Heat treatment method for eliminating coarse grain defect of large-section hot work die steel forge piece

Abstract

The invention provides a heat treatment method for eliminating coarse grain defects of a large-section hot work die steel forge piece, and belongs to the technical field of steel forge piece manufacturing. The heat treatment method for eliminating the coarse grain defect of the large-section hot work die steel forge piece comprises the steps that the large-section hot work die steel forge piece is sequentially subjected to stepped heating treatment, stepped cooling treatment, tempering heating treatment and stepped cooling treatment. Compared with a traditional heat treatment process, the method has the advantages that benign transformation of internal structures of the forge piece in all temperature sections is promoted, positive influence is generated on elimination of coarse grain defects of the large-section hot work die steel forge piece, the plasticity of the material is remarkably improved, and the method has important engineering significance and commercial value.

Description

Technical Field

[0001] The invention belongs to the technical field of manufacturing steel forgings, and in particular relates to a heat treatment method for eliminating coarse grain defects of large-section hot-working die steel forgings. Background Art

[0002] In the production process of large-section hot-working die steel forgings, in addition to defects such as inclusions and segregation, coarse-grained defects will also occur. Coarse-grained defects refer to the presence of coarse grains or uneven grain size in the forgings. If the coarse-grained defects are not handled properly during the production and processing of large-section hot-working die steel forgings, the strength and toughness of the forgings will be reduced, and the service life will be greatly reduced, resulting in significant economic losses. In current forging production, after the forgings are formed, in order to eliminate internal coarse-grained defects, a one-stage normalizing forging post-heat treatment process is often used. Although this method can alleviate the phenomenon of coarse grains to a certain extent, the effect is limited.

[0003] Therefore, it is urgent to develop a new heat treatment method that can effectively eliminate the coarse grain defects inside large-section hot working die steel, reduce the cost of forging production, and improve production efficiency. Summary of the invention

[0004] In order to solve the above technical problems, the present invention proposes a heat treatment method for eliminating coarse grain defects of large-section hot-working die steel forgings, so as to improve the strength, toughness and service life of the forgings.

[0005] To achieve the above objectives, the present invention provides a heat treatment method for eliminating coarse grain defects of large-section hot working die steel forgings, wherein the large-section hot working die steel forgings are subjected to step-by-step heating treatment, step-by-step cooling treatment, tempering heating treatment and step-by-step cooling treatment in sequence.

[0006] Furthermore, the step-wise temperature increase treatment comprises the following steps: heating to 600±(10-30)°C at a rate of 50°C / h, keeping warm for 2h, then heating to 900±(10-30)°C at a rate of 50°C / h, keeping warm for 8-11h.

[0007] Furthermore, the step cooling treatment includes the following steps: oil cooling from 900±(10-30)°C to 500±(10-30)°C, then cooling to 90±(10-30)°C at a rate of 10-25°C / h, keeping warm for 1-12h, and then air cooling to room temperature after being taken out of the furnace.

[0008] Furthermore, the tempering temperature treatment includes the following steps: heating to 380±(10-30)°C at a rate of 50°C / h, keeping warm for 4 hours, and then heating to 670±(10-30)°C at a rate of 50°C / h, keeping warm for 34-38 hours.

[0009] Furthermore, the step cooling treatment includes the following steps: reducing the temperature from 670±(10-30)°C to 400±(10-30)°C at a rate of 10-25°C / h, then reducing the temperature to 150±10°C at a rate of 15°C / h, and then air cooling to room temperature after being taken out of the furnace.

[0010] Furthermore, the large-section hot working die steel forging is a 55NiCrMoV7 large-section hot working die steel forging, a 5CrNiMo large-section hot working die steel forging or a 5CrMnMo large-section hot working die steel forging.

[0011] The invention also proposes a large-section hot-working die steel forging obtained by the heat treatment method.

[0012] The term "large-section hot-working die steel forging" in the present invention refers to a hot-working die steel forging with a cross-section of not less than 600 mm.

