JP3191008B2 - Hot tool 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 hot tool steel for a hot die used for die casting, hot press forging, and the like.

[0002]

2. Description of the Related Art As a die material for die casting and hot press forging, except for some warm forging and precision casting dies, relatively small dimensional dies, special die parts and the like. , Conventionally, S
The usual practice was to use KD6, SKD61 or the like with a heat treatment hardness of less than HRC50.

[0003]

In recent years, the development of hot working technology, for example, in the field of die casting, the application of high-pressure die casting and in the field of forging technology, the application of closed forging and high-speed forging has been advanced, and has been applied to the die surface. In many cases, thermal or mechanical loads are increased, and heat cracks are formed on the mold surface or are worn out, so that the mold cannot be used early. Among them, the most effective way to improve the wear phenomenon is to increase the heat treatment hardness.However, in the conventional mold material, when the hardness is increased, the toughness value is reduced, so that cracking and chipping are caused. The applications of molds that easily occur and can be used with a heat treatment hardness of about HRC50 or more were limited. The present invention
An object of the present invention is to provide a hot work tool steel having high toughness in a high hardness range and excellent heat crack resistance.

[0004]

Means for Solving the Problems In conventional hot tool steels such as SKD6 and SKD61, when the quenching and tempering hardness is HRC50 or more, the tempering temperature is 600 ° C or less. At this time, the distribution density of carbides finely precipitated by tempering becomes extremely large and the toughness is reduced. However, it has been found that the Si and Cr contents greatly contribute to the precipitation behavior of the carbides. Incidentally, the effect of increasing the toughness of alloy steel by limiting the Si content is described in JP-A-60-56055 and JP-A-60-56055.
-59053, 61-213348 and 61-213349. However, there is no description about the characteristic value or the like when the HRC is 50 or more in the plastic range specified in the present invention, and there is no description that limits the Si amount in relation to the Cr amount. The present invention investigates in detail the involvement of Si, Cr content with respect to the toughness value of hot work tool steel not disclosed in the above proposal,
The effect of improving the toughness value by reducing the amount of Si is larger when the hardness is equal to or higher than the HRC50 at which the tempering temperature is lower, rather than lower than the HRC50 around the HRC 42 to 48, which was the conventional hardness used. The effect of improving the toughness value by the reduction is determined by the relationship with the Cr content. For alloy steels with a low Cr content, excessive reduction is not necessary, while C alloy is considered in consideration of the hardenability of the mold.
When the amount of r is set to be high, it is necessary to keep the amount of Si low. Further, in the steels of the first and third inventions of the present application, when simply reducing the amount of Si, the surface of the mold surface is roughened. Although it is often recognized that the life of the mold is reduced, the addition of an appropriate amount of Co also has the effect of suppressing the growth of the oxide scale and preventing the skin from roughening, even with respect to the mold material in which Si is reduced. Is based on the discovery of From the above, by increasing the heat treatment hardness, even when used in a mold under severe hot working conditions, the surface of the mold such as heat cracks and abrasion is less likely to be worn,
In addition, since cracking and chipping are unlikely to occur, it has become possible to provide a hot work tool steel applicable to a wide range of applications such as large dimensions and complicated shapes.

That is, the present invention specifically relates to
With C exceeding 0.35% and less than 0.45%, Si 1.00% or less, Mn 0.
1 to 1.5%, Ni 0.1 to 1.5%, Cr 4.35 to 5.65%, W and Mo are used alone or in combination with 1/2 W + Mo 1.5 to 3.5%, V 0.5 to 1.5%
A hot tool characterized in that the Si and Cr contents satisfy the relational expression of Si <(18.7 / Cr) -3.3, the balance is made of Fe and inevitable impurities, and the quench and temper hardness is HRC50 or more. Steel, Co 0.3%
5.0% added, and in each of the above, S
It is a hot tool steel that is restricted to exceed i by 0.1%.

[0006]

Next, the reasons for limiting the range of the components of the present invention will be described. C is excellent hardenability, temper hardness of the steel of the present invention,
And maintain high-temperature hardness, and combine with carbide-forming elements such as W, Mo, V, and Cr to form carbides, and improve the crystal grain refinement effect, wear resistance, tempering softening resistance, and high-temperature hardness. It is added to give. In order to achieve the above purpose, such as to maintain the quenching and tempering hardness of HRC 50 or more, which is one of the features of the steel of the present invention, as excessive carbides lead to excessive carbide precipitation and lower toughness because it lowers toughness. The content should exceed 0.35%. Si is added in an amount of 1.0% or less in order to obtain a high toughness value with a high hardness of HRC 50 or more, which is a feature of the steel of the present invention. The details will be described collectively in the section of Cr. Mn is to improve the hardenability, too large, excessively lowering the A ₁ transformation point, excessively high annealing hardness, and less from 0.1 to 1.50% as it reduces the machinability. Ni, together with C, Cr, Mn, Mo, W, etc., imparts excellent hardenability to the steel of the present invention, and even at a slow quenching cooling rate, forms a structure mainly composed of martensite, thereby reducing the toughness. Addition of 0.1% or more is an important element for preventing the addition and also has an essential effect of improving the toughness of the matrix. Ni is added in order to obtain the above effects, but if it is too much, it excessively lowers the A ₁ transformation point, leads to a reduction in set life, excessively increases annealing hardness, and deteriorates machinability. I let you
1.50% or less.

