JPH11293385A - Steel for plastic molds with excellent machinability - Google Patents

Abstract

(57) [Problem] To provide a plastic molding die steel excellent in machinability. SOLUTION: C: 0.08 to 0.17 by weight%.
%, Si ≦ 0.60%, Mn: 0.50 to 1.20%,
Mo: 0.10 to 0.70%, Ni: 2.50 to 3.5
0%, Al: 0.85 to 1.50%, Cu: 1.80 to
2.80%, S: 0.05 to 0.15%,
Ca is Ca (wt%) / S (wt%) 0.005 to
0.055 or Zr (Zr (% by weight) / S (% by weight)) of 0.50 to 2.00, with the balance being substantially Fe for plastic molding die steel. Preferably, Cr <0.50% or V <0.03%
And

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mold steel used for a plastic molding die, which has excellent machinability.

[0002]

2. Description of the Related Art In recent years, it has been emphasized in the die industry that processing costs for die manufacturing are important, and steel for plastic forming die, which is one of the die, requires further improvement in machinability. Have been. The machinability required for steel materials is
It also refers to "excellent workability by cutting" and "life of the tool used for the cutting". In other words, in a plastic molding die requiring particularly excellent mirror finish, it is important to improve the mirror workability of the steel for the mold, and furthermore, the life of the cutting tool used for the cutting is also increased. It is required to have characteristics that can be improved.

[0003] As a plastic molding die steel related to improvement of machinability, for example, there is a sulfur free-cutting steel which has been conventionally proposed. This is an improvement in machinability by dispersing MnS inclusions in steel, and is effective as a technique for achieving improved machinability at low cost. And, in the MnS inclusions dispersed in the steel,
(1) Reducing the length / width ratio (hereinafter referred to as aspect ratio), (2) Dispersing uniformly in steel, (3)
The fact that the material is not extremely coarse is effective for further improving the machinability, and for that purpose, controlling the form of MnS inclusions is an important issue.

[0004] In particular, regarding the aspect ratio, Mn
Since S inclusions have high viscosity at high temperatures, for example, in the case of a steel material subjected to hot working, since MnS inclusions are stretched in the working direction, in addition to causing the above-described deterioration in machinability, In addition, this may cause anisotropy in mechanical properties.

[0005]

SUMMARY OF THE INVENTION
As a method of controlling the form of the nS inclusions, for example, Japanese Patent Application Laid-Open No. 62-149911 proposes a method of improving machinability by adding Zr or Ca. However, when it comes to morphological control of sulfides, it is only the improvement of the anisotropy of the mechanical properties described above. In addition, as a method of improving machinability, for example, JP-A-4-13848 and JP-A-6-279922 disclose the addition of S and T with addition of S.
Means for adding elements such as e and Bi have been proposed, but no consideration has been given to the complex addition with S, that is, the machinability involved in controlling the form of MnS inclusions.

[0006] As described above, there is much room for research on the form of MnS inclusions suitable for improving the machinability of sulfur free-cutting steel. In view of the above, an object of the present invention is to clarify effective conditions for improving machinability, and to provide a steel for a plastic molding die excellent in machinability.

[0007]

First, the present inventor paid attention to the form of MnS inclusions in the structure in order to improve the machinability of a sulfur free-cutting steel. Then, together with the preferred form control of the MnS inclusions, the element species and the content that are effective for having excellent machinability were clarified, and the present invention was reached.

That is, the steel for plastic molding dies of the present invention has a C content of 0.08 to 0.17% by weight,
≦ 0.60%, Mn: 0.50 to 1.20%, Mo:
0.10 to 0.70%, Ni: 2.50 to 3.50%,
Al: 0.85 to 1.50%, Cu: 1.80 to 2.8
0%, S: 0.05 to 0.15%, and Ca is 0.005 to 0.0 in Ca (% by weight) / S (% by weight).
50, with the balance substantially consisting of Fe.

Alternatively, in terms of% by weight, C: 0.08 to
0.17%, Si ≦ 0.60%, Mn: 0.50-1.
20%, Mo: 0.10 to 0.70%, Ni: 2.50
-3.50%, Al: 0.85-1.50%, Cu:
1.80 to 2.80%, S: 0.05 to 0.15%, Zr is 0.50 to 2.00 in terms of Zr (% by weight) / S (% by weight), and the balance is Is a steel for plastic molding dies substantially made of Fe, and in addition, Ca contained in the steel is 0.1% by Ca (% by weight) / S (% by weight).
It may be adjusted to 005 to 0.050.

