Method for heat treatment of ferritic-pearlitic steel microstructure

Abstract

The application discloses a ferrite-pearlite steel structure heat treatment method, and relates to the technical field of metal material heat treatment, and aims at solving the technical problem that the ferrite-pearlite steel processed by the existing heat treatment process has high strength but low toughness. The ferrite-pearlite steel structure heat treatment method comprises the following steps: heating and elevating a ferrite-pearlite steel workpiece to a temperature 30-60 DEG C higher than Ac3, and performing first heat preservation treatment under the temperature condition; the time of the first heat preservation treatment is at least the time required for the temperature distribution of the steel workpiece to be uniform; after the first heat preservation treatment, the steel workpiece is cooled to a temperature 0-60 DEG C higher than Ac1, second heat preservation treatment is performed under the temperature condition, and then the steel workpiece is cooled to normal temperature, the heat treatment is completed, and the time of the second heat preservation treatment is at least the time required for the temperature distribution of the steel workpiece to be uniform. The technical scheme provided by the application is used for heat treatment of the ferrite-pearlite steel workpiece, so as to improve the toughness of the steel workpiece.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for metallic materials, and in particular to a heat treatment method for ferrite-pearlite steel. Background Technology

[0002] Ferritic-pearlitic steel profiles possess advantages such as short production cycles, simple processes, and low manufacturing costs, making them one of the most widely used and consumed steel grades. Current heat treatment processes for ferritic-pearlitic steel profiles primarily involve heating the steel workpiece to austenitization and then directly cooling it. While this traditional process produces ferritic-pearlitic steel profiles with high strength, meeting strength requirements, it suffers from low impact toughness, especially poor low-temperature impact toughness, failing to meet toughness requirements and posing safety hazards during use. Therefore, a new heat treatment process is urgently needed to address these technical problems. Summary of the Invention

[0003] To address the technical problem that existing heat treatment processes result in ferritic-pearlitic steel with sufficient strength but insufficient toughness, this invention provides a heat treatment method for ferritic-pearlitic steel. Therefore, this invention is achieved through the following technical solution.

[0004] This invention provides a heat treatment method for ferrite-pearlite steel, comprising:

[0005] The ferritic-pearlitic steel workpiece is heated to 30-60°C above the Ac3 temperature and subjected to a first heat preservation treatment at this temperature. The time of the first heat preservation treatment is at least the time required to make the temperature distribution of the steel workpiece uniform.

[0006] After the first heat preservation treatment, the steel workpiece is cooled to a temperature 0-60°C above Ac1 and then subjected to a second heat preservation treatment. The duration of the second heat preservation treatment is at least the time required to make the temperature distribution of the steel workpiece uniform.

[0007] The steel workpiece after the second heat preservation treatment is cooled to room temperature to obtain the heat-treated steel workpiece.

[0008] Compared with existing technologies, in the ferrite-pearlite steel heat treatment method of the present invention, the steel workpiece is heated to the temperature of the first holding treatment, i.e., 30-60°C above the Ac3 temperature, to first obtain austenite in the steel workpiece. During the holding process at this temperature, elements such as carbon and manganese in the steel workpiece diffuse evenly, which can improve the stability of the steel workpiece. After the first holding treatment, the steel workpiece is cooled to the temperature of the second holding treatment, i.e., 0-60°C above the Ac1 temperature, and then the steel workpiece begins to form an austenite + ferrite two-phase structure. At this temperature, carbon atoms in the steel workpiece diffuse from ferrite to austenite more quickly, resulting in a higher carbon content in austenite. During the subsequent cooling process, the pearlite lamellae formed by the austenite transformation are finer, which improves the toughness of the steel workpiece while maintaining excellent strength. As the holding time is extended, the carbon content in the austenite inside the steel workpiece can be further increased, thereby forming a finer lamellar structure during subsequent cooling, which can further enhance the strength and toughness of the steel workpiece. Cooling the steel workpiece after the second heat treatment to room temperature yields the heat-treated steel workpiece. The above-mentioned technical solution of this invention solves the technical problem that existing heat treatment processes result in ferritic-pearlitic steel with sufficient strength but insufficient toughness, producing steel workpieces with high strength and good toughness.

