Smelting method for controlling sulfide inclusion of sulfur-containing 20CrMnTiH gear steel

Abstract

The invention discloses a smelting method for controlling sulfide inclusions of sulfur-containing 20CrMnTiH gear steel, which comprises the following steps of: reducing the component fluctuation of molten steel in front of a furnace by adopting measures of stably controlling the carbon content, the oxygen content and the temperature of tapping through an electric furnace end point control process; by optimizing an LF slagging system, high-alkalinity refining slag is used for promoting molten steel deoxidation and inclusion floating; by adopting the constant casting speed or keeping the casting speed to fluctuate in a narrow range, the influence of the casting speed fluctuation on the temperature gradient of the section of the casting blank is weakened, and the segregation index is further reduced; by implementing the measures, the chemical components are accurately controlled, so that the grade of sulfide inclusions in the steel is reduced, good comprehensive performance is obtained, the quality of the gear steel is improved, class A inclusions are smaller than or equal to 1.5 grade, the oxygen content is smaller than or equal to 12 ppm, the requirements of domestic main engine plants for sulfide inclusions of the sulfur-containing gear steel are met, and the quality of the automobile manufacturing industry in China is improved.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and in particular to a smelting process for controlling sulfide inclusions in sulfur-containing 20CrMnTiH gear steel. Background Art

[0002] With the continuous improvement of the level of industrial automation, the gear processing industry has gradually adopted machines to replace manual machines for production. In order to make the processing go smoothly and continuously improve production efficiency, a certain amount of sulfur is usually added to the gear to improve the cutting performance of the gear steel. On the other hand, in order to regulate the cutting performance, seismic performance and prevent crack propagation of the product, some OEMs usually set the sulfur content in the gear steel to 0.015-0.035%. However, too high sulfur content can easily lead to excessive sulfide inclusions in the rolled bar and reduce the yield rate. Due to the addition of sulfur in the gear steel, the sulfide morphology cannot meet the sulfide inspection standards of high-end users, the product has serious anisotropy, and the cutting performance is poor.

[0003] In addition, during the smelting process, the thermodynamic conditions of sulfur addition and deoxidation are different, which also increases the difficulty of smelting sulfur-containing gear steel.

[0004] Therefore, it is necessary to improve the size, quantity, shape and distribution of sulfides through appropriate smelting processes to meet the inspection requirements of the OEM and improve the cutting performance of the product. Summary of the invention

[0005] In view of this, the present disclosure provides a smelting method for controlling sulfide inclusions in sulfur-containing 20CrMnTiH gear steel, which solves the problems of sulfur-containing gear steel being unable to meet sulfide inspection standards of high-end users, severe product anisotropy and poor cutting performance due to sulfide inclusions.

[0006] To achieve the above-mentioned invention object, the smelting method for controlling sulfide inclusions in sulfur-containing 20CrMnTiH gear steel comprises:

[0007] The process flow includes electric furnace smelting, LF refining, RH vacuum degassing, continuous casting and rolling;

[0008] In the electric furnace smelting process, the endpoint control is to start tapping when the tapping conditions meet the endpoint carbon greater than 0.05%, P≤0.010%, and the temperature is 1600-1630°C.

[0009] In the present disclosure and possible embodiments, in the LF refining process, the target composition of the refined slag is controlled to be CaO: 50-55%, Al2O3: 30-35%, SiO2: 6-8%, MgO: ≤8%, FeO: ≤0.5%.

[0010] In the present disclosure and possible embodiments, in the RH vacuum degassing process, the sulfur-coated core wire is fed once to increase the sulfur before the RH is vacuumed.

[0011] In the present disclosure and possible embodiments, in the continuous casting process, a casting speed control target is set according to the superheat of molten steel to perform continuous casting of the billet, and the casting speed control target is:

[0012] Superheat <15℃, pulling speed 1.4m / min;

[0013] The superheat is 15-20℃ and the pulling speed is 1.3m / min;

[0014] The superheat is 20-30℃ and the pulling speed is 1.2m / min;

[0015] The superheat is 30-40℃ and the pulling speed is 1.0m / min;

[0016] The superheat is >40°C and the pulling speed is 0.9m / min.

