Residual heat quenching nanometer bainite steel and its preparation method

Abstract

The application discloses a kind of afterheat quenching nanometer bainite steel and preparation method thereof, and the component mass percentage is C:0.35-0.40%, Si:1.00-1.50%, Mn:0.50-0.80%, Cr:2.50-3.00%, Mo:1.00-1.50%, Al:1.50-2.00%, P<0.05%, S<0.01%, the rest is iron and inevitable impurities.The application makes the internal chemical composition and organization distribution of cast ingot uniform by high-temperature diffusion homogenization;By forging in the cooling process after homogenization, reduce the internal defects in cast ingot, low temperature interval forging makes that nanometer bainite organization starts to change temperature and change time to reduce, low in cost;Afterheat quenching and heat preservation combine high-temperature deformation and heat treatment, save energy, simplify steps, simultaneously obtain nanometer bainite organization and film-like residual austenite, improve the comprehensive mechanical properties of steel material.

Description

Technical Field

[0001] This invention belongs to the field of steel material preparation technology, and in particular relates to a residual heat quenching nano-bainitic steel and its preparation method. Background Technology

[0002] Nanobainitic steel possesses excellent comprehensive properties, and its high strength and toughness make it widely used in bridges, ships, rolling bearings, railway tracks, and vehicle armor plates. Taking mold steel as an example, mold steel is a crucial material for mold manufacturing, and its microstructure significantly influences its performance. Depending on the preparation process and alloy composition design, the microstructure of mold steel can vary. Currently, obtaining mold steel with a nanobainitic microstructure faces several challenges, such as long production cycles, low resource utilization, cumbersome production processes, and unsatisfactory overall performance of the resulting steel. These problems also exist in other application areas of nanobainitic steel.

[0003] Patent document CN116536581A discloses a hot work die steel SD400 and its preparation method. The heat treatment involves residual heat quenching followed by high-temperature tempering, ultra-fine refining, and isothermal spheroidizing annealing to obtain the banded segregation structure of the die steel. This process is lengthy and involves complex heat treatment steps, which is not conducive to energy conservation and emission reduction. Patent document CN111893391A discloses a nano-bainitic hot work die steel and its preparation method, yielding a nano-bainitic hot work die steel with an impact energy of not less than 500J, a tensile strength of not less than 1900MPa, and a hardness of not less than 52HRC. However, its heat treatment process requires repeated heating and cooling to room temperature, making the production process complex. The patent document with publication number CN107723589B provides a medium carbon nanobainitic ultra-high strength steel plate and its preparation method. By strengthening the nanobainitic structure through supercooled austenite rolling deformation, the initial transformation temperature and transformation temperature of the nanobainitic structure are reduced, shortening the preparation time. However, due to the presence of the rolling process, the resulting steel has high internal stress and poor toughness. Summary of the Invention

[0004] This invention addresses the problems of long production cycles, cumbersome processes, and low resource utilization in existing technologies by providing a residual heat quenched nano-bainitic steel and its preparation method. The ingot undergoes high-temperature homogenization treatment, followed by residual heat quenching and holding through forging deformation, and then room temperature rolling and tempering to obtain nano-bainitic steel with excellent comprehensive properties. To achieve the aforementioned objective, the technical solution adopted by this invention is as follows:

[0005] As a first aspect of the invention, it provides a nano-bainitic steel obtained by residual heat quenching, wherein the composition by mass percentage is C: 0.35-0.40%, Si: 1.00-1.50%, Mn: 0.50-0.80%, Cr: 2.50-3.00%, Mo: 1.00-1.50%, Al: 1.50-2.00%, P < 0.05%, S < 0.01%, with the remainder being iron and unavoidable impurities.

[0006] Preferably, the composition by mass percentage is C: 0.40%, Si: 1.25%, Mn: 0.50%, Cr: 2.50%, Mo: 1.25%, Al: 1.50%, P < 0.05%, S < 0.01%, with the remainder being iron and unavoidable impurities.

[0007] This invention optimizes the preparation process by further refining the Si, Mn, and Cr elements in steel materials. Adding a high Si content effectively suppresses carbide precipitation during the holding stage, ensuring the formation of nano-bainitic structure. Adding Mn improves the hardenability of the steel, lowers the bainite transformation temperature, and refines the bainitic ferrite laths. Adding a high Cr content increases the strength of the nano-bainitic steel, but to save costs, its content is controlled below 3.00%. Adding Mo refines the grain size. Adding Al acts as a deoxidizer during the smelting process and also refines the grain size.

[0008] The nano-bainitic steel used in the product has a tensile strength of not less than 1900MPa, a hardness of 52.1HRC-59.4HRC, and an unnotched impact energy of more than 530J.

[0009] As a second aspect of the present invention, a method for preparing the nano-bainitic steel is provided, comprising the following steps:

[0010] S1. Melting: Melting and casting according to the composition to initially obtain the required ingot;

[0011] S2. Homogenization treatment: The obtained ingot is heated to 1200-1250℃ at 100-120℃ / h and held for 8-10h to complete high-temperature diffusion homogenization.

[0012] S3. Forging process: After high-temperature diffusion homogenization, the material is cooled after being taken out of the furnace and then forged.

[0013] S4. Residual heat quenching and heat preservation treatment: After forging, the residual heat parts are cooled and quenched to a temperature of 320±10℃ at a rate of 5-10℃ / s, held at the temperature, and then air-cooled to room temperature after being taken out of the furnace.

[0014] S5. Room temperature rolling and tempering: The product obtained in step S4 is rolled at room temperature. After rolling, it is heated to about 300±10℃ at a rate of 100-120℃ / h, held at that temperature for 0.5h-1h, and then air-cooled to room temperature.

[0015] Preferably, in step S3, after high-temperature diffusion homogenization, the material is cooled after being taken out of the furnace and then forged. The initial forging temperature is not lower than 950℃, and the final forging temperature is not lower than 900℃. The forging is carried out 3-4 times, and the deformation amount is not less than 10% each time. The maximum temperature range between the initial forging temperature and the final forging temperature does not exceed 80℃.

[0016] Furthermore, in step S3, the initial forging temperature is 960℃.

[0017] Preferably, in step S4, after quenching, the sample is transferred to a salt bath furnace for heat preservation for 2-2.5 hours.

[0018] Preferably, in step S5, the rolling process is performed 3-4 times, with each deformation amount not less than 5%.

[0019] The principle of this invention is as follows: At high temperatures, the diffusion of atoms in the non-equilibrium structure of the ingot accelerates, and prolonged high-temperature holding further homogenizes the internal structure and composition. Before room temperature rolling, a small amount of untransformed blocky retained austenite exists. During plastic deformation, this blocky austenite tends to preferentially transform into martensite. Therefore, after room temperature rolling, the blocky retained austenite transforms into lath martensite, thereby increasing the hardness of the nano-bainitic steel.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] This invention achieves uniform chemical composition and microstructure distribution within the ingot through high-temperature diffusion homogenization. Forging during the cooling process after homogenization reduces internal defects such as porosity and shrinkage cavities during casting. Simultaneously, forging at low temperatures lowers the initiation temperature and transformation time of the nano-bainite microstructure, resulting in lower costs. Residual heat quenching and heat treatment organically combine high-temperature deformation with heat treatment, saving energy and simplifying steps, while simultaneously obtaining nano-bainite microstructure and film-like retained austenite, improving the steel's strength and wear resistance. Room temperature rolling transforms the residual blocky austenite into martensite, and finally, tempering increases dislocations within the nano-bainite steel, eliminating internal stress and improving toughness. Attached Figure Description

[0022] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0023] Figure 1 A process flow diagram of the preparation method provided by the present invention;

[0024] Figure 2 Transmission electron microscopy (TEM) image of nano-bainitic steel provided by this invention. Detailed Implementation

[0025] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0026] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0027] like Figure 1 As shown, the technical steps provided by this invention are as follows:

[0028] S1. Melting: Melting and casting according to the ingredient ratio to initially obtain the required ingot.

[0029] S2. Homogenization treatment: The obtained ingot is heated to 1200-1250℃ at a rate of 100-120℃ / h and held for 10h to complete high-temperature diffusion homogenization.

[0030] S3. Forging process: After high-temperature diffusion homogenization, the product is cooled after being taken out of the furnace and then forged. The initial forging temperature is 960℃ and the final forging temperature is not lower than 900℃. The product is forged 3-4 times, and the deformation amount is not less than 10% each time.

[0031] S4. Residual heat quenching and heat preservation treatment: After forging, the residual heat parts are cooled and quenched at a rate of 5-10℃ / s to a temperature of 320±10℃ and then transferred to a salt bath furnace for heat preservation for 2-2.5 hours. After being taken out of the furnace, they are air-cooled to room temperature.

[0032] S5. Room temperature rolling and tempering: Roll the material obtained in step four at room temperature for 3-4 times, with a deformation of not less than 5% each time. After rolling, heat the material to about 300±10℃ at a rate of 100-120℃ / h, hold for 0.5h, and then air cool it to room temperature.

[0033] This invention organically combines high-temperature deformation with heat treatment through residual heat quenching and heat preservation, saving energy, simplifying the process, and simultaneously obtaining nano-bainitic microstructure and film-like retained austenite, such as... Figure 2As shown, this improves the strength and wear resistance of steel; room temperature rolling can transform residual blocky austenite into martensite, and finally tempering can increase the internal dislocations of nanobainitic steel, eliminate internal stress, and improve toughness.

[0034] Example 1,

[0035] The composition uses the following mass percentages: C: 0.40%, Si: 1.25%, Mn: 0.50%, Cr: 2.50%, Mo: 1.25%, Al: 1.50%, P < 0.05%, S < 0.01%, with the remainder being iron and unavoidable impurities.

[0036] The preparation process of the nano-bainitic steel in this embodiment is as follows:

[0037] S1. After the above ingredients are mixed, they are melted and cast into a 30mm×30mm×30mm square billet;

[0038] S2. Heat the obtained billet to 1250℃ at 100℃ / h and hold for 10h.

[0039] S3. After heat preservation, the billet is taken out of the furnace and cooled, and then forged at 960-900℃, with a deformation amount of 15% each time.

[0040] S4. After forging, the residual heat of the part is cooled and quenched at a rate of 10℃ / s to a temperature of 320℃, then transferred to a salt bath furnace for holding for 2.5 hours, and then air-cooled to room temperature after being taken out of the furnace.

[0041] S5. After cooling to room temperature, roll the steel. Each rolling deformation is 10%. After rolling, heat the steel to about 310°C at a rate of 100°C / h and hold it at that temperature for 0.5h. Then, air cool the steel to room temperature to obtain nano-bainitic steel.

[0042] Example 2

[0043] The composition uses the following mass percentages: C: 0.35%, Si: 1.50%, Mn: 0.80%, Cr: 3.00%, Mo: 1.50%, Al: 2.00%, P < 0.05%, S < 0.01%, with the remainder being iron and unavoidable impurities.

[0044] The preparation process of the nano-bainitic steel in this embodiment is as follows:

[0045] S1, Smelting;

[0046] S2. Heat the obtained billet to 1200℃ at 120℃ / h and hold for 8h.

[0047] S3. After heat preservation, the billet is taken out of the furnace and cooled. The initial forging temperature is 950℃ and the final forging temperature is 900℃. It is forged 3 times, and the deformation amount is 10% each time.

[0048] S4. After forging, the residual heat of the parts is cooled and quenched at a rate of 5℃ / s to a temperature of 320℃ and then placed in a salt bath furnace for holding for 2.5 hours. After being taken out of the furnace, the parts are air-cooled to room temperature.

[0049] S5. After cooling to room temperature, roll the steel. Each rolling deformation is 5%. After rolling, heat the steel to about 300°C at a rate of 120°C / h and hold for 0.5h. Then, air cool the steel to room temperature to obtain nano-bainitic steel.

[0050] Comparative Example 1

[0051] The preparation process of this comparative example is exactly the same as that of Example 1, except that the percentage of chemical composition is different. Specifically: C: 0.78%, Si: 0.55%, Mn: 0.45%, Cr: 3.20%, Mo: 0.50%, Al: 1.25%, P < 0.05%, S < 0.01%, and the remainder is iron and unavoidable impurities.

[0052] Comparative Example 2

[0053] The composition uses the following mass percentages: C: 0.40%, Si: 1.25%, Mn: 0.50%, Cr: 2.50%, Mo: 1.25%, Al: 1.50%, P < 0.05%, S < 0.01%, with the remainder being iron and unavoidable impurities.

[0054] The preparation process of the nano-bainitic steel in this embodiment is as follows:

[0055] S1. Melting: Melting and casting according to the composition to initially obtain the required ingot;

[0056] S2. Homogenization treatment: The obtained ingot is heated to 1250℃ at 100℃ / h and held for 10h to complete high-temperature diffusion homogenization.

[0057] S3. Forging process: After high-temperature diffusion homogenization, the product is cooled after being taken out of the furnace and then forged. The initial forging temperature is 1000℃ and the final forging temperature is not lower than 800℃. The product is forged 3-4 times, and the deformation amount is not less than 10% each time.

[0058] S4. Residual heat quenching and heat preservation treatment: After forging, the residual heat parts are cooled and quenched at a rate of 5℃ / s to a temperature of 320℃ and then transferred to a salt bath furnace for heat preservation for 2-2.5 hours. After being taken out of the furnace, they are air-cooled to room temperature.

[0059] S5. Room temperature rolling and tempering: The material obtained in step 4 is rolled at room temperature for 3-4 times, with a deformation of not less than 5% each time. After rolling, it is heated to about 310°C at a rate of 100°C / h and held for 0.5h. Then it is air-cooled to room temperature after being taken out of the furnace.

[0060] Comparative Example 3

[0061] The composition uses the following mass percentages: C: 0.40%, Si: 1.25%, Mn: 0.50%, Cr: 2.50%, Mo: 1.25%, Al: 1.50%, P < 0.05%, S < 0.01%, with the remainder being iron and unavoidable impurities.

[0062] The preparation process of the nano-bainitic steel in this embodiment is as follows:

[0063] S1. Melting: Melting and casting according to the ingredient ratio to initially obtain the required ingot.

[0064] S2. Homogenization treatment: The obtained ingot is heated to 1200℃ at 100℃ / h and held for 10h to complete high-temperature diffusion homogenization.

[0065] S3. Forging process: After high-temperature diffusion homogenization, the product is cooled after being taken out of the furnace and then forged. The initial forging temperature is 960℃ and the final forging temperature is not lower than 900℃. The product is forged 3-4 times, and the deformation amount is not less than 10% each time.

[0066] S4. Residual heat quenching and heat preservation treatment: After forging, the residual heat parts are cooled and quenched at a rate of 5℃ / s to a temperature of 270℃ and then transferred to a salt bath furnace for heat preservation for 2-2.5 hours. After being taken out of the furnace, they are air-cooled to room temperature.

[0067] S5. Room temperature rolling and tempering: The material obtained in step 4 is rolled at room temperature for 3-4 times, with a deformation of not less than 5% each time. After rolling, it is heated to about 310°C at a rate of 100°C / h and held for 0.5h. Then it is air-cooled to room temperature after being taken out of the furnace.

[0068] The unnotched impact energy, elongation, strength, and hardness of the nano-bainitic steels prepared in Examples 1, 2, and Comparative Examples 1-3 were tested, and the results are as follows:

[0069]

[0070] According to Examples 1-2 and Comparative Examples 1-3, the nano-bainitic steel prepared in Examples 1-2 has better impact energy, elongation, tensile strength, yield strength and hardness.

[0071] As can be seen from Example 1 and Comparative Example 1, the significant decrease in Si content reduces the inhibition of carbide precipitation, resulting in a significant increase in carbide precipitation and a significantly larger precipitate size. This will lead to a certain decrease in the impact resistance and strength of the obtained steel.

[0072] According to Example 1 and Comparative Example 2, in the hot forging process of Comparative Example 2, the forging temperature range is relatively large, the transformation temperature of the nano-bainite structure does not decrease significantly, and the holding time is relatively short under the same preparation process. The bainite laths are not nano-sized, so their hardness decreases significantly.

[0073] According to Example 1 and Comparative Example 3, in Comparative Example 3, during the residual heat quenching treatment, the sample was placed in a 270°C salt bath furnace for heat preservation, which resulted in the formation of martensite structure, thus significantly increasing its hardness and decreasing its elongation.

[0074] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A residual heat quenched nano-bainitic steel, characterized in that, The composition by mass percentage is C: 0.35-0.40%, Si: 1.00-1.50%, Mn: 0.50-0.80%, Cr: 2.50-3.00%, Mo: 1.00-1.50%, Al: 1.50-2.00%, P < 0.05%, S < 0.01%, with the remainder being iron and unavoidable impurities; The tensile strength is not less than 1900MPa, the hardness is 52.1HRC-59.4HRC, and the unnotched impact energy is higher than 530J.

2. The residual heat quenched nano-bainitic steel according to claim 1, characterized in that, The composition by mass percentage is C: 0.40%, Si: 1.25%, Mn: 0.50%, Cr: 2.50%, Mo: 1.25%, Al: 1.50%, P < 0.05%, S < 0.01%, with the remainder being iron and unavoidable impurities.

3. A method for preparing nano-bainitic steel, characterized in that, Includes the following steps: S1. Melting: Melting and casting according to the composition to initially obtain the required ingot; The composition by mass percentage is C: 0.35-0.40%, Si: 1.00-1.50%, Mn: 0.50-0.80%, Cr: 2.50-3.00%, Mo: 1.00-1.50%, Al: 1.50-2.00%, P < 0.05%, S < 0.01%, with the remainder being iron and unavoidable impurities; S2. Homogenization treatment: The obtained ingot is heated to 1200-1250℃ at 100-120℃ / h and held for 8h-10h to complete high-temperature diffusion homogenization. S3. Forging process: After high-temperature diffusion homogenization, the product is cooled after being taken out of the furnace and then forged. The initial forging temperature is not lower than 950℃ and the final forging temperature is not lower than 900℃. In step S3, the temperature range between the initial forging temperature and the final forging temperature shall not exceed 80℃. S4. Residual heat quenching and heat preservation treatment: After forging, the residual heat parts are cooled and quenched to a temperature of 320±10℃ at a rate of 5-10℃ / s, held at the temperature, and then air-cooled to room temperature after being taken out of the furnace. S5. Room temperature rolling and tempering: The product obtained in step S4 is rolled at room temperature. After rolling, it is heated to 300±10℃ at a rate of 100-120℃ / h, held for 0.5h-1h, and then air-cooled to room temperature.

4. The method for preparing nano-bainitic steel according to claim 3, characterized in that, In step S3, the initial forging temperature is 960℃.

5. The method for preparing nano-bainitic steel according to claim 3, characterized in that, In step S3, forging is performed 3-4 times, with each deformation amount not less than 10%.

6. The method for preparing nano-bainitic steel according to claim 3, characterized in that, In step S4, after quenching, the sample is transferred to a salt bath furnace for heat preservation for 2-2.5 hours.

7. The method for preparing nano-bainitic steel according to claim 3, characterized in that, In step S5, the rolling process is performed 3-4 times, with each deformation amount not less than 5%.

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