Corrosion-resistant reinforced ultra-pure ferritic stainless steel and thin-walled component precision casting process

Abstract

This invention discloses a corrosion-resistant reinforced ultrapure ferritic stainless steel and its thin-walled component precision casting process. The stainless steel, by mass percentage, comprises C 0.006%–0.010%, N 0.006%–0.010%, Cr 21.10%–21.50%, Mo 2.45%–2.60%, Ti 0.29%–0.32%, Nb 0.40%–0.42%, Sn 0.10%–0.13%, Cu 0.65%–0.75%, Ce 0.045%–0.055%, W 0.65%–0.80%, B 0.006%–0.008%, as well as controlled impurities and the Fe balance, and satisfies PREN ≥ 29.5, Ce / Ti = 0.141–0.190, and Nb / Ti = 1.25–1.45. The process includes vacuum induction melting, electroslag remelting, gradient shell preparation, vacuum anti-gravity nonlinear pressure boosting casting, controlled solidification, and post-cast composite heat treatment. This invention improves the filling integrity, inclusion cleanliness, pitting corrosion resistance, and intergranular corrosion stability of thin-walled precision-cast components through Cr-Mo-W-Sn-Cu corrosion resistance synergy, Nb / Ti ratio stabilization, Ce inclusion modification, and thin-walled directional solidification control.

Description

Technical Field

[0001] This invention belongs to the field of stainless steel technology, specifically a precision casting process for corrosion-resistant reinforced ultrapure ferritic stainless steel and its thin-walled components. Background Technology

[0002] Ferritic stainless steel is a type of stainless steel with chromium as the main alloying element and a body-centered cubic matrix structure. Compared with austenitic stainless steel, ferritic stainless steel has advantages such as high thermal conductivity, low coefficient of thermal expansion, better resistance to stress corrosion, low or no nickel content, and relatively low cost. It has been widely used in heat exchangers, automotive exhaust systems, building curtain walls, chemical equipment, and other fields. With increasingly harsh service conditions such as high salt spray environments in coastal areas, chlorine-containing chemical media, and high-temperature corrosive atmospheres, ordinary ferritic stainless steel can no longer meet the requirements for high corrosion resistance applications. Therefore, ultra-pure ferritic stainless steel, characterized by ultra-low carbon, ultra-low nitrogen, and high chromium, is gradually becoming an important development direction. Ultra-pure ferritic stainless steel is usually produced by strictly controlling the content of interstitial elements such as C and N, and adding stabilizing elements such as Ti and Nb to improve its resistance to pitting corrosion, intergranular corrosion, and toughness. Existing high corrosion-resistant ultrapure ferritic stainless steels mainly employ the Cr-Mo-Nb-Ti system, increasing the PREN value (pre-corrosion resistance equivalent) by raising the Cr and Mo content. Simultaneously, some studies have explored adding microalloying elements such as Cu, Sn, W, and Ce to improve passivation film stability, inhibit pitting corrosion propagation, or reduce inclusions. On the other hand, investment casting is a near-net-shape forming process suitable for complex thin-walled components, but currently it is mainly applied to nickel-based superalloys, cobalt-based alloys, and titanium alloys. The precision casting technology for thin-walled components made of ultrapure ferritic stainless steel remains immature. Due to problems encountered in the precision casting process of ferritic stainless steel, such as poor molten steel fluidity, difficulty in feeding, coarse as-cast grains, easy coarsening of TiN inclusions, and insufficient corrosion resistance of the as-cast surface, the preparation of high corrosion-resistant ultrapure ferritic stainless steel thin-walled precision-cast components presents a technical challenge requiring coordinated control of material composition design and precision casting processes.

[0003] Existing ultra-pure ferritic stainless steel grades, such as SUS439, SUS444, SUS447J1, and S44660, are primarily designed for rolled or forged products such as plates and bars. Rolling and forging processes can break down the as-cast structure, refine grains, and reduce segregation through hot deformation and recrystallization. However, precision-cast components are near-net-shape products, typically undergoing little or no subsequent deformation processing. As-cast structure and casting defects directly affect final service performance. Therefore, directly using existing rolled ultra-pure ferritic stainless steel in thin-walled precision casting easily leads to problems such as filling difficulties, cold shuts, incomplete filling, shrinkage porosity, shrinkage cavities, coarse microstructure, and unstable performance. Furthermore, existing technologies typically rely on increasing Cr and Mo content to improve corrosion resistance. However, increased Cr content increases the viscosity of the molten steel and reduces its fluidity, while the addition of Mo narrows the solidification zone and prematurely closes the feeding channels, thus exacerbating the filling and feeding contradictions in the thin-walled precision casting process. For Ti-stabilized ultrapure ferritic stainless steel, TiN inclusions may precipitate before the molten steel solidifies, and they tend to grow under slow cooling conditions during casting, becoming preferred nucleation sites for pitting corrosion and sources of crack initiation.

[0004] Therefore, to address the shortcomings of the existing technologies, it is necessary to provide a corrosion-resistant, enhanced ultra-pure ferritic stainless steel and its precision casting process tailored to the characteristics of thin-walled precision casting. This technical solution needs to address the following technical issues: First, without solely relying on increasing the Cr and Mo content, a design scheme for an ultra-pure ferritic stainless steel alloy that balances high corrosion resistance and good casting performance should be established, ensuring the material has both a high PREN value and meets the flowability and feeding requirements in thin-walled cavities; Second, the size and distribution of TiN inclusions should be controlled to reduce their adverse effects as pitting nucleation sites and crack initiation sources. Summary of the Invention

[0005] To address the technical problems existing in the prior art, this invention provides a precision casting process for corrosion-resistant enhanced ultrapure ferritic stainless steel and its thin-walled components. This invention primarily solves the problem that existing ultrapure ferritic stainless steels, while improving corrosion resistance, cannot simultaneously satisfy the requirements for molten steel fluidity, solidification segregation tendency, and the filling and feeding capabilities of thin-walled cavities under balanced conditions.

[0006] According to one aspect of the present invention, a corrosion-resistant reinforced ultrapure ferritic stainless steel is provided, wherein the chemical composition of the stainless steel comprises, by mass percentage: C 0.006%~0.010%, N 0.006%~0.010%, Cr 21.10%~21.50%, Mo 2.45%~2.60%, Ti 0.29%~0.32%, Nb 0.40%~0.42%, Sn 0.10%~0.13%, Cu 0.65%~0.75%, Ce 0.045%~0.055%, W 0.65%~0.80%, B 0.006%~0.008%, S≤0.005%, P≤0.02%, Si≤0.3%, Mn≤0.3%, O≤0.005%, balance Fe and unavoidable impurities; And it satisfies the following relationship: PREN=Cr+3.3×Mo+0.5×W≥29.5; Ce / Ti = 0.141~0.190; Nb / Ti = 1.25~1.45; Where PREN represents pitting resistance equivalent, Cr, Mo, and W represent the mass percentage of the corresponding elements in the chemical composition of stainless steel; Ce / Ti and Nb / Ti represent the ratio of the mass percentage of the corresponding elements. Unavoidable impurities are elements other than those listed above, with the content of any single element ≤0.03% and the total amount of other elements ≤0.10%.

[0007] Existing ultrapure ferritic stainless steels typically improve pitting corrosion resistance by increasing Cr and Mo content. However, high Cr and Mo systems increase molten steel viscosity, exacerbate solidification segregation, and reduce the filling and feeding capacity of thin-walled cavities. Conversely, reducing Cr and Mo content to improve casting performance results in insufficient passivation film stability, making it difficult to meet corrosion resistance requirements in environments with high salt spray and chloride-containing media. Furthermore, Ti-stabilized ultrapure ferritic stainless steels are prone to forming coarse TiN inclusions under as-cast solidification conditions. These inclusions can become pitting corrosion nucleation sites and crack initiation sources, affecting the corrosion resistance stability and service reliability of precision-cast components. Therefore, this invention aims to achieve an alloy composition base that combines high corrosion resistance, good molten steel cleanliness, and adaptability to thin-walled precision casting without significantly increasing the total Cr and Mo content.

[0008] To solve the above-mentioned technical problems, the present invention limits the content range of each element and further limits the ratio between PREN, Ce / Ti and Nb / Ti, so as to form a matching design for corrosion resistance enhancement, inclusion control and casting formability.

[0009] This invention controls Cr to 21.10%–21.50% and Mo to 2.45%–2.60% to ensure the formation of a Cr-Mo passivation film, while avoiding further increases in Cr and Mo, which would lead to decreased steel fluidity and difficulties in solidification and feeding. Based on this, 0.65%–0.80% W is added, with PREN = Cr + 3.3 × Mo + 0.5 × W ≥ 29.5, allowing W to partially contribute to pitting corrosion resistance. This improves the pitting corrosion resistance equivalent without relying on excessively high Cr and Mo contents, balancing corrosion resistance and castability.

