JPH03134135A - Tool steel for hot working - Google Patents

Abstract

PURPOSE:To improve the high temp. strength of the tool steel without damaging its high toughness by particularly adding a specified amt. of Nb to a tool steel for hot working having a prescribed compsn. CONSTITUTION:The compsn. of a tool steel for hot working used for a press forging die or the like is formed of, by weight, 0.25 to 0.35% C, <=1.5% Si, <=2% Mn, 0.3 to 2% Ni, 2.5 to 4.5% Cr, 0.25 to 3.5% 1/2 W+Mo, 0.2 to 2% V, 0.01 to 0.2% Nb, <=0.005% S and the balance Fe with inevitable impurities. In the steel, the amt. of residual carbides in reduced at a quenching temp. higher than that of a conventional steel to suppress the flocculation of the carbides and to ensure its high temp. strength while the deterioration of toughness is prevented by adding Nb effective in refining crystalline grains. Furthermore, at the time of incorporating 0.5 to 5% Co into the steel, its galling critical load ratio is made higher.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention provides a press that has high-level high-temperature strength and toughness, has excellent heat crack resistance life, and can easily re-cut the mold surface. This relates to hot working tool steel used in forging dies and the like.

[Conventional technology]

The use of hot forging to form large parts has expanded due to the growing size of automobile parts and the shift from cast iron to steel due to increased engine horsepower. Steel for molds used in this type of application is required to have properties such as blowability.

(1) Must have the strength to withstand stress during use.

(2) Since the surface of the mold reaches a high temperature during use, it must have good high-temperature strength and softening resistance.

(3) It has excellent heat crack resistance and can be easily re-cut. (4) Because the mold is large, it has excellent hardenability and high toughness to prevent breakage.

[Problem to be solved by the invention]

Conventionally, SKT mold materials with excellent toughness have been used for this type of application, and Japanese Patent Publication No. 54-38570 and Japanese Patent Application Laid-open No. 53-80 have been used.
The mold material proposed in No. 318 is used.

However, the SKT type or the mold materials proposed in Japanese Patent Publication No. 54-38570 and Japanese Patent Application Laid-open No. 53-80318 had a problem of insufficient high temperature strength.

Also, as a tool steel for hot working, JP-A-62-112761
No., JP-A-63-162840, JP-A-57-514
No. 56, JP-A No. 62-149852, etc., have proposed steels.

However, JP-A-62-112761, JP-A-63-1
The steels proposed in No. 62840 and Special Publication No. 57-51456 both have high C and high hardness (HRC 48 or higher).
This steel inherently lacks toughness and cannot be applied to large press forging dies, which is the object of the present invention.

Also, JP-A-62-161942, JP-A-62-14
The steel proposed in No. 9852 is mainly used for die casting molds with complex shapes. Molds for this purpose are manufactured by processing the mold in an annealed state, followed by quenching and tempering, but in order to avoid dimensional changes in the mold during quenching as much as possible, gradual cooling is used as cooling during quenching. be done. Therefore, JP-A-62-161942, JP-A-6
The steel disclosed in No. 2-149852 has a composition that emphasizes hardenability, and it is difficult to apply it to the large press forging mold that is the object of the present invention.

In view of the above background, it is an object of the present invention to provide a tool steel for hot working that has particularly improved high-temperature strength.

[Means to solve the problem]

The high-temperature strength of hot work tool steel is provided by fine carbides precipitated during quenching and tempering.

However, as the temperature of the tool surface increases as the tool is used, agglomeration of carbides progresses and the strength decreases.

One of the factors that increases the rate of aggregation of carbides is that the amount of carbon is high relative to the amount of carbide-forming elements. In this case, since the activity of carbon is high, the rate of carbide aggregation becomes high. Regarding this point, the present inventor conducted studies on optimizing the amount of carbon and carbide-forming elements.

Another factor is the presence of relatively large grain size carbides that remain without solid solution in the matrix during quenching, and these large grain size residual carbides have the effect of accelerating the rate of aggregation of carbides. For this purpose, it is effective to increase the quenching temperature and reduce the amount of residual carbide, but this also causes coarsening of crystal grains and decreases toughness. In order to suppress this, in the present invention, by adding Nb, which is effective in refining grains, the amount of residual carbides is reduced at a higher quenching temperature than conventional steel, while preventing a decrease in toughness. The purpose is to suppress agglomeration and ensure high-temperature strength.

