JPH06145884A - Die for hot working excellent in plastic flow resistance - Google Patents

Abstract

PURPOSE:To provide a die for hot working excellent in plastic flow resistance while it is made of steel constituted of a compsn. approximately equal to that of general steel for a hot die or improved steel therefrom. CONSTITUTION:This die for hot working excellent in plastic flow resistance is a one having a compsn. constituted of, by weight, 0.42 to 0.55% C, <=1.20% Si, 0.1 to 1.5% Mn, 4.05 to 6.50% Cr, one or two kinds of W and Mo by 1/2W+Mo and 1.0 to 3.0% and 0.2 to 1.5% V, and the balance Fe with inevitable impurities, and in which the area ratio of carbides with >=0.15mum grain size not entering into solid solution is regulated to >=2% and the number of the same carbides in 2400mum<2> in regulated to >=200pieces. If required, Ni and Co may be added thereto. In this die for hot working, the surface part of the die is preferably applied with a nitrogen compound layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot working die mainly used for a hot press die and the like.

[0002]

2. Description of the Related Art In order to increase the durability of hot working molds,
It is generally widespread to apply nitriding treatment to SKD6, SKD61 and the like, and to apply SKD7 and SKD8, which are superior in high temperature strength to these, or their improved steels. In the case of hot press dies, heat transfer and stress from the work material, frictional action with the work material causes heat cracks and wear, or plastic flow occurs on the surface of the die, resulting in dimensional accuracy. However, in order to impart high temperature strength and wear resistance to withstand this, nitriding treatment and high strength steel are generally applied.

[0003]

However, in recent years, due to changes in hot press working techniques, conventional nitrided SK has been used.
In the fields of D6 and SKD61, the required characteristics are not increasing. Further, in the case of high-strength steel, the material cost is high, and the toughness is lower than that of SKD6 and SKD61, so that the problem of cracking may occur. In the case of hot pressing, forging technology that accelerates the forging speed in order to shorten the molding cycle is being put to practical use, and as a result, the frictional action between the die surface and the work material becomes severe, and the heat generated by sliding increases. However, there is a problem that the wear of the mold due to softening and plastic flow progresses at an early stage.

Conventionally, it is represented by SKD6 and SKD61.
In the case of 5% Cr hot work tool steel, if normal nitriding is applied to a die with normal quenching and tempering hardness of HRC40 to 55,
Although a certain effect of improving wear resistance can be obtained, a nitride layer having sufficient wear resistance may not be formed depending on the nitriding conditions. Therefore, the present inventor paid attention to the friction phenomenon between the die and the high-temperature work material, and studied in detail the influence of the properties of the nitrided layer and the amount of undissolved carbide on the friction phenomenon. as a result,
Although the composition is almost the same as that of hot die steels such as SKD6 and SKD61, the amount of carbide forming elements is mainly adjusted to disperse undissolved carbides in the matrix after tempering, and It was newly found that the properties required for hot working tool steel can be significantly improved by performing nitriding treatment.

By the way, regarding the effect of nitriding the hot working die to enhance the strength and wear resistance of the die,
JP-A-54-50421, 54-56913, 54-110916, 5
No. 5-93872. However, these are related to the mold steel that is likely to obtain a high surface hardness and a deep nitriding layer when nitriding is performed, and in terms of mold characteristics,
For the purpose of preventing the wear of the die for hot working, which is the object of the present invention, particularly the plastic surface fluidity resistance of the die surface part, for the purpose of preventing the wear of the die for the purpose of improving the strength and the wear resistance. There is no description of forming a layer or controlling the amount of undissolved carbide.

As described above, since the problem of plastic flow has become apparent due to the severe friction conditions of the mold, the conventional nitriding treatment of SKD6 and SKD61 is no longer sufficient. An object of the present invention is to provide a die for hot working which is excellent in plastic flow resistance even though it is a steel having a composition almost equivalent to that of a general hot die steel or an improved steel thereof.

[0007]

[Means for Solving the Problems] In the hot working die of the present invention, in order to disperse an appropriate amount of undissolved carbide in the matrix of the die, Cr of about 5%, W and Mo alone are used. Alternatively, the steel containing V is added in combination, and C is added to the steel containing V more than the conventional hot die steel.
Is added at a higher level.

