Austenite-bainite ductile iron material with high tensile strength and heat treatment process and application thereof

Abstract

The invention discloses a high-tensile-strength austempered ductile iron material as well as a heat treatment process and application thereof, and relates to the technical field of austempered ductile iron processing. The heat treatment process of the austempered ductile iron material comprises the steps that an austempered ductile iron casting is taken and heated to 920-960 DEG C for normalizing treatment; and the austempered ductile iron casting is subjected to isothermal quenching treatment, the isothermal quenching temperature is 370-400 DEG C, the austempered ductile iron casting is air-cooled to the room temperature, and the austempered ductile iron material with the high tensile strength is obtained. The structure of the austempered ductile iron material is as follows: more than or equal to 98% of acicular bainite and austenite, and the balance of martensite and transformations thereof; the spheroidization rate of the austempered ductile iron material is larger than or equal to 92%, and the number of graphite is larger than or equal to 200 / mm < 2 >. The tensile strength of the steel plate is 920-1180 MPa, the hardness is 270-360 HBW, the elongation is 8%-12.5%, and the steel plate has a good application prospect in an SRT walking system.

Description

Technical Field

[0001] The invention relates to the technical field of austempered ductile iron processing, and more specifically to an austempered ductile iron material with high tensile strength and a heat treatment process and application thereof. Background Art

[0002] In the rail transit industry, the SRT running system is one of the main components; it requires materials to ensure that each casting can work well in harsh environments such as strong winds, high temperatures, high humidity, vibration, noise, corrosion, and detergent pollution. At present, the upper control arm, lower control arm, support frame, steering knuckle and other locomotive parts of the SRT running system in the rail transit industry are all made of austempered ductile iron (ADI) materials. ADI materials have extremely high strength, hardness and wear resistance, but with the development of the SRT running system, the requirements for materials have increased. It is necessary to study ADI materials that improve their tensile properties while meeting their mechanical properties.

[0003] The mechanical properties of Austempered ductile iron can be improved by adjusting the composition ratio of raw materials and / or optimizing the heat treatment process. For example, patent CN112795722A proposes an Austempered ductile iron isothermal quenching technology, which adjusts the composition of the alloy liquid raw materials, heats the castings at different temperatures and isothermally quenches them, and controls the insulation time, so that the castings have good mechanical properties and can be widely used in gears and other structural parts. However, although the Austempered ductile iron material has good compressive fatigue strength and tooth root bending fatigue strength through the adjustment of process parameters, it has failed to effectively improve the tensile properties of the Austempered ductile iron material. Summary of the invention

[0004] The purpose of the present invention is to overcome the defects and shortcomings of the above-mentioned austempered ductile iron material and provide a heat treatment process for austempered ductile iron material with high tensile strength. The mechanical properties of the obtained austempered ductile iron material meet the requirements of the rail transit running system and the tensile performance is greatly improved.

[0005] Another object of the present invention is to provide an austempered ductile iron material with high tensile strength.

[0006] Another object of the present invention is to provide an application of austempered ductile iron material with high tensile strength in a rail transit running system.

[0007] The above-mentioned purpose of the present invention is achieved through the following technical solutions:

[0008] The present invention provides a heat treatment process for an austempered ductile iron material, comprising the following steps:

[0009] S1, take the Austempered ductile iron casting and heat it to 920-960℃ for normalizing treatment, and the holding time is 1-2.0h;

[0010] S2, subjecting the austempered ductile iron casting treated in step S1 to isothermal quenching at a temperature of 370-400° C. for 1-1.5 h, and air-cooling to room temperature to obtain an austempered ductile iron material with high tensile strength.

[0011] The present invention controls the normalizing temperature parameters to make the material completely austenitized as soon as possible, thereby increasing the proportion of acicular bainite and residual austenite after subsequent quenching treatment. At the same time, a mixed structure of upper bainite and lower bainite is precipitated during the isothermal quenching process, and the proportion of upper bainite is appropriately increased by controlling the quenching temperature, thereby obtaining an austenitic ductile iron material with excellent tensile properties.