Preferably, in weight%, Cr <0.50
% Or V <0.03%, it is possible to achieve more excellent machinability. Also, in weight%, (Mo
+ W / 2): It is also possible to partially replace Mo with W in the range of 0.10 to 0.70%.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, the most important feature of the present invention is to improve the free-cutting property by paying attention to the morphological control of MnS inclusions in a sulfur free-cutting steel. It is to clarify the element type and the content which are effective in having excellent machinability, together with the morphological control to preferable MnS inclusions.

First, MnS inclusions forming the basis of the present invention will be described. First, the present inventor examined elemental elements that are effective in controlling the preferred form of MnS inclusions in the sulfur free-cutting steel that is the subject of the present invention.
And Zr were found to have a remarkable effect. That is, in the sulfur free-cutting steel of the present invention, by adding a proper amount of Ca in combination with the amount of S contained therein, MnS inclusions are controlled to (Ca, Mn) S inclusions having high deformation resistance. As a result, it is possible to suppress the deterioration of machinability due to increasing the aspect ratio of inclusions.

More specifically, for example, in the case of a conventional sulfur free-cutting steel manufactured by a smelting method, the MnS inclusions contained therein tend to be segregated and distributed in a solidification step related to casting. As a result, it is considered that the machinability deteriorates due to the aggregation of the inclusions. However, if the (Ca, Mn) S inclusion controlled by the method of the present invention,
Since segregation generation such as MnS inclusions can be avoided, as a result, the machinability can be improved by making the inclusions uniform and fine.

That is, in the cutting process to the mold shape,
Uniform and fine inclusions reduce the shear stress as a source of stress concentration in the shear deformation part due to the cutting and smoothen the generation and connection of micro cracks, resulting in excellent machinability You can do it. Therefore, the above (Ca,
Mn) It is important to properly adjust the amount of Ca of the present invention in the morphological control to S inclusions with respect to the amount of S in steel. In the present invention, specifically, Ca (% by weight) / It is adjusted to 0.005 to 0.050 in S (% by weight).

Ca (% by weight) / S (% by weight) is 0.00
If it is less than 5, inclusions having a large aspect ratio increase, so that the effect of the present invention by adding Ca cannot be sufficiently obtained. When Ca (% by weight) / S (% by weight) exceeds 0.050, the generation of CaS in the steel becomes remarkable, which adversely affects the machinability. That is, if an excessive amount of Ca is added with respect to the amount of S, (Ca, Mn) S inclusions effective for improving the machinability become CaS and are generated in a large amount in the steel, and the mechanical properties ( Ductility, toughness, etc.). In addition, as a condition which can achieve the control of the inclusion form of the present invention more accurately, it is preferable to adjust the Ca content to 0.0003 to 0.0040%.

The effects of the addition of Ca of the present invention described above can also be obtained by adding Zr. In this case, it is important to control the form of the inclusions into (Zr, Mn) S inclusions, and the specific content in the present invention is Zr (% by weight) / S
(% By weight) is adjusted to 0.50 to 2.00. Preferably, the Zr content is adjusted to 0.10 to 0.15%.

In order to obtain the effect of the present invention by adding Zr more remarkably, it is effective to adjust the content of O or N in steel. That is, O and N present in the sulfur free-cutting steel are also easily combined with Zr in the steel to easily form an acid / nitride such as ZrO ₂ or ZrN. As a result, (Zr, Mn) An obstacle to the generation of S inclusions. Therefore, it is effective to reduce the O or N content in the steel, and specifically, O ≦ 20 ppm or N ≦ 150 ppm. More preferably, O ≦
10 ppm or N ≦ 100 ppm.

These effects of the present invention can be achieved by simultaneous addition of Ca and Zr by intervening Ca or Zr into MnS inclusions. In this case, with respect to the amount of S in the steel, Ca (% by weight) / S (% by weight) is adjusted to 0.005 to 0.050, and Zr is adjusted.
(% By weight) / S (% by weight) is adjusted to 0.50 to 2.00, and it is desirable to combine the above adjustments for Ca, Zr, or O and N contents in steel.