That is, the first invention of the present invention is, by weight%,
C 0.42% to 0.55%, Si 1.20% or less, Mn 0.1 to 1.5%, Cr
4.05 to 6.50%, 1 or 2 types of W and Mo 1 / 2W + Mo
1.0 to 3.0%, V 0.2 to 1.5%, the balance Fe and unavoidable impurities, and the area ratio of undissolved carbides having a particle size of 0.15 μm or more is 2% or more, and the number of the carbides in 2400 μm ² is 20
A die for hot working excellent in plastic flow resistance, characterized in that it is 0 or more. The second invention is C 0.4% by weight.
2% to 0.55%, Si 1.20% or less, Mn 0.1 to 1.5%, Ni 0.1 to
1.5%, Cr 4.05 to 6.50%, 1 or 2 of W and Mo 1 /
2W + Mo: 1.0 to 3.0%, V: 0.2 to 1.5%, balance Fe and unavoidable impurities, the area ratio of undissolved carbides having a particle size of 0.15 μm or more is 2% or more, and the number of the carbides in 2400 μm ² Is 200 or more and is a hot working die having excellent plastic flow resistance.

The third invention is, in% by weight, C 0.42% to 0.55.
%, Si 1.20% or less, Mn 0.1 to 1.5%, Cr 4.05 to 6.50%,
One or two of W and Mo is 1.0 to 3.0% at 1/2 W + Mo, V
0.2 to 1.5%, Co 0.3 to 5.0%, balance Fe and unavoidable impurities, and the area ratio of undissolved carbides having a particle size of 0.15 μm or more is 2% or more, and the number of the carbides in 2400 μm ² is 20.
A die for hot working excellent in plastic flow resistance, characterized in that it is 0 or more. The fourth invention is C 0.4% by weight.
2% to 0.55%, Si 1.20% or less, Mn 0.1 to 1.5%, Ni 0.1 to
1.5%, Cr 4.05 to 6.50%, 1 or 2 of W and Mo 1 /
1.0 to 3.0% at 2W + Mo, V 0.2 to 1.5%, Co 0.3 to 5.0%,
Consisting of balance Fe and unavoidable impurities, particle size 0.15 μm
The area ratio of the above undissolved carbides is 2% or more, 2400μ
A hot working die having excellent plastic flow resistance, characterized in that the number of the carbides in m ² is 200 or more. The fifth invention is the first to fourth inventions having a nitrogen compound layer.
It is a mold for hot working which is excellent in wear resistance according to any one of the inventions.

[0010]

The reason for limiting the undissolved carbide of the mold of the present invention will be described. Generally, wear and plastic deformation of the hot working mold surface occur due to various factors.However, due to heat generation due to friction between the mold surface and the high temperature work material, the mold surface layer softens and the plastic Flowing is one of the major factors, and when frictional heat is large, it is often observed that the transformation point of the die is exceeded and, as a result, a re-quenched layer is formed on the surface layer of the die. In order to prevent such heat generation due to friction between the die surface and the workpiece, it is preferable to form a compound of carbide or nitrogen having a relatively large particle size on the die surface. That is, the carbide or nitrogen compound on the surface of the mold has the effect of reducing the area where the matrix of the mold and the metal of the material to be processed are in direct contact with each other.

The slidability with respect to the work material and the seizure resistance can be improved by increasing the amount of carbide dispersed in the base material.
It is well known for high speed tool steels containing a large amount of primary carbides having a grain size of 2 μm or more. However, in the case of general hot die steels or these improved steels, most of the carbides form a solid solution when they are quenched at a normal quenching temperature of 1020 ° C. The fine carbides precipitated during the subsequent tempering have a small effect of suppressing the heat generation due to the above-mentioned friction.

