JP3327853B2 - Free cutting stainless steel mold steel with excellent rust resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stainless steel mold steel having excellent rust resistance and machinability, which is most suitable for use as a mold for plastic molding.

[0002]

2. Description of the Related Art Molds for molding plastic products, which require high dimensional accuracy due to the progress of precision of molded products, are required to prevent rusting from impairing the appearance or reducing the surface strength. Excellent rust resistance is required so as not to occur.

In order to prevent the generation of rust, when the molding is interrupted, the cooling water must be stopped and the moisture on the surface of the mold must be wiped off so that dew does not form on the surface of the mold. After completion of the molding, the water inside the water hole must be drained, and after the water has been drained, air must be passed through to completely remove the water inside the mold.

When a mold is stored for a long period of time, it is necessary to clean the mold surface and then apply grease. If grease is applied without cleaning the mold surface, rust may occur due to dirt on the mold surface. Further, the molding of the flame-retardant material requires corrosive gas to be generated, so that it is necessary to prevent the mold from rusting. When the molding is stopped and completed, the mold must be cleaned. In order to prevent the occurrence of rust, a regular management system is important, and training of workers is required.

Conventionally, applications requiring rust resistance include S
US403, SUS410, SUS420J2, SUS
Martensitic stainless steel such as 440C is used. The martensitic stainless steel represented by SUS440C has sufficient hardness to be used as a mold, but cannot be said to be excellent in rust resistance. Further, a molding die manufactured from high-strength martensitic stainless steel has a problem that even if it has little rust, local pitting corrosion called pitting corrosion occurs, and this easily becomes a starting point of destruction.

To solve this problem, see, for example,
001 and JP-A-10-18002 propose a high-hardness martensitic stainless steel having excellent pitting corrosion resistance. On the other hand, there is a strong demand from the die processing industry to reduce processing costs and shorten the delivery time of products for the purpose of cost reduction, and for die that combines not only rust resistance but also excellent machinability. Steel is required.
JP-97829 / A and JP-A-4-2745 disclose steels with improved machinability.

[0007]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Application Laid-open No. Hei 10-1
The martensitic stainless steel proposed in Japanese Patent Application Laid-Open No. 8001 and Japanese Patent Application Laid-Open No. Hei 10-18002 has a Cr content of 11.0% or more and less than 15.0%, and Mo and W contain Mo + 1 /.
1.0 to 3.0% at 2 W, 1.0 to 3.0% for Ni and Cu
It is characterized by containing 4.0% and satisfying Ni / Cu ≦ 0.2. However, this steel is made of Cr, Mo, W, N
Rust resistance and pitting corrosion resistance are excellent by the addition of i and Cu, but the machinability has not been improved, and the addition of Ni, W, and Mo increases the cost. There is a point.

The martensitic stainless steel proposed in the above-mentioned JP-A-3-97829 and JP-A-4-2745 is improved in machinability by adding S which is a free-cutting element. In particular, it is a steel that compensates for the rusting property deteriorated by the sulfide generated by the addition of S by increasing the Cr content and the balance between Cr and C. Certainly high C
Although r-formation can be expected to have a remarkable effect on rusting, the addition of Cr increases the cutting resistance and degrades machinability.
Although the effect of increasing Cr is compensated for by the addition of S, it cannot be said that the machinability has been sufficiently improved.

It is already clear that increasing the Cr content and adding Ni and Mo to martensitic stainless steel are effective means for improving rust resistance. The addition of Ni increases the cutting resistance and deteriorates the machinability, so it is not advisable as a means to provide both rust resistance and machinability. The addition of Ni and Mo not only deteriorates the machinability, The cost is increased by the addition. Further, in martensitic stainless steel, there is a method of using MnS as a means for improving machinability,
MnS has a problem that it promotes the corrosion progress rate and deteriorates rust resistance.

[0010]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and has both excellent rust resistance and excellent machinability. It is an object of the present invention to provide a stainless steel mold steel that can be supplied at a low cost by reducing the amount of Ni and Mo that are usually added to improve rust resistance. Is by weight: C: 0.10 to 0.40%, Si: 0.
50% or less, Mn: 0.50 to 1.50%, P: 0.0
30% or less, S: 0.080 to 0.200%, Cr: 1
1.00 to 14.00%, Cu: 0.30 to 1.00
%, Ni: 0.30% or less, Mo: 0.30% or less, A
l: 0.010-0.060%, N: 0.010-0.
040%, O: 0.020% or less, the balance being Fe and unavoidable impurities.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail by a method using MnS as a means for improving machinability in martensitic stainless steel. But,
As described above, this method has a problem that MnS promotes the corrosion progress rate and deteriorates rust resistance.

