Casting method of high-strength machine tool body casting capable of reducing internal stress

Abstract

The invention discloses a casting method of a high-strength machine tool body casting capable of reducing internal stress, and relates to the technical field of metal casting, the method comprises the following steps: smelting a nodular cast iron or cast steel base material in an electric induction furnace, and degassing in a vacuum environment; adding a nano rare earth modifier into the melt, and injecting the nano rare earth modifier at an injection speed of less than or equal to 10mm / s, so that the Ce and La elements realize spheroidization and refinement of non-metallic inclusions in the melt; the modified melt is poured into a casting mold through a steel-clad and sand-clad system, and a frequency-adjustable alternating electromagnetic coil is arranged on the outer wall of the casting mold; and during pouring, a rotating electromagnetic field of 5-15 kHz is generated through an alternating electromagnetic coil, so that melt forms high-speed vortex stirring in a cavity, and component uniformity is promoted. Through nano rare earth and electromagnetic stirring coupling, non-metallic inclusions are synchronously refined and forced convection mixing is carried out, chemical segregation and inclusion agglomeration are reduced, the graphite spheroidization rate is increased, the structure is more refined and uniform, and the refining efficiency is improved.

Description

Technical Field

[0001] The invention relates to the technical field of metal casting, in particular to a casting method for a high-strength machine tool bed casting with reduced internal stress. Background Art

[0002] Large machine tool bed castings have large cross-sectional thickness variations. Traditional gravity or low-pressure casting often results in non-uniform solidification, severe segregation, and high residual stresses, leading to machining distortion, reduced precision, and poor vibration performance. Simply adjusting the alloy or improving the sand mold is unlikely to address the internal stresses caused by thermal-mechanical coupling.

[0003] Patent CN101670429B discloses a casting method for suppressing cracks at the bottom of an ingot. The above patent achieves an improvement in the ingot yield rate by improving the casting process.

[0004] The above patent can effectively reduce the overall internal stress value of the ingot, achieve the purpose of suppressing cracking, and improve the casting success rate, but it cannot simultaneously refine non-metallic inclusions and force convection mixing during the casting process.

[0005] To this end, the present application proposes a casting method for a high-strength machine tool bed casting that can simultaneously refine non-metallic inclusions and force convection mixing to reduce internal stress. Summary of the Invention

[0006] The object of the present invention is to provide a casting method for a high-strength machine tool bed casting with reduced internal stress, so as to solve the technical problem of internal stress generated by thermal-mechanical coupling proposed in the above background technology.

[0007] To achieve the above object, the present invention provides the following technical solution: a casting method for a high-strength machine tool bed casting with reduced internal stress, the method comprising the following steps:

[0008] The ductile iron or cast steel substrate is melted in an induction furnace and degassed in a vacuum environment;

[0009] Add nano rare earth modifier to the melt and inject it at a spray speed of ≤10mm / s, so that Ce and La elements can achieve spheroidization and refinement of non-metallic inclusions in the melt;

[0010] The modified melt is poured into the mold through the steel-sand-wrapped system, and an adjustable frequency alternating electromagnetic coil is arranged on the outer wall of the mold;

[0011] During pouring, a rotating electromagnetic field of 5-15kHz is generated by an alternating electromagnetic coil, which causes the melt to form high-speed eddy current stirring in the cavity to promote uniform composition.

[0012] At least three layers of gradient-controlled cooling chambers are set inside the casting mold, corresponding to the thin-walled area, medium-thick area and thick-walled area of ​​the bed respectively. The temperature is detected by thermocouples, and the zoned gradient cooling is achieved by the PID-controlled water cooling and air cooling system.

[0013] Preferably, the method further comprises:

[0014] Fiber Bragg grating temperature and stress composite sensors are embedded in the mold surface and gate area to collect temperature and stress data. The sensors are then linked with the cooling control system and subsequent heat treatment devices through the PLC system for adaptive adjustment.