[0017] In the present disclosure and possible embodiments, in the rolling process, the starting rolling temperature is set to 980-1050°C, the descaling pressure is set to 28-30 MPa, the final rolling temperature is set to 800-950°C, and the steel with a specification of <Φ85mm is pile cooled.

[0018] In the present disclosure and possible embodiments, the chemical composition of the molten steel for smelting 20CrMnTiH gear steel is, by weight percentage:

[0019] C: 0.18%~0.23%, Si: 0.17%~0.23%, Mn: 0.80%~1.15%, P≤0.025%, S: 0.015%~ 0.030%, Ti: 0.04% to 0.07%, Cr: 1.00 to 1.35%, Mo≤0.03%, Ni≤0.30%, Cu≤0.20%.

[0020] In the present disclosure and possible embodiments, the electric furnace smelting adopts scrap steel and molten iron for smelting, and the proportion of molten iron added is about 40%. After the scrap steel is preheated in the vertical furnace, the scrap steel is added into the vertical furnace once every 3 to 5 minutes by the electric furnace secondary system;

[0021] The scrap steel is preheated to 600-800℃ by the flue gas in the furnace, and then pushed into the smelting area of ​​the furnace by the steel pusher. The carbon gun on the furnace wall is turned on to spray carbon powder into the furnace, and the oxygen flow rate is adjusted to 2500Nm 3 / h for oxygen blowing to assist melting, and adding lime and dolomite for slagging operation;

[0022] Argon is supplied throughout the steel tapping process with a flow rate of 600-750NL / min, the steel is tapped in a circular flow, and the tapping time is controlled at 5-6min. When 15-20t of steel is tapped, baked alloys are added in sequence at a baking temperature of 600-800°C. The baked alloys include aluminum wire segments, medium-carbon ferromanganese, high-carbon ferrochrome and low-carbon ferrochrome, and ladle lime and synthetic slag are added.

[0023] In the present disclosure and possible embodiments, in the LF furnace refining process, the LF arrival temperature is 1510-1530°C, the furnace is kept at a slightly positive pressure, submerged arc heating is adopted, and large argon blowing is prohibited during the refining process to prevent nitrogen absorption;

[0024] After the initial power supply is turned on for 2 to 3 minutes to form liquid slag, carbon powder is added to start carbonization. The whole process is white slag operation. When the temperature reaches 1560-1600℃, a full analysis sample is taken. According to the analysis results, lime, medium carbon ferromanganese, low carbon ferrochrome, ferrotitanium and silicon carbide are added to adjust the composition. After refining, the ladle is transferred to the RH vacuum degassing process in a way that the liquid surface is strictly prohibited from being exposed during the transfer process.

[0025] In the present disclosure and possible embodiments, in the RH vacuum degassing process, after the ladle is placed on the ladle car, the argon gas is opened and blown, the flow rate is 300-450NL / min, the sulfur cored wire is fed, and after entering the working position, the jacking operation is started, the immersion tube is inserted to a depth greater than 400mm, the argon gas is turned off, the vacuum degree is <100Pa, the duration is 10-15min, the argon gas is restored after breaking the air, the silicon calcium wire and the covering agent are fed, and the static argon blowing time is >10mi n; The internal control requirements for the chemical composition of the outgoing station are: C: 0.193~0.208%, Si: 0.18~0.22%, Mn: 0.99~1.03%, P≤0.025%, S: 0.017~0.027%, Cr: 1.13~1.17%, Ti: 0.04~0.06%, Ni≤0.05%, Cu≤0.05%, B≤0.0004%, Alt: 0.010%~0.035%.

[0026] In the present disclosure and possible embodiments, in the continuous casting process, the molten steel after vacuum degassing is hoisted to the casting position, the superheat of the molten steel is controlled to be less than 40°C, a continuous slag detection device is used for the continuous casting ladle, and the protective slag is low-carbon titanium steel protective slag;

[0027] The current of crystallization electromagnetic stirring was 340A and the frequency was 2 Hz; the current of terminal electromagnetic stirring was 360A and the frequency was 7 Hz, and the stirring mode was an alternating mode of 10s-3s-10s.

