A continuous spheroidizing annealing method for chromium-nickel-molybdenum alloy structural steel

Abstract

The application discloses a continuous spheroidizing annealing method for chromium-nickel-molybdenum alloy structural steel, and belongs to the technical field of metal material heat treatment processes, and the operation steps are: charging, preheating, heating to a two-phase region, cooling stage one, isothermal stage, cooling stage two and furnace discharge; the application eliminates a small amount of bainite+martensite organization of a part of hot-rolled state residues through preheating section treatment, is rapidly heated to the Ac1-Ac3 two-phase region for heat preservation, a part of austenitization is carried out, carbon and manganese, chromium, nickel, molybdenum and other alloy elements diffuse, a part of small-size carbides with irregular shapes can be spheroidized in advance, in addition, flaky and rod-shaped carbides are broken and become nucleation points for spheroidization in the subsequent cooling and isothermal process, then the cooling speed is controlled to be below Ar1 for isothermal treatment, and the growth of spheroidal carbides is promoted; after the round steel is spheroidized and annealed, the surface is not obviously oxidized and burned, and the requirement of raw materials for forming such as cold forging, cold heading and cold extrusion of gear, bolt, shaft pin, connecting rod and other parts is met.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for metallic materials, and relates to a continuous spheroidizing annealing heat treatment method for chromium-nickel-molybdenum alloy structural round steel. Background Technology

[0002] In the manufacturing processes of large-scale equipment in modern petrochemical, wind power, and transportation industries, drill pipes, pins, connecting rods, piston rods, fasteners, and other related structural components serve as crucial foundational elements for precision positioning, automatic control, drive mechanisms, and motion conversion. Their quality directly impacts the accuracy and service life of the machinery. These components require good comprehensive mechanical properties and high-temperature dimensional stability. Based on the requirements of high hardness, high strength, and high wear resistance in their working parts, medium-carbon alloy steels based on chromium-molybdenum or chromium-nickel-molybdenum systems are generally selected. Their chemical composition gives them high hardness, strength, and wear resistance, making them suitable for manufacturing parts requiring fatigue and wear resistance.

[0003] With the advancement of energy conservation and environmental protection efforts, cold forming of steel has gained widespread application in recent years due to its ability to reduce energy consumption and the smoke and waste gas generated during high-temperature heat treatment, achieving higher dimensional accuracy and stability, and significantly reducing material waste. However, for hot-rolled round steel in the chromium-nickel-molybdenum system, its high hardness (measured at 360-410 HBW) necessitates spheroidizing annealing before cold forming. Spheroidizing annealing is primarily used for high-carbon steel, with standardized processes and clear operations, resulting in significant spheroidizing effects. However, its application in hypoeutectoid steel is less common, and domestic research on spheroidizing annealing processes for medium-carbon steel focuses primarily on cold-heading wire rods, with limited research on large-diameter round steel. Due to the large deformation during cold working, prestress cannot be applied before spheroidizing annealing to prevent deformation, making the process more difficult to control compared to cold-heading wire rods. Based on literature and other steel mill production practices, no precedents or mature standards are available for reference, necessitating further exploration and research into its spheroidizing process. Summary of the Invention

[0004] To address the aforementioned problems, the present invention aims to overcome the deficiencies in the prior art by proposing a continuous spheroidizing annealing heat treatment method for chromium-nickel-molybdenum alloy structural round steel.

[0005] The technical solution of this invention is: a continuous spheroidizing annealing heat treatment method for chromium-nickel-molybdenum alloy structural round steel, as described in this invention; the preparation steps are as follows:

[0006] Step 1: The chromium-nickel-molybdenum alloy structural steel is in the state of hot-rolled round steel before annealing;

[0007] Step 2: Preheating stage: The above-mentioned chromium-nickel-molybdenum alloy structural round steel is laid on the inlet roller table and conveyed into the nitrogen-sealed chamber through the roller conveyor, and rapidly heated to a specific temperature range below Ac1.

