CN113502435B

CN113502435B - Oil casing pipe steel for improving low-temperature impact toughness and preparation method thereof

Info

Publication number: CN113502435B
Application number: CN202110733910.0A
Authority: CN
Inventors: 吴刚; 刘志璞; 姜学锋; 董志鹏; 文小明; 韩宇; 许学力; 李霞; 陈红远; 马思远; 吕建勋
Original assignee: Bengang Steel Plates Co Ltd
Current assignee: Bengang Steel Plates Co Ltd
Priority date: 2021-06-30
Filing date: 2021-06-30
Publication date: 2022-09-02
Anticipated expiration: 2041-06-30
Also published as: CN113502435A

Abstract

The invention relates to steel for an oil casing pipe for improving low-temperature impact toughness and a preparation method thereof, wherein the steel for the oil casing pipe comprises the following components in percentage by mass: c: 0.03-0.07%, Si: 0.15-0.30%, Mn: 1.50-2.0%, S: less than or equal to 0.005 percent, P: less than or equal to 0.010 percent, Al: 0.015 to 0.055%, O: 0.0002-0.0010%, N: less than or equal to 0.0015 percent, Ca: 0.0020-0.0040%, Nb: 0.050 to 0.10%, Ti: 0.03-0.06%, Cr: 0.25-0.45%, Ni: 0.10-0.30%, Cu: 0.10-0.50%, Nb + V + Ti is less than or equal to 0.15%, Re: 0.05-0.15%, and the balance of Fe and other inevitable impurities, wherein Re is a mixture of Ce: 48%, La: 32%, Pr: 5%, Nd: 5%, Pm + Sm + Eu + Gd: 8 percent, and the balance of Fe and other inevitable impurities. Can obviously improve the impact toughness resistance of the steel in the low-temperature environment.

Description

Oil casing pipe steel for improving low-temperature impact toughness and preparation method thereof

Technical Field

The invention relates to the technical field of production of weathering resistant steel, in particular to oil casing steel for improving low-temperature impact toughness and a preparation method thereof.

Background

With the development of the petroleum industry in China, oil casings used for deep well exploitation put higher requirements on the performance of steel, and due to the special application field and use environment (the underground temperature is generally below-40 ℃ and even lower), steel plates are required to have good low-temperature impact toughness, excellent forming performance, welding performance, higher strength and the like, the oil casings play roles in sealing off stratums and preventing the well wall from collapsing in the drilling process, and are disposable consumption drilling pipes, and the API Spec 5CT has strict requirements on the production and use of the oil casings due to the complex use environment of the oil casings.

For a long time, most of domestic oil sleeves depend on import, high-grade hot rolled coils obtain excellent comprehensive performance along with the development of microalloying and controlled rolling and controlled cooling technologies of steel, and the hot rolled coils are used to manufacture straight seam Electric Resistance Welding (ERW) sleeves to replace traditional seamless steel pipes abroad in the nineties, so that success is achieved, and good economic benefits are obtained. According to the statistics of the American Petroleum institute, the straight-seam electric resistance welding casing pipe accounts for more than 60% of the total amount of the petroleum casing pipe, according to the statistics of Japan, the casing pipe produced by the ERW welding method accounts for 70%, and the yield ratio of the straight-seam electric resistance welding casing pipe to the seamless casing pipe in other industrially developed countries is at least 3: 7, the straight welded casing has the advantages of high geometric dimension precision, uniform wall thickness, and 10-15% higher collapse resistance and crushing resistance than the same steel grade seamless casing in underground service.

As the oil casing pipe is generally used in a low-temperature environment, the oil casing pipe is required to have high strength and good low-temperature impact toughness, welding performance and forming performance by users, along with the technical progress of steel-making technology, the content of oxygen, nitrogen, hydrogen, sulfur and phosphorus in steel is controlled to be lower, but the phenomena of overproof inclusion and overlarge component fluctuation still exist in the steel, the continuous and long-strip-shaped non-metallic inclusion defects can be generated after the continuous casting billet is rolled, some quality problems such as edge crack, warping and the like can be caused, the defects have great influence on the toughness of the steel, particularly the low-temperature impact toughness, and at present, the means for improving the low-temperature impact toughness is limited by eliminating the adverse influence of the inclusion and harmful gas in the steel.

Therefore, there is a need for an oil casing steel with improved low-temperature impact toughness and a method for preparing the same, which can significantly improve the impact toughness of the steel in a low-temperature environment.