[0009] Furthermore, in the heat treatment method for ferrite-pearlite steel structure of the present invention, the time required for the first heat treatment is:

[0010] When the thickness of the steel workpiece is no more than 200mm, the required time is 0.5 to 4 hours;

[0011] For each 25mm increase in thickness on the basis of a 200mm steel workpiece, the required time increases by 0.5 to 1 hour from the original 0.5 to 4 hours.

[0012] Furthermore, in the heat treatment method for ferrite-pearlite steel structure of the present invention, the time for the second heat treatment is:

[0013] When the thickness of the steel workpiece is no more than 200mm, the second heat preservation treatment time is 0.5 to 6 hours.

[0014] Furthermore, in the heat treatment method for ferritic-pearlitic steel structure of the present invention, for every 25mm increase in thickness of the steel workpiece based on 200mm, the second heat treatment time is increased by 1 to 2 hours based on 0.5 to 6 hours.

[0015] Furthermore, in the heat treatment method for ferrite-pearlite steel structure of the present invention, the cooling rate during the process of cooling the steel workpiece to a temperature 0 to 60°C above Ac1 is 20 to 300°C / h.

[0016] Furthermore, in the heat treatment method for ferrite-pearlite steel structure of the present invention, during the process of heating the steel workpiece to a temperature 30-60°C higher than the Ac3 temperature, the heating rate is 20-300°C / h.

[0017] Furthermore, in the heat treatment method for ferritic-pearlitic steel structure of the present invention, during the process of cooling the steel workpiece after the second heat treatment to room temperature, the cooling method is furnace cooling and / or air cooling.

[0018] Furthermore, in the heat treatment method for ferrite-pearlitic steel of the present invention, before heating the steel workpiece to a temperature 30-60°C higher than the Ac3 temperature, the method further includes:

[0019] Obtain a round bar from the steel workpiece, and use the thermal expansion method to detect the upper limit austenite temperature Ac3 and / or the lower limit austenite temperature Ac1 of the round bar.

[0020] Furthermore, in the heat treatment method for ferritic-pearlitic steel structure of the present invention, the alloy grade of the ferritic-pearlitic steel workpiece includes Q235, Q345, S355, 30Mn, 40MnVN, 38MnVS, 48MnV, 30MnVS, 15CrMo or 20CrMo. Attached Figure Description

[0021] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings:

[0022] Figure 1 A schematic diagram of the heat treatment of ferrite-pearlite steel structure provided for an embodiment of the present invention;

[0023] Figure 2 A schematic diagram of the traditional heat treatment process for ferritic-pearlitic steel. Detailed Implementation

[0024] To make the technical problems to be solved, the technical solutions, and the beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

[0025] Ferritic-pearlitic steel has advantages such as short production cycle, simple process, and low manufacturing cost, making it one of the most widely used and consumed steel grades. Current heat treatment processes for ferritic-pearlitic steel mainly involve heating the steel workpiece to austenitization and then directly cooling it. While this traditional process produces ferritic-pearlitic steel with high strength, meeting strength requirements, it suffers from low impact toughness, especially poor low-temperature impact toughness, failing to meet toughness requirements and posing safety hazards during use.

[0026] To solve the above-mentioned technical problems, the present invention provides a heat treatment method for ferrite-pearlite steel, comprising:

[0027] The ferritic-pearlitic steel workpiece is heated to 30-60°C above the Ac3 temperature, and a first heat preservation treatment is performed at this temperature. The time of the first heat preservation treatment is at least the time required to make the temperature distribution of the steel workpiece uniform.

[0028] After the first heat preservation treatment, the steel workpiece is cooled to a temperature 0-60°C above Ac1 and then subjected to a second heat preservation treatment. The duration of the second heat preservation treatment is at least the time required to make the temperature distribution of the steel workpiece uniform.

[0029] The steel workpiece after the second heat preservation treatment is cooled to room temperature to obtain the heat-treated steel workpiece.

