Austenite grain size uniformity control method for gear steel

Abstract

The invention provides an austenite grain size uniformity control method for gear steel. According to the method, during smelting, the content of smelting components Al is controlled to be 0.030%-0.050%, and the content of N is controlled to be 0.006%-0.012%. The manufacturing process comprises the steps of LD converter, LF external refining, RH vacuum degassing, continuous casting, stepping heating furnace heating, high-pressure water descaling, rough rolling, pre-finish rolling, PSM rolling mill diameter reducing and sizing, water cooling, cold bed feeding, cutting and sizing, slow cooling in a slow cooling pit, finishing, collecting, packaging and weighing, and checking and warehousing. According to the method, it is guaranteed that the carburized austenite grain size and the quenched austenite grain size and uniformity of the 20CrMo gear steel with the specification of phi 20-50 mm both meet the requirements of the gear steel.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of special steel smelting, and particularly relates to a method for controlling the austenite grain size uniformity of gear steel 20CrMo with a specification of Phi 20-50 mm. BACKGROUND

[0002] Gears are important parts commonly used in automobiles and are mainly used for power transmission and speed change. During operation, the meshing tooth surfaces of gears not only roll but also slide, and are also subjected to alternating stress, so the working conditions are very complex. The 20CrMo gear steel is mainly used for the speed change gears of medium-sized automobiles and the main and passive gears of rear axles of light automobiles, and has good mechanical properties and hardenability, small heat treatment distortion and high brittle fracture resistance. Therefore, it is very important to ensure the hardenability, purity and grain size of the 20CrMo steel in production. The hardenability of the steel mainly depends on the chemical composition, and the grain size is mainly controlled through heat treatment. In the heat treatment process, the austenite grain of the 20CrMo gear steel is required to be not less than 6 levels and not mixed, only in this way can the fatigue life of the gear meet the requirements.

[0003] According to market demand, the 20CrMo steel with a specification of Phi 20-50 mm for gears is developed and produced. When the carburized austenite grain size (heating temperature 930 DEG C + or - 10 DEG C, carburizing + holding for 6 hours, cooling in the furnace) and the quenched austenite grain size (heating temperature 910 DEG C + or - 10 DEG C, holding for 3 hours, quenching cooling) of the gear steel 20CrMo are detected according to the national standard GB / T 6394-2017, it is found that the austenite grain size measured by the carburizing method is 8.0 levels, the grain is uniform and not mixed, the austenite grain size measured by the quenching method is not uniform and mixed, and cannot be graded, which cannot meet the use requirements of the gear steel.

[0004] Therefore, in order to meet the requirements of the gear steel carburizing and quenching austenite grain size uniformity (the grain size is not less than 6 levels and not mixed), it is urgent to develop a new method for controlling the austenite grain size uniformity of the gear steel 20CrMo. SUMMARY

[0005] In order to solve the technical problems in the background art, the application provides a method for controlling the austenite grain size uniformity of gear steel.

[0006] In order to achieve the above purpose, the application adopts the following technical scheme: a method for controlling the austenite grain size uniformity of gear steel, wherein the content of Al is controlled to be 0.030%-0.050% and the content of N is controlled to be 0.006-0.012% during smelting.

[0007] Al has the effect of refining the grain in the steel and preventing the mixed grain size of austenite, but Al plays the above role because Al can generate AlN with N in the steel, and AlN can pin the grain boundary in the steel to prevent grain growth, thereby refining the grain and preventing the mixed grain size of austenite. Tests show that the Al content in the finished product of the newly developed gear steel 20CrMo steel is 0.015% to 0.019%, the N content is 0.0040% to 0.0050%, the carburized austenite grain size does not occur mixed grain, and the quenched austenite grain size occurs mixed grain, indicating that even if there is Al and N in the steel, but the Al and N in the steel must reach a certain fraction, in order to prevent the gear steel 20CrMo carburized and quenched austenite grain size from appearing mixed grain, and the grain uniformity is good. Through research, it is found that when the Al content in the finished product of 20CrMo steel reaches ≥0.020%, and the N content reaches 0.006% to 0.012%, a sufficient amount of AlN can be generated in the steel, which can prevent the mixed grain of the carburized austenite grain size and the quenched austenite grain size of 20CrMo steel.

