Straight rolling forming method for plastic mold flat steel with thickness of more than 150 mm

Abstract

The invention belongs to the field of hot working of metal materials, and particularly discloses a straight rolling forming method for flat steel of a plastic mold with the thickness of 150 mm or above. The method comprises the following steps that a steel ingot is heated, and primary soaking is conducted after the temperature reaches the temperature rising temperature; the steel ingot obtained after primary soaking is cooled, secondary soaking is conducted after the temperature reaches the rolling temperature, and a temperature gradient with the temperature gradually decreasing from the center to the surface is formed on the section of the steel ingot; and the steel ingot obtained after secondary soaking is rolled, the plastic mold flat steel with the thickness larger than 150 mm is obtained, in the flat rolling pass, the absolute rolling reduction of each pass is not lower than 40 mm, and the relative deformation is not lower than 8%. According to the method, it can be guaranteed that the core of the steel has enough deformation, so that the defects of looseness and pores are eliminated, a compact structure is obtained, and the flaw detection quality, the structure compactness and the performance of the prepared plastic mold flat steel with the thickness of 150 mm or above all meet the requirements for preparing large molds.

Description

Technical Field

[0001] This invention belongs to the field of hot working of metal materials, and specifically relates to a method for direct rolling of plastic mold flat steel with a thickness of 150mm or more. Background Technology

[0002] Plastic mold steel is a type of steel specifically used to manufacture plastic molding dies (such as injection molds, blow molds, compression molds, etc.). It directly determines the precision, surface quality, and service life of plastic products and is the "heart material" of the plastics industry.

[0003] The increasing demand for large-scale integrated molds has driven the expansion of plastic mold flat steel towards wider and thicker dimensions. Large steel mills generally use forging to produce flat steel with a thickness of 150mm or more, which results in high cost, low efficiency, and poor dimensional accuracy.

[0004] Therefore, there is an urgent need to develop a direct rolling process for producing plastic mold flat steel with a thickness of 150mm or more. Summary of the Invention

[0005] The main objective of this invention is to address the shortcomings of the existing technology by providing a direct rolling process for plastic mold flat steel with a thickness of 150mm or more, which can produce plastic mold steel with a thickness of 150mm or more that meets the needs of large mold preparation through rolling.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a method for direct rolling of plastic mold flat steel with a thickness of 150mm or more, which includes the following steps: The steel ingot is heated until it reaches the set temperature, and then it undergoes a homogenization process. After the steel ingot is homogenized once, it is cooled down and then homogenized again after the temperature reaches the rolling temperature, so as to form a temperature gradient on the cross section of the steel ingot from the core to the surface. The steel ingots after secondary homogenization are rolled to obtain plastic mold flat steel with a thickness of more than 150mm. In the flat rolling passes, the absolute reduction of each pass is not less than 40mm and the relative deformation is not less than 8%.

[0007] As a further embodiment, the rolling temperature is between 1180-1200°C, and the heating temperature is 30-60°C higher than the rolling temperature.

[0008] As a further implementation method, the heat equalization time is 5-10 hours.

[0009] As a further implementation method, the secondary homogenization time is 1-4 hours.

[0010] As a further implementation method, the rolling speed during the rolling process does not exceed 5 m / s.

[0011] As a further embodiment, the dimensions of the steel ingot and the flat steel are controlled such that the compression ratio at the smallest cross-section of the steel ingot is not less than 3.

[0012] As a further embodiment, the plastic mold flat steel is 40Cr13 plastic mold flat steel.

[0013] As a further embodiment, the steel ingot is a flat ingot or a rectangular ingot.

[0014] As a further embodiment, the heating temperature, the first homogenization time, and the second homogenization time all increase with the increase of the size of the steel ingot.

[0015] As a further implementation method, high-pressure water descaling is performed on the steel ingot before rolling after secondary homogenization.

[0016] By adopting the above technical solution, the present invention has the following beneficial effects compared with the prior art: This invention produces plastic mold flat steel with a thickness of 150mm or more by rolling. By heating and cooling, a temperature gradient with a gradually decreasing temperature from the core to the surface is formed on the cross-section of the steel ingot before rolling. Combined with the control of the reduction amount per pass during rolling, sufficient deformation is ensured in the core of the steel, thereby eliminating porosity and obtaining a dense structure. The resulting plastic mold flat steel with a thickness of 150mm or more meets the requirements for manufacturing large molds in terms of flaw detection quality, density, and performance. Attached Figure Description

[0017] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the description of the present invention will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other embodiments can be obtained based on these drawings without creative effort.

