JPH0273951A - Hot-working tool steel excellent in thermal fatigue-resisting characteristic and die for high-temperature and heavy-load working - Google Patents

Abstract

PURPOSE:To provide a hot-working tool steel most suitably used for metal mold for casting and forging die by incorporating specific weight percentages of C, Si, Mn, Cr, Mo, V, Nb, Zr and/or Ce, and N to Fe. CONSTITUTION:The title steel has a composition in which 0.25%-0.4%, by weight, C, 0.10-1.2% Si, 0.10-1.2% Mn, 4.0-6.0% Cr, 1.0-3.0% Mo, 0.3-1.0% V, 0.01-0.3% Nb, 0.01-0.2% Zr and/or 0.001-0.02% Ce, and <=0.01% N are incorporated to Fe. Moreover, an Fe oxide layer containing Cr oxide and Si oxide is formed on the surface of the above steel, and further, an Fe oxide layer containing Zr oxide is formed on the above layer. By this method, the hot- working tool steel most suitably used for metal mold for casting and forging die can be provided.

Description

Detailed Description of the Invention [Industrial Field of Application] The present invention relates to hot work tool steel used for high temperature, high load processing molds such as die casting molds and forging press molds, as well as to the hot work tool steel as a base material. This article relates to molds for high-temperature, high-load machining.

[Prior art] A1 die casting and hot forging are processing methods in which processing is repeated many times over a long period of time. The product is rapidly heated by repeated contact with the product, and is rapidly cooled by spraying a coolant each time the product is taken out, which tends to cause rough skin due to oxidation and thermal fatigue cracks due to repeated thermal stress.

Moreover, the rapid heating/cooling cycle time per shot is
Recently, there has been a tendency for the length of time to be shortened more and more, and the usage conditions for molds have become even more severe. As a result, the mold is exposed to conditions where it is more likely to crack due to excessive thermal stress, resulting in a shortened lifespan. On the other hand, another requirement in the field of die casting and the like is to improve the quality of the molded product, and the tolerance standards for roughness on the surface of the mold are becoming higher.

Conventionally, the tool steel used to manufacture the above molds was 5KD.
61 has been widely used, but as mentioned above, mold usage conditions have become harsher and roughness tolerance standards have also been strengthened, and 5K
Since D61 cannot meet these demands, there is a demand for a mold for high-temperature, high-load machining with excellent durability and a tool steel for use in the mold.

[Problems to be Solved by the Invention] However, looking at the improved steels of 5KD61 that have been proposed so far, although the mechanical properties such as the strength and wear resistance of the base material have been improved, the direct heat cracking Since no improvements have been made to the characteristics of the oxidized film on the surface that causes this problem, it is often not possible to achieve a sufficient lifespan improvement effect in actual molds, and the problem is that it has almost no effect on rough skin. There is. Therefore, a few improved steels have been proposed in which a protective oxide film is formed on the surface, but these do not necessarily have sufficient crack resistance because the composition distribution in the film is not optimal, or Because the base material does not have sufficient crank progression resistance or toughness, even though it has excellent durability under low-stress, high-cycle thermal fatigue conditions, it cannot be used in high-temperature, high-load applications where heat cracks occur early in use (i.e. (low cycle fatigue conditions),
The problem is that there is almost no improvement in service life, and in some cases large cracks occur.

For example, the hot working tool steel of JP-A No. 50-102517 is a tool steel whose main feature is that Y and Ce are added alone or together. Since a protective film is formed, it is possible to improve seizure resistance, heat check resistance, corrosion resistance, etc. at high temperatures. Also, Unexamined Patent Publication 1983-
The hot working tool steel of No. 207359 has rare earth elements such as Y and Sc added to form an oxidation protective film, and Nb and Ti.
, Zr is added to refine the crystal grains and increase high-temperature strength, and the addition of these elements traps nitrogen and oxygen in the steel to suppress the formation of rare earth element oxides and nitrides. , which exhibits the effect of rare earth element addition. However, according to the research of the present inventors, CO and Ni are added to the former, and c.

