KR910001585B1 - Steel for mold for injection plastic and the method for heat treatment - Google Patents

Abstract

steel for plastic injection mould consists of 0.05-0.3 wt.% C, 0.1- 1.0 wt.% Si, 0.2-2.5 wt.% Mn, 1.5-7.0 wt.% Ni, 0.1-3.0 wt.% Cr, 0.1- 2.0 wt.% Mo, 0.5-5.0 wt.% Al and balance Fe with the standard impurities including up to 0.02 wt.% S, up to 0.02 wt.% P, up to 0.01 wt.% N, up to 0.01 wt.% O and up to 0.05 wt.% H. A method for heat treating the steel comprises air cooling the forged steel treated to solution at 850-950 deg.C, quenching at 500-600 deg.C, and aging at 500-700 deg.C after second solution heat treatment.

Description

Plastic Injection Molded Steel for Hard Surface and Heat Treatment Method

FIG. 1 shows the hardness change HMAX after aging treatment according to the amount of (nickel + aluminum) and hardness increase ΔH.

2 is an enlarged photograph of the surface after corrosion processing.

3 is a comparison chart of the machining resistance of this cast steel and conventional steel (NAK80).

The present invention relates to high hard surface PREHARDEN mold steel used for plastic injection.

Plastic injection mold steel has been increasing year by year due to the rapid increase in demand of various plastic products, and the quality is required to be high quality, so it is necessary to have a durable, excellent machinability and excellent mirrorability and weldability. Plastic injection mold steel can be classified into steel grade to be heat treated after mold processing and pre-hardened steel grade to be mold processed after heat treatment, but in the former case, high hardness can be obtained, but heat treatment causes mold dimensional change and quenching crack. You can.

In the latter case, the mold is processed after the heat treatment, so that a relatively high hardness can be obtained and the dimensional change and the quenching crack can be prevented, which is suitable for high-precision molds such as optical and electronic products, and the cost of manufacturing the mold is considerable. The demand is gradually increasing compared to the former because it can reduce processing costs and shorten mold manufacturing time. Most of the pre-hardened plastic mold steels developed in advance are steels age-hardened by alloying elements.

Aging hardenable plastic injection mold steel is a slow cooling by air cooling after solution treatment. Aging hardenable alloy element is sufficiently dissolved in iron machine and cooled by air cooling after solution treatment. It is excellent in mirroring, machinability and corrosion processability, and it has the advantage that there is no dimensional change of the processed mold by air cooling after aging treatment after welding repair.

The age hardenable plastic mold steel can maintain the same hardness as the hardness before welding due to the simple aging treatment after growth welding, and the internal stress is relaxed by the aging treatment, so that the microstructure is stable. Change rarely occurs.

A representative example of aging hardenable plastic injection mold steel having such characteristics is NAK 55 of Daedong Special Steel in Japan. Although NAK 55 has excellent machinability and weldability, it has high sulfur (S) content of more than 0.1% and is sulphide. Manganese sulfide inclusions are drawn in the processing direction during hot working to form an annealing (MnS) inclusion, which promotes anisotropy of impact value and elongation, and inferior in mirrorability and corrosion processability with poor cleanliness.

It also contains 1% Cu, which is an impurity to be suppressed in other alloys, and therefore requires dedicated furnaces and ladles, and it is not possible to mutually use furnaces and ladles with other steel grades.

NAK 55 has an oxygen content of 70PPM or more and forms an oxidizing element and an oxide, which impairs cleanliness, thus degrading the specularity and containing hydrogen of 50PPM or more, which may cause hydrogen embrittlement.

NAK 80 was developed by Japan's Daedong Special Steel Co., Ltd., which has better performance than NAK 55, and NAK 80 has lower sulfur content than NAK 50, which improves anisotropy of impact value and elongation and improves cleanness. Although it has corrosion and machinability, it contains 1% of copper, which is the same as NAK 55, and requires a dedicated furnace and ladle, making it impossible to use furnaces and ladles with other steel grades.

Therefore, in the present invention, the cross-sectional hardness distribution is uniform by the addition of the age hardening element, and the dimensional change and the quenching crack of the mold processed by the simple aging treatment after welding repair do not occur at all, and have almost the same hardness distribution as before welding or by welding It shows excellent welding repairability with almost no hardness drop, almost no dimension and performance change during use, and limited sulfur content to 0.02% or less to suppress the formation of manganese sulfide inclusions, thereby removing anisotropy of impact value and elongation. The aging hardenable plastic injection mold steel has been developed which is superior to the conventional hardenable plastic injection mold steel, and its composition and limitations are as follows.

First, carbon cannot be avoided as a basic element of steel tank, and when it is 0.05% or less, it is impossible to obtain proper hardness. When 0.3% or more, hardening is so high that the hardness value is too high during heat treatment, making machining impossible. Therefore, the appropriate carbon content is set at 0.05-0.3%.

