JPH1161331A - Hot tool steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain hot tool steel in which machinability is improved without deteriorating hardness and heat resistance checking by specifying composition consisting of C, Si, Mn, Cr, Mo, V, S, Te, Ca and Fe and also limiting the cleanliness of nonmetallic inclusions in the steel. SOLUTION: The hot tool steel contains 0.30-0.08% C, 2.0% or less Si, 3.0% or less Mn, 10% or less Cr, 5.0% or less Mo, 3.0% or less V, 0.005-0.050% S, 0.003-0.040% Te provided 0.080% or less S+T, and 0.0002-0.0030% Ca; and, as necessary, further contains one kind or more from 4.0% or less Ni, 3.0% or less Cu, and 5.0% Co;5.0% or less W; one kind or more from 2.0% or less Nb, 2.0% or less Ti, 4.0% or less Ta, and 1.0% or less Al; 0.010% or less B; 0.05% or less REM, and the balance essentially Fe; in such hot tool steel, nonmetallic inclusions in the steel are set to 0.010-0.150 in the d60×400 cleanliness stipulated in JISG0555.

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 which is used as a material for a mold for hot forming a metal, and provides a hot tool steel having improved machinability without substantially reducing hardness. I do.

[0002]

2. Description of the Related Art As materials for manufacturing tools and molds for hot forming, steels which have been stipulated in JIS standards such as SKD61, SKD8 and SKT4, 3Cr-1Mo steel,
Semi-high-speed steels and the like have been used.

[0003] Of course, it is desired that these steels have good machinability so that the mold can be easily manufactured.
In recent years, as a result of advances in numerically controlled cutting methods, automation and speeding up of die machining have been progressing, and accordingly, demands for improved machinability have been increasing.

When importance is attached to machinability, measures such as working in a low hardness state at the expense of strength and adding inclusion forming elements represented by S are taken. However, lowering the hardness is not preferred because it lowers the wear resistance and heat checkability and shortens the life of the mold. Although the effect of adding an inclusion-forming element cannot be expected with a small amount, the addition of a large amount results in the generation of coarse inclusions, which serve as starting points for cracks and lower the heat check resistance. The forging die causes large cracks, and the die-casting type causes problems such as the formation of a product in which cracks are transferred to the surface on which the design is applied.

[0005] Therefore, it is desirable to improve machinability without reducing the heat check resistance of hot tool steel.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to advance the above-mentioned state of the art and to make the heat check resistance worse by advantageously controlling the components and morphology of nonmetallic inclusions. An object of the present invention is to provide a hot work tool steel which can improve a machinability without causing a drop in hardness and can easily produce a long-life mold.

[0007]

The hot work tool steel of the present invention comprises:
As a basic alloy composition, C: 0.30 to 0.80
%, Si: 2.0% or less, Mn: 3.0% or less, Cr:
10% or less, Mo: 5.0% or less, V: 3.0% or less, S: 0.005 to 0.050%, Te:
0.003-0.040% and Ca: 0.0002-
0.0030%, provided that S + Te: 0.080
% Or less, and the balance is substantially Fe, and the nonmetallic inclusions in the steel are 0.010 to 0.150 as d60 × 400 cleanliness defined in JIS G0555.
%.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The hot work tool steel of the present invention can contain at least one of the following alloy components in each group in addition to the above basic alloy components.

(1) Ni: 4.0% or less, Cu: 3.0
% Or less and Co: 5.0% or less, (2) W: 5.0% or less, (3) Nb: 2.0% or less, Ti: 2.0% or less, Ta: 4.0% or less, and one or more kinds of Al: 1.0% or less; (4) B:
0.010% or less, and (5) REM: 0.50%
Less than.

The aspect ratio of the non-metallic inclusions, that is, the ratio of the major axis to the minor axis, is preferably 2.5 or less.

The function of each alloy component and the reason for limiting the composition of the hot work tool steel will be described below.

C: 0.30 to 0.80% C gives hardness and wear resistance. In order to ensure sufficient hardness and wear resistance as a hot work tool steel, it is necessary to add 0.30% or more. Excessive addition lowers the workability, so the upper limit is 0.80%.