[0030] Using the above technical solution, in the heat treatment method for ferrite-pearlite steel structure of the present invention, after heating the steel workpiece to a temperature 30-60°C above Ac3, austenite can be obtained first. During the heat treatment at this temperature, carbon, manganese, and other elements in the steel workpiece diffuse evenly, which can improve the stability of the steel workpiece. After the first heat treatment, the steel workpiece is cooled to the temperature of the second heat treatment, that is, heat treatment is performed at a temperature 0-60°C above Ac1. The steel workpiece begins to form an austenite + ferrite two-phase structure. At this temperature, carbon atoms in the steel workpiece diffuse from ferrite to austenite more quickly, and the carbon content of austenite is higher. During the subsequent cooling process, the pearlite lamellae formed by the austenite transformation are finer, which makes the steel workpiece not only have excellent strength but also improve its toughness. As the heat treatment time is extended, the carbon content in the austenite inside the steel workpiece can be further increased, thereby forming a finer lamellar structure during the subsequent cooling process. This structure can further improve the strength and toughness of the steel workpiece. Cool the steel workpiece after the second heat treatment to room temperature to obtain the heat-treated steel workpiece.

[0031] It should be understood that in the ferrite-pearlite steel heat treatment method of the present invention, in order to ensure sufficient diffusion of elements such as carbon and manganese in the ferrite of the steel workpiece into austenite and to improve the heat treatment efficiency, the time required for the first holding treatment should be controlled. For example, when the thickness of the steel workpiece is not greater than 200 mm, the time required for the first holding treatment should be at least the time required to achieve uniform temperature distribution in the steel workpiece. For example, the time required for the first holding treatment can be 0.5 to 4 hours. At this time, the microstructure of the steel workpiece is entirely austenite, and the carbon and manganese elements in the steel workpiece are uniformly diffused. This state can improve the stability of the steel workpiece. To obtain even better performance, after the temperature distribution of the steel workpiece is uniform, the holding time can be extended by 1 to 5 hours to ensure sufficient and uniform diffusion of carbon and manganese elements in the steel workpiece, which can further improve the stability of the steel workpiece.

[0032] As another example, when the thickness of the steel workpiece is 200mm, holding it at a temperature 30-60℃ higher than Ac3 for 3-4 hours can make the temperature distribution of the steel workpiece uniform. At this time, elements such as carbon and manganese in the steel workpiece diffuse evenly. To obtain even better performance, the holding time can be extended to 5 hours to allow the carbon and manganese in the steel workpiece to diffuse fully, which can further improve the stability of the steel workpiece.

[0033] As another example, when the thickness of the steel workpiece is 150mm, holding it at a temperature 30-60℃ higher than Ac3 for 2-3 hours can make the temperature distribution of the steel workpiece uniform. At this time, elements such as carbon and manganese in the steel workpiece diffuse evenly. To obtain even better performance, the holding time can be extended to 3 hours to allow the carbon and manganese in the steel workpiece to diffuse fully, which can further improve the stability of the steel workpiece.

[0034] As another example, when the thickness of the steel workpiece is 100mm, holding the workpiece at a temperature 30-60℃ higher than Ac3 for 1-2 hours can make the temperature distribution of the steel workpiece uniform. At this time, elements such as carbon and manganese in the steel workpiece diffuse evenly. To obtain even better performance, the holding time can be extended to 2 hours to allow the carbon and manganese in the steel workpiece to diffuse fully, which can further improve the stability of the steel workpiece.

[0035] As another example, when the thickness of the steel workpiece is 50 mm, holding the workpiece at a temperature 30–60 °C above Ac3 for 0.5–1 hour can make the temperature distribution of the steel workpiece uniform. At this time, elements such as carbon and manganese in the steel workpiece diffuse evenly. To obtain even better performance, the holding time can be extended to 0.5 hours to allow the carbon and manganese elements in the steel workpiece to diffuse fully, which can further improve the stability of the steel workpiece.

[0036] Under the above processing technology, the stability of the obtained steel workpiece can achieve the best effect.

[0037] It should also be understood that when the thickness of the steel workpiece is greater than 200mm, the time for the first heat preservation treatment needs to be increased. For example, for every 25mm increase in thickness on top of the 200mm thickness, the time required for the first heat preservation treatment should be increased by 0.5 to 1 hour from the original 0.5 to 4 hours. To obtain even better performance, the heat preservation time can be extended by 5 to 8 hours after the temperature distribution of the steel workpiece is uniform.

[0038] As another example, when the thickness of the steel workpiece is 225 mm, holding the workpiece at a temperature 30–60 °C above Ac3 for 3.5–4.5 hours can ensure a uniform temperature distribution, allowing for the uniform diffusion of elements such as carbon and manganese within the workpiece. To achieve even better performance, the holding time can be extended to 5.5 hours to allow for full diffusion of carbon and manganese, further improving the stability of the steel workpiece.

