A smelting method for sulfur-containing gear steel

Abstract

A smelting method for sulfur-containing gear steel belongs to the field of metallurgy technology and includes top and bottom combined blowing converter primary refining, LF refining, RH vacuum refining, and billet continuous casting. No aluminum deoxidizer is added during the converter process; LF refining uses silicon carbide for deoxidation, and no aluminum deoxidation or reducing white slag desulfurization is required. Instead, the molten steel is simply heated to 1640-1650°C and a covering agent is added for insulation; RH refining utilizes deep vacuum carbon deoxidation technology. After deoxidation, aluminum particles are added for final deoxidation based on data measured by the molten steel oxygen determination equipment, controlling the AlS content in the exiting molten steel to 0.020-0.035%. After the molten steel has been in pure circulation for ≥5 minutes, titanium-iron alloy is added. After re-pressurization, no sulfur line is required and no calcium treatment is required. The present invention utilizes vacuum carbon deoxidation technology to reduce Al2O3 inclusions generated after deoxidation of aluminum products, eliminate the sulfur addition operation, and avoid CaS inclusions generated by calcium treatment in traditional processes, resulting in cleaner molten steel.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and in particular relates to a smelting method of sulfur-containing gear steel. Background Art

[0002] As competition in my country's steel industry intensifies, production costs are facing further pressure. Gear steel, however, is in high demand across a wide range of industries. Therefore, improving gear steel production processes and reducing the types and amounts of raw and auxiliary materials added during the production process can lower costs and achieve a low-carbon, environmentally friendly approach. This is crucial for both enhancing the competitiveness of steel companies and protecting the environment.

[0003] To meet the high machining precision requirements of workpieces, sulfur is added to gear steel to improve cutting performance and enhance machinability. At the same time, the amount of sulfur added must be strictly controlled to reduce inclusion content to maintain the required fatigue resistance of gear steel. The addition of aluminum as a deoxidizer and calcium as a modifier causes the formation of high-melting-point inclusions such as Al2O3 and CaS in the molten steel, reducing its purity and hindering product performance. Therefore, improving the smelting process and controlling these inclusions at the source to reduce the number of high-melting-point inclusions in the molten steel plays a crucial role in improving product quality and material properties. Summary of the Invention

[0004] The present invention provides a smelting method for sulfur-containing gear steel using carbon and silicon instead of aluminum deoxidizers under high vacuum, preserving the initial sulfur content in the molten steel and eliminating the need for sulfur addition or calcium treatment. This method reduces the types and quantities of raw and auxiliary materials added, saves costs, and cleans the molten steel. Compared to existing processes, aluminum is used for deoxidation and desulfurization in the LF furnace, while only partial inclusion removal is achieved through RH degassing. After atmospheric pressure is restored, sulfur addition and calcium treatment are performed on the molten steel. This method reduces the addition of raw and auxiliary materials during the smelting process, lowers production costs, reduces inclusion formation, and ultimately cleans the molten steel.

[0005] The present invention is based on the fact that under high vacuum conditions, deoxidation is achieved by the reaction of carbon and oxygen to generate carbon monoxide gas which is discharged with the vacuum pump. The equilibrium reaction equation is (1):

[0006] [C]+[O]=CO(g) (1)

[0007] [C]%·[O]%=ppCO / K=mppCO

[0008] The equilibrium constant K is a function of temperature. At 1600°C and ppCO=1 atm, the value of m(ap / K) is 0.0020~0.0025. The deeper the vacuum, the more the reaction equilibrium shifts to the right. Therefore, the deoxidation ability of carbon under deep vacuum conditions is very strong, even exceeding that of deoxidizing elements silicon, manganese, and is equivalent to aluminum.

[0009] Under high vacuum, the partial pressure of CO gas can be reduced, making the reaction forward and achieving the purpose of carbon deoxidation. Because the product CO is a gas and is discharged with the vacuum pump, the rule that carbon is the most ideal deoxidizer under vacuum conditions is utilized.

[0010] Carbon deoxidation can avoid excessive aluminum deoxidation and reduce the formation of Al2O3 inclusions, so there is no need to treat the steel with calcium. Excessive calcium in the molten steel is avoided because calcium has a greater affinity for sulfur than iron and manganese. When calcium is present in the molten steel, CaS is generated first. The reaction equation is (2):

[0011] [Ca] + [S] = (CaS) (2)

[0012] The melting point of CaS is 1723℃, and it appears as a solid inclusion in the molten steel.