[0008] Step 3: Heating stage: Heat the above-mentioned chromium-nickel-molybdenum alloy structural round steel to the measured critical temperature range of the two-phase region;

[0009] Step 4: Two-phase region insulation stage: The chromium-nickel-molybdenum alloy structural steel is kept at a temperature within the determined critical temperature range of the two-phase region;

[0010] Step 5: Cooling Stage 1: The chromium-nickel-molybdenum alloy structural steel is cooled from the two-phase region holding temperature to a temperature below Ar1 at a certain cooling rate;

[0011] Step Six: Isothermal Stage: The chromium-nickel-molybdenum alloy structural steel is subjected to isothermal spheroidization treatment at a temperature below Ar1;

[0012] Step 7: Cooling Stage 2: The isothermal treated chromium-nickel-molybdenum alloy structural steel is furnace cooled to a certain temperature range, and then removed from the furnace and air-cooled.

[0013] Furthermore, in step one, the alloy structural steel comprises, by mass percentage: C: 0.38–0.45%, Si: 0.17–0.40%, Mn: 0.60–1.20%, Cr: 0.90–1.45%, Mo: 0.15–0.60%, Ni: 0.20–0.75%, P: ≤0.020%, S: ≤0.010%, Cu ≤0.25%, Al: 0.010–0.050%, with the remainder being matrix Fe and unavoidable impurities;

[0014] The hot-rolled microstructure is mainly degraded pearlite + equiaxed ferrite, with a small amount of bainite / martensite + retained austenite. The hardness is 360-410HB and the diameter range is 50-80mm.

[0015] Furthermore, in step two, the temperature is heated at a rate of 150-200℃ / h to a specific temperature range below Ac1, characterized by 710-730℃. The purpose is to cause the carbides to aggregate, preventing them from dissolving into austenite during subsequent heating, while promoting the elimination of the original small amount of martensite and retained austenite blocky structures in the hot-rolled round steel.

[0016] This ensures that during the austenite transformation, the presence of undissolved carbides or the concentration gradient within the austenite promotes the formation of as many spherical carbides as possible. The holding time for this section is 60-90 minutes, and rapid heating helps reduce oxidation loss and the depth of the decarburized layer on the round steel surface.

[0017] Furthermore, in step three, the temperature is increased to the two-phase region at a rate of 180-200℃ / h. Due to the high content of alloying elements such as manganese, chromium, nickel, and molybdenum, the two-phase region is very narrow. The holding temperature in the two-phase region is controlled at 750-770℃. Within this temperature range, as carbon and other alloying elements diffuse, the plate-like or short rod-shaped carbides dissolve and their distribution becomes more uniform. After the holding period, some of the angular carbides have become spheroidized, and at the same time, a large number of undissolved carbides in the matrix can serve as nucleation sites for the spheroidized pearlite in the subsequent isothermal stage.

[0018] However, at higher temperatures, most carbides are incorporated into the matrix, leaving fewer carbide particles with increased spacing. During subsequent cooling, these particles are difficult to aggregate and tend to form lamellar pearlite. At lower temperatures, carbon and other alloying elements diffuse slowly, resulting in insufficient carbide dissolution. During subsequent cooling, most carbides retain their original positions and shapes, leading to an uneven overall structure with locally coarse particles.

[0019] Furthermore, in step four, the holding time in the two-phase region is controlled between 175 and 232 minutes. If the holding time is too short, the amount of carbide precipitation will decrease, and most of it will be distributed on the grain boundaries. If the holding time is too long, a large amount of carbide will be incorporated into the austenite, and there will be a lack of nuclei for cementite spheroidization during the subsequent cooling process, thus forming a coarse lamellar pearlite structure.

[0020] Furthermore, in step five, the cooling rate of the first cooling stage is 15-30℃ / h. Within this temperature range, a segregation eutectoid transformation can occur, resulting in a better spheroidization effect.

[0021] However, when the cooling rate is too high, the phase transition point Ar1 drops significantly, and the carbide cores do not have enough time to grow, which easily leads to the formation of lamellar pearlite. When the cooling rate is too low, the spherical carbides are prone to fusion or engulfment, forming large areas of spherical structures with elongated shapes, local shapes, and irregular distribution.

