JPH05163551A - Powder high-speed tool steel - Google Patents

Abstract

PURPOSE:To provide a powder high-speed tool steel resistant to high-temp. temper softening capable of coping with the high speed at which the tool is used and also having high toughness. CONSTITUTION:The powder high-speed tool steel contains by weight, 0.7-2.0% C, <=1.0% Si, <=0.6% Mn, 3.0-6.0% Cr, 14-20% of (W+2Mo), <=5.0% V, 2.0-7.0% Nb where Nb/V >=0.5, <=15.0% Co and the balance Fe with inevitable impurities. In this case, the Ceq=0.24+0.033.W+0.063.Mo+0.2V+0.1.Nb is controlled to -0.30 to 0.05.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a powder for use in cutting tools and forging tools, which has excellent wear resistance and high toughness, especially under high-speed use conditions requiring hardness and wear resistance at high temperatures. It relates to high speed tool steel.

[0002]

2. Description of the Related Art High-speed tool steels used for cutting tools and forging tools are required to satisfy the two requirements of high hardness and excellent wear resistance and toughness. As a method of improving the toughness of the molten high speed tool steel, a method of adding a trace amount of an element such as Nb and refining the crystal grains to improve the toughness (for example, JP-A-58-73753 and JP-A-58-11).
No. 7863), and a method of uniformly refining MC type carbide mainly composed of Nb by adding Nb and rare earth elements in combination (Japanese Patent Publication No. 61-896).

On the other hand, the most general method for improving wear resistance is to increase the amount of carbide in powder high speed tool steel in which carbide is uniformly and finely distributed and crystal grains can be refined. is there. For example, Japanese Examined Sho 57-2142
And JP-A-55-148747, the amount of M ₆ C type carbides mainly containing W and Mo is increased by mainly increasing the W equivalent, and the wear resistance is improved by increasing the hardness. Is. Further, in the powder high speed tool steel, it is considered to contain Nb for the purpose of refining the crystal grains and further preventing the crystal grains from being coarsened even if the quenching temperature is raised {Metall. Trans. 19A (1988) P1395 to 1401, Japanese Patent Laid-Open No. 1-212736}.

[0004]

However, the above-mentioned Japanese Patent Laid-Open No. 58-58
-73753 and 58-117863 melted high speed tool steels have Nb
When N is excessively added, coarse NbC carbides mainly composed of Nb are crystallized, and M ₆ C type carbides mainly composed of W and Mo are also
Since a coarse carbide is crystallized during solidification, there is a problem that the effect of improving the toughness by refining the crystal grains is diminished and the toughness is rather lowered. Further, in the above powder high-speed tool steel, the amount of carbide and the hardness of the tool have been increased for the purpose of improving wear resistance, but the toughness is lowered, and the tool is broken or chipped. Was a problem.

Further, Japanese Patent Laid-Open No. 55-148747 proposes a powder high-speed steel containing Nb.
The main purpose is to add b as a substitute for V to form a hard carbide. Furthermore, Metall.Trans.1
The high speed tool steel disclosed in 9A (1988) P1395 to P1401 and Japanese Patent Application Laid-Open No. 1-212736 makes it possible to increase the quenching temperature without coarsening the crystal grains by adding Nb. According to the inventor's consideration, the alloying element amount, particularly the W equivalent, is low, so that the high temperature temper softening resistance is insufficient under severe tool use conditions.

Therefore, it has been difficult for the conventional high-speed tool steel described above to meet the recent tool usage conditions in which high speed is required. Therefore, an object of the present invention is to provide a powder high-speed tool steel having high toughness and high-temperature tempering softening resistance characteristics capable of accommodating high-speed tool use conditions.

[0007]

In recent years, as the use conditions of tools have become faster, increasing the hardness of the tools has become an important factor. Especially when using the tool, the temperature of the tool becomes high,
The tempering softening resistance characteristics are the most important.

The present invention has been made in view of this knowledge, and the following two points are the basic technical ideas. It has been found that in order to maximize the temper softening resistance, it is effective to regulate C-Ceq within a specific range in terms of chemical composition. The amount of C needs to be determined in consideration of the amounts of other carbide forming elements, and is adjusted by C-Ceq. In order to obtain high temper softening resistance, C-Ceq
It is necessary to regulate C and secure the amount of C that forms a solid solution in the matrix.

[0009] When many quenching temperatures are used as a solid solution of many alloying elements in the matrix, the crystal grains become coarse. However, by containing Nb and controlling the Nb / V ratio, Prevents coarsening of crystal grains, secures fine crystal grains, and prevents deterioration of toughness. Nb forms MC carbides like V, but in order to form fine NbC of 1 μm or less, which is effective for preventing coarsening of crystal grains,
It must contain more Nb than V in atomic ratio.
In weight ratio, Nb / V should be 0.5 or more.

It has been found that the above characteristics can be satisfied for the first time by satisfying the following component balance. That is, the present invention uses C by weight.
0.7-2.0%, Si≤1.0%, Mn≤0.6%, Cr 3.0-6.0%,
W or further Mo with W + 2Mo of 14 to 20% and V ≦ 5.0
%, Nb 2.0 to 7.0%, but Nb / V ≧ 0.5, the balance is Fe and inevitable impurities, and C-Ceq is -0.30 to 0.05 (Ceq
= 0.24 + 0.033 · W + 0.063 · Mo + 0.2V + 0.1 · Nb) is a powder high-speed tool steel.

In the present invention, C 0.7 to 2.0% by weight and S
i ≦ 1.0%, Mn ≦ 0.6%, Cr 3.0 to 6.0%, W or further Mo is W + 2Mo, 14 to 20%, and V ≦ 5.0%, Nb 2.0 to 7.
0%, but Nb / V ≧ 0.5, Co 15.0% or less, the balance is Fe and inevitable impurities, and C-Ceq is -0.30 to 0.05 (C
eq = 0.24 + 0.033 · W + 0.063 · Mo + 0.2V + 0.1 · Nb) is a powder high-speed tool steel.

[0012]

The reason for limiting the components will be described below. C forms a hard carbide with Cr, W, Mo, V, and Nb added at the same time, and contributes to the improvement of wear resistance. Further, it has a function of improving the secondary hardening by solid solution in the matrix during quenching. However, if the amount is too large, the amount of carbon solid-dissolved in the matrix increases remarkably and the toughness decreases. Therefore, the amount of C must be determined in consideration of the contents of Cr, W, Mo, V and Nb. In the present invention, the range of 0.7 to 2.0% and the value of C-Ceq are -0.3.
The amount of C is adjusted so as to satisfy the relationship of 00.05. By satisfying this relationship, one condition for achieving high high temperature temper softening resistance is achieved.

Si and Mn are added as deoxidizing agents, but if added in a large amount, there is a problem such as impairing toughness.
It is limited to 0% or less and Mn 0.6% or less. Cr is added in an amount of 3 to 6% for the purpose of enhancing the hardenability and the secondary hardening of the temper. If it is less than 3%, the above effect is small. On the contrary, if it is more than 6%, the amount of M ₂₃ C ₆ type carbides mainly composed of Cr is extremely increased and the toughness of the whole is impaired. Reduce resistance.

Further, in order to impart wear resistance, it is necessary to disperse a large amount of hard carbide and increase the matrix hardness. In the present invention, the amounts of W and Mo are important elements for the above purpose. W or even Mo with W + 2Mo 14
~ 20% If it is less than 14%, the above effect is small. However, if W + 2Mo exceeds 20%, the amount of connected carbides increases, the alloying elements that form a solid solution in the matrix also increase, and the toughness decreases, so that W or even Mo of W + 2Mo is 14%.
~ 20%

V is also an element effective in enhancing wear resistance. For the purpose of abrasion resistance, it is desirable to contain as much as possible. However, if it exceeds 5%, coarse MC type carbides are likely to crystallize, which impairs toughness and grindability of the tool.
Below. Nb is one of the most important elements in the present invention. When Nb is limited to a specific component range, a hard carbide mainly composed of 1 to 5 μm Nb effective for wear resistance and a fine carbide of 1 μm or less are crystallized.

The present inventors have found a range of components in which this fine NbC suppresses the growth of crystal grains and effectively suppresses the coarsening of crystal grains even if the quenching temperature is raised. This fine NbC is closely related to the Nb amount and the Nb / V ratio, and the Nb amount and the Nb / V ratio.
If the ratio is low, fine NbC is hardly crystallized,
The amount of Nb was adjusted so that Nb ≧ 2% and Nb / V ≧ 0.5. However, when Nb exceeds 7%, extremely coarse NbC is crystallized and impairs toughness and grindability, so it was set to 7% or less.

Co is an extremely effective element for improving the temper softening resistance of the steel of the present invention. It forms a solid solution in the matrix, delays the precipitation and agglomeration of carbides, and has the effect of significantly improving the hardness and strength at high temperatures, which is extremely important for applications where the contact part between the tool and the work such as cutting tools and end mills is particularly hot. It is an additive element. However, if Co exceeds 15.0%, the toughness decreases due to crystallization of the Co single phase due to solid solution, so the content was made 15.0% or less. Further, in order to remarkably improve the tempering softening resistance by adding Co, Co is preferably contained at 4% or more.

[0018]

【Example】

(Example 1) In Table 1, nitrogen gas atomized powder was HIPed.
The chemical compositions of eight types of experimental materials produced by the method of (hot isostatic pressing) are shown below. Each material is HIP
And forging to about 16 mm square, the forged material is
After annealing at 0 ° C. and austenitizing for 15 minutes under the heat treatment conditions shown in Table 2, a hot bath quenching at 550 ° C. was performed to obtain a sample. The tempering temperature was 560 ° C. In addition, ΔC in Table 1 is a value of C−Ceq.

The hardness of this sample after tempering, the grain size by the intercept method (after quenching) and each sample at 650 ° C.
After heating and holding for a period of time, the hardness upon cooling with air (referred to as tempering softening resistance) was measured. In addition, in order to evaluate the toughness of this sample, a bending test was performed on a test piece with a fulcrum distance of 50 mm after performing the above-mentioned heat treatment of quenching and tempering.
The results are shown in Table 2.

