Railway wagon swing bolster casting method based on directional solidification

Abstract

The invention relates to the field of railway vehicle part manufacturing, discloses a railway wagon swing bolster casting method based on directional solidification, and aims to solve the problems of low feeding efficiency and easy generation of shrinkage cavities and cracks caused by special wall thickness distribution of European type swing bolsters. The scheme is characterized in that a casting mold is poured in the posture that a center plate face faces downwards and a bottom face A area faces upwards; a built-in feeding bag of which the modulus is 1.15-1.25 times that of the center plate is arranged in the center of the center plate; easily-cut insulated risers are arranged at the discrete hot spots, and partition chillers are embedded between the adjacent risers to physically isolate the thermal field. And a stepped bottom pouring system comprising a sawtooth sand collecting bag and an ingate riser is adopted for mold filling. According to the technical scheme, directional solidification and partitioned precise feeding can be achieved, shrinkage cavities and shrinkage porosity caused by thermal field interference are eliminated, the yield is increased to 75% or above, and the reliability of castings is remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of railway vehicle component manufacturing, specifically relating to a method for casting railway freight car bolsters based on directional solidification. Background Technology

[0002] As a backbone of the modern integrated transportation system, railway freight plays an irreplaceable strategic supporting role in ensuring the stability of the global logistics supply chain and promoting regional economic integration. With the continuous growth in demand for heavy-haul, high-frequency, and cross-border intermodal transport, the manufacturing quality and service life of the bolster, a key load-bearing component of freight car bogies, have become core indicators for measuring the safety level of railway freight transport. During operation, the bolster is subjected to complex alternating loads and random impacts, and the density of its internal structure directly affects the fatigue strength and fracture resistance of the component. In the global rail transit system, due to differences in track conditions, axle load standards, and service environments, the structural design of the bolster exhibits significant regional characteristics. Specifically, the bolster structure used in Eurasia (such as Russia and other 1520mm gauge regions) differs fundamentally from the common designs used in China, Europe, America, and East Asia: Eurasian standard bolsters generally have special built-in shrinkage bulges on the back of their inner core, and their lower wall thickness exhibits a gradient distribution that gradually thins from the central planar area to the sloping sides and ends. In contrast, the back of the core plate of mainstream domestic and European and American rocker bolsters usually does not have a feeding liner, and its wall thickness distribution pattern shows that it is thickest at the rounded corner where the end meets the bevel, and gradually thins towards the two bevels. This completely reversed wall thickness gradient distribution pattern makes the traditional casting process logic face a severe technical adaptation challenge when facing European and Asian standard rocker bolsters.

[0003] In existing casting practices, technicians have attempted to improve the production of Eurasian standard rocker bolsters through various technological means. For example, existing technical solutions, such as patent publication number CN105195682A, propose a downward-facing mandrel orientation for Russian standard rocker bolsters, along with a side gating system and feeding risers placed longitudinally between the lower web and the mandrel, attempting to eliminate internal shrinkage porosity while meeting specific geometric requirements. Additionally, there are technical solutions such as publication number CN102554135A, which focus on using an integrated sand core structure to avoid flash and burrs generated by traditional multi-sand core combinations, aiming to improve the surface accuracy and dimensional stability of castings. These existing technologies have indeed solved some surface geometric forming problems and improved the casting yield to a certain extent under specific historical periods and application environments. However, from the perspective of deeper thermophysical solidification characteristics and the feeding mechanism of large-section castings, the above solutions still exhibit inherent limitations when dealing with the special thermal distribution of Eurasian rocker bolsters, which are "heavy in the middle and light at the ends."

[0004] The reason for this is that existing process systems often rely on enhancing local feeding capabilities or optimizing sand core forming accuracy to address casting defects, while neglecting the coupling interference effect of the overall thermal field of the casting during dynamic evolution. In Eurasian standard rocker bolsters, the core plate area forms a main hot spot with extremely high energy density and complex geometry due to the built-in feeding package. Under the traditional multi-riser feeding logic, in order to cover multiple hot spots distributed throughout the machine, the process design usually requires arranging a series of feeding systems along the longitudinal direction of the casting. However, this simple, discrete "additive" design often leads to overlapping pressure gradients between different feeding areas during actual solidification, causing the flow of molten metal in the feeding channels to exhibit a non-linear, turbulent state. Due to the lack of effective blocking of heat conduction paths between adjacent hot spots, the cooling curves of different areas tend to flatten. This not only fails to form an ideal directional solidification sequence, but also induces severe micro-shrinkage porosity and secondary shrinkage cavities due to the premature closure of the feeding channels or excessive accumulation of heat at the thickness interface. Furthermore, the current technology's utilization of the gravitational field gradient is still in the empirical stage, failing to deeply decouple and reconstruct the orientation guidance from the natural wall thickness gradient. This results in redundant riser systems, low process yield, and, due to improper control of the temperature difference field gradient during cooling, easily induces inconspicuous thermal cracks at stress-bearing corners. This deep-seated technical contradiction makes it difficult for the current technology to guarantee the microstructure density of the core plate and its transition region when dealing with Eurasian-type bolsters with reverse wall thickness distribution patterns. Consequently, it cannot meet the stringent technical requirements of high fatigue life and high reliability for key cast steel components in international interconnected transportation.

[0005] In summary, existing technologies either focus only on local adaptation of static geometry or only on optimization of a single dimension (such as sand core forming), failing to construct a systematic casting solution that can accurately match the special structural characteristics of Eurasian bolsters from a global perspective of gravity field guidance, zoned thermal isolation, and precise feeding control. Therefore, how to fully explore and utilize the wall thickness characteristics of Eurasian bolsters, and through scientific posture transformation and physical isolation methods, construct a casting process that can effectively eliminate thermal interference, achieve efficient directional feeding, and also consider material utilization, has become a common technical problem and key challenge that urgently needs to be solved in the current field of railway freight car cast steel parts manufacturing. (Invention Content)

[0006] To achieve the above objectives, this invention provides a method for casting railway freight car bolsters based on directional solidification. It aims to address the technical defects of traditional casting processes for Eurasian-type railway freight car bolsters, such as low feeding efficiency, susceptibility to shrinkage porosity and porosity, low yield, and hot cracking, caused by the unique wall thickness distribution pattern—namely, the presence of an internal feeding pot in the center plate area and a decrease in overall wall thickness from the center to both ends. This invention constructs a systematic control system integrating orientation-guided solidification, internal center feeding, independent zone compensation, and physical isolation of the thermal field, ensuring the density of the internal structure and the reliability of the mechanical properties of the casting.

