Manufacturing method of 300t-grade stainless steel ingot for rotating wheel forge piece

Abstract

The invention relates to a manufacturing method of a 300t-grade stainless steel ingot for a runner forge piece, belongs to the technical field of iron-based molten steel ingot casting, and solves the problems that element segregation is difficult to control in the existing large-scale stainless steel ingot preparation process and the N element narrow interval of a double-vacuum steel ingot is difficult to control. The invention discloses a manufacturing method of a 300t-grade stainless steel ingot for a rotating wheel forge piece. The manufacturing method comprises the following steps that S1, a steel ingot auxiliary tool is designed, wherein the steel ingot auxiliary tool comprises a base plate, a steel ingot mold and a heat preservation cap; s2, preparing steel raw materials according to the target composition of the steel ingot; s3, designing a molten steel distribution scheme; s4, subpackage smelting according to a molten steel distribution scheme: after crude molten steel is obtained through electric furnace smelting, refining is conducted through an LF + VOD + VD refining process, and stainless steel molten steel with qualified chemical components is obtained; s5, the molten stainless steel obtained in the step S4 is poured into a stainless steel ingot under vacuum; and S6, controlling cooling until the steel ingot is solidified and formed, wherein the comprehensive cooling rate of the steel ingot is 18-28 DEG C / h. And low-segregation and high-quality preparation of the 300t-grade large stainless steel ingot is realized.

Description

Technical Field

[0001] The invention relates to the technical field of iron-based molten steel ingot casting, and in particular to a method for manufacturing a 300t-grade stainless steel ingot for a runner forging. Background Art

[0002] To meet the needs of 500MW-class impulse turbines, the large runner hub forgings have an outer diameter of nearly φ5000mm and a thickness of nearly 1000mm, which requires 300t-class stainless steel ingots to forge. The runner is one of the core forgings of high-head impulse hydropower units, and the quality of the large stainless steel ingots used in its manufacture directly determines the success or failure of the forging. The existing technology lacks a manufacturing method for producing 300t-class large stainless steel ingots that meet quality requirements. Summary of the invention

[0003] In view of the above analysis, an embodiment of the present invention aims to provide a method for manufacturing a 300t-grade stainless steel ingot for a runner forging, so as to solve at least one of the problems in the prior art, namely, the lack of a high-quality 300t-grade stainless steel ingot preparation method that meets the requirements of large runner forgings, the difficulty in controlling element segregation in the preparation process of large stainless steel ingots, and the difficulty in controlling the narrow range of the N element in the double vacuum ingot.

[0004] The embodiment of the present invention discloses a method for manufacturing a 300t stainless steel ingot for a runner forging, which is characterized by comprising the following steps:

[0005] S1. Design of ingot auxiliary tools: Calculate the required ingot weight according to the required weight of the forging, and design corresponding auxiliary tools according to the ingot weight, casting method and target ingot composition, including a chassis, an ingot mold and a heat preservation cap, wherein the chassis is provided with a truncated cone-shaped groove;

[0006] S2. Prepare steel raw materials according to the target composition of the ingot;

[0007] S3. Design molten steel distribution plan;

[0008] S4. Subcontracting smelting according to the molten steel allocation plan: After the crude molten steel is obtained by electric furnace smelting, the LF+VOD+VD refining process is used for refining to obtain stainless steel molten steel with qualified chemical composition;

[0009] S5. The molten stainless steel obtained in step S4 is cast into a stainless steel ingot under vacuum;

[0010] S6. Control cooling until the steel ingot solidifies and forms, wherein the comprehensive cooling rate of the steel ingot is 18-28°C / h. After forming, the steel ingot is demoulded when the surface temperature at the upper 1 / 3 of the ingot body is 640-660°C.

[0011] Furthermore, the target steel ingot composition, in percentage by mass, includes: [C]=0.02-0.04%, [Si]=0.30-0.60%, [Mn]=0.50-0.80%, [P]≤0.025%, [S]≤0.010%, [Cr]=12.00-13.50%, [Ni]=4.50-6.00%, [Mo]=0.40-0.80%, [V]≤0.10%, [Cu]≤0.50%, [W]≤0.10%, [H]≤0.0002%, [O]≤0.005%, and [N]=0.01-0.03%. Residual elements [Sb]≤0.0015, [Sn]≤0.015, [As]≤0.020, [Ti]≤0.015, and the total amount of Cu+W+V≤0.5, with the balance being Fe and unavoidable impurities.

