High-strength high-toughness nodular cast iron crankshaft casting process

Abstract

The application discloses a high-strength and high-toughness nodular cast iron crankshaft casting process, which comprises the following steps: step one, component optimization; step two, smelting and pretreatment; step three, spheroidizing and inoculation treatment; step four, pouring system design; step five, solidification control; step six, isothermal quenching; step seven, surface strengthening; step eight, finishing and detection; the application improves the hardenability by nickel element, forms upper bainite through isothermal quenching, combines three times of inoculation to refine graphite, and realizes the balance between strength and toughness. The stepped pouring system and the cold iron layout effectively eliminate shrinkage holes, and the process yield is increased to 75%. The ion nitriding and laser shock superposition significantly improve the surface hardness, wear resistance and fatigue strength, and meet the demand of high-load working conditions. Compared with forged steel crankshafts, the weight is reduced by 12%, the cost is saved by 35%, and the green manufacturing requirement is met.

Description

TECHNICAL FIELD

[0001] The application relates to the casting technology field of nodular cast iron crankshafts, in particular to a nodular cast iron crankshaft casting process with high strength and high toughness. BACKGROUND

[0002] At present, nodular cast iron crankshafts are widely used in medium and low load engines, but the elongation and impact toughness thereof still have a gap compared with forged steel crankshafts. For example, the elongation of a QT900-5 crankshaft prepared by Chinese patent ZL201310335464.3 is only about 5%, while the elongation of a forged steel crankshaft is generally more than 9%. In addition, the traditional casting process is prone to defects such as shrinkage and porosity, and the insufficient hardenability leads to the difficulty in adopting an efficient heat treatment process. Therefore, it is of great significance to develop a nodular cast iron crankshaft casting process with high strength, high toughness and strong process stability. SUMMARY

[0003] The application aims to provide a nodular cast iron crankshaft casting process with high strength and high toughness to solve the problems in the background technology.

[0004] To achieve the above-mentioned purpose, the application provides the following technical scheme: a nodular cast iron crankshaft casting process with high strength and high toughness, comprising the following steps:

[0005] Step one, component optimization: the components of the nodular cast iron crankshaft are optimized;

[0006] Step two, smelting and pretreatment: smelting is carried out by using a medium-frequency induction furnace, and the furnace charge includes scrap steel, pig iron and alloy elements; slag is removed when the molten iron temperature rises to 1400 DEG C, and then metal magnesium particles with surface passivation are sprayed to carry out desulfurization, and the sulfur content is controlled to be less than or equal to 0.015%;

[0007] Step three, spheroidizing and inoculation treatment: spheroidizing treatment is carried out by using the impingement method, 1.3-1.5% of rare earth magnesium spheroidizing agent (FeSiMg8RE5) is added, and the treatment temperature is 1430-1460 DEG C; and then three times of inoculation is carried out;

[0008] Step four, design of pouring system: a stepped open pouring system is used, and multiple inner gates and blind risers are arranged;

[0009] Step five, solidification control: air flow impact molding is used, the water content of the molding sand is 4.5-5.0%, and the plane hardness of the mold is greater than or equal to 90; the pouring temperature is controlled to be 1290-1360 DEG C, and after pouring, the cooling rate is controlled by using the conformal cooling channel, and the cooling speed of the key parts reaches 5-8 DEG C / s, so as to refine the pearlite structure;

[0010] Step six, isothermal quenching: the casting blank is heated to 880℃ for 3 hours, and then quickly transferred to a molten salt bath at 230-400℃ for isothermal quenching for 1-2.5 hours to obtain upper bainite structure 2-3 levels. The process can make the tensile strength of the crankshaft ≥900MPa, the elongation ≥10%, and the impact toughness ≥190J / cm 2 ;

[0011] Step seven, surface strengthening: using ion nitriding process, ammonia and carbon dioxide mixed gas is introduced at 530℃ for 4 hours, forming a 0.01-0.012mm compound layer and a 0.10-0.13mm diffusion layer, the surface hardness is ≥700HV, the fatigue strength is increased by more than 15%; at the same time, the laser shock peening is carried out on the fillet transition area;

[0012] Step eight, finishing and detection: after casting, the crankshaft is sandblasted, polished and trimmed to remove surface burrs and defects; then strict detection is carried out, including metallographic structure, mechanical properties and fatigue performance test to ensure that all indexes of the crankshaft meet the design requirements; in addition, metallographic structure analysis is also needed for the key parts of the crankshaft to verify whether the microstructure meets the expected high strength and high toughness standards.

