Casting method for LC1 valve casting

Abstract

The present invention belongs to the technical field of the casting of valve parts. Disclosed is a casting method for an LC1 valve casting. The casting method comprises the steps of mold fabrication, modeling, coating application, mold closing, smelting, pouring, mold opening, sand removal, casting head cutting, polishing, a heat treatment, shot blasting and inspection. The procedures of electric arc furnace (EAF) and ladle furnace (LF) methods are used for performing smelting; by means of special control over chemical components, the content of carbon is specially controlled to be smaller than or equal to 0.20 wt%, the content of manganese is specially controlled to be 0.60-0.70 wt%, the content of phosphorus is specially controlled to be smaller than or equal to 0.025 wt%, the content of sulfur is specially controlled to be smaller than or equal to 0.015 wt%, the content of silicon is specially controlled according to a standard value, the content of molybdenum is specially controlled to be 0.55-0.65 wt%, and the content of nickel is specially controlled to be within the range of 0.30-0.40 wt%; and compared with a heat treatment method consisting of normalizing and tempering, a heat treatment method consisting of quenching and tempering results in high low-temperature impact toughness. By means of the casting method in the present invention, the low-temperature impact toughness of an LC1 valve casting is greatly improved, and the special requirement wherein the low-temperature impact value at -52°C should be larger than or equal to 27 J is met.

Description

A casting method for LC1 valve castings Technical Field

[0001] The invention relates to a casting method for an LC1 valve casting, and belongs to the technical field of valve part casting. Background Art

[0002] LC1 castings, as carbon-molybdenum steel valve parts, are widely used in the construction of petrochemical plants. Its manufacturing standard ASTMA352 / A352M-2021 stipulates that the molybdenum content of LC1 castings should be between 0.45-0.65wt%. Molybdenum can refine the grain size of steel, improve the hardenability of steel, and effectively inhibit the growth of austenite grains at high temperatures. Therefore, the standard stipulates that the minimum value of two specimens and the minimum average value of three tests for LC1 castings under a test temperature of -59°C are 18J, and the minimum value of a single specimen is 14J. In addition, the standard stipulates that LC1 castings can be supplied in a normalized and tempered or liquid quenched and tempered state, with a minimum tempering temperature of 590°C.

[0003] However, with the expansion of petrochemical plant construction areas, especially in high-altitude and cold regions, the application environment of LC1 castings has become more demanding. Currently, many petrochemical plants are built in high-altitude and cold regions, requiring valves to operate normally in low-temperature environments of -52°C.

[0004] To meet these extreme environmental demands and prevent catastrophic failures caused by brittle fracture of LC1 castings at -52°C, users have set higher low-temperature impact strength requirements for LC1 castings, requiring a minimum of 27J at -52°C. This requirement far exceeds the standards specified in the ASTM A352 / A352M-2021 standard, making it difficult for domestic foundries to meet this demanding requirement. Domestic foundries generally fail to meet the 27J requirement, typically only achieving 20J. Current solutions primarily focus on heat treatment, but these efforts have been ineffective. Summary of the Invention

[0005] In order to solve the above technical problems, the purpose of the present invention is to provide a casting method for LC1 valve castings, which can improve the purity of molten steel, significantly reduce the number and size of inclusions in the steel, and thereby improve the impact toughness of the material, especially the low-temperature impact toughness.

[0006] The technical problem to be solved by the present invention is achieved by adopting the following technical scheme: the casting method of the present invention mainly includes mold making, shaping, painting, box assembling, smelting, pouring, box unpacking, sand cleaning, cutting pouring head, grinding, heat treatment, shot blasting and inspection.

[0007] The smelting adopts the EAF (electric arc furnace) + LF (refining furnace) method to improve the purity of the molten steel, significantly reduce the number and size of inclusions in the steel, and thus improve the impact toughness of the material, especially the low-temperature impact toughness.

