JP5186878B2 - Steel for plastic molds and plastic molds - Google Patents

Description

  The present invention relates to a steel for plastic molds and a plastic mold.

  In recent years, plastic molded products have been used in various fields. In general, a plastic molded product is molded into a desired shape using, for example, a plastic mold such as an injection mold or a press mold.

  Conventionally, SUS420J2, SUS440C, SKD11, etc. are known as general-purpose steel types for plastic molding dies.

  In addition, SUS420J2 is mass%, C: 0.26-0.40%, Si: 1.00% or less, Mn: 1.00% or less, P: 0.040% or less, S: 0.030% Hereinafter, the steel contains a chemical component of Cr: 12.00 to 14.00%.

  Moreover, SUS440C is mass%, C: 0.95-1.20%, Si: 1.00% or less, Mn: 1.00% or less, P: 0.040% or less, S: 0.030% Hereinafter, it is steel containing a chemical component of Cr: 16.00 to 18.00%.

  Moreover, SKD11 is the mass%, C: 1.40-1.60%, Si: 0.40% or less, Mn: 0.60% or less, P: 0.030% or less, S: 0.030% Hereinafter, the steel contains chemical components of Cr: 11.00 to 13.00%, Mo: 0.80 to 1.20%, and V: 0.20 to 0.50%.

  Other steels for plastic molds include, for example, Patent Document 1, weight percent, C: 0.80% or less, Si: 0.01% or more and less than 1.40%, Mn: 0.05% or more 2.0% or less, Ni: 0.005% to 1.00%, Cr: 13.0% to 20.0%, Mo + 1 / 2W: 0.20% to 4.0%, V: 0 0.01% or more and 1.00% or less, N: 0.36% or more and 0.80% or less, O: 0.02% or less, and Al: 0.80% or less, with the balance being substantially Fe and A steel for plastic molds made of inevitable impurities is disclosed.

  In Patent Document 1, as a method of manufacturing the steel, various raw materials weighed so as to be the chemical components are melted in a pressure furnace to cast an ingot (steel ingot), and the ingot is hot forged. Is described.

JP 2007-9321 A

  However, conventional steel has room for improvement in the following points as a material for plastic molding dies.

  That is, glass fibers and the like may be added to the plastic material from the viewpoint of improving the strength of the molded product in addition to the main component resin. This type of additive wears the mold and causes a decrease in the mold life.

  In recent years, plastic molded products have been reduced in thickness in order to reduce the size, and there is a tendency that a large amount of the above additives is added to improve the strength. Therefore, the mold is likely to be significantly worn.

  However, SUS420J2 which is a general-purpose steel type has few C and N, and sufficient hardness cannot be obtained even if heat treatment is performed. Therefore, there is a problem that the wear amount of the mold increases and the wear resistance is poor.

  Next, the plastic molded product is often applied to a part such as a housing where the appearance is important. For this reason, a mold for molding the mold is required to have excellent specularity so that a good surface skin can be imparted to the molded product.

  However, SUS440C, a general-purpose steel type, contains a large amount of C. For this reason, coarse carbides precipitate when manufactured by a melting method. Coarse carbides have a problem that they drop off from the mold surface when the mold is polished and the like, and are easy to form a recess and the like, and the mirror surface property of the mold is lowered.

  Next, the mold is required to have good mold maintenance. Therefore, it is desirable that the mold has a corrosion resistance sufficient to ensure mold maintenance.

  However, the general-purpose steel type SKD11 contains a large amount of C. For this reason, carbide containing Cr is generated, and Cr is easily consumed. Therefore, there is a problem that the corrosion resistance required for mold maintenance cannot be satisfied.

  In contrast to these general-purpose steel types, the steel for plastic molding die of Patent Document 1 is a steel material excellent in wear resistance, specularity and corrosion resistance.

  However, the steel of Patent Document 1 requires a pressure furnace at the time of manufacturing the ingot. Because it contains a lot of N, if it is not sufficiently pressurized during ingot production, many blows are generated in the ingot due to N blow, making subsequent hot forging difficult, producing healthy steel. This is because it cannot be done. Moreover, the said void also causes a crack and a crack.

  As described above, the steel of Patent Document 1 has a problem that the degree of freedom in manufacturing is small and the manufacturability is inferior, such as requiring a special device.

  Therefore, the problem to be solved by the present invention is that the plastic molding metal has good manufacturability of steel materials compared to conventional steel types, and has excellent wear resistance, specularity and corrosion resistance when applied to a mold. To provide mold steel. Another object is to provide a plastic mold using the same.