[0013] Preferably, the chemical composition of the nodular cast iron crankshaft in step one is: C 3.6-3.8%, Si 2.1-2.3%, Mn 0.25-0.35%, P≤0.038%, S≤0.015%, Mg 0.034-0.039%, RE 0.027-0.029%, Cu 0.5-0.7%, Ni 0.3-0.5%, and the balance is Fe. Among them, the nickel element can refine the grain and improve the hardenability, providing conditions for subsequent isothermal quenching.

[0014] Preferably, the chemical composition of the nodular cast iron crankshaft is: C 3.6%, Si 2.1%, Mn 0.25%, P 0.038%, S 0.015%, Mg 0.034%, RE 0.027%, Cu 0.5%, Ni 0.3%, and the balance is Fe.

[0015] Preferably, the chemical composition of the nodular cast iron crankshaft is: C 3.8%, Si 2.3%, Mn 0.35%, P 0.05%, S 0.03%, Mg 0.039%, RE 0.029%, Cu 0.7%, Ni 0.5%, and the balance is Fe.

[0016] Preferably, the chemical composition of the spheroidal graphite cast iron crankshaft is as follows in terms of percentage by weight: C 3.7%, Si 2.2%, Mn 0.30%, P 0.045%, S 0.025%, Mg 0.036%, RE 0.028%, Cu 0.6%, Ni 0.4%, and the balance of Fe.

[0017] Preferably, the three inoculations in the step three are as follows: 0.7-0.9% of 75FeSi is added into the tundish for primary inoculation, 0.12-0.15% of SRC high-efficiency inoculant is added into the stream for secondary inoculation during pouring, and 0.05% of bismuth-based inoculant is added into the sprue cup for tertiary inoculation in the late pouring stage, so as to ensure that the spheroidization level is 1-2 grade and the graphite size is 5-8 grade.

[0018] Preferably, the riser diameter D = 1.2T in the step four, T is the diameter of the hot spot circle, and the riser neck is short and thin and wide to realize self-adaptive feeding; cold iron is arranged at key positions (such as main journal), and the size of the cold iron is determined according to the hot spot analysis to ensure that the solidification sequence is reasonable.

[0019] Preferably, in the step seven: the flow ratio of the mixed gas of ammonia and carbon dioxide is 1:0.3; the laser power density is 3-5 GW / cm 2 , the pulse width is 10-20 ns, a compressive stress layer with a depth of 1.0 mm is formed, and the residual compressive stress is greater than or equal to -300 MPa.

[0020] Compared with the prior art, the beneficial effects of the present application are:

[0021] The present application improves the hardenability by increasing the content of nickel, forms upper bainite through isothermal quenching, refines graphite through three inoculations, and realizes the balance between strength and toughness. The stepped pouring system and the cold iron layout effectively eliminate shrinkage holes, and the process yield is increased to 75%. The surface hardness, wear resistance and fatigue strength are significantly improved through the superposition of ion nitriding and laser shock, which meets the demand of high load working condition. Compared with forged steel crankshafts, the weight is reduced by 12%, the cost is saved by 35%, and the green manufacturing requirement is met. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is the process flow chart of the present application. DETAILED DESCRIPTION

[0023] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0024] Please refer to Figure 1The application provides a high-strength and high-toughness nodular cast iron crankshaft casting process, which comprises the following steps.

