JPH0293043A - Tool steel for molding plastics - Google Patents

Abstract

PURPOSE:To obtain the title tool steel in which air-cooling quenching is permitted and having excellent corrosion resistance by specifying the contents of C, Si, Mn, Cr, Mo, acid soluble Al and whole nitrogen and incorporating at least two kinds among specific % of V, Nb and W and the balance Fe. CONSTITUTION:The tool steel contains 0.25 to 0.50% C, 0.005 to 0.5% Si, 0.1 to 2.0% Mn, 8.0 to 10.0% Cr, 0.05 to 2.0% Mo, 0.005 to 0.050% acid soluble Al and 0.002 to 0.035% whole nitrogen, contains at least two kinds among 0.01 to 0.50% V, 0.005 to 0.25% Nb and 0.01 to 0.25% W and the balance substantial Fe. Al and N form AlN to suppress the coarsening of austenite at the time of quenching. V, Nb and W form carbide to suppress the formation of Cr-series carbide. The steel has high transformation temp., in which air-cooling quenching is permitted and furthermore has excellent corrosion resistance, so that it is extremely good as a tool steel for molding plastics.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention is applicable to home appliances such as VTR top covers, OA
Concerns tool steel for molds used for molding plastics such as equipment, engineering plastics, daily necessities, toys, etc.

[Prior art] Conventionally, plastic molds are made by processing steel materials supplied by material manufacturers and then quenching and processing them.
Heat-treated types that involve tempering were common. However, since many plastic molds have deep engravings and complex shapes, dimensional changes due to quenching distortion are likely to occur during post-processing heat treatment. For this reason, there is an increasing demand for materials that are tempered by material manufacturers and only processed by mold manufacturers, so-called brihardon mold materials. Therefore, there are currently two types of tool steel for plastic molds: a heat-treated type and the aforementioned brihardon type.

JIS G4051S45 for the Burihadon series mentioned above.
CSJIS G4105 SCM440 (0.4%
C-1%Cr-0.2%MO), and 0.15%C-3
%Ni-1%Cu-1%Al-0.1%S is the main type, and the heat treated type is JIS G4404.
SKD61 (0.4%C-5%Cr-1%MO-
1%V), and 5KDII (1,5%C-12%C"
-1%Mo-0.4%V) is the main component. In addition, if the plastic to be molded is corrosive, such as vinyl chloride, Jl5G4, a precipitation hardening type with good corrosion resistance, may be used.
303 (0.05%C-4%Ni-17%Cr-4%
Cu-0,30%Nb) is used.

[Problem to be solved by the invention] By the way, the mold industry mainly supplies products to the electrical appliance, electronics, and automobile industries, which are highly competitive, and requires lower costs, higher precision, and shorter delivery times. Due to demand, automation of mold processing is progressing. With this demand for automation, electric discharge machining is increasingly being adopted as a method for machining molds, and cutting with a so-called cutting tool is becoming less common.

When machining molds using wire electrical discharge machining, rapid heating and
Rapid cooling generates thermal stress, so if internal strain from quenching and tempering remains in the mold of the workpiece, this internal strain and the thermal stress from wire electrical discharge machining will overlap, causing significant strain in the mold material. This causes dimensional changes or cracks, and internal strains are sequentially released by cutting by electrical discharge machining, which also causes dimensional changes.

Furthermore, the material of a member heated by electrical discharge machining is likely to change due to the influence of the heat. This tendency is particularly noticeable in the case of the Brihardon type, which does not undergo heat treatment after machining, but similar problems occur in the case of the heat treatment type as well, since precision machining is often performed by wire electric discharge machining after heat treatment.

Therefore, in mold materials for plastic molding, s,
Compared to free-cutting mold materials containing elements such as PB, mold materials with lower internal strain and less change in material quality even when exposed to high temperatures during electrical discharge machining are increasingly required.

