JPH09217147A - Hot tool steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot tool steel excellent in processability in which the processability is improved without deteriorating the service lives of tools and suitable for a die steel. SOLUTION: This steel has a compsn. contg. one or >= two kinds among, by mass, 0.20 to 0.80% C, <0.10 Si, <3%Mn 0.003 to <0.030% S, 0.003 to <0.10% Te, <=10.0% Cr, <=5.0% Mo and <=3.0% V, and the balance Fe with inevitable impurities, and in which the content of S+Te is regulated >=0.006<0.040%. Furthermore, <=0.0030% Ca, <=2.0% Mb, <=2.0% Ti, <=4.0% Ta, <=0.50% rare earth metals, <=5.0% W, <=5.0% Co, <=4.0% Ni and <=3.0% Cu may be incorporated therein.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to hot work tool steel used in press dies, forging dies, die casting dies, punches and the like.

[0002]

2. Description of the Related Art Conventionally, steels for dies and punches used for hot working such as hot pressing, hot forging, die casting, etc. have high temper softening resistance, impact resistance and heat check resistance. Excellent SKD61, SKD8, SKT4, etc. J
IS standard steel, 3% Cr-1Mo steel, and semi-high speed steel have been widely used. Further, JP-A-60-56055, JP-A-60-59052, and JP-A-60-56.
According to Japanese Patent Publication No. 053, attention is paid to the fact that, in a mold or the like, microcracks are generated on the surface due to thermal fatigue (hereinafter referred to as heat check), and these propagate and spread to reach the end of life, thus reducing impact resistance. It has been studied to suppress the occurrence of heat check, which is the starting point of cracking, without causing it. This is because the conventional method for improving heat check resistance is
The material hardness is increased by increasing the amount of alloying elements and increasing the quenching temperature, and although heat check resistance is improved, there is a drawback that impact resistance decreases, so it is necessary to maintain impact resistance. It was considered in mind. Also, from the viewpoint of improving the service life, if the heat check as the starting point does not occur, even if the impact resistance is the same as the conventional one, it will not lead to chipping or cracking and the service life will be improved. Is. As a result, not only is the reduction of the Si content effective in improving the heat check resistance,
It is disclosed in the above-mentioned publication that it is also effective in improving impact resistance. The hot work tool steel having a reduced Si content has excellent characteristics when used as a mold, such as heat check resistance and impact resistance, but has a machinability when used as a conventional steel. Since it is lower than that, there is a problem that the cutting process requires cost and time. Further, with the recent progress of numerically controlled cutting and automation of die machining, there is an increasing demand for improvement of machinability of die steel.

It is known that it is effective to add free-cutting elements such as Pb and S in order to improve the machinability of steel. However, when these free-cutting elements are added, the impact value of the rope, particularly the impact value in the T direction, is significantly reduced,
When a free-cutting element is added to a material with a low Si content, Si
The impact resistance is lower than that of a material containing no free-machining element that has not been reduced. Therefore, if a minute heat check occurs, it is enlarged immediately, and there is a problem that the life of the mold or the like is shortened.

[0004]

In view of the above situation,
The object of the present invention is to improve the machinability without impairing the tool life of a high-performance steel having a low Si content, and to provide a hot work tool steel excellent in machinability suitable for die steel. To provide.

[0005]

MEANS FOR SOLVING THE PROBLEMS The hot work tool steel of the present invention comprises (1) a chemical composition of mass%, C: 0.20% or more and a value of 0.
80% or less, Si: less than 0.10%, Mn: 3.0% or less, S: 0.003% or more and less than 0.030%, Te:
0.003% or more and less than 0.010%, Cr: 10.0%
Hereinafter, any one or more of Mo: 5.0% or less and V: 3.0% or less, the balance Fe and unavoidable impurities, and S + Te: 0.006% or more and less than 0.040% Is characterized by. (2) The chemical composition is% by mass, and C 2 is 0.20% or more.
80% or less, Si: less than 0.10%, Mn: 3.0% or less, S: 0.003% or more and less than 0.030%, Te:
0.003% or more and less than 0.010%, Ca: 0.003
0% or less, Cr: 10.0% or less, Mo: 5.0% or less, V: 3.0% or less, any one kind or two or more kinds,
The balance consists of Fe and unavoidable impurities, and S + Te:
It is characterized by being 0.006% or more and less than 0.040%. (3) In addition to the above (1) or (2), further mass%
Then, Nb: 2.0% or less, Ti: 2.0% or less, Ta:
It is characterized by containing any one kind or two kinds or more of 4.0% or less. (4) In addition to the above (1) or (2), further mass%
And REM: 0.50% or less is contained. (5) In addition to the above (1) or (2), further mass%
In addition, W: 5.0% or less and Co: 5.0% or less are included. (6) In addition to the above (1) or (2), further mass%
And, it is characterized by containing any one or two of Ni: 4.0% or less and Cu: 3.0% or less.

