阅读说明：本技术 汽车覆盖件模具用球墨铸铁的正火热处理方法 (Normalizing heat treatment method of nodular cast iron for automobile panel die ) 是由 徐达义 韦开保 周建强 汪继松 于 2021-07-12 设计创作，主要内容包括：本发明的目的是提供一种汽车覆盖件模具用球墨铸铁的正火热处理方法,将汽车覆盖件模具放入热处理炉中,开启热处理炉,升温速率为10℃/min；当温度升至870-930℃时,保温2小时,再开炉门并随炉冷却30分钟；再关紧炉门以10℃/min升温至720℃,并保温1小时之后冷却,该热处理工艺正火过程效率快,同时所得汽车覆盖件模具的组织均匀度好,强度高。(The invention aims to provide a normalizing heat treatment method of nodular cast iron for an automobile panel mould, which comprises the steps of putting the automobile panel mould into a heat treatment furnace, starting the heat treatment furnace, and increasing the temperature at a rate of 10 ℃/min; when the temperature is increased to 870-930 ℃, preserving the heat for 2 hours, opening the furnace door and cooling the furnace for 30 minutes; and then the furnace door is tightly closed, the temperature is raised to 720 ℃ at a speed of 10 ℃/min, the temperature is kept for 1 hour, and then the furnace is cooled.)

1. A normalizing heat treatment method of nodular cast iron for an automobile panel mould is characterized by comprising the following steps: the method comprises the following steps:

the method comprises the following steps: placing the automobile panel mould into a heat treatment furnace, starting the heat treatment furnace, and raising the temperature at a rate of 10 ℃/min;

step two: when the temperature is increased to 870-930 ℃, preserving the heat for 2 hours, opening the furnace door and cooling the furnace for 30 minutes;

step three: then the furnace door is closed tightly, the temperature is raised to 720 ℃ at the speed of 10 ℃/min, and the temperature is kept for 1 hour and then the product is cooled.

2. The method for normalizing heat treatment of spheroidal graphite cast iron for automobile panel molds according to claim 1, wherein: and cooling after the heat preservation in the third step is as follows: and taking out the automobile panel mould, and air-cooling to room temperature under the room temperature condition.

3. The method for normalizing heat treatment of spheroidal graphite cast iron for automobile panel molds according to claim 1, wherein: the temperature rose to 930 ℃ in step two.

4. The method for normalizing heat treatment of spheroidal graphite cast iron for automobile panel molds according to claim 2, wherein: and after the automobile panel die is cooled in the air to the room temperature, microscopic structure observation, tensile port shape analysis and tensile strength and elongation detection are carried out on the automobile panel die.

5. The method for normalizing heat treatment of spheroidal graphite cast iron for automobile panel molds according to claim 1, wherein: the first step also comprises the step of firstly proportioning the ductile cast iron raw material of the automobile panel mould.

6. The method for normalizing heat treatment of spheroidal graphite cast iron for automobile panel molds according to claim 5, wherein: the nodular cast iron comprises the following chemical components in percentage by weight: 3.6 to 3.8 percent of C, 2.3 to 2.5 percent of Si, 0.45 to 0.55 percent of Mn, 0.1 to 0.3 percent of Ni, 1.0 to 1.2 percent of Cu, 0.04 to 0.06 percent of Mg, less than or equal to 0.05 percent of P, less than or equal to 0.012 percent of S and the balance of Fe.

Technical Field

The invention relates to the field of automobile panel moulds, in particular to a normalizing heat treatment method of nodular cast iron for an automobile panel mould.

Background

The heat treatment process is an important basic technology in the mechanical industry, important connecting parts such as shafts, gears, connecting rods and the like and important dies are subjected to heat treatment, the service life of the parts is doubled or even dozens of times as long as the materials are properly selected and the heat treatment is carried out, and the effect of achieving double results with half the effort can be achieved if the materials are properly selected and the heat treatment is carried out each time. The heat treatment has very important significance for exerting the potential performance of the metal material, improving the metal quality, saving the material, reducing the energy consumption, prolonging the service life of the product and promoting the economic development.

At present, the mechanical manufacturing industry in China still has some imperfect places when implementing the metal heat treatment technology, which has the influence on the heat treatment effect of the metal material and the performance of the material. The method mainly has the following defects: 1. the energy consumption is high, and the energy utilization rate is low; 2. the heat treatment production process and equipment fall behind; 3. poor product quality, low production efficiency, serious pollution and the like.

