High-strength low-cost gray cast iron and preparation method and application thereof

Abstract

The invention relates to the technical field of metallurgical materials, in particular to high-strength low-cost gray cast iron and a preparation method and application thereof.The gray cast iron is prepared from, by mass, 3.0%-3.2% of C, 1.7%-1.9% of Si, 0.7%-1.1% of Mn, smaller than or equal to 0.05% of P, 0.06%-0.09% of S, 0.35%-0.55% of Cu, 0.25%-0.35% of Cr, 0.05%-0.08% of Mo, smaller than or equal to 0.02% of Ti and the balance Fe and inevitable impurities. The invention further discloses a smelting method of the gray cast iron. The smelting method comprises the following steps of molten iron smelting, molten iron inoculation and pouring. According to the smelting method of the high-strength low-cost gray cast iron, a large amount of scrap steel and scrap iron are used, the cost of the gray cast iron material can be greatly reduced, but the mechanical property is not reduced, the mechanical property of the material is guaranteed through combination of Mn, Cu, Cr and a small amount of Mo, CE is controlled to be 3.5-3.8%, the good graphitization degree can be guaranteed, and the liquidity of molten iron can be guaranteed; and the shrinkage of molten iron is reduced to adapt to the production of complex cylinder bodies and cylinder covers.

Description

Technical Field

[0001] The present invention relates to the technical field of metallurgical materials, and particularly relates to a high-strength and low-cost gray cast iron and a preparation method and application thereof. Background Art

[0002] Gray cast iron is widely used in castings such as diesel engine cylinder blocks and cylinder heads due to its good performance. With the continuous upgrading of diesel engines, the material requirements for cylinder blocks and cylinder heads are getting higher and higher. The strength of some cylinder block bodies is required to be 300 MP or above, while in the face of a fierce market environment, the material cost required by production enterprises is getting lower and lower.

[0003] In order to ensure the performance of high-grade materials, the main furnace charges generally added in smelting are scrap steel, pig iron and clean return iron with good quality, less iron filings are used, and more precious alloys are added, such as copper, nickel, molybdenum, tin, antimony, vanadium, etc. In recent years, the prices of pig iron, scrap steel and alloy materials have continued to soar, and the smelting cost remains high.

[0004] A gray cast iron and its smelting method disclosed in Patent Application CN109182890A use scrap iron and scrap steel as the main furnace charges. Taking the total mass of gray cast iron as 100%, the non-metallic alloying elements include C: 2.8 - 3.2%, Si: 2.2 - 2.4%, P: ≤0.2% and S: ≤0.15%; the metallic alloying elements include Cr: 0.25 - 0.35% and at least one of Mn, Cu, Mo, Ni, Sn and Sb, and the total content of metallic alloying elements except Cr is 1.5 - 1.6%. (In the application, the detected element contents (wt%) are C Si Mn P S Ni 2.81 2.28 1.12 0.032 0.02 0.09; the element contents (wt%) of Cu Mo Sn Sb Cr Fe are 0.22 0.14 0.004 0.002 0.26 the balance). The gray cast iron smelted by this method is for machine tool base parts. The low C content may cause serious shrinkage in complex cylinder blocks and cylinder heads, and the Mo and Ni contents are still relatively high, and the cost is not low.

[0005] A high-strength nickel-containing gray cast iron and its casting method disclosed in Patent Application CN107267855A. The main furnace charge: the mass fraction of scrap steel is 60 - 80%, and the mass fraction of return iron is 20 - 80%. The composition control is C: 2.9 - 3.4%, Si: 1.3 - 1.7%, S: 0.05 - 0.1%, Mn: 0.6 - 1.0%, Cu: 0.4 - 0.9%, Cr: 0.15 - 0.4%, Ni: 0.1 - 0.3%, P < 0.07%. For the castings produced (the sampling thickness is between 25 - 35 mm), the strength is greater than 300 MPa, and the hardness is between 200 - 230 HBW. This method is suitable for small and medium-sized castings with a sampling thickness of 25 - 35 mm. For cylinder block castings with a slightly larger displacement (the sampling position is 35 - 45 mm), the material may not be able to reach 300 MPa. In addition, due to the addition of relatively high nickel and copper alloys, the cost is not low.

