JP6647771B2 - Mold steel and mold - Google Patents

Description

  The present invention relates to a mold steel and a mold excellent in both high-temperature strength and heat conduction performance.

High-temperature strength is required for injection molding dies, die-casting dies, hot press (also called hot stamping and die quench) dies such as resin and rubber. In these molds (including parts constituting a part of the mold), high-temperature strength is a necessary and important characteristic in suppressing the wear of the mold when shots are repeatedly performed.
Conventionally, JIS SKD61, SUS420J2, maraging steel, etc., which are representative steel types of tool steel for hot dies, have been widely used as materials for dies requiring high-temperature strength.

  In these molds, it is common practice to cool the mold by providing a cooling circuit inside and circulating cooling water therethrough.However, the cooling efficiency of cooling water at that time is good or bad. This is important because it directly affects product productivity.

  In other words, if the cooling efficiency is low, the product molding cycle time will be longer and the product productivity will be lower.On the other hand, if the cooling efficiency is higher, the product molding cycle time can be shortened and the productivity can be increased by increasing the cycle. And product cost can be reduced.

A simple way to increase the cooling efficiency is to bring the cooling circuit closer to the mold surface (design surface) of the mold.
However, in such a case, the mold becomes smaller due to a shorter distance between the cooling circuit and the molding surface, and a larger thermal stress is generated. Cracks) are likely to occur, which causes the mold life to be shortened.
Therefore, even if the cooling circuit is brought closer to the molding surface, there is naturally a limit there.

Another way to increase the cooling efficiency is to increase the thermal conductivity of the mold, i.e. to use mold steel with high thermal conductivity.
If the heat conductivity of the mold is high, the temperature decrease of the mold can be accelerated even under the same cooling circuit and cooling conditions, and the cooling efficiency can be increased.
The high thermal conductivity is also necessary for suppressing fatigue cracks (so-called heat check) by reducing the thermal stress in the mold and reducing seizure by rapidly lowering the temperature.

  However, mold steels such as JIS SKD61, SUS420J2, and maraging steel that are conventionally used have high temperature strength, but contain a large amount of elements such as Si, Cr, Ni, and Co that are easily dissolved in the matrix. Therefore, the heat conductivity is low, and it is difficult to increase the cooling efficiency by the heat conductivity.

As another method for increasing the cooling efficiency, it is conceivable that the cooling circuit is formed in a mold in an infinitely complex manner inside and outside the mold, and the cooling capacity is increased by the overall shape and layout of the cooling circuit.
However, the conventional mold manufacturing method is generally to ingot steel by making ingots, and then forging or rolling to make blocks or rectangular bars, which are then machined to form the shape of the mold, and then A mold is manufactured by performing heat treatment such as quenching and tempering. Under such a manufacturing method, it is not possible to process and form a cooling circuit having an intricately meandering shape inside and outside the mold. In reality it is difficult.

Under these circumstances, in recent years, a technique of forming a mold by a layered manufacturing method (three-dimensional layered manufacturing method) has attracted attention.
The additive manufacturing method is a processing method in which three-dimensional model data is materialized by attaching materials. In the additive manufacturing method, first, a shape represented by three-dimensional CAD data is converted into a plurality of surfaces orthogonal to a predetermined axis. Calculates the cross-sectional shape of a slice generated by slicing, slices are actually produced, stacked and laminated, and the shape expressed by a computer is materialized.

In the additive manufacturing method, there are a case where a powder is used as a material and a case where a plate is used.
In the method using powder, the powder is spread in layers (the thickness of one layer is, for example, several tens of μm), and a certain area is irradiated with heat energy, for example, a laser beam or an electron beam, so that the powder layer is melt-solidified or sintered. Then, the whole shape is formed by further stacking these.

On the other hand, in additive manufacturing using a plate as a material, individual parts (plates) generated by slicing three-dimensional shape data in CAD are actually manufactured by machining, etc., and the parts are stacked and diffusion bonded etc. Forms the entire three-dimensional shape.
For example, Patent Documents 1 and 2 disclose an example of manufacturing a mold by this kind of additive manufacturing method.

  Specifically, Patent Document 1 below discloses an invention relating to “metal powder for powder sintering and lamination, a method for producing a three-dimensionally shaped object using the same, and a three-dimensionally shaped object to be obtained”. Irradiating a light beam to the powder material of the mold metal component to sinter or melt-solidify the powder at a predetermined location to form a solidified layer, and further form a solidified layer on the solidified layer obtained by this; Is repeated to produce a three-dimensionally shaped object.

