A spheroidizing annealing process for low-carbon alloy steel warm forging

Abstract

The application provides a spheroidizing annealing process for a low-carbon alloy steel warm forging piece, wherein a holding interval of a subcritical section is added between Ac1+(20-30) DEG C and Ar1-(10-20) DEG C of conventional isothermal spheroidizing annealing, so that a microstructure state with low hardness, high spheroidizing rate and uniform distribution can be obtained in a short time. After annealing by the process, the hardness can reach 120-135 HB, and the spheroidizing rate is greater than or equal to 95%; the deformation resistance of the structure is greatly reduced, and the cold extrusion process with a large deformation amount can be resisted without cracking. Compared with the current commonly used periodic spheroidizing annealing, the production cycle can be shortened by 6-10 hours, and the application can be widely used in the spheroidizing annealing process before cold extrusion forming of low-carbon steel.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology, specifically to a spheroidizing annealing process for low-carbon alloy steel warm forgings. Background Technology

[0002] 20MnCr5 is widely used in the automotive parts industry. As a low-carbon alloy steel, it is typically pre-treated with normalizing to achieve good machinability. With advancements in forging technology, this type of low-carbon alloy steel is increasingly being formed using cold forging. Cold extrusion forming can achieve higher precision and reduce material consumption. To meet the demands of cold extrusion forming, spheroidizing annealing of low-carbon alloy steel is also beginning to be applied in production.

[0003] Spheroidizing annealing is a common heat treatment process. For low-carbon alloy steel like 20MnCr5, the main purpose of this process is to reduce the product's hardness and increase its plasticity, thereby reducing the metal's resistance to deformation. Ordinary spheroidizing annealing involves heating the product to the critical temperature Ac1+(20~30)℃, holding it for a period of time, and then slowly cooling it; or it can be isothermal spheroidizing annealing, which transforms lamellar pearlite into spheroids. Holding at Ac1+(20~30)℃ for a period of time, and then cooling to Ar1-(10~20)℃ and holding again, this annealing process only forms spheroids in the pearlite regions of the microstructure, and cannot achieve a uniform distribution of spheroidal carbides. Such a microstructure is not conducive to continuous and uniform plastic deformation of the workpiece, and is not the ideal metallographic structure required for cold extrusion forming.

[0004] To facilitate further diffusion of spheroidized carbides within the ferrite matrix, cyclic spheroidizing annealing, i.e., repeated isothermal annealing, is commonly used in current production applications. While this yields uniformly distributed spheroids, it results in a long production cycle. To reduce production costs and save energy and time, the industry has been continuously searching for new annealing processes.

[0005] Therefore, it is necessary to provide a new technical solution. Summary of the Invention

[0006] To address the technical problems existing in the prior art, this invention discloses a spheroidizing annealing process for low-carbon alloy steel warm forgings, the specific technical solution of which is as follows:

[0007] This invention provides a spheroidizing annealing process for low-carbon alloy steel forgings, which adds a sub-temperature holding stage between Ac1+(20~30)℃ and Ar1-(10~20)℃ in conventional isothermal spheroidizing annealing. The specific steps are as follows:

[0008] Step 1: Place the forged blanks in the pit furnace charge basket. Use a crane to lift the charge basket into the pit furnace, then seal the furnace cover and evacuate until the furnace pressure reaches -0.06 to 0.10 MPa. Then fill with nitrogen and control the furnace pressure at 0.06 to 0.12 MPa.

[0009] Step 2: Heat the workpiece in the furnace to 780-790℃ and hold it at this temperature for 2 hours;

[0010] Step 3: Cool down to 710-720℃ and keep warm at this temperature for 4 hours;

[0011] Step 4: Cool down to 680-690℃ and keep at this temperature for 2 hours;

[0012] Step 5: Turn off the heating and let it cool in the furnace to below 500°C before removing it from the furnace.

[0013] Furthermore, the low-carbon alloy steel is of grade 20MnCr5.

[0014] Furthermore, the cooling rate in steps 3 and 4 is 20–40 °C / h.

