阅读说明：本技术 一种无缝钢管及其制备方法 (Seamless steel pipe and preparation method thereof ) 是由 彭俊 唐科 姚火能 朱萍 向鑫 万五霞 潘先明 黎福华 饶金元 于 2020-06-15 设计创作，主要内容包括：本发明属于材料领域,特别涉及一种无缝钢管及其制备方法。本发明提供的无缝钢管的制备方法包括将管坯进行热轧和调质处理的操作；其中,所述热轧包括将所述管坯进行均匀加热处理、穿孔处理、轧管处理、减径处理,得到热轧后的钢管；所述调质处理依次包括淬火处理和回火处理。本发明提供的无缝钢管的制备方法大大缩短了钻杆接头的生产周期并有效降低生产成本,材料同时具有高强度和高韧性,适用于钻杆接头加工,性能优于现有产品,可以有效提高产品使用寿命。(The invention belongs to the field of materials, and particularly relates to a seamless steel tube and a preparation method thereof. The preparation method of the seamless steel pipe comprises the steps of carrying out hot rolling and quenching and tempering on a pipe blank; wherein the hot rolling comprises the steps of uniformly heating the tube blank, perforating, rolling a tube and reducing to obtain a hot-rolled steel tube; the quenching and tempering sequentially comprises quenching treatment and tempering treatment. The preparation method of the seamless steel pipe provided by the invention greatly shortens the production period of the drill rod joint and effectively reduces the production cost, and the material has high strength and high toughness, is suitable for machining the drill rod joint, has better performance than the existing product and can effectively prolong the service life of the product.)

1. A method for manufacturing a seamless steel pipe, the method comprising the steps of subjecting a pipe blank to hot rolling and thermal refining; wherein the content of the first and second substances,

the hot rolling comprises the steps of uniformly heating the tube blank, perforating, tube rolling and reducing to obtain a hot-rolled steel tube;

the quenching and tempering sequentially comprises quenching treatment and tempering treatment.

2. The method according to claim 1, wherein the tube blank is prepared by sequentially subjecting a production raw material to a melting treatment, a refining treatment, a vacuum treatment, and a casting treatment;

preferably, the production raw material is one or a mixture of high-quality steel scraps and hot charging molten iron;

preferably, the smelting treatment is carried out by adopting an electric furnace or a converter to obtain molten smelting steel;

preferably, in the refining treatment, a ladle furnace is adopted to refine the molten steel, and Ar is blown in the whole process and stirred to obtain refined molten steel;

further preferably, Ca wires or Ca-Si wires are added into the molten steel during the Ar blowing process for calcification treatment so that the content of Ca in the molten steel is 0.0001-0.005% by mass to improve the uniformity of the molten steel, and inclusion modification treatment is carried out so that inclusions are in a fine dispersion state;

preferably, in the vacuum treatment, the refined molten steel is placed in a vacuum furnace to be subjected to vacuum degassing treatment, wherein the vacuum degree in the furnace is less than or equal to 0.5 torr, and the molten steel after vacuum degassing is obtained;

preferably, in the pouring treatment, the molten steel subjected to vacuum degassing is poured in a protective manner in the whole continuous casting process, the superheat degree of the molten steel is controlled to be less than or equal to 30 ℃ in the pouring process, and the molten steel is electromagnetically stirred by a crystallizer, so that no secondary pollution is caused in the pouring process, and the tube blank is obtained;

further preferably, the tube blank is a solid continuous casting round blank, the diameter of the tube blank is 155mm-230mm, and the length of the tube blank is 2-4 m;

further preferably, the chemical elements in the tube blank comprise, by weight: c: 0.35-0.38%; si: 0.1-0.5%; mn: 0.6-1.5%; cr: 0.8-1.5%; mo: 0.20-0.50%; nb: less than or equal to 0.20 percent; p is less than or equal to 0.015 percent; s is less than or equal to 0.020%, and the balance of Fe and inevitable impurities; preferably, the calcium-containing material also comprises Ca, and the weight percentage of Ca element is 0.0001-0.005%.

