JPH028347A - Tool steel for warm and hot working - Google Patents

Abstract

PURPOSE:To obtain the title tool steel having good cold strength and wear resistance and having excellent high temp. strength and toughness by specifying the compsn. constituted of C, Si, Mn, Cr, W, Mo, V, Co, S, Nb and Fe. CONSTITUTION:The tool steel for warm and hot working is constituted of, by weight, 0.45 to 0.7% C, <=0.60% Si, <=1.50% Mn, 3.00 to 5.50% Cr, 2.00 to 3.50% 1/2 W+Mo, 0.80 to 1.60% V, 0.30 to 5.00% Co, <=0.005% S and <=0.20% Nb, furthermore of <=1.80% Ni and/or <=0.10% N according to necessary and the balance Fe with inevitable impurities. The tool steel has improved toughness in the direction of hot working and is provided with excellent high temp. strength and oxidation film characteristics. In the steel, settling, heat cracks, large cracks and wear are hard to generate and excellent tool service life is given.

Description

[Detailed description of the invention] [Industrial application field] The present invention is characterized by having good room temperature strength and wear resistance, and sufficient hardenability, and particularly excellent high temperature strength and toughness. This invention relates to tool steels for warm and hot working.

[Conventional technology]

Conventionally, hot work tool steel 5KD, which has particularly high high temperature strength, has been used for molds for applications that require particularly excellent dimensional accuracy of forged products, such as molds for warm forging and hot precision press forging.
8 and high-speed tool steel type material (for example, 5KH51) with high strength at room temperature to high temperature are used.

However, in the case of 5KD8, it is difficult to obtain high room-temperature strength (initial hardness) due to the low to medium C content, so if the processing temperature is low, it may cause settling and wear early, and even if the processing temperature is high, In some cases, the strength of the inner layer supporting the surface layer was not sufficient.

On the other hand, when conventional high-speed tool steel (e.g. 5KH51) is applied to molds in this field, large cracks tend to occur due to lack of toughness, and heat cracks easily occur due to insufficient softening resistance at high temperatures. Due to the above problems, sufficient mold life is not always obtained.

Furthermore, since conventional steel contains a large amount of carbide-forming elements, a string-like distribution of carbides occurs in the steel material along the hot working direction, which promotes the occurrence of the large cracks and heat cracks described above.

[Problem to be solved by the invention]

Regarding improvement of these problems of conventional steel 5KD8.5KH51, Japanese Patent Publication No. 55-2466 and Japanese Patent Publication No. 57-2634
No. 2, JP 62-112761, JP 55-49
Various proposals have been made, such as No. 148 and Japanese Patent Publication No. 58-17250, and although certain improvement effects have been obtained, they still do not fully satisfy the requirements for mold life. Each problem will be described in the Examples section.

The steel of the present invention improves high-temperature strength and toughness, which are problems of conventional steel, and suppresses the string-like distribution of carbides, making it resistant to setting at high temperatures and creating a shape material that is resistant to the propagation of cracks in the hot working direction. It is an attempt to develop and solve problems.

[Means to solve the problem]

The chemical composition of the steel of the present invention was determined by systematically examining the amount and balance of carbide-forming elements, optimizing both high-temperature strength and toughness, and reducing the tendency of the material to form a string-like distribution of carbides. This is a dispersed distribution of carbides.

In addition, it is characterized by refinement of crystal grains due to Nb addition and further N addition. That is, especially when the heating temperature of the mold material is high, it is effective to raise the quenching temperature in order to increase the softening resistance, but the toughness decreases due to coarsening of crystal grains.

Addition of Nb is particularly effective in suppressing this.

This effect becomes even more remarkable when Ni is included.

That is, in combination with the essential toughness improvement of the matrix by Ni addition, it is possible to impart particularly excellent toughness.

In addition, CO is added to provide wear resistance in warm and hot conditions.
As the temperature rises during use, a dense and highly adhesive oxide film is formed on the mold surface, and the lubricating and heat-insulating effects of this, combined with the strength of the base material and an appropriate amount of carbide distribution, improves the resistance to warm and hot conditions. It has significantly improved abrasion resistance and roughening resistance.

That is, in the present invention, C0.45 to 0.7%, S
i 0.60% or less, Mn 1.50% or less, Cr 3.
00-5.50%.

2. One or two of W and Mo is 1/2W+Mo.
00-3.50%, V 0.80-1,60%, Co
0.30-5.00%.

A temperature control material characterized by containing S 0.005% or less, Nb 0.20% or less, and in some cases Ni 1.8% or less and N 0.10% or less, with the balance consisting of Fe and unavoidable impurities. It is a tool steel for cold and hot working.