[0008] However, the aluminum content of the 20CrMo smelting composition is controlled to be not less than 0.020%, and it cannot guarantee that the finished product aluminum content is ≥0.020%. If the finished product aluminum content is lower than 0.020%, the quenched austenite grain size of 20CrMo will appear mixed grain. For 20CrMo steel, due to the low carbon content, according to the carbon-oxygen balance product formula in the steelmaking process, the low carbon content means that the oxygen content in the steel will be high, and the aluminum loss in the steel will also be large. Tests show that the aluminum loss of the aluminum content of the 20CrMo smelting composition and the corresponding finished product aluminum composition is the smallest 0.006% and the largest 0.008%. Therefore, in order to guarantee that the Al content in the finished product steel is not less than 0.020%, it is necessary to guarantee that the smelting composition Al content in the steelmaking process is not less than 0.030%, but the Al content in the steelmaking process is too high, which may cause nozzle clogging, affect the process pouring and increase the Al2O3 type inclusions in the steel, therefore, the smelting composition Al content in the steel is preferably controlled between 0.030% and 0.050% to meet the Al content in the finished product steel not less than 0.020%.

[0009] When the smelting composition Al content is controlled to 0.030% to 0.050%, the finished product Al content ≥0.020% can be met, at this time, attention should be paid to the control of the N content in the steel. When the N content in the steel reaches 0.006% to 0.012%, N and Al in the steel can fully generate AlN particles, thereby guaranteeing that the 20CrMo steel does not have mixed grain of carburized austenite grain size detection or quenched austenite grain size detection.

[0010] Specifically, the smelting method comprises the following steps in sequence: (1) LD converter performs carbon pulling operation, process ensures P removal effect, 1 min after tapping, add aluminum iron, then add other alloy, tapping slag, Al content in ladle is controlled at 0.035%~0.055%, N content is controlled at 0.006%~0.012%; (2) LF refining furnace adds refining slag, creates white slag and keeps all the time, at the same time, the liquidity of top slag is ensured, FeO+MnO≤0.50%, according to the content of Al, once aluminum wire can be added in the early stage of refining, and the operation of increasing Al is strictly prohibited in the later stage, the content of Al is controlled at 0.035%~0.055%; (3) Nitrogen / argon control in the process of LF refining: in the early stage, open large nitrogen / argon, the flow is 300-500 NL / min, in the later stage, gradually reduce, before tapping, the flow of nitrogen / argon is 150-200 NL / min in the process of power transmission, the content of N is controlled at 0.006%~0.012%; (4) RH adopts nitrogen circulating gas, the process strictly prohibits the addition of alloy and the return of LF furnace to increase temperature, the vacuum treatment time is ≥15 min, the limit vacuum degree is ≤67Pa; after RH treatment, manganese nitride cored wire can be fed to adjust the content of N to 0.006~0.012%, the continuous soft blowing time is ≥20 min, the addition of carbon additive or other alloy is strictly prohibited during soft blowing, and the content of Al is controlled at 0.035%~0.055%; (5) The whole type tundish is used for continuous casting, the tundish is cleaned, argon is filled and evacuated before casting, the argon blowing time is ≥2 minutes, the whole process is protected casting, and the casting is started outside the tank; the casting speed is 0.70-0.80 m / min, the specific water consumption is 0.15-0.20 L / Kg, in order to ensure the uniformity of composition and the quality of the casting blank, the crystallizer electromagnetic stirring and the solidification end electromagnetic stirring are used; the tundish is left with steel at the end of casting; the composition of continuous casting is measured, the content of Al is controlled to 0.030%~0.050%, and the content of N is controlled to 0.006%~0.012%.