Second, when the silicon (Si) 0.10-1.00% content of silicon (Si) is less than 0.10%, steelmaking cannot be refined due to peroxide of molten steel, so that the content of 0.10% or more is used when deoxidizing and refining ordinary silicon (1.0). When it is more than%, it is difficult to produce homogeneous products due to high fluidity during ingot manufacturing, and its hardness is excessively high due to solidification and hardening.

Third, the manganese steel (Mn) is less than 0.2%, it is difficult to operate because peroxide reaction easily occurs during steelmaking. In addition, if too low, hardenability during heat treatment is difficult to operate. If too low, the hardenability during heat treatment is low, so that an appropriate hardness value is not obtained. Manganese (Mn) is required to be 2.5% or less so that an appropriate hardenability can be obtained. If it is more than 2.5%, it is difficult to control the alloy composition due to the volatility due to the high vapor pressure during vacuum refining.

Fourth, nickel (Ni) is necessary to obtain aluminum (Al) and precipitate Ni ₃ Al to obtain the appropriate age hardening effect. Since aluminum (Al) is at least 0.5%, nickel (Ni) needs to have a lower limit of 1.5% or more, and if it is 7.0% or more, the austenite stabilization zone is widened, so that the hardenability becomes poor during heat treatment, and thus it is difficult to obtain hardness (HRC 40-48). .

Fifth, aluminum (Al) is an element that forms a precipitate of Ni ₃ Al in the ratio of nickel (Ni) and 3: 1 and eventually strengthens the matrix, and at least 0.5% or more and 5.0% or less is required.

Sixth, chromium (Cr) is difficult to obtain less than 0.1% in the use of ordinary scrap iron, and when more than 3.0% the hardenability is too high, the hardness value is too high.

Seventh, molybdenum (Mo) has the same properties as chromium (Cr) and increases the toughness of the material.

Eighth, when other sulfur (5) or phosphorus (P) is more than 0.02%, harmful non-metallic inclusions are generated to impair corrosion processability. Nitrogen (N) oxygen (0) and hydrogen (H) also have the same adverse effects, which are required to be 0.01%, 0.01% and 0.05% or less, respectively.

In addition, in the present invention, the conventional age-hardenable plastic injection mold steel contains copper and requires a special melting furnace and ladle, and it is impossible to mutually use melting furnaces and ladles with other steel grades. Allowing mutual use of furnaces and ladles.

Hereinafter, the gist of the present invention will be described by way of examples.

Example 1

As a method of controlling aluminum addition, forced deoxidation with aluminum after oxidative convulsion, the aluminum content is adjusted to 0.01-0.5%, and after the oxidation and reduction refining, aluminum is added to the ladle to adjust the aluminum content to 0.5-3.0% and vacuum degassing treatment. The aluminum is adjusted to 0.5-5.0% after degassing in a melting furnace that mixes alloying elements other than aluminum and maintains the vacuum at 0.5 Torr or less. In addition, after degassing, the hydrogen content is limited to 1.0 PPM or less, and through vacuum injection of molten steel, carbon (C) 0.05-0.3%, silicon (Si) 0.3%, manganese (Mn) 1.0-2.5%, nickel (Ni) 2.0-4.5%, chromium (Cr) 0.1-3.0%, molybdenum (Mo) 0.5%, vanadium (V) 0.1%, aluminum (Al) 0.5-1% after producing a steel ingot 4 hours at 1250 ℃ After the forging, the forging ratio was forged over 3S, and the forging finish temperature was 1000 ° C.

After forging, keep solution for 0.5-2 hours per inch of product thickness for solution treatment, air cooling, tempering at 500-600 ℃, secondary solution treatment at 900-1000 ℃, immersion in oil bath and aging treatment at 500-700 ℃ HRC 40-48) was compared with the age hardening plastic injection mold steel (hereinafter referred to as conventional steel).

The cross-sectional hardness distribution was uniform to HRC 40-48 due to the aging hardening characteristics and the slow cooling rate during heat treatment.The cleanness was improved compared to the existing steel, and the Ra (average amplitude of the surface irregularities after mirror finishing) was measured. In the case of about 0.014 μm, the steel of the present invention was superior to 0.005-0.008 μm.

Example 2

This example relates to surface corrosion processing for expressing various patterns on the surface of a workpiece after fabrication of the mold of the present invention as compared with the corrosion resistance of the existing steel (NAK 80) and the present invention as shown in FIG. Compared with the existing steel, steel has higher cleanliness and homogeneous structure, so there is almost no difference in corrosion processing speed, so the corrosion processing surface is very homogeneous.

That is, the corrosion surface portion of the picture lubricates the steel of the present invention, but the existing steel is rough, causing scratches on the plastic product produced through the corrosion-treated mold surface.