[0008] Before casting, a spectrometer is used to quickly test the chemical composition. To improve the low-temperature impact toughness of LC1, the chemical composition is specifically controlled. The chemical composition of LC1 valve castings is designed to be as follows, by mass percentage: carbon content is specifically controlled to ≤0.20wt%, manganese content is specifically controlled to 0.60-0.70wt%, phosphorus content is specifically controlled to ≤0.025wt%, sulfur content is specifically controlled to ≤0.015wt%, silicon content is controlled according to standard values, and molybdenum content is specifically controlled to 0.55-0.65wt%. In addition to the special control of elements specified in the standard, nickel content is controlled within the range of 0.30-0.40wt%. Nickel not only increases strength but also significantly improves the steel's plasticity and toughness, particularly its low-temperature impact toughness.

[0009] The quenching + tempering heat treatment method is used, which has higher low-temperature impact toughness than the normalizing + tempering heat treatment method.

[0010] Quenching heat treatment method: Casting charging temperature ≤ 200°C, heating rate 120°C / h. When the temperature reaches 750±10°C, hold for at least 1 hour, then raise the temperature to 900±10°C. The holding time at 900±10°C is calculated based on the maximum wall thickness of the casting, at a rate of 25.4mm / h, for a minimum of 2 hours. Cooling method: Water cooling. The time from fully opening the furnace door to fully immersing the casting in water shall not exceed 60 seconds. The water temperature before quenching shall be ≤ 35°C, and the water temperature in the water tank shall not exceed 50°C during quenching.

[0011] Tempering heat treatment method: Temper immediately after quenching, the tempering temperature is 650±10℃, the heating rate is 120℃ / h. The holding time at 650±10℃ is calculated based on the maximum wall thickness of the casting, the holding time is 25.4mm / h, and the minimum is 3h. The cooling method is air cooling.

[0012] As a preferred example, before the pouring step, a sampling tool is used to sample the molten steel after melting and a spectrometer is used to detect the chemical composition of the sampled molten steel sample; the sampling tool includes an external fixed tool and an internal rotating tool, the fixed tool includes a guide cylinder and a sample-carrying spherical crown, the bottom of the guide cylinder is connected to the inner cavity of the sample-carrying spherical crown, and a water outlet is provided on the side of the sample-carrying spherical crown; the rotating tool includes a rotating rod and a sampling spherical crown, the rotating rod is rotatably arranged inside the guide cylinder, the bottom of the rotating rod is connected to the sampling spherical crown, the side of the sampling spherical crown is provided with a water inlet, and the sampling spherical crown is rotatably arranged inside the sample-carrying spherical crown, and the outer spherical surface of the sampling spherical crown fits with the inner spherical surface of the sample-carrying spherical crown.

[0013] As a preferred example, the outer spherical surface area of ​​the sampling spherical crown is larger than the side water outlet area of ​​the sample-carrying spherical crown.

[0014] As a preferred example, the guide cylinder is provided with a plurality of sample-carrying spherical crowns in sequence from top to bottom, and the rotating rod is provided with a plurality of sampling spherical crowns in sequence from top to bottom.

[0015] As a preferred embodiment, it further comprises a sample carrying platform, wherein a plurality of hemispherical sample grooves are provided on the sample carrying platform, and the diameter of the hemispherical sample grooves is equal to the diameter of the sample-carrying spherical crown.

