JP2013177669A - Steel for forming die having excellent thermal conductivity, mirror polishability, weatherability, toughness, and machinability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel for a forming die which has high thermal conductivity and excellent mirror polishability and also has excellent weatherability, toughness, and machinability.SOLUTION: Steel for a forming die has composition including, in mass%, 0.040<C<0.100, 0.03<Si<0.27, 0.30<Mn<1.80, 0.30<Cu<0.61, and 4.00<Cr<9.00, and further including at least one of 0.04<Mo<1.00 and 0.02<V<0.50, with the balance of Fe and inevitable impurities.

Description

  TECHNICAL FIELD The present invention relates to a molding die steel excellent in heat conduction performance, mirror polishing, weather resistance, toughness, and machinability, and more particularly to a molding die steel suitable as a plastic molding die material.

  Various materials are required for mold materials for molding plastic products. Especially when plastic products are smooth and glossy and require aesthetics, when the mold is constructed It is required that the molding surface be highly mirror-polished so that the molding surface can be finished into a mirror surface.

  Furthermore, in recent years, there has been a strong demand for improving the productivity of plastic products, and in order to achieve this, there is a strong demand for a high cycle molding of plastic products, that is, a reduction in cycle time per molding cycle. It is required that mold materials to be molded satisfy these requirements.

On the other hand, there is an increasing demand for reducing the cost required for molds, and in order to meet this demand, reduction of material cost and processing cost is strongly demanded.
In addition, if the mold life is long, the die cost per product can be reduced (that is, the product cost can be reduced), so that the toughness necessary for extending the life of the mold is also required.

There are various plastic products here, and one of them is the four-frame frame of the TV screen, which requires a beautiful appearance. In recent years, this frame has become a larger TV screen. Along with this trend, the size of the mold is inevitably increased.
For example, a mold for molding such a molded product may be a large mold having a width of over 1 m and a thickness of several tens of cm or more.
Such a large mold material must have a high hardenability.
The actual situation is that conventional mold materials do not sufficiently meet these requirements.

In order to finish the mold, specifically, the molding surface into a beautiful mirror surface, it is necessary that the mold material has high mirror polishing properties.
For this purpose, it is necessary to reduce the amount of C added to the mold material.

  When the amount of C is large, the amount of carbide generated in the steel material also increases. Carbide is likely to appear on the surface of a mold manufactured from such a steel material. In this case, when the mold surface is mirror-polished, the carbide is dropped, and a hole (pinhole) is formed as a drop mark. When the plastic product is molded, the holes are transferred to the product side, causing a problem that the appearance of the product surface is lost and the product value is lost.

However, if the amount of C is reduced, it becomes difficult to obtain the hardness necessary for the mold.
Therefore, a material has been developed in recent years, in which the amount of C is reduced and the intermetallic compounds of Cu, Ni, and Al are deposited as a means of ensuring hardness, and the hardness of the mold is ensured by precipitation hardening. Has been.

For example, the following Patent Document 1 discloses an invention about “corrosion-resistant plastic molding die steel”, in which the amount of C is reduced to 0.02 to 0.2%, and intermetallic compounds of Cu, Ni, and Al during tempering. The point which increased hardness by precipitating is disclosed.
However, the one disclosed in Patent Document 1 is such that a large amount of Ni and Al are added at the same time to precipitate a large amount of intermetallic compounds. In this case, the cost increases due to an increase in the amount of alloy components added. In addition to this, the toughness is reduced by the embrittlement effect due to the precipitation of Ni and Al intermetallic compounds.
If the toughness is lowered, the mold is liable to be cracked and the like, leading to a reduction in the mold life.

In addition, the one disclosed in Patent Document 1 does not mention the point of enhancing the cooling performance of the mold, which is important for the high cycle (shortening of cycle time) of product molding, and particularly takes measures for that. It is not done.
Specifically, the content of Si, which plays an important role in cooling the mold, is large (in the claims, 1.5% or less, but in the examples, 0.3% is the lower limit, and less than this is disclosed) Absent).
If Si is contained in a large amount of 0.3% or more, the cooling performance of the mold after injection is insufficient, and it is difficult to realize a higher cycle than before.
In addition, the one disclosed in Patent Document 1 accelerates the lack of mold toughness due to the large amount of precipitation of Ni-Al intermetallic compounds.

  Further, the following Patent Document 2 discloses an invention relating to “a steel material for a high-strength mold excellent in machinability”, in which a C amount is 0.005 to 0.1 as a mold steel material for molding plastic products and the like. In addition, it is disclosed that the hardness is increased by the precipitation effect of Cu and the precipitation of intermetallic compounds by Ni and Al.

The one disclosed in Patent Document 2 also precipitates a large amount of Ni and Al intermetallic compounds.
Further, although Cu is also set to 3.5% or less in the claims, the lower limit in the examples is 0.77%, and the Cu amount in each of the examples is larger than this.
If the amount of Cu is large, striped segregation is likely to occur in the steel, and if such segregation occurs, the mirror polishability of the steel will be impaired.

Furthermore, even in the one disclosed in Patent Document 2, there is no mention of enhancing the cooling performance of the mold, which is important for achieving a high cycle of product molding, and no special measures are taken for this.
Specifically, even the one disclosed in Patent Document 2 contains a large amount of Si (in the claims, it is 1.5% or less, but in the examples, 0.28% is the lower limit and less than this is disclosed) Absent).

In addition, there exist some which were disclosed by following patent document 3 and patent document 4 as another prior art with respect to this invention.
Patent Document 3 relates to steel for plastic molding dies, containing Mo: 0.05 to 1.00%, V: 0.03 to 0.50%, and adding Al and Ni at the same time to age these intermetallic compounds. However, it is different from the present invention in that Cu is not added and the amount of Cr is 4.0% or less and the amount is small.

The thing disclosed in Patent Document 4 is mainly related to mold steel used for plastic molding, and contains Mo alone or in combination with W (Mo + 1 / 2W): 1.00% or less, and also contains Cr and Cu. However, it is fundamentally different from the present invention in that the C content is as high as 0.10% or more.
Furthermore, the Cr content is as low as 4.00% or less, and the Cu content is as large as 0.61% or more (in the claims, it is said to be 0.50% or more, but those disclosed in the examples are at least 0.61% and others) Are different from the present invention in that both are more than this).

JP-A-11-140591 JP 2000-297353 A JP-A-6-346185 JP-A-2-263953

  The present invention has been made for the purpose of providing a steel for a mold for molding having high heat conduction performance and excellent specular polishing properties, and excellent weather resistance, toughness, and machinability. Is.

Thus, in the present invention, the mass% is 0.040 <C <0.100, 0.03 <Si <0.27, 0.30 <Mn <1.80, 0.30 <Cu <0.61, 4.00 <Cr <9.00, and 0.04 < It contains at least one of Mo <1.00, 0.02 <V <0.50, and has a composition of the balance Fe and inevitable impurities.
In addition, normally, in the steel for forming molds, the following components and their ranges can be included as inevitable impurities.
P ≦ 0.03, S ≦ 0.003, Ni ≦ 0.30, Al ≦ 0.10, V ≦ 0.02, W ≦ 0.30, O ≦ 0.01, N ≦ 0.02, Co ≦ 0.30, Nb ≦ 0.004, Ta ≦ 0.004, Ti ≦ 0.004, Zr ≦ 0.004, B ≦ 0.0001, Ca ≦ 0.0005, Se ≦ 0.03, Te ≦ 0.005, Bi ≦ 0.01, Pb ≦ 0.03, Mg ≦ 0.02

  According to a second aspect of the present invention, the method according to the first aspect further includes at least one of 0.30 <W ≦ 4.00 and 0.30 <Co ≦ 3.00 by mass%.

  According to a third aspect of the present invention, in any one of the first and second aspects, at least one of 0.004 <Nb ≦ 0.100, 0.004 <Ta ≦ 0.100, 0.004 <Ti ≦ 0.100, 0.004 <Zr ≦ 0.100 in mass% is further added. It is characterized by containing.

  A fourth aspect of the present invention is characterized in that, in any one of the first to third aspects, 0.0001 <B ≦ 0.0050 is further contained in mass%.

  According to a fifth aspect of the present invention, in any one of the first to third aspects, the mass% further contains 0.30 <Ni ≦ 1.50.

  A sixth aspect of the present invention is the method according to any one of the first to fifth aspects, wherein the mass% is 0.003 <S ≦ 0.050, 0.0005 <Ca ≦ 0.2000, 0.03 <Se ≦ 0.50, 0.005 <Te ≦ 0.100, 0.01 <Bi ≦ 0.30. , 0.03 <Pb ≦ 0.50.

According to a seventh aspect of the present invention, in any one of the first to sixth aspects, the average hardness at room temperature is in the range of 35 to 45 HRC.
Here, the average hardness means that three randomly selected steel parts were cut out and polished, and five indentations were made in each part with a Rockwell hardness meter, and the measured values (total 15 indentations) at the three parts were averaged. Means hardness.

  An eighth aspect of the present invention is characterized in that the thermal conductivity measured by the laser flash method is 26 W / m / K or more at 100 ° C. in the seventh aspect.

  According to a ninth aspect of the present invention, in any one of the seventh and eighth aspects, the mass% is 4.60 <Mn + Cr + 0.5Ni <6.20.

  According to a tenth aspect of the present invention, in any one of the seventh to ninth aspects, the mass% is 0.19 <0.5Mo + V <0.53.

Effects and effects of the invention

The present invention is characterized in that the amount of C added is small and the hardness of steel is secured by aging precipitation of Cu while effecting secondary hardening based on the addition of Mo and V. .
In the conventional steel that uses aging precipitation to secure the hardness of the steel, it is proposed that Ni and Al are added together with Cu to age the Ni-Al intermetallic compound. In the present invention, the aging precipitation of Ni—Al intermetallic compounds is not used as a mechanism for ensuring the hardness and strengthening of steel.

  In the case of adding Ni and Al at the same time, the cost is inevitably increased, and at the same time, the toughness is reduced by the precipitation of the Ni-Al intermetallic compound. Naturally, the decrease in toughness is disadvantageous for the extension of the mold life.

Another feature of the present invention is that the mirror polishability of steel is improved by suppressing the addition of C to a small amount and suppressing the addition amounts of Cu and Mn to a low level.
Cu and Mn are elements that are easily segregated in stripes in steel. When these segregations occur, dark and thin parts such as Cu and Mn occur alternately in the steel.
In this case, since the mechanical properties such as hardness and toughness are different between the dark portion and the thin portion of Cu or Mn, stripe-like irregularities are formed in the steel when mirror polishing.

Therefore, when a metal mold is made of such steel, the striped irregularities are transferred to a molded product such as a plastic product, which greatly impairs the aesthetic appearance of the product and lowers the commercial value.
In the present invention, the upper limit of these Cu and Mn is restricted to prevent such problems from occurring.
Cu also reduces toughness with increasing amounts.
In the present invention, high toughness is ensured by suppressing the upper limit of the amount of Cu added.

Another feature of the present invention is that the amount of Si added is low, and this makes it a great feature that the heat conductivity of the steel is ensured to be high.
When a mold for injection molding is constructed using steel with high thermal conductivity (high thermal conductivity), the cooling performance of the mold is improved, and the heat drawing of the mold during injection molding is improved, so that molding is performed. The time per cycle can be shortened. That is, product molding by injection molding can be made high cycle, and productivity can be increased.

Another feature of the present invention is that it has excellent corrosion resistance and weather resistance by lowering the amount of C and adding a large amount of over 4.0% of Cr in addition to enhancing the mirror polishability of steel. Yes.
Even if steel is excellent in mirror polishability, if it is left for a while after polishing, corrosion pits will occur on the polished mirror surface. Even if it is mirror finished, it will be polished again afterwards. Will have to do.
Accordingly, steels that require mirror polishability are required to have high weather resistance and corrosion resistance that do not easily rust even if the polished surface is left for a while.

  Others The present invention has another feature in that the machinability of steel is enhanced by adding a certain amount of Si or more by adding Cu.

  The steel of the present invention is particularly suitable as a mold material for molding a plastic product. However, as a mold material other than a plastic mold, for example, a mold material for manufacturing (molding) a rubber product. Also suitable.

Next, the reasons for limiting the chemical components of the present invention will be described in detail below.
[C]: 0.040 <C <0.100
When C ≦ 0.040, it is difficult to obtain a hardness of 35 HRC or more necessary for ensuring high specularity particularly when the tempering temperature is high. When 0.100 ≦ C, the corrosion resistance is lowered and the weldability is also poor. Further, when 0.100 ≦ C, the amount of carbides increases, resulting in a decrease in mirror polishability. A preferable range is 0.055 <C <0.095, which is excellent in balance among hardness, specularity, corrosion resistance, and weldability.

  Fig. 1 shows that eight types of 0.22Si-1.10Mn-0.50Cu-4.05Cr-0.55Mo-0.12V-C square bars with different C contents were exposed for 72 hours in a humid environment at 50 ° C and 98% humidity. It shows the amount of rust generated later. The square bar used was obtained by soaking at 900 ° C. for 3 hours, quenching, tempering at 450 to 560 ° C. for 5 hours and tempering to 38 to 42 HRC, and then machining. For the sake of adjusting the hardness, the tempering temperature was increased for the higher C steel. The area ratio of rust was obtained by image processing.

  Specifically, the color image of the rusted square bar surface using the image analysis software (here, “WinROOF” manufactured by Mitani Corporation) is used, the metallic luster is white, rust is generated The binarization conversion was performed so that the existing area became a black area, and the area ratio of the black area in the entire image was obtained to obtain the area ratio of rust.

  If the corrosion resistance is low, a corroded part is likely to occur on the surface of the mirror-polished mold, and such a mold must be re-polished. Re-polishing is not preferred due to cost and delivery time issues. Accordingly, the mold is required to have good corrosion resistance.

  As shown in FIG. 1, the amount of rust generated increases as the amount of C increases. That is, the corrosion resistance is reduced. This is because the amount of Cr that forms carbides increases, and as a result, the amount of Cr that contributes to corrosion resistance decreases. Therefore, it is preferable that the amount of C is small. The amount of rust changes drastically when the C content is 0.1%, and the corrosion resistance changes greatly at this point. Therefore, in the present invention, C <0.10.

  In addition, although the basic composition of 8 steel types is Fe-0.22Si-1.10Mn-0.51Cu-4.05Cr-0.55Mo-0.12V, it cannot be made exactly the same in manufacturing. For this reason, a difference is generated in the basic components within an unavoidable range. However, the basic component was controlled to be almost equal to the target value (for example, Si was in the range of 0.21 to 0.23 with respect to the target 0.22, Mn was controlled to 1.08 to 1.12 with respect to the target 1.10, etc.). Therefore, only the influence of C can be evaluated by comparing these eight steel types.

[Si]: 0.03 <Si <0.27
When Si ≦ 0.03, machinability is significantly deteriorated. When 0.27 ≦ Si, the decrease in thermal conductivity is large. A preferable range is 0.05 <Si <0.26, which is excellent in the balance between machinability and thermal conductivity.
Figure 2 and Table 1 show that 18 kinds of 0.071C-1.10Mn-0.48Cu-4.02Cr-0.53Mo-0.12V-Si steels with different Si contents were soaked at 900 ° C for 3Hr, quenched and tempered at 510 ° C for 5Hr. The machinability after annealing is shown with respect to the Si content.

The material for machinability evaluation is a square bar with a hardness of 39 to 42 HRC and a shape of 55 mm x 55 mm x 200 mm. The life (machinability) is defined when the maximum wear amount of the side flank of the cutting tool reaches 300 μm. Judged. The larger the cutting distance in FIG.
When Si ≦ 0.03, the cutting distance is extremely small. 0.03 <Si is necessary to stably suppress the wear of the cutting tool. If 0.05 <Si, wear can be more stably suppressed.

  In addition, although the basic composition of 18 steel types is Fe-0.071C-1.10Mn-0.48Cu-4.02Cr-0.53Mo-0.12V, it cannot be made exactly the same in manufacturing. For this reason, a difference is generated in the basic components within an unavoidable range. However, as in the case of the steel of FIG. 1, other elements such as Mn were almost equal to the target basic component. Therefore, only the influence of Si could be evaluated by comparing these 18 steel types.

FIG. 3 and Table 2 show the thermal conductivity at 100 ° C. with respect to the amount of Si after 54 types of invention steels were soaked at 900 ° C. for 3 Hr, quenched, and tempered at 510 ° C. for 5 Hr. Steel is the following three groups.
Steel S1 Group: 0.072C-1.10Mn-0.47Cu-0.52Mo-0.11V --- 4.05Cr --- Si Steel Steel S2 Group: 0.072C-1.10Mn-0.47Cu-0.52Mo-0.11V --- 5.21 Cr --- Si steel S3 group: 0.072C-1.10Mn-0.47Cu-0.52Mo-0.11V --- 7.98Cr --- Si steel Cr is an important element that affects corrosion resistance. It was evaluated together with the influence of Si.

The material for thermal conductivity evaluation was a small disk having a hardness of 39 to 42 HRC and a shape of φ10 mm × 2 mm. The measurement was performed by a laser flash method at 100 ° C. That is, the laser beam emitted from the laser oscillator is irradiated at right angles to the test piece, and at that time, the amount of heat radiated from the back of the test piece is measured with an infrared detector to determine the specific heat and the thermal diffusivity. The thermal conductivity (= specific heat × thermal diffusivity × density) was calculated.
Higher thermal conductivity is preferable because of excellent cooling ability when it becomes a mold. Although the thermal conductivity differs depending on the Cr level, the tendency to increase the thermal conductivity with the decrease of Si is the same.

In any steel type, Si amount of 0.27 can be regarded as an inflection point, and the thermal conductivity increases rapidly when Si <0.27. That is, Si <0.27 is necessary to keep the thermal conductivity of the component system high. If Si <0.26, high thermal conductivity can be obtained more stably. When Si ≦ 0.05, the thermal conductivity tends to be saturated.
In addition, although the element amount of each steel type does not correspond with a basic component strictly, it is the same as the case of FIGS. Therefore, only the influence of Si can be evaluated by comparing within each group (18 steel types), and only the influence of Cr can be evaluated by comparing between groups.

  A criterion for determining whether or not the cooling ability when the mold is formed is that the thermal conductivity of the steel at 100 ° C. is 26 W / m / K or more. In the present invention, a steel material having a thermal conductivity of 28 W / m / K or more at 100 ° C. is preferable, but the cooling ability is considerably large even if the thermal conductivity is 26 W / m / K or more. Although the thermal conductivity is low in a component system that is highly alloyed due to problems such as strength and corrosion resistance, it is still preferable to have a thermal conductivity of 26 W / m / K or more at 100 ° C.

  In resin injection molding, which is an application field of the present invention, there is a strong need for productivity improvement. For that purpose, it is necessary to shorten the solidification time of one product. That is, the mold must be cooled quickly. Therefore, the cooling circuit inside the mold has been optimized. However, the cooling hole may not be installed due to the problem of the mold structure. Also, if the cooling hole is too close to the mold surface, it will contribute to the early cracking of the mold.

  On the other hand, there is an attempt to dramatically improve the cooling ability of the mold by providing a cooling circuit at a place where a cooling hole cannot be conventionally installed by laminating (sintering or joining) powder and plates. However, special equipment is required for manufacturing, which is expensive. Also, if the cooling hole is too close to the mold surface, it will contribute to the early cracking of the mold.

In this invention, the above subject can be solved and a metal mold | die can be cooled efficiently. That is, by increasing the thermal conductivity of the mold, a sufficient cooling effect can be obtained without making the cooling holes extremely close to the mold surface. For this reason, the problem of early cracking of the mold is unlikely to occur. In addition, no special equipment is required for mold manufacture, and it is possible to manufacture a mold in the same process as before. Thus, the balance between the thermal conductivity and other characteristics is a major feature of the present invention.
Needless to say, if the steel of the present invention is applied to a method of “manufacturing a metal mold by laminating (sintering or joining) powders and plates”, an even greater cooling effect can be obtained.

[Mn]: 0.30 <Mn <1.80
When Mn ≦ 0.30, the hardenability is insufficient and the hardness and toughness are insufficient. When 1.80 ≦ Mn, the thermal conductivity is remarkably lowered. Further, segregation becomes significant when 1.80 ≦ Mn. Mn tends to segregate during solidification, and significant segregation has an adverse effect on the mirror polishability when it becomes a mold. The reason for this is that the mechanical properties such as hardness and toughness are different between the thick and thin parts of Mn, so the way of shaving when mirror polishing is different between the concentrated part and the diluted part of Mn, and as a result, it corresponds to segregation. This is because a striped pattern (unevenness) is formed on the mold surface.
When the unevenness is transferred to the product, the surface quality of the product is greatly deteriorated and the commercial value is lost. That is, a good product cannot be produced with a mold having unevenness due to segregation on the surface.
A preferable range is 0.65 <Mn <1.50, which is excellent in balance between hardenability, thermal conductivity, and mirror polishing.

[Cu]: 0.30 <Cu <0.61
Cu is aged in steel by heating at 400 to 600 ° C., increases the strength of the steel, improves the machinability, and lowers the impact value. When Cu ≦ 0.30, the effect of increasing the strength due to aging precipitation of Cu is small. When 0.61 ≦ Cu, cracking during hot working is likely to occur. In addition, when 0.61 ≦ Cu, the impact value decreases significantly. A preferable range is 0.40 <Cu <0.55 which is excellent in balance between high strength, hot workability and impact value.

  Fig. 4 shows the results after 15 kinds of 0.070C-0.22Si-1.10Mn-4.02Cr-0.43Mo-0.09V-Cu steel with different Cu contents were soaked at 900 ° C for 3 hours, quenched and tempered at 550 ° C for 5 hours. HRC hardness is shown with respect to Cu content. In order to obtain a hardness of 35 HRC or higher necessary for ensuring high specularity, 0.30 <Cu is necessary. If 0.40 <Cu, the hardness can be obtained more stably.

  FIG. 5 shows the impact value evaluated by conducting a Charpy impact test using a JIS No. 3 impact test piece after quenching the steel of the same invention as FIG. 4 for 3 Hr soaking at 900 ° C. and then tempering at 500 ° C. for 5 Hr. Shows against quantity. A higher impact value is desirable because large cracks are less likely to occur when a mold is formed. Steels containing 1% or more of Cu already exist, but in these steels, a reduction in impact value becomes a problem. Similarly, in this steel to which Cu is added, low Cu was investigated to solve this problem. The impact value rises due to the decrease of Cu, and the impact value increases dramatically at the inflection point around the Cu amount of about 0.6 in the region where the Cu amount is smaller than this. In the present invention, Cu <0.61 was determined to be an appropriate range of Cu addition amount. If Cu is less than 0.55, a high impact value can be obtained more stably.

  The basic component of 15 steel types is Fe-0.070C-0.22Si-1.10Mn-4.02Cr-0.43Mo-0.09V, but it cannot be made exactly the same in manufacturing. For this reason, a difference is generated in the basic components within an unavoidable range. However, as in the case of FIGS. 1 to 3, other elements such as Si and Mn were almost equal to the target basic component. Therefore, only the influence of Cu could be evaluated by comparing these 15 steel types.

  Low C steel is difficult to obtain strength when tempering temperature is high. This is because the degree of strengthening due to secondary precipitation of carbide is small. Cu aging precipitation is an effective means for securing the strength of low C steel. In particular, the addition of Cu is effective for securing the strength when the tempering temperature must be increased due to problems in subsequent processes such as surface treatment.

  Existing steels that utilize Cu aging precipitation often contain 1-3% Cu. In the present invention, since a large amount of Cu is not used in this way, it is difficult to achieve a hardness exceeding 45 HRC, but it is possible to obtain 35 HRC or more necessary for ensuring high mirror surface properties. Further, steel containing 1 to 3% of Cu has a remarkable decrease in toughness, but the present invention secures toughness by making Cu less than 0.61%.

  In order to secure the strength of the low C steel, it is also effective to precipitate an intermetallic compound composed of Ni and Al. For this reason, there are Ni-Al based steels (Cu is an impurity level) and Cu-Ni-Al based steels. Such steels often contain 2-3% Ni and 1-2% Al. The present invention differs from existing Cu-Ni-Al steels in that it does not contain a large amount of both Ni and Al.

[Cr]: 4.00 <Cr <9.00
When Cr ≦ 4.00, the effect of improving the corrosion resistance is small. In 9.00 ≦ Cr, the decrease in thermal conductivity is significant. A preferable range is 4.03 <Cr <6.00, which is excellent in the balance between corrosion resistance and thermal conductivity. When corrosion resistance is important, the thermal conductivity is slightly lowered, but 6.00 ≦ Cr <8.60 is preferable.

Fig. 6 shows the results of rusting after exposure of eight types of 0.08C-0.22Si-1.10Mn-0.49Cu-0.47Mo-0.12V-Cr steel bar to 72Hr in a humid environment of 98% humidity at 50 ° C. The amount generated is shown. The square bar was soaked after heating at 900 ° C. for 3 hours, tempered at 510 ° C. for 5 hours and tempered to 38 to 42 HRC, and then machined. The area ratio of rust was determined by image processing according to the same method as described above.
If the corrosion resistance is low, a corroded part is likely to occur on the surface of the mirror-polished mold, and such a mold must be re-polished. Re-polishing is not preferred due to cost and delivery time issues. Therefore, the mold is required to have good corrosion resistance.

  As the amount of Cr decreases, the amount of rust generated increases, that is, the corrosion resistance decreases. Therefore, it is preferable that the amount of Cr is large. The inflection point at which the amount of rust changes abruptly is around 4% of the Cr content, and it can be seen that the corrosion resistance changes greatly at this point. From the above, 4.00 <Cr was judged to be a preferable range.

  In addition, although the basic composition of 8 steel types is Fe-0.08C-0.22Si-1.10Mn-0.49Cu-0.47Mo-0.12V, it cannot be made exactly the same in manufacturing. For this reason, a difference is generated in the basic components within an unavoidable range. However, as in the case of FIGS. 1 to 5, other elements such as Si and Mn were almost equal to the target basic component. Therefore, only the influence of Cr could be evaluated by comparing these 8 steel types.

[Mo]: 0.04 <Mo <1.00
When Mo ≦ 0.04, it is difficult to obtain a required hardness of 35 HRC or more, particularly when the tempering temperature is high. When 1.0 ≦ Mo, the fracture toughness value decreases significantly. A preferable range is 0.10 <Mo <0.90 which is excellent in balance between hardness and fracture toughness value.

[V]: 0.02 <V <0.50
When V ≦ 0.02, it is difficult to obtain a hardness of 35 HRC or more necessary for ensuring high specularity, particularly when the tempering temperature is high. When 0.50 ≦ V, the impact value and mechanical fatigue strength are significantly reduced. The reason for this is that the number of carbides, nitrides, and carbonitrides that are the starting points for cracks increase. In addition, these foreign substances not only become the starting point of cracking, but may fall off during mirror polishing to form holes (pinholes). A preferable range is 0.05 <V <0.40, which is excellent in the balance of hardness, impact value, mechanical fatigue strength, and mirror polishability.

[Chemical component of claim 2]
Since the steel of the present invention is low C, it is difficult to ensure strength depending on the tempering temperature. In such a case, W or Co may be selectively added to maintain the strength. W increases the strength by precipitation of carbides. Co increases strength by solid solution in the base material, and at the same time contributes to precipitation hardening through changes in carbide morphology. In particular,
0.30 <W ≦ 4.00
0.30 <Co ≦ 3.00
It is sufficient to contain at least one element.
If any element exceeds a predetermined amount, saturation of characteristics and significant cost increase are caused. Preferred ranges are 0.40 ≦ W ≦ 3.00 and 0.40 ≦ Co ≦ 2.00.

[Chemical component of claim 3]
In the steel of the present invention, there are not so many dispersed particles that suppress the growth of austenite crystal grains during quenching. For this reason, if the quenching heating temperature becomes high or the quenching heating time becomes long due to unexpected equipment troubles, there is a concern about deterioration of various characteristics due to coarsening of crystal grains. In preparation for such a case, it is possible to selectively add Nb, Ta, Ti, and Zr, and to suppress the coarsening of the austenite crystal grains with fine precipitates formed by these elements. In particular,
0.004 <Nb ≦ 0.100
0.004 <Ta ≦ 0.100
0.004 <Ti ≦ 0.100
0.004 <Zr ≦ 0.100
It is sufficient to contain at least one element.
When any element exceeds a predetermined amount, carbides, nitrides, and oxides are excessively generated, and the impact value and the mirror polishability are lowered.

[Chemical component of claim 4]
In recent years, the size of molds tends to increase as parts become larger and integrated. Large molds are difficult to cool. For this reason, when quenching a large metal mold of steel with low hardenability, ferrite, pearlite, and coarse bainite are precipitated during quenching, and various properties deteriorate.
The steel of the present invention has a considerably high hardenability, and there are few such concerns. However, even when a very large mold is processed by a quenching method with low cooling strength, B can be added to further improve the hardenability. In particular,
0.0001 <B ≦ 0.0050
Containing.
Note that when B forms BN, the effect of improving the hardenability is lost, and therefore B needs to be present alone in the steel. Specifically, it is sufficient that nitride is formed with an element having an affinity for N stronger than B, and B and N are not bonded. An example of such an element includes claim 3. Even if the element of claim 3 exists at the impurity level, it has an effect of fixing N, but depending on the amount of N, the range of claim 3 may be added.

[Chemical component of claim 5]
Addition of Ni is also effective as a means for improving hardenability and low temperature toughness. In the steel of claim 4, the formed nitride may fall off during the polishing of the mold to form a hole (pinhole), which greatly deteriorates the surface quality of the mold and consequently the surface quality of the product. Cause. On the other hand, when Ni is added, no nitride is formed, which is suitable for suppressing pinholes and improving hardenability. In particular,
0.30 <Ni ≦ 1.50
Containing.
When Ni exceeds a predetermined amount, the cost increases and the thermal conductivity decreases. Ni is easily segregated at the time of solidification, and the remarkable segregation becomes a striped pattern on the surface when the mold is polished, thereby deteriorating the mirror polishability. This is the same as Mn described above. A preferable range is 0.40 <Ni <1.20, which is excellent in balance between hardenability, thermal conductivity, and mirror polishing.

[Chemical component of claim 6]
The steel of the present invention has lower Si than steel with very good machinability (0.4 <Si). For this reason, there is a concern that machining and drilling into the mold shape will be difficult. In such a case, S, Ca, Se, Te, Bi, and Pb may be selectively added to improve machinability. In particular,
0.003 <S ≦ 0.050
0.0005 <Ca ≦ 0.2000
0.03 <Se ≦ 0.50
0.005 <Te ≦ 0.100
0.01 <Bi ≦ 0.30
0.03 <Pb ≦ 0.50
It is sufficient to contain at least one element.
If any element exceeds a predetermined amount, machinability is saturated, hot workability (when a mold material is manufactured), impact value and mirror surface polishability are reduced.

[Chemical component of claim 9]
In the present invention, the composition having the minimum Mn + Cr + 0.5Ni is Mn + Cr + 0.5Ni = 4.31, but if hardenability is particularly required, 4.60 <Mn + Cr + 0.5Ni. This can further reduce the risk of precipitation of ferrite, pearlite, and coarse bainite during quenching cooling.
In the present invention, Mn + Cr + 0.5Ni = 11.53 when the composition has the maximum Mn + Cr + 0.5Ni, but Mn + Cr + 0.5Ni <6.20 if thermal conductivity is particularly required. A particularly excellent range for the balance between hardenability and thermal conductivity is 4.60 <Mn + Cr + 0.5Ni <6.20. A more preferable range is 4.80 <Mn + Cr + 0.5Ni <6.05. If it is this range, an appropriate hardening structure can be obtained stably and the heat conductivity in 100 degreeC will be 28 W / m / K or more.

FIG. 7 shows the impact value (2 mmU notch) of the Charpy impact test at room temperature in a state where 17 types of invention steel were soaked at 900 ° C. for 3 Hr soaking and tempered at 520 ° C. for 5 Hr to 39 to 41 HRC. It shows against 0.5Ni amount. All 17 steel types are as follows.
Steel L: 0.071C-0.20Si-0.48Cu-0.51Mo-0.12V --- 4.01Cr-0.31Mn-0.02Ni Steel Steel M: 0.068C-0.21Si-0.50Cu-0.47Mo-0.11V --- 8.99 Cr-1.79Mn-1.50Ni steel R1 group: 0.068C-0.22Si-0.47Cu-0.46Mo-0.13V --- 4.05Cr-Mn-Ni steel R2 group: 0.070C-0.24Si-0.54Cu-0.52 Mo-0.10V --- 5.50Cr-Mn-Ni steel R3 group: 0.068C-0.20Si-0.50Cu-0.55Mo-0.09V --- 8.20Cr-Mn-Ni steel Steel L has minimum Mn + Cr + 0.5Ni The steel M has a composition that maximizes Mn + Cr + 0.5Ni. Further, the steel R1 to R3 groups are made of 15 steel types in which Mn and Ni are arbitrarily added within the prescribed range of claim 9. The components of each group 5 steel grade did not exactly match the target basic component values, but were almost the same as the basic components. This is the same as in the case of the eight types of steel shown in FIG.

Here, quenching of the impact test piece is a process simulating a large cross-section type. That is, the cooling rate was 15 ° C./min from 900 ° C. to 600 ° C., and 3 ° C./min from 600 ° C. to room temperature. A material that can obtain a high impact value even with such slow quenching is excellent in hardenability and can be used safely in large molds.
FIG. 7 shows that steel L has a relatively high impact value of 22 J / cm ^2, and it can be seen that the component system of the inventive steel is excellent in hardenability. There are many steels of 15 J / cm ² or less among commercially available materials. Here, when attention is paid to 4.60 <Mn + Cr + 0.5Ni, it can be regarded as approximately 25 J / cm ² or more, and it is clear that the hardenability is a particularly good region. When 4.80 <Mn + Cr + 0.5Ni, a high impact value can be obtained more stably.

FIG. 8 shows the relationship between thermal conductivity at 100 ° C. and Mn + Cr + 0.5Ni. There are 17 steel types as in FIG. In general, the thermal conductivity decreases with an increase in alloying elements. Steel M is also relatively high at 24.6 W / m / K, and it can be seen that the component system of the inventive steel is excellent in thermal conductivity. There are many steels of 24 W / m / K or less among commercially available materials. Here, when attention is paid to Mn + Cr + 0.5Ni <6.20, it can be considered that it is approximately 28 W / m / K or more, and it is clear that it is a region having a particularly high thermal conductivity.
When Mn + Cr + 0.5Ni <6.05, high thermal conductivity can be obtained more stably.
On the other hand, depending on needs such as corrosion resistance and nitriding, 4.50 <Cr and 6.20 ≦ Mn + Cr + 0.5Ni are acceptable. In that case, the hardenability can be said to be somewhat excessive, but the impact value is high, and even larger molds can be hardened with peace of mind. However, the thermal conductivity is lower than that of steel with Mn + Cr + 0.5Ni <6.20. Nevertheless, if the thermal conductivity is 26 W / m / K or more at 100 ° C., the cooling ability as a mold is sufficiently large. That is, for 6.20 ≦ Mn + Cr + 0.5Ni, a component system having a thermal conductivity of 26 W / m / K or more at 100 ° C. may be selected.

[Chemical component of claim 10]
In the present invention, 0.5Mo + V = 0.06 is obtained for a composition having a minimum of 0.5Mo + V, but in order to obtain a stable hardness, 0.19 <0.5Mo + V. This makes it easier to obtain a hardness of 35 HRC or higher. In the present invention, 0.5Mo + V = 0.99 for the composition having the maximum of 0.5Mo + V, but when the fracture toughness value, impact value and mechanical fatigue strength are particularly required, 0.5Mo + V <0.53. To do. A range particularly excellent in the balance of the above characteristics is 0.19 <0.5Mo + V <0.53. A more preferable range is 0.24 <0.5Mo + V <0.48. If it is this range, the hardness more than 35HRC can be obtained stably, and there will be no remarkable fall of fracture toughness value, impact value, and mechanical fatigue strength.
On the other hand, there is a case where tempering at a high temperature is unavoidable due to a subsequent process such as nitriding. In such a case, 0.53 ≦ 0.5 Mo + V may be satisfied.

FIG. 9 shows the HRC hardness at room temperature after quenching the invention steel at 900 ° C. for 3 Hr and then tempering at 535 ° C. for 5 Hr with respect to the amount of 0.5 Mo + V. The material is
Steel L2: 0.069C-0.20Si-1.11Mn-0.46Cu-4.04Cr --- 0.05Mo-0.03V Steel M2: 0.073C-0.24Si-1.15Mn-0.50Cu-4.11Cr --- 0.99Mo-0.49 Steel V Group V: 0.066C-0.24Si-1.13Mn-0.53Cu-4.06Cr --- 0.14Mo-V Steel Group V2: 0.070C-0.21Si-1.13Mn-0.48Cu-4.09Cr--0.38 Mo-V steel V3 group: 0.073C-0.20Si-1.15Mn-0.47Cu-4.03Cr --- 0.64 Mo-V steel V4 group: 0.069C-0.23Si-1.12Mn-0.51Cu-4.06Cr-- -All 22 types of 0.88 Mo-V steel. Steel L2 has a composition that minimizes 0.5Mo + V, and steel M2 has a composition that maximizes 0.5Mo + V. Further, the steel V1 to V4 group is composed of 20 steel types in which V is arbitrarily added within the specified range of claim 10.
FIG. 9 shows that steel L2 also exceeds 35 HRC, and that the component system of the inventive steel can stably obtain the hardness required for the mold. Here, when focusing on 0.19 <0.5 Mo + V, it can be regarded as a region beyond the inflection point of hardness increase, and it is clear that this is a desirable region. When 0.24 <0.5 Mo + V, hardness (approximately 36 HRC or more) can be obtained more stably.

FIG. 10 shows the relationship between the impact value and Mo + V. There are 22 types of steel as in FIG. Compared to FIGS. 7-9, the correlation is not simple. This is because the influence of refinement of the structure, matrix embrittlement, and crystallized substances is superimposed. When increasing the amount of Mo, the structure becomes finer up to a certain addition amount, so the impact value is increased. On the other hand, since the increase in solid volume causes embrittlement of the matrix, the impact value decreases with the excessive addition of Mo. When V is added, the crystal grains become finer up to a certain addition amount, so that the impact value is increased. When V is excessively added, coarse crystallized substances mainly composed of V, C, and N are formed during solidification in the production of the steel ingot, and this is the starting point, so the impact value is lowered. Furthermore, in steels with excessive V, VC precipitates at the γ grain boundaries during quenching cooling, which also contributes to a reduction in impact value.
However, depending on the amount of Mo and V, although the impact value is lowered, both levels exceed 25 J / cm ² . There are many steels of 15 J / cm ₂ or less among commercially available materials. It can also be seen that the inventive steel is stable and high toughness.
Although it can be considered that the impact value of the invention steel is stable in the range of 26 to 32 J / cm ² , when attention is paid to Mo + V <0.53, it can be regarded as a region where the impact value is stable. This tendency becomes more remarkable at Mo + V <0.48.

  By the way, an injection mold for plastic products is provided with many water cooling holes for the purpose of shortening the production time (high cycle). The inside of the water cooling hole is in a corrosive environment caused by water, and in addition, tensile stress acts. The sources of tensile stress are thermal stress when resin is injected, and mechanical stress due to mold clamping during mold clamping and injection.

As described above, when tensile stress continues to act in a corrosive environment, a crack is generated starting from the corroded portion and progresses toward the design surface (molded surface). When the crack reaches the design surface, water leaks and resin injection molding cannot be performed.
This phenomenon is called water-cooled hole cracking. Molds that have cracked water-cooled holes will be replaced, leading to increased mold costs and reduced productivity. That is, water-cooled hole cracking is a serious trouble and must be avoided.

  For the above reasons, it is important to evaluate the sensitivity of water-cooled hole cracking in mold steel. Below, the test which simulated water-cooled hole crack is demonstrated.

FIG. 11 shows a method for testing whether a steel material is easily cracked when a tensile stress is applied in a corrosive environment. The test piece has a cylindrical shape with a diameter of 6 mm, and is provided with a notch near the center. The diameter of the notch is 4 mm.
The material is the same alloy component as invention steel 1, comparative steel 2, and comparative steel 3 described later.
After supporting the test piece in a cantilever state, a bending force is applied to the test piece by hanging a weight on the end side opposite to the fixed side. At this time, tensile stress always acts on the upper side of the notch portion. In this state, water continues to be dripped onto the notch.
As described above, a situation in which tensile stress acts in a corrosive environment with water is created. This simulates the water-cooled hole cracking of the mold.

  In this test method, the time from when the weight is suspended until the test piece breaks is evaluated. It can be determined that the longer the time to break, the better the mold material that is less likely to cause water-cooled hole cracking.

  In the experiment, test pieces are set one by one on five test machines, and the evaluation of the same steel type is performed in parallel on all five machines. Then, the time at which one of the five pieces breaks is recorded as “breaking time”, and the test is terminated (even if the remaining four pieces are not broken).

  FIG. 12 shows the rupture time when 44 [N] is loaded on three steel grades tempered to 39HRC. Invention steel 1 has a rupture time approximately 1.3 times that of comparative steel 2 and 200 times or more that of comparative steel 3. That is, it can be determined that the inventive steel 1 is an excellent mold material that is difficult to break when subjected to tensile stress in a corrosive environment, and therefore hardly causes water-cooled hole cracking.

  As described above, the steel of the present invention has a feature that hardly causes water-cooled hole cracking. This is due to the fact that the corrosion resistance is high and the corroded portion is difficult to occur, and the toughness is high and the crack is difficult to progress rapidly. In addition, the steel material becomes brittle and breakage is promoted by hydrogen that penetrates from water, but the type and amount of precipitates (carbide, MnS, aging-precipitated Cu, etc.) that trap and detoxify this hydrogen are appropriate. This is also a major reason why the steel of the present invention is difficult to break.

Further, the steel of the present invention is characterized in that it is easy to adjust the heat treatment hardness and is difficult to deviate from strict hardness standards. Here, a case where a hardness standard in a narrow range of 37 to 39 HRC is required will be described as an example.
The material is the same alloy component as invention steel 1 and comparative steel 1 described later.

  FIG. 13 shows the change in hardness of the invention steel 1 and the comparative steel 1 with respect to the tempering temperature. In the comparative steel 1, in order to satisfy the 37-39 HRC standard, it is necessary to soak the steel material in a range of 10 ° C of 560-570 ° C. Therefore, the tempering condition to be set is 565 ° C. × 5 Hr.

  Generally, the heat treatment furnace has a temperature fluctuation of 5 to 15 ° C. during soaking. Further, even when the temperature fluctuation during soaking is very small, a temperature difference of 5 to 15 ° C. depending on the position in the furnace occurs. When the temperature difference between the two is added, a temperature difference of about 30 ° C. at maximum is generated.

Therefore, even if the comparative steel 1 aims at soaking at 565 ° C., it is actually heated to 550 to 580 ° C. This heating condition corresponds to a condition giving 36 to 41 HRC as seen in FIG. That is, it is very difficult to temper the comparative steel 1 in a narrow range of 37 to 39 HRC, and the hardness in the cross section becomes 36 to 41 HRC.
If the hardness varies depending on the part of the steel material, it is not preferable because the machinability and the specular polishing are deteriorated.

  On the other hand, the invention steel 1 may be heated to a temperature range of 551 ° C. or lower in order to satisfy the 37-39 HRC standard. The tempering conditions to be set are, for example, 530 ° C. × 5 Hr. Invented steel is actually heated to 515 to 545 ° C. due to the aforementioned furnace temperature variation problem, but still a hardness of approximately 38 HRC is obtained.

  Thus, the steel of the present invention has a characteristic that the hardness can be easily managed in a narrow range. By adjusting the balance of C-Cr-Mo-V-Cu, the amount of carbide containing Cr, Mo and V, and the amount of precipitated Cu are optimized, and the change in hardness with respect to the tempering temperature is moderated. It is an effect.

It is the figure which showed the relationship between the generation | occurrence | production state of rust, and C amount. It is the figure which showed the relationship between tool wear amount and Si amount. It is the figure which showed the relationship between thermal conductivity and Si amount. It is the figure which showed the relationship between tempering hardness and Cu amount. It is the figure which showed the relationship between an impact value and Cu amount. It is the figure which showed the relationship between the generation | occurrence | production state of rust, and the amount of Cr. It is the figure which showed the relationship between an impact value and Mn + Cr + 0.5Ni. It is the figure which showed the relationship between thermal conductivity and Mn + Cr + 0.5Ni. It is the figure which showed the relationship between HRC hardness and 0.5Mo + V. It is the figure which showed the relationship between an impact value and 0.5Mo + V. It is explanatory drawing explaining the method of the test which simulated the water cooling hole crack. It is the figure which showed the result of the test which simulated water-cooled hole crack. It is the figure which showed the relationship between tempering hardness and tempering temperature.

Forty types of steel shown in Table 3 (the blanks in Table 3 indicate that the chemical component is at the impurity level) were dissolved in the atmosphere and cast into 7 ton ingots. Each steel ingot was forged into a block shape of 210 × 1020 × 3500 (mm) at a surface temperature of 900 to 1250 ° C. after homogenized heat-treated steel at 1200 to 1300 ° C.
This block was reheated to 900 ° C., held for 3 hours, then immersed in oil at 40 to 100 ° C. and quenched. Furthermore, the hardness was tempered to 35 to 43 HRC by maintaining 5 hours in a temperature range of 350 to 560 ° C.
The materials cut from the block center after tempering were evaluated for machinability, impact value, thermal conductivity, specularity, weldability, corrosion resistance, sensitivity to water-cooled hole cracking, and hardness variations. The production cost was also evaluated.

The thermal conductivity is a value measured at 100 ° C. by a laser flash method. The larger this number, the better the cooling ability when it becomes a mold.
Mirror surface polishability is the upper limit count that does not cause defects (unery, spider, pinhole, etc.) on the surface when the steel material is polished by changing the count of the abrasive. Larger numbers indicate that the abrasive grains are smaller and can be polished more finely, and are preferred because they can be used for higher quality molds.
Other properties are important because they are not as good as thermal conductivity and mirror polish, but are related to mold manufacturability, maintainability and cost. These are indicated by relative comparison symbols. The evaluation decreases as ◎ → ○ → △ → x.

  The machinability was judged based on the state of wear of the cutting tool when the cutting distance and the feed rate were common and the cutting distance was 1000 mm. If the wear amount of the cutting tool is small (≦ 150 μm) and steady wear, ◎, if the steady wear amount is large (<300 μm), ○, if the wear amount further increases (≧ 300 μm) and abnormal wear is also seen In addition to abnormal wear, the case where chipping occurred was marked as x.

The impact value was judged by the value at room temperature using a 2 mm U notch test piece (JIS No. 3). That is, the impact value is 40 J / cm ² or more ◎, less than 30-40 less than J / cm ² ○, 20-30 less than J / cm ² △, was × less than 20 J / cm ^2.

  Weldability was judged from the results of multi-layering with an appropriate welding rod corresponding to the amount of C, cutting the welded portion, and examining the hardness distribution and cracks. That is, if there is no crack and there is no part where the hardness is remarkably reduced, ◎, if there is no crack but there is a region with reduced hardness, ○, if there is no crack but there is a region with greatly reduced hardness, Δ, The case where cracking occurred was marked as x.

