JP7392330B2 - Mold steel and molds - Google Patents

Description

The present invention relates to a mold steel and a mold suitable for application to molds used for plastic injection molding and the like.

For example, a mold for injection molding a plastic product (including parts that make up a part of the mold) is made by melting → refining → casting → homogenization heat treatment → hot working → normalizing → annealing (tempering) → quenching/tempering. It is manufactured through the following steps: → Cutting → Mirror polishing. Materials for molds are required to have various properties, and materials used in molds for injection molding plastic products are particularly required to have high mirror polishability. In addition, the mold must have enough corrosion resistance that it will not rust even if it is left unused, and a high impact value that will prevent it from breaking during use.

(About mirror polishability)
The mold plays the role of transferring the surface shape and properties (surface roughness and pattern) to the product. Among these, if the surface of the product is required to be smooth, it is necessary to polish the surface smoothly. This is called mirror polishing.

One of the factors that worsens mirror polishability is "pinholes." Pinholes are small holes or scratches that occur on the polished surface of a mold. If a mold with pinholes is used as is, the pinholes will be transferred to the surface of the product, resulting in poor surface quality and loss of commercial value. Therefore, pinholes must not be generated during mirror polishing.

There are two types of pinholes: pinholes caused by abrasive particles from polishing (due to polishing) and pinholes caused by foreign matter inside the steel material falling out (caused by steel). In order to reduce pinholes caused by steel, mold steel with fewer foreign substances (carbides, oxides, and nitrides) is required. In the refining and casting of steel materials, efforts are being made to reduce such foreign substances. Furthermore, if the hardness of the mold is low, pinholes are likely to occur, so it is essential to adjust the chemical components (especially C) to ensure a certain level of hardness. However, if the amount of C is too large, the amount of carbides that become foreign substances will also increase, making it difficult to balance this with hardness.

(About corrosion resistance)
If a mirror-polished mold is left unused for a period of time, such as before it is used or during production interruptions, rust may develop on the mold surface. If a mold with rust is used as is, the rust will be transferred to the surface of the product, resulting in poor surface quality and loss of commercial value. Therefore, it is necessary to re-polish the rusted mold, but the cost and man-hours required for this polishing are enormous. Molds need to have enough corrosion resistance that they will not rust even if left unused.

The corrosion resistance of a mold is almost determined by the amount of Cr. Steel with a Cr content of 0.2 to 3% is often used in injection molds. Since low Cr steel has very low corrosion resistance, molds made of such steel are likely to rust if left unattended. Therefore, 5% Cr die steel (such as SKD61) is sometimes used, but 5% Cr steel lacks corrosion resistance in a high temperature and humid environment. If sufficient corrosion resistance is desired, stainless steel (12%≦Cr) is used for the mold. For example, it is an expensive steel such as SUS420J2 or SUS630. However, in many cases, high corrosion resistance with a Cr content of 12% or more is not necessary. With stainless steel, you pay more for excessive corrosion resistance.

As described above, a Cr content of 5% is insufficient, but a Cr content of 12% or more is excessive. There is also 8-17% Cr steel with a high C content to obtain high hardness, but such steel has a large amount of carbides, which has the disadvantage of reducing mirror polishability and impact value. Furthermore, since stainless steel with a high C content consumes Cr as carbide, its corrosion resistance is not as high as expected from the Cr content.

(About impact value)
The mold is also required to not crack during injection molding. The reason for this is to avoid production interruption due to replacement of broken molds and increase in manufacturing costs due to manufacturing new molds. The higher the impact value, the lower the risk of the mold cracking during injection molding. Therefore, efforts are being made to increase the impact value of molds by optimizing the steel composition and quenching method. If the impact value (U notch, notch bottom radius 1 mm, notch bottom height 8 mm) at room temperature (21 to 27° C.) is 50 J/cm ² or more, the risk of the mold cracking during injection molding becomes extremely low. However, the impact value of injection molds is often 10 to 80 J/cm ² at 38 HRC. For this reason, it has not been possible to stably and sufficiently lower the risk of cracking. Note that the impact value here is the value obtained by dividing the absorbed energy [J] by the cross-sectional area [0.8 cm ² ] of the test piece.

