Production method of high-strength nodular cast iron

Abstract

The invention provides a production method of high-strength nodular cast iron and belongs to the technical field of metal material processing. Comprising the following steps: proportioning: proportioning the following raw materials in percentage by weight: 60-70% of pig iron, 20-30% of foundry returns and 10-15% of waste steel; pig iron is added to the bottom of the furnace, foundry returns are added, and finally waste steel is added; after the molten iron is uniformly molten, heating to 1420-1440 DEG C, and sampling to analyze chemical components; inoculation and nitrogen increasing treatment: spheroidizing by adopting a pouring method, sequentially adding pure antimony, a rare earth magnesium spheroidizing agent and a mixture of ferromanganese nitride and a ferrosilicon inoculant, adding the rest of molten iron and the inoculant in stages, and adding an antimony-containing inoculant during pouring; and after pouring is completed, the box is opened for cooling when the temperature of the casting is reduced to 800-900 DEG C. According to the method, the nitrogen increasing process is matched with microalloying, the problems of high alloy adding amount and high cost in the prior art are solved, the alloy using amount can be remarkably reduced, and meanwhile the mechanical property of cast iron is guaranteed.

Description

Technical Field

[0001] The invention belongs to the technical field of metal material processing, and specifically relates to a production method of high-strength ductile iron, and is particularly suitable for the production of high-strength ductile iron of grades such as QT500-7, QT600-3, and QT700-2. Background Art

[0002] The traditional production method of high-strength ductile iron usually uses alloying. For example, the production of QT500-7, QT600-3, and QT700-2 mainly involves adding alloying elements such as manganese, copper, antimony, and tin to promote the formation of a certain amount of pearlite in the matrix to improve mechanical properties such as tensile strength, yield strength, and hardness. However, this method has the following drawbacks: 1. The amount of alloy added is large, resulting in higher production costs; 2. Excessive addition of alloying elements may have an adverse effect on other properties of cast iron (such as toughness).

[0003] Therefore, there is an urgent need for a low-cost, high-performance method for producing high-strength ductile iron. Summary of the Invention

[0004] The technical problem solved by the present invention is to provide a method for producing high-strength ductile iron. The present invention aims to solve the problems of high alloy addition and high cost in the prior art and proposes a method for producing high-strength ductile iron through a nitrogen addition process combined with trace alloying. This method can significantly reduce the amount of alloy used while ensuring the mechanical properties of the cast iron.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is: A method for producing high-strength ductile iron comprises the following steps: Step 1) Batching: Raw materials are mixed by weight: 60% to 70% pig iron, 20% to 30% recycled materials, and 10% to 15% scrap steel; Step 2) Adding and Melting: Pig iron is added to the bottom of the furnace, followed by recycled material, and finally scrap steel. After the molten iron is evenly melted, the temperature is raised to 1420°C to 1440°C, and samples are taken for chemical composition analysis. Step 3) Inoculation and nitrogen enrichment treatment: spheroidization is performed by the flushing method, and pure antimony, rare earth magnesium spheroidizer, nitrided manganese iron and ferrosilicon inoculant mixture are added in sequence. The remaining molten iron and inoculant are added in stages, and the antimony-containing inoculant is added during pouring; Step 4) Cooling treatment: After pouring is completed, wait until the casting temperature drops to 800℃~900℃ before unpacking and cooling.

[0006] In step 3) above, the specific steps of the spheroidization treatment using the punching method are as follows: a. Add pure antimony at one side of the spheroidizing pit at the bottom of the ladle. The addition amount is 0.008% to 0.015% and the particle size is 15 to 30 mm. b. Add 1.2% to 1.4% of rare earth magnesium nodulizer; c. Mix 0.05% to 0.06% ferromanganese nitride and 0.2% to 0.3% ferrosilicon inoculant, cover the spheroidizing agent, and compact appropriately; d. Covered with 0.3% ductile iron chips and iron plates; e. When the molten iron is about 2 / 3 of the way out, add 0.3% to 0.4% of ferrosilicon inoculant and 0.02% of ferromanganese nitride mixture along with the remaining 1 / 3 of the molten iron; f. During pouring, 0.1% to 0.15% antimony-containing inoculant is injected into the mold cavity along with the molten iron.

[0007] To further limit the above solution, the components of the rare earth magnesium spheroidizer include, by weight percentage, 0.4% to 0.6% rare earth, 5% to 6% magnesium, 44% to 47% silicon, 0.8% to 1.2% calcium, and the balance iron.

[0008] To further limit the above solution, the composition of the manganese ferronitride includes, by weight percentage: a manganese content of not less than 88%, a nitrogen content of 9% to 10%, an oxygen content of less than 0.05%, and a particle size of 5 to 10 mm.

[0009] To further limit the above solution, the particle size of the 0.2% to 0.3% ferrosilicon inoculant is 5-10 mm.

[0010] To further define the above solution, the antimony-containing inoculant comprises, by weight percentage, 6.21% antimony, 68% silicon, and the remainder iron, with a particle size of 0.7 to 1 mm.

