background of the present applicationIn the prior art, the steel for the oil well pipe cannot simultaneously satisfy high strength and high toughness, and high strength and SSC resistance. The inventors have conducted intensive studies on various factors affecting the SSC resistance of high strength steel of 125ksi grade in order to solve the above problems. The inventors have discovered that (1) the major contributors to poor SSC resistance of high strength steels (above 125ksi strength grade) are not normally considered inclusions that initiate the origin and root of SSC corrosion, nor are normally considered coarse M' s₂₃C₆And (3) carbide. However, the uniform dispersion distribution of carbides plays an important role in improving the SSC resistance. (2) The superiority and inferiority of the SSC resistance of high-strength steel have a correlation with the dislocation density, which is a transmission channel of hydrogen, and the high dislocation density is a main factor causing the inferiority of the SSC resistance of the high-strength steel (higher than 125ksi strength level). However, in order to obtain high strength of steel, a high dislocation density is inevitably present in the structure, and these two are in a conflicting relationship.

Therefore, based on the above findings, the inventors believe that improving the SSC resistance of high strength steel requires improving the uniform dispersion distribution of carbides in the high strength steel and/or reducing the dislocation density of the high strength steel, mainly by three approaches: firstly, hydrogen is prevented from entering the material; secondly, hydrogen entering the interior of the material is prevented from diffusing and accumulating; thirdly, the critical hydrogen content of non-SSC generation is improved by tissue design. On the basis, the application provides steel for oil well pipes, a preparation method thereof and an oil well pipe.

In an exemplary embodiment of the present application, there is provided a steel for oil well pipes, the steel for oil well pipes comprising a chemical composition, in weight percent, of: 0.4 to 0.6 percent of C, less than or equal to 0.3 percent of Si, less than or equal to 0.5 percent of Mn, 0.8 to 1.2 percent of Cr, 0.6 to 0.9 percent of Mo, 0.1 to 0.3 percent of V, less than or equal to 0.05 percent of Nb, less than or equal to 0.05 percent of Al, less than or equal to 0.03 percent of N, less than or equal to 0.005 percent of S, less than or equal to 0.01 percent of P, 0.7 to 1.5 percent of Cu, the weight percentage of Ni and Cu is more than or equal to 0.5 and less than or equal to 1, and the balance of Fe and inevitable impurities.

The invention can improve the strength of the steel for the oil well pipe by increasing the carbon content; the carbon content is increased, so that the strength can be improved by a solid solution strengthening effect, and by increasing the types (carbide-Cr carbide, vanadium carbide, molybdenum carbide, nitride-vanadium nitride and Cu-rich phase) and the number of precipitated phases, on one hand, the precipitated phases play a role in precipitation strengthening, on the other hand, the precipitated phases can also serve as hydrogen traps to increase the density of the hydrogen traps, so that hydrogen entering the material is captured, the hydrogen is uniformly dispersed and distributed in a material matrix, the hydrogen diffusion and aggregation are inhibited, and the SSC resistance is improved; meanwhile, the toughness of the steel for the oil well pipe is effectively improved by increasing the molybdenum content. By combining the effects, the SSC resistance of the steel for the oil well pipe is improved. By balancing the chemical components of the steel for the oil well pipe, the double contradiction relationship between high strength and high toughness and between high strength and SSC resistance is solved.

To further illustrate the advantageous effects of the present application, the following description will be made of the actions of the above elements:

(1) action of C

C has a dual role in the steel for oil well pipes of the present invention: firstly, solid solution strengthening; secondly, forming carbonitride to play a role in precipitation strengthening. The reduction of strength after high-temperature tempering is effectively compensated by containing more C than the prior art steel for the oil well pipe, so that the strength grade of 125ksi (yield strength higher than 862MPa) can be ensured. In order to obtain the effect, the content of C needs to be ensured to be more than 0.4 percent; however, if the C content exceeds 0.6%, the steel tends to crack due to enhanced hardenability. Therefore, the upper limit of the C content is 0.6%. For example, the content of C may be 0.4%, 0.42%, 0.44%, 0.46%, 0.48%, 0.5%, 0.52%, 0.54%, 0.56%, 0.58%, 0.6%.

(2) Effect of Cr

Cr is an element that enhances hardenability and forms Cr carbide 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 0.8% of Cr; on the other hand, since the Cr content exceeds 1.2%, the Cr content in the steel system is saturated, and thus the Cr content is controlled to be 0.8 to 1.2%, for example, 0.8%, 0.85%, 0.9%, 0.95%, 1.0%, 1.05%, 1.1%, 1.15%, 1.2%.

(3) Effect of Mo

Mo plays two roles in the oil well pipe steel of the present invention: firstly, Mo carbide is formed to improve tempering resistance; secondly, Mo can play a role in improving the temper brittleness, so that the steel disclosed by the invention has high impact toughness. In order to achieve these dual effects, the content of Mo is controlled to be 0.6-0.9%, for example, the content of Mo is 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%.

(4) Action of V

V has a dual role in the steel for oil well pipes of the present invention: on one hand, V, C and N form carbonitrides to play a role in precipitation strengthening so as to ensure the strength level; on the other hand, the carbonitride of V is used as a hydrogen trap to capture hydrogen entering the material, so that the hydrogen is uniformly dispersed and distributed in the material matrix, and hydrogen embrittlement fracture caused by local hydrogen enrichment is avoided. The ratio of V in the steel of the present invention is preferably controlled to 0.1 to 0.3%, for example, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, in accordance with the ranges of C and N in the steel of the present invention.

(5) Effect of Nb

Nb is a component added as necessary, and it combines with C and N to form carbonitride, and plays a role of refining grains and improving toughness, and the content of Nb in the steel for oil well pipes of the present invention does not need to exceed 0.05% in accordance with the ranges of the components of C and N in the steel of the present invention, for example, the content of Nb is 0, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%.

(6) Effect of Al

Al is added according to the requirement, and if deoxidation is required, not more than 0.05% of Al can be added; in addition, Al can be added to form AlN with N in the steel, so that the function of grain refinement can be achieved. For example, the Al content is 0, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%.

(7) Effect of N

N is a component added according to needs, and N and C together form carbonitride with V, Al and Nb, so that the crystal grains can be refined, the toughness is improved, and the anti-SSC performance is improved by acting as a hydrogen trap. In order to reduce the smelting cost of steel, the content of N is less than or equal to 0.03 percent optimally. For example, the content of N is 0, 0.01%, 0.02%, 0.03%.