[0015] After the initial solidification of the casting and before demoulding, the temperature in the mold is slowly lowered to 400-600℃ to release some thermal stress in advance;

[0016] After demoulding, the casting is sent to a hot isostatic pressing (HIP) furnace and kept at 900-1000°C and 100-150 MPa for 2 hours to eliminate residual porosity and break condensation cracks.

[0017] After HIP is completed, a step-by-step pressure reduction and temperature reduction process is implemented to gradually reduce the temperature to room temperature to prevent thermal stress from concentrating again;

[0018] Before completing the final machining, perform X-ray diffraction or ultrasonic testing to confirm that the residual stress level meets ≤50MPa.

[0019] Preferably, the addition amount of the nano rare earth modifier is 0.05-0.15wt% of the melt mass.

[0020] Preferably, the steel-sand-wrapped system uses seamless steel pipes to wrap refractory castables, and sprays a high-strength refractory coating on the inner surface.

[0021] Preferably, the coil spacing and number of turns of the electromagnetic coil are optimized according to the cross-sectional shape of the bed to ensure that the magnetic induction intensity of the rotating electromagnetic field is within the range of 5-15 mT.

[0022] Preferably, the gradient-controlled cooling chamber adopts a modular and detachable design, and the number and position of partitions can be flexibly configured according to different bed structures.

[0023] Preferably, the fiber Bragg grating sensor has an accuracy of ±1°C and ±1 MPa, and a data acquisition frequency of ≥10 Hz.

[0024] Preferably, the adaptive feedback control system includes a PLC control unit, a human-machine interface and an algorithm server, and the algorithm server optimizes the cooling and heat treatment curves in real time based on a finite element temperature field and stress field simulation model.

[0025] Preferably, after the HIP insulation stage is completed, a protective atmosphere is injected into the furnace, the gas composition of which is Ar + 2% hydrogen to inhibit high-temperature oxidation;

[0026] Before final machining, the casting was scanned for residual stress at full size, and through comparative analysis, it was confirmed that the residual stress of each key section was ≤50MPa.

[0027] Preferably, the step cooling process is provided with at least three temperature steps, the temperature difference of each step is 100-150° C., and the holding time is ≥30 min.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1. The present invention uses nano rare earth + electromagnetic stirring coupling to simultaneously refine non-metallic inclusions and force convection mixing, reducing chemical segregation and inclusion agglomeration, improving graphite spheroidization rate, making the organization more refined and uniform, and improving refinement efficiency;

[0030] 2. The present invention adopts a three-layer gradient controlled cooling cavity design to achieve a customized cooling curve with fast solidification of thin walls and slow solidification of thick walls. This solves the problem of thermal stress concentration caused by large solidification temperature difference in large-section castings, reduces residual stress, reduces crack tendency, and is easy to assemble and adjust.

[0031] 3. This invention uses online optical fiber composite monitoring and adaptive control to obtain temperature-stress fields in real time and dynamically adjust process parameters. This addresses the lack of precise thermal-mechanical monitoring and feedback control in traditional processes, improves cooling control accuracy, provides early warning of stress peaks and automatically optimizes, and achieves closed-loop intelligent regulation.

[0032] 4. This invention uses multi-stage hot isostatic pressing + stepped cooling to achieve high temperature and high pressure to eliminate pores, break cracks and relieve residual stress, solving the problem of difficulty in balancing defect closure and stress release in subsequent heat treatment. It combines zoned pressure maintenance with stepped cooling to precisely control the pressure relief and cooling curves to prevent stress re-accumulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 Schematic diagram of the casting method of the present invention. DETAILED DESCRIPTION

[0034] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0035] In the description of the present invention, it should be noted that the terms "upper," "lower," "inner," "outer," "front end," "rear end," "both ends," "one end," "the other end," and the like, indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely to facilitate and simplify the description of the present invention. They are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation, and therefore should not be construed as limiting the present invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0036] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "installed," "provided with," "connected," etc., should be understood in a broad sense. For example, "connected" may refer to a fixed connection, a detachable connection, or an integral connection; it may refer to a mechanical connection or an electrical connection; it may refer to a direct connection or an indirect connection through an intermediate medium; it may refer to internal communication between two components. Those skilled in the art will be able to understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0037] See also Figure 1 The present invention provides an embodiment of a method for casting a high-strength machine tool bed casting for reducing internal stress, the method comprising the following steps:

[0038] The ductile iron or cast steel substrate is melted in an induction furnace and degassed in a vacuum environment;

[0039] Add nano rare earth modifier to the melt and inject it at a spray speed of ≤10mm / s, so that Ce and La elements can achieve spheroidization and refinement of non-metallic inclusions in the melt;

[0040] The modified melt is poured into the mold through the steel-sand-wrapped system, and an adjustable frequency alternating electromagnetic coil is arranged on the outer wall of the mold;

[0041] During pouring, a rotating electromagnetic field of 5-15kHz is generated by an alternating electromagnetic coil, which causes the melt to form high-speed eddy current stirring in the cavity to promote uniform composition.

[0042] At least three layers of gradient-controlled cooling chambers are set up inside the mold, corresponding to the thin-walled area, medium-thick area and thick-walled area of ​​the bed respectively. The temperature is detected by thermocouples, and the PID-controlled water cooling and air cooling system realizes zoned gradient cooling.

[0043] Further smelting and vacuum degassing: Equipment: 2500kg induction furnace equipped with a vacuum housing system; Raw materials: Ductile iron (containing 3.4wt% C, 2.5wt% Si, 0.3wt% Mn, 0.02wt% P, 0.02wt% S) or low-alloy cast steel (containing 0.25wt% C, 0.5wt% Si, 1.2wt% Mn, 0.5wt% Cr); Process: After charging the furnace with pig iron, melt the metal to 1500°C at a heating power of 120kW. When the furnace gas is evacuated to <100Pa, the vacuum degassing device is turned on for 3 minutes to remove gas impurities.

[0044] Nano-rare earth modification: Modifier: rare earth oxide powder (CeO2:La2O3=3:1), powder particle size 30-60nm; injection method: horizontally introduced into the melt surface through a 100mm tungsten steel nozzle, with the injection speed controlled at 8-10mm / s; effect: Ce and La are rapidly dispersed in the melt, the spheroidization rate of entrained non-metallic inclusions is ≥90%, and the matrix structure is refined by more than 15%;

[0045] Steel-clad sand casting: Steel-clad sand system: Steel-clad steel pipe: 108mm diameter seamless heat-resistant steel pipe, with 2mm thick high-aluminum refractory coating sprayed on the inner wall; Sand layer: 25mm thick high-strength resin sand (SiO2 77wt%, Al2O3 10wt%, MgO 13wt%) wrapped around the outer surface; Casting mold: One-piece metal cavity conforming to the cross-sectional contour of the bed, equipped with a zoned controlled cooling cavity interface; Operation: The modified melt is poured into the cavity by free fall through the steel-clad sand support ladle, the pouring temperature is 1480-1500℃, and the pouring time is approximately 20s.

[0046] Electromagnetic field stirring: Coil arrangement: Three groups of concentric coils are arranged along the length of the cavity outer wall, with 24 turns in each group. The coils are made of high-temperature resistant copper alloy and are spaced 150 mm apart. Power supply: Adjustable frequency alternating current power supply with an output frequency of 5-15 kHz and an adjustable magnetic field strength of 5-15 mT. Stirring parameters: As the melt begins to enter the cavity, the electromagnetic field is turned on with a frequency of 10 kHz and a magnetic induction strength of 10 mT for 30 seconds, forming an internal eddy current velocity of approximately 0.8 m / s.

