CN120079721B

CN120079721B - A preparation method and product of an ultra-large size 9Cr ferrite heat-resistant seamless steel pipe

Info

Publication number: CN120079721B
Application number: CN202510578045.5A
Authority: CN
Inventors: 张新文; 江宏亮; 俞杰; 刘永; 印卫; 单文瑞; 汪德伟
Original assignee: Jiangsu Yonggang Group Co Ltd; Jiangsu Lianfeng Energy Equipment Co Ltd
Current assignee: Jiangsu Yonggang Group Co Ltd; Jiangsu Lianfeng Energy Equipment Co Ltd
Priority date: 2025-05-07
Filing date: 2025-05-07
Publication date: 2025-09-05
Anticipated expiration: 2045-05-07
Also published as: CN120079721A

Abstract

The present invention belongs to the technical field of seamless steel pipe manufacturing, and specifically relates to a method for preparing an ultra-large-sized 9Cr-based ferrite heat-resistant seamless steel pipe and its product. The method for preparing the ultra-large-sized 9Cr-based ferrite heat-resistant seamless steel pipe comprises the following steps in sequence: a continuous casting step, a forging step, and a pipe making step. The continuous casting step uses blast furnace molten iron and scrap steel as raw materials, and produces continuous casting pipe billets through an electric furnace, LF refining, VD vacuum treatment, and round billet continuous casting machine casting; the forging step produces pipe billets through processes such as blanking, forging, annealing, and through-hole processing; and the pipe making step produces seamless steel pipes through processes such as heating, perforation, pipe rolling, pipe expansion, heat treatment, and lathe processing. The prepared 9Cr-based seamless steel pipe has uniform composition and high purity, thus having high high-temperature strength, solving the problems of unqualified flaw detection, uneven wall thickness, eccentricity, etc. caused by alloy segregation in the prior art.

Description

Preparation method of oversized 9Cr ferrite heat-resistant seamless steel pipe and product thereof

Technical Field

The invention belongs to the technical field of production and manufacturing of seamless steel pipes, and particularly relates to a preparation method of an oversized 9Cr ferrite heat-resistant seamless steel pipe and a product thereof.

Background

In recent years, 9 Cr-based ferritic heat-resistant steel has been widely used for critical parts such as super-critical and ultra-critical power station boiler superheaters and reheaters due to its excellent high temperature strength, oxidation resistance and creep resistance. With the development of the parameters of the power station to higher temperature and higher pressure, higher requirements are put on the specifications and performances of the heat-resistant steel pipes, and particularly, the requirements of oversized seamless steel pipes with large calibers and thick walls are increasingly increased.

However, the conventional preparation method is difficult to meet the production requirement of the ultra-large 9Cr ferrite heat-resistant seamless steel pipe, and mainly has the following problems:

(1) The quality of the casting blank is difficult to ensure, the oversized casting blank is easy to have defects of center loosening, segregation and the like, the quality of the steel pipe is influenced, particularly alloy element enrichment caused by segregation is easy to generate cracks at the 1/2 radius of the wall thickness of the steel pipe, and flaw detection is failed.

(2) The perforating difficulty is high, and the problems of non-penetration, eccentricity and the like easily occur when the oversized tube blank is perforated.

(3) The rolling deformation is uneven, and the uniformity of the wall thickness of the oversized steel pipe is difficult to ensure by the traditional rolling process.

(4) The compression ratio is not enough, and when a large-specification continuous casting round tube blank is adopted, the compression ratio is more than or equal to 3.0.

(5) The heat treatment process is complex, and the problems of uneven structure, substandard performance and the like easily occur in the heat treatment process of the oversized steel pipe.

The patent with publication number CN116083781A published in 2023, 5 and 9 discloses a manufacturing method of a P92 heat-resistant steel large-size continuous casting round pipe blank without a high-temperature ferrite structure, which has the technical scheme that the superheat degree of a tundish is controlled to be 25-35 ℃, the drawing speed of the continuous casting round pipe blank is controlled to be 0.22-0.26 m/min, the cooling water quantity of a crystallizer is controlled to be 3600-3800L/min, the specific water quantity of secondary cooling water is controlled to be 0.11-0.13L/kg, and three-section composite electromagnetic stirring is adopted. Through component control and continuous casting process parameter control, the large-specification P92 heat-resistant steel continuous casting round tube blank is ensured to completely eliminate high-temperature ferrite tissues, but the specification is limited to phi 690mm. And cannot be applied to larger-scale production.

The patent with publication number CN115044823A published by 9 months 13 of 2022 discloses a production process of ultra-supercritical high-pressure boiler steel P92 continuous casting large round billets, molten steel is subjected to continuous casting, temperature rise is carried out at a speed of less than or equal to 80 ℃ per hour at a temperature of more than or equal to 550 ℃, a slow cooling annealing process is carried out, the degree of superheat of the continuous casting large round billets with the hardness of less than 230HBW after annealing is obtained, the degree of superheat in the continuous casting process is between 30 ℃ and 45 ℃, two sections of water cooling are adopted to match with the electromagnetic stirring of a crystallizer, the casting flow stirring and the terminal electromagnetic stirring to control central cracks, but the maximum specification can only reach phi 700mm, the upper limit of the central crack length is 90mm, and the continuous casting large round billets cannot be suitable for larger-specification production and cannot meet higher requirements.

The patent with publication number CN118926493A published 11/12/2024 discloses a casting method of ultra-large specification ultra-supercritical P92 round steel, and the surface of the produced ultra-large specification P92 round steel has no obvious surface cracks, and the internal cracks meet the performance requirement below the level of 2.5 through casting process, straightening and annealing process control. Solves the problems that the production specification of the P92 round steel is smaller under the prior art condition, and the increasing demand of the market on the large-specification P92 round steel with the phi of 800-1000 mm cannot be met. However, the patent does not relate to means and effects of controlling segregation of chemical elements, and the central crack is large, the maximum specification can only reach phi 1000mm, and the process of preparing large-specification seamless steel pipes cannot be satisfied, in particular to a process of perforation, rolling and uniform forming.

In view of the above problems, there is a need to develop a method for preparing a 9 Cr-based ferritic heat-resistant seamless steel pipe with ultra-large specification, so as to solve the problems of failure in flaw detection, uneven wall thickness, and eccentricity caused by component enrichment in the prior art, thereby affecting the performance of the steel pipe such as high-temperature strength, and the like, and simultaneously improving the process universality, so as to be capable of meeting the preparation of the seamless steel pipe with large specification.

Disclosure of Invention

The invention aims to provide a preparation method of an oversized 9Cr series ferrite heat-resistant seamless steel pipe and a product thereof, wherein raw materials are selected and each step of working procedure is accurately controlled, so that the 9Cr series seamless steel pipe with uniform components and pure height is prepared, has higher high-temperature strength, and solves the problems of unqualified flaw detection, uneven wall thickness, eccentricity and the like caused by component enrichment in the prior art.

In order to achieve the above object, the present invention provides the following technical solutions:

The preparation method of the ultra-large 9Cr ferrite heat-resistant seamless steel pipe sequentially comprises the following steps of continuous casting, forging and pipe making, and a specific flow chart for preparing the seamless steel pipe is shown in figure 1.

The continuous casting process specifically comprises the following steps:

A1, smelting in an electric furnace, namely adding molten iron and scrap steel into the electric furnace, adding quicklime to produce steel slag, simultaneously supplying oxygen, adding lime in batches when the temperature is lower than 1570 ℃ to control the alkalinity of the steel slag to be 2-4, blowing oxygen to a steel slag interface, and rapidly stirring, and sampling and detecting that chemical components in the steel slag have tapping conditions 1 when the temperature reaches 1630 ℃. Pouring molten steel into a ladle, and adding Al, mn, cr alloy and slag-forming material to mix uniformly when the molten steel in the ladle reaches 25-50 tons, so as to obtain molten steel;

A2, LF refining, namely transferring molten steel into an LF refining furnace, electrifying and heating, introducing argon, controlling the argon flow to be 200-400NL/min, adjusting the argon flow to be 120-250NL/min when the temperature of the molten steel is more than or equal to 1650 ℃, adding alloy, supplementing quicklime or synthetic slag according to the fluidity of slag, controlling the alkalinity of a system to be 2.5-4.0, adjusting the argon flow to be 60-150NL/min, blowing argon to remove impurities, sampling and detecting that the Al and S content in the molten steel has tapping condition 2 after 10min, and obtaining refined molten steel;

a3, VD vacuum refining, namely transferring refined molten steel into a VD vacuum furnace, vacuumizing to below 67Pa within 8min, maintaining for 15-20min, detecting that the hydrogen content is lower than 1.0ppm, adding nitrogen according to component requirements after the vacuum is broken, and feeding a calcium line to obtain cast molten steel;

hoisting the casting molten steel to a continuous casting station, enabling the molten steel to flow into a tundish from a ladle, adding a carbonless covering agent and carbonized rice hulls when the weight of the casting molten steel in the tundish reaches 20 tons, pouring the molten steel into a crystallizer when the weight of the casting molten steel in the tundish reaches 30 tons, and starting an automatic liquid level control system, electromagnetic stirring and secondary cooling water opening into a pouring mode to obtain the continuous casting round pipe blank;

And A5, annealing, namely conveying the continuous casting round tube blank into an annealing furnace for annealing, wherein the annealing temperature is 780 ℃, and the heat preservation time is 27-43 hours, so as to obtain the annealed continuous casting round tube blank.

