JP6359241B2 - Corrosion-resistant plastic molding steel with excellent specularity - Google Patents

Description

  The present invention relates to a mold and a tool for molding a plastic product that requires high surface smoothness such as an optical disk.

  A mold for molding a plastic product that requires surface smoothness such as an optical disk is required to have high specularity. Further, since this mold is required to have corrosion resistance against the corrosive gas generated during plastic molding, SUS420J2 equivalent steel defined by JIS is used as a steel for plastic molding. By the way, SUS420J2-equivalent steel as plastic molding steel is currently used with segregation of carbonitrides remaining.

  As described above, a mold molded from SUS420J2 equivalent steel, which is a plastic mold steel, is polished to enhance the specularity, but the carbide in the equivalent steel is hard and difficult to polish. Since the segregated carbonitride and its periphery remain without being polished, the periphery of the carbonitride becomes slightly wavy. Moreover, many carbonitrides in the segregation part are coarse and are missing to form holes. Therefore, in a plastic mold made of SUS420J2 equivalent steel, large undulations and holes are generated in the carbonitride segregation part of the mold. For this reason, it turned out that the part of the carbonitride segregation part is used in a low mirror surface state as a plastic mold.

  By the way, in order to suppress the lack of carbonitrides as described above, the size of carbonitrides is finely controlled by reducing the C content and increasing the N content in the steel component composition. A plastic mold steel has been proposed (see, for example, Patent Document 1). However, this Patent Document 1 does not consider the swell due to segregation, and therefore it must be said that the specularity is insufficient.

  Furthermore, in order to prevent agglomeration of carbides, a method of obtaining a steel material by a powder metallurgy method has been proposed (for example, see Patent Document 2). However, in this patent document 2, since the component composition of steel contains a lot of C and carbide forming elements, coarsening of the carbide is inevitable, and carbide may be lost during polishing, and the specularity may be lowered. .

JP 2007-9321 A Japanese Patent Laid-Open No. 11-43747

  As described above, a mold for molding a plastic product that requires surface smoothness such as an optical disk is required to have high specularity, and the mold also has corrosion resistance against corrosive gas generated during plastic molding. is necessary. In order to satisfy these requirements, SUS420J2 equivalent steel is used as the steel for plastic molding dies, but since this steel has hard carbides and carbonitrides precipitated, the metal mold for the plastic molding die is used. The specularity of the mold surface was low. Therefore, the inventor was able to develop the present invention through earnest research.

  The problem to be solved by the present invention is to obtain a steel composed of a novel chemical component as a steel for molds for plastic molding, and in the carbides and carbonitrides precipitated and contained in the steel, The number of carbides per 100 μm square of the grain size, the plane parallel to the rolling direction of the steel (hereinafter referred to as the rolling surface), the maximum number and the minimum number of carbides per 50 μm square in the 300 μm square of the rolled surface It is to provide a new steel for molding metal molds with high specularity and corrosion resistance by setting the difference in a certain range.

  The means of the present invention for solving the above-mentioned problems is, in the first means, in mass%, C: 0.20 to 0.40%, Si: 0.2 to 1.0%, Mn: 1. 0% or less, Cr: 12.0 to 15.0%, Mo + W / 2: 0.4% or less, V: less than 0.1 to 1.0%, N: less than 100 ppm, the balance being inevitable impurities and Steel made of Fe. And, as carbides and carbonitrides precipitated and contained in this steel, the particle size of carbonitride is 5 μm or less, desirably 4 μm or less, and the number of carbides per 100 μm square on the rolled surface of the steel is 160 or less. Further, the difference in the number of the maximum number of carbides per 50 μm square in the 300 μm square of the rolled surface of the steel and the number of the minimum parts is 30 or less. Mold steel.

In the second means , by mass%, C: 0.20 to 0.40%, Si: 0.2 to 1.0%, Mn: 1.0% or less, Cr: 12.0 to 15.0% Mo + W / 2: 0.4% or less, V: less than 0.1 to 1.0%, N: less than 100 ppm, P: 0.030% or less, S: 0.010% or less, desirably 0.005% or less , Al: 0.05% or less, desirably 0.03% or less, O: 0.0100% or less, desirably 0.0050% or less, and the balance is steel made of inevitable impurities and Fe. And, as carbides and carbonitrides precipitated and contained in this steel, the particle size of carbonitride is 5 μm or less, desirably 4 μm or less, and the number of carbides per 100 μm square on the rolled surface of the steel is 160 or less. Further, the difference in the number of the maximum number of carbides per 50 μm square in the 300 μm square of the rolled surface of the steel and the number of the minimum parts is 30 or less. Mold steel.

