Method for controlling cleanliness of molten steel in whole process

Abstract

The invention belongs to the technical field of steelmaking, and particularly relates to a method for controlling the cleanliness of molten steel in a whole process, which carries out whole-process control on the aspects of metallurgical raw material use, converter end point control, LF refining white slag, LF and VD refining staged variable flow bottom argon blowing, continuous casting anti-oxidation protective pouring and the like. By controlling the key steps, the purposes that the total oxygen content in the bearing steel is reduced to 5 ppm or below, the oxygen content in the gear steel is stably controlled to 10 ppm or below, and the defect rate of inclusions in the steel is reduced to 0.5% from 2.0% are achieved, and stable control over the inclusions is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of steelmaking, and in particular relates to a method for controlling the cleanliness of molten steel throughout the entire process. Background Art

[0002] With the development of economy and the improvement of living standards, people's requirements for steel quality are getting higher and higher. How to smelt higher purity clean steel has become the focus of research by technicians in the metallurgical industry. All enterprises have invested a lot of resources in operation and process optimization to continuously improve the cleanliness of steel. The main factors affecting the cleanliness of molten steel are endogenous inclusions and exogenous inclusions. Exogenous inclusions mainly include auxiliary materials and refractory materials used in metallurgy. These foreign impurities are generally large in size and can greatly affect the performance and quality of steel. The source of foreign inclusions is mainly residual harmful elements in refractory materials and metallurgical raw materials. Endogenous inclusions are mainly residual elements, gases, and non-metallic inclusions in molten steel. Therefore, it is very necessary to control the factors affecting the cleanliness of high-quality steel throughout the process to improve the cleanliness of molten steel, reduce the content of gas and residual elements in steel, and reduce the level of non-metallic inclusions. Summary of the invention

[0003] The object of the present invention is to provide a method for controlling the cleanliness of molten steel throughout the entire process. The present invention controls the entire process from the raw material control stage, the converter and electric furnace smelting stage, the refining furnace smelting stage, the VD or RH vacuum treatment stage and the continuous casting stage. By controlling each key step, the cleanliness of the molten steel is improved, and the gas and residual element content and the level of non-metallic inclusions in the steel are effectively reduced.

[0004] The technical problem to be solved by the present invention is achieved by adopting the following technical solution: A method for controlling the cleanliness of molten steel in the whole process, comprising the following steps:

[0005] S1. Raw material control

[0006] Calculated by weight percentage, molten iron ≥ 80%, high-quality scrap steel 10-20%; low-titanium high-carbon ferrochrome, low-titanium carburizer, low-titanium basic refining slag and high-purity silicon carbide are used to reduce the Ti content in the steel;

[0007] Calculated by weight percentage, in the dry material of the tundish wall: SiO2≤5%, MgO≥90%, TiO2≤0.2%, to ensure the quality of the tundish refractory material and avoid inclusions caused by the tundish refractory material falling off;

[0008] S2, converter or electric furnace smelting

[0009] Steel tapping conditions: The tapping elements C, Si, Mn, P, S, Cr, Ni, Mo, Cu, Ti, Ca, and Alt meet the requirements. The tapping temperature is 145-175°C higher than the liquidus temperature to ensure the temperature at the refining position and avoid insufficient deoxidation caused by long-term temperature increase during the refining process.

[0010] Automatic slag detection and pre-deoxidation of molten steel are performed during the steel tapping and alloying process to reduce the initial oxygen content of the molten steel; automatic slag detection is the use of automatic slag detection technology during the converter tapping process to prevent the converter slag from entering the ladle and causing the molten steel to rephosphorize, or affecting the slag composition during the refining process;

[0011] S3, refining furnace smelting

[0012] The temperature is controlled to be 15-35℃ higher than the liquidus temperature to ensure the refining effect and the removal effect of inclusions;