Corrosion resistance (weather resistance) was judged from the rust when a mirror-polished material was left for 1 month in a coastal and rainy environment. In other words, ◎ if there is almost no rust or few spot-like corroded parts, ◯ if the spot-like corroded parts are conspicuous, △ if the corroded parts are connected and the rust spreads over a wide area, rust area further The case where it spreads and there are few metal gloss parts was set as x.
The results are shown in Table 4.

  The sensitivity of water-cooled hole cracking was evaluated by the test method described above. All materials were tested with a load of 90% of the bending break strength. The breaking time in this case was evaluated as the sensitivity of water-cooled hole cracking.

Hardness variation is the difference between the maximum value and the minimum value of the HRC hardness at five locations on the surface of the block material (near the four corners and at the center).
The hardness of each part of the block material does not become the same due to the influence of the temperature variation in the furnace. Here, the hardness variation was evaluated as an index of the “ease of adjusting the hardness” described above. As the hardness variation is smaller, it means that the hardness falls within a narrow range even if the furnace temperature fluctuates, and the steel material is easy to adjust the hardness.

First, the invention steel will be described. What should be noted is the high thermal conductivity, which is stably 26 W / m / K or more. In particular, 28 W / m / K or more is secured except for steel 23 to steel 26. In other words, it is difficult for the mold to cool down.
In addition, the mirror polishability also clears the count of 8000 or more, and can be used for a mold having a high surface quality level.
The other characteristics are qualitative evaluation by symbols, but the invention steel has no “x”, and it is obvious that the various characteristics are well balanced. There are some “Δ” in machinability, impact value, and cost, but there is no problem in view of balance with other characteristics.
That is, the present invention is a steel that has high thermal conductivity and high specular polishing properties as well as excellent other characteristics and cost performance. Also, the average hardness at room temperature is in the range of 35 to 45 HRC.

Furthermore, all the inventive steels have a fracture time exceeding 80 Hr in a test simulating water-cooled hole cracking. There is no invention steel that breaks in several hours or tens of hours, and it is considered that water-cooled hole cracking is unlikely to occur.
Moreover, the hardness variation is within 3 or less. In particular, except the invention steel 22 to the invention steel 26, all are within 2 hardness variations. That is, it is possible to cope with a case where a narrow hardness standard is required.

Hereinafter, the comparative steel will be described.
Comparative steel 1 is excellent in mirror polishability and has high thermal conductivity and machinability. On the other hand, there are difficulties in impact value and corrosion resistance, and cracking and rust become a problem.
Comparative steel 2 is excellent in mirror polishing and has good weldability. On the other hand, there are difficulties in thermal conductivity and impact value, and insufficient cooling capacity and cracking of the mold become problems.
The comparative steel 3 is a steel material with a fairly good balance. However, since the thermal conductivity is low, the cooling ability of the mold is insufficient. This is a fatal defect in recent years when high cycle is required. In addition, the cost is not low, and it is expensive for the steel material characteristics.
The comparative steel 4 has high thermal conductivity and good machinability. On the other hand, there are difficulties in corrosion resistance and mirror polishing, and the application range is considerably limited.
The comparative steel 5 is excellent in mirror polishing and has good corrosion resistance. On the other hand, there are difficulties in machinability and thermal conductivity, and there are problems of difficulty in mold processing and insufficient cooling ability of the mold.
The comparative steel 6 is excellent in mirror polishing and has good corrosion resistance. On the other hand, there are difficulties in machinability, weldability, and thermal conductivity, which makes it difficult to process and repair the mold, and insufficient cooling ability of the mold.
The comparative steel 7 is excellent in mirror polishing, and has good weldability and corrosion resistance. On the other hand, there is difficulty in thermal conductivity, and insufficient cooling ability of the mold becomes a problem. In addition, since the impact value is low, there is a concern about cracking of the mold.

Furthermore, there are comparative steels with extremely short break times of less than 40 hours in a test that simulates water-cooled hole cracking. Such steel is considered to have a high risk of causing water-cooled hole cracking.
Further, there are comparative steels having hardness variations exceeding 3, and such steel materials are difficult to cope with when a narrow hardness standard is required.

  As described above, the comparative steel has problems in characteristics and cost. The invention steel has high thermal conductivity and high specular polishing properties while ensuring a hardness of 35 HRC or more, and is excellent in other characteristics and cost performance. This is an effect of optimizing C, Si, Cu, and Cr and not containing a large amount of both Ni and Al at the same time.

Claims (10)

In mass%
0.040 <C <0.100
0.03 <Si <0.27
0.30 <Mn <1.80
0.30 <Cu <0.61
4.00 <Cr <9.00
And more
0.04 <Mo <1.00
0.02 <V <0.50
A mold steel having excellent thermal conductivity, specular polishing, weather resistance, toughness and machinability, characterized by comprising at least one of the following, and the balance Fe and inevitable impurities. In Claim 1, in mass%
0.30 <W ≦ 4.00
0.30 <Co ≦ 3.00
A mold steel excellent in heat conduction performance, mirror polishability, weather resistance, toughness and machinability, characterized by further containing at least one of the following. In any one of Claims 1 and 2,
0.004 <Nb ≦ 0.100
0.004 <Ta ≦ 0.100
0.004 <Ti ≦ 0.100
0.004 <Zr ≦ 0.100
A mold steel excellent in heat conduction performance, mirror polishability, weather resistance, toughness and machinability, characterized by further containing at least one of the following. In any one of Claims 1-3, In mass%
0.0001 <B ≦ 0.0050
A mold metal having excellent thermal conductivity, specular polishing, weather resistance, toughness, and machinability, further comprising: In any one of Claims 1-3, In mass%
0.30 <Ni ≦ 1.50
A mold metal having excellent thermal conductivity, specular polishing, weather resistance, toughness, and machinability, further comprising: In any one of Claims 1-5 in mass%
0.003 <S ≦ 0.050
0.0005 <Ca ≦ 0.2000
0.03 <Se ≦ 0.50
0.005 <Te ≦ 0.100
0.01 <Bi ≦ 0.30
0.03 <Pb ≦ 0.50
A mold steel excellent in heat conduction performance, mirror polishability, weather resistance, toughness and machinability, characterized by further containing at least one of the following.   The molding according to any one of claims 1 to 6, wherein the average hardness at room temperature is in the range of 35 to 45 HRC, and is excellent in heat conduction performance, mirror polishability, weather resistance, toughness and machinability. Steel for molds.   8. The thermal conductivity measured by the laser flash method according to claim 7 is 26 W / m / K or more at 100 ° C., excellent in thermal conductivity performance, mirror polishability, weather resistance, toughness and machinability Mold steel for molding. In any one of Claims 7 and 8, in mass%
4.60 <Mn + Cr + 0.5Ni <6.20
A mold steel excellent in heat conductivity, mirror polishing, weather resistance, toughness and machinability, characterized by being In any one of Claims 7-9 in mass%
0.19 <0.5Mo + V <0.53
A mold steel excellent in heat conductivity, mirror polishing, weather resistance, toughness and machinability, characterized by being

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