JPH0949057A - Age hardened steel for die casting mold - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an age hardened steel for a die casting mold excellent in heat check resistance, having a long mold service life and improved in the dimension varying rate at the time of aging heat treatment by regulating its compsn. into the one in which the content of Ni is reduced and a suitable amt. of Cr is added. SOLUTION: This age hardened steel for a die casting mold has a compsn. contg., by weight, <=0.03% C, <=0.10% Si, <=0.10% Mn, 9.0 to 11.0% Ni, 0.10 to 5.0% Cr, 5.0 to 8.0% Mo, 0.10 to 1.0% Ti and 0.05 to 0.15% Al, also satisfying 30<=Co(%)×Mo(%)<=50, and the balance Fe. The reduction in the Ni content increases the austenitic transformation point (As point) and contributes to the improvement of its heat check resistance, but, simultaneously, it causes deterioration in its toughness, thus the content of Co(%)×Mo(%) remarkably controlling the effect on precipitation at the time of age hardening treatment is regulated to secure its toughness value. The dimension varying rate at the time of heat treatment is made small by the addition of Cr, but, on the other hand, the As point reduces, and there is a fear that its heat check resistance is hindered, therefore it is regulated to the upper limit or below.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to age-hardening steel for die casting, and more specifically, it has excellent heat checkability.
In addition, the present invention relates to an age hardening steel for die casting molds having a small aging change rate.

[0002]

2. Description of the Related Art
5% Cr, 1 to 1.5% mainly as a steel material for die casting
A hot work tool steel containing% Mo was used. When a die casting die is manufactured from this hot work tool steel, it is subjected to quenching and tempering treatments to develop the required hardness and strength.

By the way, when the quenching and tempering treatments are carried out, large heat treatment distortions and deformations occur, so that roughing is performed once before the quenching and tempering treatments, and finishing work is performed after the quenching and tempering treatments. In this case, the number of processing steps increases, and the time required for manufacturing the mold also increases.

Therefore, application of an age-hardened steel represented by 18% Ni maraging steel to a die-casting type has been studied. When manufacturing a die cast mold with this kind of age-hardening steel, unlike the quenching and tempering treatment in the hot work tool steel described above, there is little deformation during heat treatment, so that it is possible to omit some processing steps. The number can be shortened and the time required for manufacturing the mold can be shortened.

[0005] However, the general age hardening steel 1
In the case of the 8% Ni maraging steel, the resistance to the heat check generated on the surface portion in the case of forming the die-cast type, that is, the generation of cracks due to the repeated action of thermal stress associated with rapid heating and cooling is not sufficient. In addition, there is a problem that the mold life is significantly shorter than that of the conventional hot work tool steel.

Although the deformation during the heat treatment is smaller than that of the conventional hot work tool steel, isotropic shrinkage due to the heat treatment is unavoidable, and if the shrinkage deformation (size change rate) is large, the die casting type is When it is constructed, the dimensional variation becomes large, and it becomes difficult to apply it to such a die casting mold in practice.

More specifically, when a die casting mold is made of age-hardened steel, the mold shape and dimensions before aging treatment are determined in consideration of shrinkage due to heat treatment. Moreover, it becomes difficult to secure the dimensional accuracy of the die casting mold.

Various techniques have heretofore been proposed for improving the fracture strength of maraging steel. For example, JP-A-6-158228 discloses that the steel composition is Cr: 6.
0-15%, Ni: 4.0-12%, Mo: 0.3-
3.0%, Ti: 1.0 to 3.0%, Al: 0.01 to
It is disclosed that 2.00% improves the delayed fracture resistance of maraging aged steel.

However, this Japanese Patent Laid-Open No. 6-158228
Because the one disclosed in Japanese Patent Publication No. contains no Co,
Hardening during aging treatment is insufficient, and this maraging steel is mainly intended for structural members such as bolts and plates, and has a large dimensional change rate during heat treatment, so it is not suitable for die casting. Have difficulty.

On the other hand, Japanese Unexamined Patent Publication No. 6-248389 discloses that
The steel composition in the maraging aging steel for die casting molds is Ni: 12-14%, Mo: 4.5% -6.0%,
Co: 7.5-9.5%, Ti: 0.5-1.0%,
C: ≤ 0.03%, Si: ≤ 0.1%, Mn: ≤ 0.1
%, P: ≤ 0.01%, S: ≤ 0.01%, Cr: ≤
0.05%, N: 0.01%, Al: 0.02 to 0.
It is disclosed that the composition comprising 20% and the balance Fe has an increased softening resistance and improved heat check resistance.

