Steel for a plastic molding die

Abstract

To provide a steel for plastic molding die which possesses enough hardness, wear resistance and corrosion resistance, and is excellent in high-precision processability and mirror polishing properties. The steel for a plastic molding die contains not more than 0.80 wt % C, not less than 0.01 wt % and less than 1.40 wt % Si, not less than 0.05 wt % and not more than 2.0 wt % Mn, not less than 0.005 wt % and not more than 1.00 wt % Ni, not less than 13.0 wt % and not more than 20.0 wt % Cr, not less than 0.20 wt % and not more than 4.0 wt % Mo+1/2 W, not less than 0.01 wt % and not more than 1.00 wt % V, not less than 0.36 wt % and not more than 0.80 wt % N, not more than 0.02 wt % O, not more than 0.80 wt % Al, and the remainder substantially including Fe and unavoidable impurities.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a steel for a plastic molding die.

[0003] 2. Description of Related Art

[0004] These days, various plastic moldings are used in a variety of areas. The plastic moldings are generally molded in a desired shape by the use of, for example, a plastic molding die such as an injection molding die.

[0005] Incidentally, from the viewpoint of improving strength of moldings, there is a case that a filler such as glass fiber is added to a material for the plastic moldings in addition to a resin being a main ingredient. This sort of additive wears away the die, lowering dimensional accuracy of plastic moldings to be obtained and reducing the lifetime of the die.

[0006] In addition, there is a case that the plastic molding material generates a corrosive gas due to decomposition of the resin during the course of kneading or the like. When compressed to be high temperature and high pressure in the die, this sort of corrosive gas rots the die, giving rise to surface roughness, a burr and the like in the plastic moldings to be obtained.

[0007] Therefore, for a material for the plastic molding die, it is necessary to use a metallic material which is excellent in hardness, wear resistance and corrosion resistance.

[0008] Conventionally, as the metallic material of this sort, there is known a martensitic stainless steel such as SUS440C and an improved version thereof.

[0009] In addition, for example, Japanese Patent Gazette No. 3438121 discloses an alloy for a plastic molding die containing 0.25 wt % to 1.0 wt % C, 1.0 wt % maximum Si, 1.6 wt % maximum Mn, 0.10 wt % to 0.35 wt % N, 1.0 wt % maximum Al, 2.8 wt % maximum Co, 14.0 wt % to 25.0 wt % Cr, 0.5 wt % to 3.0 wt % Mo, 3.9 wt % maximum Ni, 0.04 wt % to 0.4 wt % V, 3.0 wt % maximum W, 0.18 wt % maximum Nb, and 0.20 wt % maximum Ti, in which the sum of concentrations of C and N is at least 0.5 wt % and 1.2 wt % maximum, and the remainder including Fe and unavoidable impurities.

[0010] However, as the alloy described in the above-mentioned Japanese Patent Gazette No. 3438121 is prone to generate coarse crystallized carbonitrides in the manufacturing stage, there arises a problem, resulting from a difference between hardness of the generated coarse crystallized carbonitrides and hardness of a matrix phase of the alloy, that finishing accuracy is unfavorable as developing unevenness at the time of diesinking working.

[0011] In addition, for the plastic molding die, it is often the case that the inner surface of the die is mirror polished from the viewpoint of making states of moldings' surfaces excellent; however, the alloy described in the above-mentioned Japanese Patent Gazette No. 3438121 has a problem that mirror polishing properties are degraded due to the coarse crystallized carbonitrides.

[0012] It sounds logical that the alloy described in the above-mentioned Japanese Patent Gazette No. 3438121 is made less prone to generate the coarse crystallized carbonitrides simply by decreasing the C-content in the alloy; however, decreasing the C-content causes a problem that wear resistance is decreased.

SUMMARY OF THE INVENTION

[0013] An object of the invention is to overcome the problems described above and to provide a steel for a plastic molding die which possesses enough hardness, wear resistance and corrosion resistance, and is excellent in high-precision processability and mirror polishing properties.

[0014] To achieve the objects and in accordance with the purpose of the present invention, a steel for aplastic molding die includes not more than 0.80 wt % C, not less than 0.01 wt % and less than 1.40 wt % Si, not less than 0.05 wt % and not more than 2.0 wt % Mn, not less than 0.005 wt % and not more than 1.00 wt % Ni, not less than 13.0 wt % and not more than 20.0 wt % Cr, not less than 0.20 wt % and not more than 4.0 wt % Mo+1/2 W, not less than 0.01 wt % and not more than 1.00 wt % V, not less than 0.36 wt % and not more than 0.80 wt % N, not more than 0.02 wt % O, not more than 0.80 wt % Al, and the remainder substantially including Fe and unavoidable impurities.

