JPH1088290A - Age-hardening steel for die casting die - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an age-hardening steel for die casting die, excellent in heat check resistance and capable of effectively prolonging the die life when applied to a die casting die. SOLUTION: The age-hardening steel for die casting die has a composition consisting of, by weight, <=0.03% C, <=0.10% Si, <=0.10% Mn, 9.0-11.0% Ni, <=0.10% Cr, 2.0-8.0% Mo, 8.1-12.0% Co, 0.10-1.0% Ti, 0.05-0.15% Al, further one or >=2 kinds selected from the group consisting of 0.1-1.0% Nb, 0.1-1.0% V, and 0.1-1.0% Ta, and the balance essentially Fe.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an age-hardened steel for a die-casting die, and more particularly to an age-hardened steel for a die-casting die having excellent heat check resistance.

[0002]

2. Description of the Related Art
5% Cr, 1 to 1.5% mainly as a steel material for die casting
Hot tool steel as a general-purpose material containing% Mo was used. When manufacturing a die casting mold with this hot tool steel,
The required hardness and strength are developed by quenching and tempering.

[0003] Incidentally, this general-purpose hot tool steel has a problem that martensite transformation during quenching and heat treatment deformation accompanied by decomposition of martensite during tempering are large. In the case of manufacturing, it is necessary to perform roughing once before quenching and tempering, and to perform finishing after quenching and tempering. 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-casting die using this kind of age-hardened steel, unlike the quenching and tempering treatment in the above-mentioned hot tool steel, there is little deformation during heat treatment, and therefore, it is possible to omit some processing steps, thus making the manufacturing process The number can be reduced, and the time required for mold production can be shortened.

[0005] However, the general age hardening steel 1
In the case of 8% Ni maraging steel, heat resistance generated on the surface in the case of forming a die-casting die, that is, resistance to crack generation due to repetitive action of thermal stress accompanying rapid heating and cooling is not sufficient. Therefore, there is a problem that the life of the mold is significantly shorter than that of the conventional hot work tool steel.

Various techniques have been proposed for improving the breaking strength of maraging steel. For example, JP-A-6-158228 discloses that the steel composition is Cr: 6.
0 to 15.0%, Ni: 4.0 to 12.0%, Mo:
0.3 to 3.0%, Ti: 1.0 to 3.0%, Al:
It is disclosed that when the content is 0.01 to 2.00%, the delayed fracture resistance of the maraging steel is improved.

However, the method disclosed in Japanese Patent Application Laid-Open No.
Because the one disclosed in Japanese Patent Publication No. contains no Co,
Hardening during aging treatment is insufficient, and this maraging steel is intended only for structural members such as bolts and plates, and is difficult to apply to a die-casting type.

On the other hand, JP-A-6-248389 discloses that
The steel composition in the maraging steel for die casting
i: 12 to 14%, Mo: 4.5% to 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: ≦ 0.0
5%, N: ≦ 0.01%, Al: 0.02 to 0.20
%, The softening resistance is increased as a composition consisting of the balance Fe,
It discloses that the heat check resistance is improved. However, this has a problem that the heat check resistance is not sufficient because the Ni content is set high.

On the other hand, Japanese Patent Application Laid-Open No. 63-145,753 discloses that 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 to 20%, Cr: ≦ 0.10
%, Mo: 1 to 10%, Co: ≦ 18%, Al: ≦ 1.
0%, 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 heat check resistance is not sufficient.

[0010] Application of age hardened steel to a die-casting type has also been proposed in Japanese Patent Application Laid-Open No. 7-243003. The age hardened steel disclosed in this publication has a composition of steel: C: ≦ 0.03%, Si: ≦ 0.10%, Mn: ≦
0.10%, P: ≦ 0.010%, S: ≦ 0.010
%, Ni: 6.0 to 11.0%, Cr: ≤ 0.30%,
Mo: 4.0 to 9.0%, Co: 7.0 to 11.0%,
Ti: 0.10 to 1.0%, Al: 0.05 to 0.15
%, B: ≦ 0.010%, Zr: ≦ 0.10%, Ca:
≤0.10%, and the balance substantially consisting of Fe is intended to improve the heat check resistance, but the degree of improvement is still not satisfactory.

