Cleanness control method of steel CT70 for coiled tubing

Abstract

The invention discloses a method for controlling the cleanliness of steel CT70 for coiled tubing, and belongs to the technical field of ferrous metallurgy steelmaking processes. According to the method, generation and removal of the inclusions in the actual production process such as a converter process, an LF refining process, an RH refining process and a casting machine process are combined for analysis, a removal process for reducing generation of the inclusions in the production process and optimizing the inclusions is formulated, and secondary oxidation in the casting process of a casting machine is prevented; and high-cleanliness stable control over the low inclusions of the steel for the coiled tubing is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of steel metallurgy and steelmaking technology, and in particular relates to a method for controlling the cleanliness of CT70 steel for continuous oil pipes. Background Art

[0002] Coiled tubing is a flexible tubing that can be wound on a large diameter drum. It has high strength and high toughness. The length of a single tube can reach several thousand meters, and the working pressure can reach 60MPa. During operation, it is subjected to complex stresses such as pulling, pressing, bending, squeezing, swelling, and scouring, as well as a corrosive working environment. Each time it is wound and released, it undergoes 3 bends and 3 straightenings. In order to ensure that the product has good plasticity, toughness, fatigue resistance and corrosion resistance, the industry requires that all types of non-metallic inclusions in coiled tubing are ≤1.0; therefore, improving the cleanliness of molten steel and effectively controlling inclusions are the key to solving the quality control of high-quality steel. Molten steel cleanliness generally refers to the content, composition, morphology and distribution of inclusions in steel.

[0003] Liu Dexiang et al. published a study on the cleanliness control of X70 pipeline steel (Liu Dexiang et al., Study on the Cleanliness Control of X70 Pipeline Steel, Industrial Heating, March 2017, Vol. 46, No. 3). The method is to perform deep desulfurization on molten iron by spraying CaO+Mg powder through molten iron pretreatment, and remove the sulfur content of molten iron to about 0.0020%; the converter adopts double slag method and slag blocking steel tapping operation, and deoxidation and alloying are performed during steel tapping; the LF refining process produces low-oxidizing white slag, and bottom blowing argon is carried out for stirring, and submerged arc positive pressure operation is adopted to prevent molten steel from being exposed. The RH refining vacuum degree is maintained within 300Pa for more than 15min; the continuous casting process adopts non-oxidation protection casting throughout the process, and electromagnetic stirring is performed. Through the control of the above processes, the number of inclusions is reduced, and the cleanliness control level of molten steel is finally improved. The shortcoming of this document is that each area is controlled. Although the cleanliness control of molten steel can be improved to a certain extent, it cannot guarantee the comprehensiveness of the cleanliness control of continuous oil pipe steel.

[0004] Liu Shuai et al. disclosed a smelting method for improving the purity of molten steel of acid-resistant and corrosion-resistant pipeline steel (Liu Shuai et al., A smelting method for improving the purity of molten steel of acid-resistant and corrosion-resistant pipeline steel, application publication number CN 116855843A, hereinafter referred to as document 2), which is to obtain inclusions mainly composed of MgO·Al2O3 and MgO by magnesium metallurgical treatment of molten steel after refining molten steel, and float and remove them by static stirring and calming effect to improve the purity of molten steel; reduce the sulfur content in molten steel by smelting high-temperature steelmaking; produce a low melting point, high fluidity, and low oxidation refining slag system during the refining process to enhance the adsorption of inclusions, modify and refine oxide inclusions, disperse hydrogen atom adsorption points, use fine inclusions to refine grains, hinder the expansion of microcracks, and achieve the performance of acid-resistant and corrosion-resistant pipeline steel against hydrogen-induced cracking. The shortcoming of this document is that it adopts magnesium metallurgical treatment and controls various areas. Although it can improve the cleanliness control of molten steel to a certain extent, it cannot guarantee the comprehensiveness of the cleanliness control of steel for continuous oil pipes.

[0005] Zhang Chuanguo et al. disclosed a CT70 grade continuous oil pipe steel (Zhang Chuanguo et al., A CT70 grade continuous oil pipe steel, authorized publication number CN 101487101 B, hereinafter referred to as document 3), which can obtain a continuous oil pipe steel with a strength level of 70ksi (kilopounds per square inch) and good plasticity through smelting, casting, slab reheating, controlled rolling, cooling and coiling processes and reasonable process parameters. The inadequacy of this document is that the development of the product is introduced in detail, and the control means of molten steel cleanliness are not mentioned. Summary of the invention