  Patent Literature 2 below discloses an invention relating to “a mold nest, a method of manufacturing a mold nest, and a resin molding mold”, in which a nest having a spiral cooling path therein is manufactured. On the basis of the slice data, grooves for forming cooling paths are formed in the plurality of metal plates, the grooved metal plates are laminated in a predetermined order, and diffusion-bonded, and the obtained metal block is formed. The point of shaping is disclosed.

  The above-mentioned additive manufacturing method is to form the entire shape by stacking the materials, and it is possible to easily process and form even a complicated cooling circuit winding endlessly vertically and horizontally which can not be achieved by cutting work, Even if the cooling circuit is not intentionally brought closer to the molding surface of the mold than necessary, the cooling efficiency can be more effectively improved than that of the mold produced by cutting by conventional machining.

In a mold made by additive manufacturing, the cooling circuit can be freely made into a complicated shape.Thus, even if the mold material is made of maraging steel or precipitation hardening stainless steel, it can be made by additive manufacturing. Although the cooling circuit can be made into a complicated shape, the cooling effect can be increased by the shape effect of the cooling circuit, but it is difficult to increase the cooling efficiency to a sufficient level due to the low thermal conductivity of the material itself. .
Also, naturally, when a mold is manufactured by a conventional general manufacturing method without depending on additive manufacturing, the efficiency of cooling (heat exchange) becomes further insufficient.

On the other hand, as steel having high heat conduction performance (high heat conductivity), there are carbon steel and steel for machine structural use. These steels have a low content of elements such as Si, Cr, Ni, and Co which are easily dissolved in the matrix, and exhibit high thermal conductivity because they are low alloy steels.
However, these steels have a problem that their high-temperature strength is low and their life when formed into a mold is short.
That is, regardless of whether or not a mold is formed by the additive manufacturing method, a mold steel that can realize sufficient high-temperature strength and heat conduction performance when formed into a mold has been conventionally provided. Did not.

  As another prior art to the present invention, Patent Document 3 listed below discloses an invention relating to “hot tool steel”, in which C: 0.28 to 0.55%, Si: 0.15 to 0.80%, Mn: 0.40 to 0.85%, P: 0.020% or less, S: 0.018% or less, Cr: 2.5 to 5.7%, Mo: 1.4 to 2.8%, V: 0.20 to 0.90%, W: 0.01 to 1.65%, Co: 0.03 to 0.89%, Ni: 0.01 to 1.65%, the balance substantially consisting of Fe and unavoidable impurities. N of the unavoidable impurities is restricted to 0.009% or less, Ti is regulated to 0.003% or less, and B is regulated to 0.012% or less. A hot work tool steel is disclosed in which the cleanliness of inclusions is JIS dA 0.005% or less, d (B + C) 0.020% or less, and the directionality of the martensite structure after heat treatment is in the range of 17 to 33%. ing.

  As still another prior art, Patent Literature 4 listed below discloses an invention relating to “a method for manufacturing steel for crushing knife and crushing blade”, wherein C: 0.3 to 0.5%, Si: 0.2 to 0.5%, Mn. : 0.1 to 1.0%, Cr: 4.0 to 6.0%, one or two of Mo and W are Mo + 1 / 2W: 0.8 to 2.5%, and one or two of V and Nb are V + 1 / 2Nb: Disclosed is a steel for a crushing knife, which contains 0.3 to 1.0% as a basic component, and the balance is Fe and inevitable impurities.

  As still another prior art, Patent Literature 5 listed below discloses an invention relating to “a hot forging die and a method for manufacturing the same”, wherein C: 0.32 to 0.42%, Si: 0.3% or less, and Mn: 0.3 to 0.3%. 1.5%, Ni: 0.5% or less, Cr: 4.0-6.0%, V: 0.2-1.0%, Mo + 1 / 2W: 0.8-2.0%, and N: 0.005-0.04%, the balance being Fe and inevitable A hot forging die made of impurities is disclosed.