In the future development trend, the development prospect of heat treatment mainly has the following big aspects: a new process for heat treatment of metal materials; a new device for heat treatment of metal materials; novel auxiliary materials for heat treatment.

Normalizing, which is a heat treatment method that is mature in process and is generally applied, is a heat treatment process in which steel or a casting is heated to a proper temperature above a critical point Ac3 or Acm, is kept for a certain time, and is cooled in air to obtain a pearlite structure. The normalizing temperature of the nodular iron casting is generally 850-950 ℃, and the general heat preservation time is determined according to the size of the workpiece. In the normalizing and heat-preserving process, the original ferrite and pearlite structures in the nodular iron casting are subjected to austenite transformation recrystallization, while graphite nodules are hardly changed, and the workpiece structure is completely transformed into an austenite structure after the heat preservation is finished. And then, in the air cooling process, the austenite generates pearlite transformation, and finally the normalized structure of ferrite and a large amount of pearlite structures is obtained.

The appearance and performance of the automobile are influenced by the quality of the mould, the quality is high, mass production can be realized, and the performance meets the set requirements. In addition, the quality of the automobile stamping die is not only dependent on the previous design, but also related to the quality of the blank casting. After a blank casting is machined by a machine tool, the dimensional accuracy of a stamping part can be ensured only by meeting the drawing requirements. The existing automobile panel die can generate weak deformation due to insufficient strength after high-frequency impact, the dimensional precision is influenced, and the performance of the automobile panel die can be improved by designing a heat treatment process scheme. The prior art patent CN110157868A discloses a heat treatment process of a nodular cast iron piston casting, which is characterized in that the nodular cast iron piston casting is annealed twice before austenite is strengthened, so that the problems that the nodular cast iron piston casting has a large chilling tendency and large internal stress and pure ferrite and pearlite are difficult to obtain in the casting process are solved, and the high-toughness nodular cast iron piston casting is obtained. However, the prior heat treatment process of the nodular cast iron for the automobile panel mould has a plurality of unsatisfactory places, the normalizing process efficiency of the traditional heat treatment process of the automobile panel mould is relatively low, and meanwhile, the obtained automobile panel mould has poor tissue uniformity, low strength and relatively large improvement space for comprehensive performance.

Disclosure of Invention

In order to solve the problems, the invention provides a normalizing heat treatment method of nodular cast iron for an automobile panel mould, the normalizing process of the heat treatment process has high efficiency, and the obtained automobile panel mould has good tissue uniformity and high strength, thereby solving the problems in the background technology.

The invention aims to provide a normalizing heat treatment method of nodular cast iron for an automobile panel mould, which comprises the following steps:

the method comprises the following steps: placing the automobile panel mould into a heat treatment furnace, starting the heat treatment furnace, and raising the temperature at a rate of 10 ℃/min;

step two: when the temperature is increased to 870-930 ℃, preserving the heat for 2 hours, opening the furnace door and cooling the furnace for 30 minutes;

step three: then the furnace door is closed tightly, the temperature is raised to 720 ℃ at the speed of 10 ℃/min, and the temperature is kept for 1 hour and then the product is cooled.

The further improvement lies in that: and cooling after the heat preservation in the third step is as follows: and taking out the automobile panel mould, and air-cooling to room temperature under the room temperature condition.

The further improvement lies in that: the temperature rose to 930 ℃ in step two.

The further improvement lies in that: and after the automobile panel die is cooled in the air to the room temperature, microscopic structure observation, tensile port shape analysis and tensile strength and elongation detection are carried out on the automobile panel die.

The further improvement lies in that: the first step also comprises the step of firstly proportioning the ductile cast iron raw material of the automobile panel mould.

The further improvement lies in that: the nodular cast iron comprises the following chemical components in percentage by weight: 3.6 to 3.8 percent of C, 2.3 to 2.5 percent of Si, 0.45 to 0.55 percent of Mn, 0.1 to 0.3 percent of Ni, 1.0 to 1.2 percent of Cu, 0.04 to 0.06 percent of Mg, less than or equal to 0.05 percent of P, less than or equal to 0.012 percent of S and the balance of Fe.

