CN111945062B - Smelting method of low-carbon steel for mechanical structure pipe - Google Patents

Smelting method of low-carbon steel for mechanical structure pipe Download PDF

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CN111945062B
CN111945062B CN202010675948.2A CN202010675948A CN111945062B CN 111945062 B CN111945062 B CN 111945062B CN 202010675948 A CN202010675948 A CN 202010675948A CN 111945062 B CN111945062 B CN 111945062B
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魏巍
李虹
刘宪民
宋平
刘奇
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Shigang Jingcheng Equipment Development And Manufacturing Co ltd
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Abstract

A low-carbon steel for a mechanical structure tube and a smelting method thereof comprise the following components in percentage by mass: 0.14 to 0.17 percent of C, 0.28 to 0.33 percent of Si, 1.35 to 1.45 percent of Mn, less than or equal to 0.012 percent of P, less than or equal to 0.003 percent of S, 0.020 to 0.040 percent of Al, 0.70 to 0.80 percent of Cr, 0.40 to 0.60 percent of W, 0.10 to 0.20 percent of Ni, 0.10 to 0.15 percent of Cu, 0.35 to 0.50 percent of Mo, 0.03 to 0.06 percent of Nb, 0.03 to 0.07 percent of V, 0.0015 to 0.0030 percent of Ca, and the balance of Fe and inevitable impurities; the method comprises the following steps: (1) a BOF converter smelting process, (2) an LF refining process, (3) a VD vacuum treatment process, and (4) a continuous casting process. The low-carbon steel strictly controls the contents of harmful residual elements such as P, S and the like and gas components such as O, N, H and the like so as to improve the strength, hardness, wear resistance, atmospheric corrosion resistance and high-temperature hydrogen, nitrogen, ammonia and other corrosion resistance of the steel; the produced steel product meets the requirements of high-quality high-strength mechanical structure pipes. Has the characteristics of high strength, high hardness, good wear resistance, good corrosion resistance and the like.

Description

Smelting method of low-carbon steel for mechanical structure pipe
Technical Field
The invention relates to low-carbon structural steel and a smelting method thereof, in particular to a smelting method of low-carbon steel for a mechanical structural pipe.
Background
The low-carbon steel for the mechanical structural pipe is low-carbon structural steel of the Swedish SS steel material standard. The low-carbon steel grade for the mechanical structural pipe has wide application range in foreign countries, and can be used as the mechanical structural pipe or steel structural members such as welding, riveting, bolting and the like. The components can be adjusted according to different purposes, and the low-carbon steel for the mechanical structural tube can be processed into special steel such as rivet screw steel, bridge steel, pressure vessel steel, ship steel, boiler steel and the like. The low-carbon steel for the mechanical structural pipe is usually smelted by adopting a converter, an electric furnace or an open hearth furnace and other processes as a primary smelting furnace and is hot-rolled into bars, steel plates, steel strips or profiles. At present, only individual steel enterprises in China can produce low-carbon steel grades for mechanical structure pipes, and the low-carbon steel grades are limited in use amount in China due to the influence of factors such as small yield, high application specialty and the like.
Disclosure of Invention
The invention aims to provide low-carbon steel for a high-strength mechanical structural pipe; the invention also provides a smelting method of the low-carbon steel for the mechanical structure pipe.
In order to solve the technical problems, the invention comprises the following components in percentage by mass: 0.14 to 0.17 percent of C, 0.28 to 0.33 percent of Si, 1.35 to 1.45 percent of Mn, less than or equal to 0.012 percent of P, less than or equal to 0.003 percent of S, 0.020 to 0.040 percent of Al, 0.70 to 0.80 percent of Cr, 0.40 to 0.60 percent of W, 0.10 to 0.20 percent of Ni, 0.10 to 0.15 percent of Cu, 0.35 to 0.50 percent of Mo, 0.03 to 0.06 percent of Nb, 0.03 to 0.07 percent of V, 0.0015 to 0.0030 percent of Ca, and the balance of Fe and inevitable impurities.
