Molds occupy an increasingly important place in modern manufacturing, especially in the automotive and electrical manufacturing industries, where more than 70% of the parts are manufactured using molds. However, at present, China's high-quality molds rely heavily on imports. The main reason for the analysis is not our high-quality steel-making steel level, but the failure to recognize the improvement of the quality of the entire mold steel is a system control process. In addition to metallurgical quality, forging, pre-heat treatment, machining, and final heat treatment in the manufacturing process will affect the internal organization and stress state of the mold, thus determining the final performance of the mold. According to Luo Baihui, in the manufacturing process of the mold, the service life of the mold and the precision, quality and surface properties of the mold are related to the mold design, manufacturing precision and machine tool and operation conditions, and the mold material and its heat treatment process. There is also a close relationship. According to relevant statistics, the early failure of the mold is about 10% due to improper material selection and internal defects, and about 50% caused by improper heat treatment. Therefore, the correct selection of mold steel with good quality and proper heat treatment , is of great significance. The characteristics of die steel mainly include three aspects: performance, process performance and metallurgical quality.
1. Requirements for working performance of die steel
1 hardness
The working state of the mold is complicated during work. For example, the hot working mold usually undergoes alternating stress under the exchange temperature field, so it should have good resistance to softening or plastic deformation, and can still be used in long-term working environment. Maintain the shape and dimensional accuracy of the mold. Hardness is one of the raw properties of die steel. The hardness of the cold work die is generally selected to be above 58 HRC, and the hot work die is especially a mold requiring high heat fatigue resistance, and the hardness is usually about 45 HRC. For ordinary plastic molds, the general hardness is about 35HRC.
2 strength and toughness
The parts are shaped to expose the mold to huge loads, twists and other loads, especially modern high-speed stamping, high-speed precision forging and liquid forming technologies, as well as the development of one-shot forming technology. The molds are subjected to greater loads, often due to the strength of the steel. And the toughness is not enough, causing the edge or partial collapse, chipping or fracture of the cavity to fail early, so the mold should have high hardness and toughness after heat treatment.
3 wear resistance
When the part is formed, the material moves relative to the surface of the mold cavity, causing wear on the surface of the cavity, which causes the dimensional accuracy, shape and surface roughness of the mold to change and fail. Wear is a complicated process with many influencing factors. In addition to the external conditions acting on the mold, it depends largely on the chemical composition inhomogeneity, microstructure, and mechanical properties of the steel.
4 fatigue performance
When the mold is working, it is subjected to the alternating stress of mechanical shock and thermal shock. During the working process of the hot mold, the thermal alternating stress causes the mold to be thermally cracked more obviously. Cracks are caused by stress and temperature gradients, often forming shallow and fine cracks on the surface of the cavity, and its rapid propagation and expansion lead to mold failure. In addition, the chemical composition of steel and the unevenness of the structure, metallurgical defects such as non-metallic inclusions, pores, microcracks, etc. in the steel can cause the fatigue strength of the steel to decrease, because under the action of alternating stress, first These weak areas produce fatigue cracks and develop into fatigue damage.
5 adhesiveness
The surface of the tool part is due to the mutual use of two metal atoms or the diffusion of single phase. There are often some metal to be adhered, especially the surface of some cutting, shearing tools and stamping tools may cause adhesion or crusting. Phenomenon, this will affect the sharpness of the cutting edge and the change of local tissue and chemical composition, causing the blade to partially crack or adhere to the metal to scratch the mold, making the surface of the workpiece rough. Therefore, good anti-adhesion is also very important.
6 polishing and etching performance
With the widespread use of molds, especially plastic molds, low surface roughness values ​​(sometimes even specularities) are necessary, and low surface roughness values ​​affect mold life and production efficiency and product quality. High surface quality can reduce corrosion (especially local pitting corrosion); reduce the risk of cracking, the chemical composition, microstructure, hardness and carbide distribution of the polished steel must be uniform. Large carbides, especially when they are segregated and banded, are extremely detrimental to surface finish. It is particularly important that the steel does not contain large oxide inclusions or segregation that does not deform, so the smelting and deoxidation processes must be strictly controlled. Vacuum arc remelting and electroslag remelting have a good effect, and this process has now become the main production mode of advanced plastic mold steel. Even simple vacuum degassing helps to eliminate large oxide inclusions. These smelting processes not only reduce the oxide content, but also make the oxides finer and more uniform, while controlling the smelting and deoxidation processes, as well as changing inclusions. Type, softening and having good plastic toughness to improve polishing performance.
