阅读说明：本技术 一种高强度高韧性等温淬火球铁材料及其制备方法和应用 (High-strength high-toughness isothermal quenching ductile iron material and preparation method and application thereof ) 是由 袁福安 金通 康明 霍卯田 余金科 晏克春 涂欣达 于 2021-06-30 设计创作，主要内容包括：本发明公开了一种高强度高韧性等温淬火球铁材料及其制备方法和应用,其制备方法包括以下步骤：将原料采用常规的方法制备成球墨铸铁试件；将球墨铸铁试件置于气体渗碳炉中,升温至760～780℃,并保温6～10分钟,整个升温保温过程控制炉气碳势为0.6～0.7；将炉气碳势升至0.8～0.9,然后将炉温升温至奥氏体化温度并保温；对球墨铸铁试件进行等温淬火处理,得到高强度高韧性等温淬火球铁材料。本发明通过对球铁成分的选择和控制,能够稳定生产出抗拉强度R-m≥1200MPa且延伸率≥8％的高强度高韧性等温淬火球铁材料,和现有等温淬火球铁材料相比,克服了强度提高的同时韧性降低的不足。(The invention discloses a high-strength high-toughness isothermal quenching ductile iron material, a preparation method and application thereof, wherein the preparation method comprises the following steps: preparing the raw materials into a nodular cast iron test piece by a conventional method; placing the nodular cast iron test piece in a gas carburizing furnace, heating to 760-780 ℃, and preserving heat for 6-10 minutes, wherein the carbon potential of furnace gas is controlled to be 0.6-0.7 in the whole heating and preserving process; raising the carbon potential of furnace gas to 0.8-0.9, then raising the furnace temperature to austenitizing temperature and preserving heat; and carrying out isothermal quenching treatment on the nodular cast iron test piece to obtain the isothermal quenched nodular cast iron material with high strength and high toughness. The invention can stably produce the tensile strength R by selecting and controlling the components of the ductile iron m Compared with the existing isothermal quenching ductile iron material, the high-strength high-toughness isothermal quenching ductile iron material with the strength of more than or equal to 1200MPa and the elongation of more than or equal to 8 percent overcomes the defects of improved strength and reduced toughness.)

1. A preparation method of a high-strength high-toughness isothermal quenching ductile iron material is characterized by comprising the following steps:

(1) casting a nodular cast iron test piece comprising the following components in percentage by weight: 3.75-3.9% of C, 2.3-2.4% of Si, 0.2-0.3% of Mn, 0.6-0.8% of Ni, 0.3-0.4% of Mo, 0.6-0.8% of Cu, less than or equal to 0.03% of P, less than or equal to 0.02% of S, 0.025-0.05% of Mg, 0.025-0.05% of rare earth element Re, and the balance of Fe;

(2) placing the nodular cast iron test piece in a gas carburizing furnace, heating to 760-780 ℃, and preserving heat for 6-10 minutes, wherein the carbon potential of furnace gas is controlled to be 0.6-0.7 in the whole heating and preserving process;

(3) raising the carbon potential of furnace gas to 0.8-0.9, then raising the furnace temperature to austenitizing temperature and preserving heat;

(4) and carrying out isothermal quenching treatment on the nodular cast iron test piece to obtain the isothermal quenched nodular cast iron material with high strength and high toughness.

2. The method for producing a high-strength high-toughness austempered ductile iron material as set forth in claim 1, characterized in that: the preheating time from room temperature to 760-780 ℃ is 40-60 min.

3. The method for producing a high-strength high-toughness austempered ductile iron material as set forth in claim 1, characterized in that: the austenitizing temperature is 880-900 ℃.

4. The method for preparing the high-strength high-toughness austempered ductile iron material of claim 3, characterized by comprising the steps of: the time for raising the furnace temperature to the austenitizing temperature is 35-40 min, and the time for keeping the temperature at the austenitizing temperature is 85-90 min.

5. The method for producing a high-strength high-toughness austempered ductile iron material as set forth in claim 1, characterized in that: the isothermal quenching temperature is 340-360 ℃, and the time is 85-90 min.

6. The method for producing a high-strength high-toughness austempered ductile iron material as set forth in claim 1, characterized in that: the nodular cast iron test piece comprises the following components in percentage by weight: c3.75, Si 2.38, Mn 0.2, Ni 0.6, Mo 0.4, Cu 0.68, P0.03, S0.02, Mg 0.025, rare earth element Re 0.05 and the balance of iron.

7. The method for producing a high-strength high-toughness austempered ductile iron material as set forth in claim 1, characterized in that: the nodular cast iron test piece comprises the following components in percentage by weight: c3.88, Si 2.4, Mn 0.28, Ni 0.7, Mo 0.3, Cu 0.8, P0.02, S0.01, Mg 0.05, rare earth element Re 0.025 and the balance of iron.

8. The method for producing a high-strength high-toughness austempered ductile iron material as set forth in claim 1, characterized in that: the nodular cast iron test piece comprises the following components in percentage by weight: c3.9, Si 2.3, Mn 0.3, Ni 0.66, Mo 0.36, Cu 0.6, P0.02, S0.01, Mg 0.04, rare earth element Re0.03 and the balance of iron.