Disclosure of Invention

The present invention is directed to solving one of the technical problems of the prior art or the related art.

Therefore, the invention provides the steel for the oil casing pipe with improved low-temperature impact toughness and the preparation method thereof.

In view of the above, an aspect of the present invention provides a steel for an oil casing pipe with improved low-temperature impact toughness, including, by mass: c: 0.03 to 0.07%, Si: 0.15-0.30%, Mn: 1.50-2.0%, S: less than or equal to 0.005%, P: less than or equal to 0.010 percent, Al: 0.015 to 0.055%, O: 0.0002-0.0010%, N: less than or equal to 0.0015 percent, Ca: 0.0020-0.0040%, Nb: 0.050 to 0.10%, Ti: 0.03-0.06%, Cr: 0.25-0.45%, Ni: 0.10 to 0.30%, Cu: 0.10-0.50%, Nb + V + Ti is less than or equal to 0.15%, Re: 0.05-0.15%, and the balance of Fe and other inevitable impurities, wherein Re is formed by Ce: 48%, La: 32%, Pr: 5%, Nd: 5%, Pm + Sm + Eu + Gd: 8 percent, and the balance of Fe and other inevitable impurities.

Further, the thickness of the steel for oil casing pipes is 2.0mm to 16.0 mm.

Further, the tensile strength of the steel for oil casing pipes is 700MPa to 740MPa, the yield strength is 640MPa to 670MPa, and the elongation after fracture is 29 percent to 33 percent.

Further, the preparation method comprises the following steps: molten iron pretreatment → converter smelting → ladle furnace refining → continuous casting → heating → 2300mm hot continuous rolling → controlled cooling → coiling → delivery, wherein calcium treatment is performed during the ladle furnace refining, and rare earth treatment is performed during the continuous casting.

Further, the converter smelting adopts a 180-ton top-bottom combined blown converter, adopts a sliding plate to block slag and tap, and has deoxidation and alloying in the tapping process, wherein the oxygen content is strictly limited to 0.0020-0.0030%.

Further, the ladle furnace refining comprises: and (2) when the molten steel reaches a refining treatment position, blowing argon gas at the bottom for stirring, adding submerged arc slag, refining slag, lime and the like, electrifying the three electrodes, heating for 10min, measuring the temperature, sampling, adding alloy elements for adjustment, adding aluminum particles and calcium carbide white slag according to the slag condition, electrifying for more than 18min, and adjusting the alloy components.

Further, the calcium treatment performed during the ladle furnace refining process includes: measuring the temperature of the adjusted alloy components, sampling, feeding a calcium-iron wire when the component temperature of the molten steel meets the target requirement, wherein the addition amount of the calcium-iron wire is 400-500 m, and performing soft argon blowing treatment after the feeding of the calcium-iron wire is finished, wherein the soft argon blowing time is more than or equal to 10min, so that impurities are fully floated and removed.

Further, the continuous casting includes: the molten steel is transported to a slab caster for casting, in the continuous casting process, the speed of feeding the rare earth wires is set according to the difference of the drawing speed, 200g to 400g of the rare earth wires are added into each ton of steel, the rare earth wires are fed into a crystallizer, the rare earth recovery rate is ensured to reach more than 70%, the superheat degree is controlled to be 10 ℃ to 25 ℃, the drawing speed is controlled to be 1.0m/min to 1.2m/min, and the secondary cooling water of the crystallizer is cooled by adopting an inter-cooling mode and gas-water spray.

Further, the heating includes: heating the continuous casting slab to 1218-1242 ℃ in a stepping heating furnace, wherein the heating time is 120-150 min, and the heat preservation time is controlled to be more than or equal to 40 min.

Further, the 2300mm hot continuous rolling comprises rough rolling and finish rolling, wherein the thickness of an intermediate billet is controlled to be 52 mm-58 mm, the initial rolling temperature of the rough rolling is more than or equal to 1150 ℃, 3+5 rolling is adopted, the initial rolling temperature of the finish rolling is 950 ℃ -990 ℃, the final rolling temperature of the finish rolling is 840 ℃ -880 ℃, the coiling temperature is 540 ℃ -580 ℃, the compression ratio is more than or equal to 5.0, the reduction ratios of the finish rolling F1 and F2 are more than or equal to 40%, and the reduction ratios of the rest passes are controlled to be 25% to 35%.