[0025] Step one, component optimization: the component of the nodular cast iron crankshaft is optimized; the chemical component of the nodular cast iron crankshaft is as follows in terms of percentage by weight: C 3.6-3.8%, Si 2.1-2.3%, Mn 0.25-0.35%, P≤0.038%, S≤0.015%, Mg 0.034-0.039%, RE 0.027-0.029%, Cu 0.5-0.7%, Ni 0.3-0.5%, and the balance is Fe; wherein, the nickel element can refine grains and improve hardenability, and provides conditions for subsequent isothermal quenching;

[0026] Step two, smelting and pretreatment: a medium-frequency induction furnace is used for smelting, and the furnace charge comprises scrap steel, pig iron and alloy elements; slag is removed when the molten iron temperature rises to 1400 DEG C, and then surface passivated magnesium particles are sprayed to carry out desulfurization, and the sulfur content is controlled to be ≤0.015%;

[0027] Step three, spheroidizing and inoculation treatment: the spheroidizing treatment is carried out by using the impingement method, 1.3-1.5% of rare earth magnesium spheroidizing agent (FeSiMg8RE5) is added, and the treatment temperature is 1430-1460 DEG C; then three times of inoculation is carried out; the three times of inoculation is specifically as follows: 0.7-0.9% of 75FeSi is added into the iron runner for the first inoculation, 0.12-0.15% of SRC high-efficiency inoculant is added into the stream for the second inoculation during pouring, and 0.05% of bismuth-based inoculant is added into the sprue cup for the third inoculation in the late pouring stage, so that the spheroidizing level is ensured to be 1-2 levels, and the graphite size is ensured to be 5-8 levels;

[0028] Step four, pouring system design: a stepped open pouring system is used, and a plurality of inner gates and blind risers are arranged; the riser diameter D is equal to 1.2T, T is the diameter of the hot spot circle, and the riser neck is short, thin and wide to realize self-adaptive feeding; cold iron is arranged at key positions (such as the main journal), the cold iron size is determined according to the hot spot analysis, and the solidification sequence is ensured to be reasonable;

[0029] Step five, solidification control: airflow impact molding is used, the water content of the molding sand is 4.5-5.0%, and the plane hardness of the mold is greater than or equal to 90; the pouring temperature is controlled to be 1290-1360 DEG C, and the cooling rate is controlled by using the conformal cooling channel after pouring, and the cooling speed of the key position reaches 5-8 DEG C / s, so as to refine the pearlite structure;

[0030] Step six, isothermal quenching: the cast blank is heated to 880 DEG C and kept for 3 hours, and then is quickly transferred to a 230-400 DEG C molten salt bath for isothermal quenching for 1-2.5 hours, so that the upper bainite structure is obtained and the level is 2-3; the process can make the tensile strength of the crankshaft greater than or equal to 900 MPa, the elongation greater than or equal to 10%, and the impact toughness greater than or equal to 190 J / cm 2 ;

[0031] Step seven, surface strengthening: using ion nitriding process, at 530℃, ammonia and carbon dioxide mixed gas is passed in, heat preservation for 4 hours, forming 0.01-0.012mm of compound layer and 0.10-0.13mm of diffusion layer, surface hardness ≥700HV, fatigue strength is increased by more than 15%; at the same time, the laser shock reinforcement is carried out on the fillet transition area; the flow ratio of ammonia and carbon dioxide mixed gas is 1:0.3; laser power density is 3-5GW / cm 2 , pulse width is 10-20ns, forming depth 1.0mm of compressive stress layer, residual compressive stress ≥-300MPa;

[0032] Step eight, finishing and detection: after casting, the crankshaft is sand cleaned, polished and trimmed to remove burrs and defects on the surface; then strict detection is carried out, including metallographic structure, mechanical properties and fatigue performance test, to ensure that all indexes of the crankshaft meet the design requirements; in addition, metallographic structure analysis is also needed for the key parts of the crankshaft to verify whether the microstructure meets the expected high strength and high toughness standards.