The steel of the present invention is easy to machine, the microstructure is homogeneous, and the steel of the present invention is face cut and end milled compared to the conventional steel (NAK 80) as shown in FIG. The machining power is low at the same processing speed, so the machinability is easy and the tool life is extended.

In addition, it is shown in Table 1 when comparing the surface roughness value (Ra) of the mirror lettuce.

TABLE 1

The reason for the numerical limitation of each component of the present invention as described above is that carbon cannot be avoided as a basic element of the forging tank, and when quenching is over 0.05%, the hardening is so high that the hardness value is obtained at the time of heat treatment. When the content is 0.05-3.0% and the silicon content is 0.01% or less, steelmaking cannot be refined due to peroxidation of molten steel. Therefore, when the silicon is deoxidized and refined, it contains 0.1% or more, and when it is 1.0% or more, fluidity is obtained when manufacturing the ingot. As it is difficult to produce homogeneous products, and the hardness is too high due to solid solution hardening and hardening, the silicon is 0.10-1.00%, and when the steel is 0.2% or less, it is easy to cause oxidative reaction during steelmaking. Manganese requires 2.5% or less to obtain the proper hardening capacity. If it is higher than that, the vapor pressure of manganese is high, resulting in high volatility. Because funds adjustment is difficult also nickel is because the need to create the aluminum and precipitates Ni ₃ Al to obtain an appropriate age hardening effect, and if the aluminum because it is the minimum of 0.5% nickel, and requires at least 1.5% lower than 7.0%, austenite stabilizing zone It becomes harder to obtain hardness value (HRC40-48) due to weakened hardenability during heat treatment, and chromium is hardly obtained at 0.1% or less when used with ordinary scrap metal. This is because it has the same properties as chromium and increases the toughness of the material, so if it is less than 0.1%, the hardenability is low, and if it is 2.0% or more, the hardness is too high. If sulfur is more than 0.02%, harmful nonmetallic inclusions are generated, which impairs corrosion processability. .

Existing age-hardenable plastic injection mold steel contains 1-2% of copper, which is a standard steel impurity in other steel grades, and therefore requires dedicated furnaces and ladles, and it cannot be used with other steel grades. This problem is solved by replacing copper with other alloying elements.

Therefore, the present invention is a mold steel for producing a high precision, high-mirror mold, mechanical properties are very excellent and can be widely used in mass production through improved productivity.

Claims (4)

Carbon (C) 0.05-0.3%, Silicon (Si) 0.1-1.0%, Manganese (Mn) 0.2-2.5%, Nickel (Ni) 1.5-7.0%, Chromium (Cr) 0.1-3.0%, Molybdenum (Mo) 0.1-2.0%, aluminum (Al) 0.5-5.0%, the remainder is iron (Fe), 0.02% or less sulfur (S), 0.02% or less phosphorus (P), 0.01% or less nitrogen (N), A high-molecular plastic injection mold steel, characterized by steelmaking with a composition (ideal weight%) made of standard steel impurities such as 0.01% or less of oxygen (0) and 0.05% or less of hydrogen (H). The high injection-molding plastic injection mold steel according to claim 1, wherein the carbon content does not exceed 0.15% so that the aging hardenability by nickel (Ni) and aluminum (Al) is improved. The high-molecular plastic injection mold steel according to claim 1, wherein the carbon content is more than 0.15% so that the hardness increase due to aging hardening and the decrease in hardness due to tempering are mutually offset. Carbon (C) 0.05-0.3%, Silicon (Si) 0.1-1.0%, Manganese (Mn) 0.2-2.5%, Nickel (Ni) 1.5-7 0%, Chromium (Cr) 0.1-3.0%, Molybdenum (Mo ) 0.1-2.0%, aluminum (Al) 0.5-5.0%, the remainder is iron (Fe), 0.02% or less sulfur (S), 0.02% or less phosphorus (P), 0.01% or less nitrogen (N) , Steel (0.01) or less, oxygen (O), 0.05% or less, hydrogen (H), etc. (steel weight) of standard steel impurities, such as steel (Ni) and aluminum (Al) is based on iron (Fe) The precipitate was formed by aging treatment in a degassing process in a vacuum refining furnace of 0.5TORR or less to limit the hydrogen content to 1.0PPM or less, the final control of the alloying elements, and then to produce a steel ingot for 4 hours at 1250 ℃. After maintenance, forging ratio is 3S or more, 0.5-2 hours per inch of product thickness at 850-950 ℃, air-cooled after solution treatment, tempered at 500-600 ℃, and secondary solution treatment at 900-1000 ℃ after 25 ℃ Sedimentation in oil bath and 500-700 Gogyeongmyeon plastic injection mold Steel Heat treatment method according to claim that the aging treatment at.

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