JPH07179988A - Hot tool steel excellent in high temperature strength - Google Patents

Abstract

PURPOSE:To produce a hot tool steel excellent in high temp. strength by preparing a steel having a specific composition specified in C, Mo, W, and V contents and performing hardening and tempering under respectively specified temp. conditions. CONSTITUTION:A steel consisting of, by weight ratio, 0.30-0.50% C, 0.10-1.50% Si, 0.10-1.00% Mn, 0.1-1.5% Ni, 2.0-4.0% Cr, 2.0-4.0% Mo, 0.1-1.0% V, 0.1-1.0% W, and the balance Fe with impurity elements, is prepared. This steel is hardened at 1060-1100 deg.C and tempered at 500-620 deg.C. By this method, the hot tool steel excellent in strength at high temp. can be obtained. The steel has toughness equal to or higher than that of conventional steel even if subjected to hardening at a temp. higher than heretofore and can secure superior strength at high temp.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot work tool steel having an excellent high temperature strength and a toughness equal to or higher than that of a conventional steel, and an excellent high temperature strength.

[0002]

2. Description of the Related Art Conventionally, hot work tool steels specified by JIS that do not add W such as SKD61, SKD62, SKD7, etc., or add a small amount even if added, have a temperature of 1000 to 1050 ℃ except SKT3 and SKT4. Is being hardened. This is because in the case of SKD4, SKD5, and SKD8, in which a large amount of W is added, quenching is performed at a temperature exceeding 1100 ° C in order to form a solid solution with W-based inclusions, but with other steel types, quenching is performed. When the temperature is raised, the crystal grains become coarse,
This is because it was necessary to suppress the quenching temperature to 1050 ° C. or less in order to increase the influence of the grain boundary carbides that precipitate during tempering, reduce the toughness, and cause large cracks during use as a mold.

By suppressing the quenching temperature to 1050 ° C. or lower, the required toughness was secured, and it was possible to secure a material that was resistant to cracking under repeated impact loads applied during use as a mold. is there.

[0004]

As in the prior art, when the quenching temperature is lowered, the solid solution of carbides in the matrix becomes insufficient, and the degree of secondary hardening obtained during tempering decreases, resulting in improved strength. The effect cannot be obtained sufficiently. However, the high temperature strength can be increased by simply increasing the quenching temperature using conventional steel, but the crystal grains become coarse and the toughness decreases, the impact resistance decreases, and during use as a mold. There is a risk of large cracks. Therefore, conventionally, quenching has not normally been performed at a temperature exceeding 1050 ° C., which is the upper limit of the quenching temperature at which the required toughness can be secured.

The present invention provides a hot work tool steel excellent in high-temperature strength that can obtain toughness equal to or higher than that of conventional steel even if quenching is performed at a higher temperature than before, and that can secure excellent high-temperature strength. The purpose is to provide.

[0006]

The inventors of the present invention have noticed that the decrease in toughness of conventional steel is caused by grain boundary embrittlement, and the behavior and composition of carbide precipitation at the grain boundary causing grain boundary embrittlement. I investigated in detail about the relationship with. As a result, the following findings were obtained.

Cr carbide precipitates at grain boundaries from the tempering temperature of a relatively low temperature, but when the Cr addition amount is suppressed to 4% or less, at the tempering temperature of about 600 ° C., precipitation at grain boundaries occurs. The amount is not so great as to have a great effect on toughness. Even if the Cr addition amount is suppressed to 4% or less, the tempering temperature is
If it exceeds 620 ° C, the precipitation amount of carbides on the grain boundaries increases, and the toughness decreases significantly. Since carbides do not precipitate at grain boundaries unless the temperature of Mo, W, etc. is relatively high, Mo is increased compared to SKD61, SKD62, and W is added, but the solid solution temperature of W-based carbides is high. It is better to suppress it in terms of toughness. By increasing the quenching temperature to 1060 to 1100 ° C, the solid solution of carbide proceeds, so that the toughness of the matrix is improved. Further, the toughness of the matrix is further improved by adding a small amount of Ni. By increasing the quenching temperature to 1060 to 1100 ° C, the crystal grains become slightly coarser, but in order to suppress the decrease in toughness due to grain boundary carbides, the effect should be kept small by taking measures based on the findings of the above. You can When Cr content is reduced compared to conventional SKD61 and SKD62, Mo content is increased, and V and W are added together, carbide precipitates at a higher temperature than SKD61 and SKD62, and secondary hardening is obtained. Excellent performance when used at high temperatures.