[0016] Compared with the prior art, the beneficial effects of the present invention are: (1) The smelting adopts the EAF (electric arc furnace) + LF (refining furnace) method process to improve the purity of the molten steel, significantly reduce the number and size of inclusions in the steel, and thus improve the impact toughness of the material, especially the low-temperature impact toughness; (2) In order to improve the low-temperature impact toughness of LC1, in addition to special control of the contents of carbon, manganese, phosphorus, sulfur and molybdenum, the nickel content is controlled in the range of 0.30-0.40wt%. While improving the strength, the nickel element can significantly improve the plasticity and toughness of the steel, especially the low-temperature impact toughness; (3) The quenching + tempering heat treatment method is adopted, which has higher low-temperature impact toughness than the normalizing + tempering heat treatment method; (4) By adopting the casting method of the present invention, the low-temperature impact toughness of LC1 is greatly improved, and the low-temperature impact value of -52°C is required to meet the special requirement of ≥27J; (5) A specially designed molten steel sampling tool provided by the present invention can be used to perform layered sampling of the molten steel of the valve casting after smelting and before pouring, and the composition can be detected by a spectrometer, so as to facilitate more accurate control of the chemical composition of the LC1 casting. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a flow chart of the heat treatment process of the casting method of the present invention; Figure 2 is a schematic diagram of the main structure of the molten steel sampling tool before pouring in an embodiment of the present invention; Figure 3 is a schematic diagram of the cross-sectional structure of the molten steel sampling tool before sampling in an embodiment of the present invention; Figure 4 is a schematic diagram of the cross-sectional structure of the molten steel sampling tool before pouring in an embodiment of the present invention after sampling; Figure 5 is a schematic diagram of the structure of the molten steel sampling tool and the sample carrier platform before pouring in an embodiment of the present invention.

[0018] In the figure: 1. Sampling tool; 101. Guide cylinder; 102. Rotating rod; 103. Sample-carrying spherical crown; 104. Sampling spherical crown; 105. Water inlet; 106. Water outlet; 107. Rotating wristband; 108. Clamping ring; 109. Clamping groove; 2. Sample carrying platform; 201. Sample groove; 202. Limiting protrusion. DETAILED DESCRIPTION

[0019] The embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and are not to be construed as limiting the present invention.

[0020] The disclosure below provides many different embodiments or examples for realizing different structures of the present invention. In order to simplify the disclosure of the present invention, the components and settings of specific examples are described below. Of course, they are merely examples and are not intended to limit the present invention. In addition, the present invention may repeat reference numerals and / or reference letters in different examples, and such repetition is for the purpose of simplicity and clarity and does not in itself indicate the relationship between the various embodiments and / or settings discussed. In addition, the present invention provides examples of various specific methods and materials, but those of ordinary skill in the art will appreciate the application of other methods and / or the use of other materials.

[0021] The present invention provides a casting method for an LC1 valve casting, which mainly includes mold making → shaping → painting → box assembling → smelting → pouring → box unpacking → sand cleaning → cutting pouring head → polishing → heat treatment → shot blasting → inspection.

[0022] Specifically, mold making: Mold design: Design the mold structure according to the shape and size of LC1 valve casting products.

[0023] Mold making: Make the molds for casting LC1 valve castings using metal or other suitable materials.

[0024] Modeling: Prepare the mold: Install the prepared mold on the mold table.

[0025] Mold loading: Place the core material in the mold and ensure that the core material matches the mold.

[0026] Brushing paint: Brushing paint: Apply a layer of paint on the surface of the core mold to increase the separation between the mold and the casting.

[0027] Closing the mold: Closing the two parts of the mold to form a sealed chamber for pouring molten steel.

[0028] Melting: Prepare the metal: Prepare the LC1 carbon-molybdenum steel to be cast, usually by melting the carbon-molybdenum steel and heating it to a liquid state.

[0029] Pouring: Pouring Metal: Pour the molten carbon-molybdenum steel into the mold, filling the entire cavity.

[0030] Unpacking: Open the mold: Open the mold and take out the solidified casting.

[0031] Sand cleaning: Cleaning residual sand: removing residual sand from the casting surface and mold.

[0032] Cutting gates and risers: Cutting gates and risers: Cutting off the gates and risers connected to the casting to make the final casting shape complete.

[0033] Grinding: Grinding surface: Grinding castings to remove surface roughness and improve surface quality.

[0034] Heat treatment: Heat treatment method: As needed, the casting is heat treated to change its performance and structure.

[0035] Shot blasting: Surface treatment: Use shot blasting equipment to clean the surface and improve surface quality and corrosion resistance.