3. The method according to claim 1 or 2,

in the uniform heating treatment, the heating temperature is 1150-1280 ℃, the heat preservation time is 2-10 hours, the thorough burning of the tube blank is ensured, and the heated tube blank is obtained;

preferably, the tube blank is subjected to uniform heating treatment by using a medium-diameter annular furnace.

4. The method of claim 3,

in the punching treatment, a conical puncher is adopted to perform expanding punching on the heated tube blank to obtain a punched capillary tube;

preferably, the hole expansion rate of the perforation treatment is 0-5%; the rolling angle and the feeding angle of the perforation treatment are both 10-15 degrees;

preferably, the outer diameter of the perforated capillary is 156 mm and 240mm, and the wall thickness difference of the section of the capillary is less than 5 mm.

5. The method of claim 4,

in the rolling treatment, an Alssel tube mill is adopted to roll the perforated tubular billet to obtain a rolled steel tube;

preferably, the total reducing rate of the pipe mill is controlled to be not more than 30%, the feeding angle of the pipe rolling treatment is controlled to be less than or equal to 15 degrees, the rolling angle is controlled to be less than or equal to 5 degrees, and the size of a core rod used in the pipe rolling treatment is 40-120 mm.

6. The method of claim 5,

in the reducing treatment, reducing the rolled steel pipe by adopting a ten-frame two-roller micro-tension reducing mill to obtain a reduced steel pipe;

preferably, the motor superposition ratio is controlled to be less than or equal to 50%, and the reducing rate of the reducing mill is controlled to be not more than 20%.

7. The method of claim 6,

the quenching treatment comprises the following steps: heating the reduced steel pipe to 800-950 ℃, preserving heat for 1-3 hours, placing the steel pipe on a quenching machine, and controlling the quenching time to be less than or equal to 5min to obtain a quenched steel pipe;

preferably, water is used as a quenching medium, forced cooling is carried out in a mode that the inner surface and the outer surface of the steel pipe are sprayed with water simultaneously in the quenching process, and the steel pipe continuously rotates in the cooling process.

8. The method of claim 7,

the tempering treatment comprises the following steps: and after the quenching treatment is finished and the steel pipe is cooled, putting the quenched steel pipe into a furnace, heating to 500-700 ℃, preserving the heat for 2-8 hours, discharging from the furnace, and air cooling to obtain the seamless steel pipe.

9. A seamless steel pipe produced by the method according to any one of claims 1 to 8,

the seamless steel pipe has high strength and high toughness, Rp0.2 is 890-1180Mpa, Rm is more than or equal to 1000Mpa, A is more than or equal to 14 percent, Z is more than or equal to 55 percent, KV8 is more than or equal to 70J, and the grain size is more than or equal to 8 grade.

10. The seamless steel pipe according to claim 9,

the thickness-diameter ratio of the seamless steel pipe is less than or equal to 0.35, the outer diameter is 79-230mm, and the inner diameter is 30-133 mm.

Technical Field

The invention belongs to the field of materials, and particularly relates to a seamless steel pipe and a preparation method thereof.

Background

The seamless steel pipe is a round, square or rectangular steel material having a hollow cross section and no seam at the periphery. The seamless steel pipe is made up by making steel ingot or solid pipe blank into tubular billet through perforation, then hot rolling, cold rolling or cold drawing. The seamless steel pipe has a hollow cross section and is widely used as a pipeline for transporting fluid, and compared with solid steel such as round steel, the steel pipe has the same bending strength and torsion strength and lighter weight, is a steel material with an economic cross section, and is widely used for manufacturing structural members and mechanical parts.

The drill rod is one of the important parts for oil drilling and production, and consists of a pipe body and a joint, wherein the joint is a section of hollow round steel with the length of about 400mm, and at present, the traditional drill rod joint manufacturing process comprises the following steps:

hot rolling bar → cutting-off → die forging → single hardening and tempering → turning → finished product.