[0028] The present invention has the following beneficial effects:

[0029] The smelting method for controlling sulfide inclusions in sulfur-containing 20CrMnTiH gear steel of the present invention adopts a stable control measure of the carbon content, oxygen content and temperature of the steel tapping through an electric furnace endpoint control process to reduce the fluctuation of the composition of molten steel in front of the furnace; optimizes the LF slag making system and uses high-basicity refined slag to promote the deoxidation of molten steel and the floating of inclusions; adopts a constant billet drawing speed or keeps the billet drawing speed within a narrow range of fluctuations to weaken the influence of the billet drawing speed fluctuation on the temperature gradient of the billet cross section, and further reduces the segregation index; the present invention accurately controls the chemical composition through the implementation of the above measures, thereby reducing the level of sulfide inclusions in the steel, and obtains good comprehensive performance, improves the quality of gear steel, achieves Class A inclusions ≤1.5, and oxygen content ≤12ppm, meets the requirements of domestic main engine factories for sulfide inclusions in sulfur-containing gear steel, and improves the quality of my country's automobile manufacturing industry. DETAILED DESCRIPTION

[0030] The present disclosure is described below based on embodiments, but it is worth noting that the present disclosure is not limited to these embodiments. In the detailed description of the present disclosure below, some specific details are described in detail. However, for the parts that are not described in detail, those skilled in the art can also fully understand the present disclosure. At the same time, unless the context clearly requires, the words "including", "comprising" and the like in the entire specification and claims should be interpreted as including rather than exclusive or exhaustive meanings; that is, the meaning of "including but not limited to".

[0031] The present disclosure describes in detail the smelting method of controlling sulfide inclusions in sulfur-containing 20CrMnTiH gear steel of the present invention through specific examples, and the method steps are as follows:

[0032] 1. The chemical composition of molten steel for smelting 20CrMnTiH gear steel is as follows:

[0033] C: 0.18%~0.23%, Si: 0.17%~0.23%, Mn: 0.80%~1.15%, P≤0.025%, S: 0.015%~ 0.030%, Ti: 0.04% to 0.07%, Cr: 1.00 to 1.35%, Mo≤0.03%, Ni≤0.30%, Cu≤0.20%.

[0034] 2. The process flow of electric furnace smelting → LF → RH → continuous casting → rolling is adopted, in which:

[0035] (1) Electric furnace smelting:

[0036] The electric furnace uses scrap steel and molten iron for smelting, with the ratio of molten iron being about 40%. After the scrap steel is preheated in the vertical furnace, a car of scrap steel is added into the vertical furnace every 3 to 5 minutes by the electric furnace's secondary system. A total of 6 to 7 cars of scrap steel are added during the entire smelting process, with a scrap steel volume of 70 to 75 tons.

[0037] After the scrap steel is preheated to 600-800℃ by the flue gas in the furnace, it is pushed into the smelting area of ​​the furnace by the steel pusher. The carbon gun on the furnace wall is opened to spray 17.0-22.0kg / t of carbon powder into the furnace. According to the situation in the furnace, the oxygen gun at the furnace door is opened and the flow rate is adjusted to 2500Nm3 / h for oxygen blowing to assist melting. Lime 38.0-42.0kg / t and dolomite 24-26kg / t are added for slag making.

[0038] This type of steel requires a sulfur content of 0.015% to 0.030%, but too high a sulfur content can easily lead to excessive sulfide inclusions in the rolled bar and reduce the yield rate, while a low sulfur content cannot meet the machining requirements, so it needs to be strictly controlled within the internal control range of 0.017% to 0.027%. In addition, the content of oxygen, aluminum, silicon, carbon and manganese in the steel will also affect the morphology of inclusions, so the chemical composition needs to be precisely controlled.

[0039] In order to accurately control the chemical composition, in the electric furnace endpoint control process disclosed in the present invention, in order to reduce the original oxygen content of the molten steel, reduce the number of Al2O3 inclusions after deoxidation and the amount of aluminum used for deoxidation, measures are taken to increase the endpoint carbon content and control the tapping temperature, so as to reduce the dissolved oxygen content in the steel and reduce the fluctuation of the molten steel composition in front of the furnace, while reducing the deoxidation burden of the LF refining process and improving production efficiency. It is designed that when the tapping conditions meet the endpoint carbon greater than 0.05%, P≤0.010%, and the temperature is 1600-1630°C, tapping begins.