[0006]

The reasons for limiting the chemical composition of the hot work tool steel of the present invention will be described below. C: 0.20% or more and 0.80% or less C is an essential element for imparting hardness and wear resistance required for steel as a mold. For that purpose, it is necessary to contain 0.2% or more of C. However, if C is contained excessively, the impact resistance is lowered, so the upper limit of the C content is 0.80%.
And Si: less than 0.10% In the steel of the present invention, the toughness and heat check resistance of the steel can be improved by reducing the Si content. In particular, when the Si content is less than 0.1%, the above-mentioned characteristics are remarkably improved, so the Si content is less than 0.1%. Mn: 3.0% or less Mn has a deoxidizing action at the time of melting of steel, and combines with S to form MnS, which prevents red hot embrittlement of steel caused by S. Further, it is an element effective for enhancing the hardenability of steel and ensuring the hardness of steel. However, if Mn is excessively contained, the workability at the time of annealing is deteriorated and the impact resistance is impaired, so the upper limit of the content is made 3.0%. S: 0.003% or more and less than 0.030%, Te: 0.0
03% or more and less than 0.010%, S + Te: 0.006%
Above 0.040% S and Te both form compounds with Mn and intervene in the steel as non-metallic inclusions. These non-metallic inclusions act as a stress concentration source at the time of cutting steel, reduce cutting resistance and improve the crushability of cutting chips, thereby improving the machinability of steel. Further, by adding Te to S in combination with S, the elongation of the sulfide-based nonmetallic inclusions during the forging of the steel is suppressed, so that the T direction impact value due to the formation of the nonmetallic inclusions It has the effect of suppressing the decrease. In order to obtain the effect of improving the machinability and the effect of suppressing the decrease in the impact value in the T direction, the content rates of S and Te are each 0.003% or more, and S + Te is more than 0.
006% or more is required. However, since excessive addition impairs the cleanliness of the steel and reduces the toughness of the steel, S and Te are independently less than 0.030% and less than 0.010%, and S + Te is less than 0.040% in combination. Content rate. Cr: 10.0% or less, Mo: 5.0% or less, V: 3.
0% or less Cr, Mo, and V all form strong carbides, strengthen the steel material, and are effective in improving wear resistance, so at least one of them is added. However, excessive addition causes a drop in the impact resistance and workability of the steel, so Cr: 10.0
%, Mo: 5.0%, V: 3.0% are the upper limits of the content rates, respectively. Ca: 0.0030% or less Ca has the effect of suppressing the elongation of sulfide-based nonmetallic inclusions during the forging of steel and preventing the reduction of the impact value in the T direction.
Further, it also contributes to the improvement of the machinability of steel, so that it is preferable to add it. However, if added excessively, the impact resistance of the steel is rather lowered, so the upper limit of the content is made 0.0030%. Nb: 2.0% or less, Ti: 2.0% or less, Ta: 4.
0% or less Nb, Ti, and Ta all form fine carbides in the steel and refine the crystal grains of the steel to improve the impact resistance. However, even if added excessively, the effect is saturated, so Nb: 2.0% or less, Ti: 2.0% or less, and Ta:
You may add in the content rate of 4.0% or less. REM: 0.50% or less REM fixes impurities such as O and P in steel to improve the cleanliness of the matrix and improve impact resistance. However, if it is added excessively, a flaw in the steel will occur, so the content rate is 0.50%.
You may add in the following. W: 5.0% or less W forms a strong carbide in the steel and is effective for strengthening the matrix and improving wear resistance, so W may be added. However, if added excessively, the impact resistance and workability of the steel will deteriorate, so the upper limit of the content is made 5.0%. Co: 5.0% or less Co enhances the temper softening resistance of steel and strengthens the steel base,
It may be added because it is effective in improving impact resistance. But,
If it is added excessively, not only the workability of steel is impaired but also the impact resistance is lowered, so the upper limit of the content is 5.0%.
And Ni: 4.0% or less, Cu: 3.0% or less Ni and Cu are both effective in strengthening the steel matrix and improving impact resistance, and thus may be added. However, if added excessively, not only the workability of the steel is impaired, but rather the upper limit of the content rate that causes a decrease in impact resistance is Ni: 4.0%,
Cu: 3.0%.

[0007]

Embodiments of the present invention will be described below. Using an induction furnace, the steel shown in Table 1 was melted into a 2t steel ingot. This steel ingot was forged at 1200 ° C. to form a steel piece having a width of 240 mm and a thickness of 150 mm, annealed to obtain a test piece material, and rough processing was performed. As the test piece material, an impact test piece material, a heat check test piece material, and a machinability test piece material obtained from the forging direction (L direction) and the direction perpendicular to the forging (T direction) were obtained.