Preferably, the addition amount of the molten iron is 85% of the total mass of the molten iron and the scrap steel.

The molten iron comprises the following chemical components in percentage by weight, wherein the balance of ：C,≥3.5%;Mn,≤1.0%;Si,0.20%-0.60%;Ni,≤0.05%;Cu,≤0.05%;P,≤0.10%;S,≤0.03%;Mo,≤0.05%;Ti,≤0.060%;As,≤0.005%;Sn,≤0.005%;Bi,≤0.005%;Pb,≤0.003%;Sb,≤0.003%; is Fe and unavoidable impurities.

Preferably, the temperature of the molten iron is 1300-1380 ℃.

The steel scraps are self-produced steel scraps, and the chemical components of the steel scraps comprise, by weight, 0.10% -0.60% of C, less than or equal to 2.0% of Mn, less than or equal to 1.0% of Si, less than or equal to 0.15% of P, less than or equal to 0.025% of S, and the balance of Fe and unavoidable impurities.

Preferably, in the step A1, the addition amount of the quicklime is 30-60 kg/ton of molten iron.

Preferably, in the step A1, the Al, mn, cr alloys include aluminum-iron alloy, manganese metal, micro-carbon ferrochrome and low-carbon ferrochrome.

In some preferred schemes, the electric furnace smelting process is not electrified, and the heat released by the chemical reaction of oxygen supplied by an oxygen lance and carbon is used as a heat source to provide heat required for steelmaking.

Preferably, in the step A1, the slag forming material comprises quicklime and synthetic slag, and the addition amounts of the slag forming material are 8.9-9.1 kg/ton of molten steel and 1.9-2.1 kg/ton of molten steel respectively.

Preferably, in the step A1, the tapping condition 1 is that C is less than or equal to 0.04%, P is less than or equal to 0.005% and S is less than or equal to 0.015% in molten steel.

Preferably, in the step A2, the chemical components of the synthetic slag comprise, in weight percent ：CaO,45.0-55.0%;Al₂O₃,27.0-35.0%;SiO₂,≤6.0%;MgO,≤8.0%;Fe₂O₃,≤2.0%;TiO₂,≤0.03%;H₂O,≤0.5%.

Preferably, in the step A2, the quicklime or the synthetic slag is supplemented according to the slag fluidity, and the concrete operation method comprises the steps of adding the synthetic slag in a proper amount when the binary basicity of the slag is more than 4.0, increasing the slag fluidity, and adding the quicklime in a proper amount when the binary basicity of the slag is less than 2.5, so as to improve the inclusion absorbing capacity of the slag.

Preferably, in the step A2, the tapping condition 2 is that the Al content in the molten steel is 0.008-0.010 percent and S is less than or equal to 0.0030 percent.

Preferably, in the step A3, the flow rate of nitrogen before the breaking is 400 NL/min, and the flow rate of nitrogen after the breaking is 80-100 NL/min.

Preferably, the addition amounts of the carbon-free covering agent and the carbonized rice husk are 500kg and 200kg respectively.

By taking blast furnace molten iron as a main raw material and adopting LF refining and VD vacuum refining to control P, S, as, bi and other residual elements in steel, the purity of molten steel is improved from the source, the precipitation of brittle phases of grain boundaries is obviously reduced, the high-temperature service performance is improved, the damage of inclusions to plasticity is reduced, the uniform deformability of the material is improved, and the continuous casting round tube blank with high purity, uniform components and small center defect is produced by combining with the optimization of a continuous casting process, so that the elongation and the lasting strength of the steel tube are improved. This is probably because carbon content in molten iron can be removed by oxidation reaction, quicklime can be added in an oxidizing atmosphere, and phosphorus content in steel can be removed by interfacial reaction in the electric furnace smelting process. S content in molten steel can be removed and inclusion can be adsorbed by adding quicklime serving as a slag-forming material. 2 bottom air bricks are adopted in the LF refining process to blow argon in the whole process, so that floating of inclusions, uniformity of temperature and components are ensured, secondary oxidation of molten steel is prevented, and the alkalinity control (2.5-4.0) is conducive to forming low-melting-point slag to adsorb impurities. Meanwhile, the flow of argon is controlled during LF refining, argon stirring is properly regulated in the early stage to promote deoxidation and alloying, the medium argon strength is kept in the medium-stage alloy removal stage in the refining, the argon flow in the later stage of refining is reduced, and the large turnover oxidization of molten steel is avoided. In addition, proper amount of synthetic slag is added in the LF refining process, so that the slag can be ensured to have better fluidity and adsorptivity under the condition of maintaining high alkalinity. The VD vacuum refining stage reduces the risk of hydrogen induced cracking through vacuum degassing, controls the content of Al, not only inhibits AlN precipitation, but also prevents the risk of early failure of products in a high-temperature high-pressure water vapor environment, the VD vacuum treatment removes the hydrogen content in steel to below 1.0ppm, and then Ca wires are fed into the steel to promote the denaturation of inclusions, improve the purity of the steel, and simultaneously meet the basic conditions required by the forward running of casting production.

Preferably, the electromagnetic stirring adopts three sections of electromagnetic stirring of M-EMS (crystallizer electromagnetic stirring), S-EMS (casting flow electromagnetic stirring) and F-EMS (terminal electromagnetic stirring).

In some preferred schemes, continuous casting round tube blanks with different specifications are selected from electromagnetic stirrers with different specifications. For continuous casting round tube blanks of the same specification, electromagnetic stirrers used by the S-EMS and the F-EMS have the same size and specification. The electromagnetic stirrer with different specifications is that when the required continuous casting round tube blank is in the specification of phi 900, the specification of the electromagnetic stirrer is phi 1665 multiplied by phi 1200 multiplied by 1100mm, when the required continuous casting round tube blank is in the specification of phi 1000, the specification of the electromagnetic stirrer is phi 1765 multiplied by phi 1300 multiplied by 1100mm, and when the required continuous casting round tube blank is in the specification of phi 1100, the specification of the electromagnetic stirrer is phi 1865 multiplied by phi 1400 multiplied by 1100mm.

In some preferred embodiments, casting molten steel is poured into a crystallizer, the superheat degree of the molten steel is controlled between 20 and 30 ℃, and the casting is performed at a constant pull rate of 0.10 to 0.40m/min.

In some preferred schemes, when the diameter of the required continuous casting round tube blank is in the specification of phi 900mm, the drawing speed is 0.15-0.17m/min, when the diameter of the required continuous casting round tube blank is in the specification of phi 1000mm, the drawing speed is 0.13-0.16mm/min, and when the diameter of the required continuous casting round tube blank is in the specification of phi 1100mm, the drawing speed is 0.11-0.14m/min.

In the step A4, the dummy bar in the crystallizer adopts a chain type integral dummy bar, the large reduction is started when the distance from the red billet (the casting billet which is still provided with a higher temperature just pulled out of the crystallizer of the continuous casting machine) is 1.0-2.0 m, the upper pressure limit is set to be 350-400 tons, after the red billet passes through the withdrawal and straightening machine, the red billet is continued for 1.0-1.5 m, and then the red billet is gradually converted into hot billet pressure, and the pressure is set to be 50-90 tons. When the casting is finished, the upper pressure limit is restored to 350-400 tons when the distance from the tail part is about 6-8 meters.