[0018] Figure 1 A flowchart of the method for direct rolling of plastic mold flat steel with a thickness of 150mm or more provided by the present invention; Figure 2 A low-magnification image of a 200x710mm flat steel bar produced using the method of this invention; Figure 3 This is a low-magnification image of a 190x710mm flat steel bar produced using conventional methods. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0020] Specific embodiments of the invention are disclosed herein as needed; however, it should be understood that the embodiments disclosed herein are merely examples of the invention that may be implemented in various alternative forms. In the following description, various operating parameters and components are described in several contemplated embodiments. These specific parameters and components are provided as examples only and are not intended to be limiting.

[0021] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0022] Plastic mold steel has very high requirements for the density and porosity of the steel core. From a process perspective, the core of the steel ingot inevitably contains porosity defects, which need to be eliminated through subsequent pressure processing. The pressing of the core to eliminate these defects requires sufficient deformation. The thicker the flat steel, the more difficult it is for pressure to penetrate to the core. Insufficient core deformation means the porosity defects cannot be eliminated. Therefore, flat steel thicker than 150mm is generally produced by forging. However, forging thick flat steel has problems such as high cost, low efficiency, and poor dimensional accuracy.

[0023] In view of this, the present invention aims to propose a method for producing plastic mold flat steel with a thickness of 150mm or more by rolling. In order to ensure that the core of the steel has sufficient deformation, the control of the rolling temperature and reduction of the steel ingot is the most critical. Conventional steel ingot rolling temperature is to ensure that the steel ingot is fully heated and the internal and external temperatures are consistent. In the present invention, the internal temperature of the steel ingot is required to be high and the surface temperature is low to form a temperature gradient difference. Combined with the reduction of each pass, it is necessary to ensure that the core of the steel has sufficient deformation, thereby eliminating the defects of looseness and porosity and obtaining a dense structure.

[0024] Specifically, based on the aforementioned objectives, the present invention provides a method for direct rolling of plastic mold flat steel with a thickness of 150mm or more, such as... Figure 1As shown, the method includes the following steps: S1, heating the steel ingot, and performing a first homogenization after the temperature reaches the heating temperature; S2, cooling the steel ingot after the first homogenization, and performing a second homogenization after the temperature reaches the rolling temperature, so as to form a temperature gradient on the cross section of the steel ingot from the core to the surface, with the temperature gradually decreasing; S3, rolling the steel ingot after the second homogenization to obtain a plastic mold flat steel with a thickness of more than 150mm, wherein, in the flat rolling pass, the absolute reduction of each pass is not less than 40mm, and the relative deformation is not less than 8%.

[0025] This invention produces plastic mold flat steel with a thickness of 150mm or more by rolling. By heating and cooling, a temperature gradient with a gradually decreasing temperature from the core to the surface is formed on the cross-section of the steel ingot before rolling. Combined with the control of the reduction amount per pass during rolling, sufficient deformation is ensured in the core of the steel, thereby eliminating porosity and obtaining a dense structure. The resulting plastic mold flat steel with a thickness of 150mm or more meets the requirements for manufacturing large molds in terms of flaw detection quality, density, and performance.

[0026] Specifically, heating temperature is the most critical control point of this invention. The higher the temperature of the steel ingot, the lower its deformation strength and the easier it is to deform. This is why steel ingot rolling requires heating to a certain temperature. During the rolling process, the force acts on the surface of the steel ingot. Therefore, the force is always transmitted from the surface to the core, and the force attenuates more towards the core. If we look at the cross-section of the steel ingot, the deformation gradually decreases from the surface to the core in each rolling pass. Especially when rolling thick flat steel, the force is transmitted a long distance from the surface to the core, making it even more difficult for the core to have sufficient deformation. To ensure sufficient deformation in the core, temperature must be considered. Since the higher the temperature of the steel ingot, the easier it is to deform, by establishing a temperature gradient that gradually decreases from the core to the surface on the cross-section of the steel ingot, the core deformation will naturally be greater under the same reduction during rolling. This ensures that the core has sufficient deformation to eliminate porosity and improve the density of the microstructure.

[0027] To establish such a temperature gradient, a preheating process is first required. For plastic mold steel, the preheating temperature must be higher than the rolling temperature. First, the steel ingot is heated to the preheating temperature for homogenization to ensure that the temperature of the core and surface is consistent. Then, it is cooled to the rolling temperature for a second homogenization. Because temperature conduction takes time, the core of the ingot cools the slowest, while the surface cools the fastest, thus creating a temperature gradient across the ingot's cross-section that gradually decreases in temperature from the core to the surface.

