A process for refining and smelting low-silicon and low-oxygen economical gear steel 20MnCr5(H) in an electric furnace

Abstract

The invention discloses a process for refining and smelting low-silicon and low-oxygen economical gear steel 20MnCr5 (H) in an electric furnace. The silicon content of the steel molten steel is controlled by reasonable electric furnace oxidation, the silicon content is added to the maximum extent during alloying of the alloy steel ladle, a silicon-free deoxidizer is used, and a precipitation deoxidation and diffusion deoxidation combined deoxidation process is adopted to make the finished steel [Si] ≤ 0.12%, the oxygen content does not exceed 12ppm, and the non-metallic oxide inclusions do not exceed 1.0 level. The invention starts from the key processes of oxidation of electric furnace steel materials, alloying of steel tapping, and LF refining and deoxidation, optimizes the process operation, reduces the silicon and oxygen content of the steel molten steel, improves the product performance, and prolongs the product service life. The key technology for economically producing high-quality MnCr gear steel is developed, and the construction of a resource-saving society is promoted; it is of great significance to promote the application of economical gear steel, reduce the consumption of alloy resources, develop green electric furnace metallurgy, and realize the low-silicon and low-oxygen production of economical gear steel.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and in particular relates to a process for refining and smelting low-silicon and low-oxygen economical gear steel 20MnCr5 (H) in an electric furnace. Background Art

[0002] Gears are one of the most critical parts of various instruments and machines, and are mechanical elements that transmit motion and power. Gear steel, as a material for manufacturing gears, must not only meet the high hardness of the gear surface, but also meet the toughness of the core. High-strength and toughness carburized gear steel contains alloy elements such as Ni, Mo, and Cr, which are scarce and expensive, so varieties that avoid or save these alloy elements should be selected as much as possible. Mn-Cr carburized gear steel is a generally recognized economical gear steel that can meet the performance requirements of light, medium and heavy-duty automobile carburized gears and is in line with the development trend of automobile gear steel. However, the alloy elements of Mn-Cr gear steel are composed of Si, Cr, and Mn, which are all elements that promote the oxidation of the carburized layer grain boundaries. In particular, Si has a stronger affinity with oxygen, which is 10 times that of Mn and Cr. It is very easy to oxidize the grain boundaries in the carburized layer during the gear carburizing process, thereby reducing the contact fatigue performance and impact strength of the carburized gear. For this reason, it is best to ensure that the Si content in the Mn-Cr carburized gear steel is ≤0.12%. Since low oxygen content is an important means to ensure the life of gears, and Si is a relatively economical deoxidation element, how to balance low Si content control and low oxygen steel smelting has become an important metallurgical issue.

[0003] Publication No. CN102676727B and CN103276151A both involve low-Si smelting, but they are all converter processes, and the varieties are different, and the oxygen control effect is not outstanding. In order to ensure the metallurgical quality of Mn-Cr gear steel, promote the application of economical gear steel, reduce alloy resource consumption, develop green electric furnace metallurgy, and realize the low-silicon and low-oxygen production of economical gear steel represented by 20MnCr5(H), it is of great practical significance. Summary of the invention

[0004] In order to overcome the defects of the above-mentioned prior art, the purpose of the present invention is to provide a process method for refining and smelting low-silicon and low-oxygen economical gear steel 20MnCr5 (H) in an electric furnace, and to develop key technologies for economically producing high-quality MnCr gear steel. The present invention starts with the three key processes of oxidation of electric furnace steel material, alloying of steel tapping, and LF refining and deoxidation, optimizes and strictly operates the process to reduce the silicon and oxygen content of molten steel, improve product performance, extend product service life, reduce the consumption of precious alloys, and promote the construction of a resource-saving society.

