Smelting preparation method of gray cast iron
Through multi-stage inoculation and heat treatment technology, the shrinkage and incomplete inoculation problems caused by MnS inclusions in gray cast iron smelting were solved, the density and mechanical properties of the castings were improved, and efficient gray cast iron production was achieved.
Patent Information
- Application Number
- CN202510939472.1
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-07-08
- Publication Date
- 2025-09-19
AI Technical Summary
During the smelting process of gray cast iron, shrinkage caused by MnS inclusions and incomplete inoculation treatment result in poor density and low mechanical properties of the castings. In addition, single-stage inoculation makes it difficult to balance nucleation stability and final microstructure optimization.
A multi-stage inoculation treatment is adopted, including the first inoculation to form MnS inclusions as the graphite nucleation core, the second inoculation through silicon calcium barium zirconium inoculant to generate oxide/sulfide particles as the graphite heterogeneous nucleation substrate, the third inoculation through silicon strontium inoculant to generate high melting point compounds to promote the formation of type A graphite, and combined with medium frequency induction furnace and air pressure insulation pouring technology for multi-stage heat treatment.
It improves the tensile strength and hardness of gray cast iron, reduces the leakage rate, improves the density and mechanical properties of castings, solves the problems of shrinkage and white cast iron, and ensures the uniform structure and efficient production of castings.
Smart Images
Figure SMS_1
Abstract
Description
Technical Field
[0001] The invention relates to the technical field of gray cast iron materials, and in particular to a smelting preparation method of gray cast iron. Background Art
[0002] Gray cast iron is a type of cast iron with flake graphite. In automobile engines, gray cast iron is often used to manufacture engine cylinder blocks because its good wear resistance and thermal conductivity can meet the working requirements of the engine.
[0003] In existing gray cast iron smelting, scrap steel and pig iron are essential materials. These materials often contain a certain amount of manganese. During the smelting process, these materials react with sulfide impurities in the furnace and the pig iron, forming coarse MnS inclusions that clog interdendritic spaces during the late solidification phase, blocking the liquid metal's shrinkage-feeding pathway and causing negative pressure cavities in micro-regions. During gray cast iron eutectic solidification, the expansion force generated by graphite precipitation can compensate for shrinkage. However, MnS envelops graphite nuclei, reducing the amount of effective graphite and weakening the self-feeding ability. An unbalanced sulfur-manganese ratio (Mn / S > 10) promotes shrinkage defects. Uninoculated gray cast iron suffers from microstructural instability, poor mechanical properties, and the development of white cast in thin-walled areas. Single-stage inoculation struggles to balance basic nucleation stability with optimized final microstructure. This results in floating and shrinking graphite in the grown gray cast iron, impacting the density of the casting and increasing the leakage rate. Summary of the Invention
[0004] The present invention provides a smelting preparation method for gray cast iron to solve the technical problems that coarse MnS inclusions are easily formed during the production and preparation of gray cast iron, resulting in shrinkage and incomplete inoculation treatment leading to poor density.