9. A high-strength high-toughness austempered ductile iron material characterized by being prepared by the method for preparing a high-strength high-toughness austempered ductile iron material according to any one of claims 1 to 8.

10. Use of the high strength and high toughness austempered ductile iron material of claim 9.

Technical Field

The invention relates to the technical field of cast iron materials, in particular to a high-strength high-toughness isothermal quenching ductile iron material and a preparation method and application thereof.

Background

Isothermal quenching ductile iron materials (ADI) are cast iron materials obtained by carrying out heat treatment on ductile iron with certain components through an isothermal quenching process, and have high comprehensive mechanical properties such as strength, elongation and impact value, so that the isothermal quenching ductile iron materials (ADI) are commonly used for replacing automobile parts such as ductile iron, cast steel, forged steel, aluminum alloy and the like, and the aims of light weight and cost reduction are fulfilled.

The national standard GB/T24733-2009 isothermal quenching nodular iron castings discloses mechanical property indexes (tensile strength MPa/percent elongation/hardness HBW): 800/10/270-340, 900/8/270-340, 1050/6/310-380, 1200/3/340-420, 1400/1/380-480, it can be seen that as strength and hardness increase, the toughness and elongation of ADI gradually decrease, and how to maintain high elongation at high strength is a problem.

Chinese patent document CN106947911A discloses an austempered ductile iron material with high strength, high toughness and high wear resistance, which comprises the following components: 3.3 to 3.8 percent of C, 2.5 to 2.8 percent of Si, 0.2 to 0.3 percent of Mn, less than or equal to 0.02 percent of Cr, 0.50 to 0.80 percent of Ni, 0.25 to 0.40 percent of Mo, 0.7 to 1.0 percent of Cu, less than or equal to 0.025 percent of P, less than or equal to 0.016 percent of S, 0.03 to 0.05 percent of residual Mg, 0.006 to 0.01 percent of Re, and the balance of iron. The isothermal quenching ductile iron material has more alloy element addition amount, higher cost and less than 4.1 percent of elongation when the mechanical property is more than 1200 MPa.

Chinese patent document CN106947912A discloses an austempered ductile iron and a casting method thereof, wherein the austempered ductile iron contains the following components by weight percentage: 3.14-3.86% of C, 2.32-2.76% of Si, 0.32-0.98% of Mn, 0.01-0.04% of Mg, 0.45-0.67% of Cu, 0.51-0.78% of Ni, 0.2-0.4% of Mo, 0.15-0.35% of Cr, 0.005-0.027% of Sb, 0.025-0.057% of Re, 0.005-0.02% of Ce, less than 0.05% of P, less than 0.01% of S and the balance of Fe and other inevitable impurities.

Chinese patent document CN104878275A discloses a production process of a high-strength high-elongation ductile iron casting, which comprises the following steps: adding 18-22 parts of scrap steel into an electric furnace for smelting, adding 58-62 parts of scrap returns, and finally adding 0.5 part of texture modifier and 18-22 parts of pig iron for smelting to form base iron; secondly, adding a nodulizer into a nodulizing chamber of a nodulizing bag, covering an inoculant on the nodulizer, covering a silicon steel sheet on the inoculant, and finally adding a tissue modifier into the nodulizing bag; thirdly, the base iron with the temperature controlled at 1460-1480 ℃ in the electric furnace is reacted in a spheroidizing ladle to form final iron; and fourthly, pouring the final molten iron into the casting mould, and cooling to obtain the ductile iron casting. The elongation percentage of the ductile iron casting can reach 11% -12%, but the tensile strength is less than 650 MPa.

In summary, it is difficult to achieve both high tensile strength and high elongation in the austempered ductile iron material of the related art.

Disclosure of Invention

An object of the present invention is to provide an austempered ductile iron material which has both high elongation and high tensile strength, thereby having excellent mechanical properties, particularly toughness, as compared with conventional ductile cast irons in the above-mentioned prior art documents and the like, and cast parts and automobile structural parts of the austempered ductile iron material, and a method for producing the austempered ductile iron material.

In a first aspect, the invention provides a preparation method of a high-strength high-toughness isothermal quenching ductile iron material, which comprises the following steps:

(1) casting a nodular cast iron test piece comprising the following components in percentage by weight: 3.75-3.9% of C, 2.3-2.4% of Si, 0.2-0.3% of Mn, 0.6-0.8% of Ni, 0.3-0.4% of Mo, 0.6-0.8% of Cu, less than or equal to 0.03% of P, less than or equal to 0.02% of S, 0.025-0.05% of Mg, 0.025-0.05% of rare earth element Re, and the balance of Fe;

(2) placing the nodular cast iron test piece in a gas carburizing furnace, heating to 760-780 ℃, and preserving heat for 6-10 minutes, wherein the carbon potential of furnace gas is controlled to be 0.6-0.7 in the whole heating and preserving process;

(3) raising the carbon potential of furnace gas to 0.8-0.9, then raising the furnace temperature to austenitizing temperature and preserving heat;

(4) and carrying out isothermal quenching treatment on the nodular cast iron test piece to obtain the isothermal quenched nodular cast iron material with high strength and high toughness.