However, this material has a problem that the heat check resistance is not sufficient because the Ni content is set to be high, and the dimensional change rate is not sufficient.

On the other hand, in Japanese Patent Laid-Open No. 63-145753, the steel composition is C: ≤ 0.03%, Si: ≤ 0.10%,
Mn: ≦ 0.10%, Cu: ≦ 0.10%, Ni: 7 to
20%, Cr: ≦ 0.10%, Mo: 2 to 6%, Co:
5-18%, Al: 0.50%, Ti: 0.8-2.
5%, B: 0.0005 to 0.005%, P: ≦ 0.0
025%, S: ≦ 0.0025%, P + S: ≦ 0.00
A maraging steel having an improved delayed fracture resistance is disclosed as a composition comprising 30% and the balance of Fe, and Japanese Patent Application Laid-Open No. 62-228455 discloses a steel composition wherein C: ≦ 0.0
3%, Si: ≦ 0.10%, Mn: ≦ 0.10%, C
u: ≦ 0.1%, Ni: 7-20%, Cr: ≦ 0.1
%, Mo: 1 to 10%, Co: ≤ 18%, Al: ≤ 1
%, Ti: ≦ 2.5%, P: ≦ 0.002%, S: ≦
0.0015%, P + S: ≦ 0.0030%, balance Fe
A maraging steel having improved fatigue characteristics as a composition comprising is disclosed.

However, these maraging steels also have a problem that the dimensional change rate during heat treatment is not sufficiently small.

[0014]

The invention of the present application has been made to solve such a problem. Thus, the invention of the present application relates to age-hardening steel for die casting, in which C: ≤% by weight.
0.03%, Si: ≦ 0.10%, Mn: ≦ 0.10
%, Ni: 9.0-11.0%, Cr: 0.10-5.
0%, Mo: 5.0 to 8.0%, Co: 5.0 to 8.0
%, Ti: 0.10 to 1.0%, Al: 0.05 to 0.
It is characterized in that the composition is 15%, 30 ≦ Co (%) × Mo (%) ≦ 50, and the balance is substantially Fe.

The present inventor, while studying the technical means for improving the heat check resistance and suppressing the dimensional change rate during heat treatment in the age-hardening steel for die casting, in the reduction of the amount of Ni and the appropriate amount of Cr. It was found that the addition is effective in reducing the size change rate during the aging treatment.

Thus, the reduction of the Ni content raises the austenite transformation point (As point) and contributes to the improvement of heat check resistance. If the temperature at the As point is low, the surface layer of the mold is likely to austenite from the martensite state due to the heating action of the molten metal when the die casting mold is used, which promotes crack generation, but if the As point is raised, this can be prevented. The heat check resistance can be improved.

However, the reduction of the Ni content also leads to the reduction of the toughness. Therefore, in the present invention, the toughness value is ensured by controlling the amount of Co (%) × Mo (%) that greatly affects the precipitation effect during age hardening treatment. Although the dimensional change rate during heat treatment decreases with the addition of an appropriate amount of Cr, the addition of Cr may lower the As point and hinder the heat check resistance. Therefore, in the present invention, the amount of Cr added is regulated to a certain value or less.

According to the present invention, it is possible to obtain an age-hardened steel for die casting which is excellent in heat check resistance, and its useful life can be greatly extended. The age-hardening steel of the present invention has a small dimensional change rate during heat treatment,
It is possible to reduce the number of processing steps at the time of manufacturing the die casting mold and to improve the dimensional accuracy of the die casting mold.

In the present invention, the more desirable content of Cr is 0.30 to 1.0% (claim 2), and the N content is preferably as small as possible. In a system containing Ti, when N increases, TiN inclusions are precipitated, which deteriorates the low cycle fatigue characteristics in the die casting type.

Here, the low cycle fatigue is a phenomenon in which the mold is largely cracked under a large mold clamping pressure. Since an increase in the amount of N also deteriorates the toughness, it is desirable to reduce the amount as much as possible, but an extremely low N causes an increase in manufacturing cost, so the upper limit was made 0.0050% (claim 3).

The TiN inclusions have a size of 10 μm.
If more than 2% is contained, it becomes a starting point of fracture and deteriorates the low cycle fatigue characteristics, causing large die-cast type cracks. Therefore, in the present invention, 1
The amount of TiN inclusions exceeding 0 μm was set to 2% or less of the whole (claim 4).