[0013] Compared with the prior art, the present invention has the following advantages and technical effects:

[0014] Compared with the traditional heat treatment process, the present invention promotes the benign transformation of the internal structure of the forging in each temperature range by performing a step-by-step heating and cooling treatment on the forging, has a positive effect on the elimination of coarse-grained defects in large-section hot-working die steel forgings, and significantly improves the plastic properties of the material, which has important engineering significance and commercial value. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0016] Figure 1 The process flow chart of the heat treatment method for eliminating the coarse grain defects of the large-section hot-working die steel forging in Example 1;

[0017] Figure 2 The process flow chart of the heat treatment method for eliminating the coarse grain defects of the large cross-section hot working die steel forging in Example 2;

[0018] Figure 3 The microstructure diagram of the forging obtained by the conventional heat treatment method in Comparative Example 1 at different magnifications;

[0019] Figure 4 The microstructure diagrams of the forgings obtained by heat treatment in Example 1 at different magnifications. DETAILED DESCRIPTION

[0020] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0021] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0022] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0023] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present invention description and examples are exemplary only.

[0024] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0025] An embodiment of the present invention provides a heat treatment method for eliminating coarse-grained defects of large-section hot-working die steel forgings, wherein the large-section hot-working die steel forgings are subjected to step-by-step heating treatment, step-by-step cooling treatment, tempering heating treatment, and step-by-step cooling treatment in sequence.

[0026] In a preferred embodiment of the present invention, the step-by-step temperature increase treatment includes the following steps: heating to 600±(10-30)°C (T1) at a rate of 50°C / h, keeping warm for 2 hours, then heating to 900±(10-30)°C (T2) at a rate of 50°C / h, keeping warm for 8-11 hours.

[0027] Step-by-step heating treatment can effectively reduce the temperature difference between the inner and outer surfaces of the forging, prevent excessive stress in the core of the forging, and avoid cracking of the forging in the middle and early stages. Especially for hot-working die steel forgings with large cross-sections, the large cross-section brings a greater temperature difference, which increases the risk of cracking. Therefore, it is chosen to keep the material at the elastic-plastic transition temperature (600±(10~30)℃) for 2h to make the temperature difference between the inside and outside of the forging uniform before heating to the process temperature. Step-by-step heating can also reduce the temperature averaging time of the workpiece in the high-temperature section, and can better obtain fine and uniform austenite grains.

[0028] In a preferred embodiment of the present invention, the step cooling treatment includes the following steps: oil cooling from 900±(10~30)℃ to 500±(10~30)℃(T3), then cooling to 90±(10~30)℃(T4) at a rate of 10~25℃ / h, keeping warm for 1~12h, and then air cooling to room temperature after taking out of the furnace.

[0029] The step-by-step cooling during quenching is mainly to ensure the transformation of supercooled austenite to martensite. However, during the cooling process, due to factors such as rapid temperature changes and uneven cooling, the internal stress of the forging will increase, resulting in the risk of cracking. Therefore, rapid oil quenching is performed in the high temperature section, and then when the temperature is reduced to 500±(10~30)℃, the workpiece is kept at a certain cooling rate and continues to be deeply cooled, so as to fully release the internal stress while ensuring the temperature difference between the inside and outside of the workpiece.

[0030] In a preferred embodiment of the present invention, the tempering temperature treatment includes the following steps: heating to 380±(10~30)℃(T5) at a rate of 50℃ / h, keeping warm for 4h, then heating to 670±(10~30)℃(T6) at a rate of 50℃ / h, keeping warm for 34~38h.

[0031] The step-by-step heating in the tempering stage ensures the temperature uniformity of the workpiece on the one hand, and controls the workpiece to be kept warm at 380±(10~30)℃ on the other hand, so that the core of the workpiece can complete the complete transformation of the supercooled austenite.

[0032] In a preferred embodiment of the present invention, the step cooling treatment includes the following steps: reducing the temperature from 670±(10~30)℃ to 400±(10~30)℃ (T7) at a rate of 10-25℃ / h, then reducing the temperature to 150±10℃ (T8) at a rate of 15℃ / h, and then taking the product out of the furnace and air cooling it to room temperature.

[0033] Step-by-step cooling is used to effectively reduce the internal and external temperature difference during the cooling process of large-section module forgings, reduce the hardness difference from the surface to the core of the module forgings, and on the other hand, fully eliminate quenching stress and reduce forging deformation.