When the strength of the isothermal quenching ductile iron material prepared by the method reaches 1388MPa, the elongation can still be kept at 8 percent.

Preferably, the preheating time from room temperature to 760-780 ℃ is 40-60 min.

Preferably, the austenitizing temperature is 880-900 ℃.

Preferably, the time for raising the furnace temperature to the austenitizing temperature is 35-40 min, and the time for keeping the temperature at the austenitizing temperature is 85-90 min.

Preferably, the isothermal quenching temperature is 340-360 ℃, and the time is 85-90 min.

Preferably, the nodular cast iron test piece comprises the following components in percentage by weight: c3.75, Si 2.38, Mn 0.2, Ni 0.6, Mo 0.4, Cu 0.68, P0.03, S0.02, Mg 0.025, rare earth element Re 0.05 and the balance of iron. The tensile strength of the isothermal quenching ductile iron material prepared by the preferred scheme is 1320MPa, and the elongation is 9%.

Preferably, the nodular cast iron test piece comprises the following components in percentage by weight: c3.88, Si 2.4, Mn 0.28, Ni 0.7, Mo 0.3, Cu 0.8, P0.02, S0.01, Mg 0.05, rare earth element Re 0.025 and the balance of iron. The tensile strength of the isothermal quenching ductile iron material prepared by the optimal scheme is 1388MPa, and the elongation is 8%.

Preferably, the nodular cast iron test piece comprises the following components in percentage by weight: c3.9, Si 2.3, Mn 0.3, Ni 0.66, Mo 0.36, Cu 0.6, P0.02, S0.01, Mg 0.04, rare earth element Re0.03 and the balance of iron. The tensile strength of the isothermal quenching ductile iron material prepared by the optimal scheme is 1232MPa, and the elongation is 10%.

In a second aspect, the invention provides a high-strength high-toughness austempered ductile iron material prepared by the preparation method of the high-strength high-toughness austempered ductile iron material.

In a third aspect, the invention provides an application of the high-strength high-toughness austempered ductile iron material.

Compared with the prior art, the invention has the following advantages:

the invention can stably produce the tensile strength R by selecting the components of the nodular cast iron and controlling the parameters of the isothermal quenching process_mThe high-strength high-toughness isothermal quenching ductile iron material with the elongation rate of more than or equal to 1200MPa and more than or equal to 8 percent solves the problem that the high strength and the high toughness of the existing isothermal quenching ductile iron material are difficult to combine.

Detailed Description

In order to facilitate understanding of those skilled in the art, the technical solutions of the present invention will be described in further detail with reference to specific examples, but those skilled in the art will understand that the following examples are some examples of the present invention, rather than all examples, and are only used for illustrating the present invention, and should not be construed as limiting the scope of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention. The examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer.

The invention has strict requirements on casting and heat treatment when preparing the high-strength high-toughness isothermal quenching ductile iron material. In the casting aspect, higher nodularity, reasonable chemical components and metallographic structure are required, and in the heat treatment aspect, the temperature and the heat preservation time of austenitizing and isothermal quenching are strictly controlled.

The test piece of spheroidal graphite cast iron for austempered ductile iron material of the present invention can be prepared by a known method, for example, by mixing and melting raw materials such as scrap, scrap and various auxiliary materials to prepare molten iron having a desired composition, and adding a predetermined amount of a spheroidizing agent (e.g., Fe-Si-Mg alloy, yttrium-based heavy rare earth composite spheroidizing agent) to the molten iron, and casting into a test piece of spheroidal graphite cast iron of a desired shape. The chemical components of the nodular cast iron test piece need to be controlled, and the weight percentage of each component is as follows: 3.75 to 3.9 percent of C, 2.3 to 2.4 percent of Si, 0.2 to 0.3 percent of Mn, 0.6 to 0.8 percent of Ni, 0.3 to 0.4 percent of Mo, 0.6 to 0.8 percent of Cu, less than or equal to 0.03 percent of P, less than or equal to 0.02 percent of S, 0.025 to 0.05 percent of Mg, 0.025 to 0.05 percent of rare earth element Re, and the balance being iron.

Melting:

scrap steel, ferrosilicon, pig iron and various auxiliary materials can be mixed and then heated in a medium frequency induction furnace to 1350 ℃ to be melted into molten iron, and the molten iron is added with required alloy after slag removal.

Inoculation:

the inoculation is preferably carried out by adopting an in-package infusion method. A usual Fe-Si alloy may be used as an inoculant. The inoculation mode is in-ladle inoculation, stream inoculation or in-mold inoculation.

The casting method of the nodular cast iron test piece comprises the following steps:

the ductile iron test piece of the present invention may be prepared by gravity casting or the like, and the casting mold may be a green sand mold, a shell mold, a self-hardening mold, or other widely used permeable casting mold composed of sand grains. The pouring temperature is 1400-1480 ℃.

