JP2000144332A - Steel for die for plastic excellent in corrosion resistance and mirror finishing property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce steel for a die for plastics excellent in corrosion resistance and mirror finishing properties. SOLUTION: This steel for a die for plastics excellent in corrosion resistance and mirror finishing properties has a compsn. contg., by weight, 0.15 to 0.24% C, <=1.0% Si, <=1.0% Mn, 12.00 to 17.00% Cr, 0.08 to 0.20% N, and the balance Fe with inevitable impurities and has <=4% ferrite area ratio. The steel for a die for plastics excellent in corrosion-resistance and mirror finishing properties contains <=2.0% Mo or 0.02 to 0.30% S in addition to the above steel as well.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plastic mold steel having excellent corrosion resistance and mirror finish.

[0002]

2. Description of the Related Art Conventionally, iron-based alloys having a Cr content of 12% by weight or more are mainly used as mold steels for corrosion-resistant plastics for processing chemically corrosive molding materials. . This iron-based alloy can be used quite economically due to the low load of the mold, but it is not sufficient for tools with extremely corrosive materials and plastics containing abrasive additives. It has the disadvantage of not being able to provide a lifetime. In order to solve such a drawback, for example, as disclosed in Japanese Patent Application Laid-Open No. 8-253846, martensitic iron containing Cr for plastic molds having high corrosion resistance and being heat-treated is mainly used. Alloys have been proposed.

[0003]

On the other hand, steel for plastic molds is required to have a certain degree of hardness in addition to high corrosion resistance and mirror finish. Resin containing chlorine may generate corrosive chlorine gas during high-temperature molding, which often causes mold corrosion. Conventionally, this was dealt with by hard chrome plating or the like, but plating was disadvantageous in terms of accuracy and life. In addition, an increase in Cr in the mold steel is effective for improving the corrosion resistance. Therefore, SUS420J2
Etc. may be used. However, excessive addition of Cr forms coarse carbides with C, and conversely, corrosion resistance,
Impairs mirror finish. JP-A-8-25384 described above
As described in Japanese Patent Publication No. 6, although the addition of N is effective for improving corrosion resistance and the like, it is necessary to simultaneously reduce C in order to obtain higher corrosion resistance and mirror finish.

[0004]

Means for Solving the Problems In order to solve the above-mentioned problems, the inventors of the present invention have made intensive developments, and as a result, have reduced carbon content and added N, which has a negative effect on mirror surface finish. Is reduced and refined, and it is recognized that corrosion resistance and mirror finish are improved. Also, since C can be reduced, coarse carbide is free and Cr can be increased more than conventional SUS420J2, thereby improving corrosion resistance. I was able to plan.

The gist of the invention is as follows: (1) By weight%, C: 0.15 to 0.24%, Si:
1.0% or less, Mn: 1.0% or less, Cr: 12.00
１７17.00%, N: 0.08 to 0.20%, balance Fe
And inevitable impurities, and the area ratio of ferrite: 4
%, Which is excellent in corrosion resistance and mirror finish. (2) The steel for plastic molds having excellent corrosion resistance and mirror finish as described in (1) above, which contains Mo: 2.0% or less by weight. (3) The steel for plastic molds excellent in corrosion resistance and mirror finish as described in (1) or (2) above, which contains S: 0.02 to 0.30% by weight%. It is in.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the component composition range in the present invention will be described below. C: 0.15 to 0.24% C is necessary for improving hardness and strength. However, if it is less than 0.15%, this effect cannot be sufficiently obtained, and if it exceeds 0.24%, carbides are crystallized and the corrosion resistance and the mirror polishing property deteriorate. Therefore, the range is 0.1
5 to 0.24%. Si: 1.0% or less Si is mainly a deoxidizing element. However, if it exceeds 1.0%, the annealing hardness becomes large. Therefore, the range is set to 1.0% or less.

