TWI390043B - Hot working die steel for aluminum die-casting - Google Patents

Description

Hot die steel for aluminum die casting

The present invention relates to a hot-formed steel used for a die-casting mold; in particular, it is a crack caused by a water-cooled hole which is a major cause of suppressing a large-scale cracking of a die-casting mold, and can be subjected to a high cycle of die-casting in accordance with the manufacture of a die-cast product. Use hot die steel. The hot die steel for die casting of the present invention is particularly suitable for use as a material for an aluminum die casting mold.

In the aluminum die-casting mold, there is a problem that cracks (heat check) are generated on the cavity surface due to thermal fatigue. The term "hot crack" refers to the fact that when the cooling water is poured into the cavity surface after the mold is opened, the rapid cooling of the cavity surface and the temperature difference between the inside and the heating state cause tensile stress on the cavity surface. Repeating the thermal fatigue caused by this condition causes cracks on the cavity surface.

For this thermal crack, it will be advantageous to increase the hardness of the mold.

On the other hand, in recent years, there has been a demand for shortening the manufacturing cycle of aluminum die-cast products (high cycle). In order to achieve this, in order to shorten the nip time of the mold, there is a tendency to enhance the water cooling of the aluminum cast product in the mold. This enhancement of water cooling, in particular, is carried out by bringing the water-cooled holes close to the cavity surface. In this case, the thermal stress generated on the surface of the water-cooled hole during the casting of the aluminum product will increase, and the phenomenon of cracking from the water-cooled hole will cause a problem.

The rupture caused by such water-cooled holes is not only caused by the thermal stress of the repeated load during die-casting, but also by the composite delayed fracture phenomenon caused by the cracking of the thermal stress and the stress corrosion induced by the surface of the water-cooled hole.

This cracking from the water-cooled hole is more likely to occur when the hardness of the mold is higher. Therefore, it is advantageous to reduce the hardness of the mold for such cracks generated from the water-cooled holes.

That is, although the hardness of the mold is improved for the thermal crack, it is not good for the crack generated from the water-cooled hole; on the contrary, reducing the hardness of the mold is favorable for the crack generated from the water-cooled hole, but it is disadvantageous to the hot crack and causes the heat crack resistance to deteriorate. .

In order to suppress the above-mentioned cracking from the water-cooled pores, it is preferable to make the hardness of the mold to be HRC 45 to 40.

As the current aluminum die-casting mold, a 5Cr-based hot-die steel represented by JIS-SKD61 is mainly used. In recent years, in order to suppress thermal cracks generated on the cavity surface, the hardness is increased, and as the aluminum die-cast product is highly cycled, the risk of cracking of the mold from the water-cooled holes is also increased.

In the case of the above JIS-SKD61, C is about 0.4%, and the hardness in the quenched state is, for example, HRC53.

Therefore, for the purpose of suppressing cracking from the water-cooled pores, in order to lower the hardness to HRC 45 or less, it is necessary to perform tempering at a high temperature of 600 ° C or higher. However, if tempering is performed at such high temperatures, the corrosion resistance of the steel is remarkably lowered.

This material contains about 5% of Cr, which is originally a material with good corrosion resistance. However, when tempering is performed at a high temperature of 600 ° C or higher, the Cr contained therein is almost tempered at a high temperature and precipitated in the form of Cr carbide, so that Cr contained in the steel can no longer contribute to improvement of corrosion resistance.

In short, the hot mold steel represented by JIS-SKD61, which is an aluminum die-casting mold, is mainly used today, and the problem of cracking from the water-cooled hole cannot be satisfactorily solved.

In order to satisfactorily solve the problem of cracking from the water-cooled hole and the problem of thermal cracking of the cavity surface, it is considered that the rust generated in the water-cooled hole is prevented, and the hardness inside the mold in which the water-cooled hole is located is reduced, and the other is On the other hand, it is effective to increase the hardness of the cavity surface of the mold which generates hot cracks. However, to date, no material has been provided to satisfy these characteristics.

Further, in the following Reference 1, the invention disclosed is that the inner diameter surface of the water-cooled hole of the die-casting mold is made to have a lower hardness than the surface of the mold, thereby achieving both the prevention of cracking of the water-cooled hole and the heat-resistant cracking of the surface of the mold.

The steel disclosed in the reference 1 is quenched and tempered by the conventionally used JIS-SKD61 to be tempered to a higher hardness, and then heated to the surface of the water-cooled hole by induction heating, burner heating, and thunder. The tempering by firing or the like is localized to have a low hardness.