Preferably, the components are as follows by mass percent: 0.14 to 0.16 percent of C, 0.28 to 0.33 percent of Si, 1.35 to 1.45 percent of Mn, less than or equal to 0.010 percent of P, less than or equal to 0.002 percent of S, 0.025 to 0.035 percent of Al, 0.70 to 0.80 percent of Cr, 0.40 to 0.42 percent of W, 0.11 to 0.13 percent of Ni, 0.11 to 0.13 percent of Cu, 0.35 to 0.37 percent of Mo, 0.03 to 0.05 percent of Nb, 0.03 to 0.05 percent of V, 0.0015 to 0.0025 percent of Ca, and the balance of Fe and inevitable impurities.
The method of the invention is as follows: (1) BOF converter smelting process: the molten iron is required to have P less than or equal to 0.110wt%, S less than or equal to 0.020wt%, Si 0.20-0.60 wt% and temperature 1300-1450 ℃; leaving 2-5 t of steel slag in the converter after the smelting of the previous furnace is finished; the smelting period of the converter is 40-45 min; tapping in a converter, wherein the tapping is performed at the temperature of 1585-1615 ℃, and the tapping is performed at the temperature of 0.04-0.06% for [ C ], not more than 0.007% for [ P ];
steel sand aluminum, low-carbon ferromanganese, low-carbon ferrochromium, silicon-manganese alloy, ferromolybdenum, ferrotungsten and lime are added in sequence in the tapping processRefining the composite slag, strictly prohibiting slag discharging in the tapping process, and feeding an aluminum wire after tapping; the refining composite slag comprises the following components: 45-55% of CaO and SiO2≤5.0％、Al2O3 35～45％、CaF2≤5.0％、MgO 3.0-8.0％；
(2) An LF refining procedure: the refining time is more than or equal to 70min, and the white slag retention time is more than or equal to 30 min; in the LF refining process, the consumption of silicon carbide is 3-5 kg/t of steel, and the consumption of aluminum particles is 0.3-0.7 kg/t of steel; adjusting the W content according to the component requirement after LF smelting for 30min, and adjusting the component content according to the component requirement in the refining process;
controlling range of LF slag system: 52-58 wt% of CaO and Al2O3 25～30wt％、SiO2 8～12wt％、MgO≤6wt％、(TFe+MnO)≤0.5wt％、R 5.5～6.5；
(3) VD vacuum treatment process: carrying out VD high vacuum degassing treatment at 67Pa or below for 12-15 min, and controlling the flow of argon in the VD process to be 150-500 NL/min; after the VD is broken, feeding a calcium wire according to 0.12-0.15 kg/t of steel, and adding a pre-melted steel cladding covering agent according to 1.0-1.5 kg/t; the soft blowing time is controlled to be 30-40 min, and the argon flow is less than or equal to 17L/min during soft blowing; controlling the superheat degree of a ladle of the continuous casting furnace to be 50-60 ℃;
(4) and (3) continuous casting process: the residual steel amount of the tundish during continuous casting and ladle transferring is more than or equal to 26t, the residual steel amount of the ladle after pouring is more than or equal to 5t, and the fluctuation range of the liquid level of the crystallizer is controlled to be +/-3 mm; controlling the continuous casting speed to be 0.80-0.85 m/min, controlling the crystallizer water quantity to be 2400-2700L/min and controlling the specific water quantity to be 0.19-0.21L/kg; electromagnetic stirring of a crystallizer and electromagnetic stirring of the tail end are adopted, wherein the electric stirring current of the crystallizer is 300-350A, the frequency is 2.0-2.5 HZ, the electric stirring current of the tail end is 300-320A, and the frequency is 7.0-8.0 HZ; controlling the overheating of a continuous casting tundish at 20-30 ℃, and controlling the cutting quantity of a head billet of a continuous casting secondary casting blank to be more than or equal to 1500mm and the cutting quantity of a tail billet to be more than or equal to 3000 mm; and (3) cooling the casting blank at the lower line, wherein the lower line temperature is more than or equal to 600 ℃, and the unstacking temperature is less than or equal to 100 ℃.