The disadvantages of this process are:

1. the process is many, production cycle is long, with high costs, length is fixed.

2. Meanwhile, due to the influence of the production process, generally, only 100 left and right joints can be produced by a single die forging machine one day, and the requirements of a drill rod factory are difficult to meet.

3. The drill rod joint with the same specification has different lengths according to the requirements of users, and the universality of the joint blank is poor due to the fixed length of the die-forged joint blank.

4. The forged joint blank is subjected to heat treatment by one piece, the performance consistency of the whole batch is poor, and the detection frequency is high.

Note 1: and (2) performing single-piece heat treatment, namely generally placing a plurality of (generally more than ten) connector blanks after die forging on a tray, then placing the blanks and the tray into a quenching furnace for heating, placing the heated connector blanks and the tray into quenching liquid or an oil pool for quenching treatment, after quenching treatment, placing the connector blanks and the tray into a tempering furnace for tempering, and taking out after tempering, namely the whole heat treatment process.

Note 2: the swaged joint blank is typically 400-800mm in length, varying in length according to customer requirements.

Therefore, the traditional drill rod joint production process has low efficiency, long production period and poor performance uniformity, so that a new production process is needed to replace the traditional drill rod joint production process.

Disclosure of Invention

In order to solve the problems in the prior art, the invention provides a seamless steel pipe and a preparation method thereof, wherein the seamless steel pipe has high strength and high toughness and a larger thickness-diameter ratio, can be directly turned after being broken, and is very suitable for producing a common drill rod joint.

In order to achieve the purpose, the invention adopts the following technical scheme:

a method for manufacturing a seamless steel pipe, the method comprising the steps of subjecting a pipe blank to hot rolling and thermal refining; wherein the content of the first and second substances,

the hot rolling comprises the steps of uniformly heating the tube blank, perforating, tube rolling and reducing to obtain a hot-rolled steel tube;

the quenching and tempering sequentially comprises quenching treatment and tempering treatment.

In the above method for manufacturing a seamless steel tube, as a preferred embodiment, the tube blank is manufactured by sequentially performing a melting process, a refining process, a vacuum process, and a casting process on production raw materials;

preferably, the production raw material is one or a mixture of high-quality steel scraps and hot charging molten iron;

preferably, the smelting treatment is carried out by adopting an electric furnace or a converter to obtain molten smelting steel;

preferably, in the refining treatment, a ladle furnace is adopted to refine the molten steel, and Ar is blown in the whole process and stirred to obtain refined molten steel;

further preferably, Ca wires or Ca-Si wires are added into the molten steel during the Ar blowing process for calcification treatment so that the content of Ca in the molten steel is 0.0001-0.005% by mass to improve the uniformity of the molten steel, and inclusion modification treatment is carried out so that inclusions are in a fine dispersion state;

preferably, in the vacuum treatment, the refined molten steel is placed in a vacuum furnace to be subjected to vacuum degassing treatment, wherein the vacuum degree in the furnace is less than or equal to 0.5 torr, and the molten steel after vacuum degassing is obtained;

preferably, in the pouring treatment, the molten steel subjected to vacuum degassing is poured in a protective manner in the whole continuous casting process, the superheat degree of the molten steel is controlled to be less than or equal to 30 ℃ in the pouring process, and the molten steel is electromagnetically stirred by a crystallizer, so that no secondary pollution is caused in the pouring process, and the tube blank is obtained;

further preferably, the tube blank is a solid continuous casting round blank, the diameter of the tube blank is 155mm-230mm, and the length of the tube blank is 2-4 m;

further preferably, each chemical element in the tube blank comprises the following components in percentage by weight: c: 0.35-0.38%; si: 0.1-0.5%; mn: 0.6-1.5%; cr: 0.8-1.5%; mo: 0.20-0.50%; nb: less than or equal to 0.20 percent; p is less than or equal to 0.015 percent; s is less than or equal to 0.020%; the balance of Fe and inevitable impurities. Preferably, the calcium-containing material also comprises Ca, and the weight percentage of Ca element is 0.0001-0.005%.