(8) Effect of Cu

Cu is a key alloying element that has multiple roles in the steel of the invention: firstly, the SSC resistance of the steel is improved, and the performance is as follows: (1) cu and H₂S forms Cu sulfide which is attached to the surface of the material, and hydrogen can be effectively prevented from entering the material; (2) a nano-sized Cu-rich phase is formed in the tissue after tempering and serves as a hydrogen trap to capture hydrogen entering the material, so that the hydrogen is uniformly dispersed and distributed in the material matrix, and hydrogen embrittlement and breakage caused by local hydrogen enrichment are avoided. Secondly, the strength is improved, and the performance is 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. And thirdly, Cu is an austenite forming element, a certain content of reverse transformed austenite can be formed after high-temperature tempering, and the uniformly distributed austenite can contain more hydrogen content, so that the SSC resistance is improved. In order to reliably obtain the multiple effects of Cu in the steel of the invention, the minimum content of Cu should be higher than 0.7%, while too much Cu content tends to produce hot embrittlement, so the maximum content of Cu should not exceed 1.5%. For example, the Cu content is 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%.

(9) Action of Ni

Ni is an element added along with Cu, on one hand, Ni can effectively inhibit the hot brittleness problem caused by independently adding Cu, on the other hand, Ni can improve the toughness of steel, the content of Ni cannot be too small, too small Ni cannot effectively inhibit the hot brittleness damage, but excessive Ni can form more austenite, and the strength level cannot be ensured, so the content control principle of Ni in the steel is as follows: the weight percentage of Ni/Cu is more than or equal to 0.5 and less than or equal to 1. For example, Ni/Cu is 0.5, 0.6, 0.7, 0.8, 0.9, 1.

The steel for the oil well pipe obtained according to the chemical components has high strength and high toughness, the grade is higher than 125ksi, the yield strength at room temperature is larger than 862MPa, and the tensile strength is larger than 950 MPa; the impact energy of the full-size V-shaped notch is more than 130J at the temperature of 0 ℃.

In some embodiments, the metallographic structure of the steel for oil well pipes includes tempered sorbite capable of reducing dislocation density and austenite having a stronger hydrogen capacity than a martensite structure of a body centered cubic structure (BCC) in a face centered cubic structure (FCC) austenite structure. Namely, the critical hydrogen content of SSC can be increased by introducing a proper amount of austenite which is dispersed and distributed in the structure design, so that the SSC resistance is improved, the volume of the austenite is controlled to be 3-5% of the total volume of the metallographic structure, and the volume of the tempered sorbite is 95-97% of the total volume of the metallographic structure. Avoiding too much austenite content leads to a decrease in strength and too little austenite leads to a decrease in SSC resistance. In some embodiments, the metallographic structure of the steel for oil well pipes may be considered to consist of tempered sorbite and austenite, containing only very minor amounts of hetero-phases.

The steel for oil well pipes obtained according to the above chemical composition has excellent SSC resistance, and is soaked in a solution A saturated with H, as referred to Method A in NACE TM0177 standard₂The steel can not be broken after being maintained for at least 720 hours under the stress of 733MPa loaded in the S environment.

In another exemplary embodiment of the present application, there is provided a method of manufacturing the above steel for oil well pipes, the method including: step S1, mixing the chemical components of the steel for the oil well pipe to obtain raw materials, and smelting and pouring the raw materials to obtain a steel ingot; step S2, preserving heat of the steel ingot, forging the steel ingot in an austenite single-phase region, and cooling the steel ingot to room temperature to obtain a cooled steel ingot; and step S3, carrying out heat treatment on the cooled steel ingot to obtain the steel for the oil well pipe.

According to the chemical components of the steel for the oil well pipe, the corresponding process is adopted in the preparation method, so that the steel for the oil well pipe with high strength and high toughness and SSC resistance is obtained; meanwhile, the composition of a metallographic structure is controlled by implementing a cooling process in the heat treatment process, so that the dislocation density is reduced, and the SSC resistance is improved. The steel for the oil well pipe, which is prepared by the preparation method, can improve the strength of the steel for the oil well pipe by increasing the carbon content; the carbon content is increased, so that the strength can be improved by a solid solution strengthening effect, and by increasing the types (carbide-Cr carbide, vanadium carbide, molybdenum carbide, nitride-vanadium nitride and Cu-rich phase) and the number of precipitated phases, on one hand, the precipitated phases play a role in precipitation strengthening, on the other hand, the precipitated phases can also serve as hydrogen traps to increase the density of the hydrogen traps, so that hydrogen entering the material is captured, the hydrogen is uniformly dispersed and distributed in a material matrix, the hydrogen diffusion and aggregation are inhibited, and the SSC resistance is improved; meanwhile, the toughness of the steel for the oil well pipe is effectively improved by increasing the molybdenum content. By combining the effects, the SSC resistance of the steel for the oil well pipe is improved. By balancing the chemical components of the steel for the oil well pipe, the double contradiction relationship between high strength and high toughness and between high strength and SSC resistance is solved.

The above-mentioned smelting method is not particularly limited, and any smelting method commonly used in the art may be used in the present invention. The preferable smelting mode is one of vacuum induction smelting, electric arc furnace smelting and converter smelting.

One skilled in the art can use the present invention with reference to temperatures and times commonly used in the art. In some embodiments, in step S2, the temperature is 1150-1200 ℃, and the time for heat preservation is not less than 4 hours, preferably 4-12 hours.

The forging process of the present invention may refer to forging processes commonly used in the art. In some embodiments, in step S2, the forging includes initial forging and finish forging, and in order to obtain good mechanical properties of the steel for oil well pipes, the temperature of the initial forging is controlled to be 1100 to 1150 ℃, and the time of the initial forging is controlled to be 5 to 60 min; the final forging temperature is 950-1000 ℃, and the final forging time is 5-60 min; in order to enable the metallographic structure to be uniform and compact, the forging ratio of forging is preferably greater than 8, and the forging ratio of forging is preferably 8-20; the cooling is preferably air-cooled, and the cooling rate is preferably 0.5 ℃/s to 5 ℃/s.

In some embodiments, in step S3, the heat treating comprises: step S31, heating the cooled steel ingot to a first preset temperature, and carrying out first heat preservation at the first preset temperature to obtain a first steel ingot; step S32, heating the heat-insulated steel ingot to a second preset temperature, carrying out second heat insulation at the second preset temperature to obtain a second steel ingot, and cooling the second steel ingot to room temperature to obtain a third steel ingot; step S33, tempering the third steel ingot to obtain a fourth steel ingot; and step S34, cooling the fourth steel ingot to room temperature to obtain the steel for the oil well pipe. Through the two-stage heat preservation, the structure is more uniform, and undissolved carbide is completely dissolved into the matrix; then tempering is carried out to completely separate out carbide and Cu-rich phase.

Controlling the temperature and time of the heat treatment process according to the chemical composition of the steel for an oil well pipe of the present invention, in some embodiments, the first preset temperature is 600 to 700 ℃, and preferably the time coefficient of the first heat preservation is 2.0 to 4.0; the heat preservation time is the maximum thickness of the sample multiplied by the heat preservation time coefficient, wherein the unit of the heat preservation time is minutes, and the unit of the maximum thickness of the sample is mm; in order to obtain complete austenitization and the optimal size of crystal grains, the second preset temperature is controlled to be 810-860 ℃, and the second heat preservation time is preferably 60-90 minutes.

