Smelting and Production Method of QT800-2 Ductile Iron Piston Skirt Castings

Abstract

This invention provides a smelting and production method for QT800-2 ductile iron piston skirt castings, belonging to the field of casting technology. Specifically: Step 1) Set the chemical composition before furnace preparation; Step 2) The raw material composition and weight percentage are as follows: pig iron 20%~30%, carbon scrap steel 40%~50%, and the remainder is ductile iron remelting material; Step 3) Melting; Step 4) During spheroidizing treatment: Place 1.1%~1.3% of spheroidizing agent on the side of the spheroidizing pit of the spheroidizing ladle, mix 0.02%~0.04% of tin and 0.008%~0.015% of antimony with the spheroidizing agent, spread it evenly to the bottom of the spheroidizing pit and compact it appropriately, cover it with 0.2% calcium barium inoculant, spread it evenly and compact it appropriately, cover it with silicon steel sheet and spread it evenly, and finally cover it with iron plate and press several pieces of pig iron around it; Step 5) Pouring; Step 6) When the casting temperature drops to 800℃~850℃ after pouring, open the box and air cool, and sprinkle a layer of dry sand on the casting. By employing appropriate chemical compositions and adjusting the smelting process, castings with perfect mechanical properties can be obtained without altering the molding conditions or prematurely opening the mold.

Description

Technical Field

[0001] This invention belongs to the field of casting technology, specifically relating to a smelting and production method for QT800-2 ductile iron piston skirt castings. Background Technology

[0002] The piston skirt, a key component of a certain type of marine diesel engine, requires a material with high strength, high thermal fatigue strength, high wear resistance, and a certain degree of toughness. The material grade is QT800-2, making its production extremely difficult. For a long time, it has been necessary to import foreign products, which has seriously restricted my country's independent production.

[0003] In the past, the production of high-grade ductile iron QT800-2 using resin sand often resulted in insufficient tensile strength, excessive ferrite content in the matrix, or insufficient elongation. When producing as-cast ductile iron QT800-2 using resin sand, the good insulating effect of the resin sand often led to excessive ferrite content in the matrix and insufficient tensile strength. Furthermore, increasing the alloy content or prematurely opening the casting mold could cause cracking and insufficient elongation in the material properties. Ordinary QT800-2 can only meet the requirements for conventional mechanical properties, failing to achieve the desired thermal fatigue performance and wear resistance. Therefore, improvements are necessary. Summary of the Invention

[0004] The technical problem solved by this invention is to provide a melting and production method for QT800-2 ductile iron piston skirt castings. The purpose of this invention is to obtain perfect castings and qualified mechanical properties by using reasonable chemical composition and adjusting the melting process without changing the molding conditions or prematurely opening the mold.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] Smelting and production method of QT800-2 ductile iron piston skirt casting

[0007] Step 1): Set the pre-furnace chemical composition as shown in Table 1 below:

[0008] Table 1 Chemical composition before furnace operation

[0009] C Si Mn P S Cr Ni Mo Cu Ti 3.70～3.90 1.1～1.2 0.75～0.85 ≤0.08 ≤0.015 ＜0.05 ＜0.05 0.4～0.5 0.75～0.85 ≤0.05

[0010] The carbon equivalent of the casting is approximately 4.3–4.5, and the final silicon content is approximately 1.8–2.0. It is alloyed with manganese, molybdenum, copper, tin, and antimony.

[0011] Step 2): The composition and weight percentage of raw materials are as follows: pig iron 20%-30%, carbon scrap steel 40%-50%, and the remainder is ductile iron remelting material;

[0012] Step 3): Melting method: First, add 20% to 30% pig iron, 40% carbon scrap steel, and ferrosilicon, ferromanganese, ferromolybdenum, and copper according to the lower limit of the process. After the pig iron and carbon scrap steel in the furnace are completely melted, add the remaining carbon scrap steel and ductile iron return material. After complete melting, remove the surface slag, raise the temperature to above 1420℃, and take spectral and chemical samples 100mm to 120mm below the surface of the molten iron for chemical composition analysis. If the composition meets the set pre-furnace chemical composition, the temperature can be raised to the tapping temperature; otherwise, add alloys or carbon additives according to the composition difference.