[0011]

The invention of the present application has been made to solve such a problem. Thus, the invention of the present application provides an age-hardened steel for a die-casting type on the basis of C:
≦ 0.03%, Si: ≦ 0.10%, Mn: ≦ 0.10
%, Ni: 9.0 to 11.0%, Cr: ≦ 0.10%,
Mo: 2.0 to 8.0%, Co: 8.1 to 12.0%,
Ti: 0.10 to 1.0%, Al: 0.05 to 0.15
%, And one or more members selected from the group consisting of Nb, V, and Ta are Nb: 0.1 to 1.0%, V: 0.1 to
1.0%, Ta: 0.1 to 1.0%, with the balance substantially consisting of Fe (claim 1).

The present inventors conducted research on improving the heat check resistance of the age-hardened steel for die-casting, and found that the amount of Ni was reduced and one or two of Nb, Ta, and V were reduced.
It has been found that more than one addition is effective. The reduction in the amount of Ni results in an increase in the austenite transformation point (As point) at which the martensite structure changes to the austenite structure, contributing to an improvement in heat check resistance.

[0013] If the temperature at the As point is low, the surface of the mold tends to austenite from the martensitic state due to the heating action of the molten metal during use of the die casting mold, which promotes the generation of cracks. Can be prevented,
Heat check resistance can be improved.

The addition of one or more of Nb, Ta, and V improves the toughness of the steel by making the crystal grains of the steel finer, and prevents the metal grains from becoming coarse on the surface of the mold. The difference between the crystal grains in the mold surface layer and the inside, that is, the difference in structure, is reduced, thereby contributing to an improvement in heat check resistance. Nb,
The addition of Ta and V compensates for a decrease in Ni due to the crystal grain refinement effect and a decrease in toughness accompanying an increase in Co.

Age-hardened steel has a higher material cost than general-purpose hot work tool steel, and in order to absorb the increase in cost, it is required to improve the mold life, that is, to further improve the heat check resistance. You.

The heat check which becomes a problem when the age-hardened steel is applied to the die-casting type is caused by the repetition of the temperature rise and cooling of the surface layer in contact with the molten metal and the crystal grain difference between the surface layer and the inside as described above. Therefore, the present inventors have focused on the thermal conductivity, the coefficient of thermal expansion, and the addition of Nb, Ta, and V in the age-hardened steel, and studied their effects on heat resistance check.

As a result, the thermal conductivity has a greater influence on the heat resistance check than the thermal expansion coefficient.
It has been found that the addition of a and V improves the heat check resistance, and that increasing the content of the component Co contained in the age-hardened steel can also effectively increase the thermal conductivity.

Here, when the thermal conductivity is increased, the heat check resistance is improved because the heat conduction is improved from the surface layer of the mold to the inside, so that the temperature rise of the surface layer of the mold is suppressed. As a result, it is considered that the heat check resistance is improved even under the same As point.

The present invention has been made based on such knowledge, and it has been found that the reduction of the amount of Ni raises the As point, and the addition of at least one of Nb, Ta, and V results in the crystallization between the mold surface layer and the interior. The difference in grain size is reduced, the Co content is increased, the thermal conductivity is increased, and the problem of toughness reduction caused by the reduction of Ni content and the increase of Co is solved by Nb, Ta,
It is characterized by the fact that it has been solved by the addition of V, the optimization of Mo and other components, and the age-hardened steel of the present invention has good heat check resistance when applied to a die-casting die. The service life can be greatly extended, and the increase in material cost can be absorbed by the extension of the service life.