The metallographic structure of the steel for oil well tubing according to the present invention is controlled by controlling the cooling process, and in order to further balance the relative proportions of tempered sorbite and retained austenite, it is preferable that the cooling in step S32 be oil cooling at a rate of 5 to 20 ℃/S.

According to different chemical compositions of raw materials, particularly the change of carbon content, different temperature conditions can influence the generation amount of carbide precipitated phases in the tempering process of heat treatment, and the tempering temperature of the heat treatment is correspondingly adjusted in order to further improve the SSC resistance of the steel under the premise of high carbon content. In some embodiments, the tempering temperature is controlled to be 680-730 ℃, and the tempering time coefficient of the tempering treatment is 3.0-9.0; the tempering time is the maximum thickness of the sample x the tempering time coefficient, the tempering time is in minutes, and the maximum thickness of the sample is in mm. The carbon content of the steel for oil well pipes of the present invention is high, and the tempering temperature should be as high as possible in order to achieve strength and toughness of 125ksi grade and to have excellent SSC resistance. Because the carbon content is high, tempering is carried out at 680-730 ℃, so that the carbon can be fully reacted with V, Nb and the like to form corresponding carbide to play a full precipitation strengthening role. Through high-temperature tempering, the dislocation density under the high carbon content is reduced, the influence of the high anti-SSC performance caused by the high dislocation density is further controlled, and the steel for the oil well pipe has the anti-SSC performance equivalent to that of the steel for the oil well pipe with the low carbon content, namely, the high strength, the high toughness and the high anti-SSC performance of the steel for the oil well pipe with the high carbon content are ensured.

In still another exemplary embodiment of the present application, there is provided an oil country tubular good which is manufactured using the above-described oil country tubular good steel. The oil well pipe having the steel for oil well pipe has high strength and high toughness and also has excellent SSC resistance.

In an exemplary embodiment of the present application, there is provided an acid corrosion resistant steel, which comprises the following chemical components by weight percent: 0.05-0.15% of C, less than or equal to 0.3% of Si, less than or equal to 0.6% of Mn, 0.8-1.2% of Cr, 0.6-0.9% of Mo, less than or equal to 0.3% of V, 0.01-0.05% of Nb, less than or equal to 0.05% of Al, less than or equal to 0.005% of S, less than or equal to 0.01% of P, 0.7-2% of Cu, the weight percentage of Ni and Cu is more than or equal to 0.5 and less than or equal to 1% of Ni/Cu, and the balance of iron and inevitable impurities.

Based on the characteristic of low carbon content, the acid corrosion resistant steel is endowed with higher SSC resistance; and by increasing the types of precipitated phases (carbide-Cr carbide, vanadium carbide and molybdenum carbide, nitride-vanadium nitride and Cu-rich phase), on one hand, the precipitated phases play a role in precipitation strengthening; meanwhile, a Cu element is added into the chemical components, so that a 'membrane' barrier for preventing hydrogen from entering can be formed on the surface of the material, and the SSC resistance of the acid corrosion resistant steel is further improved. The Nb element and the C form carbide, so that crystal grains can be refined, and the toughness of the acid corrosion resistant steel is improved. By balancing the chemical components of the acid corrosion resistant steel, the dual contradiction relationship between high strength and high toughness and between high strength and SSC resistance is solved.

The effect of each element is as follows:

(1) action of C

C is the most effective element for enhancing hardenability in the acid corrosion resistant steel, a martensite structure is easily obtained due to high C content, and the C content in the steel is not more than 0.15% in order to obtain a bainite structure. Meanwhile, C plays a role in solid solution strengthening, and the content of C is not less than 0.05%. Therefore, the content of C in the steel is 0.05-0.15%. For example, the content of C may be 0.05%, 0.07%, 0.09%, 0.1%, 0.12%, 0.15%.

(2) Effect of Cr

Cr has the functions of general corrosion resistance and solid solution strengthening in the acid corrosion resistant steel, and can form a small amount of Cr carbide with C to play a role in precipitation strengthening. In order to obtain this effect, it is necessary to contain at least 0.8% of Cr, and on the other hand, the Cr content exceeds 1.2%, and the effect of Cr in the acid corrosion resistant steel of this system is saturated, so that the Cr content is controlled to 0.8 to 1.2%, for example, the Cr content is 0.8%, 0.85%, 0.9%, 0.95%, 1.0%, 1.05%, 1.1%, 1.15%, 1.2%.

(3) Effect of Mo

Mo has two functions in the acid corrosion resistant steel of the invention: firstly, Mo carbide is formed to improve tempering resistance; and Mo can play a role in improving the temper brittleness, so that the toughness of the acid corrosion resistant steel is improved. In order to achieve the dual effect of Mo, the content of Mo is controlled to be 0.6-0.9%, for example, the content of Mo is 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%.

(4) Action of V

V is a component added to the acid corrosion resistant steel of the present invention as needed. The V and C form carbide to play a role in precipitation strengthening, so that the strength is improved. On the other hand, the SSC resistance is improved: the carbide of V is used as a hydrogen trap to capture hydrogen entering the material, so that the hydrogen is uniformly dispersed and distributed in the material matrix, and hydrogen embrittlement fracture caused by local hydrogen enrichment is avoided. However, since V carbide is easily precipitated only when tempered at a temperature higher than 500 ℃, it is preferable that the acid corrosion resistant steel contains not more than 0.3% V, for example, 0.1%, 0.15%, 0.2%, 0.25%, 0.3% V, in order to ensure that the function of V is sufficiently exhibited at the conventional tempering temperature. For example, the acid corrosion resistant steel contains not more than 0.3% of V, and when the tempering temperature is below 500 ℃, even if V carbide is not precipitated, V does not have negative influence on the acid corrosion resistant steel.

(5) Effect of Nb

Nb is combined with C to form carbide, so that on one hand, the effect of refining grains is achieved, and the toughness of the steel is improved; on the other hand, the SSC resistance is improved: the carbide of Nb is used as a hydrogen trap to capture hydrogen entering the material, so that the hydrogen is uniformly dispersed and distributed in the material matrix, and hydrogen embrittlement fracture caused by local hydrogen enrichment is avoided. According to the composition range of C in the steel, the content of Nb in the steel is controlled to be 0.01-0.05%. For example, the Nb content is 0.01%, 0.02%, 0.03%, 0.04%, 0.05%.

(6) Effect of Al

Al is a component added according to need, and if deoxidation is needed, Al can be added in an amount of not more than 0.05%. For example, the Al content is 0, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%.

