JP3252969B2 - Hot tool steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention has a particularly excellent high-temperature strength,
The present invention also relates to a hot work tool steel characterized by having sufficient hardenability and toughness. More specifically, as a trend in the use of molds in recent years, in aluminum die-casting technology, a rise in the temperature of a molten metal of a cast alloy or laminar flow filling and aggravation of a temperature rise of a mold surface layer due to pressurization of a molten metal after filling, In order to improve the dimensional accuracy in hot forging technology, the temperature rise of the mold thin part due to the thinning of the mold is severe, or the mold is strongly cooled with the increase in molding speed as in a multi-stage high-speed hot forging machine. Of the working environment of the mold, it is used for applications where the temperature rise of the mold surface layer is extremely higher than before and the thermal stress becomes excessive. The present invention relates to a tool steel for hot working used as a mold in which generation and growth hardly proceed.

[0002]

2. Description of the Related Art In recent years, in the field of aluminum die casting,
For the production of wear-resistant parts, aluminum alloys having a high Si content are partially applied. In this case, a high Si aluminum alloy is partially applied. In this case, the melting point of the high Si aluminum alloy is high, so that
Since the molding is performed at a temperature of about 00 ° C., the temperature of the mold surface layer is high. Also, for the production of high-strength parts, molding of die-cast products to be subjected to heat treatment after die-casting has been performed. In this case, heat treatment is performed to ensure the soundness of the die-cast casting, that is, the absence of casting defects. For this reason, the application of a die casting machine for laminar filling a molten metal into a mold cavity and mechanically applying pressure to the molten metal has been advanced. In this case, the contact time between the molten metal and the mold surface is longer than before, and mechanical stress is superimposed on the mold, so that not only the temperature rise temperature of the mold surface layer is high but also excessive thermal stress acts.

On the other hand, in the field of hot forging, in order to improve the dimensional accuracy of a product, the shape of a mold is reduced in thickness, and the temperature rise in a thin portion is significantly higher than in the past. In addition, since the molding cycle is fast, when the mold is subjected to strong water cooling, the heat cycle applied to the surface layer of the mold becomes excessive. Conventionally, these fields are based on SKD of JIS steel.
61, SKD7 and SKD8 and various proposals for improving these have been made, and although a certain effect has been obtained, the mold life has not yet been sufficiently satisfied.

[0004]

In order to improve the performance of a mold material in response to the advance of these hot working techniques, it is necessary to improve the high temperature strength and the toughness. Regarding improvement of the problems of JIS steel, Japanese Patent Application Laid-Open No. 54-6807 (JP-B-56-54379),
JP-A-54-50421, JP-A-55-24931 (JP-B-61-54864), JP-A-55-1640
No. 59 (JP-B-57-19743), JP-A-62-1
No. 12761, Japanese Patent Application Laid-Open Nos. 62-149852 and 2-43346, and Japanese Patent Application No. 3-99429 by the same applicant as the present application has been proposed. As a result of destructive inspection of the waste mold for the above application using the above mold material, the phenomenon commonly observed is that the softening of the mold material is locally limited to a relatively deep range of the mold surface layer, for example, up to around 3 mm. It was found that heat cracks were generated and propagated in this part, and plastic flow of the mold surface layer was occurring.

[0005] Further, in a mold used after nitriding treatment to improve the mold life, depending on the mold material, the nitride layer is diffused during use and the nitridation depth becomes deep, and the boundary between the nitride layer and the base material is increased. , Peeling is easy to occur, the mold surface is easily damaged,
It was also found that the effect of improving the mold life by the nitriding treatment was not sufficient. An object of the present invention is to provide a mold having a sufficient quenching property and toughness, having excellent strength particularly in a high temperature range, and having no problem of large cracks even when applied to a mold having a relatively large size, and having a mold by nitriding treatment. An object of the present invention is to provide a mold material having sufficient nitriding properties to obtain an effect of improving the life.

