Continuous casting process method of S30432 stainless steel continuous casting billet

Abstract

The invention discloses a continuous casting process method of an S30432 stainless steel continuous casting billet, and solves the technical problems of poor product quality and production cost increase caused by Nb segregation in a molten steel solidification process. The continuous casting process method comprises the following steps: S1, preparing and treating molten steel; s2, adjusting a tundish; s3, initial solidification forming of a crystallizer; s4, carrying out secondary cooling; s5, throwing and straightening; s6, fixed-length cutting is carried out; s7, ejection and finishing are conducted; wherein in the step S2, the superheat degree of the tundish is controlled to be 20-30 DEG C. By optimizing the tundish superheat degree, the cooling water proportion, the pulling speed and the electromagnetic stirring parameters, the problem of Nb segregation in the molten steel solidification process is solved, the product quality is improved, the resource utilization efficiency is improved, and the production cost can also be reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of stainless steel production and processing, and particularly relates to a continuous casting process method of S30432 stainless steel continuous casting billets. BACKGROUND

[0002] S30432 stainless steel is an austenitic heat-resistant stainless steel, also called CODE CASE2328-1, Super304H, 10Cr18Ni9NbCu3BN, the material is designed for ultra-supercritical thermal power generating units, the chemical composition adds about 3% Cu element and trace N element on the basis of traditional 304H stainless steel, and is stabilized by Nb element, which significantly improves the high temperature performance of the material, so that it still maintains excellent creep strength and steam oxidation resistance in the environment of 600-620 DEG C, and has good processing forming property and welding performance, and is mainly used for manufacturing ultra-supercritical thermal power generating unit boiler high temperature section components (such as superheater, reheater pipeline, etc.), which can withstand extreme conditions of 620 DEG C and 30 MPa or more.

[0003] However, in the related art, during the production and processing of S30432 stainless steel, especially during the continuous casting operation, there is a problem of Nb segregation in the solidification process of molten steel, which not only reduces the subsequent hot working plasticity of the continuous casting billet, but also produces serious porosity, shrinkage and center cracks, has a very adverse effect on the performance and quality of the continuous casting billet, and wastes the expensive Nb resource, increases the production cost, and needs to be improved. SUMMARY

[0004] In order to solve all or part of the above problems, the purpose of the present application is to provide a continuous casting process method of S30432 stainless steel continuous casting billet, which optimizes the tundish superheat, cooling water ratio, drawing speed and electromagnetic stirring parameters, improves the problem of Nb segregation in the solidification process of molten steel, improves the product quality, improves the resource utilization efficiency, and also reduces the production cost.

[0005] The application provides a continuous casting process method of S30432 stainless steel continuous casting billet, which comprises the following steps:

[0006] S1, molten steel preparation and treatment;

[0007] S2, tundish adjustment;

[0008] S3, crystallizer initial condensation forming;

[0009] S4, secondary cooling;

[0010] S5, billet drawing and straightening;

[0011] S6, fixed-length cutting;

[0012] S7, billet discharging and finishing;

[0013] In S2, the superheat of the intermediate ladle is controlled at 20-30℃.

[0014] Optionally, in S3, the pulling speed is controlled to be 0.6-0.65 m / min.

[0015] Optionally, in S4, the cooling water intensity of the secondary cooling zone is controlled to be 0.20-0.22 L / Kg.

[0016] Optionally, in S3, the electromagnetic stirring parameters of the crystallizer are controlled as follows: current 200-250A, frequency 1-2Hz.

[0017] Optionally, in S4, the electromagnetic stirring parameters for controlling the secondary cooling zone are: current 200-250A, frequency 1-2Hz.

[0018] Optionally, in S4, the electromagnetic stirring parameters at the end of solidification are controlled as follows: current 400-450A, frequency 1-2Hz.

[0019] Optionally, in S1, the composition of the molten steel by weight percentage is:

[0020] C: 0.07-0.13%, Si≤0.30%, Mn≤1.00%, P≤0.030%, S≤0.010%, Cr: 17.00-19.00%, Ni: 7.50-10. 50%, Nb: 0.30-0.60%, Cu: 2.50-3.50%, N: 0.05-0.12%, Al: 0.003-0.030%, B: 0.001~0.010%.

