JPH05148589A - Hot tool steel - Google Patents

Abstract

PURPOSE:To decrease the anisotropy of toughness and to prevent the generation of chipping arising when the above-mentioned steel is used as a mold by spheroidizing steel while decreasing A inclusions by addition of C and decrease of S and further preventing the generation of striped segregation by decrease of V. CONSTITUTION:This steel consists of the compsn. which contains, by weight, 0.20 to O.60% C, 0.10 to 1.00% Si, <=2.00% Mn, <=0.005% to 0.50% V, <=0.001 to 0.010% Ca and the balance Fe and unavoidable impurities.

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 a small impact value anisotropy and excellent toughness regardless of the material direction.

[0002]

2. Description of the Related Art Hot forging is a hot working process in which the deformation resistance of a material is low, and therefore has an advantage of being able to carry out large working at once. However, since burr is generated, the yield is low and the dimensional accuracy is usually poor. Therefore, the development of a processing method that is free of burrs and has excellent dimensional accuracy even in hot forging has been actively carried out.

As one of the methods under development, there is a method called closed forging. Unlike ordinary presses, this method uses a press equipped with multiple rams and uses a different ram to press and fill the material to be molded into the die cavity that has been previously impressed. By properly controlling the material flow,
This is a processing method that gives optimum deformation.

The burr-free forging such as the closed forging is greatly superior to the normal hot forging in terms of material yield and dimensional accuracy, but since it is machined in a closed state, it does not work on tools. The burden increases, the mold cracks, chipping occurs,
In some cases, it became unusable at an early stage, which was a problem.
Therefore, it has been desired to develop a material which has excellent toughness and is resistant to cracking and chipping as compared with JIS-SKD61 which is most often used for dies during hot forging.

[0005]

When JIS-SKD61 was used for the above-mentioned burr-free forging, the reason why chipping is likely to occur was investigated, and the following was found out. That is, SKD6
It was found that the impact value of 1 was significantly different depending on the direction, and the impact value was remarkably low in the direction perpendicular to the rolling direction of the material, and as a result, many cracks occurred preferentially in the direction of poor toughness.

An object of the present invention is to provide a hot work tool steel having a small anisotropy in JIS-SKD61 toughness, ensuring excellent toughness irrespective of the direction, hardly cracking or chipping, and having a long life. And

[0007]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies in order to find the cause of cracking and chipping,
The following findings were obtained. That is, in SKD61 which is a conventional steel, it is considered that the striped segregation extending in the rolling direction and the A-type inclusions have different impact values depending on the direction, and by removing the causes of decreasing the toughness, We thought that the anisotropy of toughness would be improved.

In order to consider a measure for reducing the striped segregation, the area where the striped pattern was observed by a microscope was subjected to detailed surface analysis by EPMA. As a result, it was found that there were regions with high V content and regions with low V content. Therefore, as a result of experimentally dissolving a material having a lower V content than SKD61, it was found that the degree of occurrence of striped segregation was reduced.

Further, it is considered that the A-type inclusions are easily deformed by working and become elongated in the rolling direction, which is a cause of increasing anisotropy. Since it is known that the A-type inclusions are sulfide-type inclusions, S is reduced as much as possible in order to reduce the inclusions as much as possible, and further, the inclusions are spherical without being extended after rolling. We examined the composition of the system. As a result, they have found that when a small amount of Ca is added, the inclusions are spheroidized, and the impact value in the direction perpendicular to the rolling direction can be remarkably improved without becoming elongated after rolling.

Further, as a result of studying an appropriate component system for obtaining the necessary hardenability, SKD61 was prepared by adding Ni, which is not positively added to SKD61, in the range of 0.10 to 4.00%. By comparison, it was confirmed that the hardenability can be adjusted in a wide range, it can be applied to a larger mold, and the influence on the toughness is small.

The hot work tool steel of the present invention completed on the basis of the knowledge obtained above has a weight ratio of C: 0.20 to 0.60%, Si: 0.1
0 to 1.00%, Mn: 2.00% or less, S: 0.005% or less, Cr: 3.0 to 6.
0%, Ni: 0.10 to 4.00%, Mo: 1.00 to 3.00%, V: 0.10 to 0.
The gist is that it contains 50% and Ca: 0.001 to 0.010%, and the balance is Fe and inevitable impurity elements.

The reasons for limiting the components of the hot work tool steel of the present invention will be described below. C: 0.20 to 0.60% C is an essential element for forming carbides by combining with Mo, V, etc. and ensuring the hardness and wear resistance required for hot work tool steel. is necessary. However, if contained in a large amount, the toughness decreases and cracking and chipping easily occur, so the upper limit was made 0.60%.

Si: 0.10 to 1.00% Si is an element effective as a deoxidizer, and it is necessary to contain Si in an amount of 0.10% or more in order to facilitate production. However, even if it is contained in a large amount, the above effect is saturated, the toughness is lowered, and cracking and chipping easily occur, so the upper limit was made 1.00%.

Mn: 2.00% or less Since Mn is an element effective in improving the hardenability, it may be added in an amount necessary according to the size of the mold. However, if it is contained in a large amount, the machinability deteriorates, and it becomes difficult to manufacture the mold, so the upper limit was made 2.00%.

S: 0.005% or less S exists as a sulfide-based non-metallic inclusion in the steel, and this non-metallic inclusion belongs to the A-type inclusions in the JIS standard and is deformed by working. Cause the anisotropy of. Therefore, in the present invention, it is necessary to reduce the inclusions as much as possible in order to improve the toughness, and the upper limit was made 0.005%.