[0039] As another example, when the thickness of the steel workpiece is 250 mm, holding the workpiece at a temperature 30–60 °C above Ac3 for 4–5 hours can make the temperature distribution of the steel workpiece uniform. At this time, elements such as carbon and manganese in the steel workpiece diffuse evenly. To obtain even better performance, the holding time can be extended to 6 hours to allow the carbon and manganese in the steel workpiece to diffuse fully, which can further improve the stability of the steel workpiece.

[0040] When the thickness of the steel workpiece is 300mm, holding it at a temperature 30-60℃ higher than Ac3 for 5-6 hours can ensure a uniform temperature distribution, allowing for the uniform diffusion of elements such as carbon and manganese. To obtain even better performance, the holding time can be extended to 8 hours to allow for full diffusion of elements such as carbon and manganese, further improving the stability of the steel workpiece.

[0041] After the first heat treatment, the steel workpiece is cooled to a temperature 0-60°C above Ac1 and then subjected to a second heat treatment. It should be understood that in the technical solution of this invention, the second heat treatment time is at least the time required to achieve uniform temperature distribution on the steel workpiece under the second heat treatment temperature. For example, when the thickness of the steel workpiece is no greater than 200mm, the second heat treatment time is 0.5-6 hours. To ensure the steel workpiece has optimal strength, toughness, and impact resistance, the heat treatment time can be extended by 1-5 hours after the temperature distribution of the steel workpiece is uniform during the second heat treatment. With the extension of the heat treatment time, the carbon content in the austenite further increases, which is beneficial for the transformation of austenite into a finer pearlite lamellar structure during the subsequent cooling process, thereby improving the strength, toughness, and impact resistance of the steel workpiece.

[0042] For example, when the thickness of the steel workpiece is 200 mm, holding the workpiece at a temperature 0–60 °C above Ac1 for 4–6 hours can make the temperature distribution of the steel workpiece uniform and increase the carbon content in the austenite. Subsequent cooling yields a steel workpiece with a pearlitic lamellar structure. To obtain even better performance, the holding time can be extended to 3–5 hours, resulting in an even higher carbon content in the austenite. Subsequent cooling makes the pearlitic lamellar structure more compact, which can further improve the strength and toughness of the steel workpiece.

[0043] For example, when the thickness of the steel workpiece is 150 mm, holding the workpiece at a temperature 0–60 °C above Ac1 for 2–4 hours can make the temperature distribution of the steel workpiece uniform and increase the carbon content in the austenite. Subsequent cooling yields a steel workpiece with a pearlitic lamellar structure. To obtain even better performance, the holding time can be extended to 1–3 hours, resulting in an even higher carbon content in the austenite. Subsequent cooling makes the pearlitic lamellar structure more compact, which can further improve the strength and toughness of the steel workpiece.

[0044] For example, when the thickness of the steel workpiece is 100 mm, holding the workpiece at a temperature 0–60°C above Ac1 for 1–2 hours can make the temperature distribution of the steel workpiece uniform and increase the carbon content in the austenite. Subsequent cooling yields a steel workpiece with a pearlitic lamellar structure. To obtain even better performance, the holding time can be extended to 1–2 hours, resulting in an even higher carbon content in the austenite. Subsequent cooling makes the pearlitic lamellar structure more compact, which can further improve the strength and toughness of the steel workpiece.

[0045] For example, when the thickness of the steel workpiece is 50 mm, holding the workpiece at a temperature 0–60 °C above Ac1 for 0.5–1 h can make the temperature distribution of the steel workpiece uniform and increase the carbon content in the austenite. Subsequent cooling yields a steel workpiece with a pearlitic lamellar structure. To obtain even better performance, the holding time can be extended to 0.5–1 h to further increase the carbon content in the austenite. Subsequent cooling makes the pearlitic lamellar structure denser, which can further improve the strength and toughness of the steel workpiece.

[0046] Under the above processing technology, the tensile strength and impact toughness of the obtained steel workpiece can both achieve the best results.