[0013] Step 4): Spheroidization treatment:

[0014] Selection of spheroidization and inoculation methods: The spheroidization method is selected, with a spheroidization temperature of 1430℃~1550℃;

[0015] Spheroidizing and inoculation: Place 1.1%–1.3% spheroidizing agent on the side of the spheroidizing pit of the spheroidizing ladle. Mix 0.02%–0.04% tin and 0.008%–0.015% antimony with the spheroidizing agent, spread it evenly to the bottom of the spheroidizing pit, and compact it appropriately. Cover it with 0.2% calcium-barium inoculant, spread it evenly, and compact it appropriately. Cover it with silicon steel sheets the size of sunflower seed shells and spread them evenly. Finally, cover it with a pressing iron plate and press several pieces of pig iron around it. When exiting the furnace, use 0.3%–0.4% silicon-manganese-zirconium inoculant for inoculation.

[0016] Step 5): Specific operation method of casting: After the spheroidizing inoculation is completed, remove the slag from the surface of the molten iron, place slag-blocking cotton at the ladle nozzle, and transport it to the sand box and align it with the pouring cup. At the same time, prepare for instant inoculation using an instant inoculant funnel. When the molten iron is poured out, open the instant inoculant funnel to allow the instant inoculant to flow evenly onto the stream of molten iron flowing out from the ladle nozzle, and perform instant inoculation on all the molten iron. The instant inoculant uses silicon manganese zirconium or antimony-containing inoculant with a dosage of 0.1% to 0.2% and a particle size of 0.2 mm to 0.7 mm.

[0017] Step 6): After pouring, use a thermocouple pre-embedded in the sand mold to measure the temperature of the casting in real time. When the casting temperature drops to 800℃-850℃, according to long-term statistics, it takes about 3-5 hours after pouring to open the mold and air cool. When the casting is close to dark red and about 600℃, sprinkle a layer of dry sand about 10mm to 20mm thick on the casting for heat preservation.

[0018] In step 3) above, the furnace exit temperature is 1480℃~1510℃.

[0019] In step 4) above, the spheroidizing agent is a rare earth magnesium alloy spheroidizing agent.

[0020] In step 4) above, the particle size of the silicon manganese zirconium inoculant is 3-7 mm, the particle size of the 0.02% to 0.04% tin is 10 mm to 20 mm, and the particle size of the 0.008% to 0.015% antimony is 10 mm to 20 mm.

[0021] In step 6) above, the thickness of the dry sand sprinkled on the casting is 10mm to 20mm.

[0022] Advantages of this invention compared to existing technologies:

[0023] 1. This scheme adopts a reasonable chemical composition and adjusts the smelting process to produce qualified QT800-2 ductile iron piston skirt castings without changing the molding conditions or opening the mold in advance. After mechanical property testing, magnetic particle testing and ultrasonic testing, the casting qualification rate reaches 95%.

[0024] 2. In this scheme, during the spheroidizing and inoculation process, the addition of alloyed tin and antimony can lead to burn-off at temperatures exceeding 1000℃, causing the tin and antimony to vaporize and affecting the chemical composition of the molten metal. Therefore, 0.02%–0.04% of alloyed tin (10mm–20mm particle size) and 0.008%–0.015% of antimony (10mm–20mm particle size) are mixed with the spheroidizing agent, spread evenly at the bottom of the spheroidizing pit, and appropriately compacted. During the spheroidizing reaction, the tin and antimony are thoroughly stirred and mixed, ensuring the quality of the molten metal.

[0025] 3. In this design, the piston skirt casting has a relatively thin wall, with the smallest being around 10mm. Therefore, delaying the opening of the casting mold will cause the casting performance to fail to meet requirements. Furthermore, the rapid cooling rate after opening the mold can easily lead to cracks in the casting, affecting its quality and causing carbide formation, which hinders machining. Therefore, this method involves opening the mold and air-cooling the casting approximately 3-5 hours after pouring, when the casting temperature reaches 800℃~850℃. A 10-20mm thick layer of dry sand is then sprinkled on the casting. This prevents the casting from being directly exposed to air after opening the mold, allowing it to cool slowly and effectively preventing defects such as cracks. Detailed Implementation

[0026] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0027] The embodiments of the present invention are described in detail below.

[0028] Smelting and production method of QT800-2 ductile iron piston skirt casting

[0029] Step 1): Set the pre-furnace chemical composition as shown in Table 1 below: Table 1 Pre-furnace Chemical Composition

[0030] C Si Mn P S Cr Ni Mo Cu Ti 3.70～3.90 1.1～1.2 0.75～0.85 ≤0.08 ≤0.015 ＜0.05 ＜0.05 0.4～0.5 0.75～0.85 ≤0.05

[0031] The carbon equivalent of the casting is 4.3 to 4.5, the final silicon is 1.8 to 2.0, and it is alloyed with manganese, molybdenum, copper, tin and antimony.