[0010] This invention incorporates 0.10%–0.13% Sn and 0.65%–0.75% Cu to complement the Cr-Mo-W passivation system. Cr, Mo, and W primarily enhance the passivation film's resistance to chloride ion damage, while Sn and Cu improve the stability of the film / substrate interface and inhibit pitting corrosion propagation. Consequently, the material exhibits high stability during both the pitting initiation and propagation stages, rather than solely relying on PREN values ​​to improve corrosion resistance.

[0011] This invention controls both C and N to within the range of 0.006%–0.010%, and limits the Nb / Ti ratio to 1.25–1.45, ensuring an appropriate proportion of Nb in the Ti-Nb composite stabilization system. On one hand, Nb and Ti jointly fix residual C and N, reducing Cr carbonitride precipitation and the formation of Cr-depleted zones, thus lowering the risk of intergranular corrosion. On the other hand, it avoids relying solely on high Ti content to fix C and N, which could lead to TiN coarsening and precipitation. By jointly limiting the range of Ti and Nb content and the Nb / Ti ratio, the adverse effects of coarse TiN inclusions on pitting corrosion and crack initiation can be reduced while ensuring resistance to intergranular corrosion.

[0012] Preferably, the average size of TiN inclusions in stainless steel is ≤2μm, the maximum size is ≤5μm, and the area fraction of TiN inclusions on the two-dimensional metallographic section of the matrix structure of ferritic stainless steel is ≤0.05%.

[0013] Ti-stabilized ultrapure ferritic stainless steel is prone to the precipitation and growth of TiN inclusions under casting cooling conditions, which in turn become preferential nucleation sites for pitting corrosion and crack initiation sources (there is electrochemical inhomogeneity and interfacial micro-gaps between TiN and the ferrite matrix; local Cr depletion, stress concentration, or discontinuous passivation film are more likely to occur around coarse TiN, and pitting corrosion nuclei will preferentially form in chloride-containing media). Thin-walled precision castings usually do not undergo large deformation processing, and coarse TiN is difficult to break up and disperse through rolling or forging. Therefore, the size and number of inclusions directly affect the corrosion resistance, toughness, and fatigue reliability of the final component. This invention reduces the scale of interfacial defects caused by individual inclusions by controlling the average size of TiN to ≤2μm and the maximum size to ≤5μm; and reduces the number of pitting corrosion initiation sites per unit area by controlling the area fraction to ≤0.05%. The combination of Ce purification, Ti / Nb stabilization, and low C and N design in the invention allows the invention to not only meet chemical composition standards, but also improve corrosion resistance and service reliability through synergistic control of composition and inclusion structure.

[0014] Preferably, the stainless steel has a cast microstructure, in which isometric crystals account for ≥50% of the area and the average grain size is ≤500μm.

[0015] The ferritic stainless steel (Cr≥21%) of this invention typically solidifies directly from the liquid phase to a ferritic structure after precision casting, lacking a solid-state phase transformation refinement mechanism similar to that of austenitic stainless steel. This leads to problems such as coarse columnar grains, anisotropy, and insufficient filling in thin-walled structures. For near-net-shape thin-walled components, subsequent heat treatment cannot fundamentally eliminate the performance defects caused by the coarse and columnar grains in the as-cast state. Therefore, this solution controls the equiaxed grain area ratio to ≥50%, increasing the number of nucleation points during solidification and suppressing the preferential growth of columnar grains. This reduces the orientation of the microstructure and improves the uniformity of mechanical properties and localized corrosion. Furthermore, an average grain size ≤500μm reduces segregation and stress concentration at coarse grain boundaries, improving the performance stability of the as-cast components. Subsequent test examples, conducted under uniform spiral flow channels, shell preheating temperatures, and pouring pressures, verified the alloy's filling ability in thin-walled cavities, reducing the risks of cold shuts, incomplete filling, and missing material at thin-walled sharp corners. This invention simultaneously limits castability and as-cast microstructure quality, solving comprehensive technical problems in thin-walled precision casting scenarios, rather than focusing on a single material corrosion resistance index. It ensures fluidity and other technical issues in casting scenarios while meeting corrosion resistance requirements.

[0016] Another aspect of the present invention provides a precision casting process for corrosion-resistant reinforced ultrapure ferritic stainless steel thin-walled components, comprising the following steps: S1, according to the chemical composition of corrosion-resistant reinforced ultrapure ferritic stainless steel, is batched and then subjected to vacuum induction melting and electroslag remelting in sequence to obtain a remelted ingot; wherein, during the melting to remelting process, the O in the remelted ingot is controlled to be ≤0.005% and S to be ≤0.005%, and the Ce / Ti in the remelted ingot is maintained at 0.141 to 0.190; S2, prepare a wax model corresponding to the thin-walled component, and prepare a gradient shell on the outer surface of the wax model that matches the solidification characteristics of high Cr-Mo-W alloying of corrosion-resistant reinforced ultrapure ferritic stainless steel; wherein, the gradient shell is set to allow the thin-walled area to dissipate heat preferentially, and the thick-walled area or riser area to solidify later. S3, dewaxing and baking treatment of gradient shell; S4. After the remelted ingot is melted into molten metal, the molten metal is poured into the cavity of the gradient shell using a vacuum anti-gravity casting method. During the molten metal filling process, a nonlinear pressure curve is used to control the filling flow rate. The nonlinear pressure curve includes a slow start stage, a fast rise stage, and a slow filling stage that proceed in sequence. This allows the molten metal to enter the pouring channel with low disturbance in the early stage of filling, quickly form continuous filling in the thin-walled area, and maintain feeding pressure in the later stage of filling. S5. During the solidification of the molten metal, the cooling rate of the molten metal in the range of liquidus temperature to solidus temperature is controlled so that the growth of TiN inclusions, Cr / Mo / W dendrite segregation and the tendency of hot cracking in thin-walled areas are synergistically suppressed. After the molten metal solidifies and cools, the gradient shell is cleaned and the gating and riser are cut off to obtain the precision casting billet. S6 involves sequentially performing high-temperature solution treatment, low-temperature stress relief treatment, and surface passivation treatment on the precision-cast billet to reduce the segregation of Cr, Mo, W, Sn, and Cu in the as-cast state, release residual casting stress, and avoid holding the billet at 700℃~850℃ for more than 30 minutes, which would lead to the precipitation of σ or χ phases, thus obtaining corrosion-resistant reinforced ultrapure ferritic stainless steel thin-walled components.

[0017] The stainless steel material in this invention has a high Cr-Mo-W corrosion-resistant system, a Sn-Cu-Ce microalloying system, and a Ti-Nb ultra-low C / N stabilization system. If ordinary melting, general shell, linear pressure casting, or single annealing treatment is used, problems such as ineffective consumption of Ce by O / S, TiN growth, discontinuous thin-wall filling, shrinkage porosity, dendrite segregation, and insufficient as-cast passivation ability are likely to occur. As a result, although the nominal composition meets the requirements, the high PREN design and inclusion control design cannot be converted into the actual corrosion resistance and service reliability of thin-walled components.

[0018] To address the aforementioned issues, this application employs vacuum induction melting and electroslag remelting in S1 to lock in low O, low S, and Ce / Ti ratios, allowing Ce to be used for purifying and modifying inclusions instead of being consumed by oxygen and sulfur impurities. In S2, a gradient shell is used to pre-establish a heat flow direction where thin walls preferentially dissipate heat, while thick walls or risers delay solidification, compensating for the fluidity and feeding sensitivity of high Cr-Mo-W steel. In S4, vacuum anti-gravity casting and a nonlinear pressure curve of slow start-fast rise-slow charge are used, enabling the molten steel to enter the cavity with low disturbance and to achieve optimal heat dissipation. The thin-walled region spreads rapidly and continuously, maintaining feeding pressure at the end. In S5, the cooling rate of the liquid-solid zone is controlled, simultaneously managing TiN growth, dendrite segregation, and hot cracking tendency. In S6, high-temperature solution treatment at 1080℃–1100℃ reduces Cr, Mo, W, Sn, and Cu dendrite segregation, while low-temperature stress relief treatment at 280℃–350℃ releases residual casting stress. This is combined with pickling and passivation to restore the surface passivation film, preventing the induction of σ or χ phase precipitation during sensitization at 700℃–850℃ (over 30 minutes). Therefore, the technical effect of this invention is to improve the integrity of thin-walled mold filling and feeding quality, reduce gas entrapment, oxide inclusions, coarse TiN, and localized corrosion sources induced by segregation, and enable the material's pitting corrosion resistance, intergranular corrosion stability, and as-cast component reliability to be effectively realized in precision-cast thin-walled components.

[0019] Preferably, in step S1, vacuum induction melting includes the following steps: Fe, Cr, Mo, W and Cu raw materials are loaded into a crucible and melted under a vacuum of ≤0.1 Pa; After melting, refine at 1600℃~1620℃ for 20min~25min; The temperature was then lowered to 1520℃~1540℃, argon gas was introduced, and an intermediate alloy containing Nb, Ti, Sn, Ce and B was added in sequence. Electroslag remelting uses a CaF2-Al2O3-CaO slag system and is carried out under argon protection. The remelting current is 3000A~3500A, the voltage is 28V~32V, and the remelting rate is 7kg / h~9kg / h.