[0013] The white slag retention time is greater than 20 minutes to fully absorb the inclusions in the steel; calculated by weight percentage, the final refined slag contains: FeO: ≤0.5%; CaO: 49%~54%; SiO2: 6.5%~8.5%; Al2O3: 28%~35%; MgO: 4.5%~6.5%; the slag ratio and addition amount basically confirm the final slag composition, and the control of the slag system composition can effectively ensure that the inclusions float up and are absorbed by the slag;

[0014] During the power supply process, sampling and before steel tapping, argon blowing is performed in stages to ensure that inclusions fully float up; stirring during the power supply process can promote the circulation of molten steel while ensuring that inclusions float up and that the temperature of slag and steel is transferred; sampling and stirring, and sampling and temperature measurement are more representative, ensuring process temperature control and promoting the removal of inclusions; soft blowing before steel tapping promotes the floating of inclusions and avoids pollution caused by mixing steel and slag;

[0015] S4, VD or RH vacuum treatment

[0016] VD vacuum degassing: vacuum degree is less than 67Pa and maintained for ≥15min, stirring with a small amount of argon after breaking the air, and soft blowing of argon for ≥20min;

[0017] RH vacuum degassing: vacuum pure treatment time ≥ 12 minutes, vacuum degree less than 67Pa and maintained for ≥ 12 minutes; stirring with a small amount of argon after breaking the air, soft argon blowing time ≥ 10 minutes;

[0018] By controlling the VD vacuum degassing and RH vacuum degassing parameters, H and oxygen in molten steel can be effectively removed;

[0019] S5, continuous casting

[0020] Use double-layer tundish covering agent for pouring: low titanium alkaline tundish covering agent is used for the lower layer, and low carbon covering agent is used for the upper layer; the tundish adopts double-layer tundish covering agent, which plays the following roles: 1) heat insulation to prevent heat dissipation; 2) absorb floating inclusions; 3) isolate air to prevent oxygen in the air from entering the molten steel; the lower layer uses low titanium alkaline tundish covering agent, and the upper layer uses low carbon covering agent, which ensures that the covering agent in contact with the molten steel does not pollute the molten steel while keeping the heat;

[0021] Carry out full protection pouring.

[0022] Preferably, in step S1, in the low-titanium high-carbon ferrochrome, Ti≤0.030% by weight;

[0023] In low titanium carburizer, TiO2≤200ppm;

[0024] In low titanium basic refining slag, TiO2≤0.030%;

[0025] The purity of high-purity silicon carbide is above 95%.

[0026] Preferably, in step S2, the pre-deoxidation of molten steel is specifically as follows: during the steel tapping process, 1.0-2.0 kg / t of aluminum is added to the ladle for pre-deoxidation, the steel tapping time is ≥3.5 min, 0.8-2.0 kg / t of aluminum block + aluminum ingot is used, and during the alloying of the ladle, the alloy is added according to the determined lower limit of the composition to leave a certain surplus for the refining process, 10-14 kg / t of slag material is added, and the slag material ratio and addition amount, and the slag system composition are controlled to ensure that the inclusions float up and are adsorbed by the slag.

[0027] Preferably, in step S3 of the present invention, the segmented argon blowing is specifically as follows: 40-150 NL / min during the power supply process, 600-1000 NL / min for sampling, and 10-30 NL / min before steel tapping.

[0028] Preferably, in step S4, the small amount of argon gas is 5-20 NL / min.

[0029] Preferably, in step S5 of the present invention, the full protection pouring is specifically as follows: before pouring, the pre-argon blowing operation is started, the air is expelled from the tundish by filling the tundish with argon gas, so that an oxygen-deficient space is formed in the tundish molten pool; during the production process, argon is blown throughout the tundish;

[0030] The long shroud seal of the large bag and the cover seal of the middle bag are adopted;

[0031] An annular argon gas pipeline is installed at the flush position of the upper edge of the impact zone, and a hole is opened inside the pipeline to ensure that the outflowing argon gas forms a protective atmosphere at the steel liquid surface in the impact zone and isolates the air.