[0012] In some embodiments, in step S1, the heating method is heating with the furnace or instantaneous heating. Heating with the furnace means heating the material together with the furnace at room temperature to the required temperature; instantaneous heating means opening the furnace door and placing the material into the furnace after the furnace is heated to the required temperature.

[0013] In some embodiments, the heating method is to heat the temperature along with the furnace, and the heating rate is ≤120-160°C.

[0014] In some embodiments, in step S2, the quenching medium of the isothermal quenching treatment includes the following components in mass percentage: 50-70% sodium nitrite and 30-50% potassium nitrate.

[0015] The invention protects an austempered ductile iron material with high tensile strength, which is obtained by a heat treatment process of the austempered ductile iron material with high tensile strength.

[0016] An austempered ductile iron material, the structure of which is: acicular bainite and austenite ≥ 98%, and the remainder is martensite and its transformation products;

[0017] The spheroidization rate of the austempered ductile iron material is ≥92%, and the number of graphites is ≥200 / mm 2 .

[0018] The austempered ductile iron material of the present invention controls the organizational structure, spheroidization rate and the number of graphites so that the hardness is within a suitable range, while significantly improving the tensile strength, yield strength and elongation.

[0019] In some embodiments, the acicular bainite content in the microstructure is ≥ 85%.

[0020] In some embodiments, the chemical composition of the austempered ductile iron material comprises, by mass percentage:

[0021] C 3.5-3.8%, Si 2.4-2.9%, Mn 0.15-0.3%, P≤0.03%, S≤0.02%, Mo 0.1-0.3%, Ni 0.1-0.8%, Cu 0.1-0.8%, Mg 0.03-0.06%, and the balance is Fe.

[0022] The present invention controls the chemical composition of the material so that the material has better mechanical properties. Adding a certain content of Mn can improve the hardenability and improve the basic mechanical properties of the material; adding a certain content of Mg can ensure the spheroidization rate of the material and increase the number of graphite balls per unit area. At the same time, it is necessary to control the chemical composition ratio of the material. If the content of Mn is too high, carbides and segregation are likely to occur, resulting in a significant reduction in material performance. If the Mg content is too high, the shape of the graphite balls is easily distorted, the spheroidization rate is reduced, and the shrinkage tendency of the material is increased.

[0023] In some embodiments, the tensile strength is 920-1180 MPa, the hardness is 280-360 HBW, and the elongation is 8%-12.5%.

[0024] In some embodiments, the casting is made of austempered ductile iron through a heat treatment process of normalizing and then austempering.

[0025] In some embodiments, the method for preparing the austempered ductile iron casting is as follows: taking steel raw materials, performing smelting and pouring, and performing inoculation and spheroidization treatment to obtain the austempered ductile iron casting.

[0026] The invention discloses an application of austempered ductile iron material with high tensile strength in a rail transit running system.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The invention discloses a heat treatment process for austempered ductile iron material with high tensile strength, which controls the normalizing temperature parameters to make the material completely austenitized as soon as possible, thereby increasing the proportion of acicular bainite and residual austenite after subsequent quenching treatment. At the same time, a mixed structure of upper bainite and lower bainite is precipitated during the isothermal quenching process, and the proportion of upper bainite is appropriately increased by controlling the quenching temperature, thereby obtaining an austempered ductile iron material with excellent tensile properties.

[0029] The austempered ductile iron material of the present invention controls the organization structure, spheroidization rate and the number of graphites so that the hardness is within a suitable range and the tensile performance is significantly improved. The tensile strength is 920-1180MPa, the hardness is 270-360HBW and the elongation is 8%-12.5%. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1This is the microstructure metallographic diagram of the high tensile strength austempered ductile iron material after the heat treatment process of Example 1.