[0015] The steel for a plastic molding die consistent with the present invention is made to have the above-described composition, in which, especially, the C-content is decreased while the N-content is increased, so that required hardness is secured. Accordingly, the steel for a plastic molding die has enough hardness and wear resistance.

[0016] In addition, the N-content being increased, nitrogen is solubilized in a matrix phase of the steel and fine carbonitrides are formed, so that the steel for a plastic molding die is also excellent in corrosion resistance.

[0017] Further, the C-content being decreased, coarse crystallized carbonitrides are less prone to generate in the manufacturing stage. In addition, insoluble carbonitrides at the time of hardening decrease, and fine carbonitrides obtained by hardening and tempering are uniformly dispersed, so that the steel for a plastic molding die is excellent especially in high-precision processability and mirror polishing properties.

[0018] Additional objects and advantages of the invention are set forth in the description which follows, are obvious from the description, or may be learned by practicing the invention. The objects and advantages of the invention may be realized and attained by the steel for a plastic molding die in the claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] A detailed description of one preferred embodiment of a steel for a plastic molding die embodied by the present invention is provided below. The steel for a plastic molding die consistent with the present invention is characterized as containing elements as provided below, and the remainder substantially including Fe and unavoidable impurities. Hereinafter, types of the contained elements, and reasons for specifying their contents are described.

[0020] (1) C: not more than 0.80 wt %

[0021] C is an element which is necessary for securing strength and wear resistance, and generates carbides by combining with carbide-forming elements such as Cr, Mo, W, V and Nb. In addition, C is an element also necessary for securing hardness by solubilizing in a matrix phase of the steel at the time of hardening to form a martensitic structure.

[0022] An excessive content of C, however, increases a tendency to combine with the above-mentioned carbide-forming elements and a large amount of carbides are crystallized, so that coarse carbides come to remain. From the viewpoint of preventing this, the C-content is specifically not more than 0.80 wt %, preferably not more than 0.65 wt %, and more preferably less than 0.25 wt %.

[0023] In the present invention, it is desirable to decrease an amount of the coarse crystallized carbides and an amount of insoluble carbides at the time of hardening by decreasing the C-content as much as possible and uniformly disperse fine carbides obtained by hardening and tempering since hardness can be improved by increasing a content of N.

[0024] (2) Si: not less than 0.01 wt % and less than 1.40 wt %

[0025] Si functions, similarly to Al to be described later, as a deoxidation element; however, Al reacts with N and generates AlN to decrease an amount of N solubilized in the matrix phase and concurrently the generated coarse AlN degrades high-precision processability and mirror polishing properties. Accordingly, it is preferable to use Si as the deoxidation element to reduce a content of Al in the steel. Specifically, a content of Si is not less than 0.01 wt %, preferably not less than 0.05 wt %, and more preferably not less than 0.10 wt %.

[0026] An excessive content of Si, however, leads to decline in hot workability and toughness. From the view point of preventing this, the Si-content is specifically less than 1.40 wt %, preferably not more than 0.75 wt %, and more preferably not more than 0.25 wt %.

[0027] (3) Mn: not less than 0.05 wt % and not more than 2.0 wt %

[0028] Mn is added as an element for improving hardenability. In addition, in a case where S is contained unavoidably, Mn is effective at curbing decline in toughness. A content of Mn is specifically not less than 0.05 wt %.

[0029] An excessive content of Mn, however, leads to decline in hot workability, so that the Mn-content is not more than 2.0 wt %.

[0030] (4) Ni: not less than 0.005 wt % and not more than 1.00 wt %

[0031] Ni increases a solution amount of N. Specifically, a content of Ni is not less than 0.005 wt %.

[0032] An excessive content of Ni, however, increases residual austenite to cause changes in dimension with time, so that the Ni-content is specifically not more than 1.00 wt %.

[0033] (5) Cr: not less than 13.0 wt % and not more than 20.0 wt %

[0034] Cr increases the solution amount of N while improving corrosion resistance. In addition, Cr forms carbonitrides. Specifically, a content of Cr is not less than 13.0 wt %.

[0035] An excessive content of Cr, however, increases a residual austenite phase even though the steel is subjected to a subzero treatment, leading to decline in hardness, and also gives a cost increase. Accordingly, the Cr-content is specifically not more than 20.0 wt %.