[0006] A preparation method of microalloyed ultra-high-strength high-carbon equivalent gray cast iron disclosed in Patent Application CN103074538A. The gray cast iron involved in this technology is mainly for ultra-high-strength products such as cylinder heads. The main raw materials are scrap steel and high-quality carburizers, and a variety of alloy raw materials need to be additionally added. In addition, the structure is strengthened by adding alloying elements such as RE-Ca-Si-V-Ti-N strengthening agents and Zr-Mn-Si strengthening agents. Therefore, the raw material cost is higher.

[0007] The current melting method is basically electric furnace melting. To reduce costs, most also adopt the synthetic process of scrap steel + carburizer. Some methods have been summarized in the production of high-strength small and medium-sized parts and machine tool base parts. However, the production method for cylinder blocks with a slightly larger displacement (the sampling wall thickness is 35 - 45 mm) still relies on the addition of large amounts of alloys. Especially, there is a lack in the production of complex high-grade large-displacement cylinder block castings for National VI. Therefore, it is very necessary to develop a high-strength and low-cost gray cast iron suitable for the production of complex high-grade large-displacement cylinder blocks and cylinder head castings for National VI. Summary of the Invention

[0008] Aiming at the deficiencies of the existing technology, one of the purposes of the present invention is to provide a high-strength and low-cost gray cast iron, which can be used for complex high-grade large-displacement cylinder blocks and cylinder head castings (the sampling wall thickness is 35 - 45 mm) and the material cost is significantly reduced compared with the existing process.

[0009] Its composition by mass percentage is: C: 3.0 - 3.2%, Si: 1.7 - 1.9%, Mn: 0.7 - 1.1%, P: ≤0.05%, S: 0.06 - 0.09%, Cu: 0.35 - 0.55%, Cr: 0.25 - 0.35%, Mo: 0.05 - 0.08%, Ti: ≤0.02%, and the balance is Fe and unavoidable impurities.

[0010] The second object of the present invention is to provide a method for preparing the high-strength and low-cost gray cast iron, including:

[0011] (1) Melting molten iron: Fill the medium-frequency furnace with iron materials. After the melting level drops, add part of the recarburizer, and then successively add all the added iron materials, recarburizer, and alloy to melt into molten iron. Detect the composition of the molten iron and adjust it to the target value of the original molten iron to obtain the original molten iron;

[0012] (2) Molten iron inoculation and pouring: Add 0.35 - 0.5% inoculant based on the total mass of the tapped iron to the original molten iron for the first inoculation treatment. After refining the molten iron, tap the iron and pour it to obtain the gray cast iron;

[0013] Among them, the composition of the original molten iron is calculated by mass percentage as follows: C: 3.0 - 3.2%, Si: 1.5 - 1.7%, Mn: 0.7 - 1.1%, P: ≤0.05%, S: 0.06 - 0.09%, Cu: 0.35 - 0.55%, Cr: 0.25 - 0.35%, Mo: 0.05 - 0.08%, Ti: ≤0.02%, and the balance is Fe and unavoidable impurities.

[0014] According to an embodiment of the present invention, the part of the recarburizer accounts for one-third of the total amount of the recarburizer.

[0015] According to an embodiment of the present invention, the iron materials include 50 - 70% scrap steel and 30 - 50% return scrap by mass percentage; the scrap steel includes corner scrap steel and baled scrap steel; the return scrap contains 5 - 20% iron chip return scrap.

[0016] Generally, the Mn content of the general corner scrap steel is relatively high (0.5 - 1.1%) while the Ti content is relatively low, and the Mn content of the baled scrap steel is low and the Ti content is high. Therefore, when formulating the ingredients, mainly use corner scrap, and formulate the ingredients on the principle of not adding ferromanganese during melting and the Ti not exceeding the composition requirements. The machining iron chip amount of conventional cylinder blocks and cylinder heads is 10 - 18%, and on-site, part of the iron chips are actually added according to the situation to reduce the material cost.

[0017] According to an embodiment of the present invention, in step (1), after obtaining the original molten iron, place the original molten iron at 1490 - 1500 °C for 10 - 15 min and perform slag skimming treatment.

[0018] According to an embodiment of the present invention, the inoculant in step (1) is a calcium-silicon-barium inoculant or a rare-earth inoculant, and the particle size of the inoculant is 2 - 8 mm; the inoculation method is in-stream inoculation or bottom inoculation in the ladle.