[0008]

[Table 1]

These test piece materials were subjected to the heat treatments shown in Table 2 to give JIS No. 3 impact test pieces, each having a diameter of 15 mm.
× Heat check test piece with height of 5 mm, 100 mm × 4
The machinability test piece of 0 mm × 200 mm was precisely processed.

[0010]

[Table 2]

The hardness of the impact test piece was measured on the Rockwell C scale. A Charpy impact test was performed on the impact test piece to obtain a Charpy value. After heating and cooling the heat check test piece at 650 ° C. and room temperature 1000 times by high frequency heating and water cooling, each heat crack length observed in the cross section at the center of the height of the test piece (heat crack from the surface of the test piece Depth to the tip of
Was measured. The heat check resistance of the test piece was evaluated by the average value of the heat crack length.

A machinability test piece was made into a carbide end mill (UTi2
Cutting width: 1 mm, cutting depth: 3.5 m
m, cutting speed: 40 m / min, feed: 0.035 mm
/ Dry cutting was performed as a blade, and the cutting length until the flank wear width became 0.3 mm was determined as the tool life. For each steel type system, the tool life of the existing steel was set to 100, and the ratio to the tool life of each test piece was calculated to obtain a machinability index, which was used to evaluate the machinability of the test piece.

The results of each test are shown in Table 3. According to Table 3,
The example of the present invention has the same hardness as that of the comparative example.
Comparative example of conventional rope (No. 1, 7, 1)
0, 13, 16), the Charpy values are high in both the L and T directions, the average length of the heat check is small, and the machinability index and the tool life ratio are large. In addition, comparative examples of low Si content materials (No. 2, 3, 4, 5, 6,
8, 9, 11, 12, 12, 14, 15, 17, 18), the Charpy value may be slightly inferior, but
It can be seen that the average check length is the same, the machinability index is large, and the tool life is the same. This is because the embodiment of the present invention retains sufficient impact resistance for ensuring the tool life. From this, it is clear that the working examples of the present invention are not inferior in tool life to the comparative examples and are excellent in machinability.

[0014]

[Table 3]

Further, Comparative Examples 1, 2, 7, 8 in Table 1
For 10, 11, 13, 14, 16, 17 and Examples 2, 5, 11, 15, 16, 16, 19, forging dies for the stamping parts shown in the table below were prepared and actually stamped. Forging was performed and the tool life of each mold was investigated. The tool life was evaluated for each steel type by using a tool life ratio in which the number of stamping times was 1 until the die made of Kuraiki steel in which Si was not reduced was discarded. The results are shown in Table 4.

[0016]

[Table 4]

[0017]

As described in detail above, according to the hot work tool steel of the present invention, the machinability is improved without impairing the tool life,
It is possible to provide a hot work tool steel having excellent machinability, which is particularly suitable as a die steel.

Claims (6)

[Claims] 1. The chemical composition is% by mass, C: 0.20% or more and 0.80% or less, Si: less than 0.10%, Mn: 3.0% or less, S: 0.003% or more, 0. Less than 030%, Te: 0.003% or more and less than 0.010%, Cr: 10.0% or less, Mo: 5.0% or less, V: 3.
0% or less of any one or two or more, balance Fe and unavoidable impurities, and S + Te: 0.006% or more and less than 0.040%, a hot work tool steel. 2. The chemical composition is% by mass, C: 0.20% or more and 0.80% or less, Si: less than 0.10%, Mn: 3.0% or less, S: 0.003% or more, 0. Less than 030%, Te: 0.003% or more and less than 0.010%, Ca: 0.0030% or less, Cr: 10.0% or less, Mo: 5.0% or less, V: 3.
0% or less of any one or two or more, balance Fe and unavoidable impurities, and S + Te: 0.006% or more and less than 0.040%, a hot work tool steel. 3. In addition to the chemical composition according to claim 1, Nb: 2.0% by mass.
% Or less, Ti: 2.0% or less, Ta: 4.0% or less, and any one kind or two or more kinds is included, and the hot work according to claim 1 or claim 2. Tool steel. 4. In addition to the chemical composition according to claim 1 or 2, further, in mass%, REM: 0.
50% or less is included, The hot work tool steel of any one of Claim 1 or Claim 2 characterized by the above-mentioned. 5. In addition to the chemical composition according to claim 1 or 2, W: 5.0% in mass%
Below, Co: 5.0% or less is included, The hot work tool steel of any one of Claim 1 or Claim 2 characterized by the above-mentioned. 6. In addition to the chemical composition according to claim 1 or 2, Ni: 4.0 in mass% is further added.
% Or less, Cu: 3.0% or less and any one or two of them are contained. 3. The hot work tool steel according to claim 1 or 2.