In the above method for producing a seamless steel pipe, as a preferred embodiment, in the uniform heating treatment, a medium-diameter annular furnace is used to heat the pipe blank;

preferably, the heating temperature is 1150-1280 ℃, and the heat preservation time is 2-10 hours, so as to ensure that the tube blank is completely burnt, and obtain the heated tube blank.

The above-described method for manufacturing a seamless steel pipe, as a preferred embodiment,

in the punching treatment, a conical puncher is adopted to perform expanding punching on the heated tube blank to obtain a punched capillary tube;

preferably, the hole expansion rate of the perforation treatment is 0-5%; the rolling angle and the feeding angle of the perforation treatment are both 10-15 degrees;

preferably, the outer diameter of the perforated capillary is 156 mm and 240mm, and the wall thickness difference of the section of the capillary is less than 5 mm.

The above-described method for manufacturing a seamless steel pipe, as a preferred embodiment,

in the rolling treatment, an Alssel tube mill is adopted to roll the perforated tubular billet to obtain a rolled steel tube;

preferably, the total reducing rate of the pipe mill is controlled to be not more than 30%, the feeding angle of the pipe rolling treatment is controlled to be less than or equal to 15 degrees, the rolling angle is controlled to be less than or equal to 5 degrees, and the size of a core rod used in the pipe rolling treatment is 40-120 mm.

The above-described method for manufacturing a seamless steel pipe, as a preferred embodiment,

in the reducing treatment, reducing the rolled steel pipe by adopting a ten-frame two-roller micro-tension reducing mill to obtain a reduced steel pipe;

preferably, in order to ensure the consistency of the wall thickness of the whole length of the steel pipe, the motor superposition ratio is controlled to be less than or equal to 50 percent, and simultaneously, in order to improve the inner hole precision, the reducing rate of the reducing mill is controlled to be not more than 20 percent.

The above-described method for manufacturing a seamless steel pipe, as a preferred embodiment,

the quenching treatment comprises the following steps: heating the reduced steel pipe to 800-950 ℃, preserving heat for 1-3 hours, placing the steel pipe on a quenching machine, and controlling the quenching time to be less than or equal to 5min to obtain a quenched steel pipe;

preferably, water is used as a quenching medium, forced cooling is carried out in a mode that the inner surface and the outer surface of the steel pipe are sprayed with water simultaneously in the quenching process, and the steel pipe continuously rotates in the cooling process, so that the uniform and consistent transformation of the section structure in the quenching process is ensured.

The above-described method for manufacturing a seamless steel pipe, as a preferred embodiment,

the tempering treatment comprises the following steps: and after the quenching treatment is finished and the steel pipe is cooled, putting the quenched steel pipe into a furnace, heating to 500-700 ℃, preserving the heat for 2-8 hours, discharging from the furnace, and air cooling to obtain the seamless steel pipe.

The seamless steel pipe obtained by the preparation method has high strength and high toughness, Rp0.2 is 890-1180Mpa, Rm is more than or equal to 1000Mpa, A is more than or equal to 14 percent, Z is more than or equal to 55 percent, KV8 is more than or equal to 70J, and the grain size is more than or equal to grade 8.

As a preferable embodiment, the seamless steel tube obtained by the preparation method has the thickness-diameter ratio of less than or equal to 0.35, the outer diameter of 79-230mm and the inner diameter of 30-133 mm.

The preparation method of the seamless steel pipe provided by the invention greatly shortens the production period and effectively reduces the production cost, and the obtained seamless steel pipe has larger maximum thickness-diameter ratio, high strength and high toughness, is particularly suitable for processing common drill pipe joints, has better performance than the existing products, and can effectively prolong the service life of the products.

