JPH0129861B2 - - Google Patents

Description

[Detailed description of the invention]

purpose of invention

[Industrial application field]

The present invention relates to a steel for plastic molds that has excellent die-carving workability and low anisotropy in mechanical properties.

[Conventional technology]

In recent years, large, high-performance machines have appeared in plastic molding machines, which are helping to improve work efficiency, but the demands on molds have become increasingly strict. That is, while the molding die is subjected to a more severe load than conventional molds, it is also required to have higher durability in terms of work efficiency. On the other hand, in order to cope with the increasing complexity and precision of mold shapes, improving the die-sinking workability of mold steel itself has become a major issue. In order to improve the machinability of steel for plastic molds, machinability-improving elements such as S and Pb have been added for some time, and these have been somewhat effective. Deterioration of mechanical properties due to the addition of sexual elements is unavoidable,
In particular, steel drawn by rolling or forging has strong anisotropy in mechanical properties, which is a major cause of reduced mold durability and weld cracking. This is because MnS sulfide inclusions, which effectively improve machinability, exist in the steel in an expanded form.
It is thought that this is because stress concentration occurs there and a notch phenomenon occurs starting from the inclusion. Inclusions generated from machinability elements also have a negative effect on the carving properties, which are important for steel for plastic molds, especially the mirror finish and texturing properties. It is easily understood that if large inclusions are exposed in the cross section, it will be a problem in achieving a mirror finish. It seems that the exposure of inclusions does not pose much of a problem with graining processability, but
Those skilled in the art are well aware that subtle effects on unevenness significantly impair the aesthetic appearance of the grain. It has been found that the form of inclusions has a large effect on the effects of inclusions on mirror finish and graining properties. If inclusions are stretched out and have a length exceeding a certain limit, they will fall off the surface during mirror polishing and the marks will become scratches.
Furthermore, foreign molecules are added to the pattern components of the grain, which impairs its aesthetic appearance. By controlling the morphology of sulfide-based inclusions in mold steel used as a material for plastic molds, the present inventors have achieved good mold carving workability and are expected to improve the life of the mold. I thought I could do it,
As a result of research, it was found that S
It has been found that by adding Te and Te in a specific amount and at a specific ratio, most of the inclusions in steel, especially large inclusions, take on a nearly spherical shape. Furthermore, such mold steel not only has high machinability, but also has high properties required for plastic mold steel, such as mirror finish and grain workability, and also has low anisotropy in mechanical properties. It was confirmed that the durability after molding was also excellent.

[Problems to be solved by the invention]

The purpose of the present invention is to utilize the above-mentioned knowledge obtained by the inventors to improve die engraving workability, that is, the comprehensive characteristics of machinability, mirror finishability, and graining workability, as well as An object of the present invention is to provide steel for plastic molds that has high durability after molding. Composition of the invention

[Means to solve the problem]

The steel for plastic molding molds of the present invention has C:
0.25~0.45%, Si: 0.10~1.50%, Mn: 0.10~
1.50%, Cr: 4.0~6.0%, Mo: 0.10~1.50%,
V: 1.0~3.0%, W: 3.0~5.0% and Co: 2.0~
In addition to 6.0%, it contains S: 0.002 to 0.40% and Te: 0.001 to 0.40% in a range of %Te/%S: 0.04 to 0.5, with the remainder substantially consisting of Fe,
This is a die steel with excellent die-carving workability, characterized in that at least 80% of the sulfide inclusions with a major axis of 2μ or more present in the steel have a major/minus axis ratio of 10 or less. The first point in producing the steel for plastic molds of the present invention lies in the proper adjustment of the components. First, molten steel is prepared in a furnace in which the content of alloy components other than S and other elements imparting free machinability is adjusted to a predetermined value. It is preferable to reduce the amount of O to 0.015% or less by vacuum degassing or the like to suppress the generation of oxide inclusions. Next, %Te/%S is added to the molten steel in a container such as a furnace, ladle or tundish.
Adding Te to satisfy the condition of 0.04 to 0.5,
It is sufficient to disperse it evenly. Addition of Te can also be carried out in the injection tube. When adding Te, it is desirable to remove as much as possible large nonmetallic inclusions, which are mainly oxide-based inclusions.For this purpose, a non-oxidizing gas such as argon is introduced into the molten steel and forced stirring It is effective to do so. This operation can be performed prior to the addition of Te, or
Further, the process may be performed while adding Te.

[Effect]