As described above, mold steel with good mirror polishability, corrosion resistance between 5% Cr steel and 12% Cr steel, and high impact value has not yet been developed. For this reason, it has been difficult to obtain molds at low prices that can have surfaces that can be polished well, are resistant to rust even when stored during periods of non-use, and are resistant to cracking during use.

In addition, Patent Document 1 below describes a method of improving polishability and machinability by modifying oxides that cause pinhole formation and balancing the alloy components so as to reduce the size of steel materials for molds. is disclosed. However, the component composition in the example described in Cited Document 1 that embodies this is lower in C and higher in Al than the mold steel of the present invention, and is different from the present invention.

Japanese Patent Application Publication No. 2004-91840

Against the background of the above-mentioned circumstances, the present invention has been developed to have good mirror polishability and corrosion resistance intermediate between 5% Cr steel and 12% Cr steel after being tempered to a predetermined hardness, as well as high impact steel. This was done with the aim of providing mold steel and molds of high value.

Accordingly, claim 1 relates to steel for molds, and in mass %, 0.045≦C≦0.090, 0.01≦Si≦0.50, 0.10≦Mn≦0.60, 0 .80≦Ni≦1.10, 6.60≦Cr≦8.60, 0.01≦Mo≦0.70, 0.001≦V≦0.200, 0.007≦Al≦0.150, 0 0.0002≦N≦0.0500, with the remainder being Fe and unavoidable impurities.

In addition, in the steel for molds, the components shown below may be contained as unavoidable impurities within the following ranges.
P≦0.10, S≦0.008, Cu≦0.30, W≦0.30, O≦0.05, 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, REM≦0.10, etc.

A second aspect of the present invention is characterized in that, in the first aspect, it further contains 0.30<Cu≦ 0.58 in mass%.

The product according to claim 3 is characterized in that, in either claim 1 or 2, it further contains at least one of the following in terms of mass%: 0.30<W≦4.00, 0.30<Co≦3.00. shall be.

In claim 4, in any one of claims 1 to 3, mass% is 0.004<Nb≦0.200, 0.004<Ta≦0.200, 0.004<Ti≦0.200, It is characterized by further containing at least one member satisfying the relationship 0.004<Zr≦0.200.

A fifth aspect of the invention is characterized in that, in any one of claims 1 to 4, it further contains 0.0001<B≦0.0050 in mass%.

According to claim 6, in any one of claims 1 to 5, mass% is 0.008<S≦0.050, 0.0005<Ca≦0.2000, 0.03<Se≦0.50, It is characterized by further containing at least one of the following: 0.005<Te≦0.100, 0.01<Bi≦0.50, 0.03<Pb≦0.50.

Claim 7 relates to a mold, which is made of the steel according to any one of Claims 1 to 6, and has a hardness of 32 to 44 HRC.
In the present invention, the term "mold" includes not only the mold body, but also mold parts such as pins that are assembled into the mold body. Furthermore, molds made of the steel of the present invention that have been subjected to surface treatment or texturing are also included.

In order to solve the above problems, the present inventor investigated the influence of steel components on mirror polishability, corrosion resistance, and impact value, and found that the adjustment of C-Si-Mn-Ni-Cr-Mo-V-Al-N It has been found that the desired properties can be obtained. The present invention was made based on this knowledge, and it effectively increases corrosion resistance by containing extremely low C and approximately 7 to 8% Cr, and in particular, improves the amount of Al in stabilizing the impact value to a high level. It is characterized by the fact that the importance of Al has been discovered and the amount of Al has been defined within a specific range (0.007 to 0.150%). According to the mold steel of the present invention, after being tempered to a predetermined hardness, it has good mirror polishability and corrosion resistance between 5% Cr steel and 12% Cr steel, and also has high impact resistance. value can be realized. Furthermore, it is cheaper than conventional steel used in resin molding molds that require mirror polishability.

In addition, according to the mold of the present invention made of the steel of the present invention and whose hardness is adjusted to 32 to 44 HRC, cutting and polishing processes can be performed in the pre-hardened state after quenching and tempering. can be simplified. Further, when using a mold, wear and cracking during resin molding can be suppressed, and the life of the mold can be extended. On the other hand, even if the mold is left unused for a certain period of time, the surface of the mold is less likely to rust, making it possible to eliminate or reduce the need for re-polishing, which is required for molds made of low Cr steel.

