JPH07278737A - Preharden steel for plastic molding and its production - Google Patents

Abstract

PURPOSE:To attain the prolongation of a tool service life of a die or the like while the performance of a preharden steel is maintained. CONSTITUTION:This preharden steel for plastic molding is the one having a fundamental compsn. contg., by weight, 0.05 to 0.17% C, <=0.60% Si, 0.5 to 2.0% Mn, 2.5 to 3.5% Ni, 0.5 to 1.5% Al, 0.7 to 2.5% Cu, and the balance Fe with inevitable impurities and having a structure of lower bainitic one. As for a method for producing thereof, the same steel is subjected to lower bainite forming heat treatment in which it is heated to an austenitic region and is thereafter cooled to a lower bainitic region.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a new pre-harden type plastic forming steel having extremely excellent toughness and having machinability and hardness, and a method for producing the same.

[0002]

2. Description of the Related Art Pre-hardened steel for plastics molding, which is mainly used as a mold, has the advantages of reducing machinability, strength, and
It is excellent in wear resistance and requires moderate toughness.
However, these required properties are contradictory properties, and a steel that satisfies all the properties has not been obtained. The applicant of the present application, as a pre-hardened steel for plastic molding satisfying the above-mentioned properties to some extent, precipitates an intermetallic compound of Al and Ni, and also contains Cu in an amount of about 2%,
A low C-Mn-Ni- (Mo, W) -high Cu-Al system alloy having both hardness and machinability by finely depositing an e-Cu solid solution is disclosed in JP-A-63-114942 and 63-63. −
No. 183158.

Recently, low C-Mn-Ni-Mo (W) -Cu
Japanese Patent Laid-Open Publication No. Hei 10-199901 proposes that the occurrence of striped segregation is reduced by defining Si to 0.1% or less with respect to the Al-based alloy, and the anisotropy of strength is improved by making carbide fine particles. 5
-70888 and the like. Further, Japanese Patent Laid-Open No. 5-70887 proposes to provide both toughness and machinability by setting the grain size in the range of grain size numbers 4 to 6. The steel structures proposed by these are prepared to have a uniform upper bainite structure by defining C low, and the upper bainite structure ensures machinability.

[0004]

The above-mentioned proposals each provide one means for contributing to the performance improvement of the pre-hardened steel for forming plastics. But,
Recently, there has been an extremely high demand for die usage cost reduction, and it has become necessary to extend the life of the die while maintaining the performance of the conventional die. Has not been obtained. In view of the above-mentioned requirements, an object of the present invention is to provide a pre-hardened steel for plastic molding and a method for producing the same, which can achieve extension of the tool life of a mold or the like while maintaining the performance of the pre-hardened steel.

[0005]

Means for Solving the Problem] The present inventor has studied in detail the relationship between the composition and structure of a low C-Mn-Ni- (Mo, W) -Cu-Al alloy and the machinability and toughness. However, we have found a steel for plastic molding dies that has both machinability and toughness by preparing a lower bainite structure instead of the upper bainite structure that was considered to be an essential structure for improving machinability. . That is, according to the present invention, C 0.05 to 0.
17%, Si 0.60% or less, Mn 0.5 to 2.0%, Ni 2.5 to 3.5
%, Al 0.5 to 1.5%, Cu 0.7 to 2.5%, the balance Fe and inevitable impurities, and the structure is a lower bainite structure.

Particularly preferably, C 0.08-0.
17%, Si 0.60% or less, Mn 1.25 to 1.70%, Ni 2.6 to
Restricted to 3.5%, Al 0.95 to 1.50%, Cu 1.8 to 2.5%.

According to another invention of the present invention, one of the above steels is used as a basic composition, and a part of Fe is replaced with Mo or W alone or in a composition of 1/2 W + Mo 0.7% or less, and the structure becomes a lower bainite structure. It is a pre-hardened steel for plastic molding characterized by the above. Another invention of the present invention is a pre-hardened steel for plastic molding, characterized in that any one of the above steels is used as a basic composition and a part of Fe is replaced by 0.5% or less of Cr and the structure is a lower bainite structure. .