[0006] In order to solve the above technical problems, the purpose of the present invention is to provide a method for controlling the cleanliness of continuous oil pipe steel CT70, wherein the content of aluminum inclusions in the finished hot-rolled coil of the above-mentioned continuous oil pipe steel CT70 ranges from 2 to 16 ppm, the total oxygen content is controlled to be less than 30 ppm, and the nitrogen content is controlled to be less than 40 ppm; the fineness and coarseness of the hot-rolled coil inclusions are controlled to be level 1.0 for A, B, C, and D.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] The present invention provides a method for controlling the cleanliness of CT70 steel for coiled tubing, comprising the following steps:

[0009] 1) Composition design: The composition of the coiled tubing steel CT70 is C: 0.06-0.10%, Si: 0.10-0.30%, Mn: 0.60-0.90%, P: ≤0.018%, S: ≤0.005%, Cu: 0.20-0.30%, Ni: 0.10-0.25%, Cr: 0.45-0.65%, Mo: 0.10-0.30%, Nb≤0.05%, and the rest is Fe and unavoidable impurities;

[0010] 2) KR hot metal desulfurization: The hot metal temperature in the KR hot metal desulfurization process is required to be greater than 1350°C, and the silicon content in the hot metal is required to be 0.45-0.65%. The deep desulfurization process is adopted in the KR hot metal desulfurization process, and the stirring time is ≥15min; after the stirring is completed, the hot metal is allowed to stand for a period of time and then the slag is removed once, and the slag removal rate is greater than 90%. After the first slag removal, the hot metal is allowed to stand for ≥5min and then the hot metal is removed for a period of time, and the slag removal rate is greater than 95%. After the slag removal is completed, the temperature is sampled and measured, and the temperature is ≥1300°C. The sulfur content in the hot metal leaving the KR station is less than 0.002%, and deep desulfurized hot metal is obtained;

[0011] 3) Converter smelting: wherein the converter molten iron is the deep desulfurized molten iron obtained in step 2) and scrap steel, the sulfur content in the scrap steel is less than 0.005%; the amount of scrap steel added is 15-30 tons / furnace, and the total amount of converter molten iron is 250-270 tons / furnace; during the converter smelting process, the amount of lime added is 5-10 tons / furnace, the amount of dolomite added is 3-8 tons / furnace, and the amount of iron-containing material added is 0-6 tons / furnace; the converter smelting terminal temperature is controlled to be greater than 1630°C, the converter dephosphorization rate is 95%-96%, and the oxygen content of the terminal molten steel is controlled to be 600-800ppm; the steel tapping adopts a slide block Slag mode, reduce the amount of slag from the steel plate as much as possible, reduce the total amount of inclusions in the molten steel, and control the slag thickness to <50mm; add small pieces of lime, synthetic slag and aluminum iron to the ladle during the steel tapping process, the order of material addition is aluminum iron-alloy-synthetic slag-lime, the amount of aluminum iron and synthetic slag added is adjusted according to the end C content, aluminum iron is added before 60t of steel is tapped from the converter, and alloy is added after 100t of steel is tapped, among which the amount of copper plate added is 0.5-0.8 tons / furnace, the amount of nickel plate added is 2.8-5.2 tons / furnace, and the amount of ferrochrome added is 1.8-2.5 tons / furnace, as shown in the following table:

[0012]

[0013]

[0014] 4) LF smelting: LF on-site temperature measurement, on-site temperature 1540-1560℃, inlet argon flow rate 600-800 / min / line, first heating time 10-15 minutes, deoxidation alloying during heating, argon flow rate ≤400NL / min / line during heating; lime and slag-making agent are added in batches, with a total amount of not less than 2.5t / time to ensure good fluidity of refined slag, refining final rolling (FeO)+MnO)≤0.5%, white slag time ≥20min, according to the component content, low carbon ferromanganese is added in an amount of 90-130kg / furnace, ferroniobium is added in an amount of 30-60kg / furnace, ferromolybdenum is added in an amount of 400-900kg / furnace, and basicity is controlled at 6-8; LF treatment time is ≥35min in total, LF off-site measurement temperature is 1560-1590℃;

[0015] 5) RH vacuum treatment: RH in-situ oxygen determination and temperature measurement, in-situ temperature 1570-1600℃, vacuum degree requirement ≤2.6mbar, after 3 minutes of vacuum cycle treatment, add 20-60kg / furnace of alloy titanium iron, and the vacuum treatment cycle gas flow rate is set to 80-260Nm 3 / h, circulating pure degassing time ≥7min, RH off-site measurement temperature 1540-1560℃; after RH vacuum treatment, calcium treatment is carried out, and 200-300kg / furnace of modifier is evenly added to the top slag surface to ensure that the calcium treatment time is ≥8min;

[0016] 6) CCM process: When the casting machine is pouring, the tundish is purged with argon before pouring. During the pouring process, argon is continuously blown into the tundish while adding the covering agent. After the pouring of the large ladle is completed, the slag detection equipment is used to avoid slag in the large ladle. During the casting process of the casting machine, the overheating range of the tundish is controlled at 25-35°C, the production pulling speed is 0.9-1.5m / min, and the argon blowing flow rate of the stopper rod during the pouring process is 4.0-10.0L / min.