[0005] In order to solve the above technical problems, the present invention provides a smelting preparation method of gray cast iron, comprising the following steps: Step S1: 50 parts by mass of scrap steel, 40-47 parts of sulfur-manganese pig iron, 1.6-2.0 parts of a recarburizer, 0.04-0.05 parts of Ti, 0.03-0.04 parts of Cr, and 1 part of SiC are sequentially added into a smelting furnace and smelted in a medium frequency induction furnace; Step S2: Inoculate once, add 0.07-0.09 parts by mass of FeS into the furnace; Step S3: keeping the molten iron at 1530-1550°C for 10-15 minutes; Step S4: placing 0.15-0.2 parts by mass of Nb into a ladle, and then transferring the molten iron into the ladle; Step S5: secondary inoculation, adding 0.5-0.6 parts by mass of a silicon calcium barium zirconium inoculant into the casting ladle, wherein the mass percentage of the silicon calcium barium zirconium inoculant is 40%-50% Si, 0.5%-0.7% Ba, 0.6%-1.0% Ca, 0.5%-0.8% Zr, and the balance is Fe and other trace impurities; Step S6: using a gas pressure heat preservation pouring furnace and controlling the flow rate for pouring; Step S7: inoculating three times, adding 0.2-0.4 parts by mass of a silicon strontium inoculant to the molten iron for instant inoculation, wherein the mass percentage of the silicon strontium inoculant is 75%-76% Si, 1.5%-1.6% Sr, 0.08%-0.1% Ca, 0.1%-0.15% Al, and the balance Fe; Step S8: The casting is subjected to multi-stage heat treatment, water cooling at 800-860°C for 4 hours, oil cooling at 920-970°C for 5 hours, and finally air cooling at 750-800°C for 3 hours to obtain gray cast iron.
[0006] Preferably, in the above technical solution, the recarburizer in step S1 needs to be graphitized at 2600-3000°C.
[0007] Preferably, in the above technical solution, the Mn content in the sulfur-manganese pig iron in step S1 does not exceed 0.5%, and the S content in the sulfur-manganese pig iron does not exceed 0.03%.
[0008] Preferably, in the above technical solution, the smelting temperature in step S1 is 1500-1560°C.
[0009] Preferably, in the above technical solution, the particle size of FeS in step S2 is 8-20 mm.
[0010] Preferably, in the above technical solution, the particle size of the silicon calcium barium zirconium inoculant in step S5 is 3-6 mm.
[0011] Preferably, in the above technical solution, the pouring time in step S6 is controlled within 9-11 seconds, and an intermediate injection pouring system is used.
[0012] Preferably, in the above technical solution, the particle size of the silicon strontium inoculant in step S7 is 0.5-0.6 mm.
[0013] Compared with the prior art, the present invention has the following beneficial effects: The invention increases the tensile strength and hardness of the gray cast iron by adding a certain percentage of Nb during the gray cast iron smelting process.
[0014] The present invention performs a sulfur-addition treatment during the first inoculation process, combining Mn and S to form MnS inclusions that serve as graphite nuclei, while suppressing the negative effects of sulfur-induced white cast metal. Furthermore, the manganese-sulfur balance is controlled to prevent excessive MnS precipitation, which reduces effective nuclei and strength, and to avoid an increase in free sulfur, which can lead to D-type graphite and shrinkage.
[0015] The present invention uses a second inoculation method in which a silicon-calcium-barium-zirconium inoculant is directly added to a ladle and mixed with molten iron. Elements such as Ca, Ba, and Zr in the inoculant react with oxygen and sulfur in the molten iron to form oxide / sulfide particles, which serve as a substrate for heterogeneous graphite nucleation. Local high silicon concentration increases carbon activity, weakens cementite stability, promotes graphite precipitation, reduces the tendency of carbides in thin-walled areas, and uniformly grays the edges of the casting. Furthermore, D / E-type supercooled graphite is converted into evenly distributed A-type graphite, improving the strength of gray cast iron.
[0016] This invention overcomes the aging shortcomings of in-laddition inoculation through a third inoculation. Strontium forms a high-melting-point compound with oxygen and sulfur, serving as a heterogeneous nucleation substrate for graphite. This promotes the formation of type A graphite, reduces the undercooling of the eutectic transition, and avoids the formation of type D / E supercooled graphite. Furthermore, the low aluminum and calcium content helps reduce the risk of subcutaneous pores and slag inclusions, thereby increasing the density of the casting and significantly reducing leakage. DETAILED DESCRIPTION
[0017] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described 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.