In the method disclosed in the reference 1, any method requires local heating, and the water-cooling aperture must be made into a size that the burner can enter, and the shape of the water-cooled hole must be limited.

[Reference 1] Japanese Patent Laid-Open No. Hei 6-315753

The present invention has been made in view of the above circumstances, and it is an object of the invention to provide a hot die steel for die casting which is excellent in heat crack resistance and can suppress cracking from water-cooled holes.

The inventors of the present invention conducted intensive studies on the above problems, and as a result, found that the above problems can be solved by the following hot die steel for die casting.遂Complete the invention.

The gist of the present invention is as follows: 1. A hot-die steel for die-casting, which is obtained by quenching a steel containing the following components and tempering at a temperature of 500 ° C or lower; the component contained is C in mass% : 0.1 to 0.3%, Si: 0.1 to 1.5%, Mn: 0.3 to 2%, Cr: 6 to 12%, P: 0.05% or less, S: 0.01% or less, Mo: 1 to 3%, V: 0.5 to 1.5%, s-Al: 0.005 to 0.025%, N: 0.005 to 0.025%, O: 0.005% or less, and the balance being Fe and unavoidable impurities.

2. The hot-die steel for die-casting according to item 1, which further contains at least one selected from the group consisting of Ni: 2.0% or less and Cu: 1% or less in mass%.

3. The hot die steel for die casting according to item 1 or 2, which further contains Co in a % by mass or less: 5% or less.

4. The hot-die steel for die-casting according to any one of items 1 to 3, further comprising a mass selected from the group consisting of Ti: 0.2% or less, Zr: 0.2% or less, and Nb: 0.2% or less in mass% At least one of them.

In the hot-die steel for die-casting of the present invention, the content of C is reduced, and the content of Cr and Mo is increased and quantified. Therefore, when used as a die-casting mold, it is possible to effectively suppress the generation of water-cooled holes. It is broken and can impart excellent heat crack resistance to the die casting mold. The hot die steel for die casting of the present invention is particularly preferably used as a material for an aluminum die casting mold.

Cr is known as an element for improving corrosion resistance. However, in the conventional JIS-SKD61, when the hardness is used to temper at a high temperature of 600 ° C or higher, the Cr for improving the corrosion resistance is precipitated by carbide, and the effect is almost lost. On the other hand, when the tempering temperature is reduced to the extent that the Cr carbide does not precipitate, the hardness becomes 50 HRC or more, and when it is used as a die for die casting, cracking from the water-cooled hole is likely to occur.

Of course, by reducing the C content and tempering at a low temperature of 500 ° C or lower, the target hardness can be obtained. However, in this case, the hardness of the cavity surface is also lowered, which causes a problem that the heat-resistant crack resistance is deteriorated.

Here, in the hot die steel for die casting of the present invention, Mo is appropriately added while reducing the C content.

By reducing the C content and performing low-temperature tempering at 500 ° C or lower, it is possible to obtain a hardness of less than HRC 45 which is less likely to cause cracking from the water-cooled pores.

Further, when Mo is added in an appropriate amount, when a mold for die casting is used, heat from a molten metal (for example, aluminum melt) during die casting can be used to locally harden the cavity surface of the mold.

Specifically, when the mold is used in the casting of the die-cast product, the cavity surface is heated by the heat of the melt (600 to 650 ° C in the case of the aluminum melt), and the carbide type is precipitated. It can play the role of hardening the cavity surface locally.

That is, the hot die steel for die casting of the present invention has an effect that the hardness of the cavity surface is hardened by age hardening in use of the mold. By this effect, the thermal crack of the cavity surface can be satisfactorily suppressed.

In other words, the hot-die steel for die-casting of the present invention can be skillfully utilized in the case of using a mold for die-casting, whereby the cavity surface is aged and hardened by heat from the melt, and as a result, the interior can be maintained at a low level. Hardness, while the cavity surface locally increases the hardness of the mold. This point is an excellent effect of the conventional hot die steel for die casting of the present invention.

In the present invention, the amount of Cr added as a corrosion-resistant element is more than that of JIS-SKD61. In the present invention, since the tempering is performed at a low temperature of 500 ° C or lower after the quenching treatment, the added Cr does not precipitate as a carbide, but is solidified in a matrix, and the steel can be effectively improved. Corrosion resistance. In other words, when the hot die steel for die casting of the present invention is used as a die for die casting, the rust of the water-cooled hole can be suppressed, and the stress corrosion cracking due to the rust can be satisfactorily suppressed. It is accompanied by cracks from the water-cooled holes.

