JP2000087178A - Prehardened steel for metal mold for plastic molding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new prehardened-type steel for metal mold for plastic molding, extremely excellent in mirror finish characteristics and machinability and also having superior toughness. SOLUTION: This steel has a composition which consists of, by weight, 0.07-0.16% C, <=60% Si, 0.60-1.50% Mn, 2.50-3.50% Ni, 0.30-1.40% Al, 1.80-2.50% Cu, and the balance Fe with impurities and in which the amounts of S, O, and N as impurities are regulated to <=0.004%, <=0.0015%, and <=0.0030%, respectively, or further, <=0.60% Cr and Mo and W, independently or in combination, in the amounts satisfying 1/2W+Mo = 0.10 to 0.70% are properly added to the steel for metal mold.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a new pre-hardened type steel for plastic molding dies having extremely excellent mirror finish and machinability and having excellent toughness.

[0002]

2. Description of the Related Art As steel for high-grade plastic molding dies, (1) good mirror finish and no generation of pinholes or other fine pits, (2) good grain workability, (3) corrosion resistance, Good rust resistance, (4) good strength and wear resistance, high toughness, and (5) good machinability are required. However, in recent years, measures to shorten the mold manufacturing period and to prevent breakage from thin corners of the mold due to the complicated shape of the mold have been strongly proposed as an important customer requirement, and molds that combine excellent machinability and toughness The development of steel for service became an issue.

Conventionally, as a mold material for this kind of application, Japanese Patent Publication No.
JP-A-23764 and JP-B-59-37744 disclose steels for plastic molding dies. This steel for molds has almost satisfactory characteristics in terms of mirror finish and machinability, but has insufficient points in terms of combining machinability and toughness, which are particularly strongly required in recent years. .
Japanese Unexamined Patent Publication (Kokai) No. 60-204869 discloses a mold steel having particularly excellent mirror finish, grain workability and toughness. However, the mold steel does not necessarily have sufficient properties in terms of machinability. Did not have. Further, Japanese Patent Application Laid-Open No. 63-11494
No. 2 provides a mold steel excellent in machinability, but the mold steel leaves room for improvement in terms of mirror finish (pit resistance) and rusting properties. .

[0004]

As described above, conventionally, increasing the machinability of a mold steel has led to a reduction in toughness, and both of these characteristics have been combined with an excellent mirror surface. It was not easy to achieve the finish. An object of the present invention is to provide a mold steel that simultaneously satisfies the various properties required for the high-grade plastic molding die steel.

[0005]

SUMMARY OF THE INVENTION The present invention provides, in weight percent,
C: 0.07 to 0.16%, Si: 0.60% or less, M
n: 0.60 to 1.50%, Ni: 2.50 to 3.50
%, Al: 0.30 to 1.40%, Cu: 1.80 to
2.50%, the balance being Fe and impurities, with S being 0.004% or less, O being 0.0015% or less,
N is 0.0030% or less, plastic molded pre-hardened steel for molds, or 0.60% or less of Cr and 1/2 W + Mo: 0 Mo or W alone or in combination with the steel for molds. This is a plastic molded prehardened mold steel to which .10 to 0.70% is appropriately added.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail. The steel of the present invention has low C-medium to low Mn-Ni-Mo.
(W) -an alloy having a high Cu-Al system as a basic component,
A uniform upper bainite structure is formed by quenching, and further, a high-temperature tempering of 500 ° C. or more is performed to form a Cu—Fe solid solution, Ni—Al intermetallic compound, Mo
(W) Precipitates carbides, imparts strength by aging (precipitation) hardening, causes moderate embrittlement, imparts good machinability, and moderates breakage from thin mold parts during use. This is to maintain toughness. Also,
By including Cr, Mo, W, Cu, and Ni, and by limiting the amount of S to a relatively small amount, excellent corrosion resistance and rust resistance can be obtained.
, Hardly turns into martensite, hardly hardens due to electric discharge machining, and finishing is easy in this respect as well.

The most fundamental matter of the present invention is to conversely limit the Cu content to a high value (1.80 to 2.50%) to combine excellent cutting performance and low cutting surface roughness, which were difficult in the past. Has been successful. In addition, by limiting O and N to be low, it is possible to suppress the generation of nonmetallic inclusions such as Al ₂ O ₃ or AlN which are harmful to the mirror surface finish and rust resistance, and to achieve the mirror surface finish and rust resistance. Is to improve the performance.

The steel of the present invention is supplied in a pre-hardened (tempered) state having a hardness of HRC 37 or higher (tempered at 400 ° C. or more after quenching), and is directly engraved, polished, and further mirror-finished. It is used after being subjected to a graining process. Therefore, it does not require heat treatment by the user, and even if it is processed into a complex mold with a thin part as a very good machinability and plastic mold, cracks may occur from the thin corners in use. In addition, the combination of high hardness, uniform matrix, and essentially excellent cleanliness of steel quality results in extremely excellent mirror finish and grain workability. , Is a new plastic molding die steel that enables long life.

