A method for preparing high-strength and high-toughness ferrite ductile iron

Abstract

The invention discloses a preparation method of high-strength and high-toughness ferrite ductile iron, belongs to the field of ductile iron material preparation, and aims to develop a ductile iron material with strength-plasticity balance. The invention improves the toughness and strength of the ductile iron by increasing the ferrite content in the matrix and realizing silicon solid solution strengthening. In the smelting process, raw materials such as pure pig iron and scrap steel are used, and spheroidizing inoculation is carried out by a punching method. Secondary strengthening inoculation is carried out in the casting process, and finally a ductile iron material with 3.2%≤C≤4.0%, 2.0%≤Si≤3.3%, Mn≤0.2%, S≤0.03%, P≤0.02%, Ti≤0.03%, Cr≤0.03%, and 0.002%≤Sb≤0.008% is obtained. The ductile iron material obtained by the invention has high tensile strength and high elongation at the same time, and has the advantages of small hardness fluctuation range and good processability.

Description

Technical Field

[0001] The invention belongs to the technical field of material processing, relates to ductile iron production technology, and specifically relates to a preparation method of high-strength and high-toughness ferrite ductile iron. Background Art

[0002] Ductile iron, as a widely used structural material, has the characteristics of excellent mechanical properties and good casting properties. It can meet the comprehensive performance requirements of materials in a wide range. Compared with alloy steel with the same mechanical property level, ductile iron has a lower price. However, ordinary cast ductile iron such as QT400-18 and QT700-2 cannot obtain high strength and toughness at the same time, which limits the application range of ductile iron. The development of ductile iron should not be limited to ductile iron materials of traditional grades. Traditional ductile iron materials cannot meet the current development demand for ductile iron materials. Therefore, it is necessary to develop a new type of ductile iron material to make it have high strength and high toughness, which will promote the further expansion of the application range of ductile iron. However, simultaneously improving the strength and toughness of ductile iron is one of the difficult problems in the field of materials science, and the improvement of material strength is inevitably accompanied by a decrease in toughness. Therefore, the present invention proposes to control the element content and casting process of ductile iron, increase the ferrite content inside the matrix and realize the idea of ​​silicon solid solution strengthening while improving the toughness and strength of ductile iron. Summary of the invention

[0003] Purpose of the invention: The purpose of the present invention is to make the ductile iron material have good tensile strength and good toughness at the same time, so a preparation method is provided to achieve stable production of high-strength and high-toughness ferritic ductile iron, and when the wall thickness is ≤30mm, the tensile strength is ≥450MPa and the elongation is ≥18%; when the wall thickness is 30 mm-60mm, the tensile strength is ≥430MPa and the elongation is ≥14%; when the wall thickness is greater than 60mm, the tensile strength is ≥420MPa and the elongation is ≥12.5%.

[0004] Technical solution: A method for preparing high-strength and high-toughness ferrite ductile iron, comprising the following steps:

[0005] Step 1: prepare the raw material composition and weight percentage as follows: 40-70% high-purity pig iron, 30-65% medium-low carbon scrap steel, 0-20% recycled materials, and the rest are other additives;

[0006] Step 2: Add the above-mentioned high-purity pig iron, recycled materials and medium-low carbon scrap steel into an electric furnace in proportion for smelting. After complete melting, remove the surface slag. The smelting temperature is 1320°C-1480°C and stand for 4-10 minutes.

[0007] Step 3: Take samples for analysis at 80mm-120mm below the molten iron surface, and adjust the composition of the molten iron by using carburizer, ferrosilicon and ferromanganese. The chemical composition of the molten iron in the furnace after adjustment is as follows: 3.2%≤C≤3.8%, 1.8%≤Si≤2.5%, Mn≤0.2%, P≤0.03%, Ti≤0.03%, Cr≤0.03%;

[0008] Step 4: Control the S content in the molten iron to ≤ 0.03%;

[0009] Step 5, a spheroidizing and inoculating process is performed by flushing method, and a rare earth spheroidizing agent accounting for 0.8%-1.2% of the weight of the molten iron is evenly spread on the bottom of the spheroidizing bag, and a 0.03-0.05mm thick iron foil is completely covered thereon, and a silicon-barium-calcium inoculant accounting for 0.2%-0.8% of the weight of the molten iron and a pure Sb accounting for 0.001%-0.008% of the weight of the molten iron are evenly spread on the iron foil, and then the pressed iron is evenly covered thereon;

[0010] Step 6, the molten iron in the furnace is heated to a temperature of 1450-1480°C and then allowed to stand for 5-10 minutes, and after slagging, pure Sb accounting for 0.001%-0.008% of the weight of the molten iron and wrapped with 0.02-0.03mm iron foil is added into the electric furnace;

