JP2001214238A - Powder hot tool steel excellent in heat crack resistance and wear resistance and hot die - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide hot tool steel capable of simultaneously attaining the improvement of the lives of a die and a tool and the reduction of the production cost by improving its resistance to heat cracks and wear and its machinability. SOLUTION: This powder hot tool steel has a composition containing, by mass, >0.35 to <0.45% C, <1.0% Si, 0.2 to 1.5% Mn, 0.05 to 0.30% S, 2.35 to 5.65% Cr and one or two kinds of W and Mo by (1.2W+Mo): 1.5 to 3.5%, 0.5 to 1.5% V, in which the contents of Si and Cr satisfy the relational inequality of Si<; (18.7/Cr) -3.3, and the balance Fe with inevitable impurities. Alternatively, the powder hot tool steel contains <=1.5% Ni or <=5.0% Co as well. Then, the powder hot tool steel has hardness of >50 HRC, and the hot die is composed of the same powder hot tool steel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for forming non-ferrous metals such as iron, copper, aluminum, and magnesium, including casting including die casting and low pressure casting, hot forging, and the like.
The present invention relates to a hot tool steel for a hot die used for hot extrusion molding and the like, and a hot die for the hot tool steel.

[0002]

2. Description of the Related Art Conventionally, in forming non-ferrous metals such as iron or copper, aluminum, magnesium, etc., a heat including a die casting, a low pressure casting and the like, a hot forging, a hot extrusion and a hot die used for a hot extrusion. As a tool steel,
JIS-SKD6 and JIS-SKD61 have been mainly used. However, recent advances in hot working technology, such as the high-pressure die casting method in the field of die casting and the application of precision forging in the field of forging technology, have increased the thermal or mechanical load acting on the die surface, Heat cracks are generated or abraded on the mold surface, resulting in JIS-SK
Molds using D6 or JIS-SKD61 often became unusable early.

Therefore, JIS-SKD6 and JIS-SKD61 having improved high-temperature strength and toughness have been proposed and put to practical use in order to enhance heat crack resistance and wear resistance. For example, JP-A-54-6807 discloses a low C-5.
A hot tool steel with improved high-temperature strength and toughness of Cr-medium to high Mo (W) -VN has been proposed. Tokuhei 3-533
No. 84 proposes a hot work tool steel in which the toughness is improved by regulating the Si content to less than 0.10% to improve the heat check resistance.

[0004]

The above-mentioned hot tool steel with improved high-temperature strength and toughness has a great effect when used as a mold or a tool, but the tool or the mold is formed by cutting. In the case of manufacturing, the improvement effect of high temperature strength and toughness achieved by optimizing the composition is distorted, and the machinability is reduced as compared with the conventional JIS-SKD6 and JIS-SKD61, and the cutting cost is reduced. The difficulty is that it gets bigger. Normally, molds and tools are made by processing hot tool steel in an annealed state into a rough shape, then quenching and tempering to a predetermined hardness, and finally finishing mainly by electric discharge machining, but it has high temperature strength and toughness. Hot tool steel that achieves the improvement of the composition by optimizing the composition is the same as conventional JIS-SKD6 or JIS-SKD
As compared with No. 61, the machinability is inferior and the cutting cost tends to increase. This makes it difficult to apply the conventionally proposed hot tool steel with improved high-temperature strength and toughness.

[0005] An object of the present invention is to improve the resistance to heat cracks and abrasion, which are problems when using a mold or a tool, to improve the life of the mold, and to manufacture a mold or a tool. Another object of the present invention is to provide a hot work tool steel capable of improving the machinability, which is a problem, and reducing the machining cost. In addition, even in a state having a predetermined hardness, the machinability is improved, and cutting that has been difficult in the past can be performed. That is, heat that can be processed without using electric discharge machining that requires a large amount of processing time is required. It is to provide a tool steel.

