JP4337763B2 - Steel for plastic molds - Google Patents

Description

  The present invention relates to a steel for plastic molds, and more particularly to a steel for molds used for plastic injection molds, etc., and has a long cutting tool life in the manufacture of molds. The present invention relates to a steel for plastic molds having high efficiency and particularly excellent flat surface millability.

  A mold used for manufacturing a product having a special shape is an indispensable processing member (tool) for molding a material such as metal or plastic. In addition to general mechanical properties, steel used in molds requires excellent machinability so that the shape of the mold inner surface matched to the shape of the product can be processed accurately and easily. . In particular, plastic products can be molded by an injection molding method in which heated resin is press-fitted into a mold. Therefore, products having relatively complicated shapes can be manufactured, and higher machinability is required. .

As steels for plastic molding dies, the following materials have been proposed even in the past.
In Patent Document 1, 0.5 to 2.5% Si is contained, and an oxide film having a low melting point mainly composed of SiO ₂ is allowed to act as a lubricant between the cutting tool and the workpiece, Steel with improved machinability is disclosed.

  Patent Document 2 defines the existence form of double structure inclusions (inclusions in which oxide inclusions and sulfide inclusions are in contact with steel having a Si content of 1.0% or less). Steel with improved machinability is disclosed.

Patent Document 3 contains at least one of Ti and Zr in an amount of 0.05% or more, and prevents the segregation of impurities by defining the ratio between the effective Ti equivalent defined by the Ti and Zr contents and the S content. A cast steel is disclosed. In this cast steel, machinability is improved by containing 0.02% or more of S.
JP-A-9-49067 JP 2003-3234 A JP 2002-877309 A

  There is a strong demand for speeding up the development of new products, and accordingly, there is a strong demand for speeding up the production of molds used in new products. Plane milling occupies a large specific gravity in mold making, and improving machinability is important for speeding up.

On the other hand, from the viewpoint of cost reduction as well, there is a demand for manufacturing by continuous casting to improve the steelmaking yield. In order to produce high-quality mold steel by continuous casting, it is possible to suppress CO bubbles generated during casting by using so-called Al killed steel in which oxygen in the steel is killed. It is preferable from the viewpoint of ensuring quality soundness. However, Al produces hard Al ₂ O ₃ and tends to cause sharpness and poor sharpness, so that addition is an element that should be suppressed from the viewpoint of machinability. A manufacturing technique that suppresses the formation of hard Al ₂ O ₃ while ensuring the soundness of the inner quality of the mold steel is important. Further, S is an element that improves machinability, but the continuous cast material promotes center segregation and causes two-piece cracking.

Therefore, when manufacturing steel for plastic molds by continuous casting, it is necessary to determine the steel composition in consideration of the above.
The object of the present invention is to provide a new steel for plastic molds which is strongly demanded today, which overcomes the drawbacks of the prior art.

As a result of earnest research and development to solve the above problems, the present invention further provides a steel type in which an oxide film mainly composed of SiO _{2 and} having a low melting point acts as a lubricant between a cutting tool and a workpiece. It has been found that the above-mentioned problems can be solved by development.

That is, Al is a minimum addition that ensures the soundness of the quality of the mold steel by continuous casting. Furthermore, for the adverse effect on the machinability of Al addition, O (oxygen) to suppress the content suppresses the formation of Al ₂ O _3. In order to suppress the O (oxygen) content, a vacuum degassing process such as RH is utilized, the order of addition of deoxidizing elements is devised, and argon shielding is performed during casting to prevent oxidation of the molten steel.

Furthermore, in order to prevent double cracking, S is controlled to be low within a range that does not impair the machinability, and rapid cooling is performed immediately below the casting mold for continuous casting to ensure fine and fine dispersion of sulfide inclusions to ensure free-cutting properties. .
Thus, according to the present invention, the machinability can be ensured without using any environmentally harmful elements such as Pb that have been used for improving the machinability.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
(1) By mass%, C: 0.30 to 0.55%, Si: 1.10 to 2.5%, Mn: 0.10 to 1.00%, Cr: 0.02 to 1.0% V: 0.003-0.50%, P: 0.025% or less, S: 0.005-0.030%, Al: 0.007-0.06%, N: 0.006% or less, O (oxygen): 0.0001 to 0.005% is contained, the balance consists of Fe and impurities, sulfide inclusions have an average length of 20 to 150 μm, and the distribution density is 1 per square mm. Steel for plastic molds, characterized by having more than one piece.