[0017] Furthermore, the steel model for the coiled tubing is CT70.

[0018] Furthermore, the total oxygen content in the finished hot-rolled coil of the CT70 continuous oil pipe steel is controlled to be less than 30ppm, and the nitrogen content is controlled to be less than 40ppm; the non-metallic inclusions of the hot-rolled coil are controlled to be A, B, C, D fine series and coarseness to be 1.0 level.

[0019] Furthermore, KR deep desulfurization treatment is adopted. The sulfur content of KR molten iron arriving at the station is 0.005%. The heating amount of desulfurizer in the treatment process is 2.2 tons, the stirring time is 15 minutes, the number of slag removal is more than 2 times, and the slag removal rate is more than 95%; the molten iron entering the converter is deep desulfurized water, and the scrap steel is scrap steel with a sulfur content of less than 0.003%, the addition amount is 25 tons, the total loading amount of molten iron and scrap steel is controlled at 280 tons, the temperature of the molten iron entering the furnace is 1339℃, and the silicon content of the molten iron is 0.49%; the converter smelting process 8.5 tons of lime, 7.1 tons of dolomite and 1.3 tons of iron balls were added to the process slag; the oxygen content of the converter was 760ppm, the carbon content was 0.06% and the final temperature was 1637℃. After tapping, 900-1100kg of aluminum iron, 500kg of synthetic slag and 700kg of lime were added; before tapping, the alloy ratio was adjusted, 0.7 tons of copper plate, 2.7 tons of nickel plate and 2.0 tons of ferrochrome were added, 1 ton of top slag lime was added during tapping and 300kg of modifier was added after tapping. kg; LF in-situ temperature is 1549℃, slag-making agent is added in the refining process to make suitable refined white slag, and the S content in the steel after refining is 0.0010% through slag-steel reaction. In the LF refining process, aluminum particles are added to the slag surface, and aluminum wire is fed into the ladle for composite deoxidation. Argon is kept unobstructed during the refining process; alloying is performed by adding 100kg of low-carbon ferromanganese, 45kg of ferroniobium, and 600kg of ferromolybdenum, and the basicity of the white slag is 6.5; RH in-situ oxygen determination and temperature measurement, in-situ temperature 15 85℃, after deep vacuum treatment for 18min, 200kg of deoxidation and component aluminum iron were added, the vacuum degree of deep vacuum treatment was less than 2.6Mbar, after the aluminum iron was added and circulated for 3min, 45kg of alloy titanium iron was added, the decarburization and vacuum treatment time was a total of 38min, and the RH off-site measurement temperature was 1555℃; the overheat of the casting machine during pouring was 34℃, the production pulling speed was 1.1m / min, the argon blowing flow rate of the stopper rod during pouring was 5.1NL / min, the flow control stopper rod was stable during pouring, and the casting was stable.

[0020] Furthermore, KR deep desulfurization treatment is adopted. The sulfur content of KR molten iron arriving at the station is 0.004%. The heating amount of desulfurizer in the treatment process is 1.8 tons, the stirring time is 13 minutes, the number of slag removal is more than 2 times, and the slag removal rate is more than 97%; the molten iron entering the converter is deep desulfurized water, and the scrap steel is scrap steel with a sulfur content of less than 0.003%, the addition amount is 23 tons, the total loading amount of molten iron and scrap steel is controlled at 283 tons, the temperature of the molten iron entering the furnace is 1330℃, and the silicon content of the molten iron is 0.45%; the slag in the converter smelting process is 1.8 tons, the slag is 2.8 tons, and the slag is 1.8 tons. 8.2 tons of lime, 7.0 tons of dolomite and 1.2 tons of iron balls were added to the raw material; the oxygen content at the end of the converter was 660ppm, the carbon content was 0.05%, and the end temperature was 1625℃. 1000-1200kg of aluminum iron, 500kg of synthetic slag and 770kg of lime were added during the steel-making process; alloy proportioning was carried out before steel-making in the converter, 0.68 tons of copper plate, 2.8 tons of nickel plate and 1.9 tons of ferrochrome were added, 1 ton of top slag lime was added during the steel-making process, and 280 tons of modifier were added at the end of steel-making. kg; LF in-situ temperature is 1545℃, slag-making agent is added during the refining process to make suitable refined white slag, and the S content in the steel after refining is 0.0010% through slag-steel reaction. In the LF refining process, aluminum particles are added to the slag surface, and aluminum wire is fed into the ladle for composite deoxidation. Argon is kept unobstructed during the refining process; alloying is performed by adding 105kg of low-carbon ferromanganese, 43kg of ferroniobium, and 610kg of ferromolybdenum, and the basicity of the white slag is 6.9; RH in-situ oxygen determination and temperature measurement, in-situ temperature is 158 3℃, after deep vacuum treatment for 15min, 200kg of deoxidation and component aluminum and iron were added, the vacuum degree of deep vacuum treatment was less than 2.6Mbar, after the aluminum and iron were added and circulated for 3min, 44kg of alloy titanium and iron were added, the decarburization and vacuum treatment time was 39min in total, and the RH off-situ measurement temperature was 1550℃; the overheat of the casting machine during pouring was 30℃, the production pulling speed was 1.2m / min, the argon blowing flow rate of the stopper rod during pouring was 5.2L / min, the flow control stopper rod was stable during pouring, and there was no flocculation phenomenon.

