CN114769535B - Method for reducing carbon segregation limit of carbon steel in round billet by high-frequency pulse current and terminal electromagnetic stirring composite technology - Google Patents
Method for reducing carbon segregation limit of carbon steel in round billet by high-frequency pulse current and terminal electromagnetic stirring composite technology Download PDFInfo
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- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/10—Supplying or treating molten metal
- B22D11/11—Treating the molten metal
- B22D11/114—Treating the molten metal by using agitating or vibrating means
- B22D11/115—Treating the molten metal by using agitating or vibrating means by using magnetic fields
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/12—Accessories for subsequent treating or working cast stock in situ
- B22D11/122—Accessories for subsequent treating or working cast stock in situ using magnetic fields
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/16—Controlling or regulating processes or operations
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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Abstract
The invention provides a method for reducing carbon segregation of carbon steel in round billets by using a high-frequency pulse current and terminal electromagnetic stirring composite technology. Meanwhile, in order to prevent secondary remelting crystallization of refined solidification structures, a terminal electromagnetic stirring technology is adopted to continuously reduce the average temperature gradient (G) of the core part of the casting blank and break up the remelting crystallization, so that the carbon segregation of the center of the casting blank is reduced, and the carbon segregation is controlled to be less than or equal to 0.03%.
Description
Technical Field
The invention belongs to the field of alloys, and particularly relates to a method for reducing carbon segregation limit of carbon steel in round billets by a high-frequency pulse current and terminal electromagnetic stirring composite technology.
Background
The medium carbon steel is used as steel for manufacturing the crankshaft, bears the impact load and transmits power in an engine, has very bad service conditions, bears periodic load in the working process, and generates sliding friction with a bearing at a very high relative speed under the action of pressure, thereby generating higher temperature and abrasion. Therefore, the materials for manufacturing the parts have better comprehensive mechanical properties such as strength, plasticity, toughness and the like, and the uniformity of the mechanical properties is directly related to the component segregation degree of the materials. Carbon is a main solute element in molten steel, the carbon element can be pushed to the center along a solid-liquid interface along with solidification in the solidification process, carbon segregation is formed during final solidification, the influence on the quality of a finished product is large, and the segregation condition of the carbon element is one of important indexes for judging continuous casting billets.
Through patent search and market research, the domestic metallurgical manufacturers aiming at the problem mostly adopt two modes: the first method for controlling carbon segregation by adjusting technological parameters (including electromagnetic stirring parameters) of a continuous casting machine is disclosed in patent publication No. CN 111230061A, which is published in 6/5 of 2020, and the method for reducing carbon segregation of a medium carbon steel round billet is characterized in that the production conditions of columnar crystals in the solidification process of a casting blank and the solid phase rate of the casting blank at the tail end electromagnetic stirring position can meet the electromagnetic stirring requirement by reasonably matching the technological parameters of the continuous casting machine. The disadvantages of the process are: firstly, the process window is narrower, and the carbon segregation control stability effect is poor; secondly, as the tooling equipment of each metallurgical enterprise is different, the popularization and repeatability of the technological parameters are poor; thirdly, the requirement of high-quality medium carbon steel on casting blank carbon element segregation cannot be met. The second is to control carbon segregation by means of the external equipment of the continuous casting machine body, such as an on-line pressing device, and published patent publication number CN 10673502a in 5/31 of 2017, and discloses a continuous casting billet solidification end single-point and continuous heavy pressing process. The core of the process is that a reduction roller for implementing large reduction is set at the sector section, so that the effect of transmitting the strain quantity to the core of the casting blank is achieved, the shrinkage cavity of the casting blank is closed, and the carbon segregation of the casting blank is reduced. The disadvantages of the process are: the pressing process is applied to continuous casting round billets, and the problem of increased ovality of the round billets cannot be avoided.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides a method for reducing carbon segregation of carbon steel in round billets by using a high-frequency pulse current and tail end electromagnetic stirring composite technology, which has larger tolerance to fluctuation of technological parameters of continuous casting processes and good control stability of carbon segregation; the method can meet the requirement of high-quality medium carbon steel on the carbon element segregation of the casting blank, and the carbon segregation is controlled to be less than or equal to 0.03 percent.