[0033] Example 1:

[0034] A high-strength and high-toughness nodular cast iron crankshaft casting process, comprising the following steps:

[0035] Composition optimization

[0036] The composition of the nodular cast iron crankshaft is optimized; the chemical composition of the nodular cast iron crankshaft is as follows: C 3.6%, Si 2.1%, Mn 0.25%, P 0.038%, S 0.015%, Mg 0.034%, RE 0.027%, Cu 0.5%, Ni 0.3%, and the balance is Fe;

[0037] Melting and pretreatment

[0038] Scrap steel and pig iron are added to the medium-frequency furnace in proportion, and the temperature is raised to 1400℃ to remove slag. Desulfurization is carried out by spraying passivated magnesium particles (ventilation flow rate 2.0m 3 / h), and the sulfur content is reduced to 0.013%.

[0039] Spheroidizing and inoculation

[0040] 1.4% of rare earth magnesium spheroidizing agent is added to a 2t ladle, and the treatment temperature is 1450℃. Then 0.8% of 75FeSi, 0.13% of SRC inoculant and 0.05% of bismuth-based inoculant are added in turn.

[0041] Pouring and solidification

[0042] The casting temperature is 1320℃ with a step gating system. A graphite chill with dimensions of φ80x50mm is placed at the main journal area. The mold hardness is 92 for the flat surface and 87 for the side surface.

[0043] Heat treatment

[0044] The blank is heated to 880℃ for 3 hours and then transferred to a 300℃ salt bath for isothermal quenching for 2 hours. Subsequently, ion nitriding is performed at a furnace pressure of 500Pa for 4 hours.

[0045] Surface strengthening

[0046] The fillet area is subjected to laser shock processing with a spot diameter of 2mm, a pulse energy of 15J, and a coverage rate of 100%. Finishing and detection

[0047] After casting, the crankshaft is subjected to sand cleaning, grinding, and trimming to remove surface burrs and defects. Subsequently, strict detection is performed, including metallographic structure, mechanical properties, and fatigue performance tests to ensure that all indicators of the crankshaft meet the design requirements. In addition, metallographic structure analysis is performed on the key parts of the crankshaft to verify whether the microstructure meets the expected high strength and high toughness standards.

[0048] Example 2:

[0049] A high-strength and high-toughness nodular cast iron crankshaft casting process, comprising the following steps:

[0050] Component optimization

[0051] The composition of the nodular cast iron crankshaft is optimized. The chemical composition of the nodular cast iron crankshaft is as follows: C 3.8%, Si 2.3%, Mn 0.35%, P 0.05%, S 0.03%, Mg 0.039%, RE 0.029%, Cu 0.7%, Ni 0.5%, and the balance is Fe.

[0052] Melting and pretreatment

[0053] Scrap steel and pig iron are added to the intermediate frequency furnace in proportion, and the temperature is raised to 1400℃ for slagging. Desulfurization is performed by spraying passivated magnesium particles (ventilation flow rate 2.0m 3 / h) to reduce the sulfur content to 0.013%.

[0054] Spheroidizing and inoculation

[0055] 1.4% of rare earth magnesium spheroidizing agent is added to a 2t ladle at a treatment temperature of 1450℃. Subsequently, 0.8% of 75FeSi, 0.13% of SRC inoculant, and 0.05% of bismuth-based inoculant are added in sequence.

[0056] Pouring and solidification

[0057] The casting temperature is 1320℃ with a step gating system. A graphite chill with dimensions of φ80x50mm is placed at the main journal area. The mold hardness is 92 for the flat surface and 87 for the side surface.

[0058] Heat treatment

[0059] The blank is heated to 880℃ and held for 3 hours before being transferred to a 300℃ salt bath for isothermal quenching for 2 hours. Subsequently, ion nitriding is performed at a furnace pressure of 500Pa for 4 hours.