Next, the reasons for limiting the components of the steel of the present invention will be described. C keeps the quenched structure of the steel of the present invention as a massive upper bainite structure with good machinability, and
-It is a basic additive element for providing a substrate for causing precipitation hardening based on precipitation of Fe, Ni-Al intermetallic compounds, and Mo and W carbides. If the content is too large, the matrix is martensite-structured to reduce machinability, and excessive carbides are formed to reduce machinability. Further, the adverse effects such as the hardening of the electric discharge machined surface and the increase in the number of polishing finishing steps are caused. Therefore, the content is set to 0.16% or less. Therefore, the content is set to 0.07% or more. Desirably, it is 0.1% or more.

Mn is added to enhance the bainite hardenability of the steel of the present invention, to suppress the formation of ferrite, and to provide an appropriate quenching and tempering (aging) hardness. If the content is too large, the bainite transformation temperature is lowered, the bainite structure is excessively refined, and the machinability is reduced by increasing the viscosity of the matrix. Therefore, the content is set to 1.50% or less. Therefore, it is set to 0.60% or more.

Si is added depending on the purpose and application in order to increase the corrosion resistance to the atmosphere during use. If the content is too large, ferrite is formed and the machinability is reduced. Ni enhances the bainite hardenability of the steel of the present invention, suppresses the formation of ferrite, and precipitates a Ni-Al intermetallic compound during tempering (aging) to obtain the required hardness and moderate ductility. To improve the machinability. If the amount is too large, the bainite transformation temperature is lowered, the bainite structure is excessively refined, and the machinability is reduced by increasing the viscosity of the matrix. Therefore, the content is set to 3.50% or less. Therefore, it is set to 2.50% or more.

[0012] Cr is added depending on the purpose and application in order to increase the corrosion resistance of the steel of the present invention, increase the hardness during nitriding, and suppress rusting during polishing or storage in a mold.
If the content is too large, the content is set to 0.60% or less in order to refine the bainite structure and maintain the characteristics of the steel of the present invention particularly aimed at high machinability. W and Mo are 500 ° C of the steel of the present invention.
In the high temperature tempering (aging) treatment, the precipitation of fine carbides, precipitation (aging) hardening, and the formation of the strength of the steel of the present invention, and the enhancement of corrosion resistance to the atmosphere during use, the purpose and application Is added by Generally, Mo alone is added, but depending on the purpose and application, Mo,
W composite addition or single addition is performed. In the case of this application, a large amount of addition is not required, and if it is too large, the machinability is lowered. Therefore, 1 / 2W + Mo: 0.70% or less, and if it is too low, the effect of the above addition cannot be obtained.
1/20 + Mo is set to 0.10% or more.

[0013] In the tempering (aging) treatment of the steel of the present invention, Cu causes precipitation (aging) hardening due to fine precipitation of the Fe-Cu intermetallic compound, and together with the massive upper bainite matrix, the extremely excellent coating of the steel of the present invention. It is one of the most important additional elements for forming the machinability and obtaining the required hardness, and also provides excellent corrosion resistance. If the content is too large, the hot workability is reduced, the bainite is refined, and the machinability is rather reduced.
If it is too low, the effect of the above-mentioned addition cannot be obtained, so it is made 1.80% or more.

Al raises the bainite transformation temperature to make it easier to obtain a desirable massive upper bainite structure, and causes precipitation (aging) hardening due to fine precipitation of Ni-Al intermetallic compound in the tempering (aging) treatment. Together with these, it is an important additive element for forming the extremely excellent machinability of the steel of the present invention and for obtaining the required hardness. Further, an effect of increasing the nitriding hardness at the time of nitriding is brought about. If it is too large, the ductility will be excessively reduced, and if it is too low, the effect of the addition will not be obtained, so that it is made 0.30% or more.

S forms sulfide-based inclusions and causes pitting and pits. To obtain the highest degree of mirror surface, S0.
004% or less. O is an element forming alumina-based inclusions harmful to mirror finish and rust resistance.
0015% or less. N is an element producing aluminum nitride (AlN) that is harmful to mirror finish and rust resistance, and is set to 0.0030% or less.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on embodiments. Table 1 shows examples of the steel of the present invention and chemical compositions of conventional steels. Conventional steel L is SCM440.

[0017]

[Table 1]

Table 2 shows that the conventional steel L (S) was heat-treated before and after HRC39 of the steel of the present invention, but was cut by an end mill.
The machinability (tool life) index based on CM440 (HRC35) and the maximum surface roughness ratio of the cut surface are shown.