The specific technical scheme of the invention is as follows:
a method for reducing carbon segregation limit of carbon steel in round billets by using a high-frequency pulse current and terminal electromagnetic stirring composite technology comprises the following specific steps:
after molten steel enters a continuous casting machine for casting, when a continuous casting blank passes through high-frequency pulse current and terminal electromagnetic stirring, an induction coil and the terminal electromagnetic stirring are started.
The molten steel is medium carbon steel molten steel;
preferably, the molten steel is medium carbon 40Cr steel; the molten steel comprises the following components in percentage by mass: 0.37-0.44% of C, 0.20-0.30% of Si, 0.60-0.70% of Mn, less than or equal to 0.015% of P, less than or equal to 0.015% of S and 0.90-1.00% of Cr; the balance being Fe and unavoidable impurities.
When the molten steel is medium carbon 40Cr steel, the converter converting and refining furnace slagging and deoxidizing process is carried out according to the conventional high grade steel, and the following processes and parameters are controlled: 1) Ensuring the uniformity of the element components of molten steel in the steel ladle before the single-furnace continuous casting process is operated; 2) Before the single-furnace continuous casting process is operated, the temperature of molten steel in the steel ladle is ensured to be 20-50 ℃ so as to avoid production accidents of a continuous casting machine and damage equipment used by a high-frequency pulse current technology.
The high-frequency pulse current controls the voltage of the induction coil equipment to be 210-243V;
the terminal electromagnetism controls the terminal electromagnetism stirring current parameter to be (300-400) A and the frequency to be (5-8) HZ;
the production process parameters of the continuous casting machine are as follows: the pulling speed is (0.7-1.0) m/min, the superheat degree is (25-35) DEG C, the specific water quantity of secondary cooling is (0.25-0.3) L/Kg, and the water quantity of crystallizer is (115-125) m 3 /h。
Preferably, the invention also provides a relation between the production pulling speed of the continuous casting machine, the control of the electromagnetic stirring current at the tail end and the voltage control in the high-frequency pulse current control, and the relation is as follows:
when the pulling speed is more than or equal to 0.70m/min and less than or equal to 0.75m/min, the voltage is controlled at 210V, and the electromagnetic stirring current at the tail end is controlled at 400A;
when the pulling speed is more than 0.75m/min and less than or equal to 0.80m/min, the voltage is more than or equal to 230V and less than or equal to 235V, and the electromagnetic stirring current at the tail end is controlled to be 400A;
when the pulling speed is more than 0.80m/min and less than or equal to 0.85m/min, the voltage is more than 240V and less than or equal to 243V, and the electromagnetic stirring current at the tail end is controlled to be 400A;
when the pulling speed is more than 0.85m/min and less than or equal to 0.90m/min, 235V is more than or equal to 240V, and the electromagnetic stirring current at the tail end is controlled to be 400A;
when the pulling speed is more than 0.90m/min and less than or equal to 0.95m/min, 220V is more than or equal to 223V, and the electromagnetic stirring current at the tail end is controlled to be 350A;
when the pulling speed is more than 0.95m/min and less than or equal to 1.00m/min, the voltage is more than 210V and less than or equal to 215V, and the electromagnetic stirring current at the tail end is controlled to be 300A.
