JP2001152246A - Method for producing steel for plastic molding die excellent in toughness, mirror finishing property and machinability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing steel for a plastic molding die excellent in toughness, mirror finishing properties and crimping properties and moreover excellent in a machinability. SOLUTION: In this method for producing steel for a plastic molding die, steel containing, by weight, 0.01 to 0.20% C, <=1.0% Si, 0.5 to 2.5% Mn, 1.5 to 3.5% Ni, 0.3 to 2.5% Cu, 0.3 to 1.5% Al, <=15 ppm O, <=150 ppm N, and the balance Fe is hot-worked, is cooled to <=1173 K at a cooling rate of 0.5 to 10 K/s to a lower bainitic region, is held at >=1.8 Ks and is thereafter subjected to again treatment at 723 to 873 K. Moreover, the steel may contain 0.01 to 0.30% S, <=0.3% Pb, <=0.3% Bi, <=0.3% Se, <=0.3% Te, <=0.005% Zr, <=0.01% B, <=0.02% Ca, 0.05 to 0.04% Cr, 0.1 to 1.0% Mo, <=0.5% W, 0.6% V, 0.6% Ti and <=0.6% Nb as well.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a mold steel for molding plastic parts and plastic products, and more particularly to a mold steel for plastic injection molding and the like, and further having toughness and mirror finish. The present invention relates to a method for producing a steel for a plastic molding die, which exhibits excellent characteristics in graining workability and also has excellent machinability in cutting and the like.

[0002]

2. Description of the Related Art Conventionally, molds for molding plastic parts and plastic products are used to manufacture various parts and products ranging from automobile parts, office equipment parts, precision machine parts, electric parts, optical equipment parts, and the like. For this purpose, mirror finishing and grain finishing are used. In addition, in the field of plastic molding die steel in recent years, the use environment of the die has become severe due to the shortening of the injection molding cycle, and the plastic molding die which is more excellent in toughness from the viewpoint of improving the life of the die. Demand for steel is increasing.

On the other hand, transparent plastic products such as optical lenses and syringes and other medical equipment are required to have an extremely smooth surface, and the mold steel used for the molding has a high mirror finish. Has been requested. In addition, in the field of steel for plastic molding dies, strict conditions such as simplification of die production, cost reduction by extending the life of cutting tools, and high precision are required as the ratio of die production cost rises. Therefore, more excellent machinability is required for mold steel. Further, in order to reduce the material cost in the mold manufacturing cost, a cheaper steel for a plastic mold is desired.

[0004]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned problems in the prior art, and is intended to produce an inexpensive steel for a plastic molding die excellent in toughness, mirror finish and machinability. It is intended to provide a way. Conventional precipitation hardening type plastic molding die steel has a problem that it has poor toughness because it has a base structure mainly composed of upper bainite with emphasis on machinability. Further, precipitation hardening type plastic mold steel is usually subjected to hot working and aging treatment after solution treatment. However, in the above heat treatment step, heating is performed again after hot working, and the cost is increased.

For example, Japanese Unexamined Patent Publication No. 7-278737 proposes to adjust the base structure to lower toughness of lower bainite in order to achieve improvement in toughness in plastic molding steel. Also, JP-A-63-162
No. 811 proposes that the reheating is omitted in the heat treatment step, thereby reducing the manufacturing cost and improving the grain formability by suppressing the formation of martensite to form a bainite structure.

As described above, in the prior art, the above-mentioned Japanese Patent Application Laid-Open No. 7-278737 is to obtain a base structure having excellent toughness by heating to an austenite region and then cooling to a lower bainite region. Since the holding in the steel is not taken into account, the lower bainite of the entire steel material is not yet sufficient. Further, since the solution treatment is performed again after the hot working, there is a problem that the manufacturing cost is increased. Further, in the above-mentioned JP-A-63-162811, after performing hot working at 973 to 1173K at 10 to 40%, and holding at a temperature range of 723 to 823K for 1 to 100 hours, martensite is formed. By suppressing the base to have a bainite structure, the graining processability is improved and the aging treatment is achieved at the same time, and the manufacturing cost is reduced by omitting reheating. However, since the heat treatment is performed in the range of 723 to 823K so as to combine the bainite formation and the aging treatment of the base, the upper bainite is mainly used, and although the machinability is excellent, the toughness is not sufficient.

[0007]

As a result of the present inventors' investigation on the above problems, the cooling rate after hot working is controlled to cool down to the lower bainite region, and to hold in the lower bainite region. As a result, it has been found that by forming the entire steel material into a lower bainite structure, a steel for a plastic molding die having excellent toughness can be obtained, and the solution treatment step can be omitted. Further, the amount of oxygen and the amount of nitrogen in the steel for plastic molding molds are O: 15 ppm or less by weight,
By adjusting the N content to 150 ppm or less, not only non-metallic inclusions are significantly reduced, and not only a steel for a plastic molding die having excellent mirror surface properties is obtained, but also excellent machinability can be obtained. This has led to the completion of the present invention.

The gist of the invention is as follows: (1) C: 0.01 to 0.20% by weight, Si:
1.0% or less, Mn: 0.5 to 2.5%, Ni: 1.5
-3.5%, Cu: 0.3-2.5%, Al: 0.3-
1.5%, O: 15 ppm or less, N: 150 ppm or less After hot-working steel consisting of the balance Fe and unavoidable impurities, 1173 K or less is cooled to a lower bainite region at a cooling rate of 0.5 K / s or more, 1.8Ks in that area
A method for producing a steel for a plastic molding die having excellent toughness, mirror finish, and machinability, comprising performing aging treatment at a temperature of 723 K to 873 K after the above holding. (2) Further, by weight ratio, S: 0.01 to 0.30%,
Pb: 0.3% or less, Bi: 0.3% or less, Se: 0.
It is characterized by containing one or more of 3% or less, Te: 0.3% or less, Zr: 0.005% or less, B: 0.01% or less, and Ca: 0.02% or less. The method for producing a steel for a plastic molding die excellent in toughness, mirror finish and machinability according to the above (1).

(3) Further, in a weight ratio, Cr: 0.05
To 4.0%, Mo: 0.1 to 1.0%, W: 0.5% or less, wherein (1) or (2) is contained. A method for producing a steel for a plastic molding die having excellent toughness, mirror finish and machinability as described above. (4) Further, by weight ratio, V: 0.6%, Ti: 0.6
%, Nb: one or more of 0.6% or less are contained, and the plastic molding metal having excellent toughness, mirror finish and machinability according to the above (1) to (3), In the manufacturing method of mold steel.

Next, the reasons for limiting the composition range of the chemical components in the steel for plastic molding dies having excellent toughness, mirror finish and machinability according to the present invention will be described. C:
With respect to 0.01 to 0.20% by weight, C is an element necessary for securing the hardenability of the invention steel and the hardness of the quenching phase, and is required to be 0.01% or more. However, if the content is too large, the weldability is impaired and the base is made martensitic to reduce machinability, so the upper limit is made 0.20%. S
For i: 1.0% by weight or less, Si is an indispensable element as a deoxidizing material at the time of smelting, but if too much, the toughness after age hardening is reduced, so the upper limit is 1.0% by weight.

Mn: 0.5-2.5% by weight, M
n is added to ensure deoxidation and hardenability,
In order to obtain the effect, the lower limit is 0.5% by weight. However, if added in an excessively large amount, the toughness is reduced, and the amount of martensite in the matrix is increased, resulting in a reduction in machinability and grain workability. Therefore, the upper limit is set to 2.5% by weight. N
i: 1.5 to 3.5% by weight, Ni enhances bainite hardenability, suppresses ferrite formation, precipitates Ni-Al intermetallic compounds during aging treatment, and secures hardness. Therefore, at least 1.5% by weight or more is necessary because it is an effective component for improving the grain formability required for a plastic mold.
Excessive addition has the problem of lowering the thermal conductivity and lengthening the injection molding cycle, and also impairs machinability, so the upper limit is 3.5% by weight.

Cu: 0.3-2.5% by weight, C
Since u brings about precipitation hardening due to fine precipitation together with Ni and Al, and is also effective in improving machinability, at least 0.3% by weight or more is necessary. However, excessive addition lowers the hot workability and also lowers machinability and toughness, so the upper limit is 2.5% by weight. Al: 0.3
About 1.5% by weight, Al is Ni at the time of aging treatment.
Since it is indispensable to generate an Al-based intermetallic compound and obtain a desired hardness, at least 0.3% by weight or more is necessary. However, excessive addition lowers hot workability and toughness and combines with O and N to form oxide or nitride non-metallic inclusions, which adversely affects mirror finish and machinability. The upper limit is 1.5% by weight.

Regarding the reasons for limiting the components of O and N, in a plastic molding die molten steel excellent in mirror finish and machinability according to the present invention, the O content at the time of melting is set to O: 15 ppm or less by weight. It is necessary. Here, the reason why the O content is 15 ppm or less is that if the O content increases more than this, the amount of oxide-based inclusions increases, and the mirror surface finish and machinability deteriorate. It is necessary to set the N amount to 150 ppm or less by weight.
The reason why the N content is set to 150 ppm or less is that N forms nitride with Al and refines austenite grains to obtain a uniform structure. However, by adding excessively, hard and coarse nitride is reduced. The reason for this is that, when it is formed, it easily falls off at the time of mirror finishing, thereby lowering the mirror workability and lowering the machinability.

Machinability, which is an important property as plastic molding die steel, can be achieved by regulating O and N. Further, by weight ratio, S: 0.01 to 0.30%,
Pb: 0.3% or less, Bi: 0.3% or less, Se: 0.
One or more of 3% or less, Te: 0.3% or less, Zr: 0.005% or less, B: 0.01% or less, and Ca: 0.02% or less may be contained. . S is effective in improving machinability, and 0.01% or more is required to show the effect. However, if a large amount of S is added, coarse sulfide-based inclusions are formed, which impairs toughness and causes pitting corrosion and excessive pits.
The upper limit is 0.30% by weight. Also, Pb, Bi, S
e, Te, Zr, B and Ca are also effective elements for improving machinability, and may be added as necessary. However, excessive addition lowers hot workability and impairs toughness, so that Pb, Bi, Se and Te are 0.3% by weight, Zr is 0.005% by weight, and B is 0.01% by weight. The upper limits are 0.02% by weight for Ca and Ca, respectively.

Regarding the reasons for limiting the components of Cr, Mo and W, Cr: 0.05 to 4.0% by weight, Mo: 0.1 to
1.0% by weight, W: 0.5% by weight or less,
Since r, Mo, and W are effective for improving the hardenability of the mold and improving the hardness and toughness, one or more of these alloy components may be contained. Especially C
Since r and Mo are effective components for improving the corrosion resistance, the lower limit of Cr is set to 0.05% by weight and the lower limit of Mo is set to 0.1% by weight in order to make the effect effective. However, if it is added in excessive amount, it causes a problem of precipitating carbide and lowering toughness, lowering thermal conductivity and lengthening a plastic injection molding cycle,
Cr is 4.0% by weight, Mo is 1.0% by weight, W is 0.5% by weight.
The upper limit is wt%.

The reasons for limiting the components of V, Ti, and Nb are as follows: V: 0.6% or less, Ti: 0.6% or less, Nb:
When the content is set to 0.6% or less, V, Ti, and Nb refine the crystal grains to obtain an effect of improving the toughness. However, if a large amount is added, the solution hardening hardness is increased more than necessary, and conversely, the toughness is increased. Lower
In addition, since the machinability is also reduced, 0.6% by weight
It was as follows.

The method for producing a plastic mold steel excellent in toughness, mirror finish and machinability according to the present invention is characterized in that after hot rolling or forging, 1173K or less is reduced to 0.5K / s or more. It is characterized by cooling to a lower bainite region at a cooling rate, holding at least 1.8 Ks in that region, and then performing aging treatment at a temperature of 723K to 873K. Here, cooling after 1173K at a rate of 0.5K / s or more after hot working is performed when the cooling rate at a temperature of 1173K or less is less than 0.5K / s and the base structure is a structure mainly composed of ferrite and pearlite. And the toughness decreases.

The reason why the holding time in the lower bainite region is set to 1.8 Ks or more is that the lower bainite transformation region is located below the bainite nose in the CCT curve of steel, that is, at a lower temperature side than the tip of the bainite nose. This is because the holding of not less than .8 Ks allows the entire steel material to have a sufficiently lower bainite structure.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention are shown in the following examples. Steels for plastic molding dies having the chemical components shown in Table 1 are smelted by a normal smelting method to prepare test materials. Here, the invention steels 1 to 6 are plastic molding die steels satisfying the components described in any one of the first to fourth aspects. On the other hand, the comparative steel 1 has O and N different from those in the claims, but the other components are equivalent to the invention steel 1. In Comparative Steel 2, O and N are different from those in the claims, but the other components are equivalent to those of Invention Steel 4.

In Table 1, the notation "-" of the component value indicates that it is an unavoidable impurity. Subsequently, the test materials shown in Table 1 were formed into a predetermined shape by hot rolling or forging, and then 1173 under the conditions shown in Table 2.
After cooling to a lower bainite region below K and holding it at a predetermined temperature, it was subjected to aging treatment at a temperature of 773K to produce steel for a pre-hardened plastic molding die. FIG. 1 is a view showing a heat treatment pattern of the present invention.
Further, after the hot working of the inventive steels 2 and 5 shown in Table 1, the cooling rate of 1173K or less was cooled at a cooling rate less than the cooling rate described in the claims.
9 and 10.

Further, Comparative Examples Nos. 11 to 14 are pre-hardened plastic molding die steels in which inventive steels 1, 3, 4, and 6 are manufactured by a conventional process, respectively. FIG. 2 is a diagram showing an example of a conventional heat treatment pattern. It is. Table 2 summarizes the results of evaluating the hardness, structure, impact value, mirror finish, machinability, and grain workability of the pre-hardened plastic molding die steel treated under the above heat treatment conditions. Table 2
In, the structure is lower bainite: L. B, upper bainite: U.S. B, ferrite: α, and pearlite: P
Notation as In addition, about the impact value, it processed into a 2 mm U notch Charpy test piece, and evaluated at room temperature.

Regarding the mirror finish, in the test specimens of this test, the mirror finish was very good: 、, the mirror finish was good: 、, the mirror finish was slightly inferior: Δ, Poor mirror finish: x, and evaluated. Regarding machinability, an end mill workability test was performed on each test material under the conditions shown in Table 3. The end mill workability test was evaluated based on the cutting length (m) up to tool breakage. Regarding the grain formability, each specimen was subjected to graining under the same conditions, and as a result, uneven grain was generated: if grain unevenness was present, and if grain unevenness was not generated: grain unevenness was absent. An evaluation was performed.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

As is clear from the results shown in Table 2, Examples 1 to 6 of the present invention produced by the method for producing steel for a plastic molding die according to the present invention are Comparative Examples 9 to 9.
On the other hand, by adjusting the structure to lower bainite, it was clarified that the toughness was excellent. Furthermore, the inventive examples Nos. 1 to 6 are comparative examples Nos. 7, 8, 9, 1
With respect to 0, it can be seen that the mirror surface finish is excellent.
In Comparative Examples Nos. 7 and 8, the amounts of O and N significantly exceed the component ranges of the present invention, and there are many hard and coarse nonmetallic inclusions, and the mirror finish is extremely poor. Further, Comparative Examples Nos. 9 and 10
Since the base structure is a structure in which ferrite, pearlite and upper bainite are mixed, the mirror finish is reduced.

The inventive examples Nos. 1 to 6 correspond to the comparative example Nos.
For Nos. 7 and 8, good results were obtained in the end mill workability test. In Comparative Examples 7 and 8, the amounts of O and N exceeded the components of the present invention, so that the machinability was reduced. Further, the present invention examples Nos. 1 to 6 do not cause uneven grain by adjusting the base structure to lower bainite,
The grain workability was good. In Comparative Examples Nos. 9 and 10, since the base structure is a structure in which ferrite, pearlite, and upper bainite are mixed, uneven grain occurs, and the grain formability is reduced.

[0028]

As described above, the present invention controls the cooling rate after hot working, cools down to the lower bainite region, holds the lower bainite region, and performs the lower bainite forming treatment to obtain a structure having excellent toughness. And the solution treatment step can be omitted, and the amount of O is 15 ppm by weight%.
Adjusted below, and 1% by weight for N content
Adjusted to 50 ppm or less, has excellent mirror finish by reducing non-metallic inclusions, machinability, by providing a method of manufacturing a plastic mold steel excellent in graining workability, This has an extremely beneficial effect of simultaneously reducing the manufacturing cost and increasing the life of the mold.

[Brief description of the drawings]

FIG. 1 is a view showing a heat treatment pattern of the present invention.

FIG. 2 is a diagram showing an example of a conventional heat treatment pattern.

Claims (4)

[Claims] C: 0.01 to 0.20%; Si: 1.0% or less; Mn: 0.5 to 2.5%; Ni: 1.5 to 3.5%; Cu: 0.3 to 2.5%, Al: 0.3 to 1.5%, O: 15 ppm or less, N: 150 ppm or less, After hot working steel consisting of the balance Fe and unavoidable impurities, 1173K or less Cooling to the lower bainite region at a cooling rate of 0.5 K / s or more, and 1.8 Ks in that region
A method for producing a steel for a plastic molding die having excellent toughness, mirror finish, and machinability, comprising performing aging treatment at a temperature of 723 K to 873 K after the above holding. 2. The weight ratio of S: 0.01 to 0.1.
30%, Pb: 0.3% or less, Bi: 0.3% or less, S
e: 0.3% or less, Te: 0.3% or less, Zr: 0.0
The toughness, specularity and machinability according to claim 1, wherein one or more of B: 0.01% or less and Ca: 0.02% or less are contained. Method for producing excellent steel for plastic molding dies. 3. The method according to claim 1, wherein the weight ratio of Cr: 0.05 to
The toughness according to claim 1, wherein one or more of 4.0%, Mo: 0.1 to 1.0%, and W: 0.5% or less are contained. A method for producing steel for plastic molding dies having excellent mirror finish and machinability. 4. In a weight ratio, V: 0.6%, T:
i: 0.6%, Nb: one or more of 0.6% or less
The method for producing a steel for a plastic molding die having excellent toughness, specularity and machinability according to any one of claims 1 to 3, characterized by containing at least one kind.