  As still another prior art, Patent Document 6 listed below discloses an invention relating to a “die for hot working”, wherein C: 0.30% or more and less than 0.50%, Si: 0.10-0.5%, Mn: 0.30%. 1.0%, P: 0.02% or less, S: 0.005% or less, Cr: 4.0 to 8.0%, Mo: 0.2% to less than 1.5%, V: 0.05 to 1.0%, Al: 0.03% or less, N: 0.0150% or less And O: 0.0030% or less, the balance being Fe and impurities, wherein Ni and W as impurities each have a chemical composition of less than 0.7% and a tensile strength of 900 MPa or more. For hot working, a nitrided layer having a hardening depth exceeding 200 μm is provided on at least a surface in contact with the workpiece, and a hardness at a position where the depth of the nitrided layer is 30 μm or more is Vickers hardness of 900 or less. A mold is disclosed.

  As still another prior art, Patent Document 7 listed below discloses an invention of "steel for hot forging die", wherein C: 0.25 to 0.45%, Si: 0.50% or less, Mn: 0.20 to 1.0%. , P: 0.015% or less, S: 0.005% or less, Ni: 0.5 to 2.0%, Cr: 2.8 to 4.2%, Mo: 1.0 to 2.0%, V: 0.1 to 0.5%, the balance being Fe and inevitable A hot forging die steel comprising impurities is disclosed.

As still another prior art, Patent Document 8 listed below discloses an invention relating to “hot tool steel”, wherein C: 0.25 to 0.40%, Si: 0.50% or less, Mn: 0.30 to 1.00%, P: 0.015% or less, S: 0.005% or less, Ni: 0.50 to 2.00%, Cr: 2.70 to 5.50%, Mo: 1.00 to 2.00%, V: 0.40 to 0.80%, B: 0.0005 to 0.0100%, Al: 0.015 to 0.10 %, N: 0.015% or less, the balance being an alloy steel comprising Fe and unavoidable impurities, a fracture toughness (KQ) at room temperature of 250 kgf / mm ^3/2 or more, and a high temperature (600 ° C.) A hot work tool steel having a proof stress (0.2% PS) of 60 kgf / mm ² or more is disclosed.

  However, these patent documents 3 to 8 are different from the present invention in that no examples of chemical compositions satisfying the claims of the present invention are disclosed, and further, no mention is made of the thermal conductivity.

International Publication WO2011 / 149101 JP 2010-194720 A JP 2003-268486 A JP 2007-297691 A JP 2008-308745 A JP 2010-65280 A JP-A-6-256897 JP-A-8-269625

The present invention is the background of the above circumstances, when applying the layered manufacturing method for manufacturing a mold, the high temperature strength of JIS SKD61 equivalent of about feasible high thermal conductivity while maintaining the mold steel And a mold.

Claim 1 relates to a mold steel, which is expressed by mass% of 0.15 <C < ≦ 0.30 , 0.20 <Si <0.52, 5.32 <Cr <5.72,
-0.05814 × [Cr] +0.4326 <Mn <-0.2907 × [Cr] + 2.4628..Formula (1)
(However, [Cr] in the formula (1) represents the content% of Cr)
0.72 <Mo <1.60,0.20 <V ≦ 0.58, made from a powder of the steel balance to have a composition of Fe and inevitable impurities, characterized in that it is used as a material for shaping a mold by laminate shaping method .

  Claim 2 is characterized in that in claim 1, 0.10 <Al <1.20 is further contained in mass%.

Those of claim 3, in any one of claims 1, characterized in that it further contains 0 .30 <Cu ≦ 1. 5 mass%.

  According to a fourth aspect of the present invention, in any one of the first to third aspects, the composition further contains 0.0001 <B ≦ 0.0050 by mass%.

  Claim 5 relates to any one of claims 1 to 4, wherein 0.003 <S ≦ 0.250, 0.0005 <Ca ≦ 0.2000, 0.03 <Se ≦ 0.50, 0.005 <Te ≦ 0.100, 0.01 <Bi ≦ 0.50, It is characterized by further containing at least one of 0.03 <Pb ≦ 0.50.

  Claim 6 further includes at least one of 0.004 <Nb ≦ 0.100, 0.004 <Ta ≦ 0.100, 0.004 <Ti ≦ 0.100, 0.004 <Zr ≦ 0.100 by mass% in any one of claims 1 to 5. It is characterized by doing.

  A seventh aspect of the present invention is characterized in that, in any one of the first to sixth aspects, at least one of 0.10 <W ≦ 4.00 and 0.10 <Co ≦ 3.00 by mass% is further contained.