The technical scheme provided by the embodiment of the invention can have the following beneficial effects:

through calcium treatment and rare earth treatment, inclusions in steel are fully floated and removed, the effects of modification inclusion, metallurgy control, fine grain strengthening and pure molten steel are achieved in the steel, the harmful effect of the inclusions in the steel is eliminated, and finally, a composite compound of rare earth and calcium sulfide ((Ca, RE) S), rare earth and calcium oxide ((Ca, RE) O) or rare earth and calcium composite oxysulfide ((Ca, RE) (O, S)) which are distributed in a fine, uniform and dispersed mode is formed in the steel and is distributed in a spherical mode and a point mode, and the distribution effectively improves the low-temperature impact toughness of the oil casing.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

Drawings

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and together with the description, serve to explain the principles of the invention.

In order to more clearly illustrate the embodiments or technical solutions in the prior art of the present invention, the drawings used in the description of the embodiments or prior art will be briefly described below, and it is obvious for those skilled in the art that other drawings can be obtained based on these drawings without creative efforts.

FIG. 1 is a schematic flow chart illustrating a method for manufacturing steel for oil casing pipes with improved low-temperature impact toughness according to an embodiment of the present invention;

FIG. 2 is a schematic representation of the metallographic structure of an 8mm thick steel plate according to an embodiment of the invention;

FIG. 3 illustrates the exemplary non-metallic inclusion grade of FIG. 2;

FIG. 4 shows a metallographic structure of a prior art 8mm thick steel plate;

FIG. 5 shows the non-metallic inclusion grade of FIG. 4;

FIG. 6 shows SEM topography at-60 ℃ for impact fractures of an 8.0mm thick steel plate of one embodiment of the invention;

FIG. 7 shows SEM topography at-80 ℃ for impact fractures of an 8.0mm thick steel plate of one embodiment of the invention;

FIG. 8 shows SEM morphology impact fractures at-100 ℃ for an 8.0mm thick steel plate of one embodiment of the invention.

Detailed Description

Reference will now be made in detail to the exemplary embodiments, examples of which are illustrated in the accompanying drawings. When the following description refers to the accompanying drawings, like numbers in different drawings represent the same or similar elements unless otherwise indicated. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Rather, they are merely examples of apparatus and methods consistent with certain aspects of the invention, as detailed in the appended claims.

Example 1

The embodiment provides steel for an oil casing pipe for improving low-temperature impact toughness, which comprises the following components in percentage by mass: c: 0.03-0.07%, Si: 0.15-0.30%, Mn: 1.50-2.0%, S: less than or equal to 0.005 percent, P: less than or equal to 0.010 percent, Al: 0.015 to 0.055%, O: 0.0002-0.0010%, N: less than or equal to 0.0015%, Ca: 0.0020-0.0040%, Nb: 0.050 to 0.10%, Ti: 0.03-0.06%, Cr: 0.25-0.45%, Ni: 0.10-0.30%, Cu: 0.10-0.50%, Nb + V + Ti is less than or equal to 0.15%, Re: 0.05-0.15%, and the balance of Fe and other inevitable impurities, wherein Re is formed by Ce: 48%, La: 32%, Pr: 5%, Nd: 5%, Pm + Sm + Eu + Gd: 8 percent, and the balance of Fe and other inevitable impurities.

Through the two treatments of calcium and rare earth, the inclusion is fully spheroidized, and the low-temperature impact toughness of the steel plate is improved.

Further, the thickness of the steel for oil casing pipes is 2.0mm to 16.0 mm.

The steel for oil casing pipes having different thicknesses is produced according to the use requirements of users.

Further, the tensile strength of the steel for oil casing pipes is 700MPa to 740MPa, the yield strength is 640MPa to 670MPa, and the elongation after fracture is 29% to 33%.

The steel for oil casing pipes produced according to the chemical composition of the embodiment has high strength and excellent toughness.

Example 2

Fig. 1 is a schematic flow chart illustrating a method for manufacturing steel for oil casing pipes with improved low-temperature impact toughness according to an embodiment of the present invention.

