A cooling method for super-large die steel

Abstract

The present invention discloses a cooling method for oversized die steel. The method involves heating a rectangular block of oversized die steel to austenitizing temperature and maintaining the temperature. Once the temperature within the oversized die steel is uniform, the steel is removed from the furnace. The central region of the largest surface of the oversized die steel is then immediately subjected to localized forced cooling, while the remaining portion is air-cooled. The entire oversized die steel, after localized forced cooling, is then placed in a cooling medium for overall cooling. By combining localized forced cooling at a high temperature with overall cooling after the temperature is lowered, the present invention addresses defects such as edge cracking, deformation, and ridged deformation in the central region of the oversized die steel caused by thermal and structural stresses during the cooling process.

Description

Technical Field

[0001] The present invention relates to the field of heat treatment of metal materials, and more specifically to a cooling method for super-large die steel, especially a cooling method for rectangular metal materials, which can reduce material defects caused by thermal stress and structural stress. Background Art

[0002] As industries such as automobiles, home appliances, and electromechanical products have increasingly higher requirements for product quality, the production efficiency and quality control requirements for stamping and injection molded parts have also increased accordingly. The promotion and application of technologies such as "one mold with multiple cavities" and large-scale integral forming of parts have also put forward realistic requirements for the large-scale mold steel modules used to make molds.

[0003] The mold steel modules produced by steel mills are typically rectangular. After being forged into rectangular billets through ingot casting, the forged large modules need to undergo heat treatment to ensure that they meet performance requirements. While the heat treatment process varies significantly depending on the material, these heat treatment processes all share a common process: cooling. Based on the mold steel's product performance requirements and the material's CCT curve, the mold steel is typically heated to the austenitizing temperature and then rapidly cooled in a coolant to obtain a quenched martensite structure. Subsequent tempering results in tempered martensite, tempered troostite, or other fine, uniform structures. Therefore, increasing the cooling rate during quenching is crucial for achieving satisfactory product performance.

[0004] As we all know, after a metal block is heated to the austenitizing temperature, thermal stress and structural stress will appear during the cooling process due to the combined effects of factors such as structural changes and uneven temperature distribution. If the stress exceeds the fracture strength of the material, cracks will appear. This is also a cooling defect that is very easy to occur in large-sized metal blocks during the cooling process.

[0005] Large modules usually have large cross-sectional dimensions, with length and width exceeding 300mm, and even ultra-large modules with cross-sectional dimensions reaching 800×1200mm. During the rapid cooling process, the temperature difference between the surface and the interior of such large-sized modules is very large, thus forming large thermal stress, especially at the edges, which are usually the areas with the fastest cooling rate and form a large temperature gradient with the internal area. The stress is the greatest here, which is also the root cause of cracks at the edges during the cooling process.

[0006] During the cooling process of the super-large module, a ridge is formed along the length of the middle part of the largest outer surface of the rectangular parallelepiped. The main reason for this phenomenon is that the cooling rate of the middle part of the rectangular surface is lower than that of the edge parts. The rapid cooling of the edge parts produces a large cold contraction, while the cooling rate of the middle area of ​​the rectangle is lower, which hinders the contraction of the edge parts, causing tensile stress to be generated in the edge parts and greater compressive stress to be generated in the central area. During the initial water entry stage, the temperature of the workpiece is high and the deformation resistance is small, so the central part of the outer surface deforms under the action of compressive stress, causing the metal in this part to protrude and deform, forming a ridge.

[0007] During module quenching heat treatment, rapid cooling is required to achieve good hardness and produce a martensitic structure. However, to avoid cracking caused by high stress at the module's corners and edges, the cooling rate must be reduced. These two requirements create conflicting control strategies for the module's cooling rate during quenching heat treatment. To resolve this conflict, two common methods are used: one is to use quenching oil or other quenching media that reduce the cooling rate. However, due to the slow cooling rate, this method makes it difficult to achieve the necessary martensitic structure both inside and outside the module, resulting in the module's performance and lifespan failing to meet engineering requirements. Another method combines alternating air cooling with water quenching. A brief air cooling period is inserted during the water cooling process, allowing the module's surface or corner temperatures to rise during the air cooling process through internal heat transfer, thereby alleviating cracking caused by excessive thermal stress. However, cracks typically appear on the module's edges during the quenching process during the initial quenching process, when the temperature difference between the inner and outer surfaces is large, creating significant thermal stress and leading to cracking. This method requires air cooling before cracks occur, and under actual operating conditions, requires very high precision. To address the problem of edge cracking in rectangular modules during quenching and rapid cooling, two methods are commonly used: one is to reduce the cooling rate, such as by using a dedicated cooling medium; the other is to alternate air and water cooling, using air cooling to allow the edges to rise in temperature due to heat transfer within the module, thereby reducing the temperature difference between the edge and the interior of the module and thus reducing thermal stress. The above method can solve the problem of edge cracks occurring during the quenching and rapid cooling of large modules. However, there is currently no technology that can solve the problem of boss deformation in the middle of the surface of the rectangular module due to rapid cooling.

[0008] Therefore, it is urgent to develop a cooling method suitable for large-scale metal materials, which can quickly cool at the maximum cooling rate while avoiding edge cracking, deformation and ridge deformation in the middle caused by thermal stress and structural stress. Summary of the Invention

[0009] In view of the above-mentioned defects existing in the prior art, the purpose of the present invention is to provide a cooling method for oversized mold steel. By combining local strong cooling in the high-temperature stage with overall cooling after the temperature is lowered, the problem of defects such as edge cracking, deformation and ridge deformation in the middle of oversized mold steel caused by thermal stress and structural stress during the cooling process is solved; wherein the local strong cooling in the high-temperature stage can reduce the cooling speed of the edge of the oversized mold steel, thereby avoiding cracks in the edge. At the same time, the local strong cooling in the high-temperature stage can improve the material strength of the middle area during the rapid cooling process, and can also avoid the phenomenon of ridge deformation in the middle of the oversized mold steel.

[0010] To achieve the above object, the present invention adopts the following technical solutions:

[0011] The present invention provides a cooling method for super-large die steel, comprising the following steps:

[0012] S1, heating the rectangular super-large-size mold steel to the austenitizing temperature and keeping it warm, taking it out of the furnace after the temperature inside the super-large-size mold steel is uniform, and then immediately starting step S2;

[0013] S2, using a water mist spray method to perform local strong cooling on the super-large die steel, and the remaining part of the super-large die steel is air-cooled;

[0014] S3, overall cooling, putting the oversized die steel processed in step S2 into a cooling medium for overall cooling.

[0015] Preferably, in the step S1, after the super-large die steel is taken out of the furnace, the step S2 is started within a time of t≤1 min.

[0016] Preferably, in step S1, the length of the rectangular super-large specification mold steel is 4000-6000 mm, the width is 800-1500 mm, and the thickness is 600-1000 mm.

[0017] Preferably, in step S2, the local strong cooling includes:

[0018] Selecting the two largest surfaces of the oversized die steel as local cooling surfaces;

[0019] Then, the middle areas of the two local cooling surfaces are sprayed by water mist spraying, wherein the middle areas of the two local cooling surfaces correspond to each other.

[0020] Preferably, the length of the middle region is 40-80% of the corresponding length of the local cooling surface, and the width of the middle region is 40-80% of the corresponding width of the local cooling surface.

[0021] Preferably, the intersection point of the diagonals of the middle region coincides with the intersection point of the diagonals of the local cooling surface.

[0022] Preferably, in step S2, during the water mist spraying process, the surface heat transfer coefficient of the local cooling surface is 3000 to 12000 W·m -2 ℃ -1 ;

[0023] The spraying time of the water mist spray is: t=αB, wherein t is the spraying time, in min; α is the cooling coefficient, in the range of 0.04 to 0.08; and B is the minimum thickness of the oversized die steel, in mm.

[0024] Preferably, during the water mist spraying process, a water mist spraying system is used to spray the local cooling surfaces on both sides of the super-large mold, and the distance between the nozzle of the water mist spraying system and the local cooling surface is 200-300 mm.

[0025] Preferably, in the step S3, during the overall cooling, the cooling medium is water, and the overall cooling time is 45 to 120 minutes.

[0026] The present invention has the following beneficial effects:

[0027] 1. The cooling method of the oversized die steel of the present invention solves the problems of edge cracking, deformation, and ridge deformation in the middle of oversized die steel caused by thermal stress and structural stress during the cooling process by combining local strong cooling in the high-temperature stage with overall cooling after the temperature is lowered.

[0028] 2. The cooling method of the oversized die steel of the present invention can reduce the cooling rate of the edges of the oversized die steel by local strong cooling during the high temperature stage, thereby preventing cracks from forming on the edges. At the same time, the local strong cooling during the high temperature stage can improve the material strength of the middle region during the rapid cooling process, thereby preventing the occurrence of ridge deformation in the middle region of the oversized die steel.

[0029] 3. The cooling method of the oversized die steel of the present invention can slow down the cooling of the edge areas of the oversized die steel that are prone to cracking, thereby preventing cracks from occurring on the edge areas. By performing concentrated cooling in the middle areas of the two largest surfaces of the oversized die steel, the heat in the middle area of ​​the oversized die steel can be transferred via the smallest heat transfer path, achieving rapid cooling of the middle area. The cooling method of the present invention can simultaneously achieve quenching cooling at the maximum cooling rate while ensuring that quenching cracks do not occur on the edge areas.

[0030] 4. The cooling method of the oversized mold steel of the present invention can enable the middle area of ​​the oversized mold steel to obtain a larger tensile stress at the initial stage of cooling, thereby avoiding the generation of ridges on the surface of the middle area along the axial direction of the oversized mold steel, thereby coordinating the surface stress of the oversized mold steel in the subsequent overall cooling process, avoiding the occurrence of large stresses and defects such as deformation and cracking. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Other features, objects and advantages of the present invention will become more apparent upon reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0032] Figure 1 Schematic diagram of the middle area of ​​the local cooling surface in step S2 of the cooling method for super-large die steel of the present invention. DETAILED DESCRIPTION

[0033] In order to better understand the above technical solution of the present invention, the technical solution of the present invention is further described below with reference to embodiments.

[0034] The cooling method of the super-large-sized mold steel of the present invention adopts a method that combines local strong cooling in the high-temperature stage with overall cooling after the temperature is lowered, so that the super-large-sized mold steel can be cooled at the maximum cooling rate while ensuring that the edges of the super-large-sized mold steel do not have defects such as edge cracks and deformation, and convex edge deformation in the middle.

[0035] A cooling method for an oversized die steel of the present invention comprises the following steps:

[0036] S1, heating the rectangular super-large size mold steel to the austenitizing temperature and keeping it warm, and taking it out of the furnace after the temperature inside the super-large size mold steel is uniform, and then immediately starting step S2;

[0037] First, the rectangular super-large-size mold steel is heated to the austenitizing temperature of the super-large-size mold steel according to the process requirements and kept warm for a period of time. After the internal temperature of the super-large-size mold steel is uniform, it is taken out of the furnace, and then step S2 is immediately started. Preferably, after the super-large-size mold steel is taken out of the furnace, step S2 is started within t≤1min; wherein the length of the super-large-size mold steel of the rectangular parallelepiped is 4000~6000mm, the width is 800~1500mm, and the thickness is 600~1000mm.

[0038] S2, using water mist spraying to perform local strong cooling on the oversized die steel, and air cooling on the remaining parts of the oversized die steel;

[0039] After the super-large mold steel is taken out of the furnace, the water mist spray method is used to locally strong cool the super-large mold steel. The specific process is: select the two largest surfaces of the super-large mold steel as the local cooling surface among the six surfaces of the super-large mold steel, and select the middle area A as the spraying area on the local cooling surface. Figure 1 As shown, the dimensions of the central region A are 40% to 80% of the corresponding length and width dimensions of the local cooling surface, that is, the length of the central region A is 40% to 80% of the corresponding length of the local cooling surface, and the width of the central region A is 40% to 80% of the corresponding width of the local cooling surface. In a further embodiment, the diagonal intersection of the central region A coincides with the diagonal intersection of the local cooling surface. Then, a water mist spray method is used to spray the central regions A of the two local cooling surfaces of the oversized die steel. The central regions A of the two local cooling surfaces correspond to each other. In order to further improve the cooling efficiency, the areas of the central regions A of the two local cooling surfaces are the same. During the water mist spraying process, the surface heat transfer coefficient of the local cooling surface of the oversized die steel is 3000 to 12000 W·m -2 ℃ -1 During the water mist spray process, the cooling medium and spray pattern are determined by the surface heat transfer coefficient. The water mist spray time is: t = αB, where t is the spray time, measured in minutes. α is the cooling coefficient, ranging from 0.04 to 0.08. The specific value of α is selected based on the steel type, with a large value selected for steels with low thermal conductivity and a small value selected for steels with low thermal conductivity. B is the minimum thickness of oversized mold steel, measured in mm. Because the water mist spray system relies on high-pressure gas to spray water from a small nozzle, the distance between the nozzle and the local cooling surface is an important indicator for controlling cooling intensity. The closer the distance between the nozzle and the local cooling surface, the greater the cooling intensity, but the cooling area of ​​a single nozzle is reduced. Therefore, to ensure cooling intensity in the central area A, it is necessary to control the distance between the nozzle and the local cooling surface of the water mist spray system to 200 to 300 mm.

[0040] S3, overall cooling, the oversized die steel processed in step S2 is put into a cooling medium for overall cooling; the cooling medium can be water or other cooling medium; when the cooling medium is water, the overall cooling time is 45 to 120 minutes; after the cooling is completed, the oversized die steel has no defects such as cracks and central bulges on the surface.

[0041] The cooling method for super-large die steel of the present invention will be further described below with reference to specific examples.

[0042] Example 1

[0043] In this embodiment, P20 plastic mold steel with a specification of 610×1000×4000mm is taken as an example for cooling;

[0044] The first step is to heat the plastic mold steel to 1050℃ and then keep it warm to make the temperature inside the plastic mold steel uniform;

[0045] The second step is to quickly transfer the plastic mold steel from the heating furnace to the cooling area, and place the largest surface of the plastic mold steel (as the local cooling surface) vertically. The whole process takes no more than 1 minute.

[0046] The third step is to use a water mist spray system to spray the middle area of ​​the two largest surfaces of the plastic mold steel. The distance between the nozzle and the spray surface is 200-300 mm, and the water flow on each side is 80 L / min. Ensure that the heat transfer coefficient of the plastic mold steel surface is between 3000 and 12000 W·m -2 ℃ -1 The central area measures 600 x 3000 mm, and the water mist spraying time is 25 minutes (t = αB = 0.04 x 610 ≈ 25 minutes, where the minimum thickness B of the plastic mold steel is 610 mm). The plastic mold steel is then completely immersed in water for overall cooling. After approximately 50 minutes of cooling, the water is removed. After cooling, the plastic mold steel has no surface defects such as cracks or central bulges.

[0047] During the cooling process, the maximum stress occurs about 20 seconds after the entire cooling process enters the water, and the maximum stress is located at the center surface of the module, approximately 380 MPa. This reduces the stress by about 100 MPa compared to direct water quenching. At the same time, it reduces the cooling time compared to multiple air cooling and water cooling cycle cooling methods, simplifies the operation process, and improves equipment utilization.

[0048] Example 2

[0049] This example is the quenching and rapid cooling process of a large rectangular P40 plastic mold steel module with dimensions of 800×1300×6000mm.

[0050] In the first step, a large P40 plastic mold steel module with a size of 800×1300×6000mm was heated to 1040℃ and kept warm until the temperature inside the module was fully uniform.

[0051] The second step is to quickly remove the large module from the heating furnace and transfer it to the cooling area, with the largest surface (local cooling surface) placed vertically. The whole process takes no more than 1 minute.

[0052] In the third step, a water mist spray system is used to spray the middle area of ​​the two largest surfaces of the large module. The distance between the nozzle and the spray cooling surface is controlled within the range of 200 to 300 mm. The water flow per surface is 80 L / min. The spray area is rectangular and measures 900 × 4500 mm. The spraying time is 40 minutes (t = αB = 0.05 × 800 ≈ 40 minutes, where the minimum thickness B of the plastic mold steel is 800 mm). After that, the plastic mold steel is completely immersed in water for overall cooling. After cooling for about 80 minutes, the water is removed. After cooling, the plastic mold steel surface is free of defects such as cracks and central bulges.

[0053] Example 3

[0054] This example is the quenching and rapid cooling process of a large 718 plastic mold steel module with a rectangular shape of 860×1100×5000mm.

[0055] In the first step, a large 718 plastic mold steel module with a size of 860×1100×5000mm was heated to 1050℃ and kept warm until the temperature inside the module was fully uniform.

[0056] The second step is to quickly remove the large module from the heating furnace and transfer it to the cooling area, with the largest surface (local cooling surface) placed vertically. The whole process takes no more than 1 minute.

[0057] In the third step, a water mist spray system is used to spray the middle area of ​​the two largest surfaces of the large module. The distance between the nozzle and the spray cooling surface is controlled within the range of 200 to 300 mm. The water flow per side is 80 L / min. The spray area is rectangular and measures 950 × 4000 mm. The spray time is 50 minutes (t = αB = 0.06 × 800 ≈ 50 minutes, where the minimum thickness B of the plastic mold steel is 800 mm). After that, the plastic mold steel is completely immersed in water for overall cooling. After cooling for about 90 minutes, the water is removed. After cooling, the plastic mold steel surface is free of defects such as cracks and central bulges.

[0058] To sum up, the cooling method of the present invention adopts a method that combines local strong cooling in the high-temperature stage with overall cooling after the temperature is lowered, so that the cooling rate of the edge portion is reduced to avoid cracks in the edge portion, and the middle region is locally strong cooled at high temperature. On the one hand, the temperature of the middle region can be quickly reduced, the strength of the material can be improved, and the middle region does not have ridge deformation during the overall cooling process. At the same time, the middle region adopts local strong cooling to generate tensile stress in the middle region, so that the tensile stress generated in the edge portion is balanced with it in the subsequent overall cooling process, reducing the surface stress of the oversized mold steel, thereby solving the problems of edge cracking, deformation and ridge deformation in the middle portion caused by thermal stress and tissue stress in the cooling process of oversized mold steel.

[0059] Those skilled in the art should recognize that the above embodiments are merely intended to illustrate the present invention and are not intended to limit the present invention. As long as they are within the spirit of the present invention, any changes or modifications to the above embodiments will fall within the scope of the claims of the present invention.

Claims

1. A cooling method for super large die steel, characterized in that: The following steps are involved: S1, heating the rectangular super-large-size mold steel to the austenitizing temperature and keeping it warm, taking it out of the furnace after the temperature inside the super-large-size mold steel is uniform, and then immediately starting step S2; S2, using water mist spraying to perform local strong cooling on the super-large die steel, and the remaining part of the super-large die steel is air-cooled, The local strong cooling includes: Selecting the two largest surfaces of the oversized die steel as local cooling surfaces; Then the middle areas of the two local cooling surfaces are sprayed with water mist, where the middle areas of the two local cooling surfaces correspond to each other. The length of the middle region is 40-80% of the corresponding length of the local cooling surface, and the width of the middle region is 40-80% of the corresponding width of the local cooling surface. The intersection of the diagonals of the central region coincides with the intersection of the diagonals of the local cooling surface. During the water mist spraying process, the surface heat transfer coefficient of the local cooling surface is 3000 to 12000 W·m -2 ℃ -1 ; The spraying time of the water mist spray is: t=αB, where t is the spraying time, in min; α is the cooling coefficient, in the range of 0.04 to 0.08; B is the minimum thickness of the oversized die steel, in mm; S3, overall cooling, putting the oversized die steel processed in step S2 into a cooling medium for overall cooling.

2. The cooling method for super-large die steel according to claim 1, characterized in that: In the step S1, after the super-large die steel is taken out of the furnace, the step S2 is started within a time of t≤1 min.

3. The cooling method for super-large die steel according to claim 1, characterized in that: In step S1, the length of the rectangular super-large specification mold steel is 4000-6000 mm, the width is 800-1500 mm, and the thickness is 600-1000 mm.

4. The cooling method for oversized die steel according to claim 1, characterized in that: During the water mist spraying process, a water mist spraying system is used to spray the local cooling surfaces on both sides of the super-large mold, and the distance between the nozzle of the water mist spraying system and the local cooling surface is 200-300 mm.

5. The cooling method for super-large die steel according to claim 1, characterized in that: In the step S3, during the overall cooling, the cooling medium is water, and the overall cooling time is 45 to 120 minutes.

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