[0007] By setting an appropriate amount of Cr, temper softening resistance and high-temperature strength are improved, wear resistance is improved by forming carbides by combining with C, hardenability is improved, and rapid nitriding property is improved. 4.35
% Or more. However, Cr is HRC50 like steel of the present invention.
When used as a mold having the above high quenching and tempering hardness, 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 used as a mold having a high quenching and tempering hardness of HRC 50 or more like the steel of the present invention, the tempering temperature is around 600 ° C or lower, but in this temperature range, the tempering is extremely difficult in the matrix. The distribution density of the special carbides that precipitate finely 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 which precipitates in the previous stage, and conversely, by suppressing the content of Cr and Si, a proper amount of cementite carbide is precipitated to reduce the toughness of the matrix. The distribution density of carbide can be suppressed. For this reason, the Si content is set to 1.0% or less and the Cr is set to 5.65% or less. However, since Cr and Si act in combination with the above-mentioned actions, 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%), the Si content must be 0.1% or less. However, Si is Co
If this is too low, the oxide film tends to be too thick, so that it is desirable to exceed 0.1%. The same applies to the case where Co is supplemented by addition.

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 and Mo precipitate fine special carbides at the time of tempering 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 at 1.5 to 3.5% based on the strength according to the use conditions of the mold and the tool and the high temperature strength. V forms carbides which are hard to form a solid solution and has an effect on improvement of wear resistance and seizure resistance. It forms solid carbides in a matrix at the time of quenching heating and precipitates fine carbides which are hard to agglomerate at the time of tempering. It is an important element for increasing the softening resistance in a high temperature range and providing a large 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, a huge carbide is generated, the distribution tendency of the string-like carbide along the hot working direction is increased, and the growth of cracks along that direction is promoted. Since the effect of the above addition, such as early softening, cannot be obtained, the content is set to 0.50% or more.

Co forms an extremely dense protective oxide film with good adhesion at the time of temperature rise during use, thereby preventing metal contact with a counterpart material, preventing a rise in the temperature of the mold surface and providing excellent properties. This results in high abrasion resistance. However,
If this oxide film is too thick, it may cause the rough surface of the mold surface to have the opposite 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. Therefore, the addition of Co is particularly effective for improving the properties of the protective oxide film. Co is added to impart the above-mentioned effects, but if it is too large, the toughness is reduced, so the content is made 5.00% or less. If it is too low, the effect of the above-mentioned addition is not obtained, so it is made 0.30% or more.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on embodiments. 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 1 is JIS SKD6
Comparative steel 3 is disclosed in JP-A-60-5605.
No. 5, No. 5, No. 5, No. 5, No. 5, No. 5, No. 5, No. 5, No. 5, No. 5, No. 5, No. 5, No. 5, No. 5, No. 5, No. 5, No. 5, No. 5 and No. 5, No. 5, No. 5, No.
It is a hot tool steel included in the claims of JP-A-61-213349. The comparative steels 5 to 8 have the Si content at each level, but do not satisfy Si <(17.8 / Cr) -3.3. Figure 1 shows that each sample in Table 1 was heated at 1020 ° C, oil quenched or semi-cooled for 15 minutes, then tempered, and HRC was changed.
The relationship between the hardness and the Charpy impact value when tempering is performed to the respective hardnesses of 46 to HRC 54 is shown. Here, the semi-cooling quenching for 15 minutes means a treatment in which the temperature is lowered to an intermediate temperature of 520 ° C. between the quenching temperature (1020 ° C.) and room temperature (20 ° C.) in 15 minutes.
The test piece is a U-notch test piece having a depth of 2 mm. Half cold 1
The reason for performing the impact test with 5 min quenching is that when the actual mold is cooled, when the dimensions are relatively large, the actual quenching cooling rate is lower than the specimen treated in the same way, and This is because the toughness value of the inter-tool steel is easily affected by the quenching cooling rate. The impact value of oil quenching of SKD61 is
Hardness exceeding HRC50, causing a sharp drop, and semi-cool 1
The impact value of quenching for 5 min is significantly lower than that of oil quenching which is in a completely quenched state. In Comparative Steel 2, the impact value of oil quenching is higher than that of SKD61, and when the quenching cooling rate is reduced (semi-cooled 15 min), the decrease in toughness is significantly small. This is mainly due to the hardenability improving effect of the addition of Ni. However, also in the case of the comparative steel 1, a sharp drop in impact value is observed at a hardness of HRC 50 or more. The impact value of oil quenching of Comparative Steel 3 is higher than that of Comparative Steel 1 (SKD61). Comparative steel 3 has a reduced Si content in comparative steel 1 (SKD61), and has a higher toughness value than SKD61. In addition, the drop in impact value at a hardness exceeding HRC50 is moderate. Incidentally, as described above, the steel is disclosed in
In this publication, one of the examples of No. 055 was blown as a target component. In this publication, the impact value of HRC46 is described, but the impact value of HRC50 or more is not described.