(2) The plastic molding die steel according to (1), further containing B: 0.01% or less by mass%.
(3) The steel for plastic molds according to (1) or (2), further containing, by mass%, Zr: 0.02% or less.

  (4) The plastic according to any one of (1) to (3), further containing, by mass%, Mg: 0.02% or less and / or Ca: 0.02% or less. Steel for molding dies.

  (5) Further, any one of (1) to (4), characterized by containing, by mass%, Cu: 1.0% or less, Ni: 2.0% or less, Mo: 1.0% or less Steel for plastic molds as described in 1.

  (6) The steel for plastic molds according to any one of (1) to (5), which is produced by continuous casting.

  Since the plastic mold forming steel of the present invention can greatly improve the cutting efficiency, it is possible to shorten the lead time for mold manufacture. Furthermore, there is little damage to the tool, so it is possible to expect a reduction in the overall mold production cost. Moreover, since it can be manufactured by continuous casting, it is suitable for mass production as compared with the steel for molds manufactured by the conventional casting method, and the cost can be greatly reduced.

  The feature of the steel for plastic molds of the present invention (hereinafter referred to as the steel of the present invention) is a further development of the conventional high-Si steel, while properly managing the Al content and O content. Furthermore, the S content is to be specified as high for steel for plastic molding dies.

The appropriate range of the content of each element of the steel of the present invention and the basis thereof will be described below. In addition, below, content of each element is shown by the mass%.
C: C is an element effective for increasing the strength of steel. However, if the content is less than 0.30%, it is difficult to ensure the strength of the steel. On the other hand, if it exceeds 0.55%, the toughness and machinability deteriorate, so the C content was set to 0.30 to 0.55%. When the machinability is particularly important, the range of 0.30 to 0.40% is preferable. This is because by lowering the upper limit of C, a certain amount of ferrite is secured while improving the machinability while securing the hardness of the steel.

Si: Si is an oxide film formed on the surface of the cutting waste in the steel of the present invention, mainly composed of a SiO ₂ —FeO system having a low melting point, and the oxide film is between the cutting tool and the workpiece, It is an important element that acts as a lubricant. If the Si content is less than 1.1%, this effect is small. On the other hand, if it exceeds 2.5%, not only the lubricating effect is saturated but also the toughness of the steel is deteriorated. Therefore, it was 1.1 to 2.5%.

  Mn: Mn is an element effective for improving the hot workability and hardenability of steel. However, if the Mn content is less than 0.10%, those effects cannot be obtained. On the other hand, if the Mn content exceeds 1.00%, the hardenability of the steel becomes too high, and on the contrary, the machinability is impaired. Therefore, the Mn content is set to 0.10 to 1.00%.

  Cr: Cr is an element effective for improving the hardenability of steel. If the Cr content is less than 0.02%, the effect cannot be obtained. On the other hand, if the Cr content exceeds 1.0%, the machinability is impaired. Therefore, the Cr content is set to 0.02 to 1.0%.

  V: V is an element effective for improving the temper softening resistance of steel. Further, V forms an oxide having a low melting point, and therefore has an effect of helping the effect of improving the machinability of Si. When the V content is less than 0.003%, these effects cannot be obtained. On the other hand, when the content exceeds 0.50%, the machinability and toughness of steel are reduced. Therefore, the V content is set to 0.003 to 0.50%. Preferably, it is 0.003 to 0.20%.

  P: P is usually present as an impurity in steel and harms the toughness of the steel. As a range in which P can be reduced by an ordinary industrial refining method, the P content is set to 0.025% or less.

  S: S is an element effective for improving machinability. As described below, in the present invention, machinability is ensured by sulfide inclusions. For this reason, S content needs to be 0.005% or more. However, S is detrimental to the toughness of steel, and promotes center segregation in a continuous cast material and easily causes two-piece cracking. Therefore, the S content is set to 0.030% or less. More preferably, it is less than 0.020%.

  Al: Al is added as a deoxidizer for molten steel. When considering industrial mass production by continuous casting, if Al is not contained, carbon (C) and oxygen (O) in the steel react in the solidification process during continuous casting, and carbon monoxide ( CO) bubbles are generated and the soundness of the steel is impaired. Therefore, it is necessary to contain Al that suppresses the generation of carbon monoxide (CO) bubbles. Therefore, the Al content is set to 0.007% or more. More preferably, the Al content is more than 0.02%.