[0036] (6) Mo+1/2 W: not less than 0.20 wt % and not more than 4.0 wt %

[0037] Mo and W increase the solution amount of N, and improve hardenability. In order to obtain these effects, a content of Mo and W is specifically not less than 0.20 wt % for Mo+1/2 W.

[0038] An excessive content of Mo and W, however, promotes generation of crystallized carbonitrides to lower an impact value, so that the content of Mo and W is specifically not more than 4.0 wt % for Mo+1/2 W.

[0039] (7) V: not less than 0.01 wt % and not more than 1.00 wt %

[0040] V increases the solution amount of N. In addition, V forms carbonitrides, and by a pin-in effect thereof, crystal grains are fined to improve strength. Specifically, a content of V is not less than 0.01 wt %.

[0041] An excessive content of V, however, increases a tendency to generate coarse carbonitrides, and degrades high-precision processability and mirror polishing properties. Accordingly, the V-content is specifically not more than 1.00 wt %.

[0042] (8) N: not less than 0.36 wt % and not more than 0.80 wt %

[0043] N is an interstitial element which contributes to improvement in hardness of a martensitic structure. In order to obtain this effect, a content of N is specifically not less than 0.36 wt %. N of the content can be added by dissolving under pressure according to Sieverts' law.

[0044] An excessive content of N, however, causes incrassation of N in solidification, and a blow-hole resulting from N (hereinafter, referred to as an "N blow") is prone to generate, making it difficult to curb the N blow by the application of pressure. Accordingly, the N-content is specifically not more than 0.80 wt %.

[0045] (9) O: not more than 0.02 wt %

[0046] O is an element which is unavoidably contained in a molten steel. When a content of O is high, coarse oxides are generated with Si and Al, and through the mediation of the coarse oxides, toughness, high-precision processability and mirror polishing properties are degraded. Accordingly, it is desirable for the O-content to be low as much as possible. Specifically, the O-content is not more than 0.02 wt %, and preferably not more than 0.01 wt %.

[0047] (10) Al: not more than 0.80 wt %

[0048] Al functions, similarly to Si, as a deoxidation element; however, when a content of Al is excessively high, coarse AlN is prone to generate to degrade high-precision processability and mirror polishing properties significantly. Accordingly, the Al-content is specifically not more than 0.80 wt %.

[0049] In addition to the above-described essential elements, the steel for a plastic molding die consistent with the present invention may further include one or more than one arbitrary element selected from the elements cited below. Hereinafter, reasons for specifying contents of the elements are described.

[0050] <1> P: not more than 0.030 wt % [0051] S: not more than 0.030 wt %

[0052] P and S are unavoidably contained in the steel. P is segregated to a crystal grain boundary and S forms sulfides, both of which lower toughness. Accordingly, contents of P and S are favorably not more than 0.030 wt %, respectively.

[0053] <2> Cu: not less than 0.001 wt % and not more than 0.50 wt % [0054] Co: not less than 0.001 wt % and not more than 0.50 wt % [0055] B: not less than 0.0005 wt % and not more than 0.010 wt %

[0056] All of Cu, Co and B contribute to improvement in hardenability. Specifically, a content of Cu is not less than 0.001 wt %, a content of Co is not less than 0.001 wt %, and a content of B is not less than 0.0005 wt %.

[0057] However, if the contents of Cu, Co and B are made excessively high, the effect of hardenability is only saturated and a cost increase is brought about. Accordingly, the Cu-content is specifically not more than 0.50 wt %, the Co-content is specifically not more than 0.50 wt %, and the B-content is specifically not more than 0.010 wt %.

[0058] <3> Se: not less than 0.001 wt % and not more than 0.30 wt % [0059] Te: not less than 0.001 wt % and not more than 0.30 wt % [0060] Ca: not less than 0.001 wt % and not more than 0.10 wt % [0061] Pb: not less than 0.001 wt % and not more than 0.20 wt % [0062] Bi: not less than 0.001 wt % and not more than 0.30 wt %

[0063] Se, Te, Ca, Pb and Bi contribute to improvement in machinablity. Specifically, a content of Se is not less than 0.001 wt %, a content of Te is not less than 0.001 wt %, a content of Ca is not less than 0.001 wt %, a content of Pb is not less than 0.001 wt %, and a content of Bi is not less than 0.001 wt %.