[0012] Preferably, the bottom plate of the steel ingot auxiliary tool in step S1 is 600 mm to 1000 mm thick.

[0013] Exemplarily, the upper diameter of the truncated cone-shaped groove on the chassis is ≤φ2000mm, the lower diameter is ≥φ1500mm, the upper diameter is greater than the lower diameter, and the depth is 300mm to 500mm.

[0014] Preferably, the steel ingot mold in step S1 is a 24-sided plum blossom ingot with a taper of 7% to 11%.

[0015] Furthermore, the ingot mold has a height-to-diameter ratio of 0.8 to 1.3 and a wall thickness of 200 mm to 500 mm.

[0016] It should be noted that the taper of the thermal insulation cap in step S1 is 8% to 12%, and the necking ratio is 85% to 98%.

[0017] Exemplarily, in step S2, the steel raw materials are all scrap steel, ferrochrome and nickel alloy; wherein the ferrochrome grade is FeCr65C0.25, and the nickel alloy grade is Ni9990.

[0018] Furthermore, in step S4, the N content after VD refining is controlled to be in the range of 30-100 ppm, and the N element content is adjusted to 150-450 ppm after the VD vacuum is broken.

[0019] Preferably, step S5 adopts tundish pouring, and the tundish pouring speed is 5-8 t / min; argon is used for protection during the whole pouring process, and the vacuum degree in the vacuum chamber is kept not higher than 133 Pa during pouring.

[0020] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0021] 1. The present invention arranges an inverted truncated cone-shaped groove on the chassis of the steel ingot auxiliary tool, and reasonably matches the design parameters of the chassis thickness, the taper and the height-to-diameter ratio of the steel ingot mold, and the neck reduction ratio of the insulation cap, so as to control the longitudinal cooling rate of the steel ingot to ≤20mm / min, which is the basis for controlling the average cooling rate of the steel ingot.

[0022] 2. The present invention achieves an average cooling rate of 18-28°C / h for the steel ingot by matching the structural parameters of the steel ingot auxiliary tool, formulating the molten steel distribution plan and coordinating the temperature gradient of the casting process, thereby effectively improving the uniformity of the steel ingot and controlling the degree of element segregation to be low.

[0023] 3. The present invention optimizes the molten steel distribution scheme and controls the average cooling rate of the steel ingot to achieve stable control of the segregation index of the chemical composition of 300t-level high-viscosity stainless steel super-large ingots, such as C, P, S, Cr, Mo and other easily segregated elements, especially the C segregation index is controlled at 0.90%-1.10%, and the total inclusion level is as low as below 2.5 levels, laying a good foundation for the performance of the final product.

[0024] 4. In view of the characteristics of high viscosity of molten stainless steel, difficulty in floating of inclusions in the steel, and low purity of the molten steel, the present invention improves the purity of the molten steel by controlling the quality of the raw materials and the alloy quality entering the furnace and adopting VOD+VD strong deoxidation refining means; at the same time, by controlling the N content in the range of 30-100ppm after VD refining, and re-adjusting the N element content to 150-450ppm after VD vacuum breaking to balance the amount of N element reduced again due to the vacuum environment during subsequent vacuum pouring, the stable control of the N element in a narrow range is achieved, and the formation of nitride inclusions in the steel is effectively reduced, thereby improving the purity and mechanical properties of the steel.

[0025] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The drawings are only for the purpose of illustrating specific embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0027] Figure 1 This is a physical picture of the stainless steel ingot of the present invention;

[0028] Figure 2 This is a structural diagram of the steel ingot auxiliary tool of the present invention;

[0029] Figure 3 It is a schematic diagram of the stainless steel ingot of the present invention.

[0030] Reference numerals:

[0031] 1-insulation cap; 2-refractory brick; 3-ingot mold; 4-chassis; 5-riser; 6-ingot body, 7-sprue; 8-truncated cone groove. DETAILED DESCRIPTION

[0032] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0033] The manufacturing process of 300t grade (including ingots close to 300t but not exceeding 400t) stainless steel ingots for runner forgings is very difficult. First of all, stainless steel is a high-alloy steel. The viscosity of stainless steel liquid is greater than that of ordinary low-alloy steel. The unevenness of some alloy elements that have a greater impact on the performance is more prominent in the ingot, resulting in difficulty in ensuring the homogeneity of the ingot; and due to the high viscosity of stainless steel liquid, it is difficult for inclusions in the steel to float up, resulting in lower purity of the molten steel than other grades of ingots, requiring strong deoxidation refining. Strong deoxidation refining makes it more difficult to control the narrow range of N elements in the final product, affecting the reliability and durability of the final runner product; the specific heat capacity of stainless steel liquid is slightly larger and the thermal conductivity is lower, resulting in slow temperature drop of the molten steel and prolonged solidification time, resulting in larger central segregation and core defects of the ingot than other grades of ingots.