Different electromagnetic stirrer sizes are selected through continuous casting round tube blanks of different specifications, and the solidification structure of the continuous casting round tube blanks is optimized by combining constant pull speed and low superheat degree pouring, so that the uniformity of chemical components on the cross section of the continuous casting round tube blanks is ensured, the element segregation is reduced, the processing with higher compression ratio is allowed, and the forging cracking rate is reduced. On one hand, three sections of electromagnetic stirring are adopted, M-EMS breaks initial grains, S-EMS inhibits dendrite growth, F-EMS refines equiaxed grains, so that center segregation is reduced, on the other hand, continuous casting round tube blanks with different specifications correspond to different pulling speeds and electromagnetic stirring, the optimal electromagnetic stirring effect is ensured, the stay time of molten steel in a crystallizer is prolonged, and component diffusion is promoted. The particles in the molten steel are enabled to generate rotary motion under the action of electromagnetic force, so that component segregation and temperature gradient in the molten steel are broken, and uniform distribution of components is promoted. Meanwhile, the fluidity and solidification process of molten steel can be improved by electromagnetic stirring, and the generation of internal defects is reduced. The components in the steel are uniform through low superheat degree, reasonable drawing speed, electromagnetic stirring and the like, and the high-quality continuous casting round tube blank with loose center, small center crack and small segregation can be obtained.

In some preferred embodiments, the phi 900 and phi 1100 gauge continuous casting round tube blank is straightened, see fig. 2. Phi 1000 specification P92 continuous casting low-magnification photo, see figure 3. Phi 900 specification P92 continuous casting longitudinal section sample low-power photograph, see figure 4.

Preferably, in the step A5, the temperature of the annealing furnace is 530 ℃ during hot feeding of the continuous casting round tube blank.

The forging process specifically comprises the following steps:

b1, forging, namely cutting, heating and forging the annealed continuous casting round tube blank to obtain a tube blank;

B2, annealing;

B3, through holes.

Preferably, in the step B1, the total heating time is more than or equal to 30 hours, wherein the temperature of the high-temperature section is 1220-1250 ℃, and the heat preservation time is more than or equal to 10 hours.

In some preferred schemes, the specific steps of heating are that the temperature is lower than 200 ℃ when the material is fed into a furnace, the material is heated to 500 ℃ at the speed of less than or equal to 50 ℃ per hour after being kept for 2 hours, then heated to 850 ℃ at the speed of less than or equal to 70 ℃ per hour and kept for 4 hours, then heated to 1100 ℃ at the speed of less than or equal to 80 ℃ per hour and kept for 2-3 hours, then heated to 1200 ℃ at the speed of less than or equal to 100 ℃ per hour and kept for 8-15 hours, and the total heating time is 36.5-42.5 hours.

Preferably, in the step B1, the specific forging step comprises the steps that the surface temperature of the continuous casting round tube blank after being discharged from a heating furnace is 1160-1190 ℃, and the surface temperature is more than or equal to 850 ℃ after being drawn to a target diameter by one fire.

Preferably, the hot drawing method comprises the specific steps of firstly tapping the heated continuous casting round tube blank, then pressing down one end of the continuous casting round tube blank under large pressure, starting from a second anvil, gradually forging each anvil to the middle position on the basis of the last anvil, turning around the whole anvil, continuing to use the principle of tapping firstly, pressing down greatly, and finally throwing round, and blowing out surface iron scales by using compressed air in the forging process.

Through the forging process, the residual stress of the steel in the forging process can be reduced to the maximum extent, the problem of eccentricity of the continuous casting round tube blank can be avoided, the structure of the steel is more compact, and good conditions are provided for subsequent tube making. The method is characterized in that the heating speed is controlled in the forging heating process, the heat stress is reduced due to slow heating, the alloy elements are promoted to diffuse by a subsequent high-temperature section, grains are dynamically recrystallized and refined in a first hot drawing process, the oxide skin is purged by compressed air to avoid pressing in defects, the forging residual stress is eliminated by annealing, the porosity is close to zero, and therefore the residual stress and tissue compactness are optimized, and the high-temperature breaking time is further improved.

In some preferred embodiments, the forging process is schematically illustrated in FIG. 5.

Preferably, in the step B2, the specific condition of annealing is that the furnace temperature before charging is 550 ℃, the heating speed is less than or equal to 70 ℃ per hour, the annealing heat preservation temperature is 780 ℃, and the heat preservation time is 3.5min/mm thickness.

In some preferred schemes, the specific operation steps of the step B3 include sawing one end part, enabling a sawing surface to be vertical, turning the end part of the sawing surface, marking a center point on an end surface, turning two sections of the tube blank in the length direction, ensuring that the diameters of the turned parts are the same, hanging the tube blank on a lathe, placing the turned parts on a supporting roller, punching holes on the end surface by taking the geometric center point as a circular point, and punching diameters of steel tubes with different specifications are different.

Through the through hole process, the accuracy of punching positioning is ensured, and the central crack is ensured to be removed. The center point can be accurately positioned on the saw section, and the center crack is ensured to be cleared completely in the drilling range. The accurate punching process can ensure that the problems of stress concentration, crack expansion and the like cannot occur when the steel pipe bears internal and external pressure in the subsequent processing and using processes, thereby improving the breaking time and the elongation. Meanwhile, the defects such as central cracks and the like are removed, so that the overall quality and safety of the steel pipe can be improved.

In some preferred embodiments, the tube blank punching apparatus is schematically shown in fig. 6.

The tubing process specifically comprises the following steps:

C1, heating;

C2, pipe making, namely perforating, pipe rolling, leveling and preliminary heat treatment are carried out on the heated pipe blank to prepare a blank pipe;

and C3, heat treatment, finishing and detection, namely turning, sampling and detecting the inner surface and the outer surface of the capillary after heat treatment, carrying out UT and MT flaw detection, and warehousing after passing the inspection, thus obtaining the finished steel pipe.

Preferably, in the step C1, the heating includes a heating section and a high temperature section, and the total heating time is 35-43 hours.

Preferably, the heating section is divided into two zones, wherein the first zone is a preheating zone with the temperature of 550-700 ℃ and the heat preservation time is more than or equal to 3 hours, the second zone is a heating zone with the temperature of 850-1100 ℃ and the heat preservation time is more than or equal to 8 hours. In this stage, the temperature rising speed is controlled at 50-80 ℃ per hour to reduce the thermal stress and the tissue stress.

Preferably, the high temperature section is divided into two areas, the temperature of the soaking first area is 1200-1230 ℃, the heat preservation time is more than or equal to 10 hours, the temperature of the soaking second area is 1240-1270 ℃, and the heat preservation time is more than or equal to 15 hours. In this stage, the temperature rise rate from zone one to zone two was 100℃per hour.

By designing the heating process in the pipe making process, the enrichment of alloy elements, particularly Cr, W and Mo, is reduced, and the equipment load during perforation, pipe rolling and leveling is reduced, so that the risk of internal cracking of the steel pipe in the processing process is solved. The heating section aims to slowly heat the casting blank in the low-temperature section, reduce thermal stress and tissue stress and reduce cracking risk. The purpose of the high temperature section is to make the internal temperature and the surface temperature of the casting blank consistent, so that the alloy elements are fully dissolved and uniformly diffused. In addition, the temperature rising speed in the high temperature section is high, the heat preservation time is long, the round billet tube is facilitated to be fully austenitized, and strong carbide elements Cr, W, mo and the like are fully dissolved and uniform, so that the subsequent processes of perforation, rolling and the like are facilitated.

Preferably, in the step C2, the specific conditions of perforation, pipe rolling and leveling are that the temperature before perforation is higher than 1150 ℃, two-roller oblique rolling and one mandrel are adopted, the rotating speed of the perforation roller is 5-6r/min, the surface temperature after perforation is 1020-1150 ℃, the temperature after pipe rolling is 1020-1100 ℃, and the temperature after leveling is 950-1000 ℃.

In some preferred schemes, when the temperature of the rear surface of the perforated or rolled pipe is reduced to below 850 ℃, the temperature is returned to the furnace in time for temperature compensation, the temperature compensation time is 1-2 hours, and the upper limit of the temperature is 1200 ℃.

By controlling the temperature drop in the deformation process of the three main steel pipes, namely perforation, rolling and leveling, the thermal stress and the tissue stress cracking caused by overlarge temperature drop are ensured to the greatest extent, so that the cracking time and the high-temperature elongation are improved. The two-roller oblique rolling combined core rod process can reduce deformation heat loss by controlling the rotating speed. The temperature is controlled to be more than or equal to 950 ℃ after the homogenization, so that the dynamic recrystallization capability is maintained, and the work hardening is eliminated. When the temperature is lower than 850 ℃, the austenite uniformity can be recovered by timely returning to the furnace and supplementing the temperature. The processing temperature drop is reduced through the accurate control of the temperature, and the performance of the steel pipe is improved.