However, if the transformation temperature of the mold material is low, it will be heated above the transformation temperature of the material during electrical discharge machining.
An austenitic phase is partially formed (1), which is rapidly cooled during quenching to become a martensitic structure, resulting in internal stress accompanying the transformation, which causes internal strain. Furthermore, in the case of a mold material whose tempering temperature is low, it is difficult to effectively release its internal strain by tempering, and moreover, the hardness of the portion heated above the tempering temperature decreases.

Therefore, this type of mold material is required to have a high transformation temperature and be capable of high temperature tempering.

In addition, the tool steel used for plastic molding molds is
- Air cooling is required for quenching in boat fishing. In oil quenching, when quenching something with such a complex shape,
The management of oil temperature and cooling rate is complicated, making it difficult to ensure the hardness of the quenched material and easily resulting in an uneven structure.In addition, water quenching results in uneven quenching and quench cracking. This is because it is easy to occur. In particular, for heat-treated mold materials, air-cooling quenching is essential because, as mentioned above, many molds for plastic molding have deep carvings and complex shapes.

However, it cannot be said that the above-mentioned conventional materials fully satisfy the characteristics required for the above-mentioned plastic molds.

In other words, 5 4 is used as a Burihadon type.
5C% and SCM440 generally have low hardenability, so if the plate thickness is large, air-cooling hardening can be used to achieve the specified hardness (
28-40HRc) is difficult to secure. In addition, SCM440 is a material that is used as-quenched in some cases, and the upper limit of the tempering temperature is 500℃.
Therefore, it is not suitable for high temperature tempering.

0.15%C-3%Ni-1%Cu-1%Al-1%S
This type of steel is heated to 850 to 900°C, then air-cooled and quenched, and then aged at 500°C to ensure hardness. The disadvantage is that the temperature is as low as about 650°C.

Although SICD 61 and 5KDII for heat treatment maintain certain properties, there is a demand for products with even better properties from the viewpoint of improving productivity.

Precipitation hardening type steel also does not have sufficient properties other than corrosion resistance.

In addition, materials suitable for all of the hardened type, heat treated type, and corrosion resistant type have not yet been developed.

The present invention was made in view of the above circumstances, and an object of the present invention is to provide a tool steel for plastic molding that has a high transformation point temperature, can be tempered at high temperatures, can be hardened by air cooling, and has excellent corrosion resistance. do.

[Means and effects for solving the problem] The plastic molding tool steel according to the present invention has a carbon content of 0.2% by weight.
5 to 0.50%, silicon 0.005 to 0.5%, manganese 0.1 to 2.0%, chromium 8.0 to 10
．． 0%, molybdenum 0.05 to 2.0%, acid soluble aluminum 0.005 to 0.050%, total nitrogen 0
．． 002 to 0.035%, and 0.01 to 0.5
0% V, 0.005-0.25% Nb, and 0.005-0.25% Nb.
01 to 0.25% of at least two types of W,
It is characterized in that the remainder consists of iron and unavoidable impurities.

This invention will be explained in detail below.

As mentioned above, the properties particularly required for tool steel used in plastic molds are: ■ high transformation point temperature, ■ ability to be tempered at high temperatures, ■ ability to be air-cooled and quenched, and ■ corrosion resistance. There are four things that are excellent.

First, regarding the transformation point temperature, that is, A (1 point), the following equation is widely known in relation to the components added to steel.

Ac+ (”C)-723+22S 1-14Mn-1
4,4Ni+23.3Cr As is clear from this formula, it is effective to add a large amount of Si or C to raise the transformation point temperature. It is an element that also improves corrosion resistance, and it is realistic to increase the amount of Cr added to raise the transformation temperature.For example, 5US420J2 ( C 0.26-0.40%, C
Therefore, in the present invention, the Cr content is basically increased. However, if Crff1 is too large, α+
The upper limit of the Cr content is set at 10% because the α phase deteriorates hot workability and becomes δ ferrite at room temperature, causing a decrease in hardness.