On the other hand, when the Al content is high, Al ₂ O ₃ inclusions, which are Al deoxidation products, remain in the steel. Since Al ₂ O ₃ inclusions are hard, they are harmful to machinability. The smaller the Al content in the steel, the fewer the Al ₂ O ₃ inclusions, so the smaller the Al in the steel, the better. Therefore, sol. The total Al content including Al is set to 0.06% or less. If the Al content is 0.06% or less, reduction of oxygen in the steel by vacuum degassing, prevention of oxygen intrusion into the steel by argon sealing during casting, and rapid cooling immediately below the casting mold for continuous casting Al ₂ O ₃ It is possible to suppress the adverse effect of Al ₂ O _{3 on} the cutting edge by finely dispersing inclusions such as.

N: N is an element that inevitably invades from the atmosphere. N has an effect of reducing toughness, so is 0.006% or less.
O: O forms soft oxide inclusions and is effective in improving the machinability of steel. In order to obtain this effect, the O content needs to be 0.0001% or more. On the other hand, when an oxide is excessively present, a large amount of Al ₂ O ₃ is generated and the machinability is impaired, so the O content needs to be 0.005% or less. Therefore, the O content is set to 0.0001 to 0.005%.

In the present invention, if necessary, at least one of the following elements may be appropriately contained.
B: B has the effect of improving the hardenability of the steel. Further, since the oxide of B has a low melting point, it also has an effect of helping the effect of improving the machinability of Si. However, if the B content exceeds 0.01%, the toughness of steel and the weldability are lowered. In order to obtain the machinability improving effect, the B content is preferably 0.0001% or more. Therefore, when B is contained, the content is 0.01% or less, preferably 0.0001 to 0.01%.

  Zr: Zr has an effect of improving the machinability of steel by changing the form of sulfide. However, when the Zr content exceeds 0.02%, the effect is saturated and an effect commensurate with the cost cannot be obtained. In order to obtain this effect, the Zr content is preferably 0.005% or more. Therefore, when Zr is contained, the content is made 0.02% or less, preferably 0.005 to 0.02%.

  Ca, Mg: Ca and Mg have the effect of finely dispersing inclusions and improving machinability. In order to obtain this effect, the Ca and Mg contents are each preferably 0.005% or more. However, if the content of Ca and / or Mg exceeds 0.02%, the cleanliness of the steel deteriorates and the surface quality after mold processing is impaired. Therefore, when Ca and / or Mg is contained, the content is 0.02% or less, preferably 0.005 to 0.02%.

  Cu, Ni, Mo: Cu, Ni, and Mo have the effect of improving the hardenability of the steel. Further, Mo is effective in preventing temper embrittlement. In order to obtain this effect, the Cu, Ni, and Mo contents are preferably set to 0.01% or more. However, if the Cu content exceeds 1.0%, if the Ni content exceeds 2.0%, or if the Mo content exceeds 1.0%, the machinability of the steel decreases. Accordingly, when Cu, Ni and / or Mo are contained, Cu is 1.0% or less, Ni is 2.0% or less, Mo is 1.0% or less, preferably Cu is 0.01 to 1.0%, Ni is 0.01 to 2.0%, and Mo is 0.01 to 1.0%.

In the steel of the present invention, the balance is impurities and Fe. Examples of impurities in the steel of the present invention include the following.
Bi, Be, Pb, Te, Nd: Even if these elements are contained in the steel as impurities, they do not affect the machinability of the steel. However, when it contains excessively, properties other than machinability are affected. That is, when the contents of Bi and Be are excessive, the toughness of the steel is lowered, and when the content of Pb is excessive, the cutting surface becomes rough. Further, when the Te content is excessive, the high temperature ductility of the steel is impaired. Furthermore, there is a problem that Nd is expensive. In recent years, there is a strong demand to suppress the addition of these elements from the environmental viewpoint. Therefore, these elements may be Bi: 0.05% or less, Be: 0.10% or less, Pb: 0.05% or less, Te: 0.05% or less, and Nd: 0.1% or less. preferable.

  Ti, Nb: Even if these elements are contained in the steel as impurities, there is no influence on the machinability of the steel. However, when it contains excessively, a carbide | carbonized_material with high hardness may accelerate | stimulate tool wear. Therefore, Ti and Nb are preferably 0.02% or less and 0.05% or less, respectively.

In the steel of the present invention, not only the steel composition as described above but also the form and distribution of sulfide inclusions are defined as follows.
The sulfide inclusions in the steel of the present invention have an average length of 20 to 150 μm and a distribution density of 1 or more per square mm. Here, the sulfide inclusions mean inclusions mainly composed of MnS. When sulfide inclusions are present in the steel, the machinability of the steel is good. The mechanism is that the presence of sulfide inclusions helps to reduce the pressure at the cutting edge by assisting crack initiation and crack propagation between the chip and the base metal when the cutting edge is cut into the base metal. This is considered to increase the cutting speed limit and reduce the wear of the blade.