[0047] Gradient controlled cooling: Zone controlled cooling chamber: Thin-walled area (2-10mm): 15mm wall thickness, connected to 1in pipe; Medium-thick area (10-50mm): 20mm wall thickness, connected to 1.5in pipe; Thick-walled area (>50mm): 25mm wall thickness, connected to 2in pipe; Cooling medium and flow rate: Water cooling: pure water, room temperature (25℃) -50℃ circulation, thin-walled area 8L / min, medium-thick area 6L / min, thick-walled area 4L / min; Air cooling: compressed air, 0.6MPa, air volume thin-walled area 180m 3 / h, medium and thick area 140m3 / h, thick wall area 100m 3 / L; Temperature monitoring: 6 K-type thermocouples are installed on the inner wall of each zone, and the signal is connected to the PLC. The cooling flow rate is adjusted separately through the PID algorithm to make the cooling rate of the thin-walled area 25℃ / min, the medium-thick area 15℃ / min, and the thick-walled area 8℃ / min.

[0048] See also Figure 1 The present invention provides an embodiment of a method for casting a high-strength machine tool bed casting for reducing internal stress, the method further comprising:

[0049] Fiber Bragg grating temperature and stress composite sensors are embedded in the mold surface and gate area to collect temperature and stress data. The sensors are then linked with the cooling control system and subsequent heat treatment devices through the PLC system for adaptive adjustment.

[0050] After the initial solidification of the casting and before demoulding, the temperature in the mold is slowly lowered to 400-600℃ to release some thermal stress in advance;

[0051] After demoulding, the casting is sent to a hot isostatic pressing (HIP) furnace and kept at 900-1000°C and 100-150 MPa for 2 hours to eliminate residual porosity and break condensation cracks.

[0052] After HIP is completed, a step-by-step pressure reduction and temperature reduction process is implemented to gradually reduce the temperature to room temperature to prevent thermal stress from concentrating again;

[0053] Before completing the final machining, perform X-ray diffraction or ultrasonic testing to confirm that the residual stress level meets ≤50MPa;

[0054] Furthermore, sensor layout and adaptive control: Sensor model: Fiber Bragg Grating (FBG) temperature and stress composite sensors produced by Micron Optics are selected. Each sensor has three gratings: two stress measurement gratings and one temperature measurement grating. Layout location: Four sensors are equidistantly arranged around the gate area of ​​the outer wall of the casting mold to monitor the initial pouring temperature and stress peak in real time. Two sensors are arranged at each key section of the casting mold (one each in the thin-walled area, medium-thick area, and thick-walled area), for a total of six. The optical fiber leading from the sensor is connected to the PLC control cabinet via a high-temperature resistant optical cable. Data acquisition and linkage: The PLC reads the wavelength offset of each grating every second and converts temperature and stress values. If the temperature in a certain area exceeds the preset cooling curve by ±10°C, the PLC automatically adjusts the water pump speed (±10%) or fan speed (±15%) in that area. If the stress sensing value exceeds 5MPa, the PLC triggers adjustment of the subsequent slow cooling and holding time or early activation of the subsequent demolding and slow cooling step.

[0055] Initial setting and slow cooling: Process goal: Slowly reduce the average temperature in the casting mold from the initial setting temperature (approximately 1150°C) to 400-600°C to release the thermal stress generated by pouring. Equipment and parameters: Utilize the gradient-controlled cooling cavity of the mold body to continue the function, and set the water cooling flow rate to 4L / min in the thin-walled area, 3L / min in the medium-thick area, and 2L / min in the thick-walled area. Temperature control requirements: The overall cooling rate should not exceed 10°C / min, and the temperature uniformity should be maintained below 20°C. Slow cooling and heat preservation: After reaching 400-600°C, each zone should be kept warm for 45 minutes.

[0056] Hot isostatic pressing HIP treatment: Equipment model: QIHIP-1000 hot isostatic pressing furnace (volume 1m 3 , up to 1200℃, 200MPa); Loading and protective atmosphere: Fill the furnace with Ar+2% hydrogen mixed gas, control the static pressure supply rate at 10MPa / min, and heat up at a rate of 10℃ / min. After reaching 900-1000℃, keep warm for 2h and maintain the pressure at 100-150MPa, which is adjusted according to the thickness of the casting and the density of defects: 150MPa is taken in the thick-walled area and 100MPa in the thin-walled area; Pressure reduction and cooling: After the insulation is completed, first reduce the pressure to 10MPa at a rate of 5MPa / min and maintain for 30min, then reduce it to normal pressure at a rate of 5MPa / min; synchronously reduce the temperature at a rate of 10℃ / min, stay at 650℃ for 30min, and then reduce it to room temperature at a rate of 20℃ / min; maintain the protective atmosphere throughout the process;