Preferably, in the step C2, the specific condition of the preliminary heat treatment is that the temperature of the capillary tube before charging is 550+/-20 ℃ and the furnace temperature is 550 ℃, the temperature is raised to 930+/-10 ℃ and the capillary tube is kept for 6-9 hours for normalizing.

Preferably, in the step C3, the specific step of heat treatment is to heat-insulate the capillary tube for 5-7 hours at 1040-1080 ℃ for normalizing, and heat-insulate the capillary tube for 7-8 hours at 750-790 ℃ for cold tempering.

Through the control of the heat treatment process, the structure uniformity is promoted, the precipitation of harmful phases is avoided, and the high-temperature strength and the elongation of the steel pipe are improved. The high-temperature normalizing stage is completely austenitized, carbide is dissolved, then the medium-temperature tempering promotes a uniform martensite lath structure, the content of residual austenite is greatly reduced, grains are refined, the creep resistance of the steel pipe is improved, the creep rate and the content of harmful phases are reduced, and the high-temperature strength of the steel pipe is improved.

The invention relates to a seamless steel pipe, which comprises P91 steel grade, P92 steel grade and P9 steel grade.

The P91 steel comprises the chemical components of ：C,0.08-0.12%;Si,0.20-0.40%;Mn,0.30-0.50%;P,≤0.012%;S,≤0.0030%;Cr,8.20-9.00%;Ni,≤0.40%;Mo,0.85-1.05%;Al,≤0.015%;V,0.16-0.25%;Nb,0.04-0.10%;N,0.050-0.075%,W,≤0.05%;B,≤0.001%,Pb,≤0.003%;Sn,≤0.010%;As,≤0.010%;Sb,≤0.003%;Bi,≤0.003%;Pb+Sn+As+Sb+Bi,≤0.025%; percent by weight, and the balance of Fe and unavoidable impurities.

The P92 steel comprises the following chemical components in percentage by weight, wherein the balance of ：C,0.08-0.12%;Si,0.20-0.40%;Mn,0.30-0.60%;P,≤0.012%;S,≤0.0030%;Cr,8.50-9.00%;Ni,≤0.40%;Mo,0.30-0.35%;Al,≤0.015%;V：0.16-0.18%;Nb,0.04-0.09%;N,0.050-0.075%;W,1.55-1.75%;B：0.0015-0.0035%;Pb,≤0.003%;Sn,≤0.010%;As,≤0.010%;Sb,≤0.003%;Bi,≤0.003%;Pb+Sn+As+Sb+Bi,≤0.025%; is Fe and unavoidable impurities.

The P9 steel comprises the following chemical components in percentage by weight, wherein the chemical components comprise ：C,≤0.15%;Si：0.25-1.00%;Mn：0.30-0.60%;P,≤0.012%;S,≤0.0030%;Cr：8.50-10.00%;Ni,≤0.60%;Mo,0.90-1.10%;Al,≤0.015%;V,≤0.020%;Nb,≤0.020%,N,≤0.012%,W,≤0.050%,B,≤0.001%,Pb,≤0.003%;Sn,≤0.010%;As,≤0.010%;Sb,≤0.003%;Bi,≤0.003%;Pb+Sn+As+Sb+Bi,≤0.025%; parts by weight of Fe and unavoidable impurities.

The second aspect of the invention provides a product prepared by the preparation method of the oversized 9Cr series ferrite heat-resistant seamless steel pipe.

Compared with the prior art, the invention has the advantages that:

1. the invention provides a preparation method of an oversized 9Cr series ferrite heat-resistant seamless steel pipe, which prepares the 9Cr series seamless steel pipe with uniform and pure components and high temperature strength by selecting specific Fe materials and various middle supplementary materials and combining three procedures of continuous casting procedure, forging procedure and pipe making procedure, and simultaneously, the compression ratio still meets the requirements when preparing a large-sized seamless steel pipe.

2. According to the invention, the blast furnace molten iron is used as a main raw material, LF refining and VD vacuum refining are adopted to control P, S, as, bi and other residual elements in steel, so that the purity of molten steel is improved from the source, the precipitation of brittle phases of grain boundaries is obviously reduced, the long-term service performance at high temperature is improved, the damage of inclusions to plasticity is reduced, the uniform deformability of the material is improved, and the continuous casting round tube blank with high purity, uniform components and small center defect is produced by combining with the optimization of a continuous casting process, so that the elongation and the lasting strength of the steel tube are improved.

3. According to the invention, electromagnetic stirrers with different sizes are selected for continuous casting round tube blanks with different specifications, and the constant pulling speed and low superheat degree pouring are combined, so that the solidification structure of the continuous casting round tube blank is optimized, the uniformity of chemical components on the cross section of the continuous casting round tube blank is ensured, the carbon segregation is reduced, the processing with higher compression ratio is allowed, and the rolling cracking rate is reduced.

4. Through the forging process, the invention not only can furthest reduce the residual stress of the steel in the forging process, but also can avoid the eccentric problem of the continuous casting round tube blank, so that the structure of the steel is more compact, and good conditions are provided for subsequent tube making.

5. The invention ensures the accuracy of punching positioning and the clearance of the central crack through a through hole process.

6. According to the invention, by designing the heating process in the pipe making process, the enrichment of alloy elements, particularly Cr, W and Mo, is reduced, and the equipment load during perforation, pipe rolling and leveling is reduced, so that the risk of internal cracking of the steel pipe in the processing process is solved, and the high-temperature strength is further improved.

7. The invention ensures thermal stress and tissue stress cracking caused by overlarge temperature drop to the maximum extent by controlling the temperature drop in the deformation process of the three main steel pipes, namely perforation, rolling and leveling, thereby improving the cracking time and the high-temperature elongation.

8. The invention promotes the uniformity of the structure through the control of the heat treatment process, avoids the precipitation of harmful phases, and further improves the high-temperature strength and the elongation of the steel pipe.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below.

FIG. 1 is a specific flow chart of the seamless steel tube preparation of the invention;

FIG. 2 shows the straightening of the head blank of a continuous casting round tube blank with the specifications of phi 900 (left) and phi 1100 (right);

FIG. 3 is a photograph of a phi 1000 specification P92 macrostructure;

FIG. 4 is a photograph of a P92 longitudinal section of the phi 900 specification;

FIG. 5 is a schematic diagram of a forging process;

FIG. 6 is a schematic diagram of a tube blank punching apparatus;

FIG. 7 is a photograph showing the structure of a steel pipe according to example 1 after heat treatment;

FIG. 8 is a photograph showing the structure of a steel pipe according to example 2 after heat treatment.

Wherein, the device comprises a 1-pliers, a 2-blank, a 3-anvil, a 4-forging machine, a 5-second anvil, a 6-first anvil, a 7-punching device, an 8-drill rod, a 9-supporting roller 1, a 10-roller way, a 11-supporting roller 2, a 12-blank, a 13-supporting roller 3, a 14-fixing device and a 15-geometric center.

Detailed Description

The following description of the technical solutions in the embodiments of the present invention will be clear and complete, and it is obvious that the described embodiments are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

The test materials, reagents, etc. used in the examples described below are all of the existing type and are commercially available unless otherwise specified. Those of skill in the art may be able to implement the methods according to the general techniques or conditions disclosed in the art without specifying the specific techniques or conditions in the examples.

The molten iron comprises the following chemical components in percentage by weight, wherein the chemical components comprise ：C,≥3.5%;Mn,≤1.0%;Si,0.20%-0.60%;Ni,≤0.05%;Cu,≤0.05%;P,≤0.10%;S,≤0.03%;Mo,≤0.05%;Ti,≤0.060%;As,≤0.005%;Sn,≤0.005%;Bi,≤0.005%;Pb,≤0.003%;Sb,≤0.003%; of Fe and unavoidable impurities in balance.

The steel scraps are self-produced steel scraps, and the steel scraps comprise, by weight, 0.10% -0.60% of C, less than or equal to 2.0% of Mn, less than or equal to 1.0% of Si, less than or equal to 0.15% of P, less than or equal to 0.025% of S, and the balance of Fe and unavoidable impurities.

Example 1 (preparation of SA 335P 92 Steel pipe with specification ID 933X 8500X 44mm, continuous casting round pipe blank with specification phi 1000 mm)

The embodiment provides a preparation method of an oversized 9Cr ferrite heat-resistant seamless steel pipe, which sequentially comprises the following steps of continuous casting, forging and pipe making.

