JPH11158579A - Steel for plastic molding die - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel small in the difference in hardness between the surface layer part and the core part, free from the generation of welding cracks even if preheating and postheating are not executed and also good in machinability by composing a steel for a plastic molding die of specified weight % of C, Mn, Si, sol.Al, O, and the balance Fe with inevitable impurities. SOLUTION: A steel for a plastic molding die is composed of, by weight, 0.05 to 0.25% C, 0.25 to 2.0% Mn, 0.20 to 0.40% Si, 0.010 to 0.030% sol.Al, <=0.0020% 0, and the balance Fe with inevitable impurities. Furthermore, one or >= two kinds among 0.1 to 3.0% Cr, 0.5 to 1.0% Mo and 0.05 to 0.6% V are incorporated therein to increase its hardenability. Moreover, one or >=two kinds among 0.001 to 0.10% S, 0.03 to 0.2% Pb, 0.0005 to 0.01% Ca, 0.003 to 0.2% Zr, 0.01 to 0.15% Te and 0.01 to 0.20% Bi are incorporated therein to make its machinability excellent.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for a metal mold for plastic molding, and more particularly, to a plastic molding having a mold surface excellent in mirror finish, in which the occurrence of minute irregularities on the mold surface after mirror finishing is suppressed. The present invention relates to steel capable of providing a mold.

[0002]

2. Description of the Related Art The following performance is required for steel materials used for manufacturing plastic molding dies. That is, first, it has appropriate strength and toughness and has high hardness, and the difference in hardness between the surface layer portion and the core portion is small. The segregation is small, non-metallic inclusions are small, and the structure is uniform. When processed, the mold surface is finished to a mirror surface without generating fine irregularities (pinholes). It is.

[0003] Furthermore, even if the welding cracking sensitivity is low and the repair work is performed by, for example, overlay welding at the time of mold production,
The material must not cause welding cracks. Also, one of the necessary conditions is that the material has good machinability and can be used for making a mold smoothly. Conventionally, as a steel material for a plastic molding die, for example, S55C
, And medium-low carbon steels such as SCM445 are widely used.

However, since these steels are not necessarily designed as mold steels, they only partially satisfy the above-mentioned properties required for mold steels. For example, SCM445 does not have good weld cracking susceptibility. If build-up welding is performed for mold design change or repair at the time of mold production, welding is performed when build-up welding is performed without preheating or post-heating. It will crack. Therefore, in the case of this material, it is necessary to perform pre-heating and post-heating at the time of overlay welding, and it is difficult to say that the material is appropriate in terms of man-hours and thermal economy. .

As a steel specialized for use in a mold, there is, for example, a pre-hardened steel disclosed in Japanese Patent Application Laid-Open No. 61-34162. This steel is a material that does not cause quenching cracks even when subjected to water quenching and has a small difference in hardness within the steel material. It is hard to say that it is steel.

On the other hand, Japanese Unexamined Patent Publication No. Hei 3-177536 discloses a steel for a plastic molding die capable of overlay welding without causing a welding crack without performing preheating or post-heating. Japanese Unexamined Patent Publication No. 8-13088 discloses a steel for plastic molding dies, which is an improved material of the steel described in the above-mentioned Japanese Patent Application Laid-Open No. 3-177536 and has significantly improved machinability.

All of these steels are reported by Ito et al. In "Journal of the Japan Welding Society", 37, (1968), 9, and generally approved by the following formula: Pc (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B + H / 60 + t / 600 (where H is gas volume (cc) conversion, t is material thickness (mm), and others are elemental amounts (% by weight)) When the welding crack susceptibility index of the mold steel calculated based on the above is less than 0.3, it has been developed with reference to the knowledge that the preheating temperature for eliminating welding cracks can be lowered to around room temperature. It is a thing.

[0008] The design concept of these steels is that Si, which occupies a large specific gravity in the Pc value calculated by the above formula, is used.
Is characterized by restricting the content of as small as possible, thereby suppressing segregation during ingot making as much as possible and suppressing the occurrence of minute irregularities on the mold surface after mirror finishing. Further, in general, non-metallic inclusions such as Al ₂ O ₃ are generated during ingot formation, and the
Is designed based on the technical idea of reducing the content of as much as possible.

That is, the characteristics of these steels are as follows.
From the relationship of the values, it is best to set the Si content to 0.25% by weight or less and not to contain Al at the same time. Even if Al is contained as an impurity, the content of the steel is preferably controlled to 0.030% by weight or less to suppress generation of Al ₂ O ₃ inclusions as much as possible. .

