JPS60221555A - Extremely high-tension steel having superior resistance to melt fracture due to al - Google Patents

Abstract

PURPOSE:To obtain a steel having superior resistance to melt fracture due to molten Al by adding specified amounts of Ni, Cr, Mo and Ti to a low carbon steel or further adding Co. CONSTITUTION:An alloy steel contg., by weight, <0.05% C, 0.1-1.0% Al, 10- 20% Ni, 1-7% Cr, 3-7% Mo and 0.5-3% Ti or further contg. 3-30% Co is used as an extremely high tension steel having superior resistance to melt fracture due to molten Al and suitable for use as the material of dies for die-casting Al or a welding rod for repairing the dies. The steel causes hardly melt fracture due to molten Al and has superior thermal fatigue resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an ultra-high tensile strength steel with excellent Ag erosion resistance, and more specifically, it can be suitably used as a welding rod for repairing Aβ die-cast molds and AIl die-cast molds. Regarding ultra-high tensile strength steel.

Traditionally, AA' Guicast molds are mainly made of hot work tool steel such as 5KD6 steel or 5KD61 steel, or 18%Ni steel.
Manufactured from maraging steel. When using the former hot work tool steel, the material steel is annealed, rough carved, quenched, tempered, and then finished carved. However, hot work tool steel has a high C content and a low mass. Since the effect is large, heat treatment deformation and dimensional change during the manufacturing process are significant, and therefore, heat treatment and die carving are not easy, so the manufacturing process is complicated. Moreover, the AA die-cast mold obtained in this way suffers from significant melting loss, especially in molten Al.
It also has poor thermal fatigue resistance. Furthermore, in addition to the toughness at an ultra-high strength level, the strength and toughness at high temperatures, the weldability is insufficient.

On the other hand, 18% Ni maraging steel is known as a steel with excellent toughness at an ultra-high strength level, and when using this steel, it must be solution treated, die carved, and then aged. , an Al die-casting mold is manufactured. Therefore, when using 18% Ni maraging steel, two die-cutting steps, rough-cutting and finishing-cutting, are unnecessary, and one tempering operation can be omitted from the aging treatment, so the manufacturing process can be simplified. It's relatively easy. However, the Aj2 Gui cast mold obtained in this way also has poor Al corrosion resistance and low high temperature strength, as described above.
In particular, at temperatures above 400°C, the resistance to softening during over-aging is low, as well as sharply decreasing.

In addition, conventionally, repair of Al die-cast molds is usually done by
This is done by scraping the cracks that have formed in the mold and welding with 18%Ni maraging steel, but 18%N
As mentioned above, maraging steel is inferior in Al corrosion resistance and high temperature strength.

The present invention has been made in view of the above, and has not only excellent toughness at an ultra-high strength level and high-temperature strength, but also particularly excellent aluminum corrosion resistance and thermal fatigue resistance.
It is an object of the present invention to provide an ultra-high tensile strength steel that can be suitably used as an Il die-cast mold and a welding rod for repairing the same.

The ultra-high tensile strength steel with A1 erosion resistance according to the present invention is regulated to 0.05% or less of CO, ANo. 1 to 1.0%, Ni 10 to 20%, Cr 1 to 5% by weight. , Mo3-7%, Ti0.5-3%, the balance being iron and inevitable impurities.

That is, according to the steel of the present invention, first, the C content is reduced as much as possible, and the Ni and Cr contents are set within a predetermined range, and a martensitic matrix having high strength and high toughness is formed at room temperature by cooling after solution treatment. A single structure is formed, while a predetermined amount of MOLTi and An is contained together with Ni to precipitate an intermetallic compound in the matrix martensite phase after aging treatment, thereby increasing its strength and toughness. %Ni Provides excellent moldability, heat treatability, and high strength and toughness similar to maraging steel.

Furthermore, in the steel of the present invention, by adding a predetermined amount of the above-mentioned alloying elements and preferably Co, the As point is increased and the maximum condition for age hardening is shifted to the high temperature side, thereby improving the thermal fatigue resistance, that is, at high temperatures. The strength, toughness, and softening resistance of the steel were improved, and furthermore, the addition of Cr succeeded in significantly improving the A1 melting resistance.

Next, the reason for limiting the chemical composition in the high-strength steel according to the present invention will be explained in detail.

It is well known that C generally has the effect of increasing the strength of steel, but on the other hand, when the C content is high, it comes to have the drawbacks seen in the hot work tool steel described above.
In the present invention, C is regarded as an impurity element, and 0.
0.5% or less.

Since Si, Mn, P, and S are also impurity elements, it is better to reduce them as much as possible, and in the present invention, Si is 1.
0% or less, Mn is 0.1% or less, P is o, 01% or less,
S shall be 0.01% or less. Moreover, since O and N form nonmetallic inclusions and become starting points for cracks during processing, it is preferable to suppress their content to 50 ppm or less.

In the steel of the present invention, Ni is an essential element for forming a martensitic matrix together with Cr, and 10% or more of Ni is added to form a high-strength lath martensitic phase with excellent toughness. It is necessary. However, when added in a large amount exceeding 20%, the retained austenite phase is stabilized and no single martensite group is formed when cooled to room temperature. Therefore, in the steel of the present invention, the amount of Ni added is in the range of 10 to 20%.