The mold steel and mold of the present invention as described above are suitable for use in injection molding and blow molding of resins (plastics and vinyl), molding and processing of rubber, molding and processing of carbon fiber reinforced plastics, etc. It is.

Next, the reasons for limiting each chemical component in the present invention will be explained below. In addition, all the values of each chemical component are mass %.
“Regarding the chemical components of claim 1”
0.045≦C≦0.090
One of the features of the present invention is that the range of the amount of C is defined very narrowly in order to balance various properties. When C<0.045, undissolved carbides during quenching decrease and crystal grains tend to become coarse. It is difficult to obtain HRC of 32 or more when the tempering temperature is high or when applied to powder additive manufacturing. When C<0.045, delta ferrite precipitates, which adversely affects mirror polishability and impact value. In addition, the martensitic transformation point becomes high, resulting in a coarse quenched structure, resulting in a decrease in impact value.
On the other hand, when 0.090<C, weldability decreases. Also, the thermal conductivity decreases significantly. The dependence of hardness on tempering temperature becomes obvious, making it difficult to adjust tempering hardness. In addition, carbides increase, which adversely affects mirror polishability.

0.01≦Si≦0.50
When Si<0.01, machinability during machining is significantly degraded. If the undissolved carbide during quenching is VC, its amount decreases and crystal grains tend to become coarse. When the tempering temperature is high, it is difficult to stably obtain HRC of 32 HRC or higher.
On the other hand, when 0.50<Si, the thermal conductivity decreases significantly. Delta ferrite precipitates and adversely affects mirror polishability and impact value.
The preferred range of Si is 0.05≦Si≦0.46, more preferably 0.10≦Si≦0.42.

0.10≦Mn≦0.60
When Mn<0.10, hardenability is insufficient and hardness is insufficient due to the inclusion of ferrite. In addition, hardenability is insufficient and toughness is reduced due to the inclusion of bainite. Since the martensitic transformation point becomes high and a coarse quenched structure is formed, the impact value decreases. In particular, the impact value at temperatures below room temperature decreases.
On the other hand, when 0.60<Mn, the annealing property deteriorates significantly, and the heat treatment for softening becomes complicated and takes a long time, increasing the manufacturing cost. Also, the thermal conductivity decreases significantly. When the tempering temperature is high, the impact value at room temperature decreases (this is noticeable when Si and P are high).
A preferable range of Mn is 0.15≦Mn≦0.55, more preferably 0.20≦Mn≦0.50.

0.80≦Ni≦1.10
When Ni<0.80, hardenability is insufficient and hardness is insufficient due to the inclusion of ferrite. In addition, hardenability is insufficient and toughness is reduced due to the inclusion of bainite. Since the martensitic transformation point becomes high and a coarse quenched structure is formed, the impact value decreases. In particular, the impact value at temperatures below room temperature decreases. The effect of increasing hardness due to precipitation of compounds with Al is small.
On the other hand, when 1.10<Ni, the annealing property deteriorates significantly, and the heat treatment for softening becomes complicated and takes a long time, increasing the manufacturing cost. The decrease in thermal conductivity is also large. Also, the cost will increase significantly.
A preferable range of Ni is 0.84≦Ni≦1.08, more preferably 0.88≦Ni≦1.06.

6.60≦Cr≦8.60
When Cr<6.60, the amount of secondary hardening is insufficient and it is difficult to stably obtain HRC of 32 HRC or more. High temperature strength decreases. Hardenability is insufficient, leading to a decrease in toughness due to the inclusion of bainite. Since the martensitic transformation point becomes high and a coarse quenched structure is formed, the impact value decreases. Corrosion resistance deteriorates, so molds are prone to rust when left unused. In addition, the water cooling holes inside the mold become noticeably rusted, leading to cracks.
On the other hand, when 8.60<Cr, the thermal conductivity decreases significantly. The dependence of hardness on tempering temperature becomes obvious, making it difficult to adjust tempering hardness. Delta ferrite precipitates and adversely affects mirror polishability and impact value.
The preferred range of Cr is 7.20≦Cr≦8.40, more preferably 7.80≦Cr≦8.20.

