Spheroidizing Annealing Method for Medium and Low Carbon Alloy Cold Heading Steel

Abstract

The present invention relates to a spheroidizing annealing method for medium and low carbon alloy cold heading steel. After pickling, phosphating and saponification treatments, the medium and low carbon alloy cold heading steel is subjected to drawing deformation. The wire rod after cold drawing is subjected to spheroidizing annealing treatment. The wire rod is heated to 700 - 720 °C and held for 6 h, then cooled at a rate of 5 - 10 °C / h to 680 °C, and then cooled at a rate of 100 °C / h to 250 - 300 °C and held for 2 h, and then the alloy steel wire after spheroidizing is slowly cooled and taken out of the furnace. By improving the spheroidizing annealing process, a moderately coarsened carbide particle distribution state is obtained, the material strength is reduced, the plasticity is increased, and the comprehensive mechanical properties are significantly improved. The spheroidizing structure of the medium and low carbon alloy cold heading steel after spheroidizing annealing is good and will not crack during the subsequent cold heading process; it has the characteristics of simple process and good effect. By controlling the cooling and temperature rates, the distribution of the carbide particles after spheroidizing in the steel is changed, making it more evenly distributed in the matrix and inside the grains, reducing the segregation of carbides at the grain boundaries, thereby enhancing the plasticity.

Description

Technical Field

[0001] The present invention relates to the technical field of spheroidizing annealing, and more particularly to a spheroidizing annealing method for medium and low carbon alloy cold heading steel. Background Art

[0002] Spheroidizing annealing is annealing performed to spheroidize the carbides in steel, resulting in a structure of spherical or granular carbides uniformly distributed on a ferrite matrix. Spheroidizing annealing is mainly used for eutectoid steel and hypereutectoid steel to obtain a spheroidized structure similar to granular pearlite (since it is not necessarily of eutectoid composition, it is called a spheroidized structure), thereby reducing hardness, improving machinability, and preparing the microstructure for quenching. The spheroidized structure not only has better plasticity and toughness than the lamellar structure, but also has a slightly lower hardness. When machining a workpiece with a spheroidized structure, the cutting tool can avoid cutting hard and brittle cementite and pass through the soft ferrite, thus extending the service life of the cutting tool and improving the machinability of the steel.

[0003] In the existing technology, the raw materials for producing high-strength standard parts of grade 8.8 and above are usually medium carbon steel or medium and low carbon alloy cold heading steel in the hot-rolled state. During the cold heading process of standard parts, the raw materials often need to withstand a large total deformation amount, which requires the raw materials to have good plasticity and as low hardness as possible. However, the room-temperature microstructure of the hot-rolled cold heading steel wire rod is a ferrite and lamellar pearlite structure with relatively high strength and hardness, which is not conducive to the cold heading process of standard parts, easily leads to cold heading cracking, and reduces the utilization rate of raw materials.

[0004] During the wire rod reforming process, spheroidizing annealing treatment needs to be carried out after cold drawing to reduce hardness, improve plasticity, and enhance machinability. The products obtained by using the existing spheroidizing annealing process have high strength and low plasticity, and their cold-formed parts also have quality problems such as high hardness. Summary of the Invention

[0005] Aiming at the deficiencies of the existing technology, the purpose of the present invention is to provide a spheroidizing annealing method for medium and low carbon alloy cold heading steel, which improves the spheroidizing annealing process to obtain a moderately coarsened carbide particle distribution state, reduces the material strength, increases the plasticity, and significantly improves the comprehensive mechanical properties.

[0006] The above object of the present invention is achieved by the following technical solutions:

[0007] A spheroidizing annealing method for medium and low carbon alloy cold heading steel. After the medium and low carbon alloy cold heading steel is pickled, phosphated and saponified, it is subjected to drawing deformation. The wire rod after cold drawing is subjected to spheroidizing annealing treatment. The wire rod is heated to 700 - 720 °C, held for 6 h, then cooled to 680 °C at a rate of 5 - 10 °C / h, and then cooled to 250 - 300 °C at a rate of 100 °C / h and held for 2 h. Then the alloy steel wire after spheroidizing is slowly cooled and taken out of the furnace.

[0008] In a preferred embodiment of the present invention, it can be further configured that: the diameter of the wire rod after cold drawing is 5.5 - 15 mm.

[0009] In a preferred embodiment of the present invention, it can be further configured that: the chemical composition and weight percentage content of the medium and low carbon alloy cold heading steel are: C 0.12% - 0.22%, Si 0.15% - 0.32%, Mn 0.24% - 0.55%, Cr ≤ 0.20%, P ≤ 0.03%, S ≤ 0.05%, AI ≤ 0.06%, Ni ≤ 0.25%, Cu ≤ 0.20%, and the rest are Fe and impurity elements.

[0010] In a preferred embodiment of the present invention, it can be further configured that: in the step of drawing deformation, cold drawing is performed in one or two passes.