In order to solve the above-described problems, the steel for plastic molds according to the present invention is, in mass%, C: 0.350% to 0.570%, Si: 0.10% to 0.50%, Mn : 0.10% to 1.00%, Cu: 0.05% to 0.20%, Ni: 0.05% to 0.30%, Cr: 10.00% to less than 14.00% , Mo: less than 0.20%, V: 0.01% or more and 0.10% or less, N: 0.080% or more and 0.150% or less, O: 0.0100% or less, Al: 0.0500% or less , C + N: 0.500% or more and 0.650% or less, with the balance being made of Fe and inevitable impurities.

  Here, the steel for plastic molding die may further contain B: 0.0005% or more and 0.0020% or less in mass%.

  Further, the steel for plastic molding die is in mass%, Nb: 0.001% to 0.300%, Ta: 0.001% to 0.300%, Ti: 0.20% or less, and , Zr: One or more elements selected from 0.001% to 0.300% may be further contained.

  Moreover, the steel for plastic molding die is mass%, Se: 0.001% to 0.300%, Te: 0.001% to 0.300%, Ca: 0.002% to 0.000. 100% or less, Pb: 0.001% or more and 0.200% or less, and Bi: 0.001% or more and 0.300% or less, further including one or more elements selected from good.

  On the other hand, a plastic molding die according to the present invention uses the above-described steel for plastic molding as a material, and has a gist that the hardness is 55 HRC or more.

  The steel for plastic molds according to the present invention has the above chemical composition, so that it is more manufacturable in steel than conventional steel types, and has excellent wear resistance and specularity when applied to a mold. Corrosion resistance is good.

  When particularly important elements are mentioned, the plastic molding die steel according to the present invention has a reduced N content. Therefore, during ingot production, N blow can be suppressed even under non-pressurization. Less ingot is obtained. Therefore, it is easy to perform hot forging and hot rolling thereafter, and sound steel can be manufactured.

  Moreover, since the content of C + N is balanced within a specific range, when it is applied to a mold by quenching and tempering, sufficient hardness can be obtained and excellent wear resistance can be exhibited.

  Further, since the C content is reduced, coarse carbides are hardly precipitated, and the mirror surface property of the mold can be improved.

  In addition, since the C content is reduced, the amount of carbide containing Cr is small, the consumption of Cr is suppressed, and good corrosion resistance can be ensured to the extent necessary for mold maintenance.

  The plastic molding die according to the present invention uses the above-mentioned steel for plastic molding die, and the hardness is not less than a specific value.

  Therefore, it is excellent in abrasion resistance and can improve the durability of the mold. Moreover, since it is excellent in specularity, it is possible to mold a plastic molded article having a good surface skin. Moreover, since corrosion resistance is favorable, it can contribute to the improvement of type | mold maintenance property.

  In the following, steel for plastic molds (hereinafter sometimes referred to as “steel for molds”) and plastic mold (hereinafter referred to as “main molds”) according to an embodiment of the present invention may be used. .) Will be described in detail.

1. Steel for molds The steel for molds contains the following elements, and the balance consists of Fe and inevitable impurities. The types, component ratios and reasons for limitation of the additive elements are as follows. In addition, the unit of a component ratio is the mass%.

C: 0.350% or more and 0.570% or less C is an element necessary for ensuring strength and wear resistance, and is bonded to carbide forming elements such as Cr, Mo, W, V, and Nb. It is an element that produces carbide. C is also an element necessary for ensuring hardness by solid solution in the matrix during quenching and martensite organization.

  In the present steel for molds, the N content may be lowered, and from the viewpoint of securing hardness and obtaining sufficient wear resistance, the lower limit of the C content is 0.350% or more. .

  However, when the C content is excessive, it is easy to combine with the above-mentioned carbide forming elements, and a carbide containing a large amount of Cr or Mo is precipitated, the solid solution amount of Cr or Mo in the parent phase is lowered, and the corrosion resistance is lowered. . Therefore, the upper limit of C content is 0.570% or less.

Si: 0.10% or more and 0.50% or less Si is mainly added as a deoxidizer or a nitride alloy for adding nitrogen. In order to obtain the effect, the lower limit of the Si content is set to 0.10% or more.

  However, when the Si content is excessive, hot workability and toughness are reduced. From the viewpoint of preventing this, the upper limit of the Si content is 0.50% or less.