The method can be used for round billets with all cross section sizes, particularly for round billets with end face sizes larger than or equal to phi 300mm, the round billets with large cross sections are affected by solidification heat transfer, primary billet shells are thicker, effective cooling lengths are shorter, therefore, the solidification rate of a large round billet continuous casting machine is low, coarse columnar crystals and equiaxed crystals can be formed in the casting blank solidification process, and serious carbon segregation and punctiform loose defects are caused by billet material residues. Particularly, carbon segregation at the 1/2R position tends to be a main cause of exceeding the standard of the finished product carbon, and therefore, control of carbon segregation at the 1/2R position becomes a technical difficulty in production of carbon steel in large round billets. Many metallurgical workers are devoted to the study of the mechanism and control measures of carbon segregation at the 1/2R position of a round billet continuous casting machine, and although the mechanism of forming such carbon segregation has been already clarified, the control means of carbon segregation at the 1/2R position has been slow in progress, and there is almost no effective method for fundamentally suppressing such deviation. Currently, there is some consensus on control means, such as: in the aspects of reducing the carbon component target, preventing carburetion, stabilizing pouring, soft argon blowing, low superheat degree and the like, but the measures are limited by factors such as long steelmaking process flow, multiple variable factors, complex control and the like in the production process, the process making requirement is difficult to be met by 100%, and even if a small number of enterprises execute in place, the carbon segregation at the 1/2R position cannot be effectively controlled. The invention realizes synergistic effect by high-frequency pulse current and terminal electromagnetic stirring, can reduce carbon segregation limit of carbon steel in round billets, is also applicable to the end face size of large round billets with the diameter of more than or equal to phi 300mm, and can control the carbon segregation to be less than or equal to 0.03%.
The action mechanism of the invention is as follows: the carbon steel solidification mechanism in the round billet is that the front edge of dendrites moves from the surface layer to the center in the solidification process of the casting blank, and in the process of dendrite growth, the dendrite growth is influenced by an average temperature gradient (G) and an average solidification rate (V), a space exists between secondary dendrites, and the larger the dendrite space is, the more serious the solidification segregation is, so that the carbon content in a CET transition zone of the casting blank is higher. When the casting blank is solidified near tail sound, the solidified dendrite is developed to obstruct the flow of molten steel in the interior, and the feeding of the center of the casting blank is difficult, so that the center carbon content is higher. Based on the solidification mechanism, the invention utilizes the high-frequency oscillating magnetic field formed by the high-frequency pulse current in the metal melt near the solid-liquid interface of the casting blank to realize the magneto supercooling effect, reduce the average temperature gradient (G) and refine the solidification structure, thereby reducing the inter-dendrite spacing and reducing the carbon content segregation in the CET transition zone of the casting blank. Meanwhile, in order to prevent secondary remelting crystallization of refined solidification structures, a terminal electromagnetic stirring technology is adopted to continuously reduce the average temperature gradient (G) of the core of the casting blank and break up the remelting crystallization, so that carbon segregation of the center of the casting blank is reduced.
Compared with the prior art, the invention innovatively provides a method for controlling carbon element segregation, which firstly leads the pulse magneto oscillation technology to the production field of round billet continuous casting machines and provides a stronger reference platform for producing high-quality 40Cr steel by the round billet continuous casting machines in the same industry; the process method also has the characteristic of sharing square/round billets, and overcomes the defect that an online pressing process scheme cannot be suitable for round billets; compared with the traditional process conditions (adjusting the basic process parameters of the continuous casting machine), the process method of the invention has the advantages of stable and reliable effect; the process method provides a new idea for the high-efficiency production of the round billet continuous casting machine.
Drawings
FIG. 1 is a schematic diagram of a sample preparation with extremely poor carbon segregation.
Detailed Description
Terms and definitions of the invention:
1) Definition of medium carbon steel: the carbon steel or alloy structural steel with carbon content ranging from 0.2% to 0.60% is generally specified by reference to GB/T3077-2015 and GB/T699-2015; for the convenience of the person skilled in the art to understand the present invention, the implementation steps and embodiments of the present invention are represented by 40Cr steel.
2) Carbon segregation is extremely poor: and taking points on the cross section of the casting blank to analyze the difference between the highest value and the lowest value of the carbon content.
3) High frequency pulse current technique: the direction and intensity of the current are changed in unit time by using a frequency conversion technology, so that a high-frequency oscillating magnetic field is formed.
4) Carbon segregation extremely poor sample preparation and analysis method: every 10mm of sampling points are spaced on the cross section of the casting blank in any 2 vertical diameter directions, the sampling point depth is not less than 5mm, the single-point sampling weight is not less than 3 g, and the sampling drill bit is 5mm, as shown in figure 1. And after the sampling is finished, sending the sample to a CS analyzer for assaying and analyzing the content of the carbon element.