[0034] A specific embodiment of the present invention discloses a method for manufacturing a 300t grade stainless steel ingot for a runner forging, comprising the following steps:

[0035] S1. Design of ingot auxiliary tools: Calculate the required ingot weight according to the required weight of the forging, and design corresponding auxiliary tools according to the ingot weight, casting method and target ingot composition, including a chassis, an ingot mold and a heat preservation cap, wherein the chassis is provided with a truncated cone-shaped groove;

[0036] S2. Prepare steel raw materials according to the target composition of the ingot;

[0037] S3. Design molten steel distribution plan;

[0038] S4. Subcontracting smelting according to the molten steel allocation plan: After the crude molten steel is obtained by electric furnace smelting, the LF+VOD+VD refining process is used for refining to obtain stainless steel molten steel with qualified chemical composition;

[0039] S5. The molten stainless steel obtained in step S4 is cast into a stainless steel ingot under vacuum;

[0040] S6. Control cooling until the steel ingot solidifies and forms, wherein the comprehensive cooling rate of the steel ingot is 18-28°C / h. After forming, the steel ingot is demoulded when the surface temperature at the upper 1 / 3 of the ingot body is 640-660°C.

[0041] Furthermore, the target steel ingot composition, in percentage by mass, includes: [C]=0.02-0.04%, [Si]=0.30-0.60%, [Mn]=0.50-0.80%, [P]≤0.025%, [S]≤0.010%, [Cr]=12.00-13.50%, [Ni]=4.50-6.00%, [Mo]=0.40-0.80%, [V]≤0.10%, [Cu]≤0.50%, [W]≤0.10%, [H]≤0.0002%, [O]≤0.005%, and [N]=0.01-0.03%. Residual elements [Sb]≤0.0015, [Sn]≤0.015, [As]≤0.020, [Ti]≤0.015, and the total amount of Cu+W+V≤0.5, with the balance being Fe and unavoidable impurities.

[0042] Specifically, in step S1, Figure 2 , 3 As shown, the position of the truncated cone-shaped groove 8 on the chassis 4 corresponding to the stainless steel ingot is the nozzle 7, the weight of the nozzle is 3% to 5% of the weight of the ingot, the position of the ingot mold 3 corresponding to the stainless steel ingot is the ingot body 6, and the position of the insulation cap 1 corresponding to the stainless steel ingot is the riser 5; the runner forging is forged from the ingot body part of the stainless steel ingot, and the materials at the riser and nozzle of the stainless steel ingot are discarded.

[0043] Preferably, the thickness of the chassis in the steel ingot auxiliary tool is 600mm-1000mm; the steel ingot mold is a 24-sided plum blossom ingot with a taper of 7%-11%, a height-to-diameter ratio of 0.8-1.3, and a wall thickness of 200mm-500mm; the taper of the insulation cap is 8%-12%, and the necking ratio is 85%-98%;

[0044] It should be noted that the taper design of the insulation cap is 8% to 12% to achieve material savings; if the necking ratio is less than 85%, it will affect the flow of molten steel at the upper end of the ingot, causing a dead zone, and unable to form sequential solidification, affecting the quality of the ingot; since the riser material is discarded in subsequent products, if the necking ratio is greater than 98%, it will cause material waste.

[0045] Furthermore, the upper diameter of the truncated cone groove is ≤ Lower diameter ≥ The upper diameter is greater than the lower diameter, and the depth is 300 mm to 500 mm. The size of the truncated cone groove affects the longitudinal solidification speed of the ingot, thereby affecting the segregation degree of the ingot.

[0046] Exemplarily, the material of the steel ingot auxiliary tools is HT200; the insulation cap 1 is composed of a cast iron HT200 outer shell and a refractory brick 2 lining.

[0047] Preferably, in step S2, the steel raw materials are all scrap steel, ferrochrome and nickel alloy; wherein the ferrochrome grade is FeCr65C0.25, and the nickel alloy grade is Ni9990.