0.01≦Mo≦0.70
When Mo<0.01, ferrite precipitates due to insufficient hardenability, which adversely affects mirror polishability and impact value. When the contribution of secondary hardening is small and the tempering temperature is high, it becomes difficult to stably obtain HRC of 32 or more. High temperature strength is insufficient. Poor effect on improving corrosion resistance.
On the other hand, when 0.70<Mo, the annealing properties are greatly reduced, and the heat treatment for softening becomes complicated and takes a long time, increasing the manufacturing cost. Moreover, the fracture toughness decreases, making the mold more likely to crack. In addition, material costs will rise significantly.
The preferred range of Mo is 0.10≦Mo≦0.65, more preferably 0.20≦Mo≦0.60.

0.001≦V≦0.200
When V<0.001, the amount of nitrides and carbonitrides decreases, so the effect of suppressing coarsening of crystal grains during quenching is poor, leading to a decrease in impact value due to coarsening of grains. When the contribution of secondary hardening is small and the tempering temperature is high, it becomes difficult to stably obtain HRC of 32 or more.
On the other hand, when 0.200<V, there is not enough C to combine with a large amount of V, so there is no practical benefit to adding too much, and only increases costs. When the amount of C is close to the upper limit specified in the claims, coarse nitrides and carbonitrides increase and serve as starting points for cracks, resulting in a decrease in impact value.
A preferable range of V is 0.008≦V≦0.180, more preferably 0.015≦V≦0.150.

0.007≦Al≦0.150
One of the features of the present invention is that the range of Al content is narrowly defined. The biggest reason to avoid Al<0.007 is to stabilize the impact value at a high level. If the amount of Al is too small, the impact value will be significantly low. Furthermore, since the amount of AlN is reduced, the effect of suppressing coarsening of crystal grains during quenching is poor, resulting in a decrease in impact value due to coarsening of grains.
On the other hand, when 0.150<Al, AlN increases excessively and becomes coarse, so the number of starting points for fracture increases and the impact value becomes less than 50 J/cm ² . Also, the number of large foreign particles that fall off during mirror polishing increases, making pinholes more likely to occur. In addition, the thermal conductivity decreases significantly.
A preferable range of Al is 0.050<Al≦0.150, more preferably 0.050<Al≦0.120.

Conventionally, it has been thought that mold steel used for injection molding and blow molding of resins (plastics and vinyl), molding and processing of rubber, and molding and processing of carbon fiber reinforced plastics should contain less Al. The reason for this is that if the amount of Al is too large, a large amount of oxides and nitrides will be generated, which will reduce the mirror polishability and impact value. However, in the steel of the present invention, which has a special composition system, a peculiar phenomenon occurs in that if the amount of Al is too small, the impact value becomes extremely low. Based on this discovery, the amount of Al is defined within a narrow range in the present invention.

FIG. 1 shows the effect of the amount of Al on the impact value at 24° C. (U notch, notch bottom radius 1 mm, notch bottom height 8 mm). The amount of Al was varied using steel of 0.060C-0.30Si-0.35Mn-0.95Ni-7.95Cr-0.45Mo-0.10V-0.012N as the basic components. These steels were quenched from 870°C and tempered to 36.5HRC. As shown in FIG. 1(A), when Al=0.003, the impact value is very low at 14 to 27 J/cm ² . When Al=0.007, 50 J/cm ² can be almost ensured, although the variation is noticeable. Furthermore, if there is too much Al, the impact value starts to decrease due to an increase in inclusions, as shown in FIG. 1(B). Based on such data, the claimed range is 0.007≦Al≦0.150. Further, if 0.050<Al≦0.150, the impact value can be stably set to 50 J/cm ² or more.

FIG. 2 is a diagram of micrographs showing the structure of steel materials when the amount of Al is changed. The amount of Al was changed using steel of 0.063C-0.29Si-0.31Mn-0.96Ni-7.98Cr-0.45Mo-0.088V-0.0274N as the basic component. These steels were quenched from 870°C and tempered to 36.5HRC. As shown in the figure, it can be seen that when Al=0.056, the crystal grains are finer than when Al=0.008. If the crystal grains are fine, a smooth surface can be obtained by mirror polishing.
That is, the preferable range of Al in consideration of impact value and mirror polishability is 0.050<Al≦0.150.