Here, the effects of the present invention will be illustrated. FIG. 1 is a 400-fold microstructure photograph showing the form of each inclusion in the steel for plastic molding dies (1) and the comparative steel (2) adjusted to the composition of the present invention. In addition, these samples are the first samples described in the following examples.
Sample No. in the table. After forging at a forging ratio of 5 to a steel ingot corresponding to 5 or 13 and adjusted to a predetermined component,
The hardness of the RC is adjusted, and it can correspond to the pre-hardened die steel.

FIG. 1 shows that in the steel for plastic molding die of the steel (1) of the present invention, the inclusions in its structure are finely and uniformly distributed. And it turns out that the aspect ratio of an inclusion is small compared with the comparative steel (2) which applied the forging ratio. As described above, the present invention can achieve a steel for plastic molding dies having excellent machinability as a pre-hardened steel.

Further, as a means for uniformly dispersing the MnS inclusions in the steel without agglomeration and enlargement, for example, stirring casting by a melting method may be applied. Regarding the method of stirring, for example, any method such as electromagnetic stirring, mechanical stirring, and chemical stirring is applied, and the fine and
It works in a good direction for homogenization.

Next, other elements constituting the present invention will be described. The steel for plastic molding dies of the present invention,
In addition to the addition of S and Mn, the addition of Ca or Zr achieves the fundamental effect. However, since these fundamental effects of the present invention are not affected to a large extent by the other constituent elements shown below, they must be mutually adjusted together with Ca and Zr in view of the adverse effects on MnS inclusions. Is important. The results are shown below.

C keeps the quenched structure of the steel for plastic molding dies of the present invention in the upper bainite structure excellent in machinability, and reduces Cu-Fe, Ni-Al intermetallic compounds and Mo and W carbides in tempering. It is an element for obtaining a substrate for causing precipitation hardening based on precipitation. If the content is too large, the matrix will have a martensitic structure and excessive carbides will be formed, resulting in deterioration of machinability. Further, the electric discharge machining property is deteriorated, that is, the mirror finish is reduced, and the increase in electric discharge hardening causes an increase in the number of polishing steps after electric discharge machining. Therefore, the upper limit is set to 0.17%. If C is too low, precipitation of ferrite is caused, and sufficient tempering (aging) hardness cannot be obtained. Therefore, the lower limit is set to 0.08 in consideration of the relationship with Al and other alloy compositions of the present invention. %
And

The content of Si is adjusted according to its purpose and application in order to increase the corrosion resistance to the atmosphere when used as a plastic molding die. If it is too large, ferrite is formed, so the content is made 0.60% or less.

Mn is an element that enhances hardenability, adjusts the bainite hardness in accordance with the required hardness level, and suppresses the formation of ferrite. Is an important element that generates MnS inclusions. To obtain the effect, the content of S described later and the content of 0.50% or more are required. However, if the content is too large, the machinability and toughness are rather deteriorated. And

Mo is an element which secures strength by precipitating fine carbides by tempering (aging) in the steel for a plastic molding die of the present invention. It also has the effect of increasing the corrosion resistance to the atmosphere during use as a plastic mold. Therefore, in the present invention, in order to obtain these effects, the content is set to 0.10% or more. However, if the amount is too large, the machinability deteriorates, so the upper limit is set to 0.70%.

Ni is one of the most important elements in constituting the steel for a plastic molding die of the present invention. Ni of the present invention enhances bainite hardenability, suppresses the formation of ferrite, and precipitates Ni-Al intermetallic compounds during tempering (aging). As a result, excellent high hardness can be obtained, and at the same time, ductility can be appropriately reduced and excellent machinability can be achieved. If the content is too large, the bainite transformation temperature is lowered, the bainite structure is excessively refined, and the viscosity of the matrix is increased to reduce the machinability. Therefore, the upper limit is set to 3.50%. On the other hand, if it is too low, the above effect cannot be obtained, so the lower limit is made 2.50%.