The role of each component element and the reason for limiting the composition range in the steel for plastic forming molds of the present invention are shown below. C: 0.25-0.45% In order to ensure hardness and wear resistance as steel for plastic molds, it is necessary to add 0.25% or more. If added in large amounts, the toughness would decrease and become unsuitable for practical use, so it was limited to 0.45% or less. Si: 0.10-1.50% In addition to deoxidizing during melting, it is an effective element for strengthening the base, and should be added at 0.10% or more. If the amount is too large, ground scratches will increase, which is contrary to the purpose of the present invention, and machinability will decrease, so it should be within 1.50%. Mn: 0.10-1.50% Mn is an effective element for deoxidizing during melting and strengthening the base, and must be added at 0.10% or more. If it becomes too large, toughness and machinability decrease, so the upper limit is 1.50%. Cr: 4.0-6.0% It is an effective element for toughening the matrix and ensuring hardenability, wear resistance, and oxidation resistance, and is added in an amount of 4.0% or more. On the other hand, if it is too large, the toughness decreases and practicality is lost, so it should be kept at 6.0% or less. Mo: 0.10-1.50% V: 1.0-3.0% W: 3.0-5.0% All of these are strong carbide-forming elements that ensure heat treatment hardness and wear resistance. Mo is
Add 0.10% or more, V 1.0% or more, and W 3.0% or more. Adding a large amount of Mo makes manufacturing difficult and reduces toughness, so Mo content should be 1.50% or less.
V was limited to 3.0% or less, and W was limited to 5.0% or less. Co: 2.0 to 6.0% Effective for strengthening the base and ensuring wear resistance,
Add 2.0% or more. If the amount is too large, the toughness will decrease, so the upper limit is set at 6.0%. S: 0.002-0.40% S is essential for the formation of MnS-based inclusions, which are effective inclusions for improving machinability, and requires addition of 0.002% or more. The machinability improves as the amount increases, but it impairs the cleanliness of the steel and impairs its toughness, so it should be 0.40% or less. Te: 0.001 to 0.40% Te is an important element in adjusting the morphology of MnS-based inclusions and providing free machinability by itself, and is added in an amount of 0.001% or more. If added in too large a quantity, hot workability will deteriorate, so it should not exceed 0.40%. %Te/%S: 0.04-0.5 In order to improve the morphology of sulfide inclusions,
It is necessary that the ratio of %Te/%S is 0.04 or more, but if it exceeds 0.5, the effect will be saturated and the hot workability will also decrease, so the ratio of %Te/%S should be 0.04.
~0.5 range. Morphology and distribution of sulfide inclusions As mentioned above, the machinability, mirror finish and graining properties, and anisotropy of mechanical properties of steel for plastic molds are influenced by sulfide inclusions in the steel. Much depends on the form and distribution of objects. The present inventors have confirmed that relatively large sulfide inclusions with a major axis of 2μ or more influence these characteristics, and large inclusions have an extremely rope-like shape with a major axis ratio of 10 or less. If they have an unstretched form, they will not show any adverse effects, and the condition is satisfied that such inclusions account for 80% or more of the total number of sulfide inclusions. .

【Example】

Steel having the alloy composition shown in Table 1 (balance Fe) was produced. For melting, after adjusting the alloying elements to a predetermined amount in a basic electric furnace, Te is added to the ladle according to the amount of S in the molten steel and dispersed uniformly. Agglomerated. The above sample material was hot forged at a forging ratio of about 10 to produce a rough mold. Continuing
Quenched under the conditions of 1150℃ x 1 hour, 620℃ x 3 hours,
After tempering, a sample was taken from the rough shape, and the strength anisotropy was examined by an impact test (JIS No. 3 Shapey test piece). The morphology and distribution of sulfide inclusions were investigated on the test pieces after the impact test.
The results are summarized in Table 2.

【table】

[Table] As shown in Table 2, the conventional steel prepared for comparison has low impact properties in the direction perpendicular to the forging direction.
The impact value is only 1/6 of that in the forging direction, and the mechanical properties are highly anisotropic. On the other hand, in all the steels of the present invention, there is little decrease in impact properties in the direction perpendicular to the forging direction, and the impact values are 1/2 or more of the impact values in the forging direction. This is supported by the form and amount of sulfide inclusions in the steel. In the comparison steel, relatively spherical sulfide inclusions with a length ratio of 10 or less accounted for 20% of the total.
The other sulfide inclusions are elongated ones with a major axis ratio of 10 or more, whereas in the steel of the present invention, sulfide inclusions are relatively spherical with a major axis ratio of 10 or less. occupies the majority. Next, a mold for plastic injection molding was die-sinked using a rough mold for a mold manufactured from the sample materials shown in Table 1, and a part of the cavity surface was textured. Table 3 shows the time required for die engraving of each sample material as a ratio of the time to that of comparative steel.

[Table] When the textured surface was observed, the comparison steel had more than 20 scratches caused by expanded sulfide inclusions, but no scratches were found on the steel according to the present invention. From the above results, compared to conventional steel, S and
It can be seen that all of the steels of the present invention in which the amount of Te was adjusted were added, the time required for die engraving was short, and the texturability was also high. Effects of the Invention The steel for plastic forming molds of the present invention is produced by adding S and Te in appropriate amounts and in a specific quantitative ratio range to steel with a specific alloy composition to adjust the morphology of sulfide-based inclusions. It has good machinability, mirror finishing properties, and graining properties, so it can meet the high requirements for molds. This steel also has low anisotropy in mechanical properties, giving the mold excellent durability.

Claims (1)

[Claims] 1 C: 0.25-0.45%, Si: 0.10-1.50%, Mn:
0.10~1.50%, Cr: 4.0~6.0%, Mo: 0.10~1.50
%, V: 1.0-3.0%, W: 3.0-5.0% and Co:
2.0-6.0% plus %Te/%S: S: 0.002-0.40% and Te: 0.001-0.40% in the range of 0.04-0.5
, the remainder is essentially Fe, and at least 80% of the sulfide inclusions with a major axis of 2 μ or more present in the steel have a major axis ratio of 10 or less. Steel for plastic molds with excellent engraving properties.