JPS6234828B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a pre-hardened steel for plastic molds, which has the greatest feature of having a small surface roughness during cutting and excellent mirror finish, and also has excellent texturability and toughness. It is. Steel for plastic molds should: (1) have good machinability; (2) Good mirror finish and no pinholes or other minute pits. (3) Good texturing properties. (4) Good corrosion resistance and rust resistance. (5) High strength and wear resistance. The following characteristics are required. High hardness in terms of (2) and (5). Regarding (2) and (4), the steel must be of a composition that is essentially easy to clean. Regarding (3), it is required that the hardenability is sufficiently high. In this case, it is particularly advantageous in terms of mold accuracy, delivery time, and cost to meet the above requirements with prehardened steel that does not require heat treatment by the user. Conventionally, high-hardness pre-hardened steel has problems such as requiring a long time to carve the die by cutting, and the life of the cutting tool is short, increasing the number of tool changes. A method has been adopted to improve machinability by adding free-cutting elements. S, Se, Te, Pb, Ag, etc. are generally used as free-cutting elements. However, if the amount of free-cutting elements necessary to obtain sufficient machinability is added, this addition will result in the above (2).
A problem occurred in which the characteristics (3), (4), and (5) deteriorated to some extent. The steel of the present invention has a weight ratio of C0.04 to 0.30% and Si0.1 to
1.5%, Mn0.2~2.0%, Cr0.1~3.5%, Mo0.1~
3.0%, Cu0.2~3.0%, S0.05~0.20%, Al0.05~
Addition of 0.50% and (O+N) of 0.007 to 0.012%
By adding 0.005 to 0.20% Zr after controlling the sulfide to a low level, a large number of fine ZrO ₂ and ZrN, which have an effective inoculation effect on sulfides, are generated, which makes the sulfides extremely fine and uniformly distributed. This significantly improves the uneven machinability, large surface roughness, and anisotropy of mechanical properties that have traditionally been a problem with sulfur free-cutting steel.
By suppressing the amount and heat-treating, Cu-Fe intermetallic compounds and carbides such as Mo and Cr are finely precipitated on a uniform bainite structure base, giving it strength and hardness, resulting in maximum mirror finish and grain workability. Features: In addition, to improve machinability and anisotropy of mechanical properties, Se0.05~0.15%,
Te0.05~0.15%, Ca0.001~0.010%, Mg0.001~
It is possible to add 0.010%. Next, the reason for limiting the component range will be explained. C is a basic additive element for making the quenched structure of the steel of the present invention into a bainitic structure with excellent machinability, mirror finish, and graining workability, and further for bringing about precipitation hardening due to Mo and Cr carbides. However, if the C content is too high, it will form a matrix martensite structure, reducing machinability and mirror finish, and will significantly increase the number of grinding steps, so it should be 0.30% or less, and if it is too low, it will lead to the formation of ferrite, resulting in insufficient tempering hardness. Since it will be difficult to obtain a profit, it should be set at 0.04% or more. Si is contained as a deoxidizing agent and is effective in improving oxidation resistance and hardenability. However, if it is less than 0.1%, no deoxidizing effect can be obtained in steel manufacturing, and if it exceeds 1.5%, the effect of improving oxidation resistance will be reduced and machinability will deteriorate, making it impractical. Mn is essential for improving hardenability, adjusting bainite hardness according to the required hardness level, preventing ferrite formation, and forming MnS-based inclusions between S and imparting high machinability. It is a containing element. Too much will reduce machinability, so
The content should be 2.0% or less, and if it is too low, the above effects will not be obtained and hot workability will be impaired, so the content should be 0.2% or more. Cr increases corrosion resistance and hardenability, and during tempering,