  In the third means, by mass%, C: 0.20 to 0.40%, Si: 0.2 to 1.0%, Mn: 1.0% or less, Cr: 12.0 to 15.0% Mo + W / 2: 0.4% or less, V: less than 0.1 to 1.0%, N: less than 100 ppm, Ni: 0.04 to 0.40%, with the balance being inevitable impurities and Fe It is steel. And, as carbides and carbonitrides precipitated and contained in this steel, the particle size of carbonitride is 5 μm or less, desirably 4 μm or less, and the number of carbides per 100 μm square on the rolled surface of the steel is 160 or less. Further, the difference in the number of the maximum number of carbides per 50 μm square in the 300 μm square of the rolled surface of the steel and the number of the minimum parts is 30 or less. Mold steel.

In the 4th means , by mass%, C: 0.20-0.40%, Si: 0.2-1.0%, Mn: 1.0% or less, Cr: 12.0-15.0% Mo + W / 2: 0.4% or less, V: less than 0.1 to 1.0%, N: less than 100 ppm, P: 0.030% or less, S: 0.010% or less, desirably 0.005% or less Al: 0.05% or less, preferably 0.03% or less, O: 0.0100% or less, preferably 0.0050% or less, Ni: 0.04 to 0.40%, the balance being inevitable impurities and Steel made of Fe. And, as carbides and carbonitrides precipitated and contained in this steel, the particle size of carbonitride is 5 μm or less, desirably 4 μm or less, and the number of carbides per 100 μm square on the rolled surface of the steel is 160 or less. Further, the difference in the number of the maximum number of carbides per 50 μm square in the 300 μm square of the rolled surface of the steel and the number of the minimum parts is 30 or less. Mold steel.

  In the fifth means, by mass%, C: 0.20 to 0.40%, Si: 0.2 to 1.0%, Mn: 1.0% or less, Cr: 12.0 to 15.0% , Mo + W / 2: 0.4% or less, V: less than 0.1 to 1.0%, N: less than 100 ppm, and one or more of Ti, Nb, Ta, and Zr are 0 0.01 to 0.30% of the steel, the balance being inevitable impurities and Fe. And, as carbides and carbonitrides precipitated and contained in this steel, the particle size of carbonitride is 5 μm or less, desirably 4 μm or less, and the number of carbides per 100 μm square on the rolled surface of the steel is 160 or less. Further, the difference in the number of the maximum number of carbides per 50 μm square in the 300 μm square of the rolled surface of the steel and the number of the minimum parts is 30 or less. Mold steel.

  In the sixth means, by mass%, C: 0.20 to 0.40%, Si: 0.2 to 1.0%, Mn: 1.0% or less, Cr: 12.0 to 15.0% Mo + W / 2: 0.4% or less, V: less than 0.1 to 1.0%, N: less than 100 ppm, P: 0.030% or less, S: 0.010% or less, desirably 0.005% or less Al: 0.05% or less, desirably 0.03% or less, O: 0.0100% or less, desirably 0.0050% or less, and one or two of Ti, Nb, Ta, and Zr The steel contains 0.01 to 0.30% of the above, and the balance is inevitable impurities and Fe. And, as carbides and carbonitrides precipitated and contained in this steel, the particle size of carbonitride is 5 μm or less, desirably 4 μm or less, and the number of carbides per 100 μm square on the rolled surface of the steel is 160 or less. Further, the difference in the number of the maximum number of carbides per 50 μm square in the 300 μm square of the rolled surface of the steel and the number of the minimum parts is 30 or less. Mold steel.

In the seventh means, by mass%, C: 0.20 to 0.40%, Si: 0.2 to 1.0%, Mn: 1.0% or less, Cr: 12.0 to 15.0% Mo + W / 2: 0.4% or less, V: less than 0.1 to 1.0%, N: less than 100 ppm , Ni: 0.04 to 0.40%, and Ti, Nb, Ta, Zr It is steel which contains 0.01 to 0.30% of 1 type or 2 types or more, and the balance consists of inevitable impurities and Fe. And, as carbides and carbonitrides precipitated and contained in this steel, the particle size of carbonitride is 5 μm or less, desirably 4 μm or less, and the number of carbides per 100 μm square on the rolled surface of the steel is 160 or less. Further, the difference in the number of the maximum number of carbides per 50 μm square in the 300 μm square of the rolled surface of the steel and the number of the minimum parts is 30 or less. Mold steel.