[0006]

Means for Solving the Problems The present inventor has studied the improvement of machinability, heat crack resistance, and abrasion resistance, and in order to improve machinability, it is necessary to add an appropriate amount of S. To be most effective It is most effective to use powder metallurgy to suppress the decrease in toughness due to the addition of S. To improve heat crack resistance and abrasion resistance, It has been newly found that it is effective to achieve high toughness by optimizing and use at high hardness, and it is more preferable to set the hardness to more than 50HRC, which is particularly difficult to set with conventional steel. The present invention has been reached.

That is, according to the present invention, C: 0.3% by mass.
More than 5 and less than 0.45%, Si: less than 1.0%, Mn:
0.2-1.5%, S: 0.05-0.30%, Cr:
2.35 to 5.65%, one or two types of W and Mo (1 / 2W + Mo): 1.5 to 3.5%, V: 0.5 to
1.5%, the amount of Si and Cr is Si <(18.7 / Cr)
It is a powdered hot tool steel which satisfies the relational expression of -3.3 and is excellent in heat crack resistance and abrasion resistance comprising the balance of Fe and inevitable impurities. Desirably, the powdered hot tool steel described above is Ni: 1.5% or less. Desirably, the powdered hot tool steel is Co: 5.0% or less.

[0008] Further, the present invention desirably includes 50 HRC.
It is a powdered hot tool steel excellent in heat crack resistance and abrasion resistance as described above, having a hardness exceeding.
And it is a hot metal mold which consists of these powdered hot tool steels of the present invention.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, important features of the present invention reside in that 0.05 to 0.30% of S is added, and that powder metallurgy is employed as a method for producing a material. S is combined with Mn to form MnS, and when present in steel, machinability is improved. The effect is remarkable when added at 0.05% or more. In general, hot tool steel is produced by casting a steel ingot having a predetermined composition and hot working the steel ingot. MnS generated in the steel ingot is plastically deformed in the forging direction at the time of hot working to have a linear form. If it is linear, it easily becomes a starting point of fracture, and the toughness in the direction perpendicular to the forging direction is greatly reduced. This is also the reason that JIS-SKD6 and JIS-SKD61 regulate the S content to 0.03% or less.

However, when starting the production of a steel ingot produced by consolidating alloy particles containing 0.05 to 0.30% of S by hot isostatic pressure, MnS exists in the form of fine spheres. It is hard to be linear even by hot working. Therefore, the toughness in the direction perpendicular to the forging direction is not significantly reduced. However, excessive addition of S exceeding 0.30% causes MnS having a large grain size to be present and lowers the toughness value. Therefore, the upper limit is set to 0.30%. In addition, as a manufacturing method of the alloy particles, a water atomizing method and a gas atomizing method are preferable. In order to bring the density of the steel ingot close to the theoretical density, the compaction of the alloy particles is carried out at 980 ° C. or higher after encapsulation in a steel plate capsule or the like.
It is desirable to carry out at a hot isostatic pressure of MPa or more. Further, it is preferable that the ingot is subjected to hot working with a press, a hammer, a rolling mill or the like at a temperature of 900 to 1250 ° C. after forging and forging and forming.

Next, the reasons for defining the amounts of alloying elements will be described. C maintains excellent hardenability, temper hardness, and high-temperature hardness of the steel of the present invention, combines with carbide-forming elements such as W, Mo, V, and Cr to form carbides, It is added in order to provide a softening effect, abrasion resistance, tempering softening resistance, and high-temperature hardness. If it is too large, precipitation of excessive carbides is caused and the toughness is reduced, so the content is made less than 0.45%.
Further, in order to achieve the above-mentioned object, for example, to maintain the quenching and tempering hardness exceeding 50 HRC, which is a feature of the steel of the present invention, the content should exceed 0.35%.