[0036] Inspection: Quality inspection: Quality inspection of castings, including inspection of size, shape and material properties.

[0037] Melting: The EAF (electric arc furnace) + LF (refining furnace) method is used for melting to improve the purity of the molten steel, significantly reduce the number and size of inclusions in the steel, and thus improve the impact toughness of the material, especially the low-temperature impact toughness.

[0038] Before pouring, a spectrometer is used to quickly detect the chemical composition. In order to improve the low-temperature impact toughness of LC1, the chemical composition is specially controlled: Carbon content: The standard stipulates ≤0.25wt%, which is specially controlled at ≤0.20wt%. Carbon can increase the hardness and strength of steel, but will reduce its plasticity and toughness, so the carbon content is controlled to the lower limit.

[0039] Manganese content: The standard stipulates 0.50-0.80wt%, and is specially controlled at 0.60-0.70wt%. Manganese can improve the hardenability of steel, but it is very easy to cause the austenite grains to coarsen at high temperatures and reduce the low-temperature impact toughness of steel. The manganese content is controlled at an intermediate value.

[0040] Phosphorus content: The standard stipulates ≤0.040wt%, and is specially controlled at ≤0.025wt%. Phosphorus will increase the brittleness of steel, so the phosphorus content needs to be strictly controlled.

[0041] Sulfur content: The standard stipulates ≤0.045wt%, and the special control is ≤0.015wt%. Sulfur element will increase the brittleness of steel, so the sulfur content needs to be strictly controlled.

[0042] Silicon content: The standard stipulates ≤0.060wt%, and is controlled according to the standard value.

[0043] Molybdenum content: The standard stipulates 0.45-0.65wt%, and is specially controlled at 0.55-0.65wt%. The molybdenum element can refine the grains of steel and improve the hardenability of steel. It can effectively inhibit the growth of austenite grains at high temperatures, and control the molybdenum content at the upper limit.

[0044] In addition to special control of the elements specified in the standard, the nickel content is controlled within the range of 0.30-0.40wt%. While increasing the strength, the nickel element can significantly improve the plasticity and toughness of the steel, especially the low-temperature impact toughness.

[0045] Referring to Figure 1, the quenching + tempering heat treatment method has higher low-temperature impact toughness than the normalizing + tempering heat treatment method.

[0046] Quenching heat treatment method: Casting charging temperature ≤ 200°C, heating rate 120°C / h. When the temperature reaches 750±10°C, hold for at least 1 hour, then raise the temperature to 900±10°C. The holding time at 900±10°C is calculated based on the maximum wall thickness of the casting, at a rate of 25.4mm / h, for a minimum of 2 hours. Cooling method: Water cooling. The time from fully opening the furnace door to fully immersing the casting in water shall not exceed 60 seconds. The water temperature before quenching shall be ≤ 35°C, and the water temperature in the water tank shall not exceed 50°C during quenching.

[0047] Tempering heat treatment method: Temper immediately after quenching, the tempering temperature is 650±10℃, the heating rate is 120℃ / h. The holding time at 650±10℃ is calculated based on the maximum wall thickness of the casting, the holding time is 25.4mm / h, and the minimum is 3h. The cooling method is air cooling.

[0048] Example: 1) Using EAF (Electric Arc Furnace) + LF (Refining Furnace) process to carry out smelting; 2) Controlling the chemical composition of the LC1 valve casting, as shown in Table 1 below: Table 1 3) Heat Treatment: Quenching Heat Treatment Method: Castings were loaded into the furnace at a temperature of 150°C, with a heating rate of 120°C / h. When the temperature reached 750°C, it was held for 1.5 hours, then raised to 900°C. The temperature was held at 900°C for 3 hours. Cooling was performed using water, with a time interval of 40 seconds between the furnace door being fully opened and the casting being completely immersed in water. The water temperature before quenching was 25°C, and during quenching, the water temperature in the water tank was 40°C.

