A casting method for reducing carbon element segregation in large castings

Abstract

The present invention relates to the technical field of casting of castings, and particularly relates to a casting method for reducing carbon element segregation in large castings. The casting steps include: controlling the carbon element in the refined molten steel, and controlling the carbon element content to the standard carbon content according to the steel grade; transferring the molten steel with qualified carbon content to the ladle; degassing the molten steel in the ladle, soft blowing for 5 minutes after degassing, and then adding a carbon wire to the upper molten steel in the ladle for carbon increase; adding a carbon-free protective slag to the ladle, and the fixed carbon in the carbon-free protective slag ≤ 2%; adopting bottom casting method for casting, and the casting temperature is 1515 - 1525 °C; after casting is completed, use a ring-shaped air cooling to cool down the ingot mold. The present invention effectively reduces the problem of carbon element segregation in large castings through the design of casting process, auxiliary material addition and ring-shaped air cooling device.

Description

Technical Field

[0001] The present invention relates to the technical field of casting of castings, and particularly relates to a casting method for reducing carbon element segregation in large castings. Background Art

[0002] Large steel ingots are mainly used in the manufacture of large machinery, including medium carbon steel and high carbon steel. The demand for large forgings is increasing, and at the same time, the quality requirements for forgings are also getting higher. Large forged steel ingots are raw materials for processing forgings, and their quality is particularly important for the quality of the final forgings. Due to the large size and high tonnage of large steel ingots themselves, the solidification process is relatively slow. Depending on the size of the steel ingot, the solidification time ranges from several hours to dozens of hours. During the solidification process of molten steel, solute selection crystallization occurs, resulting in the enrichment of low melting point elements such as C, P, and S before solidification, making the chemical composition of different regions of the steel ingot uneven and causing segregation phenomena.

[0003] Chinese Patent Application (CN 114921615 A) discloses a method for reducing segregation of non-vacuum casting carbon steel and alloy steel ingots, including the following steps: (a) melting molten steel with molten iron and scrap steel as raw materials; (b) adding aluminum blocks and a composite deoxidizer for preliminary deoxidation when pouring the molten steel, and the addition amount of the aluminum blocks is determined according to the measured oxygen content of the molten steel; (c) performing LF refining on the preliminarily deoxidized molten steel through a ladle; the LF refining includes adding lime to form slag in the ladle and then performing slag surface diffusion deoxidation; (d) performing vacuum degassing on the molten steel after LF refining; (e) performing argon blowing on the molten steel after vacuum degassing to uniform the composition and temperature of the molten steel in the ladle; (f) injecting the molten steel into a hanging ingot mold at 30 - 40°C and dry without cracks; when the molten steel is poured to 2 / 3 of the ingot mold riser, add a high-efficiency heat-generating agent.

[0004] However, there are top casting methods and bottom casting methods for casting large steel; when using the bottom casting method to cast large steel ingots over 40 tons, during solidification, the carbon element in the molten steel will move from bottom to top in the molten steel, thereby causing carbon element segregation in the steel ingot. Carbon segregation generated during casting in a non-vacuum state will be more serious. When detecting the carbon element of 55# steel, it is found that the carbon content at the bottom of the casting is nearly half less than that at the top riser, and moreover, as the ingot type of the casting increases, this carbon segregation tendency becomes more obvious. Summary of the Invention

[0005] Aiming at the technical problem that carbon element segregation easily occurs in large castings, the present invention provides a casting method for reducing carbon element segregation in large castings.

[0006] The technical solution of the present invention is as follows:

[0007] A casting method for reducing carbon element segregation in large castings, including the following steps:

[0008] (1) Control the carbon element in the refined molten steel, and control the carbon element content to the standard carbon content according to the steel grade; reducing the carbon content in the molten steel can ensure subsequent supplementation;

[0009] (2) Transfer the molten steel with qualified carbon content to the ladle;

[0010] (3) Degasify the molten steel in the ladle, soft blow for 5 minutes after degassing, and then add carbon wire to the upper molten steel in the ladle for carbon increase;

[0011] (4) Add carbon-free protective slag to the ladle, and the fixed carbon in the carbon-free protective slag ≤ 2%;

[0012] (5) Cast the molten steel in step (4) into the ingot mold by the bottom casting method, and the casting temperature is 1515 - 1525 °C;

[0013] (6) After casting is completed, use a ring-shaped air cooling to cool down the ingot mold.