[0010]

By the way, the above-mentioned steel is melted in a melting furnace, the molten steel is placed in a ladle, and then ingoted using a mold. After annealing, quenching and tempering are performed to produce a desired mold, and the mirror-finished mold surface has a size of 5 to 15 μm.
Some minute irregularities may be scattered in a pinhole shape. Therefore, the steel has appropriate hardness and good machinability, and it does not cause weld cracking during overlay welding without pre-heating and post-heating. May be present, and as a result, these minute irregularities will be transferred to the plastic molded product, which will be a factor of deteriorating the surface quality of the plastic molded product,
The solution is strongly required.

The present invention satisfies the above-mentioned requirements, and has low weld cracking susceptibility to the extent that no weld cracks occur even at room temperature. In addition, the occurrence of the fine irregularities is almost recognized on the mirror-finished mold surface. With the aim of providing no new plastic mold steel.

[0012]

Means for Solving the Problems In the course of intensive research to achieve the above object, the present inventors
The minute irregularities generated on the mold surface were analyzed. That is, the surface of the steel was mirror-finished, and the minute irregularities were observed under a microscope at a magnification of 3000 times. As a result, it was found that a large number of clusters were distributed in a homogeneous matrix in the microstructure of the uneven portion.

[0013] Therefore, EP and EP
MA analysis was performed to investigate each composition. As a result, the matrix contained Al ₂ O ₃ : about 63% by weight, MnO: about 26% by weight.
Weight percent of the non-metallic inclusions, and the clusters are composed of non-metallic inclusions whose main components are Al ₂ O ₃ : about 30% by weight, SiO ₂ : about 39% by weight, and MnO: about 31% by weight. Turned out to be a thing.

From this, the present inventors have obtained the knowledge that the fine irregularities are composed of nonmetallic inclusions containing MnO, SiO ₂ , and Al ₂ O ₃ as main components. The non-metallic inclusions were formed in the ingot, and it was determined that the hard non-metallic inclusions appeared on the mold surface and formed minute irregularities during mirror finishing. Next, the present inventors presumed that the formation of the nonmetallic inclusions in the steel ingot was clearly a result of insufficient deoxidation.

Therefore, the oxygen content of each ingot of the same steel type but different lots is analyzed, and the AS
The number of B + C inclusions was measured by the TA-A method. The result is shown in FIG. As is clear from FIG. 1, steel having a high oxygen content, that is, steel having insufficient deoxidation, generates large B + C-based inclusions. A large number of pinhole-shaped minute irregularities were observed on the mold surface of the steel. However, in the case of steel having a low oxygen content, the amount of B + C-based inclusions generated is small,
In fact, it has been found that the regulation of the oxygen content to 0.002% by weight or less can suppress the occurrence of minute irregularities.

By the way, it is considered that this non-metallic inclusion is formed into a steel ingot through the following process. That is, this steel has a reduced content of Si that exhibits a deoxidizing effect in order to reduce the susceptibility to welding cracks, and an Al alloy that also exhibits a deoxidizing effect in order to suppress the formation of Al ₂ O ₃ -based inclusions. Is reduced to zero or as low as possible. This causes insufficient deoxidation during melting in the melting furnace, and as a result, the refractory bricks forming the inner wall of the melting furnace and the ladle are eroded, and become foreign matters, which cause ingots to form ingots. Mixed in.

The present invention is a steel developed based on the findings and considerations described above. That is, the design concept is to make the composition capable of deoxidation so that the oxygen content of the obtained steel ingot at the time of melting the steel is 0.0020% by weight or less. For this purpose, the content of Si is increased to such an extent that the upper limit of the Pc value becomes 0.3, thereby exhibiting a deoxidizing effect. Based on the idea that it is effective to contain Al in order to exert its deoxidizing effect, and through various experiments, the steel for plastic molds of the present invention has been developed.