Additional features and advantages of the invention will be set forth in the detailed description which follows.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention are described in further detail below. It should be understood that the detailed description and specific examples, while indicating the present invention, are given by way of illustration and explanation only, not limitation.

The first aspect of the present invention provides a method for producing a seamless steel pipe, comprising the operations of subjecting a pipe blank to hot rolling and heat-refining; wherein the content of the first and second substances,

the hot rolling comprises the steps of uniformly heating the tube blank, perforating, tube rolling and reducing to obtain a hot-rolled steel tube;

the quenching and tempering sequentially comprises quenching treatment and tempering treatment.

According to the first aspect of the invention, the tube blank may be a continuously cast round blank. The tube blank can be prepared by sequentially carrying out smelting treatment, refining treatment, vacuum treatment and pouring treatment on production raw materials.

The production raw material can be one or a mixture of high-quality steel scraps and hot charging molten iron.

The smelting treatment preferably adopts an electric furnace or a converter to obtain molten smelting steel.

In the refining treatment, the molten steel is refined by adopting a ladle furnace, and Ar is blown and stirred in the whole process to obtain refined molten steel.

Preferably, Ca wire or Ca-Si wire is added to the molten steel during the Ar blowing to perform calcification treatment so that the Ca content in the molten steel is 0.0001-0.005% by mass to improve the homogeneity of the molten steel, and the inclusion is modified to be in a fine dispersion state.

In the vacuum treatment, the refined molten steel is placed in a vacuum furnace to be subjected to vacuum degassing treatment, wherein the degree of vacuum in the furnace is less than or equal to 0.5 torr (1 torr is 133.322 Pa), and the molten steel after vacuum degassing is obtained.

In the pouring treatment, the molten steel subjected to vacuum degassing is poured in a continuous casting whole-process protection mode, the superheat degree of the molten steel is controlled to be less than or equal to 30 ℃ in the pouring process, electromagnetic stirring is carried out by a crystallizer, secondary pollution in the pouring process is avoided, and the obtained continuous casting round billet (tube billet) is low in loosening level; the tube blank is a solid blank with the diameter of 155mm-230mm and the length of 2-4 m.

The tube blank (continuous casting round blank) produced by the process comprises the following components in percentage by weight: c: 0.35-0.38%; si: 0.1-0.5%; mn: 0.6-1.5%; cr: 0.8-1.5%; mo: 0.20-0.50%; nb: less than or equal to 0.20 percent; p is less than or equal to 0.015 percent; s is less than or equal to 0.020%; the balance of Fe and inevitable impurities; preferably, the calcium-containing material also comprises 0.0001-0.005% of Ca by weight.

According to the first aspect of the present invention, the hot rolling may include subjecting the pipe blank to a uniform heating treatment, a piercing treatment, a pipe rolling treatment, a reducing treatment to obtain a hot-rolled steel pipe.

In the uniform heating treatment, a medium-diameter annular furnace is preferably adopted to heat the tube blank. The heating temperature is 1150-minus one and 1280 ℃, the heat preservation time is 2-10 hours, the thorough burning of the tube blank is ensured, and the heated tube blank is obtained.

Further, the temperature of the heat treatment may be in a range between any one or two of 1150, 1170, 1190, 1200, 1220, 1240, 1250, 1270 and 1280 ℃, for example, 1150-; the incubation time may be in the range of any one or two of 2, 3, 4, 5, 6, 7, 8, 9 and 10 hours, for example 2 to 5 hours, 7 to 10 hours, 3 to 8 hours or 5 to 8 hours.

In the piercing treatment, it is preferable to perform diameter-expanding piercing of the heated tube blank with a conical piercing machine to obtain a pierced hollow billet. The hole expanding rate of the perforation treatment is 0-5%, so that the size uniformity is ensured, and the size range of the blank capable of producing the steel pipe is increased. Wherein, the rolling angle and the feeding angle of the perforation treatment are both 10-15 degrees. The angle rolling of the rolling angle and the feeding angle can eliminate tangential deformation and torsional deformation in the rolling process, improve the quality of the inner surface of the steel pipe, and reduce the defects of rolling instability and generation. The invention realizes that the difference of the interface thick wall of the capillary is less than 5mm by controlling the diameter expansion rate, the rolling angle and the angle of the feeding angle.