[0006]

The steel of the present invention focuses on the importance of the balance between the amounts of Cr, W, Mo and V given to the types and amounts of special carbides precipitated during tempering for imparting high-temperature strength. In addition, the chemical composition is optimized in terms of both high-temperature strength and toughness. In particular, the setting of the amount of Cr is based on the result of finding that it is important from the viewpoint of the balance between high-temperature strength and hardenability, and furthermore, the nitriding property. Further, Co is added for imparting hot wear resistance, and a dense and highly adherent oxide film is formed on the mold surface by increasing the temperature during use. This significantly improves the resistance to the generation and growth of heat cracks in the part.

[0007] That is, the present invention relates to a hot working machine.
Intermediate tool steel containing 0.36 to 0.45% C by weight, Si 0.
5% or less, Mn 1.0% or less, Ni 0.1 to 1.0%, Cr over 4.0% and less than 4.35%, one or two of W and Mo are 1 / 2W + Mo
1.9-3.5%, V 0.5-1.6%, and in some cases C
containing o 0.5 to 5.0%, Ri Do a balance of Fe and unavoidable impurities, is hot work tool steel which is characterized in that to regulate the N which is an impurity below 0.013%.

[0008]

Next, the reasons for limiting the component range of the steel of the present invention will be described. C maintains the excellent hardenability, temper hardness, and high temperature hardness of the steel of the present invention, and combines with carbide-forming elements such as W, Mo, V, Nb and Cr to form carbides, It is added in order to provide finer crystal grains, abrasion resistance, tempering softening resistance, and high-temperature hardness. If too much, excessive precipitation of carbides is caused and toughness is reduced.
% Or less, and the content is 0.36% or more for the above purpose.

[0009] Si is added to impart oxidation resistance in the heating temperature range of hot working or heat treatment during the production of a mold material. However, the addition of 0.5% or more lowers the toughness value during quenching and tempering. % Or less. Mn is to improve the hardenability, too large, excessively lowering the A ₁ transformation point, excessively high annealing hardness, and 1.0% or less since lowering machinability.

Ni is added to improve hardenability. In the case of quenching tool steel, upper bainite is generated when the quenching cooling rate is slow, and the toughness value during quenching and tempering is reduced, but the formation of upper bainite is suppressed by the addition of Ni. For this effect, 0.1% or more is added, but excessive addition lowers the high-temperature strength, so it is made 1.0% or less.

[0011] Cr can improve tempering softening resistance and high-temperature strength by setting an appropriate addition amount, improve wear resistance by combining with C to form carbide, improve hardenability, and improve nitriding property. It has an effect and is the most important element in the limited range of the component in the present invention. Cr is
As the amount of addition increases, the high-temperature strength decreases, but on the other hand, it has the same effect as Ni, and has the effect of suppressing a decrease in toughness during quenching and tempering when the quenching and cooling rate is slow. Further, in the nitriding treatment, the bonding force with nitrogen is large, and the nitriding layer has an effect of improving the nitriding property which makes it difficult for softening and diffusion due to heating during use to progress. From the above three points, it has been found that the most appropriate range of Cr is more than 4.0% and less than 4.35%. Details will be described in the section of Examples.

The setting of the amounts of W and Mo is important for maintaining the high temperature strength and the softening resistance required for the use of the steel of the present invention.
W, Mo, and V precipitate fine special carbides at the time of aging treatment to increase softening resistance and high-temperature strength. However, excessive addition leads to excessive carbide precipitation and lowers toughness,
One or two types are added at 1/2 W + Mo in an amount of 1.9 to 3.5% based on the strength according to the use conditions of the mold and the tool and the high-temperature strength.

V is an important element for forming a carbide which hardly forms a solid solution to increase the resistance to abrasion resistance and seizure resistance and to provide a high temperature proof stress. Also, it improves the toughness and refining crystal grains, increasing the A ₁ transformation point, coupled with excellent high temperature yield strength, those that result the effect of improving the heat crack resistance. The setting of the amount of V is very important for the combination of excellent toughness and high-temperature strength, which are features of the steel of the present invention. If it is too large, it will generate huge carbides, increase the distribution tendency of string-like carbides along the hot working direction, and promote the generation and propagation of cracks along that direction. Since the effect of the above addition is not obtained, such as leading to early softening of the surface, 1.90
% Or more.