Further, when the hot die steel for die casting of the present invention is used as a die for die casting, the cavity surface of the mold is hardened by secondary hardening (aging hardening) by precipitation of Mo carbide to ensure heat crack resistance. Hardness above HRC45.

Next, the reasons for limiting the chemical components in the present invention are detailed below. In addition, in the following, "%" means "% by mass".

C: 0.1~0.3%

C is an essential element for ensuring important hardness and wear resistance in the performance of the mold.

In the usual hot mold steel, it contains about 0.4% of C. In the present invention, in order to obtain a hardness of HRC45 or less at a low temperature tempering of 500 ° C or less, the C content is lower than that of a normal hot mold steel, and the range is 0.1. ~0.3%, preferably 0.15~0.25%.

Si: 0.1 to 1.5% Si is an essential element as a deoxidizing element in steel making.

Further, by increasing the content thereof, the machinability and the temper softening resistance can be improved.

However, since the impact toughness is lowered when the amount of addition is large, the addition range is set to 0.1 to 1.5%, preferably 0.1 to 0.5%.

Mn: 0.3 to 2% Mn is an essential component for ensuring hardenability and hardness, and the amount thereof is set to 0.3% or more.

Further, when Mn is excessively added, the hardenability is too high, and a large amount of residual γ is generated during quenching, so that the impact value is lowered, or the hardness cannot be lowered even if tempering is performed, so the upper limit is made 2%. Further, the upper limit of the amount of addition of Mn is preferably 1%.

Cr: 6 to 12% Cr is an element which improves hardenability and improves corrosion resistance of water-cooled pores.

In order to obtain an effect of improving corrosion resistance, it is necessary to add 6% or more, and it is preferable to add 8% or more.

However, if added in a large amount, the temper softening resistance is lowered and the mold performance is lowered. Therefore, it is limited to 12%. Further, the upper limit of the Cr content is preferably set to 10%.

P: ≦ 0.05% Since the P element will lower the impact value, it is preferable to reduce the content thereof, and in the case of unavoidable inclusion, it is preferable to reduce it to 0.05% or less.

S: ≦ 0.01% Since the S element forms MnS to lower the impact value, it is preferable to reduce the content thereof.

In the case of inevitable inclusion, it is preferable to reduce it to 0.01% or less.

Mo: 1 to 3%, since Mo forms carbides, the substrate is strengthened and abrasion resistance is required, and it is necessary to ensure hardenability.

Further, when the hot die steel for die casting of the present invention is used as a die for die casting, the carbide of Mo is precipitated by heat from the melt (in the vicinity of the aluminum melt at about 600 ° C), and the hardness of the mold is increased.

In the present invention, in order to prevent cracking from the water-cooled hole, the hardness of the mold after quenching and tempering is set to be lower than HRC 45, and the temperature of the cavity surface is increased in the die casting (in the case of aluminum melt, it is around 600 ° C). ), the hardness above HRC 45 can be obtained, and the heat crack resistance can be improved.

In order to obtain such effects, it is necessary to add 1% or more, and it is preferable to add 1.5% or more.

However, even if it is added excessively, since the effect is saturated and it is not economical, the upper limit of the addition amount is set to 3%. Further, the upper limit of the amount of Mo added is preferably 2.5%.

V: 0.5 to 1.5% V element is precipitated by forming carbides during tempering, which enhances the reinforcing substrate and improves wear resistance.

Further, by forming fine carbides during quenching, it is possible to suppress the coarsening of the crystal grains and suppress the decrease in the impact value.

In order to achieve these effects, it is necessary to add 0.5% or more.

On the other hand, if it is excessively added, coarse crystallization of carbonitride is formed during solidification, and the toughness is lowered, so the upper limit is 1.5%. Further, the upper limit of the amount of addition of V is preferably set to 1%.

s-Al: 0.005~0.025% Al element is used as a deoxidizing element in steel making, and is N-bonded with steel to finely disperse in the form of nitride, which can suppress coarse grain size during quenching heating. Chemical.

In order to obtain these effects, it is necessary to add 0.005% or more.

However, even if the excess is added, since the effect is saturated, the upper limit is 0.025%.

N: 0.005 to 0.025% N forms a nitride with Al and V in the steel, and by finely dispersing, it is possible to suppress coarsening of crystal grains during quenching heating, and is an effective element for preventing a decrease in impact value.