0.0002≦N≦0.0500
When N<0.0002, the amount of AlN decreases, so the effect of suppressing coarsening of crystal grains during hardening is poor, and the coarsening of grains causes a decrease in impact value.
When 0.0500<N, the refining time and cost required for N addition increase, leading to an increase in material cost. The impact value decreases because coarse AlN increases and becomes the starting point for cracks. Moreover, coarse AlN also reduces mirror polishability.
A preferable range of N is 0.0010≦N≦0.0400, more preferably 0.0020≦N≦0.0300.

“Regarding the chemical components of claim 2”
0.30<Cu≦ 0.58
The steel of the present invention, which has a low C content, has few carbides that suppress the movement of grain boundaries, so the grains tend to become coarse during quenching. Therefore, it is effective to add Cu, which has an excellent solute drag effect, to prevent grain coarsening during hardening. In the steel of the present invention, hardenability is somewhat insufficient when Mn--Ni--Cr is small, but Cu also has the effect of increasing hardenability. Further, in the present invention, since there are few elements such as C--Mo--V that contribute to secondary hardening, the tempering hardness does not become extremely high, but the tempering hardness can be considerably increased by utilizing precipitation hardening of Cu.
On the other hand, if Cu is too large, there will be problems such as increased cost and cracking during hot working.

“Regarding the chemical components of claim 3”
As a tool steel, the steel of the present invention, which contains less Mo and V, does not have very high high temperature strength. Selective addition of W and Co is effective in ensuring high-temperature strength. W increases strength through precipitation of carbides and solid solution. Co increases the strength by solid solution in the base metal, and at the same time contributes to precipitation hardening through a change in carbide morphology. Furthermore, these elements also have the effect of suppressing the growth of austenite crystal grains during quenching due to the solute drag effect. in particular,
0.30<W≦4.00
0.30<Co≦3.00
What is necessary is to contain at least one kind (one element) of the following.
Exceeding a predetermined amount of either element results in saturation of properties and significant cost increase.

“Regarding the chemical components of claim 4”
Selective addition of Nb-Ta-Ti-Zr is also effective in suppressing the growth of austenite crystal grains during hardening. Carbides, nitrides, and carbonitrides generated by bonding with these elements suppress movement of grain boundaries. in particular,
0.004<Nb≦0.200
0.004<Ta≦0.200
0.004<Ti≦0.200
0.004<Zr≦0.200
It is sufficient to contain at least one of the following.
When any of the elements exceeds a predetermined amount, carbides, nitrides, and oxides are excessively produced, which becomes a starting point for mold failure.

“Regarding the chemical components of claim 5”
B has the effect of improving hardenability and strengthening grain boundaries. As a result, the impact value is stabilized at a high level. in particular,
0.0001<B≦0.0050
It is sufficient if it contains.
Note that if the added B forms BN, the original purpose of B addition cannot be achieved. Therefore, it is preferable to form a nitride with an element that has a stronger affinity for N than for B, fix N, and avoid combining B and N. Examples of such elements include Nb, Ta, Ti, and Zr. These elements have the effect of fixing N even if they are present at an impurity level.
Additionally, B addition is effective in improving machinability. In order to improve machinability, BN may be formed. BN has properties similar to graphite and reduces cutting resistance while improving chip breakability. Note that B, a compound of Fe and B, BN, etc. may coexist in the steel. In that case, hardenability, impact value, machinability, etc. are improved depending on the state of B in the steel.

“Regarding the chemical components of claim 6”
It is also effective to selectively add S-Ca-Se-Te-Bi-Pb to improve machinability. in particular,
0.008<S≦0.050
0.0005<Ca≦0.2000
0.03<Se≦0.50
0.005<Te≦0.100
0.01<Bi≦0.50
0.03<Pb≦0.50
It is sufficient to contain at least one of the following.
When any of the elements exceeds a predetermined amount, hot workability and impact value are significantly reduced.

According to the present invention as described above, after being tempered to a predetermined hardness, it has good mirror polishability and corrosion resistance between 5% Cr steel and 12% Cr steel, and also has a high impact value. We can provide mold steel and molds.

FIG. 3 is a diagram showing the relationship between Al content and impact value. It is a figure of the micrograph which showed the structure of the steel material when changing the amount of Al.