[0022] Furthermore, in step six, the isothermal spheroidizing temperature is 705-720℃, and the holding time is 312-420min. If the temperature is too high, the carbide precipitation rate will accelerate, forming lamellar pearlite. If the temperature is too low, the residual carbide, which serves as the spheroidizing core, will not have sufficient driving force for growth, leading to the aggregation of local carbide particles and an increase in hardness.

[0023] Furthermore, in step seven, during the second cooling stage, after the round steel isothermal spheroidization is completed, it is slowly cooled in the furnace to 530-580℃. If the cooling rate is too fast, the temperature difference between the inside and outside of the round steel is large, and the curvature of the round steel is prone to exceed the standard. If the cooling rate is too slow, the surface burn-off and decarburization tend to increase, and it has a great impact on production efficiency.

[0024] The beneficial effects of this invention are as follows: This invention utilizes a preheating stage to eliminate a small amount of residual bainite + martensite structure from the hot-rolled state, improving the uniformity of the microstructure. Rapid heating to the Ac1-Ac3 two-phase region followed by heat preservation partially austenitizes the microstructure, allowing diffusion of carbon and alloying elements such as manganese, chromium, nickel, and molybdenum. Some irregularly shaped small-sized carbides can be pre-spheroidized. Furthermore, lamellar and rod-shaped carbides dissolve and become nucleation sites for spheroidization during subsequent cooling and isothermal processes. Then, the cooling rate is controlled to below Ar1 for isothermal treatment, promoting the growth of spheroidized carbides and ensuring their uniform distribution. This technology offers fast heating speed and high efficiency. After spheroidizing annealing, the round steel exhibits a spheroidization rate ≥80%, hardness ≤170HB, total decarburized layer ≤0.5D, and no significant oxidation or burn-off on the surface, meeting the requirements for raw materials in cold forging, cold heading, and cold extrusion of components such as gears, bolts, shafts, and connecting rods. Attached Figure Description

[0025] Figure 1 This is an operation flowchart of the present invention;

[0026] Figure 2 This is a diagram of the spheroidized microstructure of the continuous furnace spheroidizing annealing process in Embodiment 1 of the present invention;

[0027] Figure 3 This is a diagram of the spheroidized microstructure of the continuous furnace spheroidizing annealing process in Embodiment 2 of the present invention;

[0028] Figure 4 This is a diagram of the spheroidized microstructure of the continuous furnace spheroidizing annealing process in Embodiment 3 of the present invention. Detailed Implementation

[0029] The specific technical solution of the present invention will be further described in detail below with reference to specific examples.

[0030] Example 1

[0031] The mass percentage of the round steel is as follows: C: 0.42%, Si: 0.25%, Mn: 0.98%, Cr: 1.35%, Ni: 0.39%, Mo: 0.39%, P: 0.008%, S: 0.002%, Al: 0.030%, Cu: 0.05%, with the balance being iron. Before spheroidizing annealing, the round steel is in a hot-rolled state, with a diameter range of 50-80mm. The microstructure is mainly degraded pearlite + equiaxed ferrite, with a small amount of bainite / martensite + retained austenite. The hardness is 360-410 HBW.

[0032] This embodiment includes the following steps:

[0033] 1. Loading: Hot-rolled round steel bars are hoisted and arranged on the roller table at the inlet of the continuous annealing furnace. After being laid out and neatly arranged, the material height is 160mm. It is then conveyed into the continuous annealing furnace through the inlet roller conveyor, and the roller speed is set to 3.5m / h.

[0034] 2. Preheating: Heat to 730℃ (below Ac1) at a heating rate of 180℃ / h, and hold at this temperature for 70 minutes.

[0035] 3. Heating to the two-phase region: Increase the temperature to 760℃ in the two-phase region at a heating rate of 180℃ / h, and hold at this temperature for 210 minutes;

[0036] IV. Cooling Stage 1: The alloy structural steel in the two-phase region is cooled to 720°C below Ar1 at a cooling rate of 20°C / h.

[0037] V. Isothermal Stage: The round steel is held at 720℃ for 360 minutes to undergo isothermal spheroidization treatment;

[0038] VI. Cooling Stage Two: The round steel bars after isothermal spheroidization treatment are cooled to 560℃ in a continuous annealing furnace at a cooling rate of 30℃ / h;

[0039] 7. Unloading: After slow cooling, the round steel bars are unloaded from the furnace through the outlet sealed chamber to the unloading platform. After cooling to near room temperature, they are moved horizontally or hoisted into the storage location.

