JPH11335775A - Steel for metal mold for plastic molding, excellent in mirror-finish characteristics and machinability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for metal mold for plastic molding, excellent in mirror-finish characteristics as well as machinability such as rendering of long cutting tool life and high machining efficiency. SOLUTION: A steel has a composition containing, by weight, 0.05-0.20% C, <=1.0% Si, 0.5-2.5% Mn, 2.5-3.5% Ni, 0.5-2.5% Cu, and 0.5-1.5% Al as principal alloy components, also containing <=15 ppm 0 and <=150 ppm N, and having the balance Fe with inevitable impurities. Moreover, the size of non-metallic inclusions except sulfide contents is <=50 μm when estimated by the extreme value statistical method (ROOT AREA max). This steel is excellent in machinability as well as mirror-finish characteristics as shown in a graph in the figure.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mold steel for molding plastic parts and plastic products, and more particularly to a mold steel for injection molding of plastics, which further extends the life of a cutting tool and performs cutting. The present invention relates to steel for plastic molding dies which has excellent machinability such as high efficiency and also has excellent performance in mirror finish.

[0002]

2. Description of the Related Art Molds for molding plastic parts and plastic products are used to manufacture various parts and products ranging from automobile parts to office equipment parts, precision machine parts, electric parts, optical equipment parts, and the like. For this purpose, processing such as mirror finishing or embossing is added and used.

[0003] Above all, transparent plastic products such as optical lenses and syringes and other medical equipment are required to have extremely smooth surface. Therefore, the mold steel used for the molding is required to have a high mirror finish.

In recent years, there has been a tendency to produce various kinds and small quantities of steel for metal molds for plastic molding, and at the same time, as the ratio of mold production cost to the total cost has increased, simplification of mold production and cutting have been required. Strict conditions such as cost reduction and high precision due to extension of tool life have been required.
Therefore, more excellent machinability is required for mold steel.

Conventionally, various proposals have been made for plastic molding die steel, and a plastic molding pre-hardened die steel having a hardness of about HRC 40 used in a pre-hardened state by age hardening, 63-
183158, Japanese Patent Publication No. 5-12422).

However, the pre-hardened steel used in general dies includes non-metallic inclusions such as Al ₂ O ₃ which is a deoxidation product at the time of steel making and MnS which imparts free-cutting properties.
They drop off or are excavated during mirror polishing to generate pinholes, which adversely affect the mirror finish.

Therefore, in order to solve this problem, steel for plastic molding dies (Japanese Patent Application Laid-Open No. 63-162837) in which non-metallic inclusions are reduced, and formation of non-metallic inclusions are also proposed. HRC3
Plastic molded pre-hardened mold steel excellent in mirror finish and machinability reduced to about 0 (Japanese Patent Laid-Open No. 19754/1990)
No. 8).

[0008]

The above-mentioned JP-A-63-1
The conventional steels for plastic molding dies proposed in JP-A-83158 and JP-B-5-12422 are not yet sufficient in mirror finish and machinability, and further improvement in mirror finish and machinability is desired. There was a problem of solving these problems as an issue.
Further, a plastic melting mold steel proposed in Japanese Patent Application Laid-Open No. 63-162837 is a special melting method in which a raw material is melted in a vacuum and then remelted by an electroslag melting method, an electrobeam melting method, or the like. In this case, there is a problem that the production cost is increased.

In addition, the steel for plastic molding dies proposed in Japanese Patent Application Laid-Open No. 2-197548 needs to reduce the S and Al components in order to suppress the formation of non-metallic inclusions. , Graining processability decreases. Also, lowering the hardness promotes the removal of non-metallic inclusions and adversely affects the mirror finish and the abrasion resistance when using a mold.

The present invention has been made in view of the above-mentioned problems in the prior art, and an object of the present invention is to provide a plastic molding die steel excellent in mirror finish and machinability.