Since a large amount of . For this reason, molds for low-cycle fatigue applications at high temperatures and high loads do not necessarily have a long life and have the problem of large cracks. Moreover, in all of the above-mentioned known techniques, the amount of nitrogen exceeds 0.01%, which is the normal atmospheric melting level, and although the latter uses Zr etc. for nitrogen fixation, not only Zr etc. but also rare earth elements are used. Since the rare earth elements react with nitrogen to form nitrides, there is a problem in that the added rare earth elements do not contribute to improving the properties or composition of the oxidation protective film.

That is, in the field of the present invention, the crack resistance of the oxide film that inevitably forms on the surface of the mold when it is used, and the crank progression resistance and toughness of the base material are important, and both properties must be excellent at the same time. The roughening resistance, heat check resistance, and cracking resistance will be poor, and there will be cases where the mold cannot withstand use as a mold.

The present invention has been made with attention to these circumstances, and by devising the alloy composition using 5KD61 as the basic composition, the present invention provides a hot work tool steel with excellent durability in high temperature and high load machining, and the hot work tool steel. The object of the present invention is to provide a mold for high-temperature, high-load processing, which has a dense oxide film with excellent crack resistance on its surface.

[Means for Solving the Problems] The hot work tool steel of the present invention that achieves the above objects has the following characteristics: C+0.25-0.4% Si: 0.10-12% Mn: 0.10-1. 2% Cr:
4.0 to 6.0% Mo+1.0 to 3.0% V: 0.3 to 1.0% Nb: 0.01 to 0.3% Zr: 0
．． 01-0.2% and/or Ce: 0.001-0.
02% N: 0.01% or less Fe and unavoidable impurities: The main point is that the mold for high-temperature, high-load machining of the present invention To,
having an Fe oxide layer containing Cr oxide and Si oxide,
Furthermore, the gist lies in that an Fe oxide layer containing Zr oxide is provided above the Fe oxide layer.

[Function] In order to obtain a mold suitable for high-temperature, high-load machining, we established the policy that it is necessary to review the base steel composition as well as study the composition of the oxide film itself, which is deeply related to the base steel composition. 5KD61
The basic composition is used to achieve high MO and low This improved the crack resistance of the oxide film that forms on the mold surface during use, and succeeded in significantly improving heat fatigue resistance and skin roughness resistance.

The reasons for limiting the composition of the steel according to the present invention will be explained in order below.

C: Combines with carbide-forming elements such as Cr, Mo, ■, Nb, and Zr to precipitate hard fine carbides and improve basic properties such as high-temperature strength, softening resistance, and wear resistance. Upper limit 0.4%
This is to prevent deterioration in toughness due to the formation and segregation of coarse carbides with Cr and V, and to suppress deterioration in oxidation resistance. However, if the temperature is too low, the required hardness cannot be obtained because a complete martensitic structure cannot be obtained, and the hardenability of the center of the material decreases, and the prior austenite grains become coarse and the toughness decreases.
% or more.

Si: Provides appropriate oxidation resistance to prevent surface roughness of the mold, and also promotes carbide precipitation (secondary hardening) during high temperature tempering to increase high temperature strength. It also functions as a deoxidizing agent, but adding too much leads to a decrease in toughness, so the content should be 1% or less. On the other hand, if it is less than 0.1%, the thickness of the oxide film will increase, and the cracking and peeling of the film will become more severe, making the skin rougher and at the same time, the deoxidizing effect will be reduced.
1% or more.

Mn: Added as a deoxidizing agent and at the same time added at least 0.1% to ensure hardenability. Excessive addition impairs toughness and stiffness, so the content should be 1.2% or less.

Cr: has the function of improving hardenability, strengthening the matrix, improving softening resistance, and improving wear resistance due to carbide precipitation, and also has the function of imparting appropriate oxidation resistance similarly to Si.

In order to effectively exhibit these effects, 4% or more of Cr is added. However, if it is too large, the toughness will be impaired due to the formation of coarse carbides, so it should be kept at 6% or less.