[0040] The key properties of the round steel bars subjected to spheroidization annealing in a continuous heat treatment furnace according to this example are shown in Table 1. The spheroidization rate is above 88%, the hardness of all parts of the round steel is <170HBW, and the total decarburized layer is <0.5%D, meeting the relevant technical agreements and specifications. Figure 1 The spheroidized structure of the continuous furnace spheroidizing annealing process in Example 1 shows a good spheroidizing effect.

[0041] Table 1. Properties of round steel subjected to spheroidizing annealing according to the process flow of Example 1.

[0042]

[0043] Example 2

[0044] The mass percentage of the round steel is as follows: C: 0.41%, Si: 0.26%, Mn: 1.00%, Cr: 1.35%, Ni: 0.38%, Mo: 0.40%, P: 0.009%, S: 0.001%, Al: 0.025%, Cu: 0.03%, with the balance being iron. Before spheroidizing annealing, it is a hot-rolled round steel with a diameter of 80mm. The microstructure is mainly degraded pearlite + equiaxed ferrite, with a small amount of bainite / martensite + retained austenite. The hardness is 386HBW.

[0045] This embodiment includes the following steps:

[0046] 1. Loading: The round steel bars are hoisted and arranged on the roller table at the inlet of the continuous annealing furnace. After being laid out and neatly arranged, the material height is 160mm. It is then conveyed into the continuous annealing furnace through the inlet roller conveyor, and the roller speed is set to 3.0m / h.

[0047] 2. Preheating: Heat to 720°C below Ac1 at a heating rate of 150°C / h, and hold at this temperature for 90 minutes;

[0048] 3. Heating to the two-phase region: Increase the temperature to 770℃ in the two-phase region at a heating rate of 180℃ / h, and hold at this temperature for 232min;

[0049] IV. Cooling Stage 1: The round steel bars in the two-phase region are cooled to 710℃ below Ar1 at a cooling rate of 15℃ / h.

[0050] V. Isothermal Stage: The round steel is held at 710℃ for 420 minutes to undergo isothermal spheroidization treatment;

[0051] VI. Cooling Stage Two: The round steel bars after isothermal spheroidization treatment are cooled to 580℃ in a continuous annealing furnace at a cooling rate of 20℃ / h;

[0052] 7. Unloading: After slow cooling, the round steel bars are unloaded from the furnace through the outlet sealed chamber to the unloading platform. After cooling to near room temperature, they are moved horizontally or hoisted into the storage location.

[0053] The key properties of round steel subjected to spheroidization annealing in a continuous heat treatment furnace according to this example are shown in Table 2. The spheroidization rate is not less than 90%, the hardness of all parts of the round steel is <170HBW, and the total decarburized layer is <0.5%D, meeting the relevant technical agreements and specifications. Figure 2 The spheroidized structure of the continuous furnace spheroidizing annealing process in Example 2 shows a good spheroidizing effect.

[0054] Table 2 shows the properties of round steel subjected to spheroidizing annealing according to the process flow in Example 2.

[0055]

[0056] Example 3

[0057] The mass percentage of the round steel is as follows: C: 0.43%, Si: 0.27%, Mn: 1.00%, Cr: 1.35%, Ni: 0.40%, Mo: 0.40%, P: 0.008%, S: 0.001%, Al: 0.030%, Cu: 0.03%, with the balance being iron. Before spheroidizing annealing, it is a hot-rolled round steel with a diameter of 50mm. The microstructure is mainly degraded pearlite + equiaxed ferrite, with a small amount of bainite / martensite + retained austenite. The hardness is 392HBW.

[0058] This embodiment includes the following steps:

[0059] 1. Loading: The round steel bars are hoisted and arranged on the roller table at the inlet of the continuous annealing furnace. After being laid out and neatly arranged, the material height is 160mm. It is then conveyed into the continuous annealing furnace through the inlet roller conveyor, and the roller speed is set to 4.0m / h.