[0018] Example 1:
[0019] The present invention provides a smelting preparation method for gray cast iron, comprising the following steps: Step S1: 50 parts by weight of scrap steel, 40-47 parts of sulfur-manganese pig iron, 1.6-2.0 parts of a recarburizer, 0.04-0.05 parts of titanium, 0.03-0.04 parts of chromium, and 1 part of silicon carbide are sequentially added to a smelting furnace and smelted at a temperature of 1500-1560°C using a medium-frequency induction furnace. The recarburizer must be graphitized at 2600-3000°C. The Mn content in the low-sulfur manganese pig iron should not exceed 0.5%, and the S content in the low-sulfur manganese pig iron should not exceed 0.03%. The graphitized recarburizer at 2600-3000°C arranges carbon atoms into a graphite structure, which enhances nucleation ability. According to thermodynamic equilibrium, the manganese-sulfur product in gray cast iron should be controlled within the range of 0.03-0.04, achieving the highest tensile strength. If the proportion of scrap steel is high, the sulfur content is likely to exceed the standard (S>0.08%), resulting in MnS>0.128 (far exceeding 0.04), and low-sulfur manganese pig iron must be added to neutralize it. The appropriate proportion of low-sulfur manganese pig iron can avoid exacerbating MnS segregation and reduce sulfide inclusions.
[0020] Medium-frequency induction furnaces utilize an induction coil to generate an alternating magnetic field, which in turn creates eddy currents within the metal. This resistance heating effect melts / heats the metal without direct contact with the coil, generating heat evenly from the inside out. With precise temperature control, high energy efficiency, and environmental friendliness, medium-frequency induction furnaces have become a core component of modern casting, forging, and heat treatment.
[0021] Step S2: Incubate once, add 0.07-0.09 parts by mass of FeS with a particle size of 8-20 mm into the furnace.
[0022] Gray cast iron requires a sulfur content of 0.07%-0.10% to optimize inoculation and graphite morphology. If the sulfur content of the base iron is less than 0.05%, sulfur needs to be added to the appropriate range to achieve a controlled manganese-sulfur balance. Mn combines with sulfur to form MnS inclusions, which serve as graphite nuclei and suppress the negative effects of sulfur. If the Mn / S ratio is greater than 10, excessive MnS precipitation reduces the number of effective nuclei and lowers strength. If the Mn / S ratio is less than 5, free sulfur increases, leading to D-type graphite and shrinkage.
[0023] Step S3: The molten iron is kept at 1530-1550°C for 10-15 minutes to eliminate heredity and refine the grains; if the temperature exceeds 1550°C, the critical point of gray cast iron is reached and its nucleation ability will decrease.
[0024] Step S4: placing 0.15-0.2 parts by mass of Nb in a casting ladle, and then transferring the molten iron into the casting ladle; adding an appropriate amount of Nb can hinder dislocation movement through the hard phase, improve strength, fix free nitrogen, reduce grain boundary impurities, enhance the bonding strength of eutectic groups, and thus increase tensile strength.
[0025] Step S5: Secondary inoculation, adding 0.5-0.6 parts by mass of a silicon calcium barium zirconium inoculant into the casting ladle, wherein the mass percentage of the silicon calcium barium zirconium inoculant composition is 40%-50% Si, 0.5%-0.7% Ba, 0.6%-1.0% Ca, 0.5%-0.8% Zr, and the balance is Fe and other trace impurities; the particle size of the silicon calcium barium zirconium inoculant is 3-6 mm.
[0026] Calcium reacts with oxygen and sulfur to form CaO and CaS, whose structure closely matches the interlayer spacing of graphite, creating an efficient graphite nucleation substrate. This removes oxide inclusions from the molten iron, improves its cleanliness, and reduces slag inclusion defects. Barium delays the oxidation of inoculant elements, extending the effective nucleation time to 20 minutes or more, making it suitable for large castings with long pouring cycles. It remains active at high temperatures, compensating for nucleus loss caused by overheating of the molten iron during electric furnace smelting. Zirconium fixes and dissolves nitrogen in the molten iron to form ZrN, eliminating the risk of nitrogen pores. It also forms ZrC as a heterogeneous core, refining austenite dendrites, enhancing matrix strength, and improving tensile strength. A stable and durable nucleation core system is established in a high-temperature smelting environment to resist nucleus burnout and inoculation decay caused by overheating of the molten iron.
