JP2002129284A - Steel for hot-forging die and hot-forging die - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for hot-forging die and the hot-forging die using the steel having excellent high temperature strength and also, excellent nitriding characteristic. SOLUTION: This steel for hot-forging die is composed by mass% of 0.3-0.5% C, <=1.0% Si, <=1.0% Mn, 0.5-1.5% Ni, >0.3 to 3.6% Cr, 2.0-4.0% single of Mo and W or complex of (Mo+1/2W), 0.2-1.5% V, 0.4-0.7% Al and the balance Fe with inevitable impurities and further, composed of <=0.5% Co in addition to the above. Further, in the steel for hot-forging die, N is regulated to <=0.100%, or further, P, S and O being the inevitable impurities are desired to be regulated to <=0.025%, <=0.010% and <=0.005% respectively. Then, this hot-forging die is manufactured by using this steel for hot-forging die and has a nitrided layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot forging die steel having excellent high-temperature strength and nitriding characteristics, and a hot forging die using the same.

[0002]

2. Description of the Related Art The forming surface (mold surface) of a hot forging die is heated by heat transfer from a high-temperature work material and frictional heat with the work material, and the forging surface pressure and the work material are reduced. Flow wear occurs due to the frictional stress of. In order to enhance the durability of the mold, the steel for the mold needs to have high-temperature strength and softening resistance. For this reason, JIS SK which has increased high-temperature strength and softening resistance with respect to JIS SKD61 steel of 5% by mass Cr (hereinafter referred to as “% by mass”) by increasing the Mo amount with 3% Cr.
D7 steel is used in some cases.

In recent years, in order to further increase the durability of a hot forging die, application of a nitriding treatment has been advanced. The nitride layer is lubricated,
It has an insulating effect and is effective not only when lubricating coolant is not sufficiently distributed, such as when aiming for products with more complex shapes by near net shape forging,
Since the hardness of the nitrided layer itself is high, the effect of suppressing the flow wear is also large.

[0004]

When a hot forging die is subjected to nitriding treatment, Cr is an alloying element that causes a mutual reaction with N and forms a high hardness of the nitrided layer. For this reason,
High nitriding hardness cannot be obtained with mold steel having low Cr. Further, when the mold is used at an elevated temperature, N in the nitrided layer diffuses into the base material, so that the nitriding hardness decreases. C
In a mold steel having a low r, the interaction with N is small, so that N is easily diffused at an early stage, and the nitriding hardness tends to decrease with respect to the number of hot forging.

[0005] From the above two points, the 3% Cr having low Cr
SKD7 steel is 5% Cr represented by SKD61 steel
Compared with hot tool steels of the series, there is a disadvantage in terms of durability especially when used in a hot forging die subjected to nitriding treatment.

Accordingly, the present invention is to provide a hot forging die steel having excellent high-temperature strength and also having excellent nitriding properties, and a hot forging die using the same. Aim.

[0007]

In order to solve the above-mentioned problems, for example, a nitriding steel to which Al which is an element having a strong interaction with N is added is known for use as a structural member or the like. Regarding this, as a hot working tool material, a proposal has been made to improve the nitriding characteristics by adding Al.

Therefore, the present inventors have proposed the above proposed A
After investigating and examining the l addition means,
It has been found that the addition changes and influences the high temperature strength and the toughness by changing the quenching transformation characteristics in addition to the interaction with N. Further studies revealed that, as a hot forging die steel capable of achieving excellent high-temperature strength and toughness in addition to the improvement of nitriding properties, the steel had a higher Al content than previously reported. And found that there is a narrower range and an optimum Al content range, and that the hot forging die steel itself has an optimum composition according to an appropriate Al content in its basic composition. Reached the present invention.

That is, the first hot forging die steel of the present invention has a mass percentage of C: 0.3 to 0.5% and Si: 1.0%.
%, Mn: 1.0% or less, Ni: 0.5 to 1.5
%, Cr: more than 3.0% and 3.6% or less, Mo and W
Alone or in combination (Mo + 1 / 2W): 2.0-4.
0%, V: 0.2 to 1.5%, Al: 0.4 to 0.7
%, The balance being Fe and unavoidable impurities, and is a hot forging die steel having both excellent high-temperature strength and excellent nitriding properties and excellent wear resistance.

[0010] The second hot forging die steel of the present invention contains, by mass%, C: 0.3 to 0.5%, and Si: 1.0%.
Mn: 1.0% or less, Ni: 0.5 to 1.5%,
Cr: more than 3.0% and 3.6% or less, Mo and W are used alone or in combination (Mo + 1 / 2W): 2.0 to 4.0.
%, V: 0.2 to 1.5%, Al: 0.4 to 0.7%,
Co: 5.0% or less, with the balance being Fe and unavoidable impurities.