[0048] It is worth noting that the auxiliary tool parameter design in steps S3 to S5 and step S1 comprehensively affects the average cooling rate in step S6; the truncated cone groove on the bottom plate mainly affects the longitudinal cooling rate of the steel ingot, and the molten steel distribution scheme mainly affects the heat brought into the steel ingot mold during the pouring process;

[0049] The simulation software was used to numerically simulate the solidification process of the ingot until the simulation results met the requirements of longitudinal cooling rate ≤ 20 mm / min and average cooling rate 18-28 °C / h, and the optimized auxiliary tool parameters, molten steel distribution plan, and molten steel pouring temperature gradient of each package were obtained.

[0050] Specifically, in step S3, the molten steel distribution plan is determined according to the weight of the ingot. For ingots with a total weight of 250t to 300t, 2 to 4 bags of molten steel are used for production distribution, wherein the molten steel distribution plan for 2 bags is 100t to 150t and 150t to 200t, the molten steel distribution plan for 3 bags is 105t to 140t, 105t to 140t and 40t to 70t, and the molten steel distribution plan for 4 bags is 70t to 90t, 70t to 90t, 70t to 90t and 40t to 80t.

[0051] Further, in step S4, the whole scrap steel is used as raw material to produce crude molten steel in an electric furnace, and then transferred to the LF refining furnace for slag deoxidation to adjust the chemical composition of the molten steel, and then transferred to the VOD furnace for oxygen blowing and decarburization, and then vacuum VD treatment is performed. The N content after VD refining is in the range of 30-100ppm, and the N element is continued to be adjusted to 150-450ppm after breaking the air, and the steel is soft-blown for about 25 minutes for pouring; the N element in the steel ingot is stably controlled in a narrow range of 0.01-0.03%, effectively reducing the formation of nitride inclusions in the steel, thereby improving the purity and mechanical properties of the steel ingot. The vacuum degree of the VOD process is controlled at 6KPa-13KPa, and the vacuum degree of the VD process is controlled below 267Pa.

[0052] Preferably, during the smelting process in step S4, the selected ferrochrome alloy and nickel alloy are conveyed to the refining bag by a high-level silo belt or the like, so as to reduce the influence of foreign inclusions, further improve the purity of the molten steel, and thus reduce the segregation degree of the steel ingot.

[0053] Preferably, in step S4, [Sb]=0.0009%, [Sn]=0.005%, [As]=0.005%, [Cu]=0.06%, and [W]=0.10% in the rough steel made in the electric furnace to ensure the purity of the steel.

[0054] Furthermore, in the step S5, the qualified stainless steel molten steel in each package smelted in the step S4 is poured into the tundish at a speed of 8-10 t / min, and the liquid level of the molten steel in the tundish is ensured to be stable during the process of pouring the molten steel into the tundish;

[0055] The pouring speed of the molten steel in the tundish to the ingot mold is 5-8t / min, and the pouring temperature is distributed in a step-by-step manner within the range of 1500°C-1580°C; the superheat of the poured molten steel is selected according to the chemical composition and physical state of the molten steel, and then the stepped distribution temperature is determined according to the pouring order of each ladle of molten steel, specifically, when there are 2 ladles of molten steel in step S3, the pouring temperature gradient of the tundish is high and medium-high; when there are 3 ladles of molten steel in step S3, the pouring temperature gradient of the tundish is medium-high, low, and high; when there are 4 ladles of molten steel in step S3, the pouring temperature gradient of the tundish is medium-high, low, low, and high design principles; by designing different molten steel amounts and the existence of a temperature gradient, the heat entering the ingot mold is in a balanced state, and the superheat of the molten steel is kept stable in a certain range.

[0056] Specifically, in step S5, argon is used to protect the pouring throughout the pouring process to prevent secondary oxidation, and the vacuum degree in the vacuum chamber is maintained at no higher than 133 Pa during pouring.