The continuous casting process specifically comprises the following steps:

a1, smelting in an electric furnace, namely adding scrap steel into the electric furnace, adding 1350-1380 ℃ molten iron, adding quicklime to produce steel slag, simultaneously supplying oxygen, adding lime in batches to control the alkalinity of the steel slag to be 2-4 when the temperature is lower than 1570 ℃, blowing oxygen to a steel slag interface to stir rapidly, sampling and detecting C less than or equal to 0.04%, P less than or equal to 0.005% and S less than or equal to 0.015% in the molten steel when the temperature reaches 1630 ℃, pouring the molten steel into a ladle, and adding Al, mn, cr alloy and slag forming materials to mix uniformly when the molten steel reaches 25 tons to obtain smelted molten steel;

a2, LF refining, namely transferring molten steel into an LF refining furnace, electrifying and heating, introducing argon, controlling the argon flow to be 300NL/min, when the temperature of the molten steel is more than or equal to 1650 ℃, adjusting the argon flow to be 200NL/min, adding alloy, supplementing quicklime or synthetic slag according to the fluidity of slag, controlling the alkalinity of a system to be 2.5-4.0, adjusting the argon flow to be 100NL/min, blowing argon to remove impurities, and controlling the Al content of a ladle to be 0.008-0.010% after 10min before LF ladle hanging, wherein S in the molten steel is less than or equal to 0.0030%, thereby obtaining refined molten steel;

a3, VD vacuum refining, namely transferring refined molten steel into a VD vacuum furnace, vacuumizing to below 67Pa within 8min, keeping the hydrogen content below 1.0ppm after 20min, blowing nitrogen to increase nitrogen before breaking, adding ferroboron after breaking, and feeding a calcium line to obtain cast molten steel;

A4, continuously casting round tube blanks, namely hanging casting molten steel to a continuous casting station and flowing the casting molten steel into a tundish from a ladle, adding a carbonless covering agent and carbonized rice hulls when the weight of the casting molten steel of the tundish reaches 20 tons, pouring the molten steel into a crystallizer when the weight of the casting molten steel of the tundish reaches 30 tons, starting a liquid level automatic control system, electromagnetic stirring and secondary cooling water starting pouring modes, controlling the specification of casting flow and a tail end electromagnetic stirrer to be phi 1765 multiplied by 1300 multiplied by 1100mm, controlling the superheat degree of the molten steel between 20-30 ℃ during normal pouring, pouring at a constant pull rate, controlling the pulling rate to be 0.13m/min, adopting a chain type integral guide bar, starting a large reduction amount when the distance from a red billet is 1.0 meter, setting the upper pressure limit to be 350 tons, continuing to be 1.0-1.5 meters after the red billet passes through a withdrawal and then gradually converting the red billet into hot billet pressure, designing the pressure to be 80 tons, and recovering the upper pressure limit to be 380 tons when the pouring is finished and obtaining continuous casting round tube blanks of phi 1000 specification after the pouring is finished;

and A5, annealing, namely conveying the continuous casting round tube blank into an annealing furnace at the temperature of 530 ℃ for annealing, wherein the annealing temperature is 780 ℃, and the heat preservation time is 43 hours, so as to obtain the annealed continuous casting round tube blank.

The addition amount of the molten iron is 85% of the total mass of the molten iron and the scrap steel.

In the step A1, the addition amount of the quicklime is 55 kg/ton of molten iron.

In the step A1, the Al, mn and Cr alloys comprise aluminum-iron alloy, manganese metal, micro-carbon ferrochrome and low-carbon ferrochrome.

The addition amounts of the aluminum-iron alloy, the manganese metal, the micro-carbon ferrochrome and the low-carbon ferrochrome are respectively 1.6kg/t, 3.5kg/t, 95.8 kg/t and 6.01kg/t.

In the step A1, the oxygen supply amount at the time of oxygen supply was 58Nm ³/t.

In the step A1, the slag-forming materials are quicklime and synthetic slag, and the addition amounts of the slag-forming materials are 9 kg/ton of molten steel and 2 kg/ton of molten steel respectively.

In the step A2, the concrete operation method for supplementing the quicklime or the synthetic slag according to the slag fluidity comprises the steps of adding the synthetic slag in a proper amount when the binary basicity of the slag is more than 4.0, increasing the slag fluidity, and adding the quicklime in a proper amount when the binary basicity of the slag is less than 2.5, so as to improve the capability of absorbing inclusion of the slag.

In the step A2, the alloy is 76.7kg/t of low-carbon ferrochrome, 0.3kg/t of electrolytic nickel, 2.7kg/t of ferrovanadium, 0.5kg/t of silicomanganese alloy, 1.1kg/t of ferrosilicon alloy, 1.1kg/t of manganese metal, 5.5kg/t of ferromolybdenum alloy, 0.67kg/t of ferroniobium, 0.6kg/t of nickel bean, 60.12kg/t of micro-carbon ferrochrome and 23.1kg/t of ferrotungsten.

In the step A3, the ferroboron is 0.15kg/t.

In the step A3, the flow rate of nitrogen before the breaking is 400 NL/min, and the flow rate of nitrogen after the breaking is 90 NL/min.

The addition amounts of the carbon-free covering agent and the carbonized rice husk are 500kg and 200kg respectively.

The electromagnetic stirring adopts three sections of electromagnetic stirring of M-EMS, S-EMS and F-EMS.

The forging process comprises the following specific steps:

B1, forging, namely cutting a continuous casting round tube blank annealed with the specification of phi 1000mm into single length of 2.2m, heating in a trolley furnace, heating to a temperature of less than 200 ℃ when the tube blank is fed into the furnace, heating to 500 ℃ at a speed of 50 ℃ per hour after heat preservation for 5 hours, heating to 850 ℃ at a speed of 70 ℃ per hour, heating to 1100 ℃ at a speed of 80 ℃ per hour, heating to 1200 ℃ at a speed of 100 ℃ per hour, heating for 10 hours, and directly drawing to a target diameter phi 770+/-2 mm after heating. The heated continuous casting round tube blank is tapped first, residual stress is released, and oxidized iron scales are removed. Then pressing down from one end by a large amount, starting from a second anvil, gradually turning around each anvil on the basis of the previous anvil when the anvil is forged to the middle position, continuing to adopt the principle of tapping firstly and pressing down by a large amount, and finally throwing round;

b2, annealing, namely charging the tube blank into a furnace, wherein the furnace temperature is 550 ℃ before charging, the heating speed is 70 ℃ per hour, the annealing heat preservation temperature is 780 ℃, and the heat preservation time is 45 hours;

And B3, turning light in the length direction of 1/3/, 2/3 and the length of the end surface of the tube blank by about 150mm respectively, and drawing vertical lines in the vertical and horizontal directions of the end surface, wherein the intersection point is the geometric center point of the tube blank. And (3) hanging the pipe blank on a lathe, placing 1/3/, 2/3 parts on a supporting roller, and punching the pipe blank by taking a geometric center point as a circular point on the end surface, wherein the diameter of phi 770 specification punching is phi 180mm.

The pipe making process comprises the following specific steps:

And C1, heating, namely heating the tube blank in a heating furnace, wherein the heating furnace is divided into a heating section and a high-temperature section, the heating section is divided into two areas, the first area is a preheating area, the temperature is 630 ℃, the heat preservation time is 3 hours, the second area is a heating area, the temperature is 1000 ℃, and the heat preservation time is 8 hours. The high temperature section is divided into two areas, namely a soaking area, a soaking area and a soaking area, wherein the temperature is 1230 ℃, the heat preservation time is 10 hours, the temperature is 1270 ℃, the heat preservation time is 15 hours, and the total heating time is 37 hours;

In the step C2, the specific conditions of perforation, pipe rolling and leveling are that the temperature before perforation is higher than 1150 ℃, two-roller oblique rolling and one core rod are adopted, the rotating speed of a perforation roller is 5.5r/min, the surface temperature after perforation is 1100 ℃, the temperature after pipe rolling is 1060 ℃, and the temperature after leveling is 970 ℃.

In the step C2, the specific condition of the preliminary heat treatment is that the temperature of a capillary tube before charging is 550+/-20 ℃, the furnace temperature is set to 550 ℃, and the heat preservation is carried out for 6 hours under the condition that the temperature is increased to 930 ℃ for normalizing.

The target size after perforation is phi 780 multiplied by 170mm, the target size after tube rolling is phi 920 multiplied by 100mm, the target size after uniform adjustment is phi 1020 multiplied by 78.5mm, the target size after tube expanding is phi 1055 multiplied by 76mm, wherein the inner diameter of the steel tube is 933mm, the wall thickness is 44mm, the turning allowance of the inner wall is 15mm respectively, and the turning allowance of the outer wall is 17mm respectively.