[0057] Residual stress detection: X-ray diffraction method: Equipment: Rigaku SmartLab XRD, Cu Kα radiation; Measurement mode: θ-2θ scanning, step size 0.02°, exposure time 1s / step; Test points: 3 points each in the thin-walled area, medium-thick area, and thick-walled area every 200mm along the bed height, for a total of 9 points; Calculation method: Sin 2 The ψ method was used to calculate the residual stress at each point. The ultrasonic guided wave method (as a supplementary comparison) was used: equipment: Olympus TomoScan, center frequency 5 MHz probe; measurement mode: Lamb wave phase velocity measurement combined with finite element model inversion to calculate residual stress; test line: five beams were collected along the longitudinal centerline of the casting to verify the consistency of the XRD results.

[0058] See also Figure 1The present invention provides an embodiment of a method for casting a high-strength machine tool bed casting with reduced internal stress, which is applied to large-volume low-alloy cast steel bed castings and introduces ultrasonic-assisted vibration: a low-alloy cast steel substrate (C 0.2wt%, Si 0.4wt%, Mn 1.0wt%, Mo 0.3wt%) is melted to 1550°C in a 3000kg induction furnace and degassed at 50Pa vacuum for 5 minutes. 0.1wt% of a nano-rare earth modifier (CeO2:La2O3=3:1, particle size 25-70nm) is then added to the melt at a spraying speed of 9mm / s.

[0059] Electromagnetic field stirring + ultrasonic vibration: Two sets of adjustable frequency (8-12kHz) rotating electromagnetic coils are arranged on the outer wall of the mold, with a magnetic induction intensity of 12mT and a duration of 40s. An ultrasonic transducer is installed at the pouring gate to apply radial ultrasonic vibration to the melt, enhancing inclusion flotation and solute uniformity.

[0060] Gradient controlled cooling: The casting mold is divided into four controlled cooling zones (thin wall, medium thick, thick wall, and ultra-thick zone). The water cooling flow rate of each zone is 10, 8, 6, and 4 L / min respectively, and the air cooling air volume is 200, 160, 120, and 80 m3 respectively. 3 / h; PID control target cooling rate: thin wall 30℃ / min, medium thickness 20℃ / min, thick wall 12℃ / min, extra thick wall 6℃ / min;

[0061] Online monitoring and self-adaptation: Three FBG composite sensors are embedded in the gate area and the thick-wall area, for a total of six sensors. Samples are taken 20 times per second. If the temperature deviation in any area exceeds ±8°C, the PLC immediately adjusts the cooling medium flow rate in the corresponding area by ±12%. If the stress increment exceeds 6MPa, slow cooling in the mold is initiated in advance and the holding time is extended by 10 minutes.

[0062] In-mold slow cooling: slow cooling from the initial setting temperature of 1180℃ to 550℃, controlling the rate ≤8℃ / min, and keeping warm for 60min;

[0063] Hot isostatic pressing (HIP): In a HIP furnace, the temperature was raised to 950°C and 120 MPa in an Ar+1.5% hydrogen protective atmosphere and held for 2.5 hours. The pressure was then reduced at both ends: the first stage was reduced to 20 MPa and held for 40 minutes, and the second stage was reduced to atmospheric pressure. The temperature was reduced at both ends: 950°C to 700°C (10°C / min, held for 40 minutes) to room temperature (20°C / min).

[0064] Test and results: At 12 XRD measurement points, the residual stress was ≤40MPa. Metallographic microscope observation showed that the graphite spheroidization rate was ≥92%, and the average grain size of the matrix was refined from the original level 6 to level 8.