[0031] Figure 2 This is a graph for evaluating the spheroidization rate of the high tensile strength austempered ductile iron material after the heat treatment process of Example 4. DETAILED DESCRIPTION

[0032] The present invention is further described below in conjunction with specific embodiments, but the embodiments do not limit the present invention in any form. Unless otherwise specified, the raw materials and reagents used in the embodiments of the present invention are conventionally purchased raw materials and reagents.

[0033] Industrial pig iron, ordinary scrap steel and ductile iron recycled materials are used as raw materials; the castings are obtained by the traditional smelting-pouring method, and after inoculation and spheroidizing treatment, austenitic ductile iron castings are obtained.

[0034] The chemical composition of the austempered ductile iron casting is shown in Table 1 in terms of mass percentage.

[0035] Table 1

[0036]

[0037]

[0038] Example 1

[0039] A heat treatment process for austempered ductile iron material with high tensile strength comprises the following steps:

[0040] S1, take the above-mentioned austempered ductile iron casting, pre-grind the surface oxide scale, then wash and dry it with alcohol, put it into a heat treatment furnace, heat it to 920℃ and keep it for 1.5h, and the heating rate is 130℃;

[0041] S2, take out the austempered ductile iron casting treated in step S1 and immediately put it into mixed salt, and then perform isothermal quenching treatment, the isothermal quenching temperature is 370°C, the time is 1 hour, and then take it out of the furnace and air-cool it to room temperature to obtain an austempered ductile iron material with high tensile strength; the mixed salt is compounded by the following components in mass percentage: 60% sodium nitrite and 40% potassium nitrate.

[0042] Example 2

[0043] A heat treatment process for austempered ductile iron material with high tensile strength comprises the following steps:

[0044] S1, take the above-mentioned austempered ductile iron casting, pre-grind the surface oxide scale, then wash and dry it with alcohol, put it into a heat treatment furnace, heat it to 930°C and keep it for 1h, and the heating rate is 140°C;

[0045] S2, take out the austempered ductile iron casting treated in step S1 and immediately put it into mixed salt, and then perform isothermal quenching treatment, the isothermal quenching temperature is 380°C, the time is 1.2h, and then take it out of the furnace and air-cool it to room temperature to obtain an austempered ductile iron material with high tensile strength; the mixed salt is compounded by the following components in mass percentage: 60% sodium nitrite and 40% potassium nitrate.

[0046] Example 3

[0047] A heat treatment process for austempered ductile iron material with high tensile strength comprises the following steps:

[0048] S1, take the above-mentioned austempered ductile iron casting, pre-grind the surface oxide scale, then wash and dry it with alcohol, put it into a 940℃ heat treatment furnace, heat it instantly, and keep it warm for 1.4h;

[0049] S2, take out the austempered ductile iron casting treated in step S1 and immediately put it into the mixed salt, and then perform isothermal quenching treatment, the isothermal quenching temperature is 390°C, the time is 1.4h, and then take it out of the furnace and air-cool it to room temperature to obtain an austempered ductile iron material with high tensile strength; the mixed salt is compounded by the following components in mass percentage: 60% sodium nitrite and 40% potassium nitrate.

[0050] Example 4

[0051] A heat treatment process for austempered ductile iron material with high tensile strength comprises the following steps:

[0052] S1, take the above-mentioned austempered ductile iron casting, pre-grind the surface oxide scale, then wash and dry it with alcohol, put it into a 950℃ heat treatment furnace, heat it instantly, and keep it warm for 1.5h;

[0053] S2, take out the austempered ductile iron casting treated in step S1 and immediately put it into the mixed salt, and then perform isothermal quenching treatment, the isothermal quenching temperature is 400°C, the time is 1.5 hours, and then take it out of the furnace and air-cool it to room temperature to obtain an austempered ductile iron material with high tensile strength; the mixed salt is compounded by the following components in mass percentage: 60% sodium nitrite and 40% potassium nitrate.