The inventors of the present invention have conducted intensive studies on the formation of MnS in order to combine rust resistance and excellent machinability. It is effective, and it has been found that it is effective to form AlN by positively adding an appropriate amount of Al.

More specifically, by precipitating AlN finely, the precipitation nuclei of MnS are increased, and MnS can be finely dispersed and precipitated. In addition, AlN suppresses coarsening of austenite crystal grains to reduce the crystal grain size, and since the hardness can be eliminated by reducing the crystal grain size, AlN can be used as a mold steel for plastic molding. Work effectively. In this case, although the amount of AlN is increased, it has been found that the machinability is not affected because most of AlN is combined with MnS, and the present invention has been completed.

That is, the free-cutting stainless steel for molds having excellent rust resistance according to the present invention has a C content of 0.10 to 0.10% by weight.
0.40%, Si: 0.50% or less, Mn: 0.50%
1.50%, P: 0.030% or less, S: 0.080-
0.200%, Cr: 11.00 to 14.00%, C
u: 0.30 to 1.00%, Ni: 0.30% or less, M
o: 0.30% or less, Al: 0.010 to 0.060
%, N: 0.010-0.040%, O: 0.020%
Hereinafter, the balance consists of Fe and inevitable impurities.

[0015] The steel of the present invention has the above-described structure, and is therefore made of SUS403, SUS410, SUS420J.
It is more excellent in machinability and rust resistance than martensitic stainless steels such as No. 2. That is, high C
It is a stainless steel for molds that has improved rust resistance without being changed to r and has excellent machinability. Therefore, the stainless steel for molds can be supplied at low cost without increasing the amounts of Cr, Ni, and Mo that are usually added to improve rust resistance.

Next, the reasons for limiting the components of the steel of the present invention will be described below.
10 to 0.40%. C has a hardness of 30 after quenching and tempering
It is necessary to maintain hardness as high as HRC, and 0.10% or more is necessary to obtain the required strength as a mold. When the amount of C increases, hard carbides increase, and the cutting edge is worn out, thereby reducing the machinability. Therefore, the upper limit is set to 0.40%.

Si: 0.50% or less. Si is an element effective as a deoxidizing agent, and if too large, the crystal grain size becomes coarse, so the upper limit was made 0.50%. Mn: 0.5
0 to 1.50%. Mn is an element necessary for forming MnS, and its lower limit is set to 0.50% in order to secure a solid solution in a matrix to secure strength. If it is too large, the corrosion resistance and oxidation resistance deteriorate, so the upper limit is 1.50.
%.

P: 0.030% or less. P has many harmful effects such as a decrease in toughness, an increase in hot cracking susceptibility and an increase in pitting susceptibility, so that it is desirable to reduce P as much as possible. S: 0.080 to 0.2
00%. S is necessary for directly improving machinability by forming MnS, and the lower limit is set to 0.080% in order to obtain a sufficient effect. If it is too large, pitting corrosion sensitivity increases and hot workability deteriorates, so the upper limit was made 0.200%.

Cr: 11.00 to 14.00%.
Cr is a basic component of stainless steel, and the greater the amount of Cr, the better the rust resistance, oxidation resistance, and pitting resistance. At least 11.00% is necessary to maintain the passivity, and the lower limit is set to 11.00%. If it is too large, the machinability deteriorates, so the upper limit is made 14.00%. Cu: 0.30
1.00%. Cu is added to improve rust resistance. If it is less than 0.30%, the effect of addition is poor, so the lower limit is made 0.30%. On the other hand, if the content is too large, it promotes grain boundary embrittlement at a high temperature and hinders hot workability.
0%.

Ni: 0.30% or less. Ni is effective in increasing toughness and preventing a decrease in hot workability.
If it is a small amount, it may be contained. However, if it increases, the cutting resistance increases and the machinability deteriorates, so the upper limit is made 0.30%. Mo: 0.30% or less. Mo is an element effective for improving rust resistance and pitting corrosion resistance, and may be contained in a small amount. However, when it increases, it has the function of increasing the cracking sensitivity, and the upper limit is made 0.30%.