Al is one of the most important elements for constituting the steel for plastic molds of the present invention. That is, Al raises the bainite transformation temperature to make it easier to obtain a desirable upper bainite, and also causes precipitation (aging) hardening due to fine precipitation of the Ni-Al intermetallic compound by tempering (aging), so that a high hardness level is obtained. And excellent machinability can be achieved. Further, Al also has an effect of improving the nitriding hardness when a nitriding treatment is performed.
Therefore, in the present invention, the lower limit is set to 0.85% as a content for sufficiently obtaining the above effects. However, if the content is too large, the tendency to form alumina-based inclusions increases, which lowers manufacturability and lowers mirror finish and corrosion resistance. Therefore, the upper limit is set to 1.50%.

Cu is one of the most important elements for constituting the steel for plastic molds of the present invention. That is, in the tempering (aging) treatment, Cu becomes Fe—C
Since the precipitation (aging) hardening is caused by the fine precipitation of the u intermetallic compound, a plastic molding die having a high hardness of 43 HRC or more can be achieved in combination with the upper bainite base and the precipitation of the Ni-Al intermetallic compound. . Also,
Since the addition of Cu is also effective in achieving excellent corrosion resistance, in order to achieve the above effects, the content is made 1.80% or more. However, when the content is too large, the hot workability is reduced, and the machinability is rather deteriorated in order to make bainite finer, so the upper limit is 2.8.
0%.

S is indispensable as an important element for forming MnS inclusions which are the basis for achieving the machinability of the present invention together with Mn. In order to obtain the effect of improving machinability according to the present invention, the content of Mn is required to be 0.50% or more, and the content is required to be 0.05% or more. However, excessive content causes deterioration of hot workability and toughness, so the upper limit is 0.15%,
Preferably, it is 0.10%.

Further, in the steel for a plastic molding die according to the present invention, adjustment of the Cr content is effective for further improving the machinability. If the Cr content is too large, the bainite structure is refined, and the content of the Cr is 0.50% in a plastic molding die steel aiming particularly at high machinability.
It is more preferably less than 0.20%. The addition of Cr increases the corrosion resistance of the plastic molding die and increases the hardness when subjected to nitriding. Therefore, depending on the purpose or application, 0.50%
It is possible to contain less than.

The inclusion of V in the plastic molding die steel of the present invention can form a coarse carbide in addition to the formation of ferrite, so that it is preferable in terms of machinability, particularly in maintaining excellent mirror workability. Is preferably V <0.03%, more preferably V ≦ 0.02%.
In particular, 2.50% or more of Ni or 0.85% or more of Al,
And, in the present invention of a high alloy type containing 1.80% or more of Cu, reduction of V is important in reducing the inhibition of the machinability improvement effect by these elements.

Although W is an element effective for obtaining the same effect as Mo, its content is effective for improving the softening resistance of a plastic molding die. In this case, it is possible to achieve a sufficient effect with the plastic molding die steel of the present invention, not only when W is added alone, but also when combined with Mo, which is particularly effective for achieving high hardness. ,
The content of W is converted into W / 2 to replace a part of Mo. That is, preferably, (Mo + W /
In the case of the plastic molding die steel satisfying 0.10 to 0.70% in the value of 2) and not containing Mo, W
What is necessary is just to adjust content to 0.20-1.40%.

[0034]

EXAMPLE First, a steel ingot adjusted to a predetermined chemical composition was melted and hot forged at a forging ratio of 5. After quenching the hot forged material,
A tempering treatment at 520 ° C. was performed to adjust the hardness to 40 HRC, and this was used as an evaluation sample. Table 1 shows the chemical composition of each sample.
The sample No. No. 2 is obtained by mechanical stirring during casting.

[0035]

[Table 1]

Evaluation of physical properties and characteristics of each evaluation sample is as follows. First, the degree of dispersion and the average aspect ratio of inclusions distributed in the structure of each sample were measured by image analysis. The average aspect ratio was set to an actual measurement area of 4.7 mm ² at the measurement site, and the degree of dispersion was evaluated by the degree of dispersion of the divided regions by the Voronoi division method. In other words, the smaller the degree of dispersion, the more uniformly the inclusions in the structure are dispersed. In this example, in Comparative Example No. to which Ca and Zr were not added. Assuming that the dispersion degree of Comparative Sample No. 13 is 1, this Comparative Sample No. It is shown by the ratio of dispersion to 13.