[0054] Example 5

[0055] A heat treatment process for austempered ductile iron material with high tensile strength comprises the following steps:

[0056] S1, take the above-mentioned austempered ductile iron casting, pre-grind the surface oxide scale, then wash and dry it with alcohol, put it into a heat treatment furnace, heat it to 960°C and keep it for 1.0h, and the heating rate is 130°C;

[0057] S2, take out the austempered ductile iron casting treated in step S1 and immediately put it into the mixed salt, and then perform isothermal quenching treatment, the isothermal quenching temperature is 360°C, the time is 1 hour, and then take it out of the furnace and air-cool it to room temperature to obtain an austempered ductile iron material with high tensile strength; the mixed salt is compounded by the following components in mass percentage: 60% sodium nitrite and 40% potassium nitrate.

[0058] Example 6

[0059] A heat treatment process for austempered ductile iron material with high tensile strength comprises the following steps:

[0060] S1, take the above-mentioned austempered ductile iron casting, pre-grind the surface oxide scale, then wash and dry it with alcohol, put it into a heat treatment furnace, heat it to 960°C and keep it for 1.0h, and the heating rate is 130°C;

[0061] S2, take out the austempered ductile iron casting treated in step S1 and immediately put it into the mixed salt, and then perform isothermal quenching treatment, the isothermal quenching temperature is 370°C, the time is 1 hour, and then take it out of the furnace and air-cool it to room temperature to obtain an austempered ductile iron material with high tensile strength; the mixed salt is compounded by the following components in mass percentage: 60% sodium nitrite and 40% potassium nitrate.

[0062] Comparative Example 1

[0063] A heat treatment process for austempered ductile iron material with high tensile strength comprises the following steps:

[0064] S1, take the above-mentioned austempered ductile iron casting, pre-grind the surface oxide scale, then wash and dry it with alcohol, put it into a heat treatment furnace, heat it to 980°C and keep it for 1.5h, and the heating rate is 130°C;

[0065] S2, take out the austempered ductile iron casting treated in step S1 and immediately put it into the mixed salt, and then perform isothermal quenching treatment, the isothermal quenching temperature is 370°C, the time is 1 hour, and then take it out of the furnace and air-cool it to room temperature to obtain an austempered ductile iron material with high tensile strength; the mixed salt is compounded by the following components in mass percentage: 60% sodium nitrite and 40% potassium nitrate.

[0066] Comparative Example 2

[0067] A heat treatment process for austempered ductile iron material with high tensile strength comprises the following steps:

[0068] S1, take the above-mentioned austempered ductile iron casting, pre-grind the surface oxide scale, then wash and dry it with alcohol, put it into a heat treatment furnace, heat it to 920℃ and keep it for 1.5h, and the heating rate is 130℃;

[0069] S2, take out the austempered ductile iron casting treated in step S1 and immediately put it into the mixed salt, and then perform isothermal quenching treatment, the isothermal quenching temperature is 350°C, the time is 1 hour, and then take it out of the furnace and air-cool it to room temperature to obtain an austempered ductile iron material with high tensile strength; the mixed salt is compounded by the following components in mass percentage: 60% sodium nitrite and 40% potassium nitrate.

[0070] Performance Testing

[0071] 1. Characterization of tissue structure

[0072] The high tensile strength austempered ductile iron materials of Examples 1-4 were subjected to metallographic structure testing, and the results are shown in Table 2. Among them, the metallographic structure of the austempered ductile iron material of Example 1 is as follows: Figure 1 The spheroidization rate evaluation diagram of the austempered ductile iron material of Example 4 is shown in FIG. Figure 2 shown.

[0073] Table 2

[0074]

[0075] The results in Table 2 show that the needle-shaped bainite and austenite of the high tensile strength austempered ductile iron material of the present invention are ≥98%, the spheroidization rate is ≥92%, and the number of graphites is ≥200 / mm 2 .