[0060] 2. Preheating: Heat to 710℃ below Ac1 at a heating rate of 200℃ / h, and hold at this temperature for 60 minutes;

[0061] 3. Heating to the two-phase region: Increase the temperature to 750℃ in the two-phase region at a heating rate of 200℃ / h, and hold at this temperature for 232min;

[0062] IV. Cooling Stage 1: The round steel bars in the two-phase region are cooled to 705℃ below Ar1 at a cooling rate of 30℃ / h.

[0063] V. Isothermal Stage: The round steel is held at 705℃ for 312 minutes to undergo isothermal spheroidization treatment;

[0064] VI. Cooling Stage Two: The round steel bars after isothermal spheroidization treatment are cooled to 530℃ in a continuous annealing furnace at a cooling rate of 20℃ / h;

[0065] 7. Unloading: After slow cooling, the round steel bars are unloaded from the furnace through the outlet sealed chamber to the unloading platform. After cooling to near room temperature, they are moved horizontally or hoisted into the storage location.

[0066] The key properties of the round steel bars subjected to spheroidization annealing in a continuous heat treatment furnace according to this example are shown in Table 3. The spheroidization rate is above 90%, the hardness of all parts of the round steel is <170HBW, and the total decarburized layer is <0.5%D, meeting the relevant technical agreements and specifications. Figure 3 The spheroidized structure of the continuous furnace spheroidizing annealing process in Example 3 shows a good spheroidizing effect.

[0067] Table 3 shows the properties of round steel subjected to spheroidizing annealing according to the process flow of Example 3.

[0068]

[0069] Based on the development trend of low-carbon emissions and high-precision forming in equipment manufacturing industries such as petrochemicals, wind power, and general machinery, this invention studies a continuous spheroidizing annealing process suitable for cold-forming chromium-nickel-molybdenum alloy steels. Through phase transformation point determination under different heating and cooling rate conditions, control of different holding temperatures and holding times in the austenite + ferrite two-phase region, control of isothermal temperature and time below Ar1, and control of cooling rate after isothermal treatment, the resulting alloy steel has a spheroidization rate ≥80%, hardness ≤170HB, and total decarburized layer ≤0.5D. It can meet the material requirements for cold forging, cold heading, and cold extrusion of parts such as gears, bolts, shafts, and connecting rods, and can significantly reduce material loss and energy consumption during parts manufacturing.

[0070] Furthermore, with the rapid development of equipment manufacturing in the energy and transportation sectors, the requirements for material performance will be further enhanced in order to improve production efficiency and sustainability. This invention can significantly improve the mechanical properties, processing performance, and stability of such steel, making it more suitable for specific engineering and application fields.

Claims

1. A continuous spheroidizing annealing method for chromium-nickel-molybdenum alloy structural steel, characterized in that: The mass percentage of the chromium-nickel-molybdenum alloy structural steel is as follows: C: 0.42%, Si: 0.25%, Mn: 0.98%, Cr: 1.35%, Ni: 0.39%, Mo: 0.39%, P: 0.008%, S: 0.002%, Al: 0.030%, Cu: 0.05%, with the balance being iron. Before spheroidizing annealing, it is a hot-rolled round steel with a diameter range of 50-80mm. The microstructure is degraded pearlite + equiaxed ferrite, with a small amount of bainite / martensite + retained austenite. The hardness is 360-410HBW. The operating steps are as follows:

1. Loading: Hot-rolled round steel bars are hoisted and arranged on the roller table at the inlet of the continuous annealing furnace. After being laid out and neatly arranged, the material height is 160mm. It is then conveyed into the continuous annealing furnace through the inlet roller conveyor, and the roller speed is set to 3.5m / h.

2. Preheating: Heat to 730℃ (below Ac1) at a heating rate of 180℃ / h, and hold at this temperature for 70 minutes.

3. Heating to the two-phase region: Increase the temperature to 760℃ in the two-phase region at a heating rate of 180℃ / h, and hold at this temperature for 210 minutes; IV. Cooling Stage 1: The alloy structural steel in the two-phase region is cooled to 720°C below Ar1 at a cooling rate of 20°C / h. V. Isothermal Stage: The round steel is held at 720℃ for 360 minutes to undergo isothermal spheroidization treatment; VI. Cooling Stage Two: The round steel bars after isothermal spheroidization treatment are cooled to 560℃ in a continuous annealing furnace at a cooling rate of 30℃ / h; 7. Unloading: After slow cooling, the round steel bars are unloaded from the furnace through the outlet sealed chamber to the unloading platform. After cooling to near room temperature, they are moved horizontally or hoisted into the storage location.