[0021] Compared with the prior art, the beneficial technical effects of the present invention are:

[0022] Based on the above technical solution, the cleanliness control method of CT70 steel for continuous oil pipes provided is combined with the analysis of the generation and removal of inclusions in the actual production processes such as converter process, LF furnace, RH refining process, and casting machine process. A process for reducing the generation of inclusions in the production process and optimizing the removal of inclusions is formulated to prevent secondary oxidation during the casting process of the casting machine, and to achieve stable control of high cleanliness of CT70 steel for continuous oil pipes with low inclusions. DETAILED DESCRIPTION

[0023] In order to improve the control level of internal inclusions in steel, the inventor specifically analyzed the characteristics of the methods in the above-mentioned documents 1-3, and found that the common point of the above three documents is to control a certain point in some processes. Although the cleanliness control of molten steel can be improved to a certain extent, it cannot guarantee the comprehensiveness of the cleanliness control of IF steel. The present invention analyzes the generation and removal of inclusions in the actual production process such as the converter process, LF refining process, RH refining process, and casting process, and formulates a process to reduce the generation of inclusions in the production process and optimize the removal of inclusions, prevent secondary oxidation in the casting process of the casting machine, and achieve high cleanliness and stable control of CT70 steel for continuous oil pipes with low inclusions.

[0024] Compared with document 1, the present invention is a method for improving the cleanliness of molten steel by controlling the whole process of steelmaking, LF, RH and continuous casting. The oxygen content of the converter end point is precisely controlled by process means, and the oxygen content of the molten steel at the converter end point is less than 800ppm; the LF process is heated and argon is blown throughout the process according to the conditions of the molten steel in place to ensure the floating of inclusions, but it does not cause the exposure of the molten steel and cause secondary oxidation; the RH vacuum treatment can be deoxidized by adding a small amount of carbon powder to reduce the amount of aluminum and iron added at the end of RH decarburization, which is conducive to reducing oxide inclusions in the molten steel; while the casting machine is doing a good job of protecting the pouring, according to the flocculant situation during the tundish pouring process, the stopper rod argon blowing flow is manually controlled to improve the crystallizer liquid level fluctuation, reduce the slag rolling phenomenon caused by the liquid level fluctuation, and thus improve the cleanliness control level in the steel.

[0025] Compared with Document 2, the present invention reduces the active oxygen of molten steel and increases the terminal carbon content through the converter end point, thereby reducing the content of deoxidation products in the molten steel; argon is blown throughout the LF and RH to ensure the fluidity of the molten steel, and the molten steel is not exposed to the outside, the white slag (TFe+MnO) is ≤1.5%, and the RH is vacuum treated throughout the whole process, which effectively improves the cleanliness of the molten steel; in addition to the argon purging of the tundish before pouring, the casting machine operation continuously blows argon into the tundish through the argon pipeline installed on the tundish cover while adding the covering agent during the entire pouring; the pouring process is under constant pulling speed control, and the stopper rod position is observed and manually adjusted by the stopper rod argon blowing flow control method to improve flocculence, and the crystallizer liquid level fluctuation is effectively controlled, rather than the flow control being consistent for the entire pouring.

[0026] Compared with Document 3, the present invention is a control method for achieving clean molten steel through precise control of steelmaking, refining and continuous casting processes and process innovation, which not only improves the cleanliness control of the head and tail billets, but also significantly improves the cleanliness control level of the molten steel during the entire casting. In order to improve the defects on the surface of the product roll caused by inclusions, the casting machine innovates the control of the molten steel pouring superheat during the casting process. The original process of continuous oil pipe steel pouring superheat control range is 15-30°C, and the superheat control range designed by the present invention is 25-45°C. By increasing the casting superheat of the casting machine, the meniscus generated during the solidification of the billet causes secondary melting, avoiding the meniscus from capturing inclusions, thereby promoting the floating of inclusions in the molten steel in the crystallizer, effectively reducing the number of subcutaneous inclusions in the continuous oil pipe steel billet, and greatly improving the surface quality control of the finished coil.