According to the eighth aspect of the present invention, a test piece for measuring thermal conductivity made from a steel block having the above-described composition manufactured by smelting according to any one of the first to seventh aspects is evaluated by a laser flash method. The thermal conductivity at 2 ° C. is 24.0 W / m / K or more.

Claim 9 relates to a mold (including parts constituting a part of the mold), and is manufactured by an additive manufacturing method using a steel powder according to any one of claims 1 to 8. It is characterized by the following.

  The present invention reduces the thermal conductivity of high alloy steels such as maraging steel and stainless steel under the circumstances where mold steel having both high temperature strength and high thermal conductivity has not been provided. While reducing the content of alloying components to be reduced, the content of alloying elements that increase high-temperature strength relative to steel for machine structural use is increased, and by appropriately balancing these alloying components, the high-temperature strength required for wear control is reduced to JIS. High thermal conductivity can be realized while maintaining the same level as SKD61.

The invention applies to powder materials for use in manufacturing molds in molding by layered manufacturing method. Specifically, in the present invention the die steel leave without a flour powder, it is possible to manufacture a mold in layered manufacturing method using these materials.
In the additive manufacturing method, particularly the additive manufacturing method using powder, when heat energy is applied to a layer in which the powder is laid to solidify the powder, the powder is melt-solidified or sintered.
At that time, the powder is rapidly cooled from a high temperature state such as a molten state, and quenching is automatically performed. The quenching takes place rapidly under a high cooling rate. That is, quenching is performed sequentially and simultaneously in the process of laminating and molding the powder.

Since quenching is performed at a high cooling rate as described above, quenching can be performed well during additive manufacturing, even if the content of the hardenability improving component as a steel component is kept low in advance.
The steel for molds of the present invention is used as a material for additive manufacturing.However, as a reference example, as a material for forming a mold shape by cutting a lump of steel by machining to manufacture a mold. Can also be used. At this time, heat treatment conditions such as quenching may be determined according to the contained elements.

Next, the reasons for limiting each chemical component in the present invention will be described below.
In addition, the value of each chemical component is mass%.
1) <Chemical component of claim 1>
0.15 <C ≦ 0.30
When 0.15 <C, when the mold manufactured by molding from the ingot by cutting is heat-treated, a hardness of 30 to 57 HRC required for the mold can be obtained. In addition, 30 to 57 HRC can be obtained in a mold as manufactured by additive manufacturing. Furthermore, when the mold after the lamination molding is heat-treated, 30 to 57 HRC is obtained. In any of these production methods, hardness is insufficient when C ≦ 0.15. On the other hand , excessive addition lowers the thermal conductivity.

0.20 <Si <0.52
When Si ≦ 0.20, the machinability deteriorates remarkably. When Si ≦ 0.20, it is difficult to secure hardness (ie, strength). On the other hand, when 0.52 ≦ Si, the thermal conductivity significantly decreases. A particularly preferred range is 0.28 <Si <0.52.

5.32 <Cr <5.72
When Cr ≦ 5.32, the corrosion resistance is insufficient. Further, when Cr ≦ 5.32, the hardenability when quenching a mold manufactured by molding from the ingot material or by quenching a mold manufactured by additive molding is insufficient. On the other hand, when 5.72 ≦ Cr, the thermal conductivity decreases.

-0.05814 × [Cr] +0.4326 <Mn <-0.2907 × [Cr] + 2.4628..Formula (1)
When Mn ≦ −0.05814 × [Cr] +0.4326, hardenability is insufficient. Lack of hardenability is remarkable especially when Cr is low. When −0.2907 × [Cr] + 2.4628 ≦ Mn, the thermal conductivity decreases. The decrease in thermal conductivity is remarkable especially when Cr is high. As for the lower limit of the amount of Mn, a particularly preferred range is 0.19 <Mn.

The reason for defining the Mn content as a function of the Cr content, that is, in relation to the Cr content, as shown in the equation (1), is as follows.
The thermal conductivity of a steel containing 0.32C-0.50Si-1.25Mo-0.58V as a basic component when Cr and Mn were changed was investigated. These steel blocks were manufactured by smelting, and φ11 mm × 100 mm cylinders cut out of the steel blocks were quenched at a cooling rate of 1030 ° C. to 20 ° C./min, and adjusted to 43 HRC by tempering at 550 to 620 ° C. Test pieces of φ10 mm × 2 mm for measuring thermal conductivity were prepared from the tempered cylinder. The thermal conductivity was measured at 25 ° C. by a laser flash method. The thermal conductivity was evaluated as x when the thermal conductivity was less than 24.0 W / m / K, and as ○ when the thermal conductivity was 24.0 W / m / K or more.