[0014] Further, the standard carbon content in step (1) = (maximum carbon content - minimum carbon content) / 4 + minimum carbon content; the maximum and minimum carbon contents are determined according to the steel grade.

[0015] Further, the carbon increase amount of medium carbon steel in step (3) is 0.02% - 0.04% of the standard carbon content, and the carbon increase amount of high carbon steel is 0.07% - 0.08% of the standard carbon content.

[0016] Further, in step (3), degassing is carried out by the negative pressure formed by the high-temperature and high-pressure steam jet, so that the pressure in the vacuum tank is far lower than the atmospheric pressure, below 67 Pa, and maintained for 25 - 35 minutes. Then, the molten steel in the ladle is blown with argon from the bottom of the ladle to make the molten steel in the ladle flow up and down, and finally remove H, N, and O in the molten steel.

[0017] Further, the addition amount of carbon-free protective slag in step (4) is 0.9 - 1.1 kg / t; at the same time, add 1.1 - 1.5 kg / t of heating agent.

[0018] Further, the carbon-free protective slag in step (4) is based on 15% - 20% of SiO2, 45% - 55% of CaO, and 20% - 30% of Al2O3, and the total amount of 2% - 10% of alkaline earth oxides (Na2O, Li2O, K2O) and fluorides (CaF2, NaF) is used as a flux; the fixed carbon with a content ≤ 1% is used as a skeleton.

[0019] Further, in step (5), the ingot mold is an ingot mold with a single-sided taper of 2.0%.

[0020] Further, step (6) includes:

[0021] a. An annular air-cooling device is installed outside the ingot mold;

[0022] b. Immediately after the molten steel casting is completed, ventilation is carried out for air-cooling, and the ventilation time is controlled to be 4 - 6 h according to the size of the ingot;

[0023] c. After the air-cooling is completed, the annular air-cooling device is removed, and mold cooling is carried out, and the internal preheating of the ingot is used to eliminate stress.

[0024] The beneficial effects of the present invention are as follows:

[0025] (1) The present invention effectively reduces the problem of carbon element segregation in large castings through the design of the casting process, auxiliary material addition, and annular air-cooling device.

[0026] (2) The casting temperature of the molten steel is reduced by 5 - 8 °C compared with the existing casting temperature. Reducing the casting temperature reduces the liquid shrinkage of the molten steel; it helps the casting to cool down below the solidification point faster and reduces the occurrence of segregation.

[0027] (3) The addition of no carbon protection slag and heating agent slows down the solid shrinkage time of the molten steel and provides molten steel supplement for the large shrinkage generated during the solid-liquid transformation of high-carbon steel.

[0028] (4) The design of the annular air-cooling device realizes sequential solidification during casting, and at the same time accelerates cooling to slow down the diffusion of carbon elements and alloy elements, thereby reducing the segregation of large steel ingots. Description of the Drawings

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0030] Figure 1 is a sectional view of the annular air-cooling device.

[0031] Figure 2 is a sampling point diagram for measuring the carbon content of the ingot mold.

[0032] In the figure, 1 - ingot mold, 2 - base, 3 - runner, 4 - middle pouring tube, 5 - annular air-cooling device. Detailed Embodiments

[0033] To enable those skilled in the art to better understand the technical solutions in the present invention, the following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0034] Example 1

[0035] A casting method for a medium-carbon steel 35# steel ingot specifically includes the following steps:

[0036] (1) Control the carbon element of the refined molten steel. According to the steel number 35#, determine the carbon content range to be 0.32% - 0.39%; control the standard carbon content in the molten steel to be (0.39% - 0.32%) / 4 + 0.32% = 0.355%;

[0037] (2) Transfer the molten steel with qualified carbon content to the ladle;

[0038] (3) Through the negative pressure formed by the high-temperature and high-pressure steam injection, make the pressure in the vacuum tank far lower than the atmospheric pressure, maintain it at 60 Pa for 35 min, then make the molten steel in the ladle flow up and down through the bottom argon blowing in the tank, finally remove H, N, and O in the molten steel, complete the degassing treatment of the molten steel in the ladle, softly blow for 5 min after degassing, and then add carbon wire to the upper molten steel in the ladle for carbon increment, and the carbon increment is 0.2% of the standard carbon content;

[0039] (4) Add to the ladle. The addition amount of the carbon-free protective slag is 0.9 kg / t, and at the same time add 1.2 kg / t of the heat-generating agent; the carbon-free protective slag includes 20% of SiO2, 55% of CaO, 20% of Al2O3, 2% of Li2O, 2.85% of CaF2, and 0.15% of fixed carbon;