Co forms a very dense protective oxide film with good adhesion at the time of temperature rise during use, thereby eliminating metal contact with the mating material, preventing temperature rise on the mold surface, and providing excellent properties. This results in high abrasion resistance. However, if this oxide film becomes too thick, it may cause the rough surface of the mold surface to have an adverse effect, but Co has the effect of suppressing the formation speed and thickness of the oxide film. In the case of steel having a small amount of Si such as the steel of the present invention, the oxide film becomes too thick. Therefore, the addition of Co is particularly effective in improving the properties of the protective oxide film. Co
Is added to impart the above effect, but if it is too much, the toughness is reduced, so the content is made 5.00% or less. If it is too low, the effect of the above addition is not obtained, so it is made 0.50% or more.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to embodiments and drawings. Table 1 shows inventive steels 1 to 9 and comparative steels 11 to 18 subjected to a high-temperature tensile test, an impact test and a nitriding property test.
Shows the chemical composition of Invention steels 1 to 9 are comparative steels 11 to 18.
In order to clarify the difference from the above, N which is mixed as an impurity is analyzed and displayed. In Table 1, comparative steels 11-18
Japanese Patent Application Laid-Open No. Sho 55-2
No. 4931 (JP-B-61-54864) and JP-A-55-55.
No. 164059 (JP-B-57-19743), JP-A-62-112761, and JP-A-62-149852.
And Japanese Patent Application Laid-Open No. 2-43346, which is made from the examples proposed in Japanese Patent Application No. 3-99429.

FIG. 1 shows 1070 of the steel of the present invention and the comparative steel of Table 1
After heating to ℃, quenching for 15 minutes in a half-cooling period, and then tempering to a hardness of HRC50, the high-temperature strength is shown by the amount of Cr contained in each sample. High temperature strength at 700 ℃
Immediately after the test specimen was heated and held for 10 minutes, a tensile test was performed and evaluated by tensile strength. In the present invention steel indicated by a white circle and the comparative steel indicated by a cross in FIG.
Although it is in the range of 8 to 0.95%, there is a certain tendency that the high-temperature strength decreases as the Cr amount increases. The steel of the present invention having a V content of 1% or more indicated by a black circle is located at a high high-temperature strength level based on the relationship between the high-temperature strength and the Cr amount indicated by a broken line.

[0017]

[Table 1]

FIG. 2 shows the steels of the present invention and the comparative steels shown in Table 1 which were subjected to the same heat treatment as described above, and the impact values by the Charpy impact test were arranged by the Cr content of each sample. The reason for setting the quenching cooling rate to 15 min of semi-cooling is that the mold size targeted by the steel of the present invention is 100 m in terms of the diameter of a round bar.
m or more, the half-cooling time of the central part is about 15 minutes in the general heat treatment method. The behavior of the impact value with respect to the contained Cr amount tends to be opposite to the high temperature strength, and the impact value increases as the Cr amount increases. Inventive steels having a V content of 1% or more indicated by black circles have slightly lower values as compared with samples having a V content of 0.8 to 0.95% indicated by broken lines. The impact value of the comparative steel 11 is much lower than the others. This is because only the comparative steel 11 did not contain Ni, and the formation of upper bainite during quenching proceeded as compared with other samples.

FIG. 3 shows that the steel of the present invention and the comparative steel in Table 1 were subjected to the same heat treatment as described above, and then subjected to nitriding at 570 ° C. for 90 minutes in a salt bath, and further to heating at 700 ° C. for 20 minutes.
This is the result of measuring the nitriding depth after air cooling and organizing it by the Cr amount of the sample. In this experiment, the application of the mold nitriding treatment became popular in recent hot forging dies, but when the initial nitride layer diffused inside and deepened due to the temperature rise of the mold surface during use,
The heat cracks that have propagated in the nitrided layer merge and separate from the boundary between the nitrided layer and the base, so that the intended effect of improving the mold life may not be obtained. Because it is easy to go deep and peel easily,
700 ° C, which is considered to be the sum of the mold surface temperature and cycle number
This is an experiment conducted because it is an important factor as a nitriding characteristic of the mold material that the nitride layer does not easily become deep under heating conditions of about × 20 min. The depth of the nitrided layer immediately after nitriding is 0.05-
It was 0.08 mm.