In order to obtain such effects, it is necessary to add 0.005% or more.

However, even if it is excessively added, since its effect is saturated, the upper limit is 0.025%.

O: ≦0.005% O forms an oxide system to reduce the impact value. In order to suppress the decrease in the impact value, the O content must be 0.005% or less.

Ni: ≦2%, Cu≦1% Ni and Cu are effective for improving the hardenability and strengthening the substrate, and may be added as needed.

However, even if the excess is added, since the effect is saturated, it is disadvantageous in terms of economic efficiency, so the upper limit is set to 2% and 1%, respectively.

Co: ≦ 5% Co is an element which increases strength by solid solution strengthening and can be added as needed.

However, even if it is excessively added, since the effect is saturated, it is disadvantageous in terms of economic efficiency, so the upper limit is set to 5%.

Ti: ≦ 0.2%, Zr: ≦ 0.2%, Nb: ≦ 0.2%, etc., such as Ti (CN), Zr (CN), Nb (CN) and these composite carbonitrides are finely precipitated. An element that prevents coarsening of crystal grains during quenching heating. In order to refine the crystal grains to ensure the toughness, it may be added as needed.

However, if the excess is added, the coarse carbonitride is crystallized during solidification, and the impact value is lowered, so the upper limit is made 0.2%.

Further, in the case of such a composite addition, the total amount thereof is preferably 0.5% or less.

Next, embodiments of the present invention will be described in detail below.

Hereinafter, the present invention is exemplified by the steel of the present invention and comparative steel, but it is needless to say that the present invention is not limited thereto.

The various steels shown in Table 1 were melted in a 150 Kg vacuum high frequency induction furnace so that the ingot was forged at 1200 ° C to form a 60 x 60 mm section angle bar.

The corner bar was cut into a length of 500 mm, heated to 1030 ° C, and then quenched in an oil bath.

Then, tempering was carried out twice at 450 ° C for 1 hour. For each corner bar subjected to tempering, the hardness of the 1/4H portion (the portion of the surface and one half of the center portion) was measured, and the T direction (width direction of the corner bar) of the test piece with a 2 mm U notch was performed. Charpy impact test, and cut 10×10×10mm pieces from 1/4H part, immersed the surface in sandpaper, and immersed in industrial water at 20°C for 24 hours to confirm the corrosion resistance test of rust occurrence. .

In addition, the evaluation of the corrosion resistance was performed as "○" in the case where no rust occurred, and "x" in the occurrence of rust.

Moreover, in order to simulate the thermal experience in the casting of the aluminum die-cast product by repeated heating, the mixture was heated from room temperature to 650 ° C for 4 seconds by high-frequency heating, and then water-cooled as 1 cycle, and the above-mentioned corner bars were tempered at 450 ° C. Further, 1000 cycles were repeated, and the surface hardness after the measurement was measured.

The above evaluation results are shown in Table 2.

Moreover, the inventive steel No. 2 of Table 1 was heated to 1030 ° C, and then quenched in an oil bath, and tempered twice at 450 ° C for 1 hour; and after heating the conventional steel A to 1030 ° C, oil was applied. Bath quenching, tempering twice at 450 ° C × 1 hour; and tempering 2 times under the condition of 630 ° C × 1 hour; respectively, for the purpose of cracking as a water-cooled hole The resistance to delay characteristics of the sensitivity index is evaluated.

Here, the evaluation of the delayed damage resistance characteristics was carried out as follows.

That is, industrial water (for rusting) was dropped on the notch portion of the test piece having a ring-shaped notch of 0.1 R, and the relationship between the flexural stress and the failure time was investigated.

Further, the evaluation of the delayed fracture resistance was performed by comparing the ratio of the static flexural stress (0 hour fracture stress) to the fracture stress at 200 hours.

Further, the mixture was heated from room temperature to 650 ° C for 4 seconds, and then water-cooled as one cycle. After 10,000 cycles were repeated, the length of the thermal crack generated on the surface was measured, and the index as the heat-resistant crack resistance was evaluated.

The above evaluation results are shown in Table 3.

Further, in Table 3, the target value of the delayed fracture resistance characteristic was set to 0.7 or more.

As shown in the results of Table 2, the inventive steel No. 1 to No. 11 had a hardness of HRC 40 to 44 at 450 ° C tempering, and a hardness of HR ° 46 to 49 after repeated heating at 650 ° C, that is, hardness. improve.