That is, in the present invention, C0.25 to 0.35% by weight,
SL 1.5% or less, Mn 2% or less, Ni 0.3-2%
, Cr2.5-4.5%, 1/2W +Mo 0.25
-3.5%, V 0.2-2%, Nb 0.01-0.
2%, S 0.005% or less, and in some cases,
This is a hot working tool steel characterized by comprising 0.5 to 5% Co, the balance Fe and inevitable impurities.

[Effect]

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

C is W, M0. ■It is added to combine with carbide-forming elements such as Nb and Or to form carbides and provide tempering softening resistance and high-temperature strength, but if it is too large, it will increase the amount of residual carbides and may also precipitate during tempering. This speeds up the growth and aggregation of finely precipitated carbides that provide high-temperature strength. For this reason, it is necessary to keep the content low considering the balance with the amount of carbide-forming elements, and the upper limit is set at 0.35%. If it is too low, hardenability and tempering hardness will be insufficient.
The content shall be 0.25% or more.

Si is 1.5 because it forms a protective oxide film due to temperature rise during use, and also reduces toughness and thermal conductivity.
0% or less.

Mn improves hardenability, but too much Mn results in A.

It lowers the transformation point excessively, increases the annealing hardness excessively, and reduces machinability, so it is set to 2.00% or less.

Ni is C, Cr, Mn, M0. It is an important additive element that, together with W and the like, imparts excellent hardenability to the steel of the present invention, forms a martensite-based structure even at a slow quenching cooling rate, and prevents a decrease in toughness.

It also contributes to improving the essential toughness of the base.

Ni is required to be 0.3% or more to obtain the above effects, but if it is too large, A. excessively lowers the transformation point, reduces fatigue resistance and life, and excessively increases annealing hardness. Since this decreases machinability, it is set to 2.00% or less.

Cr has the effect of improving tempering softening resistance and high temperature strength by setting an appropriate amount of addition, improving wear resistance by combining with C to form carbide, and improving hardenability.

If it is too low, the oxidation resistance will be insufficient and the surface will be rough during use.In order to obtain the above-mentioned addition effects as well as the excellent toughness that is a characteristic of the steel of the present invention, the content should be 2.5% or more. If the content is too high, carbides that tend to aggregate when the temperature rises are formed, reducing softening resistance and high-temperature strength, so the content should be 4.50% or less.

W and Mo have the effect of increasing softening resistance and high-temperature strength by precipitating fine carbides during tempering, so they should be added in as large a quantity as possible depending on the amount of C, but if they are too large, they may interfere with the amount of C. In relation to this, the amount of carbides becomes too large, and these are arranged like strings in the hot working direction, making it easier for cracks to extend in the hot working direction.Also, the amount of fine carbides precipitated during tempering becomes too large, which deteriorates the toughness. 1/2 to reduce
The W+Mo content should be 3.5% or less, and if it is too low, the effect of the above addition cannot be obtained, so the content should be 0.25% or more.

(2) forms carbides that are difficult to form a solid solution, and has the effect of improving wear resistance and seizure resistance. It is an important element for increasing softening resistance in high temperature ranges and obtaining high high temperature strength.

In addition, it refines the crystal grains to improve toughness, and
It raises the transformation point and, together with excellent high-temperature strength, has the effect of improving heat crack resistance.

If the amount is too large, huge carbides will be generated, increasing the distribution tendency of string-like carbides along the hot working direction, and promoting the extension of cracks along the hot working direction, so the upper limit is set to 2%. On the other hand, if the content is too low, the above effects of addition may not be obtained, such as early softening of the mold surface, so the content should be 0.20% or more.

Nb has the effect of suppressing grain coarsening during quenching at high temperatures in order to increase softening resistance and high-temperature strength, and provides both the excellent high-temperature strength and toughness that are the characteristics of the steel of the present invention. It is an important element for

If the content is too large, carbides that are difficult to form a solid solution are formed and the toughness is reduced, so the content should be 0.20% or less. If it is too low, the effect of the above addition cannot be obtained, so the content should be 0.02% or more.