[0027] The specific scheme adopted by the present invention is that the converter uses a computer simulation program to simulate and calculate the converter endpoint control based on the data collection of the auxiliary gun process, avoids the spot blowing operation, realizes the control of the oxygen content in the terminal molten steel within the range of 600-800ppm, controls the tapping temperature to be greater than 1630℃, avoids the control of the terminal molten steel overoxidation, and achieves the purpose of low oxygen, low nitrogen, and low inclusion stable control of the tapping molten steel; the tapping process adopts the slag blocking operation to avoid the slag lowering operation, and the ladle top slag is subjected to a modification treatment at the end of tapping. In order to reduce the heat loss of molten steel during the tapping process, the ladle turnover adopts 4 turnovers to ensure that the temperature drop during the tapping process is controlled at less than 60℃. During the LF furnace treatment process, the argon flow rate is controlled at 600-800 / min / line, the white slag is ≤0.5%, the basicity is controlled at 6-8, the soft blowing time is greater than 10min, and the molten steel is not exposed to the outside; during the vacuum treatment of the RH furnace, the alloy is fine-tuned, and the titanium-iron alloy is added in a cycle of ≥3min. After the vacuum treatment, calcium treatment is carried out, and 200-300kg / furnace of modifier is evenly added to the surface of the top slag, and finally the iron oxide content in the top slag is less than 5%, which effectively improves the oxygen transfer at the slag-molten steel interface, avoids secondary pollution of the molten steel, and improves the slag's ability to absorb inclusions. When the casting machine is pouring, the tundish is purged with argon before pouring. During the pouring process, argon is continuously blown into the tundish while adding the covering agent. After the pouring of the large ladle is completed, the slag detection equipment is used to avoid slag in the large ladle. During the casting process of the casting machine, the overheating range of the tundish is controlled within 25-45℃; the argon blowing control of the stopper rod is based on the trend of the stopper rod during the casting process. The argon blowing flow rate of the stopper rod is manually adjusted. When the flocculent flow is serious, the argon blowing amount is gradually increased, and the argon blowing flow control is reduced when the stopper rod descends.

[0028] The following takes CT70 steel as an example to specifically illustrate the content of the present invention. These embodiments are only used to describe the content of the present invention for the purpose of facilitating understanding, and do not limit the content of the present invention.

[0029] Example 1

[0030] KR deep desulfurization treatment is adopted. The sulfur content of KR molten iron at the station is 0.005%. The heating amount of desulfurizer in the treatment process is 2.2 tons, the stirring time is 15 minutes, the number of slag removal is more than 2 times, and the slag removal rate is more than 95%. The molten iron entering the converter is deep desulfurized water, and the scrap steel is self-produced scrap steel (low sulfur, sulfur content is less than 0.003%), the addition amount is 25 tons, the total loading amount of molten iron and scrap steel is controlled at 280 tons, the temperature of the molten iron entering the furnace is 1339℃, and the silicon content of the molten iron is 0.49%. 8.5 tons of lime, 7.1 tons of dolomite, and 1.3 tons of iron balls are added to the slag during the converter smelting process. The oxygen content of the converter is 760ppm, the carbon content is 0.06%, and the terminal temperature is 1637℃. After tapping, 900-1100kg of aluminum iron, 500 grams of synthetic slag, and 700kg of lime are added. Before tapping the converter, alloy proportioning is performed, 0.7 tons of copper plate, 2.7 tons of nickel plate, 2.0 tons of ferrochrome are added, 1 ton of top slag lime is added during tapping, and 300 kg of modifier is added after tapping. The LF in-situ temperature is 1549℃, slag-making agent is added during the refining process to make suitable refined white slag, and the S content in the steel after refining is 0.0010% through slag-steel reaction. Aluminum particles are added to the slag surface during the LF refining process, and aluminum wire is fed into the ladle for composite deoxidation. Argon gas is kept unobstructed during the refining process. Refining and alloying, adding 100kg of low-carbon ferromanganese, 45kg of ferroniobium, and 600kg of ferromolybdenum, the basicity of white slag is 6.5; RH in-situ oxygen determination and temperature measurement, the in-situ temperature is 1585℃, after deep vacuum treatment for 18min, add 200kg of deoxidation and component aluminum iron (the vacuum degree of deep vacuum treatment is less than 2.6Mbar), after the addition of aluminum iron, 3min of circulation, and then add 45kg of alloy titanium iron, the decarburization and vacuum treatment time is 38min in total, and the RH out-of-situ measurement temperature is 1555℃. The overheat of the casting process of the casting machine is 34℃, the production pulling speed is 1.1m / min, the flow rate of the plug rod argon blowing in the casting process is 5.1NL / min, the flow control plug rod is stable in the casting process, and the casting is stable.