In the step C3, the heat treatment comprises the specific steps of normalizing the capillary tube for 5.5 hours at 1070 ℃ and then preserving the heat for 7.5 hours at 770 ℃ for cold tempering.

The P92 steel comprises the following chemical components in percentage by weight, wherein the balance of ：C,0.08-0.12%;Si,0.20-0.40%;Mn,0.30-0.60%;Cr,8.50-9.00%;P,≤0.012%;S,≤0.0030%;Ni,≤0.40%;Mo,0.30-0.35%;Al,≤0.015%;V：0.16-0.18%;Nb,0.04-0.09%;N,0.050-0.075%;W,1.55-1.75%;B：0.0015-0.0035%;Pb,≤0.003%;Sn,≤0.010%;As,≤0.010%;Sb,≤0.003%;Bi,≤0.003%;Pb+Sn+As+Sb+Bi,≤0.025%; is Fe and unavoidable impurities.

The steel pipe ID933×44mm represents an inner diameter pipe, that is, an inner hole diameter of 933mm and a wall thickness of 44mm, a compression ratio from a continuous casting round pipe blank to a forged bar of 1.69, a compression ratio from a forged bar to a steel pipe of 1.88, and a total of 3.57, and satisfies a standard requirement of a compression ratio of 3 or more.

Example 2 (preparation of P91 Steel pipe with specification ID885×4500×43.5mm, continuous casting round pipe blank with specification phi 900 mm)

The embodiment is different from the embodiment 1 in that A4, a continuous casting round tube blank is formed by hanging casting molten steel to a continuous casting station and flowing into a tundish from a ladle, when the weight of the casting molten steel in the tundish reaches 20 tons, a carbonless covering agent and carbonized rice hulls are added, when the weight of the casting molten steel in the tundish reaches 30 tons, the casting is carried out in a crystallizer, an automatic liquid level control system is started, electromagnetic stirring and a secondary cooling water starting casting mode are carried out, the specification of casting flow and a terminal electromagnetic stirrer is phi 1665 x phi 1200 x 1100mm, the superheat degree of the molten steel is controlled between 20-30 ℃ during normal casting, the constant pull rate is 0.15m/min, a chained integral dummy bar is adopted, a large reduction is started when the distance from a red blank is 1.0 meter, the upper pressure limit is set to 350 tons, after the red blank head passes through a withdrawal straightener, the casting is continued for 1.0-1.5 meters, then the casting is gradually converted into hot blank pressure, the pressure is designed to 80 tons, when the casting is finished, the upper pressure limit is restored to 380 tons when the distance from the tail is about 6-8 meters, and the specification of the continuous casting round tube blank is 900 tons after casting is finished;

and A5, annealing, namely conveying the continuous casting round tube blank into an annealing furnace at the temperature of 530 ℃ for annealing, wherein the annealing temperature is 780 ℃, and the heat preservation time is 27 hours, so as to obtain the annealed continuous casting round tube blank.

The forging process comprises the following specific steps:

B1, forging, namely cutting a continuous casting round tube blank annealed with the specification of phi 900mm into single length of 1.2m, heating in a trolley furnace, heating to a temperature of less than 200 ℃ when the tube blank is fed into the furnace, heating to 500 ℃ at a speed of 50 ℃ per hour after heat preservation for 5 hours, heating to 850 ℃ at a speed of 70 ℃ per hour after heat preservation for 4 hours, heating to 1100 ℃ at a speed of 80 ℃ per hour after heat preservation for 2 hours, heating to 1200 ℃ at a speed of 100 ℃ per hour, and heat preservation for 8 hours, wherein the total heating time is not less than 36.5 hours, and directly drawing to a target diameter phi 730+/-2 mm after heating. The heated continuous casting round tube blank is tapped first, residual stress is released, and oxidized iron scales are removed. Then pressing down from one end by a large amount, starting from a second anvil, gradually turning around each anvil on the basis of the previous anvil when the anvil is forged to the middle position, continuing to adopt the principle of tapping firstly and pressing down by a large amount, and finally throwing round;

B2, annealing, namely charging the tube blank into a furnace, wherein the furnace temperature is 550 ℃ before charging, the heating speed is 70 ℃ per hour, the annealing heat preservation temperature is 780 ℃, and the heat preservation time is 43 hours;

And B3, turning light in the length direction of 1/3/, 2/3 and the length of the end surface of the tube blank by about 150mm respectively, and drawing vertical lines in the vertical and horizontal directions of the end surface, wherein the intersection point is the geometric center point of the tube blank. And (3) hanging the pipe blank on a lathe, placing 1/3/, 2/3 parts on a supporting roller, and punching the pipe blank by taking a geometric center point as a circular point on the end surface, wherein the diameter of phi 730 is 150mm.

The pipe making process comprises the following specific steps:

And C1, heating, namely heating the tube blank in a heating furnace, wherein the heating furnace is divided into a heating section and a high-temperature section, the heating section is divided into two areas, the first area is a preheating area, the temperature is 630 ℃, the heat preservation time is 3 hours, the second area is a heating area, the temperature is 1000 ℃, and the heat preservation time is 8 hours. The high temperature section is divided into two areas, namely a soaking area, a soaking area and a soaking area, wherein the temperature is 1230 ℃, the heat preservation time is 8 hours, the temperature is 1250 ℃, the heat preservation time is 15 hours, and the total heating time is 35 hours;

In the step C2, the specific conditions of perforation, pipe rolling and leveling are that the temperature before perforation is higher than 1150 ℃, two-roller oblique rolling and one core rod are adopted, the rotating speed of a perforation roller is 5.5r/min, the surface temperature after perforation is 1070 ℃, the temperature after pipe rolling is 1040 ℃, and the temperature after leveling is 970 ℃.

In the step C2, the specific condition of the preliminary heat treatment is that the temperature of a capillary tube is 550+/-20 ℃ before charging, the furnace temperature is 550 ℃, and the heat preservation is carried out for 6 hours under the condition that the temperature is increased to 930 ℃ for normalizing.

The target size after perforation is phi 750 multiplied by 160mm, the target size after tube rolling is phi 870 multiplied by 100mm, the target size after uniform adjustment is phi 960 multiplied by 78mm, the target size after tube expanding is phi 1006 multiplied by 75.5mm, wherein the inner diameter of the steel tube is 885mm, the wall thickness is 43.5mm, the turning allowance of the inner wall is 15mm respectively, and the turning allowance of the outer wall is 17mm respectively.

In the step C3, the heat treatment comprises the specific steps of normalizing the capillary tube at 1050 ℃ for 6.5 hours, and then preserving the capillary tube at 750 ℃ for 7.5 hours and performing space-time cold tempering.

The P91 steel comprises the chemical components of ：C,0.08-0.12%;Si,0.20-0.40%;Mn,0.30-0.50%;P,≤0.012%;S,≤0.0030%,Cr,8.20-9.00%;Ni,≤0.40%;Mo,0.85-1.05%;Al,≤0.015%;V,0.16-0.25%;Nb,0.04-0.10%;N,0.050-0.075%,W,≤0.05%;B,≤0.001%,Pb,≤0.003%;Sn,≤0.010%;As,≤0.010%;Sb,≤0.003%;Bi,≤0.003%;Pb+Sn+As+Sb+Bi,≤0.025%; percent by weight, and the balance of Fe and unavoidable impurities.

Steel pipe ID885 represents an inner diameter pipe, the compression ratio of the continuous casting round pipe blank to the forged bar is 1.52, the compression ratio of the forged bar to the steel pipe is 1.82, and the total is 3.34, thereby satisfying the standard requirement of the compression ratio of 3 or more.