Next, the reasons for limiting each chemical component in the present invention will be described in detail. C: ≤0.03% C precipitates carbides such as TiC and Mo ₂ C at the grain boundaries and significantly reduces toughness, so the upper limit was made 0.03%. Further, 0.01% or less is desirable to secure toughness.

Si: ≤0.10% Si deteriorates the toughness, so it must be 0.10% or less. Mn: ≦ 0.10% Mn is bound to S to form MnS inclusions and deteriorates the toughness, so the content is regulated to 0.10% or less.

Ni: 9.0-11.0% Ni is an indispensable element that forms a matrix with high toughness by forming a solid solution with Fe. However, the precipitation amount increases as the addition amount increases, so that aging shrinkage occurs. The rate (size change rate) increases. Also, as the addition amount increases, the austenite transformation point As
Points, and the heat check resistance deteriorates. Since the mold surface is exposed to a high-temperature molten aluminum of 600 to 650 ° C., the As point must be equal to or higher than the reached surface temperature. For that purpose, Ni must be 11.0% or less. However, if the addition amount is less than 9.0%, the required toughness cannot be secured. From the above points, in the present invention, the Ni content is set to 9.0 to 11.0%.

Cr: 0.10 to 5.0% Addition of 0.10% or more of Cr can reduce the aging size change rate. However, if over 5.0% is added excessively, the heat transformation point and toughness decrease, so 0.1
It was set to 0 to 5.0%. A more desirable range of Cr is 0.3
~ 1.0%.

Mo: 5.0 to 8.0% 5% or more is necessary to obtain the age hardening of HRC 40 or more required for the die casting mold. However, if it exceeds 8.0%, the toughness deteriorates, the Ms point lowers, and the cost increases. Therefore, it is set to 5.0 to 8.0%.

Co: 5.0-8.0% As the amount of Co increases, the solid solubility of Mo decreases, so that age hardening is effective. However, if it is less than 5%, the effect is small and the As point is lowered. If it exceeds 8%, the toughness deteriorates,
Further, the cost is high, so the range is set to 5.0 to 8.0%.

Ti: 0.10 to 1.0% Ti is a hardening element that forms Ni ₃ Ti and contributes to age hardening. However, if added excessively, TiN inclusions precipitate and toughness and low cycle fatigue deteriorate. Let Further, the Ti segregation zone becomes a retained austenite band due to the lowering of the Ms point, and the heat check resistance is lowered by lowering the austenite transformation point. Therefore, the upper limit is set to 1.0%. Although the toughness is improved and the austenite transformation point is increased as the Ti content is reduced, if the content is less than 0.10%, the age hardenability becomes insufficient, so the lower limit was made 0.10%.

Al: 0.05 to 0.15% Al is added as a deoxidizer, and contributes to age hardening together with Ti. However, if it is less than 0.05%, the age-hardening property is insufficient, and conversely, if it exceeds 0.15%, the toughness is lowered, so in the present invention, it is set to 0.05 to 0.15%.

N: ≦ 0.0050% As the amount of N increases, TiN as inclusions precipitates and the low cycle fatigue characteristics deteriorate. Mold damage is caused by thermal stress during heating and cooling, but in the case of large molds, low cycle fatigue leading to large cracks becomes a problem. In addition, since the toughness is also deteriorated, the N content should be as low as possible, but an extremely low N content leads to an increase in manufacturing cost, so the upper limit is 0.0050%.
And

TiN inclusions having a circle equivalent diameter of more than 10 μm account for 2% or less of the whole. If any of the inclusions in the material has a size of more than 10 μm, mold clamping force, pouring stress and thermal stress act. It becomes a fracture starting point at a point and deteriorates low cycle fatigue characteristics,
Since it causes large cracks in the mold, it is necessary to make the TiN inclusion particles exceeding 10 μm 2% or less of the whole.

The solubility product of 30 ≦ Co (%) × Mo (%) ≦ 50 Co (%) × Mo (%) shows the age hardening property, and if it is reduced, the aging hardness also decreases. As a result, the decrease in toughness caused by the reduction of the Ni content is suppressed and the toughness is secured. Therefore, the upper limit of this value is set to 50. On the other hand, when Co (%) × Mo (%) is less than 30, the softening resistance at high temperature is not sufficient and the heat check resistance rapidly deteriorates.
And

[0033]

Next, embodiments of the present invention will be described in detail. Table 1
The age-hardening steel having the chemical composition shown in Table 1 was subjected to atmospheric induction melting and agglomeration (350 mmφ), kept at 930 to 1130 ° C. and water cooled, and further kept at 880 to 980 ° C. and air cooled to 1/2 R.
From the T direction of the (radius) part (the direction perpendicular to the rolling direction) to JIS3
No. Charpy impact test piece, 15mmφ (diameter) × 5mm
A heat check test piece of t (thickness) and a low cycle fatigue test piece were collected and subjected to a Charpy impact test, a heat check test, and a low cycle fatigue test. Further, a block material having a size of 210 L (length) × 130 W (width) × 45 mmt (thickness) was sampled from the central portion and dimension measurement → aging treatment → dimension measurement was carried out to measure the size change rate.