MO=One of the important elements for increasing thermal fatigue resistance and toughness. MO forms a carbide Mo2C which is finer than the above-mentioned Cr carbide, and solidly dissolves in the matrix to strengthen it, so it has the effect of increasing wear resistance, hardness, and high temperature strength.
At the same time, it is effective in improving toughness. Improving high-temperature strength reduces plastic strain that occurs during thermal cycling, while improving toughness increases the resistance to propagation of cracks in the oxide film to the base, so thermal fatigue resistance can be significantly improved. However, if excessively added, it will cause a decrease in toughness, so 3
% or less. However, if it is too small, the mutual effect of precipitation of fine carbides and solid solution strengthening cannot be obtained sufficiently, so it is set to 1% or more.

(2) Solid solution during the niostenite treatment precipitates as hard fine carbides (VC) during tempering, thereby increasing high-temperature wear resistance, tempering softening resistance, and high-temperature strength. Further, during the same treatment, grains become finer due to the presence of fine VC which is partially undissolved, and toughness is also improved. In order to form undissolved carbides, it is necessary to add 0.3% or more of V in addition to the above-mentioned amount of C. However, if it exceeds 1%, coarse carbides are likely to be formed, leading to a decrease in toughness, so 1%
The following shall apply.

During the Nb + austenite treatment, NbC forms a more solid solution than the VC and is finer than the VC, which contributes to the refinement of crystal grains. Also, Nb precipitates during tempering.
Since C also has extremely small dimensions, it improves toughness and strength, and improves the heat fatigue resistance of wood-like wood by a mechanism similar to that of MO. For grain refinement, it is necessary to add 0.01% or more, but excessive addition impairs oxidation resistance, so it is limited to 0.3% or less.

Zr: Along with Ce, Zr is an extremely important element in improving the thermal fatigue resistance of the steel of the present invention. Zr easily combines with N to form ZrN, which has an unfavorable effect on maintaining toughness, but when N is limited to a certain amount or less, precipitation of ZrN is suppressed and Z is dissolved in the matrix. Zr dissolved in solid solution accelerates the outward diffusion of Si in the steel by heating, forming a Si-rich protective film on the base surface.Also, some Zr oxide is produced in the film, but its presence The crack resistance of the film can only be improved by using this method, and since the coefficient of thermal expansion is between that of Fe and Si oxides, it has the effect of reducing the thermal stress of the film. Due to the above effects,
Suppresses the formation of excessive oxide film and improves skin roughness resistance and film crack resistance. However, if added in excess of 1%, excessive Zr will precipitate as ZrN, impairing toughness, so it should be limited to 1% or less. Also, if it is too small, a Si-rich film will not be formed, so the content should be 0.01% or more.

Similar to Ce/Zr, it suppresses the formation of an excessive oxide film and increases the thickness of the Cr- and Si-rich protective film on the base surface. In order to exhibit a sufficient effect, it is necessary to add 0001% or more. However, when added in excess of 0.02%,
Toughness decreases due to nitride formation and segregation, and
If the Cr- and Si-rich film becomes too thick, the thermal stress due to the difference in thermal expansion and elastic modulus with the base will increase, making it more likely to peel off, so the content should be set to 0.02% or less.

N: Suppression of the amount of N is extremely important to exhibit the effect of improving thermal fatigue resistance by Zr and Ce. If the amount of N is large, it will combine with Zr and Ce to form nitrides, so Zr will form a solid solution.
, the amount of Ce decreases, making it impossible to obtain a good protective film, and at the same time, the precipitation of these nitrides results in a decrease in toughness. Therefore, it is limited to 0.01% or less.

[Example] Table 1 shows a comparison of the chemical components of the steel of the present invention and the conventional steel (SKD61). These steels are quenched at 1000-1025℃ and tempered at 590-610℃ to increase the hardness to H.
It was adjusted to around RC48 and subjected to a high temperature low cycle tension/compression fatigue test and a fluidized bed thermal fatigue test. The high-temperature, low-cycle fatigue test was conducted under a strain waveform with compressive strain retention, and the test temperature was 600°C. In the fluidized bed thermal fatigue test, test pieces were alternately immersed in a fluidized bed at 700°C and 150°C, and the length of developing cracks was measured.

These temperature and strain patterns are examples of simulating the surface environment of an aluminum casting mold for a certain product.