Further, with respect to the mold steel of the present invention,
N: hot forging die steel with 0.100% or less;
Alternatively, P, S, and O, which are inevitable impurities,
P: 0.025% or less, S: 0.010% or less, O:
The content is preferably set to 0.005% or less.

A hot forging die manufactured using the steel for a hot forging die according to the present invention is excellent in toughness and high-temperature strength. With a layer, sufficient wear resistance can be achieved.

[0013]

DETAILED DESCRIPTION OF THE INVENTION The feature of the present invention is that the addition of Al improves the nitriding property, which is a weak point of 3% Cr-based steel such as SKD7 steel applied to a hot forging die.
When the other effects and effects of the addition of Al were found, and the effects of improving the high-temperature strength and reducing the toughness were studied intensively, the optimum amount of Al added and Al
The present inventors have also found out the optimum main composition of the hot-forging die steel itself to which the addition has been made.

That is, the present invention provides an SK having excellent high-temperature strength.
Al is added based on a 3% Cr-based material in order to improve the nitriding characteristics while making use of the characteristics of D7 steel.
Further, as described later, the hot forging die steel of the present invention has a Cr content of 3.6% or less.
The above-mentioned materials already have sufficient nitriding properties, so that the effect of the addition of Al of the present invention is not only meaningless, but also to suppress a decrease in high-temperature strength due to an increase in the amount of Cr.

First, Al which is a feature of the present invention will be described. The present inventors have found that the addition of Al to the hot work tool steel increases the martensite transformation start temperature, that is, the Ms point, and the bainite transformation start temperature Bs. These temperature changes cause a change in the quenched structure, that is, a change in the quenched transformation substructure, and change the structure and characteristics of the tempered state which is a heat treatment state used as a mold.

In hot work tool steel, special carbides such as Cr, Mo or W, V carbides are precipitated by tempering, and secondary hardened and impart high-temperature strength. Here, by utilizing the above-mentioned martensitic transformation start temperature Ms point and the change in the quenched structure due to the increase in bainite transformation start temperature Bs,
The precipitation distribution behavior of the special carbide can be changed to an extremely fine uniform state, that is, Cr, Mo,
The high-temperature strength can be improved even with the amounts of W and V.

On the other hand, however, the change in the tempered structure described above leads to a decrease in toughness. In other words, depending on the set value of C, which is a basic constituent element of the hot work tool steel, which greatly affects the change of the Ms point, the toughness is excessively lowered, and is no longer practical as a hot forging die steel. It will be unbearable.
Therefore, when Al is added to hot forging die steel, C
Corresponding setting of the content is important. Further, the addition of Ni is effective to make up for the above-mentioned drawback to toughness.

As described above, the present inventors have conducted intensive studies on the effects of the respective constituent elements when Al is added to the hot forging die steel. They found the proper composition and arrived at the present invention. Hereinafter, the reasons for limiting the components of the steel of the present invention will be described.

C secures the required tempering hardness as a hot forging die steel, and is added for the same reason as in the SKD7 steel. However, if it is too high, it is a coarse carbide that does not form a solid solution with the matrix during quenching. Is generated, and becomes a starting point of cracking when used as a hot forging die steel. What is important here is that if the amount of C added is too low, Al
In combination with the addition, the Ms point and the Bs point are excessively increased, and as a result, the precipitation of Cr, Mo or W, V carbide in tempering becomes too fine and uniform, leading to a decrease in toughness. And

[0020] Si is a deoxidizing agent at the time of steel making, but if it is too much, ferrite is formed, so it is set to 1.0% or less.

Mn has the effect of enhancing hardenability, suppressing the formation of ferrite, and obtaining an appropriate quenching and tempering hardness.
If the amount is too large, the viscosity of the base is increased and the machinability is reduced. Preferably, it is at least 0.2%.

Cr precipitates fine carbides by tempering to impart strength. However, compared to W, Mo, and V-based precipitated carbides, Cr agglomerates at a lower temperature and in a shorter time. Resistance decreases. Although Cr also has a function of obtaining hardening by nitriding, it lowers the high-temperature strength as described above. Therefore, the present invention is characterized in that the hardening by nitriding is mainly obtained by adding Al. Therefore,
In the present invention, Cr is set to 3.6% or less. Note that Cr has an effect of suppressing a decrease in toughness due to coarsening of the lower structure due to formation of a bainite structure during quenching. Since the addition of Al is an element that promotes the above embrittlement, the Cr of the present invention should be added in excess of 3.0% from this point as well.