[0060] Surface strengthening

[0061] The fillet area is subjected to laser shock processing with a spot diameter of 2mm, a pulse energy of 15J, and a coverage rate of 100%. Finishing and detection

[0062] After casting is completed, the crankshaft is subjected to sand cleaning, grinding, and trimming to remove surface burrs and defects. Subsequently, strict detection is performed, including metallographic structure, mechanical properties, and fatigue performance tests to ensure that all indicators of the crankshaft meet the design requirements. In addition, metallographic structure analysis is performed on the key parts of the crankshaft to verify whether the microstructure meets the expected high strength and high toughness standards.

[0063] Example 3:

[0064] A high-strength and high-toughness nodular cast iron crankshaft casting process, comprising the following steps:

[0065] Component optimization

[0066] The components of the nodular cast iron crankshaft are optimized. The chemical composition of the nodular cast iron crankshaft is as follows: C 3.7%, Si 2.2%, Mn 0.30%, P 0.045%, S 0.025%, Mg 0.036%, RE 0.028%, Cu 0.6%, Ni 0.4%, and the balance is Fe.

[0067] Melting and pretreatment

[0068] Scrap steel and pig iron are added to the intermediate frequency furnace in proportion, and the temperature is raised to 1400℃ to remove slag. Desulfurization is performed by spraying passivated magnesium particles (ventilation flow rate 2.0m 3 / h) to reduce the sulfur content to 0.013%.

[0069] Spheroidizing and inoculation

[0070] 1.4% of rare earth magnesium spheroidizing agent is added to a 2t ladle at a treatment temperature of 1450℃. Subsequently, 0.8% of 75FeSi, 0.13% of SRC inoculant, and 0.05% of bismuth-based inoculant are added in sequence.

[0071] Pouring and solidification

[0072] The casting temperature is 1320℃ with a step gating system. A graphite chill with dimensions of φ80x50mm is placed at the main journal position. The mold hardness is 92 for the flat surface and 87 for the side surface.

[0073] Heat treatment

[0074] The blank is heated to 880℃ for 3 hours and then transferred to a 300℃ salt bath for isothermal quenching for 2 hours. Subsequently, ion nitriding is performed at a furnace pressure of 500Pa for 4 hours.

[0075] Surface strengthening

[0076] The fillet area is subjected to laser shock processing with a spot diameter of 2mm, a pulse energy of 15J, and a coverage rate of 100%. Finishing and detection

[0077] After casting, the crankshaft is subjected to sand cleaning, grinding, and trimming to remove surface burrs and defects. Subsequently, strict detection is performed, including metallographic structure, mechanical properties, and fatigue performance tests to ensure that the crankshaft meets the design requirements. In addition, metallographic structure analysis is performed on the key parts of the crankshaft to verify whether the microstructure meets the expected high strength and high toughness standards.

[0078] The detection results of the nodular cast iron crankshaft prepared according to the above embodiments 1-3 are as follows:

[0079] Metallographic structure: spheroidization level 2, graphite size 6, pearlite 90%, upper bainite 2.

[0080] Mechanical properties: tensile strength 980MPa, elongation 12.5%, impact toughness 195J / cm 2 , hardness 290HBW.

[0081] Fatigue performance: bending fatigue torque 3500N·m, increased by more than 20% compared with forged steel crankshafts.