[0031] Example 2

[0032] KR deep desulfurization treatment is adopted. The sulfur content of KR molten iron at the station is 0.004%. The heating amount of desulfurizer in the treatment process is 1.8 tons, the stirring time is 13 minutes, the number of slag removal is more than 2 times, and the slag removal rate is more than 97%. The molten iron entering the converter is deep desulfurized water, and the scrap steel is self-produced scrap steel (low sulfur, sulfur content is less than 0.003%), the addition amount is 23 tons, the total loading amount of molten iron and scrap steel is controlled at 283 tons, the temperature of the molten iron entering the furnace is 1330℃, and the silicon content in the molten iron is 0.45%. 8.2 tons of lime, 7.0 tons of dolomite, and 1.2 tons of iron balls are added to the slag during the converter smelting process. The oxygen content of the converter is 660ppm, the carbon content is 0.05%, and the terminal temperature is 1625℃. 1000-1200kg of aluminum iron, 500 pieces of synthetic slag, and 770kg of lime are added during the steel tapping process. Before tapping the converter, alloy proportioning is performed, 0.68 tons of copper plate, 2.8 tons of nickel plate, 1.9 tons of ferrochrome are added, 1 ton of top slag lime is added during tapping, and 280 kg of modifier is added after tapping. The LF in-situ temperature is 1545℃, slag-making agent is added during the refining process to make suitable refined white slag, and the S content in the steel after refining is 0.0010% through slag-steel reaction. Aluminum particles are added to the slag surface during the LF refining process, and aluminum wire is fed into the ladle for composite deoxidation. Argon gas is kept unobstructed during the refining process. Refining and alloying, adding 105kg of low-carbon ferromanganese, 43kg of ferroniobium, and 610kg of ferromolybdenum, the basicity of white slag is 6.9; RH in-situ oxygen determination and temperature measurement, the in-situ temperature is 1583℃, after deep vacuum treatment for 15min, 200kg of deoxidation and component aluminum iron are added (the vacuum degree of deep vacuum treatment is less than 2.6Mbar), after the addition of aluminum iron, 44kg of alloy titanium iron is added after 3min of circulation, the decarburization and vacuum treatment time is 39min in total, and the RH out-of-situ measurement temperature is 1550℃. The overheat degree of the casting process of the casting machine is 30℃, the production pulling speed is 1.2m / min, the flow rate of argon blowing of the plug rod in the casting process is 5.2L / min, the flow control plug rod is stable in the casting process, and there is no flocculent phenomenon, so the flow rate of argon blowing of the plug rod of this furnace is not adjusted.

[0033] The chemical composition content of CT70 obtained by testing in Example 1-2 is shown in Table 1, and the inspection results of hot-rolled coil inclusions are shown in Table 2.

[0034] Table 1 Chemical composition of CT70

[0035] Examples C Si Mn P S Ti Alt Nb Mo Cu Cr Ni N O one 0.07 0.19 0.74 0.010 0.001 0.015 0.045 0.014 0.12 0.22 0.48 0.14 0.0035 0.0025 two 0.07 0.18 0.75 0.009 0.001 0.014 0.048 0.014 0.11 0.22 0.49 0.15 0.0028 0.0024

[0036] Table 2 Hot rolled coil inclusion inspection

[0037]

[0038] From Table 1 and Table 2, the steelmaking, refining and continuous casting processes adopt the method of the present invention, the chemical composition of the finished product and the detection and control of the inclusions of the hot rolled coils all meet the target requirements, the nitrogen content and oxygen content are controlled at a low level, the inclusion aluminum content ranges from 2 to 16 ppm, the total oxygen content is controlled to be less than 25 ppm, and the molten steel achieves the purpose of high cleanliness control. The inclusion control of the hot rolled coils of different thicknesses is controlled at level 0 except for the D-type fine system 1.0. Therefore, the high cleanliness control level of CT70 steel for continuous oil pipes is achieved by the method of the present invention.

