A heat treatment method for low alloy steel forgings

Abstract

This invention relates to the field of steel forging manufacturing technology, and more particularly to a heat treatment method for manufacturing low-alloy steel forgings, comprising: heating the forging to a preset temperature and holding it at that temperature to obtain a forging to be quenched; immersing the forging in a quenching liquid to reduce its temperature to the martensitic transformation temperature range to output the quenched forging; obtaining a residual stress distribution map of the quenched forging, determining the residual tensile stress region and residual compressive stress region of the quenched forging, as well as the minimum cooling temperature of the quenched forging; determining the damage-risk surfaces of the quenched forging; determining the cooling rate of each surface based on the area ratio and shortest straight-line distance of each non-damage-risk surface; and continuing to cool the quenched forging according to the minimum cooling temperature and cooling rate to output the finished low-alloy steel forging. This invention improves the uniformity of hot working of steel forgings.

Description

Technical Field

[0001] This invention relates to the field of steel forging manufacturing technology, and in particular to a heat treatment method for manufacturing low alloy steel forgings. Background Technology

[0002] Low-alloy high-strength steel, due to its excellent combination of strength and toughness and cost advantages, is widely used in the manufacture of key components for low-temperature environments in fields such as wind power and petrochemicals, such as large wind turbine bearing housings and flanges. Currently, in order to obtain high strength while ensuring good low-temperature toughness, the industry's technical trend is to pursue a microstructure dominated by lower bainite across the entire cross-section of the forging. Lower bainite has a fine lath structure and highly dispersed carbides, and its strength, toughness, and crack propagation resistance are all superior to upper bainite, characterized by coarse laths, or brittle lamellar martensite. Large forgings have previously... During the smelting and forging process, microscopic compositional segregation, porosity, and structural inhomogeneity can occur. These inhomogeneous structures can lead to differences in thermal conductivity and phase transformation kinetics in different areas of the forging. Under uniform cooling conditions, these inhomogeneous structures can exhibit abnormally rapid or slow cooling, resulting in an uneven temperature field across the entire workpiece. Ultimately, this leads to asynchronous structural transformation, forming a mixed structure of upper bainite, lower bainite, and martensite. Therefore, there is an urgent need for a heat treatment method to overcome the influence of the structural inhomogeneity of the forging itself and ensure that a uniform and singular lower bainite structure is obtained across the entire cross-section in the manufacture of low-alloy steel forgings.

[0003] Chinese Patent Publication No. CN114182176A discloses a high-strength, high-toughness low-alloy steel ring forging and its manufacturing method. The method involves designing the raw material composition ratio, using fine-grained elements to refine the product's grain size and enhance precipitation strengthening, forging and rolling the billet to strengthen dislocations, and finally performing performance heat treatment. The resulting low-alloy steel ring forging meets the requirements for high strength and high toughness. However, the method suffers from several drawbacks. These include thermal stress during the smelting and forging process of large forgings, leading to differences in thermal conductivity and phase transformation kinetics across different regions of the forging. Consequently, under uniform cooling conditions, residual stress within the forging causes a decrease in uniformity after quenching, and residual stress also results in oxide scale residue on the surface of the quenched forging. Summary of the Invention

[0004] Therefore, the present invention provides a heat treatment method for manufacturing low alloy steel forgings to overcome the problems in the prior art, which are caused by thermal stress in large forgings during smelting and forging, resulting in differences in thermal conductivity and phase transformation kinetics in different regions of the forgings. Consequently, under uniform cooling conditions, the residual stress inside the forgings leads to a decrease in the uniformity of the forgings after quenching, and the residual stress inside the forgings leads to oxide scale residue on the surface of the quenched forgings.

[0005] To achieve the above objectives, the present invention provides a heat treatment method for manufacturing low-alloy steel forgings, comprising:

[0006] The forging is heated to a preset temperature and held at that temperature to obtain the forging to be quenched;

[0007] The forging to be quenched is placed in the quenching liquid to output a quenched forging with a temperature reduced to the martensitic transformation temperature range.

[0008] Obtain the residual stress distribution diagram of the quenched forging;

[0009] The residual tensile stress region and residual compressive stress region of the quenched forging are determined based on the residual stress distribution diagram.

[0010] The minimum cooling temperature of the quenched forging is determined based on the residual tensile stress region and the residual compressive stress region.

[0011] The damage risk surface of the quenched forging is determined based on the residual tensile stress amplitude in the residual tensile stress region, the shortest straight-line distance between the residual tensile stress region and the surface of the quenched forging, the thickness of the quenched forging, and the material yield strength of the quenched forging.

[0012] The cooling rate of each surface is determined based on the area ratio of each non-damage risk surface and the shortest straight distance, wherein the surface area of ​​the quenched forging after removing the damage risk surface is determined as the non-damage risk surface.

[0013] The quenched forging is further cooled according to the minimum cooling temperature and the cooling rate to output a low alloy steel forging product. The method of further cooling is determined according to the thickness of the quenched forging and the real-time temperature.

[0014] Further, determining the residual tensile stress region and the residual compressive stress region based on the residual stress distribution diagram includes:

[0015] The three-dimensional closed region inside the quenched forging where the residual tensile stress is greater than a preset second stress threshold is defined as the residual tensile stress region.

[0016] The three-dimensional closed region with residual compressive stress less than a preset first stress threshold is defined as the residual compressive stress region.

[0017] Further, determining the minimum cooling temperature of the quenched forging based on the residual tensile stress region and the residual compressive stress region includes:

[0018] The shortest straight-line distance from the geometric center of the residual tensile stress region to the surface of the quenched forging and the projected area of ​​the residual compressive stress region of the quenched forging on the surface of the quenched forging are obtained respectively.

[0019] The minimum cooling temperature of the quenched forging is calculated based on the shortest straight distance that meets the preset distance condition and the projected area of ​​the residual compressive stress region on the surface of the quenched forging that meets the preset projected area condition.

[0020] Wherein, the preset distance condition is that the shortest straight distance is less than a preset distance; the preset projected area condition is that the projected area of ​​the residual compressive stress region on the surface of the quenched forging is less than a preset projected area.

[0021] Furthermore, the minimum cooling temperature is the sum of the minimum critical temperature for martensitic transformation, the distance correction, and the area correction.