Mo and W are added singly or in combination to precipitate and aggregate fine carbides by tempering to impart high-temperature strength and improve softening resistance. In order to exceed the high-temperature strength of general-purpose steel SKD61, it is added in a larger amount. However, excessive addition causes a decrease in toughness. Therefore, in the present invention, (Mo + 1 / 2W) is 2.0 to 4.0%.
And

V also increases the high-temperature strength and the softening resistance similarly to Mo and W. To obtain this effect, 0.2% or more is required. However, if it is too much, it is dispersed as enormous carbides even after quenching and tempering, leading to a reduction in toughness.

[0025] Al is the most important element of the present invention.
Cr increases the nitriding hardness but lowers the high-temperature strength, whereas Al does not lower the high-temperature strength, but improves the high-temperature strength. Both hardness can be improved at once. This is as described above. Al
The nitridation hardness by the addition hardly changes with the addition of about 0.1%, but a remarkable improvement effect can be obtained from about 0.4%. Therefore, in the present invention, it is important that the Al content is 0.4% or more as the practically effective optimum content.

On the other hand, on the other hand, a change in the tempered structure due to the addition of Al leads to a decrease in toughness.
It is important to properly adjust the amount added. In addition, A
The nitriding hardness by the addition of 1 does not show a significant improvement effect even when the addition is about 1.0% compared with the addition of about 0.5%. From these matters, in the present invention, A
l The amount of addition is set to 0.7% or less. The amount of Al of the present invention, taking into account the effect of reducing toughness on the effect of improving the nitriding hardness,
The study of limiting the extent of each other led to the setting of the optimal range. As described above, the content of Al of the present invention is 0.4 to 0.7%.

Ni has the effect of lowering the Ms point and improving the hardenability, such as making the bainite structure finer. Further, while the addition of Al of the present invention lowers the toughness due to the coarsening of the bainite structure due to the action of raising the Ms point, it is also an important element for adjusting and compensating for the deterioration of the toughness. Therefore, even in the mutual adjustment with the Al addition amount of the present invention, Ni is added at 0.5% or more. Since excessive addition leads to disadvantageous properties change as a hot forging dies, such as reduction of A ₁ transformation point,
1.5% or less.

Co is desirably added because it strengthens the precipitation of carbides during tempering and increases the high-temperature strength. Here, the addition exceeding 5.0% saturates the effect and is an expensive element, so the upper limit of the addition amount is set to 5.0%. A preferable addition amount is 0.5 to 4.0%.

N has a crystal grain refining effect, but if added excessively, it combines with Al and precipitates in the form of AlN, which is an important effect of the present invention and improves the nitriding characteristics by Al during nitriding. Therefore, the content is desirably 0.100% or less.

Since unavoidable impurities P, S, and O decrease the toughness of the steel of the present invention, it is preferable that each of P:
0.025% or less, S: 0.010% or less, O: 0.0
It shall be not more than 05%.

A hot forging die manufactured using the steel for a hot forging die according to the present invention described above has excellent toughness and high-temperature strength. If a nitrided layer is provided on the molding surface by performing the nitriding treatment, sufficient wear resistance can be achieved, and the effect of improving the wear life is great. In addition,
Nitriding treatment is generally applied to hot forging dies, ion nitriding treatment, gas nitriding treatment, gas soft nitriding treatment, salt bath nitriding treatment, salt bath sulphonitriding treatment, and gas sulphonitriding treatment,
Alternatively, even if any type of nitriding treatment, such as one using a solid nitriding medium, is applied, the effect is remarkable.

[0032]

EXAMPLES Table 1 shows the chemical composition of the steels of the present invention (E, F, H, I) and comparative steels (A to D, G, J, K).

[0033]

[Table 1]

(Example 1) When a nitriding treatment was performed,
A sample for measurement was prepared in order to evaluate the hardness of the nitrided layer.
First, a 10 mm square sample was cut out of a forged material having a composition of 100 mm × 100 mm in cross section having the compositions of the steel of the present invention and the comparative steel shown in Table 1 and quenched at 1020 ° C.
The hardness was adjusted to HRC. Then, ion nitriding is performed at 500
C. × 20 h, in a gas composition ratio of N ₂ : H ₂ = 1: 1, was used as a sample for measuring the hardness distribution of the surface portion of the nitrided layer. Then, assuming the surface layer temperature of the hot forging die in use, heating was performed at 700 ° C. for 20 minutes to obtain a nitrided layer of the sample in a state as it was before the heating and in a state after the heating. The cross-sectional hardness distribution was measured with a Vickers hardness tester. Table 2 summarizes the maximum hardness in the nitrided layer.