2. A continuous spheroidizing annealing method for chromium-nickel-molybdenum alloy structural steel, characterized in that: The mass percentage of the chromium-nickel-molybdenum alloy structural steel is as follows: C: 0.41%, Si: 0.26%, Mn: 1.00%, Cr: 1.35%, Ni: 0.38%, Mo: 0.40%, P: 0.009%, S: 0.001%, Al: 0.025%, Cu: 0.03%, with the balance being iron. Before spheroidizing annealing, it is a hot-rolled round steel with a diameter of 80mm. The microstructure is degraded pearlite + equiaxed ferrite, with a small amount of bainite / martensite + retained austenite. The hardness is 386HBW. The operating steps are as follows:

1. Loading: The round steel bars are hoisted and arranged on the roller table at the inlet of the continuous annealing furnace. After being laid out and neatly arranged, the material height is 160mm. It is then conveyed into the continuous annealing furnace through the inlet roller conveyor, and the roller speed is set to 3.0m / h.

2. Preheating: Heat to 720°C below Ac1 at a heating rate of 150°C / h, and hold at this temperature for 90 minutes; 3. Heating to the two-phase region: Increase the temperature to 770℃ in the two-phase region at a heating rate of 180℃ / h, and hold at this temperature for 232min; IV. Cooling Stage 1: The round steel bars in the two-phase region are cooled to 710℃ below Ar1 at a cooling rate of 15℃ / h. V. Isothermal Stage: The round steel is held at 710℃ for 420 minutes to undergo isothermal spheroidization treatment; VI. Cooling Stage Two: The round steel bars after isothermal spheroidization treatment are cooled to 580℃ in a continuous annealing furnace at a cooling rate of 20℃ / h; 7. Unloading: After slow cooling, the round steel bars are unloaded from the furnace through the outlet sealed chamber to the unloading platform. After cooling to near room temperature, they are moved horizontally or hoisted into the storage location.

3. A continuous spheroidizing annealing method for chromium-nickel-molybdenum alloy structural steel, characterized in that: The mass percentage of the chromium-nickel-molybdenum alloy structural steel is as follows: C: 0.43%, Si: 0.27%, Mn: 1.00%, Cr: 1.35%, Ni: 0.40%, Mo: 0.40%, P: 0.008%, S: 0.001%, Al: 0.030%, Cu: 0.03%, with the balance being iron. Before spheroidizing annealing, it is a hot-rolled round steel with a diameter of 50mm. The microstructure is degraded pearlite + equiaxed ferrite, with a small amount of bainite / martensite + retained austenite. The hardness is 392HBW. The operating steps are as follows:

1. Loading: The round steel bars are hoisted and arranged on the roller table at the inlet of the continuous annealing furnace. After being laid out and neatly arranged, the material height is 160mm. It is then conveyed into the continuous annealing furnace through the inlet roller conveyor, and the roller speed is set to 4.0m / h.

2. Preheating: Heat to 710℃ below Ac1 at a heating rate of 200℃ / h, and hold at this temperature for 60 minutes; 3. Heating to the two-phase region: Increase the temperature to 750℃ in the two-phase region at a heating rate of 200℃ / h, and hold at this temperature for 232min; IV. Cooling Stage 1: The round steel bars in the two-phase region are cooled to 705℃ below Ar1 at a cooling rate of 30℃ / h. V. Isothermal Stage: The round steel is held at 705℃ for 312 minutes to undergo isothermal spheroidization treatment; VI. Cooling Stage Two: The round steel bars after isothermal spheroidization treatment are cooled to 530℃ in a continuous annealing furnace at a cooling rate of 20℃ / h; 7. Unloading: After slow cooling, the round steel bars are unloaded from the furnace through the outlet sealed chamber to the unloading platform. After cooling to near room temperature, they are moved horizontally or hoisted into the storage location.

Images