That is, in the first invention of the present invention, C: 0.30 to 0.50%, Si: 0.10 to 1.50%, Mn: 0.10 to 1.
00%, Ni: 0.1-1.5%, Cr: 2.0-4.0%, Mo: 2.0-4.0%, V:
Steel containing 0.1 to 1.0% and W: 0.1 to 1.0%, the balance of which is Fe and impurity elements, is quenched at 1060 to 1100 ° C, then 500 to 620
It is a hot work tool steel excellent in high temperature strength characterized by being tempered at ℃, the second invention is tougher than the first invention,
In order to improve machinability, Ca is further contained in an amount of 0.001 to 0.010%.

Next, the reasons for limiting the component composition and heat treatment conditions of the hot work tool steel having excellent high temperature strength in the present invention will be explained below. C: 0.30 to 0.50% C is required to be contained in an amount of 0.30% or more in order to combine with a carbide forming element and obtain the hardness and wear resistance required for hot work tool steel. However, if contained in a large amount, the toughness decreases, so the upper limit was made 0.50%.

Si: 0.10 to 1.00% Si is necessary for deoxidation during steel making, and has high temperature strength,
It is an element that has the effect of enhancing heat check resistance. Therefore, a minimum content of 0.10% is required. However, if contained in a large amount, the toughness is lowered and the workability into a mold is deteriorated, so the upper limit was made 1.00%.

Mn: 0.10 to 1.00% Mn is an element that has a deoxidizing and desulfurizing effect during steelmaking and is necessary for improving hardenability, and it is necessary to contain 0.10% or more. However, if contained in a large amount, the toughness and workability deteriorate as with Si, so the upper limit was made 1.00%.

Ni: 0.10 to 1.50% Ni is an element effective not only in improving the toughness of the matrix but also in improving the hardenability, and the content of 0.10% or more is necessary. However, if contained in a large amount, the machinability deteriorates and it becomes difficult to manufacture the mold, so the upper limit was made 1.50%.

Cr: 2.00 to 4.00% Cr is an element which combines with C to form a carbide, which is effective in improving wear resistance and contributes to improving hardenability.
It is necessary to contain 2.00% or more. However, if contained in a large amount, the amount of precipitation of grain boundary carbides increases and the toughness decreases significantly, so the upper limit was made 4.00%.

Mo: 2.0-4.0%, V: 0.1-1.0%, W: 0.1-1.
0% Mo, V, and W are elements that are essential to combine with C to form carbides, improve the high-temperature strength by secondary hardening during tempering, and ensure the necessary wear resistance. Therefore, it is necessary to add at least 2.0% of Mo, 0.1% of V, and 0.1% of W in combination. However, when contained in a large amount, excessively formed carbide precipitates at the grain boundaries and the toughness decreases, so the upper limits were set to 4.0% for Mo, 1.0% for V, and 1.0% for W. Further, since the W 3 carbide has a high solid solution temperature, it is necessary to regulate the upper limit to 1.0% or less in order to sufficiently dissolve the carbide during quenching.

Ca: 0.001 to 0.010% Ca is an element effective for making the morphology of inclusions spherical and improving toughness and machinability. It is necessary to contain 0.001% or more. However, even if contained in a large amount, the above effect is saturated and conversely the toughness decreases, so the upper limit was made 0.010%.

Next, the reasons for limiting the heat treatment conditions will be described. The lower limit of the quenching temperature is set to 1060 ° C because the effect of improving high temperature strength cannot be sufficiently obtained at a temperature lower than 1060 ° C.
This is because superior strength cannot be obtained compared to JIS steel.
When quenching is performed at a temperature exceeding ℃, the crystal grains become coarse, and it is difficult to secure toughness equivalent to or better than that of conventional steel even if the above-mentioned components and tempering conditions are optimized. This is because.

The tempering temperature range is 500 to 620 ° C.
The reason for this range is that at a temperature of less than 500 ° C, the strength improvement effect due to secondary hardening is not sufficiently obtained and the toughness is poor, and if it exceeds 620 ° C, the amount of carbides that precipitate at grain boundaries increases. It is because the toughness decreases.

[0018]

The hot work tool steel of the present invention enhances the toughness of the matrix by the addition of Ni and the high quenching temperature, and regulates the upper limit of the tempering temperature to a lower limit to reduce the amount of Cr and to precipitate the grain boundary carbides. Therefore, even if the quenching temperature is increased, the toughness does not decrease, and high temperature strength superior to that of the conventional steel can be obtained.