[0032] Preferably, the sealing gasket of the long shroud of the large ladle is an inverted conical cylindrical structure matching the inner wall of the long shroud of the large ladle, which plays a sealing role.

[0033] Preferably, in step S1 of the present invention, the high-quality scrap steel is self-circulating billet head and tail scrap steel.

[0034] Preferably, in step S5, the TiO2 in the low titanium alkaline coating agent is ≤ 0.8% by weight;

[0035] In low carbon covering agent, TC≤3%.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention adopts precise raw material control means to control residual elements such as Ti in the raw material control stage; automatic slag detection and molten steel pre-deoxidation means are adopted in the converter or electric furnace smelting stage to reduce the initial oxygen content of molten steel; the present invention adopts segmented argon control means in the refining furnace smelting stage to achieve full floating of inclusions; vacuum treatment maintenance and soft blowing flow are adopted in the VD or RH vacuum treatment stage to achieve full floating of inclusions; and pre-argon blowing, protective casting and other means are adopted in the continuous casting stage to control the air intake of molten steel.

[0037] The present invention controls the whole process from the use of metallurgical raw materials, converter endpoint control, LF refining white slag, LF and VD refining staged variable flow bottom argon blowing, continuous casting anti-oxidation protection pouring, etc. Through the control of each key step, the total oxygen content in bearing steel is reduced to below 5ppm, the oxygen content in gear steel is stably controlled to below 10ppm, and the inclusion defect rate in steel is reduced from 2.0% to 0.5%. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is the metallographic diagram of the steel grade of furnace No. 1 in the embodiment;

[0039] Figure 2 This is the metallographic diagram of the No. 1 furnace steel in the embodiment;

[0040] Figure 3 This is the metallographic diagram of the steel grade of furnace No. 2 in the embodiment;

[0041] Figure 4 This is the metallographic diagram of the steel grade of furnace No. 2 in the embodiment;

[0042] Figure 5 This is the metallographic diagram of the No. 3 furnace steel in the embodiment;

[0043] Figure 6 This is the metallographic diagram of the No. 3 furnace steel in the embodiment;

[0044] Figure 7 This is the metallographic diagram of the No. 4 furnace steel in the embodiment;

[0045] Figure 8 This is the metallographic diagram of the No. 4 furnace steel in the embodiment;

[0046] Fig. 9 This is a diagram of the inclusion control situation of each heat in the embodiment;

[0047] Fig.10 This is a schematic diagram of the structure of the long shroud sealing gasket for the large package. DETAILED DESCRIPTION

[0048] The present invention will be further described below in conjunction with the accompanying drawings and specific implementation examples, but the present invention is not limited to the scope of the description of the implementation examples.

[0049] The present invention takes GCr15 as an example to describe the implementation process of the scheme of the present invention:

[0050] 1. Process equipment and procedures

[0051] Batching (hot charging of more than 80% molten iron) → 120t converter or 120t electric furnace → 120tLF refining → 120tVD or 120tRH vacuum degassing treatment → R16.5m five-machine five-strand round billet continuous casting machine (Ф500 / 650 / 700 / 800mm) or 6-machine six-strand continuous casting machine (320*420 / 240*375mm) → slow cooling in billet warehouse → rolling → storage.

[0052] 2. GCr15 chemical composition

[0053] Internal control chemical composition standard requirements (%)

[0054]

[0055] Residual element control requirements (%)

[0056]

[0057] 3. Raw material control

[0058] 3.1 Requirements for molten iron and scrap steel entering the furnace: Calculated by weight percentage, molten iron ≥ 80%, high-quality scrap steel 10-20%.

[0059] 3.2 Requirements for raw materials and auxiliary materials (calculated by weight percentage):

[0060] (1) Low titanium and high carbon ferrochrome, Ti≤0.030%.

