Preparation method of bainite steel with large-size section, high strength, high toughness and uniform performance

Abstract

A preparation method of bainite steel with a large-size section, high strength, high toughness and uniform performance relates to the field of engineering material production, and comprises the following steps: forging a steel ingot, placing the refined steel ingot in a box-type furnace, carrying out first-stage rapid heating, carrying out second-stage slow heating, and carrying out austenitizing heat preservation; thirdly, medium cooling is conducted for three times, medium stagnation is conducted, and steel ingot heat conduction is conducted; according to the method, the surface of the large-size sample is cooled through water cooling, oil cooling or mist spraying, heat of the center of the large-size sample is conducted outwards, the surface temperature is increased, cooling is conducted through a medium, heat returning is conducted, and the steps are repeated. And cooling and heating for multiple times until the temperature of the core and the surface is uniform, and placing in air or burying sand for cooling. The obtained large-size component has high strength and plasticity, meanwhile, the content of retained austenite in the structure ranges from 10% to 15%, and the balance is bainite ferrite.

Description

Technical Field

[0001] The present invention relates to the technical field of engineering material production, and particularly to a preparation method of bainitic steel with high strength, high toughness and uniform properties in a large-size cross-section. Background Art

[0002] As an important infrastructure metal material, steel is subdivided into many categories and grades according to different application requirements and performance requirements, such as carbon steel, alloy steel, stainless steel, etc. Among them, bainitic steel has received extensive attention due to its unique performance advantages. Bainitic steel has excellent welding performance, good combination of strength and toughness, and fatigue and wear resistance, and is widely used in key components such as railway turnout frogs and large-thickness wear-resistant plates. These components often need to withstand large stresses and complex working conditions, so extremely high requirements are imposed on the performance of steel. However, the current preparation process of bainitic steel still faces many challenges in the production of large-size cross-section components.

[0003] In the traditional preparation process of bainitic steel, high-strength, high-toughness and uniform bainite structure can be obtained through isothermal treatment in a salt bath in the medium-temperature range. This method is feasible in the production of small-size specimens and precision components in the laboratory, but for large-size components, the salt bath isothermal process has obvious limitations. First, a large amount of waste salt is generated during the salt bath treatment process, causing serious environmental pollution and not meeting the environmental protection requirements of modern industrial production. Second, the equipment cost of the salt bath isothermal process is high, the operation is complex, and it is difficult to achieve large-scale industrial production.

[0004] In actual production, controlling continuous cooling is a more economical and environmentally friendly important way to produce large-size cross-section components. However, due to more heat storage inside large-size cross-section components, the cooling rate is uneven, and problems such as improper selection of cooling rate also occur from time to time. These problems make it difficult to ensure that the components produced in multiple batches have stable structures and properties, seriously affecting the wide application of bainitic steel in the field of large-size components. Summary of the Invention

[0005] The present invention aims to solve at least one of the technical problems in the related art to some extent.

[0006] To this end, the object of the present invention is to provide a preparation method of bainitic steel with high strength, high toughness and uniform properties in a large-size cross-section, obtaining stable bainitic steel without salt bath isothermal treatment, and the structure and properties of the environmentally friendly large-size cross-section specimen are stable from 10 mm away from the surface layer to the core, and the service life is stable in actual application.

[0007] To achieve the above object, the present invention provides a preparation method of bainitic steel with high strength, high toughness and uniform properties in a large-size cross-section, including the following steps:

[0008] S1: First, forge the steel ingot. Place the refined steel ingot in a box furnace, then carry out rapid heating in the first stage, followed by slow heating in the second stage, and then carry out austenitizing and holding.

[0009] S2: Conduct the first medium cooling and stagnate in the medium for 4 - 8 minutes.

[0010] S3: Take out the steel ingot for the first time and stagnate in the air for 2 - 5 minutes. Heat conduction occurs from the core to the surface of the steel ingot.

[0011] S4: Conduct the second medium cooling and stagnate in the medium for 1 - 5 minutes.

[0012] S5: Take out the steel ingot for the second time and stagnate in the air for 1 - 4 minutes. Heat conduction occurs from the core to the surface of the steel ingot.

[0013] S6: Conduct the third medium cooling and stagnate in the medium for 0.5 - 4 minutes.