[0028] It should be noted that the expressions "temperature reaches XX temperature" in this invention refer to the surface temperature of the steel ingot reaching a specified temperature, not the core temperature of the steel ingot reaching a specified temperature. For example, "temperature reaches heating temperature" means that the surface temperature of the steel ingot reaches the heating temperature, at which point the core temperature of the steel ingot is lower than the heating temperature. The core temperature of the steel ingot reaches the heating temperature after a homogenization process. "Temperature reaches rolling temperature" means that the surface temperature of the steel ingot reaches the rolling temperature, at which point the core temperature of the steel ingot is higher than the rolling temperature.

[0029] The rolling process is a crucial factor in ensuring the density of the steel ingot's core structure. During the rolling process, in the deformation stage, the absolute reduction in each pass must not be less than 40 mm, and the relative deformation must not be less than 8%. This is to ensure sufficient reduction force; only with a large initial reduction force can the reduction force transmitted to the core be guaranteed.

[0030] In some embodiments, the plastic mold flat steel is 40Cr13 plastic mold flat steel. In other words, the method of the present invention is applicable to the production of 40Cr13 plastic mold flat steel with a thickness of 150mm or more. In the field of plastic mold steel, 40Cr13 steel is the most widely used type. However, with the increasing demand for large molds in the mold industry, the demand for mold flat steel with a thickness of 150mm or more has also increased. Furthermore, as rolling mill technology matures and rolling force increases, the demand for thick flat steel continues to expand. Thick flat steel that previously required forging can now be produced through rolling, commonly known as rolling instead of forging. All 40Cr13 series steel grades involved within the spirit and principles of this invention should be included within the scope of protection of this invention.

[0031] In some embodiments, the rolling temperature is between 1180-1200°C, and the heating temperature is 30-60°C higher than the rolling temperature. The rolling temperature can typically, but is not limited to, be set to 1180°C, 1185°C, 1190°C, 1195°C, or 1200°C; the difference between the heating temperature and the rolling temperature can typically, but is not limited to, be set to 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, or 60°C. For example, for 40Cr13 plastic mold steel, the rolling temperature is typically between 1180-1200°C, therefore the heating temperature is required to be between 1210-1260°C.

[0032] In some embodiments, the initial homogenization time is 5-10 hours. Typically, but not limited to, the initial homogenization time can be set to 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, or 10 hours. Initial homogenization is to ensure uniform temperature between the core and surface of the steel ingot, and the above-mentioned initial homogenization time range can efficiently achieve this purpose.

[0033] In some embodiments, the secondary homogenization time is 1-4 hours. The secondary homogenization time can typically, but not exclusively, be set to 1 hour, 2 hours, 3 hours, or 4 hours. Secondary homogenization is used to create a temperature gradient that gradually decreases from the core to the surface across the cross-section of the steel ingot, and the aforementioned range of secondary homogenization times can achieve this purpose.

[0034] In some embodiments, the rolling speed does not exceed 5 m / s during the rolling process. The transmission of deformation also takes time; excessively high rolling speeds will only cause deformation to concentrate in the surface layer.

[0035] In some embodiments, the dimensions of the steel ingot and flat bar are controlled such that the compression ratio at the smallest cross-section of the steel ingot is not less than 3. Firstly, direct rolling of steel ingots requires matching suitable ingots. Different specifications and thicknesses of steel correspond to different sizes of ingots. Generally, the thinner the steel, the smaller the corresponding ingot; the thicker the steel, the larger the corresponding ingot. This is to ensure sufficient deformation during rolling. Only with sufficient deformation can the finished product have a dense structure, and the loose, porous defects generated in the core of the ingot during solidification can be compressed and eliminated. The amount of deformation is usually measured using the compression ratio, which is calculated by dividing the cross-sectional area at the smallest point of the steel ingot by the cross-sectional area of ​​the finished product. In actual production, a compression ratio greater than 2 is generally sufficient for common specifications such as steel with a thickness of 80mm or less. However, for thick flat steel, it is more difficult for the rolling force to penetrate to the core of the rolled material. According to simulation calculations, for 200mm thick flat steel, the core deformation must be greater than 3 to achieve the same level of deformation as the 80mm thick specification. Furthermore, actual production has shown that 200mm thick flat steel rolled from ingots with a compression ratio of 2.2 has lower grain size and hardness compared to 200mm thick flat steel rolled from ingots with a compression ratio of 3. The grain size and hardness of 200mm thick flat steel rolled from ingots with a compression ratio of 3 are very close to those of 200mm thick flat steel produced by forging. This indicates that the microstructure of 200mm thick flat steel rolled from ingots with a compression ratio of 2.2 is significantly less dense than that rolled from ingots with a compression ratio of 3. Therefore, when rolling plastic mold flat steel with a thickness of 150mm or more using matching ingots, a compression ratio ≥ 3 must be ensured.