[0020] This invention first loads Fe, Cr, Mo, W, and Cu raw materials into a crucible and melts them under a vacuum of ≤0.1 Pa. Fe, Cr, Mo, W, and Cu are the main matrix elements and corrosion-resistant strengthening elements in the alloy system of this invention. Among them, Mo and W have high melting points and are difficult to homogenize. Loading them into the furnace first and melting them under high vacuum conditions helps to improve the dissolution uniformity of high-melting-point elements, while reducing the gas content in the molten steel and minimizing the adverse effects of gaseous elements such as O, N, and H on the cleanliness and inclusion control of ultrapure ferritic stainless steel.

[0021] After melting, refining at 1600℃~1620℃ for 20min~25min ensures thorough homogenization of elements such as Cr, Mo, W, and Cu, and promotes the removal of non-metallic inclusions by flotation. This temperature range is higher than the subsequent casting and alloy addition temperatures, ensuring sufficient melt homogeneity for the high Cr-Mo-W system. Simultaneously, limiting the refining time to 20min~25min avoids excessively long high-temperature exposure, which could lead to element burn-off, secondary contamination of the molten steel, or intensified crucible reactions.

[0022] The temperature was then lowered to 1520℃~1540℃ and argon gas was introduced. Next, Nb-containing master alloys, Ti-containing master alloys, Sn-containing master alloys, Ce-containing master alloys, and B-containing master alloys were added sequentially to reduce the loss of easily oxidized and burnable elements during the high-temperature vacuum refining stage. Nb and Ti are used to form stable carbonitrides with residual C and N, reducing the risk of intergranular corrosion; Sn is used to improve the stability of the passivation film and the ability to inhibit pitting corrosion propagation in conjunction with the Cr-Mo-W-Cu system; Ce is used to purify the molten steel and modify oxygen and sulfur inclusions, reducing the promoting effect of large inclusions on TiN nucleation and pitting corrosion initiation; B is used to improve the grain boundary bonding state and grain boundary stability during casting solidification. By adding the above master alloys at a lower temperature and under argon protection, the alloy element yield, molten steel cleanliness, and Ce / Ti ratio stability can be simultaneously satisfied while maintaining equilibrium.

[0023] Furthermore, this invention employs a CaF2-Al2O3-CaO slag system for electroslag remelting, and controls the remelting current at 3000A–3500A, voltage at 28V–32V, and remelting rate at 7kg / h–9kg / h under argon protection. The CaF2-Al2O3-CaO slag system can adsorb and remove oxides, sulfides, and complex inclusions from the molten steel, reducing O and S content and decreasing the heterogeneous nucleation cores of coarse TiN inclusions during subsequent solidification. Argon protection reduces secondary oxidation and nitrogen absorption during remelting, preventing Ce from being consumed by excessive oxygen and sulfur impurities, thereby maintaining the designed Ce / Ti ratio of 0.141–0.190.

[0024] Limiting the remelting current, voltage, and remelting rate is crucial for forming a stable molten pool and achieving a reasonable solidification rate. If the remelting current or voltage is too low, the molten pool temperature will be insufficient, easily leading to incomplete remelting and compositional segregation in the high Cr-Mo-W system. If the current or voltage is too high, it may cause an excessively deep molten pool, localized overheating, element burn-off, and inclusion entrapment. If the remelting rate is too fast, the removal of inclusions and molten pool purification will be insufficient, easily retaining oxygen-sulfur inclusions and TiN nucleation cores. If the remelting rate is too slow, it will reduce production stability and increase the risk of loss of microalloying elements such as Ce and B. This invention controls the remelting rate to 7 kg / h to 9 kg / h, achieving a balance between purification effect, compositional stability, and uniform solidification structure.

[0025] Therefore, the vacuum induction melting, staged feeding, and electroslag remelting processes defined in this invention are not simply a combination of ordinary melting steps, but rather purification and composition stabilization measures specifically designed for the high Cr-Mo-W corrosion-resistant system, Ti-Nb stabilization system, and Ce-B-Sn microalloying system of this invention. The technical effects are: on the one hand, reducing the content of O, S, and composite inclusions, and reducing the ineffective consumption of Ce; on the other hand, improving the effective yield and distribution uniformity of elements such as Ti, Nb, Ce, B, and Sn, inhibiting the formation of coarse TiN inclusions, and ensuring that the molten metal has a good foundation for cleanliness, flow stability, and corrosion resistance during subsequent thin-wall precision casting.

[0026] Preferably, in step S2, the gradient shell includes an inner layer close to the cavity and an outer layer covering the outside of the inner layer; The inner layer includes a zircon powder surface layer close to the cavity surface and an Al2O3-based thermally conductive transition layer located outside the zircon powder surface layer. The total thickness of the inner layer is 3mm to 5mm. The outer layer is an Al2O3-based thermal insulation backing layer containing ZrO2, with a thickness of 8mm to 12mm; Furthermore, a copper quench block is provided on the outside of the thin-walled end, the thick-thin transition area, or the predetermined priority solidification area of ​​the corresponding wax mold; the copper quench block is a T2 pure copper plate with a thickness of 3mm to 6mm and a width of 10mm to 20mm, which is embedded in the back layer of the outer layer in an inlay manner and is flush with the outer surface of the back layer. The surface of the quench block facing the inner layer of the mold shell is directly opposite the thin-walled end, the thick-thin transition area, or the predetermined priority solidification area; the copper quench block is used to form a directional solidification path that advances from the thin-walled area to the thick-walled area or the riser area after the molten metal has been continuously filled.

[0027] High-Cr-Mo-W ultrapure ferritic stainless steel faces the problem of insufficient filling heat and easy interruption of feeding channels during thin-walled precision casting. Excessive heat dissipation in the thin-walled area can cause cold shuts or incomplete filling, while delayed solidification in the thick-walled area can easily lead to shrinkage porosity and shrinkage cavities. If the mold shell adopts only a single heat transfer structure, it is difficult to achieve a controllable solidification sequence for this alloy in thin-walled components.

[0028] This invention employs a combination of an Al2O3-based thermally conductive inner layer, a ZrO2-based thermally insulating outer layer, and a copper quench block in the thin-walled region. Instead of simply selecting refractory materials, it spatially distributes the heat flow path: the thermally conductive inner layer improves the heat exchange stability at the interface between the molten metal and the mold cavity; the thermally insulating outer layer prevents excessively rapid overall heat dissipation, which could lead to temperature loss at the thin-walled end; and the copper quench block ensures that the thin-walled region preferentially forms a solidification front and pushes the last solidified zone towards the thick-walled region or riser region. The effect is to simultaneously reduce the risks of cold shuts, incomplete filling, and shrinkage porosity, ensuring that the flow window and feeding window of the stainless steel material are matched in the thin-walled component.

[0029] Preferably, in step S2, the gradient shell is prepared by a multi-layer coating method, which includes forming a surface layer, a transition layer and a back layer in sequence. The surface layer is prepared using zircon powder and silica sol; The transition layer is prepared using Al2O3-based refractory materials and silica sol; The backing layer is made of Al2O3-based thermal insulation material containing ZrO2; The inner layer consists of the surface layer and the transition layer, while the outer layer consists of the back layer.

[0030] Thin-walled precision-cast components exhibit significant differences in wall thickness across different areas. Thin-walled regions are prone to rapid cooling, leading to cold shuts or premature closure of feeding channels, while thick-walled regions are susceptible to shrinkage porosity and porosity due to slower solidification. If the mold shell uses only a single material and thickness, its heat transfer capacity cannot simultaneously meet the requirements of rapid thin-walled forming, thick-walled feeding, and overall directional solidification. This invention employs a composite thermal design consisting of an Al2O3-based thermally conductive layer near the cavity, an outer ZrO2-based thermally insulating layer, and a copper chiller block in the thin-walled region. This transforms the mold shell from a mere forming shell into a functional structure that participates in solidification path control.

[0031] From a mechanistic perspective, the inner Al2O3-based thermally conductive layer can quickly remove heat from the contact area between the molten metal and the mold cavity, improving the uniformity of surface solidification and reducing sand adhesion, coarse surface grains, and localized overheating. The outer ZrO2-based thermal insulation layer can reduce insufficient filling and premature interruption of the feeding channels caused by excessively rapid overall heat dissipation. The copper quenching block is arranged on the outside of the thin-walled area, which allows the thin-walled area to preferentially form a stable solidification front and promotes the solidification interface to advance towards the thick-walled area or riser area in a predetermined direction, so that the final solidification zone is concentrated at the feeding location. Thus, this invention controls the direction of heat flow through a spatial combination of "thermal conduction-thermal insulation-localized quenching" to overcome the defects of filling and feeding of thin-walled components.