[0027] Step S6: Using a pressure-insulated pouring furnace, control the flow rate for pouring. The pouring time is controlled within 9-11 seconds, and an intermediate injection pouring system is used.
[0028] The center-injection pouring system features an inner gate located at the parting surface, eliminating the need for complex molding and simplifying mold design and production processes. It combines the high-temperature shrinkage compensation advantages of top pouring with the smooth filling characteristics of bottom pouring, reducing the risk of defects associated with either method. Compressed air or inert gas is introduced into the top of the pneumatic insulated pouring furnace, creating a stable pressure that propels the molten iron through a siphon into the pouring trough. Laser or float sensors monitor the pouring trough liquid level in real time, dynamically adjusting the pressure to maintain a constant liquid level and eliminate flow deviations caused by a drop in liquid level. Constant-flow pouring reduces turbulence and air entrainment, resulting in a 60% reduction in slag inclusion and porosity defects.
[0029] Step S7: Inoculate three times, adding 0.2-0.4 parts by weight of a silicon-strontium inoculant to the molten iron for instantaneous inoculation. The silicon-strontium inoculant comprises 75%-76% Si, 1.5%-1.6% Sr, 0.08%-0.1% Ca, 0.1%-0.15% Al, and the balance Fe. The particle size of the silicon-strontium inoculant is 0.5-0.6 mm.
[0030] Strontium has a strong ability to target graphitization. It selectively adsorbs at the graphite growth front, blocking the cementite (Fe3C) formation path and effectively eliminating thin-wall white spots without significantly increasing the number of eutectic clusters, thus preventing excessive shrinkage and porosity. Furthermore, it promotes the formation of type A graphite across the entire cross-section. The low aluminum content reduces the risk of subcutaneous porosity, and the low calcium content reduces slag inclusion rates. This addresses the issue of secondary inoculation being insufficient in thin-walled, sensitive areas.
[0031] Step S8: The casting undergoes a multi-stage heat treatment, water cooling at 800-860°C for 4 hours, followed by oil cooling at 920-970°C for 5 hours, and finally air cooling at 750-800°C for 3 hours to produce gray cast iron. 860°C is below the complete austenitization temperature (above Ac3) of most medium- and high-alloy steels, but above Ac1. This allows for partial austenitization of the steel, dissolving some carbides while retaining a small amount of undissolved carbides to inhibit grain growth. Rapid water cooling forms fine-grained martensite and retained austenite, eliminating as-cast segregation and providing a uniform microstructure for subsequent high-temperature austenitization. A water cooling rate (>100°C / s) prevents carbide reprecipitation. At 920-970°C, alloy carbides (such as Cr7C3 and Mo2C) are fully dissolved, increasing the matrix alloy concentration. Oil cooling balances cooling intensity with deformation control, avoiding the risk of high-stress cracking associated with water cooling. 750-800℃ is higher than Ac1 but lower than Ac3, and the residual carbides are partially dissolved in the austenite; during cooling, nano-scale alloy carbides precipitate, producing a precipitation hardening effect, and the hardness is recovered to a certain extent. The slower cooling rate reduces thermal stress and promotes uniform precipitation of carbides, obtaining a tempered bainite + dispersed carbide structure, which takes into account both wear resistance and fatigue resistance.