[0064] Excessive contents of Se, Te, Ca, Pb and Bi, however, lower toughness. Accordingly, the Se-content is specifically not more than 0.30 wt %, the Te-content is specifically not more than 0.30 wt %, the Ca-content is specifically not more than 0.10 wt %, the Pb-content is specifically not more than 0.20 wt %, and the Bi-content is specifically not more than 0.30 wt %.

[0065] <4> Ti: not more than 0.20 wt % [0066] Nb: not less than 0.001 wt % and not more than 0.30 wt % [0067] Ta: not less than 0.001 wt % and not more than 0.30 wt % [0068] Zr: not less than 0.001 wt % and not more than 0.30 wt %

[0069] Ti, Nb, Ta and Zr combine with C and N to form carbonitrides, and contribute to curbed coarsening of crystal grains. Specifically, a content of Ti is not less than 0.01 wt %, a content of Nb is not less than 0.001 wt %, a content of Ta is not less than 0.001 wt %, and a content of Zr is not less than 0.001 wt %.

[0070] Excessive contents of Ti, Nb, Ta and Zr, however, lower toughness. Accordingly, the Ti-content is specifically not more than 0.20 wt %, the Nb-content is specifically not more than 0.30 wt %, the Ta-content is specifically not more than 0.30 wt %, and the Zr-content is specifically not more than 0.30 wt %.

[0071] In addition, in the above-described steel for a plastic molding die, it is favorable that a particle size of contained carbonitrides is not more than 4.0 .mu.m, preferably not more than 3.5 .mu.m, and more preferably not more than 3.0 .mu.m, by which the steel is made excellent especially in high-precision processability and mirror polishing properties.

[0072] Incidentally, the particle size of carbonitrides indicates a representing value such that 90% or more of the total number of carbonitrides to be observed have particle sizes not larger than the representing value, when a measuring plane of a finishing-polished specimen is rotten using a corrosive liquid and observed through an optical microscope, a scanning electron microscope or the like.

[0073] Next, description will be given to one example of a production process of the above-described steel for a plastic molding die.

[0074] Cited are a production process in which the steel for a plastic molding die having the above-described composition is molten by the use of a melting furnace such as a high-frequency induction furnace capable of applying pressure, and cast into an ingot or the like, and thereafter, the ingot or the like is subjected to hot forging or hot rolling to produce a steel material having necessary dimensions, and the like.

[0075] One example of a heat treatment to which the above-described steel for a plastic molding die is subjected is as follows. Specifically, annealing can be performed, for example, by applying heat in a temperature range of 850.degree. C. to 900.degree. C. for 3 to 5 hours, then providing cooling in a furnace to the vicinity of 600.degree. C. at a velocity of 10-20.degree. C./hour, and thereafter providing air-cooling. In addition, specifically, hardening and tempering can be performed as follows: the hardening is performed, for example, by applying heat in a temperature range of 1000.degree. C. to 1200.degree. C. for 0.5 to 1.5 hours and then providing oil-cooling, and then, the steel is subjected to a subzero treatment at -196.degree. C. or -76.degree. C. for 0.5 to 1 hour, and thereafter, the tempering is performed by applying heat in a temperature range of 200.degree. C. to 700.degree. C. for 0.5 to 1.5 hours and then providing air-cooling.

EXAMPLES

[0076] Hereinafter, further detailed description on the present invention will be given employing and referring to Examples.