In the eighth means, in mass% , C: 0.20 to 0.40%, Si: 0.2 to 1.0%, Mn: 1.0% or less, Cr: 12.0 to 15.0% Mo + W / 2: 0.4% or less, V: less than 0.1 to 1.0%, N: less than 100 ppm, P: 0.030% or less, S: 0.010% or less, preferably 0.005% or less Al: 0.05% or less, desirably 0.03% or less, O: 0.0100% or less, desirably 0.0050% or less, Ni: 0.04 to 0.40%, Ti, Nb, One or more of Ta and Zr is contained in an amount of 0.01 to 0.30%, and the balance is steel made of inevitable impurities and Fe. And, as carbides and carbonitrides precipitated and contained in this steel, the particle size of carbonitride is 5 μm or less, desirably 4 μm or less, and the number of carbides per 100 μm square on the rolled surface of the steel is 160 or less. Further, the difference in the number of the maximum number of carbides per 50 μm square in the 300 μm square of the rolled surface of the steel and the number of the minimum parts is 30 or less. Mold steel.

  By adopting the above-mentioned means, the present invention can be used as a mold or a tool for molding a plastic product such as an optical disk that requires high surface smoothness, and has a high specularity and excellent corrosion resistance. The steel for plastic molds of the present invention has a small particle size of carbonitride on the steel surface, and the surface of the rolled surface is 100 μm square on the mirror surface without causing holes due to sliding of these carbonitrides when polished. The number of carbides per unit is small and there is no large amount of unevenness on the part, and the difference between the maximum part and the minimum part of the number of carbides per 50 μm square in the 300 μm square of the rolling surface is small, so the minimum part Has excellent effects such as no waviness on the mirror surface without being easily polished.

  As a mode for carrying out the invention of the present application, first, the reasons for limiting the component ranges of chemical components in the steel for plastic molds according to the invention of the present application will be described below. In addition, a chemical component is shown by the mass%.

C: 0.20 to 0.40%
C is an element necessary for improving hardness, wear resistance and hardenability by solid solution strengthening, and for this purpose, 0.20% or more is necessary. However, if C exceeds 0.40%, coarse carbides are formed, the mirror surface property of the steel for molds is deteriorated, and the corrosion resistance is lowered. Therefore, C is set to 0.20 to 0.40%.

Si: 0.2 to 1.0%
Si is an element necessary for obtaining a deoxidizer at the time of melting, hardenability of steel, and hardness of the base, and 0.2% or more is necessary for this purpose. However, when Si exceeds 1.0%, toughness and workability deteriorate. Therefore, Si is 0.2 to 1.0%.

Mn: 1.0% or less Mn is an element necessary for obtaining a deoxidizer during melting and hardenability of steel. However, if Mn exceeds 1.0%, the matrix becomes brittle. Therefore, Mn is 1.0% or less.

Cr: 12.0 to 15.0%
Cr is an element that forms a passive state, improves corrosion resistance, and enhances hardenability. For this purpose, Cr needs to be 12.0% or more. However, when Cr exceeds 15.0%, coarse carbides are formed, and the mirror surface property of the steel for molds is deteriorated. Therefore, Cr is 12.0 to 15.0%.

Mo + W / 2: 0.4% or less Mo and W form hard carbides, improve hardness and wear resistance, and improve hardenability and temper softening resistance. For this purpose, Mo + W / 2 needs to be 0.4% or less. When Mo + W / 2 exceeds 0.4%, coarse carbides and carbide segregation are formed and the specularity is deteriorated. Therefore, Mo + W / 2 is set to 0.4% or less.

V: Less than 0.1 to 1.0% V is finely dispersed and precipitated as a VC in the steel material, and the VC serves as a precipitation starting point for carbides of other elements and suppresses carbide segregation. For this purpose, V needs to be 0.1% or more. However, if V is 1.0% or more, coarse carbonitride and carbonitride segregation are formed, and the specularity is deteriorated. Therefore, V is set to less than 0.1 to 1.0%.

N: Less than 100 ppm When N is contained in an amount of 100 ppm or more, a nitride is formed in the steel, which is lost during polishing of the steel surface to form a hole, thereby reducing the specularity. Therefore, N is set to less than 100 ppm.