[0019] According to an embodiment of the present invention, the tapping temperature of the molten iron in step (2) is 1460 - 1480 °C.

[0020] According to an embodiment of the present invention, the pouring temperature in step (2) is 1380 - 1420 °C.

[0021] The third object of the present invention is to provide a high-strength and low-cost gray iron casting, including the aforementioned high-strength and low-cost gray iron;

[0022] Or, including the high-strength and low-cost gray iron obtained by the aforementioned preparation method.

[0023] According to an embodiment of the present invention, the gray iron casting includes at least one of a cylinder block and a cylinder head.

[0024] The present invention uses a large amount of scrap steel and iron filings, which can significantly reduce the cost of gray iron materials without reducing the mechanical properties.

[0025] The gray iron provided by the present invention uses a conventional combination of Cu, Cr, Mo, and Mn alloying elements. The raw materials are easy to purchase, and the amount of Mo used is small. In particular, a certain amount of iron filings and return iron are used in the main furnace charge, further reducing the material cost.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] (1) Generally, high-grade gray iron selects a lower C content or adds more precious alloys such as Mo, Cu, and Ni. It has high requirements for the casting structure and casting process design, and the cost is also relatively high. However, the present invention ensures the mechanical properties of the material through the reasonable and precise combination of alloys and components, using a combination of Mn, Cu, Cr, and a small amount of Mo. Controlling CE at 3.5 - 3.8% can ensure better graphitization degree and the fluidity of the molten iron, significantly reducing the addition amount of precious alloys. While the material properties of the gray iron reach a high level, the cost is greatly reduced. And it can reduce the shrinkage of the molten iron to adapt to the production of complex cylinder blocks and cylinder heads.

[0028] (2) Increasing the amount of iron filings in the batching reduces the cost. The reasonable combination of scrap steel from cuttings and bale press scrap can meet the target composition requirements without adding ferromanganese, further reducing the cost.

[0029] (3) The present invention improves the material properties of the material. The tensile strength of the poured standard test bar (∮30) reaches 340 - 360 MPa. When applied to complex cylinder blocks and cylinder heads of diesel engines (wall thickness is 35 - 45 mm), the tensile properties of the body reach 300 MPa and above, and the hardness is 200 - 240 HBW. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a metallographic structure photograph (graphite morphology, ×100) of the cylinder block casting obtained in Example 1 of the present invention;

[0031] Figure 2This is the metallographic structure photo (matrix structure, ×100) of the cylinder block casting obtained in Example 1 of the present invention. Detailed implementation mode

[0032] The following further illustrates the specific implementation of the present invention in combination with examples, but the implementation and protection of the present invention are not limited thereto. It should be noted that for the processes not specifically described in detail below, those skilled in the art can implement or understand them with reference to the prior art.

[0033] Example 1

[0034] This embodiment is described by taking a certain diesel engine cylinder block (grade HT300) produced in the workshop as an example.

[0035] Batching: The intermediate frequency furnace used on site is 10t, with 4000kg of scrap steel from the corners, 2000kg of pressed scrap steel, 3000kg of return iron such as gating and riser systems, 1000kg of return iron from iron filings, 122kg of ferrosilicon, 34kg of ferrochrome, 217kg of carbonaceous additive, 34kg of electrolytic copper, 12kg of sulfur additive, and 8kg of ferromolybdenum.

[0036] Melting: First, add the return iron from iron filings - fill the intermediate frequency furnace with scrap steel, start the power for melting. After the iron material drops, add 1 / 3 of the carbonaceous additive, then add scrap steel and the carbonaceous additive. After adding the scrap steel, add the return iron. When there is one batch of return iron material left, add the alloy, and melt to obtain the original molten iron.

[0037] After the molten iron is melted and cleared, raise the temperature to 1430 - 1480 °C (measured 1468 °C), take a sample to detect the composition of the original molten iron, and obtain: 3.18% C, 1.61% Si, 0.84% Mn, 0.025% P, 0.08% S, 0.53% Cu, 0.059% Mo, 0.31% Cr, 0.016% Ti, and the balance is Fe and inevitable impurities, and the composition meets the requirements. Raise the furnace temperature to 1490 - 1500 °C (measured 1496 °C), cut off the power for refining for 12 minutes, and at the same time perform slag skimming treatment. After the refining is completed, measure the temperature and pour out the iron at 1475 °C.