[0061] (2) Low titanium carburizer C ≥ 98%; S ≤ 0.1%; N ≤ 0.02%; TiO2 ≤ 200ppm; H2O ≤ 0.5%; particle size: 0.5 ~ 5mm ≥ 90%.

[0062] (3) Low titanium basic refining slag: 44% ≤ CaO ≤ 54%; 28% ≤ Al2O3 ≤ 36%; 3% ≤ MgO ≤ 7%; SiO2 ≤ 6%; TiO2 ≤ 0.03%; Fe2O3 ≤ 2%; H2O ≤ 0.5%; particle size: 5-40 mm, ≤ 5% of those < 5 mm.

[0063] (4) High-purity silicon carbide 65% ≤ TSi ≤ 75%; 20% ≤ TC ≤ 32%; TC + TSi ≥ 95%; H2O ≤ 0.5%; particle size ≤ 10 mm, 8 mm < and ≤ 10 mm ≤ 10%.

[0064] (5) The middle package covering agent is 60%≤SiO2≤75%; 15%≤CaO≤25%; Al2O3≤5%; MgO≤5%; TiO2≤0.25%; Fe2O3≤1.5%; TC≤3%; H2O≤0.7%; particle size 0~2mm.

[0065] (6) The low titanium alkaline covering agent in the middle package is 30%≤Al2O3≤40%; SiO2≤6%; 45%≤CaO≤55%; MgO≤8%; Fe2O3≤1.5%; TiO2≤0.8%; H2O≤0.5%; particle size 0-5mm, melting point 1350-1450℃.

[0066] (7) The dry material of the intermediate ladle wall has SiO2≤5%, MgO≥90%, and TiO2≤0.2%.

[0067] 4. Converter smelting

[0068] 4.1 Ladle: Ladle baking temperature ≥ 800℃, do not use new ladle, new slag line ladle and repaired ladle, the previous casting of the ladle is strictly prohibited to produce Ti-containing steel.

[0069] 4.2 Steel tapping conditions: Steel tapping carbon 0.15% ≤ C ≤ 0.30%, Steel tapping Si ≤ 0.15%, Steel tapping Mn ≤ 0.25%, Steel tapping P ≤ 0.015%, End point [Ni] ≤ 0.20%, End point [Mo] ≤ 0.10%, End point [Cu] ≤ 0.20%, End point [Pb] ≤ 0.002%, End point [As] ≤ 0.025%, End point [Sn] ≤ 0.02%, End point [Sb] ≤ 0.007%, End point [As+Sn+Sb] ≤ 0.075%. Steel tapping temperature 1600~1630℃.

[0070] 4.3 Steelmaking alloying process: During the steelmaking process, the amount of aluminum added in the ladle for pre-deoxidation is 1.0-2.0 kg / t steel, the steelmaking time is ≥3.5 min, and the aluminum block + aluminum ingot is 0.8-2.0 kg / t steel. The composition requirements after steelmaking are: [C] composition 0.80%-0.95%, [Al] composition 0.04%-0.05%, [Mn] composition 0.23-0.25%, [Cr] composition 1.36-1.40%, and the slag-making material is added at about 10-14 kg / t. The automatic slag detection technology is used in the steelmaking process.

[0071] 5. Refining furnace smelting

[0072] 5.1 Arrival temperature: (1480±10)℃.

[0073] 5.2 The retention time of white slag is greater than 20 minutes. Calculated by weight percentage, in the final slag of refining, FeO: ≤0.5%; CaO: 49% to 54%; SiO2: 6.5% to 8.5%; Al2O3: 28% to 35%; MgO: 4.5% to 6.5%.

[0074] 5.3 Argon blowing: 40-150NL / min during power supply, 600-1000NL / min for sampling, 10-30NL / min before steel tapping.

[0075] 6.VD or RH vacuum treatment

[0076] VD or RH vacuum degassing: vacuum degree is less than 67Pa and maintained for ≥15min. Stir with a small amount of argon after breaking the air. Soft argon blowing time is ≥20min, and the small amount of argon is 5-20NL / min.