[0011] Compared with the prior art, the advantages and positive effects of the present application are that: by the control method, the content of Al is controlled at 0.030%~0.050%, and the content of N is controlled at 0.006~0.012%, so that the gear steel 20CrMo product meeting the uniform and non-mixed crystal of carburized austenite grain size and quenched austenite grain size can be produced. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 It is the quenched austenite grain size microstructure diagram in example 1 of the present application.

[0013] Figure 2 It is the carburized austenite grain size microstructure diagram in example 1 of the present application. DETAILED DESCRIPTION

[0014] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described below through embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be combined with each other.

[0015] Numerous specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways than those described herein, and therefore the invention is not limited to the specific embodiments disclosed in the following specification. Example 1

[0016] This embodiment provides a method for controlling the uniformity of austenitic grain size in gear steel. Table 1 shows the composition requirements for 20CrMo.

[0017]

[0018] Six heats of 20CrMo steel were produced in a continuous casting process, including one heat of 20CrMo gear steel, with the following production details: The LD converter performs carbon removal operation to ensure P removal effect. One minute after tapping, aluminum iron is added, followed by manganese nitride and other alloys. After tapping and slag removal, samples are taken for testing. The Al content of the molten steel in the ladle is 0.036%, and the N content is 0.0063%.

[0019] Refining slag is added to the LF refining furnace to produce white slag, which is maintained at the same time to ensure the fluidity of the top slag. The content of FeO + MnO is 0.41%. Based on the Al content, aluminum wire is added once in the early stage of refining. In the later stage, aluminum addition is strictly prohibited. Sampling and testing showed that the Al content was 0.042%.

[0020] Initially, nitrogen (argon) gas was supplied at a high flow rate of 410 NL / min, which was gradually reduced later. During the power supply process before tapping, the nitrogen (argon) gas flow rate was 178 NL / min, and the nitrogen content was 0.0067% when sampled and tested.

[0021] The RH process uses nitrogen circulating gas. Adding alloys and returning to the LF furnace for heating are strictly prohibited during the process. The vacuum treatment time is 16 minutes, with an ultimate vacuum of 58 Pa. After the RH treatment, 300 meters of manganese nitride cored wire are fed in to adjust the N content to the target value of 0.0090%. Continuous soft blowing takes 22 minutes. During soft blowing, adding carbonizers or other alloys is strictly prohibited. Sampling and testing show that the Al content is 0.036% and the N content is 0.0094%.

[0022] The continuous casting uses an integral tundish. The tundish is thoroughly cleaned and purged with argon gas before casting begins, with an argon blowing time of 3 minutes. The entire casting process is protected, and casting begins outside the ladle. The continuous casting speed is 0.75 m / min, and the specific water content is 0.17 L / kg. To ensure the uniformity of composition and the quality of the billet, advanced technologies such as electromagnetic stirring in the crystallizer and electromagnetic stirring at the end of solidification are used. At the end of casting, residual steel is left in the tundish for operation. The composition of the continuous casting is measured, with an Al content of 0.030% and a N content of 0.0091%.

[0023] The steel was continuously cast into 240mm×300mm billets, and then rolled into Φ20mm 20CrMo round bars. The Al and N content of the finished product was determined, and the Al content was 0.022% and the N content was 0.0089%. The grain size of the carburized austenite and the grain size of the quenched austenite were tested, and no mixed grain phenomenon was found, which met the customer's requirements. The specific test results are shown in Table 2. Example 2

[0024] This embodiment provides a method for controlling the uniformity of austenite grain size in gear steel. Five heats of 20CrMo steel were produced in a continuous casting process, with one heat of 20CrMo gear steel produced as follows: (1) The LD converter performs carbon removal operation, ensuring the P removal effect. After tapping the steel for 1 minute, aluminum iron is added, followed by manganese nitride and other alloys. After tapping the steel, slag is removed, and samples are taken for testing. The Al content of the molten steel in the ladle is 0.036%, and the N content is 0.0067%.