[0036] In some embodiments, the steel ingot is a flat ingot or a rectangular ingot. The cross-section of a flat ingot is distinctly a flat rectangle with a large length-to-width ratio, typically ≥2:1. The cross-section of a rectangular ingot is close to a square or a rectangle with a shorter side, with a smaller length-to-width ratio, generally around 1:1 to 1.5:1.

[0037] In some embodiments, the heating temperature, the first homogenization time, and the second homogenization time all increase with the size of the steel ingot. The larger the steel ingot, the longer the time required for temperature conduction from the core to the surface. Therefore, the time required for the heating and homogenization stages (i.e., the first homogenization) and the cooling and homogenization stages (i.e., the second homogenization) is also longer, and the upper limit of the required time must be taken. Since large steel ingots have large cross-sections, the heating temperature must also be at its upper limit to form a sufficiently wide temperature gradient; the opposite is true for small steel ingots. In other words, the larger the steel ingot, the upper limit is taken for the heating temperature, the homogenization time after heating (i.e., the first homogenization time), and the homogenization time after cooling (i.e., the second homogenization time); the smaller the ingot, the lower limit is taken for the heating temperature, the homogenization time after heating, and the homogenization time after cooling, thereby ensuring a large temperature gradient with a gradually decreasing temperature from the core to the surface across the steel ingot cross-section.

[0038] In some embodiments, high-pressure water descaling is performed on the steel ingot before rolling after secondary homogenization.

[0039] The following describes the process methods mentioned in this invention with specific embodiments, but this invention is not limited to the following embodiments.

[0040] Example 1: (1) Steel ingot preparation: The steel ingot used is a flat ingot, weighing 6t, with a large end size of 805 / 590mm and a small end size of 750 / 450mm. The rolled section is a flat steel with a specification of 150x710mm and a compression ratio of 3.

[0041] (2) Heating process: The rolling temperature of 40Cr13 plastic mold steel is set at 1190±10℃. Although the cross-sectional area of ​​6t flat ingot is also large, it is a flat ingot with a large width-to-thickness ratio. Therefore, the heating temperature and the soaking time after heating should be taken to the middle value. The heating temperature is set at 1230±10℃ and the soaking time is set at 7h. After the soaking is completed, the temperature is reduced to 1190±10℃ and then the soaking treatment is carried out again. The soaking time is taken to the middle value and set at 3 hours.

[0042] (3) Rolling process: High-pressure water descaling is performed before rolling. The taper of the ingot is removed in the first four passes of rolling. The first two passes are flat rolling with a reduction of 65 mm and relative deformation of 11% and 12.4% respectively. The third and fourth passes are vertical rolling with a reduction of 20 mm. Vertical rolling of flat steel is mainly to ensure dimensional accuracy and edge shape. No special requirements are made for the reduction and relative deformation. The fifth to eighth passes are flat rolling with a reduction of 60 mm in the fifth and sixth passes and relative deformation of 13% respectively. The reduction for passes 7 and 8 is 50mm, with relative deformation of 14.7% and 17.2% respectively. Passes 9 and 10 use vertical rolling with a reduction of 20mm. Passes 11 and 12 use flat rolling with a reduction of 40mm, with relative deformation of 16.7% and 20% respectively. Passes 13 and 14 use vertical rolling with a reduction of 20mm. Passes 15 and 16 use flat rolling and are shaping passes, with no reduction requirement.

[0043] Tests showed that the resulting flat steel had no loose or porous defects and had a dense structure.

[0044] Example 2: (1) Steel ingot preparation: The steel ingot used is a rectangular ingot, weighing 8t, with a large end size of 840 / 745mm and a small end size of 750 / 600mm. The rolled section is a flat steel with a specification of 200x710mm and a compression ratio of 3.

[0045] (2) Heating process: The rolling temperature of 40Cr13 plastic mold steel is set at 1190±10℃. Since the cross-sectional area of ​​the 8t rectangular ingot is large, the heating temperature and the soaking time after heating should be set to the upper limit. The heating temperature is set at 1250±10℃ and the soaking time is set at 9h. After the soaking is completed, the temperature is reduced to 1190±10℃ and then soaking is performed again. The soaking time is set to the upper limit of 3.5 hours.