The BOF converter smelting process of the method comprises the following steps: performing slag splashing protection by blowing nitrogen from the bottom, wherein the slag splashing time is 3-5 min, and the nitrogen flow is 24000-27000 m during slag splashing3H; the converter molten iron scrap steel proportion is: the iron water amount is 80-85%, and the scrap steel amount is 15-20%; 40-50 kg/t steel lime, 15-25 kg/t light-burned dolomite and 35-E limestone are added in the smelting process of the converter45kg/t steel, oxygen consumption 45-55 m3T, oxygen supply time is 14-20 min; the consumption of nitrogen is 20-35 m3T, the converter gas can be recovered by 100-150 m3/t。
The BOF converter smelting process of the method comprises the following steps: adding 7-8 kg/t of steel by lime and 8-9 kg/t of refined composite slag in the tapping process, and strictly prohibiting slag discharging in the tapping process; and feeding an aluminum wire according to the steel weight of 0.3-0.5 kg/t after tapping.
The LF refining process of the method comprises the following steps: the consumption of lime in the refining process is 6-8 kg/t steel, and the consumption of the refined composite slag is 4-6 kg/t steel; the flow rate of argon in the LF process is controlled to be 300-800 NL/min, and the argon consumption is 0.3-0.5 m3/t。
The method of the invention comprises the following VD vacuum treatment procedures: the argon consumption in the VD vacuum treatment process is 0.05-0.08 m3And (t) consuming 20-25 kg/t of steel by steam.
Adopt the produced beneficial effect of above-mentioned technical scheme to lie in: the invention redesigns the components of the steel, adjusts the contents of C, Si, Mn, Cr, W, Ni, Cu, Mo, Nb, V and Ca, strictly controls the contents of harmful residual elements such as P, S and the like and gas components such as O, N, H and the like, and improves the strength, the hardness and the wear resistance of the steel, and the capability of resisting atmospheric corrosion and corrosion of hydrogen, nitrogen, ammonia and the like at high temperature; thereby ensuring that the produced low-carbon steel products meet the requirements of high-quality high-strength mechanical structural pipes. The invention has the characteristics of high strength, high hardness, good wear resistance, good corrosion resistance and the like.
The method adopts a converter-LF-VD-continuous casting process to produce the steel casting blank for the high-strength mechanical structural pipe, has simple process flow, can ensure that the finished product [ P ] is less than or equal to 0.012 percent by using the converter as a primary smelting furnace, can ensure that the finished product [ S ] is less than or equal to 0.003 percent by using an LF refining slag system to be stable, has high molten steel purity, lower gas content [ O ] is less than or equal to 10ppm, [ N ] is less than or equal to 60ppm and [ H ] is less than or equal to 1.2ppm, has stable continuous casting process, good casting blank quality at low power, and has the characteristics of high product strength, high hardness, good wear resistance, good corrosion resistance and the like.
Detailed Description
The present invention will be described in further detail with reference to specific embodiments.
Examples 1 to 12: the low-carbon steel for the mechanical structural pipe adopts a BOF (100t) -LF (120t) -VD (120t) -CC (arc radius R14m) process flow, and the process parameters of each process are as follows.
(1) BOF converter smelting process: the molten iron is required to have P less than or equal to 0.110wt%, S less than or equal to 0.020wt%, Si 0.20-0.60 wt% and temperature 1300-1450 ℃. 2-5 t of steel slag is left in the converter after the previous smelting is finished, slag splashing protection is carried out by blowing nitrogen at the bottom, the slag splashing time is 3-5 min, and the nitrogen flow is 24000-27000 m during slag splashing3H is used as the reference value. The converter molten iron scrap steel proportion is: the amount of the molten iron is 80-85%, and the amount of the scrap steel is 15-20%. Before smelting, converter slag near a converter cap needs to be cleaned, if splashing occurs in the blowing process, converter slag remained on the converter cap needs to be cleaned for the second time, and the fact that the converter cap is cleaned up and has no residues is guaranteed so that steel can be tapped. In the smelting process of the converter, 40-50 kg/t of steel lime, 15-25 kg/t of light-burned dolomite and 35-45 kg/t of steel of limestone are added, and the oxygen consumption is 45-55 m3T, and the oxygen supply time is 14-20 min. The smelting period of the converter is 40-45 min, and the consumption of nitrogen is 20-35 m3T; the converter gas can be recycled, namely the amount of the converter gas recycled in the smelting process is 100-150 m3T is calculated. Tapping required for converter tapping [ C ]]0.04-0.06 wt%, tapping [ P ]]Not more than 0.007 wt% and at the temperature of 1585-1615 ℃; the blowing end point of the converter realizes double hit of components and temperature, and molten steel peroxidation is avoided.
