KR0151662B1 - Method for manufacturing plastic die steel - Google Patents

Abstract

The composition of the high toughness free-cut mirror plastic mold steel of the present invention is based on the percentage of C: 0.07-0.25%, Si: 0.15-0.90%, Mn: 0.70-2.0%, P: 0.030% or less, S: 0.05-0.20%, Cu : 0.20-0.80%, Ni: 2.0-4.0%, Cr: 0.6-1.5%, Mo: 0.2-0.8%, V: 0.05-0.20%, W: 0.05-0.18%, Al: 0.5-2.0%, Ca: Precipitation of intermetallic compounds by Ni, Cu, and Al, by V, W, Mo, and Cr in order to further improve the hardness and strength of AISI P21, which is a low carbon Ni-Al plastic mold steel with 0.001-0.010% of component High carbon, high toughness and high hardness as precipitates of fine carbonitride, and the addition of free cutting elements, S, Ca to improve the machinability to improve the processing time, cost and mold life of the mold. In particular, the present invention relates to an excellent toughness free-cutting plastic mold steel suitable for precision machining and a method of manufacturing the same because the alloying elements of the above have improved workability by deriving an optimum heat treatment condition and simplifying heat treatment to achieve a sufficient effect. will be.

Description

High toughness free cutting mirror plastic mold steel and its manufacturing method.

1 is a 100-fold microstructure photograph of steel (B) according to the present invention,

Figure 2 is a photograph showing the chip treatment after the machinability evaluation of the steel according to the present invention.

The present invention relates to a low carbon-Ni-Cu-Al-S-based high toughness free-cut mirror free hardened steel used for plastic injection and molding.

Recently, plastic products have become more compact, lighter, more advanced, and more sophisticated, as well as more applicable to automobiles, home appliances, and general necessities. Accordingly, the types of plastic materials for injection molding are also diversified, and as the quality and precision of plastic products using the plastic mold steel are advanced, wear resistance and endurance are required for the properties of the existing plastic mold steel, but the conventional high-quality plastic mold steel is low carbon. Since Ni-Cu-Al, Low Carbon-Ni-Al, and Low Carbon-Ni-Cu-Al-S are the predominant, these steels are complex in shape due to their bias in free machinability, specularity, discharge workability and water retention. In the precision processing molds with many meat parts, the edges of the thin parts are often broken and the life of the mold is shortened.

In addition, in case of heat treatment of large mold steel, the heat treatment time should be long to improve the hardness by aging hardening of conventional steel, and the reheat treatment was performed 3-4 times, which put a heavy burden on the cost of special steel manufacturers.

The present invention aims to provide a material that maintains the advantages of cutting, water retention, specularity, and discharge workability while solving the problems of the conventional steel, and improves the hardness uniformity, toughness improvement, and wear resistance of the mold steel, and is most suitable for precision cutting. It is for manufacturing the steel for molding free cutting specular plastic mold.

The steel of the present invention is Mo, V, W for the addition of Ca, toughness, abrasion, uniform hardness for suppressing the miniaturization, homogenization and softening of MuS emulsification in low carbon-Mn-Ni-Cu-Cr-Al-S-based mold steel In addition, induction of upper bainitization for suppressing cutting resistance of the matrix structure itself was carried out, and Ni-Al, Cu-Al intermetallic compound and Mo. It is characterized by the hardness improvement and toughness improvement by precipitation of V, W composite microcarbide.

Steel according to the present invention has the following component ranges. That is, by weight ratio C 0.07-0.25%, Si 0.15-0.90%, Mn 0.70-2.0%, P≤0.030%, S 0.05-0.20%, Cu 0.20-0.80%, Ni 2.0-4.0%, Cr 0.6-1.5% , Mo 0.2-0.8%, V 0.05-0.20%, W 0.05-0.18%, Al 0.5-2.0%, Ca 0.001-0.010%. The remaining amount is Fe, which is a common trace impurity in steelmaking.

Hereinafter, a statement regarding the reason for limiting the chemical composition of the steel of the present invention.

Carbon (C) is a basic additive element that forms the upper bainite, which is the basic structure of the steel of the present invention, while increasing the hardness and strength by solidifying it at the base, and when the carbon content is large, mardenite is organized and excess carbide precipitates to reduce cutting resistance. As it increases, the machinability deteriorates and the machined surface hardens during discharge processing. Therefore, it is required to be 0.25% or less.

If the carbon content is low, the high carbon capacity during the heat treatment is low, so the basic hardness essential for the mold steel is low, and the precipitation of Cr, V, W microcarbide, which makes the hardness homogeneous and increases the wear resistance, is short, resulting in short mold life. Therefore, it is necessary to make it to 0.07% or more.