[0040] Argon is supplied throughout the whole process of steel tapping, with a flow rate of 600-750NL / min. Steel is tapped in a circular flow, and the molten steel is prohibited from spreading. The tapping time is controlled at 5-6min. When 15-20t of steel is tapped, the baked alloys are added in sequence, with a baking temperature of 600-800℃ to reduce the entry of foreign gas into the molten steel. Among them, 1.0-1.5kg / t of aluminum wire segments, 9.0-11.0kg / t of medium carbon ferromanganese, 9.0-10.0kg / t of high carbon ferrochrome, and 7.0-8.5kg / t of low carbon ferrochrome are added, and 600kg / furnace of ladle lime and 500kg / furnace of synthetic slag are added.

[0041] (2) LF refining:

[0042] In this LF refining, in order to accurately control the chemical composition, an optimized LF slag making system was adopted, and high basicity refining slag was used to promote the deoxidation of molten steel and the floating of inclusions, as follows:

[0043] The molten steel after leaving the station is sent to the LF furnace for refining. The LF arrival temperature is 1510-1530℃. Ensure that the LF furnace cover is in close contact with the ladle, maintain a slightly positive pressure in the furnace, and use submerged arc heating to prevent arc light leakage. During the refining process, the argon flow rate is continuously adjusted, and large argon blowing is prohibited to prevent nitrogen absorption.

[0044] After the initial power supply forms liquid slag for 2 to 3 minutes, carbon powder is added to start carbonization. The whole white slag operation is carried out. When the temperature reaches 1560-1600℃, a full analysis sample is taken. Then 3.5-4.5kg / t of lime, 4.5-5.5kg / t of medium carbon ferromanganese, 8.0-10.0kg / t of low carbon ferrochrome, 1.1-1.5kg / t of ferrotitanium and 150kg / furnace of silicon carbide are added to adjust the composition. After refining, the liquid surface is strictly prohibited from being exposed during the transfer of the ladle, and the RH process is transferred.

[0045] In order to ensure the castability of molten steel and low oxygen content and RH treatment, calcium wire needs to be fed to deform non-metallic inclusions to generate low melting point 12CaO7Al2O3, and the refined slag composition is controlled within the range of Table 1.

[0046] Table 1 Target composition of refined slag

[0047]

[0048]

[0049] (3) RH vacuum degassing:

[0050] After the ladle is placed on the ladle car, the argon gas is turned on to blow it open, with a flow rate of 300-450NL / min, and 240-280m / furnace of sulfur cored wire is fed.

[0051] This type of steel was previously produced by sulfurizing the steel with a sulfur feeding line after RH vacuum treatment, but the sulfur recovery rate was low, the sulfur content in the finished product was unstable and poorly uniform, and CaS inclusions were easily produced after calcium feeding, leading to continuous casting nodules. In this RH vacuum degassing process, sulfur is added to the steel with a sulfur feeding line once before RH vacuuming, so that during the RH treatment, sulfur can be evenly distributed in the molten steel to prevent sulfides from aggregating to form large-sized inclusions, and sulfur, as a surface active element, can improve the ability to remove large-sized inclusions during the RH treatment.

[0052] After entering the working position, start the jacking operation, the insertion depth of the immersion tube is greater than 400mm, turn off the argon, the vacuum degree is <100Pa, the duration is 10-15min, restore the argon in time after breaking the air, feed 200m / furnace of silicon-calcium wire and 60kg / furnace of covering agent, and the static argon blowing time is >10min. The chemical composition of the station is guaranteed to meet the internal control requirements C: 0.193-0.208%, Si: 0.18-0.22%, Mn: 0.99-1.03%, P≤0.025%, S: 0.017-0.027%, Cr: 1.13-1.17%, Ti: 0.04-0.06%, Ni≤0.05%, Cu≤0.05%, B≤0.0004, Alt: 0.010-0.035%.

[0053] (4) Continuous casting:

[0054] The molten steel after vacuum degassing is hoisted to the casting position, and the superheat of the molten steel is controlled to be less than 40°C. The continuous casting ladle adopts a continuous slag detection device to prevent slag from falling. The electromagnetic stirring parameters are shown in Table 2, and the protective slag is low-carbon titanium steel protective slag.

[0055] Table 2 Electromagnetic stirring parameters

[0056]

[0057] According to different superheat degrees, the pulling speed in Table 3 is used for production, and a constant pulling speed is ensured.