[0022] Wherein, the distance correction amount is the product of the shortest straight-line distance and the distance correction coefficient; the area correction amount is the product of the projected area and the area correction coefficient.

[0023] Furthermore, the surfaces of quenched forgings with surface damage risk coefficients greater than a preset coefficient are defined as the damage-risk surfaces of the quenched forgings.

[0024] The surface damage risk coefficient is the product of the first ratio and the second ratio.

[0025] The first ratio is the ratio of the residual tensile stress amplitude to the yield strength of the material, and the second ratio is the ratio of the shortest straight distance to the thickness of the quenched forging.

[0026] Further, determining the cooling rate of each surface based on the area percentage of each non-damage-risk surface and the shortest straight-line distance includes:

[0027] The sum of the product of the area ratio of the non-damage risk surface and the area ratio weight coefficient and the product of the shortest straight distance and the straight distance weight coefficient is determined as the evaluation parameter of the corresponding surface of the quenched forging.

[0028] If the evaluation parameter is less than the preset evaluation parameter, it is determined that the degree of obstruction of the damage risk surface of the quenched forging to the lower bainite transformation process of the quenched forging does not meet the conditions, and the cooling rate of the corresponding surface of the quenched forging is reduced.

[0029] The cooling rate of the corresponding surface of the quenched forging is positively correlated with the evaluation parameters of the corresponding surface.

[0030] Furthermore, the area ratio of the non-damage risk surface is the ratio of the total area of ​​the non-damage risk surface to the surface area of ​​the quenched forging.

[0031] Furthermore, if the quenched forging meets the preset cooling conditions, then the quenched forging is further cooled using atomization cooling.

[0032] If the quenched forging does not meet the preset cooling conditions, a combination of air cooling and conventional cooling is used to further cool the quenched forging.

[0033] The preset cooling conditions are that the thickness of the quenched forging is less than or equal to the preset thickness, and the average real-time temperature of the quenched forging is greater than the preset temperature.

[0034] Furthermore, the average real-time temperature is the ratio of the sum of the temperature values ​​of all temperature sampling points of the quenched forging within a unit temperature monitoring cycle to the number of temperature sampling points.

[0035] Furthermore, in response to the atomization cooling method in the continued cooling process, if the vertical flow velocity of the cooling medium on the surface of the quenched forging is less than a preset flow velocity, it is determined that the oxide scale removal effect does not meet the requirements, and the atomization pressure is increased.

[0036] The atomization pressure is negatively correlated with the vertical flow velocity of the cooling medium.

[0037] Compared with the prior art, the beneficial effects of the present invention are as follows: During the quenching and cooling test, the present invention obtains a residual stress distribution map of the quenched forging, and determines the residual tensile stress region and residual compressive stress region based on the residual stress distribution map. The residual tensile stress region is a three-dimensional closed region within the quenched forging where the residual tensile stress is greater than a preset second stress threshold, and the residual compressive stress region is a three-dimensional closed region within the quenched forging where the residual compressive stress is less than a preset first stress threshold. When the residual tensile stress is greater than the preset second stress threshold, it indicates that a tensile stress concentration region has formed inside the quenched forging, posing a risk of cracking or insufficient phase transformation. When the residual compressive stress is less than the preset first stress threshold, it indicates that the compressive stress protection layer on the surface of the quenched forging is insufficient and inadequate to effectively inhibit crack initiation and propagation. Furthermore, based on the spatial distribution relationship and relative intensity between the residual tensile stress region and the residual compressive stress region, the safe temperature window and cooling path of the quenched forging in the subsequent cooling stage are determined.

[0038] Furthermore, this invention obtains the shortest straight-line distance between the residual tensile stress region and the surface of the quenched forging, and the projected area of ​​the residual compressive stress region on the surface of the quenched forging, respectively. The shortest straight-line distance represents the physical depth of the residual tensile stress region from the surface of the quenched forging. When this shortest straight-line distance is less than a preset distance, it indicates that the residual tensile stress region is close to the outer surface of the workpiece, placing it in a high-risk position where quenching cracks are easily induced by the superposition of surface tensile stress generated by subsequent cooling. The projected area represents the size of the coverage area of ​​the residual compressive stress region on the surface of the quenched forging. When this projected area of ​​the residual compressive stress region on the surface of the quenched forging is less than a preset projected area, it indicates that during the quenching process, the surface of the quenched forging typically forms compressive stress due to rapid cooling, while the interior of the quenched forging generates tensile stress due to cooling lag and phase transformation differences. If the residual tensile stress region is too close to the surface, and although a surface compressive stress layer exists, it is insufficient to completely cover the residual compressive stress region, further drastic cooling will cause the temperature difference between the surface and the interior to increase. The increased difference between the gradient and the phase transformation process leads to a further increase in tensile stress, which may break through the protection of the surface compressive stress layer, thereby inducing microcracks or even macroscopic cracks near the surface. Therefore, the minimum cooling temperature of the quenched forging is determined by the sum of the minimum critical temperature of martensitic transformation, the distance correction, and the area correction. The distance correction is the product of the shortest straight distance and the distance correction coefficient, which is the corresponding increase in the minimum critical temperature of martensitic transformation caused by the reduction of the unit shortest straight distance. This means that the smaller the shortest straight distance, the larger the required increase in the positive value of the minimum cooling temperature correction. The area correction is the product of the projected area and the area correction coefficient, which is the corresponding increase in the minimum critical temperature of martensitic transformation caused by the reduction of the projected area of ​​the unit residual compressive stress region. This means that the smaller the projected area, the larger the required increase in the positive value of the minimum cooling temperature correction. Furthermore, by establishing a deterministic mathematical relationship between the minimum cooling temperature and the geometric characteristics of the stress state of the quenched forging monitored in real time, the cooling efficiency of the quenched forging is improved.