Cr: 3.0 to 6.0% Cr is an essential element for ensuring the high temperature strength and hardenability required for hot work tool steel, and it is necessary to contain 3.00% or more. However, if contained too much, the toughness decreases,
Since cracking and chipping are likely to occur, the upper limit was made 6.0%.

Ni: 0.10 to 4.00% Ni is only recognized as an impurity in SKD61, but it can improve hardenability without lowering toughness, and by adding an appropriate amount, Ni compared to SKD61. The range of adjusting the hardenability is expanded, and it becomes easy to obtain the hardenability according to the size of the mold. To obtain the above effect,
It is necessary to contain 0.10% or more. However, if contained in a large amount, the machinability deteriorates and it becomes difficult to process the die by cutting, so the upper limit was made 4.00%.

Mo: 1.00 to 3.00% Mo is an element that forms carbides and has the effect of improving wear resistance, which is an essential property for hot work tool steels. is necessary. However, even if added in a large amount, the above effect is saturated, and since carbide increases more than necessary and causes toughness deterioration, the upper limit is 3.
It was set to 00%.

V: 0.10 to 0.50% V forms a carbide and has the effect of improving wear resistance, so that it is contained in a large amount in the conventional hot work tool steel. However, if V is added at about 1% like SKD61, striped segregation in the rolling direction grows and the anisotropy of toughness increases, so it is necessary to suppress the addition amount, and the upper limit was made 0.50%.
However, it is necessary to add a small amount to ensure wear resistance, and the lower limit was made 0.10%.

Ca: 0.001 to 0.010% Ca is an important element for the present invention, and it extends in the working direction by rolling and reduces the impact value in the direction perpendicular to rolling.
Spheroidal inclusions have the effect of reducing the difference in impact value (toughness anisotropy) between the rolling direction and the direction orthogonal to the rolling direction.
It is necessary to contain at least 1%. However, if contained in a large amount, the anisotropy of toughness decreases, but the toughness value itself decreases, so the upper limit was made 0.010%.

[0021]

EXAMPLES Next, the characteristics of the steel of the present invention will be clarified by examples in comparison with comparative steel and conventional steel. Table 1 shows the chemical components of the test materials used in the examples. In Table 1, 1 to
Steel No. 5 is the steel of the present invention, Steel Nos. 6 to 10 are comparative steels partially outside the composition range of the present invention, Steel Nos. 11 and 12 are conventional steels corresponding to JIS-SKD61.

[0022]

[Table 1]

The test materials having the components shown in Table 1 were melted in a high-frequency induction furnace for making test materials, and then forged into a rod having a diameter of 70 mm, and the impact value was measured to determine the toughness. An evaluation was conducted on the directionality. Since the impact value changes depending on the hardness,
It is easier to understand the effect if the hardness is unified. Therefore, Table 1
The relationship between the heat treatment temperature and the hardness of the steel shown in Table 1 was investigated. Then, as an impact test piece, a JIS No. 3 test piece was prepared, and heat treatment was performed under the condition that the hardness was H _R C42, 48 based on the above-mentioned survey result, and the impact value in the rolling direction and the rolling orthogonal direction was measured. did.
The results are shown in Table 2.

[0024]

[Table 2]

As is clear from Table 2, the comparative steel is 6
When comparing ~ 10 steel with the steel of the present invention, 6 or 7 steel is C or
Since the content of Si is high, the impact value is low.
9 steel has a high S or V content, so the anisotropy is large and the impact value in the direction perpendicular to the rolling direction is inferior. The impact value is low. In addition, since the conventional steels 11 and 12 have high Si, S, and V contents,
The impact value in the direction perpendicular to the rolling is extremely inferior.

On the other hand, the steels 1 to 5 of the present invention are
By adding Ca in an appropriate amount and reducing S as much as possible, A-type inclusions are reduced and spheroidization is achieved, and V is also reduced to suppress the occurrence of striped segregation, thereby improving the toughness anisotropy. Can be reduced to 3kgf even in the direction perpendicular to the rolling direction.
An impact value of m / cm ² or more could be obtained.

Next, a die for producing a connecting rod is prepared by melting the steels 1 to 5 of the present invention shown in Table 1 and the conventional steels 11 and 12 and then forging them to an appropriate size. It was produced and evaluated for durability. As a result, it was confirmed that the steel of the present invention has a significantly reduced damage due to chipping and a significantly improved life compared with the conventional steel. Further, in the steel of the present invention, as described above, since the hardenability can be controlled by adding an appropriate amount of Ni, it is possible to adjust the hardenability to an appropriate one according to the size of the mold.

[0028]

As described above, the hot work tool steel of the present invention has a smaller anisotropy of toughness as compared with the conventional steel, and the toughness does not largely decrease even in the direction perpendicular to the rolling. Therefore, it can be used very effectively when the burden on the die is required to be increased due to the improvement of the forging method which has been actively performed in recent years. Therefore, an excellent effect can be obtained when developing a processing technique for improving the dimensional accuracy, yield, etc. of hot forging.

Claims (1)

[Claims] 1. A weight ratio of C: 0.20 to 0.60%, Si: 0.10.
~ 1.00%, Mn: 2.00% or less, S: 0.005% or less, Cr: 3.0 to 6.0
%, Ni: 0.10 to 4.00%, Mo: 1.00 to 3.00%, V: 0.10 to 0.5
A hot work tool steel characterized by containing 0% and Ca: 0.001 to 0.010% or less, and the balance being Fe and inevitable impurity elements.