Mn: 0.10% or more and 1.00% or less Mn is added as an element that improves hardenability. Moreover, when S is inevitably contained, Mn is effective in suppressing a decrease in toughness. In order to obtain the effect, the lower limit of the Mn content is 0.10% or more.

  However, when the Mn content is excessive, hot workability is reduced. From the viewpoint of preventing this, the upper limit of the Mn content is 1.00% or less.

Cu: 0.05% or more and 0.20% or less Cu is an element that stabilizes austenite. In order to obtain the effect, the lower limit of the Cu content is set to 0.05% or more.

  However, when the Cu content is excessive, the amount of retained austenite increases, which causes aging of the mold dimensions. Moreover, hot workability is also reduced. From the viewpoint of preventing this, the upper limit of the Cu content is 0.20% or less.

Ni: 0.05% or more and 0.30% or less Ni is an austenite generating element. In order to obtain the effect, the lower limit of the Ni content is set to 0.05% or more.

  However, when the Ni content is excessive, the amount of retained austenite increases, which causes aging of the mold dimensions. From the viewpoint of preventing this, the upper limit of the Ni content is set to 0.30% or less.

Cr: 10.00% or more and less than 14.00% Cr is an element that improves corrosion resistance. In order to obtain the effect, the lower limit of the Cr content is set to 10.00% or more.

  However, when the Cr content is excessive, the C solid solution amount and the N solid solution amount of austenite at the quenching temperature are decreased, and it is difficult to ensure hardness. From the viewpoint of preventing this, the upper limit of the Cr content is less than 14.00%.

Mo: Less than 0.20% Mo is unavoidably added from raw material scrap and the like. Removing Mo more than necessary causes an increase in manufacturing cost. Therefore, the upper limit of the Mo content is less than 0.20%.

V: 0.01% or more and 0.10% or less V is an element that increases the dissolution amount of N. In order to obtain the effect, the lower limit of the V content is set to 0.01% or more.

  However, when the V content is excessive, coarse carbonitrides are likely to be generated, and the specularity is lowered. From the viewpoint of preventing this, the upper limit of the V content is 0.10% or less. The upper limit of the V content is preferably 0.05% or less.

N: 0.080% or more and 0.150% or less N is an interstitial element that contributes to improving the hardness of the martensite structure. Compared with C, which is the same interstitial element, the gamma stabilization ability is large. Further, N contributes to improvement of corrosion resistance in a solid solution state. In order to obtain the effect, the lower limit of the N content is set to 0.080% or more.

  However, when the N content is excessive, the nitrogen gas ejection limit is exceeded due to the concentration of nitrogen during solidification. Therefore, at the time of manufacturing an ingot under non-pressurization, many voids are likely to be generated in the ingot, and the productivity of the steel is lowered. From the viewpoint of preventing this, the upper limit of the N content is set to 0.150% or less.

O: 0.0100% or less O is an element inevitably contained in molten steel. However, when O becomes excessive, Si and Al and coarse oxides are formed, and these become inclusions, which deteriorate toughness and specularity. From the viewpoint of preventing this, the upper limit of the O content is 0.0100% or less.

-Al: 0.0500% or less Al is an element added as a deoxidizer like Si. However, when the Al content is excessive, coarse nitrides are likely to be generated, and the specularity is lowered. From the viewpoint of preventing this, the upper limit of the Al content is set to 0.0500% or less.

C + N: 0.500% or more and 0.650% or less C + N (the sum of C content and N content) ensures hardness, obtains sufficient wear resistance, and is used for coarse carbonitrides. This is a condition necessary for balancing generation and suppressing specularity. The amount of C + N dissolved at the quenching temperature is related to the hardness of the martensite structure after quenching. It is also involved in the amount of secondary curing in high temperature tempering. C and N that could not be dissolved at the quenching temperature exist as carbonitrides. Therefore, it grows into a coarse carbide and reduces specularity. If carbonitride is present in an amount larger than the amount of carbonitride necessary to suppress the coarsening of crystal grains, the demerit of reducing specularity increases.

  In order to secure hardness and obtain sufficient wear resistance, the lower limit of the sum of the C content and the N content is 0.500% or more.

  On the other hand, the upper limit of the sum of the C content and the N content is set to 0.650% or less in order to suppress the formation of coarse carbonitrides and ensure specularity.