On the other hand, as mentioned above, the processing method for plastic molding molds is shifting from cutting using a cutting tool to wire-cut electrical discharge machining, etc., and the mold material is subjected to thermal history during processing. In order to eliminate the disadvantages caused by this thermal history, it is necessary to perform high-temperature tempering to precipitate stable carbides and reduce internal strain.

However, although high Cr steels such as the above-mentioned 5US420J2 do have a high transformation temperature, when high-temperature tempering is performed, aggregated and coarsened Cr-based M23C6 type carbides precipitate, making it difficult to secure the desired hardness. The problem is that it is difficult to Conventionally, it has not been possible to obtain this type of steel that can be tempered at high temperatures.

The reason for this was that they had only considered tool steel as a mold material for plastic molding. In other words, conventional tool steels are intended to maintain wear resistance through undissolved carbides during quenching or secondary hardening carbides precipitated during low-temperature tempering; However, if such high C steel is tempered at a high temperature of about 600 to 700°C, which is the wire electric discharge machining temperature, carbides will aggregate and coarsen as described above, resulting in a decrease in hardness. It is.

In conventional tool steel, low-temperature tempering of 150 to 300°C is common, and in such low-temperature tempering, martensite is not sufficiently tempered to maintain the necessary hardness. The situation is extremely unstable. It is clear that such tool steels, due to their hardening mechanism, cannot maintain the required hardness when heated above the tempering temperature. In addition, although this type of tool steel was developed on the premise that the temperature would not rise during machining, it is thought that the temperature may rise due to friction etc. during use, which is a factor in shortening the service life due to a decrease in hardness during use. It also becomes.

The inventor of the present application does not believe in the conventional idea of hardening the steel with undissolved carbides or secondary hardening carbides in tool steel, but rather in order to maintain stable carbides even by high temperature tempering in a region where Cm is further reduced. We carried out intensive research from the perspective of what should be done. In other words, high-temperature tempering has the advantage of sufficiently releasing the strain generated during quenching, but the hardness of martensite inevitably decreases. However, if stable carbides can be precipitated by high-temperature tempering, the required hardness can be ensured by precipitation hardening.

From this point of view, in order to enable high-temperature tempering without softening the steel, it is first necessary to eliminate undissolved carbides as much as possible at the quenching heating temperature,
It is necessary to set the amounts of Cm and carbide-forming elements so that carbides can be sufficiently dissolved in solid solution at around 1200° C., which is an industrially possible quenching temperature. Second,
In order to prevent Cr-based carbides from agglomerating and becoming coarse due to high-temperature tempering, it is necessary to add elements such as V, Nb, and W, which have a much stronger bonding force with C than Cr.

Regarding high-temperature tempering of cold-work tool steels, see the paper by Ito et al. 55, No. 4, pages 248-256).

The steel presented in this article has a composition as shown in Table 1.

As shown in this Table 1, the steel shown here is Cl
a is 0.6 to 1.43%, which is higher than that of the present invention, and
V is the only element for producing stable carbides. Since there is a large amount of Cm in this way, as mentioned above, high-temperature tempering tends to cause carbide agglomeration and coarsening, and since V is the only carbide stabilizing element, it is thought that the amount of stable carbide produced is small. In this paper, the steel shown here was not tempered at a high temperature of 600 to 700°C as intended in the present invention.
By tempering at a high temperature of about 00 to 700℃, ■
Since Cr and C, which are added in an overwhelmingly large amount, are mainly combined, it is presumed that V-based carbides are small and the softening resistance is reduced. In fact, as shown in Figure 3, all of the steels shown in Ito et al.'s paper reach a peak in hardness at a tempering temperature of 520°C, and the hardness drops sharply at 560°C. . Therefore, the tempering temperature is 600℃.

It is clear that the hardness decreases significantly when the temperature rises to 700°C.