That is, the steel for a plastic molding die of the present invention has a C content of 0.05 to 0.25% by weight and a Mn of 0.25%.
To 2.0% by weight, Si: 0.20 to 0.40% by weight, so
1-Al: 0.010 to 0.030% by weight, O: 0.00
20% by weight or less, with the balance being Fe and unavoidable impurities. In the present invention, the composition described above is further added with Cr: 0.1 to 3.0% by weight and Mo: 0.1% by weight.
5% to 1.0% by weight, V: 0.05% to 0.6% by weight of one or more kinds of steel for plastic mold metal molds with enhanced hardenability, and further, S: 0.5% by weight. 001 to 0.10
% By weight, Pb: 0.03 to 0.2% by weight, Ca: 0.00
0.05 to 0.010% by weight, Zr: 0.003 to 0.2% by weight, Te: 0.01 to 0.15% by weight, Bi: 0.01 to
The present invention provides a plastic molding die steel excellent in machinability, containing one or more kinds of 0.20% by weight.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION In the above composition, C is a component for ensuring the hardness of steel, and its content is 0.0.
It is set to 5 to 0.25% by weight. If the content is less than 0.05% by weight, for example, when tempering at a temperature of 600 ° C. or more in order to remove heat treatment residual stress during overlay welding, sufficient hardness as a mold, for example, HB 163 or more, is obtained. Can not. On the other hand, if the content is more than 0.25% by weight, the Pc value increases, the weld cracking sensitivity increases, and the machinability decreases.

Mn functions as a deoxidizing agent at the time of melting and is necessary for ensuring hardenability.
It is a component for lowering the base hardness of steel during build-up welding to suppress welding cracks. Its content is 0.25 to 2.0% by weight.
Is set to If the content is less than 0.25% by weight, the above-mentioned effects cannot be sufficiently exerted, and 2.0% by weight or less.
When the content is more than 10% by weight, the machinability is lowered and P
The c value is increased to cause weld cracking of the steel.

Si functions not only as a deoxidizing agent but also as a component for improving hardenability, and its content is 0.2.
It is set to 0 to 0.40% by weight. If the content is less than 0.20% by weight, insufficient deoxidation at the time of smelting will be caused, and the refractory bricks of the melting furnace and the ladle will be eroded. Is mixed to generate minute irregularities on the mold surface after mirror finishing. But,
If it exceeds 0.40% by weight, the aforementioned Pc value is reduced to 0.3.
As the size becomes larger, the susceptibility to weld cracking increases, and weld cracks occur more frequently.

Sol-Al functions as a deoxidizing agent in the same manner as in the case of Si, and its content is 0.010-0.03.
It is set to 0% by weight. If the content is less than 0.010% by weight, insufficient deoxidation at the time of smelting is caused, and thus the refractory brick is mixed into the obtained steel ingot, so that minute irregularities on the mold surface after mirror finishing are generated. I will be.

However, if the content is more than 0.030% by weight, Al ₂ O ₃ non-metallic inclusions are formed at the time of agglomeration, and the mold surface of the steel is remarkably deteriorated. Between them and AlN to cause cracking of the steel. O is a component that is inevitably mixed during melting and ingot making of steel, and is most desirably completely absent. However, as described above, when the content in the obtained steel increases, the mold surface becomes large. , The occurrence of minute irregularities increases, so the upper limit of the content must be regulated to 0.0020% by weight. By containing Si and Al in the above range, the O content in the steel is reduced to 0.0020.
It is possible to restrict the amount to not more than% by weight.

The inevitable impurities in the steel of the present invention include, for example, P, N, Cu, Ni, W, Co, and the like.
030% by weight or less, N: 0.020% by weight or less, Cu:
0.25% by weight or less, Ni: 0.25% by weight or less, W:
It is preferable to limit the content to 0.10% by weight or less and Co: 0.10% by weight or less. For example, in the case of P, weld cracking sensitivity is increased to cause weld cracking, and N also combines with Al as described above to cause cracking. Therefore, it is necessary to reduce P as much as possible.

The steel of the present invention has the above composition,
r, Mo, and V may be blended. In that case, Cr is effective for securing the hardenability of steel, and its content is preferably set to 0.1 to 3.0% by weight. If the content is less than 0.1% by weight, the above-mentioned effects cannot be sufficiently obtained, and if the content is more than 3.0% by weight, the machinability is lowered and the cost is increased.

Mo enhances the hardenability of steel,
It is effective for increasing the tempering softening resistance at a temperature of 600 ° C. or higher, and its content is preferably set to 0.5 to 1.0% by weight. If the content is less than 0.5% by weight, the above effects cannot be sufficiently obtained. If the content is more than 1.0% by weight, the machinability is lowered and the cost is increased.