[0077] The steels having the chemical compositions listed in Table 1 (the steels consistent with Examples 1 to 16, and the steels consistent with Comparative Examples 1 to 6) were molten by the use of a high-frequency induction furnace capable of applying pressure, and then cast into 50 kg to produce squared bars 60 mm per side through hot forging. TABLE-US-00001 TABLE 1 C Si Mn P S Ni Cr Mo W Mo + 1/2W V Al O N Others Example 1 0.80 0.81 0.10 0.017 0.030 1.00 18.1 0.7 1.10 1.25 0.13 0.210 0.015 0.60 Ca = 0.05 Example 2 0.71 1.00 1.70 0.026 0.021 0.53 20.0 0.4 0.20 0.50 0.69 0.440 0.008 0.71 Ti = 0.13 Example 3 0.54 0.45 0.40 0.003 0.005 0.05 17.3 1.1 0.01 1.11 0.03 0.005 0.013 0.36 -- Example 4 0.35 1.30 0.80 0.021 0.011 0.81 16.6 0.5 1.90 1.45 0.44 0.460 0.017 0.41 -- Example 5 0.48 0.69 1.10 0.011 0.023 0.65 13.0 0.1 0.10 0.15 0.25 0.800 0.001 0.47 Nb = 0.10 Example 6 0.60 0.42 0.05 0.030 0.026 0.47 14.6 0.2 3.10 1.75 0.98 0.680 0.012 0.80 Cu = 0.23 Example 7 0.12 0.15 0.40 -- -- 0.02 15.2 1.9 0.01 1.91 0.01 0.010 0.005 0.41 -- Example 8 0.42 0.25 1.10 0.023 0.013 0.77 13.7 1.3 0.10 1.35 0.51 0.390 0.020 0.77 Pb = 0.05 Example 9 0.19 0.05 2.00 0.007 0.017 0.17 16.3 0.1 3.90 2.05 0.72 0.570 0.011 0.63 -- Example 10 0.01 0.15 0.50 0.007 0.004 0.50 17.9 1.0 0.01 1.01 0.05 0.150 0.003 0.50 -- Example 11 0.24 0.35 1.30 0.013 0.019 0.23 19.3 0.3 2.10 1.35 0.37 0.560 0.008 0.36 -- Example 12 0.15 0.15 0.30 0.011 0.021 0.21 15.4 0.5 1.30 1.17 0.45 0.230 0.012 0.36 Co = 0.30 Example 13 0.30 0.26 0.20 0.027 0.015 0.43 17.2 0.2 2.40 1.40 0.32 0.460 0.009 0.41 B = 0.007 Example 14 0.10 0.41 0.40 0.023 0.003 0.63 18.3 1.2 0.40 1.43 0.23 0.342 0.006 0.42 Se = 0.10 Te = 0.15 Example 15 0.25 0.23 0.60 0.017 0.007 0.32 14.3 0.8 3.30 2.48 0.69 0.246 0.003 0.36 Ta = 0.21 Zr = 0.14 Example 16 0.15 0.37 0.10 0.006 0.011 0.83 15.6 0.6 0.60 0.90 0.31 0.187 0.001 0.46 Bi = 0.07 Comparative 1.05 0.60 0.40 0.030 0.020 0.05 16.9 0.5 0.01 0.49 0.01 0.020 0.040 0.02 -- Example 1 Comparative 0.37 1.00 0.39 0.011 0.007 0.20 13.5 0.1 0.02 0.11 0.25 0.014 0.012 0.01 -- Example 2 Comparative 0.54 0.45 0.40 0.010 0.020 0.07 17.3 1.1 0.05 1.13 0.05 0.020 0.030 0.20 -- Example 3 Comparative 0.24 0.25 0.32 0.010 0.020 0.05 13.0 0.1 2.30 1.25 0.23 1.000 0.009 0.12 -- Example 4 Comparative 0.63 1.50 1.50 0.015 0.025 0.70 13.2 3.0 2.50 4.25 1.11 0.600 0.020 0.38 -- Example 5 Comparative 0.33 0.50 0.50 0.030 0.070 0.04 15.0 1.0 2.90 2.45 0.13 0.700 0.070 0.40 -- Example 6

[0078] Next, as shown in Table 2, the respective steels consistent with Examples and Comparative Examples were hardened at a temperature ranging from 1030.degree. C. to 1150.degree. C. Further, the steels consistent with Examples 1 to 16 were subjected to a subzero treatment at -76.degree. C. or -196.degree. C. and then, tempered at a temperature ranging from 200.degree. C. to 475.degree. C. Properties of specimens consistent with Examples and Comparative Examples were assessed as follows. TABLE-US-00002 TABLE 2 Hardening Tempering Subzero temperature temperature treatment Example1 1030.degree. C. 200.degree. C. -196.degree. C. Example2 1030.degree. C. 200.degree. C. -196.degree. C. Example3 1030.degree. C. 200.degree. C. -76.degree. C. Example4 1030.degree. C. 200.degree. C. -196.degree. C. Example5 1030.degree. C. 200.degree. C. -76.degree. C. Example6 1100.degree. C. 450.degree. C. -76.degree. C. Example7 1100.degree. C. 450.degree. C. -196.degree. C. Example8 1150.degree. C. 450.degree. C. -196.degree. C. Example9 1100.degree. C. 400.degree. C. -76.degree. C. Example10 1100.degree. C. 400.degree. C. -76.degree. C. Example11 1150.degree. C. 400.degree. C. -196.degree. C. Example12 1050.degree. C. 300.degree. C. -196.degree. C. Example13 1075.degree. C. 250.degree. C. -76.degree. C. Example14 1100.degree. C. 400.degree. C. -196.degree. C. Example15 1150.degree. C. 475.degree. C. -196.degree. C. Example16 1075.degree. C. 200.degree. C. -76.degree. C. Comparative 1030.degree. C. 200.degree. C. not performed Example 1 Comparative 1030.degree. C. 200.degree. C. not performed Example 2 Comparative 1030.degree. C. 200.degree. C. not performed Example 3 Comparative 1030.degree. C. 200.degree. C. not performed Example 4 Comparative 1030.degree. C. 200.degree. C. not performed Example 5 Comparative 1030.degree. C. 200.degree. C. not performed Example 6