[0058] Table 3 Casting speed control targets

[0059] Superheat℃ Blank <15 15～20 20～30 30～35 35～40 ＞40 Pulling speed m / min Middle billet 1.4 1.3 1.2 1.0 1.0 0.9

[0060] In this continuous casting process, in order to accurately control the chemical composition, a constant billet drawing speed is adopted or the billet drawing speed is kept within a narrow range of fluctuations to weaken the influence of the billet drawing speed fluctuation on the temperature gradient of the billet cross section and further reduce the segregation index.

[0061] (5) Rolling:

[0062] Since the sulfide inclusions in sulfur-containing steel are generally MnS, controlling the deformation of MnS inclusions at 1000-1050°C can reduce the aspect ratio of MnS, which is beneficial to refine the morphology of inclusions and reduce the influence of MnS deformation on steel properties. Therefore, in this rolling process, the starting rolling temperature is set at 980-1050°C, high-pressure water descaling is used before rolling, the descaling pressure is 28-30MPa, the final rolling temperature is 800-950°C, and steel with specifications <Φ85mm is pile cooled.

[0063] Example 1

[0064] According to the method of the present invention, sulfur-containing 20CrMnTiH gear steel is smelted in an electric furnace as follows:

[0065] 1. The chemical composition of 20CrMnTiH is calculated by weight percentage:

[0066] C: 0.18~0.23%, Si: 0.17~0.37%, Mn: 0.80~1.15%, P≤0.025%, S≤0.015~0.030%, Cr: 1.00~1.35%, Ti: 0.04~0.07%, Mo≤0.03%, Ni≤0.30%Cu≤0.20%, V≤0.02%, Nb≤0.03%, B≤0.0008%, As≤0.012%, Sn≤0.012%, Pb≤0.010%, Sb≤0.010%, Bi≤0.010%, Ca≤0.0015%, and the rest is Fe.

[0067] 2. The process flow of electric furnace smelting → LF → RH → continuous casting of medium billet → heating → rolling is adopted, in which:

[0068] (1) Electric furnace smelting:

[0069] The electric furnace uses scrap steel and molten iron for smelting, with 47.6t of molten iron and 75.7t of scrap steel added. After the scrap steel is preheated to 600-800℃ by the flue gas in the furnace, it is pushed into the smelting area in the furnace by the steel pusher, and the carbon gun on the furnace wall is opened to spray 17.9kg / t of carbon powder into the furnace. According to the situation in the furnace, the oxygen gun at the furnace door is opened, and the flow rate is adjusted to 2500Nm3 / h for oxygen blowing to assist melting, and 40kg / t of lime and 25kg / t of dolomite are added for slag making.

[0070] When the final carbon content of the steel is 0.06%, P is 0.010%, and the temperature is 1600℃, steel tapping begins.

[0071] Argon is supplied throughout the whole process of steel tapping, with a flow rate of 600-750NL / min. Steel is tapped in a circular flow, and the molten steel is prohibited from spreading. The tapping time is controlled at 5-6min. When 15-20t of steel is tapped, the baked alloys are added in sequence, with a baking temperature of 600-800℃ to reduce the entry of foreign gas into the molten steel. Among them, 1.35kg / t of aluminum wire segments, 9.1kg / t of medium carbon ferromanganese, 9.1kg / t of high carbon ferrochrome, and 8.0kg / t of low carbon ferrochrome are added, and 600kg / furnace of ladle lime and 500kg / furnace of synthetic slag are added.

[0072] (2) LF refining:

[0073] The molten steel after leaving the station is sent to the LF furnace for refining. The temperature of LF at the station is 1524℃. Ensure that the LF furnace cover is in close contact with the ladle, maintain a slight positive pressure in the furnace, use submerged arc heating to prevent arc light leakage, and constantly adjust the argon flow rate during the refining process. It is forbidden to blow argon in large quantities to prevent nitrogen absorption. After the initial power supply is 2 minutes to form liquid slag, carbon powder is added to start carbonization. The whole white slag operation is carried out. When the temperature reaches 1560-1600℃, a full analysis sample is taken, and then 4.0kg / t of white ash, 4.9kg / t of medium carbon ferromanganese, 9.2kg / t of low carbon ferrochrome, 1.4kg / t of ferrotitanium, and 150kg / furnace of silicon carbide are added to adjust the composition. After refining, it is strictly forbidden to expose the liquid surface during the transfer of the ladle, and it is transferred to the RH process.