[0040] (5) Use the bottom casting method to cast the molten steel in step (4) into the ingot mold. The ingot mold is a small taper ingot mold, the unilateral taper of the ingot mold is 2.0%, and the casting temperature is 1515 °C;

[0041] (6) After the casting is completed, use the annular ring air cooling to cool down the ingot mold. The specific steps include:

[0042] a. Install an annular air cooling device outside the ingot mold;

[0043] b. Immediately ventilate for air cooling after the molten steel casting is completed, and control the ventilation time to be 6 h according to the size of the ingot;

[0044] c. After the air cooling is completed, remove the annular air cooling device and perform mold cooling to eliminate stress by using the internal preheating of the ingot.

[0045] Comparative Example 1

[0046] Use the same device as in Example 1 to cast a 35# steel ingot; control the carbon element in the refined molten steel, with a carbon content of 0.35%, and directly cast the molten steel using the bottom casting method, with a casting temperature of 1530°C; after casting, cool down, and the cooling time is 6 h.

[0047] Take points on the longitudinal section of the center of the steel ingot cast in Example 1 to measure the carbon content at different positions, and analyze the results of carbon element segregation. Take eleven points on the horizontal line at the upper end of the steel ingot (100 mm from the top surface) for carbon content detection, denoted as M10 - M20; the eleven sampling positions are equally spaced;

[0048] Take five points 100 mm below the sampling positions of M10 - M20 for carbon content detection, denoted as M31 - M35, and the five point positions are equally spaced;

[0049] Take nine points on the horizontal line at the lower end of the steel ingot (100 mm from the bottom surface) for carbon content detection, denoted as S10 - S18, and the intervals between S10 - S18 are equal;

[0050] Take six points on the central axis of the steel ingot for carbon content detection, denoted as H04 - H09, and the intervals between H04 - H09 are equal; M15, M33, and S14 are on the same central axis of the steel ingot;

[0051] The sampling positions of the steel ingot in Comparative Example 1 are the same as those of the steel ingot in Example 1, and the specific positions are as Figure 2 shown; the carbon content in Comparative Example 1 is the carbon content before improvement, and the carbon content in Example 1 is the carbon content after improvement. The specific measurement results are shown in Table 1.

[0052] Table 1 Carbon content at different sampling positions on the steel ingot

[0053] Position M10 M11 M12 M13 M14 M15 M16 M17 M18 M19 M20 Carbon content before improvement (%) 0.36 0.38 0.40 0.45 0.51 0.50 0.49 0.46 0.39 0.37 0.35 Carbon content after improvement (%) 0.36 0.38 0.39 0.41 0.44 0.46 0.43 0.40 0.39 0.37 0.35 Position M31 M32 M33 M34 M35 H04 H05 H06 H07 H08 H09 Carbon content before improvement (%) 0.34 0.39 0.46 0.38 0.35 0.39 0.36 0.35 0.34 0.33 0.32 Carbon content after improvement (%) 0.34 0.38 0.42 0.37 0.35 0.38 0.36 0.35 0.34 0.33 0.32 Position S10 S11 S12 S13 S14 S15 S16 S17 S18 / / Carbon content before improvement (%) 0.34 0.32 0.29 0.27 0.25 0.28 0.30 0.33 0.35 / / Carbon content after improvement (%) 0.36 0.33 0.32 0.31 0.29 0.30 0.31 0.33 0.35

[0054] Through the detection of the carbon content at different positions of the steel ingots in Comparative Example 1 and Example 1 in Table 1, it can be seen that in Example 1, by controlling the carbon element of the refined molten steel, adding auxiliary materials, reducing the casting temperature, and annular air cooling, the carbon element content at each point in the steel ingot is more stable, and the carbon element segregation in the steel ingot is reduced.

[0055] Example 2

[0056] A casting method for an 80# steel ingot, specifically including the following steps:

[0057] (1) Control the carbon element in the refined molten steel to make the carbon element content reach the standard carbon content; the standard carbon content = (0.85% - 0.77%) / 4 + 0.77% = 0.81%;

[0058] (2) Transfer the molten steel with qualified carbon content to the ladle.

[0059] (3) Through the negative pressure formed by the high-temperature and high-pressure steam injection, make the pressure in the vacuum tank far lower than the atmospheric pressure, maintain it at 65 Pa for 35 min, and then make the molten steel in the ladle flow up and down by blowing argon through the bottom of the ladle in the tank, finally remove H, N, and O in the molten steel, complete the degassing treatment of the molten steel in the ladle, soft blow for 5 min after degassing, and then add carbon wire to the upper molten steel in the ladle for carbon increase, and the carbon increase amount is 0.08% of the standard carbon content.