[0057] In summary, the present invention arranges an inverted truncated cone-shaped groove on the chassis of the ingot auxiliary tool, and reasonably matches the design parameters of the chassis thickness, the taper and the height-to-diameter ratio of the ingot mold, and the neck reduction ratio of the insulation cap, so as to control the longitudinal cooling rate of the ingot to be ≤20 mm / min; further, the molten steel distribution scheme is coordinated with the temperature gradient of the casting process to achieve an average cooling rate of the ingot of 18-28 ° C / h, effectively improve the uniformity of the ingot, control the degree of element segregation to be low, and achieve the chemical composition of the 300t-level high-viscosity stainless steel super-large ingot, such as C, P, S, Cr, Mo and other segregation-prone elements. The segregation index is stably controlled, especially the C segregation index is controlled at 0.90%-1.10%, and the inclusion level is as low as below 2.5, which lays a good foundation for the performance of the final product. By controlling the N content in the range of 0.0015%-0.0045% after VD refining and readjusting the N content to 150-450ppm after VD vacuum breaking to balance the amount of N element reduced again due to the vacuum environment during subsequent vacuum pouring, stable control of the N element in a narrow range can be achieved, effectively reducing the formation of nitride inclusions in steel, thereby improving the purity and mechanical properties of the steel.

[0058] The method for manufacturing a 300t grade stainless steel ingot of the present invention is described below with reference to specific embodiments.

[0059] Example 1

[0060] This embodiment provides a method for manufacturing a 290t stainless steel ingot for a runner forging. Figure 1 As shown, the following steps are included:

[0061] S1. Design of ingot auxiliary equipment: including chassis, ingot mold and insulation cap;

[0062] The total weight of the steel ingot is 290t, and the weight percentages of each part are as follows: nozzle accounts for 2.4%, riser accounts for 21.03%, and ingot body accounts for 76.57%. Among the auxiliary tools used, the bottom plate is 800mm thick and the upper diameter of the center nozzle is Lower diameter Depth 430mm; the ingot mold is a 24-sided plum blossom ingot with a taper of 10.3%, a height-to-diameter ratio of 0.96, and a wall thickness of 380mm; the insulation cap has a taper of 10.3% and a diameter reduction ratio of 95.59%.

[0063] S2. Prepare steel raw materials according to the target composition of the ingot;

[0064] Select the raw materials for the furnace, and make the molten steel in the electric furnace with [Sb] = 0.0009%, [Sn] = 0.005%, [As] = 0.005%, [Cu] = 0.06%, [W] = 0.10%. The lime in the slag-making material should be active lime, and its activity is 325ml / 4N-HCL. The ferrochrome alloy uses the grade FeCr65C0.25, and the nickel alloy uses the grade Ni9990.

[0065] S3. Design molten steel distribution plan;

[0066] A three-package molten steel distribution plan is adopted, namely 105t, 135t and 50t.

[0067] S4. To smelt stainless steel liquid with qualified chemical composition, use all scrap steel as raw material to produce crude molten steel in electric furnace, transfer to LF refining furnace for slag deoxidation, adjust the chemical composition of molten steel, and then transfer to VOD furnace for oxygen blowing and decarburization. After completion, vacuum VD treatment is performed. The N content ranges from 30-100ppm after VD refining. After breaking the air, continue to adjust the N element to 150-450ppm, and soft blowing takes about 25 minutes to produce steel. The vacuum degree of VOD process is controlled at 10KPa, and the vacuum degree of VD process is controlled below 267Pa.

[0068] The composition of qualified stainless steel liquid is calculated by weight and includes: [C] = 0.031%, [Si] = 0.38%, [Mn] = 0.55%, [P] = 0.015%, [S] = 0.004%, [Cr] = 12.66%, [Ni] = 4.63%, [Mo] = 0.59%, [V] = 0.06%, [Cu] = 0.05%, [W] = 0.09%, [H] = 0.00015%, [O] = 0.0024%, [N] = 0.0263%. The residual elements [Sb] = 0.0009%, [Sn] = 0.005%, [As] = 0.005%, [Cu] = 0.06%, [W] = 0.10%, and the total amount of Cu + W + V is ≤ 0.5, and the balance is Fe;

[0069] S5. Use tundish to cast stainless steel ingots under vacuum;

[0070] Argon is used to protect the pouring throughout the pouring process to prevent secondary oxidation. The vacuum degree in the vacuum chamber is kept no higher than 67Pa when pouring in the tundish. The pouring speed is 7.2t / min, the pouring temperature of the first ladle is 1551℃, the pouring temperature of the second ladle is 1532℃, and the pouring temperature of the third ladle is 1573℃.

[0071] S6. Control cooling until the ingot solidifies and forms;

[0072] The average cooling rate of the steel ingot is 23.5℃ / h, and the surface temperature of the upper 1 / 3 of the ingot body is 650℃ when demoulding.