Produces fine carbide precipitation and appropriately adjusts the hardness value during precipitation hardening. If it is too high, it will lead to the formation of ferrite, so it should be set at 3.5% or less, and if it is too low, the above effects cannot be obtained, so it should be set at 0.1% or more. Mo is an element that precipitates fine carbides during high-temperature tempering treatment at 500° C. or higher, brings precipitation hardening, builds strength, and improves corrosion resistance against the atmosphere during use. If it is too high, machinability and toughness will deteriorate, so the content should be 3.0% or less, and if it is too low, the above effects cannot be obtained, so the content should be 0.1% or more. Cu is an essential element for precipitation hardening and obtaining the required hardness during high-temperature tempering treatment at temperatures above 500°C, and has the effect of increasing corrosion resistance and suppressing ferrite formation. If it is too high, machinability will deteriorate, so the content should be 3.0% or less, and if it is too low, the above effects cannot be obtained, so the content should be 0.2% or more. S is an essential element for improving machinability.
Too much will impair toughness and workability during delivery, so 0.2
% or less, and if it is too low, the above effects cannot be obtained, so it should be 0.05% or more. Al is an element that increases the inoculation effect of Zr on sulfides and makes the sulfides finer, thereby improving machinability and mirror finish. However, if it is less than 0.05%, the above effect will be small, and if it is too much, the amount of ZrO ₂ produced will be insufficient, and hard non-metallic inclusions Al ₂ O ₃ that are harmful to machinability due to reoxidation and nitrification during casting will be produced. It is set to 0.5% or less because it facilitates the generation of AlN and AlN. Zr is an essential element for improving machinability and mirror finish. The added Zr becomes ZrO ₂ or ZrN, which has an effective inoculation effect against sulfides. This inoculation effect results in very good uniformity in the shape, size and distribution of the sulphides. This uniformity significantly improves machinability and mirror finish. moreover
By allowing 70% or more of each of ZrO ₂ and ZrN to coexist with sulfide, it is possible to suppress the decrease in tool life during cutting. However, if there is too much Zr, the above effect will be saturated, and ZrO ₂ and ZrN, which do not coexist with sulfides and exist independently in the matrix, will increase, which will accelerate mechanical wear of the tool and reduce machinability, as well as reduce the polishing ability. It should be kept at 0.20% or less since it deteriorates the mirror finish due to the occurrence of pits. If the content is too low, the above effects cannot be obtained, so the content should be 0.005% or more. O and N are constituent elements of ZrO ₂ and ZrN, and are essential elements for the inoculating action against sulfides by the Zr compounds ZrO ₂ and ZrN, which is a feature of the steel of the present invention. In order to obtain good machinability, it is necessary for 70% or more of each of ZrO ₂ and ZrN to coexist with sulfide, and to further refine the sulfide, Zr0.005
-0.20%, (O+N) must also be kept as low as 0.007-0.012% and must be strictly controlled. (O+N)
If Zr is less than 0.007%, the inoculation effect is insufficient, resulting in uneven sulfide shape distribution and coarsening of the sulfide, and coarse Zr carbides are formed independently on the matrix, shortening tool life. If (O+N) exceeds 0.012%, oxides or nitrides that do not coexist with sulfides and exist independently in the matrix increase, which may deteriorate machinability and mechanical properties. Se improves machinability and corrosion resistance. 0.15%
If it exceeds 0.03%, the toughness will be significantly reduced, and if it is less than 0.03%, the above effect will be small. Like Se, Te improves machinability and corrosion resistance. If it exceeds 0.15%, the toughness decreases significantly;
If it is less than %, the above effect will be small. The effects of Ca and Mg help the inoculation effect of Zr compounds on S′de, making the shape and distribution of S′de uniform, and suppressing the elongation of sulfide during rolling processing by producing CaS and MgS. This improves mechanical anisotropy. If the amount is too high, deoxidation by Ca and Mg will proceed excessively, making it impossible to obtain the inoculation effect, so each should be kept at 0.010% or less. If the content is too low, the above effects cannot be obtained, so each content should be 0.001% or more. Se, Te, Ca, and Mg all have high vapor pressures, so when adding them, use methods such as injection injection, alloy bullets, or addition at the time of tapping.
Since Mg is active, it is also important to deoxidize it in advance, and the oxygen content in molten steel is controlled to 60 ppm before Mg is added. Examples of the steel of the present invention are shown below. Table 1 shows the chemical composition of the test materials (all Fe Bal). Table 2 also shows the proportion of Zr compounds coexisting with sulfides.