Si: 2.0% or less Si is necessary as a deoxidizing element, but from the viewpoint of heat check resistance, the amount of addition is preferably low, and from this viewpoint, the upper limit is 2.0%.

Mn: 3.0% or less Necessary as a deoxidizing element for steel, and also necessary for ensuring hardenability and hardness. Excessive addition causes a reduction in workability during annealing, so the upper limit is 3.0%.

Cr: 10.0% or less Carbides are formed to strengthen the matrix and improve wear resistance. Excessive addition lowers the workability, so the upper limit is 1
0.0%.

Mo: 5.0% or less Also forms a carbide, which is useful for strengthening the matrix and improving wear resistance. If an excessive amount is added, the workability is reduced.
Therefore, the upper limit is set to 5.0%.

V: 3.0% or less Carbide is also formed, and plays a role of strengthening the matrix and improving wear resistance. If the addition is too large, the processability is also lowered, so the upper limit was set at 3.0%.

S: 0.005 to 0.050%, Te:
0.003 to 0.040%, provided that S + Te: 0.0
80% or less S and Te form Mn (S, Te), which serves as a stress concentration source during cutting to enhance machinability. In order to obtain such an effect, at least S: 0.005% and T
e: Addition of 0.003% or more is required. However,
If added excessively, the hot workability at the time of production decreases or the heat check resistance decreases, so that S: 0.050% or less, Te: 0.040% or less, S + Te: 0.080%
The following are the upper limits. By the combined addition of S and Te, the sulfide-based inclusions are highly spheroidized as compared with the case of adding S alone, and a decrease in heat check resistance can be suppressed.

Ca: 0.0002% or more and 0.0030%
Hereinafter, Ca forms a fine oxide, and this oxide serves as a nucleus for sulfide formation. Therefore, by adding Ca to S and Te in a complex manner, sulfides are refined and a decrease in heat check resistance is suppressed. Ca is also useful for improving hot workability during production. In order to obtain such effects, addition of at least 0.0002% is necessary. On the other hand, even if added excessively, the effects are saturated, so the upper limit is made 0.0030%.

Situation of non-metallic inclusions: dA60 × 400 defined as JIS G0555, with a purity of 0.010-0.
The 150% non-metallic inclusion improves the machinability by, for example, being a source of stress concentration during chip generation and exerting a lubricating action between the tool and the chip during cutting. To get this effect,
The purity must be at least 0.010%.
However, if it is excessively present, the heat check resistance deteriorates. Therefore, the content is restricted to 0.150% or less. Note that dA60
× 400 = ×% means that the surface to be inspected is enlarged 400 times and 60%
The number of lattice points in the visual field occupied by the A-based inclusions (mainly sulfides) when measured for each visual field was x% of the total lattice points.
It means that it is.

The function of the optionally added alloy component and the reason for limiting the composition range are as follows.

Ni: 4.0% or less, Cu: 3.0% or less, Co: 5.0% or less All are effective in improving hardenability and strengthening the matrix, and are added as required. If added excessively, Ni and Co lower the workability, and Cu also lowers the impact resistance.

W: 5.0% or less W forms carbide and is effective for strengthening the matrix and improving wear resistance. Excessive addition lowers the workability, so the upper limit is made 5.0%.

Nb: 2.0% or less, Ti: 2.0% or less, T
a: 4.0% or less, Al: 1.0% or less Nb, Ti, Ta
Forms fine carbides, and Al forms fine nitrides, all of which contribute to the refinement of crystal grains and, consequently, the improvement of impact resistance. Since the effect is saturated even if added excessively, the above upper limits are respectively set.

B: 0.010% or less B is an element having a large effect of improving hardenability, and is added as necessary. Since it is not preferable for hot workability and toughness, the addition is limited to 0.010% or less. REM: 0.50% or less Impurities such as O and P are fixed, the cleanliness of the base is increased, and the impact resistance is improved. If the amount is too large, a ground flaw is generated. Therefore, the upper limit is 0.50% or less.