[0039] Furthermore, this invention determines the damage-risk surface of the quenched forging based on the surface damage risk coefficient. The surface damage risk coefficient is the product of the ratio of residual tensile stress amplitude to material yield strength and the ratio of the shortest straight distance to the thickness of the quenched forging. The residual tensile stress amplitude represents the stress level in the tensile stress concentration area and is the direct driving force for crack initiation and propagation. The material yield strength represents the ability of the quenched forging material itself to resist plastic deformation and damage; it is an inherent mechanical property indicator of the material. The larger the ratio of residual tensile stress amplitude to material yield strength, the higher the risk of damage. This indicates that the closer the tensile stress is to or exceeds the yield strength of the material, the higher the risk of local yielding or cracking. The smaller the ratio of the shortest straight distance to the thickness of the quenched forging, the closer the tensile stress region is to the surface of the quenched forging, the more obvious the stress concentration effect on the surface, and the easier it is for cracks to initiate or propagate from the surface. Therefore, surface areas with surface damage risk coefficients exceeding the preset coefficient are identified as damage risk surfaces that require special attention. This is to facilitate subsequent evaluation of the cooling rate of the quenched forging surface, in order to alleviate stress concentration, promote stress relaxation, and prevent crack formation.

[0040] Furthermore, this invention determines the cooling rate of each surface of the quenched forging based on evaluation parameters of the surface of the quenched forging. The evaluation parameters are the sum of the product of the shortest straight-line distance and the straight-line distance weighting coefficient, and the product of the area ratio of the non-damage-risk surface and the area ratio weighting coefficient. The straight-line distance weighting coefficient represents the resistance stress to the lower bainite transformation process per unit straight-line distance when a sudden change in movement occurs at the surface damage site of the quenched forging; that is, the larger the shortest straight-line distance, the greater the required cooling rate. The area ratio weighting coefficient represents the resistance stress to the lower bainite transformation process per unit area of ​​the non-damage-risk surface when a sudden change in movement occurs at the surface damage site of the quenched forging; that is, the area ratio of the non-damage-risk surface represents the area ratio of the non-damage-risk surface. The larger the ratio, the greater the required cooling rate. In the low-temperature stage of quenching cooling, such as the martensitic transformation zone and below, the material plasticity decreases. Reducing the cooling rate can directly reduce the temperature gradient between the surface and the core of the forging, as well as between different areas of the surface, thereby reducing the new thermal stress caused by uneven thermal shrinkage. Therefore, by reducing the cooling rate of the surface area with the smaller evaluation parameter of the corresponding surface of the quenched forging, the comprehensive risk of each surface can be quantitatively diagnosed through the evaluation parameter. Then, by adjusting the cooling rate in the opposite direction, the cooling intensity of the damaged risk surface of the quenched forging can be locally weakened, thereby actively inhibiting the initiation and propagation of quenching cracks. Ultimately, while improving the product qualification rate, the uniformity and reliability of the forging performance can be improved.

[0041] Furthermore, this invention, based on the fact that when the thickness of the quenched forging is less than or equal to a preset thickness, it indicates that the quenched forging is a thin-walled or medium-thickness component, and the cross-sectional temperature distribution of the quenched forging is relatively uniform during the cooling process, resulting in a relatively low tendency for quenching cracks; when the average real-time temperature of the quenched forging is greater than the preset temperature, it indicates that the forging is still in a relatively high temperature range, the material still has good plasticity, and the subsequent cooling process still needs to complete a sufficient martensitic transformation, therefore, the cooling method is determined to be atomized cooling. This further utilizes the adjustable and moderate cooling intensity characteristics of atomized cooling to ensure sufficient microstructure transformation while avoiding excessively rapid cooling. New thermal stress and structural stress concentration; when the thickness of the quenched forging is greater than the preset thickness, and the average real-time temperature of the quenched forging is less than or equal to the preset temperature, it indicates that the quenched forging is a thick cross-section component, and the overall temperature of the quenched forging has dropped to a low level. If a strong cooling method is continued at this time, residual stress or even cracks are easily generated due to the excessive temperature difference between the inside and the surface of the quenched forging. Therefore, the cooling method is determined to be a combination of air cooling and wind cooling. The lower cooling rate further promotes the uniformity of cross-sectional temperature, slows down the structural transformation process, thereby reducing the stress gradient and cracking risk, and ensuring the integrity and performance uniformity of the internal structure of the forging.

[0042] Furthermore, in response to the atomization cooling method in the continued cooling process, the present invention compares the vertical flow velocity of the cooling medium on the surface of the quenched forging with a preset flow velocity. The vertical flow velocity of the cooling medium directly characterizes the momentum transfer efficiency of the cooling medium in the normal direction of the forging surface. The vertical flow velocity of the cooling medium reflects the dissipation of kinetic energy and the flow state of the cooling medium after impacting the surface. The vertical flow velocity of the cooling medium is affected by the physical state of the forging surface, namely the surface roughness and oxide scale. If there is residual tensile stress on the surface or subsurface of the forging, the direction of action of the residual tensile stress is to open the material surface layer, which is equivalent to mechanically improving the bonding strength between the oxide scale and the forging. The increased atomization pressure reduces the peeling driving force caused by the phase transformation shrinkage of the forging, making it more difficult for the oxide scale to fall off under the impact of the cooling medium. Therefore, when the vertical flow velocity of the cooling medium on the surface of the quenched forging is less than the preset flow velocity, it indicates that the removal of oxide scale on the surface of the forging does not meet the requirements. Increasing the atomization pressure means that the impact momentum and dynamic pressure carried by individual droplets and droplet groups increase. Droplets with higher kinetic energy continuously impact the oxide scale surface. When the shear stress generated by the droplet impact force exceeds the bonding strength between the oxide scale and the forging enhanced by the residual tensile stress, the fastening effect of the oxide scale can be overcome, and the oxide scale can be broken and washed off from the substrate, thereby improving the surface quality of the low alloy steel forging product. Attached Figure Description

[0043] Figure 1This is an overall flow chart of the heat treatment method for manufacturing low alloy steel forgings according to an embodiment of the present invention;

[0044] Figure 2 This is a CCT curve of the material of the quenched forging in the heat treatment method for manufacturing low alloy steel forgings according to an embodiment of the present invention;

[0045] Figure 3 This is a flowchart illustrating the determination of residual tensile stress region and residual compressive stress region in a heat treatment method for manufacturing low alloy steel forgings according to an embodiment of the present invention.

[0046] Figure 4 This is a flowchart illustrating the process of further cooling a quenched forging in a heat treatment method for manufacturing low-alloy steel forgings, as described in an embodiment of the present invention. Detailed Implementation

[0047] To make the objectives and advantages of the present invention clearer, the present invention will be further described below with reference to embodiments; it should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention.