[0035]

[Table 2]

Table 2 shows that the comparative steel A (S
The nitriding hardness of KD61) exceeds 1000 HV,
% Cr-based comparative steel B (SKD7) nitriding hardness 920HV
Is considerably advantageous in terms of abrasion resistance. However,
In the case of a hot forging die, the heating temperature of the workpiece is 1000 ° C.
And the surface temperature of the mold rises to about 700 ° C., so that the surface of the mold is softened during use. Therefore, in this embodiment, the measurement sample as nitrided is 700 ° C. × 20 m
In heating, the softening characteristics of the nitrided layer are evaluated by measuring the nitriding hardness thereafter.

As a result, the comparative steel A decreased to 750 HV, while the comparative steel B was 850 HV, and the nitriding hardness was reversed. This behavior is based on the softening resistance of the base material rather than the difference in softening resistance of the nitrided layer. In other words, the SKD61 of the comparative steel A facilitates the softening of the base metal more easily than the SKD7 of the comparative steel B, so that the hardness of the base metal is lowered and the nitriding hardness is also reduced. On the other hand, when steels C to K in which Al was added to a 3% Cr-based steel were evaluated, it was found that the steels other than the comparative steels C and D having a small amount of Al added had 11
Hardness was obtained as-nitrided at 00 HV or more, which is higher than that of Comparative Steel A. Furthermore, high hardness is maintained even after heating at 700 ° C. In addition, the comparative steel J of 4% Cr is:
As in Comparative Steel A, the softening resistance of the base metal is inferior to that of the 3% Cr-based alloy, so that the nitriding hardness after heating at 700 ° C. cannot be said to be sufficient.

FIG. 1 shows the distribution of the surface section hardness of the steels A to G as nitrided. As the addition amount of Al, comparative steel A (S
To exceed the nitriding hardness of KD61), 0.3% of Al
The addition is insufficient, and it is necessary to add 0.4% or more of Al. Also, even if Al contains 1.0%, there is no difference in the nitriding hardness between those containing 0.7% Al, and therefore, in terms of the effect of improving the nitriding hardness, it exceeds 0.7%. It is not necessary to add Al. A of about 0.1%
l may also be contained by deoxidation treatment in ordinary melting and refining, but with such an amount of Al, there is no effect on the nitriding hardness, so that the effect of improving the nitriding properties of the present invention can be obtained. Is in short supply.

(Example 2) Next, toughness was evaluated. In the evaluation, the cross-sectional dimensions having the compositions of the inventive steel and the comparative steel of A to K in Table 1 were 100 mm × 100 mm.
A 10 mm square piece cut out from the forged material was used as a measurement sample.

First, the transformation start temperature at the time of quenching of the measurement sample was measured using a thermal expansion measuring device. Heating and holding during quenching was performed at 1020 ° C. × 30 min, and cooling rates were set at three rates of 100, 30, and 10 ° C./min. Furthermore, after performing the same quenching process, 47HRC
Each sample adjusted to a hardness of was prepared as a 2 mm U notch Charpy test piece, and an impact test was performed at room temperature. Table 3 shows the results.

[0041]

[Table 3]

As shown in Table 3, when the quenching cooling rate of the comparative steel B (SKD7) is lower than 30 ° C./min, the transformation starting temperature becomes 400 ° C. or higher. The transformation structure in this case is martensite or bainite depending on the temperature,
The higher the transformation start temperature, the coarser the unit structure size of these substructures, and the lower the toughness value after tempering. In other words, in the case of the comparative steel B, when the hot forging die produced is large, or when the die is of a complicated shape that may cause quenching cracking, quenching and cooling must be performed slowly. 30 ° C /
When the mold cannot provide a cooling rate higher than min, the toughness is reduced, resulting in a mold having high crack sensitivity.

On the other hand, in the comparative steel C, even when the quenching and cooling rate is reduced, the rise in the transformation start temperature is suppressed, and the decrease in toughness is small. On the other hand, in the steel E of the present invention to which Al is added, the transformation start temperature is higher than that of the comparative steel C. However, if the cooling rate is about 30 ° C./min, the transformation start temperature is 400. It is considerably lower than ℃ and the toughness value is secured. From these, A of the present invention
It can be seen that, when adding l amount, it is necessary to adjust the basic composition of the hot forging die steel as the base to the composition of comparative steel C or inventive steel E.