  In addition to the essential elements described above, the present mold steel may further optionally contain one or more elements selected from the following elements. The component ratio of each element, reasons for limitation, etc. are as follows.

-B: 0.0005% or more and 0.0020% or less B is an element effective for suppressing burning cracks. In order to obtain the effect, the lower limit of the B content is set to 0.0005% or more.

  However, when the B content is excessive, coarse nitrides are likely to be generated, and the specularity is lowered. From the viewpoint of preventing this, the upper limit of the B content is set to 0.0020% or less.

Nb: 0.001% or more and 0.300% or less, Ta: 0.001% or more and 0.300% or less, Ti: 0.20% or less, Zr: 0.001% or more and 0.300% or less Nb, Ta , Ti and Zr are elements effective to combine with C and N to form carbonitrides, suppress coarsening of crystal grains, and improve specularity. In order to obtain the effect, the lower limit of the Nb content is 0.001% or more. Similarly, the lower limit of the Ta content is 0.001% or more. The lower limit of the Ti content is not particularly limited. The lower limit of the Zr content is 0.001% or more.

  However, when the contents of Nb, Ta, Ti, and Zr are excessive, machinability is deteriorated. From the viewpoint of preventing this, the upper limit of the Nb content is set to 0.300% or less. Similarly, the upper limit of the Ta content is set to 0.300% or less. The upper limit of the Ti content is 0.20% or less. The upper limit of the Zr content is 0.300% or less.

Se: 0.001% to 0.300%, Te: 0.001% to 0.300%, Ca: 0.002% to 0.100%, Pb: 0.001% to 0.200 % Or less, Bi: 0.001% or more and 0.300% or less Se, Te, Ca, Pb, and Bi are elements effective for improving the machinability. In order to obtain the effect, the lower limit of the Se content is 0.001% or more. Similarly, the lower limit of the Te content is 0.001% or more. The lower limit of the Ca content is 0.002% or more. The lower limit of the Pb content is 0.001% or more. The lower limit of Bi content is 0.001% or more.

  However, when the contents of Se, Te, Ca, Pb, and Bi are excessive, the toughness is reduced. From the viewpoint of preventing this, the upper limit of the Se content is 0.300% or less. Similarly, the upper limit of Te content is set to 0.300% or less. The upper limit of the Ca content is 0.100% or less. The upper limit of the Pb content is 0.200% or less. The upper limit of Bi content is 0.300% or less.

  The chemical components of the mold steel are as described above. In the steel for molds, the particle size of the carbonitride contained is preferably 4.0 μm or less from the viewpoint of improving the specularity.

  The particle size of the carbonitride refers to the total number of carbonitrides observed by corroding the measurement surface of the finish polished sample with a corrosive liquid and observing this measurement surface with an optical microscope, a scanning electron microscope, or the like. The typical value of the particle size in which 90% or more is the value or less is indicated.

  The present steel for molds only needs to exhibit a hardness of 55 HRC or more by heat treatment or the like when finally used as a mold. Therefore, it is sufficient that the steel for molds has a hardness that does not interfere with mold processing such as cutting, drilling, and tapping before it becomes a final mold.

  If the material hardness is excessively high, a special tool or the like may be required, or troubles such as a reduction in tool life may occur. Further, if the material hardness is excessively low, chips may continue and troubles such as not being divided may occur.

  In consideration of the above, the steel for molds should be low in hardness at the time of mold processing. For example, spheroidizing annealing may be performed. In this case, the hardness after spheroidizing annealing is preferably less than 20 HRC.

  However, when the chip crushability is poor, the present steel for mold may be pre-hardened from the viewpoint of adjusting the hardness to be suitable for mold processing. In this case, the pre-hardened hardness is preferably 30 HRC or more and 40 HRC or less.

Next, an example of the manufacturing method of this steel for metal mold | dies is demonstrated.
A steel having the above composition is melted under no pressure to cast an ingot. At this time, a non-pressurized melting furnace such as a vacuum induction furnace or an atmospheric induction furnace can be used as the melting furnace. In the present steel for molds, since the N content is reduced, N blow can be suppressed even when the ingot is manufactured even under non-pressurization, and an ingot with less voids can be obtained.

  Next, the obtained ingot is hot forged and / or hot rolled to adjust to a steel material having a required size. In this steel for molds, since an ingot with few voids is obtained, it is easy to perform hot forging and hot rolling, cracks and cracks are suppressed, and sound steel can be produced.