V is a component that enhances temper softening resistance and refines crystal grains to improve toughness, and its content is preferably set to 0.05 to 0.6% by weight. If the content is less than 0.1% by weight, the above-mentioned effects cannot be sufficiently exerted. If the content is more than 0.6% by weight, machinability and toughness are reduced. Further, the steel of the present invention comprises S, Pb, Ca, Zr, Te, Bi.
One or two or more free-cutting elements can be added to improve the machinability. In this case, if these elements are added in too large a quantity, ground scratches and black spots will occur in the obtained steel.
001 to 0.10% by weight, in the case of Pb 0.03 to 0.2
% By weight, 0.0005 to 0.010% by weight for Ca,
0.003 to 0.2% by weight for Zr, 0.01 to 0.15% by weight for Te, 0.01 to 0.2% for Bi
It is preferable to regulate each content to 0% by weight.

The steel of the present invention is prepared by, for example, preparing a molten steel having the above-mentioned composition in a vacuum degassing melting furnace, placing the molten steel in a ladle,
Furthermore, it can be manufactured by casting into a mold and ingot. At that time, the amount of nonmetallic inclusions generated in the obtained steel can be reduced by applying the usual heat retention enhancement during ingot making.

[0029]

EXAMPLES In a vacuum degassing and melting furnace, molten steels of various compositions were prepared by changing the compounding amounts of the respective components, and the molten steels were placed in a ladle and further ingot. Next, the steel ingot was forged, quenched and tempered after appropriate annealing treatment. The chemical composition of each steel ingot was analyzed, and the Pc value was calculated based on the analysis result. The results are shown in Tables 1 and 2. The characteristics of each steel were examined according to the following specifications. The results are also shown in Tables 1 and 2.

Hardness: The steel material is soaked at a temperature of 970 ° C., quenched, tempered at a temperature of 610 ° C., and the HB of the surface layer and the core is measured. Machinability: end mill diameter 10mm, cutting width 10mm, depth of cut 5
An end mill cutting test was performed under the conditions of mm and cutting oil Yushilon No. 3 (trade name), and the relative value when the value of the conventional example (SCM445) was set to 1 was determined.

Measurement of nonmetallic inclusions: B + C inclusion numbers are measured in accordance with the ASTA-A method. Measurement of minute irregularities: The core of a steel material was polished with # 2000 emery, the processed surface (10000 mm ² ) was observed with a microscope, and the number of minute irregularities existing in an area of 2500 mm ² was measured.

[0032]

[Table 1]

[0033]

[Table 2]

The following is clear from Tables 1 and 2. (1) Each of the steels of the examples has extremely small nonmetallic inclusions on the polished surface, a small number of minute irregularities, and has particularly excellent mirror finish. (2) On the other hand, Comparative Example 1 having a high O content, sol-A
Comparative Example 2 with low l-content, Comparative Example 3 with high Si-content
As is evident from the above, any one of these components deviates from the composition range of claim 1, the size of the nonmetallic inclusions is large, the number of fine irregularities is large, and the mirror surface property is deteriorated. I have.

(3) One or two of Cr, Mo, and V
3. The steel of claim 2 containing at least one species, and S, Pb, C
In the case of the steel of claim 3 containing one or more of a, Zr, Te, and Bi, the same tendency as above is observed.

[0036]

As is clear from the above description, the steel of the present invention has a small difference in hardness between the surface layer and the core, does not cause weld cracking even without preheating and postheating, and has high machinability. And the mold surface after the mirror finishing has almost no microscopic irregularities made of nonmetallic inclusions, and is extremely useful as a material for a plastic molding die.

[Brief description of the drawings]

FIG. 1 shows the oxygen content of steel and B measured by the ASTA-A method.
It is a graph which shows the relationship with + C system inclusion number.

Claims (3)

[Claims] 1. C: 0.05 to 0.25% by weight, Mn:
0.25 to 2.0% by weight, Si: 0.20 to 0.40% by weight, sol-Al: 0.010 to 0.030% by weight, O:
A steel for a plastic molding die, wherein 0.0020% by weight or less, the balance being Fe and unavoidable impurities. 2. Cr: 0.1 to 3.0% by weight, M
The plastic molding die steel according to claim 1, wherein one or more of o: 0.5 to 1.0% by weight and V: 0.05 to 0.6% by weight are contained. 3. S: 0.001 to 0.10% by weight, Pb:
0.03 to 0.2% by weight, Ca: 0.0005 to 0.010
Wt%, Zr: 0.003 to 0.2 wt%, Te: 0.0
The plastic molding die steel according to claim 1 or 2, which contains one or more of 1 to 0.15% by weight and Bi: 0.01 to 0.20% by weight.