[0021] As can be seen from the above technical solution, the continuous casting process method for S30432 stainless steel continuous casting billets provided by the present invention has the following advantages:

[0022] This continuous casting process improves Nb segregation during steel solidification by optimizing tundish superheat, cooling water ratio, casting speed, and electromagnetic stirring parameters, thereby enhancing product quality, increasing resource utilization efficiency, and reducing production costs.

[0023] Other features and advantages of the present invention will be set forth in the following description. Attached Figure Description

[0024] The accompanying drawings are provided to further understand the technical solutions of the present invention and constitute a part of the specification. They are used together with the embodiments of the present invention to explain the technical solutions of the present invention, and do not constitute a limitation on the technical solutions of the present invention.

[0025] Figure 1 This is a process flow diagram of the continuous casting process in an embodiment of the present invention;

[0026] Figure 2 This is a cross-sectional view of a φ390mm continuously cast round billet of S30432 stainless steel.

[0027] Figure 3 This is a schematic diagram of the cross-section sampling of a φ390mm continuously cast round billet of S30432 stainless steel. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be noted that, unless otherwise specified, the embodiments and features described in the embodiments of the present invention can be arbitrarily combined with each other.

[0029] like Figure 1 , Figure 2 , Figure 3 The figure shown is an embodiment of the present invention, which discloses a continuous casting process method for S30432 stainless steel continuous casting billets, including the following steps:

[0030] S1, Steel preparation and treatment;

[0031] S2, intermediate package adjustment;

[0032] S3, initial crystallization and forming in the crystallizer;

[0033] S4, secondary cooling;

[0034] S5, throwing and straightening;

[0035] S6, fixed-length cutting;

[0036] S7, billet production and finishing.

[0037] S30432 stainless steel is based on 304H stainless steel, with the addition of Cu, Nb, N, and B elements to achieve a strengthening effect. Among them:

[0038] The addition of Cu causes the precipitation of a copper-rich phase, thereby enhancing the high-temperature durability of the material.

[0039] The addition of Nb, accompanied by the precipitation of niobium carbide, increases the material's resistance to intergranular corrosion. Another effect is to improve the high-temperature creep strength of steel.

[0040] The addition of nitrogen will produce two strengthening mechanisms, solid solution and precipitation, which will improve the high-temperature performance of the material.

[0041] The addition of trace amounts of boron (B) improves the thermoplasticity of steel while inhibiting the precipitation of methyl monoxide (M) at grain boundaries. 23 C6 improves the steel's resistance to intergranular corrosion.

[0042] Due to the high affinity of Nb for C and N, Nb-containing austenitic stainless steel is prone to Nb segregation during solidification. This reduces the plasticity of subsequent hot working and can even lead to severe porosity, shrinkage cavities, and central cracks, which have a very adverse effect on the performance and quality of the cast billet. It also wastes expensive Nb resources and increases production and application costs. Therefore, it is necessary to focus on controlling Nb segregation during solidification.

[0043] However, the superheat of the tundish, the cooling water ratio, the casting speed, and the electromagnetic stirring parameters are all closely related to the control of element segregation during solidification. If you want to improve the quality of the product, you need to focus on controlling and optimizing the superheat of the tundish, the cooling water ratio, the casting speed, and the electromagnetic stirring parameters during continuous casting.

[0044] To suppress Nb segregation, a low superheat is also necessary. High superheat slows the solidification rate of molten steel, leading to excessive columnar crystal growth and a tendency for bridging. This isolates the molten steel in the central region, making feeding difficult, preventing solute elements from escaping, and resulting in severe central segregation. Reducing superheat can decrease defects such as central segregation, porosity, and shrinkage cavities, ensuring the performance and quality of the cast billet.

[0045] However, if the tundish superheat is too low, the molten steel's fluidity will decrease, the surface tension will increase, increasing the risk of slag entrapment in the crystallizer. It will also affect the floating of non-metallic inclusions, and in severe cases, block the nozzle, affecting production continuity. This depends on factors such as the characteristics of the steel grade, equipment conditions, and smooth production.

[0046] Therefore, in S2, controlling the superheat of the tundish to 20-30℃ is the core, and low superheat is the key. With the help of electromagnetic stirring, segregation and defects in the center of the billet can be effectively suppressed, the quality of the billet can be improved, and the performance of the subsequent one-fired finished products can be guaranteed.

[0047] At low superheat, to ensure the molten steel is poured out of the ladle and tundish normally, and to avoid excessive tundish filler and low billet yield, the casting speed should not be set too low. Currently, after the carbon steel continuous casting round billet equipment is put into operation, the casting speed range for the φ390mm crystallizer specification is 0.50-0.70 m / min (designed maximum casting speed 0.72 m / min).

[0048] However, high casting speeds mean shorter residence time of the billet in the mold, larger flow rate of molten steel per unit time, and slower cooling. This results in longer diffusion and migration time of Nb elements in the liquid steel, which can exacerbate segregation. Therefore, based on the solidification characteristics of the steel grade and practical experience, in S3, the casting speed is controlled at 0.6-0.65 m / min, supplemented by low superheat and effective electromagnetic stirring to suppress segregation.

[0049] Specific water intensity is a crucial indicator for measuring the cooling intensity of the secondary cooling zone. In stainless steel continuous casting, excessive secondary cooling intensity causes the billet to cool too quickly, easily leading to Nb segregation, surface cracks, and central defects. The core of optimized control lies in a appropriately weakened and uniformly distributed secondary cooling strategy, combined with electromagnetic stirring. This aims to reduce thermal stress, improve feeding conditions, and enhance the internal quality of the billet, especially the density and uniformity of the central region, laying the foundation for the performance of the subsequent single-fired product. Therefore, in S4, the cooling water intensity in the secondary cooling zone is controlled at 0.20-0.22 L / kg.

[0050] Electromagnetic stirring technology controls the flow, heat transfer, and solidification of molten steel through electromagnetic stirring, increasing heat dissipation, thus reducing superheat and improving Nb segregation and central defects. Therefore, the current and frequency are the core control parameters for electromagnetic stirring in the crystallizer (M-EMS), secondary cooling zone (S-EMS), and final solidification stage (F-EMS): higher current results in stronger stirring force; higher frequency results in deeper penetration, thereby affecting the final billet quality.

[0051] For electromagnetic stirring (M-EMS) in crystallizers, if the current is too low, the stirring force will be insufficient to break up dendrites and promote the flow of molten steel and reduce segregation. If the current is too high, the stirring force will be too strong, greatly increasing the risk of slag entrapment and deteriorating the surface quality.

[0052] Taking all factors into consideration, electromagnetic stirring in crystallizers typically employs medium-intensity current and low frequency because its penetration depth is large, and its effective area is closer to the interior of the billet shell, thus promoting grain refinement. If high frequency is used, the penetration depth is shallow, and the effective area is concentrated on the surface of the molten steel, resulting in a weaker effect on breaking up dendrites at the solidification front.

[0053] Therefore, in S3, the electromagnetic stirring parameters of the crystallizer are controlled as follows: current 200-250A, frequency 1-2Hz, in order to prevent the Nb-enriched molten steel at the end of solidification from flowing towards the center, thereby reducing segregation and central defects.

[0054] For electromagnetic stirring in the secondary cooling zone (S-EMS), the solidified billet shell already has a certain thickness (1 / 3 to 1 / 2 of the billet thickness). Applying electromagnetic force to stir the paste-like zone (solid-liquid two-phase zone) allows the Nb-enriched molten steel to mix with the surrounding solution, reducing segregation.

[0055] If the current is too low, the stirring force will be insufficient, and the effect of reducing segregation will be small. If the current is too high, the stirring force will be too strong, which will easily cause the Nb-enriched molten steel to be carried away by forced convection, forming an Nb-depleted zone (negative segregation zone), which is a bright white defect. The higher the current, the wider and more obvious the defect will be.

[0056] Taking all factors into consideration, the electromagnetic stirring in the secondary cooling zone uses a medium-intensity current and low frequency. This allows for greater penetration depth, reaching the central region of the mushy zone and helping to reduce segregation. If a high frequency is used, the penetration depth is shallower, primarily affecting the outer layer of the mushy zone near the solidified billet shell. This results in faster reciprocating motion of the molten steel, making it more prone to producing bright white band defects.

[0057] Therefore, in S4, the electromagnetic stirring parameters for controlling the secondary cooling zone are: current 200-250A and frequency 1-2Hz, which can reduce segregation and prevent defects such as white bright bands.

[0058] For solidification end electromagnetic stirring (F-EMS), at this time solidification is about to end, the main function of applying electromagnetic stirring is to strongly stir and squeeze the residual molten steel rich in Nb and inclusions between dendrites, promote the residual molten steel to fill the voids formed by solidification shrinkage (central looseness, shrinkage cavity), break dendrites, make the Nb element in the residual molten steel more evenly dispersed in the final solidified structure, and promote the floating or dispersion of inclusions.

[0059] If the current is too low, the stirring force will be insufficient, failing to effectively stir and squeeze the residual molten steel, resulting in minimal effect on improving center defects and end segregation. If the current is too high, the stirring force will be too strong, with a low probability of causing overheating in the center region (heat generation during stirring) and expanding center defects.

[0060] Taking all factors into consideration, electromagnetic stirring at the end of solidification using a relatively high-intensity current and low frequency is preferable because its large penetration depth effectively targets the residual molten steel in the core of the billet, thus helping to reduce segregation and central defects. If a high-frequency current is used, the penetration depth is shallower, resulting in a less effective improvement in the quality of the billet's core.

[0061] Therefore, in S4, the electromagnetic stirring parameters at the end of solidification are controlled as follows: current 400-450A, frequency 1-2Hz, to ensure product quality.

[0062] In S1, the composition of the molten steel is controlled by weight percentage as follows:

[0063] C: 0.07-0.13%, Si≤0.30%, Mn≤1.00%, P≤0.030%, S≤0.010%, Cr: 17.00-19.00%, Ni: 7.50-10. 50%, Nb: 0.30-0.60%, Cu: 2.50-3.50%, N: 0.05-0.12%, Al: 0.003-0.030%, B: 0.001~0.010%.

[0064] The continuous casting process of S30432 stainless steel billet in this embodiment improves the problem of Nb segregation during the solidification of molten steel by optimizing the tundish superheat, cooling water ratio, casting speed and electromagnetic stirring parameters, thereby improving product quality, increasing resource utilization efficiency and reducing production costs.

[0065] To illustrate this embodiment more clearly, the specific embodiment is as follows:

[0066] Example 1

[0067] S1, Steel preparation and treatment, where the composition of the molten steel is controlled by weight percentage as follows:

[0068] C: 0.076%, Si: 0.26%, Mn: 0.79%, P: 0.030%, S: 0.001%, Cr: 17.75%, Ni: 8.89%, Nb: 0.38%, Cu: 2.70%, N: 0.111%, Al: 0.005%, B: 0.007%;

[0069] S2, intermediate tundish adjustment, and in this step, the superheat of the intermediate tundish is controlled at 25°C, while the pulling speed is controlled at 0.63m / min;

[0070] S3, the crystallizer initially solidifies and forms the crystal, and the electromagnetic stirring parameters of the crystallizer in this step are controlled as follows: current 230A, frequency 1Hz;

[0071] S4, secondary cooling, and in this step, the cooling water intensity of the secondary cooling zone is controlled at 0.21L / Kg, while the electromagnetic stirring parameters of the secondary cooling zone are controlled as follows: current 230A, frequency 1Hz, and the electromagnetic stirring parameters at the end of solidification are controlled as follows: current 430A, frequency 1Hz.

[0072] S5, throwing and straightening;

[0073] S6, fixed-length cutting;

[0074] S7, billet production and finishing.

[0075] After sampling and testing, the S30432 stainless steel φ390mm continuously cast round billet produced in Example 1 had a center crack grade of 0.5 (e.g., Figure 2 As shown), there are no other defects and the product requirements are met.