  In order to help the starting point of crack generation or the expansion of cracks, the average length of sulfide inclusions is 20 μm or more. However, if the average length of sulfide inclusions exceeds 150 μm, the appearance of the product surface is impaired as a ground on the mold surface. Therefore, the average length of sulfide inclusions is 20 to 150 μm.

  In addition, the number of sulfide inclusions should be large, and the distribution density should be 1 or more per square mm. Preferably, it is 1.5 or more. Although the upper limit of the number of sulfide inclusions is not particularly specified, when the steel of the present invention is produced, the number of sulfide inclusions is usually 8 or less.

Here, the average length is defined as follows.
In general, inclusions mainly composed of MnS are stretched by rolling. The average length referred to here is the major axis length of the stretched MnS.

The average length is measured by the following measurement.
From the surface of the steel after production, 1/4 position of the plate thickness, 1/2 position of the plate thickness, 3/4 position of the plate thickness, from the back surface, each sample having a plate thickness of 5 mm, width 20 mm × length 20 mm, 10 pieces are collected from each position. Ten micrographs are taken at random for each sample. At this time, shooting is performed at a magnification of 200 times. Then, the size of the sulfide inclusions is measured with a scale attached to the microscope, and the major axis length is measured one by one and then arithmetically averaged to obtain the average length.

  As for the distribution density, a sample is taken similarly to the average length, a micrograph is taken at 200 times, and the number of sulfide inclusions in the field of view is counted. Then, the distribution density is obtained by dividing the number of inclusions by the area of the visual field.

Al ₂ O ₃ inclusions present in the same manner as sulfide inclusions are not particularly limited, but the distribution density of Al ₂ O ₃ inclusions having an equivalent circle average diameter of 30 μm or less and an equivalent circle average diameter of 1 μm or more. Is preferably 30 or less per square mm.

Since Al ₂ O ₃ inclusions are very hard and adversely affect the blade, coarse Al ₂ O ₃ inclusions cause chipping of the blade. Therefore, it is preferable that the equivalent circle average diameter is 30 μm or less. .
Further, Al ₂ O ₃ inclusions having a size of 1 μm or more have a great influence on the chipping of the blade edge and the wear of the blade. For this reason, the distribution density of Al ₂ O ₃ inclusions of 1 μm or more is set to 30 or less per square mm.

The circle equivalent average diameter is an average value of the circle equivalent diameters of the Al ₂ O ₃ inclusions.
Al ₂ O ₃ inclusions are produced in the steelmaking process. That is, it occurs when oxygen and Al in steel are combined in molten steel. (It is often called Al deoxidation or Al killing that Al is added to molten steel and oxygen and Al in the steel are combined.) Furthermore, the solubility of the liquid state and the solid state is also observed during solidification of the steel. Al ₂ O ₃ is generated due to the difference.

Coarse Al ₂ O ₃ inclusions are caused by agglomeration of Al ₂ O ₃ produced in molten steel. In order to prevent the formation of Al ₂ O ₃ inclusions, the oxygen in the steel is reduced as much as possible by vacuum degassing, and an oxidizing element such as Mn and Si is added first, and finally Al is added. After the addition of Al, continuous casting may be performed immediately and solidified rapidly. In this way, even when Al added to stably realize continuous casting is added to the steel, the generation of coarse Al ₂ O ₃ inclusions as described above can be prevented, and Al as described above can be prevented. _There is no need to provide restrictions on ₂ O ₃ inclusions.

  In this way, as a secondary effect of rapid cooling associated with continuous casting, it is possible to achieve fine, precise and appropriate dispersion of sulfide inclusions, and horizontal milling workability than steel produced by the ingot-making method Can be obtained.

  In addition, the steel for plastic molds having such inclusions can be obtained by using a normal casting method and quenching after casting, but can be easily formed by using a continuous casting method. Can do.

Here, typical steps for producing the steel of the present invention are as follows.
Blast furnace hot metal → Combined converter blowing → Vacuum degassing (RH) → Continuous casting (quick cooling immediately under the mold, electromagnetic stirring, under freezing point pressure) → Slab measurement → Heating furnace (1100 ° C heating) → Thick plate rolling (900 ° C finishing) → (Normalized at 880 ° C)

  Tables 1 and 2 show the chemical compositions of the test materials. Here, Table 1 shows the chemical composition of the test material having a steel composition that satisfies the composition of the steel of the present invention. The chemical composition of the test material which has the steel composition which does not have the composition of this invention steel except G is shown.