In the present invention, the undissolved carbide is positively formed by adjusting the compounding amount of the composition which is almost the same as that of the general hot die steel. In the mold obtained by quenching and tempering the steel having the composition of the present invention, the grain size of 0.1
The area ratio of undissolved carbides of 5 μm or more is less than 2%, and
When the number of the carbides in 2400 μm ² is less than 200,
Since it is not possible to suppress heat generation due to the frictional action between the die surface and the work material, the area ratio of undissolved carbides with a grain size of 0.15 μm or more is 2% or more in the die that has been quenched and tempered. And the number of said carbides in 2400 μm ² is 20
Limited to 0 or more. Further, the mold for hot working of the present invention not only disperses the above-mentioned undissolved carbide in a specific amount but also forms a nitrogen compound by nitriding treatment to further suppress the heat generation effect due to friction with the workpiece. Be effective.

The steel according to the present invention is the conventional SKD6 or SKD61.
In comparison with, the amount of undissolved carbide is increased to exert plastic flow resistance, but in order to increase the amount of undissolved carbide, the amount of C in the die material and the carbide forming element are required. It is necessary to increase Cr, W, Mo, V which is Of these, W, Mo, and V are not only high in alloy cost, but also the carbides generated during steel ingot melting do not change due to subsequent thermal treatment, so it is difficult to disperse them uniformly, and for this application Not suitable.

Therefore, it is necessary to increase the amount of undissolved carbides by adjusting the amounts of C and Cr. SKD6, SK
For D61, carbides present in annealed state before quenching is the MC mainly of M ₂₃ C ₆ and V mainly of Cr, almost all of the M ₂₃ C ₆ in the quenching heating base Dissolves in the inside. On the other hand, the amount of undissolved carbides can be increased by forming M ₇ C ₃ whose main component is Cr, which is less likely to form a solid solution, than M ₂₃ C ₆ . Specifically, it is necessary to keep the Cr / C ratio Cr / C of the die material low. The SKD61 has a Cr / C value of around 13.4, but in the material of the mold of the present invention, the total amount of carbides is set while the C content is set higher so that the Cr / C value is lower than this. The Cr amount is also set to a high value so as to increase at the same time.

Next, the reasons for limiting the component range of the hot working mold of the present invention will be described. C is the most important element that imparts excellent plastic flow resistance of the mold of the present invention, and W, M
It forms carbides by combining with carbide-forming elements such as o, V, and Cr, and part of them remains as undissolved carbides in the matrix during quenching, increasing plastic flow resistance, and compounding during nitriding. It facilitates the formation of the layer and contributes to the improvement of the plastic flow resistance of the nitriding-treated mold of the present invention. To obtain the above effect, C must be at least 0.42%. If it is too much, it causes excessive precipitation of carbides and reduces toughness, so 0.55%
Limited to: Si is added in an amount of 1.2% or less in order to impart the oxidation resistance of the mold of the present invention. Mn improves the hardenability, but if it is too much, the A1 transformation point is excessively decreased, the annealing hardness is excessively increased, and the machinability is deteriorated.
It is set to 1 to 1.50%.

By adding an appropriate amount of Cr, temper softening resistance and high temperature strength are improved, plastic flow resistance is improved by combining with C to form carbides, hardenability and nitriding treatment. Has the effect of easily forming a nitrogen compound, and it is necessary to add 4.05% or more. Cr is a particularly important element in forming a nitrogen compound by the nitriding treatment of the present invention. During the nitriding reaction in the nitriding treatment, Cr combines with nitrogen to form an extremely fine dispersed nitride in the diffusion layer, which improves the hardness and high temperature strength of the surface of the mold of the present invention and also has the effect of improving plastic flow resistance. Give. If it is too large, the high temperature strength of the steel of the present invention will be reduced, so it is made 6.50% or less.

The setting of the amounts of W and Mo is important for maintaining the high temperature strength and softening resistance required for the use of the steel of the present invention.
W and Mo combine with C to form carbides, part of which contributes to plastic flow resistance as undissolved carbides during quenching, and those which form a solid solution in the remaining matrix are fine special carbides during tempering. To enhance softening resistance and high temperature strength. W
If one or two of Mo and Mo are less than 1.0% at 1/2 W + Mo, the above effect cannot be obtained. On the contrary, excessive addition causes precipitation of coarse carbides and lowers toughness. 2 types are limited to 1.0-3.0% with 1 / 2W + Mo.