In the present invention, the more preferable content of Co for effectively increasing the thermal conductivity is 10 to 12% (claim 2). However, since the toughness decreases when the content of Co increases, one or more of Nb, Ta, and V are added.
It is desirable to set the Mo amount low. In that sense, a desirable range of the Mo amount is 2 to 4%.

The thermal conductivity is 0.23 J / cm ·
It has been confirmed that the temperature is preferably set to sec. However, if the component system has a too high thermal conductivity, the toughness is reduced.
It is desirable that the temperature be 3 J / cm · sec · ° C. or less.

In the present invention, one or two or more selected from the group consisting of Nb, V, and Ta are used in an amount of Nb: 0.1 to
1.0%, V: 0.1-1.0%, Ta: 0.1-1.
It is necessary to contain it in the range of 0%. These N
b, V, and Ta are crystal grain refining elements, and by including them, the heat check resistance can be further improved by refining the crystal grains. In addition, coarsening of the crystal grains on the surface of the mold is prevented, the difference in the size of the crystal grains between the surface and the inside is reduced as compared with conventional steel, and the heat check resistance is improved.

Next, the reasons for limiting each chemical component in the present invention will be described in detail. C: ≦ 0.03% If C is contained in excess of 0.03%, TiC, Mo ₂ C
And the like, precipitates at the grain boundaries to significantly lower toughness and inhibit age hardening, so the upper limit is 0.03%.
And

Si: ≦ 0.10% Since the toughness is deteriorated when the content of Si exceeds 0.10%, the upper limit is set to 0.10%.

Mn: ≦ 0.10% Since Mn combines with S to form MnS inclusions and deteriorate the toughness, Mn is regulated to 0.10% or less.

Ni: 9.0 to 11.0% Ni is an indispensable element that forms a matrix with high toughness by forming a solid solution in Fe, but the austenite transformation point (As point) decreases as the amount of addition increases. And the heat check resistance is reduced. The mold surface is, for example, 600 to 650 ° C.
The As point must be equal to or higher than the attained surface temperature because it is exposed to the high-temperature molten aluminum.
i needs to be 11.0% or less. Also Ni
If the content exceeds 11.0%, the thermal conductivity also decreases, so the upper limit is also set to 11.0% in this sense.
It is necessary to However, if the addition amount is less than 9.0%, the required toughness cannot be ensured, and the age hardening property also decreases, so in the present invention, the Ni content is set to 9.0 to 11.0%. .

Cr: ≦ 0.10% If the Cr content exceeds 0.10%, the toughness deteriorates, and As
Points also drop. In addition, since the thermal conductivity also decreases, in the present invention, the content thereof is regulated to 0.10% or less as an impurity.

Mo: 2.0 to 8.0% Mo is a component necessary for ensuring age hardening. However, when the content is less than 2.0%, the age hardening property is insufficient, and the martensite transformation point is also reduced. On the other hand, if the content exceeds 8.0%, Ni ₃ Mo, Fe ₂ Mo, etc. are precipitated to deteriorate the toughness and the thermal conductivity. Therefore, in the present invention, Mo
In the range of 2.0 to 8.0%. A more desirable range is 2.0 to 4.0%.

Co: 8.1 to 12.0% Co is an essential component for ensuring age hardening and for further increasing the thermal conductivity. However, when the content is less than 8.1%, the age hardening property is insufficient, and the thermal conductivity is also insufficient. The martensitic transformation point also decreases. Therefore, in the present invention, the lower limit is set to 8.1%. On the other hand, if the content exceeds 12.0%, the toughness is deteriorated, and the dimensional change rate (age dimensional change rate) during aging treatment is increased. Since the cost is further increased, the upper limit is set to 12.0% in the present invention.

Ti: 0.10 to 1.0% Ti is a hardening element that forms Ni ₃ Ti and contributes to age hardening. However, if it is added in excess of 1.0%, the amount of TiN inclusions increases, Deteriorates toughness. On the other hand, 0.10
%, The age hardening property is insufficient. Therefore, in the present invention, the content range is set to 0.10% to 1.0%.