[0011] The advantages of the present invention compared with the prior art are: 1. This solution significantly reduces the amount of alloy by combining the nitrogen addition process with micro-alloying. The resulting ductile iron has excellent mechanical properties and reduces production costs. It is particularly suitable for grades such as QT500-7, QT600-3, and QT700-2. The process is simple and easy to promote industrialization. 2. This solution uses trace antimony (0.008%-0.015%) and ferromanganese nitride (0.05%-0.06%) to synergistically optimize pearlite formation, resulting in small pearlite plate spacing, uniform matrix structure, and excellent mechanical properties. 3. This solution ensures uniform distribution of nitrogen by adding inoculant and ferromanganese nitride in stages; 4. This solution strictly controls the cooling temperature to avoid cracking or uneven performance of the castings. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is the metallographic structure × 100 image of QT700-2 produced by traditional methods; Figure 2 Metallographic structure of QT700-2 produced by traditional method × 500; Figure 3 Metallographic ×200 image of QT700-2 produced by the nitrogen addition method of the present invention; Figure 4 Metallographic ×500 image of QT700-2 produced by the nitrogen addition method of the present invention. DETAILED DESCRIPTION

[0013] The following is a combination of the embodiments of the present invention Figure 1-4 The technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. All other embodiments derived by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts are within the scope of protection of the present invention.

[0014] The embodiment of the present invention is as follows: A method for producing high-strength ductile iron comprises the following steps: Step 1) Batching: Raw materials are mixed by weight: 60% to 70% pig iron, 20% to 30% recycled material (ductile iron), and 10% to 15% scrap steel; Step 2) Charging and Melting: 60% to 70% pig iron is added to the bottom of the furnace, followed by 20% to 30% recycled material, and finally 10% to 15% scrap steel. After the molten iron is evenly melted, the temperature is raised to 1420°C to 1440°C for sampling. A cast steel sampling spoon coated with alcohol-based paint is used to dig 100mm to 200mm below the molten iron surface and cast spectroscopic and chemical samples. Step 3) Inoculation and nitrogen enrichment treatment: spheroidization is performed by the flushing method, and pure antimony, rare earth magnesium spheroidizer, nitrided manganese iron and ferrosilicon inoculant mixture are added in sequence. The remaining molten iron and inoculant are added in stages, and the antimony-containing inoculant is added during pouring; The specific steps of the spheroidization treatment using the flushing method are as follows: a. According to the ductile iron grade and molten iron weight, pure antimony is added to one side of the spheroidizing pit at the bottom of the casting ladle. The addition amount is 0.008% to 0.015%, and the particle size is 15 to 30 mm. b. Add 1.2% to 1.4% of rare earth magnesium nodulizer; The components of the rare earth magnesium nodulizer include, by weight percentage, 0.4% to 0.6% rare earth, 5% to 6% magnesium, 44% to 47% silicon, 0.8% to 1.2% calcium, and the balance iron.

[0015] c. Mix 0.05%-0.06% ferromanganese nitride with 0.2%-0.3% ferrosilicon inoculant and cover it on the spheroidizing agent, and compact it appropriately; The composition of the ferromanganese nitride includes, by weight percentage: a manganese content of not less than 88%, a nitrogen content of 9%-10%, an oxygen content of less than 0.05%, and a particle size of 5-10 mm; The particle size of the 0.2% to 0.3% ferrosilicon inoculant is 5 to 10 mm.

[0016] d. Covered with 0.3% ductile iron chips and iron plates; e. When the molten iron is about 2 / 3 of the way out, add 0.3% to 0.4% of ferrosilicon inoculant and 0.02% of ferromanganese nitride mixture along with the remaining 1 / 3 of the molten iron; f. During pouring, 0.1% to 0.15% antimony-containing inoculant is injected into the mold cavity along with the molten iron.

[0017] The antimony-containing inoculant comprises, by weight percentage, 6.21% antimony, 68% silicon, and the remainder iron, with a particle size of 0.7 to 1 mm.

[0018] Step 4) Cooling treatment: After pouring is completed, wait until the casting temperature drops to 800℃~900℃ before unpacking and cooling.

[0019] The present invention combines nitrogen addition technology with micro-alloying to produce high-strength ductile iron of different grades, which can significantly reduce the amount of alloy used. The obtained ductile iron has excellent mechanical properties, reduced production costs, simple process and is easy to promote industrialization.

[0020] Nitrogen increases the undercooling of the eutectic transformation in ductile iron, shortening the primary austenite dendrites, reducing the spacing between the secondary dendrite arms, and refining the eutectic clusters. Nitrogen dissolved in austenite reduces the undercooling of the eutectoid transformation and expands the eutectoid transformation temperature range. Nitrogen's most significant effect in ductile iron is increasing the undercooling of the eutectoid transformation, promoting pearlite formation and stabilizing the pearlite structure. As nitrogen incorporation increases, the eutectoid transformation temperature decreases, and the difference between the eutectoid transformation start and end temperatures increases. This element is generally considered an alloying element. These effects of nitrogen on the eutectoid transformation make it an element that promotes pearlite formation and refines the pearlite structure. Adding manganese nitride near the time of its release from the furnace effectively increases the dissolved nitrogen content and reduces the presence of combined nitrogen. Only dissolved nitrogen can effectively enhance and refine the pearlite structure. However, when using high-content recarburizers to increase nitrogen, the molten nitrogen is easily lost in the form of gas during the smelting process. Even if a certain amount of nitrogen is dissolved, it will react with elements such as silicon and titanium to form nitrides over a long period of time to form combined nitrogen, which makes it impossible to effectively utilize nitrogen to increase and refine pearlite.

[0021] Table 1 below shows the chemical composition of three typical high-strength ductile irons produced by this method: Table 1 Chemical composition Through experiments, it was found that the matrix structure of various brands of high-strength ductile iron produced by this method is pearlite or troostite with small interlamellar spacing and bull's-eye ferrite. Among them, the pearlite content of QT500-7 is 40%-50%, the pearlite content of QT600-3 is 50%-60%, and the pearlite content of QT700-2 is not less than 60%. The matrix structure of each brand of ductile iron is uniform and the mechanical properties are good.

[0022] Comparison of mechanical properties between the method of the present invention and the traditional alloying method: as shown in Table 2 and Table 3 below, Table 2 Table 3 like Figure 1-4 The following is a comparison of the metallographic structures produced by the traditional method and this solution. Figure 1 、 Figure 2 、 Figure 3 、 Figure 4 , it can be seen that the traditional alloying method produces linear carbides between grains of QT700-2 and coarse pearlite interlamellar spacing, while the QT700-2 produced using the nitrogen addition method only has a few point-like alloy compounds between grains. Even after magnification 500 times, the pearlite interlamellar spacing is very small. In addition, through comparative production, it was found that the traditional alloying method produces ductile iron castings with a greater tendency to shrink, while the ductile iron produced using the nitrogen addition method has a much lower shrinkage tendency.

[0023] 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.

[0024] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A method for producing high-strength ductile iron, characterized in that: The following steps are involved: Step 1) Batching: Raw materials are mixed by weight: 60% to 70% pig iron, 20% to 30% recycled materials, and 10% to 15% scrap steel; Step 2) Adding and Melting: Pig iron is added to the bottom of the furnace, followed by recycled material, and finally scrap steel. After the molten iron is evenly melted, the temperature is raised to 1420°C to 1440°C, and samples are taken for chemical composition analysis. Step 3) Inoculation and nitrogen enrichment treatment: spheroidization is performed by the flushing method, and pure antimony, rare earth magnesium spheroidizer, nitrided manganese iron and ferrosilicon inoculant mixture are added in sequence. The remaining molten iron and inoculant are added in stages, and the antimony-containing inoculant is added during pouring; Step 4) Cooling treatment: After pouring is completed, wait until the casting temperature drops to 800℃~900℃ before unpacking and cooling.

2. The method for producing high-strength ductile iron according to claim 1, wherein: In step 3) above, the specific steps of the spheroidization treatment using the punching method are as follows: a. Add pure antimony at one side of the spheroidizing pit at the bottom of the ladle. The addition amount is 0.008% to 0.015% and the particle size is 15 to 30 mm. b. Add 1.2% to 1.4% of rare earth magnesium nodulizer; c. Mix 0.05% to 0.06% ferromanganese nitride and 0.2% to 0.3% ferrosilicon inoculant, cover the spheroidizing agent, and compact appropriately; d. Covered with 0.3% ductile iron chips and iron plates; e. When the molten iron is about 2 / 3 of the way out, add 0.3% to 0.4% of ferrosilicon inoculant and 0.02% of ferromanganese nitride mixture along with the remaining 1 / 3 of the molten iron; f. During pouring, 0.1% to 0.15% antimony-containing inoculant is injected into the mold cavity along with the molten iron.

3. The method for producing high-strength ductile iron according to claim 2, wherein: The components of the rare earth magnesium nodulizer include, by weight percentage, 0.4% to 0.6% rare earth, 5% to 6% magnesium, 44% to 47% silicon, 0.8% to 1.2% calcium, and the balance iron.

4. The method for producing high-strength ductile iron according to claim 2, wherein: The composition of the ferromanganese nitride includes, by weight percentage: a manganese content of not less than 88%, a nitrogen content of 9% to 10%, an oxygen content of less than 0.05%, and a particle size of 5 to 10 mm.

5. The method for producing high-strength ductile iron according to claim 2, wherein: The particle size of the 0.2% to 0.3% ferrosilicon inoculant is 5 to 10 mm.

6. The method for producing high-strength ductile iron according to claim 2, wherein: The antimony-containing inoculant comprises, by weight percentage, 6.21% antimony, 68% silicon, and the remainder iron, with a particle size of 0.7 to 1 mm.

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