[0047] It should also be understood that, in order to achieve optimal impact resistance in steel workpieces, when the thickness of the steel workpiece is greater than 200 mm, the time for the second heat treatment needs to be increased. Specifically, for every 25 mm increase in thickness beyond 200 mm, the second heat treatment time should be increased by 1-2 hours beyond the initial 0.5-6 hours. To obtain even better performance, after the temperature distribution of the steel workpiece is uniform following the second heat treatment, the heat treatment time can be extended by 5-8 hours. For example, when the thickness of the steel workpiece is 225 mm, heat treatment at a temperature 0-60°C above Ac1 for 5-6 hours can achieve uniform temperature distribution and increase the carbon content in the austenite. Subsequent cooling yields a steel workpiece with a pearlitic lamellar structure. To obtain even better performance, the heat treatment time can be extended by another 5-6 hours to further increase the carbon content in the austenite. Subsequent cooling further densifies the pearlitic lamellar structure, which can further improve the strength and toughness of the steel workpiece.

[0048] For example, when the thickness of the steel workpiece is 250 mm, holding the workpiece at a temperature 0–60 °C above Ac1 for 6–7 hours can result in a more uniform temperature distribution and increased carbon content in the austenite. Subsequent cooling yields a steel workpiece with a pearlitic lamellar structure. To obtain even better performance, the holding time can be extended to 6–7 hours, further increasing the carbon content in the austenite. Subsequent cooling makes the pearlitic lamellar structure more compact, which can further improve the strength and toughness of the steel workpiece.

[0049] For example, when the thickness of the steel workpiece is 300 mm, holding the workpiece at a temperature 0–60 °C above Ac1 for 7–8 hours can make the temperature distribution of the steel workpiece uniform and increase the carbon content in the austenite. Subsequent cooling yields a steel workpiece with a pearlitic lamellar structure. To obtain even better performance, the holding time of the steel workpiece can be extended to 8 hours, further increasing the carbon content in the austenite. Subsequent cooling makes the pearlitic lamellar structure denser, which can further improve the strength and toughness of the steel workpiece.

[0050] It should also be understood that, in order to prevent the steel workpiece from cracking or deforming during the cooling process, the cooling rate needs to be controlled during the cooling process. For example, the cooling rate during the process of cooling the steel workpiece to a temperature 0 to 60°C above Ac1 is 20 to 300°C / h. In another example, the cooling rate during the process of cooling the steel workpiece to a temperature 0 to 60°C above Ac1 can be 20°C / h, 150°C / h, or 300°C / h.

[0051] It should also be understood that, during the process of heating the steel workpiece to a temperature 30–60°C above Ac3, in order to control the rate of grain growth in the steel workpiece and thus ensure that the heat-treated steel workpiece has strong rigidity and toughness, the heating rate should also be controlled. For example, the heating rate is 20–300°C / h, and in another example, the heating rate can be 20°C / h, 100°C / h, 150°C / h, 200°C / h, or 300°C / h.

[0052] It should also be understood that during the process of cooling the steel workpiece after the second heat preservation treatment to room temperature, in order to avoid the steel workpiece becoming brittle, losing toughness, or transforming into other structures after cooling, the cooling method should be controlled. For example, the cooling method is furnace cooling and / or air cooling.

[0053] As one possible implementation, the heat treatment method for ferrite-pearlitic steel structure of the present invention, before heating the steel workpiece to a temperature 30-60°C above the Ac3 temperature, further includes:

[0054] Obtain a round bar from the steel workpiece, and use the thermal expansion method to detect the upper limit austenite temperature Ac3 and / or the lower limit austenite temperature Ac1 of the round bar.

[0055] By adopting the above technical solution, the relationship between linear strain and time and temperature during the thermal cycling process of steel workpieces can be measured by the thermal expansion method. The upper limit austenite temperature Ac3 and the lower limit austenite temperature Ac1 of the steel workpieces can be obtained. Round bars are more regular than other shapes, easier to process and measure. The round bars of steel workpieces have a higher thermal conductivity during the testing process, which can reach thermal equilibrium more quickly, thereby saving testing time.

[0056] It should be understood that the ferritic-pearlitic steel workpiece has a variety of alloy grades, and the present invention can select one or more of them for heat treatment. For example, the alloy grades of the ferritic-pearlitic steel workpiece include Q235, Q345, S355, 30Mn, 40MnVN, 38MnVS, 48MnV, 30MnVS, 15CrMo or 20CrMo.