(7) Effect of Cu

Cu is a key alloying element that has multiple roles in the acid corrosion resistant steel of the present invention: firstly, the SSC resistance of the steel is improved, and the performance is as follows: (1) cu and H₂S forms a Cu sulfide 'membrane barrier' which is attached to the surface of the material and can effectively prevent hydrogen from entering the material; (2) a nano-sized Cu-rich phase is formed in the tissue after tempering and serves as a hydrogen trap to capture hydrogen entering the material, so that the hydrogen is uniformly dispersed and distributed in the material matrix, and hydrogen embrittlement and breakage caused by local hydrogen enrichment are avoided. Secondly, the strength is improved, and the performance is 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. In order to reliably obtain the multiple effects of Cu in the steel of the present invention, the minimum Cu content should be higher than 0.7%, while excessive Cu content is liable to cause hot embrittlement, so that the maximum Cu content should not exceed 2.0%. For example, the Cu content is 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%.

(8) Action of Ni

On one hand, Ni can improve the toughness of steel, the content of Ni cannot be too small, the damage of thermal brittleness cannot be inhibited by too small Ni, but the hardenability is increased by excessive Ni, and martensite is easily generated. Therefore, the principle of controlling the Ni content in the steel is as follows: the weight percentage of Ni/Cu is more than or equal to 0.5 and less than or equal to 1. For example, Ni/Cu is 0.5, 0.6, 0.7, 0.8, 0.9, 1.

The acid corrosion resistant steel obtained according to the chemical components has high strength and high toughness which are higher than 125ksi level, and the yield strength of the acid corrosion resistant steel at room temperature is larger than 862MPa, and the tensile strength of the acid corrosion resistant steel is larger than 980 MPa; the impact energy of the full-size V-shaped notch is more than 130J at the temperature of 0 ℃.

Because the dislocation density of martensite is high, the SSC resistance sensitivity of the martensite is higher than that of a bainite structure and a ferrite structure, namely, the more martensite is in a metallographic structure, the higher the dislocation density is, and the poorer the SSC resistance performance is, in order to obtain the high-strength acid corrosion resistant steel with excellent SSC resistance, the invention controls the structure of the acid corrosion resistant steel to be complete bainite by reducing the C content so as to avoid excessive negative effects caused by the fact that the dislocation density is too high and the SCC resistance performance is resisted.

The acid corrosion resistant steel obtained by the chemical composition has excellent SSC resistance, and is soaked in solution A in saturated H according to NACE TM0177 standard Method A₂The steel can not be broken after being maintained for at least 720 hours under the stress of 733MPa loaded in the S environment.

In another exemplary embodiment of the present application, there is provided a method of manufacturing the above-described acid corrosion resistant steel, including: step S1, mixing the chemical components of the acid corrosion resistant steel to obtain raw materials, and smelting and pouring the raw materials to obtain a steel ingot; step S2, preserving heat of the steel ingot, forging the steel ingot in an austenite single-phase region, and cooling the steel ingot to room temperature to obtain a cooled steel ingot; and step S3, carrying out heat treatment on the cooled steel ingot to obtain the acid corrosion resistant steel.

According to the chemical components of the acid corrosion resistant steel, a corresponding process is adopted in the preparation method, so that the acid corrosion resistant steel with high strength and high toughness and SSC resistance is obtained; meanwhile, the composition of a metallographic structure is controlled by controlling a cooling process of heat treatment, so that the dislocation density is reduced, and the SSC resistance is improved. The acid corrosion resistant steel prepared by the preparation method is characterized by low carbon content, and has higher SSC resistance; and by increasing the types of precipitated phases (carbide-Cr carbide, vanadium carbide and molybdenum carbide, nitride-vanadium nitride and Cu-rich phase), on one hand, the precipitated phases play a role in precipitation strengthening; meanwhile, a Cu element is added into the chemical components, so that a 'membrane' barrier for preventing hydrogen from entering can be formed on the surface of the material, and the SSC resistance of the acid corrosion resistant steel is further improved. The Nb element and the C form carbide, so that crystal grains can be refined, and the toughness of the acid corrosion resistant steel is improved. By balancing the chemical components of the acid corrosion resistant steel, the dual contradiction relationship between high strength and high toughness and between high strength and SSC resistance is solved.

One skilled in the art can use the present invention with reference to temperatures and times commonly used in the art. In some embodiments, in step S2, the temperature is 1150-1200 ℃, and the time for heat preservation is not less than 4 hours, preferably 4-20 hours.

The forging process of the present invention may refer to forging processes commonly used in the art. In some embodiments, the forging includes an initial forging and a final forging in step S2, wherein the temperature of the initial forging is 1100-1150 ℃, and the time of the initial forging is 5-60 min; the final forging temperature is 950-1000 ℃, and the final forging time is 5-60 min; preferably, the forging ratio of forging is more than 8, and the forging ratio of forging is 8-20; the cooling is preferably air cooling, and the cooling speed is preferably 0.5-5 ℃/s.

In some embodiments, in step S3, the heat treating comprises: step S31, heating the cooled steel ingot to a first preset temperature, and carrying out first heat preservation at the first preset temperature to obtain a first steel ingot; step S32, heating the heat-insulated steel ingot to a second preset temperature, carrying out second heat insulation at the second preset temperature to obtain a second steel ingot, and cooling the second steel ingot to room temperature to obtain a third steel ingot; step S33, tempering the third steel ingot to obtain a fourth steel ingot; and step S34, cooling the fourth steel ingot to room temperature to obtain the acid corrosion resistant steel.

Controlling the temperature and time of the heat treatment process according to the chemical composition of the acid corrosion resistant steel, wherein in some embodiments, the first preset temperature is 600-700 ℃, the time coefficient of the first heat preservation is 2.0-4.0, the heat preservation time is the maximum thickness of the sample multiplied by the heat preservation time coefficient, the unit of the heat preservation time is minutes, and the unit of the maximum thickness of the sample is mm; in order to obtain complete bainite formation and the optimal size of crystal grains, the second preset temperature is controlled to be 890-930 ℃, and the second heat preservation time is preferably 60-90 minutes.

According to the different chemical compositions of the raw materials, especially the change of the carbon content, the tempering temperature of the heat treatment is correspondingly adjusted. The carbon content of the acid corrosion resistant steel is low, and in order to achieve the strength and toughness of 125ksi grade and have excellent SSC resistance, the tempering temperature should be correspondingly reduced, and the tempering time should be increased. Controlling the tempering temperature of the tempering treatment to be 400-600 ℃, and controlling the tempering time of the tempering treatment to be 6-10 hours. The tempering temperature is increased, and the tempering time can be properly reduced. The dislocation density can be improved by tempering at the low temperature of 400-600 ℃, so that the strength is improved, excessive reduction of the SSC resistance caused by the improvement of the dislocation density is avoided, and the high strength and high SSC resistance of the acid corrosion resistant steel are improved.

The metallographic structure of the steel for an oil well pipe of the present invention is controlled by controlling the cooling process, and in order to obtain complete bainite, it is preferable that the cooling in step S32 is slow cooling with heat-retaining cotton at a cooling rate of 0.2 ℃/S to 5 ℃/S.