【table】

【table】

[Table] Figures 1 and 2 (invention steel C) and Figures 3 and 4 (conventional steel A) are micrographs showing the casting structures of the invention steel and conventional steel. The figure shows that the distribution of sulfides is almost uniform in both the steel of the present invention and the conventional steel, and that Zr compounds coexist with sulfides. However, it can be seen that the size of sulfides is significantly finer in the steel of the present invention than in the conventional steel. This is due to the increase in the inoculating effect of Zr on sulfides obtained by adding Al. FIGS. 5 and 6 are micrographs showing the morphology of sulfides in inventive steel C and conventional steel A with a forging ratio of 20, respectively. In the steel of the present invention, the sulfide content is extremely small, the shape and distribution are uniform, and the sulfide form is favorable for machinability and mirror finish. Table 3 shows the quenching and tempering (H
The machinability in _R C32) is expressed as an index with the machinability of conventional steel A as 100. Therefore, a large index indicates good machinability. It can be seen that the steel of the present invention is clearly superior to the conventional steel in terms of machinability. 7 and 8 are graphs showing the finished surface roughness of conventional steel A and steels C, D, and E of the present invention in cutting tests, respectively. It can be seen from Figures 7 and 8 that the finished surface roughness of the steel of the present invention is significantly improved. These results are attributable to the addition of Al and Zr and the control of (O+N)%, which not only made the sulfide distribution uniform but also made the sulfide extremely fine.

【table】

[Table] Table 4 shows the mirror finish (pit resistance) of the steel of the present invention.
compared with conventional steel. The sample was quenched and tempered to H _R C32 with a size of 50 mm square, and after aging treatment, a mirror finish was applied using a grinder → paper → diamond compound method, and the number of minute pits was counted using a 10x magnifying glass.

[Table] It can be seen that the steel of the present invention has extremely excellent mirror finish properties. This is because the addition of Al lowered the (O+N) concentration and reduced the absolute amount of nonmetallic inclusions, and the addition of Al, which was mentioned earlier, made the inoculants ZrO ₂ and ZrN for sulfides finer and reduced the amount of sulfide. This is due to the miniaturization of objects. As detailed above, the steel of the present invention has an extremely excellent finished surface in cutting processing, and its greatest feature is that it has excellent mirror finish properties. In addition to improving the efficiency of mold creation, the uniform fine distribution of sulfide inside and outside improves grain workability and mechanical properties, eliminating the need for concerns such as rust, making it tough and long lasting. This is a free-cutting steel for pre-hardened molds for plastic molding.

[Brief explanation of the drawing]

Figures 1 and 2 are micrographs of the metallographic structures of the invention steel C, and Figures 3 and 4 are micrographs of the metal structure of the conventional steel A. Figure 5 is the metal structure of the invention steel C, and Figure 6 is the conventional steel A. Micrographs of metal structures showing the morphology of sulfides at a forging ratio of 20, Figure 7 shows the finished surface roughness of conventional steel A, and Figures 8 A to C show the finished surface roughness of inventive steels C, D, and E, respectively. This is a graph representing

Claims (1)

[Claims] 1. C0.04~0.30%, Si0.1~1.5% by weight,
Mn0.2~2.0%, Cr0.1~3.5%, Mo0.1~3.0%,
Cu0.2~3.0%, S0.05~0.20%, Al0.05~0.50%,
Contains Zr0.005~0.20%, (O+N)0.007~0.012%, and further contains 70% each of Zr compounds ZrO ₂ and ZrN
A steel for plastic forming pre-hardened molds having excellent machinability, characterized in that the above coexists with sulfides, and the remainder consists of inevitable impurities and Fe. 2 C0.04~0.30%, Si0.1~1.5% by weight,
Mn0.2~2.0%, Cr0.1~3.5%, Mo0.1~3.0%,
Cu0.2~3.0%, S0.05~0.20%, Al0.05~0.50%,
Contains one or two of Zr0.005-0.20%, (O + N) 0.007-0.012%, Se0.05-0.15%, Te 0.05-0.15%, and further contains Zr compounds ZrO ₂ and ZrN
A steel for plastic forming pre-hardened molds with excellent machinability, characterized in that 70% or more of each coexists with sulfides, and the remainder consists of unavoidable impurities and Fe. 3 C0.04~0.30%, Si0.1~1.5% by weight,
Mn0.2~2.0%, Cr0.1~3.5%, Mo0.1~3.0%,
Cu0.2~3.0%, S0.05~0.20%, Al0.05~0.50%,
Contains one or two of Zr0.005-0.20%, (O+N) 0.007-0.012%, Ca0.001-0.010%, Mg0.001-0.010%, and further contains 70% each of Zr compounds ZrO ₂ and ZrN A steel for plastic forming pre-hardened molds having excellent machinability, characterized in that the above coexists with sulfides, and the remainder consists of inevitable impurities and Fe.