[0011]

According to the present invention, as a result of detailed studies of the above problems in the prior art by the present inventors, the oxygen content and the nitrogen content in the steel for plastic molding dies are expressed by weight ratio. , O: 15 ppm or less,
N: adjusted to 150 ppm or less, and the size of nonmetallic inclusions excluding sulfide inclusions was determined by the extreme value statistical method (route AREAm
ax), characterized in that it is not more than 50 μm when estimated, whereby a plastic molding die steel excellent in mirror finish and machinability can be obtained.
As disclosed in Japanese Patent Application Laid-Open No. 3-162837, a special melting method in which a raw material is melted in vacuum and then redissolved by an electroslag melting method, an electrobeam melting method, or the like is not employed. 2-197548
As described in the above publication, excellent mirror surface finish can be achieved without reducing components that affect the machinability and grain formability, such as S and Al components, in order to suppress the generation of nonmetallic inclusions. The present invention has been found to have excellent machinability as well as to have the present invention.

According to the first aspect of the present invention, there is provided a plastic mold steel excellent in mirror finish and machinability according to the present invention for solving the above-mentioned problems.
0.20%, Si: 1.0% or less, Mn: 0.5-2.
5%, Ni: 2.5 to 3.5%, Cu: 0.5 to 2.5
%, Al: 0.5 to 1.5%, O: 15 ppm or less,
N: 150 ppm or less, the balance consisting of Fe and unavoidable impurities, and the size of nonmetallic inclusions excluding sulfide inclusions is 50 μm or less when estimated by the extreme value statistical method (route AREAmax). This is a steel for plastic molding dies which is excellent in characteristic mirror finish and machinability.

According to the second aspect of the present invention, C:
0.05 to 0.20%, Si: 1.0% or less, Mn:
0.5 to 2.5%, Ni: 2.5 to 3.5%, Cu:
0.5-2.5%, Al: 0.5-1.5%, S: 0.
01 to 0.30%, O: 15 ppm or less, N: 150p
pm or less, the balance consisting of Fe and unavoidable impurities, and the size of nonmetallic inclusions excluding sulfide inclusions was 5 when estimated by the extreme value statistical method (route AREAmax).
It is a steel for plastic molding dies excellent in mirror finish and machinability, characterized in that the thickness is 0 μm or less.

According to the third aspect of the present invention, C:
0.05 to 0.20%, Si: 1.0% or less, Mn:
0.5 to 2.5%, Ni: 2.5 to 3.5%, Cu:
0.5 to 2.5%, Al: 0.5 to 1.5%, O: 15
ppm or less, N: 150 ppm or less, and C
r: 0.05 to 2.5%, Mo: 0.1 to 0.4%,
W: The content of at least one selected from 0.5% or less, the balance being Fe and unavoidable impurities, and the size of nonmetallic inclusions excluding sulfide inclusions is determined by the extreme value statistical method (route AREAmax). 50 μm when estimated by
The present invention is a steel for a plastic molding die excellent in mirror finish and machinability, characterized in that:

According to a fourth aspect of the present invention, C:
0.05 to 0.20%, Si: 1.0% or less, Mn:
0.5 to 2.5%, Ni: 2.5 to 3.5%, Cu:
0.5 to 2.5%, Al: 0.5 to 1.5%, O: 15
ppm, N: 150 ppm or less, V: 0.5% or less, Ti: 0.5% or less, Nb: 0.5
% At least one selected from the group consisting of
And the size of nonmetallic inclusions, excluding sulfide-based inclusions, is determined by the extreme value statistical method (route AREA
max), which is not more than 50 μm when estimated by plastics and is excellent in mirror finish and machinability.