3.5-4 kg/t of steel grit aluminum, 10-13 kg/t of low-carbon ferromanganese, 10-13 kg/t of low-carbon ferrochrome, 5-7 kg/t of silicon-manganese alloy, 5-6 kg/t of ferromolybdenum, 5-6 kg/t of ferrotungsten, 7-8 kg/t of lime steel and 8-9 kg/t of refined composite slag are sequentially added in the tapping process; strictly forbidding slag discharging in the tapping process, and feeding an aluminum wire according to the steel weight of 0.3-0.5 kg/t after tapping; adjusting Cu, Ni, V and Nb in the subsequent LF refining process to ensure that the Al content in the LF in-place steel is 0.020-0.040%; the oxygen content of the molten steel is low, and good conditions are provided for the subsequent LF deoxidation and desulfurization. The lime is high-quality refined lime, and the composition and performance indexes of the lime are (wt): CaO is more than or equal to 90 percent, MgO is less than or equal to 5.0 percent, and SiO2Less than or equal to 2.0 percent, less than or equal to 0.030 percent of S, less than or equal to 4 percent of ignition loss, more than or equal to 320 percent of activity and more than or equal to 90 percent of granularity of 10-50 mm. The refining composite slag comprises the following components in percentage by weight: 45-55% of CaO and SiO2≤5.0％，Al2O3 35～45％，CaF2≤5.0％，MgO 3.0～8.0％。
The converter process can ensure that the finished product [ P ] is less than or equal to 0.012 wt% and [ S ] is less than or equal to 0.003 wt%, and the converter deoxidizes and alloys after tapping, preliminarily adjusts the components of molten steel, has lower oxygen content of the molten steel, and provides good conditions for LF deoxidation and desulfurization.
The specific process parameters of each embodiment are shown in tables 1-3, and the main component indexes of the refined composite slag are shown in table 4.
Table 1: BOF converter smelting process parameters of each example
Figure GDA0003200217070000041
Table 2: BOF converter smelting process parameters of each example
Figure GDA0003200217070000042
Figure GDA0003200217070000051
Table 3: BOF converter smelting process parameters of each example
Figure GDA0003200217070000052
Figure GDA0003200217070000061
Table 4: main component index (wt%) of the refined composite slag in each example
Examples CaO SiO2 Al2O3 CaF2 MgO
1 50.0 3.8 37.4 4.3 5.5
2 47.2 4.4 41.3 4.8 6.7
3 49.3 4.7 40.5 4.4 4.2
4 53.4 3.2 38.4 4.5 3.8
5 52.7 3.3 40.0 3.6 7.4
6 50.4 4.0 39.2 4.1 6.0
7 50.5 4.3 43.7 5.0 5.2
8 45.0 4.5 42.5 3.9 5.0
9 46.7 3.0 45.0 3.2 6.4
10 55.0 4.6 36.2 4.0 3.0
11 45.6 5.0 35.6 3.7 4.8
12 54.2 4.8 35.0 3.0 8.0
(2) An LF refining procedure: after LF refining is in place, small-gear and low-current power is supplied for electric slagging, double air bricks are used for a steel ladle, lime and refined composite slag are added according to the melting condition and the fluidity of refined slag, the using amount of the lime is 6-8 kg/t steel and the using amount of the refined composite slag is 4-6 kg/t steel in the refining process, the refining time is more than or equal to 70min, and the holding time of the white slag is more than or equal to 30 min; the indexes of the main components of the refining composite slag are shown in Table 4. In the LF refining process, the consumption of silicon carbide is 3-5 kg/t of steel, the consumption of aluminum particles is 0.3-0.7 kg/t of steel, diffusion deoxidation is carried out, and a good deoxidation effect is ensured. The flow rate of argon in the LF process is controlled to be 300-800 NL/min, and the argon consumption is 0.3-0.5 m3And t, ensuring good air permeability and stirring effect, and promoting the progress of metallurgical reactions such as deoxidation and desulfurization, component adjustment, uniform temperature, inclusion floating and the like. And adjusting the W content according to the internal control components after LF smelting for 30min, and accurately adjusting the contents of Cu, Ni, V, Nb and other components according to the internal control components in the refining process to ensure that all components of the molten steel meet the internal control requirement.