[0032] Preferably, in step S4, the nonlinear pressure curve includes a slow start phase, a rapid rise phase, and a slow charge phase. The pressure increase rate during the slow start-up phase is 1 kPa / s to 3 kPa / s, and the duration is 3 s to 5 s. The pressure rise rate during the rapid rise phase is 5 kPa / s to 10 kPa / s, and the duration is 5 s to 8 s. The pressure increase rate during the slow charging phase is 1 kPa / s to 2 kPa / s, and is maintained until the cavity is completely filled. Specifically, when the minimum wall thickness of the thin-walled component is 1.5 mm to 2.0 mm, the pressure rise rate during the rapid rise stage is 8 kPa / s to 10 kPa / s; when the minimum wall thickness of the thin-walled component is greater than 2.0 mm and less than 2.5 mm, the pressure rise rate during the rapid rise stage is 6 kPa / s to 8 kPa / s; and when the minimum wall thickness of the thin-walled component is 2.5 mm to 3.0 mm, the pressure rise rate during the rapid rise stage is 5 kPa / s to 7 kPa / s.

[0033] During vacuum anti-gravity casting, if the pressure is increased too quickly, the molten metal entering the mold cavity will cause scouring, splashing, air entrapment, and oxide inclusions. If the pressure is increased too slowly, cold shuts, incomplete filling, or incomplete end-filling can easily occur due to rapid heat dissipation in thin-walled areas. Ordinary linear pressure increases are difficult to balance initial stability, main filling efficiency, and end-filling defect control. Therefore, this invention divides the filling pressure curve into three continuous stages: slow start, fast increase, and slow filling, and defines the pressure increase rate and duration for each stage.

[0034] The slow-start stage, with a pressure increase of 1 kPa / s to 3 kPa / s, allows the molten metal to smoothly enter the gating system and mold cavity, reducing initial impact and ceramic shell erosion. The rapid-rise stage, with a pressure increase of 5 kPa / s to 10 kPa / s, rapidly fills the thin-walled areas of the main body before the molten metal cools excessively, reducing the risk of cold shuts. The slow-fill stage, with a pressure decrease to 1 kPa / s to 2 kPa / s, allows for gentle filling in complex thin-walled, sharp-cornered, and locally thick-to-thin transition areas at the end, avoiding excessive flow velocity that could cause turbulence, gas entrainment, and increased susceptibility to hot cracking. This segmented nonlinear pressure control reflects targeted regulation of the defect formation mechanism at different filling stages. The pressure parameters of this invention are not arbitrary process optimizations, but rather a specific filling control scheme established for the precision casting of thin-walled ferritic stainless steel.

[0035] Preferably, in step S4, the initial pouring temperature of the molten metal is controlled to be 120°C to 150°C higher than the liquidus temperature of the molten metal; wherein, the liquidus temperature of stainless steel is 1420°C to 1450°C and the solidus temperature is 1370°C to 1390°C. Furthermore, the average cooling rate of the molten metal after filling is controlled to be 1℃ / s to 5℃ / s within the range of liquidus temperature to solidus temperature.

[0036] When the pouring temperature and solidification cooling rate are mismatched, thin-walled ultrapure ferritic stainless steel precision castings are prone to imbalances in filling integrity, grain control, inclusion growth, and feeding quality. Too low a pouring temperature can lead to incomplete pouring at the thin-walled end and cold shuts; too high a pouring temperature will prolong the residence time in the liquid and pasty regions, promoting grain coarsening, increased segregation, and TiN inclusion growth. Therefore, this invention limits the initial pouring temperature to 120°C–150°C above the liquidus and further limits the average cooling rate in the region between the liquidus and solidus.

[0037] The superheat of 120°C to 150°C above the liquidus provides the necessary filling heat for the thin-walled cavity, while avoiding excessive superheat that could lead to shell reaction, grain coarsening, and difficulty in feeding. The solidification cooling rate of 1°C / s to 5°C / s maintains a relative balance between nucleation and grain growth, preventing columnar grain coarsening, segregation, and TiN growth due to excessively slow cooling, and avoiding insufficient feeding, thermal stress concentration, and increased tendency for hot cracking due to excessively rapid cooling. This feature, combined with the gradient shell, quench block, and pressure curve, ensures a match between the filling temperature window and the solidification structure window.

[0038] Preferably, step S6 includes the following processing stages: High-temperature solution treatment stage: heat to 1080℃~1100℃, hold for 1.5h~2h, and then cool to room temperature by water quenching; Low-temperature stress relief treatment stage: After the high-temperature solution treatment stage is completed and cooled to room temperature, the temperature is heated to 280℃~350℃ and held for 1h~2h, and then cooled to room temperature by air cooling. Pickling and passivation stage: Pickling solution containing 200g / L to 300g / L HNO3 and 30g / L to 50g / L HF is used to remove the surface oxide scale at a temperature of 40℃ to 60℃ for 10min to 20min. Then, passivation solution containing 300g / L to 400g / L HNO3 is used to passivate the surface at a temperature of 50℃ to 70℃ for 30min to 60min.

[0039] In fine-cast ultrapure ferritic stainless steel, problems such as dendritic segregation, residual stress, local Cr / Mo enrichment or depletion, and uneven surface oxide film may exist. These factors weaken the uniformity of the passivation film and increase the susceptibility to intergranular corrosion or pitting corrosion. For high-alloy ferritic stainless steel containing more than 21% Cr and about 2.5% Mo, holding at 700℃ to 850℃ for more than 30 minutes can easily promote the precipitation of brittle intermetallic compounds rich in Cr / Mo, such as σ phase and χ phase, which consumes passivation film forming elements and causes room temperature embrittlement. Therefore, this invention changes the post-cast treatment to a composite system of "high-temperature solution treatment - low-temperature stress relief - pickling and passivation" instead of the above-mentioned holding treatment (more than 30 minutes) within the sensitization range.

[0040] High-temperature solution treatment at 1080℃~1100℃ for 1.5h~2h promotes the redispersion of elements such as Cr, Mo, W, Cu, and Sn between and within dendrites, reducing microsegregation in the as-cast state and allowing for the redissolution of unfavorable precipitates or a more uniform distribution of passivation film-forming elements. Subsequent water quenching inhibits the re-precipitation of σ phase, χ phase, and Cr / Mo enriched phase during cooling. Low-temperature stress relief treatment at 280℃~350℃ for 1h~2h releases residual thermal and residual stresses in thin-walled precision castings, while avoiding the rapid precipitation temperature range of σ and χ phases in high-Cr-Mo ferritic stainless steel. This composite system makes the heat treatment of this invention not a general annealing, but a staged treatment scheme designed for the segregation of the as-cast microstructure, residual stress, and the recovery of corrosion resistance, which supports its contribution to the final corrosion resistance and elongation of thin-walled precision casting components.

[0041] The beneficial effects of this invention are as follows: 1. This invention controls Cr and Mo within a narrow range suitable for thin-walled precision casting, and introduces W, Sn, and Cu to form a Cr-Mo-W-Sn-Cu composite corrosion-resistant system. Without simply increasing the total amount of Cr and Mo, PREN is maintained above 29.5. While maintaining the required flowability for filling thin-walled cavities (spiral flowability length ≥ 430 mm), high pitting corrosion resistance is achieved, avoiding a significant reduction in filling and feeding capacity due to excessive increase in Cr and Mo content.

[0042] 2. This invention constrains the size, quantity, and distribution of TiN inclusions by controlling ultra-low C and N content, stabilizing Ti-Nb composites, and controlling rare earth modification with Ce / Ti ratios of 0.141 to 0.190, thereby reducing the precipitation of Cr carbonitrides and the tendency for Cr depletion at grain boundaries, and thus reducing pitting corrosion sources, crack sources, and intergranular corrosion susceptibility.

[0043] 3. This invention constructs a gradient shell by combining a zircon powder surface layer, an Al2O3-based thermally conductive transition layer, an Al2O3-based thermally insulating back layer containing ZrO2, and a copper chiller block. The copper chiller block is a T2 pure copper plate with a thickness of 3mm to 6mm and a width of 10mm to 20mm, which is embedded in the back layer in an inlay manner. Combined with a vacuum anti-gravity slow start-fast rise-slow fill nonlinear pressure curve, the thin-walled area can be continuously filled and solidified in a predetermined direction, thereby improving the molding integrity rate of thin-walled components and reducing defects such as shrinkage porosity, shrinkage cavity, air entrapment, and hot cracking.

[0044] 4. This invention reduces Cr, Mo, W, Sn, and Cu dendrite segregation and casting residual stress by controlling the cooling rate in the liquid-solid zone, high-temperature solution treatment, low-temperature stress relief, and pickling passivation treatment. It also avoids the precipitation of σ or χ phases caused by holding at 700℃~850℃ for more than 30 minutes and restores the quality of the surface passivation film. This results in more stable overall performance of precision-cast thin-walled components in pitting corrosion, salt spray, intergranular corrosion, and room temperature tensile tests. Attached Figure Description

[0045] The invention will now be further described with reference to the accompanying drawings.