In yet another exemplary embodiment of the present application, there is provided an oil country tubular good which is manufactured using the above-described acid corrosion resistant steel. The oil well pipe with the acid corrosion resistant steel has high strength, high toughness and excellent SSC resistance.

The present application is described in further detail below with reference to specific examples, which should not be construed as limiting the scope of the invention as claimed.

Example 1

The steel comprises the following chemical components in percentage by weight: 0.45% of C, 0.21% of Si, 0.42% of Mn, 0.99% of Cr, 0.76% of Mo, 0.11% of V, 0.01% of Nb, 0.025% of Al, 0.018% of N, 0.001% of S, 0.009% of P, 0.99% of Cu and 0.62% of Ni, wherein Ni/Cu is 0.62%; the balance being Fe.

The preparation method comprises the following steps:

(1) mixing the raw materials according to the chemical components, and carrying out vacuum induction smelting and pouring to obtain a steel ingot;

(2) keeping the temperature of the steel ingot at 1160 ℃ for 8 hours;

(3) forging a steel ingot in an austenite single-phase region: the initial forging temperature is 1100 ℃, the initial forging time is 20min, the final forging temperature is 980 ℃, the final forging time is 15min, the forging ratio is 10, air cooling is carried out to the room temperature after forging, and the cooling speed is 5 ℃/s.

(4) Cutting a cylindrical steel sample with the diameter of 20mm and the thickness of 20mm for heat treatment: firstly, heating a steel sample to a first preset temperature of 650 ℃ along with a furnace, and preserving heat, wherein the heat preservation time coefficient is 2.0, and the heat preservation time is 40 minutes; then continuously heating to a second preset temperature of 820 ℃, preserving heat for 60 minutes, and cooling the discharged oil to room temperature at a cooling speed of 15 ℃/s; tempering the steel sample at 680 ℃ for 180 minutes, wherein the tempering time coefficient is 9.0; and discharging the tempered steel sample from the furnace, and cooling to room temperature at the cooling speed of 12 ℃/s.

Example 2

Unlike example 1, the tempering temperature in step (4) was 710 ℃, the tempering coefficient was 6.0, and the tempering time was 120 minutes.

Example 3

Unlike example 1, the tempering temperature in step (4) was 730 ℃, the tempering coefficient was 3.0, and the tempering time was 60 minutes.

Example 4

Different from the embodiment 1, the chemical components of the steel are as follows according to weight percentage: 0.48% of C, 0.19% of Si, 0.32% of Mn, 1.10% of Cr, 0.81% of Mo, 0.12% of V, 0.01% of Nb, 0.01% of N, 0.001% of S, 0.009% of P, 1.27% of Cu and 0.70% of Ni, wherein Ni/Cu is 0.55%, and the balance is Fe.

Example 5

Unlike example 4, the tempering temperature was 710 ℃, the tempering coefficient was 6.0, and the tempering time was 120 minutes.

Example 6

Unlike example 4, the tempering temperature was 730 ℃, the tempering coefficient was 3.0, and the tempering time was 60 minutes.

Example 7

Different from the embodiment 1, the chemical components of the steel are as follows according to weight percentage: 0.6% of C, 0.3% of Si, 0.5% of Mn, 1.20% of Cr, 0.9% of Mo, 0.3% of V, 0.05% of Nb, 0.03% of N, 0.005% of S, 0.01% of P, 1.5% of Cu and 1.5% of Ni, wherein the ratio of Ni to Cu is 1, and the balance of Fe.

Example 8

Different from the embodiment 1, the chemical components of the steel are as follows according to weight percentage: 0.4% of C, 0.21% of Si, 0.42% of Mn, 0.8% of Cr, 0.6% of Mo, 0.1% of V, 0.01% of Nb, 0.025% of Al, 0.018% of N, 0.001% of S, 0.009% of P, 0.0.7% of Cu and 0.35% of Ni, wherein Ni/Cu is 0.5%; the balance being Fe.

Example 9

Different from the embodiment 1, the chemical components of the steel are as follows according to weight percentage: 0.45% of C, 0.21% of Si, 0.42% of Mn, 0.99% of Cr, 0.76% of Mo, 0.11% of V, 0.018% of N, 0.001% of S, 0.99% of Cu and 0.62% of Ni, wherein Ni/Cu is 0.62%; the balance being Fe.

Example 10

In contrast to example 1, the tempering temperature was 750 ℃.

Example 11

In contrast to example 1, the tempering temperature was 650 ℃.

Example 12

Unlike example 1, in step (3), the time for the initial forging was 5min, and the time for the final forging was 5 min.

Example 13

Unlike example 1, in step (3), the time for the initial forging was 60min, and the time for the final forging was 60 min.

Example 14

Unlike example 1, in step (3), the forging ratio was 20.

Example 15

Unlike example 1, in step (3), the forging ratio was 5.

Example 16

Unlike example 1, in step (3), the forging ratio was 25.

Example 17

Unlike example 1, in step (3), the cooling rate was 0.5 ℃/s.

Example 18

In step (4), the first preset temperature was 600 ℃ and the second preset temperature was 810 ℃ unlike in example 1.

Example 19

Unlike the embodiment 1, in the step (4), the first preset temperature is 700 ℃ and the second preset temperature is 860 ℃.

Example 20

Unlike in example 1, in step (4), the second preset temperature was 750 ℃.

Example 21

Unlike in example 1, in step (4), the second preset temperature was 900 ℃.

Comparative example 1

The steel comprises the following chemical components in percentage by weight: 0.24% of C, 0.20% of Si, 0.49% of Mn, 1.0% of Cr, 0.80% of Mo, 0.021% of Nb, 0.023% of Al, 0.0023% of S, 0.005% of P and the balance of Fe.

The preparation method comprises the following steps:

(2) keeping the temperature of the steel ingot at 1160 ℃ for 8 hours;

(4) Cutting a cylindrical steel sample with the diameter of 20mm for heat treatment: firstly, heating a steel sample to 910 ℃ along with a furnace, and preserving heat for 60 minutes; water cooling to room temperature at the cooling speed of 15 ℃/s; tempering the steel sample at 690 deg.C for 60min at 9 deg.C; and discharging the tempered steel sample from the furnace, cooling to room temperature at the cooling speed of 12 ℃/s.

Comparative example 2

Different from the comparative example 1, the chemical components of the steel are as follows by weight percentage: 0.25% of C, 0.20% of Si, 0.44% of Mn, 1.0% of Cr, 0.80% of Mo, 0.10% of V, 0.020% of Nb, 0.025% of Al, 0.0023% of S, 0.005% of P and the balance of Fe.

Comparative example 3

Different from the comparative example 1, the chemical components of the steel are as follows by weight percentage: 0.16% of C, 0.24% of Si, 0.44% of Mn, 1.03% of Cr, 0.79% of Mo, 0.11% of V, 0.022% of Al, 0.003% of S, 0.005% of P and the balance of Fe.