[0074] (3) RH vacuum degassing:

[0075] After the ladle is placed on the ladle car, turn on the argon gas to blow it open, with a flow rate of 300-450NL / min. After entering the working position, start the jacking operation, the immersion tube insertion depth is greater than 400mm, turn off the argon gas, the vacuum degree is <100Pa, the duration is 12min, and the argon gas is restored in time after breaking the air. 200m / furnace of silicon-calcium wire and 60kg / furnace of covering agent are fed. The static argon blowing time is >10min. The chemical composition of the outgoing station is C: 0.191%, Si: 0.22%, Mn: 0.99%, P: 0.001%, S: 0.018%, Cr: 1.16%, Ti: 0.05%, Ni: 0.03%, Cu: 0.05%, B: 0.0005%, Alt: 0.028%.

[0076] (4) Continuous casting:

[0077] The molten steel after vacuum degassing is hoisted to the casting position, the molten steel superheat is 30℃, the pulling speed is 1.25m / min, and the continuous slag detection device is used for the continuous casting ladle to prevent the ladle from slag. The electromagnetic stirring parameters are shown in Table 4, and the protective slag is low-carbon titanium steel protective slag. Under this parameter, the stirring force of the molten steel in the crystallizer is moderate, and by changing the flow state of the molten steel, the floating time of inclusions can be extended to achieve the purpose of purifying the molten steel.

[0078] Table 4 Electromagnetic stirring parameters

[0079]

[0080] 3. The finished product composition and non-metallic inclusion indexes of the sulfur-containing 20CrMnTiH gear steel of Example 1 are shown in Tables 5 and 6 respectively:

[0081] Table 5 Chemical composition of this example %

[0082] Example C Si Mn P S Cr Mo Ni Cu Alt O 1 0.20 0.23 1.01 0.001 0.023 1.17 0.01 0.03 0.05 0.020 0.0009

[0083] As shown in Table 5, by adopting the smelting method of the present invention, the chemical composition meets the set range requirements.

[0084] Table 6 Non-metallic inclusion indicators of Example 1 and Comparative Example

[0085]

[0086] Note: The standard number for non-metallic inclusion index is ASTM E45 A method, and the evaluation of DS inclusions refers to GB / T 10561.

[0087] From the comparative data in Table 6, it can be seen that the Class A sulfide fine inclusions in Example 1 of the present invention can be controlled at levels 1.0-1.5, while the Class A sulfide fine inclusions in the comparative example using the conventional process are controlled at levels 2.0-2.5. Therefore, the smelting method of the present invention can reduce the inclusion level by about 1 level.

[0088] The above-described embodiments are only embodiments of the present disclosure, and the descriptions thereof are relatively specific and detailed, but they cannot be construed as limiting the scope of the present disclosure. It should be noted that, for a person of ordinary skill in the art, without departing from the concept of the present disclosure, several variations, equivalent substitutions, improvements, etc. may be made, all of which fall within the scope of protection of the present disclosure. Therefore, the scope of protection of the present disclosure shall be subject to the attached claims.

Claims

1. A smelting method for controlling sulfide inclusions in sulfur-containing 20CrMnTiH gear steel, characterized in that: include: The process flow includes electric furnace smelting, LF refining, RH vacuum degassing, continuous casting and rolling; In the electric furnace smelting process, the endpoint control is to start tapping when the tapping conditions meet the endpoint carbon greater than 0.05%, P≤0.010%, and the temperature is 1600-1630°C.

2. The smelting method according to claim 1, characterized in that: In the LF refining process, the target composition of the refined slag is controlled to be CaO: 50-55%, Al2O3: 30-35%, SiO2: 6-8%, MgO: ≤8%, and FeO: ≤0.5%.

3. The smelting method according to claim 2, characterized in that: In the RH vacuum degassing process, the sulfur-coated core wire is fed once before the RH is vacuumed to increase the sulfur.

4. The smelting method according to claim 3, characterized in that: In the continuous casting process, the casting speed control target is set according to the superheat of the molten steel to carry out the continuous casting of the medium billet. The casting speed control target is: Superheat <15℃, pulling speed 1.4m / min; The superheat is 15-20℃ and the pulling speed is 1.3m / min; The superheat is 20-30℃ and the pulling speed is 1.2m / min; The superheat is 30-40℃ and the pulling speed is 1.0m / min; The superheat is >40°C and the pulling speed is 0.9m / min.