  FIG. 1A shows the effect of Cr and Mn on the thermal conductivity. It can be seen that the thermal conductivity becomes lower and becomes “x” as both Cr and Mn increase. Conversely, the smaller the amount of both Cr and Mn, the higher the thermal conductivity becomes, and the result becomes “○”. The straight line estimated as the boundary between x and ○ is Mn = −0.2907 × [Cr] +2.4628, and ○ is stably obtained in a region below this straight line. Therefore, the upper limit of Mn is defined as Mn <−0.2907 × [Cr] +2.4628. As described above, the range of Cr is 5.32 <Cr <5.72.

The impact value of a steel containing 0.32C-0.50Si-1.25Mo-0.58V as a basic component when Cr and Mn were changed was investigated. These steel blocks were manufactured by smelting, and 11 mm × 11 mm × 60 mm square bars cut out of the steel blocks were cooled from 1030 ° C. to 550 ° C. at 20 ° C./min, and then cooled to room temperature at 3 ° C./min. Hardened. This quenching pattern simulates the cooling history inside the mold when quenching a large mold created by machining from a block of ingot material. The square bar was adjusted to 45 HRC by tempering at 550-610 ° C. An impact test piece of 10 mm × 10 mm × 55 mm was prepared from the tempered square bar. It is a JIS No. 3 test piece with a notch radius of 1 mm.
Impact values were measured at 25 ° C. The impact value was evaluated less than 20 J / cm ² ×, as ○, a 20 J / cm ² or more.

  FIG. 1B shows the effect of Cr and Mn on the impact value. It can be seen that the more Cr and Mn are both, the higher the impact value is and becomes “○”. Conversely, the smaller the Cr and the Mn are, the lower the impact value is, and the result is “X”. The straight line estimated to be the boundary between ○ and × is Mn = −0.05814 × [Cr] +0.4326, and ○ is stably obtained in a region above this straight line. Therefore, the lower limit of Mn is defined as Mn> −0.05814 × [Cr] +0.4326. As described above, the range of Cr is 5.32 <Cr <5.72.

0.72 <Mo <1.60
If Mo ≦ 0.72, it is difficult to secure hardness by secondary hardening at the time of tempering, and the high-temperature strength becomes insufficient. On the other hand, when 1.60 ≦ Mo, the decrease in fracture toughness value is large. A preferred range is 0.72 <Mo <1.51. A more preferred range is 1.10 <Mo <1.51.

0.20 <V ≦ 0.58
When V ≦ 0.20, coarsening of austenite crystal grains in the case of quenching becomes a problem. Further, when V ≦ 0.20, it is difficult to secure hardness by secondary hardening at the time of tempering, and the high-temperature strength becomes insufficient.
On the other hand , excessive addition tends to saturate the above-mentioned effects and causes an increase in cost. Further, when prepared in the usual method (dissolution → refining → casting → hot working), coarse VC crystallizing is increased during solidification, a possibility increases that become starting points of fracture in the case of a mold . A particularly preferred range is 0.32 <V ≦ 0.58 .

In the steel of the present invention, the following components can be usually contained as inevitable impurities in the following amounts.
N ≦ 0.05
P ≦ 0.05
S ≦ 0.003
Cu ≦ 0.30
Ni ≦ 0.30
Al ≦ 0.10
W ≦ 0.10
O ≦ 0.01
Co ≦ 0.10
Nb ≦ 0.004
Ta ≦ 0.004
Ti ≦ 0.004
Zr ≦ 0.004
B ≦ 0.0001
Ca ≦ 0.0005
Se ≦ 0.03
Te ≦ 0.005
Bi ≦ 0.01
Pb ≦ 0.03
Mg ≦ 0.02

2) <Chemical component of claim 2>
The steel of the present invention may undergo quenching after additive manufacturing. To suppress coarsening of austenite grains during quenching
0.10 <Al <1.20
Can be contained.
Al combines with N to form AlN, and has an effect of suppressing the movement of austenite crystal grain boundaries (that is, grain growth).
In addition, since Al forms nitrides in steel and contributes to precipitation strengthening, it also has an effect of increasing the surface hardness of the nitrided steel material. It is effective to use a steel material containing Al for a mold (including parts constituting a part of the mold) which is subjected to nitriding treatment for higher wear resistance.