[0048] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0049] It should be noted that in the description of this invention, the terms "upper", "lower", "left", "right", "inner", "outer", etc., which indicate directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings. This is only for the convenience of description and is not intended to indicate or imply that the device or element must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation of this invention.

[0050] Furthermore, it should be noted that, in the description of this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; 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; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0051] Please see Figure 1 The diagram shown is an overall flow chart of a heat treatment method for manufacturing low-alloy steel forgings according to an embodiment of the present invention. The heat treatment method for manufacturing low-alloy steel forgings according to an embodiment of the present invention includes:

[0052] Step S1: Heat the forging to a preset temperature and hold it at that temperature to obtain the forging to be quenched;

[0053] Step S2: Place the forging to be quenched into the quenching liquid to output a quenched forging with a temperature reduced to the martensitic transformation temperature range.

[0054] Step S3: Obtain the residual stress distribution diagram of the quenched forging;

[0055] Step S4: Determine the residual tensile stress region and residual compressive stress region of the quenched forging based on the residual stress distribution diagram;

[0056] Step S5: Determine the minimum cooling temperature of the quenched forging based on the residual tensile stress region and the residual compressive stress region;

[0057] Step S6: Determine the damage risk surface of the quenched forging based on the residual tensile stress amplitude of the residual tensile stress region, the shortest straight-line distance between the residual tensile stress region and the surface of the quenched forging, the thickness of the quenched forging, and the material yield strength of the quenched forging.

[0058] Step S7: Determine the cooling rate of each surface based on the area ratio of each non-damage risk surface and the shortest straight distance, wherein the surface area of ​​the quenched forging after removing the damage risk surface is determined as the non-damage risk surface.

[0059] Step S8: The quenched forging is further cooled according to the minimum cooling temperature and the cooling rate to output a low alloy steel forging product. The method of further cooling is determined according to the thickness of the quenched forging and the real-time temperature.

[0060] As will be understood by those skilled in the art, the operating principle and process of an ultrasonic 3D scanner are conventional technical means familiar to them, and therefore the operating principle and process of an ultrasonic 3D scanner will not be described in detail here.

[0061] Please refer to the chemical composition table of the forging. The material of the forging in this embodiment of the invention is S690QL. Based on existing production experience and the rules of the influence of alloying elements such as nickel, manganese, niobium, and vanadium on strength and low-temperature impact, the room temperature strength and low-temperature impact performance of S690QL material are improved by adding an appropriate amount of nickel and replacing nickel with manganese, and by micro-alloying of alloying elements such as niobium and vanadium.

[0062] Please continue to refer to the chemical composition table of the forging. The chemical elements of the forging of S690QL material in this embodiment of the invention include 0.18 to 0.20 carbon, 0.20 to 0.40 silicon, 1.40 to 1.70 manganese, less than or equal to 0.015 phosphorus, less than or equal to 0.005 sulfur, 0.02 to 0.06 niobium, 0.07 to 0.12 vanadium, less than or equal to 0.05 titanium, 0.020 to 0.050 aluminum, 0.70 to 1.00 chromium, 0.70 to 1.00 nickel, less than or equal to 0.50 copper, 0.15 to 0.30 molybdenum, less than or equal to 0.005 boron, less than or equal to 0.15 zirconium, less than or equal to 2.0 hydrogen, less than or equal to 25 oxygen, less than or equal to 100 nitrogen, and less than or equal to 0.80 carbon equivalent.

[0063] Specifically, the formula for calculating carbon equivalent is: Carbon equivalent = Carbon + Manganese / 6 + (Chromium + Molybdenum + Vanadium) / 5 + (Nickel + Copper) / 15.

[0064] Table 1 - Chemical Composition of Forgings

[0065] ;

[0066] Specifically, carbon equivalent is used to assess the weldability and hardenability of steel.

[0067] In practice, this invention improves the grain refinement effect of the material by adding a small amount of microalloying elements such as niobium and vanadium. At the same time, reducing the content of alloying elements such as nickel, molybdenum, and chromium enables the designed material to have higher strength and low-temperature resistance.

[0068] Specifically, the process of heating the forging to a preset temperature and holding it at that temperature is carried out in a temperature-controlled trolley-type resistance furnace.

[0069] Specifically, the holding time for the forging is 5 minutes.

[0070] Optionally, the preset temperature range is [890℃, 930℃].

[0071] Preferably, the preset temperature in the preferred embodiment is 910°C.

[0072] Specifically, the quenching fluid used to quench the forgings is a 10% concentration PAG polymer aqueous solution with a minimum volume of 0.15L.

[0073] Specifically, a centrifugal fan is used to perform the air-cooling process on the quenched forgings.

[0074] Specifically, mist cooling nozzles are used to perform the mist cooling process on quenched forgings;

[0075] Specifically, a flow sensor positioned above the quenched forging is used to obtain the flow rate of the cooling medium.

[0076] Specifically, the residual stress distribution map of the quenched forging is obtained by using an ultrasonic 3D scanner positioned above the quenched forging.

[0077] Specifically, the martensitic transformation temperature range is [180℃, 420℃].

[0078] Please see Figure 2 As shown, this is a CCT curve of the material of the quenched forging in the heat treatment method for manufacturing low alloy steel forgings according to an embodiment of the present invention. The horizontal axis of the CCT curve of the composition of the quenched forging is time, and the vertical axis is temperature. The right side of the figure shows the legend of the transformation critical lines of ferrite, pearlite, bainite and austenite, the coordinate point where martensite begins to transform, the coordinate point where martensite transforms to 50%, and the coordinate point where martensite transforms to 90%. The austenitizing temperature is 900℃, and the grain size of the quenched forging is 9.0 ASTM.

[0079] Please continue to refer to Figure 2 As shown in the figure, the legends of the transformation critical lines on the right side of the figure represent the critical line when the ferrite transformation amount reaches 1%, the critical line when the pearlite transformation amount reaches 1%, the critical line when the bainite transformation amount reaches 1%, and the critical line when the austenite transformation amount reaches 1%.

[0080] Specifically, the starting line of the hardness curves for ferrite, pearlite, and bainite indicates the critical cooling temperature at which these soft phases begin to precipitate.

[0081] In practice, this invention predicts the final microstructure and hardness by overlaying the actual cooling curve with the CCT diagram, providing theoretical verification for the expected results of the heat treatment process of forgings.

[0082] Please see Figure 3 The diagram shows a flowchart illustrating the determination of residual tensile stress regions and residual compressive stress regions in a heat treatment method for manufacturing low-alloy steel forgings according to an embodiment of the present invention. The determination of residual tensile stress regions and residual compressive stress regions based on the residual stress distribution diagram includes:

[0083] The three-dimensional closed region inside the quenched forging where the residual tensile stress is greater than a preset first stress threshold is defined as the residual tensile stress region.

[0084] The three-dimensional closed region with residual compressive stress less than a preset first stress threshold is defined as the residual compressive stress region.

[0085] Optionally, the preset range of the first stress threshold is [-600MPa, -100MPa]; the preset range of the second stress threshold is [100MPa, 560MPa].

[0086] Preferably, the first preset stress threshold is -200 MPa in a preferred embodiment; the second preset stress threshold is 200 MPa in a preferred embodiment.

[0087] In practice, this invention obtains a residual stress distribution map of the quenched forging during the quenching and cooling test. Based on the residual stress distribution map, residual tensile stress region and residual compressive stress region are determined. The residual tensile stress region is a three-dimensional closed region inside the quenched forging where the residual tensile stress is greater than a preset second stress threshold. The residual compressive stress region is a three-dimensional closed region inside the quenched forging where the residual compressive stress is less than a preset first stress threshold. When the residual tensile stress is greater than the preset second stress threshold, it indicates that a tensile stress concentration region has formed inside the quenched forging, posing a risk of cracking or insufficient phase transformation. When the residual compressive stress is less than the preset first stress threshold, it indicates that the compressive stress protection layer on the surface of the quenched forging is insufficient and cannot effectively inhibit crack initiation and propagation. Furthermore, based on the spatial distribution relationship and relative intensity between the residual tensile stress region and the residual compressive stress region, the safe temperature window and cooling path of the quenched forging in the subsequent cooling stage are determined.

[0088] Specifically, determining the minimum cooling temperature of the quenched forging based on the residual tensile stress region and the residual compressive stress region includes:

[0089] The shortest straight-line distance from the geometric center of the residual tensile stress region to the surface of the quenched forging and the projected area of ​​the residual compressive stress region of the quenched forging on the surface of the quenched forging are obtained respectively.

[0090] The minimum cooling temperature of the quenched forging is calculated based on the shortest straight distance that meets the preset distance condition and the projected area of ​​the residual compressive stress region on the surface of the quenched forging that meets the preset projected area condition.

[0091] Wherein, the preset distance condition is that the shortest straight distance is less than a preset distance; the preset projected area condition is that the projected area of ​​the residual compressive stress region on the surface of the quenched forging is less than a preset projected area.

[0092] Specifically, the minimum cooling temperature is the sum of the minimum critical temperature for martensitic transformation, the distance correction, and the area correction.

[0093] Wherein, the distance correction amount is the product of the shortest straight-line distance and the distance correction coefficient; the area correction amount is the product of the projected area and the area correction coefficient.

[0094] Specifically, the minimum critical temperature for martensitic transformation is 200℃.

[0095] Optionally, the preset distance can be selected from [0.5mm, 2.0mm]; the preset area can be selected from [60mm2, 240mm2].

[0096] Preferably, the preset distance is 1.0 mm; the preset area is 150 mm2.

[0097] Specifically, the distance correction factor is the increase in the minimum critical temperature for martensitic transformation caused by the reduction in the unit shortest straight distance, and the unit of the distance correction factor is ℃ / mm.

[0098] Specifically, the area correction factor is the increase in the minimum critical temperature for martensitic transformation caused by the decrease in the projected area of ​​a unit residual compressive stress region, and the unit of the area correction factor is ℃ / mm2.

[0099] In a specific embodiment, the shortest straight-line distance from the geometric center of the current residual tensile stress region to the surface of the quenched forging is 0.8 mm, and the projected area of ​​the residual compressive stress region on the surface of the quenched forging is 140 mm². Comparing the shortest straight-line distance and the projected area, it is found that the shortest straight-line distance is less than the preset distance and the projected area is less than the preset projected area. Based on material test and engineering data optimization, the distance correction factor is -50℃ / mm and the area correction factor is -0.1℃ / mm². The calculated minimum cooling temperature is 200℃ + (0.8 mm × -50℃ / mm + 140 mm² × -0.1℃ / mm²) = 146℃.

[0100] In practice, this invention obtains the shortest straight-line distance between the residual tensile stress region and the surface of the quenched forging, and the projected area of ​​the residual compressive stress region on the surface of the quenched forging, respectively. The shortest straight-line distance represents the physical depth of the residual tensile stress region from the surface of the quenched forging. When this shortest straight-line distance is less than a preset distance, it indicates that the residual tensile stress region is close to the outer surface of the workpiece, and is in a high-risk position where quenching cracks are easily induced by the superposition of surface tensile stress generated by subsequent cooling. The projected area represents the size of the coverage area of ​​the residual compressive stress region on the surface of the quenched forging. When this projected area of ​​the residual compressive stress region on the surface of the quenched forging is less than the preset projected area, it indicates that during the quenching process, the surface of the quenched forging usually forms compressive stress due to rapid cooling, while the interior of the quenched forging generates tensile stress due to cooling lag and phase transformation differences. If the residual tensile stress region is too close to the surface, and although the surface compressive stress layer exists, it is insufficient to completely cover the residual compressive stress region, further drastic cooling will cause the temperature difference between the surface and the interior to increase. The increased difference between the gradient and the phase transformation process leads to a further increase in tensile stress, which may break through the protection of the surface compressive stress layer, thereby inducing microcracks or even macroscopic cracks near the surface. Therefore, the minimum cooling temperature of the quenched forging is determined by the sum of the minimum critical temperature of martensitic transformation, the distance correction, and the area correction. The distance correction is the product of the shortest straight distance and the distance correction coefficient, which is the corresponding increase in the minimum critical temperature of martensitic transformation caused by the reduction of the unit shortest straight distance. This means that the smaller the shortest straight distance, the larger the required increase in the positive value of the minimum cooling temperature correction. The area correction is the product of the projected area and the area correction coefficient, which is the corresponding increase in the minimum critical temperature of martensitic transformation caused by the reduction of the projected area of ​​the unit residual compressive stress region. This means that the smaller the projected area, the larger the required increase in the positive value of the minimum cooling temperature correction. Furthermore, by establishing a deterministic mathematical relationship between the minimum cooling temperature and the geometric characteristics of the stress state of the quenched forging monitored in real time, the cooling efficiency of the quenched forging is improved.

[0101] Specifically, the surfaces of quenched forgings with surface damage risk coefficients greater than a preset coefficient are defined as the damage risk surfaces of the quenched forgings.

[0102] The surface damage risk coefficient is the product of the first ratio and the second ratio.

[0103] The first ratio is the ratio of the residual tensile stress amplitude to the yield strength of the material, and the second ratio is the ratio of the shortest straight distance to the thickness of the quenched forging.

[0104] Specifically, the residual tensile stress amplitude is the maximum value of the residual tensile stress among all sampling points within the residual tensile stress region.

[0105] Optionally, the preset coefficient can be selected in the range of [0.03, 0.08]; the residual tensile stress amplitude can be selected in the range of [300MPa, 550MPa].

[0106] Preferably, the preset coefficient is 0.05 in the preferred embodiment; the residual tensile stress amplitude is 450 MPa in the preferred embodiment.

[0107] Specifically, the dimensions of the quenched forging are 10mm × 10mm × 55mm.

[0108] Specifically, based on the material of the forging being S690QL, the selectable range of the material yield strength of the forging is [690MPa, 770MPa].

[0109] Preferably, the yield strength of the material is 720 MPa.

[0110] In practice, this invention determines the damage-risk surface of the quenched forging based on the surface damage risk coefficient. The surface damage risk coefficient is the product of the ratio of residual tensile stress amplitude to material yield strength and the ratio of the shortest straight distance to the thickness of the quenched forging. The residual tensile stress amplitude represents the stress level in the tensile stress concentration area and is the direct driving force for crack initiation and propagation. The material yield strength represents the quenched forging material's inherent ability to resist plastic deformation and damage; it is an inherent mechanical property indicator of the material. The larger the ratio of residual tensile stress amplitude to material yield strength, the better. This indicates that the closer the tensile stress is to or exceeds the yield strength of the material, the higher the risk of local yielding or cracking. The smaller the ratio of the shortest straight distance to the thickness of the quenched forging, the closer the tensile stress region is to the surface of the quenched forging, the more obvious the stress concentration effect on the surface, and the easier it is for cracks to initiate or propagate from the surface. Therefore, surface areas with surface damage risk coefficients exceeding the preset coefficient are identified as damage risk surfaces that require special attention. This is to facilitate subsequent evaluation of the cooling rate of the quenched forging surface, in order to alleviate stress concentration, promote stress relaxation, and prevent crack formation.

[0111] Specifically, determining the cooling rate of each surface based on the area percentage of each non-damage-risk surface and the shortest straight-line distance includes:

[0112] The sum of the product of the area ratio of the non-damage risk surface and the area ratio weight coefficient and the product of the shortest straight distance and the straight distance weight coefficient is determined as the evaluation parameter of the corresponding surface of the quenched forging.

[0113] If the evaluation parameter is less than the preset evaluation parameter, it is determined that the degree of obstruction of the damage risk surface of the quenched forging to the lower bainite transformation process of the quenched forging does not meet the conditions, and the cooling rate of the corresponding surface of the quenched forging is reduced.

[0114] The cooling rate of the corresponding surface of the quenched forging is positively correlated with the evaluation parameters of the corresponding surface.

[0115] Specifically, the area ratio of the non-damage risk surface is the ratio of the total area of ​​the non-damage risk surface to the surface area of ​​the quenched forging.

[0116] Optionally, the area percentage of the non-damage-risk surface can be selected from [85%, 95%].

[0117] Preferably, the area of ​​the non-damage-risk surface accounts for 90% in the preferred embodiment.

[0118] Specifically, the straight-line distance weighting coefficient is the stress that hinders the transformation of lower bainite at a unit straight-line distance when a sudden change in action occurs at the surface damage site of the quenched forging, and the unit is MPa / mm.

[0119] Specifically, the area proportion weighting coefficient is the stress that hinders the transformation of lower bainite when the surface damage site of the quenched forging undergoes abrupt change in motion in the area of ​​a unit non-damage risk surface, and the unit is MPa.

[0120] Optionally, the preset evaluation parameters can be selected within the range of [100MPa, 300MPa].

[0121] Preferably, the preferred embodiment of the preset evaluation parameter is 220 MPa.

[0122] In practice, when the evaluation parameter is less than the preset evaluation parameter value by less than 10 MPa, the cooling rate of the corresponding surface of the quenched forging is adjusted to 95% of the current cooling rate. When the evaluation parameter is less than the preset evaluation parameter value by more than 10 MPa, the cooling rate is reduced by 5.0 °C / min for every 1 MPa exceeding 10 MPa. In a specific embodiment, the current cooling rate of the corresponding surface of the quenched forging is 50.0 °C / min, and the current evaluation parameter is 209 MPa. Therefore, the reduced cooling rate is 50.0 °C / min × 95% - (1 MPa / 1 MPa) × 5.0 °C / min = 42.5 °C / min.

[0123] Specifically, the cooling rate of the corresponding surface of the quenched forging is adjusted by a number of mist-cooling nozzles that are equally spaced on the side wall of the cooling chamber that cools the quenched forging, and by a proportional valve set on the mist-cooling nozzle.

[0124] In implementation, this invention determines the cooling rate of each surface of the quenched forging based on evaluation parameters of the surface. The evaluation parameters are the sum of the product of the shortest straight-line distance and the straight-line distance weighting coefficient, and the product of the area ratio of the non-damage-risk surface and the area ratio weighting coefficient. The straight-line distance weighting coefficient represents the resistance stress to the lower bainite transformation process per unit straight-line distance when a sudden change in movement occurs at the surface damage site of the quenched forging; that is, the larger the shortest straight-line distance, the greater the required cooling rate. The area ratio weighting coefficient represents the resistance stress to the lower bainite transformation process per unit area of ​​the non-damage-risk surface when a sudden change in movement occurs at the surface damage site of the quenched forging; that is, the area ratio of the non-damage-risk surface per unit area represents the area ratio of the non-damage-risk surface. The larger the ratio, the greater the required cooling rate. In the low-temperature stage of quenching cooling, such as the martensitic transformation zone and below, the material plasticity decreases. Reducing the cooling rate can directly reduce the temperature gradient between the surface and the core of the forging, as well as between different areas of the surface, thereby reducing the new thermal stress caused by uneven thermal shrinkage. Therefore, by reducing the cooling rate of the surface area with the smaller evaluation parameter of the corresponding surface of the quenched forging, the comprehensive risk of each surface can be quantitatively diagnosed through the evaluation parameter. Then, by adjusting the cooling rate in the opposite direction, the cooling intensity of the damaged risk surface of the quenched forging can be locally weakened, thereby actively inhibiting the initiation and propagation of quenching cracks. Ultimately, while improving the product qualification rate, the uniformity and reliability of the forging performance can be improved.

[0125] Specifically, if the quenched forging meets the preset cooling conditions, then the quenched forging is further cooled using atomization cooling.

[0126] If the quenched forging does not meet the preset cooling conditions, a combination of air cooling and conventional cooling is used to further cool the quenched forging.

[0127] The preset cooling conditions are that the thickness of the quenched forging is less than or equal to the preset thickness, and the average real-time temperature of the quenched forging is greater than the preset temperature.

[0128] Specifically, the combination of air cooling and wind cooling is used to further cool the quenched forgings. First, the centrifugal fan is started to blow on the surface of the quenched forgings at a preset wind speed. When the temperature of the forgings reaches the preset cooling temperature, the centrifugal fan is turned off, and the quenched forgings are placed in still air for natural convection cooling, i.e., air cooling, until the quenched forgings reach room temperature.

[0129] Specifically, when the quenched forging does not meet the preset cooling conditions, i.e., the shortest straight distance from the geometric center of the residual tensile stress region to the surface of the quenched forging is greater than or equal to the preset distance, and the projected area of ​​the residual compressive stress region on the surface of the quenched forging is greater than or equal to the preset projected area, the preset wind speed of the axial flow fan is set to 3.0 m / s.

[0130] Optionally, the preset cooling temperature can be selected within a range of [150℃, 250℃].

[0131] Preferably, the preset cooling temperature in this embodiment is 200°C.

[0132] As will be understood by those skilled in the art, the operating principle and process of centrifugal fans are conventional technical means well known to them, and therefore the operating principle and process of centrifugal fans will not be described in detail here.

[0133] Optionally, the preset thickness can be selected from [5mm, 15mm]; the preset temperature can be selected from [300℃, 450℃].

[0134] Preferably, the preset thickness is 10 mm; the preset temperature is 350°C.

[0135] Specifically, the average real-time temperature is the ratio of the sum of the temperature values ​​of all temperature sampling points of the quenched forging within a unit temperature monitoring cycle to the number of temperature sampling points.

[0136] Specifically, the unit temperature monitoring cycle is the time interval after the start of the continued cooling phase.

[0137] In practice, this invention is based on the following: when the thickness of the quenched forging is less than or equal to a preset thickness, it indicates that the quenched forging is a thin-walled or medium-thickness component. During the cooling process, the cross-sectional temperature distribution of the quenched forging is relatively uniform, and the tendency to quench cracking is relatively low. When the average real-time temperature of the quenched forging is greater than the preset temperature, it indicates that the forging is still in a relatively high temperature range, the material still has good plasticity, and the subsequent cooling process still needs to complete a sufficient martensitic transformation. Therefore, the cooling method is determined to be atomization cooling. Furthermore, the adjustable and moderate cooling intensity of atomization cooling is utilized to ensure sufficient microstructure transformation while avoiding excessively rapid cooling. New thermal stress and structural stress concentration; when the thickness of the quenched forging is greater than the preset thickness, and the average real-time temperature of the quenched forging is less than or equal to the preset temperature, it indicates that the quenched forging is a thick cross-section component, and the overall temperature of the quenched forging has dropped to a low level. If a strong cooling method is continued at this time, residual stress or even cracks are easily generated due to the excessive temperature difference between the inside and the surface of the quenched forging. Therefore, the cooling method is determined to be a combination of air cooling and wind cooling. The lower cooling rate further promotes the uniformity of cross-sectional temperature, slows down the structural transformation process, thereby reducing the stress gradient and cracking risk, and ensuring the integrity and performance uniformity of the internal structure of the forging.

[0138] Specifically, in response to the atomization cooling method in the continued cooling process, if the vertical flow velocity of the cooling medium on the surface of the quenched forging is less than a preset flow velocity, it is determined that the oxide scale removal effect is not satisfactory, and the atomization pressure is increased.

[0139] The atomization pressure is negatively correlated with the vertical flow velocity of the cooling medium.

[0140] Optionally, the preset flow velocity can be selected within a range of [0.2 m / s, 0.6 m / s].

[0141] Preferably, the preset flow rate is 0.4 m / s.

[0142] In implementation, when the vertical flow velocity of the cooling medium is less than the preset flow velocity by less than 0.05 m / s, the atomization pressure is adjusted to 1.1 times the current atomization pressure. When the vertical flow velocity of the cooling medium is less than the preset flow velocity by more than 0.05 m / s, the atomization pressure is increased by 0.01 MPa for every 0.01 m / s exceeding 0.05 m / s. In a specific embodiment, the current vertical flow velocity of the cooling medium is 0.36 m / s, and the current atomization pressure is 0.80 MPa. Therefore, the increased atomization pressure is 0.80 MPa × 1.1 + (0.01 m / s / 0.01 m / s) × 0.01 MPa = 0.89 MPa.

[0143] Specifically, the atomization pressure is adjusted by regulating the opening of the proportional valve set on the mist-cooled nozzle array.

[0144] In practice, this invention responds to the atomization cooling method in the continued cooling process by comparing the vertical flow velocity of the cooling medium on the surface of the quenched forging with a preset flow velocity. The vertical flow velocity of the cooling medium directly characterizes the momentum transfer efficiency of the cooling medium in the normal direction of the forging surface. The vertical flow velocity of the cooling medium reflects the dissipation of kinetic energy and the flow state of the cooling medium after impacting the surface. The vertical flow velocity of the cooling medium is affected by the physical state of the forging surface, namely the surface roughness and oxide scale. If there is residual tensile stress on the surface or subsurface of the forging, the direction of action of the residual tensile stress is to open the material surface layer, which is equivalent to mechanically improving the bonding strength between the oxide scale and the forging. The increased atomization pressure reduces the peeling driving force caused by the phase transformation shrinkage of the forging, making it more difficult for the oxide scale to fall off under the impact of the cooling medium. Therefore, when the vertical flow velocity of the cooling medium on the surface of the quenched forging is less than the preset flow velocity, it indicates that the removal of oxide scale on the surface of the forging does not meet the requirements. Increasing the atomization pressure means that the impact momentum and dynamic pressure carried by individual droplets and droplet groups increase. Droplets with higher kinetic energy continuously impact the oxide scale surface. When the shear stress generated by the droplet impact force exceeds the bonding strength between the oxide scale and the forging enhanced by the residual tensile stress, the fastening effect of the oxide scale can be overcome, and the oxide scale can be broken and washed off from the substrate, thereby improving the surface quality of the low alloy steel forging product.

[0145] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the scope of protection of the present invention.

Claims

1. A heat treatment method for manufacturing low-alloy steel forgings, characterized in that, include: The forging is heated to a preset temperature and held at that temperature to obtain the forging to be quenched; The forging to be quenched is placed in the quenching liquid to output a quenched forging with a temperature reduced to the martensitic transformation temperature range. Obtain the residual stress distribution diagram of the quenched forging; The residual tensile stress region and residual compressive stress region of the quenched forging are determined based on the residual stress distribution diagram. The minimum cooling temperature of the quenched forging is determined based on the residual tensile stress region and the residual compressive stress region. The damage risk surface of the quenched forging is determined based on the residual tensile stress amplitude in the residual tensile stress region, the shortest straight-line distance between the residual tensile stress region and the surface of the quenched forging, the thickness of the quenched forging, and the material yield strength of the quenched forging. The cooling rate of each surface is determined based on the area ratio of each non-damage risk surface and the shortest straight distance, wherein the surface area of ​​the quenched forging after removing the damage risk surface is determined as the non-damage risk surface. The quenched forging is further cooled according to the minimum cooling temperature and the cooling rate to output a low alloy steel forging product. The method of further cooling is determined according to the thickness of the quenched forging and the real-time temperature. Determining the minimum cooling temperature of the quenched forging based on the residual tensile stress region and the residual compressive stress region includes: The shortest straight-line distance between the residual tensile stress region and the surface of the quenched forging and the projected area of ​​the residual compressive stress region of the quenched forging on the surface of the quenched forging are obtained respectively. The minimum cooling temperature of the quenched forging is calculated based on the shortest straight distance that meets the preset distance condition and the projected area of ​​the residual compressive stress region on the surface of the quenched forging that meets the preset projected area condition. Wherein, the preset distance condition is that the shortest straight distance is less than a preset distance; the preset projected area condition is that the projected area of ​​the residual compressive stress region on the surface of the quenched forging is less than a preset projected area; The minimum cooling temperature is the sum of the minimum critical temperature for martensitic transformation, the distance correction, and the area correction. Wherein, the distance correction amount is the product of the shortest straight-line distance and the distance correction coefficient; the area correction amount is the product of the projected area and the area correction coefficient; The surfaces of quenched forgings with a surface damage risk coefficient greater than a preset coefficient are defined as the damage risk surfaces of the quenched forgings. The surface damage risk coefficient is the product of the first ratio and the second ratio. Wherein, the first ratio is the ratio of the residual tensile stress amplitude to the yield strength of the material, and the second ratio is the ratio of the shortest straight distance to the thickness of the quenched forging; The step of determining the cooling rate of each surface based on the area ratio of each non-damage-risk surface and the shortest straight-line distance includes: The sum of the product of the area ratio of the non-damage risk surface and the area ratio weight coefficient and the product of the shortest straight distance and the straight distance weight coefficient is determined as the evaluation parameter of the corresponding surface of the quenched forging. If the evaluation parameter is less than the preset evaluation parameter, it is determined that the degree of obstruction of the damage risk surface of the quenched forging to the lower bainite transformation process of the quenched forging does not meet the conditions, and the cooling rate of the corresponding surface of the quenched forging is reduced. The cooling rate of the corresponding surface of the quenched forging is positively correlated with the evaluation parameters of the corresponding surface.

2. The heat treatment method for manufacturing low-alloy steel forgings according to claim 1, characterized in that, The step of determining the residual tensile stress region and the residual compressive stress region based on the residual stress distribution diagram includes: The three-dimensional closed region inside the quenched forging where the residual tensile stress is greater than a preset second stress threshold is defined as the residual tensile stress region. The three-dimensional closed region with residual compressive stress less than a preset first stress threshold is defined as the residual compressive stress region.

3. The heat treatment method for manufacturing low-alloy steel forgings according to claim 1, characterized in that, The area ratio of the non-damage risk surface is the ratio of the total area of ​​the non-damage risk surface to the surface area of ​​the quenched forging.

4. The heat treatment method for manufacturing low-alloy steel forgings according to claim 3, characterized in that, If the quenched forging meets the preset cooling conditions, then the quenched forging is further cooled using atomization cooling. If the quenched forging does not meet the preset cooling conditions, a combination of air cooling and conventional cooling is used to further cool the quenched forging. The preset cooling conditions are that the thickness of the quenched forging is less than or equal to the preset thickness, and the average real-time temperature of the quenched forging is greater than the preset temperature.

5. The heat treatment method for manufacturing low-alloy steel forgings according to claim 4, characterized in that, The average real-time temperature is the ratio of the sum of the temperature values ​​of all temperature sampling points of the quenched forging within a unit temperature monitoring cycle to the number of temperature sampling points.

6. The heat treatment method for manufacturing low-alloy steel forgings according to claim 5, characterized in that, In response to the atomization cooling method in the continued cooling process, if the vertical flow velocity of the cooling medium on the surface of the quenched forging is less than the preset flow velocity, it is determined that the oxide scale removal effect is not satisfactory, and the atomization pressure is increased. The atomization pressure is negatively correlated with the vertical flow velocity of the cooling medium.

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