[0014] S7: Take out the steel ingot for the third time and cool it to room temperature in the air.

[0015] Furthermore, in S1, the reaction conditions for the rapid heating stage in the first stage: rapidly heat up 600°C - 700°C in 20 - 30 min; the reaction conditions for the slow heating stage in the second stage: slowly heat up to 890°C - 980°C within 60 min - 120 min; the austenitizing holding time is 2 - 3 h.

[0016] Furthermore, in S1, the cross-sectional size of the forged steel ingot is not less than 110mm×180mm and not more than 200mm×300mm, and the length size is less than 200mm.

[0017] Furthermore, in S1, the composition of the forged steel ingot includes, by weight percentage, 0.15 - 0.60% carbon, 0.4 - 2.0% silicon, 1.0 - 3.4 manganese, and the balance is iron and other inevitable impurities.

[0018] Furthermore, in S7, after the steel ingot is cooled to room temperature in the air, samples are taken from the core to a position 10 - 60 mm away from the surface, and finally bainite structure is obtained, with the retained austenite between 10% - 15%. The yield strength of the steel nail is not less than 980 MPa, the tensile strength is not less than 1250 MPa, and the elongation is not less than 10.0%.

[0019] Furthermore, the cooling medium uses water or oil.

[0020] Beneficial effects:

[0021] 1. Solve the problem of large differences in the microstructure and properties between the surface layer and the core of large-size cross-section components in terms of temperature;

[0022] 2. Solve the problem that due to insufficient supercooling degree, other phases such as ferrite are obtained before the bainite phase transformation in the internal structure of large-size cross-section components, thereby affecting the performance.

[0023] 3. Solve the environmental protection temperature brought by conventional austempering.

[0024] This application can obtain stable bainite steel without salt bath isothermal treatment. The structure and performance of the environmental protection large-size cross-section specimen are stable from 10 mm away from the surface to the core, and the service life is stable in practical applications.

[0025] Additional aspects and advantages of the present invention will be given in part in the following description, become apparent in part from the following description, or be understood through the practice of the present invention. Brief Description of the Drawings

[0026] The above-mentioned and / or additional aspects and advantages of the present invention will become apparent and easy to understand from the following description of the embodiments in conjunction with the drawings, where:

[0027] Figure 1 It is a temperature change diagram for the preparation method of bainite steel with high strength, high toughness and uniform performance of large-size cross-section according to the present invention;

[0028] Figure 2 Parts a and b in the middle figure are the microstructural diagrams of the sample in Example 1 of the present invention at different magnification ratios. Detailed Embodiments

[0029] The embodiments of the present invention will be described in detail below. The examples of the embodiments are shown in the drawings, where the same or similar reference numerals denote the same or similar elements or elements with the same or similar functions from beginning to end. The embodiments described below with reference to the drawings are exemplary and are intended to explain the present invention, and should not be construed as limiting the present invention.

[0030] The preparation method of bainite steel with high strength, high toughness and uniform performance of large-size cross-section according to the embodiments of the present invention will be described below with reference to the drawings.

[0031] Example 1

[0032] Process: The first cooling medium water stagnates for 5 min, the heat return time in the air is 3 min to 4 min, the second water entry stagnates for 2.5 min, the air heat return time is 3 min, and the third water entry time is 1.7 min. At this time, the surface temperature of the sample is 320 - 360 °C. Finally, air cooling is carried out. Samples are taken at positions 15 - 60 mm away from the surface, and the sample performance is shown in the following table.

[0033]

[0034] Where: δ is the elongation; σ 0.2 is the yield strength; σ bis the tensile strength; A KU is the U-notch impact energy; V RA is the volume fraction of retained austenite.

[0035] Example 2

[0036] Process: The first immersion in water lasts for 4 - 5 min, the heat return time in air is 3 - 4 min, the second immersion in water lasts for 2 - 3 min, the heat return time in air is 2 - 3 min, the third immersion in water lasts for 1 - 2 min, and the surface temperature of the sample is 320 - 360 °C at this time. Finally, air cooling is carried out. At a distance of 15 - 60 mm from the surface, the sample properties are as shown in the following table.