[0057] To better understand the present invention, the following specific embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0058] Unless otherwise specified, all raw materials used in the following examples are commercially available.

[0059] Example 1

[0060] This embodiment provides a heat treatment method for ferritic-pearlitic steel, including:

[0061] S100, obtain Q345 steel for wind turbine flanges, and take a φ8x12mm Q345 steel round bar from the forging;

[0062] S200, using the thermal expansion method, the upper limit austenite temperature Ac3 of Q345 steel round bar is 810℃ and the lower limit austenite temperature Ac1 is 695℃.

[0063] S300, obtain a Q345 steel flange blank with a thickness of 200mm, place the blank in a heating furnace and heat it to 850℃, and hold it at 850℃ for 3.5h to make the temperature distribution of the blank uniform; the heating rate is 50℃ / h;

[0064] After the heat treatment in step S300 is completed, the billet is cooled to 720℃ for 1 hour and then kept at 720℃ for 5 hours to ensure uniform temperature distribution. The billet is then removed from the furnace and air-cooled to room temperature to complete the heat treatment of the Q345 steel flange billet.

[0065] In this embodiment, two sample blocks of the same size were taken from the center of the heat-treated Q345 steel flange blank and processed into tensile specimens and Charpy V-type impact specimens. The room temperature tensile mechanical properties of the tensile specimens were tested, and the -50℃ low temperature impact properties of the Charpy V-type impact specimens were tested. The test results are shown in Table 1.

[0066] Example 2

[0067] This embodiment provides a heat treatment method for ferritic-pearlitic steel, including:

[0068] S100, obtain Q345 steel for wind turbine flanges, and take a φ8x12mm Q345 steel round bar from the forging;

[0069] S200, using the thermal expansion method, the upper limit austenite temperature Ac3 of Q345 steel round bar is 810℃ and the lower limit austenite temperature Ac1 is 695℃.

[0070] S300: Obtain a Q345 steel flange blank with a thickness of 200mm, place the blank in a heating furnace and heat it to 850℃, and hold it at 850℃ for 3.5h to ensure uniform temperature distribution, and then continue to hold it for 4.5h; the heating rate is 50℃ / h.

[0071] After the heat treatment in step S300 is completed, the billet is cooled to 720℃ for 1 hour and then kept at 720℃ for 5 hours to ensure uniform temperature distribution. After that, the billet is kept at 720℃ for another 5 hours. After the heat treatment is completed, the billet is removed from the furnace and air-cooled to room temperature to complete the heat treatment of the Q345 steel flange billet.

[0072] In this embodiment, two sample blocks of the same size were taken from the center of the heat-treated Q345 steel flange blank and processed into tensile specimens and Charpy V-type impact specimens. The room temperature tensile mechanical properties of the tensile specimens were tested, and the -50℃ low temperature impact properties of the Charpy V-type impact specimens were tested. The test results are shown in Table 1.

[0073] Example 3

[0074] This embodiment provides a heat treatment method for ferritic-pearlitic steel, including:

[0075] S100, obtain 15CrMo steel slab, and take φ8x12mm 15CrMo steel round bar from the forging;

[0076] S200, using the thermal expansion method, the upper limit austenite temperature Ac3 of 15CrMo steel round bar is 850℃ and the lower limit austenite temperature Ac1 is 740℃.

[0077] S300, obtain a 15CrMo steel slab with a thickness of 150mm, place the slab in a heating furnace and heat it to 910℃, and hold it at 910℃ for 2.5h to make the temperature distribution of the slab uniform, and then continue to hold it for 3h; the heating rate is 100℃ / h.

[0078] After the heat treatment in step S300, the slab is cooled to 770°C for 1 hour and then kept at 770°C for 3.5 hours to ensure uniform temperature distribution. After that, the heat treatment is continued for another 3 hours. After the heat treatment is completed, the slab is furnace cooled to room temperature to complete the heat treatment of the 15CrMo steel slab.

[0079] Example 4

[0080] This embodiment provides a heat treatment method for ferritic-pearlitic steel, including:

[0081] S100, obtain 15CrMo steel slab, and take φ8x12mm 15CrMo steel round bar from the forging;

[0082] S200, using the thermal expansion method, the upper limit austenite temperature Ac3 of 15CrMo steel round bar is 850℃ and the lower limit austenite temperature Ac1 is 740℃.