[0038] Inoculation: Adopt one-time inoculation, use the calcium-silicon-barium inoculant on the production site (particle size 2 - 8mm, main components: Si: 72 - 75%, Ba: 2 - 4%, Ca: 1 - 2%, Al: 1.0 - 1.5%, and the balance is Fe), and the addition amount is 0.4%, which is added into the molten iron with the flow when pouring out the iron.

[0039] Pouring: The inoculated molten iron is transported for pouring after slag skimming. Measure the temperature at 1403 °C before pouring, and pour the standard test bar and the cylinder block produced on site according to the normal operation pouring standard.

[0040] The metallographic structure of the cylinder block castings produced according to the above steps was detected. The metallographic photos of the graphite morphology and matrix structure are respectively as follows Figure 1 and Figure 2 shown.

[0041] It can be seen from Figure 1 that the graphite in the obtained cylinder block castings is flake graphite. Further in accordance with the American standard ASTM A-247 and the Chinese standard GB / T 7216-1987 "Metallographic Structure Standard for Gray Iron", it can be determined that the graphite in the castings belongs to type A graphite; according to GB / T 7216-1987, the graphite length grade can be determined to be grade 4.

[0042] It can be seen from Figure 2 that the obtained castings are a composite structure of pearlite and ferrite; after further measurement, the area ratio of pearlite is slightly greater than 98%, and the rest is ferrite.

[0043] The final molten iron composition controlled by the above method is as follows in the table:

[0044] Component C％ Si% Mn% P％ S％ Cu% Cr％ Mo% Ti% Fe% and others Content 3.12 1.89 0.85 0.025 0.08 0.52 0.31 0.059 0.016 Remainder

[0045] The tensile strength and hardness of the test bars and castings produced according to the above method are as follows:

[0046]

[0047]

[0048] It can be seen from the above that the gray iron provided by this application case has reached the grade of HT350, the tensile strength of the casting body has reached 303, 323 MPa, and the hardness is between 207-223 HBW. The melting material cost is much lower than normal, so high-strength and low-cost cylinder blocks and cylinder heads are obtained.

[0049] Example 2

[0050] This embodiment is described by taking a certain diesel engine cylinder block (grade HT300) produced in the workshop as an example.

[0051] Batching: The intermediate frequency furnace used on site is 10t, with 4200 kg of scrap steel from cuttings, 1800 kg of baled scrap steel, 2500 kg of return iron such as gating systems and risers, 1500 kg of return iron from iron filings, 111 kg of ferrosilicon, 29 kg of ferrochrome, 216 kg of recarburizer, 35 kg of electrolytic copper, 12 kg of sulfurizing agent, and 7 kg of ferromolybdenum.

[0052] Smelting: First, add iron filings and return scrap iron, fill the medium-frequency furnace with scrap steel, turn on the power for smelting. After the iron materials drop, add 1 / 3 of the recarburizer, then add scrap steel and the recarburizer. After adding scrap steel, add return scrap iron. When there is one batch of return scrap iron left, add the alloy. After melting and clearing, obtain the original molten iron.

[0053] After the molten iron is melted and cleared, raise the temperature to 1430 - 1480 °C (measured 1456 °C), take samples to detect the composition of the original molten iron, and obtain: 3.20% C, 1.56% Si, 0.94% Mn, 0.026% P, 0.08% S, 0.54% Cu, 0.056% Mo, 0.30% Cr, 0.013% Ti, with the balance being Fe and unavoidable impurities. The composition meets the requirements. Raise the furnace temperature to 1490 - 1500 °C (measured 1492 °C), cut off the power for refining for 14 minutes, and at the same time carry out slag skimming treatment. After the refining is completed, measure the temperature and tap the iron at 1468 °C.

[0054] Inoculation: Adopt one-time inoculation, use the calcium-silicon-barium inoculant on the production site (particle size 2 - 8 mm, main components: Si: 72 - 75%, Ba: 2 - 4%, Ca: 1 - 2%, Al: 1.0 - 1.5%, with the balance being Fe), and the addition amount is 0.42%. Add it into the molten iron along with the flow when tapping the iron.

[0055] Pouring: The inoculated molten iron is transferred for pouring after slag skimming. Measure the temperature at 1395 °C before pouring, and pour the standard test bars and the cylinder blocks produced on site according to the normal operation pouring standards.