[0073] Table 1 Results of non-metallic inclusion testing of forgings in Example 1

[0074]

[0075]

[0076] Note: T- tangential test sample; L- radial test sample; M- riser end test sample; S- nozzle end test sample.

[0077] Table 2 Results of element content testing of forging samples in Example 1

[0078] Sampling No. C / ％ P / ％ S / ％ Cr / ％ Mo / % O / ％ N / ％ L-0° 0.029 0.013 0.002 12.98 0.59 0.0013 0.020 L-180° 0.027 0.013 0.002 12.96 0.59 0.0020 0.019 T-0° 0.030 0.014 0.002 12.94 0.58 0.0024 0.018 T-180° 0.031 0.013 0.002 12.96 0.58 0.0019 0.019 M outer circle 0.031 0.014 0.002 12.86 0.57 0.0021 0.028 M Center 0.033 0.014 0.002 12.90 0.58 0.0025 0.030 S outer circle 0.033 0.014 0.002 12.88 0.58 0.0020 0.029 S Center 0.030 0.014 0.002 12.89 0.58 0.0019 0.029

[0079] Note: T- tangential test sample; L- radial test sample; M- riser end test sample; S- nozzle end test sample.

[0080] Comparative Example 1

[0081] This embodiment provides a method for manufacturing a 290t stainless steel ingot for a runner forging. The steps are the same as those in Embodiment 1, except that the parameters of the ingot auxiliary tool are matched as follows:

[0082] The total weight of the steel ingot is 290t, and the weight percentages of each part are as follows: nozzle accounts for 6.37%, riser accounts for 18.53%, and ingot body accounts for 75.1%. Among the auxiliary tools used, the bottom plate is 500mm thick and the upper diameter of the center nozzle is Lower diameter Depth 200mm; the ingot mold is a 24-sided plum blossom ingot with a taper of 10%, a height-to-diameter ratio of 1.0, and a wall thickness of 300mm; the insulation cap has a taper of 8% and a necking ratio of 90%.

[0083] And in step S6, the average cooling rate of the steel ingot solidification molding is controlled to be 15°C / h, and the surface temperature of the upper 1 / 3 of the steel ingot body is 720°C when demoulding.

[0084] The segregation is serious, the uniformity of the ingot is poor, and the C element segregation index is greater than 2.0%.

[0085] Table 3 Results of element content testing at the riser of forgings in comparative example 1

[0086] Sampling No. C / ％ P / ％ S / ％ M outer circle 0.040 0.018 0.005 M Center 0.038 0.017 0.003

[0087] Note: T- tangential test sample; L- radial test sample; M- riser end test sample; S- nozzle end test sample.

[0088] Comparative Example 2

[0089] This embodiment provides a method for manufacturing a 290t stainless steel ingot for a runner forging, comprising the following steps:

[0090] S1. Design of ingot auxiliary equipment: including chassis, ingot mold and insulation cap;

[0091] The auxiliary tool parameters are the same as those in Example 1.

[0092] S2. Prepare steel raw materials according to the target composition of the ingot;

[0093] Select the raw materials for the furnace, and make the molten steel in the electric furnace with [Sb] = 0.0009%, [Sn] = 0.005%, [As] = 0.005%, [Cu] = 0.06%, [W] = 0.10%. The lime in the slag-making material should be active lime, and its activity is 325ml / 4N-HCL. The ferrochrome alloy uses the grade FeCr65C0.25, and the nickel alloy uses the grade Ni9990.

[0094] S3. Design molten steel distribution plan;

[0095] A three-package molten steel distribution plan is adopted, namely 160t, 100t and 30t.

[0096] S4. To smelt stainless steel liquid with qualified chemical composition, use all scrap steel as raw material to produce crude molten steel in electric furnace, transfer to LF refining furnace for slag deoxidation, adjust the chemical composition of molten steel, and then transfer to VOD furnace for oxygen blowing and decarburization. After completion, vacuum VD treatment is performed. The N content ranges from 30-100ppm after VD refining. After breaking the air, continue to adjust the N element to 150-450ppm, and soft blowing takes about 25 minutes to produce steel. The vacuum degree of VOD process is controlled at 10KPa, and the vacuum degree of VD process is controlled below 267Pa.