5) The high-quality medium carbon steel has extremely poor requirement on the carbon segregation of the casting blank: through market research and feedback of some automobile industry and engineering machinery clients, the carbon segregation limit requirement of the medium carbon steel casting blank is less than or equal to 0.03 percent.
6) The basic conditions of the continuous casting machine used in the invention are shown in table 1.
TABLE 1 basic overview of the continuous casting machine according to the invention
Comparative example 1
The existing production technology only adopts terminal electromagnetic stirring, and the conditions are as follows: the pulling speed is 0.75m/min, the superheat degree (25-35) DEG C, the specific water quantity of the secondary cooling is 0.25L/Kg, and the water quantity of the crystallizer is 120m 3 The carbon segregation limit of the cross section of the 40Cr steel casting with the parameters of end electromagnetic stirring of 400A8HZ is shown in Table 2. From data analysis, the average carbon segregation was substantially 0.05% (see patent application number CN 111230061 a).
TABLE 2 carbon segregation limit of 40Cr steel casting cross section under original process conditions
Table 2 shows that the control stability of the prior art production process on the C-range is not high, and only individual production can reach the carbon segregation range of less than or equal to 0.03%, and most of the prior art production processes are characterized in that the carbon segregation range is more than or equal to 0.03%; however, the customer requires stability, and this approach is not satisfactory for the market because it is unstable.
The following examples of the invention are controlled according to the following process:
step one: the converter blowing and refining process of the medium carbon 40Cr steel and the slag-forming and deoxidizing process of the refining furnace are carried out according to the conventional excellent steel, but the following detailed process scheme needs to be set: 1) Ensuring the uniformity of the element components of molten steel in the steel ladle before the single-furnace continuous casting process is operated; 2) Before the single-furnace continuous casting process is operated, ensuring that the temperature of molten steel in a ladle is required to be 20-50 ℃ so as to avoid production accidents of a continuous casting machine and damage equipment (hereinafter referred to as induction coils) used in a high-frequency pulse current technology;
step two: the continuous casting process of the medium carbon 40Cr steel is carried out according to the conventional excellent special steel, namely after a continuous casting machine starts casting, when a continuous casting blank passes through an induction coil and a tail end to be subjected to electromagnetic stirring, the induction coil and the tail end are started to be subjected to electromagnetic stirring.
Step three: and after the process parameters are adjusted, taking a casting blank sample for analysis, wherein the length of the casting blank sample is not less than 300mm.
To ensure accuracy and repeatability of the smelting process data described in the present invention, specific production parameters of specific examples are listed below:
example 1
A method for reducing carbon segregation limit of carbon steel in round billets by using a high-frequency pulse current and terminal electromagnetic stirring composite technology comprises the following specific steps:
in the production process, a high-frequency pulse current and terminal electromagnetic stirring composite technology is used according to the method, and specific parameters are as follows:
1) The medium carbon 40Cr steel composition requirements are shown in Table 3.
TABLE 3 Steel grade Table of example 1, balance Fe and unavoidable impurities
2) Production process parameters of the continuous casting machine: the pulling speed is 0.75m/min, the superheat degree (25-35) DEG C, the specific water quantity of the secondary cooling is 0.25L/Kg, and the water quantity of the crystallizer is 120m 3 And/h, the electromagnetic stirring parameter at the tail end is 400A8HZ, and the voltage of the induction coil device is 210V and 230V respectively.
3) Sampling analysis results: the carbon segregation of the 40Cr steel produced according to the above process parameters is extremely poor as shown in Table 4. From the experimental results, the voltage of the induction coil equipment is 210V and 230V, and the carbon element segregation limit of the cross section of the casting blank is relatively low.
TABLE 4 example 1 high frequency pulse electric control of carbon segregation limit at different induction coil device voltages
Comparative example 2
A method for reducing carbon segregation limit of carbon steel in round billets by using a high-frequency pulse current and terminal electromagnetic stirring composite technology comprises the following specific steps:
in the production process, a high-frequency pulse current and terminal electromagnetic stirring composite technology is used according to the method, and specific parameters are as follows:
1) The medium carbon 40Cr steel composition requirements are shown in Table 5.