Example 3 (preparation of P9 Steel pipe with specification of phi 965X 5500X 95mm, continuous casting round pipe blank with specification of phi 1100 mm)

The difference between the embodiment and the embodiment 1 is that A3 and VD vacuum refining is that the refined molten steel is transferred into a VD vacuum furnace, vacuumized to 67Pa or lower within 8min, kept for 20min, and then detected that the hydrogen content is lower than 1.0ppm, argon is blown in before the breaking, and calcium lines are fed after the breaking, so as to obtain casting molten steel;

A4, continuously casting round tube blanks, namely hanging casting molten steel to a continuous casting station and flowing the casting molten steel into a tundish from a ladle, adding a carbonless covering agent and carbonized rice hulls when the weight of the casting molten steel of the tundish reaches 20 tons, pouring the casting molten steel into a crystallizer when the weight of the casting molten steel of the tundish reaches 30 tons, starting a liquid level automatic control system, electromagnetic stirring and secondary cooling water starting pouring modes, controlling the specification of a casting flow and a tail end electromagnetic stirrer to be phi 1865x1400 x 1100mm, controlling the superheat degree of the molten steel to be 20-30 ℃ during normal pouring, pouring at a constant pull rate, controlling the pull rate to be 0.11m/min, adopting a chain type integral type guide bar, starting a large reduction amount when the distance from a red billet is 1.0 meter, setting the upper pressure limit to be 380 tons, continuing to be 1.0-1.5 meters after the red billet passes through a withdrawal straightening machine, then gradually converting the red billet into hot billet pressure, designing the pressure to be 90 tons, and recovering the upper pressure limit to 400 tons when the casting is about 6-8 meters from the tail end, and obtaining continuous casting round tube blanks of phi mm specification after the casting is completed;

and A5, annealing, namely conveying the continuous casting round tube blank into an annealing furnace at the temperature of 530 ℃ for annealing, wherein the annealing temperature is 780 ℃, and the heat preservation time is 48 hours, so as to obtain the annealed continuous casting round tube blank.

The forging process comprises the following specific steps:

B1, forging, namely cutting a continuous casting round tube blank annealed with the specification of phi 1100mm into single length of 2.5m, heating in a trolley furnace, heating to a temperature lower than 200 ℃ when the continuous casting round tube blank is fed into the furnace, heating to 500 ℃ at a speed of 50 ℃ per hour after heat preservation for 2 hours, heating to 850 ℃ at a speed of 70 ℃ per hour after heat preservation for 4 hours, heating to 1100 ℃ at a speed of 80 ℃ per hour, heating to 1200 ℃ at a speed of 100 ℃ per hour, and heat preservation for 15 hours, wherein the total heating time is 42.5 hours, and directly drawing to a target diameter phi 770+/-2 mm after heating. The heated continuous casting round tube blank is tapped first, residual stress is released, and oxidized iron scales are removed. Then pressing down from one end by a large amount, starting from a second anvil, gradually turning around each anvil on the basis of the previous anvil when the anvil is forged to the middle position, continuing to adopt the principle of tapping firstly and pressing down by a large amount, and finally throwing round;

B2, annealing, namely charging the tube blank into a furnace, wherein the furnace temperature is 550 ℃ before charging, the heating speed is 70 ℃ per hour, the annealing heat preservation temperature is 780 ℃, and the heat preservation time is 46 hours;

And B3, turning light in the length direction of 1/3/, 2/3 and the length of the end surface of the tube blank by about 150mm respectively, and drawing vertical lines in the vertical and horizontal directions of the end surface, wherein the intersection point is the geometric center point of the tube blank. And (3) hanging the pipe blank on a lathe, placing 1/3/, 2/3 parts on a supporting roller, and punching the pipe blank by taking a geometric center point as a circular point on the end surface, wherein the diameter of phi 770 specification punching is phi 150mm.

The pipe making process comprises the following specific steps:

And C1, heating, namely heating the tube blank in a heating furnace, wherein the heating furnace is divided into a heating section and a high-temperature section, the heating section is divided into two areas, the first area is a preheating area, the temperature is 630 ℃, the heat preservation time is 3 hours, the second area is a heating area, the temperature is 1000 ℃, and the heat preservation time is 8 hours. The high temperature section is divided into two areas, namely a soaking area, a soaking area and a soaking area, wherein the temperature is 1230 ℃, the heat preservation time is 10 hours, the temperature is 1240 ℃, the heat preservation time is 20 hours, and the total heating time is 42 hours;

In the step C2, the specific conditions of perforation, pipe rolling and leveling are that the temperature before perforation is higher than 1150 ℃, two-roller oblique rolling and one core rod are adopted, the rotating speed of a perforation roller is 5.5r/min, the surface temperature after perforation is 1070 ℃, the temperature after pipe rolling is 1050 ℃, and the temperature after leveling is 970 ℃.

In the step C2, the specific condition of the preliminary heat treatment is that the temperature of a capillary tube before charging (550+/-20) DEG C, the furnace temperature is 550 ℃, and the heat preservation is carried out for 8.5 hours under the condition that the temperature is increased to 930 ℃ for normalizing.

In the step C3, the heat treatment comprises the specific steps of normalizing the capillary tube at 1040 ℃ for 5.5 hours and then preserving the capillary tube at 740 ℃ for 7.5 hours and performing space-time cold tempering.

The perforated target size is phi 790 multiplied by 185mm, the rolled target size is phi 860 multiplied by 130mm, the averaged target size is phi 940 multiplied by 120mm, the expanded target size is phi 975 multiplied by 105mm, wherein the outer diameter of the finished steel pipe is phi 965mm, the wall thickness is 95mm, and the grinding allowance of the inner wall and the outer wall is 5mm respectively.

The P9 steel comprises the following chemical components in percentage by weight, wherein the chemical components comprise ：C,≤0.15%;Si：0.25-1.00%;Mn：0.30-0.60%;P：≤0.012%;S≤0.0030%;Cr：8.50-10.00%;Ni,≤0.60%;Mo,0.90-1.10%;Al,≤0.015%;V,≤0.020%;Nb,≤0.020%,N,≤0.012%,W,≤0.050%,B,≤0.001%,Pb,≤0.003%;Sn,≤0.010%;As,≤0.010%;Sb,≤0.003%;Bi,≤0.003%;Pb+Sn+As+Sb+Bi,≤0.025%; parts by weight of Fe and unavoidable impurities.

The steel pipe phi 965 multiplied by 95mm represents an inner diameter pipe, namely an outer diameter is 965mm, a wall thickness is 95mm, a compression ratio from a continuous casting round pipe blank to a forging rod is 2.04, a compression ratio from the forging rod to the steel pipe is 1.56, and the total is 3.60, thereby meeting the standard requirement of the compression ratio of more than 3.

Comparative example 1

The comparative example is different from example 1 in that the added amount of molten iron is 50% of the total mass of molten iron and scrap steel. Residual elements in molten steel in the smelting process are difficult to control, so that the content of partial residual elements is high, such As 0.25% of Cu, 0.018% of P, 0.03% of As and 0.015% of Sb, and the concentration of the low-melting-point elements in the grain boundary affects the high-temperature strength of the steel.

Comparative example 2

The difference between the comparative example and the example 1 is that A3 and VD vacuum refining is that refined molten steel is transferred into a VD vacuum furnace, vacuumized to 67Pa or lower within 8min, kept for 5min, the hydrogen content in the molten steel is 2.5ppm, and after the molten steel is broken, ferrochromium nitride cored wire is added to increase nitrogen, and then calcium wire is fed to obtain the casting molten steel. Because the alloy content in the steel is high, the product specification is large, a hydrogen white point is easy to form, nitrogen is increased by adopting a wire feeding mode after the steel is broken, secondary oxidation is serious, the purity of the steel is reduced, and the high-temperature strength of the product is affected.

Comparative example 3

This comparative example differs from example 1 in that cast electromagnetic stirring and end electromagnetic stirring were not employed. In the continuous casting process, liquid metal is rapidly cooled in a crystallizer to form a layer of fine equiaxed crystal area, and after entering a secondary cooling strong convection heat conduction area, the surface is cooled to form columnar crystals produced along the radial direction towards the circle center. When the columnar crystal grows to the central area of the round billet, the temperature distribution of the residual liquid metal tends to be uniform, the heat radiation directivity is weakened, an isotropic thermal environment is formed, and an equiaxial crystal area is formed. In the process of columnar crystal growth, the content of carbon element and alloy element is continuously increased, but the content of each element in the equiaxed crystal area tends to be stable until the content of the solidification end is increased again, so that the chemical components at the junction of columnar crystal and equiaxed crystal (CET area) and the solidification end are higher. The example 1 shown uses cast-iron electromagnetic stirring and end electromagnetic stirring to attenuate the segregation of CET and solidification end chemical components. In the production process of the comparative example, the casting flow electromagnetic stirring and the end electromagnetic stirring are not adopted, so that the chemical components at the two parts are segregated greatly, and the high-temperature strength of the material is affected.

Comparative example 4

The difference between this comparative example and example 1 is that in the step A4, when a phi 900 gauge round billet is produced, a phi 1100 gauge electromagnetic stirrer (phi 1865 x phi 1400 x 1100 mm) was used for the cast strand stirrer and the end electromagnetic stirrer. When the electromagnetic stirrer is too large, the magnetic leakage rate can be obviously increased, the stirring effect is affected, strong carbide elements such as W, mo, cr and the like can be enriched, serious component segregation is formed, and finally the high-temperature strength of the material is affected.