[0046] Unless otherwise stated, all accompanying drawings in this application are obtained or drawn based on the alloy composition and process regime of Example 1; wherein Figures 1-3 This is a schematic diagram drawn based on the process flow, gradient shell structure, and vacuum anti-gravity nonlinear pressurization parameters of Example 1. Figures 4-7 The results are representative of the thin-walled precision-cast specimens from Example 1, obtained after metallographic preparation, SEM / EDS, and industrial CT analysis following as-cast cleaning. Figures 8-10 The results are representative of those obtained after completing the corrosion resistance test under the conditions of Test Example 2. Figure 11 The comparison results are obtained by elemental segregation scanning of the precision-cast sample in Example 1 before and after composite heat treatment. The terms "Example" and "Comparative Example" in each figure are subject to the corresponding figure numbers and specific groups in the test examples below.

[0047] Figure 1 This is a flowchart of the precision casting process for the corrosion-resistant reinforced ultrapure ferritic stainless steel thin-walled components of the present invention. Figure 2 This is a schematic diagram of the gradient shell cross-section and directional solidification heat design of the present invention; Figure 3 This is a schematic diagram of the nonlinear pressure curves of the three stages of slow start-up, fast rise, and slow filling during the vacuum anti-gravity casting process of the present invention. Figure 4 This is a metallographic diagram of the ferritic stainless steel obtained in Example 1 of the present invention under as-cast conditions; Figure 5 This is a schematic diagram of the SEM morphology of TiN inclusions in the as-cast sample of Example 1 of the present invention; Figure 6 This is a schematic diagram of EDS composition analysis of TiN inclusions in the as-cast sample of Example 1 of the present invention; Figure 7 This is a schematic diagram of industrial CT inspection and local defect analysis of a thin-walled precision-cast component according to Embodiment 1 of the present invention; Figure 8 This is a schematic diagram comparing the polarization curves obtained by potentiodynamic polarization test in 3.5wt% NaCl solution of Example 1 and Comparative Example 1 of the present invention; Figure 9 This is a schematic diagram comparing the local corrosion morphology obtained after 500 hours of neutral salt spray testing of Example 1 and Comparative Example 3 of the present invention. Figure 10 This is a comparative schematic diagram of the macroscopic appearance of the samples obtained after 500h neutral salt spray test in Examples 1, 5, Comparative Example 1 and Comparative Example 3 of the present invention. Figure 11 This is a schematic diagram of the segregation scanning results of Cr, Mo, and W elements in the precision-cast sample before and after composite heat treatment in Example 1 of the present invention. Detailed Implementation

[0048] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0049] The specific conditions for obtaining the accompanying drawings in this application are as follows: Figure 4 The sample was taken from the middle section of the thin-walled section with a minimum wall thickness of 2.0 mm in Example 1. After being inlaid, polished and etched step by step, the sample was observed and photographed under bright field conditions of an optical microscope. Figure 5 The morphology of TiN inclusions was observed in the polished cross section of the same as-cast sample under the backscattered electron mode of a scanning electron microscope. Figure 6 To Figure 5 The compositional confirmation results were obtained from EDS point analysis and area scanning of selected TiN inclusions. Figure 7 The results of industrial CT inspection and analysis of local cross-sectional defects are shown for the thin-walled precision-cast component of Example 1 after cleaning and removal of the gating system and risers. Figure 8 The potentiodynamic polarization curves of the samples of Example 1 and Comparative Example 1 were obtained under the conditions of 3.5wt% NaCl solution, 25℃±2℃, scan rate of 1mV / s, and saturated calomel electrode (SCE) as reference electrode. Figure 9 The local corrosion morphology of the samples of Example 1 and Comparative Example 3 after being subjected to 5 wt% NaCl and 35℃±2℃ neutral salt spray for 500 h as specified in ASTM B117 is photographed at the same magnification. Figure 10 The macroscopic appearance of 50mm×100mm×1mm salt spray samples from Examples 1, 5, Comparative Example 1, and Comparative Example 3 after 500h neutral salt spray test; Figure 11 The results of Cr, Mo, and W element segregation obtained by SEM-EDS line scanning at the same cross-sectional position before and after composite heat treatment of the precision-cast sample in Example 1 are shown.

[0050] In the following examples and comparative examples, stepped thin-walled specimens were prepared. The specimens included thin-walled sections with a minimum wall thickness of 1.5 mm to 2.8 mm, thick-thin transition sections, and riser feeding sections. At least 5 specimens were prepared for each group, and samples from the same furnace were taken for microstructure, inclusions, fluidity, casting defects, corrosion resistance, and mechanical properties testing.

[0051] The overall preparation process of this invention is as follows: Figure 1 As shown, it connects the ingredient distribution, vacuum induction melting, electroslag remelting, gradient shell preparation, vacuum anti-gravity casting, controlled solidification, composite heat treatment, and pickling and passivation into a continuous process chain. Figure 1The process shown corresponds to the specific operation steps of Examples 1 to 5.

[0052] Example 1: The stainless steel composition of this embodiment is C 0.008%, N 0.008%, Cr 21.20%, Mo 2.52%, Ti 0.30%, Nb 0.40%, Sn 0.12%, Cu 0.70%, Ce 0.050%, W 0.72%, B 0.007%, S 0.003%, P 0.016%, Si 0.20%, Mn 0.20%, O 0.003%, with the balance being Fe and unavoidable impurities; its PREN is 29.88, Ce / Ti is 0.17, and Nb / Ti is 1.33. After the above composition is prepared, Fe, Cr, Mo, W, and Cu are melted under a vacuum of ≤0.1 Pa, refined at 1600℃~1620℃ for 22 min, then cooled to 1530℃ and purged with argon gas, and Nb, Ti, Sn, Ce, and B master alloys are added sequentially. Electroslag remelting was performed using a CaF2-Al2O3-CaO slag system, with a remelting current of 3200A, a voltage of 30V, and a remelting rate of 8kg / h. A wax model with a minimum wall thickness of 2.0mm was prepared. A 4mm thick zircon powder surface layer / Al2O3-based thermally conductive inner layer and a 10mm thick ZrO2-containing thermally insulating back layer were formed outside the wax model. Copper chillers were placed at the thin-walled ends and on the outer side of the thick-thickness transition zone. The pouring temperature was approximately 135℃ above the liquidus line. The vacuum anti-gravity pouring pressure curve was as follows: a slow start phase of 2kPa / s for 4s, a rapid rise phase of 8.5kPa / s for 6s, and a slow charging phase of 1.5kPa / s maintained until full. The average cooling rate between the liquidus and solidus lines was controlled at 3℃ / s. After cleaning, the casting was solution-treated at 1090℃ for 1.8h and water-quenched, then stress-relieved at 320℃ for 1.5h and air-cooled. Finally, it underwent pickling and passivation treatment.

[0053] The structure and directional solidification path of the gradient shell in Example 1 can be found in [reference needed]. Figure 2 . Figure 2 The structure consists of a zircon powder top layer, an Al2O3-based thermally conductive transition layer, an Al2O3-based thermally insulating back layer containing ZrO2, and a copper chiller block, forming a thermal design that combines thermal conductivity, thermal insulation, and localized chilling. This design allows the thin-walled region to preferentially dissipate heat and solidify after continuous filling, pushing the final solidified area towards the thick-walled region or the riser region. This structure corroborates the results in Table 1, where the defect volume fraction is only 0.18% and the thin-walled filling integrity rate reaches 98.5%.

[0054] The nonlinear pressure curve used in Example 1 can be found in [reference needed]. Figure 3 . Figure 3The slow start-up phase is used to reduce initial air entrapment and scouring, the fast rise phase is used to complete the main body filling before the thin-walled region cools down, and the slow filling phase is used to maintain the end feeding pressure. When this pressure curve is combined with the gradient shell, Example 1 simultaneously exhibits a 462mm flowability length, a high equiaxed crystal area ratio, and a low defect volume fraction in Table 1, indicating that the process parameters can support the stability of thin-walled precision casting.

[0055] Example 2: The stainless steel composition of this embodiment is C 0.006%, N 0.006%, Cr 21.10%, Mo 2.47%, Ti 0.29%, Nb 0.40%, Sn 0.10%, Cu 0.65%, Ce 0.045%, W 0.75%, B 0.006%, S 0.004%, P 0.018%, Si 0.25%, Mn 0.25%, O 0.004%, with the balance being Fe and unavoidable impurities; its PREN is 29.63, Ce / Ti is 0.16, and Nb / Ti is 1.38. In terms of process, the electroslag remelting current is 3000A, the voltage is 28V, and the remelting rate is 7kg / h; thin-walled components with a minimum wall thickness of 2.6mm are prepared, with a total inner shell thickness of 3mm and an outer shell thickness of 8mm; the casting temperature is about 120℃ above the liquidus line, the pressure rise rate during the rapid rise stage is 6kPa / s and lasts for 7s, and the average cooling rate in the liquid-solid zone is 2℃ / s; the solution treatment is water quenching after holding at 1080℃ for 1.5h, and the low-temperature stress relief treatment is air cooling after holding at 300℃ for 1.5h. The remaining steps are the same as in Example 1.