[0013] In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is:

[0014] A smelting method for sulfur-containing gear steel, comprising top and bottom combined blowing converter primary refining, LF refining, RH vacuum refining and billet continuous casting;

[0015] (1) Top and bottom combined blowing converter primary smelting process: the sulfur content at the end of smelting is controlled at 0.025wt%≤S≤0.045wt%, and no slag washing material and aluminum product deoxidation are added during the tapping process;

[0016] (2) LF refining process: add 4-6 kg / t steel lime, add 0.3-0.5 kg / t steel silicon carbide for weak deoxidation, do not add aluminum deoxidizer, do not produce reducing white slag desulfurization; use submerged arc method to raise the temperature to between 1640 and 1650 ° C, and add covering agent for heat preservation;

[0017] (3) RH vacuum refining process: vacuum carbon deoxidation treatment is adopted. After carbon deoxidation is completed, aluminum particles are added for final deoxidation according to the oxygen content of the molten steel. After adding aluminum particles, the Als content in the molten steel is controlled in the range of 0.020-0.035% (the calculation basis of aluminum addition is: aluminum addition amount = deoxidized aluminum + alloyed aluminum); after the addition of aluminum particles, the molten steel is circulated for 5-8 minutes according to the amount of aluminum particles added. If the amount of aluminum particles added is large, the molten steel circulation time should be set as the upper limit, and then titanium-iron alloy is added; after the RH returns to atmospheric pressure, no sulfur wire is fed and no calcium treatment is performed.

[0018] In the RH vacuum refining process, the soft blowing volume is controlled to 5-10Nm 3 / h, time ≥15min.

[0019] The billet continuous casting process adopts a ladle long nozzle and an immersed nozzle with micro-positive pressure protection casting to perform oxidation-free casting throughout the entire process; the superheat of the ladle is 20-40°C, the constant casting speed is controlled at 1.2±0.1m / min, and the crystallizer is electromagnetically stirred.

[0020] The beneficial effects of the above-mentioned technical solution are as follows: By avoiding the addition of slag wash material and aluminum deoxidizer in the converter process, the present invention reduces the aluminum content in the molten steel, saving on slag and deoxidizer. In the LF refining process, lime usage is reduced by 7-9 kg / t of steel and the addition of aluminum deoxidizer is eliminated, reducing the cost of raw and auxiliary materials. In the RH vacuum refining process, carbon deoxidation and alloying are completed. After returning to atmospheric pressure, unlike conventional processes, no sulfur wire is added and no calcium treatment is performed, saving on sulfur wire and calcium alloy wire. Because sulfur and calcium feeding are not performed in the RH process, production time can be shortened by 7-10 minutes compared to conventional processes. Furthermore, the use of deep vacuum carbon deoxidation during the production process reduces the large amount of Al2O3 inclusions generated after aluminum deoxidation and the CaS inclusions generated by the further calcium treatment after the sulfur addition process, resulting in cleaner molten steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The morphology and energy spectrum of Al2O3 inclusions in the ingot produced by the original process;

[0022] Figure 2 The morphology and energy spectrum of Al2O3 inclusions in the ingot produced by the present invention;

[0023] Figure 3 The morphology and energy spectrum of Al2O3-SiO2-MnO-CaS composite inclusions in the ingot produced by the original process;

[0024] Figure 4 This is the morphology and energy spectrum of Al2O3-SiO2-MgO-CaS composite inclusions in the ingot produced by the present invention. DETAILED DESCRIPTION

[0025] The present invention will be further described in detail below with reference to specific embodiments.

[0026] The present invention adopts vacuum carbon deoxidation to control the sulfur content in molten steel, reduce the addition of raw and auxiliary materials in the converter, LF and RH processes, and reduce production costs.