[0025] (2) Add refining slag to the LF refining furnace to produce white slag and maintain it, while ensuring the fluidity of the top slag. The content of FeO+MnO is 0.39%. According to the Al content, aluminum wire is added once in the early stage of refining. In the later stage, aluminum addition is strictly prohibited. Sampling and testing show that the Al content is 0.044%.

[0026] (3) In the early stage, the nitrogen (argon) gas flow rate was increased to 407 NL / min, and then gradually decreased. During the power supply process before tapping the steel, the nitrogen (argon) gas flow rate was 169 NL / min. The sample was tested and the N content was 0.0071%.

[0027] (4) Nitrogen circulating gas is used for RH. Alloys are strictly prohibited from being added during the process and the temperature is raised by returning to the LF furnace. The vacuum treatment time is 17 min and the ultimate vacuum degree is 55 Pa. After the RH treatment is completed, 300 meters of manganese nitride cored wire is fed in to adjust the N content to the target value of 0.0090%. The continuous soft blowing time is 21 min. During the soft blowing, it is strictly prohibited to add carbon additives or other alloys. Sampling and testing are performed. The Al content is 0.041% and the N content is 0.0091%.

[0028] (5) The continuous casting adopts an integral tundish. The tundish is cleaned and purged with argon before casting. The argon blowing time is 2 minutes. The casting process is protected throughout and casting is started outside the tundish. The continuous casting speed is 0.75m / min and the specific water content is 0.17L / Kg. At the same time, in order to ensure the uniformity of composition and the quality of billet, advanced crystallizer electromagnetic stirring and solidification end electromagnetic stirring are adopted. When the casting is finished, the tundish is left with residual steel for operation. The composition of the continuous casting is measured. The Al content is 0.036% and the N content is 0.0087%.

[0029] The steel was continuously cast into 240mm×300mm billets, and then rolled into Φ35mm 20CrMo round bars. The Al and N content of the finished product was determined, and the Al content was 0.028% and the N content was 0.0083%. The grain size of the carburized austenite and the grain size of the quenched austenite were tested, and no mixed grain phenomenon was found, which met the customer's requirements. The specific test results are shown in Table 2. Example 3

[0030] This embodiment provides a method for controlling the uniformity of austenite grain size in gear steel. Nine heats of 20CrMo steel were produced in a continuous casting process, with one heat of 20CrMo gear steel produced as follows: (1) The LD converter performs carbon removal operation, ensuring the P removal effect. After tapping the steel for 1 minute, aluminum iron is added, followed by manganese nitride and other alloys. After tapping the steel, slag is removed, and samples are taken for testing. The Al content of the molten steel in the ladle is 0.037%, and the N content is 0.0063%.

[0031] (2) Add refining slag to the LF refining furnace to produce white slag and maintain it, while ensuring the fluidity of the top slag. The content of FeO+MnO is 0.43%. According to the Al content, aluminum wire is added once in the early stage of refining. In the later stage, aluminum addition is strictly prohibited. Sampling and testing show that the Al content is 0.056%.

[0032] (3) In the early stage, the nitrogen (argon) flow rate was increased to 407 NL / min, and then gradually decreased. During the power supply process before tapping the steel, the nitrogen (argon) flow rate was 171 NL / min. The sample was tested and the N content was 0.0067%.

[0033] (4) Nitrogen circulating gas is used for RH. Alloys are strictly prohibited from being added during the process and the temperature is raised by returning to the LF furnace. The vacuum treatment time is 16 min and the ultimate vacuum degree is 49 Pa. After the RH treatment is completed, 300 meters of manganese nitride cored wire is fed in to adjust the N content to the target value of 0.0090%. The continuous soft blowing time is 23 min. During the soft blowing, it is strictly prohibited to add carbon additives or other alloys. Sampling and testing are performed. The Al content is 0.054% and the N content is 0.0097%.