[0007] This invention provides a method for casting railway freight car bolsters based on directional solidification, achieved through the following technical means:

[0008] A method for casting railway freight car bolsters based on directional solidification is characterized by the following engineering implementation steps: First, a gravity-guided positioning system is constructed to define the pouring posture of the bolster mold in a gravity field with the center plate facing down and the bottom A area facing up. This posture setting ensures that the center plate, which is the hottest point in the entire casting, is located at the lowest level of the mold. Through this arrangement, utilizing the initial static pressure gradient of the molten metal during the filling process and the quenching effect of the cold molding sand at the bottom, a first cooling front is established at the bottom center plate of the casting, thereby constructing a bottom-up directional solidification reference thermal field.

[0009] Secondly, a built-in center compensation system is installed. An integrated core-plate compensation package, integrally formed with the core-plate structure, is installed in the geometric center region of the bolster cavity corresponding to the core plate. This built-in core-plate compensation package is located inside the inner cavity sand core, which is composed of high-strength resin sand. Its effective volume is calibrated according to the geometric modulus of the core plate's thermal section. The modulus M is calculated according to M = V / A (where V is the thermal section volume and A is the heat dissipation area), and the modulus M of the built-in core-plate compensation package is required to be... 补缩包 Satisfy 1.15×M 心盘 ≤M 补缩包 ≤1.25×M 心盘 By shortening the feeding distance and utilizing the natural thermal barrier effect of the sand core to slow down the cooling rate of the molten metal inside the feeding ladle, direct molten metal compensation for the shrinkage porosity in the core region of the core plate can be achieved.

[0010] Furthermore, this invention constructs a zoned independent compensation system. At the critical hot spots on the top and sides of the bolster casting—namely, at three discrete geometric inflection points: area A on the bottom surface, the middle of the inclined surface, and the inner side of the end plane—easily cut insulating risers are independently installed. These easily cut insulating risers are covered by riser sleeves made of high-performance heat-generating and insulating material. The riser sleeves, by weight percentage, comprise: 15%-25% aluminum powder, 10%-15% iron oxide powder, 20%-30% expanded perlite, 15%-20% hollow microspheres, 10%-15% refractory fiber, and the balance being binder. Upon contact with molten metal, the riser sleeves undergo an oxidation-reduction reaction to generate auxiliary heat, maintaining their thermal conductivity between 0.15 and 0.25 W / (m·K), ensuring that each zoned hot spot still has sufficient liquid metal replenishment capacity after the main structure solidifies.

[0011] In a preferred embodiment of the present invention, a chill is embedded in the wall thickness transition region between any two adjacent easily cuttable insulating risers, i.e., the non-feeding, thermally sensitive zone. The chill is made of chromite ore or alloy steel with a high heat storage coefficient, and its heat storage coefficient b should be no less than 2000 J / ( m 2·K·s 1 / 2 The chill, through physical contact with the casting surface, forcibly creates a low-temperature zone between two adjacent feeding zones, thereby generating a thermal blocking effect in the originally continuous thermal field. This arrangement ensures that the feeding fluid dynamics path of each easily cut insulating riser is confined within a preset geometric range, eliminating interference from feeding channel intersections caused by overlapping isotherms, and resulting in a layered advancement of the solidification front perpendicular to the casting wall.

[0012] Furthermore, an easy-cutting blade is integrated between the root of the easily cut insulating riser and the casting body. The easy-cutting blade is made of a high-temperature resistant, low-thermal-conductivity ceramic material, with a thickness of 3mm to 6mm. A circular flow hole is formed at the geometric center of the easy-cutting blade, with a diameter set to 35% to 45% of the riser root diameter. This structure utilizes the diameter reduction effect to stabilize the molten metal flow rate during the filling stage and maintain a constant pressure head gradient during the feeding stage. Simultaneously, in subsequent cleaning processes, stress concentration is induced through mechanical impact loads, achieving integrated brittle fracture and separation of the riser and the casting matrix.

[0013] Furthermore, the gating system of the present invention adopts a stepped bottom-pouring layout. The gating system comprises a pouring cup, a horizontal sprue, a serrated sand collection bag, an ingate, and an ingate riser. The serrated sand collection bag is located at the connection point between the horizontal sprue and the ingate. The internal cavity of the serrated sand collection bag has a continuous serrated structure with an included angle of 45 to 60 degrees, designed to utilize the local turbulence and centrifugal force generated when the fluid passes through a nonlinear boundary to capture and retain non-metallic inclusions and scouring sand particles in the molten metal at the root of the serrations. The ingate is tangentially connected to the ingate riser located at the bottom edge of the casting, and the cross-sectional area of ​​the ingate riser is set to 1.8 to 2.2 times the cross-sectional area of ​​the ingate. Through the sudden expansion effect of the cross-sectional area, the Reynolds number of the molten metal entering the core region is reduced, ensuring a smooth filling process without splashing.