[0005] In order to solve the problem of low silicon and low oxygen content control of economical gear steel 20MnCr5 (H) under electric furnace refining conditions, the present invention realizes the smelting of low silicon and low oxygen economical gear steel 20MnCr5 (H) by controlling the silicon content of steel tapped from the electric furnace, adding sufficient FeMn and FeCr Si-increasing alloys in the ladle alloying, using silicon-free Al and CaC2 powder deoxidizers, and adopting precipitation deoxidation and diffusion deoxidation combined deoxidation process, thereby achieving good metallurgical effect.

[0006] A process for refining and smelting low-silicon, low-oxygen economical gear steel 20MnCr5(H) in an electric furnace adopts the following technical solutions:

[0007] ① Silicon-controlled technology for steelmaking in electric furnace

[0008] The average steel-tapping temperature of the electric furnace is 1650°C. According to the silicon deoxidation state diagram, the [Si] equilibrium value corresponding to the highest equilibrium [O] content at this temperature is 0.04-0.05%. Production practice shows that the content will not decrease even when the molten steel is overoxidized. Therefore, the present invention requires that the electric furnace oxygen blowing oxidation control the silicon content of the molten steel to be ≤0.05%.

[0009] ②LF furnace controls silicon addition process

[0010] The slag is controlled during production operation, and the maximum slag amount does not exceed 100kg; the amount of FeMn alloy and FeCr alloy added in the ladle alloying process is increased, the composition of Mn and Cr is made as close to the specification requirements as possible, the strong oxidizing property of the electric furnace molten steel is used to oxidize the silicon content in FeMn and FeCr, and the amount of silicon added in the steel alloying process is reduced; the amount of FeMn alloy added in the ladle alloying accounts for more than 75.0%, and the amount added in the LF furnace accounts for less than 25.0%; the amount of FeCr alloy added in the ladle alloying accounts for more than 75.0%, and the amount added in the LF furnace accounts for less than 25.0%.

[0011] During the LF smelting process, the silicon content in the molten steel will increase, which is called silicon increase. The reasons for silicon increase in this stage are: first, the silicon contained in the added Mn and Cr alloy materials FeMn and FeCr enters the molten steel; second, after adding aluminum for deoxidation, aluminum reduces SiO2 in the slag and refractory materials, and silicon enters the molten steel, thus causing silicon increase in the molten steel.

[0012] The SiO2 in the ladle mainly comes from the slag from the electric furnace and the refined slag. When the amount of slag from the electric furnace is different, the amount of silicon added to the molten steel will be different. Practice shows that the silicon increase in LF molten steel is linearly related to the amount of slag from the electric furnace. The larger the slag amount, the more serious the silicon increase in the molten steel. After theoretical calculation and practical verification, when the slag amount is 100kg, the silicon increase in the molten steel will not exceed 0.0096%. Therefore, the production operation must control the slag, and the maximum slag amount shall not exceed 100kg.

[0013] The alloy materials added to 20MnCr5H gear steel are mainly FeMn and FeCr. The addition time is the maximum addition during ladle alloying, and the total amount is not less than 75%. The rest is added during the LF furnace smelting process. Under normal circumstances, the theoretical calculation of the amount of silicon added by FeMn and FeCr during ladle alloying is 0.04-0.05%, and the amount of silicon added by FeMn and FeCr during the LF process is about 0.02%. In this way, it is difficult to ensure that the total amount of silicon in the steel does not exceed the upper limit of the specification of 0.12%. Since FeMn and FeCr are added in the reducing atmosphere of the LF furnace, the silicon content will not be oxidized, and will directly enter the molten steel, causing silicon increase. Therefore, the addition of FeMn and FeCr alloys should be completed as much as possible during the ladle alloying process, so that the Mn and Cr components are basically close to the specification requirements. In this way, based on the fact that the affinity between [Si] and [O] is greater than that between [Mn] and [O] and [Cr] and [O], and that silicon will be oxidized before manganese and chromium, the strong oxidizing property of electric furnace molten steel can be used to oxidize the silicon content in FeMn and FeCr, thereby reducing the silicon increase in molten steel.