The outer diameter of the perforated capillary is 156 mm and 240mm, and the wall thickness difference of the section of the capillary is less than 5 mm.

Further, the outer diameter of the perforated capillary is in the range between any one or two of 156, 160, 170, 180, 190, 200, 210, 220, 230 and 240mm, for example, 190mm, 200mm, 240mm, 160mm, 210mm, 180mm and 220mm or 170 mm and 230 mm.

The hole expansion rate in the perforation treatment of the step is 0-5%, so that the adopted process is equal-diameter or hole expansion perforation, the process has high requirements on adjustment, and if the adjustment is not proper, uneven deformation is easy to generate to cause surface defects; but the process can use the blank with smaller diameter to produce the steel pipe with larger outer diameter; compared with the mode of producing the steel pipe with the reduced outer diameter by using the blank with larger diameter, the method saves the uniform heating time of the pipe blank and simultaneously ensures that the specification of the pipe blank is more flexible.

In the rolling treatment, an asel tube mill is preferably used to roll the perforated hollow billet to obtain a rolled steel tube. In order to improve the dimensional accuracy and the surface quality of the steel pipe, avoid cracks and increase the rolling stability, the total reducing ratio of the pipe mill is controlled to be not more than 30 percent, the feeding angle of the pipe rolling treatment is controlled to be not more than 15 degrees, the rolling angle is controlled to be not more than 5 degrees, and the size of the core rod used in the pipe rolling treatment is 40-120mm (can be any one or the range between 40, 50, 60, 70, 80, 90, 100, 110 and 120mm, such as 40-80mm, 90-120mm, 50-100mm, 70-110mm or 60-100 mm).

In the reducing treatment, it is preferable to reduce the diameter of the rolled steel pipe by a ten-stand two-roll micro-tension reducing mill to obtain a reduced steel pipe.

In order to ensure the consistency of the wall thickness of the whole length of the steel pipe, different motor superposition ratios are selected according to the size, the motor superposition ratio is controlled to be less than or equal to 50 percent, elliptical or inner hexagonal inner holes are avoided to improve the inner hole precision, and the reducing rate of the reducing mill is controlled to be not more than 20 percent.

The maximum thickness-diameter ratio (wall thickness/outer diameter) of the steel pipe rolled by the process can reach 0.35.

According to the first aspect of the present invention, the thermal refining includes quenching and tempering in this order.

The quenching treatment can be as follows: and (3) heating the reduced steel pipe to 800-950 ℃, preserving heat for 1-3 hours, and then placing the steel pipe on a quenching machine, wherein the quenching time is controlled to be less than or equal to 5min, so as to obtain the quenched steel pipe. Water can be used as a quenching medium, forced cooling is preferably carried out in a mode of simultaneously spraying water on the inner surface and the outer surface in the quenching process, the steel pipe continuously rotates in the cooling process, and the uniform and consistent section structure transformation in the quenching process is ensured. When the quenching temperature is lower than 800 ℃, the steel cannot be fully quenched and cannot obtain the steel with the performance of the invention, and when the quenching temperature is higher than 950 ℃, the steel is easy to crack and has coarse grains.

Further, the temperature of the quenching treatment may be in a range between any one or two of 800, 820, 840, 860, 880, 900, 920, 940 and 950 ℃, for example, 800-; the incubation time may be in the range of any one or two of 1, 1.5, 2, 2.5 and 3 hours, for example 1 to 2 hours, 1.5 to 3 hours, 1.5 to 2.5 hours or 2 to 3 hours.