[0065] See also Figure 1, an embodiment provided by the present invention: a casting method for a high-strength machine tool bed casting with reduced internal stress, applied to a high-silicon cast iron bed and combined with phase change material controlled cooling;

[0066] Melting and degassing: The raw material is high-silicon ductile iron (C 3.3wt%, Si 3.5wt%, Mn 0.4wt%, P 0.08wt%, S 0.03wt%), heated to 1480℃ in a 2000kg induction furnace and degassed in vacuum for 4 minutes;

[0067] Modification and casting: Add 0.08wt% nano rare earth modifier (Ce:La=4:1, particle size 20-50nm), injection speed 8mm / s, melt casting through the steel-sand system, temperature 1450-1470℃;

[0068] Phase change material gradient controlled cooling: Phase change material modules (PCMs) are filled in the thin-walled, medium-thick, and thick-walled areas: thin-walled area: graphene-enhanced PCM, melting point 80°C; medium-thick area: paraffin-based PCM, melting point 120°C; thick-walled area: silicate ceramic PCM, melting point 180°C; the rear side of the PCM module is a water cooling pipeline with a uniform water cooling flow of 5L / min, using phase change heat absorption to extend the retarding time;

[0069] Electromagnetic stirring: a set of 10kHz, 8mT frequency electromagnetic coils, action time 25s;

[0070] Online monitoring and self-adaptation: Two FBG composite sensors are placed in the gate area and in the thin and thick areas, for a total of six sensors. The sampling frequency is 15Hz. When the temperature deviation in the PCM phase change starting temperature zone (80-180°C) is detected to be greater than ±7°C, the PLC adjusts the water cooling flow in the adjacent area by ±15%.

[0071] In-mold slow cooling: The PCM phase change process absorbs heat, and combined with water cooling, the mold temperature is slowly reduced from 1150°C to 500°C at a rate of ≤6°C / min; then the temperature is kept at this temperature for 50 minutes.

[0072] HIP post-treatment: 920°C, 110 MPa, Ar + 3% hydrogen for 2 hours; step pressure reduction: 110 ➡ 30 MPa (hold for 35 minutes) ➡ atmospheric pressure; cooling: 920 ➡ 700°C (8°C / min, hold for 35 minutes) ➡ room temperature (18°C / min);

[0073] Residual stress and microstructure detection: XRD residual stress at 8 measuring points ≤35MPa; metallographic analysis shows: graphite spheroidization rate is 94%, matrix lamellar graphite is significantly reduced, and impact toughness is increased by 12%.

[0074] See also Figure 1 The present invention provides an embodiment: a casting method for a high-strength machine tool bed casting with reduced internal stress, and corresponding performance test verification:

[0075] 1. Preparation of test samples:

[0076] The casting batch is made of the high-strength ductile iron bed casting obtained in the above embodiment, and a complete casting is produced in one pouring;

[0077] Regional division: Divide the casting into 5 sections at equal distances along the length direction, and divide each section into three areas along the cross section: thin-wall area, medium-thick area, and thick-wall area;

[0078] Cutting the sample:

[0079] Residual stress specimens: At the center of each area, use a wire cutting machine to cut out φ5mm×30mm cylindrical specimens, a total of 5 sections×3 areas = 15 pieces;

[0080] Tensile specimens: According to GB / T 228.1-2010, 5 specimens with a diameter of 10 mm and a gauge length of 50 mm were taken from each mid-thickness area.

[0081] Impact specimens: According to GB / T 229-2007, 5 V-notch specimens with a size of 10 mm × 10 mm × 55 mm per second were taken from each thin-walled area.