[0011] In a preferred embodiment of the present invention, it can be further configured that: when performing spheroidizing annealing treatment, nitrogen is introduced, and at the same time, the exhaust valve is opened, and the temperature is raised to 750 - 780 °C along with the furnace, and the heating time is 150 min; the medium and low carbon alloy cold heading steel is held at 700 - 720 °C in the spheroidizing annealing furnace for 6 h, and a protective atmosphere is conveyed into the spheroidizing annealing furnace during the constant temperature process.

[0012] In a preferred embodiment of the present invention, it can be further configured that: the flow rate adjustment range of the nitrogen delivery is 25 - 30 m 3 / h.

[0013] In a preferred embodiment of the present invention, it can be further configured that: the flow rate adjustment range of the protective atmosphere is 12 - 18 m 3 / h.

[0014] In a preferred embodiment of the present invention, it can be further configured that: the protective atmosphere is methanol high-temperature cracking gas, or natural gas catalytic decomposition gas.

[0015] In summary, the present invention includes at least one of the following beneficial technical effects:

[0016] The present invention discloses a spheroidizing annealing method for medium and low carbon alloy cold heading steel. By improving the spheroidizing annealing process, a moderately coarsened carbide particle distribution state is obtained, the material strength is reduced, the plasticity is increased, and the comprehensive mechanical properties are significantly improved. The spheroidized structure of the medium and low carbon alloy cold heading steel after spheroidizing annealing is good and will not crack during subsequent cold heading; it has the characteristics of simple process and good effect. By controlling the cooling and temperature rates, the distribution of carbide particles after spheroidization in the steel is changed, making it more uniformly distributed in the matrix and inside the grains, reducing the segregation of carbides at the grain boundaries, thereby enhancing the plasticity. Detailed implementation manners

[0017] The technical solutions in the embodiments of the present application will be clearly and completely described below; obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present application.

[0018] Embodiment 1:

[0019] The present invention discloses a spheroidizing annealing method for medium and low carbon alloy cold heading steel. After pickling, phosphating and saponifying treatment, the medium and low carbon alloy cold heading steel is subjected to drawing deformation. The wire rod after cold drawing is subjected to spheroidizing annealing treatment. The wire rod is heated to 700 - 720 °C, held for 6 h, then cooled to 680 °C at a rate of 5 - 10 °C / h, and then cooled to 250 - 300 °C at a rate of 100 °C / h and held for 2 h, and then the alloy steel wire after spheroidization is slowly cooled and taken out of the furnace. The diameter of the wire rod after cold drawing is 5.5 - 15 mm.

[0020] The chemical composition and weight percentage content of the medium and low carbon alloy cold heading steel are: C 0.12% - 0.22%, Si 0.15% - 0.32%, Mn 0.24% - 0.55%, Cr ≤ 0.20%, P ≤ 0.03%, S ≤ 0.05%, AI ≤ 0.06%, Ni ≤ 0.25%, Cu ≤ 0.20%, and the rest are Fe and impurity elements.

[0021] In the step of drawing deformation, cold drawing is carried out in one or two passes. When carrying out the spheroidizing annealing treatment, nitrogen is introduced, and at the same time, the exhaust valve is opened, and the temperature is raised to 750 - 780 °C along with the furnace, and the heating time is 150 min; the medium and low carbon alloy cold heading steel is held at 700 - 720 °C in the spheroidizing annealing furnace for 6 h, and a protective atmosphere is conveyed into the spheroidizing annealing furnace during the constant temperature process. The flow rate adjustment range of nitrogen delivery is 25 - 30 m3 / h. The flow rate adjustment range of the protective atmosphere is 12 - 18 m3 / h. The protective atmosphere is methanol high-temperature cracking gas or natural gas catalytic decomposition gas.

[0022] Before the process improvement, a large number of fine granular carbides were aggregated and adhered along the grain boundaries of massive ferrite, and there were very few carbide particles in the grains. The distribution position corresponded highly to the area where the original pearlite was located, indicating that although spheroidizing annealing was carried out, the original lamellar pearlite morphology disappeared and transformed into fine-grained pearlite, but the carbide particles did not achieve dispersed precipitation. Due to the aggregation of particles at the grain boundaries, the grain boundaries were weakened, which was not conducive to spheroidizing annealing. The transverse carbide accumulation was an important reason for the high annealing hardness. After the process improvement, the phenomenon of carbide grain boundary segregation basically disappeared, the overall dispersion degree increased, and some particles achieved coarsening and growth, and the spheroidizing effect was better.

[0023] The comparison results of the mechanical properties of medium and low carbon alloy cold heading steel before and after the process improvement are shown in Table 1.

[0024]

[0025]

[0026] Analysis shows that after the process improvement, the tensile strength of medium and low carbon alloy cold heading steel decreased from 385 MPa to 335 MPa, the elongation after fracture remained unchanged, but the reduction of area increased from 73% to 82%. This indicates that after the process improvement, the material strength decreased, the plasticity increased, and the comprehensive mechanical properties were better.