[0014] In terms of metallurgical and thermal parameter control, the material used in this invention is low-alloy high-strength cast steel, whose chemical composition by mass percentage is set as follows: carbon (C) 0.20%-0.25%, manganese (Mn) 1.30%-1.60%, silicon (Si) 0.40%-0.60%, chromium (Cr) ≤0.30%, nickel (Ni) ≤0.30%, copper (Cu) ≤0.30%, phosphorus (P) ≤0.020%, sulfur (S) ≤0.020%, with the balance being iron (Fe). The casting temperature is strictly controlled between 1540℃ and 1570℃, and the casting speed is calibrated to 15kg / s to 25kg / s. Throughout the solidification process, through the above-mentioned zonal control system, the Niyama criterion value Ny of all key stress sections is always maintained above 0.1. The calculation formula for the Niyama criterion is:

[0015] Ny = G /

[0016] In the formula, G represents the local temperature gradient at the solidification front (in K / mm). This represents the cooling rate (in K / s). Quantitative control of this parameter ensures the continuous opening of the interdendritic feeding channels, preventing the formation of microscopic shrinkage porosity at the microscopic level.

[0017] Furthermore, to address the tensile stress generated in the side support guide grooves and stress corners during the solid-state shrinkage of the casting, multiple sets of anti-crack strips are installed in these areas. These anti-crack strips are elongated trapezoidal protrusions, with a height set to 15% to 20% of the casting wall thickness at that location, and their arrangement is parallel to the principal tensile stress trajectory. By locally strengthening the section modulus, the anti-crack strips can effectively absorb and distribute the shrinkage load caused by the temperature gradient, thereby eliminating hot crack defects.

[0018] In the post-processing, the bolster casting obtained by the method described in this invention needs to undergo systematic normalizing and tempering treatment. The normalizing process parameters are set as follows: heating temperature 890℃ to 920℃, holding time not less than 3.5 hours, followed by cooling in still air; the tempering process parameters are set as follows: heating temperature 580℃ to 620℃, holding time not less than 4 hours. Through the synergy of heat treatment process and directional solidification structure, the average impact toughness Akv of the casting in the extreme environment of -60℃ is not less than 40J.

[0019] The present invention provides a method for casting railway freight car bolsters based on directional solidification, which has the following definite technical effects:

[0020] First, by combining position guidance with the built-in core plate compensation package, the core hot spot of the Eurasian rocker is precisely compensated by utilizing the gravitational field and local thermal barrier effect, which increases the metal processing yield from the traditional 58% level to more than 75%, significantly reducing material loss and smelting energy consumption.

[0021] Secondly, the combined application of the isolation chill and the partitioned riser system creates multiple independent and controlled solidification units inside the complex variable wall thickness casting, eliminating the risk of premature closure of the feeding channel due to thermal field interference, and ensuring that the UT flaw detection pass rate of the core plate and transition zone reaches the second level or above standard.

[0022] Furthermore, the design of the easy-cutting blade and anti-crack strip not only reduces the labor intensity and machining cost of the cleaning process, but also ensures the stability of the casting's geometric dimensions and surface integrity by effectively releasing thermal stress.

[0023] Finally, the systematic casting control system provided by this invention enables precise control of the entire process of large-section complex castings, from the filling flow field to the solidification stress field. It can stably produce railway freight car bolsters that meet the highest international standards, which has important engineering application value for improving freight safety and service life.

[0024] As a detailed embodiment of the present invention, during the forming of the built-in core feeding package, the surface of the inner cavity sand core surrounding it needs to be sprayed with a zircon powder coating with a thickness of 0.5 mm to 1.0 mm. The refractoriness of the zircon powder coating is not less than 1750℃, which is intended to prevent physical and chemical sintering phenomena caused by the high-temperature molten metal staying in the built-in feeding package for a long time, and to ensure the flatness of the inner cavity surface of the casting.

[0025] Furthermore, the effective volume of the sawtooth sand collection bag is set to 1.5 to 2.5 times the total volume of the transverse runner. Its slag-blocking mechanism lies in the following: when molten metal enters the sawtooth cavity in a turbulent state, according to the fluid continuity equation, the instantaneous change in flow velocity leads to a non-uniform distribution of the pressure field. Lighter slag, oxide film, and broken sand particles are pushed towards the leeward side of the sawtooth under the control of the eddy current and deposited due to the combination of gravity and adhesion, thus ensuring the metallurgical purity of the molten metal entering the cavity.

[0026] Furthermore, the easy-to-cut sheet needs to be preheated before installation, and its surface is coated with a graphite powder layer to prevent chemical adhesion between the molten metal and the ceramic sheet during the pouring process, ensuring that the root of the riser can be broken off at the root during the cleaning stage without the need for secondary grinding.

[0027] In summary, this invention provides a production method that can fully adapt to the special structural requirements of Eurasian-type rocker bolsters by deeply decoupling and reconstructing gravity orientation, feeding configuration, thermal field zoning, and metallurgical parameters. The absolute certainty of its technical solution and the rigor of its physical logic ensure process stability and consistency of casting quality under large-scale industrial production conditions. Attached Figure Description

[0028] Figure 1 This is a structural diagram of the novel rocking pillow product of the present invention;

[0029] Figure 2 This is a partial cross-sectional view of the central disk region and the built-in core disk compensation package of the present invention.

[0030] Figure 3 for Figure 2 Top view;

[0031] Figure 4 This is a schematic diagram of the overall process flow of the casting method of the present invention.

[0032] The attached figures are labeled as follows:

[0033] 1. Rocker casting; 2. Core plate; 3. Internal core plate feeding bag; 4. Bottom surface A area; 5. Easily cut insulating riser; 6. Riser sleeve; 7. Isolation chill; 8. Easily cut disc; 9. Flow hole; 10. Pour cup; 11. Horizontal sprue; 12. Serrated sand collection bag; 13. Ingate; 14. Ingate riser; 15. Crack prevention strip; 16. Inner cavity sand core. Detailed Implementation

[0034] Against the backdrop of engineering initiatives promoting heavy-haul railway freight and international standardization, and specifically for bolster castings with unique wall thickness gradients and built-in feeding bulge structures within the Eurasian standard system, this invention provides a directional solidification-based bolster casting method for railway freight cars. Through deep coupling of gravity, thermal, and flow fields, a complete and precisely quantifiable manufacturing process system is constructed.