According to a fifth aspect of the present invention, C:
0.05 to 0.20%, Si: 1.0% or less, Mn:
0.5 to 2.5%, Ni: 2.5 to 3.5%, Cu:
0.5-2.5%, Al: 0.5-1.5%, S: 0.
01 to 0.30%, O: 15 ppm or less, N: 150p
pm or less, Cr: 0.05-2.5%,
Mo: at least one selected from 0.1 to 0.4% and W: 0.5% or less, the size of nonmetallic inclusions excluding sulfide-based inclusions, the balance being Fe and unavoidable impurities Is 50 μm or less when estimated by the extreme value statistical method (route AREAmax), and is a plastic molding die steel excellent in mirror finish and machinability.

According to the sixth aspect of the present invention, C:
0.05 to 0.20%, Si: 1.0% or less, Mn:
0.5 to 2.5%, Ni: 2.5 to 3.5%, Cu:
0.5-2.5%, Al: 0.5-1.5%, S: 0.
01 to 0.30%, O: 15 ppm or less, N: 150p
pm or less, V: 0.5% or less, Ti:
0.5% or less, Nb: contains at least one selected from 0.5% or less, the balance of Fe and unavoidable impurities, the size of nonmetallic inclusions excluding sulfide inclusions is:
It is a steel for plastic molding dies excellent in specularity and machinability, characterized in that it is 50 μm or less when estimated by an extreme value statistical method (route AREAmax).

According to the seventh aspect of the present invention, C:
0.05 to 0.20%, Si: 1.0% or less, Mn:
0.5 to 2.5%, Ni: 2.5 to 3.5%, Cu:
0.5 to 2.5%, Al: 0.5 to 1.5%, O: 15
ppm or less, N: 150 ppm or less, and C
r: 0.05 to 2.5%, Mo: 0.1 to 0.4%,
W: at least one selected from 0.5% or less, V: 0.5% or less, Ti: 0.5% or less, N
b: containing at least one selected from 0.5% or less, the balance being Fe and unavoidable impurities, and the size of nonmetallic inclusions excluding sulfide inclusions is determined by the extreme value statistical method (route AREAmax) 50 μm when estimated by
The present invention is a steel for a plastic molding die excellent in mirror finish and machinability, characterized in that:

According to the eighth aspect of the present invention, C:
0.05 to 0.20%, Si: 1.0% or less, Mn:
0.5 to 2.5%, Ni: 2.5 to 3.5%, Cu:
0.5-2.5%, Al: 0.5-1.5%, S: 0.
01 to 0.30%, O: 15 ppm or less, N: 150p
pm or less, Cr: 0.05-2.5%,
Mo: at least one selected from 0.1 to 0.4%, W: 0.5% or less, and V: 0.5% or less;
Ti: contains at least one selected from 0.5% or less and Nb: 0.5% or less, and the balance of Fe and unavoidable impurities, the size of nonmetallic inclusions excluding sulfide-based inclusions is: It is a steel for plastic molding dies excellent in specularity and machinability, characterized in that it is 50 μm or less when estimated by an extreme value statistical method (route AREAmax).

Next, the reasons for limiting the composition range of the chemical components in the steel for plastic molding dies having excellent mirror finish and machinability according to the present invention will be described.

C: 0.05 to 0.20% by weight
C is a quenched structure obtained by performing a solution treatment on the invention steel,
It is an effective component for maintaining precipitation in a massive upper bainite structure with good machinability and effecting precipitation hardening due to precipitation of carbide when aging treatment is performed. Therefore, in order to obtain the effect effectively, the lower limit is 0.05% by weight. However, if too much is added, the base becomes martensitic,
It reduces machinability and also produces excessive carbides, further reducing machinability. In addition, since the toughness after age hardening is reduced, the upper limit is 0.20% by weight.

Si: 1.0% by weight or less, Si is an indispensable element as a deoxidizing agent at the time of melting, but if too much, the toughness after age hardening is reduced. Upper limit.

Mn: For 0.5 to 2.5% by weight, M
n is added in order to secure hardenability at the time of performing deoxidation and solution treatment, but in order to obtain the effect effectively, 0.1 is added.
The lower limit is 5% by weight. However, if added in an excessively large amount, the toughness decreases, so the upper limit is 2.5% by weight.