[0034] (5) The continuous casting adopts an integral tundish. The tundish is cleaned and purged with argon before casting. The argon blowing time is 2 minutes. The casting process is protected throughout and casting is started outside the tundish. The continuous casting speed is 0.75m / min and the specific water content is 0.17L / Kg. At the same time, in order to ensure the uniformity of composition and the quality of billet, advanced crystallizer electromagnetic stirring and solidification end electromagnetic stirring are adopted. When the casting is finished, the tundish is left with residual steel for operation. The composition of the continuous casting is measured. The Al content is 0.049% and the N content is 0.0095%.

[0035] The steel was continuously cast into 240mm×300mm billets, and then rolled into Φ50mm 20CrMo round bars. The Al and N content of the finished product was determined, and the Al content was 0.042% and the N content was 0.0090%. The grain size of the carburized austenite and the grain size of the quenched austenite were tested, and no mixed grain phenomenon was found, which met the customer's requirements. The specific test results are shown in Table 2.

[0036] Table 2. Grain size test results of 20CrMo austenitic gear steel

[0037] As can be seen from the above embodiments, by using the control method of the present invention, the Al content of the smelting composition is controlled at 0.030% to 0.050% and the N content is controlled at 0.006% to 0.012% during smelting, producing gear steel 20CrMo products with a Φ20 to 50mm specification that meet the requirements of uniform and non-mixed grain size of carburized austenite and quenched austenite.

[0038] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments for application in other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method of controlling austenite grain uniformity of a gear steel, characterized by, The method is to control the content of Al to 0.030%-0.050% and the content of N to 0.006-0.012% during smelting.

2. A method of controlling austenite grain uniformity of a gear steel, characterized by, The smelting method comprises the following steps in sequence: (1) LD converter performs carbon pulling operation, and the process ensures P removal effect, and aluminum iron is added after 1 min of tapping, and then other alloys are added, and the ladle is tapped and slagged, and the content of Al is controlled to 0.035%-0.055% and the content of N is controlled to 0.006%-0.012%; (2) LF refining furnace adds refining slag, and white slag is formed and maintained all the time, and the flowability of the top slag is ensured, FeO+MnO≤0.50%, according to the content of Al, once aluminum wire can be added in the early stage of refining, and the operation of increasing Al is strictly prohibited in the later stage, and the content of Al is controlled to 0.035%-0.055%; (3) Nitrogen / argon control during LF refining process: large nitrogen / argon is opened in the early stage, and the flow is 300-500 NL / min, and the flow is gradually reduced in the later stage, and the flow of nitrogen / argon is 150-200 NL / min during the power supply process before tapping, and the content of N is controlled to 0.006%-0.012%; (4) RH adopts nitrogen circulating gas, and the process strictly prohibits the addition of alloy and the return of LF furnace for temperature increase, and the vacuum treatment time is ≥15 min, and the limit vacuum degree is ≤67 Pa; after RH treatment, manganese nitride cored wire can be fed to adjust the content of N to 0.006-0.012%, the continuous soft blowing time is ≥20 min, and the addition of carbon additive or other alloys is strictly prohibited during soft blowing, and the content of Al is controlled to 0.035%-0.055%; (5) The whole intermediate ladle is used for continuous casting, the intermediate ladle is cleaned, argon is filled and exhausted before casting, the argon blowing time is ≥2 min, the whole process is protected casting, and the casting is started outside the tank; the casting speed is 0.70-0.80 m / min, and the specific water volume is 0.15-0.20 L / Kg, and in order to ensure the uniformity of the composition and the quality of the casting blank, electromagnetic stirring in the crystallizer and electromagnetic stirring at the solidification end are adopted; the intermediate ladle is left with steel at the end of pouring; the composition of continuous casting is measured, the content of Al is controlled to 0.030%-0.050%, and the content of N is controlled to 0.006%-0.012%.

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