[0077] 7. Continuous casting

[0078] 7.1 Use a double-layer tundish covering agent: use a low-titanium alkaline tundish covering agent for the lower layer and a low-carbon covering agent for the upper layer. No red should be visible on the slag surface.

[0079] 7.2 Protective pouring: Start the argon blowing operation 10 minutes before pouring. Fill the tundish with argon to expel the air, so that an oxygen-deficient space is formed in the tundish molten pool. Argon is blown throughout the production process.

[0080] 7.3 Use sealing gasket for long water inlet of large package, sealing of cover for middle package, argon sealing flow of long water inlet (50~150)L / min, target 100L / min, pressure (0.3-0.8)MPa, target 0.5MPa.

[0081] like Fig.10 As shown, the sealing gasket of the large ladle long shroud is an inverted conical cylindrical structure matching the inner wall of the large ladle long shroud, and a limiting flange is arranged on the upper end.

[0082] 7.4 Install an annular argon pipeline at the level position of the upper edge of the impact zone, and open a hole inside the pipeline to ensure that the outflowing argon gas forms a protective atmosphere at the steel liquid surface in the impact zone and isolates the air.

[0083] 7.5 The crystallizer liquid level fluctuation is ≤±3mm, and the distance between the slag liquid level and the upper edge of the crystallizer (water gap) is 80~120mm.

[0084] 7.6 Use an integral nozzle, and the depth of the immersion nozzle inserted into the molten steel is 80 to 120 mm.

[0085] 8. Steel quality

[0086] Table 1 shows the chemical composition of the GCr15 bearing steel prepared in this embodiment, and Table 2 shows the performance test results of the GCr15 bearing steel in the embodiment. Fig. 9 This is the inclusion detection result of GCr15 bearing steel in Example 1. The determination method is a conventional method in the art.

[0087] 8.1 Chemical composition and gases

[0088] Table 1 Chemical composition of GCr15 bearing steel (wt%, Fe balance)

[0089]

[0090]

[0091] The five harmful elements are: lead (Pb), tin (Sn), arsenic (As), antimony (Sb) and bismuth (Bi).

[0092] 8.2 Non-metallic inclusions

[0093] (1) Inspection of inclusions and microscopic pores in GCr15 bearing steel

[0094] Table 2

[0095]

[0096] (2) Statistical analysis of GCr15 bearing steel data

[0097] Statistical analysis has been conducted on 87 heats of data. The inclusion control status is as follows: Fig. 9As shown, A fine inclusions of 1.0 level account for 98.85%, and the maximum level is 1.5 level; A coarse inclusions of 0 level account for 86.21%, and the maximum level is 0.5 level. B fine inclusions of 0.5 level account for 95.40%, and the maximum level is 1.0 level; B coarse inclusions of 0 level account for 98.85%, and the maximum level is 0.5 level. C fine inclusions of 0 level account for 98.85%, and the maximum level is 1.0 level; C coarse inclusions of 0 level account for 100%. D fine inclusions of 0.5 level account for 96.55%, and the maximum level is 0.5 level; D coarse inclusions of 0 level account for 98.85%, and the maximum level is 0.5 level. Ds inclusions of 0 level account for 100%. Stable production control of the content of inclusions has been achieved.