[0076] Example 2

[0077] S1, Steel preparation and treatment, where the composition of the molten steel is controlled by weight percentage as follows:

[0078] C: 0.070%, Si: 0.19%, Mn: 0.79%, P: 0.026%, S: 0.002%, Cr: 17.79%, Ni: 8.65%, Nb: 0.41%, Cu: 2.78%, N: 0.095%, Al: 0.007%, B: 0.006%;

[0079] S2, intermediate tundish adjustment, and in this step, the superheat of the intermediate tundish is controlled at 30°C, while the pulling speed is controlled at 0.61m / min;

[0080] S3, initial crystallization in the crystallizer, and the electromagnetic stirring parameters of the crystallizer in this step are controlled as follows: current 220A, frequency 1.5Hz;

[0081] S4, secondary cooling, and in this step, the cooling water intensity in the secondary cooling zone is controlled at 0.22L / Kg, while the electromagnetic stirring parameters in the secondary cooling zone are controlled at: current 220A, frequency 1.5Hz, and the electromagnetic stirring parameters at the solidification end are controlled at: current 420A, frequency 1.5Hz.

[0082] S5, throwing and straightening;

[0083] S6, fixed-length cutting;

[0084] S7, billet production and finishing.

[0085] After sampling and testing, the S30432 stainless steel φ390mm continuous casting round billet produced in Example 1 had a Class 1 central crack and no other defects, meeting the product requirements.

[0086] For Examples 1 and 2, the sampling method involved drilling nine points at equal intervals along the radial direction of a φ390mm continuously cast round billet of S30432 stainless steel (center, R / 4, R / 2, 3R / 4, and 5mm below the surface). The drill hole diameter was 5mm, and the drill hole depth was 15mm. The sampling locations were as follows: Figure 3 As shown, the Nb content of samples from 9 points was measured simultaneously, and the average value was taken.

[0087] If the value of a single point is higher than the average, the value of the single point is divided by the average; if the value of a single point is lower than the average, the value of the average is divided by the value of the single point. The Nb segregation coefficient for each single point is obtained. As can be seen from the data in Table 1, the Nb segregation coefficient is much lower than the general industry requirement for niobium-containing heat-resistant stainless steel (≤1.10).

[0088] Table 1. Sampling and testing data of S30432 stainless steel φ390mm continuous casting round billet.

[0089] Number Nb (wt. %) Nb segregation coefficient Nb (wt. %) Nb segregation coefficient Nb (wt. %) Nb segregation coefficient Nb (wt. %) Nb segregation coefficient Nb (wt. A1 0.41 1.014 A2 0.42 1.039 A3 0.41 1.014 A4 0.42 1.039 A0 0.39 1.036 A11 0.40 1.010 A12 0.40 1.010 A13 0.40 1.010 A14 0.39 1.036

[0090] It should be noted that, unless otherwise stated, the technical or scientific terms used in this invention should have the ordinary meaning as understood by one of ordinary skill in the art.

[0091] Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly defined.

[0092] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. The present invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A continuous casting process for S30432 stainless steel continuously cast billets, characterized in that, Includes the following steps: S1, Steel preparation and treatment; S2, intermediate package adjustment; S3, initial crystallization and forming in the crystallizer; S4, secondary cooling; S5, throwing and straightening; S6, fixed-length cutting; S7, billet production and finishing; In S2, the superheat of the intermediate ladle is controlled at 20-30℃.

2. The continuous casting process method according to claim 1, characterized in that, In S3, the pulling speed is controlled at 0.6-0.65 m / min.

3. The continuous casting process method according to claim 1, characterized in that, In S4, the cooling water intensity in the secondary cooling zone is controlled at 0.20-0.22 L / Kg.

4. The continuous casting process method according to claim 1, characterized in that, In S3, the electromagnetic stirring parameters for controlling the crystallizer are: current 200-250A, frequency 1-2Hz.

5. The continuous casting process method according to claim 4, characterized in that, In S4, the electromagnetic stirring parameters for controlling the secondary cooling zone are: current 200-250A, frequency 1-2Hz.

6. The continuous casting process method according to claim 5, characterized in that, In S4, the electromagnetic stirring parameters at the end of solidification are controlled as follows: current 400-450A, frequency 1-2Hz.

7. The continuous casting process method according to claim 1, characterized in that, In S1, the composition of the molten steel by weight percentage is as follows: C: 0.07-0.13%, Si≤0.30%, Mn≤1.00%, P≤0.030%, S≤0.010%, Cr: 17.00-19.00%, Ni: 7.50-10. 50%, Nb: 0.30-0.60%, Cu: 2.50-3.50%, N: 0.05-0.12%, Al: 0.003-0.030%, B: 0.001~0.010%.

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