[0083] S300: Obtain a 15CrMo steel slab with a thickness of 150mm, place the slab in a heating furnace and heat it to 880℃, and hold it at 880℃ for 3 hours to ensure uniform temperature distribution, and then continue to hold it for 3 hours; the heating rate is 100℃ / h.

[0084] After the heat treatment in step S300 is completed, the slab is cooled to 800℃ for 1 hour and then kept at 800℃ for 3.5 hours to ensure uniform temperature distribution. After that, the heat treatment is continued for another 3 hours. After the heat treatment is completed, the slab is furnace cooled to room temperature to complete the heat treatment of the 15CrMo steel slab.

[0085] Example 5

[0086] This embodiment provides a heat treatment method for ferritic-pearlitic steel, including:

[0087] S100, obtain 15CrMo steel slab, and take φ8x12mm 15CrMo steel round bar from the forging;

[0088] S200, using the thermal expansion method, the upper limit austenite temperature Ac3 of 15CrMo steel round bar is 850℃ and the lower limit austenite temperature Ac1 is 740℃.

[0089] S300: Obtain a 250mm thick 15CrMo steel slab, place the slab in a heating furnace and heat it to 910℃, hold it at 910℃ for 4.5h to ensure uniform temperature distribution, and then continue holding it for 6h; the heating rate is 100℃ / h.

[0090] S400, after the heat preservation in step S300, the slab is cooled to 740℃ for 1 hour, and then kept at 740℃ for 6 hours to make the temperature distribution of the slab uniform. After the heat preservation is completed, the slab is furnace cooled to room temperature to complete the heat treatment of the 15CrMo steel slab.

[0091] Two sample blocks of the same size were taken from the center of the 15CrMo steel slab after heat treatment in Examples 3 to 5 above, and were processed into tensile specimens and Charpy U-shaped impact specimens respectively. The room temperature tensile mechanical properties of the tensile specimens were tested, and the 0℃ low temperature impact properties of the Charpy U-shaped impact specimens were tested. The test results are shown in Table 2.

[0092] Comparative Example 1

[0093] This comparative example provides a heat treatment method for ferritic-pearlitic steel, which employs a traditional heat treatment process, including:

[0094] S100, obtain Q345 steel for wind turbine flanges, and take a φ8x12mm Q345 steel round bar from the forging;

[0095] S200, using the thermal expansion method, the upper limit austenite temperature Ac3 of Q345 steel round bar is 810℃ and the lower limit austenite temperature Ac1 is 695℃.

[0096] S300: Obtain a Q345 steel flange blank with a thickness of 200mm, place the blank in a heating furnace and heat it to 850℃, and hold it at 850℃ for 4 hours; the heating rate is 50℃ / h; after the holding period is completed, remove the blank from the furnace and air cool it to room temperature to complete the heat treatment of the Q345 steel flange blank.

[0097] Two identical specimen blocks were taken from the center of the heat-treated Q345 steel flange blank in this comparative example and processed into tensile specimens and Charpy V-type impact specimens. The room temperature tensile mechanical properties of the tensile specimens were tested, and the -50℃ low temperature impact properties of the Charpy V-type impact specimens were tested. The test results are shown in Table 1 below.

[0098] Table 1. Test results of mechanical properties of Q345 steel flange blanks after heat treatment in Examples 1, 2 and Comparative Example 1:

[0099]

[0100]

[0101] Comparative Example 2

[0102] This comparative example provides a heat treatment method for ferritic-pearlitic steel, which employs a traditional heat treatment process, including:

[0103] S100, obtain 15CrMo steel slab, and take φ8x12mm 15CrMo steel round bar from the forging;

[0104] S200, using the thermal expansion method, the upper limit austenite temperature Ac3 of 15CrMo steel round bar is 850℃ and the lower limit austenite temperature Ac1 is 740℃.

[0105] S300: Obtain a 15CrMo steel slab with a thickness of 150mm, place the slab in a heating furnace and heat it to 870℃, and hold it at 870℃ for 4 hours; the heating rate is 100℃ / h; after the holding period, the slab is furnace cooled to room temperature to complete the heat treatment of the 15CrMo steel slab.