[0056] Example 3: The stainless steel composition of this embodiment is C 0.010%, N 0.010%, Cr 21.50%, Mo 2.60%, Ti 0.32%, Nb 0.42%, Sn 0.13%, Cu 0.75%, Ce 0.055%, W 0.80%, B 0.008%, S 0.003%, P 0.014%, Si 0.18%, Mn 0.18%, O 0.003%, with the balance being Fe and unavoidable impurities; its PREN is 30.48, Ce / Ti is 0.17, and Nb / Ti is 1.31. In terms of process, the electroslag remelting current is 3500A, the voltage is 32V, and the remelting rate is 9kg / h; the total thickness of the inner shell layer is 5mm and the outer shell layer is 12mm; the casting temperature is about 150℃ above the liquidus line, the pressure rise rate during the rapid rise stage is 10kPa / s and lasts for 5s, and the average cooling rate of the liquid-solid zone is 4.5℃ / s; the solution treatment is water quenching at 1100℃ for 2h, and the low-temperature stress relief treatment is air cooling at 350℃ for 2h. The remaining steps are the same as in Example 1.

[0057] Example 4: The stainless steel composition of this embodiment is C 0.007%, N 0.009%, Cr 21.25%, Mo 2.55%, Ti 0.30%, Nb 0.40%, Sn 0.11%, Cu 0.72%, Ce 0.048%, W 0.70%, B 0.007%, S 0.003%, P 0.015%, Si 0.20%, Mn 0.16%, O 0.003%, with the balance being Fe and unavoidable impurities; its PREN is 30.02, Ce / Ti is 0.16, and Nb / Ti is 1.33. Compared with Example 1, this example prepares a thin-walled component with a minimum wall thickness of 1.5 mm. Copper quench blocks are placed at the end of the thin wall, corners, and the thick-thin transition zone. The pressure rise rate during the rapid charging stage is 9.5 kPa / s and lasts for 6 s, the pressure rise rate during the slow charging stage is 1.2 kPa / s, and the average cooling rate in the liquid-solid zone is controlled at 5 °C / s. The solution treatment is water quenching at 1080 °C for 1.5 h, and the low-temperature stress relief treatment is air cooling at 300 °C for 1 h. The remaining steps are the same as in Example 1.

[0058] Example 5: The stainless steel composition of this embodiment is C 0.008%, N 0.007%, Cr 21.35%, Mo 2.50%, Ti 0.29%, Nb 0.41%, Sn 0.11%, Cu 0.68%, Ce 0.052%, W 0.78%, B 0.007%, S 0.003%, P 0.015%, Si 0.22%, Mn 0.17%, O 0.003%, with the balance being Fe and unavoidable impurities; its PREN is 29.99, Ce / Ti is 0.18, and Nb / Ti is 1.41. Compared with Example 1, this example prepares a thin-walled component with a minimum wall thickness of 2.2 mm, a total inner shell thickness of 4 mm, and an outer shell thickness of 9 mm; the pressure rise rate during the rapid rise stage is 7 kPa / s and lasts for 8 s, and the average cooling rate of the liquid-solid zone is controlled at 2.5℃ / s; the solution treatment is water quenching at 1090℃ for 2 h, and the low-temperature stress relief treatment is air cooling at 320℃ for 2 h. The remaining steps are the same as in Example 1.

[0059] Comparative Example 1: The difference from Example 1 is that: the synergistic corrosion-resistant system of W, Sn and Cu is not added, only Cr 21.00%, Mo 2.45%, Ti 0.30%, Nb 0.40%, Ce 0.050% and the same C, N, B and impurity control are retained, W is 0%, Sn is 0.02% and Cu is 0.20%, and its PREN is 29.09, which is lower than the limit of this invention; the melting, molding, casting and heat treatment processes are the same as those of Example 1.

[0060] Comparative Example 2: The difference from Example 1 is that a high alloying scheme with simply increased Cr and Mo content is adopted, with Cr at 23.50%, Mo at 3.20%, and W at 0.70%, while the remaining microalloying elements and process steps are basically the same as in Example 1.

[0061] Comparative Example 3: The difference from Example 1 is that the Ce addition is reduced to 0.015%, while Ti remains at 0.30%, and the Ce / Ti ratio is approximately 0.05, which is lower than the range defined in this invention; the remaining components, shell, casting, and heat treatment processes are the same as in Example 1.

[0062] Comparative Example 4: The difference from Example 1 is that: C is increased to 0.018%, N is increased to 0.018%, Ti is reduced to 0.12%, Nb is reduced to 0.07%, and Ce is 0.022%, where Ce / Ti is about 0.18. However, the absolute contents of Ti, Nb and Ce are all lower than the range of the present invention, and Nb / Ti is about 0.58, which is lower than the preferred range of the present invention. Therefore, the same low C / N-Ti / Nb-Ce synergistic stabilization control effect as the present invention cannot be formed; the rest of the process is the same as in Example 1.

[0063] Comparative Example 5: The difference from Example 1 is that the same alloy composition is used, but the shell is a common homogeneous Al2O3 shell, without a ZrO2 heat insulation back layer and a copper chiller block; the vacuum anti-gravity casting adopts linear pressure increase, and the pressure increase rate is fixed at 4kPa / s until it is full; the remaining melting and heat treatment steps are the same as in Example 1.

[0064] Comparative Example 6: The difference from Example 1 is that the same alloy composition, the same shell, and the same nonlinear pressure curve are used, but after casting, only ordinary annealing treatment of holding at 950℃ for 1 hour and air cooling is performed, without high-temperature solution treatment at 1080℃~1100℃, low-temperature stress relief and pickling passivation treatment.

[0065] Test Example 1: This test case was used to evaluate the thin-wall casting molding capability, as-cast microstructure, and inclusion control effect of the examples and comparative examples. After remelting each group of remelted ingots, a spiral flowability test was conducted under superheat conditions 120℃~150℃ higher than the corresponding liquidus temperature of the molten metal. The spiral flowability test used a ceramic shell spiral channel with a circular cross-section of 6mm in diameter and a total spiral length of 600mm. The shell preheating temperature was 1050℃±20℃. The casting process was carried out according to the corresponding casting regime for each group. Examples 1-5 and Comparative Examples 1-4 and Comparative Example 6 used vacuum anti-gravity casting and a slow-start-fast-rise-slow-fill nonlinear pressure curve. Comparative Example 5 used a fixed 4kPa / s linear pressure increase. After the molten metal solidified, the length from the spiral channel inlet to the solidified end of the molten metal was measured as the flowability length. Simultaneously, the thin-walled components with the smallest wall thickness were subjected to visual, X-ray, or industrial CT inspections, and the thin-walled filling integrity rate and defect volume fraction were statistically analyzed. The cleaned precision-cast blanks were subjected to as-cast metallographic observation. Image analysis software was used to statistically analyze the equiaxed grain area ratio and average grain size. SEM / EDS was used to identify TiN inclusions in the as-cast samples, and the average size, maximum size, and two-dimensional area fraction of TiN were statistically analyzed in at least 10 random fields of view. The test results are shown in Table 1.

[0066] The test results for spiral flow length and thin-wall filling integrity rate are shown in Table 1. Figures 4-7 Example 1 is used as the representative figure: The results of the as-cast microstructure observation are as follows. Figure 4 As shown, the SEM morphology of the TiN inclusions is as follows: Figure 5 As shown, the EDS component confirmation results are as follows: Figure 6 As shown, the CT inspection and local defect analysis methods for thin-walled precision-cast components are as follows: Figure 7 As shown. Figures 4-7 The conditions for obtaining the TiN inclusions are as follows: take the middle section of the thin-walled section of the precision-cast billet in Example 1, and perform metallographic observation after grinding, polishing and etching. The morphology and composition of the TiN inclusions are confirmed by SEM / EDS, and the cleaned thin-walled component is subjected to industrial CT scanning. Figures 4-7 These correspond to the equiaxed crystal area ratio, average grain size, TiN size / area fraction, and defect volume fraction in Table 1, respectively.

[0067] Table 1. Casting, microstructure, and inclusion test results of the examples and comparative examples:

[0068] As shown in Table 1, the spiral flow length of Examples 1 to 5 is not less than 430 mm, the thin-walled filling integrity rate is 95.5% to 98.8%, and the defect volume fraction is not higher than 0.32%, indicating that the present invention still maintains suitable filling and feeding capabilities for thin-walled cavities under the Cr-Mo-W corrosion-resistant system. Among them, although Example 4 is a more demanding component with a minimum wall thickness of 1.5 mm, a 95.5% filling integrity rate was still obtained through a high rapid rise rate and local quenching, proving that the nonlinear pressure curve and gradient shell can alleviate the contradiction between thin-walled filling and directional solidification.

[0069] As can be seen from Table 1, the continuous filling length in the spiral channel of the embodiment reaches more than 430 mm, which is consistent with the test results of 430 mm to 470 mm of Embodiments 1 to 5. This shows that the present invention has the flow window required for thin-walled filling while maintaining a high PREN design. Figure 7 The CT scan and local cross-section of the thin-walled component shown further indicate that no through-type cold shuts or large shrinkage cavities were observed in the 1.5mm to 2.8mm thin-walled area, only a small number of scattered micro-defects. This result corresponds to the data in Table 1 where the defect volume fraction is no higher than 0.32%.