Tests were conducted to evaluate the mirror polishability, corrosion resistance, and impact value of the invention steel and comparative steel (22 steel types in total) shown in Table 1.
Note that Comparative Steel 1 is a commercially available product, and is a mold steel commonly used for injection molding and blow molding of resins (plastics and vinyl). Comparative steel 2 is JIS SKD61, which is a 5Cr die steel. Comparative steel 3 is JIS SUS420J2, which is a high-strength stainless steel. Comparative steel 4 is a high-strength stainless steel known as JIS SUS630. These comparative steels fall outside the claimed scope of the present invention in at least four major elements.

Each of the 22 steel types shown in Table 1 was cast into a 150 kg ingot to produce a steel ingot. After homogenizing the steel ingot at 1240°C for 24 hours, the steel ingot was hot forged into a bar with a rectangular cross section of 60 mm x 45 mm, and the steel bar was cooled to 100°C or lower. Subsequently, the steel bar was normalized by heating it to 1020°C and cooling it to 100°C or less. Furthermore, the steel bar was tempered. The tempering conditions were that Comparative Steel 1 and Comparative Steel 4 were held at 600°C for 12 hours, and the other steels were held at 680°C for 8 hours. Various test pieces were prepared from this tempered material.

<Evaluation of mirror polishability>
A plate of 51 mm x 31 mm x 101 mm was cut out from the tempered material and tempered to 36 to 38 HRC by quenching and tempering in a vacuum. The quenching temperature was 870°C for Inventive Steel Type 18 and Comparative Steel 1, 1030°C for Comparative Steel 2 and Comparative Steel 3, and 1050°C for Comparative Steel 4. After holding each quenching temperature for 1 hour, quenching was performed by cooling with nitrogen gas at 6 bar. For tempering, a process of holding at 500 to 650°C for 3 hours was performed multiple times.
The plate after tempering was ground to a size of 50 mm x 30 mm x 100 mm, and the surface roughness of the 50 mm x 100 mm surface was set to ▽▽▽G.

As an evaluation of mirror polishability, a 50 mm x 100 mm surface was polished with increasing abrasive grain count, and finally a mirror finish was obtained with #8000 grit. The polished surface was visually observed and the presence or absence of pinholes was evaluated according to the following criteria.
"S" if there are no pinholes due to falling foreign substances (carbides, oxides, nitrides), "A" if there are 1 or 2 pinholes, "B" if there are 3 or more pinholes .

The evaluation results were "S" for 12 types of invention steel, comparative steel 1, and comparative steel 4, "A" for comparative steel 2, and "B" for comparative steel 3. The reason why Comparative Steel 3 has more pinholes than Comparative Steel 2 is because there are more coarse Cr-based carbides and alumina (Al ₂ O ₃ ).

From the above, it was confirmed that the mirror polishability of the invented steel was equivalent to that of Comparative Steel 1, which is commonly used in molds for injection molding and blow molding of resins (plastics and vinyl). The invented steel has excellent mirror polishability. Furthermore, the market evaluation that pinholes are likely to occur in Comparative Steel 2 and Comparative Steel 3 was also supported by this experiment.

<Evaluation of corrosion resistance>
A plate of 41 mm x 26 mm x 13 mm was cut out from the tempered material and tempered to 36 to 38 HRC by quenching and tempering in a vacuum. The quenching and tempering conditions are the same as those for the mirror polishing test piece. The plate after tempering was ground to a size of 40 mm x 25 mm x 12 mm, and all six sides were polished to a mirror finish.

Corrosion resistance was evaluated by a wet test. A mirror-polished test piece was left standing in an environment with a temperature of 50° C. and a humidity of 98% for 40 hours, and the occurrence of rust was compared and evaluated according to the following criteria.
If there are no rusted spots, give an "S", if there are 1 to 3 rusty spots, get a "A", if there are 4 to 10 spots, get a "B", and if there are more than 10 spots, get a "C".

The corrosion resistance results were "S" for Comparative Steel 4, "A" for Invention Steel 18 and Comparative Steel 3, "B" for Comparative Steel 2, and "C" for Comparative Steel 1. Although the invention steel is not as good as the low C 17% Cr stainless steel (Comparative Steel 4), it is equivalent to the high C 12% Cr stainless steel (Comparative Steel 3). Furthermore, the invention steel is superior to the 5% Cr die steel (comparative steel 2). From the above, it was confirmed that the corrosion resistance of the invented steel is between 5% Cr steel and 12% Cr steel, and is quite close to that of stainless steel.

