A low-carbon steel material for low-temperature environments and its heat treatment process.

Abstract

This application relates to the field of metal casting technology, and in particular to a low-carbon steel material for low-temperature environments and its heat treatment process. This application achieves a balance between low-temperature toughness and hardenability by precisely controlling the content of microalloying elements and ensuring synergy among multiple elements, thereby significantly improving the impact toughness of the material in low-temperature environments. The application employs a composite heat treatment process of double quenching and high-temperature tempering, which effectively refines the microstructure, homogenizes the composition, and eliminates segregation, thus solving the technical problems of poor hardenability in thick sections and large differences in properties between the core and surface microstructures.

Description

Technical Field

[0001] This application relates to the field of casting technology, and more specifically, to a low-carbon steel material for low-temperature environments and its heat treatment process. Background Technology

[0002] Low-carbon cast steel is widely used in the manufacture of structural components for room-temperature environments due to its moderate cost. However, traditional low-carbon cast steel has a low alloy element content, poor hardenability, and significant differences in the core and surface microstructure of thick, large-section castings, resulting in uneven mechanical properties between the core and surface. When this type of material is used in low-temperature environments, its impact toughness decreases significantly, failing to meet the service requirements under extreme low-temperature conditions such as -50°C.

[0003] If medium- or high-alloy cast steel is selected, although the impact toughness value can be improved, the alloy cost will be significantly increased, which is not conducive to large-scale industrial applications.

[0004] Therefore, there is a need for a technical solution that can improve the impact toughness of low-carbon cast steel materials in low-temperature environments through precise composition design and optimized heat treatment processes without significantly increasing costs. Summary of the Invention

[0005] The purpose of this invention is to provide a low-carbon steel material and its heat treatment process for low-temperature environments, so as to solve the technical problems of low-carbon cast steel materials in the prior art having low impact toughness, poor hardenability of thick sections, and large differences in properties between the core and surface microstructures in low-carbon cast steel materials at -50℃.

[0006] A low-carbon steel material for use in low-temperature environments comprises the following chemical components by weight percentage: Carbon, with a content of 0.18~0.25wt%; silicon, with a content of 0.35~0.45wt%; manganese, with a content of 1.20~1.30wt%; nickel, with a content of 0.30~0.50wt%; molybdenum, with a content of 0.08~0.12wt%; niobium, with a content of 0.0013~0.0017wt%; boron, with a content of 0.0023~0.0027wt%; vanadium, with a content of 0.015~0.025wt%; chromium, with a content of 0.15~0.25wt%; phosphorus, with a content of ≤0.01wt%; sulfur, with a content of ≤0.01wt%; copper, with a content of ≤0.030wt%; the balance is iron. The carbon equivalent Ceq is 0.41~0.44wt%, and the carbon equivalent Ceq is calculated by the formula Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15.

[0007] Preferably, the nickel content is 0.33~0.47wt%.

[0008] Preferably, the low-carbon cast steel material is prepared by a smelting method that sequentially performs EAF electric arc furnace melting, LF ladle refining, and VD vacuum degassing.

[0009] This invention also provides a heat treatment process for low-carbon cast steel materials used in low-temperature environments. The low-carbon cast steel material comprises the following chemical composition by weight percentage: carbon 0.18~0.25wt%, silicon 0.35~0.45wt%, manganese 1.20~1.30wt%, nickel 0.30~0.50wt%, molybdenum 0.08~0.12wt%, niobium 0.0013~0.0017wt%, boron 0.0023~0.0027wt%, vanadium 0.015~0.025wt%, chromium 0.15~0.25wt%, phosphorus ≤0.01wt%, sulfur ≤0.01wt%, copper ≤0.030wt%, carbon equivalent (Ceq) of 0.41~0.44wt%, and the balance being iron. The process includes the following steps: First quenching: Heat the casting to 860-900℃ at a rate of 5-15℃ / min and hold for 8 hours. Then, raise the temperature to 900-940℃ at a rate of 20-30℃ / min and hold for 0.5 hours. Remove the casting from the furnace and immerse it completely in PAG quenching liquid within 60 seconds to cool to room temperature. Second quenching: The casting is heated to 840-880℃ at a rate of 5-15℃ / min and held for 8 hours before being taken out of the furnace. Within 60 seconds, the casting is completely immersed in the quenching liquid and cooled to 350-380℃. It is then returned to the furnace and held for 4 hours. After that, it is cooled to room temperature at a rate of 5-10℃ / min. High-temperature tempering: Heat the casting to 600-680℃ at a rate of 5-15℃ / min and hold for 8 hours, then cool it to below 200℃ at a rate of 5-10℃ / min and air cool it to room temperature.