[0020]

[Table 1]

[0021]

[Table 2]

Of the alloys shown in Table 1, Sample Nos. 1 to 5 are
The amount of V and the amount of Nb were changed. No. 1 and No. 2 are comparative steels containing less Nb than the steels of the present invention,
When the quenching temperature exceeded 1200 ° C and became high, the crystal grains suddenly became coarse and the transverse rupture strength became 280 kgf / mm ² or less, so the quenching temperature was set to 1200 ° C. However, as shown in Table 2, this 12
At a quenching temperature of 00 ° C, the solid solution amount of alloying elements in the matrix was insufficient, and sufficient tempering softening resistance could not be obtained. Samples No. 3 and No. 4 are steels of the present invention, and Table 2 shows that, at a quenching temperature of 1250 ° C., they have higher temper softening resistance, higher hardness and higher resistance than 280 kgf / mm ² as compared with the comparative steel. It was confirmed to have folding strength. Further, Sample No. 5 is a comparative steel in which the amount of Nb exceeded the specified range of the present invention, and giant carbide of NbC of 5 μm or more was generated, and the transverse rupture strength was remarkably lowered.

Samples Nos. 6 to 8 are obtained by changing the value of W + 2Mo by changing the amounts of W and Mo. Sample No. 6 has a low W + 2Mo value of 13.1%, and the amount of alloying elements that can form a solid solution in the matrix is insufficient. Therefore, compared with Steel sample No. 7 of the present invention, hardness after heat treatment and temper softening resistance Was low. In addition, sample No.8 is W + 2
Since the Mo value was as high as 21.1%, the transverse rupture strength was undesirably reduced to 280 kgf / mm ² or less.

(Example 2) In order to confirm the effect of the value of C-Ceq on hardness, temper softening resistance and transverse rupture strength, the amount of C shown in Table 3 was changed in the same manner as in Example 1. A sample of different chemical composition was obtained. Here, C-Ceq is shown as ΔC. Hardness after tempering, grain size, temper softening resistance and transverse rupture strength were measured for these samples in the same manner as in Example 1. The results are shown in Table 4.

[0025]

[Table 3]

[0026]

[Table 4]

From Table 4, sample No. 9 having a value of ΔC of less than -0.30 and sample No. 12 having a value of ΔC of more than 0.05 were harder than the samples No. 10 and No. 11 of the present invention. It was confirmed that either the temper softening resistance or the transverse rupture strength was inferior and not preferable.

Example 3 Samples shown in Table 5 were obtained in the same manner as in Example 1. Further, with respect to the samples shown in Table 5, the hardness after tempering, the grain size, the temper softening resistance and the transverse rupture strength were measured in the same manner as in Example 1. The results are shown in Table 6. Sample Nos. 13 to 17 are the steels of the present invention, and have hardnesses of HRC67 or higher and HRC.
It was confirmed that the material had a tempering softening resistance of 58 or more and a bending strength of 280 kgf / mm ² or more. Further, among the steels of the present invention, samples No. 15 to 17 are samples containing Co, and as the amount of Co increases, the tempering softening resistance increases, and
The addition of o improved the temper softening resistance. In addition,
When Co was added, tempering was performed three times in order to prevent the presence of residual austenite. On the other hand, in sample No. 18, which is a comparative steel containing more than 15% of Co, the temper softening resistance was high, but the transverse rupture strength was significantly reduced, which was not preferable.

[0029]

[Table 5]

[0030]

[Table 6]

[0031]

According to the present invention, the softening resistance characteristic at high temperature, which has been insufficient in the past, can be remarkably improved, so that the wear resistance at high temperature can be remarkably improved. Further, since the crystal grains remain fine and the toughness is as high as or higher than that of the conventional one, it is possible to significantly improve the service life under the high-speed use condition of the tool.

Claims (2)

[Claims] 1. C 0.7 to 2.0% by weight ratio, Si ≦ 1.0%, Mn
≤0.6%, Cr 3.0-6.0%, W or even Mo is W + 2Mo
14 to 20% and V ≦ 5.0%, Nb 2.0 to 7.0%, but Nb /
V ≧ 0.5, balance of Fe and inevitable impurities, and C
-Ceq is -0.30 to 0.05 (however, Ceq = 0.24 + 0.033 ・ W + 0.0
Powder high speed tool steel characterized by satisfying the relationship of 63 · Mo + 0.2V + 0.1 · Nb). 2. C 0.7-2.0% by weight ratio, Si ≦ 1.0%, Mn
≤0.6%, Cr 3.0-6.0%, W or even Mo is W + 2Mo
14 to 20% and V ≦ 5.0%, Nb 2.0 to 7.0%, but Nb /
V ≧ 0.5, Co ≦ 15.0%, the balance Fe and unavoidable impurities, and C-Ceq is -0.30 to 0.05 (where Ceq = 0.24).
+0.033 ・ W + 0.063 ・ Mo ＋ 0.2V + 0.1 ・ Nb) powder high speed tool steel.