<Evaluation of impact value>
A square bar measuring 11 mm x 11 mm x 55 mm was cut out from the tempered material and tempered to 36 to 38 HRC by quenching and tempering in a vacuum. The quenching and tempering conditions are the same as those for the mirror polishing test piece. An impact test piece measuring 10 mm x 10 mm x 55 mm was cut out from the square bar after tempering. The notch shape is U-shaped, the radius of the bottom of the notch is 1 mm, and the height below the notch is 8 mm. The test was conducted at room temperature (21-27°C), and the impact value was determined by dividing the absorbed energy by the cross-sectional area of 0.8 cm ² and evaluated according to the following criteria.
If the impact value exceeds 100 J/cm ² , it is ``S'', if it exceeds 50 J/cm <sup>2</sup> but not more than 100 J/cm <sup>2</sup> , it is ``A'', and if it is 50 J/cm ² or less, it is rated ``B''.

The impact value results were "S" for Invention Steel Type 12, "A" for Comparative Steel 2 and Comparative Steel 4, and "B" for Comparative Steel 1 and Comparative Steel 3. Comparative steel 1 is a type that precipitates intermetallic compounds of Ni and Al and is brittle. Comparative Steel 3 has a low impact value because it has many coarse carbides.
From the above, it was confirmed that the impact value of the invented steel was even higher than that of Comparative Steel 1, which is commonly used in molds for injection molding and blow molding of resins (plastics and vinyl).

<Summary of characteristics>
The results obtained are summarized in Table 2 below. The costs shown in Table 2 are based on Comparative Steel 1, which is commonly used for molds for injection molding and blow molding of resins (plastics and vinyl), as "A", and those cheaper than that as "S". , those higher than Comparative Steel 1 were evaluated as "B".

As shown in Table 2, the invention steels are "S" and "A", and there are no steels below "B". On the other hand, comparative steels include "B" and "C". From the above, it was confirmed that the invented steel has excellent mirror polishability, high corrosion resistance, and high impact value. Furthermore, it can be seen that the inventive steel has a low additive amount of expensive elements such as Cu, Ni, and Al, and achieves the above-mentioned excellent properties without increasing cost.

Although the embodiments of the present invention have been described in detail above, this is merely an example. For example, it is also effective to use the steel and mold of the present invention in combination with surface modification treatments such as shot peening, nitriding treatment, PVD treatment, CVD treatment, PCVD treatment, plating treatment, and DLC coating treatment. It is also effective as a method to increase the added value of the present invention to add texture processing to provide a specific pattern (irregularities) on the surface of the mold (including parts) of the present invention by machining or corrosion. Further, the steel of the present invention can be made into a bar or wire shape and used as a welding repair material for molds and parts. Alternatively, the present invention can be implemented with various modifications without departing from its spirit, such as making the steel of the present invention into plates or powder and manufacturing molds and parts by additive manufacturing. It is.

Claims (7)

Mass%: 0.045≦C≦0.090
0.01≦Si≦0.50
0.10≦Mn≦0.60
0.80≦Ni≦1.10
6.60≦Cr≦8.60
0.01≦Mo≦0.70
0.001≦V≦0.200
0.007≦Al≦0.150
0.0002≦N≦0.0500
Steel for molds, characterized in that the remainder has a composition of Fe and unavoidable impurities. In claim 1, 0.30<Cu≦ 0.58 in mass%
Steel for molds, further comprising:
In either of claims 1 and 2, 0.30<W≦4.00 in mass%
0.30<Co≦3.00
A steel for molds, further comprising at least one of the above. In any one of claims 1 to 3, 0.004<Nb≦0.200 in mass%
0.004<Ta≦0.200
0.004<Ti≦0.200
0.004<Zr≦0.200
A steel for molds, further comprising at least one of the above. In any one of claims 1 to 4, 0.0001<B≦0.0050 in mass%
Steel for molds, further comprising: In any one of claims 1 to 5, 0.008<S≦0.050 in mass%
0.0005<Ca≦0.2000
0.03<Se≦0.50
0.005<Te≦0.100
0.01<Bi≦0.50
0.03<Pb≦0.50
A steel for molds, further comprising at least one of the above. A mold made of the steel according to any one of claims 1 to 6, characterized in that it has a hardness of 32 to 44 HRC.

Images