[0019]

EXAMPLES The features of the present invention will be described below in comparison with comparative steels and conventional steels with reference to examples. Table 1 shows the chemical components of the test materials used in the examples.

[0020]

[Table 1]

In Table 1, steels 1 to 8 are steels of the present invention, steels 1 to 4 are steels of the first invention, and steels 5 to 8 are steels of the second invention. Steels 9 to 15 are comparative steels, and steels 16 to 18 are conventional steels SKD61, SKD7 and SKD62, respectively.

Specimens having the components shown in Table 1 were melted in a high frequency melting furnace of 20 kg, forged into a round bar having a diameter of 15 mm, and processed into test pieces. The hardness, high temperature strength (tensile strength at 700 ° C.) and impact value (JIS No. 3 U notch Charpy test) were measured to evaluate the performance of each test material. Note that the heat treatment conditions are a quenching temperature of 1080 ° C and a tempering temperature of 600 ° C. For conventional steels 16-18, the normal quenching temperature is 10
At the same time, the evaluation was performed at 30 ° C (tempering temperature 600 ° C). The evaluation results are shown in Table 2.

[0023]

[Table 2]

As is clear from Table 2, when the comparative steels, the conventional steels 9 to 18 are compared with the examples of the present invention, the 9 steels are
Since the C content is high, the toughness is inferior, and on the contrary, the 10 steel has a low C content and thus has poor hardness and high-temperature strength. 11 steel has a high Si content and thus poor toughness, 12 steel has a low Ni content and therefore a little inferior toughness, and 13 steel has a high Cr content, which increases the quenching temperature. As a result, the toughness was significantly reduced, and the 14th and 15th steels had a low Mo, V, and W content, and therefore had poor high-temperature strength.

Further, in the case of the conventional steel, when the quenching temperature is raised, the strength is slightly increased, but the toughness is greatly lowered, and when the heat treatment is performed under the conventional conditions, the high temperature strength is higher than that of the present invention. It is inferior.

On the other hand, the steels 1 to 8 of the present invention are
While adding Ni to improve the toughness of the matrix,
By optimizing the amounts of Cr, Mo, and W, even if the quenching temperature was raised, the toughness did not deteriorate, and excellent high-temperature strength could be obtained.

Next, the influence of changes in heat treatment conditions on the performance of the present invention will be clarified by another example. Table 1
Using the steels 1 and 5 of the present invention among the test materials shown in (1), the influence on the performance was investigated by changing the quenching temperature and the tempering temperature. The results are shown in Table 3.

[0028]

[Table 3]

As is clear from Table 3, even when the steels of the present invention 1 and 5 are used, when they are heat-treated at temperatures outside the predetermined quenching temperature and tempering temperature range, the high temperature strength and toughness It can be seen that both cannot obtain excellent performance.
Therefore, in order to bring out the effect of the present invention sufficiently, it is necessary to perform the heat treatment under the conditions within the regulated range.

[0030]

According to the present invention, it is possible to provide a steel having excellent high temperature strength by only slightly increasing the quenching temperature.
Therefore, when it is used as a mold, the life of the mold can be significantly improved, the time and effort required for mold replacement can be reduced, and the productivity can be greatly improved.

Claims (2)

[Claims] 1. C: 0.30 to 0.50% by weight ratio, Si: 0.10
~ 1.50%, Mn: 0.10 ~ 1.00%, Ni: 0.1 ~ 1.5%, Cr: 2.0 ~
Steel containing 60%, Mo: 2.0 to 4.0%, V: 0.1 to 1.0%, W: 0.1 to 1.0%, the balance Fe and impurity elements 1060 to 1100
A hot work tool steel with excellent high-temperature strength, characterized by being tempered at 500 to 620 ° C after being quenched at ℃. 2. C: 0.30 to 0.50% by weight ratio, Si: 0.10
~ 1.50%, Mn: 0.10 ~ 1.00%, Ni: 0.1 ~ 1.5%, Cr: 2.0 ~
4.0%, Mo: 2.0 to 4.0%, V: 0.1 to 1.0%, W: 0.1 to 1.0%, C
a: Steel containing 0.001 to 0.010% with the balance being Fe and impurity elements was quenched at 1060 to 1100 ° C, and then tempered at 500 to 620 ° C. Hot work tool steel.