The heat treatment method of the nodular cast iron test piece comprises the following steps:

austenitizing the cast nodular cast iron test piece, and rapidly cooling the austenitized nodular cast iron test piece to a temperature higher than the martensite start transformation temperature by using a salt bath for isothermal quenching so as to convert pearlite into fine acicular ferrite and carbon-rich austenite; and after isothermal quenching, naturally cooling the ductile iron casting test piece in the air to obtain the high-strength high-toughness isothermal quenching ductile iron material. Preferably, before austenitizing, slowly heating to 760-780 ℃ to avoid overlarge internal stress of the nodular cast iron test piece caused by too fast heating; in the austenitizing treatment process, the carbon potential of furnace gas is controlled to be higher than that of austenite, enough time is kept, and the phenomenon of decarburization of the austenite, which causes overlarge internal stress, is avoided.

Specifically, the heat treatment method for the nodular cast iron test piece comprises the following steps: placing the nodular cast iron test piece in a gas carburizing furnace, heating to 760-780 ℃, and preserving heat for 6-10 minutes, wherein the carbon potential of furnace gas is controlled to be 0.6-0.7 in the whole heating and preserving process; raising the carbon potential of furnace gas to 0.8-0.9, then raising the furnace temperature to austenitizing temperature and preserving heat; and carrying out isothermal quenching treatment on the nodular cast iron test piece to obtain the isothermal quenched nodular cast iron material with high strength and high toughness. When the austenitizing temperature is lower, the diffusion capacity of alloy elements and carbon atoms is poorer, carbides cannot be completely dissolved, the solid solubility of carbon in austenite is lower, the hardenability of a matrix is poorer, and meanwhile, because the supercooling degree from a high-temperature furnace to a salt bath furnace is lower, the transformation is slower in the isothermal quenching stage, the final structure is a massive lath or massive matrix structure, and the strength is lower; the austenitizing temperature is higher, the crystal grains grow to resist certain side effects generated by tensile strength, so that the tensile strength is reduced, meanwhile, the heat treatment cost is increased, and the heat treatment efficiency is reduced.

Preferably, the preheating time from room temperature to 760-780 ℃ is 40-60 min, the austenitizing temperature is 880-900 ℃, the heating time from the furnace temperature to the austenitizing temperature is 35-40 min, the heat preservation time of the ductile iron test piece at the austenitizing temperature is 85-90 min, the isothermal quenching temperature is 340-360 ℃, and the time is 85-90 min.

Preferably, the nodular cast iron test piece comprises the following components in percentage by weight: c3.75, Si 2.38, Mn 0.2, Ni 0.6, Mo 0.4, Cu 0.68, P0.03, S0.02, Mg 0.025, rare earth element Re 0.05 and the balance of iron.

Preferably, the nodular cast iron test piece comprises the following components in percentage by weight: c3.88, Si 2.4, Mn 0.28, Ni 0.7, Mo 0.3, Cu 0.8, P0.02, S0.01, Mg 0.05, rare earth element Re 0.025 and the balance of iron.

Preferably, the nodular cast iron test piece comprises the following components in percentage by weight: c3.9, Si 2.3, Mn 0.3, Ni 0.66, Mo 0.36, Cu 0.6, P0.02, S0.01, Mg 0.04, rare earth element Re0.03 and the balance of iron.

Preferably, the salt bath is prepared by using 50-55% KNO as quenching agent₃+45％～50％NaNO₂。

Example 1

The high-strength and high-toughness austempered ductile iron material prepared by the embodiment comprises the following chemical components in percentage by weight: c3.75, Si 2.38, Mn 0.2, Ni 0.6, Mo 0.4, Cu 0.68, P0.03, S0.02, Mg 0.025, rare earth element Re 0.05 and the balance of iron.

The method for preparing the high-strength high-toughness austempered ductile iron material comprises the following steps of:

1. casting a nodular cast iron test piece comprising the following components in percentage by weight: c3.75, Si 2.38, Mn 0.2, Ni 0.6, Mo 0.4, Cu 0.68, P0.03, S0.02, Mg 0.025, rare earth element Re 0.05 and the balance of iron; smelting, inoculating, spheroidizing, pouring and cooling to obtain a nodular cast iron test piece with the wall thickness of 40 mm; wherein, the smelting step adopts a conventional method, and raw materials are smelted in a medium-frequency induction furnace; the inoculation step adopts an in-package flushing method for inoculation; the casting temperature was controlled to 1420 ℃ in the casting step.