[0034] In the embodiments of the present invention, 55NiCrMoV7 large-section hot working die steel forgings are taken as examples, including but not limited to the above-mentioned forgings.

[0035] The term "large-section hot-working die steel forging" in the present invention refers to a hot-working die steel forging with a cross-section of not less than 600 mm.

[0036] The room temperature in the embodiments of the present invention refers to "25±2°C".

[0037] The technical solution of the present invention is further illustrated by the following embodiments.

[0038] Example 1

[0039] A heat treatment method for eliminating coarse grain defects in large-section hot-working die steel forgings (process flow chart see Figure 1 ), using 55NiCrMoV7 large-section hot-working die steel forgings with a size of 5500mm×2500mm×650mm, specifically including the following steps:

[0040] (1) The forgings are subjected to a step-by-step temperature treatment: the temperature is raised to 600±10℃ at 50℃ / h, kept at this temperature for 2h, and then the temperature is raised to 900±10℃ at 50℃ / h, and kept at this temperature for 11h.

[0041] (2) The forgings after the treatment in step (1) are subjected to a step cooling treatment: oil cooling from 900±10°C to 500±10°C, then cooling to 90±10°C at a rate of 15°C / h in the furnace, keeping the temperature for 12 hours, and then air cooling to room temperature after being taken out of the furnace.

[0042] (3) The forgings treated in step (2) are subjected to tempering heating treatment: the temperature is raised to 380±(10)°C at a rate of 50°C / h, and kept at that temperature for 4 hours, and then the temperature is raised to 670±10°C at a rate of 50°C / h, and kept at that temperature for 38 hours.

[0043] (4) The forgings after step (3) are subjected to a step cooling treatment: from 670±10°C to 400±10°C at a rate of 20°C / h, then to 150±10°C at a rate of 15°C / h, and then taken out of the furnace and air-cooled to room temperature.

[0044] The mechanical properties test results of 55NiCrMoV7 large-section hot working die steel forgings before and after heat treatment are shown in Table 1.

[0045] Table 1 Mechanical properties test results of forgings before and after heat treatment

[0046] <![CDATA[R m / MPa]]> Rp0.2 / MPa A / ％ Z / ％ Akv / J Forgings before heat treatment 892 763 8.5 26 10.3 / 8.4 / 12.1 Heat treated forgings 1398 1274 12.5 43 23.7 / 25.8 / 24.0

[0047] In Table 1, Rm represents tensile strength, which is measured according to the method specified in GB / T229-2020, Rp0.2 represents the stress value corresponding to when the plastic elongation reaches 0.2%, which is measured according to the method specified in GB / T229-2020, A represents elongation, which is measured according to the method specified in GB / T229-2020, Z represents cross-sectional shrinkage, which is measured according to the method specified in GB / T229-2020, and Akv represents the energy absorbed under impact load, which is measured according to the method specified in GB / T228.1-2020 (the same below).

[0048] It can be seen from Table 1 that the mechanical properties of the forgings after heat treatment are significantly improved compared with those before heat treatment, with the tensile strength increasing by 56% and the average impact energy increasing by 138%.

[0049] Example 2

[0050] A heat treatment method for eliminating coarse grain defects in large-section hot-working die steel forgings (process flow chart see Figure 2 ), using 55NiCrMoV7 large-section hot working die steel forgings with a size of 3300mm×2900mm×900mm, specifically including the following steps:

[0051] (1) Perform a step-by-step temperature treatment on the forging: heat up to 600±10℃ at 50℃ / h, keep warm for 2h, then heat up to 900±10℃ at 50℃ / h, keep warm for 8h;

[0052] (2) The forgings after the treatment in step (1) are subjected to a step cooling treatment: oil cooling from 900±10°C to 500±10°C, then cooling to 90±10°C at a rate of 15°C / h in the furnace, keeping the temperature for 10 hours, and then air cooling to room temperature after being taken out of the furnace.

[0053] (3) The forgings after the treatment in step (2) are subjected to tempering and heating treatment: the temperature is raised to 380±10°C at a rate of 50°C / h, and kept at this temperature for 4 hours, and then the temperature is raised to 670±10°C at a rate of 50°C / h, and kept at this temperature for 34 hours.