[0014] ③Deoxidation process

[0015] High-standard 20MnCr5 (H) gear steel strictly requires that the oxygen content in the steel does not exceed 12ppm. For this reason, a deoxidation process combining precipitation deoxidation and diffusion deoxidation must be adopted.

[0016] Due to the limitation of [Si]≤0.12% for 20MnCr5H gear steel, silicon-containing alloys cannot be added, so FeSi and CaSi in commonly used precipitating deoxidizers cannot be used. FeMn must be added due to the manganese content requirement, but due to its weak deoxidation ability, it cannot be used as a precipitating deoxidizer for this type of steel. Research has determined that aluminum with the strongest deoxidation ability should be used as the precipitating deoxidizer.

[0017] The slag used for diffusion deoxidation must have a very low ∑FeO content (∑FeO<0.5~1.0%), so deoxidizers must be added to the slag in the furnace in stages and batches to remove unstable oxides in the slag and keep the slag at a very low ∑FeO. Due to the steel grade requirement of [Si]≤0.12%, silicon-containing deoxidizers FeSi powder and CaSi powder cannot be used, and C powder used in combination with FeSi powder cannot be used. As a new type of deoxidizer, CaC2 has strong deoxidation and desulfurization capabilities. Al powder is also a good diffusion deoxidizer. Research and experiments have shown that when calcium and aluminum are used for deoxidation at the same time, the oxygen in the steel can be reduced to a very low value, and the mechanical properties of the steel can be improved. This is because when calcium and aluminum are used for deoxidation at the same time, spherical calcium aluminate (12CaO·7Al2O3) with a low melting point can be formed. It has a small specific surface area, so the floating speed is very fast, so the amount of inclusions in the molten steel is significantly reduced. The small amount of calcium aluminate inclusions remaining in the steel will not be deformed during rolling, and will not form a sharp-angle chain distribution like Al2O3 inclusions. It can improve the isotropy of the steel and has good processing performance and mechanical properties. In addition, adding calcium alloy to the molten steel deoxidized with a large amount of aluminum can remove Al2O3 inclusions in the steel. Because calcium can reduce Al2O3, spherical calcium aluminate is produced. Therefore, the combined diffusion deoxidation of calcium and aluminum can not only reduce the sulfur and oxygen content in steel, but also change the composition and morphology of inclusions.

[0018] A low-silicon, low-oxygen economical gear steel 20MnCr5(H) prepared by the above method, wherein [Si]≤0.12%, oxygen content does not exceed 12ppm, and non-metallic oxide inclusions do not exceed level 1.0.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention provides an economical process method for refining and smelting low-silicon and low-oxygen economical gear steel 20MnCr5 (H) in an electric furnace, through reasonable electric furnace oxidation silicon control, maximum addition during alloying of the alloy steel ladle, use of silicon-free Al and CaC2 powder deoxidizers, and a combined deoxidation process of precipitation deoxidation and diffusion deoxidation, so that the finished steel meets [Si]≤0.12%, oxygen content not exceeding 12ppm, non-metallic oxide inclusions not exceeding level 1.0, meeting the requirements of reducing the consumption of precious alloy resources in product production and long-life operation. DETAILED DESCRIPTION

[0020] The present invention is further described below in conjunction with specific examples, but the present invention is not limited in any way. To avoid redundant description, the raw materials in the following examples are all commercially available products unless otherwise specified, and the methods used are all conventional methods unless otherwise specified.

[0021] Example

[0022] The invention provides a process for refining and smelting low-silicon, low-oxygen economical gear steel 20MnCr5(H) in an electric furnace. The chemical components and mass percentage contents of the 20MnCr5(H) steel are as follows: C 0.17%-0.21%, Si≤0.12%, Mn1.10%-1.30%, P≤0.025%, S≤0.020-0.035%, Cr 1.00%-1.30%, Ni, Mo≤0.05%, Cu≤0.10%, and the rest are Fe and unavoidable impurities.