Aspect ratio of non-metallic inclusions (ratio of major axis to minor axis): 2.5 or less When there are elongated non-metallic inclusions, a deep heat check tends to occur. If the aspect ratio of the nonmetallic inclusions is 2.5 or less, the heat check resistance hardly decreases, so the aspect ratio is measured to be 2.5 or less. This can be realized by appropriately combining the amounts of the inclusion forming elements S, Te and Ca.

[0027]

【Example】

[Example 1] Hot tool steels having the alloy compositions shown in Tables 1A and 1B were melted in a vacuum induction furnace. These are the traditional SK
It is based on D61 steel and SKT4 steel.

Table 1A Alloy composition No. CSiMnCuNiCrMoVSTeCa Other SKD61 Examples 1 0.40 0.06 0.50 0.01 0.08 5.30 2.01 0.85 0.006 0.003 0.0011 -2 0.39 0.07 0.49 0.01 0.08 5.30 1.97 0.84 0.025 0.006 0.0013-3 0.40 0.07 0.51 0.01 0.08 5.27 1.99 0.85 0.046 0.009 0.0015 −4 0.39 0.08 0.51 0.01 0.08 5.28 1.98 0.84 0.026 0.005 0.0013 Nb: 0.12 5 0.40 0.08 0.50 0.01 0.08 5.33 2.01 0.83 0.024 0.005 0.0011 Al: 0.04 Comparative example 1 0.41 0.08 0.51 0.01 0.08 5.32 2.02 0.85 0.001 − − − 2 0.41 0.07 0.52 0.01 0.08 5.35 1.96 0.84 0.007 − − − 3 0.40 0.07 0.50 0.01 0.08 5.25 2.00 0.85 0.045 − − − 4 0.40 0.06 0.49 0.01 0.07 5.28 2.01 0.87 0.001 0.003 0.0012 − 5 0.40 0.06 0.48 0.01 0.07 5.31 2.03 0.86 0.066 0.016 0.0012- Table 1B Alloy composition No. CSiMnCuNiCrMoVSTeCa Other SKT4 Examples 11 0.56 0.34 1.02 0.15 1.56 1.03 0.48 0.10 0.007 0.015 0.0007 -12 0.55 0.36 0.98 0.17 1.57 0.99 0.50 0.12 0.015 0.033 0.0005 -13 0.54 0.34 1.03 0.13 1.54 1.00 0.49 0.11 0.035 0.035 0.0006 − 14 0.55 0.34 1.03 0.13 1.54 1.00 0.50 0.12 0.016 0.031 0.0006 Ti: 0.05 15 0.55 0.34 0.98 0.15 1.55 1.01 0.48 0.10 0.017 0.034 0.0008 Ta: 0.43 16 0.54 0.35 1.00 0.14 1.52 1.00 0.51 0.09 0.015 0.033 0.0006 B: 0.004 17 0.56 0.34 1.01 0.17 1.54 1.02 0.50 0.12 0.017 0.033 0.0007 REM: .12 18 0.53 0.33 1.00 0.16 1.53 1.00 0.48 0.11 0.015 0.032 0.0006 Co: 0.50 W: 0.14 Ti: 0.008 Al: 0.005 B: 0.005 REM: 0.05 Comparative Example 11 0.56 0.33 1.04 0.14 1.55 1.01 0.47 0.13 0.001 − − − 12 0.55 0.34 1.01 0.15 1.53 0.98 0.51 0.10 0.008 − − − 13 0.53 0.35 1.04 0.13 1.56 1.01 0.49 0.13 0.046 − − − −14 0.54 0.35 1.02 0.14 1.53 0.99 0.51 0.10 0.064 0.034 0.0008 − The ingot is heated to 1200 ° C and forged It was plate width 110 mm × thickness 45 mm.

After annealing these samples,
Three types of test pieces were roughly processed: (1) a test piece for evaluation of cleanliness and inclusion morphology, (2) a heat check test piece, and (3) a machinability test piece. For (2) and (3), quenching and tempering are performed.
The test was performed under the heat treatment conditions shown in Table 2 below.

Table 2 Steel Type Quenching Tempering Target Hardness SKD61-based 1030 ° C 615-625 ° C HRC45 SKT4-based 870 ° C 600-610 ° C An HRC43 heat-treated test piece was precisely processed. (1) has a block shape, (2) has a button shape with a diameter of 15 mm × 5 mm in height, and (3) has a plate shape with a length of 200 mm × width 100 mm × thickness 40 mm.