V forms a carbide that is difficult to form a solid solution and has an effect of improving plastic flow resistance, wear resistance and seizure resistance, and a part of V dissolves in the matrix during quenching and heating. However, it is an important element for precipitating fine carbides that are hard to agglomerate at the time of tempering to increase the softening resistance in a high temperature range and to give a large high temperature proof stress. Further, the crystal grains are refined to improve the toughness, the A ₁ transformation point is raised, and the high temperature proof stress is combined with the effect of improving the wear resistance. If it is too much, huge carbides will be generated and the distribution tendency of string-like carbides along the hot working direction will be increased, and the development of cracks will be promoted along that direction. Conversely, if it is too low, it will lead to early softening of the mold surface part. However, the effect of the above addition cannot be obtained, so the content is made 0.2% or more.

Ni, together with C, Cr, Mn, Mo, W, etc., imparts excellent hardenability to the mold of the present invention, and forms a structure mainly composed of martensite even at a slow quenching cooling rate. , Has an action of preventing deterioration of toughness, and imparts an essential toughness improving effect of the matrix, so that it is added as necessary. N
i is required to be at least 0.1% in order to obtain the above effect, but if it is too large, the A ₁ transformation point will be excessively lowered, leading to a decrease in fatigue life and an excessively high annealing hardness. If it is added, its content should be 1.5% or less to reduce the workability.

Co forms a protective oxide film that is extremely dense and has good adhesion when the temperature rises during use. This prevents metal contact with the mating material, prevents temperature rise on the die surface, and is excellent. It also provides plastic flow resistance and wear resistance. However, if this oxide film becomes too thick, it causes the roughening of the surface of the mold and has the opposite effect, but Co has the effect of suppressing the formation rate and thickness of the oxide film. Co is preferably added to impart the above effect, but if it is too much, it lowers the toughness, so it is 5.0% or less, and if it is too low, the effect of the above addition cannot be obtained, so it is 0.3% or more.

[0021]

【Example】

(Example 1) Hereinafter, the present invention will be described in detail based on examples. Materials having the compositions shown in Table 1 were prepared. Test pieces for evaluation of plastic flow resistance were prepared from these, rotated at 1540 rpm, and pressed against SNCM439 steel heated to 600 ° C with a predetermined load. The dimensions of the test piece are 5 mm in diameter and 40 mm in length,
The end surface with a diameter of 5 mm was pressed against the heated counterpart material. The load is
The load was 30 kg to 80 kg, and the load causing buckling was defined as the limit load. After the test, as a result of observing the cross section of the pressed portion of the test piece, it was confirmed that the surface layer portion was in a re-quenched state. Table 2 shows the hardness, the amount of undissolved carbide and the test result of the test piece corresponding to the mold of the present invention. The test piece was heated at 1020 ° C and then immersed in oil at 200 ° C.
50. Further, for the measurement of undissolved carbides, 10 fields of view of a structure having an area of 240 μm ² (magnification: 10,000 times) were photographed by a scanning electron microscope, and the number and area ratio of undissolved carbides having a particle size of 0.15 μm or more were measured.

[0022]

[Table 1]

[0023]

[Table 2]

From Table 2, the test piece corresponding to the mold of the present invention has a larger number of undissolved carbides than the test piece corresponding to the conventional mold.
It can be seen that, because the area ratio is large, there is little heat generation due to friction with the high temperature mating material SNCM439, and as a result, plastic flow is suppressed.

(Example 2) Using the same test piece as in Example 1, the same quenching and tempering treatment was carried out, followed by nitriding treatment. The nitriding treatment was performed by ion nitriding at 500 ° C. for 20 hours at a gas composition of N ₂ : H ₂ = 2: 8. Then, the critical load at which buckling occurs was measured in the same manner as in the example. Table 3 shows the thickness of the nitrogen compound layer on the outermost surface and the test results at this time. Table 3
As described above, the test piece corresponding to the mold of the present invention has a large number of undissolved carbides and a large area ratio, and since the nitrogen compound layer is thick, plastic flow is suppressed as compared with the comparative test piece. I understand.