Comparative example 4

Different from the comparative example 1, the chemical components of the steel are as follows by weight percentage: 0.27% of C, 0.20% of Si, 0.48% of Mn, 1.02% of Cr, 0.80% of Mo, 0.10% of V, 0.006% of Nb, 0.025% of Al, 0.0023% of S, 0.005% of P and the balance of Fe.

Comparative example 5

Unlike comparative example 1, the chemical composition of the steel for oil country tubular goods was 0.49% by weight of C, 0.24% by weight of Si, 0.41% by weight of Mn, 1.0% by weight of Cr, 0.74% by weight of Mo, 0.21% by weight of V, 0.02% by weight of Nb, 0.01% by weight of Al, 0.012% by weight of N, 0.001% by weight of S, 0.008% by weight of P, and the balance being Fe.

The procedure was the same as in example 1.

Comparative example 6

Unlike comparative example 1, the chemical composition of the steel for oil well pipes was: 0.55% of C, 0.20% of Si, 0.38% of Mn, 0.91% of Cr, 0.86% of Mo, 0.11% of V, 0.01% of N, 0.001% of S, 0.008% of P and the balance of Fe.

The procedure was the same as in example 1.

Comparative example 7

Unlike comparative example 1, the chemical composition of the steel for oil well pipes was: 0.45% of C, 0.21% of Si, 0.42% of Mn, 0.99% of Cr, 0.76% of Mo, 0.11% of V, 0.01% of Nb, 0.025% of Al, 0.018% of N, 0.001% of S, 0.009% of P, 2% of Cu and 1.24% of Ni, wherein Ni/Cu is 0.62%; the balance being Fe.

The procedure was the same as in example 1.

Comparative example 8

Unlike comparative example 1, the chemical composition of the steel for oil well pipes was: 0.45% of C, 0.21% of Si, 0.42% of Mn, 0.99% of Cr, 0.76% of Mo, 0.11% of V, 0.01% of Nb, 0.025% of Al, 0.018% of N, 0.001% of S, 0.009% of P, 0.5% of Cu and 0.31% of Ni, wherein Ni/Cu is 0.62; the balance being Fe.

The procedure was the same as in example 1.

Comparative example 9

Unlike comparative example 1, the chemical composition of the steel for oil well pipes was: 0.45% of C, 0.21% of Si, 0.42% of Mn, 0.99% of Cr, 0.76% of Mo, 0.11% of V, 0.01% of Nb, 0.025% of Al, 0.018% of N, 0.001% of S, 0.009% of P, 0.99% of Cu and 0.396% of Ni, wherein Ni/Cu is 0.4; the balance being Fe.

The procedure was the same as in example 1.

Comparative example 10

Unlike comparative example 1, the chemical composition of the steel for oil well pipes was: 0.45% of C, 0.21% of Si, 0.42% of Mn, 0.99% of Cr, 0.76% of Mo, 0.11% of V, 0.01% of Nb, 0.025% of Al, 0.018% of N, 0.001% of S, 0.009% of P, 0.99% of Cu and 1.089% of Ni/Cu, wherein the ratio of Ni/Cu is 1.1; the balance being Fe.

The procedure was the same as in example 1.

Cutting mechanical property samples from the steel samples subjected to heat treatment in the examples and the comparative examples, wherein the tensile sample specification is 5mm in diameter, 25mm in gauge length and room temperature; the impact specimen size was 10mm × 10mm × 55mm, V-notch, test temperature was 0 ℃. The samples evaluated for SSC resistance were tested according to NACE TM0177 standard, Methoda, solution A, with a loading stress of 733 MPa. The mechanical properties and SSC resistance of the steels of the examples and comparative examples are shown in Table 1.

TABLE 1

As shown in fig. 1 and 2, the microstructure obtained by the steel for an oil country tubular good in example 1 of the present invention was a tempered sorbite structure and a small amount of an austenite structure.

Example 22

The acid corrosion resistant steel comprises the following chemical components in percentage by weight: 0.091% of C, 0.24% of Si, 0.51% of Mn, 1.0% of Cr, 0.82% of Mo, 0.20% of V, 0.02% of Nb, 0.01% of Al, 0.001% of S, 0.008% of P, 0.92% of Cu, 0.64% of Ni, wherein the ratio of Ni to Cu is 0.7, and the balance of Fe.

The preparation method comprises the following steps:

(2) keeping the temperature of the steel ingot at 1180 ℃ for 5 hours;

(3) forging a steel ingot in an austenite single-phase region: the initial forging temperature is 1120 ℃, the initial forging time is 20min, the final forging temperature is 950 ℃, the final forging time is 20min, the forging ratio is 12, air cooling is carried out to the room temperature after forging, and the cooling speed is 5 ℃/s;

(4) cutting a cylindrical steel sample with the diameter of 20mm for heat treatment: firstly, heating a steel sample to a first preset temperature of 680 ℃ along with a furnace, and preserving heat, wherein the heat preservation time coefficient is 2.0, and the heat preservation time is 40 minutes; then, continuously heating to a second preset temperature of 910 ℃, preserving heat for 60 minutes, discharging, covering the steel sample with heat preservation cotton, and slowly cooling to room temperature at a cooling speed of 1.2 ℃/s; tempering the steel sample at 450 ℃ for 10 hours; and discharging the tempered steel sample from the furnace, and cooling to room temperature at a cooling speed of 5 ℃/s.

Example 23

Unlike example 22, the tempering temperature was 550 ℃ and the tempering time was 8 hours.

Example 24

Unlike example 22, the tempering temperature was 600 ℃ and the tempering time was 6 hours.

Example 25

Different from the embodiment 22, the chemical components of the steel are as follows according to weight percentage: 0.14% of C, 0.21% of Si, 0.50% of Mn, 1.0% of Cr, 0.82% of Mo, 0.03% of V, 0.02% of Nb, 0.01% of Al, 0.001% of S, 0.008% of P, 0.93% of Cu, 0.465% of Ni/Cu, and the balance of Fe.

Example 26

In contrast to example 25, the tempering temperature was 550 ℃ and the tempering time was 8 hours.

Example 27

In contrast to example 25, the tempering temperature was 600 ℃ and the tempering time was 6 hours.

Example 28

Different from the embodiment 22, the chemical components of the acid corrosion resistant steel are as follows by weight percent: 0.097% of C, 0.21% of Si, 0.51% of Mn, 1.0% of Cr, 0.82% of Mo, 0.10% of V, 0.015% of Nb, 0.02% of Al, 0.001% of S, 0.008% of P, 1.17% of Cu, 0.75% of Ni, wherein the ratio of Ni to Cu is 0.64, and the balance of Fe.

Example 29

In contrast to example 28, the tempering temperature was 550 ℃ and the tempering time was 8 hours.

Example 30

In contrast to example 28, the tempering temperature was 600 ℃ and the tempering time was 6 hours.

Example 31

Different from the embodiment 22, the chemical components of the steel are as follows according to weight percentage: 0.099% of C, 0.22% of Si, 0.51% of Mn, 1.03% of Cr, 0.83% of Mo, 0.01% of V, 0.02% of Nb, 0.03% of Al, 0.001% of S, 0.009% of P, 1.28% of Cu and 0.72% of Ni, wherein Ni/Cu is 0.56, and the balance is Fe.

Example 32

In contrast to example 31, the tempering temperature was 550 ℃ and the tempering time was 8 hours.

Example 33

In contrast to example 31, the tempering temperature was 600 ℃ and the tempering time was 6 hours.

Example 34

Different from the embodiment 22, the chemical components of the steel are as follows according to weight percentage: 0.05% of C, 0.24% of Si, 0.51% of Mn, 0.8% of Cr, 0.6% of Mo, 0.20% of V, 0.01% of Nb, 0.01% of Al, 0.001% of S, 0.008% of P, 0.7% of Cu and 0.35% of Ni0.5%, wherein the balance is Fe.

Example 35

Different from the embodiment 22, the chemical components of the steel are as follows according to weight percentage: 0.15% of C, 0.3% of Si, 0.6% of Mn, 1.2% of Cr, 0.9% of Mo, 0.30% of V, 0.05% of Nb, 0.05% of Al, 0.005% of S, 0.01% of P, 2% of Cu and 2% of Ni, wherein Ni/Cu is 1, and the balance is Fe.

Example 36

In contrast to example 22, the tempering temperature was 400 ℃.

Example 37

In contrast to example 22, the tempering temperature was 650 ℃.

Example 38

In contrast to example 22, the tempering temperature was 350 ℃.

Example 39

Unlike example 22, in step (3), the time for the initial forging was 5min, and the time for the final forging was 5 min.

Example 40

Unlike example 22, in step (3), the time for the initial forging was 60min, and the time for the final forging was 60 min.

EXAMPLE 41

Unlike example 22, the forging ratio in step (3) was 20.

Example 42

Unlike example 22, in step (3), the forging ratio was 5.

Example 43

Unlike example 22, in step (3), the forging ratio was 25.

Example 44

Unlike example 22, in step (3), the cooling rate was 0.5 ℃/s.

Example 45

In step (4), the first preset temperature was 600 ℃ and the second preset temperature was 890 ℃ unlike example 22.

Example 46

Unlike example 22, in step (4), the first preset temperature was 700 ℃ and the second preset temperature was 930 ℃.

Example 47

Unlike in example 22, in step (4), the second preset temperature was 950 ℃.

Example 48

Unlike in example 22, in step (4), the second preset temperature was 850 ℃.

Comparative example 11

The acid corrosion resistant steel comprises the following chemical components in percentage by weight: 0.24% of C, 0.20% of Si, 0.49% of Mn, 1.0% of Cr, 0.80% of Mo, 0.021% of Nb, 0.023% of Al, 0.0023% of S, 0.005% of P and the balance of Fe.

The preparation method comprises the following steps:

(2) keeping the temperature of the steel ingot at 1160 ℃ for 8 hours;

(3) forging a steel ingot in an austenite single-phase region: the initial forging temperature is 1100 ℃, the initial forging time is 20min, the final forging temperature is 980 ℃, the final forging time is 20min, the forging ratio is 10, air cooling is carried out to the room temperature after forging, and the cooling speed is 0.5 ℃/s.

(4) Cutting a cylindrical steel sample with the diameter of 20mm for heat treatment: firstly, heating a steel sample to 910 ℃ along with a furnace, and preserving heat for 60 minutes; water cooling to room temperature at the cooling speed of 1.2 ℃/s; tempering the steel sample at 690 deg.C for 60 min; discharging the tempered steel sample from the furnace, cooling to room temperature at the cooling speed of 3 ℃/s.

Comparative example 12

Different from the comparative example 11, the chemical components of the steel are as follows by weight percentage: 0.25% of C, 0.20% of Si, 0.44% of Mn, 1.0% of Cr, 0.80% of Mo, 0.10% of V, 0.020% of Nb, 0.025% of Al, 0.0023% of S, 0.005% of P and the balance of Fe.

Comparative example 13

Different from the comparative example 11, the chemical components of the steel are as follows by weight percentage: 0.16% of C, 0.24% of Si, 0.44% of Mn, 1.03% of Cr, 0.79% of Mo, 0.11% of V, 0.022% of Al, 0.003% of S, 0.005% of P and the balance of Fe.

Comparative example 14

Different from the comparative example 11, the chemical components of the steel are as follows by weight percentage: 0.27% of C, 0.20% of Si, 0.48% of Mn, 1.02% of Cr, 0.80% of Mo, 0.10% of V, 0.006% of Nb, 0.025% of Al, 0.0023% of S, 0.005% of P and the balance of Fe.

Comparative example 15

Different from the comparative example 11, the chemical components of the steel are as follows by weight percentage: 0.097% of C, 0.21% of Si, 0.51% of Mn, 1.0% of Cr, 0.82% of Mo, 0.10% of V, 0.015% of Nb, 0.02% of Al, 0.001% of S, 0.008% of P, 0.4% of Cu, 0.256% of Ni, wherein the ratio of Ni to Cu is 0.64, and the balance of Fe.

The preparation method is the same as that of example 22.

Comparative example 16

Different from the comparative example 11, the chemical components of the steel are as follows by weight percentage: 0.097% of C, 0.21% of Si, 0.51% of Mn, 1.0% of Cr, 0.82% of Mo, 0.10% of V, 0.015% of Nb, 0.02% of Al, 0.001% of S, 0.008% of P, 2.5% of Cu, 1.6% of Ni, wherein the ratio of Ni to Cu is 0.64, and the balance of Fe.

The preparation method is the same as that of example 22.

Comparative example 17

Different from the comparative example 11, the chemical components of the steel are as follows by weight percentage: 0.097% of C, 0.21% of Si, 0.51% of Mn, 1.0% of Cr, 0.82% of Mo, 0.10% of V, 0.015% of Nb, 0.02% of Al, 0.001% of S, 0.008% of P, 1.17% of Cu, 0.468% of Ni/Cu, and the balance of Fe.

The preparation method is the same as that of example 22.

Comparative example 18

Different from the comparative example 11, the chemical components of the steel are as follows by weight percentage: 0.097% of C, 0.21% of Si, 0.51% of Mn, 1.0% of Cr, 0.82% of Mo, 0.10% of V, 0.015% of Nb, 0.02% of Al, 0.001% of S, 0.008% of P, 1.17% of Cu, 1.287% of Ni, 1.1% of Ni/Cu and the balance of Fe.

The preparation method is the same as that of example 22.

Comparative example 19

Different from the comparative example 11, the chemical components of the steel are as follows by weight percentage: 0.091% of C, 0.24% of Si, 0.51% of Mn, 1.0% of Cr, 0.82% of Mo, 0.20% of V, 0.02% of Nb, 0.01% of Al, 0.001% of S, 0.008% of P, 0.92% of Cu and the balance of Fe.

The preparation method is the same as that of example 22.

Cutting a mechanical property sample from the steel sample after heat treatment, wherein the specification of a tensile sample is 5mm in diameter, 25mm in gauge length, and the test temperature is room temperature; the impact specimen size was 10mm × 10mm × 55mm, V-notch, test temperature was 0 ℃.

Samples for evaluation of SSC resistance according to NACE TM0177 standard, Methoda, solution A, in saturated H₂And 733MPa of stress is loaded in the S environment. The tempering temperatures and tempering times of the steels of the examples and comparative examples are shown in Table 2, corresponding to their relevant properties.

TABLE 2

The results of the examples show that the steel of the invention achieves a microstructure of bainite, see figure 3.

The steel of the invention solves the double contradiction relationship between high strength and high toughness, and between high strength and SSC resistance. This is mainly due to the fact that the lower dislocation density bainite structure (fig. 3) and the high density of nano-sized precipitates (fig. 4) act as favorable hydrogen traps and that the formation of Cu sulfides on the surface of the material prevents the entry of hydrogen (fig. 5, fig. 6).

From the above description, it can be seen that the above-described embodiments of the present invention achieve the following technical effects: the invention can improve the strength of the steel for the oil well pipe by increasing the carbon content; the carbon content is increased, so that the strength can be improved by a solid solution strengthening effect, and by increasing the types (carbide-Cr carbide, vanadium carbide, molybdenum carbide, nitride-vanadium nitride and Cu-rich phase) and the number of precipitated phases, on one hand, the precipitated phases play a role in precipitation strengthening, on the other hand, the precipitated phases can also serve as hydrogen traps to increase the density of the hydrogen traps, so that hydrogen entering the material is captured, the hydrogen is uniformly dispersed and distributed in a material matrix, the hydrogen diffusion and aggregation are inhibited, and the SSC resistance is improved; meanwhile, the toughness of the steel for the oil well pipe is effectively improved by increasing the molybdenum content. By combining the effects, the SSC resistance of the steel for the oil well pipe is improved. By balancing the chemical components of the steel for the oil well pipe, the double contradiction relationship between high strength and high toughness and between high strength and SSC resistance is solved.

The invention endows the acid corrosion resistant steel with higher SSC resistance based on the characteristic of low carbon content; and by increasing the types of precipitated phases (carbide-Cr carbide, vanadium carbide and molybdenum carbide, nitride-vanadium nitride and Cu-rich phase), on one hand, the precipitated phases play a role in precipitation strengthening; meanwhile, a Cu element is added into the chemical components, so that a 'membrane' barrier for preventing hydrogen from entering can be formed on the surface of the material, and the SSC resistance of the acid corrosion resistant steel is further improved. The Nb element and the C form carbide, so that crystal grains can be refined, and the toughness of the acid corrosion resistant steel is improved. By balancing the chemical components of the acid corrosion resistant steel, the dual contradiction relationship between high strength and high toughness and between high strength and SSC resistance is solved.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. The steel for the oil well pipe is characterized by comprising the following chemical components in percentage by weight:

0.4 to 0.6 percent of C, less than or equal to 0.3 percent of Si, less than or equal to 0.5 percent of Mn, 0.8 to 1.2 percent of Cr, 0.6 to 0.9 percent of Mo, 0.1 to 0.3 percent of V, less than or equal to 0.05 percent of Nb, less than or equal to 0.05 percent of Al, less than or equal to 0.03 percent of N, less than or equal to 0.005 percent of S, less than or equal to 0.01 percent of P, 0.7 to 1.5 percent of Cu, the weight percentage of Ni and Cu is more than or equal to 0.5 and less than or equal to 1, and the balance of Fe and inevitable impurities.

2. The steel for oil well pipes according to claim 1, characterized by having a yield strength >862MPa, a tensile strength >950MPa at room temperature; the impact energy of the full-size V-shaped notch is more than 130J at the temperature of 0 ℃.

3. The steel for an oil well pipe as claimed in claim 1, wherein the metallographic structure of the steel for an oil well pipe comprises tempered sorbite and austenite, the volume of the austenite is 3 to 5% of the total volume of the metallographic structure, and the volume of the tempered sorbite is 95 to 97% of the total volume of the metallographic structure.

4. The steel for oil well pipes according to claim 1, wherein the steel for oil well pipes is saturated with H₂The steel can not be broken after being maintained for at least 720 hours under the stress of 733MPa loaded in the S environment.

5. A method for producing the steel for oil well pipes according to any one of claims 1 to 4, characterized by comprising:

step S1, mixing the chemical components of the steel for the oil well pipe according to any one of claims 1 to 4 to obtain raw materials, and smelting and pouring the raw materials to obtain a steel ingot;

step S2, preserving heat of the steel ingot, forging the steel ingot in an austenite single-phase region, and cooling the steel ingot to room temperature to obtain a cooled steel ingot;

and step S3, carrying out heat treatment on the cooled steel ingot to obtain the steel for the oil well pipe.

6. The preparation method according to claim 5, wherein in the step S2, the temperature for heat preservation is 1150-1200 ℃, the time for heat preservation is not less than 4h, and preferably the time for heat preservation is 4-12 h.

7. The method according to claim 5, wherein in the step S2, the forging includes initial forging and finish forging, the temperature of the initial forging is 1100-1150 ℃, and the time of the initial forging is 5-60 min; the finish forging temperature is 950-1000 ℃, and the finish forging time is 5-60 min; preferably, the forging ratio of the forging is more than 8, and the forging ratio of the forging is 8-20; preferably, the cooling is air cooling, and the cooling speed is 0.5 ℃/s-5 ℃/s.

8. The method according to claim 5, wherein in the step S3, the heat treatment includes:

step S31, heating the cooled steel ingot to a first preset temperature, and carrying out first heat preservation at the first preset temperature to obtain a first steel ingot;

step S32, heating the heat-insulated steel ingot to a second preset temperature, carrying out second heat insulation at the second preset temperature to obtain a second steel ingot, and cooling the second steel ingot to room temperature to obtain a third steel ingot;

step S33, tempering the third steel ingot to obtain a fourth steel ingot;

and step S34, cooling the fourth steel ingot to room temperature to obtain the steel for the oil well pipe.

9. The method according to claim 8, wherein the first predetermined temperature is 600 to 700 ℃, preferably the time coefficient of the first heat preservation is 2.0 to 4.0; the second preset temperature is 810-860 ℃, and the second heat preservation time is preferably 60-90 minutes; preferably, the cooling in step S32 is oil cooling, and the cooling rate is 5 ℃/S to 20 ℃/S.

10. The method according to claim 8, wherein the tempering temperature is 680 to 730 ℃, and the tempering time coefficient is 3.0 to 9.0.

11. An oil country tubular good characterized in that it is produced using the steel for oil country tubular good according to any one of claims 1 to 4.