1゜ Part 1 table (%) Figure 1 shows a high-temperature, low-cycle fatigue life diagram.

The steel of the present invention has a longer life than 5KD61 in any strain range.

Table 2 shows a comparison of fatigue life using 0.5% as a typical strain range. The steel of the present invention is 5KD6
It can be seen that the fatigue life is 30 to 60% longer than that of No. 1.

Charpy impact values are also shown in Table 2. The steel of the present invention has a toughness equal to or higher than that of 5KD61, indicating that it has high crack propagation resistance.

Table 2 shows the formation of an oxide film on the surface and the occurrence of cracks after the high-temperature, low-cycle fatigue test. In addition, FIG. 2 (A> shows the surface state of the conventional steel F, FIG. 2 (B) shows the surface state of the present invention vi4B, FIG. 2 (C) shows the surface state of the present invention wiD, and FIG. 2 (D) shows the surface state of the present invention wiD. In conventional steel F, which shows the cross-sectional state taken along the line II-II in 8), many long cracks have occurred in the thick oxide film, and some of these cracks have significantly progressed into the base. Large peeling of the film is also observed.On the other hand, in the steels B and D of the present invention, the formation of an oxide film is slight and there are few cracks extending into the matrix.

FIG. 3 shows the elemental distribution near the surface of the high-temperature, low-cycle fatigue test piece obtained by EPMA analysis.

In the conventional @F (SKD61), the thickness of the Si oxide or Cr oxide that serves as the protective film is thin, resulting in the formation of a thick Fe oxide film with poor crack resistance. On the other hand, in the steel B of the present invention to which Zr is added, the amount of Si is large in the portion adjacent to the matrix, a good protective film is formed, and the thickness of the Fe oxide film is small. In addition, between the Fe oxide on the outer surface and the above protective film, there is Zr, which is rich in toughness and whose coefficient of thermal expansion is between the two.
Oxides are observed. In the steel of the present invention to which Ce is added,
The thickness of the Cr- and Si-rich layer adjacent to the matrix is larger than that of conventional steel, and therefore the thickness of the oxide film, which has poor crack resistance, is reduced.

Figure 4 shows the results of the fluidized bed thermal fatigue test. This shows the length of crack growth due to repeated heating and cooling. It can be seen that the steel of the present invention is extremely superior in terms of both crack initiation life and crack propagation speed.

[Effects of the Invention] The present invention is configured as described above, and improves the crack resistance of the surface oxide film by limiting the amount of N and adding Zr and/or Ce, and further improves the crack resistance of the surface oxide film. By optimizing the amount of toughness-enhancing elements such as Nb added, it was possible to improve the resistance to propagation of cracks generated in the surface oxide film to the base.

In this way, it is possible to provide a hot work tool steel that is optimal as a material for casting molds, forging molds, etc. that are repeatedly heated and cooled, and molds formed with the hot work tool steel not only have excellent durability but also have a low surface roughness. However, it was possible to bring great industrial benefits to the field of molds for high-temperature, high-load processing.

[Brief explanation of the drawing]

Fig. 1 is a graph comparing the fatigue life of the inventive steel and conventional steel, Fig. 2 (A) to (C) are photographs substituted for drawings showing the surface metallographic structure of the inventive steel and conventional steel, and Fig. 2B, D FIG. 4 is a graph showing the surface element distribution after the high temperature low cycle test, and FIG. 4 is a graph showing the progress of the crank in the fluidized bed thermal fatigue test.

Claims (2)

[Claims] (1) C: 0.25-0.4% (meaning of weight %, same below) Si: 0.10-1.2% Mn: 0.10-1.2% Cr: 4.0-6. 0% Mo: 1.0-3.0% V: 0.3-1.0% Nb: 0.01-0.3% Zr: 0.01-0.2% and/or Ce: 0.001
~0.02% N: 0.01% or less Fe and unavoidable impurities: A hot work tool steel with excellent thermal fatigue resistance. (2) C on the surface of the base steel having the components of claim (1)
A mold for high-temperature, high-load processing, characterized in that it has an Fe oxide layer containing an r oxide and a Si oxide, and further has an Fe oxide layer containing a Zr oxide on top of the Fe oxide layer.