3) <Chemical component of claim 3>
In recent years, the size of a mold tends to increase due to enlargement and integration of mold parts. Large molds are difficult to cool. For this reason, when quenching a large mold of a steel material having low quenchability, ferrite, pearlite, and coarse bainite precipitate during quenching, and various characteristics deteriorate. Such concerns may be dealt with by selectively adding Cu to enhance the hardenability. In particular,
0 .30 <Cu ≦ 1.5
The may be contained.
The C u, has the effect of increasing the hardness in aging precipitation. The preferred range is
0 .50 ≦ Cu ≦ 1.2
It is. However , when the amount exceeds a predetermined amount, segregation becomes remarkable, and the mirror polishing property is reduced.

4) <Chemical component of claim 4>
As a measure for improving the hardenability, the addition of B is also effective. Specifically, if necessary
0.0001 <B ≦ 0.0050
Is contained.
Since B has no effect of improving hardenability when BN is formed, B alone needs to be present in steel. Specifically, it is only necessary to form a nitride with an element having a higher affinity for N than B and not to combine B and N. Examples of such elements include Nb, Ta, Ti, Zr, and the like. Although these elements have the effect of fixing N even if they are present at the impurity level, depending on the amount of N, it may be preferable to add them within the range of claim 6 described later.

5) <Chemical component of claim 5>
Since the steel of the present invention has a small amount of Si, the machinability is slightly poor. As a measure for improving workability, the following S, Ca, Se, Te, Bi, and Pb may be selectively added. In particular,
0.003 <S ≦ 0.250
0.0005 <Ca ≦ 0.2000
0.03 <Se ≦ 0.50
0.005 <Te ≦ 0.100
0.01 <Bi ≦ 0.50
0.03 <Pb ≦ 0.50
May be contained.
If any of the elements exceeds a predetermined amount, it leads to saturation of machinability, deterioration of hot workability, impact value and mirror polishing property.

6) <Chemical component of claim 6>
If the quenching heating temperature is increased or the quenching heating time is prolonged due to unexpected equipment trouble or the like, deterioration of various characteristics due to coarsening of crystal grains is concerned. In preparation for such a case, Nb, Ta, Ti, and Zr are selectively added, and coarsening of austenite crystal grains can be suppressed by fine precipitates formed by these elements. In particular,
0.004 <Nb ≦ 0.100
0.004 <Ta ≦ 0.100
0.004 <Ti ≦ 0.100
0.004 <Zr ≦ 0.100
May be contained.
When any of the elements exceeds a predetermined amount, carbides, nitrides, and oxides are excessively generated, and the impact value and the mirror polishing property are reduced.

7) <Chemical component of claim 7>
An increase in C is effective for increasing the strength. However, an excessive increase in C causes deterioration of characteristics (impact value and mechanical fatigue characteristics) due to an increase in carbide. In order to increase the strength without causing such a problem, W or Co may be selectively added.
W increases the strength by fine precipitation of carbides. Co increases the strength by solid solution in the base material, and also contributes to precipitation hardening through change in carbide morphology. In particular,
0.10 <W ≦ 4.00
0.10 <Co ≦ 3.00
May be contained.
When any of the elements exceeds a predetermined amount, the characteristics are saturated and the cost is significantly increased. The preferred range is
0.30 ≦ W ≦ 3.00
0.30 ≦ Co ≦ 2.00
It is.

  According to the present invention as described above, it is possible to provide a mold steel and a mold having high thermal conductivity while maintaining high high-temperature strength required for suppressing wear equivalent to JIS SKD61.

(A) is a diagram showing the effect of Cr and Mn on thermal conductivity. (B) is a diagram showing the effect of Cr and Mn on the impact value. It is sectional drawing of the die-casting die which has a spool core of one Embodiment of this invention. It is a figure showing a worn state of a spool core.

Next, embodiments of the present invention will be described in detail below.
Powders of 34 steels having the chemical compositions shown in Table 1 were produced by a gas atomization method, and a three-dimensional additive manufacturing method using a laser irradiation was performed using the powders to form a partial mold that was part of the die casting mold 10 shown in FIG. Was manufactured. The spool core 12 has a cooling circuit 14 formed therein. Here, the cooling circuit 14 has a spiral three-dimensionally complicated shape.