First, regarding the test piece of (1), the condition of nonmetallic inclusions according to the JIS method was examined, and dA60 × 400.
The cleanliness was measured. At the same time, the aspect ratio of each nonmetallic inclusion was measured at a magnification of 400 using an image analyzer, and the average value thereof was calculated.

In the heat check test, the test piece of (2) was heated to 600 ° C. by high-frequency heating, and cooled to room temperature by spraying cold water on the side surface (circumference). This was repeated, and the average depth of the generated cracks was measured to evaluate the heat check resistance. Prior to this test, the hardness of the test piece (2) was measured.

The machinability was evaluated by cutting the edge of the test piece (3) with an end mill. The test conditions are as follows: Tool: Carbide end mill (M20 [UTi20T]), 1
Blade Cutting width: 1 mm Cutting depth: 4.0 mm Cutting speed: 100 m / min Feed: 0.035 mm / blade Cutting oil: None (dry type) Full cutting until the flank wear width of the carbide tip becomes 0.3 mm The length was measured, and the value corresponding to the total cut length of steel prepared for comparison and containing no inclusion forming element was determined as 100.

The results of the above tests are shown in Tables 3A and 3B. For reference, JIS d (B + C) 60 × 400
Cleanliness data is also provided.

Table 3A Purity Aspect Ratio Hardness Heat Machinability No. JIS dA JIS d (B + C) Checkability 60x400 60x400 HRC (μm) SKD61 series Example 1 0.0120 1.8 45.4 12.6 160 2 0.0480 1.9 45.0 12.0 190 3 0.014 0.004 2.0 45.5 12.5 240 4 0.055 0 1.9 45.2 12.6 200 5 0.049 0 2.2 45.1 12.0 180 Comparative Example 100-44.8 12.5 100 2 0.008 0 3.3 45.0 12.0 140 300 1.4 44.8 12.3 125 400 1.4 44.8 12. 3 125 5 0.213 0.008 2.8 44.9 16.3 355 Table 3B Purity Aspect ratio Hardness Heat resistance Machinability No. JIS dA JIS d (B + C) Checkability 60x400 60x400 HRC (μm) SKT4 Example 11 0.010 0 1.9 43.0 29.2 180 12 0.024 0.004 1.7 43.2 29 0.0 215 13 0.125 0 2.2 43.0 29.4 300 14 0.017 0 1.7 43.1 29.5 225 15 0.020 0 1.8 1.8 43.3 29.4 225 16 0 0.025 0.008 2.0 42.7 28.8 205 17 0.015 0 1.8 42.9 29.2 230 18 0.017 0 1.9 43.1 29.6 210 Comparative Example 110 0 -43.2 29.4 100 12 0.011 0.004 3.4 43.3 28.9 145 13 0.138 0 5.0 42.8 32.5 240 14 0.212 0 2.7 42. 8 35.4 440 steel of the present invention, resistance heating compared to the comparative example And lowering of the check of less than 10%, no less substantially, while the machinability 5
It has improved by 0% or more.

Example 2 A steel having an alloy composition shown in Table 4 was
It was melted in an arc furnace. These are the SKD6 of Example 1.
It has an alloy composition belonging to the series of KSD8 steel and semi-high-speed steel in addition to steel 1 and SKT4 steel.

Table 4 Alloy Composition No. CSiMnCuNiCrMoVSTeCa Other SKD61 system Example 21 0.39 0.07 0.52 0.06 0.08 5.35 1.98 0.85 0.035 0.005 0.0013-Comparative example 21 0.40 0.08 0.52 0.07 0.08 5.33 2.01 0.84 0.001 ---SKT4 system Example 22 0.56 0.34 1.01 0.13 1.54 1.03 0.51 0.10 0.037 0.005 0.0008 − Comparative example 22 0.56 0.35 1.03 0.15 1.52 1.04 0.48 0.11 0.002 − − − SKD8 system Example 23 0.36 0.34 0.32 0.08 0.07 4.29 0.38 2.16 0.034 0.007 0.0010 W: 4.20 Co: 4.20 Comparative example 23 0.35 0.33 0.31 0.08 0.08 4.33 0.40 2.17 0.002--W: 4.23 Co: 4.17 Semi-high speed Example 24 0.70 0.45 0.24 0.07 0.08 4.20 3.99 1.09 0.035 0.006 0.0012-Comparative example 24 0.68 0.46 0.22 0.07 0.08 4.22 4.02 1.07 0.002 --- Each ingot was heated to 1200 ° C. and forged into a billet having a width of 250 mm and a height of 150 mm.