<Particle Size of Carbonitrides>

[0079] 15 cubic millimeters of blocks were cut from the respective squared bars and subjected to the heat treatments, and then measuring planes thereof were polished using emery paper of #1500. Then, the measuring planes were finished by buffing using diamond paste of 1 .mu.m and rotten using a villela etching liquid. Then, the measuring planes were photographed using an optical microscope (magnification: 400.times., with 10.times. eyepieces), and a value, such that 90% or more of the total number of carbonitrides to be observed had particle sizes not larger than the value, was defined as a representing value. The one whose carbonitrides had the particle size not more than 4.0 .mu.m was regarded as passed.

<Hardness>

[0080] 10 cubic millimeters of blocks were cut from the respective squared bars and subjected to the heat treatments, and then measuring planes and ground planes thereof were polished using emery paper of #400. Then, hardness of the blocks was measured using a Rockwell C scale, and the one having the hardness of not less than HRC55 was regarded as passed.

<Wear Resistance>

[0081] Wear resistance was assessed using a pin-on-disk friction and wear tester. Specifically, two pins 8 mm in diameter were cut from the respective squared bars and subjected to the heat treatments, and a disk which was cut from S45C was used. Test conditions were as follows: a slipping velocity; 1.6 m/s, a slipping distance; 5,000 m, a pressing load; 10.5 kgf, and lubrication oil; not used. Before and after the test, weights of the pins were measured, and thereby weights of wear were measured. Besides, in Table 3, listed are the ratios of the wear weights of the steels consistent with Examples and Comparative Examples except Comparative Example 1, to the wear weight of the steel consistent with Comparative Example 1 (SUS440C), which is assumed to be 100. The one with the ratio which was below 130 was regarded as passed.

<Corrosion Resistance>

[0082] Rods 15 mm in diameter and 60 mm in length were made from the respective squared bars, subjected to the heat treatments, and then surfaces of which were finished using emery paper of #400. Then, based on JIS Z2371, a salt spray test was performed to check the formation of rust. Besides, in Table 3, the one which formed no rust was defined as A, the one which slightly formed rust was defined as B, the one which considerably formed rust was regarded as C, and the one which formed rust overall was defined as D, and the one which is A or B was regarded as passed.

<High-Precision Processability>

[0083] Specimens of 60 mm.times.60 mm.times.100 mm were prepared from the respective squared bars, and machined using a solid carbide end mill (with six flutes) of 10 mm in diameter as a tool under the conditions of cutting speed of 120 m/min., feed speed of 0.06 mm/rev., the width of cut of 0.5 mm, and the height of cut of 10 mm. Then, based on JIS B0633, the maximum surface roughness R.sub.y of machined surfaces thereof was measured. At this time, the one whose maximum surface roughness R.sub.y was not more than 2.0 .mu.m was regarded as passed.

<Mirror Polishing Properties>

[0084] Plates of 50 mm.times.45 mm.times.12 mm were made from the respective squared bars, subjected to the heat treatments, and then polished by a machine using a grinding stone of # 14000. Then, the plates were subjected to chemical etching and specimens were prepared. After that, based on JIS B0633, surface roughness R.sub.a of the specimens was measured. At this time, the one whose surface roughness R.sub.a was not more than 0.05 .mu.m was regarded as passed.