[0097] The composition of qualified stainless steel liquid is calculated by weight and includes: [C] = 0.031%, [Si] = 0.38%, [Mn] = 0.55%, [P] = 0.015%, [S] = 0.004%, [Cr] = 12.66%, [Ni] = 4.63%, [Mo] = 0.59%, [V] = 0.06%, [Cu] = 0.05%, [W] = 0.09%, [H] = 0.00015%, [O] = 0.0024%, [N] = 0.0263%. The residual elements [Sb] = 0.0009%, [Sn] = 0.005%, [As] = 0.005%, [Cu] = 0.06%, [W] = 0.10%, and the total amount of Cu + W + V is ≤ 0.5, and the balance is Fe;

[0098] S5. Use tundish to cast stainless steel ingots under vacuum;

[0099] Argon is used to protect the pouring throughout the pouring process to prevent secondary oxidation. The vacuum degree in the vacuum chamber is maintained at 67Pa during pouring in the tundish. The pouring speed is 7.2t / min, the pouring temperature of the first ladle is 1551℃, the pouring temperature of the second ladle is 1532℃, and the pouring temperature of the third ladle is 1573℃.

[0100] S6. Control cooling until the ingot solidifies and forms;

[0101] The average cooling rate of the steel ingot is 13℃ / h, and the surface temperature of the upper 1 / 3 of the ingot body is 750℃ when demoulding.

[0102] The segregation is serious, the uniformity of the ingot is poor, and the C element segregation index is greater than 2.0%.

[0103] Table 4 Results of element content testing at the riser of forgings in comparative example 2

[0104] Sampling No. C / ％ P / ％ S / ％ M outer circle 0.045 0.023 0.008 M Center 0.040 0.020 0.005

[0105] Note: T- tangential test sample; L- radial test sample; M- riser end test sample; S- nozzle end test sample.

[0106] Comparative Example 3

[0107] This embodiment provides a method for manufacturing a 290t stainless steel ingot for a runner forging. The steps are the same as those in Embodiment 1, except that the temperature pouring gradient in step S5 is: the pouring temperature of each ladle is 1500°C.

[0108] The average cooling rate of the steel ingot is 30℃ / h, and the surface temperature of the upper 1 / 3 of the ingot body is 650℃ when demoulding.

[0109] The poor uniformity of the steel ingot affects the mechanical properties of various parts of the forging and makes it impossible to formulate corresponding heat treatment process parameters.

[0110] Comparative Example 4

[0111] This embodiment provides a method for manufacturing a 290 t stainless steel ingot for a runner forging. The steps are the same as those in Embodiment 1, except that the N content is not controlled after VD refining and breaking in step S4.

[0112] The average cooling rate of the steel ingot is 23.4℃ / h, and the surface temperature of the upper 1 / 3 of the ingot body is 650℃ when demoulding.

[0113] After testing, the N content in various parts of the steel ingot was less than 0.01%, indicating that the VOD+VD strong deoxidation refining and the high degree of degassing during vacuum pouring did not match the two properly, resulting in the N content being unable to match other elements, ultimately affecting the performance indicators of the forgings.

[0114] It can be seen from Example 1 and Comparative Examples 1, 2 and 3 that when Example 1 adopts the auxiliary tool design parameters and the molten steel smelting and pouring scheme of the present invention, the average cooling rate of the steel ingot can be 18-28°C / h, thereby obtaining a stainless steel ingot with uniform element composition and small segregation; the auxiliary tool design of Comparative Example 1 does not meet the requirements of the present invention, the average cooling rate of the steel ingot is 15°C / h, the segregation is serious, and the uniformity of the steel ingot is poor; the molten steel distribution scheme of Comparative Example 2 does not meet the requirements of the present invention, the average cooling rate of the steel ingot is 13°C / h, the segregation is serious, and the uniformity of the steel ingot is poor; the temperature gradient is not set during the pouring process of Comparative Example 3, the average cooling rate of the steel ingot is 30°C / h, the uniformity of the steel ingot is poor, and the mechanical properties of various parts of the forging are affected;

[0115] In Comparative Example 4, the N content was not controlled during the smelting and casting process, resulting in the N content being unable to match other elements, ultimately affecting various performance indicators of the forgings.