[0035] The primary engineering step in this invention, a method for casting railway freight car bolsters based on directional solidification, lies in the precise construction of a gravity-guided positioning system. During implementation, the bolster mold is strictly confined within a gravity field, with the center plate 2 facing downwards and the bottom surface A4 facing upwards, for mold assembly and pouring. From a thermodynamic perspective, the center plate 2 region is not only the most geometrically complex intersection point of the bolster but also the area with the highest thermal modulus in the entire casting. By placing this region at the lowest level of the mold, the process system effectively utilizes the static pressure gradient formed by the molten metal during the initial filling stage. At this point, the molten metal at the bottom, while bearing the maximum liquid column load, undergoes intense heat exchange with the cold bottom molding sand, which has a very high heat capacity, thereby inducing the first cooling front at the bottom of the center plate 2 region. The establishment of this front marks the formal establishment of the bottom-up directional solidification reference thermal field inside the casting, providing the physical prerequisite for the subsequent orderly compensation of the molten metal.

[0036] Within the aforementioned directional solidification framework, this invention further incorporates a built-in central feeding system within the bolster. Specifically, in the geometric center region corresponding to the core disk 2 within the bolster's inner cavity, a built-in core disk feeding package 3, structurally integrated with the core disk 2, is integrated through the molding space of the inner cavity sand core 16. The inner cavity sand core 16 is made of high-strength resin sand, which exhibits excellent dimensional stability and collapse resistance at high temperatures. To ensure the effectiveness of this built-in feeding system, the geometric modulus M of the built-in core disk feeding package 3 is... 补缩包 The actual module M of the core plate thermal junction needs to be considered. 心盘 Precise calibration is performed. The module calculation strictly follows the relationship M=V / A. Through discretization of the three-dimensional solid model of the core plate region, the ratio of its local heat volume VV to its effective heat dissipation area AA is calculated. In the engineering practice of this invention, the module M of the built-in core plate compensation package 3... 补缩包 Precisely set in M 补缩包 Satisfy 1.15×M 心盘 ≤M 补缩包 ≤1.25×M 心盘Within the closed interval. This ratio range ensures that the molten metal in the feeding pot still has sufficient fluidity and pressure head when solidification and shrinkage begin in the core area of ​​the core plate. Since the feeding pot is embedded inside the sand core with a natural thermal barrier effect, its heat dissipation path is significantly extended. This strong temperature difference between the local high temperature zone and the bottom chilling zone forces the shrinkage cavity risk point inside the core plate 2 to migrate to the inside of the feeding pot. In order to prevent sand inclusion defects caused by prolonged hot wetting of high-temperature molten metal, the inner cavity sand core 16 is coated with a zircon powder coating with a thickness of 0.5 mm to 1.0 mm on the surface of the corresponding feeding pot. The coating uses zircon powder as aggregate and has a refractoriness of over 1750℃. It not only effectively blocks the physical penetration of molten metal into sand particles, but also inhibits interfacial reactions through its high chemical inertness, ensuring the surface smoothness of the casting cavity after cleaning.

[0037] The subsequent construction of a zoned independent compensation system aims to solve the discrete thermal points caused by uneven wall thickness on the top surface and inclined surface of the bolster. At three shrinkage-sensitive points formed by geometric transitions—area A4 on the bottom surface of the casting, the middle of the inclined surface, and the inner side of the end plane—the invention independently arranges easily cut insulating risers 5.

[0038] To verify the necessity and critical control significance of the aforementioned riser sleeve material ratio range, this invention conducted systematic orthogonal optimization of the formula and thermophysical performance benchmarking experiments during the process finalization stage. For the dosage gradients of the four core variables in the riser sleeve composite structure—active aluminum powder, iron oxide powder, expanded perlite, and hollow microspheres—three sets of control samples with typical ratios were designed. Standard test rings were prepared under the same pressing process and calcination conditions. Based on ISO 22074-5, the method for determining the thermal conductivity of casting auxiliary materials, the equivalent thermal conductivity of riser sleeves with each ratio was measured using the hot wire method during the aluminothermic reaction stabilization period (5-8 min after casting). Simultaneously, combined with finite element thermo-fluid coupling simulation, the solidification time of the hot spot region on the top surface of the casting and the closing time of the feeding channel were compared under different thermal conductivityes. Detailed experimental data are shown in Table A.

[0039] Table A. Comparison of the effects of different riser sleeve material ratios on thermal conductivity and feeding efficiency:

[0040]

[0041] The following regular conclusions can be clearly drawn from the data in the table above:

[0042] Firstly, the total amount of the exothermic agent (aluminum powder + iron oxide powder) must be strictly limited to a closed range of 25% to 40% (this invention corresponds to 15%-25% aluminum powder + 10%-15% iron oxide powder). Comparative Example 2 shows that when the total amount of the exothermic agent exceeds 45% and the proportion of insulating aggregate is compressed, although the aluminothermic reaction is initially intense, the insufficient content of expanded perlite and hollow microspheres results in excessively low porosity of the riser sleeve skeleton, causing heat to dissipate rapidly into the environment. The measured thermal conductivity soars to over 0.41 W / (m·K), leading to premature solidification of the riser neck and closure of the feeding channel before the main casting solidifies. Simultaneously, the excessive aluminothermic reaction causes a glassy sintered layer to form on the inner wall of the riser sleeve, which not only weakens the subsequent insulation capacity but also causes chemical adhesion between the residual metal of the riser and the sleeve wall, significantly increasing the difficulty of cleaning.

[0043] Secondly, the total amount of insulating aggregate (expanded perlite + hollow microspheres) needs to be maintained within a synergistic range of 35% to 50% (20%-30% + 15%-20% in this invention). Comparative Example 3 shows that when the proportion of insulating aggregate exceeds 55%, the density of the riser sleeve is too low. Although the initial thermal conductivity can be controlled at around 0.30 W / (m·K), the latent heat released by the aluminothermic reaction cannot effectively offset the radiative heat dissipation of the molten steel at the top of the riser due to the insufficient absolute content of aluminum powder and iron oxide powder. Thermal imaging monitoring shows that the liquid surface at the top of the riser in Comparative Example 3 forms a complete solid shell 90 seconds after pouring, losing the pressure head driving force for continuous feeding to the casting body. Its casting yield is only 66.2%, which has no significant advantage compared with traditional sand mold risers.

[0044] Thirdly, the 15%-25% active aluminum powder, 10%-15% iron oxide powder, 20%-30% expanded perlite, and 15%-20% hollow microspheres specified in this invention constitute the optimal compatibility range of the "heating-insulation-refractory" ternary system. Under this ratio, the riser sleeve achieves Pareto optimality in both heating rate and insulation aging: the exothermic curve rises sharply in the initial stage of the aluminothermic reaction (0~60s), rapidly establishing a high-temperature pressure head; in the middle and later stages of the reaction (60~300s), due to the hierarchical porous structure formed by the expanded perlite and hollow microspheres, the thermal conductivity is stably controlled within the golden range of 0.15~0.25 W / (m·K). This is highly consistent with the thermophysical parameters of the Fushico Kalmin300 series and other mainstream high-performance heating and insulation riser sleeves both domestically and internationally, and the raw material cost is reduced by more than 40% compared to imported products.

[0045] Data demonstrates the significance of critical control: When the thermal conductivity exceeds the upper limit of 0.25 W / (m·K) (as in Comparative Example 2), the effective feeding distance of the riser to the hot spot is shortened by more than 35%, the Niyama criterion value inside the casting drops sharply from 0.12 to below 0.06, and the percentage of micro-shrinkage volume recovers to above 1.2%. When the thermal conductivity is below the lower limit of 0.15 W / (m·K) (although not listed separately in the table, experiments have shown that if the insulating aggregate is further increased), the riser sleeve's room temperature strength cannot meet the requirements for handling and box assembly, and it is prone to plastic deformation or even leakage under the static pressure of molten steel. Therefore, limiting the thermal conductivity to 0.15~0.25 W / (m·K) in this invention is not an arbitrary choice, but rather the only feasible solution domain derived from the dual constraints of "the exothermic agent being sufficient to ignite and maintain the reaction, and the insulating agent being sufficient to lock in the heat and ensure structural strength".

[0046] In summary, the material ratio range of the riser sleeve of this invention was determined through extensive quantitative comparative experiments, which revealed its technical boundary. Any deviation from the dosage of active aluminum powder, iron oxide powder, expanded perlite, and hollow microspheres will cause the thermal conductivity to exceed the critical window of 0.15~0.25 W / (m·K), thereby disrupting the thermodynamic synchronicity of the independent compensation system and making it impossible to reproduce the high process yield of 78.4% and zero-defect internal metallurgical quality of Example 1. This set of comparative experiments, from the perspective of evidence, established the incompressibility and technical necessity of the material ratio range of this invention.

[0047] Each easily cut insulating riser 5 is equipped with a riser sleeve 6 made of high-performance heat-generating and heat-insulating material. The chemical composition of the riser sleeve 6 is precisely proportioned, with 15%-25% active aluminum powder and 10%-15% iron oxide powder. These two act as exothermic agents, instantly triggering an aluminothermic reaction and releasing a large amount of latent heat upon contact with molten metal above 1500°C. Simultaneously, 20%-30% expanded perlite and 15%-20% hollow microspheres are added as insulating aggregate, along with 10%-15% refractory fiber. This multi-component composite structure allows the riser sleeve 6 to maintain a stable thermal conductivity of 0.15 to 0.25 W / (m·K) after the reaction. Through this dual gain of heat generation and insulation, each riser section can maintain an effective liquid metal pressure head for a long period after the main cross-section structure has solidified, thus achieving precise "point-to-point" compensation for localized hot spots.

[0048] To further optimize the thermal field distribution and eliminate coupling interference between adjacent feeding systems, this invention introduces a physical thermal field isolation system. A chill 7 is embedded in the wall thickness transition region between any two adjacent easily cuttable insulating risers 5. The chill 7 is made of chromite sand with a high heat storage coefficient or a specific alloy steel block, and its heat storage coefficient b is not less than 2000 J / (m²·K·s).1 / 2 From a thermodynamic perspective, the chill 7 forcibly creates a physically extremely low temperature zone in the originally gentle temperature gradient distribution diagram, acting as a "heat flow blocking valve." This layout effectively prevents heat interference between adjacent hot spots through conduction, avoiding premature closure of the feeding channels caused by overlapping isotherms. Under this isolation mechanism, the solidification front in each independent thermal unit can maintain a layered advancement perpendicular to the casting wall, ensuring the density of the microstructure.

[0049] As a key design feature for improving cleaning efficiency and quality stability, an easy-cutting blade 8 is integrated between the root of the easily cut insulating riser 5 and the casting body. The easy-cutting blade 8 is made of mullite-based or alumina-based ceramic material, with its thickness precisely controlled between 3mm and 6mm. A flow-through hole 9, located at the center of the blade, has a diameter specified as 35% to 45% of the riser root diameter. This structural design has significant implications from a fluid dynamics perspective: during the filling stage, the flow-through hole 9 acts as a throttling mechanism, helping to eliminate flow instability; during the solidification stage, it acts as a physical neck, maintaining the molten metal feeding pressure while significantly reducing the cross-sectional area of ​​the connection between the riser and the casting. In the cleaning process after sand removal, due to the difference in thermal expansion coefficients between the ceramic material and the cast steel material, micro-stress exists at the connection interface. Only mechanical impact load induction is needed to achieve brittle fracture at the neck position of the riser, thus avoiding secondary thermal stress cracks that may result from oxy-acetylene cutting and eliminating the need for cumbersome subsequent grinding processes.

[0050] The stepped bottom-pouring gating system employed in this invention is another core line of defense for ensuring the metallurgical quality of castings. The molten metal flows sequentially through the pouring cup 10 and the runner 11, entering a specially designed serrated sand collection bag 12. The internal cavity of the serrated sand collection bag 12 has a continuous serrated structure, with the serration angle set at 45 to 60 degrees, and its effective volume is designed to be 1.5 to 2.5 times the total volume of the runner 11. When the molten metal enters this region at a high Reynolds number, the fluid generates controlled turbulence and micro-vortices at the nonlinear boundary. According to the fluid continuity equation and the principle of momentum conservation, this drastic change in flow pattern leads to a non-uniform distribution of the pressure field, causing low-density non-metallic inclusions, detached sand particles, and oxide films to be forced to gather and remain on the "leeward side" of the serrations under centrifugal force. Subsequently, the clean molten metal enters the ingate riser 14 tangentially through the ingate 13. The cross-sectional area of ​​the ingate riser 14 is set to be 1.8 to 2.2 times the cross-sectional area of ​​the ingate 13. By utilizing the sudden expansion effect of this cross-sectional area, the flow velocity of the molten metal is effectively reduced, realizing the transition from turbulent flow to stable laminar flow, and fundamentally eliminating the risk of metal splashing and air entrapment in the core plate 2 area during filling.

[0051] In terms of materials metallurgy and solidification criterion control, this invention selects optimized low-alloy high-strength cast steel with the following chemical composition (mass percentage): carbon (C) 0.20%-0.25%, manganese (Mn) 1.30%-1.60%, silicon (Si) 0.40%-0.60%, chromium (Cr) ≤0.30%, nickel (Ni) ≤0.30%, copper (Cu) ≤0.30%, phosphorus (P) ≤0.020%, sulfur (S) ≤0.020%, with the balance being iron. By strictly controlling the content of impurity elements such as P and S, combined with a casting temperature of 1540℃ to 1570℃ and a casting speed of 15kg / s to 25kg / s, an ideal solidification kinetic environment is systematically constructed. Throughout the solidification process, through the above-mentioned zonal control system, the Niyama criterion value Ny of all key stress sections is consistently maintained above 0.1. Achieving this quantitative indicator signifies that the interdendritic feeding channels remain unobstructed at the end of solidification, completely eliminating the generation of micro-shrinkage porosity from a microscopic mechanism perspective.

[0052] Furthermore, to address the issue of stress concentration and thermal cracking at the side frame guide grooves and stress-bearing corners during the solid-state shrinkage stage of the bolster, this invention symmetrically arranges multiple sets of anti-crack strips 15 in the aforementioned sensitive areas. Each anti-crack strip 15 is a long, trapezoidal protrusion with a height set to 15% to 20% of the casting wall thickness at that location, and its geometric axis remains parallel to the principal tensile stress trajectory. By locally increasing the section modulus, the anti-crack strip 15 effectively distributes the shrinkage load caused by the temperature gradient, ensuring the integrity of the casting surface and subsurface.

[0053] To transform the aforementioned as-cast microstructure into excellent service performance, this invention includes a systematic normalizing and tempering heat treatment process. The casting is normalized at 890°C to 920°C for at least 3.5 hours to eliminate casting stress and refine grains through austenitization. Subsequently, it undergoes high-temperature tempering at 580°C to 620°C for at least 4 hours to precipitate strengthening phases and further optimize matrix toughness. Castings treated with this process exhibit a stable average impact toughness (Akv) of over 40 J at -60°C.

[0054] The technical superiority of the present invention is demonstrated by quantitative data through specific engineering embodiments and comparative examples.

[0055] In a specific embodiment 1, a bolster for a 1520mm gauge Eurasian railway freight car was manufactured. The bolster material was selected from the aforementioned C-Mn low-alloy cast steel, with a casting weight of 1250kg. It was positioned with the core plate facing downwards, and the modulus ratio of the internal core plate shrinkage package was set to 1.20. Four easily cut insulating risers were arranged on the top and inclined surfaces, with a riser sleeve thermal conductivity of 0.18 W / (m·K). Three sets of steel chills were embedded between adjacent risers. The easily cut sheet thickness was 5mm, and the flow orifice diameter accounted for 40%. The casting temperature was 1550℃, and the casting speed was 20kg / s.

[0056] In Comparative Example 1, a traditional top-position casting process was used. The core plate faced upwards, and no internal feeding bag was installed; only three large open risers were installed above and on both sides of the core plate. No chills were used, and the risers were ordinary sand mold risers. A standard direct-pump gating system was used. The remaining chemical composition and casting temperature were consistent with those of Example 1.

[0057] By conducting dissection, flaw detection, and mechanical property testing on the two sets of test pieces, the comparative data shown in Table 1 below were obtained:

[0058] Table 1: Comparison of technical data between Example 1 of the present invention and Comparative Example 1 of the conventional process

[0059]

[0060] The experimental data in Table 1 clearly demonstrate that the present invention scheme adopted in Example 1 exhibits overwhelming advantages in all key technical indicators. The process yield increased significantly from 57.6% to 78.4%, meaning that under the same smelting capacity, the present invention can produce approximately 36% more qualified castings, significantly reducing energy consumption per ton of steel and material recycling costs. The perfect performance of the UT flaw detection pass rate in the core plate area is attributed to the scientific coupling of the built-in core plate feeding package 3 and the orientation guidance system, which transforms the originally most difficult-to-feed central hot spot into a controlled area during the solidification process.

[0061] Further analysis of the microstructure evolution revealed that, due to the use of a zoned independent compensation system and isolation chills 7 in Example 1, the cooling rate of each thermal unit was precisely controlled. The grain size increased from grade 4.0 to grade 7.5, and this fine-grained strengthening effect was directly reflected in the significant leap in impact toughness at -60℃. The relatively low Niyama criterion value (0.04) in Comparative Example 1 fully explains the physical root cause of its severe internal microstructure shrinkage, namely, the lack of effective thermal field partitioning leading to large-area closure of the interdendritic feeding channels during the later stages of solidification. In contrast, this invention, by maintaining a Ny value of 0.12, ensured the flowability of the molten metal between dendrites, achieving consistency between macroscopic compactness and microstructure.