[0049] Tempering heat treatment method: Temper immediately after quenching, the tempering temperature is 650℃, the heating rate is 120℃ / h. Keep at 650℃ for 4.5h, and cool by air cooling.

[0050] 4) The test results of the embodiment are shown in Table 2 below: Table 2 5) Conclusion: According to the contents of Table 1 and Table 2, by adopting a LC1 valve casting method provided by the present invention, the purity of the molten steel can be improved, the number and size of inclusions in the steel can be significantly reduced, and the impact toughness of the material can be improved, especially the low-temperature impact toughness. The low-temperature impact toughness of LC1 is greatly improved, meeting the special requirement that the low-temperature impact value of -52°C must be ≥27J.

[0051] Referring to Figures 2 to 4, in order to ensure the accuracy of the internal chemical components of LC1 carbon-molybdenum steel before the pouring step, it is necessary to sample the molten steel and perform component content analysis and control. In the embodiments of the present invention, a specially designed layered sampling tool 1 is used to sample the molten steel after smelting and a spectrometer is used to perform chemical composition detection on the sampled molten steel samples.

[0052] Specifically, the sampling tool 1 includes an external fixed tool and an internal rotating tool. Both the fixed tool and the rotating tool are made of nickel-plated carbon steel. The fixed tool includes a guide cylinder 101 and a sample-carrying spherical crown 103. A clamping ring 108 is provided on the outside of the guide cylinder 101. A clamping groove 109 is provided on the clamping ring 108 to facilitate the clamping of the sampling tool 1 as a whole. The bottom opening of the cylindrical cavity inside the guide cylinder 101 is connected to the inner cavity of the sample-carrying spherical crown 103. A water outlet 106 is provided on the side of the sample-carrying spherical crown 103. Due to the large surface tension of molten steel, it is difficult for small holes to flow into the molten steel. Therefore, the diameter of the water outlet 106 is set to at least 20 mm. The rotating tool includes a rotating rod 102 and a sampling spherical crown Body 104, a rotating bracelet 107 is fixedly connected to the top of the rotating rod 102, the rotating rod 102 is rotatably set inside the guide cylinder 101, and the bottom of the rotating rod 102 is connected to the sampling spherical crown body 104, and a water inlet 105 is set on the side of the sampling spherical crown body 104, and the sampling spherical crown body 104 is rotatably set inside the sample-loading spherical crown body 103, and the outer spherical surface of the sampling spherical crown body 104 fits with the inner spherical surface of the sample-loading spherical crown body 103. In order to ensure that the molten steel is sealed and stored inside the sample-loading spherical crown body 103, the spherical surface of the sampling spherical crown body 104 and the water outlet 106 on the side of the sample-loading spherical crown body 103 are interference fit, that is, the outer spherical surface area of ​​the sampling spherical crown body 104 is larger than the area of ​​the water outlet 106 on the side of the sample-loading spherical crown body 103.

[0053] In order to realize layered sampling of molten steel and make the obtained detection data more accurate, the guide cylinder 101 of the sampling tool 1 of the present invention is sequentially provided with multiple sample-carrying spherical crowns 103 from top to bottom, at least three, and the rotating rod 102 is sequentially provided with multiple sampling spherical crowns 104 from top to bottom.

[0054] Referring to Figure 5, the molten steel after sampling is stored in the inner cavity of the sample-carrying spherical crown 103 and the sampling spherical crown 104. In order to facilitate the spectrometer to detect the chemical composition of the molten steel, the present invention also designs a sample carrying platform 2 compatible with the sampling tool 1. A plurality of hemispherical sample grooves 201 are provided on the sample carrying platform 2. The diameter of the hemispherical sample grooves 201 is equal to the diameter of the sample-carrying spherical crown 103. The sample carrying platform 2 is made of coated sand. A limiting protrusion 202 is formed between each pair of sample grooves 201. The limiting protrusion 202 just cooperates with the groove between each pair of sample-carrying spherical crowns 103 to prevent it from moving back and forth.