2. At a carbon potential of 0.6: preheating a ductile cast iron test piece from room temperature to 780 ℃ and keeping the temperature for 6min, wherein the time for preheating from the room temperature to 780 ℃ is 50 min;

3. heating the nodular cast iron test piece from 780 ℃ to 900 ℃ and preserving heat for 90min, wherein the time for heating from 780 ℃ to 900 ℃ is 30min, and the carbon potential is maintained to be 0.9 in the whole heating and preserving process;

4. rapidly transferring the nodular cast iron test piece into a molten salt tank of industrial salt, and carrying out isothermal quenching, wherein the isothermal quenching temperature is 350 ℃, and the isothermal quenching time is 85 min;

5. and after the salt bath isothermal quenching, naturally cooling the nodular cast iron test piece in the air to obtain the high-strength high-toughness isothermal quenching ductile iron material.

The high-strength and high-toughness austempered ductile iron material obtained in the embodiment has the tensile strength of 1320MPa and the elongation at break of 9 percent, and belongs to QTD1200-8 materials.

Example 2

The high-strength and high-toughness isothermal quenching ductile iron material prepared by the embodiment comprises the following chemical components in percentage by weight: c3.88, Si 2.4, Mn 0.28, Ni 0.7, Mo 0.3, Cu 0.8, P0.02, S0.01, Mg 0.05, rare earth element Re 0.025 and the balance of iron.

The method for preparing the high-strength high-toughness austempered ductile iron material comprises the following steps of:

1. casting a nodular cast iron test piece comprising the following components in percentage by weight: c3.88, Si 2.4, Mn 0.28, Ni 0.7, Mo 0.3, Cu 0.8, P0.02, S0.01, Mg 0.05, rare earth element Re 0.025 and the balance of iron; smelting, inoculating, spheroidizing, pouring and cooling to obtain a nodular cast iron test piece with the wall thickness of 40 mm; wherein, the smelting step adopts a conventional method, and raw materials are smelted in a medium-frequency induction furnace; the inoculation step adopts an in-package flushing method for inoculation; the casting temperature is controlled to be 1450 ℃ in the casting step.

2. At a carbon potential of 0.7: preheating a ductile cast iron test piece from room temperature to 760 ℃ and keeping the temperature for 10min, wherein the time for preheating from the room temperature to 760 ℃ is 50 min;

3. heating the nodular cast iron test piece from 760 ℃ to 880 ℃ and preserving heat for 85min, wherein the time for heating from 760 ℃ to 880 ℃ is 40min, and the carbon potential is maintained to be 0.8 in the whole heating and heat preserving process;

4. rapidly transferring the nodular cast iron test piece into a molten salt tank of industrial salt, and carrying out isothermal quenching, wherein the isothermal quenching temperature is 340 ℃, and the isothermal quenching time is 88 min;

5. and after the salt bath isothermal quenching, naturally cooling the nodular cast iron test piece in the air to obtain the high-strength high-toughness isothermal quenching ductile iron material.

Tensile strength R of the high-strength high-toughness austempered ductile iron material obtained in the example_m1388MPa and 8 percent of elongation, belonging to QTD1200-8 material.

Example 3

The high-strength and high-toughness austempered ductile iron material prepared by the embodiment comprises the following chemical components in percentage by weight: c3.9, Si 2.3, Mn 0.3, Ni 0.66, Mo 0.36, Cu 0.6, P0.02, S0.01, Mg 0.04, rare earth element Re0.03 and the balance of iron.

The method for preparing the high-strength high-toughness austempered ductile iron material comprises the following steps of:

1. casting a nodular cast iron test piece comprising the following components in percentage by weight: c3.9, Si 2.3, Mn 0.3, Ni 0.66, Mo 0.36, Cu 0.6, P0.02, S0.01, Mg 0.04, rare earth element Re0.03, and the balance of iron; smelting, inoculating, spheroidizing, pouring and cooling to obtain a nodular cast iron test piece with the wall thickness of 40 mm; wherein, the smelting step adopts a conventional method, and raw materials are smelted in a medium-frequency induction furnace; the inoculation step adopts an in-package flushing method for inoculation; in the pouring step, the pouring temperature is controlled to be 1430 ℃.

2. At a carbon potential of 0.6: preheating a ductile cast iron test piece from room temperature to 780 ℃ and keeping the temperature for 8min, wherein the time for preheating from the room temperature to 780 ℃ is 50 min;

3. heating the nodular cast iron test piece from 780 ℃ to 890 ℃ and preserving heat for 87min, wherein the time for heating from 780 ℃ to 890 ℃ is 38min, and the carbon potential is maintained to be 0.9 in the whole heating and preserving process;

4. quickly transferring the nodular cast iron test piece into a molten salt tank of industrial salt, and carrying out isothermal quenching, wherein the isothermal quenching temperature is 360 ℃, and the isothermal quenching time is 90 min;

5. and after the salt bath isothermal quenching, naturally cooling the nodular cast iron test piece in the air to obtain the high-strength high-toughness isothermal quenching ductile iron material.

Tensile strength R of the high-strength high-toughness austempered ductile iron material obtained in the example_m1232MPa, elongation 10%, is QTD1200-8 material.

Comparative example 1

Unlike example 1, in this comparative example, Mo was not added and the alloying elements were insufficient.