[0010] Preferably, in the first quenching, the heating temperature of the first stage is 880~890℃, and the heating temperature of the second stage is 920~930℃.

[0011] Preferably, in the second quenching, the heating temperature is 860~870℃, and the isothermal temperature after cooling is 360~370℃.

[0012] Preferably, the high-temperature tempering temperature is 640~660℃.

[0013] This application achieves a balance between low-temperature toughness and hardenability by precisely controlling the content of microalloying elements and ensuring synergy among multiple elements, thereby significantly improving the impact toughness of the material at a low temperature of -50℃. The composite heat treatment process of double quenching and high-temperature tempering effectively refines the microstructure, homogenizes the composition, and eliminates segregation, solving the technical problems of poor hardenability of thick cross-sections and large differences in properties between the core and surface microstructures. Attached Figure Description

[0014] Figure 1 This is a schematic flowchart of the heat treatment process for low-carbon cast steel materials used in low-temperature environments, provided in the embodiments of this application. Detailed Implementation

[0015] To facilitate understanding of this application, a more complete description will be provided below with reference to the accompanying drawings. Preferred embodiments of this application are shown in the drawings. However, this application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0016] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0017] In this invention, the term "wt%" refers to weight percentage. Unless otherwise specified, all raw materials used in the following examples are commercially available. The carbon equivalent Ceq is calculated using the formula Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15.

[0018] The first aspect of this application provides a low-carbon steel material for low-temperature environments, comprising the following chemical composition by weight percentage: Carbon (C): 0.18~0.25wt%. Carbon is the main element that ensures strength. Controlling it within this range can prevent the material from becoming too brittle, while it works in conjunction with other elements to ensure hardenability.

[0019] Silicon (Si): 0.35~0.45wt%. It acts as a deoxidizer and simultaneously strengthens ferrite through solid solution.

[0020] Manganese (Mn): 1.20~1.30 wt%. Improves hardenability and works synergistically with chromium and nickel.

[0021] Nickel (Ni): 0.30~0.50wt%. It significantly reduces the ductile-brittle transition temperature and is a key element for ensuring toughness at -50℃; a further preferred range is 0.33~0.47wt%.

[0022] Molybdenum (Mo): 0.08~0.12wt%. Refines grain size, improves hardenability, and suppresses temper brittleness.

[0023] Chromium (Cr): 0.15~0.25wt%. Improves hardenability.

[0024] Vanadium (V): 0.015~0.025wt%. It forms carbonitrides during solidification, pins grain boundaries, and refines austenite grains.

[0025] Niobium (Nb): 0.0013~0.0017wt%. It works synergistically with vanadium to refine grains and improve the balance between strength and toughness.

[0026] Boron (B): 0.0023~0.0027wt%. Trace amounts of boron have the most significant effect on improving hardenability, ensuring that thick cross-section castings obtain a uniform quenched structure.

[0027] Phosphorus (P) and sulfur (S): both ≤0.01wt%. Strict control of phosphorus and sulfur content is required to ensure the purity of molten steel and its low-temperature toughness.

[0028] Copper (Cu): ≤0.030wt.

[0029] In this embodiment of the application, the carbon equivalent Ceq is 0.41~0.44wt%, and the carbon equivalent Ceq is calculated by the formula Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15.

[0030] Please refer to Figure 1 , Figure 1 This is a schematic flow chart of the heat treatment process for low-carbon cast steel materials used in low-temperature environments, provided in an embodiment of this application. The process employs a composite treatment of two quenching processes followed by high-temperature tempering to eliminate casting defects, refine grains, and achieve excellent low-temperature impact toughness. Furthermore, to achieve precise control of the above-mentioned composition and ensure the purity of the molten steel, this invention uses a smelting method of EAF (electric arc furnace) + LF (ladle refining) + VD (vacuum degassing) sequentially. Specific steps include: S1: First quenching: homogenization and grain refinement.