[0027] Based on the above concept, the smelting method of the sulfur-containing gear steel of the present invention includes top-bottom combined-blowing converter primary refining, LF refining, RH vacuum refining, and billet continuous casting. The steps of each process are as follows:

[0028] (1) Top and bottom combined blowing converter primary smelting process: The sulfur content at the end of smelting is controlled at 0.025wt%≤S≤0.045wt%. During the steelmaking process, ferrosilicon manganese, high carbon ferromanganese, recarburizer and medium carbon ferrochrome are added in sequence, and slag washing material and aluminum deoxidizer are not added.

[0029] (2) LF refining process: within 3 minutes after the start of refining heating, add 4-6 kg / t of lime to submerge the steel and heat it. Add 0.3-0.5 kg / t of silicon carbide to deoxidize the steel. Eliminate the slag-making reducing white slag desulfurization step and heat it to 1640-1650℃. After the refining is completed, add a covering agent to keep it warm. During the refining process, pay attention to the bottom blowing argon flow rate of less than 10Nm 3 / h, which avoids the steel slag from fully contacting and reacting, and also reduces the argon consumption.

[0030] (3) RH vacuum refining process: After the molten steel enters the RH refining, it is vacuum carbon deoxidized. After carbon deoxidation is completed, aluminum particles are added according to the oxygen content of the molten steel for final deoxidation. When the oxygen content in the molten steel is between 20ppm and 50ppm, 35kg to 60kg of aluminum particles are generally required to be added. After adding aluminum particles, the Als content in the RH outgoing steel is guaranteed to be between 0.020 and 0.035%. After the addition of aluminum particles, the molten steel is circulated for 5min to 8min according to the amount of aluminum particles added. If the amount of aluminum particles added is large, the molten steel circulation time should be set as the upper limit, and then titanium-iron alloy is added. After the RH returns to atmospheric pressure, unlike the conventional process, no sulfur wire is fed and no calcium treatment is performed. The soft blowing volume is controlled at 5 to 10Nm 3 / h, soft blowing time ≥15min.

[0031] (4) Billet continuous casting process: The RH furnace is equipped with a ladle containing qualified molten steel, which is hoisted to the continuous casting machine by an overhead crane. The atmosphere of the tundish is replaced with an inert gas, argon, before pouring. The replacement time is ≥3 minutes. After the atmosphere replacement is completed, the ladle starts pouring. The long nozzle and submerged nozzle of the ladle are used to protect the casting with micro-positive pressure to perform non-oxidation casting throughout the process. The nitrogen addition of the molten steel is significantly reduced, effectively inhibiting the secondary oxidation of the molten steel during the casting process. The cross-section of the billet continuous casting is 200mm×200mm. The superheat control range of the tundish is 20~40℃. The constant casting speed is controlled at 1.2±0.1m / min, and the crystallizer is electromagnetically stirred. After adopting the above process, the crystallizer liquid level is stable and the casting is obviously stable.

[0032] Example 1-6: A method for smelting sulfur-containing gear steel is described in detail as follows.

[0033] Taking a 120t converter and refined smelting steel grade H20CrMnTi-2 as an example, the parameters of each step of the smelting process of each embodiment are shown in Table 1; the composition of the ladle samples of Examples 1-6 are shown in Tables 2 to 7, and the reduced cost is shown in Table 8.

[0034] Table 1: Parameters of each step of the smelting process in each embodiment

[0035]

[0036] Table 2: Composition of ladle samples in each step of the smelting process of Example 1 (wt%)

[0037]

[0038] Table 3: Composition of ladle samples in each step of the smelting process of Example 2 (wt%)

[0039]

[0040] Table 4: Composition of ladle samples in each step of the smelting process of Example 3 (wt%)

[0041]

[0042] Table 5: Composition of ladle samples in each step of the smelting process of Example 4 (wt%)

[0043]

[0044] Table 6: Composition of ladle samples in each step of the smelting process of Example 5 (wt%)

[0045]

[0046] Table 7: Composition of ladle samples in each step of the smelting process of Example 6 (wt%)

[0047]

[0048] Tables 2 through 7 show that adding a small amount of lime and alloy to the LF process results in a slight decrease in sulfur. In the RH process, under high vacuum, carbon can remove oxygen to around 20-30 ppm, followed by the addition of aluminum particles and titanium alloy for final deoxidation.