  Next, after the hot working, one or more heat treatments are performed as necessary. Examples of the heat treatment include quenching and tempering, sub-zero treatment, and spheroidizing annealing.

  Specifically, the quenching and tempering is, for example, heating at 1000 to 1200 ° C. for 0.5 to 1.5 hours, followed by oil cooling and quenching, and then, for example, −196 ° C. or −76 as necessary. Examples thereof include a method of performing sub-zero treatment at a temperature of 0.5 to 1 hour, heating at 200 to 700 ° C. for a time of 0.5 to 1.5 hours, air cooling, and tempering.

  In addition, the spheroidizing annealing is specifically performed by, for example, a method of heating at 850 to 900 ° C. for 3 to 5 hours, furnace cooling to about 600 ° C. at a rate of 10 to 20 ° C./hour, and then air cooling. It can be illustrated.

2. Main mold The main mold uses the above-described steel for a mold as a material.

  Here, the hardness of the present steel for molds is 55 HRC or more. A hardness of 55 HRC or more can be imparted, for example, by quenching and tempering the mold steel having the above-described chemical components at an optimal temperature and time.

  In addition, the hardness of this metal mold | die is the hardness measured by the Rockwell C scale about metal mold | die material (base material) itself, and is from the part which is not influenced by the work hardening by cold work, and various surface treatments. Measured.

  The hardness of the present mold is preferably 56 HRC or higher, more preferably 58 HRC or higher, from the viewpoint of imparting wear resistance. In addition, the upper limit of the hardness of this metal mold | die is not specifically limited.

  The present mold can be manufactured, for example, by subjecting the mold steel to rough machining, quenching and tempering so that the hardness becomes 55 HRC or more, and performing precision mold processing.

  In addition, the shape of this metal mold | die is suitably selected according to the shape to give to a plastic molded product, and is not specifically limited.

Hereinafter, the present invention will be described more specifically with reference to examples.
Steels having chemical compositions shown in Table 1 were melted in a vacuum induction furnace (under no pressure), and then two 50 kg ingots were cast.

  Of the two ingots after casting, one was cut to investigate the void area ratio, and the other one was hot forged to produce a 60 mm square bar. The area ratio of voids was measured by polishing a slice of the center of the ingot, photographing 60 fields of view with an optical microscope at 100 times, and performing image analysis.

  Table 1 shows the chemical compositions of the steels according to the examples and comparative examples. The steel according to Comparative Example 1 is a general-purpose steel type SUS420J2, the steel according to Comparative Example 2 is a general-purpose steel type SUS440C, and the steel according to Comparative Example 3 is a general-purpose steel type SKD11. Comparative Examples 4 to 6 are conventional steels having a relatively high N content and a value of C + N that is outside the scope of the present application. Table 2 shows the void area ratio of the ingot and the result of whether or not hot forging is possible.

  According to Table 2, the following can be understood. That is, since the steels according to Comparative Examples 4 to 6 contain a large amount of N, when an ingot is produced under non-pressurization, a large number of voids are generated in the ingot by N blow, and subsequent hot forging becomes difficult. , Sound steel could not be manufactured. Therefore, it can be said that these steels have low manufacturing freedom and are inferior in manufacturability.

  In contrast to the steels according to Comparative Examples 4 to 6, the steels according to Examples 1 to 8 and the steels according to Comparative Examples 1 to 3 are less prone to voids during ingot production and have excellent manufacturability. It could be confirmed.

  Then, about the following evaluation, except the steel which concerns on Comparative Examples 4-6, it performed about the steel which concerns on Examples 1-8 and the steel which concerns on Comparative Examples 1-3.

  That is, each bar described above was annealed, and test pieces used for evaluation of hardness measurement, wear resistance, specularity, and corrosion resistance were cut out from the obtained bar, and various evaluations were performed.

<Hardness>
A cubic block with a side of 10 mm was cut out from each bar, the heat treatment shown in Table 3 was performed, the measurement surface and the ground contact surface were polished to # 400, and the hardness was measured with a Rockwell C scale.

<Abrasion resistance>
Wear resistance was evaluated using a pin-on-disk friction and wear tester. That is, two pins φ8 mm × 2 were cut out from each bar, and the heat treatment shown in Table 3 was performed. The disc was cut out from S45C. The test conditions were a sliding speed of 1.6 m / s, a sliding distance of 5000 m, a pressing load of 10.5 kgf, and no lubricating oil. The weight of the pin was measured before and after the test, and the wear weight was measured from this. In this evaluation, the wear weight ratio of the steel according to the example and the steel according to the remaining comparative examples when the wear amount of the steel according to comparative example 3 (SKD11) is set to 100 is shown.