Si is added in an amount of 1.0 to impart oxidation resistance.
%. The details will be collectively described in the section of Cr. M
n is a characteristic of the steel of the present invention, S: 0.05 to 0.30%
Adds MnS to improve machinability. Furthermore, hardenability is improved. If it is too large, the A1 transformation point is excessively decreased, the annealing hardness is excessively increased, and the machinability is decreased.

The effect of Cr improves temper softening resistance and high-temperature strength by setting an appropriate amount of addition, improves wear resistance by forming carbides by combining with C, improves hardenability, and provides rapid nitriding. 2.35%
Add above. 4.35 especially when quenching property is emphasized.
% Or more is desirable. However, Cr is 50 as in the steel of the present invention.
When used with a high quenching and tempering hardness exceeding HRC, it is necessary to limit the content of the steel of the present invention in order to ensure a high toughness value. This is based on the operation described below.

When the steel of the present invention is used with a high quenching and tempering hardness exceeding 50 HRC, the tempering temperature is 60.
At around 0 ° C. or even lower, in this temperature range, the distribution density of special carbides extremely finely precipitated in the matrix due to tempering is extremely large, and the toughness of the matrix is significantly reduced. On the other hand, Cr and Si have the effect of suppressing the precipitation of cementite carbide that precipitates in the previous stage, so that Cr and Si
By suppressing the content of, a proper amount of cementite carbide can be precipitated, whereby the distribution density of the special carbide that lowers the toughness of the matrix can be reduced. Therefore S
The i content is less than 1.0% and the Cr content is 5.65% or less.
Since Cr and Si act in combination with the above-mentioned effects, in order to obtain a toughness value required for a mold, Si <(18.7 /
Cr) is added so as to satisfy the relational expression of -3.3.

As can be seen from this relational expression, when it is desired to increase the Cr content (for example, Cr> 5.5%), Si
The amount must be less than 0.1%. Where Si is Co
As described in the section, if the content is too low, the oxide film tends to be too thick, so that the content is preferably 0.1% or more. The same applies to the case where the addition is made by adding Co.

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 precipitates fine special carbides during tempering to increase softening resistance and high-temperature strength. However, excessive addition causes excessive precipitation of carbides and lowers the toughness. Therefore, depending on the application, one or two of them are (1 / 2W + Mo) in the range of 1.5 to 3.5.
%Added.

V forms a carbide which hardly forms a solid solution and is effective in improving wear resistance and seizure resistance. In addition, it has an important role of increasing the softening resistance and high temperature proof stress in a high temperature range by precipitating a small amount of carbide that hardly agglomerates during tempering and partially forming a solid solution in the matrix during quenching and heating. Further, while improving the toughness by refining the crystal grains, the A1 transformation point is increased, and the effect of improving the heat crack resistance in combination with the excellent high temperature proof stress is brought. 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 the content is too large, the toughness is greatly reduced, so that the content is set to 1.5% or less. If the content is too low, the effect of the addition cannot be obtained, such as early wear of the surface layer of the mold.

Ni, together with C, Cr, Mn, Mo, W, etc., imparts excellent hardenability to the steel of the present invention, forms a martensite-based structure even at a slow quenching cooling rate, and reduces toughness. Is an important additive element for preventing the occurrence of a crack, and is preferably added in an amount of 0.1% or more to give an essential effect of improving the toughness of the matrix. However, excessive addition excessively lowers the A1 transformation point, lowers the set life, and excessively increases the annealing hardness, thereby reducing machinability.
% Or less.

Co forms an extremely dense protective oxide film having good adhesion at the time of raising the temperature during use, thereby preventing metal contact with a counterpart material, preventing a temperature rise on the mold surface and excellent. It provides wear resistance. However, if the oxide film is too thick, the surface of the mold surface becomes rough, which has 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 tends to be too thick, and therefore, the addition of Co is particularly effective in improving the properties of the protective oxide film. C
o is added to impart the above effect, but if it is too much, the toughness is reduced, so that it is made 5.0% or less. If it is too low, the above effect is not obtained, so it is preferably made 0.3% or more.