[0082] Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can modify the technical solutions described in the foregoing embodiments or make equivalent replacements to some technical features, and any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A high-strength, high-toughness ductile iron crankshaft casting process, characterized in that: Includes the following steps: Step 1: Composition Optimization: The composition of the ductile iron crankshaft is optimized; the chemical composition of the ductile iron crankshaft, by weight percentage, is: C 3.6-3.8%, Si 2.1-2.3%, Mn 0.25-0.35%, P≤0.038%, S≤0.015%, Mg0.034-0.039%, RE 0.027-0.029%, Cu 0.5-0.7%, Ni 0.3-0.5%, with the balance being Fe; Step 2, Smelting and Pretreatment: Smelting is carried out in a medium-frequency induction furnace. The furnace charge includes scrap steel, pig iron, and alloying elements. When the molten iron temperature reaches 1400℃, the slag is removed, and then passivated magnesium particles are injected for desulfurization, controlling the sulfur content to ≤0.015%. Step 3, Spheroidization and Inoculation Treatment: Spheroidization is performed using the pouring method, with 1.3-1.5% rare earth magnesium spheroidizing agent added at a treatment temperature of 1430-1460℃; followed by three inoculation processes; the three inoculation processes are as follows: 0.7-0.9% 75FeSi is added to the iron outlet trough for the first inoculation, 0.12-0.15% SRC high-efficiency inoculant is added during casting for the second inoculation, and 0.05% bismuth-based inoculant is added to the pouring cup for the third inoculation in the later stage of casting, to ensure a spheroidization level of 1-2 and a graphite size of 5-8. Step 4, Gating System Design: A stepped open gating system is adopted, with multiple endogates and concealed risers; Step 5, Solidification Control: Airflow impact molding is adopted, the moisture content of molding sand is 4.5-5.0%, and the hardness of the mold plane is ≥90; the pouring temperature is controlled at 1290-1360℃, and the cooling rate is controlled by conformal cooling channels after pouring, with the cooling rate of key parts reaching 5-8℃ / s, in order to refine the pearlite structure. Step 6, isothermal quenching: Heat the cast blank to 880℃ and hold for 3 hours, then quickly transfer it to a molten salt bath at 230-400℃ for isothermal quenching for 1-2.5 hours to obtain upper bainite structure of grade 2-3. Step 7, Surface Strengthening: Using ion nitriding process, a mixture of ammonia and carbon dioxide gas is introduced at 530℃ and held for 4 hours to form a chemical layer of 0.01-0.012mm and a diffusion layer of 0.10-0.13mm. The surface hardness is ≥700HV and the fatigue strength is increased by more than 15%. At the same time, laser shock strengthening is performed on the rounded corner transition area. Step 8, Finishing and Inspection: After casting, the crankshaft is cleaned, polished and finished to remove burrs and defects from the surface; then, rigorous inspections are carried out, including metallographic structure, mechanical properties and fatigue performance tests, to ensure that all indicators of the crankshaft meet the design requirements; in addition, metallographic structure analysis is required for key parts of the crankshaft to verify whether its microstructure meets the expected high strength and high toughness standards.

2. The high-strength, high-toughness ductile iron crankshaft casting process according to claim 1, characterized in that: The chemical composition of the ductile iron crankshaft, by weight percentage, is: C 3.6%, Si 2.1%, Mn 0.25%, P 0.038%, S 0.015%, Mg 0.034%, RE 0.027%, Cu 0.5%, Ni 0.3%, with the balance being Fe.

3. The high-strength, high-toughness ductile iron crankshaft casting process according to claim 1, characterized in that: The chemical composition of the ductile iron crankshaft, by weight percentage, is: C 3.8%, Si 2.3%, Mn 0.35%, P 0.05%, S 0.03%, Mg 0.039%, RE 0.029%, Cu 0.7%, Ni 0.5%, with the balance being Fe.

4. The high-strength, high-toughness ductile iron crankshaft casting process according to claim 1, characterized in that: The chemical composition of the ductile iron crankshaft, by weight percentage, is: C 3.7%, Si 2.2%, Mn 0.30%, P 0.045%, S 0.025%, Mg 0.036%, RE 0.028%, Cu 0.6%, Ni 0.4%, with the balance being Fe.

5. The high-strength, high-toughness ductile iron crankshaft casting process according to claim 1, characterized in that: In step four, the riser diameter D = 1.2T, where T is the diameter of the hot spot circle. The riser neck is short, thin, and wide to achieve adaptive feeding. Chills are set in key areas, and the size of the chills is determined based on the hot spot analysis to ensure a reasonable solidification sequence.

6. The high-strength, high-toughness ductile iron crankshaft casting process according to claim 1, characterized in that: In step seven: the flow ratio of the ammonia and carbon dioxide mixture is 1:0.3; the laser power density is 3-5 GW / cm². 2 The pulse width is 10-20ns, forming a compressive stress layer with a depth of 1.0mm and a residual compressive stress ≥-300MPa.

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