[0032] The present invention performs a sulfur addition treatment during the smelting process through the first inoculation, so that Mn and S are combined to form MnS inclusions as the graphite nucleation core, while suppressing the negative effect of the sulfur white cast tendency. At the same time, the manganese and sulfur balance is controlled to avoid excessive precipitation of MnS, which will reduce the effective crystal nucleus and reduce the strength, and also avoid the increase of free sulfur, which leads to D-type graphite and shrinkage. The second inoculation is an inoculation treatment method in which a silicon calcium barium zirconium inoculant is directly added to the ladle and mixed with the molten iron. The elements such as Ca, Ba, and Zr in the inoculant react with the oxygen and sulfur in the molten iron to form oxide / sulfide particles as the graphite heterogeneous nucleation substrate. The local high silicon concentration increases the carbon activity, weakens the stability of cementite, promotes the precipitation of graphite, reduces the tendency of carbides in thin-walled parts, and makes the edges of the castings uniformly gray. On the other hand, the D / E type supercooled graphite is converted into evenly distributed A-type graphite, which improves the strength of gray cast iron. The third inoculation solves the aging shortcoming of inoculation in the ladle. Strontium forms high-melting-point compounds with oxygen and sulfur, serving as a heterogeneous nucleation substrate for graphite. This promotes the formation of type A graphite, reduces the undercooling of the eutectic transition, and avoids undercooling of type D / E graphite. Furthermore, its low aluminum and calcium content helps reduce the risk of subsurface porosity and slag inclusions, thereby increasing the density of castings and significantly reducing leakage.
[0033] Example 2:
[0034] In this embodiment, the present invention provides a method for smelting and preparing gray cast iron, comprising the following steps: Step S1: 50 parts by mass of scrap steel, 40-47 parts of sulfur-manganese pig iron, 1.6-2.0 parts of a recarburizer, 0.04-0.05 parts of Ti, 0.03-0.04 parts of Cr, and 1 part of SiC are sequentially added into a smelting furnace, and smelting is performed at a temperature of 1500-1560° C. using a medium frequency induction furnace.
[0035] Step S2: Incubate once, add 0.07-0.09 parts by mass of FeS with a particle size of 8-20 mm into the furnace.
[0036] Step S3: keeping the molten iron at 1530-1550° C. for 10-15 minutes.
[0037] Step S4: Secondary inoculation, adding 0.5-0.6 parts by mass of a silicon calcium barium zirconium inoculant into the casting ladle, wherein the mass percentage of the silicon calcium barium zirconium inoculant composition is 40%-50% Si, 0.5%-0.7% Ba, 0.6%-1.0% Ca, 0.5%-0.8% Zr, and the balance is Fe and other trace impurities; the particle size of the silicon calcium barium zirconium inoculant is 3-6 mm.
[0038] Step S5: Using a pressure-insulated pouring furnace, control the flow rate for pouring. The pouring time is controlled within 9-11 seconds, and an intermediate injection pouring system is used.
[0039] Step S6: Inoculate three times, adding 0.2-0.4 parts by weight of a silicon-strontium inoculant to the molten iron for instantaneous inoculation. The silicon-strontium inoculant comprises 75%-76% Si, 1.5%-1.6% Sr, 0.08%-0.1% Ca, 0.1%-0.15% Al, and the balance Fe. The particle size of the silicon-strontium inoculant is 0.5-0.6 mm.
[0040] Step S7: The casting is subjected to multi-stage heat treatment, water cooling at 800-860°C for 4 hours, oil cooling at 920-970°C for 5 hours, and finally air cooling at 750-800°C for 3 hours to obtain gray cast iron.
[0041] In this embodiment, compared with the first embodiment, the step of adding Nb to the molten iron is omitted.
[0042] Example 3:
[0043] This example will use Example 2 as a benchmark. Under the same smelting preparation environment, through experimental comparison, the effect of adding Nb on the quality and microstructure of the final gray cast iron will be investigated, as well as the differences in gray cast iron performance and microstructure changes caused by different percentages of Nb.