[0011] Step 7: pour the pure molten iron after the static slag removal into the spheroidizing ladle at a spheroidizing temperature of 1400°C-1480°C;

[0012] Step 8: After the spheroidization reaction of the molten iron in the ladle is completed, slag treatment is carried out and pouring is carried out. The time interval from spheroidization treatment to pouring is controlled within 15 minutes;

[0013] Step nine, pouring the molten iron in the spheroidizing bag into the pouring basin, and performing secondary inoculation simultaneously during the casting process, with the amount of the inoculant being 0.1%-0.2% of the weight of the molten iron;

[0014] Step 10: When the temperature of the molten iron in the pouring basin drops to 1305°C-1350°C, pull out the plug blocking the pouring channel, complete the filling of the molten iron, and form ductile iron.

[0015] As an optimization: after spheroidizing and inoculation, the chemical composition mass percentage of ductile iron is: 3.2%≤C≤4.0%, 2.0%≤Si≤3.3%, Mn≤0.2%, S≤0.03%, P≤0.02%, Ti≤0.03%, Cr≤0.03%, 0.002%≤Sb≤0.008%.

[0016] As an optimization: the matrix ferrite of the ductile iron is ≥85%, the spheroidization rate is ≥90%, when the wall thickness is ≤30 mm, the tensile strength of the ductile iron is ≥450 MPa, and the elongation is ≥18%; when the wall thickness is 30 mm-60 mm, the tensile strength of the ductile iron is ≥430 MPa, and the elongation is ≥14%; when the wall thickness is greater than 60 mm, the tensile strength of the ductile iron is ≥420 MPa, and the elongation is ≥12.5%.

[0017] As an optimization: the composition (mass percentage) of high-purity pig iron is: C: 4.0-4.12%, Si: 0.47-0.60%, Mn≤0.05%, S≤0.02%, P≤0.02%, Ti≤0.01%, Cr≤0.01%, and the total content of other alloy elements except Fe is less than 0.05%.

[0018] As an optimization: the composition of scrap steel (mass percentage) is: Mn < 0.25%, S < 0.02%, P < 0.03%, Ti < 0.03%, Cr < 0.03%, Cu < 0.03%, Mo < 0.01%, and the sum of other alloy elements except Fe is less than 0.05%.

[0019] As an optimization: the S content in the molten iron is controlled by the process of first desulfurization and then sulfur addition. The molten iron is desulfurized by the injection method, and then the S content in the molten iron is increased in the form of S and O inoculants.

[0020] As an optimization: the spheroidizer used is a rare earth magnesium alloy spheroidizer with the composition (mass percentage) of: 5.62%≤Mg≤5.9%, 0.60%≤Al≤0.80%, 0.72%≤Ca≤1.31%, 43.00%≤Si≤46.00%, 0.40%≤RE≤0.53%.

[0021] As an optimization: the silicon, barium and calcium components used (mass percentage) are: 0.80%≤Al≤1.30%, 0.80%≤Ca≤1.50%, 71.00%≤Si≤73.00%, 2.40%≤Ba≤2.95%.

[0022] As an optimization: the pouring basin includes a slag retaining plate and a water outlet, the pouring basin is placed above the sand mold, the water outlet of the pouring basin is aligned with the runner of the sand mold and is perfectly combined, wherein the sand mold contains the runner and the casting cavity, and the slag retaining plate in the pouring basin is 1-20 cm away from the bottom of the pouring basin.

[0023] As an optimization: the sand mold and the pouring basin are both prepared by sand mold.

[0024] Beneficial effect: The present invention improves the toughness and strength of ductile iron by increasing the ferrite content inside the matrix and realizing silicon solid solution strengthening. In the smelting process, pure pig iron, scrap steel and other raw materials are used, and the punching method is adopted for spheroidizing inoculation. Secondary strengthening inoculation is carried out during the casting process, and finally a ductile iron material with 3.2%≤C≤4.0%, 2.0%≤Si≤3.3%, Mn≤0.2%, S≤0.03%, P≤0.02%, Ti≤0.03%, Cr≤0.03%, and 0.002%≤Sb≤0.008% is obtained. The ductile iron material obtained by the present invention has high tensile strength and high elongation, and has the advantages of small hardness fluctuation range and good processability. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a structural schematic diagram of the casting process of the present invention.

[0026] Figure 2 The microstructure picture of the ductile iron core prepared by Example 1.

[0027] Explanation of reference numerals: 1. ladle, 2. pouring basin, 201. water outlet, 202. slag retaining plate, 3. plug, 4. sand mold, 401 runner of sand mold, 402. casting cavity. DETAILED DESCRIPTION

[0028] The technical solutions in the embodiments of the present invention will be described clearly and completely below so that those skilled in the art can better understand the advantages and features of the present invention, thereby making a clearer definition of the protection scope of the present invention. The embodiments described in the present invention are only a part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention. Specific embodiments

[0029] A method for preparing high-strength and high-toughness ferrite ductile iron comprises the following steps:

[0030] Step 1: Mix the pig iron, recycled materials and scrap steel in proportion: the weight percentage of each raw material is: 40% high-purity pig iron, 60% medium-low carbon scrap steel, and 0% recycled materials; the remaining raw materials such as carburizer, ferrosilicon, ferromanganese, and desulfurizer are placed aside.

[0031] Step 2: Add the above pig iron, recycled materials and scrap steel into an electric furnace for smelting. After heating to 1320°C and completely melting, remove the slag on the surface and let it stand for 10 minutes.

[0032] Step 3: Take samples and analyze them at 80mm below the molten iron surface. Adjust the composition of the molten iron by using carburizer, ferrosilicon and ferromanganese. After adjustment, the chemical composition mass percentages of the molten iron in the furnace are: C: 3.2%, Si: 1.8%, Mn: 0.05%, P: 0.01%, Ti: 0.01% and Cr: 0.02%.

[0033] Step 4: Desulfurize the molten iron by injection method, and then increase the S content in the final molten iron by adding S and O inoculants, and the S content is 0.02%.

[0034] Step 5: The molten iron is spheroidized and inoculated once by the flushing method. A rare earth spheroidizer accounting for 0.8% of the weight of the molten iron is evenly spread on the bottom of the spheroidizing bag, and a 0.03 mm thick iron foil is completely covered thereon. The iron foil is evenly covered with a silicon-barium-calcium inoculant accounting for 0.2% of the weight of the molten iron and pure Sb accounting for 0.003% of the weight of the molten iron, and then pressed iron is evenly covered thereon.

[0035] Step 6: Raise the temperature of the molten iron in the furnace to 1450°C and let it stand for 5 minutes. After deslagging again, put pure Sb accounting for 0.001% of the weight of the molten iron and wrapped with 0.02 iron foil into the electric furnace.

[0036] Step 7: Pour the pure molten iron after standing and slagging into the spheroidizing ladle at a temperature of 1400°C.

[0037] Step 8: After the spheroidization reaction of the molten iron in the ladle is completed, the slag is removed and preparation for pouring is carried out. The time interval from spheroidization to pouring is controlled within 15 minutes.

[0038] Step 9: Casting process Figure 1 As shown, the molten iron in the spheroidizing bag is poured into a pouring basin and a sand mold prepared by a sand mold, the pouring basin 2 is placed above the sand mold 4, a slag retaining plate 202 is provided in the pouring basin 2, which is 1 cm away from the bottom of the pouring basin, the water outlet 201 of the pouring basin 2 is aligned with the runner 401 of the sand mold and is well combined, wherein the sand mold contains the runner and the casting cavity 402.

[0039] Step 10: Pour the molten iron in the spheroidizing bag into the pouring basin 2. When the temperature of the molten iron in the pouring basin 2 drops to 1305°C, pull out the plug 3 blocking the pouring channel 401. During the casting process, secondary inoculation is performed simultaneously, and the amount of the inoculant used is 0.1% of the weight of the molten iron, and finally the filling of the molten iron is completed.

[0040] The material components mentioned in the above process are as follows:

[0041] The composition (mass percentage) of high-purity pig iron is: C: 4.04%, Si: 0.47%, Mn: 0.04%, S: 0.010%, P: 0.013%, Ti: 0.002%, Cr: 0.009%, and the total of other alloy elements is less than 0.05%.

[0042] The composition (mass percentage) of the scrap steel is: Mn: 0.012%, S: 0.012%, P: 0.015%, Ti: 0.016%, Cr: 0.014%, Cu: 0.023%, Mo: 0.0015%, and the sum of other alloy elements is less than 0.05%.

[0043] The composition (mass percentage) of the rare earth magnesium alloy spheroidizer is: Mg: 5.62%, Al: 0.60%, Ca: 0.72%, Si: 43.00%, RE: 0.43%.

[0044] The composition (mass percentage) of the silicon-barium-calcium inoculant is: Al: 0.80%, Ca: 0.80%, Si: 71.00%, Ba: 2.40%.

[0045] The specific organization and performance index of the material obtained by the preparation method of high-strength and high-toughness ferrite ductile iron of the present invention are: matrix ferrite ≥85%, spheroidization rate ≥90%. When the wall thickness is ≤30 mm, the tensile strength is ≥450MPa, and the elongation is ≥18%. When the wall thickness is 30mm-60mm, the tensile strength is ≥430MPa, and the elongation is ≥14%. When the wall thickness is greater than 60 mm, the tensile strength is ≥420MPa, and the elongation is ≥12.5%. The target composition is C: 3.2%, Si: 2.0%, Mn: 0.2%, S: 0.02%, P: 0.02%, Ti: 0.025%, Cr: 0.02%, Sb: 0.004%.

Claims

1. A method for preparing high-strength and high-toughness ferrite ductile iron, comprising the following steps: Step 1: Mix the pig iron, recycled materials and scrap steel in proportion: the weight percentage of each raw material is: 40% high-purity pig iron, 60% medium-low carbon scrap steel; Step 2: Add the above pig iron and scrap steel into an electric furnace for smelting, heat to 1320°C until completely melted, scrape off the surface slag, and let stand for 10 minutes; Step 3: Take samples and analyze them at 80mm below the molten iron surface. Adjust the composition of the molten iron by using carburizer, ferrosilicon and ferromanganese. The chemical composition of the molten iron in the furnace after adjustment is as follows: C: 3.2%, Si: 1.8%, Mn: 0.05%, P: 0.01%, Ti: 0.01%, Cr: 0.02%; Step 4: Control the S content in the molten iron to ≤ 0.03%; Step 5: The molten iron is spheroidized and inoculated once by the flushing method. A rare earth spheroidizer accounting for 0.8% of the weight of the molten iron is evenly spread on the bottom of the spheroidizing bag, and a 0.03 mm thick iron foil is completely covered thereon. A silicon-barium-calcium inoculant accounting for 0.2% of the weight of the molten iron and pure Sb accounting for 0.003% of the weight of the molten iron are evenly spread on the iron foil, and then pressed iron is evenly covered thereon; Step 6: Raise the temperature of the molten iron in the furnace to 1450°C and let it stand for 5 minutes. After slagging again, add 0.001% pure Sb wrapped with 0.02mm iron foil into the electric furnace; Step 7: pour the clean molten iron after the static slag removal into the spheroidizing ladle at a temperature of 1400°C; Step 8: After the spheroidization reaction of the molten iron in the ladle is completed, the slag is removed and prepared for pouring. The time interval from spheroidization to pouring is controlled within 15 minutes; Step 9: During the casting process, the molten iron in the spheroidizing ladle is poured into a pouring basin and a sand mold prepared from a sand mold, the pouring basin (2) is placed above the sand mold (4), a slag retaining plate (202) is provided in the pouring basin (2), and the distance from the bottom of the pouring basin to the pouring basin is 1 cm. The water outlet (201) of the pouring basin (2) is aligned with the runner (401) of the sand mold and is well combined, wherein the sand mold contains the runner and the casting cavity (402); Step 10, pouring the molten iron in the spheroidizing bag into the pouring basin (2), and when the temperature of the molten iron in the pouring basin (2) drops to 1305° C., pulling out the plug (3) blocking the pouring channel (401), and performing secondary inoculation simultaneously during the casting process, with a flow-inoculating agent of 0.1% of the weight of the molten iron, and finally completing the filling of the molten iron; The mass percentage of high-purity pig iron is: C: 4.04%, Si: 0.47%, Mn: 0.04%, S: 0.010%, P: 0.013%, Ti: 0.002%, Cr: 0.009%, the sum of other alloying elements is less than 0.05%; The composition of scrap steel is as follows: Mn: 0.012%, S: 0.012%, P: 0.015%, Ti: 0.016%, Cr: 0.014%, Cu: 0.023%, Mo: 0.0015%, the sum of other alloying elements is less than 0.05%; The mass percentage of the components of the rare earth magnesium alloy spheroidizer is: Mg: 5.62%, Al: 0.60%, Ca: 0.72%, Si: 43.00%, RE: 0.43%; The mass percentage of the components of the silicon-barium-calcium inoculant is: Al: 0.80%, Ca: 0.80%, Si: 71.00%, Ba: 2.40%; The specific organization and performance indicators of the material obtained by the preparation method of high-strength and high-toughness ferritic ductile iron are: matrix ferrite ≥85%, spheroidization rate ≥90%; when the wall thickness is ≤30 mm, the tensile strength is ≥450MPa, and the elongation is ≥18%; when the wall thickness is 30mm-60mm, the tensile strength is ≥430MPa, and the elongation is ≥14%; when the wall thickness is greater than 60 mm, the tensile strength is ≥420MPa, and the elongation is ≥12.5%. The target composition is C: 3.2%, Si: 2.0%, Mn: 0.2%, S: 0.02%, P: 0.02%, Ti: 0.025%, Cr: 0.02%, and Sb: 0.004%.

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