[0116] In summary, the present invention arranges an inverted truncated cone-shaped groove on the chassis of the ingot auxiliary tool, and reasonably matches the design parameters of the chassis thickness, the taper and the height-to-diameter ratio of the ingot mold, and the neck reduction ratio of the insulation cap, so as to control the longitudinal cooling rate of the ingot to be ≤20 mm / min; further, the molten steel distribution scheme is coordinated with the temperature gradient of the casting process to achieve an average cooling rate of the ingot of 18-28 ° C / h, effectively improve the uniformity of the ingot, control the degree of element segregation to be low, and achieve the chemical composition of the 300t-level high-viscosity stainless steel super-large ingot, such as C, P, S, Cr, Mo and other segregation-prone elements. The segregation index is stably controlled, especially the C segregation index is controlled at 0.90%-1.10%, and the inclusion level is as low as below 2.5, which lays a good foundation for the performance of the final product. By controlling the N content in the range of 30-100ppm after VD refining and readjusting the N content to 150-450ppm after VD vacuum breaking to balance the amount of N element reduced again due to the vacuum environment during subsequent vacuum pouring, stable control of the N element in a narrow range can be achieved, effectively stabilizing the N element level of forgings, thereby improving the purity and mechanical properties of the steel.

[0117] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for manufacturing a 300t grade stainless steel ingot for a runner forging, characterized in that: The steps include: S1. Design of ingot auxiliary tools: Calculate the required ingot weight according to the required weight of the forging, and design corresponding auxiliary tools according to the ingot weight, casting method and target ingot composition, including a chassis, an ingot mold and a heat preservation cap, wherein the chassis is provided with a truncated cone-shaped groove; S2. Prepare steel raw materials according to the target composition of the ingot; S3. Design molten steel distribution plan; S4. Subcontracting smelting according to the molten steel allocation plan: After the crude molten steel is obtained by electric furnace smelting, the LF+VOD+VD refining process is used for refining to obtain stainless steel molten steel with qualified chemical composition; S5. The molten stainless steel obtained in step S4 is poured into a stainless steel ingot under vacuum; S6. Control cooling until the steel ingot solidifies and forms, wherein the comprehensive cooling rate of the steel ingot is 18-28°C / h. After forming, the steel ingot is demoulded when the surface temperature at the upper 1 / 3 of the ingot body is 640-660°C.

2. The manufacturing method according to claim 1, characterized in that: The target steel ingot composition, in percentage by mass, includes: [C]=0.02-0.04%, [Si]=0.30-0.60%, [Mn]=0.50-0.80%, [P]≤0.025%, [S]≤0.010%, [Cr]=12.00-13.50%, [Ni]=4.50-6.00%, [Mo]=0.40-0.80%, [V]≤0.10%, [Cu]≤0.50%, [W]≤0.10%, [H]≤0.0002%, [O]≤0.005%, and [N]=0.01-0.03%. Residual elements [Sb]≤0.0015, [Sn]≤0.015, [As]≤0.020, [Ti]≤0.015, and the total amount of Cu+W+V≤0.5, with the balance being Fe and unavoidable impurities.

3. The manufacturing method according to claim 1, characterized in that: The bottom plate of the steel ingot auxiliary tool in step S1 has a thickness of 600 mm to 1000 mm.

4. The manufacturing method according to claim 3, characterized in that: The upper diameter of the truncated cone-shaped groove on the chassis is ≤φ2000mm, the lower diameter is ≥φ1500mm, the upper diameter is greater than the lower diameter, and the depth is 300mm-500mm.

5. The manufacturing method according to claim 1, characterized in that: The steel ingot mold in step S1 is a 24-sided plum blossom ingot with a taper of 7% to 11%.

6. The manufacturing method according to claim 5, characterized in that: The steel ingot mold has a height-to-diameter ratio of 0.8 to 1.3 and a wall thickness of 200 mm to 500 mm.

7. The manufacturing method according to claim 1, characterized in that: The thermal insulation cap in step S1 has a taper of 8% to 12% and a necking ratio of 85% to 98%.

8. The manufacturing method according to claim 1, characterized in that: In step S2, the steel raw materials are all scrap steel, ferrochrome and nickel alloy; the ferrochrome grade is FeCr65C0.25, and the nickel alloy grade is Ni9990.

9. The manufacturing method according to claim 1, characterized in that: In step S4, the N content after VD refining is controlled to be in the range of 30-100 ppm, and the N content is adjusted to 150-450 ppm after the VD vacuum is broken.

10. The manufacturing method according to claim 1, characterized in that: Step S5 adopts tundish pouring, and the tundish pouring speed is 5-8t / min; Argon is used for protection during the whole pouring process, and the vacuum degree in the vacuum chamber is kept not higher than 133Pa during pouring.

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