[0032] Step 2): The composition and weight percentage of raw materials are as follows: pig iron 20%-30%, carbon scrap steel 40%-50%, and the remainder is ductile iron remelting material;

[0033] Step 3): Melting method: First, add 20%–30% pig iron, 40% carbon scrap steel, and ferrosilicon, ferromanganese, ferromolybdenum, and copper according to the lower limit of the process. After the pig iron and carbon scrap steel in the furnace are completely melted, add the remaining carbon scrap steel and ductile iron return material. After complete melting, remove the surface slag, raise the temperature to above 1420℃, and take spectral and chemical samples 100mm–120mm below the surface of the molten iron for chemical composition analysis. If the composition meets the set pre-furnace chemical composition, the temperature can be raised to the tapping temperature; otherwise, add alloys or carbonizers according to the composition difference. The tapping temperature is 1480℃–1510℃.

[0034] Step 4): Spheroidization treatment:

[0035] Selection of spheroidization and inoculation methods: The spheroidization method is selected, with a spheroidization temperature of 1430-1550℃;

[0036] Spheroidizing and inoculation process: 1.1%–1.3% rare earth magnesium alloy spheroidizing agent is placed on the side of the spheroidizing pit in the spheroidizing ladle. 0.02%–0.04% tin (particle size 10mm–20mm) and 0.008%–0.015% antimony (particle size 10mm–20mm) are mixed with the spheroidizing agent, spread evenly to the bottom of the pit, and appropriately compacted. 0.2% calcium barium inoculant is then placed on top, spread evenly, and appropriately compacted. Silicon steel sheets the size of sunflower seed shells are then spread evenly on top, and finally, a pressing iron plate is placed on top, with several pieces of pig iron pressed around the edges. During the spheroidizing reaction, the tin and antimony are thoroughly stirred and mixed. Upon exiting the furnace, 0.3%–0.4% silicon manganese zirconium inoculant is used for inoculation; preferably, the particle size of the silicon manganese zirconium inoculant is 3–7mm. The spheroidizing agent is a rare earth magnesium alloy.

[0037] In the above-mentioned spheroidizing and inoculation process, the addition of alloyed tin and antimony can lead to burn-off at temperatures exceeding 1000℃, causing the tin and antimony to vaporize and affecting the chemical composition of the molten metal. Therefore, mixing 0.04% tin and 0.008% antimony with the spheroidizing agent, spreading it evenly at the bottom of the spheroidizing pit, and compacting it appropriately allows the tin and antimony to be fully stirred and mixed during the spheroidizing reaction, ensuring the quality of the molten metal.

[0038] Step 5): Specific operation method for casting: After spheroidizing inoculation is completed, remove the slag from the surface of the molten iron, place slag-blocking cotton at the ladle nozzle, and transport it to the sand box and align it with the pouring cup. At the same time, prepare for instant inoculation using an instant inoculant funnel. When the molten iron is poured out, open the instant inoculant funnel to allow the instant inoculant to flow evenly onto the stream of molten iron flowing out from the ladle nozzle, and perform instant inoculation on all the molten iron. The instant inoculant uses 0.1% to 0.2% silicon manganese zirconium or antimony-containing inoculant with a particle size of 0.2 mm to 0.7 mm.

[0039] Step 6): After pouring, use a thermocouple pre-embedded in the sand mold to measure the temperature of the casting in real time. When the casting temperature drops to 800℃~850℃, according to long-term statistics, it takes about 3-5 hours after pouring to open the mold and air cool. When the casting is close to dark red, about 600℃, sprinkle a layer of dry sand about 10mm~20mm thick on the casting for heat preservation.

[0040] Because the piston skirt casting has a relatively thin wall, with the smallest being around 10mm, delaying the opening of the casting mold will cause the casting performance to fail to meet requirements. Furthermore, the rapid cooling rate after opening the mold can easily lead to cracks in the casting, affecting its quality and causing carbide formation, which hinders machining. Therefore, this method involves opening the mold and air-cooling the casting approximately 3-5 hours after pouring. When the casting approaches a dark red color (approximately 600℃), a layer of dry sand about 10mm-20mm thick is sprinkled on it for insulation. This prevents the casting from being continuously exposed to air after opening the mold, allowing it to cool slowly and effectively preventing defects such as cracks.