[0055] The present invention provides a layered molten steel sampling tool 1, which can sample LC1 molten steel after melting but before pouring, and detect the composition of the molten steel on the sample supporting platform 2 through a spectrometer, which helps to ensure that the various chemical components in the molten steel during the casting production process are effectively and accurately controlled.

[0056] In the description of the present invention, “a plurality of” means at least two, for example, two or three, unless otherwise clearly and specifically defined.

[0057] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

Claims

1. A casting method for an LC1 valve casting, comprising the following steps: Die making, modeling, painting, mold assembling, melting, pouring, mold opening, fettling, cutting of risers and gates, grinding, heat treatment, shot blasting and inspection; characterized in that during the melting process, an electric arc furnace and a refining furnace process flow are used for melting; During the melting process, the chemical composition of the LC1 valve casting is designed by mass percentage as follows: the carbon content is controlled within the range of ≤0.20 wt%, the manganese content is controlled within the range of 0.60 - 0.70 wt%, the phosphorus content is controlled within the range of ≤0.025 wt%, the sulfur content is controlled within the range of ≤0.015 wt%, the silicon content is controlled according to the standard value, the molybdenum content is controlled within the range of 0.613 - 0.65 wt%, and the nickel content is controlled within the range of 0.30 - 0.40 wt%; Before the pouring step, a sampling tooling is used to sample the molten steel after melting, and a spectrometer is used to detect the chemical composition of the sampled molten steel sample; The sampling tooling includes an external fixing tooling and an internal rotating tooling. The fixing tooling includes a guiding cylinder and a sample-carrying spherical crown. The bottom of the guiding cylinder communicates with the inner cavity of the sample-carrying spherical crown. The side of the sample-carrying spherical crown is provided with a water outlet. The rotating tooling includes a rotating rod and a sampling spherical crown. The rotating rod is rotatably arranged inside the guiding cylinder. The bottom of the rotating rod is connected with a sampling spherical crown. The side of the sampling spherical crown is provided with a water inlet, and the sampling spherical crown is rotatably arranged inside the sample-carrying spherical crown. The outer spherical surface of the sampling spherical crown fits with the inner spherical surface of the sample-carrying spherical crown; The outer spherical surface area of the sampling spherical crown is larger than the area of the water outlet on the side of the sample-carrying spherical crown; A plurality of sample-carrying spherical crowns are arranged on the guiding cylinder in sequence from top to bottom, and a plurality of sampling spherical crowns are correspondingly arranged on the rotating rod in sequence from top to bottom; It further includes a sample bearing table, and a plurality of hemispherical sample grooves are arranged on the sample bearing table. The diameter of the hemispherical sample groove is equal to the diameter of the sample-carrying spherical crown; The heat treatment adopts a quenching heat treatment method and a tempering heat treatment method; The quenching heat treatment method includes: The loading temperature of the LC1 valve casting is ≤200 °C, the heating rate is 120 °C / h. When the temperature rises to 750 ± 10 °C, keep it warm for at least 1 h; Then raise the temperature to 900 ± 10 °C, and the holding time is 25.4 mm / h. When the holding time calculated according to the maximum wall thickness of the casting is less than 2 h, the holding time is 2 h; The cooling method is water cooling; the water temperature in the water tank before quenching is ≤35 °C; The tempering heat treatment method includes: Immediately temper after quenching. The tempering temperature is 650 ± 10 °C, the heating rate is 120 °C / h, and the holding time at 650 ± 10 °C is 25.4 mm / h. When the holding time calculated according to the maximum wall thickness of the casting is less than 3 h, the holding time is 3 h, and the cooling method is air cooling.

2. The casting method of an LC1 valve casting according to claim 1, characterized in that, During the quenching heat treatment method, the water temperature in the water tank cannot exceed 50 °C.

3. A method for casting an LC1 valve casting according to claim 1, characterized in that, During the quenching heat treatment method, the time from when the furnace door is fully opened to when the LC1 valve casting is completely immersed in water does not exceed 60 s.

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