Si participates in order to act effectively as a carbonate and to have corrosion resistance in the air, but when added in a large amount, it inhibits the machinability of the material and impairs toughness, so it is limited to 0.9% or less. Since the effect is difficult to obtain and the ferrite phase is weakened and the hardness is reduced, it is necessary to contain 0.15% or more.

Mn is one of the important elements in the steel of the present invention because it improves the hardenability, regulates the bainite phase, homogenizes the hardness and increases the machinability by the formation of the MnS system.

When the Mn content is large, bainite is finely reduced, machinability is decreased, residual austenite is increased, and hot workability is deteriorated. Therefore, the Mn content is limited to 2.0% or less. It decreases and hardenability worsens, so it is limited to 0.7% or more.

S is added for the purpose of improving machinability, but when 0.2% or more is added, the hot workability is lowered and the toughness of the mold is lowered. Therefore, S should be added at 0.2% or less. When the amount of S is small, the amount of MnS inclusions bonded with Mn is small. Since it does not improve machinability, it is necessary to contain 0.05% or more.

Cu is an important element that makes Cu-Al intermetallic compound in the aging heat treatment of the present invention and makes the hardness uniform without impairing machinability. When Cu is excessively added, Cu inhibits hot workability and is 0.8% or less. Since the effect of addition is less limited to 0.2% or more.

Ni is an important element that precipitates Ni-Al intermetallic compound during aging heat treatment in the present invention steel and improves the hardness of bainite, which is a known structure. When Ni is large, bainite becomes fine and the machinability decreases, so it is limited to 4.0% or less. When the amount of Ni is small, the corrosion resistance is lowered and the hardness is hardly maintained by precipitation hardening. Therefore, the Ni content needs to be 2.0% or more.

Cr is an element that makes fine chromium carbide during heat treatment and improves the core hardness of the mold material and improves the corrosion resistance of the mold surface. When Cr amount increases, machinability of the material decreases, so it is limited to 1.5% or less. Since no effect can be obtained, it is necessary to make it 0.6% or more.

Mo is an element that increases the hardness of the material by synergistic with Cr and Ni. When the amount of Mo increases, the machinability of the material deteriorates and the added amount cannot be seen. Therefore, the amount of Mo needs to be 0.8% or less. It is difficult to control the strength and hardness necessary for the mold, so it is limited to 0.2% or more.

V is an expensive and important additive that combines with C and N to improve wear resistance and strength, while maintaining toughness while preserving the strength of the material.

However, if the content is excessive, it is not economical compared to the effect, so the upper limit is 0.2% or less, and when the amount of V is small, strength and toughness are difficult to secure, so it is limited to 0.05% or more.

W is an additive element that improves the wear resistance and maintains the hardness of the mold material processing surface. When the amount of W increases, machinability decreases due to a large amount of carbide. Therefore, W should be less than 0.18%. Therefore, it is limited to 0.05% or more.

Al is an important element that precipitates as a fine Ni-Al intermetallic compound during aging heat treatment and has high hardness in bainite matrix, and does not impair machinability.

In addition, during the nitriding process, the nitriding hardness is increased. However, when excessively added, the amount of Al ₂ O ₃ produced during steelmaking is relatively high. Therefore, special steelmaking technology is required to maintain the cleanliness of the material and the ductility decreases, so the amount of Al is limited to 2.0% or less. If less, the excellent effect cannot be obtained, so it should be added at 0.5% or more.

Ca is an important element that contributes to improved machinability and tool life along with MnS inclusions, which increases the nucleation of MnS and makes it finer, and prevents excessive stretching of MnS, thereby preventing toughness.

However, when excessively added, large inclusions are formed to decrease the specularity, so it is limited to 0.001% or less. When the amount of Ca is small, the excellent effect cannot be obtained.

Hereinafter, the present invention will be described in detail with reference to Examples.

Example 1

The steel of the present invention is a special steel of low carbon-Ni-Cu-S-Al, and has been manufactured by electric steelmaking and vacuum degassing to secure cutting property, high hardness, and specularity.

First, silicon and aluminum were properly used to prevent peroxidation during decarburization in an electric furnace, and after the deoxidation power was recovered by slag-tal acid method, the oxidation inclusions generated before vacuum degassing were sufficiently stirred and removed. Adjust the alloying components by administering (Si), manganese (Mn), copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo), vanadium (V), tungsten (W), and sulfur (S) It was. In addition, the cleanliness of steel was achieved by controlling the vacuum degree to 0.1 Torr or less during vacuum degassing. The steel was cast in accordance with the target composition to Al, Ca, and heated in two stages of 850 ℃ and 1200 ℃ and then rolled into flat iron.