FIG. 3 shows that all the samples have a large influence of the Cr content. Further, by containing 4% or more of Cr, the effect of improving the nitridation characteristics described above is large, and the addition of more Cr has the effect of improving the nitridation characteristics, but the improvement effect is small. It can be seen that the amount of Cr is
It can be seen that a Cr content in the range of -4.35% is sufficient.
The material properties to meet the mold material of interest, the high temperature strength as high temperature strength, 700 ° C. tensile strength of 40 kgf / mm ² or more, the impact value is at 2 kgf / cm ² or more, the type in the application of nitriding The steel of the present invention satisfies the above three conditions because it has a nitriding property capable of improving the life.

Next, Table 2 shows the L values of the steels of the present invention and Comparative Steels 13, 14 and 15 shown in Table 1 which were subjected to the same heat treatment as described above.
The impact value in the direction (parallel forging / stretching) and the impact value in the T direction (the same perpendicular direction) are shown in comparison. Comparative steels 13 to 15 disclosed in Japanese Patent Application Laid-Open No. 62-149852 have N added to refine the structure. Although the impact value in the L direction is equal to or higher than that of the steel of the present invention, this tendency is remarkable for the comparative steel 15 having a relatively low impact value in the T direction and particularly having a large V content.

[0022]

[Table 2]

V and N have the property of being more easily bonded than V and C. However, VN generated at the time of solidification becomes a solidification nucleus of the VC, so that VC is easily generated, and this is a string at the time of forging and stretching. Since it is easy to arrange in the forging direction in the shape of a fork, a crack in the forging direction easily advances. When the V amount of the mold material is as high as that of the steel of the present invention, the addition of N is based on the above-mentioned reason. It can be seen that it promotes. From the above, the steel of the present invention is free from N.

[0024]

As described above, the steel of the present invention has a high-temperature strength level of JIS such as JIS SKD61 or SKD7.
Higher than steel, has hardenability that gives sufficient toughness even when applied to relatively large dies, and has low softening and diffusion of the nitrided layer, so it is applied to hot working dies However, since no large cracks, heat cracks, peeling of the mold surface, etc. occur, the life of the mold can be greatly improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing the effect of V content and Cr content on high-temperature strength represented by 700 ° C. tensile strength.

FIG. 2 is a diagram showing the influence of the V content and the Cr content on the Charpy impact value.

[FIG. 3] After nitriding in a salt bath at 570 ° C. for 90 minutes, 700 ° C. ×
It is a figure which shows the relationship between the nitriding depth which performed the diffusion process for 20 minutes, and Cr content.

Claims (2)

(57) [Claims] 1. A hot work tool steel obtained by hot working.
Te, C 0.36-0.45% by weight%, Si 0.5% or less, Mn
1.0% or less, Ni 0.1 to 1.0%, Cr more than 4.0% and less than 4.35%, one or two of W and Mo are 1.9 to 3.5 at 1 / 2W + Mo.
% V contains 0.5 to 1.6%, Ri Do a balance of Fe and unavoidable impurities, hot work tool steel which is characterized in that <br/> where the N is an impurity restricted below 0.013%. 2. A hot work tool steel obtained by hot working.
Te, C 0.36-0.45% by weight%, Si 0.5% or less, Mn
1.0% or less, Ni 0.1 to 1.0%, Cr more than 4.0% and less than 4.35%, one or two of W and Mo are 1.9 to 3.5 at 1 / 2W + Mo.
%, V 0.5-1.6%, Co 0.5-5.0%, and the balance Fe
And Ri Do unavoidable impurities, the N is an impurity 0.01
Hot tool steel characterized by being restricted to 3% or less .