[Effect]

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

C is added to provide the excellent hardenability, tempering hardness, and high-temperature hardness of the present invention. If it is too large, the toughness will decrease and a string-like carbide distribution will occur, so the content should be 0.70% or less, and if it is too small, the above-mentioned effect of addition cannot be obtained, so the content should be 0.45% or more.

Si is 0.6 because it forms a protective oxide film due to temperature rise during use, and also reduces toughness and thermal conductivity.
0% or less. More preferably, it is 0.30% or less.

Mn improves hardenability, but if it is too large, it will excessively lower the A1 transformation point, excessively increase annealing hardness, and reduce machinability, so it should be kept at 1.50% or less.

Ni is C, Cr, Mn, M0. Together with W and the like, it is an important additive element that provides excellent hardenability to the present invention, forms a martensite-based structure even at a slow quenching cooling rate, and prevents a decrease in toughness. It also gives an essential toughness improvement to the base.

Ni is added to obtain the above effects, but if it is too large, it will excessively lower the A1 transformation point, leading to a decrease in fatigue life, and excessively increasing annealing hardness, decreasing machinability. Therefore, if it is included, it should be 1.80% or less.

Cr improves temper softening resistance and high-temperature strength by setting an appropriate amount of addition, improves wear resistance by combining with C to form carbides, improves hardenability, and imparts rapid nitriding properties. It is something that you have. If it is too low, the oxidation resistance will be insufficient and the surface will be rough during use.In addition to the above-mentioned effects of addition, it is added in an amount of 3.00% or more in order to obtain the excellent toughness that is a characteristic of the steel of the present invention. 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 5.50% or less.

W and Mo can be added singly or in combination, and they form carbides that are difficult to dissolve in the matrix during quenching and heating, and have the effect of improving wear resistance.They also form fine carbides during tempering. It precipitates and has the effect of increasing softening resistance and high-temperature strength.

W and Mo are added to obtain the above effect, but if they are too large, the amount of carbides becomes excessively large in relation to the amount of C, and these are arranged in a string shape in the hot working direction, resulting in hot working. Since cracks tend to extend in the processing direction and the amount of fine carbides precipitated during tempering becomes excessively large, reducing toughness, one or both of W and Mo are added to 1/2W.
+Mo is set to 3.50% or less, and if it is too low, the effect of the above addition cannot be obtained, so it is set to 2.00% or more.

The addition of W has a greater effect of increasing high temperature strength and wear resistance than the addition of Mo, while M increases the toughness.

is more advantageous.

■ is a substance that forms carbides and has the effect of improving wear resistance and seizure resistance.It dissolves solidly in the matrix during quenching and heating, precipitates fine carbides that are difficult to aggregate during tempering, and is effective in high temperature ranges. It is an important element for increasing softening resistance and providing large high-temperature proof strength.

In addition, it refines the crystal grains to improve toughness, and
This has the effect of raising the 1 transformation point and, together with excellent high-temperature yield strength, improving heat crack resistance.

The setting of the v amount is very important for the steel of the present invention to have both excellent toughness and high-temperature strength.

If it is too high, it will generate huge carbides, increase the distribution tendency of string-like carbides along the hot direction, and encourage the extension of cracks along the hot direction. Since the above-mentioned effects of addition cannot be obtained, such as causing early softening of the parts, the content should be 0.80% or more.

When the temperature rises during use, Co forms an extremely dense and highly adhesive protective oxide film, which eliminates metal contact with the other material, prevents the metal surface from rising in temperature, and provides excellent wear resistance. It brings about

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 effects, 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 0.30% or more.

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

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.

S forms sulfides and is distributed in an elongated manner in the hot working direction, reducing the toughness in the hot working direction. The effect of S on this becomes large at 0.005% or less, and even more so when S O. It was found that the effect becomes significantly greater when the temperature is 0.003% or less, and the upper limit of S is set to 0.03% or less. oos%, and a more desirable range is 0.003% or less.

N has the effect of refining crystal grains and improves the toughness of the steel of the present invention, and is added for this purpose.

It is not necessary to add a large amount, and the content should be 0.10% or less in consideration of manufacturability during melting and agglomeration.

〔Example〕

The present invention will be described in detail below 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 T-direction toughness of the steel of the present invention and the conventional steel under standard heat treatment conditions.

High-temperature strength is shown as tensile strength at 700°C using a sample taken from a 120nynφ steel material in the L direction (parallel to forging and stretching direction), and toughness in the T direction is based on a sample taken from a 120mφ steel material in the T direction (perpendicular to the forging and stretching direction). The results are shown in the results of a 100mmR Charpy impact test.

The major feature of the steels of the present invention shown in Table 1 is that the toughness in the T direction is clearly superior to that of conventional steels.