[0026]

[Table 3]

(Example 3) An example in which the present invention is applied to a hot press die will be shown below. Conventional mold equivalent material 1 shown in Table 1
Also, materials 1 to 13 equivalent to the present invention were prepared, a hot press mold was produced from this, and the results of practical tests were shown in Table 4.
Shown in. The mold is a nested mold for making automobile parts, and the dimensions are 220mm width, 330mm length and 180mm thickness. Forging temperature is 115
Crank press with a maximum capacity of 1500 tons at 0 ℃ 40
Molding was performed at the shot speed.

[0028]

[Table 4]

In Table 4, the conventional mold 1 is JIS SKD61. The heat treatment was carried out by roughing the hot-pressing die, heating it at 1020 ° C., oil-quenching by immersing it in oil at 200 ° C., and then tempering it to a hardness of HRC50. After finishing, tuftride treatment was performed. Table 4 shows the die life of these hot press dies. In this hot press work, the forming speed is high, so the sliding speed between the work piece and the mold is high, and the conventional mold, JIS SKD61, has a plastic flow on the surface of the mold due to frictional heat generation. It turns out that it is easy to proceed early. On the other hand, in the mold of the present invention, a remarkable mold life was obtained due to the synergistic effect of the effect of the undissolved carbide and the effect of the nitrogen compound.

[0030]

EFFECT OF THE INVENTION The hot working die of the present invention disperses a large amount of undissolved carbide in the matrix as compared with the conventional die, so that when the work material is formed, friction with the die is generated. Heat generation can be suppressed, and as a result, plastic flow on the surface of the mold can be prevented. The mold of the present invention has a nitrogen compound layer on the surface of the mold to further enhance the effect.

Claims (5)

[Claims] 1. By weight%, C 0.42% to 0.55%, Si 1.20%
Below, Mn 0.1-1.5%, Cr 4.05-6.50%, W and Mo 1
1.0 to 3.0% at 1 / 2W + Mo, V 0.2 to 1.5
%, Balance Fe and unavoidable impurities, particle size 0.15μ
Area ratio of undissolved carbide of m or more is 2% or more, 2400
A die for hot working having excellent plastic flow resistance, characterized in that the number of the carbides in μm ² is 200 or more. 2. By weight%, C 0.42% to 0.55%, Si 1.20%
Below, Mn 0.1 to 1.5%, Ni 0.1 to 1.5%, Cr 4.05 to 6.50
%, W and Mo 1 type or 2 types at 1/2 W + Mo 1.0 to 3.0
%, V 0.2 to 1.5%, the balance Fe and unavoidable impurities, the area ratio of undissolved carbides having a particle size of 0.15 μm or more is 2% or more, and the number of the carbides in 2400 μm ² is 200 or more. A die for hot working, which is characterized by having excellent plastic flow resistance. 3. By weight%, C 0.42% to 0.55%, Si 1.20%
Below, Mn 0.1-1.5%, Cr 4.05-6.50%, W and Mo 1
1.0 to 3.0% at 1 / 2W + Mo, V 0.2 to 1.5
%, Co 0.3 to 5.0%, balance Fe and unavoidable impurities, and the area ratio of undissolved carbides having a particle size of 0.15 μm or more is 2%.
The above is the number of the carbides in 2400 μm ² is 200 or more, and the hot working die is excellent in plastic flow resistance. 4. By weight%, C 0.42% to 0.55%, Si 1.20%
Below, Mn 0.1 to 1.5%, Ni 0.1 to 1.5%, Cr 4.05 to 6.50
%, W and Mo 1 type or 2 types at 1/2 W + Mo 1.0 to 3.0
%, V 0.2 to 1.5%, Co 0.3 to 5.0%, the balance Fe and unavoidable impurities, and the area ratio of undissolved carbides having a grain size of 0.15 μm or more is 2% or more, and the carbides in 2400 μm ² are A die for hot working with excellent plastic flow resistance, characterized in that the number is 200 or more. 5. The method according to claim 1, further comprising a nitrogen compound layer.
A die for hot working which is excellent in wear resistance according to any one of 1.