[0085] Assessment results of the properties are shown in Table 3. TABLE-US-00003 TABLE 3 Particle size of High-precision Mirror polishing carbonitrides Hardness Wear resistance Corrosion processability properties (.mu.m) (HRC) (Wear resistance ratio) resistance (Ry: .mu.m) (Ra: .mu.m) Example 1 2.6 61.3 105 B 1.42 0.0298 Example 2 2.5 61.2 103 B 1.43 0.0286 Example 3 2.7 61.0 109 B 1.41 0.0275 Example 4 2.6 58.7 114 B 1.40 0.0281 Example 5 2.8 59.6 109 B 1.40 0.0286 Example 6 2.3 62.2 105 B 1.39 0.0274 Example 7 2.7 59.0 126 B 1.37 0.0252 Example 8 2.9 60.6 103 B 1.38 0.0277 Example 9 2.4 59.7 106 B 1.37 0.0263 Example 10 2.1 55.4 124 A 1.37 0.0266 Example 11 2.6 58.8 112 B 1.36 0.0256 Example 12 2.2 58.2 102 B 1.35 0.0284 Example 13 2.4 60.3 102 B 1.37 0.0272 Example 14 2.5 57.6 112 B 1.43 0.0268 Example 15 2.4 59.6 107 B 1.37 0.0256 Example 16 2.5 59.3 104 B 1.42 0.0274 Comparative 8.2 60.2 100 C 2.56 0.0544 Example 1 Comparative 3.5 52.0 133 B 1.68 0.0304 Example 2 Comparative 5.8 58.2 115 B 2.49 0.0295 Example 3 Comparative 4.2 52.2 131 B 2.45 0.0311 Example 4 Comparative 7.8 59.6 102 B 2.51 0.5620 Example 5 Comparative 8.5 58.1 105 A 2.48 0.0525 Example 6

[0086] According to Table 3, it is apparent that in the steel consistent with Comparative Example 1, there exist coarse crystallized carbonitrides, so that it is inferior in high-precision processability and mirror polishing properties. It is also inferior in corrosion resistance.

[0087] In addition, the steels consistent with Comparative Examples 2 and 4 have a content of N smaller than the specified value of the present invention, so that they cannot obtain enough hardness and is inferior in wear resistance.

[0088] In addition, the steel consistent with Comparative Example 3 has a content of O larger than the specified value of the present invention, so that it forms coarse oxides. In addition, the steel consistent with Comparative Example 4 has a content of Al larger than the specified value of the present invention, so that it forms coarse AlN. Accordingly, they are inferior in high-precision processability.

[0089] In addition, the steel consistent with Comparative Example 5 has a content of V larger than the specified value of the present invention, so that it forms coarse VN. Accordingly, it is inferior in high-precision processability and mirror polishing properties.

[0090] In addition, the steel consistent with Comparative Example 6 has a content of O larger than the specified value of the present invention, so that it forms coarse oxides. Accordingly, it is inferior in high-precision processability and mirror polishing properties.

[0091] It was shown that, in contrast to the steels consistent with Comparative Examples 1 to 6, all of the steels consistent with Examples 1 to 16 according to the present invention possess enough hardness, wear resistance and corrosion resistance, and are excellent in high-precision processability and mirror polishing properties.

[0092] Therefore, it can be said that the steels consistent with the present invention are favorably employed as a material for a plastic molding die.

[0093] The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in the light of the above teachings or may be acquired from practice of the invention. The embodiments chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.

Claims

1. A steel for a plastic molding die comprising: not more than 0.80 wt % C; not less than 0.01 wt % and less than 1.40 wt % Si; not less than 0.05 wt % and not more than 2.0 wt % Mn; not less than 0.005 wt % and not more than 1.00 wt % Ni; not less than 13.0 wt % and not more than 20.0 wt % Cr; not less than 0.20 wt % and not more than 4.0 wt % Mo+1/2 W; not less than 0.01 wt % and not more than 1.00 wt % V; not less than 0.36 wt % and not more than 0.80 wt % N; not more than 0.02 wt % O; not more than 0.80 wt % Al; and the remainder substantially including Fe and unavoidable impurities.

2. The steel for a plastic molding die according to claim 1 further comprising at least one element selected from the group consisting of: not more than 0.030 wt % P; and not more than 0.030 wt % S.

3. The steel for a plastic molding die according to claim 2 further comprising at least one element selected from the group consisting of: not less than 0.001 wt % and not more than 0.50 wt % Cu; not less than 0.001 wt % and not more than 0.50 wt % Co; and not less than 0.0005 wt % and not more than 0.010 wt % B.

4. The steel for a plastic molding die according to claim 3 further comprising at least one element selected from the group consisting of: not less than 0.001 wt % and not more than 0.30 wt % Se; not less than 0.001 wt % and not more than 0.30 wt % Te; not less than 0.001 wt % and not more than 0.10 wt % Ca; not less than 0.001 wt % and not more than 0.20 wt % Pb; and not less than 0.001 wt % and not more than 0.30 wt % Bi.