[0076] 2. Mechanical properties test

[0077] The high tensile strength austempered ductile iron materials of Examples 1-4 were subjected to mechanical property tests, and the results are shown in Table 3.

[0078] Among them, the test methods for tensile strength, yield strength and elongation are: the test method of standard GB / T228.1, and the test conditions are room temperature.

[0079] The hardness is tested according to the standard GB / T231.1 test method.

[0080] Table 3

[0081] Tensile strength / MPa Yield strength / MPa Elongation / % Hardness / HBW Require ≥900 ≥650 ≥8 270-380 Example 1 1180 1020 8 360 Example 2 1100 980 9 330 Example 3 1020 880 10.5 315 Example 4 920 790 12.5 270 Example 5 1150 943 8.5 350 Example 6 1120 920 9 335 Comparative Example 1 1100 895 6 345 Comparative Example 2 1250 1030 4.5 385

[0082] The results show that the tensile strength of the Austempered Ductile Iron material of the present invention is 920-1180MPa, the hardness is 270-360HBW, and the elongation is 8%-12.5%. It is proved that after the Austempered Ductile Iron material is heat-treated by the heat treatment process of the present invention, the tensile strength of the material is significantly improved, and the industrial raw materials are cheap and no other precious metals are added. The preparation is also simple, and it has good application prospects in the SRT walking system.

[0083] The normalizing temperature in Comparative Example 1 is relatively high, which reduces the acicular bainite content of the treated austempered ductile iron material, and reduces the yield strength and elongation properties. The austempering temperature in Comparative Example 2 is relatively low, which reduces the proportion of upper bainite, improves the tensile strength, but significantly reduces the elongation.

[0084] The above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A heat treatment process for austempered ductile iron material, characterized in that: The following steps are involved: S1, take the Austempered ductile iron casting and heat it to 920-960℃ for normalizing treatment, and the holding time is 1-2.0h; S2, subjecting the austempered ductile iron casting treated in step S1 to isothermal quenching at a temperature of 370-400° C. for 1-1.5 h, and air-cooling to room temperature to obtain an austempered ductile iron material with high tensile strength.

2. The heat treatment process according to claim 1, characterized in that: In step S1, the heating method is heating along with the furnace or heating instantly.

3. The heat treatment process according to claim 2, characterized in that: The heating method is that when the temperature is raised along with the furnace, the heating rate is ≤120-160°C.

4. The heat treatment process according to claim 1, characterized in that: In step S2, the quenching medium of the isothermal quenching treatment includes the following components in mass percentage: 50-70% sodium nitrite and 30-50% potassium nitrate.

5. An austempered ductile iron material, characterized in that: The austempered ductile iron material is obtained by the heat treatment process of any one of claims 1 to 4.

6. The austempered ductile iron material according to claim 5, characterized in that: The structure of the austenitic ductile iron material is: acicular bainite and austenite ≥ 98%, and the remainder is martensite and its transformation products; The spheroidization rate of the austempered ductile iron material is ≥92%, and the number of graphites is ≥200 / mm 2 .

7. The austempered ductile iron material according to claim 6, characterized in that: The acicular bainite content in its structure is ≥85%.

8. The austempered ductile iron material according to claim 5, characterized in that: The chemical composition of the austempered ductile iron material includes, by mass percentage: C 3.5-3.8%, Si 2.4-2.9%, Mn 0.15-0.3%, P≤0.03%, S≤0.02%, Mo 0.1-0.3%, Ni 0.1-0.8%, Cu 0.1-0.8%, Mg 0.03-0.06%, and the balance is Fe.

9. The austempered ductile iron material according to claim 5, characterized in that: Its tensile strength is 920-1180MPa, hardness is 280-360HBW, and elongation is 8%-12.5%.

10. Use of the austempered ductile iron material according to any one of claims 5 to 9 in a rail transit running system.

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