  Steels having the respective compositions shown in Table 1 and Table 2 are steel Nos. Except for F and G, after hot metal, the above-described representative manufacturing steps including continuous casting were performed to obtain a hot-rolled steel sheet (plate thickness 70 mm), which was normalized at 880 ° C. to obtain each specimen. On the other hand, Steel No. Steel having the composition shown in F and G is not continuously cast, a cylindrical ingot is produced by a normal casting method, hot rolled to produce a steel plate having a thickness of 70 mm, A material was used. About these test materials, 10 places were observed at random with the optical microscope, and the number of inclusions etc. were measured and evaluated.

Table 3 shows the average length and distribution density of sulfide inclusions in steel having the composition shown in Table 1, and the circle-equivalent average diameter and distribution density of Al ₂ O ₃ inclusions. For these steels, all satisfy the average length and distribution density of sulfide inclusions defined in the steel of the present invention, and the equivalent circle average diameter of Al ₂ O ₃ inclusions is equivalent to the circle equivalent average diameter. Was 30 μm or less and 30 or less per square mm.

In addition, the test steel was milled using a carbide tool, the flank wear (VB (mm)) of the tool after machining was observed with a microscope, and the width of the wear mark was obtained, thereby obtaining machinability. Evaluated. This is also shown in Table 3. As shown in Table 3, in the steel of the present invention, VB is 0.08 mm or less for any of the test steels due to the fine dispersion of sulfide inclusions and the lubricating action by SiO ₂ , and the machinability is good. is there.

Table 4 shows the average length and distribution density of sulfide inclusions in steel having the composition shown in Table 2, and the circle-equivalent average diameter and distribution density of Al ₂ O ₃ inclusions.

  Steel No. A and steel no. Since B does not contain Al or has a low Al content, bubbles that were thought to be caused by the generation of CO gas during continuous casting were discovered during cutting. In these steels, elements that improve machinability satisfy the scope of the present invention, so VB is low, but in steel for plastic molds, it becomes ground that is transferred to plastic molded products, It cannot be used as steel for plastic molding dies.

Steel No. C has a low Si content and does not provide a lubricating action of the tool during cutting. Furthermore, since the S content is low and the distribution density of sulfide inclusions is low and VB is high, the machinability is poor.
Steel No. D is steel No. Like C, the Si content and S content are low, and the lubrication effect of the tool cannot be obtained. Further, since Mn is high, the hardness becomes too high and VB becomes high, so that the machinability is poor. Furthermore, since Al is low, there is a problem in soundness.

  Steel No. E contains an appropriate amount of S, and the sulfide inclusions satisfy the distribution density of sulfide inclusions effective for machinability, but the V content increases because the Si content is low. The machinability is bad.

  Steel No. Since F does not use a continuous casting method, there is no problem in the soundness of steel even when Al is not added. However, due to the difference in cooling rate during casting, the size of the sulfide inclusions became relatively large and the distribution density also became low. Therefore, VB becomes high and machinability is bad.

  Steel No. G satisfies the steel composition specified in the present invention, but does not use the continuous casting method, the size of the sulfide inclusions is relatively large, and the distribution density is also low. Therefore, VB becomes high and machinability is bad.

Claims (6)

% By mass
C: 0.30 to 0.55%,
Si: 1.1 to 2.5%
Mn: 0.10 to 1.00%,
Cr: 0.02-1.0%,
V: 0.003-0.50%,
P: 0.025% or less,
S: 0.005-0.030%,
Al: 0.007 to 0.06%,
N: 0.006% or less,
O (oxygen): 0.0001 to 0.005% contained,
The balance is made of Fe and impurities, the sulfide inclusions have an average length of 20 to 150 μm, and the distribution density is one or more per square mm.   Furthermore, B: 0.01% or less is contained in the mass%, Plastic molding metal steel of Claim 1 characterized by the above-mentioned.   The steel for plastic molds according to claim 1 or 2, further comprising, by mass%, Zr: 0.02% or less.   The steel for plastic molds according to any one of claims 1 to 3, further comprising Mg: 0.02% or less and / or Ca: 0.02% or less in mass%. .   The plastic molding die according to any one of claims 1 to 4, further comprising, by mass%, Cu: 1.0% or less, Ni: 2.0% or less, Mo: 1.0% or less. Steel.   The steel for plastic molds according to any one of claims 1 to 5, which is produced by continuous casting.