[0031] The casting is loaded into the furnace and slowly heated to 860-900℃ at a rate of 5-15℃ / min, and held for 8 hours; then the temperature is rapidly increased to 900-940℃ at a rate of 20-30℃ / min, and held for 0.5 hours before being removed from the furnace.

[0032] Cooling: After exiting the furnace, immerse the casting completely in PAG quenching liquid within 60 seconds and cool to room temperature.

[0033] Preferred parameters: To further optimize grain size and composition uniformity, the preferred heating temperature for the first stage is 880~890℃, and the preferred heating temperature for the second stage is 920~930℃.

[0034] This step aims to eliminate dendritic segregation and coarse grains in the as-cast microstructure, homogenize the composition through high-temperature diffusion, and retain fine austenite grains by utilizing the "pseudo-critical zone heating" effect, while ensuring that carbides are fully dissolved.

[0035] S2: Second quenching: Lower bainite transformation.

[0036] The casting, after its first quenching, is heated to 840-880°C at a rate of 5-15°C / min and held at that temperature for 8 hours before being removed from the furnace. Immediately after removal, the casting is fully immersed in the quenching liquid within 60 seconds, rapidly cooled to 350-380°C, and immediately returned to the furnace. It is then held at this temperature for 4 hours. After the holding period, the temperature is slowly lowered to room temperature at a rate of 5-10°C / min. The preferred heating temperature is 860-870°C, and the preferred isothermal temperature is 360-370°C.

[0037] This step utilizes a composite process of "quenching + isothermal bainitic transformation" to obtain a multiphase microstructure consisting of lower bainite, a small amount of martensite, and stabilized retained austenite. This microstructure effectively eliminates structural stress and improves the material's plasticity and toughness.

[0038] S3: High-temperature tempering: stabilizes the microstructure.

[0039] The casting is heated to 600-680℃ at a rate of 5-15℃ / min and held for 8 hours. After holding, it is cooled to below 200℃ at a rate of 5-10℃ / min and then air-cooled to room temperature. The preferred tempering temperature is 640-660℃. This process transforms metastable bainite and martensite into tempered sorbite or tempered bainite, improving the dimensional stability of the casting, reducing strength, and significantly improving plasticity, impact toughness, and fracture toughness.

[0040] The following describes in further detail a process method for repairing surface defects in sintered blanks provided in this application, using specific embodiments (Examples 2 to 6): Example

[0041] This embodiment provides a low-carbon steel material for low-temperature environments, whose chemical composition by weight percentage is: C 0.22wt%, Si 0.40wt%, Mn 1.25wt%, Ni 0.35wt%, Mo 0.10wt%, Nb 0.0015wt%, B 0.0025wt%, V 0.02wt%, Cr 0.20wt%, P 0.008wt%, S 0.008wt%, Cu 0.020wt%, with the balance being Fe. The carbon equivalent Ceq = 0.22 + 1.25 / 6 + (0.20 + 0.10 + 0.02) / 5 + (0.35 + 0.020) / 15 = 0.42wt%.

[0042] The preparation method of the low-carbon cast steel material includes the following steps: Smelting: An EAF (Electric Arc Furnace) is used for smelting. Scrap steel, pig iron, and other raw materials are added to the furnace and heated to over 1600℃ to melt them. After melting, the composition is adjusted by adding alloys such as ferrosilicon, ferromanganese, nickel plates, ferromolybdenum, ferrochrome, and ferrovanadium to adjust the content of each element to the target range. After tapping, the steel enters the LF ladle for refining. Argon blowing and stirring, along with the addition of refining slag (lime, fluorite, aluminum granules, etc.), remove inclusions from the molten steel and reduce sulfur and phosphorus content.

[0043] After refining, the ladle is transferred to a VD vacuum degassing unit and held for 15 minutes under a vacuum of less than 67 Pa to remove gases such as hydrogen, oxygen, and nitrogen from the molten steel, reducing the gas content to H ≤ 2 ppm. O ≤ 30ppm, N ≤ 50ppm. After degassing, add ferroniobium and ferroboron to adjust the Nb content to 0.0015wt% and the B content to 0.0025wt%. The temperature of the molten steel is controlled at 1540~1560℃ to ensure good fluidity.

[0044] Pouring: The treated molten steel is poured into a sand mold or metal mold to form a casting. The pouring temperature is controlled at 1500~1520℃, and the pouring speed is moderate to avoid air entrapment and slag inclusions. After the casting solidifies, the sand is cleaned and the risers and gating gates are cut to obtain the as-cast steel part.

[0045] First quenching: The casting is placed in a heat treatment furnace and slowly heated to 880℃ at a rate of 10℃ / min, and held for 8 hours. This stage utilizes high-temperature diffusion to homogenize the composition, eliminate dendritic segregation and non-equilibrium phases in the as-cast structure, and reduce microsegregation. After holding, the temperature is rapidly increased to 920℃ at a rate of 25℃ / min and held for 0.5 hours.

[0046] This stage achieves a pseudo-critical zone heating effect, allowing the initial fine grains to grow to a limited extent under short-term high temperatures, preserving relatively fine austenite grains while ensuring sufficient dissolution of carbides, providing a uniform austenite composition for subsequent quenching. This high-temperature, short-time treatment avoids the formation of boron (BN) or nitrogen (M) under prolonged high temperatures. 23 (C,B)6, thus retaining effective boron and ensuring hardenability. After the holding period, the casting is completely immersed in the PAG quenching liquid within 60 seconds and cooled to room temperature.

[0047] PAG quenching fluid is a polyalkylene glycol-based quenching medium with a cooling rate between that of water and oil. It provides uniform cooling and yields martensitic or martensitic + bainitic microstructures. After the first quenching, the casting microstructure is fine-grained martensite or martensite + bainitic, with uniform composition and no segregation, providing a high-quality initial microstructure for the subsequent second quenching.

[0048] Second quenching: The casting, after the first quenching, is slowly heated to 860℃ at a rate of 10℃ / min and held for 8 hours. This stage tempers and transforms the martensitic structure obtained from the first quenching, obtaining uniform fine-grained austenite, preparing for the second quenching. After holding at this temperature, the casting is completely immersed in the quenching liquid within 60 seconds and rapidly cooled to 365℃.

[0049] After cooling to 365℃, the casting is immediately returned to the furnace and held at 365℃ for 4 hours. This stage is the isothermal bainitic transformation stage, which involves rapidly passing through the pearlite transformation zone to avoid pearlite formation. A small amount of martensite may form in some areas first, followed by bainitic transformation during the isothermal stage, ultimately resulting in a multiphase structure of lower bainite + a small amount of martensite + stabilized retained austenite.

[0050] The casting was isothermally held at 365℃ for 4 hours to ensure complete bainite transformation (transformation rate greater than 90%), simultaneously eliminating structural stresses caused by martensitic transformation, promoting the stabilization of retained austenite, and preventing the formation of brittle martensite during subsequent cooling. After the isothermal holding period, the casting was slowly cooled to room temperature at a rate of 8℃ / min. After the second quenching, the microstructure of the casting was a multiphase structure consisting of lower bainite, a small amount of martensite, and stabilized retained austenite, with a fine and uniform microstructure.

[0051] High-temperature tempering: The casting, after its second quenching, is slowly heated to 640℃ at a rate of 10℃ / min and held for 8 hours. This stage transforms metastable bainite and martensite into tempered sorbite or tempered bainite, resulting in a stable microstructure and improved dimensional stability. Simultaneously, it reduces strength but significantly improves plasticity, impact toughness, and fracture toughness. After holding, the casting is slowly cooled to below 200℃ at a rate of 8℃ / min and then air-cooled to room temperature.

[0052] After high-temperature tempering, the microstructure of the casting is tempered sorbite or tempered bainite, which is stable and exhibits excellent plasticity and toughness. Tensile and impact specimens were cut from the casting, and mechanical properties were tested according to GB / T228 and GB / T229 standards. The test results are as follows: tensile strength Rm = 635 MPa, yield strength Rp0.2 = 507 MPa, elongation A = 24%, reduction of area Z = 68%, room temperature impact toughness KV2 = 50.3 J (56.6 J, 48.9 J, and 45.3 J for the three specimens), and Brinell hardness HB = 201. Example

[0053] This embodiment provides a low-carbon cast steel material for low-temperature environments, whose chemical composition by weight percentage is: C 0.20wt%, Si 0.38wt%, Mn 1.22wt%, Ni 0.45wt%, Mo 0.12wt%, Nb 0.0016wt%, B 0.0026wt%, V 0.022wt%, Cr 0.22wt%, P 0.009wt%, S 0.009wt%, Cu 0.025wt%, with the balance being Fe. The carbon equivalent Ceq = 0.20 + 1.22 / 6 + (0.22 + 0.12 + 0.022) / 5 + (0.45 + 0.025) / 15 = 0.44wt%.

[0054] The preparation method of the low-carbon cast steel material is the same as that in Example 2, including EAF+LF+VD smelting, casting, first quenching, second quenching, and high-temperature tempering. The first quenching process parameters are: heating to 880℃ at a rate of 10℃ / min and holding for 8 hours, then heating to 920℃ at a rate of 25℃ / min and holding for 0.5 hours, and then immersing in PAG quenching liquid to cool to room temperature within 60 seconds.

[0055] The second quenching process parameters are as follows: heat to 860℃ at a rate of 10℃ / min and hold for 8 hours; immerse in quenching liquid within 60 seconds to cool to 365℃ and return to the furnace for holding for 4 hours; then cool to room temperature at a rate of 8℃ / min. The high-temperature tempering process parameters are as follows: heat to 640℃ at a rate of 10℃ / min and hold for 8 hours; then cool to below 200℃ at a rate of 8℃ / min and air cool to room temperature.

[0056] Tensile and impact specimens were cut from the casting and their mechanical properties were tested according to GB / T228 and GB / T229 standards. The test results were as follows: tensile strength Rm=588MPa, yield strength Rp0.2=449MPa, elongation A=27%, reduction of area Z=66%, room temperature impact toughness KV2=64.5J (51.3J, 73.0J, and 69.3J for the three specimens respectively), and Brinell hardness HB=188.

[0057] Compared to Example 2, in this example, the C content is reduced to 0.20 wt%, the Ni content is increased to 0.45 wt%, and the Ceq is increased to 0.44 wt%. The lower C content reduces strength but significantly improves plasticity and toughness, with an elongation (A) of 27% and a room temperature impact toughness (KV2) of 64.5 J. The higher Ni content further lowers the ductile-brittle transition temperature and improves low-temperature toughness. The Ceq is controlled at 0.44 wt%, ensuring a balance between hardenability and weldability.

[0058] Please refer to Table 1, which shows the performance test results after implementing the process according to the embodiments of this application: Table 1: Test piece 1 635 507 24 68 50.3（56.6 / 48.9 / 45.3） 201 Test piece 2 588 449 27 66 64.5（51.3 / 73.0 / 69.3） 188 Example

[0059] This embodiment optimizes the first quenching process parameters based on Embodiment 2. The chemical composition is the same as in Embodiment 2. The first quenching process parameters are as follows: heat to 885°C at a rate of 10°C / min and hold for 8 hours, then heat to 925°C at a rate of 25°C / min and hold for 0.5 hours, and then immerse in PAG quenching liquid to cool to room temperature within 60 seconds.

[0060] The process parameters for the second quenching and high-temperature tempering were the same as in Example 2. Tensile and impact specimens were cut from the casting for mechanical property testing. The test results were as follows: tensile strength Rm = 642 MPa, yield strength Rp0.2 = 512 MPa, elongation A = 25%, reduction of area Z = 69%, room temperature impact toughness KV2 = 52.8 J, and Brinell hardness HB = 203.

[0061] Compared to Example 2, the first stage temperature of the first quenching in this example is increased by 5°C to 885°C, and the second stage temperature is increased by 5°C to 925°C. The increased first stage temperature accelerates high-temperature diffusion, resulting in better compositional homogenization and further reducing microsegregation. The increased second stage temperature promotes complete dissolution of carbides, leading to more uniform austenite composition, higher effective boron retention, and further improved hardenability. The microstructure uniformity is improved, and both strength and toughness show slight increases. Example

[0062] This embodiment optimizes the second quenching process parameters based on Embodiment 2. The chemical composition is the same as in Embodiment 2. The first quenching process parameters are the same as in Embodiment 2. The second quenching process parameters are as follows: heat to 865°C at a rate of 10°C / min and hold for 8 hours, immerse in quenching liquid within 60 seconds to cool to 367°C and return to the furnace for holding for 4 hours, then cool to room temperature at a rate of 8°C / min.

[0063] The high-temperature tempering process parameters were the same as in Example 2. Tensile and impact specimens were cut from the casting for mechanical property testing. The test results were as follows: tensile strength Rm = 630 MPa, yield strength Rp0.2 = 505 MPa, elongation A = 26%, reduction of area Z = 70%, room temperature impact toughness KV2 = 55.2 J, and Brinell hardness HB = 199.