Ni: 2.5 to 3.5% by weight of N
i enhances bainite hardenability, suppresses the formation of ferrite, precipitates a Ni-Al intermetallic compound during aging treatment, obtains high hardness, and further improves the grain formability required for a plastic mold. It is an effective ingredient for. Therefore, in order to obtain the effect effectively, the lower limit is set to 2.5% by weight. However, if too much is added,
Since the bainite transformation temperature is lowered to make the bainite structure excessively fine and the machinability deteriorates, the upper limit is 3.5% by weight.

Cu: 0.5-2.5% by weight, C
u is a component effective for causing precipitation hardening due to fine precipitation, and is a component effective for improving machinability in combination with the massive upper bainite base. Therefore, in order to obtain the effect effectively, the lower limit is 0.5% by weight.
However, if added in an excessively large amount, the hot workability is reduced and the machinability is also reduced. Therefore, the upper limit is 2.5% by weight.

Al: 0.5 to 1.5% by weight of A
l raises the bainite transformation temperature, facilitates obtaining a desirable massive upper bainite structure, and reduces the Ni-Al
It is an effective component for precipitating the intermetallic compound of the system to obtain high hardness and further improving the grain formability required for the plastic mold. Therefore, in order to obtain the effect effectively, the lower limit is 0.5% by weight. However, when added in an excessively large amount, alumina-based inclusions (Al ₂ O ₃ ) are excessively generated, which adversely affects mirror finish and machinability, so the upper limit is 1.5 wt%.

The free-cutting property, which is an important property as steel for plastic molding dies, is achieved by O and N regulations described later, but S may be added at a lower limit of 0.01% by weight. When importance is attached to machinability, it is preferable to add 0.03% by weight or more. On the other hand, when a large amount of S is added, coarse sulfide-based inclusions are formed, which impairs toughness and causes pitting and excessive pits. Therefore, the upper limit is 0.30% by weight. I do.

Regarding the reasons for limiting the components of O and N, in the plastic mold steel according to the present invention, which is excellent in mirror finish and machinability, 0:15 ppm by weight of the O content at the time of melting. It is necessary to: The reason why the O amount is set to 15 ppm or less is that if the O amount is further increased, the mirror finish and machinability are reduced. The amount of N is a weight ratio, N: 150
ppm or less. Here, the N amount is 15
The reason for setting the content to 0 ppm or less is that N forms nitride with Al,
Austenite grains are refined and a uniform structure is obtained, but when added excessively, hard coarse nitrides are generated,
This is because the machinability and the mirror workability are reduced. This affects the specularity of the steel shown in FIG.
It is clear from the graph of the relationship between the amount and the amount of N.

The reasons for limiting the components of Cr, Mo and W are as follows: Cr: 0.05 to 2.5% by weight, Mo: 0.1 to
0.4% by weight, W: 0.5% by weight or less is due to C
Since r, Mo, and W are effective components for improving the hardenability of the mold during the solution treatment and improving the hardness and toughness, one or more of these alloy components are used. May be contained. Since Cr and Mo are particularly effective components for improving corrosion resistance, the lower limit of Cr is 0.05% by weight and the lower limit of Mo is 0.1% by weight in order to make the effect effective. I do. However, if too much is added, excessive carbides are precipitated and the toughness is reduced.
Cr is 2.5% by weight, Mo is 0.4% by weight, W is 0.5% by weight.
The upper limit is wt%.

Regarding the reasons for limiting the components of V, Ti and Nb, V: 0.5% by weight or less, Ti: 0.5% by weight or less,
Nb: 0.5% by weight or less means that V, Ti, and Nb have an effect of improving crystal toughness and improving toughness.
If a large amount is added, the solution hardening becomes unnecessarily high, the toughness is reduced, and the machinability is also reduced.