Another invention of the present invention is characterized in that one of the above steels is used as a basic composition and a part of Fe is replaced by S0.3% or less and the structure is a lower bainite structure. It is steel. Another invention of the present invention is to use at least one of the above-mentioned steels as a basic composition and at least part of Fe selected from the group 4A group element and the group 5A group element.
It is a pre-hardened steel for plastics forming, characterized in that it is replaced with 0.5% or less by seeds and the structure is a lower bainite structure.

In another invention of the present invention, one of the above steels is used as a basic composition and a part of Fe is less than 0.5% Be and B
It is a pre-hardened steel for plastics molding, characterized in that it has a lower bainite structure by substituting at least one selected from 0.01% or less. In another invention of the present invention, one of the above steels is used as a basic composition and a part of Fe is added as P.
b, Bi, Se, Te, at least one selected from 0.
It is a pre-hardened steel for plastics forming, characterized in that it is replaced with a range of 5% or less and the structure is a lower bainite structure.

Further, the manufacturing method of the present invention uses the above-mentioned weight%.
C 0.05 to 0.17%, Si 0.60% or less, Mn 0.5 to 2.0
%, Ni 2.5-3.5%, Al 0.5-1.5%, Cu 0.7-2.5%
Is a basic alloying component and the balance Fe is the main component, and is then heated to an austenite region, and then a lower bainite forming heat treatment is performed to cool it to a lower bainite region.

[0012]

[Function] One of the features forming the basis of the present invention is low C-Mn-
In the Ni- (Mo, W) -Cu-Al system plastic pre-hardened mold steel, the structure is a lower bainite structure. As described above, the conventional low C-Mn-Ni- (M
When an o, W) -Cu-Al alloy is used as a steel for plastic molding pre-hardened mold, it has been prepared to have an upper bainite structure in order to secure its machinability. However, although the upper bainite structure is a structure having excellent machinability, it has a low toughness. The present inventor has found that by extending the upper bainite structure to a lower bainite structure having high toughness, the mold life can be extended.

The lower bainite structure defined in the present invention is
CCT of steel is not obtained only by determining the composition
It is necessary to perform heat treatment for transformation at the lower part of the bainite nose in the curve, that is, at a temperature lower than the tip of the bainite nose. It should be noted that when the quenching heat treatment that is faster than the bainite nose is performed, the structure becomes martensite and the machinability is significantly deteriorated, which is not preferable.

Further, a desirable composition feature of the present invention is that a low C-Mn-Ni- (Mo, Mo, which combines hardness and machinability by finely depositing a Fe-Cu solid solution with high Cu.
W) -In high Cu-Al based pre-hardened steel for plastic molding, Al is increased to improve machinability and
This is because the composition having a higher Mn has excellent toughness and machinability even if the structure is a lower bainite structure.

The reasons for defining the composition specified in the present invention will be described below. In the present invention, Mn is the most important element for the plastic forming pre-hardened die steel based on the lower bainite structure having high toughness. Mn is an element that basically enhances the toughness of the matrix, and is required to be 0.5% or more. Further, Mn is an element that enhances the bainite quenchability and makes it easier to obtain the lower bainite structure that is a feature of the structure of the present invention. Therefore, it is preferably 1.0% or more, and more preferably 1.25% or more. Further, Mn also has an effect of suppressing the formation of ferrite and giving an appropriate quenching and tempering (aging) hardness. If the amount of Mn is too large, the toughness becomes too high and the excellent machinability obtained by adding Al cannot be maintained, so the content is specified to 2.0% or less.

Al is one of the important elements that bring about precipitation (aging) hardening by fine precipitation of Ni-Al intermetallic compounds in the tempering (aging) treatment and form the excellent machinability of the present invention. Al is also an additive element for obtaining the required hardness. Also, Al has the effect of increasing the nitriding hardness during nitriding. If it is too large, the amount of alumina-based inclusions increases, which reduces the mirror finish and also reduces the pitting corrosion resistance. Since it causes excessive decrease in ductility
0.5% or less, and if it is too low, the machinability decreases, so 0.5%
That is all. Preferably, it is 0.95% or more in order to further improve machinability when the lower bainite structure is formed.