FIG. 2 shows a semi-cooled steel 15 of the invention steels 1, 2, 3, and 4.
The impact values of min quenching are shown in comparison with those of Comparative Examples 2 and 4. Compared with Comparative Steel 2, the impact value of semi-cooled 15 min was improved by the hardenability by adding Ni as described above, but did not satisfy Si <(18.7 / Cr) -3.3. Therefore, the drop in impact value at a high hardness of HRC50 or more is sharp. On the other hand, the steels 1, 2, 3, and 4 of the present invention and the comparative steel 4 satisfy the above-mentioned relational expression between Si and Cr, and therefore this decrease is gradual. The steel 3 of the present invention has the same impact value as that of the comparative steel 4 because Mo and V are added in a large amount to impart high-temperature strength, but is characterized by a long heat crack generation life described later. Particularly, in applications where the temperature of the mold surface is high, such as a die casting mold having a high molten metal temperature of a work material or a forging mold having a long contact time with the work material, W, Mo, which impart high-temperature strength are used. Increase the V amount.
Accordingly, the absolute value of the impact value tends to decrease, but no sharp decrease in the impact value with an increase in hardness is observed.

FIG. 3 shows the results of an experiment performed for setting the amounts of Cr and Si. The figure plots the Si and Cr contents of the comparative steel of Table 1 and several examples of the steel of the present invention, and shows the Charpy impact value when the hardness is HRC52 by quenching after semi-cooling for 15 min and tempering. It was done. In the figure, the reference numeral 7 indicates the inventive steel 7 and the ratio 1 indicates the comparative steel 1 in the same manner. As described above, Cr is an element that improves the hardenability, so that a large amount of a mold material having a problem of hardenability is added in a large amount. But in this case, the "action"
As described above, the precipitation distribution and behavior of the tempered carbide are affected, so that it is necessary to set the Si amount accordingly. As shown in FIG. 3, when the temper hardness is HRC52 after quenching in a semi-cooled state for 15 minutes, the impact value is low when both Cr and Si are high (Example: Comparative steel 8). When the Cr content is high, S
Even if the amount of i is low, the impact value is reduced (eg, comparative steel 5), whereas if the amount of Cr is relatively low, a certain amount of S
Even when i is contained, the impact value is relatively high (eg, Steel 7 of the present invention).
Excessive reduction of the amount of Si may lead to roughening of the mold surface due to insufficient oxidation resistance or a decrease in workability due to the cutting tool of the mold. Therefore, the Cr required in consideration of the influence of the above toughness is required. Depending on the amount, it is set so as to satisfy Si <(18.7 / Cr) -3.3 by weight%.

Next, in hot tool steel, in terms of practical performance,
The results of confirmation of the most important heat crack resistance test will be described. Regarding the test for heat crack resistance, the number of heat cycles N until the heat crack occurs
And the maximum depth D of cracks when this test was continued up to twice the number N of the thermal cycles. The test specifications are such that the operation of rapidly heating the surface of a 60 mmφ × 40 mm test piece to 600 ° C. and then rapidly cooling the surface with water at 20 ° C. is repeated. The number N of heat cycles to the occurrence of a heat crack is mainly related to the high-temperature strength, and the elongation to the inside of the heat crack, that is, the maximum depth D of the crack is mainly related to the toughness value. The properties of the surface, particularly the oxidation resistance, each affect the heat crack resistance in terms of the ease of forming and growing an oxide film. In FIG.
The results of the heat crack resistance test of the steel of the present invention and the comparative steel are shown. From each data, it can be seen that increasing the hardness clearly increases the number of cycles N until a heat crack occurs. The comparative steel 7 has a higher Mo strength than the comparative steel 1 and has an increased high-temperature strength. However, this indicates that the number of cycles N increases and this leads to an improvement in the heat crack life of the mold. But this is HR hardness on one side
It can be seen that when the temperature is increased to C52, the depth of development of the heat crack increases. When SKD61 is tempered to about HRC52 and cast using a practical die such as an aluminum die-casting die, mechanical stress such as mold clamping force acts on the die in addition to thermal stress due to heat cycle. In addition, breakage from a deeply developed heat crack is likely to occur. For this reason,
Except for small-sized molds, SKD61 is currently used with a hardness of HRC50 or less. On the other hand, as shown in FIG. 4, in the steel of the present invention, even when the hardness is increased to HRC52, the depth of heat crack propagation is small. The steel of the present invention retains a high toughness value even with the high hardness of HRC52 as described above. Therefore, the steel of the present invention is applied to a practical die and does not cause breakage.