Al: 0.010-0.060%.
Al is necessary for forming AlN and finely dispersing MnS, and the lower limit is set to 0.010%. If the content is too large, oxide-based nonmetallic inclusions increase to adversely affect the machinability, so the upper limit was made 0.060%. Also, N:
0.010 to 0.040%. N is necessary for forming AlN and finely dispersing MnS.
It is effective to refine crystal grains and improve toughness as a substitute for. The lower limit was set to 0.010%. If the amount is too large, the finished surface roughness is deteriorated.
0%.

O: 0.020% or less. O excessively increases the amount of oxide-based inclusions and adversely affects machinability. Therefore, it is desirable to reduce O as much as possible.
20%.

[0023]

EXAMPLES The steel of the present invention will be described below with reference to examples. Table 1 shows the chemical compositions of the steel of the present invention and comparative steels.
The test material was melted by a normal steelmaking method and forged at a forging ratio of 4 or more and allowed to cool. After further holding at 1020 ° C. for 1 hour,
An oil-cooled quenching treatment was performed, followed by a tempering treatment at 500 to 600 ° C. The hardness of the test material obtained by the above treatment was 29 to 36 HRC.

[0024]

[Table 1]

Table 2 shows the results of comparison of hardness, rust resistance, and machinability of the steel of the present invention and the comparative steel after quenching and tempering. The rust resistance was evaluated by a salt spray test based on JIS Z2371. The salt solution used in the salt spray test was adjusted to a salt concentration of 5 ± 1% by mass, so that the solution sprayed at 35 ° C. was in a pH range of 6.5 to 7.2. The pressure of the supply air for spraying is 1.0 kgf / c
m ² and the exposure zone where the sample in the salt spray chamber is placed is 35 ± 2 ° C.
Kept. The test time was set to 15 hours, and the rust generation state was compared. Based on the rusting area of the comparative steel J, the rusting state of other test materials was represented by a rusting index.

[0026]

[Table 2]

FIG. 1 shows the state of rusting of the steels A and B of the present invention and the comparative steels J and K in a salt spray test after a test time of 15 hours. The evaluation of the machinability is performed by performing side cutting with a two-flute end mill and determining the processing length when the amount of wear at the boundary of the tool (average of the short blade and the long blade) is 100 μm. Cutting conditions are as follows: cutting depth: 15 mm, cutting width: 1 m
m, cutting speed: 21 m / min, feed speed: 94 mm /
min, number of revolutions: 670 rpm, feed amount per tooth:
0.070 mm / blade.

The steels (A to H) of the present invention are stainless steels having excellent rust resistance and excellent machinability, and have the required material hardness as a steel for molding plastic products. It has. Comparative steel (I, J, K) is inferior to invention steel in rust resistance, and comparative steel (L, M, O, N)
Is inferior to the invention steel in machinability.

[0029]

The free-cutting stainless steel for molds having excellent rust resistance according to the present invention has the above-described structure.
It has both better machinability and rust resistance than martensitic stainless steels such as US403, SUS410, SUS420J2, etc., and also has the material hardness required for plastic mold steel. In addition, since the steel of the present invention is configured to improve rust resistance without increasing Cr, Cr, Ni, M which is usually added in order to improve rust resistance is improved.
There is an effect that it is not necessary to increase the amount of o, and therefore, it is possible to supply at low cost.

[Brief description of the drawings]

FIG. 1 is a photograph showing the state of rusting of steels A and B of the present invention and comparative steels J and K after a test time of 15 hours in a salt spray test.

Continuation of the front page (56) References JP-A-8-193249 (JP, A) JP-A-3-97829 (JP, A) JP-A-64-25950 (JP, A) JP-A-59-179761 (JP) JP-A-2-267218 (JP, A) JP-A-63-125644 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (1)

(57) [Claims] (1) C: 0.10 to 0.40% by weight,
Si: 0.50% or less, Mn: 0.50 to 1.50%,
P: 0.030% or less, S: 0.080 to 0.200
%, Cr: 11.00 to 14.00%, Cu: 0.30
1.00%, Ni: 0.30% or less, Mo: 0.30
% Or less, Al: 0.010 to 0.060%, N: 0.0
10 to 0.040%, O: 0.020% or less, the balance being F
A free-cutting stainless steel for molds having excellent rust resistance, comprising e and unavoidable impurities.