JPH042659B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a new pre-hardened type steel for plastic forming pre-hardened molds, which has extremely excellent machinability and also has appropriate mirror finish and toughness. [Prior art] Steel for high-grade plastic molds has the following characteristics: (1) Good mirror finish and low tendency to generate pinholes and other fine pits; (2) Good texturing properties; (3) It is required to have good corrosion resistance and rust resistance, (4) good strength, wear resistance, and toughness, and (5) good machinability. [Problems to be solved by the invention] However, in recent years, measures have been strongly raised as an important customer requirement to shorten the production lead time of molds and to prevent breakage of molds from thin corners due to the complicated shapes of molds.
Simply add S to MnS to shorten mold processing delivery time.
In the case of free-machining steel that has improved machinability with system sulfides, there is a strong tendency for cracks to occur at thin-walled corners due to the distribution of MnS extending long in the hot working direction and the segregation distribution of sulfides. Therefore, it was not easy to combine excellent mirror finish and breakage resistance with particularly excellent machinability. The object of the present invention is to provide an alloy that simultaneously satisfies the above-mentioned required properties. [Means for solving the problem] That is, the steel of the present invention has low C-medium to low Mn-Ni-
In (Mo)(W)-high Cu-low Al-S steel (Mo and W are added singly or in combination), MnS sulfide is uniformly and finely distributed and excessive elongation in the hot working direction is suppressed. It is characterized by the addition of Ca and Ce either singly or in combination to achieve this. In the present invention, a uniform upper bainite structure is generated by quenching, and further high-temperature tempering at 500°C or higher is performed to form a low C bainite base with Fe-Cu, Ni-
Al intermetallic compounds, Mo, and W precipitate singly or composite carbides, imparting strength through aging (precipitation) hardening, moderately embrittlement, and imparting good machinability to the matrix itself; In addition to achieving extremely excellent machinability by controlling the distribution and shape of the mold, it also maintains adequate toughness to withstand breakage from the thin corners of the mold during use, and maintains excellent mirror finish. It is. Also, Cr, Mo,
The inclusion of W, Cu, and Ni provides excellent corrosion resistance and rust resistance, and there is little hardening due to electrical discharge machining.
In this respect as well, finishing is easy. The steel of the present invention is in a pre-hardened state (tempered at 450℃ or higher after quenching) with a hardness of H _R C37 or higher.
It is supplied as is, and used as is after being subjected to die engraving, polishing, mirror finishing, and graining. Therefore, it does not require heat treatment by the user, has the highest degree of good machinability, and even when processed into a mold with a complex shape with thin walls as a plastic mold, it can be easily removed from thin corner parts during use. In addition, the fine and uniform distribution of MnS-based inclusions that provide high hardness, uniform base, and free machinability combine to provide excellent mirror finish and texturability. This is a new steel for plastic molds that provides a long service life without worrying about wear. [Example] Table 1 shows examples of the invention steel, comparative steels, and chemical compositions.

[Table] Comparative steel Q is a free-cutting steel, but it is characterized by particularly excellent toughness, and is a mold steel that does not break from thin corners during use. Comparative steel R contains Al and has machinability compared to comparative steel Q.
However, it does not contain Ca or Ce, has a strong tendency for MnS-based sulfides to elongate in the hot working direction, and is also inferior to the steel of the present invention in terms of uniformity of distribution. Table 2 shows the machinability index and maximum roughness ratio of the cut surface based on SCM440 (H _R C37.4) when cutting with an end mill, although the invention steel and comparative steel were heat treated to about H _R C39. show. Although the invention steel has a higher hardness than the comparative steel S (SCM440, H _R C37.4), it has much better machinability, and the roughness of the cut surface is clearly finer, making it suitable for subsequent polishing. This shows that it is highly effective in reducing man-hours.

【table】

[Table] Table 3 shows the cross-sectional dimensions of the steel according to the present invention: 50 mm x 150 mm
w After tempering the forged and drawn material to around H _R C39, test pieces were taken in parallel (L) and perpendicular (T) directions to the forging and drawing direction and subjected to a tensile test and a sharpie impact test (2 mmu notch). Here are the results.

【table】

[Table] The steel of the present invention is a comparative steel S with particularly excellent toughness.
(SCM440), the elongation in the tensile test,
The reduction of area and the sharpie impact value are low, which makes it easy to remove chips by shearing during cutting, which improves machinability and roughness of the cut surface compared to comparative steels Q and S.
MnS is superior to MnS and also provides free machinability.
Since the distribution of sulfides in the system is uniform and fine, the roughness of the cut surface is also finer than that of comparative steel R. On the other hand, the elongation, reduction of area and Charpy impact value are
Compared to comparative steel Q, which maintains a good level of toughness, it does not show any extreme deterioration, does not break even when used as a mold with a complex shape or thin wall, and has extremely good machinability and moderate toughness. It is characterized by having both. On the other hand, in the case of comparative steel R, which does not contain Ca or Ce, which has the effect of making the distribution of MnS sulfide-based inclusions fine and uniform and rounding the shape of the inclusions, the elongation in the T direction in the tensile test, When applied to molds with a complex shape or thin corners due to their low mechanical strength and mechanical strength impact values, chipping or breakage may occur. Table 4 shows the results of the rusting resistance test of the steel of the present invention in a salt water atmosphere. The sample was 25 mm x 25 mm, and the number of rusted samples was counted after being left in a salt water atmosphere for 2 hours after being polished to a mirror finish using paper and buffing.

[Table] Compared to comparative steel R, the size and distribution of MnS-based inclusions that promote corrosion steel formation are finer and more uniform in the steel of the present invention. The number is clearly small. Table 5 compares the mirror finish (pit resistance) of the invention steel with comparative steels R and S. The sample was 50 mm square, and after hardening and tempering to H _R C39, it was polished to a mirror finish using a grinder → paper → diamond compound polishing method, and the number of minute pits was counted using a 10x magnifying glass. It is a thing (an indexed thing).

〔Effect of the invention〕

As detailed above, the steel of the present invention is H _R C38~
Even with a high hardness of around 40, it has extremely excellent machinability, moderate T-direction toughness, excellent mirror finish, and pitting corrosion resistance.It also has excellent texturability and electrical discharge hardening resistance, and has excellent strength. The present invention provides a steel for pre-hardened molds for plastic molding, which combines wear resistance and long life.

Claims (1)

[Claims] 1% by weight, C0.07~0.16%, Si≦0.60%,
Mn0.70~1.80%, Ni2.30~3.40%, Cr≦0.80%,
And one or two types of W and Mo are 1/2W + Mo
0.10~0.70%, Al0.20~0.60%, Cu1.80~2.60%,
S0.05~0.16% and Ca0.001~0.010%
One or two types of Ce0.01-0.10% (in the case of two types)
Contains Ca0.001~0.008%, Ce0.01~0.08%),
Steel for plastic forming pre-hardened molds consisting of balance Fe and normal impurities.