[0060] (4) Add non-carbon protective slag to the ladle at an amount of 1.1 kg / t, and at the same time add a heating agent at 1.5 kg / t; the non-carbon protective slag includes 15% SiO2, 45% CaO, 30% Al2O3, 5% K2O, 4% CaF2, and 1% fixed carbon.

[0061] (5) Cast the molten steel in step (4) into the ingot mold by the bottom casting method, and the casting temperature is 1525 °C; the ingot mold is an ingot mold with a unilateral taper of 2.0%.

[0062] (6) After the casting is completed, use ring-shaped air cooling to cool down the ingot mold. The specific steps include:

[0063] a. Set up a ring-shaped air cooling device outside the ingot mold.

[0064] b. Immediately ventilate for air cooling after the molten steel casting is completed, and control the ventilation time to be 6 h according to the size of the ingot.

[0065] c. After the air cooling is completed, remove the ring-shaped air cooling device and perform mold cooling to eliminate stress by using the internal preheating of the ingot.

[0066] Although the present invention has been described in detail by referring to the accompanying drawings and in combination with the preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, those of ordinary skill in the art can make various equivalent modifications or substitutions to the embodiments of the present invention, and these modifications or substitutions should all be within the scope covered by the present invention / Any person skilled in the art within the technical scope disclosed by the present invention can easily think of changes or substitutions, and all should be covered within the protection scope of the present invention.

Claims

1. A casting method for reducing carbon element segregation in large castings, characterized in that, It includes the following steps: (1) Control the carbon element of the refined molten steel, and control the carbon element content to the standard carbon content according to the steel grade; Standard carbon content = (maximum carbon content - minimum carbon content) / 4 + minimum carbon content; the maximum and minimum carbon contents are determined according to the steel grade; (2) Transfer the molten steel with qualified carbon content to the ladle; (3) Degasify the molten steel in the ladle. After degassing, soft blow for 5 minutes, and then add carbon wire to the upper molten steel in the ladle for carbon increase; The carbon increase amount for medium carbon steel is 0.02% - 0.04% of the standard carbon content, and the carbon increase amount for high carbon steel is 0.07% - 0.08% of the standard carbon content; (4) Add carbon-free protective slag to the ladle, and the fixed carbon in the carbon-free protective slag ≤ 2%; (5) Cast the molten steel in step (4) into the ingot mold by bottom casting method, and the casting temperature is 1515 - 1525 °C; (6) After casting is completed, use ring-shaped air cooling to cool down the ingot mold.

2. The casting method for reducing carbon element segregation in large castings according to claim 1, characterized in that, In step (3), degassing is achieved by the negative pressure formed by the high-temperature and high-pressure steam injection, so that the pressure in the vacuum tank is far lower than the atmospheric pressure, below 67 Pa, and maintained for 25 - 35 minutes. Then, blow argon from the bottom of the ladle in the tank to make the molten steel in the ladle flow up and down, and finally remove H, N, and O in the molten steel.

3. The casting method for reducing carbon element segregation in large castings as described in claim 1, characterized in that, In step (4), the addition amount of carbon-free protective slag is 0.9 - 1.1 kg / t, and at the same time, add 1.1 - 1.5 kg / t of heat-generating agent.

4. A casting method for reducing carbon element segregation in large castings as described in claim 1, characterized in that, In step (4), the carbon-free protective slag is based on 15% - 20% SiO2, 45% - 55% CaO, and 20% - 30% Al2O3, and the total amount of 2% - 10% alkaline earth oxides and fluorides is used as a flux; the fixed carbon with a content ≤ 1% is used as a skeleton.

5. A casting method for reducing carbon element segregation in large castings as described in claim 1, characterized in that, In step (5), the ingot mold is an ingot mold with a unilateral taper of 2.0%.

6. The casting method for reducing carbon element segregation in large castings according to claim 1, characterized in that, Step (6) includes: a. Install a ring-shaped air cooling device outside the ingot mold; b. Immediately ventilate for air cooling after the molten steel casting is completed, and control the ventilation time to 4 - 6 hours according to the size of the ingot; c. After air cooling is completed, remove the ring-shaped air cooling device and perform mold cooling to eliminate stress using the internal heat of the ingot.

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