Controlling range of LF slag system: 52-58% of CaO and Al2O3 25～30％、SiO28-12%, MgO is less than or equal to 6%, TFe + MnO is less than or equal to 0.5%, and R is 5.5-6.5%. The LF slag system has good quick desulfurization effect, has good adsorbability on floating impurities, can ensure good purity of molten steel, and meets the production requirements of steel for mechanical structure pipes.
The specific process parameters of each example are shown in table 5, and the main components of the LF slag system are shown in table 6.
Table 5: LF refining Process parameters of the examples
Figure GDA0003200217070000071
Table 6: examples LF slag series of the examples was based on the main component (wt%)
Figure GDA0003200217070000072
Figure GDA0003200217070000081
(3) VD vacuum treatment process: VD high vacuum degassing treatment is carried out for 12-15 min at 67Pa or below; the flow rate of argon in the VD process is controlled to be 150-500 NL/min, and the argon consumption is 0.05-0.08 m3And/t, steam consumption is 20-25 kg/t steel. After the VD is broken, feeding a calcium wire according to 0.12-0.15 kg/t of steel, and adding a pre-melted steel cladding covering agent according to 1.0-1.5 kg/t; the pre-melting ladle covering agent comprises the following components in percentage by weight: 9-17% of CaO and SiO2 29～39％、Al2O3 6-14％、MgO≤6.0％、Fe2O3Less than or equal to 5.0 percent and 20 to 35 percent of C solid existing in a solid form but not in a compound form; the soft blowing time is controlled to be 30-40 min, the argon flow is less than or equal to 17L/min during soft blowing, impurities are promoted to fully float, and the superheat degree of a ladle of the continuous casting furnace is controlled to be 50-60 ℃.
The specific process parameters for each example are shown in Table 7.
Table 7: VD Process parameters for the examples
Figure GDA0003200217070000082
(4) And (3) continuous casting process: and protective pouring is carried out in the continuous casting process, the steel ladle is forbidden to slag, the steel ladle is fully poured, the residual steel amount of the steel ladle during continuous casting and ladle transferring is not less than 26t, the residual steel amount of the steel ladle at the end of pouring is not less than 5t, and the fluctuation range of the liquid level of the crystallizer is controlled within +/-3 mm. The continuous casting drawing speed is controlled to be 0.80-0.85 m/min, the water quantity of a crystallizer is 2400-2700L/min, the specific water quantity is 0.19-0.21L/kg, electromagnetic stirring and tail end electromagnetic stirring of the crystallizer are adopted, the electric stirring current of the crystallizer is 300-350A, the frequency is 2.0-2.5 HZ, the electric stirring current of the tail end is 300-320A, the frequency is 7.0-8.0 HZ, the overheating of a continuous casting tundish is controlled to be 20-30 ℃, the cutting quantity of a head billet of a continuous casting secondary casting billet is not less than 1500mm, and the cutting quantity of a tail billet is not less than 3000 mm; using a low-carbon tundish covering agent and peritectic steel crystallizer covering slag, wherein the main component requirements of the low-carbon tundish covering agent are (wt): 5-15% of C, 24-40% of CaO, and SiO2 22～38％、Al2O3≤10％、MgO≤10％、Fe2O3Less than or equal to 5.0 percent; the peritectic steel crystallizer casting powder comprises the following main components in percentage by weight: c13 +/-4.0%, CaO 26.8 +/-5.0%, and SiO2 25.5±5.0％、Al2O3 10.2±4.0％、MgO≤8.0％、Fe2O3Less than or equal to 7.0 percent. And (3) cooling the casting blank at the lower line, wherein the lower line temperature is more than or equal to 600 ℃, and the unstacking temperature is less than or equal to 100 ℃. The liquid level of the molten steel in the continuous casting process is stable, slag entrapment is avoided, the casting blank is low-power and has good surface quality, normal pouring is protected, the residual steel amount in the ladle is strictly controlled, and slag falling does not occur.