Claims

1. A method for controlling the cleanliness of molten steel in the whole process, characterized in that: The following steps are involved: S1. Raw material control Calculated by weight percentage, molten iron ≥ 80%, high-quality scrap steel 10-20%; low-titanium high-carbon ferrochrome, low-titanium carburizer, low-titanium basic refined slag and high-purity silicon carbide; Calculated by weight percentage, in the dry material of the tundish wall: SiO2≤5%, MgO≥90%, TiO2≤0.2%; S2, converter or electric furnace smelting Steel tapping conditions: The elements of C, Si, Mn, P, S, Cr, Ni, Mo, Cu, Ti, Ca and Alt meet the requirements, and the tapping temperature is 145-175℃ higher than the liquidus temperature; Automatic slag detection and pre-deoxidation of molten steel are performed during the alloying process of steel tapping; S3, refining furnace smelting Control the temperature to be 15-35℃ higher than the liquidus temperature; White slag retention time is greater than 20 minutes; Calculated by weight percentage, in the final refined slag: FeO: ≤0.5%; CaO: 49% to 54%; SiO2: 6.5% to 8.5%; Al2O3: 28% to 35%; MgO: 4.5% to 6.5%; Argon blowing is carried out in stages during the power supply process, sampling and before steel tapping; S4, VD or RH vacuum treatment VD vacuum degassing: vacuum degree is less than 67Pa and maintained for ≥15min, stirring with a small amount of argon after breaking the air, and soft blowing of argon for ≥20min; RH vacuum degassing: vacuum pure treatment time ≥ 12 minutes, vacuum degree less than 67Pa and maintained for ≥ 12 minutes, stirring with a small amount of argon after breaking the air, soft argon blowing time ≥ 10 minutes; S5, continuous casting Use a double layer of tundish covering agent for casting: the lower layer uses a low titanium alkaline tundish covering agent, and the upper layer uses a low carbon covering agent; Carry out full protection pouring.

2. The method for controlling the cleanliness of molten steel in the whole process according to claim 1, characterized in that: In step S1, in the low-titanium high-carbon ferrochrome, Ti≤0.030% by weight; In low titanium carburizer, TiO2≤200ppm; In low titanium basic refining slag, TiO2≤0.030%; The purity of high-purity silicon carbide is above 95%.

3. The method for controlling the cleanliness of molten steel in the whole process according to claim 1, characterized in that: In step S2, the pre-deoxidation of molten steel is specifically as follows: during the steel tapping process, 1.0-2.0 kg / t of steel is added to the ladle for pre-deoxidation, the tapping time is ≥3.5 min, 0.8-2.0 kg / t of steel is used for aluminum blocks + aluminum ingots, alloys are added according to the determined lower limit of the composition during alloying of the ladle, and 10-14 kg / t of slag making material is added.

4. The method for controlling the cleanliness of molten steel in the whole process according to claim 1, characterized in that: In step S3, the specific requirements for the power supply process, sampling and segmented argon blowing before steel tapping are: 40-150NL / min during the power supply process, 600-1000NL / min for sampling, and 10-30NL / min before steel tapping.

5. The method for controlling the cleanliness of molten steel in the whole process according to claim 1, characterized in that: In step S4, the small amount of argon gas is 5-20 NL / min.

6. The method for controlling the cleanliness of molten steel in the whole process according to claim 1, characterized in that: In step S5, full protection pouring specifically includes: starting the pre-argon blowing operation before pouring, filling the tundish with argon to expel the air, so that the tundish molten pool forms an oxygen-deficient space; blowing argon in the tundish throughout the production process; The long shroud seal of the large bag and the cover seal of the middle bag are adopted; An annular argon gas pipeline is installed at the flush position of the upper edge of the impact zone, and a hole is opened inside the pipeline to ensure that the outflowing argon gas forms a protective atmosphere at the steel liquid surface in the impact zone and isolates the air.

7. The method for controlling the cleanliness of molten steel in the whole process according to claim 6, characterized in that: The sealing gasket of the large ladle long shroud is an inverted conical cylindrical structure matching the inner wall of the large ladle long shroud.

8. The method for controlling the cleanliness of molten steel in the whole process according to claim 1, characterized in that: In step S1, the high-quality scrap steel is the self-circulating billet head and tail scrap steel.

9. The method for controlling the cleanliness of molten steel in the whole process according to claim 1, characterized in that: In step S5, the TiO2 in the low titanium basic middle coating agent is ≤ 0.8% by weight; In low carbon covering agent, TC≤3%.

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