In addition, in order to improve hardness and corrosion resistance, N: 0.
It may contain 1% or less, Nb: 0.1% or less for grain refinement, Al: 0.3% or less for improvement of nitriding hardness, and Cu: 5% or less for workability improvement.

[0021]

(Embodiment 1) Hereinafter, the present invention will be described in detail based on an embodiment of the present invention. Table 1 shows the chemical compositions of the steels of the present invention and the comparative steels subjected to the test. In Table 1, Comparative Steel 27 is JIS-SKD61 manufactured by a melting method. For the test, a machinability test by face milling, a Charpy impact test, and a heat crack test were performed.

[0022]

[Table 1]

The specifications of the machinability test will be described below. A sample was prepared by annealing steel having the chemical composition shown in Table 1 at 860 ° C. Attach one carbide coated chip to a 160mm diameter cutter, cutting speed 120m / m
in, feed rate 0.15 mm / rotation, cutting width 120 mm,
The cutting was performed under the condition of a cutting depth of 3 mm, and the machinability was evaluated based on the magnitude of the cut size up to the life of the chip as the life cut volume. The larger the life cutting volume, the better the machinability.

The specification of the Charpy impact test is described below.
The test piece was a JIS No. 4 test piece. The test piece was taken from a direction perpendicular to the material forging direction. After maintaining the steel having the chemical composition shown in Table 1 at an austenitizing temperature of 1020 ° C., quenching was performed at a cooling rate of half-cooling for 15 minutes. Here, semi-cooling for 15 minutes refers to a cooling rate that takes 15 minutes to cool from 1020 ° C. to 520 ° C. Thereafter, tempering was performed twice to a hardness of 51 HRC. The Charpy impact test was performed at room temperature. The quenching cooling rate of the semi-cooled 15 minutes was adopted because the actual quenching cooling rate of the mold was:
This is because the size is slower than that of the test piece cooled by the same cooling method. Since the toughness value of the hot work tool steel is greatly affected by the quenching cooling rate, in this test, the test piece was heat treated at a cooling rate close to the actual die.

The specifications of the heat crack test are described below.
The shape of the test piece is a cylinder having a diameter of 90 mm and a height of 50 mm. Three test pieces having different hardnesses were prepared from steels having the chemical compositions shown in Table 1. After the test piece was maintained at the austenitizing temperature of 1020 ° C., it was quenched at a cooling rate of half a minute for 15 minutes. Thereafter, tempering was performed twice to obtain a hardness of 51 HRC, 48 HRC, and 45 HRC, respectively. 90mm diameter of test piece 45mm diameter at center of end face
After rapidly heating the circular portion to 600 ° C. by high-frequency induction heating, cooling the heated portion of the test piece with spray water was counted as one time, and the number of times of occurrence of heat crack was evaluated.

[0026]

[Table 2]

Table 2 shows the test results. Heat crack resistance is greatly affected by chemical composition, but the most obvious is that
Setting the hardness to more than 50 HRC is a great effect. Obviously, the higher the hardness and the higher the abrasion resistance, the more advantageous. However, when the hardness is extremely high, such as exceeding 50 HRC, the Charpy impact value becomes small, and the mold is easily cracked or chipped. Therefore, it can be seen that steel which can be used with a hardness exceeding 50 HRC requires optimization of the chemical composition. The steel of the present invention has a high Charpy impact value even at a high hardness exceeding 50 HRC due to the optimized chemical composition.

The most important effect of the addition of S in the present invention will be examined. In comparison with the comparative steel 27 (JIS-SKD61) having a small S content and the comparative steel 22 in the material starting from the ingot produced by the smelting method, the comparative steel 22 has an excellent Charpy impact value and a heat crack. Although the number of cycles until generation is slow, the life cutting volume is small. The comparative steel 22 has high-temperature strength and toughness according to JIS-SKD61 by optimizing the composition.
It can be seen that although it is more improved, the machinability is greatly reduced. Comparative steel 25 and comparative steel 26 correspond to those obtained by adding S to comparative steel 22 and comparative steel 27 (JIS-SKD61), respectively. It can be seen that the improvement is very effective. However, the Charpy impact value is very small.