The specific process parameters for each example are shown in tables 8 and 9.
Table 8: continuous casting process parameters of each example
Figure GDA0003200217070000091
Table 9: continuous casting process parameters of each example
Figure GDA0003200217070000092
Figure GDA0003200217070000101
(5) The chemical compositions of the obtained low-carbon steel are shown in Table 10, and the balance is Fe and inevitable impurities; the gas conditions in the continuous casting slabs are shown in Table 11; the mechanical properties and the macroscopic scale are shown in Table 12, wherein the inspection standard of the central porosity is shown in YB/T4149-2018.
Table 10: composition of continuous casting slab of each example (wt%)
Figure GDA0003200217070000102
Figure GDA0003200217070000111
Table 11: gas content of continuous casting slabs of examples
Content (ppm) O N H
Example 1 9.8 50.3 0.7
Example 2 8.6 51.2 0.9
Example 3 9.1 49.3 1.0
Example 4 8.9 52.0 0.8
Example 5 8.2 54.1 0.9
Example 6 8.5 48.9 0.8
Example 7 9.0 50.6 1.1
Example 8 8.8 51.5 0.9
Example 9 8.5 51.2 0.9
Example 10 9.0 49.5 0.7
Example 11 8.9 52.1 0.8
Example 12 8.6 54.5 0.9
Table 12: mechanical properties and low power grade of continuous casting slabs of each example
Figure GDA0003200217070000112
Figure GDA0003200217070000121

Claims (5)

1. A smelting method of low-carbon steel for a mechanical structure pipe is characterized by comprising the following components in percentage by mass: 0.14 to 0.17 percent of C, 0.28 to 0.33 percent of Si, 1.35 to 1.45 percent of Mn, less than or equal to 0.012 percent of P, less than or equal to 0.003 percent of S, 0.020 to 0.040 percent of Al, 0.70 to 0.80 percent of Cr, 0.40 to 0.60 percent of W, 0.10 to 0.20 percent of Ni, 0.10 to 0.15 percent of Cu, 0.35 to 0.50 percent of Mo, 0.03 to 0.06 percent of Nb, 0.03 to 0.07 percent of V, 0.0015 to 0.0030 percent of Ca, and the balance of Fe and inevitable impurities; the method comprises the following steps: (1) BOF converter smelting process: the molten iron is required to have P less than or equal to 0.110wt%, S less than or equal to 0.020wt%, Si 0.20-0.60 wt% and temperature 1300-1450 ℃; leaving 2-5 t of steel slag in the converter after the smelting of the previous furnace is finished; the smelting period of the converter is 40-45 min; tapping in a converter, wherein the tapping is performed at the temperature of 1585-1615 ℃, and the tapping is performed at the temperature of 0.04-0.06% for [ C ], not more than 0.007% for [ P ];
steel grit aluminum, low-carbon ferromanganese, low-carbon ferrochrome, silicon-manganese alloy, ferromolybdenum, ferrotungsten, lime and refined composite slag are sequentially added in the tapping process, slag is strictly forbidden in the tapping process, and an aluminum wire is fed after tapping; the refining composite slag comprises the following components: 45-55% of CaO and SiO2≤5.0%、Al2O3 35～45%、CaF2≤5.0%、MgO 3.0-8.0%；
(2) An LF refining procedure: the refining time is more than or equal to 70min, and the white slag retention time is more than or equal to 30 min; in the LF refining process, the consumption of silicon carbide is 3-5 kg/t of steel, and the consumption of aluminum particles is 0.3-0.7 kg/t of steel; adjusting the W content according to the component requirement after LF smelting for 30min, and adjusting the component content according to the component requirement in the refining process;
controlling range of LF slag system: 52-58 wt% of CaO and Al2O3 25～30wt%、SiO2 8～12wt%、MgO≤6wt%、(TFe+MnO)≤0.5wt%、R 5.5～6.5；
