JPS5841341B2 - Tool steel for hot working - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is a low-alloy material that can be used as a mold material for medium-volume production in applications where the heating temperature during use is relatively low, such as Al extrusion dies, A1 die-casting dies, and shallow impression hot forging dies. in,
The present invention also relates to an economical tool steel for hot working that satisfies necessary properties.

In the case of molds such as Al extrusion dies whose billet temperature is generally low, below 500°C, and whose surface is nitrided, softening resistance and high-temperature strength in the relatively low temperature range of 600 to 650°C or below are important. In addition, sufficient toughness is required to suppress the propagation of cracks from some corners.

In addition, in order to prevent skin roughness during use and maintain a beautiful product skin, a high nitrided surface layer hardness is required.At the same time, in order to suppress the initial crack generation and progression from the nitrided layer, repeated nitriding treatment is required. A characteristic that prevents the nitrided layer from becoming excessively deep is required.

In addition, even in medium-volume production molds for applications where the temperature rise temperature during use is relatively low, such as AI die-casting molds and shallow impression forging molds, we can provide softening resistance and high-temperature strength at temperatures below 600 to 650°C. is required as a minimum essential requirement.

For example, conventionally 0.37C-5C is used for Al extrusion dies.
5KD61.0.38C of r-1,3Mo-IV
5Cr-1,3Mo-0,4V 5KD6.0.37
C5Cr-1,3W-1,3Mo-0,4V 5KD6
2, etc., but considering the above usage conditions and required properties, it is possible to use a lower alloy as a mold material for medium volume production in terms of high temperature strength, and also in terms of crack resistance. There are cases where it is more desirable.

On the other hand, for low-alloy 5KT4, the nitriding temperature (generally 5
Not only is it not possible to maintain sufficient hardness (HRC45-50) as a mold material for this purpose at temperatures below 650-600°C, which is required for a mold material for this purpose, but sufficient nitrided surface hardness cannot be obtained. It is also insufficient in terms of softening resistance and high temperature strength.

The present invention uses C-5Cr-low MoW low to medium ■ as a basic component, and has a low Mof by making C slightly high.
It has a sufficiently high softening resistance at temperatures below 650°C even under the conditions of We have developed an economical tool steel for hot working that has a small tendency for decarburization and roughening, is air-quenchable, easy to heat treat, and has excellent machinability.

Table 1 shows the chemical compositions of the steel of the present invention and the conventional steel (SKD62).

Table 2 shows the tempering softening resistance of the steel of the present invention.

It can be seen that the steel of the present invention has a softening resistance equivalent to or similar to that of conventional steel at temperatures of 600 to 650°C or lower.

This is due to the increase in the amount of precipitated carbides due to the higher C content.

Note that the comparative steel has a much lower softening resistance and cannot be applied to applications such as Al extrusion dies and AI die cast molds. Table 3 shows the high temperature strength (600° C.) of the steel of the present invention.

It can be seen that the steel of the present invention has high-temperature strength at 600°C comparable to that of the conventional steel (SKD62).

This is due to the fact that the steel of the present invention has a high C content and the softening resistance at 600 to 650°C is adjusted to the same level as 5KD62 or a similar level.

In addition, comparative steel Q (SKT4) has significantly lower high temperature strength.
The steel cannot be used in applications such as Al extrusion dies and other applications for the steel of the present invention.

Table 4 shows the hardness behavior of the steel of the present invention upon nitriding.

It can be seen that the steel of the present invention has almost the same nitriding properties as the conventional steel 5KD62.

This is due to the setting of the Cr content in the steel of the present invention; if the Cr content is increased as in the steel of the present invention, the nitrided surface hardness increases but the depth decreases. It can be seen that if the value is set lower, the depth of the nitrided layer becomes greater.

In addition, the hardness of the nitrided layer of Comparative Example Steel Q is low and cannot be used for the purpose of the present invention.

Table 5 shows the bending failure load ratio of the nitrided test pieces of the steel of the present invention.