Although these comparative examples are materials starting from a steel ingot manufactured by the smelting method, the machinability is improved by simply adding S to the steel ingot manufactured by the smelting method. Is negative for heat crack resistance. Further, since the Charpy impact value at a hardness exceeding 50 HRC is low, the hardness cannot be set to a value exceeding 50 HRC, so that improvement in wear resistance cannot be expected. The steel 1 of the present invention has a chemical composition equivalent to that of the comparative steel 25 produced by the smelting method, and is a material starting from a steel ingot produced by the powder metallurgy method.
Not only a remarkable improvement in the machinability due to the addition is observed, but also the Charpy impact value is high.

(Example 2) An example in which a mold was actually produced from the steel 1 of the present invention and the comparative steel 14 shown in Table 1 and casting was performed is shown below. Invention steel 1 is 50.5HRC by quenching and tempering.
A block having a hardness of 5 mm was finished by direct cutting. For the comparative steel 14, the annealed material was roughly processed by cutting, and after quenching and tempering to a hardness of 45.3 HRC, finishing was mainly performed by electric discharge machining. These two dies are mounted on a die casting machine with a clamping force of 350 t,
ADC12 alloy casting at 680 ° C. was performed.

[0031]

[Table 3]

The occurrence of heat crack was confirmed in the groove near the gate. As shown in Table 3, the mold of the steel 1 of the present invention has a significantly longer heat crack generation life than the mold of the comparative steel 14. Particularly, the mold of the steel 1 of the present invention was characterized in that the opening of the heat crack was smaller than that of the mold of the comparative steel 14. Since the opening of the heat crack is transferred to the product, the smaller the better. Also, the die manufacturing cost was reduced by about 14%. In particular, it is significant that the finishing process by electric discharge machining, which greatly affects the cost, is enabled by the cutting process. This has an advantage that the production of the electric discharge machining electrode can be omitted. In addition, the quenching and tempering can be omitted, thereby shortening the mold manufacturing period. From these results, the superiority of the steel of the present invention was confirmed.

As described above, the steel of the present invention has a hardness exceeding 50 HRC which is effective in improving heat crack resistance and wear resistance by optimizing the chemical composition, adding an appropriate amount of S, and employing powder metallurgy. It was confirmed that the setting was possible and that it had excellent machinability.

[0034]

According to the present invention, the resistance to heat cracks and abrasion, which is a problem when using a mold or a tool, and the machinability, which is a problem when a mold or a tool is manufactured, are dramatically improved. , And both the improvement of the life of the mold and the reduction of the mold manufacturing cost can be achieved at the same time.

Claims (5)

[Claims] (1) In mass%, C: more than 0.35 and 0.45
%, Si: less than 1.0%, Mn: 0.2 to 1.5
%, S: 0.05 to 0.30%, Cr: 2.35 to 5.
65%, W and Mo in one or two kinds (1 / 2W + M
o): 1.5-3.5%, V: 0.5-1.5%, S
A powdered hot tool steel in which the amounts of i and Cr satisfy the relational expression of Si <(18.7 / Cr) -3.3, and are excellent in heat crack resistance and wear resistance comprising the balance of Fe and inevitable impurities. 2. The heat crack resistance according to claim 1, wherein Ni is not more than 1.5% by mass%.
Powdered hot tool steel with excellent wear resistance. 3. The powdered hot tool steel according to claim 1, wherein the content of Co is 5.0% or less by mass%. 4. The powdered hot tool steel according to claim 1, which has a hardness exceeding 50 HRC. 5. A hot die comprising the powdered hot tool steel according to any one of claims 1 to 4.