[0049] Table 8: Comparison of costs of the two processes

[0050]

[0051] Note: The original process specifically refers to the use of aluminum products for deoxidation in the LF furnace, the addition of a large amount of lime and slag for desulfurization, and the RH process only for degassing to remove some inclusions. After restoring atmospheric pressure, the molten steel is sulfurized and calcium treated.

[0052] From Table 8, it can be seen that by adopting the present invention to smelt H20CrMnTi-2, 28.1 to 33.7 yuan can be saved per ton of steel. Excluding the additional SiC, carbon powder and covering agent required in the smelting process, 27.8 to 33.4 yuan can be saved per ton of steel.

[0053] To compare the inclusion levels of the molten steel under the two process conditions, cake samples were taken from three actual furnaces and analyzed as metallographic specimens. After mounting and polishing, the metallographic specimens were analyzed for microscopic inclusions under a metallographic microscope at a magnification of 500. The number of microscopic inclusions in each sample was counted, assuming an equivalent diameter of 7.5 μm, as shown in Table 9.

[0054] Table 9: Microscopic inclusion content in each sample (pieces / mm 2 )

[0055]

[0056] Table 9 shows that the present invention, by avoiding RH sulfur feed and calcium treatment, can increase the static blowing time by 5-7 minutes during production, significantly improving inclusion removal. The average inclusion count of the ingots produced by the present invention is 5.9% lower than that of the ingots produced by the original process. The ingots produced by the present invention are cleaner, primarily because carbon deoxidation eliminates inclusions, and the use of a small amount of aluminum for final deoxidation reduces Al2O3 inclusions. Furthermore, the production schedule facilitates increased soft blowing time control, further facilitating inclusion removal.

[0057] Depend on Figure 1 、 Figure 2 It can be seen that there are Al2O3 inclusions in the molten steel with different shapes. The number of Al2O3 inclusions in the ingot of the present invention is smaller. Compared with the original process, the number of such inclusions in the ingot of the present invention can be reduced by 48%, and the size of the inclusions is smaller. Figure 3 、 Figure 4 It can be seen that the ingots produced by the original process contain Al2O3-SiO2-MnO-CaS composite inclusions, especially CaS high-melting point inclusions that are not easy to float up and remove in the molten steel. However, in the ingots produced by the present invention, the size of the inclusions is reduced, and the proportion of CaS inclusions is significantly reduced.

Claims

1. A method for smelting sulfur-containing gear steel, characterized in that: It includes top and bottom blown converter primary refining, LF refining, RH vacuum refining and billet continuous casting processes; (1) Top and bottom combined blowing converter primary smelting process: the sulfur content at the end of smelting is controlled at 0.025wt%≤S≤0.045wt%, and no slag washing material and aluminum deoxidizer are added during the tapping process; (2) LF refining process: add 4-6 kg / t of steel lime for submerged arc heating, add 0.3-0.5 kg / t of steel silicon carbide for weak deoxidation, do not add aluminum deoxidizer, do not produce reducing white slag desulfurization, use submerged arc method to heat to 1640-1650℃, and add covering agent for heat preservation; the bottom blowing argon flow rate during the refining process is less than 10Nm 3 / h; (3) RH vacuum refining process: vacuum carbon deoxidation treatment is adopted. After carbon deoxidation is completed, aluminum particles are added for final deoxidation according to the oxygen content of the molten steel, and the Als content of the RH outgoing molten steel is controlled at 0.020-0.035%. After the addition of aluminum particles, the molten steel is circulated for 5-8 minutes, and then titanium-iron alloy is added. After re-pressing, no sulfur line is fed and no calcium treatment is performed.

2. The smelting method of sulfur-containing gear steel according to claim 1, characterized in that: In the RH vacuum refining process, the soft blowing volume is controlled to 5-10Nm 3 / h, soft blowing time ≥15min.

3. The smelting method of sulfur-containing gear steel according to claim 2, characterized in that: The billet continuous casting process adopts a ladle long nozzle and an immersed nozzle for casting with slight positive pressure protection to perform oxidation-free casting throughout the process.

4. The smelting method of sulfur-containing gear steel according to any one of claims 1 to 3, characterized in that: In the billet continuous casting process, the tundish superheat is controlled within a range of 20 to 40° C., the drawing speed is controlled at a constant drawing speed of 1.2±0.1 m / min, and the crystallizer is electromagnetically stirred.

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