The sample was a flat plate, and after being treated with tuffyride at 570°C for 5 hours, it was loaded and fractured using a single point loading method at the center, and the breaking load was determined and expressed as a ratio, with that of conventional steel 5KD62 set as 100.

All of the steels of the present invention exhibit better breaking loads than 5KD62.

This is due to the improvement of the toughness of the base metal based on the reduction of segregation due to low alloying and the adjustment of base metal constituent elements such as Cr and Ni. Ni addition improves the toughness of the base metal and increases the breaking load. Furthermore, an increase in the amount of Cr (reduction in the nitriding depth) also increases the bending breaking load, and a decrease in Cr increases the thickness of the nitriding layer, which has the effect of reducing the bending breaking load.

Table 6 shows the machinability of the steel of the present invention.

The tool life in turning is expressed as a ratio, with that of conventional steel set as 100.

The present invention steel is conventional steel 5KD62 without Mn and Ni addition.
It is clear that the machinability is greater.

It has been found that an increase in Ni and Mn reduces machinability, and the amount added is subject to restrictions from this point of view.

Table 7 shows the seizure critical load in the high temperature seizure resistance test of the steel of the present invention.

The sample is a cylindrical sample, which has been quenched and tempered in advance by HRC.
45) → Nitriding → 700°C after performing air oxidation treatment
The maximum load (critical load) at which seizure does not occur when the end face is pressed against a heated steel material (component material) while rotating at high speed is determined, and is expressed as an index with the value of conventional steel set as 100.

It can be seen that the steel of the present invention exhibits a seizure critical load comparable to or superior to that of the conventional steel 5KD62.

This is because the strength of the base material and the properties of the nitrided layer are similar to 5KD62, the carbon content is high and the amount of carbide is increased, and the steels M and N of the present invention have a dense oxide film due to the addition of Co. This is due to the protective effect of

Next, the reasons for limiting the composition of the steel of the present invention will be described.

The setting of C is important because it maintains the softening resistance of the steel of the present invention in the temperature range of 600 to 650° C. or lower to the same level as conventional steel 5KD62.

The action of C maintains the hardenability, temperability, and high-temperature hardness of the steel of the present invention, and also combines with carbide-forming elements such as W, Mo, V, and Cr to form carbides and refines grains. , 5KD62 in the temperature range below 600-650℃
It is added to provide high-temperature wear resistance, tempering softening resistance, and high-temperature strength similar to the above.

If the amount is too high, the toughness and high-temperature strength will also be lowered, so the content should be 0.60% or less. If the content is too low, the effect of the above addition cannot be obtained, so the content should be 0.45% or more.

Si has the effect of improving the oxidation resistance of the steel of the present invention, and is generally 0.70% or less as a standard, and the amount added is adjusted depending on the purpose and use.

If it is too large, it lowers the thermal conductivity and also causes a decrease in ductility, so the content should be 1.50% or less.

In order to supplement the hardenability of the steel of the present invention, Mn is added in consideration of the purpose, application, mold dimensions, etc.

Generally, the standard is 0.60% or less, and the amount added is adjusted in consideration of the above factors.

If it is too large, the machinability will be reduced and the A1 transformation point will also be lowered, so the content should be 1.50% or less.

Ni is added for the purpose of further improving the toughness of the steel of the present invention, depending on the use and required conditions.

If it is too large, the annealing hardness becomes excessively high and the machinability decreases, so it should be 1.50% or less.

By setting an appropriate content, Cr has two effects: resistance to temper softening, improvement of high-temperature strength, provision of appropriate oxidation properties, improvement of wear resistance by combining with C to form carbides, and direction of A1 transformation point. , which has the effect of improving hardenability and is an extremely important element.

In the case of the steel of the present invention, its setting is particularly important because it allows air-cooling quenching and provides softening resistance, high temperature strength, and custom nitriding properties similar to conventional steel 5KD62.