As shown in fig. 1, the present embodiment provides a method for preparing steel for oil casing pipe with improved low-temperature impact toughness, the method comprising the following steps:

step 1, pretreating molten iron;

step 2, smelting in a converter;

step 3, ladle furnace refining, wherein calcium treatment is carried out in the ladle furnace refining process;

step 4, continuous casting, wherein rare earth treatment is carried out in the continuous casting process;

step 5, heating;

step 6, hot continuous rolling with the thickness of 2300 mm;

step 7, controlling cooling;

step 8, coiling;

and 9, delivering goods.

Through calcium treatment and rare earth treatment, inclusions in steel are promoted to float upwards and be removed through the calcium treatment and the rare earth treatment, the effects of modification inclusion, metallurgical control, fine grain strengthening and pure molten steel are achieved in the steel, the harmful effect of the inclusions in the steel is eliminated, and finally, composite compounds of fine, uniform and dispersed calcium, rare earth sulfide ((Ca, RE) S), calcium, rare earth oxide ((Ca, RE) O) or calcium, rare earth oxysulfide ((Ca, RE) (O, S)) and the like are formed in the steel and are distributed in a spherical and dotted mode, and the distribution effectively improves the low-temperature impact toughness of the oil casing.

Furthermore, the converter smelting adopts a 180-ton top-bottom combined blown converter, a sliding plate is adopted to block slag and tap, and the oxygen content is strictly limited to 0.0020% to 0.0030% in the deoxidation alloying process during the tapping process, so as to improve the quality of the steel plate and reduce the gas content in the steel.

Specifically, in order to fully float and remove impurities in steel, argon gas is blown from the bottom for stirring, submerged arc slag, refining slag, lime and the like are added for further desulfurization, and alloy elements are added for adjustment according to the requirements of internal control standards of chemical components; according to the slag condition, adding aluminum particles and calcium carbide white slag, electrifying for more than 18min, and adjusting alloy components to ensure that the chemical components and the temperature of the refined molten steel meet the process standard requirements.

Further, the calcium treatment during the ladle furnace refining treatment includes: measuring the temperature of the adjusted alloy components, sampling, feeding calcium iron wires when the component temperature of molten steel meets the target requirement, wherein the addition amount of the calcium iron wires is 400-500 m, and carrying out soft argon blowing treatment after the feeding of the calcium iron wires is finished, wherein the soft argon blowing time is more than or equal to 10min, so that impurities are fully floated and eliminated.

After calcium treatment, the molten steel comprises the following components in percentage by mass: c: 0.03 to 0.07%, Si: 0.15-0.30%, Mn: 1.50-2.0%, S: less than or equal to 0.005%, P: less than or equal to 0.010 percent, Al: 0.015 to 0.055%, O: 0.0002 to 0.0010%, N: less than or equal to 0.0015 percent, Ca: 0.0020-0.0040%; nb: 0.050 to 0.10%, Ti: 0.03-0.06%, Cr: 0.25-0.45%, Ni: 0.10-0.30%, Cu: 0.10-0.50%; nb + V + Ti is less than or equal to 0.15 percent, and the balance is Fe and other inevitable impurities.

Further, the continuous casting includes: the molten steel is transported to a slab caster for casting, in the continuous casting process, the speed of feeding the rare earth wires is set according to the difference of the drawing speed, 200g to 400g of rare earth wires are added into each ton of steel, the rare earth wires are fed into a crystallizer, the recovery rate of the rare earth is ensured to reach more than 70 percent, the superheat degree is controlled to be between 10 ℃ and 25 ℃, the drawing speed is controlled to be between 1.0m/min and 1.2m/min, the secondary cooling water of the crystallizer adopts an intercooling mode, and the secondary cooling water is cooled by gas-water spray.

After rare earth treatment, the oil casing steel finally comprises the following components in percentage by mass: c: 0.03 to 0.07%, Si: 0.15-0.30%, Mn: 1.50-2.0%, S: less than or equal to 0.005%, P: less than or equal to 0.010 percent, Al: 0.015 to 0.055%, O: 0.0002 to 0.0010%, N: less than or equal to 0.0015 percent, Ca: 0.0020-0.0040%; nb: 0.050 to 0.10%, Ti: 0.03-0.06%, Cr: 0.25 to 0.45%, Ni: 0.10-0.30%, Cu: 0.10-0.50%; nb + V + Ti is less than or equal to 0.15%, Re: 0.05-0.15%, wherein the rare earth contains Ce: 48%, La: 32%, Pr: 5%, Nd: 5%, Pm + Sm + Eu + Gd: 8 percent, and the balance of Fe and other inevitable impurities.