Comparative example 5

The difference between the comparative example and the example 1 is that in the step B1, the internal stress is concentrated due to the direct beating with high pressure, and the high temperature strength of the finished pipe is greatly reduced due to the failure of welding in the subsequent pipe making process.

Comparative example 6

This comparative example differs from example 1 in that in step B3, the perforation is not located. Uneven wall thickness occurs in the pipe making process, and part of central cracks remain on the inner wall of the steel pipe, so that the finished pipe fails in advance.

Comparative example 7

The difference between the comparative example and the example 1 is that in the step C1, the tube blank is placed in a heating furnace for heating, the heating furnace is divided into a heating section and a high-temperature section, the heating section is divided into two sections, the first section is a preheating section, the temperature is 850 ℃, the heat preservation time is 2 hours, the second section is a heating section, the temperature is 1000 ℃, and the heat preservation time is 3 hours. The high temperature section is divided into two areas, namely a soaking area, a temperature of 1100 ℃ and a heat preservation time of 5 hours, a soaking area, a temperature of 1200 ℃ and a heat preservation time of 10 hours, and the total heating time is 20 hours.

Comparative example 8

The difference between this comparative example and example 1 is that in the step C2, the capillary is randomly placed at 300 ℃ after being produced, and no preliminary heat treatment is performed, so that an unbalanced structure of ferrite, pearlite, bainite, etc. is formed, resulting in uneven original grain size.

Performance testing

With reference to GB/T2039-2012, GB/T5310-2006 and ASME BPVC-II-Part D, seamless steel pipes were tested for elongation at 625℃under different test stresses and for high temperature tensile strength and high temperature yield strength at 10 ten thousand hours under these conditions. The results are shown in Table 1.

Table 1 measurement results

According to statistics, seamless steel pipes with different components and different diameters prepared in the embodiment 1-3 of the invention have long fracture time and higher elongation and high-temperature strength. With the example 1 with the largest specification As a control group, the comparative example 1 is not smelted by adopting a molten iron ratio of 85 percent, so that the contents of residual elements such As As, sb, P and the like are excessively enriched in grain boundaries. Comparative example 2 did not control the hydrogen content to 1.0ppm or less and hydrogen white spots appeared in the finished product. Comparative example 3 does not employ cast electromagnetic stirring and end electromagnetic stirring, resulting in greater segregation of components. Comparative example 4 did not distinguish the electromagnetic stirrer by specification, resulting in greater segregation of components. Comparative example 5 was not forged as required, resulting in formation of fine cracks inside the product. The failure of the accurate positioning of comparative example 6 causes the wall thickness of the steel pipe to be out of tolerance, which affects the high temperature strength of the steel pipe. Comparative example 7 has insufficient heat-retaining temperature and heat-retaining time, and the steel pipe has fine cracks during the piercing process. Comparative example 8 was not subjected to preliminary heat treatment, and the steel pipe had a mixed crystal phenomenon.

After the longitudinal section samples of the steel pipes prepared in example 1 and example 2 were corroded with ferric trichloride or picric acid, photographs of tissues were taken, see fig. 7 and 8, respectively. As can be seen from fig. 7 and 8, the structure was entirely of diffuse tempered sorbite, and no high-temperature ferrite was found.

Therefore, the 9Cr system seamless steel pipe prepared by the method has uniform and pure components and high temperature strength. The steel pipe described in example 1 is mainly used for main steam pipeline and reheat steam pipeline of ultra supercritical unit whose service temperature is not more than 625 deg.C, the steel pipe described in example 2 is mainly used for steam pipeline of supercritical power station boiler whose service temperature is not more than 593 deg.C, and the steel pipe described in example 3 is mainly used for petrochemical equipment whose service temperature is not more than 550 deg.C and high-temperature pressure container.

While the foregoing is directed to the preferred embodiments of the present invention, it will be appreciated by those skilled in the art that various modifications and adaptations can be made without departing from the principles of the present invention, and such modifications and adaptations are intended to be comprehended within the scope of the present invention.

Claims

1. A method for preparing an ultra-large-size 9Cr ferrite heat-resistant seamless steel pipe, characterized by comprising the following steps in sequence: a continuous casting step, a forging step, and a pipe making step;

The continuous casting process includes the following steps:

A1. Electric Furnace Smelting: After charging molten iron and scrap steel into the electric furnace, quicklime is added to form slag while supplying oxygen. When the temperature and composition meet the design requirements and the steel is ready for tapping, the molten steel is poured into the ladle and mixed with Al, Mn, Cr alloy and slag-forming materials to obtain molten steel. The slag-forming materials include quicklime and synthetic slag, with the addition amounts being 8.9-9.1 kg/ton of molten steel and 1.9-2.1 kg/ton of molten steel, respectively.

A2. LF refining: The molten steel is transferred to the LF refining furnace, and argon is introduced while the temperature is increased. When the temperature of the molten steel is ≥1650℃, alloy is added. Quicklime or synthetic slag is added according to the fluidity of the slag. Argon is blown to remove impurities to obtain refined molten steel.

A3. VD vacuum refining: The refined molten steel is transferred to a VD vacuum furnace and evacuated to below 67 Pa for 15-20 minutes until the hydrogen content in the molten steel is less than 1.0ppm. Before breaking the vacuum, nitrogen is blown into the steel for grades that require nitrogen addition, while argon is blown into the steel for grades that do not require nitrogen addition. The gas flow rate before breaking the vacuum is 400 NL/min; the gas flow rate after breaking the vacuum is 80-100 NL/min. After breaking the vacuum, calcium wire is fed to obtain cast molten steel.

A4. Continuous casting of round tube billets: The molten steel is hoisted to the continuous casting station. The molten steel flows from the ladle into the tundish and then into the crystallizer. The automatic liquid level control system, electromagnetic stirring, and secondary cooling water are activated to start the casting mode to obtain the continuously cast round tube billet.

A5. Annealing: The continuous casting round tube billet is sent to an annealing furnace for annealing to obtain the annealed continuous casting round tube billet;

Different specifications of continuous casting round tube billets require different specifications of electromagnetic stirrers; the superheat of molten steel is controlled at 20-30℃ during pouring, and the casting speed is constant, with a casting speed of 0.10-0.40m/min;

The forging process includes the following steps:

B1. Cutting, heating and forging the annealed continuous casting round tube billet to obtain a tube billet;

B2, annealing;

B3, through hole;

The pipe making process includes the following steps:

C1. Heating: Place the tube billet in a heating furnace for heating. The heating furnace is divided into a heating section and a high-temperature section. The total heating time is 35-43 hours. The heating section is divided into two zones. The first zone is a preheating zone with a temperature of 550-700°C and a holding time of ≥3 hours. The second zone is a heating zone with a temperature of 850-1100°C and a holding time of ≥8 hours. The high-temperature section is divided into two zones. The temperature of the soaking zone 1 is 1200-1230°C and a holding time of ≥10 hours. The temperature of the soaking zone 2 is 1240-1270°C and a holding time of ≥15 hours.

C2. Tube making: The heated tube blank is subjected to perforation, tube rolling, leveling, and preliminary heat treatment to form a rough tube;

C3. Heat treatment, finishing and testing: After the rough tube is heat treated, the inner and outer surfaces are turned, samples are taken for testing, and UT and MT flaw detection are performed. After passing the test, it is put into storage to obtain the finished steel pipe.

2. The method for preparing an ultra-large size 9Cr ferrite heat-resistant seamless steel pipe according to claim 1, wherein the electromagnetic stirring adopts three-stage electromagnetic stirring of M-EMS, S-EMS and F-EMS.

3. The method for preparing an ultra-large size 9Cr ferritic heat-resistant seamless steel pipe according to claim 2 is characterized in that the specific steps of heating in step B1 are: the temperature is lower than 200°C when entering the furnace, and after being kept warm for 2 hours, the temperature is increased to 500°C at a rate of ≤50°C/h and kept warm for 5 hours, then the temperature is increased to 850°C at a rate of ≤70°C/h and kept warm for 4 hours, then the temperature is increased to 1100°C at a rate of ≤80°C/h and kept warm for 2-3 hours, and then the temperature is increased to 1200°C at a rate of ≤100°C/h and kept warm for 8-15 hours, and the total heating time is 36.5-42.5 hours.

4. A product prepared according to the method for preparing an ultra-large size 9Cr ferrite heat-resistant seamless steel pipe according to any one of claims 1 to 3.