[0062] In terms of production efficiency, thanks to the engineered application of the easy-to-cut blade 8, the cleaning time for a single bolster has been reduced from 7.5 hours to 2.8 hours. This not only means a reduction in direct labor costs, but more importantly, it significantly reduces the risk of surface scratches and thermal stress caused by frequent grinding and hot cutting during the post-processing stage of the casting. The serrated sand collection bag 12 was also verified in Example 1. Through cross-sectional observation of the riser residue, inclusions were successfully captured at the bottom of the sand collection bag, and the amount of oxide film entering the casting body was reduced by about 85% compared to Comparative Example 1.

[0063] This invention, through strategic selection of the gravitational field orientation, combined with a tactical layout of a built-in central feeding package and zoned insulating risers, and supplemented by physical isolation of the thermal field using chills, achieves a comprehensive casting process innovation encompassing macroscopic geometric adaptation and microscopic microstructure control. This systematic control system not only overcomes the technical bottlenecks in the manufacturing process of Eurasian-type bolsters but also provides a high-precision, high-economic, and highly deterministic engineering paradigm for the lean manufacturing of key load-bearing castings for railway freight cars. Its social and economic benefits in improving the service safety of freight vehicles, extending maintenance cycles, and promoting green manufacturing are unparalleled by existing technologies.

[0064] All technical details described in this invention, including but not limited to specific chemical component ratios, modulus calculation ranges, riser sleeve material formulations, and heat treatment process parameters, are optimized results derived from extensive engineering experiments and finite element thermal analysis. The determinism of these parameters ensures the portability and repeatability of this invention in the production of different types of railway bolsters. In actual production, adjustments to the relevant parameters within the closed range, based on fine-tuning of the bolster tonnage, do not deviate from the core protection scope of this invention. In summary, this invention, through deep decoupling and reconstruction of the casting physical process, successfully solves various persistent problems in the directional solidification process of large-section complex castings, contributing a highly competitive solution to the quality upgrade of global railway freight equipment.

Claims

1. A method for casting railway freight car bolsters based on directional solidification, characterized in that, The following are the engineering implementation steps: First, a gravity orientation guidance system is constructed: the rocker mold is placed in a gravity field with the center plate (2) facing down and the bottom area A (4) facing up, so that the center plate (2), which is the largest hot spot of the entire casting, is at the lowest level of the mold. By utilizing the initial static pressure gradient of the molten metal during the filling process and the cooling effect of the bottom molding sand, the first cooling front is established at the center plate (2) at the bottom of the casting, thereby constructing a directional solidification reference thermal field from bottom to top. Secondly, a built-in center compensation system is set up: In the geometric center area of ​​the bolster cavity corresponding to the core plate (2), a built-in core plate compensation package (3) integrally formed with the core plate (2) structure is set up. The built-in core plate compensation package (3) is set inside the inner cavity sand core (16), and its effective volume is calibrated according to the geometric modulus of the core plate hot spot, and the modulus M of the built-in core plate compensation package (3) is... 补缩包 The thermal modulus M of the core plate (2) 心盘 Satisfy: 1.15 × M 心盘 ≤M 补缩包 ≤1.25×M 心盘 By shortening the feeding distance and utilizing the thermal barrier effect of the inner cavity sand core (16) to delay the cooling rate of the molten metal in the feeding bag, direct molten metal compensation for the shrinkage pores in the core area of ​​the core plate is achieved. Furthermore, an independent compensation system for each zone is constructed: at the discrete geometric turning points on the top and sides of the bolster casting (1), namely the bottom surface A zone (4), the middle of the inclined surface and the inner side of the end plane, easy-to-cut heat-insulating risers (5) are independently set. The easy-to-cut heat-insulating risers (5) are covered by riser sleeves (6) made of heat-generating heat-insulating material. The redox reaction generated by the riser sleeves (6) in contact with the molten metal maintains the replenishment capacity of each zone's heat nodes after the solidification of the main structure. Then, a physical isolation system for the thermal field is constructed: in the wall thickness transition area between any two adjacent easily cut heat-insulating risers (5), a blocking chill (7) with a high heat storage coefficient is embedded. The blocking chill (7) constructs a physical low temperature zone between two adjacent feeding action areas, blocks the heat conduction path to eliminate the cross interference of the feeding channel, and guides the solidification front to present a layered advancement perpendicular to the casting wall. Finally, the mold is filled by a stepped bottom pouring system: the molten metal passes through the pouring cup (10), the horizontal runner (11), the sawtooth sand collection bag (12), the ingate (13) and the ingate riser (14) in sequence, and enters the mold cavity in a laminar flow state. Under the synergistic effect of the above systems, the casting is densified and formed. The implementation details of the built-in center compensation system include: The inner cavity sand core (16) is made of high-strength resin sand, and a layer of zircon powder coating with a thickness of 0.5mm to 1.0mm is sprayed on the surface of the inner cavity sand core (16) corresponding to the inner core plate feeding bag (3). The refractoriness of the zircon powder coating is not less than 1750℃. The high refractoriness and chemical inertness of the zircon powder coating are used to prevent the high temperature molten metal from undergoing physical or chemical sintering in the inner core plate feeding bag (3) due to long-term hot wetting. This ensures that after the inner core plate feeding bag (3) completes the feeding task of the core hot spot of the core plate (2), the inner cavity surface of the casting remains flat, and forces the shrinkage risk point inside the core plate (2) to migrate to the inner core plate feeding bag (3). The sawtooth sand collection bag (12) in the stepped bottom-pouring system is set at the connection node between the horizontal pouring channel (11) and the inner pouring channel (13). Its internal cavity presents a continuous sawtooth structure with an included angle of 45 degrees to 60 degrees, and the effective volume of the sawtooth sand collection bag (12) is set to 1.5 to 2.5 times the total volume of the horizontal pouring channel (11). By utilizing the local turbulence, centrifugal force, and non-uniform pressure field distribution caused by the instantaneous change in flow velocity generated when the fluid passes through the nonlinear sawtooth boundary, non-metallic inclusions, oxide films, and scouring sand particles in the molten metal are captured and retained on the leeward side of the sawtooth root, ensuring the metallurgical purity of the molten metal entering the cavity. The isolation chill (7) generates a thermal blocking effect in the originally continuous thermal field through physical contact with the surface of the rocker casting (1), and acts as a heat flow blocking valve, so that the feeding fluid dynamic path of each easily cut heat-insulating riser (5) is limited to the preset geometric range. By forcibly creating a physical extremely low temperature zone, the isotherms of different regions are avoided from overlapping, ensuring that the solidification process of each thermal unit of the casting does not interfere with each other.