TABLE 5 Steel grade Table of comparative example 2, balance Fe and unavoidable impurities
2) Production process parameters of the continuous casting machine: the pulling speed is 0.75m/min, the superheat degree (25-35) DEG C, the specific water quantity of the secondary cooling is 0.25L/Kg, and the water quantity of the crystallizer is 120m 3 And/h, the terminal electromagnetic stirring parameter is 400A8HZ, and the induction coil device voltages are respectively 50V, 100V, 150V, 200V, 250V and 300V.
3) Sampling analysis results: the carbon segregation of the 40Cr steel produced according to the above process parameters is extremely poor as shown in Table 6. From the experimental results, the voltages of the induction coil devices are 50, 100, 150, 200, 250 and 300V, and the carbon element segregation range of the cross section of the casting blank is relatively high and unstable. As is clear from the results, the adjustment of the voltage of the induction coil device has a larger influence on the extremely poor segregation of carbon element in the cross section of the cast slab than in example 1, and the ratio of carbon element segregation to the cross section of the cast slab cannot be 0.03%.
TABLE 6 comparative example 2 high frequency pulse electric control of carbon segregation limit at different induction coil device voltages
Comparative example 3
A method for reducing carbon segregation limit of carbon steel in round billets by using a high-frequency pulse current and terminal electromagnetic stirring composite technology comprises the following specific steps:
the specific parameters are as follows:
1) The composition of the medium carbon 40Cr steel is shown in table 7;
TABLE 7 Steel grade Table of comparative example 3, balance Fe and unavoidable impurities
2) The continuous casting process parameters are as follows: production process parameters of the continuous casting machine: the pulling speed is 0.70m/min, the superheat degree (25-35) DEG C, the specific water quantity of the secondary cooling is 0.25L/Kg, and the water quantity of the crystallizer is 120m 3 And/h, the electromagnetic stirring parameter at the tail end is 400A8HZ, and the voltage of the induction coil is respectively set as follows: 150V, 170V, 190V, 200V, 250V, 270V, 290V.
3) Sampling analysis results: the carbon segregation limit of the 40Cr steel produced according to the above process parameters is shown in Table 8.
TABLE 8 carbon segregation limit at different coil voltages
Comparative example 4
A method for reducing carbon segregation limit of carbon steel in round billets comprises the following specific steps:
1) The composition of the medium carbon 40Cr steel is shown in table 9;
table 9, comparative example 4 Steel grade composition Table, balance Fe and unavoidable impurities
2) The continuous casting process parameters are as follows: the end electromagnetic stirring (i.e., according to the process parameters of comparative example 1) and the pulsed magneto-oscillation technique (according to the method of example 1, except that the end electromagnetic stirring was turned off) were separately tested, and the specific parameters are shown in table 10. From the experimental results, the carbon segregation effect of the cross section of the casting blank is not ideal.
Table 10, comparative example 4 carbon segregation limit conditions of cast blanks under different process conditions
Example 2
A method for reducing carbon segregation limit of carbon steel in round billets by using a high-frequency pulse current and terminal electromagnetic stirring composite technology comprises the following specific steps:
the specific parameters are as follows:
1) The composition of the medium carbon 40Cr steel is shown in table 11;
table 11, example 2 Steel grade composition Table, balance Fe and unavoidable impurities
2) The continuous casting process parameters are as follows: the pulling speed is 0.75m/min, the superheat degree (25-35) DEG C, the specific water quantity of the secondary cooling is 0.25L/Kg, and the water quantity of the crystallizer is 120m 3 The electromagnetic stirring parameter at the tail end is 400A8HZ, and the pulse magneto oscillation voltage is 220V;
3) Sampling analysis results: the carbon segregation limit of the 40Cr steel produced according to the above process parameters is shown in Table 12.