(3) VD vacuum treatment process: carrying out VD high vacuum degassing treatment at 67Pa or below for 12-15 min, and controlling the flow of argon in the VD process to be 150-500 NL/min; after the VD is broken, feeding a calcium wire according to 0.12-0.15 kg/t of steel, and adding a pre-melted steel cladding covering agent according to 1.0-1.5 kg/t; the soft blowing time is controlled to be 30-40 min, and the argon flow is less than or equal to 17L/min during soft blowing; controlling the superheat degree of a ladle of the continuous casting furnace to be 50-60 ℃;
(4) and (3) continuous casting process: the residual steel amount of the tundish during continuous casting and ladle transferring is more than or equal to 26t, the residual steel amount of the ladle after pouring is more than or equal to 5t, and the fluctuation range of the liquid level of the crystallizer is controlled to be +/-3 mm; controlling the continuous casting speed to be 0.80-0.85 m/min, controlling the crystallizer water quantity to be 2400-2700L/min and controlling the specific water quantity to be 0.19-0.21L/kg; electromagnetic stirring of a crystallizer and electromagnetic stirring of the tail end are adopted, wherein the electric stirring current of the crystallizer is 300-350A, the frequency is 2.0-2.5 Hz, and the electric stirring current of the tail end is 300-320A, and the frequency is 7.0-8.0 Hz; controlling the overheating of a continuous casting tundish at 20-30 ℃, and controlling the cutting quantity of a head billet of a continuous casting secondary casting blank to be more than or equal to 1500mm and the cutting quantity of a tail billet to be more than or equal to 3000 mm; and (3) cooling the casting blank at the lower line, wherein the lower line temperature is more than or equal to 600 ℃, and the unstacking temperature is less than or equal to 100 ℃.
2. A method of smelting a low carbon steel for a machine structural pipe according to claim 1, wherein the BOF converter smelting step comprises: performing slag splashing protection by blowing nitrogen from the bottom, wherein the slag splashing time is 3-5 min, and the nitrogen flow is 24000-27000 m during slag splashing3H; the converter molten iron scrap steel proportion is: the iron water amount is 80-85%, and the scrap steel amount is 15-20%; in the smelting process of the converter, 40-50 kg/t of steel lime, 15-25 kg/t of light-burned dolomite and 35-45 kg/t of steel of limestone are added, and the oxygen consumption is 45-55 m3T, oxygen supply time is 14-20 min; the consumption of nitrogen is 20-35 m3T, the converter gas can be recovered by 100-150 m3/t。
3. The method of smelting a low carbon steel for a machine structural pipe according to claim 2, wherein the BOF converter smelting step: adding 7-8 kg/t of steel by lime and 8-9 kg/t of refined composite slag in the tapping process, and strictly prohibiting slag discharging in the tapping process; and feeding an aluminum wire according to the steel weight of 0.3-0.5 kg/t after tapping.
4. A method of smelting a low carbon steel for a machine structural pipe according to claim 3, wherein the LF refining step: the consumption of lime in the refining process is 6-8 kg/t steel, and the consumption of the refined composite slag is 4-6 kg/t steel; the flow rate of argon in the LF process is controlled to be 300-800 NL/min, and the argon consumption is 0.3-0.5 m3/t。
5. A method of smelting a low carbon steel for a machine structural pipe according to any one of claims 1 to 4, wherein the VD vacuum treatment step: the argon consumption in the VD vacuum treatment process is 0.05-0.08 m3And (t) consuming 20-25 kg/t of steel by steam.
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