[0064] Compared to Example 2, the second quenching heating temperature in this example is increased by 5°C to 865°C, and the isothermal temperature is increased by 2°C to 367°C. The increased heating temperature allows for a more complete tempering transformation of the martensite structure after the first quenching, resulting in more uniform, fine-grained austenite. The increased isothermal temperature of 367°C is closer to the bainite transformation nose temperature, accelerating the bainite transformation rate and achieving a more complete transformation with a bainite transformation rate greater than 90%.

[0065] The lower bainite structure is finer and more uniform, the retained austenite has better stability, and the low-temperature toughness is further improved. The room temperature impact toughness KV2 reaches 55.2J, the elongation A reaches 26%, the reduction of area Z reaches 70%, and both plasticity and toughness are improved. Example

[0066] This embodiment optimizes the high-temperature tempering process parameters based on Embodiment 2. The chemical composition is the same as in Embodiment 2. The process parameters for the first and second quenchings are the same as in Embodiment 2. The high-temperature tempering process parameters are: heating to 650°C at a rate of 10°C / min and holding for 8 hours, then cooling to below 200°C at a rate of 8°C / min and air-cooling to room temperature.

[0067] Tensile and impact specimens were cut from the casting for mechanical property testing. The test results were as follows: tensile strength Rm = 618 MPa, yield strength Rp0.2 = 495 MPa, elongation A = 28%, reduction of area Z = 72%, room temperature impact toughness KV2 = 58.6 J, and Brinell hardness HB = 195.

[0068] Compared to Example 2, the high-temperature tempering temperature in this example is increased by 10°C to 650°C. The increased tempering temperature leads to a more complete transformation of bainite and martensite to tempered sorbite, more thorough carbide precipitation, a softer ferrite matrix, and more complete elimination of internal stress. Plasticity and toughness are significantly improved, with elongation A reaching 28%, reduction of area Z reaching 72%, and room temperature impact toughness KV2 reaching 58.6 J, the highest value among all examples. Strength is slightly reduced, but still meets the requirements of Rm≥580MPa and Rp0.2≥440MPa.

[0069] It is understood that the carbon content is not limited to 0.18~0.25wt%, but can also be 0.15~0.30wt%. The silicon content is not limited to 0.35~0.45wt%, but can also be 0.20~0.60wt%. The manganese content is not limited to 1.20~1.30wt%, but can also be 1.00~1.50wt%. The nickel content is not limited to 0.30~0.50wt%, but can also be 0.20~0.60wt%.

[0070] It is understood that the content of molybdenum is not limited to 0.08~0.12wt%, but may be 0.05~0.15wt%. The content of niobium is not limited to 0.0013~0.0017wt%, but may be 0.001~0.002wt%. The content of boron is not limited to 0.0023~0.0027wt%, but may be 0.002~0.003wt%. The content of vanadium is not limited to 0.015~0.025wt%, but may be 0.01~0.03wt%.

[0071] It is understood that the chromium content is not limited to 0.15~0.25wt%, but can also be 0.10~0.30wt%. The carbon equivalent Ceq is not limited to 0.41~0.44wt%, but can also be 0.40~0.45wt%. The smelting method is not limited to EAF+LF+VD, but can also be other smelting methods that can ensure precise control of composition and purity of molten steel.

[0072] It is understood that the first stage heating temperature of the first quenching is not limited to 860~900℃, but can also be 850~910℃. The second stage heating temperature of the first quenching is not limited to 900~940℃, but can also be 890~950℃. The quenching fluid of the first quenching is not limited to PAG quenching fluid, but can also be other quenching media with moderate cooling rate and uniform cooling.

[0073] It is understood that the heating temperature for the second quenching is not limited to 840~880℃, but can also be 830~890℃. The isothermal temperature for the second quenching is not limited to 350~380℃, but can also be 340~390℃. The isothermal holding time for the second quenching is not limited to 4 hours, but can also be 3~5 hours. The high-temperature tempering temperature is not limited to 600~680℃, but can also be 580~700℃.