[0015] The present invention has the following beneficial effects:

[0016] This invention enables low-carbon alloy steel warm forgings to achieve a microstructure with high spheroidization and uniform distribution of spherical carbides in a relatively short time. Products treated with this process exhibit a reduced matrix hardness to 120-135 HB, good plastic deformation capacity, and can withstand complex, high-deformation cold extrusion forming processes. In the actual production of such products, it can replace periodic spheroidizing annealing, reducing the production cycle.

[0017] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is the heat treatment process curve for the spheroidizing annealing process of a low-carbon alloy steel warm forging according to the present invention.

[0020] Figure 2 Metallographic photograph of a low-carbon alloy steel hollow shaft forged blank, magnification: 100x.

[0021] Figure 3 for Figure 2 Metallographic photograph of the forged blank after spheroidizing annealing by the process described in this invention, magnification: 500x. Detailed Implementation

[0022] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0023] In the description of this invention, it should be understood that the terms "upper," "lower," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0024] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0025] This invention provides a spheroidizing annealing process for low-carbon alloy steel forgings, which adds a sub-temperature holding stage between Ac1+(20~30)℃ and Ar1-(10~20)℃ in conventional isothermal spheroidizing annealing. The specific steps are as follows:

[0026] Step 1: Place the forged blanks in the pit furnace charge basket. Use a crane to lift the charge basket into the pit furnace, then seal the furnace cover and evacuate until the furnace pressure reaches -0.06 to 0.10 MPa. Then fill with nitrogen and control the furnace pressure at 0.06 to 0.12 MPa.

[0027] Step 2: Heat the workpiece in the furnace to 780-790℃ and hold it at this temperature for 2 hours;

[0028] Step 3: Cool down to 710-720℃ and keep warm at this temperature for 4 hours;

[0029] Step 4: Cool down to 680-690℃ and keep at this temperature for 2 hours;

[0030] Step 5: Turn off the heating and let it cool in the furnace to below 500°C before removing it from the furnace.

[0031] The low-carbon alloy steel is 20MnCr5.

[0032] The cooling rate in steps 3 and 4 is 20–40 °C / h.

[0033] refer to Figure 1 This is the heat treatment process curve for the spheroidizing annealing process of a low-carbon alloy steel warm forging according to the present invention. Figure 2 Metallographic photograph of a low-carbon alloy steel hollow shaft forged blank, magnification: 100x. Figure 3 for Figure 2 Metallographic photograph of the forged blank after spheroidizing annealing by the process described in this invention, magnification: 500x.

[0034] After spheroidizing annealing according to the process described in this invention, the product's hardness decreases from the original 180-190 HB to 120-135 HB, the spheroidization rate is ≥95%, and the spherical carbides in the microstructure are uniformly distributed in the ferrite (see appendix). Figure 3 This greatly reduces the deformation resistance of the tissue, giving the product good plasticity during subsequent cold extrusion, preventing product cracking, and also improving the service life of the cold extrusion die.

[0035] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. In addition, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.

[0036] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications and variations to the above embodiments within the scope of the present invention.

Claims

1. A spheroidizing annealing process for low-carbon alloy steel warm forgings, characterized in that, An additional sub-temperature holding stage is added between the conventional isothermal spheroidizing annealing processes of Ac1+ (20–30) °C and Ar1- (10–20) °C. Includes the following steps: Step 1: Place the forged blanks in the pit furnace charge basket. Use a crane to lift the charge basket into the pit furnace, then seal the furnace cover and evacuate until the furnace pressure reaches -0.06 to 0.10 MPa. Then fill with nitrogen and control the furnace pressure at 0.06 to 0.12 MPa. Step 2: Heat the workpiece in the furnace to 780-790℃ and hold it at this temperature for 2 hours; Step 3: Cool down to 710-720℃ and keep warm at this temperature for 4 hours; Step 4: Cool down to 680-690℃ and keep at this temperature for 2 hours; Step 5: Turn off the heating and allow the furnace to cool to below 500°C before removing from the furnace. The cooling rate in steps 3 and 4 is 20–40 °C / h.

2. The spheroidizing annealing process for low-carbon alloy steel warm forgings according to claim 1, characterized in that, The low-carbon alloy steel is 20MnCr5.

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