Al: 0.05 to 0.15% Al is added as a deoxidizing agent and contributes to age hardening together with Ti. However, when the content is less than 0.05%, the age hardening property is insufficient, and the deoxidizing effect is reduced. On the other hand, if it exceeds 0.15%, Ti ₃ Al precipitates and the toughness deteriorates. Therefore, in the present invention, the content is in the range of 0.05 to 0.15%.

Nb: 0.1 to 1.0% V: 0.1 to 1.0% Ta: 0.1 to 1.0% Nb, V and Ta are fine grains and crystal grains on the surface of the mold. Is a coarsening prevention element, and exhibits the required effect when added at 0.1% or more. However, if the content exceeds 1.0%, the toughness is deteriorated.
The range was 1 to 1.0%.

[0033]

Next, embodiments of the present invention will be described in detail. Table 1
Age-hardened steel having the chemical composition shown in Table 1 was melted and formed into an air by induction (350 mmφ), kept at 930 to 1130 ° C and then water-cooled, and further kept at 880 to 980 ° C and air-cooled. , 15mmφ (diameter)
× 5mmt (thickness) heat check test piece and 10m
A test piece for measuring the thermal conductivity of mφ (diameter) × 1 mmt (thickness) was collected and subjected to a Charpy impact test, a heat check test, and a thermal conductivity measurement.

[0034]

[Table 1]

The aging treatment, the heat check test, the Charpy impact test, and the measurement of the thermal conductivity were performed under the conditions shown in Table 2 and below, respectively. Heat check test: High-frequency heating at 600 ° C. for 4 seconds and water cooling for 3 seconds were repeated (2,000 repetitions). After polishing, the outer peripheral portion was measured with an optical microscope to measure the length of the crack. Charpy impact test: 2 mm U notch specimen, room temperature n
= Average of three samples was taken. Thermal conductivity measurement: Measurement was performed at room temperature to 400 ° C. using a test piece having a size of 10 mmφ × 1 mmt. These results are shown in Table 2. In addition, the relationship between the thermal conductivity and the heat check resistance was determined by plotting the thermal conductivity on the horizontal axis and the maximum length in the heat check test on the vertical axis, and shown in FIG.

[0036]

[Table 2]

As can be seen from these results, in the case of the steel of the present invention, the maximum crack length in the heat check test was 18% Ni maraging steel of the comparative steel and Nb, Ta, V
It is improved compared to steel without additives.

As is clear from FIG. 1 showing the relationship between the thermal conductivity and the heat check resistance, the heat check resistance is improved by increasing the thermal conductivity.
In particular, it can be seen that the heat check resistance is effectively improved by setting the thermal conductivity to 0.23 J / cm · sec · ° C. or more.

Although the embodiment of the present invention has been described in detail, this is merely an example, and the present invention can be implemented in variously modified forms without departing from the gist thereof.

[0040]

As described above, according to the present invention, the heat check resistance of the age-hardened steel can be increased more than before, and when this is applied to a die-casting die, the life of the die is reduced. It can be extended effectively.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between thermal conductivity and heat check resistance obtained in an example of the present invention.

Claims (2)

[Claims] 1. C: ≤0.03% Si: ≤0.10% Mn: ≤0.10% Ni: 9.0-11.0% Cr: ≤0.10% Mo: 2. 0 to 8.0% Co: 8.1 to 12.0% Ti: 0.10 to 1.0% Al: 0.05 to 0.15%, and further selected from the group of Nb, V, and Ta Nb: 0.1 to 1.0% V: 0.1 to 1.0% Ta: 0.1 to 1.0% and the balance substantially consisting of Fe Age-hardened steel for die-casting with excellent heat check properties. 2. The age-hardened steel for a die-casting type according to claim 1, wherein Co is 10 to 12%.