[0056] The composition of the final molten iron controlled by the above method is as follows in the table:

[0057] Component C％ Si% Mn% P％ S％ Cu% Cr％ Mo% Ti% Fe% and others Content 3.14 1.74 0.94 0.026 0.08 0.54 0.30 0.057 0.013 Remainder

[0058] For the test bars and castings produced by the above method, detect the tensile strength and hardness as follows:

[0059] Serial number Item Tensile strength (MPa) Hardness (HBW) 1 Test bar 357 228 2 Cylinder block (sampling position: wall thickness of bearing seat is 41 mm) 309 212 3 Cylinder head 335 225

[0060] As can be seen from the above, for the gray cast iron provided by this application case, the grade basically reaches HT350. The tensile strength of the casting body reaches 309 - 335 MPa, and the hardness is 212 - 225 HBW. The melting material cost is much lower than normal. Therefore, high-strength and low-cost cylinder blocks and cylinder heads are obtained.

[0061] The above embodiments are only the preferred embodiments of the present invention, and are only used to explain the present invention, rather than limiting the present invention. Any changes, substitutions, modifications, etc. made by those skilled in the art without departing from the spirit essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A high-strength and low-cost gray cast iron, characterized in that, Its components by mass percentage are: C: 3.0 - 3.2%, Si: 1.7 - 1.9%, Mn: 0.7 - 1.1%, P: ≤0.05%, S: 0.06 - 0.09%, Cu: 0.35 - 0.55%, Cr: 0.25 - 0.35%, Mo: 0.05 - 0.08%, Ti: ≤0.02%, and the balance is Fe and unavoidable impurities.

2. The preparation method of the high-strength and low-cost gray cast iron according to claim 1, characterized in that, It includes: (1) Melting hot metal: Fill the medium-frequency furnace with iron materials. After the melting level drops, add part of the carburizer, and then successively add all the added iron materials, carburizer, and alloys to melt into hot metal. Detect the composition of the hot metal and adjust it to the original hot metal target value to obtain the original hot metal. (2) Hot metal inoculation and pouring: Add 0.35 - 0.5% inoculant by the total mass of the tapped hot metal for the first inoculation treatment in the original hot metal. After the hot metal is refined, tap the iron and pour it to obtain the gray cast iron. Among them, the composition of the original hot metal by mass percentage is: C: 3.0 - 3.2%, Si: 1.5 - 1.7%, Mn: 0.7 - 1.1%, P: ≤0.05%, S: 0.06 - 0.09%, Cu: 0.35 - 0.55%, Cr: 0.25 - 0.35%, Mo: 0.05 - 0.08%, Ti: ≤0.02%, and the balance is Fe and unavoidable impurities.

3. The preparation method according to claim 3, characterized in that, The part of the carburizer accounts for one-third of the total amount of all carburizers.

4. The preparation method according to any one of claims 2 or 3, characterized in that, The iron materials include 50 - 70% scrap steel and 30 - 50% return scrap iron by mass percentage; the scrap steel includes scrap steel from cuttings and baled scrap steel; the return scrap iron contains 5 - 20% iron filings return scrap iron.

5. The preparation method according to any one of claims 2 or 3, characterized in that, In step (1), after obtaining the original hot metal, place the original hot metal at 1490 - 1500 °C and let it stand for 10 - 15 minutes, and perform slag skimming treatment.

6. The preparation method according to any one of claims 2 or 3, characterized in that, The inoculant in step (1) is calcium-silicon-barium inoculant or rare-earth inoculant, and the particle size of the inoculant is 2 - 8 mm; the inoculation method is in-stream inoculation or bottom inoculation in the ladle.

7. The preparation method according to any one of claims 2 or 3, characterized in that, The tapping temperature of the hot metal in step (2) is 1460 - 1480 °C.

8. The preparation method according to any one of claims 2 or 3, characterized in that, The pouring temperature in step (2) is 1380 - 1420 °C.

9. A high-strength and low-cost gray iron casting, characterized in that, It includes the high-strength and low-cost gray cast iron described in claim 1; Or, it includes the high-strength and low-cost gray cast iron prepared by the preparation method described in any one of claims 2 - 8.

10. The high-strength and low-cost gray iron casting according to claim 9, wherein, The high-strength and low-cost gray cast iron parts include at least one of a cylinder block and a cylinder head.

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