[0074] It is understood that the low-carbon cast steel material can be used to manufacture thick-section cast steel structural parts for low-temperature environments with a cross-sectional thickness ≥100mm, and can also be used to manufacture medium and small-sized cast steel parts for low-temperature environments with a cross-sectional thickness less than 100mm. The low-temperature environment is not limited to ~50℃, but can also be ~40℃, ~60℃ or other low-temperature environments.

[0075] It is understood that the low-carbon cast steel material can be applied to the manufacturing of cast steel parts that need to operate in low-temperature environments, such as polar scientific research equipment, cryogenic storage tanks, marine engineering cryogenic structural components, cryogenic valves, cryogenic pumps, and cryogenic pipelines.

[0076] In this application, the mechanical property testing methods may include: tensile strength Rm, yield strength Rp0.2, elongation A, and reduction of area Z are determined according to GB / T228 standard; room temperature impact toughness KV2 is determined according to GB / T229 standard, using V-notch specimens, and the average value of three specimens in each group is taken; hardness is determined according to GB / T231.1 standard, using a Brinell hardness tester with a load of 3000 kg and an indenter diameter of 10 mm.

[0077] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0078] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A low-carbon steel material for use in low-temperature environments, comprising the following chemical components by weight percentage: Carbon, with a content of 0.18~0.25wt%; silicon, with a content of 0.35~0.45wt%; manganese, with a content of 1.20~1.30wt%; nickel, with a content of 0.30~0.50wt%; molybdenum, with a content of 0.08~0.12wt%; niobium, with a content of 0.0013~0.0017wt%; boron, with a content of 0.0023~0.0027wt%; vanadium, with a content of 0.015~0.025wt%; chromium, with a content of 0.15~0.25wt%; phosphorus, with a content of ≤0.01wt%; sulfur, with a content of ≤0.01wt%; copper, with a content of ≤0.030wt%; the balance is iron. The carbon equivalent Ceq is 0.41~0.44wt%, and the carbon equivalent Ceq is calculated by the formula Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15.

2. The low-carbon steel material for low-temperature environments according to claim 1, characterized in that, The nickel content is 0.33~0.47wt%.

3. The low-carbon steel material for low-temperature environments according to any one of claims 1 or 2, characterized in that, The low-carbon cast steel material is prepared by a smelting method consisting of EAF electric arc furnace melting, LF ladle refining, and VD vacuum degassing.

4. A heat treatment process for low-carbon cast steel material used in low-temperature environments, wherein the low-carbon cast steel material comprises the following chemical composition by weight percentage: carbon 0.18~0.25wt%, silicon 0.35~0.45wt%, manganese 1.20~1.30wt%, nickel 0.30~0.50wt%, molybdenum 0.08~0.12wt%, niobium 0.0013~0.0017wt%, boron 0.0023~0.0027wt%, vanadium 0.015~0.025wt%, chromium 0.15~0.25wt%, phosphorus ≤0.01wt%, sulfur ≤0.01wt%, copper ≤0.030wt%, carbon equivalent Ceq of 0.41~0.44wt%, and the balance being iron; characterized in that, Includes the following steps: First quenching: Heat the casting to 860-900℃ at a rate of 5-15℃ / min and hold for 8 hours. Then, raise the temperature to 900-940℃ at a rate of 20-30℃ / min and hold for 0.5 hours. Remove the casting from the furnace and immerse it completely in PAG quenching liquid within 60 seconds to cool to room temperature. Second quenching: The casting is heated to 840-880℃ at a rate of 5-15℃ / min and held for 8 hours before being taken out of the furnace. Within 60 seconds, the casting is completely immersed in the quenching liquid and cooled to 350-380℃. It is then returned to the furnace and held for 4 hours. After that, it is cooled to room temperature at a rate of 5-10℃ / min. High-temperature tempering: Heat the casting to 600-680℃ at a rate of 5-15℃ / min and hold for 8 hours, then cool it to below 200℃ at a rate of 5-10℃ / min and air cool it to room temperature.

5. The heat treatment process according to claim 4, characterized in that, In the first quenching, the heating temperature in the first stage is 880~890℃, and the heating temperature in the second stage is 920~930℃.

6. The heat treatment process according to claim 5, characterized in that, In the second quenching, the heating temperature is 860~870℃, and the isothermal temperature after cooling is 360~370℃.

7. The heat treatment process according to any one of claims 4 to 6, characterized in that, The high-temperature tempering temperature is 640~660℃.

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