[0023] The best implementation case of smelting 20MnCr5(H) steel by the process of the present invention is furnace number 22A0997, and the Si content of the finished product is 0.08%, and the oxygen content of the finished product is 9ppm. The key implementation steps are as follows:

[0024] The electric furnace smelting is carried out by blowing oxygen, and the actual silicon content of the molten steel tapped by the electric furnace is 0.04%. The electric furnace tapping operation is slag retention, and the slag amount is 85kg. The total amount of FeMn alloy added is 831kg, of which 654kg is added to the ladle alloying, accounting for 78.70%, and 177kg is added to the LF furnace, accounting for 21.30%; the total amount of FeCr alloy added is 1003kg, of which 804kg is added to the ladle alloying, accounting for 80.16%, and 199kg is added to the LF furnace, accounting for 19.84%.

[0025] 60kg aluminum ingots are added to the electric furnace for pre-deoxidation, 200m (80kg) aluminum wire is fed into the LF furnace for precipitation deoxidation, 800kg active lime and 80kg alumina powder are added to make slag during refining, and after the liquid slag is formed, a total of 45kg CaC2 and 60kg cut aluminum are added in three batches for diffusion deoxidation.

[0026] Six non-metallic inclusion test pieces were prepared, and the inclusion detection results are shown in Table 1. The measured data show that non-metallic inclusions are well controlled.

[0027] Table 1 Test results of non-metallic inclusions in finished products of the embodiment

[0028]

[0029] For any technician familiar with the art, without departing from the scope of the technical solution of the present invention, the technical content disclosed above can be used to make many possible changes and modifications to the technical solution of the present invention, or modified into equivalent embodiments with equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention should still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A process for refining and smelting low-silicon, low-oxygen economical gear steel 20MnCr5(H) in an electric furnace, characterized in that: The following technical solutions were adopted: ① Electric furnace steel-making silicon-controlled process: The electric furnace steel-making temperature is 1640℃~1650℃, the electric furnace is blown with oxygen for oxidation, and the silicon content of the steel-making liquid is controlled to be ≤0.05%; ②LF furnace controlled silicon-increasing process: production operation controls slag discharge, and the maximum slag discharge amount does not exceed 100 kg; FeMn alloy and FeCr alloy are added in the early stage of electric furnace tapping for ladle alloying, and the amount of FeMn alloy and FeCr alloy added in the ladle alloying process is increased to make the composition of Mn and Cr close to the specification requirements, and the strong oxidizing property of electric furnace molten steel is used to oxidize the silicon content in FeMn and FeCr, thereby reducing the amount of silicon added in the alloying process of steel tapping; The amount of FeMn alloy added, the amount added in the ladle alloying accounts for more than 75.0%, and the amount added in the LF furnace accounts for less than 25.0%; the amount of FeCr alloy added, the amount added in the ladle alloying accounts for more than 75.0%, and the amount added in the LF furnace accounts for less than 25.0%; ③ Deoxidation process: A deoxidation process combining precipitation deoxidation and diffusion deoxidation is adopted, aluminum wire is selected as precipitation deoxidizer, and CaC2 and cut Al are selected as diffusion deoxidizer; The deoxidation process adopts calcium and aluminum combined diffusion deoxidation, and the diffusion deoxidizer is added in stages and batches to ensure that the slag ∑FeO is less than 0.5%~1.0%; aluminum ingots are added to the electric furnace for pre-deoxidation, aluminum wire is fed into the LF furnace for precipitation deoxidation, active lime and alumina powder are added to the refining to make slag, and CaC2 and cut aluminum are added in batches for diffusion deoxidation after the liquid slag is formed; The low-silicon, low-oxygen economical gear steel 20MnCr5(H) prepared by the method has chemical components and their mass percentage contents of C: 0.17%-0.21%, Si≤0.08%, Mn: 1.10%-1.30%, P≤0.025%, S≤0.020%, Cr: 1.00%-1.30%, Ni≤0.05%, Mo≤0.05%, Cu≤0.10%, and the rest are Fe and unavoidable impurities.

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