The plastic mold steel having excellent mirror finish and machinability according to the present invention is subjected to solution treatment at 800 to 900 ° C. after rolling or forging.
Age hardening treatment is performed at 0 to 600 ° C, and the steel is provided as a pre-hardened plastic molding die steel utilizing age hardening.

Similarly, in plastic mold steel having excellent mirror finish and machinability according to the present invention, nonmetallic inclusions other than sulfide inclusions were estimated by the extreme value statistical method (route AREAmax). The maximum non-metallic inclusion size at that time must be 50 μm or less. Here, the reason why the size of the nonmetallic inclusions is set to 50 μm or less is that when the size of the nonmetallic inclusions is larger than this,
This is because the mirror finish is reduced.

[0033]

BEST MODE FOR CARRYING OUT THE INVENTION The steel of the present invention may be manufactured in the same manner as the steel normally manufactured. For example, a steel ingot smelted in an electric furnace is rolled or forged to a desired shape. After that, it is subjected to solution treatment and age hardening treatment and used as pre-hardened steel. Embodiments of the present invention are shown in the following examples.

[0034]

EXAMPLE A steel for plastic molding dies having the chemical composition shown in Table 1 is melted by a usual melting method to prepare a test material. Here, invention steel 1 satisfies claim 1, invention steel 2 satisfies claim 2, invention steel 3 satisfies claim 3, invention steel 4 satisfies claim 4, and invention steel 5 Is claim 5
The invention steel 6 satisfies claim 6, the invention steel 7 satisfies claim 7, and the invention steel 8 is a steel for a plastic molding die that satisfies claim 8. On the other hand, the comparative steels 1 to 8 are plastic molding die steels which do not satisfy the claims 1 to 8, respectively.

[0035]

[Table 1]

In Table 1, * 1: "-" in the component value indicates an unavoidable impurity. * 2: O and N are different from the claims, but the other components are the same as those of Invention Steel 1. * 3: O and N are different from the claims, but the other components are the same as those of Invention Steel 2. * 4: O and N are different from those in the claims, but the other components are equivalent to those of Invention Steel 3. * 5: O and N are different from the claims, but the other components are equivalent to those of Invention Steel 4. * 6: O and N are different from the claims, but the other components are the same as those of Invention Steel 5. * 7: O and N are different from the claims, but the other components are the same as those of Invention Steel 6. * 8: O and N are different from the claims, but the other components are the same as those of Invention Steel 7. * 9: Ni, Al, O, and N are different from those in the claims, but the other components are equivalent to those of the invention steel 8.

Subsequently, for the test materials shown in Table 1,
Casting was carried out, and thereafter, the solution was heated to 850 ° C. to perform a solution treatment, subsequently heated to 500 ° C., and then subjected to an age hardening treatment to obtain a pre-hardened plastic molding die steel.

In the pre-hardened plastic molding die steel treated under the above heat treatment conditions, the hardness after heat treatment and nonmetallic inclusions (AlN, Al ₂ O ₃ ) estimated by the extreme value statistical method (route AREAmax) The size, mirror finish, machinability and grain workability were evaluated. Table 2 summarizes the results. Regarding the mirror finish, in this test material, the mirror finish was very good: 、, the mirror finish was good: 、, the mirror finish was slightly inferior: △, the mirror finish Good finish:
X, and each test material was evaluated.

[0039]

[Table 2]

For the machinability, an end mill workability test was performed on each test material under the conditions shown in Table 3, and the end mill workability test was evaluated based on the cutting length (m) up to the breakage of the tool. Was.

Regarding the grain workability, for each test material,
Graining was performed under the same conditions, and as a result, unevenness was observed: unevenness was present, and if no unevenness was observed: no unevenness was evaluated.