In addition, Ni can be changed to N13Mo, Ni3T by aging treatment.
i. Intermetallic compounds such as N15AI are precipitated to increase strength.

Mo needs to be added in an amount of 3% or more in order to prevent grain boundary segregation due to P and S, and to shift the aging temperature to a higher temperature side to prevent over-aging.

MO can also be converted to N 13 Mo and others by aging treatment.
It precipitates as 82Mo and contributes to the strengthening of steel, but even if it is added in excess, it becomes uneconomical, so the upper limit is set to 7.
%.

Ti is an important element that controls the strength of the steel of the present invention,
Through the aging treatment, Ni3Ti is precipitated to strengthen the steel 1, and at the same time, the As point is raised, the maximum aging condition is shifted to the high temperature and long time side, and the softening resistance is increased. In order to effectively exhibit such an effect, it is necessary to add at least 0.1%, but adding too much causes embrittlement of the steel, so the upper limit is set at 3%.

AI2, like Ti, is an element that strengthens steel.

Through aging treatment, Ni3A1 precipitates and contributes to strengthening, and also improves high-temperature oxidation resistance, but if added in a large amount, it causes embrittlement of the steel, so the addition amount should be 0.1~
The range is 1.0%.

Cr is an essential element in the steel of the present invention, in order to form a martensitic phase with excellent toughness together with Ni, and in particular to ensure the An corrosion resistance and high temperature oxidation resistance of the steel. In order to effectively express the above, it is necessary to add at least 1.0% to the steel of the present invention.

However, when added in excess, the residual austenite phase is stabilized, so the amount added is in the range of 1 to 7%.

In the steel of the present invention, it is preferable that Ni+Cr be 19% or less in order to make the steel structure at room temperature a single phase of martensite by air cooling after solution treatment. This is because when the steel structure becomes a two-phase structure containing retained austenite, the strength becomes insufficient.

In the steel of the present invention, in order to further enhance its thermal fatigue resistance, it is preferable to contain Co in an amount of 3% or more. That is,
Co reduces the solid solubility of Mo, promotes the precipitation of Ni3M0, etc., shifts the aging temperature to a higher temperature side, increases high temperature strength, and thus significantly improves thermal fatigue resistance. However, since adding too much is not preferable from an economic point of view, the upper limit is set at 30%.

According to the steel of the present invention, for example, AI! A guicast mold is manufactured by vacuum melting steel having a predetermined chemical composition to form an ingot, followed by soaking treatment, solution treatment, die carving, and aging treatment. In the case of welding rods (filler wires), after soaking treatment, hot rolling is performed to form a wire rod with a diameter of 10 to 15i+a, solution treatment is applied, and then cold wire drawing is performed to form a wire rod of an appropriate diameter. A welding rod (filler wire) can be obtained by solution treatment again.

As described above, the steel of the present invention suppresses the C content to 0.05% or less and forms a single parent phase consisting of high strength and high toughness martensite by adding the above-mentioned alloying elements, which is then subjected to aging treatment. By precipitating intermetallic compounds, the strength is weakened, and by shifting the maximum condition for age hardening to higher temperatures, the strength, toughness, and softening resistance at high temperatures are improved, increasing thermal fatigue resistance. Furthermore, by adding Cr, its An corrosion resistance has been significantly improved, making it useful as an ultra-high tensile strength steel for A1 die-casting molds. Of course, it can also be suitably used in other molds, such as molds for plastic or rubber molding. Further, the steel of the present invention is useful as a welding rod for repairing AA die-cast molds that does not require heat treatment after repair.

Examples of the present invention are listed below, but the present invention is not limited to these Examples.

EXAMPLE Steels having chemical compositions shown in Table 1 Steel Nos. 1 to 4 were prepared by vacuum melting method and made into ingots. After soaking treatment, the ingots were skinned. After bare forging at 1150-1200℃, finish forging to diameter 20 dragon steel, solution treatment by heating at 850℃ for 1 hour, after air cooling, test piece processing, 550℃
Aging treatment was performed by heating at °C for 3 hours and cooling.

Table 2 shows the room temperature strength of the forged product, the mechanical properties at 600℃,
and HRC after aging treatment at 600°C for 3 hours
shows. Table 3 also shows transformation temperature, thermal fatigue resistance, and AI! The results of the erosion test are shown.

Also, for comparison, 5KD61 steel, 18% Ni Co
The chemical compositions of additive-free maraging steel and 18% Ni 300ksi maraging steel are shown in Table 1 as comparative steels, and properties similar to those of the steel of the present invention are shown in Tables 2 and 3.

According to the steel of the present invention, it has significantly better room temperature strength and high temperature strength than Comparative Steel 5, and has excellent A1 erosion resistance, and also has better high temperature strength and A1 resistance than conventional maraging steel. It is clear that the corrosion resistance is extremely good.

Claims (2)

[Claims] (1) In terms of weight percent, GO is regulated to 0.05% or less, and 80.0% to 1.0%. Ni 10-20%, Cr 1-7%, Mo 3-7%, TiO, 5-3%. An ultra-high tensile J flint with excellent Ag erosion resistance, characterized in that the remainder consists of iron and unavoidable impurities. (2) Regulate CO,05% or less in weight%, Δk 0.1-1.0%, N110-20%, Cr 1-7%, Mo3-7%, Co 3-30%, TiO,5 ~3%, the balance being iron and unavoidable impurities.