It should be noted that the rare earth wire is a rare earth core-spun wire, and the diameter of the rare earth core-spun wire can be selected to be 2.5mm, 4.0mm or 5.0mm according to the difference of the drawing speed. That is, when the drawing speed is 1.0m/min, the diameter of the rare earth core-spun yarn is 2.5 mm; when the pulling speed is between 1.1m/min and 1.2m/min, the diameter of the rare earth core-spun yarn is 4.0 mm; when the drawing speed is more than or equal to 1.3m/min, the diameter of the rare earth core-spun yarn is 4.5mm, so that the burning loss is reduced, and the recovery rate of the rare earth is ensured.

Specifically, the walking beam furnace can transport materials flexibly, blanks are arranged on the furnace bottom or the beam at intervals, and the blanks can be heated uniformly and quickly. The furnace body is divided into a preheating section, a heating section and a soaking section, 5 flame adjusting burners are arranged on the side walls of the two sides of the heating section, a flat flame burner is arranged on the upper heating section of the soaking section, a flame adjusting burner is arranged on the lower heating section of the soaking section, an electromagnetic valve and an adjusting valve are arranged on a mixed gas pipeline of coal gas and air of the flame adjusting burner, an adjusting valve is arranged on a mixed gas pipeline of coal gas and air of the flat flame burner, and an air main pipeline and a coal gas main pipeline are arranged on the furnace top.

In order to ensure that the alloy elements are fully dissolved, the heating time is required to be sufficient, the heating temperature is uniform, so as to ensure the quality of a final product, the continuous casting billet is heated to 1218-1242 ℃, the heating time is 120-150 min, and the heat preservation time is controlled to be more than or equal to 40 min.

Further, the 2300mm hot continuous rolling comprises rough rolling and finish rolling, wherein the thickness of the intermediate billet is controlled to be 52 mm-58 mm, the initial rolling temperature of the rough rolling is more than or equal to 1150 ℃, 3+5 rolling is adopted, the initial rolling temperature of the finish rolling is 950 ℃ to 990 ℃, the final rolling temperature of the finish rolling is 840 ℃ to 880 ℃, the coiling temperature is 540 ℃ to 580 ℃, the compression ratio is more than or equal to 5.0, the reduction ratios of F1 and F2 in the finish rolling are more than or equal to 40%, and the reduction ratios of the rest passes are controlled to be 25% to 35%.

By the hot continuous rolling mode, the metallographic structure, the mechanical property and the forming property of the steel for the oil casing can be ensured, and the product quality is improved.

Further, in order to obtain even, tiny gold phase structure, guarantee steel sheet isotropy, guarantee steel sheet head, well, tail performance uniformity, avoid the cooling rate too fast, steel sheet after hot continuous rolling batches the back and requires to carry out slow cooling (keeping away from the storehouse door in the storehouse, encloses cold and avoid cold wind direct-blowing) in the hot district in the storehouse, just can remove after 32 hours, finally obtains the tissue: ferrite is added with pearlite and a small amount of bainite, and accordingly Ca and Re are combined with S, Al, Mg and O to form uniform, fine, dispersed and spherical composite inclusions to exist, so that non-metallic inclusions A, B, C and D are less than or equal to 1.0 grade and exist in steel in a punctiform and spherical manner, the damage of long-strip-shaped inclusions to the performance of the steel is avoided, and the low-temperature impact toughness (below minus 60 ℃) of the steel plate special for the oil casing is effectively improved.

The oil casing steel treated by the method has good low-temperature impact toughness and cold bending processability, meets the subsequent processing requirements, has higher finished product quality and yield in the production process, improves the economy, and can realize the light weight design of the oil well pipe special steel on the basis of the existing equipment.

Example 3

According to the above-mentioned mixture ratio and machining method, three kinds of steel for oil casing pipes were machined according to various parameters of the three kinds of steel for oil casing pipes in tables 1 and 2.

TABLE 1 chemical composition weight percentage of steel for three oil casings

TABLE 1 shows the mass percentages of the chemical components of three kinds of steel for oil casing

TABLE 2 machining parameters of three kinds of steel for oil casing

Numbering	Thickness/mm	F1 opening temperature/. degree C	Final Rolling temperature/. degree.C	Coiling temperature/. degree.C
					A	5.0	960	845	545
B	6.0	985	862	561
					C	8.0	985	880	579

The mechanical properties and bending properties of the three types of steel for oil casings processed by the above methods are shown in table 3.

TABLE 3 mechanical and bending properties of three kinds of steel for oil casing

Numbering	Yield strength/MPa	Tensile strength/MPa	Elongation A50 (%)	180 degree cold bending experiment
					A	665	731	32.5	Intact
B	650	722	30	Intact
					C	640	701	29.5	Intact

The oil casing steel obtained by the chemical components and the processing method completely meets the use requirements.

It should be noted that the final chemical components are identical to the foregoing except for rare earth, and Ce, La, Pr, Nd, Pm + Sm + Eu + Gd are all rare earth elements.

Comparative example 1

FIG. 2 is a schematic representation of the metallographic structure of an 8mm thick steel plate according to an embodiment of the invention; fig. 3 shows a typical non-metallic inclusion grade of fig. 2.

As shown in fig. 2 and 3, the metallographic structure of the steel sheet for oil casing produced by the method of example 2 was ferrite, pearlite and bainite, and it was found from the inclusion and energy spectrum analysis that B-type inclusions in the steel were of 0.5 grade, and the inclusions in the steel were present as uniform, fine, dispersed and spherical composite inclusions formed by the combination of Ca, RE, S, Al, Mg and O, thereby effectively improving the impact properties of the steel for oil casing.

FIG. 4 shows a metallographic structure of a prior art 8mm thick steel plate; fig. 5 shows the non-metallic inclusion levels of fig. 4.

As shown in fig. 4 and 5, the metallographic structure of the steel sheet without being subjected to the cleaning treatment was: ferrite + pearlite, the non-metallic inclusions in the steel are: A1.5B2.0D0.5, the non-metallic inclusions are serious and adversely affect the formability and impact properties of the steel sheet.

Comparative example 2

The impact properties of the steel plate were measured by taking V-notch impact specimens in the transverse and longitudinal directions of the steel plate, and the impact properties of samples not subjected to calcium treatment and rare earth treatment and samples subjected to calcium treatment and rare earth treatment are shown in Table 4.

TABLE 4 comparative table of transverse and longitudinal impact properties

As can be seen from Table 4, after two times of purification treatment, the transverse impact energy of the steel plate at-60 ℃ is 1.7 times that of the steel plate without calcium and rare earth treatment, and the longitudinal impact energy is equivalent; when the temperature of the steel plate is reduced to-80 ℃, the value of transverse impact energy is 1.6 times that of the steel plate without calcium and rare earth treatment, and the value of longitudinal impact energy is 1.4 times; the temperature of the steel plate is further reduced to-100 ℃, the value of transverse impact energy is 3 times that of the steel plate without calcium and rare earth treatment, and the value of longitudinal impact energy is 2 times. The steel plate after two times of purification treatment has obviously better low-temperature toughness than the untreated steel plate.

FIG. 6 shows SEM morphology impact fracture at-60 ℃ for an 8.0mm thick steel plate of one embodiment of the invention; FIG. 7 shows SEM morphology impact fracture at-80 ℃ for an 8.0mm thick steel plate of one embodiment of the invention; FIG. 8 shows SEM morphology impact fractures at-100 ℃ for an 8.0mm thick steel plate of one embodiment of the invention.

Through transmission electron microscope observation, as can be seen in fig. 6 to 8, the fracture morphology of the impact specimen at-60 ℃ presents deeper equiaxed dimple, which indicates that the impact specimen undergoes great plastic deformation before fracture; at-80 ℃, the fracture morphology is still a dimple fracture, but the dimple fracture is shallow, flat and thick compared with the former dimple fracture, which indicates that the fracture is still ductile at the temperature, so that the fracture can be judged to be ductile at the temperature above-80 ℃; when the temperature is reduced to-100 ℃, the fracture morphology presents an obvious complete cleavage pattern, the fracture is proved to reach complete brittle fracture, and the low-temperature ductile-brittle transition temperature of the special steel for the oil casing for deep well exploitation is between-80 ℃ and-100 ℃, so that the use requirement under severe low-temperature conditions can be met.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

It is to be understood that the present invention is not limited to what has been described above, and that various modifications and changes may be made without departing from the scope thereof. The scope of the invention is limited only by the appended claims.