Observation of the cross-sectional morphology of the heat crack generated on the above-mentioned heat crack test piece revealed that the surface of the test piece and the opening of the crack were also oxidized, and the steel 5 of the present invention had an oxide film. In particular, the thickness was 0.1 mm. On the other hand, the steels 11 and 12 of the present invention in which Co was added to improve the oxidation resistance had an oxide film thickness of 0.04 mm, a small crack propagation depth, and a large effect of improving the heat crack propagation resistance. Was.

[0015]

As described above, the steel of the present invention is higher in hardness than conventional materials as a hot die, specifically, HR.
With a hardness of C50 or more, it can be used without causing breakage, and it is possible to manufacture a mold that gives an excellent service life in terms of heat crack generation and wear. Moreover, the growth rate of the oxide film on the surface of the mold during use of the mold is suppressed, and the development of rough skin and heat cracks into the inside of the mold is suppressed.

[Brief description of the drawings]

FIG. 1 is a view showing the behavior of a change in Charpy impact value with respect to tempering hardness when oil hardness and half-cooling of Comparative Steels 1 to 3 are quenched for 15 minutes, and then tempered to each hardness.

FIG. 2 is a view showing Charpy impact values when the steels of the present invention 1, 2, 3, 4 and comparative steels 2, 4 are quenched for 15 minutes and then tempered to respective hardnesses.

[FIG. 3] After quenching each of the inventive steels 1, 4, 5, 6, 8, 9, 10 and comparative steels 1, 2, 5, 6, 7, 8 for half a minute for 15 minutes, the tempered hardness was HRC52. FIG. 4 is a diagram in which the Charpy impact values at the time are arranged by the amounts of Si and Cr contained in each steel.

FIG. 4 is a view showing the results of a heat crack resistance test on the steels of the present invention 1, 2, 5, 6, 11, 12 and comparative steels 1, 7.

[Table 1]

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 9/00

Claims (4)

(57) [Claims] (1) more than 0.35% C and less than 0.45% by weight,
Si 1.00% or less, Mn 0.1-1.5%, Ni 0.1-1.5%, Cr 4.
35 to 5.65%, 1/2 W + Mo 1.5 with one or two types of W and Mo
~ 3.5%, V 0.5 ~ 1.5%, and the amount of Si and Cr is Si <(18.7 /
A hot work tool steel which satisfies the relational expression of (Cr) -3.3, is composed of a balance of Fe and inevitable impurities, and is used for quenching and tempering hardness of HRC50 or more. 2. In% by weight, more than 0.35% of C and less than 0.45%,
Si 1.00% or less, Mn 0.1-1.5%, Ni 0.1-1.5%, Cr 4.
35 to 5.65%, 1/2 W + Mo 1.5 with one or two types of W and Mo
~ 3.5%, V 0.5 ~ 1.5%, Co 0.3 ~ 5.0%, and Si, Cr
The amount satisfies the relational expression of Si <(18.7 / Cr) -3.3, and the balance is composed of Fe and unavoidable impurities.
Hot work tool steel characterized by being used in 50 or more. 3. In% by weight, more than 0.35% of C and less than 0.45%,
Si over 0.1% and below 1.00%, Mn 0.1-1.5%, Ni 0.1-
1.5%, Cr 4.35-5.5%, W and Mo are 1/2 by 1 or 2 types
W + Mo 1.5 to 3.5%, V 0.5 to 1.5%, Si and Cr contents satisfy the relational expression of Si <(18.7 / Cr) -3.3, and the balance consists of Fe and unavoidable impurities, and has a quenching and tempering hardness of HRC50 or more Hot tool steel characterized by being used in: 4. The composition according to claim 1, wherein the weight percentage is more than 0.35% and less than 0.45%.
Si over 0.1% and below 1.00%, Mn 0.1-1.5%, Ni 0.1-
1.5%, Cr 4.35-5.5%, W and Mo are 1/2 by 1 or 2 types
W + Mo 1.5-3.5%, V 0.5-1.5%, Co 0.3-5.0%,
A hot work tool steel characterized by the fact that the amounts of Si and Cr satisfy the relational expression of Si <(18.7 / Cr) -3.3, the balance being Fe and unavoidable impurities, and the use of the steel with a quench and temper hardness of HRC50 or more.