P: 0.030% or less P segregates to a grain boundary and lowers toughness. Therefore, P is set to 0.030% or less.

S: 0.010% or less, desirably 0.0050% or less S forms sulfides, deteriorates specularity, and further deteriorates toughness and hot workability. Therefore, S is set to 0.010% or less, preferably 0.0050% or less.

Al: 0.05% or less, desirably 0.03% or less Al forms oxides and nitrides and deteriorates the specularity. Therefore, Al is made 0.05% or less, preferably 0.03% or less.

O: 0.0100% or less, desirably 0.0050% or less O forms oxides with other metal elements and deteriorates the specularity. Therefore, O is 0.0100% or less, preferably 0.0050% or less.

Ni: 0.04 to 0.40%
Ni is an element that enhances hardenability and corrosion resistance. For this purpose, Ni needs to be 0.04% or more. However, when Ni is contained in an amount of 0.40% or more, an austenite structure remains and causes aging of the steel material. Therefore, Ni is set to 0.04 to 0.40%.

One or more of Ti, Nb, Ta, and Zr: 0.01 to 0.30% in total
Ti, Nb, Ta, and Zr are elements that improve toughness by forming fine carbonitrides and suppressing coarsening of crystal grains. For that purpose, the content of one or more of Ti, Nb, Ta, and Zr needs to be 0.01% or more in total. However, when the content of one or more of Ti, Nb, Ta, and Zr exceeds 0.30% in total, coarse carbonitride and carbonitride segregation are formed and mirror surface Worsens sex. Therefore, one or more of Ti, Nb, Ta, and Zr is made 0.01 to 0.30% in total.

  Furthermore, the reasons for limitation of the structural requirements other than the above-described chemical components will be described below with respect to the structural requirements of the plastic mold steel in the claims of the present application.

Carbonitride particle size: 5 μm or less, desirably 4 μm or less When the carbonitride particle size is coarse, it is missing from the steel surface during polishing to form pores, thereby reducing the specularity. Therefore, the carbonitride particle size is 5 μm or less, preferably 4 μm or less.

The number of carbides per 100 μm square on the rolled surface of the steel material: 160 or less If the number of carbides per 100 μm square on the rolled surface of the steel material is too large, a large amount of fine unevenness is generated at that portion. Therefore, the number of carbides per 100 μm square on the rolled surface of the steel material is 160 or less.

Difference between the maximum number of carbides per 50 μm square and the minimum number of carbides per 300 μm square on the rolled surface of the steel: 30 or less The maximum number of carbides per 50 μm square in the 300 μm square on the rolled surface of the steel If the difference between the number and the number of the minimum part is large, the minimum part is easily polished, so that the mirror surface is wavy. Therefore, the difference between the maximum number of carbides per 50 μm square out of 300 μm square on the rolled surface of the steel material and the number of minimum parts is 30 or less.

  Further, the embodiments of the present invention will be sequentially described below with reference to the table. First, in the steel according to the present invention, for example, the following production methods (1) and (2) can be suitably applied as a method for realizing a carbide precipitation state that provides high specularity.

  (1) A manufacturing method in which a cast material composed of the chemical components of the steel of the present invention is remelted, secondarily melted, and resolidified. Generally, vacuum arc remelting (VAR) or electroslag remelting is used. It is secondarily dissolved and re-coagulated by the method (ESR). That is, in this method, solidification after re-dissolution is performed in a short time by secondary dissolution, so that solidification segregation hardly occurs and partial agglomeration of carbide can be suppressed.

  (2) This is a manufacturing method in which the steel melted and re-solidified by (1) is subjected to a soaking treatment at 1000 to 1200 ° C. for 10 hours or more. This production method is an optimum production method for controlling coarse carbides precipitated in the steel within a suitable range. This soaking process needs to be performed at a temperature higher than the quenching temperature and lower than the melting point. If the soaking process is properly performed, it is possible to reduce the formed coarse carbides and further reduce the amount of carbides to uniformly disperse. The appropriate temperature and time for the soaking process vary depending on the components.