5. The steel for a plastic molding die according to claim 4 further comprising at least one element selected from the group consisting of: not more than 0.20 wt % Ti; not less than 0.001 wt % and not more than 0.30 wt % Nb; not less than 0.001 wt % and not more than 0.30 wt % Ta; and not less than 0.001 wt % and not more than 0.30 wt % Zr.

6. The steel for a plastic molding die according to claim 3 further comprising at least one element selected from the group consisting of: not more than 0.20 wt % Ti; not less than 0.001 wt % and not more than 0.30 wt % Nb; not less than 0.001 wt % and not more than 0.30 wt % Ta; and not less than 0.001 wt % and not more than 0.30 wt % Zr.

7. The steel for a plastic molding die according to claim 2 further comprising at least one element selected from the group consisting of: not less than 0.001 wt % and not more than 0.30 wt % Se; not less than 0.001 wt % and not more than 0.30 wt % Te; not less than 0.001 wt % and not more than 0.10 wt % Ca; not less than 0.001 wt % and not more than 0.20 wt % Pb; and not less than 0.001 wt % and not more than 0.30 wt % Bi.

8. The steel for a plastic molding die according to claim 7 further comprising at least one element selected from the group consisting of: not more than 0.20 wt % Ti; not less than 0.001 wt % and not more than 0.30 wt % Nb; not less than 0.001 wt % and not more than 0.30 wt % Ta; and not less than 0.001 wt % and not more than 0.30 wt % Zr.

9. The steel for a plastic molding die according to claim 2 further comprising at least one element selected from the group consisting of: not more than 0.20 wt % Ti; not less than 0.001 wt % and not more than 0.30 wt % Nb; not less than 0.001 wt % and not more than 0.30 wt % Ta; and not less than 0.001 wt % and not more than 0.30 wt % Zr.

10. The steel for a plastic molding die according to claim 1 further comprising at least one element selected from the group consisting of: not less than 0.001 wt % and not more than 0.50 wt % Cu; not less than 0.001 wt % and not more than 0.50 wt % Co; and not less than 0.0005 wt % and not more than 0.010 wt % B.

11. The steel for a plastic molding die according to claim 10 further comprising at least one element selected from the group consisting of: not less than 0.001 wt % and not more than 0.30 wt % Se; not less than 0.001 wt % and not more than 0.30 wt % Te; not less than 0.001 wt % and not more than 0.10 wt % Ca; not less than 0.001 wt % and not more than 0.20 wt % Pb; and not less than 0.001 wt % and not more than 0.30 wt % Bi.

12. The steel for a plastic molding die according to claim 11 further comprising at least one element selected from the group consisting of: not more than 0.20 wt % Ti; not less than 0.001 wt % and not more than 0.30 wt % Nb; not less than 0.001 wt % and not more than 0.30 wt % Ta; and not less than 0.001 wt % and not more than 0.30 wt % Zr.

13. The steel for a plastic molding die according to claim 10 further comprising at least one element selected from the group consisting of: not more than 0.20 wt % Ti; not less than 0.001 wt % and not more than 0.30 wt % Nb; not less than 0.001 wt % and not more than 0.30 wt % Ta; and not less than 0.001 wt % and not more than 0.30 wt % Zr.

14. The steel for a plastic molding die according to claim 1 further comprising at least one element selected from the group consisting of: not less than 0.001 wt % and not more than 0.30 wt % Se; not less than 0.001 wt % and not more than 0.30 wt % Te; not less than 0.001 wt % and not more than 0.10 wt % Ca; not less than 0.001 wt % and not more than 0.20 wt % Pb; and not less than 0.001 wt % and not more than 0.30 wt % Bi.

15. The steel for a plastic molding die according to claim 14 further comprising at least one element selected from the group consisting of: not more than 0.20 wt % Ti; not less than 0.001 wt % and not more than 0.30 wt % Nb; not less than 0.001 wt % and not more than 0.30 wt % Ta; and not less than 0.001 wt % and not more than 0.30 wt % Zr.

16. The steel for a plastic molding die according to claim 1 further comprising at least one element selected from the group consisting of: not more than 0.20 wt % Ti; not less than 0.001 wt % and not more than 0.30 wt % Nb; not less than 0.001 wt % and not more than 0.30 wt % Ta; and not less than 0.001 wt % and not more than 0.30 wt % Zr.