[0039] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for controlling the cleanliness of CT70 steel for coiled tubing, characterized in that: The following steps are involved: 1) Composition design: The composition of the coiled tubing steel CT70 is C: 0.06-0.10%, Si: 0.10-0.30%, Mn: 0.60-0.90%, P: ≤0.018%, S: ≤0.005%, Cu: 0.20-0.30%, Ni: 0.10-0.25%, Cr: 0.45-0.65%, Mo: 0.10-0.30%, Nb≤0.05%, and the rest is Fe and unavoidable impurities; 2) KR hot metal desulfurization: The hot metal temperature in the KR hot metal desulfurization process is required to be greater than 1350°C, and the silicon content in the hot metal is required to be 0.45-0.65%. The deep desulfurization process is adopted in the KR hot metal desulfurization process, and the stirring time is ≥15min; after the stirring is completed, the hot metal is allowed to stand for a period of time and then the slag is removed once, and the slag removal rate is greater than 90%. After the first slag removal, the hot metal is allowed to stand for ≥5min and then the hot metal is removed for a period of time, and the slag removal rate is greater than 95%. After the slag removal is completed, the temperature is sampled and measured, and the temperature is ≥1300°C. The sulfur content in the hot metal leaving the KR station is less than 0.002%, and deep desulfurized hot metal is obtained; 3) Converter smelting: wherein the converter molten iron is the deep desulfurized molten iron obtained in step 2) and scrap steel, the sulfur content in the scrap steel is less than 0.005%; the amount of scrap steel added is 15-30 tons / furnace, and the total amount of converter molten iron is 250-270 tons / furnace; during the converter smelting process, the amount of lime added is 5-10 tons / furnace, the amount of dolomite added is 3-8 tons / furnace, and the amount of iron-containing material added is 0-6 tons / furnace; the converter smelting terminal temperature is controlled to be greater than 1630°C, the converter dephosphorization rate is 95%-96%, and the oxygen content of the terminal molten steel is controlled to be 600-800ppm; the steel tapping adopts a slide block Slag mode, reduce the amount of slag from the steel plate as much as possible, reduce the total amount of inclusions in the molten steel, and control the slag thickness to <50mm; add small pieces of lime, synthetic slag and aluminum iron to the ladle during the steel tapping process, the order of material addition is aluminum iron-alloy-synthetic slag-lime, the amount of aluminum iron and synthetic slag added is adjusted according to the end C content, aluminum iron is added before 60t of steel is tapped from the converter, and alloy is added after 100t of steel is tapped, among which the amount of copper plate added is 0.5-0.8 tons / furnace, the amount of nickel plate added is 2.8-5.2 tons / furnace, and the amount of ferrochrome added is 1.8-2.5 tons / furnace, as shown in the following table: 4) LF smelting: LF on-site temperature measurement, on-site temperature 1540-1560℃, inlet argon flow rate 600-800 / min / line, first heating time 10-15 minutes, deoxidation alloying during heating, argon flow rate ≤400NL / min / line during heating; lime and slag-making agent are added in batches, with a total amount of not less than 2.5t / time to ensure good fluidity of refined slag, refining final rolling (FeO)+MnO)≤0.5%, white slag time ≥20min, according to the component content, low carbon ferromanganese is added in an amount of 90-130kg / furnace, ferroniobium is added in an amount of 30-60kg / furnace, ferromolybdenum is added in an amount of 400-900kg / furnace, and basicity is controlled at 6-8; LF treatment time is ≥35min in total, LF off-site measurement temperature is 1560-1590℃; 5) RH vacuum treatment: RH in-situ oxygen determination and temperature measurement, in-situ temperature 1570-1600℃, vacuum degree requirement ≤2.6mbar, after 3 minutes of vacuum cycle treatment, add 20-60kg / furnace of alloy titanium iron, and the vacuum treatment cycle gas flow rate is set to 80-260Nm 3 / h, circulating pure degassing time ≥7min, RH off-site measurement temperature 1540-1560℃; after RH vacuum treatment, calcium treatment is carried out, and 200-300kg / furnace of modifier is evenly added to the top slag surface to ensure that the calcium treatment time is ≥8min; 6) CCM process: When the casting machine is pouring, the tundish is purged with argon before pouring. During the pouring process, argon is continuously blown into the tundish while adding the covering agent. After the pouring of the large ladle is completed, the slag detection equipment is used to avoid slag in the large ladle. During the casting process of the casting machine, the overheating range of the tundish is controlled at 25-35°C, the production pulling speed is 0.9-1.5m / min, and the argon blowing flow rate of the stopper rod during the pouring process is 4.0-10.0L / min.

2. The method for controlling the cleanliness of CT70 steel for coiled tubing according to claim 1, characterized in that: The steel type for the coiled tubing is CT70.

3. The method for controlling the cleanliness of CT70 steel for coiled tubing according to claim 2, characterized in that: The total oxygen content in the finished hot-rolled coil of the CT70 continuous oil pipe steel is controlled to be less than 30ppm, and the nitrogen content is controlled to be less than 40ppm; the non-metallic inclusions of the hot-rolled coil are controlled to be A, B, C, D fine series and coarseness to be 1.0 level.