[0037]

[0038] Example 3

[0039] Process: The first immersion in oil lasts for 5 - 6 min, the heat return time in air is 2 - 3 min, the second immersion in oil lasts for 1 - 2 min, the heat return time in air is 2 - 3 min, the third immersion in oil lasts for 1 - 2 min, and the surface temperature of the sample is 320 - 360 °C at this time. Finally, air cooling is carried out. At a distance of 15 - 60 mm from the surface, the sample properties are as shown in the following table.

[0040]

[0041] Example 4

[0042] Process: The first immersion in oil lasts for 5 min, the heat return time in air is 3 - 4 min, the second immersion in oil lasts for 2 - 3 min, the heat return time in air is 2 - 3 min, the third immersion in oil lasts for 1 - 2 min, and the surface temperature of the sample is 320 - 360 °C at this time. Finally, air cooling is carried out. At a distance of 15 - 60 mm from the surface, the sample properties are as shown in the following table.

[0043]

[0044] In the above examples, under the given composition of the forged steel ingot, according to the process given in the patent, the properties of the large-size cross-section bainitic steel reach the indexes mentioned in the patent, and at the same time, the content of retained austenite in the microstructure is also within its range.

[0045] In summary, in the present invention, by using water cooling, oil cooling or spraying, the surface of the large-size sample is cooled, and then by using the heat conduction from the core of the large-size sample to the outside to raise the surface temperature, then cooling with a medium and then heat return, and so on. By using multiple cooling and heating cycles until the core and surface temperatures are uniform, air cooling or buried sand cooling can be carried out. The large-size component obtained in this way has high strength and plasticity, and at the same time, the content of retained austenite in the structure is between 10% and 15%, and the rest is bainitic ferrite.

[0046] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A preparation method of a bainitic steel with high strength, high toughness and uniform properties in a large-size cross-section, characterized in that, It includes the following steps: S1: First, forge the ingot. Place the refined ingot in a box furnace, then perform rapid heating in the first stage, then perform slow heating in the second stage, and then perform austenitizing heat preservation. S2: Perform the first-stage medium cooling and stagnate in the medium for 4 - 8 minutes. S3: Take out the ingot for the first time and stagnate in the air for 2 - 5 minutes, and heat conduction occurs from the core to the surface of the ingot. S4: Perform the second-stage medium cooling and stagnate in the medium for 1 - 5 minutes. S5: Take out the ingot for the second time and stagnate in the air for 1 - 4 minutes, and heat conduction occurs from the core to the surface of the ingot. S6: Perform the third-stage medium cooling and stagnate in the medium for 0.5 - 4 minutes. S7: Take out the ingot for the third time. The temperature of the ingot is 270°C - 350°C, and it is cooled to room temperature in the air.

2. The preparation method of the bainite steel with high strength, high toughness and uniform performance of large cross-section according to claim 1, characterized in that, In S1, the reaction conditions for the rapid heating stage in the first stage: Rapidly heat up 600°C - 700°C in 20 - 30 minutes. The reaction conditions for the slow heating stage in the second stage: Slowly heat up to 890°C - 980°C within 60 - 120 minutes. The austenitizing heat preservation time is 2 - 3 hours.

3. The preparation method of the bainitic steel with high strength, high toughness and uniform properties in a large-size cross section according to claim 1, characterized in that In S1, the cross-sectional size of the forged ingot is not less than 110mm×180mm and not more than 200mm×300mm, and the length size is less than 200mm.

4. The preparation method of the bainitic steel with high strength, high toughness and uniform properties in a large-size cross-section according to claim 1, characterized in that In S1, the composition of the forged ingot by weight percentage includes 0.15 - 0.60% carbon, 0.4 - 2.0% silicon, 1.0 - 3.4 manganese, and the balance is iron and other inevitable impurities.

5. The preparation method of bainitic steel with high strength, high toughness and uniform properties in a large-size cross-section according to claim 1, characterized in that In S7, after the ingot is cooled to room temperature in the air, samples are taken from the core to a position 10 - 60mm away from the surface, and finally bainite structure is obtained.

6. The preparation method of the bainitic steel with high strength, high toughness and uniform properties for a large-size cross-section according to claim 1, characterized in that, The cooling medium used is water or oil.

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