[0054] (4) The forgings after step (3) are subjected to a step cooling treatment: from 670±10°C to 400±10°C at a rate of 20°C / h, then to 150±10°C at a rate of 15°C / h, and then taken out of the furnace and air-cooled to room temperature.

[0055] The mechanical properties test results of 55NiCrMoV7 large-section hot working die steel forgings before and after heat treatment are shown in Table 2.

[0056] Table 2 Mechanical properties test results of forgings before and after heat treatment

[0057] Rm / Mpa Rp0.2 / Mpa A / ％ Z / ％ Akv / J Forgings before heat treatment 913 802 8.0 24 10.2 / 7.6 / 11.2 Heat treated forgings 1386 1277 12.0 41 24.1 / 22.6 / 24.4

[0058] It can be seen from Table 2 that the mechanical properties of the forgings after heat treatment are significantly improved compared with those before heat treatment, the tensile strength is increased by 51.8%, and the average impact energy is increased by 145%.

[0059] Comparative Example 1

[0060] Traditional heat treatment includes the following steps:

[0061] (1) Heating the forging: heating to 900±(10~30)℃ at a heating rate of 50℃ / h and keeping at this temperature for 11h;

[0062] (2) Cooling the forgings after the treatment in step (1): oil cooling from 900±(10-30)°C to 90±(10-30)°C, keeping the temperature for 12 hours, and then air cooling to room temperature after taking out of the furnace.

[0063] (3) The forgings after the treatment in step (2) are subjected to tempering and heating treatment: the temperature is raised to 670±(10-30)°C at a rate of 50°C / h and kept at this temperature for 42h.

[0064] (4) The forgings after step (3) are cooled from 670±(10-30)°C to 150±10°C at a rate of 10-25°C / h, and then taken out of the furnace and air-cooled to room temperature.

[0065] Comparative Example 1 The organization diagrams of the forgings obtained by conventional heat treatment at different magnifications are shown in Figure 3 The organization diagrams of the forgings obtained by the heat treatment method of Example 1 of the present invention at different magnifications are shown in Figure 4 It can be seen that Example 1 of the present invention promotes the benign transformation of the internal structure of the forging in each temperature range by performing a step-by-step heating and cooling treatment on the forging, compared with the traditional heat treatment process of Comparative Example 1. Figure 3 and Figure 4 It can be seen that compared with the limited coarse grain elimination level of the traditional heat treatment process, the method of Example 1 of the present invention can basically eliminate the coarse grain defects inside the large-section hot working die steel forging, the grains are significantly refined, and the grain size is increased from the previous 1-1.5 level to 5 level ( Figure 3 It can be seen that the grain size is only 1.5. Figure 4 It can be seen that the grain size is level 5), and the strength and toughness of the material are significantly improved, indicating that the technical solution of the present invention has important engineering significance and commercial value.

[0066] Comparative Example 2

[0067] A heat treatment method for a large-section hot working die steel forging, using a 55NiCrMoV7 large-section hot working die steel forging with a size of 5500mm×2500mm×650mm, specifically comprising the following steps:

[0068] (1) Heating the forging: heating to 900±10℃ at a heating rate of 50℃ / h and keeping at this temperature for 11h;

[0069] (2) Cooling the forgings after the treatment in step (1): oil cooling from 900±10℃ to 90±10℃, keeping the temperature for 12h, and then air cooling to room temperature.

[0070] (3) The forgings after the treatment in step (2) are subjected to tempering and heating treatment: the temperature is raised to 670±10°C at a rate of 50°C / h and kept at this temperature for 42h.

[0071] (4) Cooling the forgings after step (3): cooling the temperature from 670±10°C to 150±10°C at a rate of 15°C / h, and then air-cooling them to room temperature.

[0072] The mechanical properties test results of 55NiCrMoV7 large-section hot working die steel forgings before and after heat treatment are shown in Table 3.