[0027] The key to the spheroidizing annealing process is to heat and hold at 20 - 30 °C above Ac1 to partially austenitize the material, and there are a large number of undissolved carbide particles retained in the austenite. During the subsequent cooling process, carbides can precipitate again with these undissolved particles as the core and undergo aggregation and growth, thus realizing the spheroidization of carbides.

[0028] Before the process improvement, conventional one-stage spheroidizing annealing was adopted. Due to factors such as the cooling rate and holding time, carbon elements could not be fully diffused in the matrix after austenitization, resulting in insufficient precipitation with undissolved particles as the core during the cooling process. After the process improvement, on the premise that the temperature of the original holding section remained basically unchanged, the holding time was shortened from the original 10 h to 6 h, and the temperature was gradually decreased to about 680 °C at an extremely slow cooling rate. For medium and low carbon alloy cold heading steel, its Ar1 = 680 °C. Therefore, during the process of slow cooling and temperature reduction, the material was basically in the austenite range, and pearlite-type transformation hardly occurred. However, due to the gradual decrease in temperature, while carbon diffused in the matrix, the solubility product of carbides also showed a downward trend, which was conducive to the precipitation of second-phase particles during the subsequent cooling process. Then, it was cooled to 250 - 300 °C at a relatively fast speed and held for 2 h, similar to the low-temperature tempering aging process. New carbides could precipitate in this temperature range, and the already precipitated carbides achieved partial aggregation and coarsening, showing a state of uniform and dispersed distribution of fine particles.

[0029] The implementation principle of this embodiment is as follows: The present invention discloses a spheroidizing annealing method for medium and low carbon alloy cold heading steel. By improving the spheroidizing annealing process, a moderately coarsened carbide particle distribution state is obtained, the material strength is reduced, the plasticity is increased, and the comprehensive mechanical properties are significantly improved. The spheroidized structure of the medium and low carbon alloy cold heading steel after spheroidizing annealing is good and will not crack during the subsequent cold heading process; it has the characteristics of simple process and good effect. By controlling the cooling and temperature rates, the distribution of the carbide particles after spheroidization in the steel is changed, making it more evenly distributed in the matrix and inside the grains, reducing the segregation of carbides at the grain boundaries, thereby enhancing the plasticity.

[0030] The embodiments of this specific implementation manner are all preferred embodiments of the present invention, and do not limit the protection scope of the present invention accordingly. Therefore, all equivalent changes made according to the structure, shape, and principle of the present invention should be covered within the protection scope of the present invention.

Claims

1. A spheroidizing annealing method for medium and low carbon alloy cold heading steel, characterized in that: The medium and low carbon alloy cold heading steel is subjected to pickling, phosphating and saponification treatments and then undergoes drawing deformation. The wire rod after cold drawing is subjected to spheroidizing annealing treatment. The wire rod is heated to 700 - 720 °C, held for 6 h, then cooled at a rate of 5 - 10 °C / h to 680 °C, and then cooled at a rate of 100 °C / h to 250 - 300 °C and held for 2 h, and then the alloy steel wire after spheroidizing is slowly cooled and taken out of the furnace; The chemical composition and weight percentage of the medium and low carbon alloy cold heading steel are: C 0.12% - 0.22%, Si 0.15% - 0.32%, Mn 0.24% - 0.55%, Cr ≤ 0.20%, P ≤ 0.03%, S ≤ 0.05%, Al ≤ 0.06%, Ni ≤ 0.25%, Cu ≤ 0.20%, and the rest are Fe and impurity elements.

2. The spheroidizing annealing method of a medium-low carbon alloy cold heading steel according to claim 1, characterized in that: The diameter of the wire rod after cold drawing is 5.5 - 15 mm.

3. The spheroidizing annealing method of a medium and low carbon alloy cold heading steel according to claim 1, characterized in that: In the step of drawing deformation, cold drawing is carried out in one or two passes.

4. The spheroidizing annealing method for medium and low carbon alloy cold heading steel according to claim 1, characterized in that: When carrying out spheroidizing annealing treatment, nitrogen is introduced, and at the same time the exhaust valve is opened, and the temperature is raised to 750 - 780 °C along with the furnace, and the heating time is 150 min; the medium and low carbon alloy cold heading steel is held at 700 - 720 °C in the spheroidizing annealing furnace for 6 h, and a protective atmosphere is conveyed into the spheroidizing annealing furnace during the constant temperature process.

5. A spheroidizing annealing method for medium and low carbon alloy cold heading steel according to claim 4, characterized in that: The flow rate adjustment range of the nitrogen delivery is 25 - 30 m 3 / h.

6. A spheroidizing annealing method for medium and low carbon alloy cold heading steel according to claim 4, characterized in that: The flow rate of the protective atmosphere is adjusted in the range of 12 - 18 m 3 / h.

7. A spheroidizing annealing method for medium and low carbon alloy cold heading steel according to claim 4, characterized in that: The protective atmosphere is methanol high-temperature cracking gas or natural gas catalytic decomposition gas.

Images