The isothermal quenching ductile iron material prepared by the comparative example comprises the following chemical components in percentage by weight: c3.76, Si 2.32, Mn 0.22, Ni 0.68, Cu 0.74, P0.02, S0.01, Mg 0.037, rare earth element Re 0.033 and the balance of iron.

The method for preparing the austempered ductile iron material of the comparative example comprises the following steps:

1. casting a nodular cast iron test piece comprising the following components in percentage by weight: c3.76, Si 2.32, Mn 0.22, Ni 0.68, Cu 0.74, P0.02, S0.01, Mg 0.037, rare earth element Re 0.033 and the balance of iron; smelting, inoculating, spheroidizing, pouring and cooling to obtain a nodular cast iron test piece with the wall thickness of 40 mm; wherein, the smelting step adopts a conventional method, and raw materials are smelted in a medium-frequency induction furnace; the spheroidizing inoculation step adopts an in-ladle flushing method for inoculation; controlling the pouring temperature to be 1430 ℃ in the pouring step;

2. at a carbon potential of 0.6: preheating a ductile cast iron test piece from room temperature to 780 ℃ and keeping the temperature for 6min, wherein the time for preheating from the room temperature to 780 ℃ is 50 min;

3. heating the nodular cast iron test piece from 780 ℃ to 900 ℃ and preserving heat for 90min, wherein the time for heating from 780 ℃ to 900 ℃ is 35min, and the carbon potential is maintained to be 0.9 in the whole heating and preserving process;

4. rapidly transferring the nodular cast iron test piece into a molten salt tank of industrial salt, and carrying out isothermal quenching, wherein the isothermal quenching temperature is 360 ℃, and the isothermal quenching time is 85 min;

5. and (3) after the salt bath austempering, naturally cooling the nodular cast iron test piece in the air to obtain the austempered ductile iron material.

Tensile strength R of the austempered ductile iron material obtained in the comparative example_m990MPa and elongation of 5%.

Comparative example 2

The isothermal quenching ductile iron material prepared by the comparative example comprises the following chemical components in percentage by weight: c3.86, Si 2.37, Mn 0.21, Ni 0.71, Mo 0.35, Cu 0.98, P0.03, S0.02, Mg 0.032, rare earth element Re 0.041, and the balance of iron.

The method for preparing the austempered ductile iron material of the comparative example comprises the following steps:

1. casting a nodular cast iron test piece comprising the following components in percentage by weight: c3.86, Si 2.37, Mn 0.21, Ni 0.71, Mo 0.35, Cu 0.98, P0.03, S0.02, Mg 0.032, rare earth element Re 0.041, and the balance of iron; smelting, inoculating, spheroidizing, pouring and cooling to obtain a nodular cast iron test piece with the wall thickness of 40 mm; wherein, the smelting step adopts a conventional method, and raw materials are smelted in a medium-frequency induction furnace; the inoculation step adopts an in-package flushing method for inoculation; controlling the pouring temperature to be 1430 ℃ in the pouring step;

2. at a carbon potential of 0.6: preheating a ductile cast iron test piece from room temperature to 780 ℃ and keeping the temperature for 6min, wherein the time for preheating from the room temperature to 780 ℃ is 50 min;

3. heating the nodular cast iron test piece from 780 ℃ to 900 ℃ and preserving heat for 90min, wherein the time for heating from 780 ℃ to 900 ℃ is 35min, and the carbon potential is maintained to be 0.9 in the whole heating and preserving process;

4. rapidly transferring the nodular cast iron test piece into a molten salt tank of industrial salt, and carrying out isothermal quenching, wherein the isothermal quenching temperature is 360 ℃, and the isothermal quenching time is 85 min;

5. and (3) after the salt bath austempering, naturally cooling the nodular cast iron test piece in the air to obtain the austempered ductile iron material.

Tensile strength R of the obtained high-strength high-toughness isothermal quenching ductile iron material_m1282MPa and the elongation rate is 9 percent.

Comparative example 3

The isothermal quenching ductile iron material prepared by the comparative example comprises the following chemical components in percentage by weight: c3.72, Si 2.35, Mn 0.24, Ni 0.65, Mo 0.37, Cu 0.77, P0.03, S0.02, Mg 0.028, rare earth element Re 0.039, and the balance of iron.

The method for preparing the austempered ductile iron material of the comparative example comprises the following steps:

1. casting a nodular cast iron test piece comprising the following components in percentage by weight: c3.72, Si 2.35, Mn 0.24, Ni 0.65, Mo 0.37, Cu 0.77, P0.03, S0.02, Mg 0.028, rare earth element Re 0.039, and the balance of iron; smelting, inoculating, spheroidizing, pouring and cooling to obtain a nodular cast iron test piece with the wall thickness of 40 mm; wherein, the smelting step adopts a conventional method, and raw materials are smelted in a medium-frequency induction furnace; the inoculation step adopts an in-package flushing method for inoculation; controlling the pouring temperature to be 1430 ℃ in the pouring step;

2. at a carbon potential of 0.6: preheating a ductile cast iron test piece from room temperature to 780 ℃ and keeping the temperature for 6min, wherein the time for preheating from the room temperature to 780 ℃ is 50 min;

3. heating the nodular cast iron test piece from 780 ℃ to 800 ℃ and preserving heat for 90min, wherein the time for heating from 780 ℃ to 800 ℃ is 35min, and the carbon potential is maintained to be 0.9 in the whole heating and preserving process;

4. rapidly transferring the nodular cast iron test piece into a molten salt tank of industrial salt, and carrying out isothermal quenching, wherein the isothermal quenching temperature is 360 ℃, and the isothermal quenching time is 85 min;

5. and (3) after the salt bath austempering, naturally cooling the nodular cast iron test piece in the air to obtain the austempered ductile iron material.