[0106] Two identical specimen blocks were taken from the center of the 15CrMo steel slab after heat treatment in this comparative example, and were respectively processed into tensile specimens and Charpy U-shaped impact specimens. The room temperature tensile mechanical properties of the tensile specimens were tested, and the 0℃ low temperature impact properties of the Charpy U-shaped impact specimens were tested. The test results are shown in Table 2 below.

[0107] Table 2 shows the mechanical property test results of 15CrMo steel slabs after heat treatment in Examples 3 to 5 and Comparative Example 2:

[0108]

[0109] Based on the above embodiments, comparative examples, and the test results in Tables 1 and 2, it can be seen that the strength of the Q345 steel flange blanks after heat treatment in Examples 1 and 2 is slightly higher than that after conventional processing in Comparative Example 1, with little overall difference. However, the -50℃ impact toughness of the Q345 steel flange blanks after heat treatment in Examples 1 and 2 is significantly improved compared to Comparative Example 1. The strength of the 15CrMo steel slabs after heat treatment in Examples 3 to 5 is slightly higher than that after conventional processing in Comparative Example 2, but the 0℃ impact toughness of the 15CrMo steel slabs after heat treatment in Examples 3 to 5 is significantly improved compared to Comparative Example 2. The technical solution of this invention solves the technical problem that ferritic-pearlitic steel treated by existing heat treatment processes has sufficient strength but insufficient toughness.

[0110] In the description of the above embodiments, specific features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or examples.

[0111] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A heat treatment method for ferrite-pearlite steel, characterized in that, include: The ferritic-pearlitic steel workpiece is heated to 30-60°C above the Ac3 temperature, and a first heat preservation treatment is performed under this temperature condition. The time of the first heat preservation treatment is at least the time required to make the temperature distribution of the steel workpiece uniform. After the first heat preservation treatment, the steel workpiece is cooled to a temperature 0-60°C above Ac1 and then subjected to a second heat preservation treatment. The duration of the second heat preservation treatment is at least the time required to make the temperature distribution of the steel workpiece uniform. The steel workpiece after the second heat preservation treatment is cooled to room temperature to obtain the heat-treated steel workpiece.

2. The heat treatment method for ferrite-pearlite steel structure according to claim 1, characterized in that, The time required for the first insulation treatment is: When the thickness of the steel workpiece is no more than 200mm, the required time is 0.5 to 4 hours; For each 25mm increase in thickness on the basis of a 200mm steel workpiece, the required time increases by 0.5 to 1 hour from the original 0.5 to 4 hours.

3. The heat treatment method for ferrite-pearlite steel structure according to claim 1 or 2, characterized in that, The duration of the second heat preservation treatment is: When the thickness of the steel workpiece is no more than 200mm, the second heat preservation treatment time is 0.5 to 6 hours.

4. The heat treatment method for ferrite-pearlite steel structure according to claim 3, characterized in that, For every 25mm increase in thickness of the steel workpiece from 200mm, the second heat preservation treatment time is increased by 1-2 hours from 0.5-6 hours.

5. The heat treatment method for ferrite-pearlite steel structure according to claim 1, characterized in that, The cooling rate during the process of cooling the steel workpiece after the first heat preservation treatment to a temperature 0-60°C above Ac1 is 20-300°C / h.

6. The heat treatment method for ferrite-pearlite steel structure according to claim 1, characterized in that, During the process of heating the steel workpiece to a temperature 30-60°C above Ac3, the heating rate is 20-300°C / h.

7. The heat treatment method for ferrite-pearlite steel structure according to claim 1, characterized in that, During the process of cooling the steel workpiece after the second heat preservation treatment to room temperature, the cooling method is furnace cooling and / or air cooling.

8. The heat treatment method for ferrite-pearlite steel structure according to claim 1, characterized in that, Before heating the steel workpiece to a temperature 30–60°C above Ac3, the process further includes: Obtain a round bar from the steel workpiece, and use the thermal expansion method to detect the upper limit austenite temperature Ac3 and / or the lower limit austenite temperature Ac1 of the round bar.

9. The heat treatment method for ferrite-pearlite steel structure according to any one of claims 1 to 8, characterized in that, The alloy grades of the ferritic-pearlitic steel workpieces include Q235, Q345, S355, 30Mn, 40MnVN, 38MnVS, 48MnV, 30MnVS, 15CrMo or 20CrMo.

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