[0041] This invention utilizes a reasonable chemical composition and adjusted smelting process to produce qualified QT800-2 ductile iron piston skirt castings without altering the molding conditions or prematurely opening the mold. After mechanical property testing, magnetic particle testing, and ultrasonic testing, the casting qualification rate reached 95%. The mechanical property values ​​of individual casting samples are shown in Table 2 below:

[0042] Table 2 shows the mechanical properties of the single-cast specimens.

[0043] Testing items tensile strength Yield strength elongation hardness National Standard Requirements ≥800 ≥480 ≥2 245-335 actual 853 557 5 266

[0044] After dissection of the casting, the mechanical properties of the casting were tested and are shown in Table 3 below:

[0045] Testing items tensile strength Yield strength elongation hardness National Standard Requirements ≥700 ≥420 ≥2 225-305 actual 773 521 6 266 .

[0046] 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 implemented 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 exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

[0047] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A melting production method of a QT800-2 nodular cast iron piston skirt casting, characterized in that: Step 1): the chemical composition of the furnace front is set as shown in Table 1: Table 1: Chemical composition of the furnace front The carbon equivalent of the casting is 4.3-4.5, the final silicon is 1.8-2.0, and manganese, molybdenum, copper, tin, and antimony are alloyed; Step 2): the raw material composition and weight percentage are as follows: 20%-30% pig iron, 40%-50% carbon scrap steel, and the rest is nodular iron return material; Step 3): melting method: first add 20%-30% pig iron, 40% carbon scrap steel, and according to the process lower limit, add ferrosilicon, ferromanganese, ferromolybdenum, and copper. After the pig iron and carbon scrap steel in the furnace are completely melted, add the remaining carbon scrap steel and nodular iron return material. After complete melting, remove the surface dross, raise the temperature to above 1420°C, take a spectrum sample and a chemical sample 100-120 mm below the surface of the molten iron to analyze the chemical composition. If the composition meets the set furnace front chemical composition, the temperature can be raised to the tapping temperature. Otherwise, add alloy or carbon additive according to the composition difference; Step 4): spheroidization treatment: Spheroidization and inoculation method selection: choose impingement method spheroidization, spheroidization temperature 1430-1550°C; Spheroidization package furnace and inoculation: put 1.1%-1.3% spheroidizing agent into the spheroidizing pit side, mix 0.02%-0.04% tin and 0.008%-0.015% antimony with the spheroidizing agent, lay flat to the bottom of the spheroidizing pit and compact appropriately, cover 0.2% calcium bar inoculant on top, lay flat and compact appropriately, cover silicon steel sheet the size of melon seeds on top, finally cover and press iron plate and press several pieces of pig iron around; use 0.3%-0.4% silicon manganese zirconium inoculant for impingement inoculation when tapping; Step 5): pouring: after spheroidization and inoculation, remove the surface dross of the molten iron, place slag stopping cotton at the package nozzle, adjust and transport to the sand box to align the pouring cup. At the same time, prepare the transient inoculant hopper for transient inoculation. When the molten iron is poured out, open the transient inoculant hopper to make the transient inoculant flow uniformly onto the molten iron stream from the pouring nozzle, and perform transient inoculation on all the molten iron. The transient inoculant uses silicon manganese zirconium or antimony-containing inoculant with a dosage of 0.1%-0.2% and a particle size of 0.2-0.7 mm; Step 6): after pouring, use a pre-embedded thermocouple in the sand mold to measure the temperature of the casting in real time. When the casting temperature drops to 800-850°C, according to long-term statistics, the casting can be opened and air cooled after 3-5 hours of pouring. When the casting approaches dark red, i.e. 600°C, a layer of dry sand is sprinkled on the casting for heat preservation. In the above step 3), the tapping temperature is 1480-1510°C.

2. The QT800-2 ductile iron piston skirt casting melting production method according to claim 1, characterized in that: In the above step 4), the spheroidizing agent is rare earth magnesium alloy.

3. The QT800-2 ductile iron piston skirt casting melting production method according to claim 1, characterized in that: In the above step 4), the particle size of the silicon manganese zirconium inoculant is 3-7 mm, the particle size of the 0.02%-0.04% tin is 10-20 mm, and the particle size of the 0.008%-0.015% antimony is 10-20 mm.

4. The QT800-2 ductile iron piston skirt casting melting production method according to claim 1, characterized in that: In the above step 6), the thickness of the dry sand sprinkled on the casting is 10-20 mm.