C is a low C-Mn-Ni-Mo (W) -Cu-Al-based plastic molding pre-hardened die steel, which maintains the bainite structure as the quenching structure and is a Cu-Fe solid solution, Ni-Al in the tempering process. It is a basic additive element for providing a substrate for causing precipitation hardening based on the precipitation of intermetallic compounds and Mo and W carbides. If the amount is too large, the matrix is martensite-structured to reduce the machinability, and excessive carbides are formed to reduce the machinability. Therefore, in the present invention,
Specified as 0.17%. Preferably it is 0.08% or more. Si is added to enhance the corrosion resistance to the atmosphere during use. If it is too large, it will lead to the formation of ferrite and reduce the machinability, so the content is made 0.6% or less. When Si is reduced, the anisotropy is reduced, the striped segregation is reduced, and excellent mirror surface workability is obtained. Therefore, it is preferably 0.4% or less, more preferably 0.1% or less.

Ni enhances the bainite hardenability, suppresses the formation of ferrite, and further, when tempering (aging), Ni-
The Al intermetallic compound is added to precipitate the Al intermetallic compound, obtain the required hardness, moderately reduce the ductility, and improve the machinability. If it is too much, it lowers the bainite transformation temperature, excessively refines the bainite structure, and increases the viscosity of the matrix to reduce the machinability, so it is 3.5% or less, and if it is too low, the effect of the above addition cannot be obtained. So 2.5% or more. It is preferably 2.6% or more.

In the tempering (aging) treatment of the steel of the present invention, Cu causes precipitation (aging) hardening due to fine precipitation of the Fe-Cu solid solution, and is necessary for imparting the basic machinability of the steel of the present invention. Is an element for obtaining the hardness of. Also Cu
Provides excellent corrosion resistance. If it is too large, the hot workability is deteriorated, bainite is made finer, and the machinability is rather deteriorated. Therefore, if it is too low, the effect of the above addition cannot be obtained, so it is made 0.7% or more.
It is preferable to set it to 1.8% or more to further improve the machinability of the Fe-Cu solid solution.

[0020] Cr enhances the corrosion resistance of the steel of the present invention, enhances the hardness during nitriding, and suppresses rust during polishing or storage in the mold, and thus may be added depending on the purpose and application. If the amount is too large, the bainite structure becomes finer and the machinability deteriorates. Therefore, it is necessary to set the content to 0.50% or less for a metal mold for plastic molding which is particularly required to be machinable.
W and Mo are elements that have the effect of precipitating fine carbides and causing precipitation (aging) hardening in the high temperature tempering (aging) treatment of the steel of the present invention exceeding 500 ° C., and enhancing corrosion resistance to the atmosphere during use, It may be added depending on the purpose and use. In the case of this application, it is not necessary to add a large amount, and if it is too large, the machinability is deteriorated. Therefore, 1 / 2W + Mo 0.70% or less is preferable. 2W + Mo 0.10% or more.

In the present invention, S is added in an amount of 0.3% or less in order to improve machinability. Although the addition of S is extremely effective in improving the machinability, the addition deteriorates the mirror surface workability. Therefore, when the mirror surface property is particularly required, its use as an impurity can be reduced to 0.004% or less. desirable.
In addition, 4A, 5A group elements V, Nb, Ta, T
At least one element selected from i, Zr, and Hf is
It is an element that has the same effect in that the crystal grains are made finer and the toughness is increased. If a large amount is added, the solution hardness and the aging hardness will be increased more than necessary, and the machinability and toughness will be deteriorated. Therefore, it is necessary to add 0.5% or less when adding.

At least one element selected from Be and B is an element having the same action as an element for improving the hardenability in a large mold. These elements have a small effect even if added in a large amount, and rather deteriorate the workability. Therefore, Be 0.5% or less and B 0.01% or less. Further, at least one element selected from Pb, Bi, Se, and Te has the same action as an element that improves machinability. Addition of a large amount of these elements deteriorates toughness, so the total amount is made 0.5% or less.