In the traditional process, the material is quenched by water spraying on the outer surface or quenching in an oil/quenching liquid pool; compared with the traditional process, the quenching treatment mode adopted in the invention has the characteristics of large quenching strength and uniform structure.

The tempering treatment may be: and after the quenching treatment and cooling are finished, putting the quenched steel pipe into a furnace, heating to 500-700 ℃, preserving heat for 2-8 hours, discharging from the furnace, and air cooling to obtain the seamless steel pipe. When the tempering temperature is lower than 500 ℃, the steel strength is too high and the impact value is low; the strength of the steel is greatly reduced when the temperature is higher than 700 ℃.

Further, the temperature of the tempering treatment may be in a range between any one or two of 500 ℃, 520 ℃, 540, 560, 600, 620, 640, 660, 680 and 700 ℃, for example, 500-; the incubation time may be in the range of any one or two of 2, 3, 4, 5, 6, 7 and 8 hours, for example 2-5 hours, 6-8 hours, 3-6 hours, 4-7 hours or 4-6 hours.

The method for producing a seamless steel pipe according to the first aspect of the present invention may further include an operation of performing flaw detection on the steel pipe.

The flaw detection may be ultrasonic flaw detection. The ultrasonic flaw detection can adopt automatic ultrasonic flaw detection lines to detect the steel pipe, detect longitudinal, transverse and oblique defects of the steel pipe according to the L2 level in GB/T5777, and require that the looseness is less than or equal to 1 level and the segregation is less than or equal to 1 level, so that the steel pipe is qualified to be a finished product steel pipe. The yield of the seamless steel pipe produced by the method of the first aspect of the present invention is 100%.

After the steel pipe is subjected to the quenching and tempering treatment (namely, the steel pipe is subjected to the tempering treatment), the performance of the steel pipe after the heat treatment can reach Rp0.2 (namely, the elongation strength when the non-proportional elongation is 0.2) which is 890-1180Mpa, Rm (namely, the tensile strength) which is not less than 1000Mpa, A (namely, the elongation after fracture) which is not less than 14 percent, Z (namely, the reduction of section) which is not less than 55 percent, KV8 (namely, the energy absorbed by a V-shaped notch sample under the impact of a 8mm pendulum edge) which is not less than 70J, and the grain size which is not less than 8 grade.

The steel pipe prepared by the method has various properties such as yield strength fluctuation range, minimum strength, impact toughness and the like which are far higher than the requirements of the industry standard, and the requirements of API 5DP on joints are as follows: rp0.2 is more than or equal to 827 and 1138MPa, Rm is more than or equal to 965MPa, A is more than or equal to 13 percent, and KV8 is more than or equal to 54J).

The seamless steel pipe prepared by the method is particularly suitable for common drill pipe joints.

A second aspect of the present invention provides a seamless steel pipe produced by the method for producing a seamless steel pipe according to the first aspect of the present invention.

According to the second aspect of the invention, the seamless steel tube has high strength and high toughness, Rp0.2 is 890-1180MPa, Rm is more than or equal to 1000MPa, A is more than or equal to 14 percent, Z is more than or equal to 55 percent, KV8 is more than or equal to 70J, and the grain size is more than or equal to grade 8. The performance parameters are obtained by detecting according to an ASTMA370 method.

The thickness-diameter ratio of the seamless steel pipe is less than or equal to 0.35 (namely, the maximum ratio is equal to 0.35), and preferably 0.28-032.

The outer diameter of the seamless steel pipe is 79-230mm, and the inner diameter of the seamless steel pipe is 30-133 mm.

Further, the outer diameter of the seamless steel tube can be in the range of any one or two of 79, 100, 120, 140, 160, 180, 200, 220 and 230mm, for example, 79-160mm, 180-; the inner diameter is in the range of any one or two of 30, 40, 50, 60, 70, 80, 90, 100, 110, 120 and 133mm, and may be, for example, 30-80mm, 70-133mm, 60-100mm, 70-120mm or 50-90 mm.