[0019]

[Table 2]

[0020] The steel of the present invention is SCM440 (HRC35.
Despite having a higher hardness than 1), the machinability is clearly superior, and the cutting surface roughness is also extremely fine.
This indicates that the effect of reducing the number of steps in the subsequent polishing process is great. Table 3 shows the cross-sectional dimensions of the steel of the present invention of 50 mmt × 150 m.
After tempering the mw forged material before and after HRC38,
The test piece is parallel (L) and perpendicular (T) to the forging direction.
Direction and tensile test and Charpy impact test (2
(mm μ notch).

[0021]

[Table 3]

The steel of the present invention has a low elongation, a draw value and a Charpy impact value in a tensile test, as compared with the conventional steel J, which is particularly excellent in toughness, which facilitates the removal of chips by shearing during cutting. The machinability and the roughness of the cutting surface are superior to those of the conventional steel J, and the conventional steel L (SC
M440), the machinability is greatly improved.
The steels A, C and E of the present invention, which have a small amount of Al, do not have an extreme decrease in elongation, drawing and Charpy impact value, do not break even when used as a mold having a complicated shape or a thin portion, and have good machinability. The feature is that it has both.

On the other hand, in the case of the steel I of the present invention having a particularly high Al content, the machinability is good, but the elongation, the drawing value and the Charpy impact value in the tensile test, particularly those in the T direction, are low, and the complex shape is low. Or for molds with thin corners,
Defects or breakage may occur. Table 4 shows the results of the rust resistance test of the steel of the present invention in a salt water atmosphere. The sample is 25m
The number of rusts generated when the paper was buffed to a mirror surface of mx 25 mm and left in a salt water atmosphere for 2 hours after the mirror buffing was counted.

[0024]

[Table 4]

Since the steel of the present invention has a low S, O and N content, there are few sulfides, oxides and nitrides which cause the formation of corrosion pits. Have In the case of the comparative steel M having a relatively high S content, the sulfide-based inclusions increase and the rusting number ratio slightly increases. Table 5 compares the mirror finish (pit resistance) of the steel of the present invention. The sample is 50 mm square, and after quenching and tempering in HRC38, it is mirror-finished by grinder → paper → diamond compound polishing method.
The number of fine pits generated is counted using a double magnifier.

[0026]

[Table 5]

In the steel of the present invention, since S, O and N are made low, the amount of inclusions is small and the number of pits is none. Comparative steel M has a high S, sulfide-based inclusions slightly increased, and the number of pits increased.

[0028]

As described in detail above, the steel according to the present invention retains particularly good machinability even at a hardness of HRC 38 or more due to an appropriate combination of the matrix composition, the matrix structure, and the precipitated (aged) product. In addition, it has excellent mirror finish, pitting corrosion resistance, discharge hardening resistance, grain hardenability, and moderate ductility, so it has a thin corner even in mold applications that receive severe stress during use. The present invention provides a plastic molding pre-hardened (for example, HRC 36-41) mold material that does not cause breakage and provides excellent mold life, and is industrially very useful.

Claims (4)

[Claims] (1) C: 0.07 to 0.16% by weight
Si: 0.60% or less, Mn: 0.60 to 1.50%,
Ni: 2.50 to 3.50%, Al: 0.30 to 1.4
0%, Cu: 1.80 to 2.50%, balance Fe and impurities, S as impurities is 0.004% or less, O
Is 0.0015% or less and N is 0.0030% or less. 2. C: 0.07 to 0.16% by weight,
Si: 0.60% or less, Mn: 0.60 to 1.50%,
Ni: 2.50 to 3.50%, Cr: 0.60% or less,
Al: 0.30 to 1.40%, Cu: 1.80 to 2.5
0%, the balance being Fe and impurities.
Is 0.004% or less, O is 0.0015% or less, and N is 0.0030% or less. 3. C: 0.07 to 0.16% by weight,
Si: 0.60% or less, Mn: 0.60 to 1.50%,
Ni: 2.50 to 3.50%, Mo or W alone or in combination, 1 / 2W + Mo: 0.10 to 0.70%, Al:
0.30 to 1.40%, Cu: 1.80 to 2.50%,
The balance consists of Fe and impurities.
004% or less, O is 0.0015% or less, N is 0.00
A plastic molded prehardened mold steel having a content of 30% or less. 4. C: 0.07 to 0.16% by weight,
Si: 0.60% or less, Mn: 0.60 to 1.50%,
Ni: 2.50 to 3.50%, Cr: 0.60% or less,
Mo, W alone or in combination, 1 / 2W + Mo: 0.10
~ 0.70%, Al: 0.30 to 1.40%, Cu:
1.80 to 2.50%, the balance being Fe and impurities, S as impurities is 0.004% or less, and O is 0.00
Pre-hardened plastic molding steel, characterized in that the content of N is not more than 15% and the content of N is not more than 0.0030%.