The invention has the beneficial effects that: the research of the invention finds that when the temperature rises, the hot-pressing stress is generated in the material, when the hot-pressing stress reaches a certain value, the austenite is deformed, so that the dislocations in the austenite are plugged to the periphery of the cementite, and the dislocations generate local stress at the periphery of the cementite, so that the cementite is unstable and is easy to decompose. The carbon in nodular cast iron exists mostly in the form of graphite nodules, which can absorb or release carbon atoms, and the decomposition of cementite necessarily produces carbon atoms, which form graphite cores at austenite grain boundaries. The graphite core has high solid solubility, and a solute concentration gradient from high to low exists between the graphite core and the graphite nodules, so that carbon in the graphite core tends to diffuse around the graphite nodules. Therefore, cementite is decomposed in the nodular cast iron structure under the normalizing condition, and granular pearlite becomes more.

1. Aiming at the problems that the conventional heat treatment process of the nodular cast iron for the automobile panel mould is low in normalizing efficiency, and the obtained automobile panel mould is poor in tissue uniformity and low in strength, the nodular cast iron for the automobile panel mould is treated by the heat treatment process, and a furnace door is opened for cooling in the heat preservation process, so that the normalizing efficiency can be improved, and the normalizing heat treatment tissue is improved, so that the normalizing efficiency and the strength of the automobile panel mould are improved, and a new process path is provided for production and manufacturing of the automobile panel mould.

2. The heat preservation temperature is increased to 930 ℃ in the normalizing heat preservation process, the decomposition speed of cementite in the nodular cast iron for the automobile panel mould can be effectively increased, and the strength of the automobile panel mould is improved.

3. And when the temperature is raised to 720 ℃ again and the temperature is kept for a period of time, compared with direct air cooling, the method is favorable for keeping the newly generated crystal grains fine.

4. And the normalizing heat preservation process adopts furnace door cooling, so that atomic diffusion can be promoted compared with furnace cooling, nucleation of crystal nuclei is facilitated, the number of new crystal grains is increased, and the normalizing efficiency is improved. On the other hand, compared with direct air cooling, the graphite after the decomposition of the cementite in the nodular cast iron structure is promoted to have more time to diffuse to the periphery of the graphite nodules, so that the decomposition of the cementite is more complete, and the normalizing heat treatment structure is improved.

5. And the processes of temperature rise, heat preservation and air cooling are carried out again, compared with the process of directly air cooling to room temperature, the space between the generated pearlite plates is reduced, and the structure is refined.

Drawings

FIG. 1 is a metallographic microstructure of a sample obtained in example 1 of the present invention at a magnification of 250.

FIG. 2 is a metallographic microstructure of a sample obtained in example 2 according to an embodiment of the present invention at a magnification of 250.

FIG. 3 is a metallographic microstructure of a sample obtained in example 3 according to an embodiment of the present invention at a magnification of 250.

FIG. 4 is a metallographic microstructure of a sample obtained in comparative example 1 according to an embodiment of the present invention at a magnification of 250.

FIG. 5 is a fracture morphology diagram under a scanning electron microscope of 500 times of a sample obtained in example 1 of the present invention.

FIG. 6 is a fracture morphology diagram under a scanning electron microscope of 500 times of a sample obtained in example 2 of the present invention.

FIG. 7 is a fracture morphology diagram under a scanning electron microscope of 500 times of a sample obtained in example 3 of the present invention.

FIG. 8 is a fracture morphology under a scanning electron microscope at 500 times of a sample obtained in comparative example 1 according to an embodiment of the present invention.

Detailed Description

In order to enhance the understanding of the present invention, the present invention will be further described by the following examples, which are only for the purpose of illustration and are not to be construed as limiting the scope of the present invention.

In examples 1 to 3 and comparative example 1, a round test bar of 40mm diameter made of spheroidal graphite cast iron was used as a mold sample for automobile panels. The weight percentage of the chemical components is as follows: 3.7% of C, 2.4% of Si, 0.50% of Mn, 0.2% of Ni, 1.1% of Cu, 0.05% of Mg, 0.05% of P, 0.012% of S and the balance of Fe.