100 kg of plastic molding steel made of the chemical components of the inventive steel shown in Table 1 was melted in a vacuum induction melting furnace. The steel material obtained by this melting is soaked in a soaking furnace heated to 1200 ° C. for 10 hours, forged into a square material of 50 mm in length and width, and then subjected to quenching treatment that is heated to 1030 ° C. and air-cooled. The tempering process of heating to 200 to 500 ° C. and air cooling was repeated twice.
On the other hand, a steel having a chemical composition outside the range of the chemical composition of the invention steel of Table 1 is used as a comparative steel, 100 kg of which is melted in a vacuum induction melting furnace, and the steel material obtained by melting is heated to 1200 ° C. It was soaked in a heating furnace for 10 hours, forged into a square material having a length and width of 50 mm, and then subjected to a quenching treatment by heating to 1030 ° C. and air cooling. The same treatment as that of the inventive steel according to the above-described embodiment of the present invention was performed, except that the annealing, that is, the annealing treatment was omitted.
The quenching and tempering hardness in these was 50 HRC or more.

  Regarding the specularity, in the above method of the present invention, after tempering, a square material having a length and width of 50 mm was cut, and the horizontal surface was polished to # 14000 by mechanical polishing to prepare a test piece. The surface center (300 μm square) after polishing was photographed with a laser scanning ultrasonic microscope, and the number of carbides per 50 μm square in the image and the size of the carbides were measured with an image analyzer. Further, the roughness index Ra and the waviness index WSm were measured on the polished surface. If Ra is 0.015 or less (Ra ≦ 0.015) and WSm is 65 μm (WSm ≦ 65 μm), even if surface roughness and undulation are transferred to the steel surface of the product, the product will not be affected. Table 2 shows the results of the evaluation with the invention steel according to the present invention as the invention steel and the comparison steel as the comparison steel.

  As can be seen in Table 1, the comparative steels outside the range of the chemical composition of the claimed invention, and the comparative steels not subjected to soaking treatment are oxide and carbon from the steel surface as seen in Table 2. There is a lack of nitride, waviness occurs on the steel surface, and the roughness index Ra and the waviness index WSm are large. In contrast, the steel of the present invention has a small roughness index Ra and a small undulation index WSm. Therefore, the plastic molding die made of the steel of the present invention has excellent specularity.

Claims (8)