The pre-hardened steel for plastic molding of the present invention is a pre-hardened steel having a hardness of HRC27 or higher.
It is supplied in a (tempered) state (generally tempered at 400 ° C or higher after quenching). The pre-hardened steel for plastic molding of the present invention is used as it is, after undergoing die-cutting processing → polishing processing, and further subjected to a high degree of mirror finishing or graining.

[0024]

【Example】

(Example 1) Alloys A to G having the chemical components shown in Table 1 were hot-rolled, then heated to an austenite region of 900 ° C, and cooling rates of alloys A and B were semi-cooling 5 minutes, alloys C and D. And E are semi-cooled for 15 minutes, and alloys F and G are subjected to bainite heat treatment under the conditions of semi-cooled 60 minutes to obtain a lower bainite structure, which is tempered in a temperature range of 540 to 590 ° C. and has a hardness of HRC 38 to 40. Was prepared. In addition,
Semi-cooling (hour) is the time required to cool from the quenching temperature to the temperature of (quenching temperature + room temperature) / 2. Further, as comparative examples, test pieces of the same composition were tested in the austenite region at 900 ° C., alloys A and B were semi-cooled for 15 minutes, alloys C, D and E were semi-cooled for 60 minutes, and alloys F and G were semi-cooled for 120 minutes. The bainite heat treatment was performed under the conditions described above to obtain an upper bainite structure, and tempering was performed in a temperature range of 500 to 560 ° C. to adjust the hardness to HRC 38 to 40.

As a typical structure of the present invention from the obtained sample, the lower bainite structure of the present invention when alloy D is used is shown in FIG. As a comparative example, the upper bainite structure when using the same alloy D is shown in FIG. The lower bainite structure in the present invention is a structure in which intragranular precipitated carbides equivalent to or more than the standard structure shown in FIG. 3 are present. Next, the mechanical properties and machinability of each sample were evaluated. The machinability was evaluated by performing a cutting test using an end mill of JISSKH59 Co-containing high-speed tool steel, measuring the wear amount of the flank of the end mill, and evaluating the cutting distance until the wear of 0.3 mm progressed.

Further, the mirror surface workability is emery number # 40.
The sample was evaluated by polishing with a diamond paste of No. 00. The evaluation of this mirror surface workability is extremely delicate,
It is difficult to quantify, and in the present example, the steel No. of the present invention having a lower bainite structure was used. On the basis of No. 7, those with less pits were evaluated as excellent, those with equivalent pits as good, those with a little inferior were acceptable, and those with which it was judged that they were unsuitable as a die were evaluated as unacceptable.

[0027]

[Table 1]

[0028]

[Table 2]

As shown in Table 2, it can be seen that the steel of the present invention prepared to have the lower bainite structure has significantly better toughness than the comparative steel prepared to have the upper bainite structure.
Regarding the mirror surface workability, there was no difference between the steel of the present invention and the comparative steel, and it had excellent mirror surface workability as a pre-hardened mold steel for plastic molding. The Mn content was 1.
In alloys A and B containing less than 25%, steel No. of the present invention having a lower bainite structure was used. The machinability of Nos. 1 and 3 was the same as Comparative Steel No. 3 having an upper bainite structure. The results were slightly lower than those of 2 and 4. On the other hand, in the alloys C to G in which the amount of Mn is 1.25% or more, the steel of the present invention prepared to have the lower bainite structure has machinability equivalent to that of the comparative steel having the upper bainite structure, but is extremely tough. You can see that it will be expensive.