S forms sulfides and is distributed in an elongated manner in the hot working direction, reducing the toughness in the hot working direction. The influence of S on this is 0.005% or less, and it has been found that the effect is large, so the upper limit of S is set at 0.005%.

Co forms an extremely dense protective oxide film with good adhesion when the temperature rises during use, which prevents metal contact with the other material, prevents the temperature rise of the metal surface, and provides excellent wear resistance. It is something that brings.

Furthermore, effects such as a heat insulation effect due to the formation of the oxide film, improvement of heat crack resistance due to the protective effect, and suppression of the formation of starting points for crack generation can be obtained.

Co is added to provide the above effect, but if it is too large it will reduce the toughness, so it should be 5.00% or less, and if it is too low, the above effect of addition cannot be obtained, so it should be 0.50% or more.

〔Example〕

Hereinafter, the present invention will be explained in detail based on examples.

Table 1 shows the chemical compositions of the invention steel, comparative steel, and conventional steel. Table 2 shows the high temperature strength and toughness of the invention steel, comparative steel, and conventional steel under standard heat treatment conditions.

Table 2 High temperature strength is shown as tensile strength at 700℃, toughness is 2
mm U notch Charpy impact test results.

The steels of the present invention (A-D) are clearly superior in high temperature strength to conventional @F (SKT4).

In addition, the steel of the present invention was used as conventional steel G (steel disclosed in Japanese Patent Publication No. 54-38570), conventional steel H (steel disclosed in Japanese Patent Publication No. 58-8031).
It is clearly superior to the steel disclosed in No. 8 D). This is because the standard quenching temperature of the steel of the present invention is higher than that of conventional steel. On the other hand, the toughness is equal to or higher than conventional steel. From these things, the mold surface is 600
When the steel of the present invention is used in a mold for applications where the temperature rises above .degree. C., it is presumed that heat cracks and plastic flow on the surface of the mold are less likely to occur.

Table 3 shows the quenching temperature, austenite grain size of the quenched structure, high-temperature strength, and toughness (
2 mm U notch Charpy impact value). From Table 3, it is clear that the higher the quenching temperature, the higher the high-temperature strength of both the present invention aD and comparative RE.
In comparison R4E, the toughness decreases as the quenching temperature increases.

This is because as the quenching temperature increases, the austenite crystal grains in the quenched structure become coarser and the structure becomes coarser. On the other hand, the steel of the present invention to which Nb is added does not lose its toughness even when the quenching temperature increases. This is because Nb prevents coarse growth of austenite crystal grains, and the importance of Nb is recognized particularly for applications where high temperature strength and softening resistance are important.

Table 4 Table 4 shows the heat crack resistance test results of the steel of the present invention. This test result shows that the surface of a 15Mφ x 25M test piece was rapidly heated to 700℃ and then rapidly cooled to 20℃ in water for 15 minutes.
This is the result of repeating 00 times.

The steel of the present invention has a smaller number of heat cracks and a smaller depth of heat cracks than conventional steel.

This is one of the major features of the steel of the present invention, and shows that heat cracks are less likely to occur, and that the heat cracks that do occur are less likely to propagate deeply, reducing the number of man-hours required to recut the mold surface.

No. 5 Maximum load without seizure (critical load) when attached
, and the seizure critical load of conventional steel F (SKT4) is calculated as 10
It is expressed as an index with 0 as the value.

This is due to the high temperature strength of the steel of the present invention, but the reason why the critical baking load of the steel C of the present invention containing Go is high is due to the addition of Co, which is formed on the sample surface by the oxidation treatment. This is because the oxide film, which has protective and lubricating effects, is dense and difficult to peel off.

〔Effect of the invention〕

Claims (1)

[Claims] 1% by weight: C 0.25-0.35%, Si 1.5% or less, Mn 2% or less, Ni 0.3-2%, Cr 2.5-4
．． 5%, 1/2W+Mo0.25-3.5%, V0.2
2% Nb, 0.01-0.2% Nb, 0.005% or less S, and the balance Fe and unavoidable impurities. 2% by weight: C 0.25-0.35%, Si 1.5% or less, Mn 2% or less, Ni 0.3-2%, Cr 2.5-4
．． 5%, 1/2W+Mo0.25-3.5%, V0.2
2% Nb, 0.01-0.2% S, 0.005% or less Co, 0.5-5% Co, and the remainder Fe and unavoidable impurities.