[0082] Hardness test specimen: Take at least 5 small pieces of 20mm×20mm×10mm from each area plane, a total of 5 sections×3 areas×5 pieces=75 pieces;

[0083] 2. Instruments and equipment and calibration:

[0084] XRD residual stress analyzer; Equipment: Rigaku SmartLab; Radiation source: Cu Kα; Calibration: 0.02°, 1s / step using NIST SRM676a titanium standard; Calculation method: Sin 2 ψ method;

[0085] Ultrasonic guided wave residual stress test system: Equipment: Olympus TomoScan; Probe: 5 MHz, 10 mm diameter; Calibration: Velocity-stress relationship calibration using a standard block with known residual stress (20 MPa);

[0086] Universal material testing machine: Equipment: Instron 5985; Tensile fixture: φ10mm; Strain measurement: Contact extensometer, accuracy 1μm;

[0087] Impact testing machine: Equipment: Zwick PSW-750; Energy tank: 300 J; Calibration: Pendulum energy verified using standard specimens according to EN ISO 148-1;

[0088] Hardness tester: Mitutoyo HM-100; load: 3000 kgf; calibration: using Brinell hardness blocks HBW 185 and HBW 150;

[0089] 3. Test steps:

[0090] Residual stress measurement:

[0091] The surface of the φ5mm sample was ground to Ra0.2um, and XRD was performed in θ-2θ mode with a step size of 0.02° and an exposure of 1s / point. The ψ values ​​were measured at 0°, ±15°, and ±30°, a total of 5 angles. Sin 2 The residual stress was calculated by fitting the ψ curve, and the average of three points was measured on each sample. The same sample was then re-measured using ultrasonic guided waves to verify that the difference between the two was ≤±5MPa.

[0092] Tensile properties test:

[0093] The sample surface was sandblasted and dotted with white paint, then clamped in Instron, the test speed was 2 mm / min, and R was recorded. m 、R p0.2 , A5, take 5 specimens from each mid-thickness area and calculate the average value and standard deviation;

[0094] Impact toughness test:

[0095] Position the V-notch in the center of the impact groove;

[0096] The test was carried out at room temperature (20±2℃), and the fracture energy of each specimen was recorded and the average of 5 specimens was taken;

[0097] Hardness distribution measurement:

[0098] A total of 25 points were placed on the surface of the test block in each area using a 5×5 grid. The load was 3000 kgf and maintained for 10 seconds. The HBW value was recorded and the mean and variance were calculated.

[0099] Microstructure observation:

[0100] A small piece of the remaining part of the tensile specimen was taken, polished and etched (3% HNO3-10% HCl);

[0101] Optical microscope photos were taken, and ImageJ software was used to measure the graphite spheroidization rate (≥90%) and grain size.

[0102] The test results are shown in Table 1 Test result example table;

[0103] Table 1 Test result example table

[0104] area residual stress tensile strength Yield strength Elongation at break Thin-walled area 38±2 - - - Zhonghou District 40±3 550±12 420±9 12.3±0.4 Thick wall area 42±3 - - -

[0105] illustrate:

[0106] “-” means that the test is not conducted in this area;

[0107] All indicators meet or exceed industry standards (residual stress ≤ 50MPa, graphite spheroidization rate 0%, tensile strength ≥ 500MPa, impact toughness ≥ 7J / cm 3 ).

[0108] Working principle: Utilize vacuum degassing and nano rare earth modifier, combined with eddy current stirring of adjustable frequency electromagnetic field, to spheroidize and disperse the non-metallic inclusions in the melt, thus ensuring high uniformity of chemical composition and graphite morphology;

[0109] By controlling the cooling cavity in three layers, namely thin-wall, medium-thick and thick-wall, and using PID to control the water cooling and air cooling flow, the local cooling rate can be differentiated, which greatly reduces the temperature gradient and alleviates the thermal stress caused by solidification shrinkage.

[0110] Fiber Bragg grating temperature and stress composite sensors are deployed to provide real-time feedback on the thermal field distribution during the pouring and initial setting stages. The PLC adaptively adjusts the cooling control and subsequent slow cooling and HIP parameters to ensure that residual stress is effectively suppressed.

[0111] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims, not the foregoing description, and all variations within the meaning and range of equivalents of the claims are intended to be included therein. Any reference sign in a claim should not be construed as limiting the claim to which it relates.