In this invention, the amount of C is lower than the range shown in this paper, and V as a carbide stabilizing element.
Since appropriate amounts of two or more of Nb and W are added, 600
In high-temperature tempering of up to 700°C, the formation of agglomerated and coarse carbides can be suppressed, and moreover, the amount of stable carbides produced can be increased. Therefore, the required hardness can be maintained even by high-temperature tempering.

In order to achieve the third requirement of "capability of air-cooling hardening", it is essential that the hardenability is good.

Therefore, the lower limits of the contents of C, Mn, and Mo were determined from the viewpoint of not reducing the hardenability.

Next, the reasons for limiting the composition will be explained.

The amount of C is 0.25 to 0.50% by weight. Cf1 is 0.
If it is less than 25%, it will not be possible to ensure the necessary hardenability, and if it is more than 0.50%, unnecessary undissolved carbides will increase. As undissolved carbides increase, toughness decreases, and high temperature tempering causes agglomeration and coarsening of carbides. Moreover, when there is a large amount of Cfa, the quenching heating temperature must be significantly increased in order to dissolve carbides into solid solution.

The amount of SiQ is 0.005 to O, D5ffi' amount %.

If the amount of Si is less than 0.005%, deoxidation will be insufficient, and if it is more than 0.05%, the hardenability will increase more than necessary and the toughness will decrease.

M n Q is 0.1 to 2.0% by weight. Set the lower limit to 0.
The reason for setting it to 1% is to ensure hardenability. Also,
If it exceeds 2.0%, the hardness will increase more than necessary and the toughness will decrease, and retained austenite during quenching will increase.

As mentioned above, Cr is an important point of this invention, and its amount is 8.0 to 10.0% by weight. 8.0%
If the amount is less, the transformation temperature will be too low and the corrosion resistance will be insufficient. In addition, if it is more than 10.0%,
As described above, Cr-based carbides tend to aggregate and coarsen, and two-phase solidification of α and γ tends to occur.

The amount of Mo is 0.05 to 2.0 weight26. The lower limit is 0.
05% is the amount necessary to ensure hardenability and MO-based carbide, and the upper limit of 2.0% is the amount that is necessary because the increase in hardness is saturated even if it exceeds this, and it is not economical. The decision was made taking into consideration.

AI and N are elements necessary to form AIN and suppress coarsening of austenite during hardening. In other words, in order to completely dissolve carbides during quenching and heating,
It is necessary to raise the quenching temperature, but this causes the austenite grains to become coarser and deteriorate the toughness of the matrix.To prevent this, AI and N, which form AIN, are
Is required.

iW soluble A I noahao, 005~0. 05ff
lffi%. If this amount is less than 0.005%, the necessary amount of AIN cannot be secured, and 0.05%
If the amount is larger, AI-based inclusions will increase.

The total amount of nitrogen is 0.002-0.035% by weight. If this amount is less than 0.002%, the required amount of AIN cannot be secured, and if it is more than 0.035%, the AIN becomes coarse and the peening effect disappears.

As mentioned above, V, Nb, and W play an important role in this invention as carbide-forming elements. In this invention, 0.01 to 0.50% by weight of V, 0
．． 005-0.25 2% Nb, 0.01-0.25
It is essential to contain two or more types of W in the proportion of 2%. In the tempering stage, M23C6 type Cr-based carbides are first formed, but if at least two of V, Nb, and W are contained within the above-mentioned range, the formation of these carbides will lead to the formation of Cr-based carbides. Generation is suppressed, and as a result, agglomeration and coarsening of carbides are prevented.

If the content of these elements is less than the lower limit mentioned above, the required amount of carbide cannot be secured, and if the upper limit is exceeded, the quenching heating temperature for completely dissolving the carbide will rise significantly.

By adjusting the components as described above, it is possible to obtain a plastic molding tool steel that has a high transformation temperature, can be tempered at high temperatures, can be air-cooled and quenched, and has excellent corrosion resistance.

[Example] Examples of the present invention will be described in detail below.

Table 2 shows the composition and thickness of the samples studied.