[0044] Table 1 Comparison of properties of gray cast iron with different percentages of Nb
[0045] The above test data demonstrates that, within the tested range, the percentage of Nb in the gray cast iron smelting process is directly proportional to its tensile strength and hardness. The data indicates that a Nb content between 0.15% and 0.20% significantly improves the tensile strength and hardness of gray cast iron. While a higher Nb content increases the tensile strength and hardness of gray cast iron, excessive accumulation of niobium-rich phases (Nb(C,N)) at a content between 0.25% and 0.30% increases the brittleness of the gray cast iron, leading to a decrease in machinability. While tensile strength and hardness increase, the overall brittleness of the gray cast iron is compromised.
[0046] Therefore, adding Nb to the gray cast iron during smelting, at a content between 0.15% and 0.20%, is most effective. The primary niobium-rich phase formed, Nb(C,N), takes the form of square / rhombus-shaped hard particles. Its strengthening effect is achieved by hindering dislocation motion through the hard phase, increasing strength, fixing free nitrogen, reducing grain boundary impurities, and enhancing eutectic cohesion.
[0047] Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments, or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.
Claims
1. A method for smelting and preparing gray cast iron, characterized in that: The following steps are involved: Step S1: 50 parts by mass of scrap steel, 40-47 parts of sulfur-manganese pig iron, 1.6-2.0 parts of a recarburizer, 0.04-0.05 parts of Ti, 0.03-0.04 parts of Cr, and 1 part of SiC are sequentially added into a smelting furnace and smelted in a medium frequency induction furnace; Step S2: Inoculate once, add 0.07-0.09 parts by mass of FeS into the furnace; Step S3: keeping the molten iron at 1530-1550°C for 10-15 minutes; Step S4: placing 0.15-0.2 parts by mass of Nb into a ladle, and then transferring the molten iron into the ladle; Step S5: secondary inoculation, adding 0.5-0.6 parts by mass of a silicon calcium barium zirconium inoculant into the casting ladle, wherein the mass percentage of the silicon calcium barium zirconium inoculant is 40%-50% Si, 0.5%-0.7% Ba, 0.6%-1.0% Ca, 0.5%-0.8% Zr, and the balance is Fe and other trace impurities; Step S6: using a gas pressure heat preservation pouring furnace and controlling the flow rate for pouring; Step S7: inoculating three times, adding 0.2-0.4 parts by mass of a silicon strontium inoculant to the molten iron for instant inoculation, wherein the mass percentage of the silicon strontium inoculant is 75%-76% Si, 1.5%-1.6% Sr, 0.08%-0.1% Ca, 0.1%-0.15% Al, and the balance Fe; Step S8: The casting is subjected to multi-stage heat treatment, water cooling at 800-860°C for 4 hours, oil cooling at 920-970°C for 5 hours, and finally air cooling at 750-800°C for 3 hours to obtain gray cast iron.
2. The method for smelting and preparing gray cast iron according to claim 1, wherein: The recarburizer in step S1 needs to be graphitized at 2600-3000°C.
3. The method for smelting and preparing gray cast iron according to claim 1, wherein: In step S1, the Mn content in the sulfur-manganese pig iron does not exceed 0.5%, and the S content in the sulfur-manganese pig iron does not exceed 0.03%.
4. The method for smelting and preparing gray cast iron according to claim 1, wherein: The smelting temperature in step S1 is 1500-1560°C.
5. The method for smelting and preparing gray cast iron according to claim 1, characterized in that: The particle size of FeS in step S2 is 8-20 mm.
6. The method for smelting and preparing gray cast iron according to claim 1, characterized in that: The particle size of the silicon calcium barium zirconium inoculant in step S5 is 3-6 mm.
7. The method for smelting and preparing gray cast iron according to claim 1, characterized in that: The pouring time in step S6 is controlled within 9-11 seconds, and an intermediate injection pouring system is used.
8. The method for smelting and preparing gray cast iron according to claim 1, characterized in that: The particle size of the silicon strontium inoculant in step S7 is 0.5-0.6 mm.