Example 2 Alloys H to Z having the chemical components shown in Table 3 were hot-rolled, then heated to an austenite region of 900 ° C., and subjected to bainite heat treatment at a cooling rate of 20 minutes for semi-cooling. , A lower bainite structure was obtained, tempering was performed in a temperature range of 540 to 590 ° C., and hardness was adjusted to HRC 38 to 40. Further, as a comparative example, a test piece having the same composition was subjected to bainite heat treatment from an austenite region of 900 ° C. under the condition of 90 minutes of semi-cooling,
An upper bainite structure was obtained, tempered in a temperature range of 500 to 560 ° C., and hardness was adjusted to HRC 38 to 40. Similar to Example 1, mechanical properties, machinability and mirror surface machinability were evaluated. The results are shown in Table 4 and Table 5.

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

[Table 5]

As shown in Tables 4 and 5, it is understood that the steel of the present invention prepared to have the lower bainite structure has significantly better toughness than the comparative steel prepared to the upper bainite. From Tables 3, 4, and 5, the sample No. using the alloy H having a low Si is shown. In Nos. 15 and 16, excellent mirror surface workability was obtained. In addition, the sample No. using the alloy I having a small amount of Cu. Nos. 17 and 18 resulted in inferior machinability as compared with the sample having a high Cu content. Further, particularly, the samples using the alloys J, R, S, T, and U with S added have extremely excellent results with respect to the machinability, and are proved to be effective as a steel having improved machinability. . The alloys J, R, S, T, and U to which S is added have excellent machinability, but have slightly poor mirror surface workability. Therefore, the amount of S is reduced particularly when mirror surface workability is required. I know I should.

[0035]

EFFECTS OF THE INVENTION The steel of the present invention has excellent toughness not found in the conventional pre-hardened steel for plastic molding and also has machinability. Therefore, other properties are not significantly deteriorated and extension of the tool life for plastic molding can be achieved, which is extremely effective. Further, since the steel of the present invention has high toughness, cracks are less likely to occur due to thermal stress caused by working of a die or the like, which makes it particularly suitable for more precise die working.

[Brief description of drawings]

FIG. 1 is a photograph showing an example of a metal microstructure of the present invention.

FIG. 2 is a photograph showing an example of a metal microstructure of a comparative example.

FIG. 3 is a photograph for explaining a lower bainite metallographic structure.

Claims (9)

[Claims] 1. C 0.05-0.17% by weight, Si 0.60% by weight
Below, Mn 0.5-2.0%, Ni 2.5-3.5%, Al 0.5-1.
A pre-hardened steel for plastic molding, which has a composition of 5%, Cu 0.7 to 2.5%, the balance Fe and inevitable impurities, and has a structure of a lower bainite structure. 2. C 0.08 to 0.17% by weight, Si 0.60% by weight
Below, Mn 1.25 to 1.70%, Ni 2.6 to 3.5%, Al 0.95
2. The pre-hardened steel for plastic molding according to claim 1, wherein the content is regulated to 1.50% and Cu 1.8 to 2.5%. 3. The pre-hardened steel for plastic molding according to claim 1, wherein a part of Fe is replaced with Mo or W alone or in a combination of not more than 1/2 W + Mo 0.7%. 4. The pre-hardened steel for plastic molding according to claim 1, wherein a part of Fe is replaced by Cr 0.5% or less. 5. The pre-hardened steel for plastic molding according to any one of claims 1 to 4, wherein a part of Fe is replaced by S 0.3% or less. 6. The pre-hardening material for plastic molding according to any one of claims 1 to 5, wherein a part of Fe is substituted with at least one element selected from 4A group elements and 5A group elements in an amount of 0.5% or less. steel. 7. A part of Fe is Be 0.5% or less and B
The pre-hardened steel for plastic molding according to any one of claims 1 to 6, which is substituted with at least one selected from 0.01% or less. 8. The plastic molding according to claim 1, wherein a part of Fe is replaced with at least one selected from Pb, Bi, Se and Te in a range of 0.5% or less. Pre-hardened mold steel. 9. By weight%, C 0.05 to 0.17%, Si 0.60%
Below, Mn 0.5-2.0%, Ni 2.5-3.5%, Al 0.5-1.
5%, Cu 0.7 to 2.5%, the balance Fe is the main steel, after heating to the austenite region, lower bainite formation heat treatment for cooling to a lower bainite region Production method.