Any unclosed voids in the steel will affect its polishing performance, so it is necessary to press and loosen metallurgical defects such as looseness in the hot working and to maintain the compactness of the structure, which can be achieved by modern forming processing technology. For example, repeated drawing techniques, rotary forging techniques, high-temperature isostatic pressing, etc. can refine the original as-cast structure, the voids in the dendrites. Electroslag remelting and vacuum arc remelting refining process are also very beneficial to steel uniformity. For defects caused by heat treatment or surface hardening, decarburization which causes uneven hardness should be avoided as much as possible. These measures, combined with proper composition design and control, produce steels with excellent mirror finish.
In addition, according to the working conditions of the mold and the difference in the environment, it should be considered that the mold steel used should have good thermal conductivity, corrosion resistance, oxidation resistance and magnetic permeability.
2, mold steel requirements in terms of process performance
1 processability
The machinability of steel mainly includes both machinability and cold thermoplastic deformation, which depends on the chemical composition of the steel, the microstructure after heat treatment and the internal quality of metallurgical production. In recent years, in order to improve the machinability of steel, Some steels incorporate free-cutting elements or change the distribution of inclusions in the steel to improve the surface quality of the mold steel and reduce the wear of the mold. In hot working, it is important for some high carbon high alloy mold steels, especially to improve the morphology and distribution of carbides, grain size and austenite alloying.
In addition to good processability, it also has good electrical workability and embossing processability.
2 hardenability and hardenability
The requirements of the two properties of the mold are different depending on the working conditions. For the mold with high hardness uniformity of the whole section, such as steel for hammer forging, it is more important to have high hardenability. Small molds with high hardness, such as blanking die steel, are more important for high hardenability.
3 heat treatment deformability
When the mold parts are heat treated, the deformation is required to be small, the directions are changed in all directions, and the structure is stable. The quenching deformation is small, except for the quenching temperature and the cooling medium. It depends mainly on the uniform composition of the steel, metallurgical quality and structural stability.
4 decarburization sensitivity
When the die steel is forged, annealed or quenched, it is heated in an unprotected atmosphere, and defects such as oxidative decarburization are generated on the surface, so that the durability of the mold is lowered. In addition to the heat treatment process and equipment, decarburization depends mainly on the chemical composition of the steel, especially the carbon content. When it contains high silicon, molybdenum and other elements, it will also aggravate decarburization.
In addition, the mirror polishing, grinding and electrochemical properties of the mold should be considered according to the conditions of use of the mold.
3. Requirements for metallurgical quality of die steel
The high metallurgical quality can exert the basic characteristics of the steel. The internal metallurgical quality of the die steel is of equal importance to its basic properties. While studying the performance, it is necessary to study the factors affecting the metallurgical quality. Generally, the internal and external quality problems of mold steel are often encountered in the following aspects:
1 chemical composition uniformity
Die steel is usually an alloy steel containing multi-element. When steel is solidified from a liquid in an ingot mold, due to the selective crystallization, various elements in the molten steel are unevenly distributed in the solidified structure to form segregation. The segregation will result in differences in organization and performance, and it is one of the important factors affecting the quality of steel. Reducing the degree of segregation of steel can effectively improve the performance of steel. In recent years, many metallurgical plants at home and abroad are working hard to produce steel with uniform composition and fine structure.
2 harmful element content
Sulfur and phosphorus form phosphides and sulfides during solidification of steel and precipitate at grain boundaries, thus producing intergranular brittleness, which reduces the plasticity of steel. Excessive S and P content will cause cracks in steel ingots during rolling. And will greatly reduce the mechanical properties of steel. Japan’s Matsuda Yuki et al. studied the effects of S and P content on the toughness and thermal fatigue properties of W(Cr) 5% hot work die steel (H13). The results show that the content of W(S, P) is from 0.025%. When 0.010% is reduced to W(P) 0.005% and W(S) 0.001%, the length and number of thermal fatigue cracks will be reduced by half. Hitachi Metals reduced the W(P) content of SKD61 steel from 0.03% to 0.001%, which increased the impact toughness of steel at 45HRC from 39.2J/cm2 to 127.5 J/cm2. In addition, reducing the S and P contents in the steel can also effectively improve the isotropic properties of the steel.
3 non-metallic inclusions in steel
The quality of the steel is not only in accordance with the technical standards, but also the content of non-metallic inclusions in the steel should be as small as possible, because the volume of non-metallic inclusions in steel is small, but for steel. The performance impact is great. Reducing non-metallic inclusions in steel is one of the main tasks of steelmaking. Generally speaking, non-metallic inclusions in steel mainly refer to compounds formed by the action of iron and other alloying elements with oxygen, sulfur, nitrogen, etc., such as FeO, MnO, Al2O3, SiO2, FeS, MnS, AlN, VN, etc. And refractory materials brought in steelmaking and casting, the latter components are mainly oxides of Si, Al, Fe, Cr, Ca, Mg, and the like. The non-metallic inclusions in steel can be classified into intrinsic inclusions and foreign inclusions from the source. The inclusions in the steel are compounds formed in the liquid and solidification process of steel.