If Al is added excessively as in Comparative Steel G, the transformation start temperature becomes too high, and the toughness is reduced by the same phenomenon as described above. Similarly, the comparative steel K having a high transformation start temperature has a low C content of the base composition itself and a high transformation temperature, and thus has a deteriorated toughness, so that Al can no longer be added. is there.

Example 3 Next, the high-temperature strength was evaluated. 10 from the forged material having a cross-sectional dimension of 100 mm x 100 mm having the compositions of the inventive steels and comparative steels of A to K in Table 1
A sample of mm square was cut out and heated and maintained at 1020 ° C. × 30.
After performing a quenching treatment at a cooling rate of 30 ° C./min for 30 minutes, a tensile test piece adjusted to a hardness of 47 HRC by tempering was prepared and tested at 700 ° C. Table 4 shows the results.

[0046]

[Table 4]

The steels E, F, H, and I of the present invention are inferior to the high-temperature strength of the comparative steel B and the comparative steel K because the toughness of the mold is taken into consideration, but the comparative steel A (SKD6) is a general-purpose steel.
High temperature strength of 1) is sufficiently exceeded. Therefore, the synergistic effect of high nitridation hardness and high high-temperature strength increases the wear life of hot forging dies when used in hot forging dies as compared to SKD61 or SKD7, each of which is insufficient. You can do it. And, as shown in Table 3, even when the quenching cooling rate is reduced, the comparative steel J with little deterioration in toughness has a high temperature strength that is not much different from the comparative steel A (SKD61).
The wear life cannot be improved from this aspect.

(Example 4) Finally, a hot forging die was manufactured and actual forging was performed, and the life of the die was evaluated. A punch having an outer diameter of 150 mm and a length of 250 mm having the compositions of the steel of the present invention (E) and the comparative steels (A to D, K) shown in Table 1 was processed and produced, and 1020 was placed in a chamber pressurized to 0.5 MPa.
After quenching with nitrogen gas cooling from the heating temperature of
Tempered to RC and tempered, and gas-nitrided at 570 ° C.
The quenching and cooling rate of these dies was 45 ° C./min.

The product formed by the hot forging by the punch is an automobile part, and the heating temperature of the workpiece is 1200.
C., the press cycle time is 45 times per minute. Table 5 shows the results of this experiment. The mold made of comparative steel A was 7
The life reached at 900 cycles, and the cause was plastic flow wear accompanied by softening of the surface layer. On the other hand, the mold using the comparative steel B has a high high-temperature strength as shown in Table 4 and should not easily soften. However, since the nitrided layer was worn out early, the mold life was longer than that of the comparative steel A. There was not much difference. In comparison with this, the mold using the steel E of the present invention improved the wear life by about 80%. The mold made of the comparative steel K had insufficient toughness, and cracks occurred early. On the other hand, the comparative steels C and D did not sufficiently exceed the wear life of the comparative steel B because the effect of the addition of Al was not sufficient.

[0050]

[Table 5]

[0051]

According to the present invention, a high life hot forging die can be achieved because the steel has both high temperature strength and nitriding hardness at a high level, which have been incompatible with conventional steels. This is effective in improving the life of the hot forging die that has been applied.
Therefore, the production cost can be reduced, and the industrial value of the present invention is high.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of the effect of the present invention.

Claims (6)

[Claims] C .: 0.3 to 0.5% by mass, S:
i: 1.0% or less, Mn: 1.0% or less, Ni: 0.5
1.5%, Cr: more than 3.0% and 3.6% or less, Mo
And W alone or in combination (Mo + 1 / 2W): 2.
0 to 4.0%, V: 0.2 to 1.5%, Al: 0.4 to
A hot forging die steel comprising 0.7%, the balance being Fe and unavoidable impurities. 2. In mass%, C: 0.3-0.5%, S
i: 1.0% or less, Mn: 1.0% or less, Ni: 0.5
1.5%, Cr: more than 3.0% and 3.6% or less, Mo
And W alone or in combination (Mo + 1 / 2W): 2.
0 to 4.0%, V: 0.2 to 1.5%, Al: 0.4 to
A hot forging die steel comprising 0.7%, Co: 5.0% or less, the balance being Fe and inevitable impurities. 3. The hot forging die steel according to claim 1, wherein N is 0.100% or less in mass%. 4. The method according to claim 1, wherein P, S, and O, which are unavoidable impurities, are contained by mass% of P: 0.025% or less, S: 0.010% or less, and O: 0.005% or less. The hot forging die steel according to any one of claims 1 to 3. 5. A hot forging die manufactured using the steel for a hot forging die according to any one of claims 1 to 4. 6. The hot forging die according to claim 5, further comprising a nitride layer.