2. The method for casting railway freight car bolsters based on directional solidification according to claim 1, characterized in that, The riser sleeve (6) in the partitioned independent compensation system adopts a multi-component composite structure, and its material by weight percentage includes: 15%-25% aluminum powder, 10%-15% iron oxide powder, 20%-30% expanded perlite, 15%-20% hollow microspheres, 10%-15% refractory fiber, and the balance binder. The aluminum powder and iron oxide powder act as exothermic agents, which trigger an aluminothermic reaction and release latent heat after contacting the molten metal. The expanded perlite, hollow microspheres and refractory fiber act as insulating aggregates, which together maintain the thermal conductivity of the riser sleeve (6) between 0.15 and 0.25 W / m·K after the reaction, thereby forming multiple independent controlled heat sinks on the top and inclined surfaces of the variable wall thickness casting, and realizing precise point-to-point compensation for thermal nodes at discrete geometric inflection points.

3. The method for casting railway freight car bolsters based on directional solidification according to claim 1, characterized in that, The chills (7) in the thermal field physical isolation system are made of chromite or alloy steel, and their heat storage coefficient b should not be less than 2000 J / ( m 2·K·s 1 / 2 ).

4. The method for casting railway freight car bolsters based on directional solidification according to claim 1, characterized in that, The root of the easily cut heat-insulating riser (5) and the body of the bolster casting (1) are integrated with an easily cut piece (8) with a thickness of 3mm to 6mm. The easily cut piece (8) is made of ceramic material and is preheated and coated with graphite powder before installation. A circular flow hole (9) is provided at the geometric center of the easily cuttable piece (8). The diameter of the flow hole (9) is set to 35% to 45% of the diameter of the riser root. During the filling stage, the flow hole (9) is used to stabilize the flow rate of the molten metal. During the feeding stage, a constant pressure head gradient is maintained. In the subsequent cleaning process, the micro-stress generated by the difference in thermal expansion coefficients between ceramic materials and cast steel materials, combined with the stress concentration induced by mechanical impact load, achieves the brittle fracture and peeling of the riser and the casting matrix at the neck position of the flow hole (9).

5. A method for casting railway freight car bolsters based on directional solidification according to claim 6, characterized in that, The ingate (13) is tangentially connected to the ingate riser (14) located at the bottom edge of the casting, and the cross-sectional area of ​​the ingate riser (14) is set to be 1.8 to 2.2 times the cross-sectional area of ​​the ingate (13); By utilizing the sudden expansion effect of the cross-sectional area to reduce the Reynolds number of the molten metal entering the core plate (2) region, the flow state is transformed from high-energy turbulence to stable laminar flow, eliminating the metal splashing and air entrapment phenomenon in the core plate (2) region during the initial filling stage, and in conjunction with the gravity orientation guidance system, a stable heat gradient distribution is formed at the bottom of the core plate (2).

6. A method for casting railway freight car bolsters based on directional solidification according to any one of claims 1 to 5, characterized in that, The bolster casting (1) is made of low-alloy high-strength cast steel, and its chemical composition by mass percentage is: carbon 0.20%-0.25%, manganese 1.30%-1.60%, silicon 0.40%-0.60%, chromium ≤0.30%, nickel ≤0.30%, copper ≤0.30%, phosphorus ≤0.020%, sulfur ≤0.020%, with the balance being iron; the pouring temperature is controlled between 1540℃ and 1570℃, and the pouring speed is calibrated to be between 15kg / s and 25kg / s; during the solidification process, through the zonal control of the above system, the Niyama criterion value Ny of all key stress sections of the bolster casting (1) is always kept above 0.1, where Ny = G / G represents the local temperature gradient at the solidification front. This represents the cooling rate, which ensures the continuous opening of the interdendritic feeding channels and prevents micro-shrinkage.

7. The method for casting railway freight car bolsters based on directional solidification according to claim 1, characterized in that, It also includes stress control and post-processing steps: Multiple sets of anti-crack strips (15) are symmetrically arranged at the side frame guide groove and the stress corner of the bolster casting (1). The anti-crack strip (15) is a long strip trapezoidal protrusion, and its height is set to 15% to 20% of the wall thickness of the casting at that location. The arrangement direction of the anti-crack strip (15) is parallel to the principal tensile stress trajectory at that location. The shrinkage load caused by the temperature difference gradient is absorbed by locally increasing the section modulus, thereby eliminating hot crack defects. After casting, the bolster casting (1) is subjected to normalizing and tempering treatment: the normalizing heating temperature is 890℃ to 920℃, the holding time is not less than 3.5 hours and then air cooling is performed; the tempering heating temperature is 580℃ to 620℃, the holding time is not less than 4 hours; through the synergistic refinement of heat treatment process and directional solidification structure, the average value of impact toughness Akv of the casting in the -60℃ environment is not less than 40J, and the ultrasonic flaw detection qualification rate of the core plate area reaches level II or above.

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