This is because the steel of the present invention not only has a high base toughness, but also has a small tendency to form a string-like distribution of residual carbides and a small tendency for striped segregation. In tools using conventional steel, large cracks along the string-like distributed carbides occur early, and heat cracks generated by thermal stress tend to extend along the string-like distributed carbides, resulting in damage to the tool surface. However, by using the steel of the present invention, the occurrence of large cracks was eliminated and the tool life due to damage to the tool surface was also improved. It also has high-temperature strength that is equal to or higher than that of conventional steel.

Next, the characteristics of the steel of the present invention and the conventional steel will be compared and the problems of each will be pointed out.

Conventional steel L (Japanese Patent Publication No. 55-2466) has low toughness in the T direction. This is because the amount of C is too high relative to the amount of V, and the VC carbides are distributed in a string-like manner in the forging direction.

Conventional steel M (Japanese Patent Publication No. 57-26342) also has low toughness in the T direction. This is because the amounts of v and C are too high, and the VC carbides tend to have a strong string-like distribution in the forging direction, and the amounts of W and Mo are too large, and the amount of carbides precipitated during tempering is too large.

Conventional steel N (JP 62-112761) does not have a string-like distribution of residual carbides, but the amounts of W and Mo are too large, resulting in lower toughness than the steel of the present invention, and the amount of V, which contributes to high-temperature strength, is lower than that of the steel of the present invention. It is too low and is lower than the steel of the present invention in this aspect as well.

Conventional steel O (Japanese Patent Publication No. 55-49148) has low toughness.

This is because the amount of Si contained is too high.

Conventional steel P (Japanese Patent Publication No. 58-17250) has low high temperature strength. This is because C promotes the agglomeration of carbides by heating at high temperatures.
This is because it contains a large amount of r.

Table 3 shows the quenching temperature, austenite grain size of the quenched structure, high temperature strength, and toughness (L
This figure shows the relationship between direction 10 nwn R Charpy impact value).

From Table 3, it is clear that the higher the quenching temperature, the higher the high-temperature strength of both the invention steel F and the comparison steel ③, but the toughness of the comparative steel 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 F 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 shows the baking critical load (ratio) in the high temperature baking test of the steel of the present invention. The sample was a cylindrical sample, and after heat treatment and polishing, it was previously subjected to air oxidation treatment at 550℃, and seizure occurred when the end surface was pressed against a steel material (other material) heated to 700℃ while rotating at high speed. The maximum load (critical load) that does not occur is calculated and expressed as an index, with the seizure critical load of conventional steel (SKH51) being 100.

It can be seen that the steel of the present invention has a clearly higher seizure critical load than the conventional steel.

This is due to the protective effect and lubricating effect of the dense, peel-resistant monooxide film formed on the surface of the steel sample of the present invention by the above-mentioned oxidation treatment, in addition to the high-temperature strength and wear resistance imparted by carbide distribution. This is one of the major features of the steel of the present invention.

Table 4 =17 〔Effect of the invention〕

Claims (1)

[Claims] 1% by weight: C 0.45 to 0.7%, Si 0.60% or less, Mn 1.50% or less, Cr 3.00 to 5.50%,
2. One or two of W and Mo is 1/2W+Mo.
00-3.50%, V0.80-1.60%, Co0.
30-5.00%, S0.005% or less, Nb0.20
% or less, the balance being Fe and unavoidable impurities. 2% by weight: C 0.45-0.7%, Si 0.60% or less, Mn 1.50% or less, Cr 3.00-5.50%,
2. One or two of W and Mo is 1/2W+Mo.
00-3.50%, V0.80-1.60%, Ni1.
8% or less, Co0.30-5.00%, S0.005%
Hereinafter, a tool steel for warm and hot working characterized by comprising 0.20% or less of Nb, the balance being Fe and unavoidable impurities. 3% by weight: C 0.45-0.7%, Si 0.60% or less, Mn 1.50% or less, Cr 3.00-5.50%,
2. One or two of W and Mo is 1/2W+Mo.
00-3.50%, V0.80-1.60%, Co0.
30-5.00%, S0.005% or less, Nb0.20
% or less, N0.10% or less, the balance being Fe and unavoidable impurities. 4% by weight: C 0.45-0.7%, Si 0.60% or less, Mn 1.50% or less, Cr 3.00-5.50%,
2. One or two of W and Mo is 1/2W+Mo.
00-3.50%, V0.80-1.60%, Ni1.
8% or less, Co0.30-5.00%, S0.005%
Below, Nb 0.20% or less, N 0.10% or less, balance F
A tool steel for warm and hot working, characterized in that it comprises e and inevitable impurities.