[0073] Table 3 Mechanical properties test results of forgings before and after heat treatment

[0074] <![CDATA[R m / MPa]]> Rp0.2 / MPa A / ％ Z / ％ Akv / J Forgings before heat treatment 889 758 7.8 24 10.2 / 9.4 / 10.1 Heat treated forgings 1086 942 10.3 30 16.4 / 14.8 / 14.6

[0075] It can be seen from Table 3 that the mechanical properties of the forgings after heat treatment are improved compared with those before heat treatment, the tensile strength is increased by 22%, and the average tensile strength is increased by 54%, which is lower than the performance results obtained by the step-by-step heating and cooling process.

[0076] Comparative Example 3

[0077] A heat treatment method for a large-section hot working die steel forging, using a 55NiCrMoV7 large-section hot working die steel forging with a size of 5500mm×2500mm×650mm, specifically comprising the following steps:

[0078] (1) Perform a step-by-step temperature treatment on the forging: heat up to 600±10℃ at 50℃ / h, keep warm for 2h, then heat up to 900±10℃ at 50℃ / h, keep warm for 11h;

[0079] (2) The forgings after the treatment in step (1) are subjected to a step cooling treatment: oil cooling from 900±10°C to 500±10°C, then cooling to 90±10°C at a rate of 15°C / h in the furnace, keeping the temperature for 12 hours, and then air cooling to room temperature after being taken out of the furnace.

[0080] The mechanical properties test results of 55NiCrMoV7 large-section hot working die steel forgings before and after heat treatment are shown in Table 4.

[0081] Table 4 Mechanical properties test results of forgings before and after heat treatment

[0082] <![CDATA[R m / MPa]]> Rp0.2 / MPa A / ％ Z / ％ Akv / J Forgings before heat treatment 901 784 8.2 25 10.5 / 10.8 / 11.1 Heat treated forgings 1493 1309 6.5 14 5.2 / 6.5 / 6.8

[0083] It can be seen from Table 4 that after only the quenching step heating and cooling process, the properties of the sample except for the tensile strength and yield strength showed a downward trend. For hot working die steel, poor toughness will greatly affect the service life of the module.

[0084] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A heat treatment method for eliminating coarse grain defects of large-section hot-working die steel forgings, characterized in that: The large-section hot-working die steel forgings are subjected to step-wise heating treatment, step-wise cooling treatment, tempering heating treatment and step-wise cooling treatment in sequence.

2. The heat treatment method for eliminating coarse grain defects of large-section hot-working die steel forgings according to claim 1, characterized in that: The step-by-step temperature treatment comprises the following steps: heating to 600±(10-30)°C at a rate of 50°C / h, keeping the temperature for 2h, then heating to 900±(10-30)°C at a rate of 50°C / h, keeping the temperature for 8-11h.

3. The heat treatment method for eliminating coarse grain defects of large-section hot-working die steel forgings according to claim 1, characterized in that: The step cooling process The method comprises the following steps: oil cooling from 900±(10-30)℃ to 500±(10-30)℃, then cooling to 90±(10-30)℃ at a speed of 10-25℃ / h, keeping the temperature for 1-12h, and then taking out of the furnace and air cooling to room temperature.

4. The heat treatment method for eliminating coarse grain defects of large-section hot-working die steel forgings according to claim 1, characterized in that: The tempering temperature treatment comprises the following steps: heating to 380±(10-30)°C at a rate of 30-70°C / h, keeping the temperature for 4 hours, then heating to 670±(10-30)°C at a rate of 50°C / h, and keeping the temperature for 34-38 hours.

5. The heat treatment method for eliminating coarse grain defects of large-section hot-working die steel forgings according to claim 1, characterized in that: The step cooling process The method comprises the following steps: reducing the temperature from 670±(10-30)°C to 400±(10-30)°C at a speed of 10-25°C / h, then reducing the temperature to 150±10°C at a speed of 15°C / h, and then taking the temperature out of the furnace and air cooling to room temperature.

6. The heat treatment method for eliminating coarse grain defects of large-section hot-working die steel forgings according to claim 1, characterized in that: The large-section hot working die steel forgings are 55NiCrMoV7 large-section hot working die steel forgings, 5CrNiMo large-section hot working die steel forgings or 5CrMnMo large-section hot working die steel forgings.

7. A large-section hot-working die steel forging obtained by the heat treatment method according to any one of claims 1 to 6.

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