Claims

1. The preparation method of the steel for the oil casing pipe for improving the low-temperature impact toughness is characterized by comprising the following steps of: molten iron pretreatment → converter smelting → ladle furnace refining → continuous casting → heating → 2300mm hot continuous rolling → controlled cooling → coiling → delivery, wherein calcium treatment is performed during the ladle furnace refining, and rare earth treatment is performed during the continuous casting;

the continuous casting includes: conveying molten steel to a slab caster for casting, setting the speed of feeding rare earth wires according to different drawing speeds in the continuous casting process, adding 200-400 g of rare earth wires into each ton of steel, adopting a mode of feeding the rare earth wires into a crystallizer, ensuring that the recovery rate of rare earth reaches more than 70%, controlling the superheat degree between 10 ℃ and 25 ℃, controlling the drawing speed between 1.0m/min and 1.2m/min, and cooling secondary cooling water of the crystallizer in an intercooling mode and carrying out gas-water spray cooling;

the steel for the oil casing pipe comprises the following components in percentage by mass: c: 0.03 to 0.07%, Si: 0.15-0.30%, Mn: 1.50-2.0%, S: less than or equal to 0.005 percent, P: less than or equal to 0.010 percent, Al: 0.015 to 0.055%, O: 0.0002-0.0010%, N: less than or equal to 0.0015 percent, Ca: 0.0020 to 0.0040%, Nb: 0.050 to 0.10%, Ti: 0.03-0.06%, Cr: 0.25-0.45%, Ni: 0.10-0.30%, Cu: 0.10-0.50%, Nb + V + Ti is less than or equal to 0.15%, RE: 0.05-0.15%, and the balance of Fe and other inevitable impurities, wherein RE is a Ce: 48%, La: 32%, Pr: 5%, Nd: 5%, Pm + Sm + Eu + Gd: 8 percent, and the balance of Fe and other inevitable impurities;

the thickness of the steel for the oil casing pipe is 2.0mm to 16.0 mm; the tensile strength of the steel for the oil casing pipe is 700MPa to 740MPa, the yield strength is 640MPa to 670MPa, and the elongation after fracture is 29 percent to 33 percent;

the calcium treatment in the ladle furnace refining treatment process comprises the following steps: measuring the temperature of the adjusted alloy components, sampling, feeding a calcium-iron wire when the component temperature of the molten steel meets the target requirement, wherein the addition amount of the calcium-iron wire is 400-500 m, and performing soft argon blowing treatment after the feeding of the calcium-iron wire is finished, wherein the soft argon blowing time is more than or equal to 10min, so that impurities are fully floated and removed.

2. The method for preparing steel for oil casing pipes with improved low-temperature impact toughness according to claim 1, wherein the converter smelting is carried out by using a 180-ton top-bottom combined blown converter, the steel is tapped by using a sliding plate for slag blocking, and the oxygen content is strictly limited to 0.0020% to 0.0030% by deoxidation alloying during the tapping process.

3. The method for preparing the steel for oil casing pipe with improved low temperature impact toughness according to claim 1, wherein the ladle furnace refining comprises: and (2) when the molten steel reaches a refining treatment position, blowing argon gas at the bottom for stirring, adding submerged arc slag, refining slag, lime and the like, electrifying a three-phase electrode, heating for 10min, measuring the temperature, sampling, adding alloy elements for adjustment, adding aluminum particles and calcium carbide white slag according to the slag condition, electrifying for more than 18min, and adjusting the alloy components.

4. The method for preparing steel for oil casing pipes with improved low-temperature impact toughness according to claim 1, wherein the heating comprises: heating the continuous casting slab to 1218-1242 ℃ in a stepping heating furnace, wherein the heating time is 120-150 min, and the heat preservation time is controlled to be more than or equal to 40 min.

5. The method for preparing the steel for oil casings with improved low-temperature impact toughness according to claim 4, wherein the 2300mm hot continuous rolling comprises rough rolling and finish rolling, wherein the thickness of the intermediate blank is controlled to be 52mm to 58mm, the initial rolling temperature of the rough rolling is not less than 1150 ℃, 3+5 rolling is adopted, the initial rolling temperature of the finish rolling is 950 ℃ to 990 ℃, the finish rolling temperature of the finish rolling is 840 ℃ to 880 ℃, the coiling temperature is 540 ℃ to 580 ℃, the compression ratio is not less than 5.0, the reduction ratios of F1 and F2 are not less than 40%, and the reduction ratios of the rest of the passes are controlled to be 25% to 35%.