[0042]

[Table 3]

The extreme value statistical method according to the present invention will be described. The extreme value statistical method is a method in which a plurality of test pieces are sampled from a certain population, and the maximum inclusions (in the present application, hard specimens) By measuring the size of non-metallic inclusions excluding sulfide-based inclusions that affect the mirror surface and machinability of the inclusions, and plotting it on the extreme probability paper,
It predicts the size of the largest inclusion (route AREAmax) present in a population or an arbitrary volume (area), and is used for evaluating inclusions in mass-produced materials in the same way as other inclusion evaluation methods. In particular, in the present supplied test material, the area of one visual field was set to 10 mm × 10 mm, and for each test material, inclusions were measured every 30 visual fields, and the predicted area was 3 mm.
0000 mm ² , the maximum inclusion size was estimated by the extreme value statistical method.

As is clear from the results shown in Table 2, the steel for plastic molding dies according to the present invention has a uniform hardness without any variation in hardness after heat treatment. As shown in Table 2, the invention steels 1 to 8 are clearly superior in mirror finish in the respective comparisons with the comparative steels 1 to 8 due to the influence of the amounts of O and N and the size of the nonmetallic inclusions. Admitted. Furthermore, from the results in Table 2, invention steels 1 to 8
Showed good results in the end mill workability test with respect to the comparative steel, and clearly showed O and N
Good results were found due to the effect of the amount and the size of the non-metallic inclusions. Also, from the results in Table 2, the invention steels 1 to
In any of Nos. 8 and 7, in terms of grain workability, the occurrence of grain unevenness was not observed and was good.

[0045]

As described above, the steel of the present invention has a
The amount of which is adjusted to 15 ppm or less by weight%,
And the N content is adjusted to 150 ppm or less and the size of nonmetallic inclusions (AlN, Al ₂ O ₃ ) is 50 μm or less. After the raw material is melted in vacuum, S, Al are used without adopting a special melting method in which the raw material is redissolved by an electroslag melting method, an electrobeam melting method, or the like.
It is possible to provide a steel for plastic molding dies having excellent mirror finish and excellent machinability without reducing components that affect machinability and grain workability such as components. It has a remarkably excellent effect that it becomes possible.

[Brief description of the drawings]

FIG. 1 is a graph showing the influence of the amounts of O and N in steel on the specularity evaluation of steel.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Nobuhiro Tsujii 3007 one-letter Nakajima character in Shima-ku, Himeji City, Hyogo Prefecture Inside Sanyo Special Steel Co., Ltd. Sanyo Special Steel Co., Ltd.

Claims (8)