4. The method for controlling the cleanliness of CT70 steel for coiled tubing according to claim 1, characterized in that: KR deep desulfurization treatment is adopted. The sulfur content of KR molten iron arriving at the station is 0.005%. The desulfurizer heating amount in the treatment process is 2.2 tons, the stirring time is 15 minutes, the number of slag removal is more than 2 times, and the slag removal rate is more than 95%. The molten iron entering the converter is deep desulfurized water, and the scrap steel is the scrap steel with sulfur content less than 0.003%. The added amount is 25 tons, and the total loading amount of molten iron and scrap steel is controlled at 280 tons. The temperature of the molten iron entering the furnace is 1339℃, and the silicon content of the molten iron is 0.49%; the slag material is added during the converter smelting process. 8.5 tons of lime, 7.1 tons of dolomite and 1.3 tons of iron balls are added; the oxygen content at the end of the converter is 760ppm, the carbon content is 0.06%, and the end temperature is 1637℃. After tapping, 900-1100kg of aluminum iron, 500kg of synthetic slag and 700kg of lime are added; before tapping, alloy proportioning is carried out, 0.7 tons of copper plate, 2.7 tons of nickel plate and 2.0 tons of ferrochrome are added, 1 ton of top slag lime is added during tapping, and 300kg of modifier is added at the end of tapping; The LF in-situ temperature is 1549℃. Slag-making agent is added during the refining process to make suitable refined white slag. Through slag-steel reaction, the S content in the steel after refining is 0.0010%. In the LF refining process, aluminum particles are added to the slag surface, and aluminum wire is fed into the ladle for composite deoxidation. Argon is kept unobstructed during the refining process. Refining is performed for alloying, 100kg of low-carbon ferromanganese, 45kg of ferroniobium, and 600kg of ferromolybdenum are added, and the basicity of the white slag is 6.5; RH is in-situ for oxygen determination and temperature measurement, and the in-situ temperature is 158 5℃, after deep vacuum treatment for 18min, 200kg of deoxidation and component aluminum iron were added, the vacuum degree of deep vacuum treatment was less than 2.6Mbar, after the aluminum iron was added and circulated for 3min, 45kg of alloy titanium iron was added, the decarburization and vacuum treatment time was 38min in total, and the RH off-site measurement temperature was 1555℃; the overheat of the casting machine during pouring was 34℃, the production pulling speed was 1.1m / min, the argon blowing flow rate of the stopper rod during pouring was 5.1NL / min, the flow control stopper rod was stable during pouring, and the casting was stable.

5. The method for controlling the cleanliness of CT70 steel for coiled tubing according to claim 1, characterized in that: KR deep desulfurization treatment is adopted. The sulfur content of KR molten iron arriving at the station is 0.004%. The heating amount of desulfurizer in the treatment process is 1.8 tons, the stirring time is 13 minutes, the number of slag removal is more than 2 times, and the slag removal rate is more than 97%. The molten iron entering the converter is deep desulfurized water, and the scrap steel is the scrap steel with sulfur content less than 0.003%. The added amount is 23 tons. The total loading amount of molten iron and scrap steel is controlled at 283 tons. The temperature of the molten iron entering the furnace is 1330℃, and the silicon content of the molten iron is 0.45%; white slag is added to the slag during the converter smelting process. 8.2 tons of ash, 7.0 tons of dolomite, and 1.2 tons of iron balls are added; the oxygen content of the converter is 660ppm, the carbon content is 0.05%, and the terminal temperature is 1625℃. 1000-1200kg of aluminum iron, 500 kg of synthetic slag, and 770kg of lime are added during the steel-making process; alloy proportioning is carried out before steel-making in the converter, 0.68 tons of copper plate, 2.8 tons of nickel plate, and 1.9 tons of ferrochrome are added. 1 ton of top slag lime is added during the steel-making process, and 280kg of modifier is added at the end of steel-making; The LF in-situ temperature is 1545℃. Slag-making agent is added during the refining process to make suitable refined white slag. The S content in the steel after refining is 0.0010% through slag-steel reaction. Aluminum particles are added to the slag surface during the LF refining process. Aluminum wire is fed into the ladle for composite deoxidation. Argon is kept unobstructed during the refining process. Alloying is performed during refining. 105kg of low-carbon ferromanganese, 43kg of ferroniobium, and 610kg of ferromolybdenum are added. The basicity of the white slag is 6.

9. RH is in-situ for oxygen determination and temperature measurement. The in-situ temperature is 1583 ℃, after deep vacuum treatment for 15 minutes, 200kg of deoxidation and component aluminum iron were added, the vacuum degree of deep vacuum treatment was less than 2.6Mbar, after the aluminum iron was added and circulated for 3 minutes, 44kg of alloy titanium iron was added, the decarburization and vacuum treatment time was 39 minutes in total, and the RH off-site measurement temperature was 1550℃; the overheat of the casting machine during the pouring process was 30℃, the production pulling speed was 1.2m / min, the argon blowing flow rate of the stopper rod during the pouring process was 5.2L / min, the flow control stopper rod was stable during the pouring process, and there was no flocculation phenomenon.

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