Tensile strength R of the austempered ductile iron material obtained in the comparative example_m1048MPa and 6% elongation.

Comparative example 4

The isothermal quenching ductile iron material prepared by the comparative example comprises the following chemical components in percentage by weight: c3.84, Si 2.38, Mn 0.28, Ni 0.66, Mo 0.33, Cu 0.72, P0.02, S0.01, Mg 0.047, rare earth element Re 0.043 and the balance of iron.

The method for preparing the austempered ductile iron material of the comparative example comprises the following steps:

1. casting a nodular cast iron test piece comprising the following components in percentage by weight: c3.84, Si 2.38, Mn 0.28, Ni 0.66, Mo 0.33, Cu 0.72, P0.02, S0.01, Mg 0.047, rare earth element Re 0.043, and the balance of iron; smelting, inoculating, spheroidizing, pouring and cooling to obtain a nodular cast iron test piece with the wall thickness of 40 mm; wherein, the smelting step adopts a conventional method, and raw materials are smelted in a medium-frequency induction furnace; the inoculation step adopts an in-package flushing method for inoculation; controlling the pouring temperature to be 1430 ℃ in the pouring step;

2. at a carbon potential of 0.6: preheating a ductile cast iron test piece from room temperature to 780 ℃ and keeping the temperature for 6min, wherein the time for preheating from the room temperature to 780 ℃ is 50 min;

3. heating the ductile cast iron test piece from 780 ℃ to 980 ℃ and preserving heat for 90min, wherein the time for heating from 780 ℃ to 980 ℃ is 35min, and the carbon potential is maintained to be 0.9 in the whole heating and preserving process;

4. rapidly transferring the nodular cast iron test piece into a molten salt tank of industrial salt, and carrying out isothermal quenching, wherein the isothermal quenching temperature is 360 ℃, and the isothermal quenching time is 85 min;

5. and (3) after the salt bath austempering, naturally cooling the nodular cast iron test piece in the air to obtain the austempered ductile iron material.

Tensile strength R of the austempered ductile iron material obtained in the comparative example_m1022MPa, and an elongation of 6%.

TABLE 1 chemical composition of examples and comparative examples

TABLE 2 Heat treatment Process parameters for the examples and comparative examples

As shown in tables 1 and 2, the addition amount of the alloying elements (Cu, Ni, Mo) was small, and the strength and toughness were reduced; the addition amount of alloy elements is high, the cost is increased, and the performance cannot be continuously improved. Comparative example 1 is different from example 1 in that the alloy element is insufficient (Mo is not added), resulting in insufficient hardenability, and strength and elongation are reduced; the difference between the comparative example 2 and the example 1 is that the alloy element is excessive (the addition amount of Cu is increased to 0.98), the material cost is increased, and the performance is not continuously improved; comparative example 3 differs from example 1 in that the temperature rise is below the required range, and example 4 differs from comparative example 1 in that the temperature rise is above the required range, all resulting in a decrease in strength and toughness.

Comparative example 5

Referring to example 1 of Chinese patent document CN106947911A, austempered ductile iron with high strength, high toughness and high wear resistance is prepared by the following steps:

1. preparing molten iron: taking pig iron, ferrosilicon, scrap steel and foundry returns as main sources of carbon, silicon and Mn, adjusting the contents of Ni, Mo and Cu by using alloys such as ferronickel, ferromolybdenum, electrolytic copper and the like, calculating the proportion of each raw material according to the requirements of the final molten iron, putting each raw material into an intermediate frequency circuit for smelting according to the requirements, measuring the components of the molten iron when the molten iron is clear and the temperature is 1320-; spheroidizing by a flushing method, placing 1 wt% of nodulizer and 1.1 wt% of inoculant in a 1 ton spheroidizing ladle, covering the nodulizer with scrap iron, flushing the nodulizer into the original molten iron, standing for 1 minute, pouring the molten iron into a pouring ladle, carrying out stream inoculation during pouring, wherein the inoculant is 0.8-1.1 wt%, and the composition of the obtained final molten iron is as follows: 3.75% of C, 2.5% of Si, 0.21% of Mn, 0.016% of Cr, 0.50% of Ni, 0.32% of Mo, 0.8% of Cu, 0.021% of P, 0.013% of S, 0.032% of residual Mg, 0.007% of Re0.007% and the balance of iron.