These samples were subjected to annealing, rough processing of the test pieces, heat treatment of quenching and tempering, and fine processing of the test pieces in the same manner as in Example 1. Each test was performed on the test pieces. Done. The heat treatment conditions are as described in Table 5. Table 5 Steel Type Quenching Tempering Target Hardness SKD61 series steel 1030 ° C 615-625 ° C HRC45 SKT4 series steel 870 ° C 600-610 ° C HRC43 SKD8 series steel 1175 ° C 660-670 C HRC45 semi- high- speed steel 1110 C 590-600 C The HRC60 test results are shown in Table 6.

Table 6 Purity Aspect Ratio Hardness Heat Machinability No. JIS dA JIS d (B + C) Checkability 60x400 60x400 HRC (μm) SKD61 series Example 21 0.108 0.004 2.1 45.1 12.5 205 Comparative example 210-0-45.3 12.9 100 SKT4 system Example 22 0.083 0 2.3 42.9 30.6 195 Comparative example 22 00 1.2 42.8 30.4 100 SKD8 system Example 23 0.091 0 2.4 45.3 6.8 200 Comparative Example 23 00 1.4 45.0 6.8 100 Semi-High Speed Example 24 0.115 1.9 60.2 19.9 190 Comparative Example 240 0-59.7 19.7 100 In this example, the heat check resistance was reduced by 10%.
The goal of less than 50% improvement in machinability has been achieved.

[0040]

The hot work tool steel of the present invention has substantially the same hardness as the conventional steel, and has significantly improved machinability without substantially reducing heat check resistance. This effect is realized by distributing an appropriate amount of inclusions having an appropriate shape and size in steel by balancing the types and the quantitative combinations of the nonmetallic inclusion forming elements.

The use of this hot tool steel makes machining easier than before, so that tools and dies having the same life as those using existing steel are reduced with higher efficiency. At a low cost.

Claims (7)

[Claims] 1. C: 0.30 to 0.80%, Si: 2.
0% or less, Mn: 3.0% or less, Cr: 10% or less, M
o: not more than 5.0% and V: not more than 3.0%
S: 0.005 to 0.050%, Te: 0.003 to
0.040% and Ca: 0.0002 to 0.0030
%, But S + Te: 0.080% or less, the balance being substantially composed of Fe, and the nonmetallic inclusions in the steel having a cleanliness of d60 × 400 defined by JIS G0555 of 0.010 to 0.150. % Hot tool steel. 2. The alloy composition according to claim 1, wherein N
i: 4.0% or less, Cu: 3.0% or less, and Co:
A hot work tool steel containing one or more kinds of not more than 5.0%, and nonmetallic inclusions in the steel are as defined in claim 1. 3. A hot work tool steel containing W: 5.0% or less in addition to the alloy composition according to claim 1 or 2, wherein nonmetallic inclusions in the steel are as defined in claim 1. . 4. In addition to the alloy composition according to claim 1, Nb: 2.0% or less, Ti: 2.0%
A hot work tool steel containing one or more of Ta: 4.0% or less and Al: 1.0% or less, wherein nonmetallic inclusions in the steel are as defined in claim 1. 5. A heat containing B: 0.010% or less in addition to the alloy composition according to any one of claims 1 to 4, wherein the nonmetallic inclusions in the steel are as defined in claim 1. Between tool steel. 6. A heat containing REM: 0.50% or less in addition to the alloy composition according to claim 1, wherein the non-metallic inclusions in the steel are as defined in claim 1. Between tool steel. 7. The hot work tool steel according to claim 1, wherein an aspect ratio of nonmetallic inclusions in the steel is 2.5 or less.