[0046] (3) Rolling process: High-pressure water descaling is performed before rolling. The taper of the ingot is removed in the first four passes of rolling. The first and second passes are flat rolling with a reduction of 70 mm and relative deformation of 9.4% and 10.4% respectively. The third and fourth passes are vertical rolling with a reduction of 45 mm. Vertical rolling of flat steel is mainly to ensure dimensional accuracy and edge shape. The reduction and relative deformation are not particularly required. The fifth to eighth passes are flat rolling with a reduction of 60 mm in the fifth and sixth passes and relative deformation of 9.8% and 10.9% respectively. The seventh and eighth passes have a reduction of 50 mm. The relative deformation amounts per pass are 10.2% and 11.4%, respectively. Passes 9 and 10 use vertical rolling with a reduction of 20 mm. Passes 11 to 14 use flat rolling. Passes 11 and 12 have a reduction of 50 mm, with relative deformation amounts of 12.8% and 14.7%, respectively. Passes 13 and 14 have a reduction of 40 mm, with relative deformation amounts of 12.8% and 14.7%, respectively. Passes 15 and 16 use vertical rolling with a reduction of 20 mm, and passes 17 and 18 use flat rolling. Passes 17 and 18 are shaping passes, and the reduction amount is not required.

[0047] To verify the advantages of this invention in producing thick flat steel compared to conventional methods, a comparative experiment was conducted. Figure 2 The image shown is a low-magnification image of a 200x710mm flat steel bar produced using the method of this invention. Figure 3 The image shows a low-magnification image of the same 710mm wide flat bar produced using the conventional method (only one homogenization process, isothermal rolling inside and outside the steel ingot). It is clear from the image that the 200mm thick flat bar produced using the method of this invention has a much higher density than the 190mm thick flat bar produced using the conventional method.

[0048] The specific embodiments described above are for illustrative purposes only and should be understood as merely specific embodiments of the present invention, and are not intended to limit the scope of protection of the present invention. In particular, it should be noted that any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for direct rolling of plastic mold flat steel with a thickness of 150mm or more, characterized in that, Includes the following steps: The steel ingot is heated until it reaches the set temperature, and then it undergoes a homogenization process. After the steel ingot is homogenized once, it is cooled down and then homogenized again after the temperature reaches the rolling temperature, so as to form a temperature gradient on the cross section of the steel ingot from the core to the surface. The steel ingots after secondary homogenization are rolled to obtain plastic mold flat steel with a thickness of more than 150mm. In the flat rolling passes, the absolute reduction of each pass is not less than 40mm and the relative deformation is not less than 8%.

2. The method for direct rolling of plastic mold flat steel with a thickness of 150mm or more according to claim 1, characterized in that, The rolling temperature is between 1180-1200℃, and the heating temperature is 30-60℃ higher than the rolling temperature.

3. The method for direct rolling of plastic mold flat steel with a thickness of 150mm or more according to claim 1, characterized in that, The heating time for one cycle is 5-10 hours.

4. The method for direct rolling of plastic mold flat steel with a thickness of 150mm or more according to claim 1, characterized in that, The second heating time is 1-4 hours.

5. The method for direct rolling of plastic mold flat steel with a thickness of 150mm or more according to claim 1, characterized in that, During the rolling process, the rolling speed shall not exceed 5 m / s.

6. The method for direct rolling of plastic mold flat steel with a thickness of 150mm or more according to claim 1, characterized in that, The dimensions of the steel ingot and the flat steel are controlled such that the compression ratio at the smallest cross-section of the steel ingot is not less than 3.

7. The method for direct rolling of plastic mold flat steel with a thickness of 150mm or more according to claim 6, characterized in that, The plastic mold flat steel is 40Cr13 plastic mold flat steel.

8. The method for direct rolling of plastic mold flat steel with a thickness of 150mm or more according to claim 1, characterized in that, The steel ingot is a flat ingot or a rectangular ingot.

9. The method for direct rolling of plastic mold flat steel with a thickness of 150mm or more according to claim 1, characterized in that, The heating temperature, the first homogenization time, and the second homogenization time all increase with the increase of the size of the steel ingot.

10. The method for direct rolling of plastic mold flat steel with a thickness of 150mm or more according to claim 1, characterized in that, Before rolling the steel ingot after secondary homogenization, high-pressure water descaling is performed.

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