[0070] Combination Figure 4 As can be seen, the as-cast microstructure of the examples exhibits a high proportion of isometric crystals and relatively uniform grain boundaries, which directly supports the statistical results in Table 1, showing an isometric crystal area ratio of 61%–74% and an average grain size of 310 μm–410 μm. Combined with… Figure 5 and Figure 6 As can be seen, the TiN inclusions are finely dispersed and confirmed by EDS to be Ti and N enriched phases, which supports the results in Table 1 showing that the average TiN size is 1.2 μm to 1.6 μm, the maximum size is 3.6 μm to 4.5 μm, and the area fraction is 0.028% to 0.042%. The above image evidence demonstrates that the Ce / Ti ratio, Nb / Ti ratio, ultra-low C / N ratio, and controlled cooling collectively suppressed the formation of coarse TiN and coarse columnar crystals, thus providing an organizational basis for subsequent improvement in corrosion resistance.

[0071] In Examples 1 through 5, the equiaxed crystal area ratio was greater than 50%, the average grain size was less than 500 μm, the average TiN size was less than 2 μm, the maximum TiN size was less than 5 μm, and the TiN area fraction was less than 0.05%, which corresponds to the limitations of this invention regarding as-cast microstructure and inclusion control. In Comparative Example 2, simply increasing the Cr and Mo content reduced the flowability length to 355 mm and increased the defect volume fraction to 1.10%, and both the grains and TiN showed significant coarsening. While excessively high Cr and Mo content can improve corrosion resistance, it can impair the quality of thin-walled precision casting. In Comparative Example 3, due to the low Ce / Ti ratio, the maximum TiN size increased to 9.8 μm and the area fraction increased to 0.084%, indicating that Ce plays a crucial role in the transformation of oxygen-sulfur inclusions and the inhibition of TiN growth. Comparative Example 5, using a conventional mold and linear pressure boosting, showed that the flow length decreased to 418 mm, the mold filling rate was only 76.0%, and the grain size increased to 700 μm, proving that the mold thermal design and nonlinear pressure curve of the present invention are necessary process conditions for obtaining the reliability of thin-walled precision casting.

[0072] Test Example 2: This test example is used to evaluate the pitting corrosion resistance, intergranular corrosion resistance, salt spray corrosion resistance, and basic mechanical properties of the examples and comparative examples. Samples obtained from each group according to their corresponding heat treatment and surface treatment regimes were processed to the same size and subjected to the following tests in sequence: potentiodynamic polarization test (3.5wt% NaCl solution, 25℃±2℃, scan rate 1mV / s, reference electrode: saturated calomel electrode SCE), critical pitting temperature test (ASTM G150 standard, 1mol / L NaCl solution), intergranular corrosion weight loss comparison test (referencing GB / T4334-2020 sulfuric acid-copper sulfate system, corrosion time 24h, post-corrosion mass loss statistics based on unit area), 500h neutral salt spray test (ASTM B117 standard, 5wt% NaCl solution, 35℃±2℃), and room temperature tensile test (GB / T 228.1-2021, small-size proportion sample, tensile rate 2mm / min); the pitting potential was determined by a current density reaching 100μA / cm. 2 The potential value at the time of salt spray test (relative to the saturated calomel electrode SCE) was measured, and the maximum pit depth was measured after the salt spray test. The tensile properties were taken as the average value of three samples. The experimental results are shown in Table 2.

[0073] Image results of corrosion resistance performance as follows Figures 8-10 As shown, where Figure 8 The “Example” in the text refers to Example 1, and the “Comparative Example” refers to Comparative Example 1, which represents the comparison results of the potentiodynamic polarization curves obtained by the two in 3.5wt% NaCl solution; Figure 9 The “Example” in the text refers to Example 1, and the “Comparative Example” refers to Comparative Example 3, which represent the comparison results of the local corrosion morphology of the two after 500h neutral salt spray test; Figure 10The terms “Example-1”, “Example-2”, “Comparative Example-1”, and “Comparative Example-2” in the text correspond to Example 1, Example 5, Comparative Example 1, and Comparative Example 3, respectively, representing the macroscopic appearance comparison results of salt spray samples of the same size after 500h of neutral salt spray test. Figures 8-10 These correspond to the pitting potential, maximum salt spray pit depth, and localized corrosion severity in Table 2, respectively. Elemental segregation scan results before and after heat treatment are shown below. Figure 11 As shown, Figure 11 Example 1 was used as a representative sample to illustrate the effect of composite heat treatment in reducing Cr, Mo, and W dendrite segregation and improving the uniformity of element distribution in the passivation film formation.

[0074] Table 2. Test results of corrosion resistance and mechanical properties of the examples and comparative examples:

[0075] Table 2 shows that the pitting potentials of Examples 1 to 5 were 520 mV to 610 mV, the critical pitting temperatures were 51℃ to 58℃, and the maximum pit depth after 500 h of salt spray was 6 μm to 11 μm. These values ​​are all superior to those of Comparative Example 1, which did not form a synergistic corrosion-resistant system of Cr-Mo-W-Sn-Cu. This indicates that W, Sn, and Cu improved the stability of the passivation film and the ability to inhibit pitting propagation without significantly sacrificing castability. The higher pitting potentials of Examples 3 and 5 are consistent with their higher PREN and lower TiN area fractions, indicating that the corrosion-resistant enhancement effect is not due to a single element, but rather the result of the synergistic effect of corrosion-resistant alloying and inclusion control.

[0076] Combination Figure 8 (Electrodynamic polarization curves of Example 1 and Comparative Example 1) It can be seen that the current density of the polarization curve of Example 1 is generally lower than that of Comparative Example 1, and the pitting potential is higher; combined with Figure 9 It is evident that Example 1 showed fewer localized corrosion spots after 500 hours of neutral salt spray testing compared to Comparative Example 3, with only a few isolated micro-pits observed in the corrosion morphology; combined with Figure 10 It is evident that the macroscopic salt spray corrosion levels of Examples 1 and 5 are significantly lower than those of Comparative Examples 1 and 3. These image results correspond to the pitting potentials (520 mV–610 mV), CPT (51 °C–58 °C), and maximum salt spray pit depths (6 μm–11 μm) in Table 2 for the examples, directly demonstrating the contribution of the Cr-Mo-W-Sn-Cu composite corrosion-resistant system, inclusion control, and surface passivation treatment to pitting and salt spray corrosion resistance.

[0077] Combination Figure 11It is evident that the content fluctuations of Cr, Mo, and W along the dendritic and interdendritic regions are significantly reduced after composite heat treatment, indicating that high-temperature solid solution treatment can reduce dendritic segregation in the as-cast state and improve the uniformity of passivation film formation element distribution. Low-temperature stress relief treatment is used to release residual stress in thin-walled precision-cast components without entering the rapid precipitation temperature range of σ and χ phases. This result explains why the examples in Table 2 have higher pitting potential, lower intergranular corrosion weight loss, and smaller salt spray pit depth compared to Comparative Example 6, and also corresponds to the beneficial effects of this invention, namely "reducing segregation, restoring passivation film quality, and improving corrosion resistance."

[0078] Although Comparative Example 2 exhibits a high pitting potential and CPT, its fluidity, defect volume fraction, grain size, and elongation after fracture (Table 1) show significant deterioration, indicating that simply increasing the Cr and Mo content cannot simultaneously satisfy the forming reliability and service stability of thin-walled precision-cast components. Comparative Example 3, due to its low Ce / Ti ratio, shows an increase in coarse TiN and composite inclusions, resulting in a pitting potential of 455 mV and a maximum salt spray pit depth of 35 μm, demonstrating a direct correlation between inclusion degradation and TiN size control and pitting resistance. Comparative Example 4, while bringing the Ce / Ti value close to the range of this invention, deviates from the limits of this invention in terms of the absolute content of Ti and Nb and the Nb / Ti ratio, and has a high C and N content, leading to an imbalance in the control of TiN and grain boundary carbonitrides, and a significant increase in intergranular corrosion weight loss to 4.80 g·m³. -2 This demonstrates that the present invention is not limited to a single proportional relationship, but relies on the overall matching of ultra-low C / N, Ti-Nb composite stabilization, and Ce denaturation control. Comparative Examples 5 and 6 respectively reflect the defects, segregation, and decreased passivation film quality caused by insufficient process thermal control and insufficient post-cast composite heat treatment, and their corrosion resistance and elongation are lower than those of Example 1.

[0079] The embodiments of the present invention have been described above. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims. All of these forms are within the protection scope of the present invention.