<Specularity>
A 50 mm × 45 mm × 12 mm plate was processed from each bar and subjected to the heat treatment shown in Table 3, followed by mechanical polishing to # 14000 to produce a test piece. About the obtained test piece, surface roughness Rzmax was measured based on JIS B0633.

  Moreover, as data relating to specularity, the particle size of carbonitride in each steel was measured in the following manner.

  That is, a cubic block having a side of 15 mm was cut out from each bar, and after the heat treatment shown in Table 3, the measurement surface was polished to # 1500. Subsequently, buffing was performed using 1 μm diamond paste, and final polishing was performed. Subsequently, the measurement surface was corroded using Villera corrosive liquid. Next, the measurement surface was photographed with an optical microscope (10 fields at 400 magnifications), and the value of the particle size at which 90% or more of the total number of observed carbonitrides was less than that value was determined as a representative value.

<Corrosion resistance>
A round bar of φ15 mm × 60 mm was processed from each bar, and after performing the heat treatment shown in Table 3, the surface was made equivalent to # 400 by finishing. Next, a salt spray test was performed in accordance with JIS Z 2371 to confirm the rusting state.

  Here, the case where rust did not occur, the case where rust occurred slightly, the case where rust occurred but did not reach the entire surface were all good, and the case where rust occurred on the entire surface was judged as bad. . This is because in the present application, it is only necessary to ensure good corrosion resistance to the extent necessary for mold maintenance.

  Comparing Table 4 shows the following. That is, in the steel (SUS420J2) according to Comparative Example 1, the C + N content is outside the specified range of the present application, among others. Therefore, it is understood that sufficient hardness cannot be obtained even when heat treatment is performed, and the amount of wear is large.

  The steel (SUS440C) according to Comparative Example 2 has a particularly high C content as compared with the specified range of the present application. For this reason, when manufactured by a melting method, coarse carbides are deposited, which drops off from the surface at the time of polishing, and the specularity is poor.

  The steel (SKD11) according to Comparative Example 3 has a particularly high C content compared to the specified range of the present application. Therefore, the corrosion resistance required for mold maintenance could not be satisfied. This is because carbide was easily generated and Cr was easily consumed. Moreover, the mirror surface property was also bad by the carbide | carbonized_material.

  Compared to the steels according to Comparative Examples 1 to 3 and Comparative Examples 4 to 6, the steels according to Examples 1 to 8 are all excellent in steel material manufacturability and excellent when applied to a mold. It was confirmed that wear resistance and specularity were obtained, and that good corrosion resistance was obtained.

  Heretofore, the embodiments and examples of the present invention have been described. The present invention is not particularly limited to these embodiments and examples, and various modifications can be made.

Claims (5)

% By mass
C: 0.350% or more and 0.570% or less,
Si: 0.10% or more and 0.50% or less,
Mn: 0.10% or more and 1.00% or less,
Cu: 0.05% or more and 0.20% or less,
Ni: 0.05% or more and 0.30% or less,
Cr: 10.00% or more and less than 14.00%,
Mo: less than 0.20%,
V: 0.01% or more and 0.10% or less,
N: 0.080% or more and 0.150% or less,
O: 0.0100% or less,
Al: 0.0500% or less,
C + N: Steel for plastic molds containing 0.500% or more and 0.650% or less, the balance being made of Fe and inevitable impurities. % By mass
B: Steel for plastic molds according to claim 1, further comprising 0.0005% or more and 0.0020% or less. % By mass
Nb: 0.001% or more and 0.300% or less,
Ta: 0.001% or more and 0.300% or less,
Ti: 0.20% or less, and
Zr: Steel for plastic molds according to claim 1 or 2, further comprising one or more elements selected from 0.001% to 0.300%. % By mass
Se: 0.001% or more and 0.300% or less,
Te: 0.001% or more and 0.300% or less,
Ca: 0.002% or more and 0.100% or less,
Pb: 0.001% or more and 0.200% or less, and
Bi: Steel for plastic molds according to any one of claims 1 to 3, further comprising one or more elements selected from 0.001% to 0.300%. . The steel for plastic molds according to any one of claims 1 to 4 is used as a material,
A plastic mold having a hardness of 55 HRC or more.