Further, since the Cr carbide was hardly precipitated at a low temperature of 450 ° C, either of them showed good corrosion resistance.

In contrast, compared with steel C, which is 0.05%, which is lower than the lower limit of 0.1% of the present invention, the hardness in the tempering at 450 °C is only HRC36, and the hardness after repeated heating at 650 °C is also lower. And the heat crack resistance is poor.

Conversely, the comparative steel b has a C of 0.38% higher than the upper limit of the present invention of 0.3%. Therefore, the hardness after tempering at 450 ° C is as high as HRC 53 and the impact value is low.

The Si content of the comparative steel c was 2%, which was higher than the upper limit of 1.5% of the present invention, and the impact value was low.

The Mn content of the comparative steel d was 2.5%, which was higher than the upper limit of 2% of the present invention, and the impact value was low.

The content of the impurity P of the comparative steel e was 0.08%, which was higher than the upper limit of 0.05% of the present invention, so the impact value was low.

Further, the content of the impurity S of the comparative steel f was 0.05%, which was higher than the upper limit of 0.01% of the present invention, so that the impact value was low.

The Cr content of the comparative steel g was 5.1%, which was lower than the lower limit of 6% of the present invention, and therefore the corrosion resistance was low.

On the contrary, the Cr content of the comparative steel h was 13.5%, which was higher than the upper limit of 12% of the present invention, so that the impact value was low.

The Mo content of the comparative steel i was 0.6%, which was lower than the lower limit of 1% of the present invention. Therefore, even if the heating was repeated at 650 ° C, the hardness could not be sufficiently improved. It shows poor heat crack resistance.

The V content of the comparative steel j was 0.3%, which was lower than the lower limit of 0.5% of the present invention, so that coarsening of the crystal grains occurred and the impact value was low.

The s-Al content of the comparative steel k is 0.003%, which is lower than the lower limit of 0.005% of the present invention, so that the coarsening of the crystal grains occurs, and the impact value is low.

The N content of the comparative steel 1 was 0.002%, which was lower than the lower limit of 0.005% of the present invention, and similarly, the crystal grains were coarsened and the impact value was low.

The O content of the comparative steel m is 0.008%, which is higher than the upper limit of 0.005% of the present invention. Therefore, the amount of the intervening material increases and the impact value is low.

Next, the conventional steel A is JIS-SKD61, and its hardness under tempering at 450 ° C is HRC 53 , and the hardness after repeated heating at 650 ° C is lowered to HRC 47. And the corrosion resistance is also poor.

Next, in Table 3, the hardness of the inventive steel No. 2 at a low temperature tempering at 450 ° C is low, but it has the same heat-resistant crack resistance as the 450 ° C tempered material of the conventional steel A having a high hardness. And excellent resistance to delayed damage.

Furthermore, compared with the case where the conventional steel A is tempered at a high temperature of 630 ° C, the tempering is performed at a low temperature, so that the corrosion resistance is high and the heat crack resistance is excellent.

As described above, it has been found that the inventive steel system has both the characteristics of suppressing cracking from the water-cooled pores and the properties of heat-resistant crack resistance which are conventionally opposite in nature.

As described above, the present invention has been described in detail with reference to the specific embodiments of the present invention, and various modifications and improvements can be made without departing from the spirit and scope of the invention. It is a saying.

The present invention is based on Japanese Patent Application No. 2005-346156, filed on Nov. 30, 2005, the disclosure of which is incorporated herein.

Claims (4)

A hot die steel for aluminum die casting, which is obtained by quenching a steel containing the following components and tempering at a temperature of 500 ° C or lower; the component contained is in mass %: C: 0.1 ~0.3%, Si: 0.1 to 1.5%, Mn: 0.3 to 2%, Cr: 6 to 12%, P: 0.05% or less, S: 0.01% or less, Mo: 1 to 3%, V: 0.5 to 1.5% s-Al: 0.005 to 0.025%, N: 0.005 to 0.025%, O: 0.005% or less, and the balance being Fe and unavoidable impurities. The hot-die steel for aluminum die-casting according to the first aspect of the invention, further comprising at least one selected from the group consisting of Ni: 2% or less and Cu: 1% or less in mass%. The hot die steel for aluminum die casting according to the first aspect of the patent application, further containing Co: 5% or less by mass%. For example, the hot die steel for aluminum die casting according to item 2 of the patent application scope is further improved. The step contains Co in a % by mass: 5% or less.