Example 1

The method comprises the following steps: placing the automobile panel mould sample into a heat treatment furnace, starting the heat treatment furnace, and raising the temperature at a rate of 10 ℃/min;

step two: when the temperature is increased to 930 ℃, preserving the heat for 2 hours, opening the furnace door and cooling the furnace for 30 minutes;

step three: then the furnace door is tightly closed, the temperature is raised to 720 ℃ at a speed of 10 ℃/min, and the temperature is kept for 1 hour;

step four: and after the heat preservation is finished, taking out the automobile panel die sample, and air-cooling to room temperature under the room temperature condition.

Example 2

The method comprises the following steps: placing the automobile panel mould sample into a heat treatment furnace, starting the heat treatment furnace, and raising the temperature at a rate of 10 ℃/min;

step two: when the temperature is increased to 900 ℃, preserving the heat for 2 hours, opening the furnace door and cooling the furnace for 30 minutes;

step three: then the furnace door is tightly closed, the temperature is raised to 720 ℃ at a speed of 10 ℃/min, and the temperature is kept for 1 hour;

step four: and after the heat preservation is finished, taking out the automobile panel die sample, and air-cooling to room temperature under the room temperature condition.

Example 3 (incubation time 870 ℃ C.)

The method comprises the following steps: placing the automobile panel mould sample into a heat treatment furnace, starting the heat treatment furnace, and raising the temperature at a rate of 10 ℃/min;

step two: when the temperature is increased to 870 ℃, preserving the heat for 2 hours, opening the furnace door and cooling the furnace for 30 minutes;

step three: then the furnace door is tightly closed, the temperature is raised to 720 ℃ at a speed of 10 ℃/min, and the temperature is kept for 1 hour;

step four: and after the heat preservation is finished, taking out the automobile panel die sample, and air-cooling to room temperature under the room temperature condition.

Comparative example 1

An automobile panel mold sample without heat treatment.

Based on the above, metallographic specimens prepared from normalized samples of the automobile panel molds of examples 1, 2 and 3 and comparative example 1 were subjected to microstructure observation and tensile fracture morphology, as shown in fig. 1 to 8, in which:

1) in fig. 1, 2 and 3 of examples 1, 2 and 3, the pearlite morphology was significantly changed from lamellar pearlite to granular pearlite, as compared with fig. 4 of comparative example 1.

2) The temperature for keeping the temperature of the normalizing treatment in example 1 was 930 ℃, the temperature for keeping the temperature of the normalizing treatment in example 3 was 900 ℃, and the temperature for keeping the temperature of the normalizing treatment in example 2 was 870 ℃. As can be seen in fig. 1, 2, 3 of comparative examples 1, 2, 3, example 1 is the most effective.

3) The test pieces of the automobile panel mold obtained in examples and comparative examples were subjected to the automobile panel mold test using a universal tester to test the tensile strength and elongation data of each sample, respectively, to verify the above analysis results, and the tensile strength and elongation value test results are shown in table 1 below. The data show that the elongation is greater in example 1 than in examples 2 and 3, without significant change in tensile strength.

TABLE 1 mechanical Properties of the samples at room temperature

4) In comparison with fig. 8 of comparative example 1 and fig. 5 of example 1, in fig. 6 and 7 of examples 2 and 3, it was found that the fracture of the normalized sample had a river pattern with cleavage facets and diverged all around, and exhibited a quasi-cleavage brittle fracture characteristic. Whereas the normalized specimens of comparative example 1 and example 1 exhibited fracture with fewer dissociated facets and fewer shear tearing ridges and more dimples present, manifested as ductile fracture. The elongation of the sample of example 1 was made higher than that of examples 2 and 3. The fact that the decomposition speed of cementite in the nodular cast iron for the automobile panel mould can be improved by improving the heat preservation temperature and the strength of the automobile panel mould is also well explained.

In conclusion, the scheme of the invention can improve the normalizing efficiency of the automobile panel mould and the mechanical property and the tissue uniformity of the automobile panel mould, thereby being beneficial to avoiding the phenomena of abrasion, fracture and the like in the use process caused by low hardness and poor tissue uniformity of the conventional automobile panel mould and prolonging the service life of the automobile panel mould.

It is to be understood that the invention is not limited to the specific embodiments described above, but is intended to cover various insubstantial modifications of the inventive process concepts and solutions, or its application to other applications without modification.