Claims

1. A corrosion-resistant reinforced ultrapure ferritic stainless steel, characterized in that: The chemical composition of the stainless steel, by weight percentage, includes: C 0.006%~0.010%, N 0.006%~0.010%, Cr 21.10%~21.50%, Mo 2.45%~2.60%, Ti 0.29%~0.32%, Nb 0.40%~0.42%, Sn 0.10%~0.13%, Cu 0.65%~0.75%, Ce 0.045%~0.055%, W 0.65%~0.80%, B 0.006%~0.008%, S≤0.005%, P≤0.02%, Si≤0.3%, Mn≤0.3%, O≤0.005%, balance Fe and unavoidable impurities; And it satisfies the following relationship: PREN=Cr+3.3×Mo+0.5×W≥29.5; Ce / Ti = 0.141~0.190; Nb / Ti = 1.25~1.45; Where PREN represents pitting resistance equivalent, Cr, Mo, and W represent the mass percentage of the corresponding elements in the chemical composition of the stainless steel; Ce / Ti and Nb / Ti represent the ratio of the mass percentage of the corresponding elements. The unavoidable impurities are elements other than those listed above, wherein the content of any single element is ≤0.03% and the total content of other elements is ≤0.10%.

2. The corrosion-resistant reinforced ultrapure ferritic stainless steel according to claim 1, characterized in that: The average size of the TiN inclusions in the stainless steel is ≤2μm, the maximum size is ≤5μm, and the area fraction of the TiN inclusions on the two-dimensional metallographic section of the matrix structure of the ferritic stainless steel is ≤0.05%.

3. The corrosion-resistant reinforced ultrapure ferritic stainless steel according to claim 1, characterized in that: The stainless steel has a cast microstructure, wherein isometric crystals account for ≥50% of the area of ​​the cast microstructure and the average grain size is ≤500μm.

4. A precision casting process for thin-walled components of corrosion-resistant reinforced ultrapure ferritic stainless steel, applicable to the corrosion-resistant reinforced ultrapure ferritic stainless steel described in any one of claims 1-3; characterized in that, Includes the following steps: S1, the raw materials are prepared according to the chemical composition of the corrosion-resistant reinforced ultrapure ferritic stainless steel, and vacuum induction melting and electroslag remelting are carried out in sequence to obtain a remelted ingot; wherein, during the melting to remelting process, the O in the remelted ingot is controlled to be ≤0.005% and S ≤0.005%, and the Ce / Ti in the remelted ingot is maintained at 0.141 to 0.190; S2, prepare a wax model corresponding to the thin-walled component, and prepare a gradient shell on the outer surface of the wax model that matches the solidification characteristics of the high Cr-Mo-W alloying of the corrosion-resistant reinforced ultrapure ferritic stainless steel; wherein, the gradient shell is configured to allow the thin-walled region to dissipate heat preferentially, and the thick-walled region or riser region to solidify later. S3, the gradient shell is dewaxed and calcined; S4, after the remelted ingot is melted into molten metal, the molten metal is poured into the cavity of the gradient shell using a vacuum anti-gravity casting method, and a nonlinear pressure curve is used to control the filling flow rate during the filling process of the molten metal; wherein, the nonlinear pressure curve includes a slow start stage, a fast rise stage and a slow filling stage that proceed in sequence, so that the molten metal enters the pouring channel with low disturbance in the early stage of filling, quickly forms continuous filling in the thin-walled area, and maintains the feeding pressure in the later stage of filling; S5, during the solidification of the molten metal, the cooling rate of the molten metal in the range from the liquidus temperature to the solidus temperature is controlled so that the growth of TiN inclusions, the segregation of Cr / Mo / W dendrites and the tendency of hot cracking in thin-walled areas are synergistically suppressed; after the molten metal solidifies and cools, the gradient shell is cleaned and the gating and riser are cut off to obtain a precision-cast billet. S6. The precision-cast billet is subjected to high-temperature solution treatment, low-temperature stress relief treatment and surface passivation treatment in sequence to reduce the segregation of Cr, Mo, W, Sn and Cu in the as-cast state, release the residual casting stress, and avoid holding at 700℃~850℃ for more than 30 minutes to prevent the precipitation of σ phase or χ phase, so as to obtain corrosion-resistant reinforced ultrapure ferritic stainless steel thin-walled component.

5. The precision casting process for a corrosion-resistant reinforced ultrapure ferritic stainless steel thin-walled component according to claim 4, characterized in that: In step S1, the vacuum induction melting includes the following steps: Fe, Cr, Mo, W and Cu raw materials are loaded into a crucible and melted under a vacuum of ≤0.1 Pa; After melting, refine at 1600℃~1620℃ for 20min~25min; The temperature was then lowered to 1520℃~1540℃, argon gas was introduced, and an intermediate alloy containing Nb, Ti, Sn, Ce and B was added in sequence. The electroslag remelting uses a CaF2-Al2O3-CaO slag system and is carried out under argon protection. The remelting current is 3000A-3500A, the voltage is 28V-32V, and the remelting rate is 7kg / h-9kg / h.

6. The precision casting process for a corrosion-resistant reinforced ultrapure ferritic stainless steel thin-walled component according to claim 5, characterized in that: In step S2, the gradient shell includes an inner layer close to the cavity and an outer layer covering the outside of the inner layer; The inner layer includes a zircon powder surface layer near the cavity surface and an Al2O3-based thermally conductive transition layer located outside the zircon powder surface layer. The total thickness of the inner layer is 3mm to 5mm. The outer layer is an Al2O3-based thermal insulation backing layer containing ZrO2, with a thickness of 8mm to 12mm; Furthermore, a copper quench block is provided on the outside of the thin-walled end, the thick-thin transition area, or the predetermined preferential solidification area corresponding to the wax mold; the copper quench block is a T2 pure copper plate with a thickness of 3mm to 6mm and a width of 10mm to 20mm, which is embedded in the back layer of the outer layer in an inlay manner and is flush with the outer surface of the back layer. The surface of the quench block facing the inner layer of the mold shell is directly opposite the thin-walled end, the thick-thin transition area, or the predetermined preferential solidification area; the copper quench block is used to form a directional solidification path that advances from the thin-walled area to the thick-walled area or the riser area after the molten metal has been continuously filled.

7. The precision casting process for a corrosion-resistant reinforced ultrapure ferritic stainless steel thin-walled component according to claim 6, characterized in that: In step S2, the gradient shell is prepared by a multi-layer coating method, which includes forming a surface layer, a transition layer and a back layer in sequence. The surface layer is prepared using zircon powder and silica sol; The transition layer is prepared using Al2O3-based refractory material and silica sol; The backing layer is made of Al2O3-based thermal insulation material containing ZrO2; The surface layer and the transition layer together constitute the inner layer, and the back layer constitutes the outer layer.

8. The precision casting process for a corrosion-resistant reinforced ultrapure ferritic stainless steel thin-walled component according to claim 7, characterized in that: In step S4, the nonlinear pressure curve includes a slow start phase, a rapid rise phase, and a slow charge phase; The pressure increase rate of the slow start phase is 1 kPa / s to 3 kPa / s, and the duration is 3 s to 5 s; The pressure rise rate of the rapid rise phase is 5 kPa / s to 10 kPa / s, and the duration is 5 s to 8 s; The voltage increase rate during the slow charging phase is 1 kPa / s to 2 kPa / s, and is maintained until the cavity is completely filled. Specifically, when the minimum wall thickness of the thin-walled component is 1.5 mm to 2.0 mm, the pressure rise rate of the rapid rise stage is 8 kPa / s to 10 kPa / s; when the minimum wall thickness of the thin-walled component is greater than 2.0 mm and less than 2.5 mm, the pressure rise rate of the rapid rise stage is 6 kPa / s to 8 kPa / s; and when the minimum wall thickness of the thin-walled component is 2.5 mm to 3.0 mm, the pressure rise rate of the rapid rise stage is 5 kPa / s to 7 kPa / s.

9. The precision casting process for a corrosion-resistant reinforced ultrapure ferritic stainless steel thin-walled component according to claim 8, characterized in that: In step S4, the initial pouring temperature of the molten metal is controlled to be 120°C to 150°C higher than the liquidus temperature of the molten metal; wherein the liquidus temperature of the stainless steel is 1420°C to 1450°C and the solidus temperature is 1370°C to 1390°C. Furthermore, the average cooling rate of the molten metal after filling is controlled to be 1℃ / s to 5℃ / s within the range of liquidus temperature to solidus temperature.

10. The precision casting process for a corrosion-resistant reinforced ultrapure ferritic stainless steel thin-walled component according to claim 9, characterized in that: Step S6 includes the following processing stages in sequence: High-temperature solution treatment stage: heat to 1080℃~1100℃, hold for 1.5h~2h, and then cool to room temperature by water quenching; Low-temperature stress relief treatment stage: After the high-temperature solution treatment stage is completed and cooled to room temperature, the temperature is heated to 280℃~350℃ and held for 1h~2h, and then cooled to room temperature by air cooling. Pickling and passivation stage: Pickling solution containing 200g / L to 300g / L HNO3 and 30g / L to 50g / L HF is used to remove the surface oxide scale at a temperature of 40℃ to 60℃ for 10min to 20min. Then, passivation solution containing 300g / L to 400g / L HNO3 is used to passivate the surface at a temperature of 50℃ to 70℃ for 30min to 60min.

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