The non-metallic inclusions in steel are considered to be cracks of a certain size in the basic sense. It destroys the continuity of the metal and causes stress concentration. Under the action of external stress, the crack extension is easy to develop and expand and lead to performance. reduce. The presence of plastic inclusions, as the forging process is extended and deformed, causes anisotropy in the steel. At the same time, the peeling during the polishing of the inclusions improves the surface roughness of the mold. Therefore, for large and important molds, it is important to improve the purity of the steel.
4, white spots
White spots are a common defect in hot rolled billets and large forgings and are one type of internal cracking of steel. The presence of white spots has a very adverse effect on the properties of steel. This effect is mainly caused by the reduction of the mechanical properties of steel, the quenching of forgings during heat treatment, or the development of more serious damage accidents during use, so in any case Under, you can not use forgings with white spots. Different steels have different sensitivity to white spots. Generally, steels that are prone to white spots are chrome steel, chrome molybdenum steel, manganese steel, manganese molybdenum steel, chromium nickel molybdenum steel, chromium tungsten steel, and the like. Among them, martensite chromium-nickel steel and chromium-nickel-molybdenum steel containing W(C) greater than 0.30%, W(Cr) greater than 1%, and W(Ni) 2.5% are most sensitive to white spots. The reason for the formation of white spots is the desolvation and precipitation of hydrogen in the steel, and the formation of silver in the longitudinal section of the steel is a bright white-coarse circular or elliptical spot. It tends to cause cracks in the interior of the forgings and billets. The mold steel 5CrNiMo, 5CrMnMo, etc. are most likely to have white spots. If the carbide elements Cr, Mo and V are added, the sensitivity of the white point can be reduced. In the production of such steels, attention must be paid to degassing and strengthening the post-forging slow cooling or dehydrogenation annealing of large forgings.
5, oxygen content
The allowable gas content in the steel is generally not specified for the die steel. As the oxygen content increases, the particles and the amount of oxides increase, the fatigue properties of the steel decrease, and hot cracks are easily generated. Some people have tested 4Cr5MoSiV1 steel, and the oxygen content is preferably not more than 1.5*10-5. Which Japan Shanyang Special Steel Co., Ltd. stipulates that the oxygen content of high purity steel is not more than 1.0*10-5. Therefore, in recent years, in order to improve the manufacturing quality of the mold. Die steel at home and abroad is gradually developing in the direction of low oxygen content.
6, carbide unevenness
Carbide is an essential component of most mold steels. In addition to the austenite-soluble carbides, there are some residual carbides that are insoluble in austenite. The size, shape and distribution of carbides have a significant influence on the performance of the die steel. The size, shape and distribution of the carbide are related to the steel smelting method, the solidification conditions of the steel ingot, and the hot working deformation conditions. The carbides of the hypereutectoid steel may form wind-like carbides at the grain boundaries or the carbides may be elongated to form banded carbides or both in the processing deformation. In the Leysite die steel, there is a primary carbide. And secondary carbides, in the process of thermal deformation, most of the reticulated eutectic carbides can be broken, and the carbides first extend along the deformation direction, resulting in a band shape. As the degree of deformation increases, the carbide becomes uniform and small. . The non-uniformity of carbide has a great influence on quenching deformation, cracking and mechanical properties of steel.
7, segregation
Segregation, which is the behavior of steel composition and tissue inhomogeneity, is a drawback often found in the inspection of low-magnification microstructures of die steel. It is formed during the solidification process of the ingot, and is related to the chemical composition of the steel and the pouring temperature. Generally divided into dendritic segregation, square segregation, point segregation and the like. Due to the presence of dendritic segregation, the mechanical properties of the different directions show significant differences. The square segregation is formed by the accumulation of a large amount of impurities and pores at the end of the columnar crystal and the equiaxed crystal region of the ingot when the ingot is crystallized. Severe square segregation has a significant impact on the quality of the steel, especially for parts with large machining volumes or mold parts with core forces. In addition to affecting the isotropic properties of the mechanical properties of the die steel, segregation also has a certain influence on the polishing performance of the mold. Therefore, there are strict regulations in foreign related standards.
8, loose
Looseness is an indication of the intimacy of steel. Most of the looseness occurs in the upper and middle parts of the ingot, where more impurities and gases are concentrated. Due to the existence of loose defects, the strength and toughness of the steel are reduced, and the roughness of the surface after processing is seriously affected. The influence in the general mold steel is not particularly large, but such as cold rolls, large modules, punches. And plastic forming mold parts have strict requirements. For example, the forging die and punch of the deep cavity require loose steel of no more than 1 or 2 grades, and the steel for plastic molds for dials or translucent parts requires no more than 1 grade.
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