  In mass%, C: 0.20 to 0.40%, Si: 0.2 to 1.0%, Mn: 1.0% or less, Cr: 12.0 to 15.0%, Mo + W / 2: 0 .4% or less, V: 0.1 to less than 1.0%, N: less than 100 ppm, the balance being steel composed of Fe and inevitable impurities, as carbides and carbonitrides precipitated and contained in the steel The carbonitride has a particle size of 5 μm or less, the number of carbides per 100 μm square on the rolled surface of the steel is 160 or less, and the number of carbides per 50 μm square in 300 μm square on the rolled surface of the steel. A steel for a corrosion-resistant plastic molding die excellent in specularity, characterized in that the difference between the number of maximum parts and the number of minimum parts is 30 or less.   In mass%, C: 0.20 to 0.40%, Si: 0.2 to 1.0%, Mn: 1.0% or less, Cr: 12.0 to 15.0%, Mo + W / 2: 0 .4% or less, V: less than 0.1 to 1.0%, N: less than 100 ppm, P: 0.030% or less, S: 0.010% or less, Al: 0.05% or less, O: 0.0. 0100% or less, the balance being steel composed of Fe and inevitable impurities, and as carbides and carbonitrides precipitated and contained in the steel, the grain size of the carbonitride is 5 μm or less, and on the rolling surface of the steel The number of carbides per 100 μm square is 160 or less, and the difference between the maximum number of carbides per 50 μm square in the 300 μm square on the rolling surface of the steel and the number of minimum parts is 30 or less. A special steel for molding metal molds with excellent specularity and corrosion resistance.   In mass%, C: 0.20 to 0.40%, Si: 0.2 to 1.0%, Mn: 1.0% or less, Cr: 12.0 to 15.0%, Mo + W / 2: 0 0.4% or less, V: 0.1 to less than 1.0%, N: less than 100 ppm, Ni: 0.04 to 0.40%, the balance being Fe and inevitable impurities, the steel The carbides and carbonitrides precipitated and contained in the carbonitride have a particle size of 5 μm or less, the number of carbides per 100 μm square on the rolled surface of the steel is 160 or less, and further on the rolled surface of the steel A corrosion-resistant plastic mold steel having excellent specularity, characterized in that the difference between the maximum number of carbides per 50 μm square in 300 μm square and the minimum number of carbides is 30 or less. In mass%, C: 0.20 to 0.40%, Si: 0.2 to 1.0%, Mn: 1.0% or less, Cr: 12.0 to 15.0%, Mo + W / 2: 0 .4% or less, V: less than 0.1 to 1.0%, N: less than 100 ppm, P: 0.030% or less, S: 0.010% or less, Al: 0.05% or less , O: 0.0. 0100% or less, Ni: 0.04 to 0.40% is contained, and the balance is steel composed of Fe and inevitable impurities, and as carbides and carbonitrides precipitated and contained in the steel, the particle size of carbonitrides Is not more than 5 μm, the number of carbides per 100 μm square on the rolled surface of the steel is not more than 160, and the number of carbides and the minimum part of the number of carbides per 50 μm square in 300 μm square on the rolled surface of the steel Corrosion-resistant plastic with excellent specularity, characterized in that the difference in the number of particles is 30 or less Chick molding die steel.   In mass%, C: 0.20 to 0.40%, Si: 0.2 to 1.0%, Mn: 1.0% or less, Cr: 12.0 to 15.0%, Mo + W / 2: 0 .4% or less, V: 0.1 to less than 1.0%, N: less than 100 ppm, and further, one or more of Ti, Nb, Ta, and Zr are 0.01 to 0.30. In which the balance is Fe and inevitable impurities, and the carbides and carbonitrides precipitated and contained in the steel have a carbonitride particle size of 5 μm or less, and 100 μm square on the rolling surface of the steel. The number of carbides per unit is 160 or less, and the difference between the maximum number and the minimum number of carbides per 50 μm square in 300 μm square on the rolled surface of the steel is 30 or less. Corrosion-resistant plastic mold steel with excellent specularity.   In mass%, C: 0.20 to 0.40%, Si: 0.2 to 1.0%, Mn: 1.0% or less, Cr: 12.0 to 15.0%, Mo + W / 2: 0 .4% or less, V: less than 0.1 to 1.0%, N: less than 100 ppm, P: 0.030% or less, S: 0.010% or less, Al: 0.05% or less, O: 0.0. 0100% or less, and further containing 0.01 to 0.30% of one or more of Ti, Nb, Ta, and Zr, with the balance being Fe and inevitable impurities, the steel The carbides and carbonitrides precipitated and contained in the carbonitride have a particle size of 5 μm or less, the number of carbides per 100 μm square on the rolled surface of the steel is 160 or less, and further on the rolled surface of the steel The difference between the maximum number and the minimum number of carbides per 50 μm square in 300 μm square is 3 Specularity excellent corrosion resistance plastic molding die steel, characterized in that at number less. In mass%, C: 0.20 to 0.40%, Si: 0.2 to 1.0%, Mn: 1.0% or less, Cr: 12.0 to 15.0%, Mo + W / 2: 0 .4% or less, V: less than 0.1 to 1.0%, N: less than 100 ppm , Ni : 0.04 to 0.40%, and one of Ti, Nb, Ta and Zr or two or more containing 0.01 to 0.30%, a steel remaining part being Fe and unavoidable impurities, as a carbide and carbonitride are contained precipitated in the steel, the grain size of the carbonitride 5μm The number of carbides per 100 μm square on the rolling surface of the steel is 160 or less, and the maximum number and the minimum number of carbides per 50 μm square in 300 μm square on the rolling surface of the steel Corrosion-resistant plastic with excellent specularity, characterized by having a difference of 30 or less Metal mold steel. In mass%, C: 0.20 to 0.40%, Si: 0.2 to 1.0%, Mn: 1.0% or less, Cr: 12.0 to 15.0%, Mo + W / 2: 0 .4% or less, V: less than 0.1 to 1.0%, N: less than 100 ppm, P: 0.030% or less, S: 0.010% or less, Al: 0.05% or less, O: 0.0. 0100% or less, Ni: 0.04 to 0.40%, further containing 0.01 to 0.30% of one or more of Ti, Nb, Ta and Zr, the balance being Fe Further, as carbides and carbonitrides precipitated and contained in the steel, the grain size of the carbonitride is 5 μm or less, and the number of carbides per 100 μm square on the rolled surface of the steel is 160 pieces. And the maximum number of carbides per 50 μm square out of 300 μm square on the rolling surface of the steel. A corrosion-resistant plastic mold steel excellent in specularity, characterized in that the difference between the number of major parts and the number of smallest parts is 30 or less.