Regarding the machinability, the degree of wear of the cutting tool was evaluated by measuring the flank wear width in accordance with JIS B0170, and the surface finish after the cutting was evaluated. Table 2 shows the measurement results.

[0038]

[Table 2]

From Table 2, it can be seen that Sample No. 1-12
Is a comparative sample No. It can be seen that the inclusions are finely and uniformly dispersed, the aspect ratio is small, and the machinability is excellent as compared with 13 to 15. Among them, the sample No. satisfying the preferred conditions of the present invention. 1, 3, 5, 11,
In particular, the sample No. 11, in addition to good surface finish,
The flank wear width is small and excellent machinability is achieved. Sample No. to which agitated casting was applied in the melting process. In the sample No. 2, the aspect ratio of the inclusion was sample No. 2. Equivalent to 8, but with excellent dispersion, the flank wear width is small and machinability is improved.

Comparative sample No. 14 and 15 are the Ca / S of the present invention due to the small content of Ca or Zr.
Or below the Zr / S ratio. In other words, in this case, since the control of the form of the inclusions of the present invention is not perfect, the characteristics required for the inclusions are not satisfied, and the flank wear width of Comparative Sample No. 2 is not satisfied. 13 or so.

[0041]

The steel for a plastic molding die of the present invention described above can satisfy not only excellent machinability but also various properties such as strength required for the plastic molding die. For example, if the steel for a plastic molding die of the present invention is supplied in a pre-hardened state of 37 HRC or more (after quenching and tempering at 480 ° C. or more), it is subjected to engraving as it is, followed by polishing, further mirror finishing or embossing. It can be used after processing.
Therefore, in addition to extending the life of the cutting tool, it is possible to improve productivity with an increase in the cutting rate, and achieve the production of plastic molds with excellent finishability and strength while achieving significant cost reduction. Because it can be done, its industrial value is great.

[Brief description of the drawings]

FIG. 1 is a metal microstructure photograph showing the form of inclusions in steel for plastic molding dies according to the present invention and a comparative example.

Continuing from the front page (72) Inventor Yoshiyuki Murakawa 2107-2 Yasugi-cho, Yasugi-shi, Shimane Pref. Metallurgy Research Laboratory, Hitachi Metals, Ltd. (72) Inventor Fumio Toyama 2107-2 Yasugi-cho, Yasugi-shi, Shimane Pref. Hitachi Metals Yasugi Factory

Claims (6)

[Claims] 1. C: 0.08 to 0.17% by weight
%, Si ≦ 0.60%, Mn: 0.50 to 1.20%,
Mo: 0.10 to 0.70%, Ni: 2.50 to 3.5
0%, Al: 0.85 to 1.50%, Cu: 1.80 to
2.80%, S: 0.05 to 0.15%,
Ca is Ca (wt%) / S (wt%) 0.005 to
0.050, the balance being substantially made of Fe, characterized by excellent machinability for plastic molding die steel. 2. C: 0.08 to 0.17% by weight
%, Si ≦ 0.60%, Mn: 0.50 to 1.20%,
Mo: 0.10 to 0.70%, Ni: 2.50 to 3.5
0%, Al: 0.85 to 1.50%, Cu: 1.80 to
2.80%, S: 0.05 to 0.15%,
Zr is 0.50 in Zr (% by weight) / S (% by weight).
2.00, and the balance is substantially made of Fe. 3. C: 0.08 to 0.17% by weight
%, Si ≦ 0.60%, Mn: 0.50 to 1.20%,
Mo: 0.10 to 0.70%, Ni: 2.50 to 3.5
0%, Al: 0.85 to 1.50%, Cu: 1.80 to
2.80%, S: 0.05 to 0.15%,
Ca is Ca (wt%) / S (wt%) 0.005 to
0.055, Zr is 0.50 to 2.00 in terms of Zr (% by weight) / S (% by weight), and the balance is substantially Fe, and the plastic is excellent in machinability. Steel for forming dies. 4. The steel according to claim 1, wherein Cr is less than 0.50% by weight. 5. The steel according to claim 1, wherein V <0.03% by weight%. 6. (Mo + W / 2): 0.1% by weight
6. The plastic molding die steel having excellent machinability according to claim 1, wherein a part of Mo is replaced with W in the range of 0 to 0.70%.