The aging treatment, heat check resistance test, and Charpy impact test were carried out under the following conditions. Aging treatment: 520 ° C. × 5 hr, AC (peak aging condition) Heat resistance check test: High frequency heating 650 ° C. × 4 sec and water cooling 3 sec, repeated 1000 times Charpy impact test: 2 mm U notch test piece, T direction sampling Room temperature test

The aging rate of change is 210 L × 13.
The 0 W × 45 mmt block test piece was subjected to each of the length (L) direction and the width (T) direction. Further, the low cycle fatigue test was conducted under the maximum stress of 123 kgf / mm ²
This was carried out by repeatedly applying the uniaxial tension of 1 axis to determine the number of repetitions until the fracture. These results are shown in Table 2.
It is shown in

[0036]

[Table 1]

[0037]

[Table 2]

In the results shown in Table 2, Comparative Example 18 having a Ni content outside the range of the present invention has a low As point and is inferior in heat check resistance.
In Comparative Example 19 containing 10%, the Charpy impact value shows a good value, but the As point is significantly low, and the heat check resistance is also greatly inferior.

The solubility product of Co and Mo Co (%) × M
Comparative Examples 20 and 21 in which the value of o (%) is lower than the range of the present invention
Heat resistance is poor, and conversely, the Charpy impact values of Comparative Examples 22 and 23, which are higher than the range of the present invention, are inferior, whereas all of the inventive examples show good values. .

In this example, the aging sizing rate (L direction),
The target value which is the standard for determining the quality of each of the As point, the average length in the heat check resistance, the maximum length, and the Charpy impact value is a dimensional change rate (absolute value): ≤0.070%, As
Points: ≧ 650 ° C., heat check resistance average length: ≦ 7.5 μm, maximum length: ≦ 55 μm, Charpy impact value: ≧ 20 J / cm ² .

Next, FIGS. 1A and 1B show the relationship between the solubility product of Co and Mo and the heat check resistance and the Charpy impact value based on the results of Table 2. From this figure, Co It can be seen that by controlling the solubility product of Mo and Mo within the range of 30 to 50, good values can be obtained for both heat check resistance and Charpy impact value.

Although the embodiment of the present invention has been described in detail above, this is merely an example, and the present invention can be carried out in variously modified modes without departing from the spirit of the invention.

[0043]

According to the present invention, it is possible to obtain an age-hardened steel for die casting which is excellent in heat check resistance, and its useful life can be greatly extended. Further, since the age-hardening steel of the present invention has a small dimensional change rate during heat treatment, it is possible to reduce the man-hours required for manufacturing the die casting mold and to improve the dimensional accuracy of the die casting mold.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a solubility product of Co (%) and Mo (%), heat check resistance and a Charpy impact value obtained in an example of the present invention.

Claims (4)

[Claims] 1. By weight%, C: ≦ 0.03% Si: ≦ 0.10% Mn: ≦ 0.10% Ni: 9.0-11.0% Cr: 0.10-5.0% Mo: 5.0 to 8.0% Co: 5.0 to 8.0% Ti: 0.10 to 1.0% Al: 0.05 to 0.15% and 30 ≦ Co (%) × Mo ( %) ≦ 50 Age-hardening steel for die casting, which is excellent in heat check resistance and aging dimensional change rate, and which consists essentially of Fe. 2. The Cr according to claim 1, which is 0.30 to 0.30.
Age-hardening steel for die casting, which has an excellent heat check resistance and an aging change rate, which is 1.0%. 3. The method according to claim 1, wherein N: ≦ 0.0.
Age-hardening steel for die casting, which is excellent in heat check resistance and aging size change rate, characterized by being regulated to 050%. 4. The TiN inclusion having a circle-equivalent diameter of more than 10 μm accounts for 2% of the entire steel according to claim 1, 2.
Age-hardening steel for die casting, which is excellent in heat check resistance, aging change rate and low cycle fatigue, characterized by the following: