JP3508943B2 - Aluminum forging die steel - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mold steel used for forging aluminum or an aluminum alloy (hereinafter referred to as "aluminum").

[0002]

2. Description of the Related Art In recent years, with the increasing need for vehicle weight reduction, parts with high specific strength have been demanded, and the application of aluminum forged members has spread. As a die material for cold / hot forging of aluminum, SKT4 / SKD61 and the like have been conventionally used except for a die having a relatively small size and a die having a special shape.

[0003]

Since aluminum forging has a low material temperature of 550 ° C. even if the material temperature of the workpiece is higher than room temperature, it is lower than that of iron-based hot forging or aluminum die casting mold.
There is no wear such as plastic flow or abrasion caused by the heat rise of the mold material, and it is scrapped due to a corner crack where stress concentration occurs at the R portion of the bottom of the mold recess. Therefore, it cannot be said that the conventional hot work tool steel, which mainly aims to suppress the wear due to the temperature rise of the die material, can sufficiently exhibit the effect of corner crack resistance.

It is well known that general hot work tool steels such as ferritic / pearlite steels and martensitic steels have a ductile brittle transition temperature (hereinafter simply referred to as transition temperature) in the vicinity of 100 ° C to 200 ° C. In aluminum forging, especially when the hardness of the mold material is 46 HRC or more, since the temperature of the material to be processed is low and the large-sized mold that does not easily raise the temperature of the mold material during the forging operation is impacted below the transition temperature. Causes a remarkable decrease in fatigue life due to a decrease in toughness. An object of the present invention is to provide an aluminum forging steel that is used at a relatively low temperature side, has high hardness, high toughness, and has a long fatigue life.

[0005]

As described above, since the aluminum forging die material is forged at a relatively low temperature, the die engraving depth is deep and the radius of curvature of the corner R shape of the die bottom is large. A crack occurs in a small area. As a result of investigating a large number of waste metal molds by the present inventor, it was found that the main cause of waste of aluminum forging die materials is fatigue fracture in which the occurrence of corner cracks is classified as low cycle fatigue. The first means for preventing fatigue fracture is to increase the hardness. For example, in a punch die that receives a compressive stress, the fatigue life increases as the hardness is increased to a high hardness region of 60 HRC or more. However, in general, in the case of a mold having a corner R portion, tensile stress concentration acts, so at 46HRC or more, the fatigue life is shortened due to insufficient toughness. The second means is to improve toughness while maintaining high hardness, but in general, when hot work tool steel is used for aluminum forging, insufficient toughness particularly below the transition temperature becomes a problem.

[0006] For example, in the case of a quench-hardening type steel represented by SKT4, if it is attempted to obtain 46HRC or more, the martensite newly formed by decomposition of retained austenite is not completely tempered, so that the toughness becomes low. Also, SKD6
Precipitation-strengthened steels represented by No. 1 aim for positive secondary hardening, so that a hardness of 46HRC or higher can be obtained even at a tempering temperature of 550 ° C or higher, so the decomposition of retained austenite proceeds and the internal strain is stable with little It becomes a structure and toughness is improved. However, these precipitation-strengthened steels have tempering temperatures of 550-600 ° C.
MC, M ₂ near the peak hardness (around 55HRC) existing between
C-type fine carbide precipitates at high density, and its influence is MC,
It extends to a hardness region of about 46 HRC in which M ₂ C type carbide is decomposed to become M ₇ C ₃ , M ₂₃ C ₆ type carbide completely. Therefore, there is a problem that the toughness cannot be further increased. The present inventor has studied a method of increasing both hardness and toughness below the transition temperature.
It was found that the involvement of the Cr content is extremely large and that the Si content needs to be controlled by the Cr content.

By the way, the effect of enhancing the toughness of alloy steel by limiting the Si content to a low level is described in JP-A-60-56055.
No. 60-59053 and No. 61-213348 are also disclosed. However, these contain HRC46 within the composition range specified in the present invention.
There is no description of the fatigue characteristics in the above case, and no description of limiting the Si content in relation to the Cr content. Further, in Japanese Patent Application Laid-Open No. 4-308059 and Japanese Patent Application No. 5-140695, Si <(18.7 /
Although the relational expression of Cr) -3.3 is specified, the former is not clear about the relationship between Si and Cr, which is the characteristic required for a mold material for low temperature processing such as a mold material for aluminum forging, and it is difficult to understand the relationship between It is a hot work tool steel for which the surface temperature of the mold material is 300 ° C or higher. The latter is also a steel limited to applications in aluminum extrusion dies.
It is premised that the mold is used while being kept at 0 ° C. or higher, and in both cases, the mold use temperature range is intended to be the transition temperature or higher.

On the other hand, the mold steel of the present invention is intended for a mold used at a temperature of 200 ° C. or lower, which is lower than the transition temperature, and has a different wear pattern from the above die casting, hot press forging or aluminum extrusion. The target is a mold having different characteristics required for the mold material. That is, in order to improve the toughness in the operating temperature range below the ductile brittle transition temperature where the lack of toughness becomes noticeable, the present invention controls the Si and Cr contents and reduces the fatigue resistance to 200 ° C or less required for an aluminum forging die. The purpose is to improve the characteristics.

Specifically, the present invention has a mold surface temperature of 200
Aluminum forging die steel used at temperatures below ℃, weight%
, C more than 0.35% and 0.60% or less , Si 0.11 to 1.50 %, M
n 1.5% or less, Cr 3.50 to 5.65%, W or Mo one or two kinds at 1/2 W + Mo of 0.2 to 4.0%, V 0.05 to 2.0%, and Si and Cr contents Si <(18.7 / Cr) -3.3, the balance consists of Fe and unavoidable impurities, quenching and tempering hardness is 46HRC or more, and impact value at room temperature is 4.8kgfm / c
It is a steel for die forging aluminum characterized by having a size of at least m ² . Preferably, it is an aluminum forging die steel used at a die surface temperature of 200 ° C. or lower, and has a weight percentage of C 0.35%.
Over 0.50%, Si 0.11-1.00 %, Mn 1.5% or less, C
r 4.35 to 5.65%, 1 or 2 types of W and Mo are 1 / 2W + M
0.5 to 3.5% in V, 0.2 to 1.5% in V, and Si and Cr contents satisfy the relational expression of Si <(18.7 / Cr) -3.3, and the balance Fe and unavoidable impurities make it hardened and tempered. Saga 46 HRC
Above, and the impact value at room temperature is 4.8 kgfm / cm ² or more, the aluminum forging die steel. Further, if necessary, a part of Fe can be substituted alone or in combination within the range of Ni 1.5% or less and Co 5.0% or less.

[0010]

Next, the reasons for limiting the component range of the present invention will be described. C maintains the excellent hardenability, tempering hardness, and high-temperature hardness of the steel of the present invention, and forms a carbide by combining with a carbide forming material such as W, Mo, V, and Cr to form a crystal grain. It has the effect of refining and giving hardness. An important function of C as the aluminum forging die steel is to increase the yield strength and prevent local plastic deformation of the bottom of the corner R portion from occurring. If it is too low, the above effects cannot be obtained, and if it is too high, excessive precipitation of carbides is caused and the toughness is lowered. Therefore, the C range is set to more than 0.35% and 0.60% or less . Desirably 0.35
% And 0.50% or less, more preferably 0.35%
It is less than 0.45%. Si is 46 HRC, which is a characteristic of the steel of the present invention.
In order to obtain a high toughness value with the above high hardness, it is added at 1.50% or less. The desirable range of Si is 1.0% or less. The details will be collectively described in the section for explaining the Cr amount. Si has the effect of improving the machinability, and in the case of a complicated die, 0.1% or more is desirable. In the present invention, it is 0.11% or more.

Mn improves the hardenability, but if it is too much, it lowers the A ₁ transformation point, excessively increases the annealing hardness, and reduces the machinability, so it is set to 1.50% or less. The desirable Mn range is 0.1 to 1.50%. By controlling the proper addition amount of Cr, Cr suppresses the formation of MC and M ₂ C (special) carbides and prevents the deterioration of toughness, and at the same time, M ₇ C ₃ , M
_It promotes the formation of ₂₃ C ₆ carbides to obtain a peak hardness of 46 HRC or more and also has an effect of improving hardenability.
Add more than%. However, Cr is 46HRC like the steel of the present invention.
In the case of applications where quenching and tempering to a high hardness as described above is used, it is necessary to limit the content in order to secure a high toughness value at the same time. This is based on the operation described below.

In the case where the steel of the present invention is used by quenching and tempering to a high hardness of 46 HRC or more, the tempering temperature is lower than 630 ° C. Since the distribution density of special carbides that precipitate extremely finely is extremely large, 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 prior to the precipitation of special carbides at temperatures around 450 ° C, which is lower than the temperature at which special carbides precipitate. By suppressing the amount, an appropriate amount of cementite carbide can be precipitated, and it becomes possible to suppress the distribution density of the special carbide that reduces the toughness of the matrix. Therefore, the Si content is 1.00% or less and the Cr content is 5.65%.
As described below, since Cr and Si act in combination with the above action, the relational expression of Si <(18.7 / Cr) -3.3% is satisfied in order to obtain the toughness value required for aluminum forging die steel. So add .

In the case of aluminum forging die steel, it is necessary to set the amount of Cr for the following two reasons. (1) If the amount of Cr is too large, the cohesive resistance of the carbide precipitated by tempering during heating becomes too small, and coarsening of the carbide tends to proceed particularly at the former austenite grain boundaries. For this reason, the Cr content is a balance with other alloy element contents,
5.65% or less. (2) When the amount of Cr is reduced, the bainite transformation moves to the short time side, and therefore the bainite nose is likely to be applied when passing through the temperature range of 200 to 400 ° C. when the cooling rate during quenching and cooling is slow, Difficult to apply to large size molds.
When the bainite structure is present, the toughness below the transition temperature is markedly reduced. For this reason, the Cr content is at least 3.50% in balance with other alloy contents. The desirable range of Cr is 4.35 to 5.65%, and more desirably 4.50 to 5.6.
5%.

In the case of the steel of the present invention, the W and Mo contents are set to slow the cohesive resistance of the tempered carbides, thereby suppressing the coarsening of the carbides at the prior austenite grain boundaries during tempering and preventing the deterioration of toughness. However, excessive addition tends to cause precipitation of special carbides and precipitation of austenite grain boundaries that occurs during quenching and cooling, which rather reduces toughness. 0.2-4.0%
Added. A desirable range is 0.5 to 3.5%, more desirably 1.0 to 3.5%. V retains undissolved carbides during quenching and heating, has an austenite grain boundary pinning effect, prevents coarsening of crystal grains, and prevents high temperature tempering brittleness caused by grain boundary segregation of P during tempering. . When V is added excessively, solidification segregation becomes remarkable, the distribution tendency of string-like carbides along the hot working direction is increased, and crack growth in that direction is promoted, so V is 2.0% or less. If it is too low, the effect of the above addition cannot be obtained, so the content is made 0.05% or more. It is preferably 0.2 to 1.5%, more preferably 0.5 to 1.0%.

Ni, together with C, Cr, Mn, Mo, W, etc., imparts excellent hardenability to the steel of the present invention and forms a martensite-based structure even at a slow quenching cooling rate.
It has the effect of preventing the deterioration of toughness, and it is a solid solution N in the matrix.
Since i contributes to the essential improvement of toughness, i is added if necessary. On the other hand, Ni has the above-mentioned effect. However, if it is too much, the A ₁ transformation point is excessively lowered, the fatigue life is shortened, the annealing hardness is excessively increased, and the machinability is lowered, so Ni is added. If it does, it should be 1.5% or less. Co forms a protective oxide film that is extremely dense and has good adhesion when the temperature rises during use, which prevents metal contact with the mating material,
It is added as necessary in order to prevent the temperature rise on the mold surface and to bring about excellent 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. In the case of a steel having a small amount of Si, such as the steel of the present invention, the oxide film becomes too thick.
It is particularly effective for improving the characteristics of the protective oxide film. Co is added to impart the above effect, but if it is too much, it lowers the toughness, so it is made 5.0% or less.

[0016]

Embodiments will be described in detail below based on embodiments of the present invention. A material for a mold having the composition shown in Table 1 was prepared, a mold shown in FIG. 3 was manufactured from this, and a workpiece was a JIS 6061 aluminum alloy, and a practical test was performed at room temperature. Diameter 90
It is engraved to make a rectangular parallelepiped with a width of 15 mm, a length of 65 mm, and a depth of 15 mm in a mold with a circle of 30 mm and a height of 30 mm, and an 8 mm round pin hole for knockout is formed in the center of the mold. ing.
The punch used is SKH51 and has a hardness of 60HRC, and is processed so as to fit into the mold shape. Life assessment, cracks generated in the lower mold of the corner R portion (radius of curvature 2 mm) was performed on the number of shots until the height of 1mm is transferred to the workpiece. The die was not intentionally heated, and forging was performed under the condition that the internal pressure was 70 kgf / m in one shot every 2 seconds. In Table 1, No.1 to 20 are invention steels have
Is a reference steel , and out of No.31 to 38, No.31 is JI
S SKD61 and Nos. 32 to 38 have Si contents at respective levels, but do not satisfy Si <(17.8 / Cr) -3.3.

[0017]

[Table 1]

The heat treatment includes roughing the die, heating it at 1020 ° C., air-quenching to cool it to 200 ° C. or less, and then changing the tempering temperature to obtain a hardness of 48 HRC to 53 HRC as shown in Table 2. It was tempered as usual. At the same time, a Charpy impact test was carried out on a test piece that was split from a sample that had been heat treated with a sample of the same size as the mold. The test piece is a U-notch test piece (JIS No. 3 test piece) having a depth of 2 mm. This is because, in the heat treatment with an actual die and a normal test piece, the cooling rate of the die during quenching cooling is significantly lower than the cooling rate of the test piece, and the toughness value of this type of steel is This is because the impact test piece is apt to be affected by the cooling rate during quenching, and a sample having the same size as the mold must be heat-treated to carry out an impact test piece. After the electric discharge machining of the mold, the machining affected layer on the surface was removed by shot blasting. No surface hardening treatment such as nitriding or carburizing was performed.

[0019]

[Table 2]

Table 2 shows the number of occurrences of cracks, which is the durable life of these molds, and the set life (the number of shots at which the height of the burr transferred to the work material due to the crack opening gradually falling off is 1 mm). ), As well as the Charpy impact value. The cracked portion is a corner R portion shown by 3 in FIG. 1, which forms 45 ° C. in the punch loading direction and extends to the side surface of the die.

The mold made of the steel of the present invention is a comparative steel No. 31.
In comparison with Nos. 38 to 38, both the number of shots with cracks and the set life are 2 to 9 times longer. Further, it can be seen that the magnitude relationship of the life depends on the Charpy impact value rather than the hardness which is said to normally determine the magnitude of the fatigue life. This is Kuroshima et al. Japan Society for Opportunity Society Volume A56 529
As described in No. 89-1390B (1990), the tensile fatigue life is highly dependent on hardness (strength) at hardnesses of 40 to 50 HRC or less, but notch susceptibility at higher hardnesses. It is speculated that such a result was obtained because the dependency due to (toughness) increased. In these papers, the toughness-dependent factor in this high hardness region is sought for inclusions, but it was also confirmed that this is because the type and distribution of the carbide of the matrix phase in the steel are modified in the present invention.

The microstructure photographs shown in FIG. 2 are the invention steel No. 9 and the comparative steel No. 31, which are electron microscope structures obtained by the extraction replica method. The black area is where the density of carbide precipitation is high. Comparative steel No. 31 has an excessive amount of fine MC and M ₂ C carbides in the matrix, but the distribution of carbides is small because MC and M ₂ C precipitates are small in the matrix of the invention steel No. 9. Inhomogeneity occurs in the steel, and it has a fine composite structure with high carbide density, which is responsible for the improvement of toughness, where there is little carbide, and for maintaining strength. Therefore, the contrast on the photograph is strong,
It is presumed that excellent strength and toughness were improved.
Cr, which is considered to be most closely related to the expression of these tissues,
The result of an experiment conducted for setting the amount of Si is shown in FIG. The toughness is low on the high Si and high Cr sides, and the toughness is high on the low Si and low Cr sides. If the Charpy value exceeds 4.0, 4.0
When the following is sorted by regression calculation, Si = (18.7 / Cr) -3.3
You can draw a border.

As described above, Cr is an element that improves the hardenability, so it is added in a large amount in large-sized steels where the hardenability is a problem. However, in this case, since it affects the precipitation distribution and the behavior of the tempered carbide as described in the section "Action", it is necessary to set the Si amount accordingly. As shown in FIG. 1, when the tempering hardness is 48.5 HRC or more, the impact value is low when both Cr and Si contents are high. One example is comparative steel No.32. If the Cr content is high,
As in the case of the comparative steel No.34, even if the Si content is low, the impact value is lowered, whereas when the Cr content is relatively low, the steel of the present invention No. 6 has a certain extent. The impact value is high even if Si is contained. Furthermore, excessive reduction of the Si amount leads to a reduction in the die cutting process. Therefore, considering the influence of the toughness, depending on the Cr amount required, Si <(18.7 /
It is recommended to set it so that Cr) -3.3 is satisfied.

[0024]

As described above, the aluminum forging die steel used at the die surface temperature of 200 ° C. or less of the present invention is
By improving toughness, fatigue resistance in the high hardness region of 46 HRC or higher is improved, crack generation due to stress concentration at corners is suppressed, and die life is improved, thereby reducing the manufacturing cost of forged aluminum products. Can contribute to
It has a great industrial effect.

[Brief description of drawings]

1] Inventive steel, reference steel Nos. 1 to 20 and comparative steel No. 3
It is the figure which arranged the Charpy impact value at the time of tempering hardness 48.5-53 after carrying out the air-cooling quenching of 1-38 respectively by the Si and Cr amount of each steel.

FIG. 2 is photographs of metal structures of the invention steel No. 9 and the comparative steel No. 31, respectively, when the tempering hardness after air-cooling quenching is 50 to 52 HRC.

FIG. 3 is a diagram showing an example of a die shape used for a practical test.

[Explanation of symbols]

1 die, 2 punch, 3 cracked part, 4 knockout pin hole

Claims (4)

(57) [Claims] 1. An aluminum forging die steel to be used at a die surface temperature of 200 ° C. or less, wherein the weight percentage exceeds C 0.35%.
For example 0.60% or less, Si 0.11~ 1.50%, Mn 1.5 % or less, Cr
3.50-5.65%, 1 type or 2 types of W and Mo is 1 / 2W + Mo
Of 0.2 to 4.0% and V of 0.05 to 2.0%, the Si and Cr contents satisfy the relational expression of Si <(18.7 / Cr) -3.3, and the balance is Fe and inevitable impurities, and the quenching and tempering hardness is Is 46 HRC
Above, and the impact value at room temperature is 4.8kgfm / cm ² or more, aluminum forging die steel. 2. An aluminum forging die steel to be used at a die surface temperature of 200 ° C. or lower, wherein the weight% is more than C 0.35% and 0.50% or less, Si 0.11 to 1.00 %, Mn 1.5% or less, Cr
4.35 to 5.65%, 1 or 2 types of W and Mo are 1/2 W + Mo
Of 0.5 to 3.5% and V of 0.2 to 1.5%, the Si and Cr contents satisfy the relational expression of Si <(18.7 / Cr) -3.3, and the balance Fe and unavoidable impurities make it hardened and tempered. Is 46 HRC
Above, and the impact value at room temperature is 4.8kgfm / cm ² or more, aluminum forging die steel. 3. The aluminum forging die steel according to claim 1, wherein a part of Fe is replaced by Ni 1.5% or less. 4. The aluminum forging die steel according to claim 1, wherein a part of Fe is replaced by Co 5.0% or less.