Claims

1. A casting method for a high-strength machine tool bed casting with reduced internal stress, characterized in that: The method comprises the following steps: The ductile iron or cast steel substrate is melted in an induction furnace and degassed in a vacuum environment; Add nano rare earth modifier to the melt and inject it at a spray speed of ≤10mm / s, so that Ce and La elements can achieve spheroidization and refinement of non-metallic inclusions in the melt; The modified melt is poured into the mold through the steel-sand-wrapped system, and an adjustable frequency alternating electromagnetic coil is arranged on the outer wall of the mold; During pouring, a rotating electromagnetic field of 5-15kHz is generated by an alternating electromagnetic coil, which causes the melt to form high-speed eddy current stirring in the cavity to promote uniform composition. At least three layers of gradient-controlled cooling chambers are set inside the casting mold, corresponding to the thin-walled area, medium-thick area and thick-walled area of ​​the bed respectively. The temperature is detected by thermocouples, and the zoned gradient cooling is achieved by the PID-controlled water cooling and air cooling system.

2. The method for casting a high-strength machine tool bed casting with reduced internal stress according to claim 1, characterized in that: The method further comprises: Fiber Bragg grating temperature and stress composite sensors are embedded in the mold surface and gate area to collect temperature and stress data. The sensors are then linked with the cooling control system and subsequent heat treatment devices through the PLC system for adaptive adjustment. After the initial solidification of the casting and before demoulding, the temperature in the mold is slowly lowered to 400-600℃ to release some thermal stress in advance; After demoulding, the casting is sent to a hot isostatic pressing (HIP) furnace and kept at 900-1000°C and 100-150 MPa for 2 hours to eliminate residual porosity and break condensation cracks. After HIP is completed, a step-by-step pressure reduction and temperature reduction process is implemented to gradually reduce the temperature to room temperature to prevent thermal stress from concentrating again; Before completing the final machining, perform X-ray diffraction or ultrasonic testing to confirm that the residual stress level meets ≤50MPa.

3. The method for casting a high-strength machine tool bed casting with reduced internal stress according to claim 1, characterized in that: The addition amount of the nano rare earth modifier is 0.05-0.15wt% of the melt mass.

4. The method for casting a high-strength machine tool bed casting with reduced internal stress according to claim 1, characterized in that: The steel-sand-wrapped system uses seamless steel pipes to wrap refractory castables, and sprays high-strength refractory coatings on the inner surface.

5. The method for casting a high-strength machine tool bed casting with reduced internal stress according to claim 1, characterized in that: The coil spacing and number of turns of the electromagnetic coil are optimized according to the cross-sectional shape of the bed to ensure that the magnetic induction intensity of the rotating electromagnetic field is within the range of 5-15 mT.

6. The method for casting a high-strength machine tool bed casting with reduced internal stress according to claim 1, characterized in that: The gradient-controlled cooling chamber adopts a modular and detachable design, and the number and position of partitions can be flexibly configured according to different bed structures.

7. The method for casting a high-strength machine tool bed casting with reduced internal stress according to claim 2, characterized in that: The fiber Bragg grating sensor has an accuracy of ±1°C and ±1 MPa, and a data acquisition frequency of ≥10 Hz.

8. The method for casting a high-strength machine tool bed casting with reduced internal stress according to claim 1, characterized in that: The adaptive feedback control system includes a PLC control unit, a human-machine interface and an algorithm server. The algorithm server optimizes the cooling and heat treatment curves in real time based on the finite element temperature field and stress field simulation model.

9. The method for casting a high-strength machine tool bed casting with reduced internal stress according to claim 2, characterized in that: After the HIP insulation stage is completed, a protective atmosphere is injected into the furnace, the gas composition of which is Ar + 2% hydrogen to inhibit high-temperature oxidation; Before final machining, the casting was scanned for residual stress at full size, and through comparative analysis, it was confirmed that the residual stress of each key section was ≤50MPa.

10. The method for casting a high-strength machine tool bed casting with reduced internal stress according to claim 1, characterized in that: The step cooling process is set with at least three temperature steps, with a temperature difference of 100-150℃ for each step and a holding time of ≥30min.

Images