[Claims] 1. C: 0.05 to 0.20 by weight
%, Si: 1.0% or less, Mn: 0.5 to 2.5%, N
i: 2.5 to 3.5%, Cu: 0.5 to 2.5%, A
l: 0.5 to 1.5%, O: 15 ppm or less, N: 15
0 ppm or less, the balance being Fe and unavoidable impurities, and the size of nonmetallic inclusions excluding sulfide inclusions is 50 μm or less when estimated by the extreme value statistical method (route AREAmax). Steel for plastic molds with excellent mirror finish and machinability. 2. C: 0.05 to 0.20 by weight
%, Si: 1.0% or less, Mn: 0.5 to 2.5%, N
i: 2.5 to 3.5%, Cu: 0.5 to 2.5%, A
l: 0.5 to 1.5%, S: 0.01 to 0.30%,
O: 15 ppm or less, N: 150 ppm or less, balance Fe
And the size of non-metallic inclusions, excluding sulfide-based inclusions, consisting of unavoidable impurities, was determined by the extreme value statistical method (route AREAm
(a) a steel for plastic molding dies having excellent specularity and machinability, which is estimated to be 50 μm or less when estimated by ax). 3. C: 0.05 to 0.20 by weight
%, Si: 1.0% or less, Mn: 0.5 to 2.5%, N
i: 2.5 to 3.5%, Cu: 0.5 to 2.5%, A
l: 0.5 to 1.5%, O: 15 ppm or less, N: 15
0 ppm or less, and Cr: 0.05 to 2.5
%, Mo: 0.1 to 0.4%, W: at least one selected from the group consisting of Fe and inevitable impurities, and excluding sulfide-based inclusions. Characterized by having a size of not more than 50 μm when estimated by an extreme value statistical method (route AREAmax), which is excellent in mirror finish and machinability. 4. C: 0.05 to 0.20 by weight
%, Si: 1.0% or less, Mn: 0.5 to 2.5%, N
i: 2.5 to 3.5%, Cu: 0.5 to 2.5%, A
l: 0.5 to 1.5%, O: 15 ppm or less, N: 15
0 ppm or less, V: 0.5% or less, T
i: contains at least one selected from 0.5% or less and Nb: 0.5% or less, and the balance of Fe and unavoidable impurities is non-metallic inclusions other than sulfide-based inclusions. A steel for plastic molding die excellent in mirror-like property and machinability, characterized in that it is 50 μm or less when estimated by a value statistical method (route AREAmax). 5. C: 0.05 to 0.20 by weight
%, Si: 1.0% or less, Mn: 0.5 to 2.5%, N
i: 2.5 to 3.5%, Cu: 0.5 to 2.5%, A
l: 0.5 to 1.5%, S: 0.01 to 0.30%,
O: contains 15 ppm or less, N: contains 150 ppm or less,
Further, Cr: 0.05 to 2.5%, Mo: 0.1 to 0.
4%, W: at least one selected from 0.5% or less, the balance consisting of Fe and unavoidable impurities, and the size of nonmetallic inclusions other than sulfide inclusions is determined by the extreme value statistical method (route AREAmax) 50
A steel for a plastic molding die having excellent mirror finish and machinability, characterized in that it is not more than μm. 6. C: 0.05 to 0.20 by weight
%, Si: 1.0% or less, Mn: 0.5 to 2.5%, N
i: 2.5 to 3.5%, Cu: 0.5 to 2.5%, A
l: 0.5 to 1.5%, S: 0.01 to 0.30%,
O: contains 15 ppm or less, N: contains 150 ppm or less,
Further, V: 0.5% or less, Ti: 0.5% or less, Nb:
The size of nonmetallic inclusions, including at least one selected from 0.5% or less and the balance being Fe and unavoidable impurities, excluding sulfide inclusions, is determined by the extreme value statistical method (route AR
EAmax) is 50 μm or less when estimated by EAmax). 7. C: 0.05 to 0.20 by weight
%, Si: 1.0% or less, Mn: 0.5 to 2.5%, N
i: 2.5 to 3.5%, Cu: 0.5 to 2.5%, A
l: 0.5 to 1.5%, O: 15 ppm or less, N: 15
0 ppm or less, and Cr: 0.05 to 2.5
%, Mo: 0.1 to 0.4%, W: at least one selected from 0.5% or less, V: 0.5% or less, Ti: 0.5% or less, Nb: 0 The size of non-metallic inclusions containing at least one selected from 0.5% or less, the balance being Fe and unavoidable impurities, and excluding sulfide inclusions was estimated by the extreme value statistical method (route AREAmax). A steel for plastic molding dies having excellent mirror finish and machinability, which is sometimes 50 μm or less. 8. C: 0.05 to 0.20 by weight
%, Si: 1.0% or less, Mn: 0.5 to 2.5%, N
i: 2.5 to 3.5%, Cu: 0.5 to 2.5%, A
l: 0.5 to 1.5%, S: 0.01 to 0.30%,
O: contains 15 ppm or less, N: contains 150 ppm or less,
Further, Cr: 0.05 to 2.5%, Mo: 0.1 to 0.
4%, W: at least one selected from 0.5% or less, further V: 0.5% or less, Ti: 0.5% or less, Nb: at least one selected from 0.5% or less And the balance of nonmetallic inclusions, excluding sulfide inclusions, consisting of Fe and unavoidable impurities, is 50 when estimated by the extreme value statistical method (route AREAmax).
A steel for a plastic molding die having excellent mirror finish and machinability, characterized in that it is not more than μm.