Mn: 1.0% or less Mn is a deoxidizing element like Si. However, if it exceeds 1.0%, the corrosion resistance is reduced. Therefore, the range is 1.
0% or less. Cr: 12.00 to 17.00% Cr has a great effect on corrosion resistance. However, if the content is less than 12.00%, the effect cannot be obtained, and if it exceeds 17.00%, carbides and ferrite phases are formed, and the corrosion resistance and the mirror finish are impaired. Therefore, the range is set to 12.00 to 17.00%
And

N: 0.08 to 0.20% N has a great effect on hardness and corrosion resistance. However, 0.08%
If the value is less than 0.0, the effect cannot be obtained.
8 to 0.20%. Ferrite area ratio: 4% or less When the ferrite area ratio exceeds 4%, the mirror finish is impaired. Therefore, it was set to 4% or less.

Mo: 2.0% or less Mo greatly contributes to improvement of corrosion resistance. But,
If the content exceeds 2.0%, the effect is saturated, so the range is set to 2.0% or less. S: 0.02 to 0.30% S is effective for improving machinability, and 0.02% is necessary for ensuring free machinability.
If it exceeds 30%, toughness and hot workability deteriorate, so the range was made 0.02 to 0.30%.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments. As a test material, the steel of the present invention (N
o1 to No6), comparative steels (No7 to No11), and conventional steels (No12), each test was carried out using ingots of 100 kg of steel in a vacuum induction melting furnace. The test method in the practice of the present invention is as follows.

Hardness measurement: Oil quenching was performed from 1050 ° C., and then the hardness when tempered at 200 ° C. was measured. -Corrosion resistance: Hardening of 1050 ° C → 200 ° C tempered material was tested for pitting corrosion resistance to chloride ions. The evaluation was made based on the corrosion loss when immersed in a mixed solution of 6% sodium chloride and 0.5% hydrogen peroxide at 25 ° C.

・ Carbide area ratio: Hardening at 1050 ° C. → 20
Using a scanning electron microscope, 5 μm
The area ratio of the above carbides was determined by image analysis. Ferrite area ratio: 1050 ° C. quenching → 200 ° C. tempered material was measured for the area ratio of the ferrite phase remaining in the martensite structure using an image analyzer. Machinability: A drill piercing test was performed as a machinability test. The evaluation was made on the assumption that the shorter the time required to reach a certain depth, the better the machinability was obtained by drilling the annealed material using a drill. The test conditions are as follows.

Drill: SKH51, φ5 straight shank drill Thrust: 414 N Circumferential speed: 18.7 m / min Cutting oil: None Drilling depth: 10 mm

[0014]

[Table 1]

[0015]

[Table 2]

As shown in Table 2, Nos. 1 to 6 are invention steels, and Nos. 1 to 3 have high hardness and excellent corrosion resistance because each component is blended in a well-balanced manner. In addition, carbides and ferrite phases, which deteriorate the mirror polishing property, are hardly produced, and also have these excellent properties. In Nos. 4 and 5, Mo was further added, and the corrosion resistance was significantly improved. No6 is S in No5 component steel
Is added, and the machinability is remarkably improved although the corrosion resistance is slightly deteriorated.

Nos. 7 to 12 are comparative steels or conventional steels. No. 7 has a low hardness because C is less than the scope of the claims, and a ferrite phase is also observed. No. 8 has high hardness due to high C, but has a large amount of carbide precipitation and poor corrosion resistance. No9
Is low in hardness and low in corrosion resistance due to low N. No10
Is poor in corrosion resistance because of low Cr. No11 is Cr
, The carbide and ferrite phases are considerable and the hardness is low. No. 12 is inferior in corrosion resistance due to low Cr.

[0018]

As described above, according to the present invention, it is possible to provide steel for plastic molds which is excellent in corrosion resistance and mirror finish and has a certain degree of hardness, and which is most suitable for plastic mold steel. Can be.

Claims (3)

[Claims] C: 0.15 to 0.24%, Si: 1.0% or less, Mn: 1.0% or less, Cr: 12.00 to 17.00%, N: 0% by weight 0.08 to 0.20%, the balance being Fe and unavoidable impurities, and having a ferrite area ratio of 4% or less, a mold steel for plastics excellent in corrosion resistance and mirror finish. 2. The plastic mold steel according to claim 1, wherein the steel contains Mo: 2.0% or less by weight. 3. The steel for a plastic mold according to claim 1, wherein the steel contains 0.02 to 0.30% by weight of S in weight%.