The thickness-diameter ratio and the specification of the seamless steel pipe cannot be achieved through a common steel pipe process in the prior art, can be achieved only through processes such as bar hole drilling or bar die forging, and the like, but can be achieved directly through the steel pipe process.

Compared with the prior art, the invention has the following beneficial effects:

1. the production process of the joint is optimized as follows: the steel pipe → cutting → turning → the finished product, compared with the traditional process, the working procedures of die forging and the like are reduced. Meanwhile, the length of the steel pipe is usually more than 8m, the steel pipe can be cut off according to the requirement of an order and then used for manufacturing joints with different lengths, and the universality of materials is obviously superior to that of the traditional process.

2. The invention adopts the method of continuous casting round billet → hot rolling (including perforation, pipe rolling and reducing) steel pipe → thermal refining to produce, the quality is good, the dimensional accuracy is high, the cost is low, the production efficiency is high, the qualification rate is high, and the product performance is excellent; in addition, the thickness-diameter ratio of the seamless steel pipe obtained by the method can reach 0.35 to the maximum, and the length of the steel pipe can reach 10m to the maximum.

3. The product specification has wide coverage range, and the production range is as follows: the outer diameter is 79-230mm, and the inner diameter is 30-133mm to cover all joint requirements.

4. The integral heat treatment of the steel pipe can meet the requirement of the same batch of materials on the uniformity of the performance, the fluctuation of the yield strength is less than or equal to 150Mpa, and the performance of the seamless steel pipe is as follows: rp0.2 is 890-1180MPa, Rm is more than or equal to 1000MPa, A is more than or equal to 14 percent, Z is more than or equal to 55 percent, KV8 is more than or equal to 70J, and the grain size is more than or equal to 8 grade, which is far superior to the standard requirements of the traditional process and the product industry.

The invention will be further illustrated with reference to the following specific examples. It should be understood that these examples are only for the purpose of the present invention and are not intended to limit the scope of the present invention. It should be understood that various changes and modifications can be made by those skilled in the art after reading the disclosure of the present invention, and equivalents fall within the scope of the appended claims.

The weight percentage of each chemical element in the pipe blank of each embodiment of the invention and the mass percentage of Ca in the molten steel are shown in Table 1:

TABLE 1

The content of each element in the tube blank	Example 1	Example 2	Example 3
				C	0.35％	0.36％	0.38％
Si	0.1％	0.3％	0.5％
				Mn	0.6％	1.0％	1.5％
Cr	0.8％	1.1％	1.5％
				Mo	0.2％	0.3％	0.5％
Nb	0.2％	0.1％	0.08％
				P	0.015％	0.013％	0.01％
S	0.010％	0.015％	0.018％
				Ca	0.003％	0.0008％	0.004％

The technological parameters for preparing the tube blank in each embodiment of the invention are shown in table 2:

TABLE 2

Procedure (ii)	Example 1	Practice ofExample 2	Example 3
				Vacuum degree torr of vacuum treatment	0.4	0.2	0.3
Superheat degree of molten steel deg.C	25	30	20
				Diameter of tube blank	155mm	195mm	230mm
Length of tube blank	2.9m	3.2m	3.6m

The parameters of the piercing process for preparing seamless steel tubes according to the examples of the present invention are shown in table 3:

TABLE 3

The parameters of the tube rolling and reducing process for preparing the seamless steel tube in the embodiments of the present invention are shown in table 4:

TABLE 4

The quenching and tempering process parameters for preparing the seamless steel pipe in each embodiment of the invention are shown in table 5:

TABLE 5

The results of the performance study of the seamless steel pipes prepared in the examples of the present invention are shown in table 6:

TABLE 6

Performance of	Example 1	Example 2	Example 3
				Rp0.2MPa	985	913	925
RmMPa	1078	1014	1025
				A％	17.5	21.5	21
Z％	55	66	64
				KV8J	95	110	108
Grain size	8	8	8