Table 12, carbon segregation in the cross section of the 40Cr steel cast slab produced in example 2 was extremely poor
Example 5
By combining the results of examples 1-2, the invention continues to expand experimental results, and searches for pulse magneto-oscillation and terminal electromagnetic stirring composite parameters under different pull-up speeds on the basis of ensuring the carbon segregation limit (the carbon segregation limit is less than or equal to 0.03%). The continuous casting machine provided by the invention is intended to improve the station time efficiency, so that pulse magneto oscillation and terminal electromagnetic stirring composite parameters under different pulling speeds are continuously tested according to the steps of the invention, and a process parameter execution table of 40Cr steel under different pulling speeds is formulated according to the test results, as shown in table 13.
Table 13, 40Cr steel process parameter execution table under different pull rate conditions
Example 3
A method for reducing carbon segregation limit of carbon steel in round billets by using a high-frequency pulse current and terminal electromagnetic stirring composite technology comprises the following specific steps:
the specific parameters are as follows:
1) The composition of the medium carbon 40Cr steel is shown in table 11;
TABLE 14 example 3 Steel grade composition Table, balance Fe and unavoidable impurities
2) The continuous casting process parameters are as follows: the pulling speed is respectively 0.80, 0.90, 1.0m/min, the superheat degree (25-35) DEG C, the specific water quantity of the secondary cooling is 0.25L/Kg, and the water quantity of the crystallizer is 120m 3 And/h, the terminal electromagnetic stirring parameter is 400A8HZ, and the corresponding pulse voltages are 232V, 238V and 213V respectively.
In conclusion, after the process is adopted, the carbon segregation limit of the cross section of the round billet 40Cr steel casting billet can be stably controlled below 0.03%, and compared with the average carbon segregation limit of 0.05% before process optimization, the carbon segregation limit is obviously reduced; after the process is adopted, the booster continuous casting machine can realize high-efficiency production on the premise of ensuring the product quality.
Claims (2)
1. A method for reducing carbon segregation limit of carbon steel in round billets by using a high-frequency pulse current and terminal electromagnetic stirring composite technology is characterized by comprising the following specific steps:
after molten steel enters a continuous casting machine for casting, when a continuous casting blank passes through high-frequency pulse current and terminal electromagnetic stirring, an induction coil and the terminal electromagnetic stirring are started;
the relation between the pulling speed of the continuous casting machine production, the control of the electromagnetic stirring current at the tail end and the voltage control in the high-frequency pulse current control is specifically as follows:
when the pulling speed is more than or equal to 0.70m/min and less than or equal to 0.75m/min, the voltage is controlled at 210V, and the electromagnetic stirring current at the tail end is controlled at 400A;
when the pulling speed is more than 0.75 and m/min and less than or equal to 0.80 and m/min, the voltage is more than or equal to 230V and less than or equal to 235V, and the electromagnetic stirring current at the tail end is controlled to be 400A;
when the pulling speed is more than 0.80 and m/min and less than or equal to 0.85 and m/min, the voltage is more than 240 and V and less than or equal to 243V, and the electromagnetic stirring current at the tail end is controlled to be 400A;
when the pulling speed is more than 0.85 and m/min and less than or equal to 0.90 and m/min, the voltage is more than 235V and less than or equal to 240V, and the electromagnetic stirring current at the tail end is controlled to be 400A;
when the pulling speed is more than 0.90 and m/min and less than or equal to 0.95 and m/min, 220V is more than or equal to 223V, and the electromagnetic stirring current at the tail end is controlled to be 350A;
when the pulling speed is more than 0.95m/min and less than or equal to 1.00m/min, 210V is more than or equal to 215V, and the electromagnetic stirring current at the tail end is controlled to be 300A;
the casting blank produced by the method is controlled to have carbon segregation less than or equal to 0.03%;
the molten steel is medium carbon 40Cr steel; the molten steel comprises the following components in percentage by mass: 0.37-0.44% of C, 0.20-0.30% of Si, 0.60-0.70% of Mn, less than or equal to 0.015% of P, less than or equal to 0.015% of S and 0.90-1.00% of Cr; the balance being Fe and unavoidable impurities.
2. The method of claim 1, wherein the method produces a round billet with a face size of ∈300mm or more.
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