5. The smelting method according to claim 4, characterized in that: In the rolling process, the starting rolling temperature is set at 980-1050°C, the descaling pressure is set at 28-30MPa, the final rolling temperature is set at 800-950°C, and the steel with specifications less than Φ85mm is pile cooled.

6. The smelting method according to any one of claims 1 to 5, characterized in that: The chemical composition of molten steel for smelting 20CrMnTiH gear steel is as follows by weight percentage: C: 0.18%~0.23%, Si: 0.17%~0.23%, Mn: 0.80%~1.15%, P≤0.025%, S: 0.015%~ 0.030%, Ti: 0.04%~0.07%, Cr: 1.00~1.35%, Mo≤0.03%, Ni≤0.30%, Cu≤0.20%.

7. The smelting method according to claim 6, characterized in that: The electric furnace smelting adopts scrap steel and molten iron, and the proportion of molten iron added is about 40%. After the scrap steel is preheated in the vertical furnace, the scrap steel is added into the vertical furnace once every 3 to 5 minutes by the electric furnace secondary system; The scrap steel is preheated to 600-800℃ by the flue gas in the furnace, and then pushed into the smelting area of ​​the furnace by the steel pusher. The carbon gun on the furnace wall is turned on to spray carbon powder into the furnace, and the oxygen flow rate is adjusted to 2500Nm 3 / h for oxygen blowing to assist melting, and adding lime and dolomite for slagging operation; Argon is supplied throughout the steel tapping process with a flow rate of 600-750NL / min, the steel is tapped in a circular flow, and the tapping time is controlled at 5-6min. When 15-20t of steel is tapped, baked alloys are added in sequence at a baking temperature of 600-800°C. The baked alloys include aluminum wire segments, medium-carbon ferromanganese, high-carbon ferrochrome and low-carbon ferrochrome, and ladle lime and synthetic slag are added.

8. The smelting method according to claim 7, characterized in that: In the LF furnace refining process, the LF station temperature is 1510-1530°C, the furnace is kept at a slightly positive pressure, submerged arc heating is adopted, and large argon blowing is prohibited during the refining process to prevent nitrogen absorption; After the initial power supply is turned on for 2 to 3 minutes to form liquid slag, carbon powder is added to start carbonization. The whole process is white slag operation. When the temperature reaches 1560-1600℃, a full analysis sample is taken. According to the analysis results, lime, medium carbon ferromanganese, low carbon ferrochrome, ferrotitanium and silicon carbide are added to adjust the composition. After refining, the ladle is transferred to the RH vacuum degassing process in a way that the liquid surface is strictly prohibited from being exposed during the transfer process.

9. The smelting method according to claim 8, characterized in that: In the RH vacuum degassing process, after the ladle is placed on the ladle car, the argon gas is opened and blown, with a flow rate of 300-450 NL / min, and the sulfur cored wire is fed. After entering the working position, the lifting operation is started, the immersion tube is inserted to a depth greater than 400 mm, the argon gas is closed, the vacuum degree is <100 Pa, and the duration is 10-15 minutes. After breaking the air, the argon gas is restored, and the silicon calcium wire and the covering agent are fed, and the static argon blowing time is >10 minutes; the chemical out of the station The internal control requirements for composition are: C: 0.193~0.208%, Si: 0.18~0.22%, Mn: 0.99~1.03%, P≤0.025%, S: 0.017~0.027%, Cr: 1.13~1.17%, Ti: 0.04~0.06%, Ni≤0.05%, Cu≤0.05%, B≤0.0004%, Alt: 0.010%~0.035%.

10. The smelting method according to claim 9, characterized in that: In the continuous casting process, the vacuum degassed molten steel is hoisted to the casting position, the superheat of the molten steel is controlled to be less than 40°C, a continuous slag detection device is used for the continuous casting ladle, and the protective slag is low-carbon titanium steel protective slag; The current of crystallization electromagnetic stirring was 340A and the frequency was 2 Hz; the current of terminal electromagnetic stirring was 360A and the frequency was 7 Hz, and the stirring mode was an alternating mode of 10s-3s-10s.

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