The components of the nodulizer and the inoculant are the same as those in the tables 1 and 2 of the example 1 of the Chinese patent document CN 106947911A:

2. and pouring the final iron liquid into the casting mold to obtain a ductile iron casting blank.

3. Carrying out austenitizing treatment on the ductile iron casting blank, wherein the austenitizing temperature is 905 ℃, the heat preservation time is 90 minutes, and the temperature deviation is +/-5 ℃, then quickly transferring the ductile iron casting blank into a molten salt tank of industrial salt under a closed protective atmosphere, cooling to a temperature higher than the martensite start transformation temperature, and carrying out salt bath isothermal quenching treatment, wherein the salt bath temperature is 270 ℃, the time is 120 minutes, and the temperature deviation is +/-5 ℃;

4. and (3) after the salt bath austempering, naturally cooling the ductile iron casting blank in the air to obtain the austempered ductile iron.

The properties of the austempered ductile iron thus obtained were examined, and the results are shown in Table 3. Wherein, the tensile strength is tested by adopting a hydraulic strength tester.

TABLE 3 Properties of austempered ductile iron prepared in comparative example 5

Tensile strength Rm	1370MPa
		Elongation after fracture	3.9％

Comparative example 5 although a higher tensile strength was obtained, the elongation after fracture was too low in comparative example 5, compared with the high-strength, high-toughness austempered ductile iron material of 1388MPa tensile strength prepared in example 2.

Comparative example 6

Referring to Chinese patent document CN106947912A, isothermal quenching nodular cast iron is prepared in example 1, and comprises the following components in percentage by weight: 3.72 percent of C, 2.51 percent of Si, 0.48 percent of Mn0.025 percent of Mg, 0.5 percent of Cu, 0.63 percent of Ni, 0.34 percent of Mo, 0.27 percent of Cr, 0.015 percent of Sb, 0.037 percent of Re0.037 percent of Ce, less than 0.05 percent of P, less than 0.01 percent of S, and the balance of iron and other inevitable impurities.

The casting method of the austempered ductile iron comprises the following steps:

(1) mixing and heating scrap steel, ferrosilicon, pig iron and carburant to melt the scrap steel, the ferrosilicon, the pig iron and the carburant into molten iron, and controlling the chemical components of the molten iron and the weight percentage of the chemical components;

(2) when the temperature of the molten iron is increased to 1470 ℃, slagging off, then sequentially adding required amounts of alloys of all components, adjusting the content of all elements in the molten iron, heating to 1530 ℃, preserving the temperature for 8min, and tapping;

(3) controlling the temperature of molten iron to be 1500 ℃, transversely placing the converter, flushing the molten iron into the converter, continuously adding a silicon-strontium inoculant with the Al content of less than or equal to 0.1 percent into half of the molten iron when the molten iron is half discharged, and stirring and inoculating, wherein the addition amount of the inoculant is 0.08 percent of the molten iron amount;

(4) standing a converter, spheroidizing by adopting a flushing method, wherein the adding amount of a spheroidizing agent is 1.5 percent of the amount of molten iron, cooling the molten iron to 1380 ℃, starting pouring, applying vibration treatment to a sand mold during pouring, and obtaining a nodular cast iron sample after pouring;

(5) heating the nodular cast iron sample to 890 ℃ for austenitizing, preserving heat for 2h, cooling to 560 ℃, preserving heat for 1h, placing in air for air cooling, heating the nodular cast iron sample to 870 ℃ for preserving heat for 2h, and then rapidly putting the nodular cast iron sample into a water tank for quenching for a short time, wherein the quenching time is 5s, then putting the nodular cast iron sample into a salt bath furnace for isothermal quenching, wherein an isothermal quenching medium in the salt bath furnace consists of 55% of sodium nitrite and 45% of potassium nitrate, the isothermal quenching temperature is 360 ℃ and the temperature is kept for 1.5-3h, cooling the nodular cast iron sample after the isothermal quenching is finished, the cooling speed is 140 and 160 ℃/h, the cooling time is 2h, rapidly heating the cooled nodular cast iron sample to 220 ℃ and keeping the temperature for 1h, and naturally cooling the insulated nodular cast iron sample in the air to obtain the nodular cast iron. The tensile strength of the nodular cast iron obtained by the comparative example is 1082MPa, and the elongation is 10%. The tensile strength of comparative example 6 was much inferior to that of the high-strength, high-toughness austempered ductile iron material of example 3, which had an elongation of 10% and a tensile strength of 1232 MPa.

Industrial applicability

The high-strength, high-toughness austempered ductile iron material of the present invention can be used for various structural members, and is particularly suitable for automotive structural members because of excellent toughness. For example, it may be used for a knuckle, a crankshaft, a support beam, a connecting rod, a brake body, a brake bracket, a shackle, a spring bracket, a turbine housing, a bracket, a differential case, an engine mount bracket, and the like.

It is noted that, herein, relational terms such as first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other identical elements in a process, method, article, or apparatus that comprises the element.

The foregoing are merely exemplary embodiments of the present invention, which enable those skilled in the art to understand or practice the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.