If it is too low, the nitrided surface hardness decreases, crack resistance occurs due to increased nitrided depth, progressability decreases, 600 to 650°C
The softening resistance and high-temperature strength cannot be maintained in the following temperature ranges, and air-cooling quenching is no longer possible, so the percentage should be 4.50% or more. If it is too high, the nitriding depth will be excessively reduced, and the high-temperature strength and softening resistance will even be reduced. It should be set at 6.50% or less.

Mo and W combine with C to form carbides, increasing wear resistance, and solid solution in the matrix during quenching, forming and dispersing fine carbides during tempering, imparting softening resistance and high-temperature strength. It is an important and essential additive element.

In the case of the steel of the present invention, by increasing the C content, the required amounts of W and Mo are reduced, thereby improving crack propagation resistance and providing economic efficiency.

In connection with controlling the amount of C to 0.45 to 0.65% in order to maintain the softening resistance and high temperature strength in the temperature range of 600 to 650°C or less at a level similar to that of conventional steel 5KD62 (1/2 W content (Mo) is required to be 0.40□ or more, with a lower limit of 0.40% and an upper limit of 0.40%, and an upper limit of 0.40% to maintain a particularly high crack growth resistance ratio, to extremely suppress segregation and the tendency to form coarse carbides, and In view of economic efficiency and the purpose of the present invention as a medium-volume production steel, the content is limited to 1.20% or less.

Like W and Mo, ■ forms fine carbides that are difficult to agglomerate during tempering, and is essential for maintaining tempering softening resistance and high-temperature strength, as well as for refining crystal grains and improving toughness. , and without V, Mo,
The softening resistance and high-temperature strength improvement effects of W are also not fully exhibited.

In order to maintain the softening resistance and high-temperature strength in the temperature range of 600 to 650'C or less at a level similar to that of conventional steel 5KD62, the lower limit was set at 0.05 in relation to the amounts of C, Mo, and W.
%, and the upper limit was set to O and SO% in order to suppress segregation and the tendency to form coarse carbides, particularly for the purpose of the present invention as a steel for high crack growth resistance, economic efficiency, and medium-volume production.

Co is an additive element for imparting extremely high wear resistance at high temperatures to the steel of the present invention.

The addition of CO strengthens the substrate and at the same time forms an extremely dense and highly adhesive protective oxide film when the mold surface is heated during use, resulting in excellent high-temperature wear resistance.

In the case of the steel of the present invention used for medium-sized production, it is not necessary to add a large amount, and too much will lead to the addition of dust, so 3.00
% or less.

In the case of the steel of the present invention, the standard addition amount is 1.00 to 1.50%.

As described above, the steel of the present invention is suitable for use in hot molds for medium-volume production where the heating temperature during use is relatively low, has excellent property improvement effects through nitriding, and has economic benefits. The present invention provides a new low-alloy hot work tool steel.

Claims (1)

[Claims] I C0.45 to 0.60%, Si 1.50% or less,
Mn 1.50% or less, Cr 4.50-6.50%, (
1/2W+Mo) 0.40-1.20%, Vo, 0
A tool steel for hot working consisting of 5 to 0.80% balance Fe. 2 C0.45~0.60%, Si1.50% or less,
Mn 1.50% or less, Cr 4.50-6.50%, (
1/2W+Mo) 0.40~1.20%, Vo,
A tool steel for hot working consisting of 0.05 to 0.80% Fil and 5% or less Fe. 3 C0.45-0.60%, Si1.50% or less,
Mn 1.50% or less, Cr 4.50-6.50%, (
1/2W+Mo) 0.40-1.20%, VO
, 05 to 0.80%, Co 3.00% or less, and the balance Fe. 4 C0.45-0.60%, Si 1.50% or less,
Mn 1.50% or less, Cr 4.50-6.50%, (
1/2W+Mo) 0.40~1.20%, Vo,
05-0.80%, Co 3.00% or less, Ni 1.5
Hot working tool steel consisting of 0% or less balance Fe.