After the rolled mold steel is maintained in a continuous heat treatment furnace for 1-2 hours per 20mm thickness for solution treatment and air-cooling, it is then aging heat-treated for 2-4 hours per 25mm thickness at 450-550 ℃, followed by air cooling for hardness (H _R C ) 38-45 high toughness free cutting plastic mold steel for age hardening was prepared.

Example 2

An example in which the steel of the present invention is carried out in comparison with a comparative steel is shown. Table 1 shows the chemical compositions of the test pieces.

A and B steels in Table 1 are the present invention steel, and C-G steels are comparative steels, all of which are conventional low carbon-Ni-Al-based plastic mold steels. In particular, in the case of G steel, AISI P21 was used as the standard of steel grades. The test materials were tested for strength, hardness distribution, impact toughness, wear resistance, machinability, and specularity.

The strength and impact toughness tests were compared by testing the processed tensile test pieces and the impact test pieces at room temperature, respectively.

In the hardness distribution test, the cross-sectional hardness of the finished product having a thickness of 50 mm of each specimen was measured at a uniform interval from the surface to the deep portion using the Rockwell C Scale.

The machinability test was conducted to measure cutting resistance and chip treatment using a copper ball dynamometer based on the material diameter Φ35.

In the mirror test, the surface roughness of the mold materials was measured using a final 1 μm diamond paste, and the standard was set to Ra (μm).

Table 2 shows the mechanical properties tested with the mold steel having the chemical composition of Table 1.

The inventive steels A and B obtained excellent results in strength, toughness and hardness deviations compared to the comparative steels.

This is due to the even distribution of fine carbides as well as aging precipitates compared to G steel, which is the basic steel.In particular, the strength and impact value are excellent, and high strength has been achieved by giving toughness and rigidity to the thin edges of precision molded products. Since the hardness is relatively high and the variation is small, the wear resistance is excellent, and the life of the mold can be greatly improved.

1 is a 100 times microstructure photograph of the steel (B) of the present invention.

As shown in this picture, the low-carbon upper bainite matrix is uniform, and the MnS nonmetallic emulsions by Mn and S added to improve the machinability are finely and shortly distributed without being stretched by Ca, thus improving impact toughness and strength. It can be seen that the improvement of machinability is apparent.

2 is a view of chip treatment after evaluating the machinability of the steel of the present invention.

The chip shape is slightly different according to the cutting speed and depth of cutting. Despite the high hardness and toughness of the plastic mold steel, the shape is controlled by the MnS which has a controlled shape.

Evaluation standard: 1-5 (good) G steel (A1S1 P21): 3 standard

Table 3 is an evaluation result table for each characteristic item required for precision plastic mold steel.

Compared to the comparative steel G steel, the present invention steels A and B showed excellent results in all properties, and can improve the strength, toughness and machinability without reducing the mirror property compared to C, E and F steel having excellent mirror properties. This was due to the uniformly distributed intermetallic compound of MnS with controlled shape, the precipitation of fine carbides, and the upper bainitization of the matrix.

As described above, the present invention is a precision mold by alloying so as to have a high strength, high toughness and uniform hardness distribution as well as the hardness, machinability, specularity, weldability required by the precision plastic mold steel, as well as the low toughness of the existing steel It is possible to prevent the cutting edge of the thin part, and to derive the simple heat treatment method to exert such characteristics, it is possible to extend the life of the mold, and the work efficiency can be excellent.

Claims (2)

As weight ratio, C: 0.07-0.25%, Si: 0.15-0.90%, Mn: 0.70-2.0%, P: 0.030% or less, S: 0.05-0.20%, Cu: 0.20-0.80%, Ni: 2.0-4.0%, Cr: 0.6-1.5%, Mo: 0.2-0.8%, V: 0.05-0.20%, W: 0.05-0.18%, Al: 0.5-2.0%, Ca: 0.001-0.010%, mostly Fe and the rest High toughness free-cut plastic mold steel, characterized by trace impurities in steelmaking As weight ratio, C: 0.07-0.25%, Si: 0.15-0.90%, Mn: 0.70-2.0%, P: 0.030% or less, S: 0.05-0.20%, Cu: 0.20-0.80%, Ni: 2.0-4.0%, Steel: Cr: 0.6-1.5%, Mo: 0.2-0.8%, V: 0.05-0.20%, W: 0.05-0.18%, Al: 0.5-2.0%, Ca: 0.001-0.010% After heating in two stages of 1200 ℃, flat iron rolling, and maintaining the rolled mold steel for 1-2 hours per 20mm thickness in a continuous heat treatment furnace, solution treatment and air-cooling, then again 25mm thickness at 450-550 ℃ A method for producing a high toughness free-cut plastic mold steel which consists of air-cooling after aging heat treatment for 2-4 hours.