Samples 1 to 9 in Table 2 are examples within the composition range of the present invention, and samples 10 to 12 are comparative examples outside the composition range of the present invention. For comparison, the compositions of conventional materials 5KD61 and S K Dll are also shown.

FIG. 1 is a diagram showing the relationship between tempering parameters on the horizontal axis and hardness (HRc) on the vertical axis. In the figure, the white circles indicate the examples in Table 2, and the squares indicate the examples in Table 2.
KD61 is shown. In addition, black circles, black triangles, and X indicate samples 10°11.12, which are comparative examples, respectively. In addition, the tempering holding time and tempering temperature in the tempering parameters are shown in the figure. In addition, the tempering parameter P is defined as P−T・(log
t+20) XIO-3.

As shown in this figure, it was confirmed that in the case of the example of the present invention, even if the tempering temperature was high, it was possible to obtain a hardness suitable for a Brihardened steel in the range shown by the broken line in the figure. For example, as shown in this figure, if you want to secure HRC-40 as a Brihardened steel, the tempering retention time is 2.
If it is desired to ensure that the tempering temperature is 600 to 650° C. and Hnc = 31, the tempering temperature can be made extremely high, such as 700° C. On the other hand, in the comparative example and the conventional material, when the tempering temperature is as high as 600 to 700°C, the hardness is extremely reduced.

FIG. 2 is a diagram showing the relationship between quenching heating temperature and hardness (HRC) in Sample 4 of Example. As shown in this figure, sufficient hardness characteristics can be obtained at a quenching heating temperature of 1200°C, so quenching from the as-rolled state is sufficient in terms of hardness characteristics. It is possible to set the tempering conditions to achieve the predetermined hardness required as a Brihardened steel.

Regarding corrosion resistance, Sample 2, which is an example of this invention,
．． 4.5.9 was compared with 5US420J2, a corrosion-resistant steel.

The results are shown in Table 3.

Table 3 Table 4 where corrosion resistance is measured in 0.5% HCI for 4 hours and in 0.5% HCI.
Evaluation was made by corrosion weight loss after holding in 5H2SO4 for 1 hour. Note that the liquid temperature was 25°C. As shown in Table 3, within the scope of the present invention, 13Cr-based corrosion resistance fr
It was confirmed that the same level of corrosion resistance as AsUs420J2 was maintained.

Table 4 lists samples 5.9, which are within the scope of this invention.
The heat treatment conditions and impact test values for 5KD61 and 5KDII are shown. Note that the impact value was determined by conducting a Charpy impact test using a test piece in which a U-notch with a length of 2 mm was formed.

As is clear from this table, 5KD61.11 has extremely low impact values at tempering temperatures of 600°C and 500°C, whereas Samples 5 and 9 showed high impact values even at 650°C. This is because, within the scope of the present invention, coarse undissolved carbides do not exist and martensite is sufficiently tempered.

[Effects of the Invention] According to the present invention, it is possible to obtain a material that has a high transformation temperature, does not aggregate or coarsen carbides even during high-temperature tempering, allows air-cooling hardening, and has excellent corrosion resistance. Therefore, a plastic molding tool steel with extremely excellent properties can be obtained.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between tempering parameters and hardness, Figure 2 is a graph showing the relationship between quenching temperature and hardness, and Figure 3 is a graph showing the relationship between tempering temperature and hardness in the literature by Ito et al. be.

Claims (1)

[Claims] In weight percent, carbon is 0.25 to 0.50% and silicon is 0.25 to 0.50%.
005 to 0.5%, manganese 0.1 to 2.0%,
Chromium: 8.0 to 10.0%, Molybdenum: 0.05%
2.0% to 2.0%, acid soluble aluminum 0.005 to 0
．． 050%, total nitrogen 0.002-0.035%, 0.01-0.50% V, 0.005-0.050%.
A plastic molding tool steel comprising at least two of 25% Nb and 0.01 to 0.25% W, with the remainder consisting of iron and inevitable impurities.