In Table 1, Comparative Example 1 is hot die steel SKD61, Comparative Example 2 is 18Ni maraging steel, Comparative Example 3 is martensitic stainless steel SUS420J2, and Comparative Example 4 is mechanical structure steel SCM435.
In addition, although each of the invention examples in Table 1 may contain an unavoidable amount of impurity components, they are not described in the table.

In FIG. 2, the die casting mold 10 has a fixed mold 16 and a movable mold 18. A cavity 20 as a molding space for the product and a runner 22 are provided between them, and they are connected by a narrow gate 24.
The spool core 12 is arranged at a position sandwiching a cylindrical biscuit portion 28 which is a final solidification position of a casting together with the plunger 26. The runner 22 extends from the biscuit portion 28.
A groove is formed in the spool core 12, and a part of the runner 22 is formed in the groove.

The mold obtained by the above procedure was heated (tempered or aged) in the range of 100 to 650 ° C. and tempered to 43 HRC. This heating also serves to remove residual stress. Depending on the chemical components, the desired characteristics can be obtained without tempering or aging. In that case, removal of residual stress is the main purpose of heating at 100 to 650 ° C. Of course, when desired characteristics can be obtained as it is, and there is no problem with residual stress, heating after the additive manufacturing is unnecessary.
Then, it was finished to the final mold shape by machining. The die is a spool core 12 of a 135-ton die casting machine. The position of the spool core 12 in the mold structure is shown in FIG. Here, FIG. 2 is a cross-sectional view of a die-casting mold structure as viewed from the side.

The cycle of die casting is a cycle of mold clamping → injection → die timer → mold opening → product removal → air blow → release agent spray → air blow (FIG. 2 shows the process of the die timer).
First, the movable mold 18 comes into contact with the fixed mold 16 to be in a mold clamped state. At this time, a cavity 20 is formed as a product molding space. In this state, a molten metal of an aluminum alloy (hereinafter, referred to as an aluminum alloy) is poured into the sleeve 30 with a ladle, and the molten metal is injected by the plunger 26 moving at a high speed.
The injected molten metal moves through the runner 22 and flows into the cavity 20 from the gate 24 in a liquid, granular, or mist state. It is easy to understand if you imagine a water gun or spray. Eventually, the cavity 20 is filled with the molten metal. Then, pressure is applied to the molten metal that fills the cavity 20 to wait until it is solidified.

  This is a process called a di-timer, and FIG. 2 shows this state. When the molten metal is solidified into a product, the movable mold 18 is moved to open the mold. The product is removed using an extrusion pin or manipulator. The mold that has been in contact with the high-temperature aluminum alloy has a high temperature, and is cooled by air blowing and spraying a release agent. This is one cycle of die casting.

  In the above process, shortening of the die timer (the process of solidifying the molten metal in the mold) was studied. If the cooling capacity of the spool core 12 is high, the die timer can be shortened because the biscuit portion 28 solidifies quickly, and therefore the overall cycle time can be shortened. Reducing the cycle time is very preferable from the viewpoint of improving productivity.

For the test, a die casting machine with a mold clamping force of 135 tons was used. From the state where the die timer was sufficiently long (the state where the biscuit part 28 was completely solidified), the die timer was shortened by one second, and the biscuit part 28 was opened when the mold was opened. It was judged as pass if solidified and failed if not. Then, the shortest die timer that passed was evaluated.
The shape of the biscuit part 28 is φ50 × 40 mm, and the distance between the water cooling hole of the cooling circuit 14 of the spool core 12 and the surface is 15 mm. The melt is ADC12 at 730 ° C., and the mass of the casting is 660 g. Further, it was also evaluated whether or not remarkable wear was observed on the spool core 12 after the 10,000 shot casting. If the high-temperature strength is insufficient, wear due to the flow of the molten metal becomes remarkable, and the life of the mold cannot be ensured.

Table 2 shows the test results. The goal is that there is no wear with a depth of 0.2 mm or more after 10,000 shots for the die timer and 10 [seconds] or less.
The die timers of Comparative Examples 1 to 3 are as long as 12 to 14 seconds. This is because the heat conductivity is as low as less than 24.0 [W / m / K], and heat exchange is difficult to be performed.
On the other hand, there was no remarkable wear on the spool core 12 after the 10,000 shot casting. This is because they have sufficient high-temperature strength.

  In the case of Comparative Example 4 having a high thermal conductivity of 38.4 [W / m / K], the die timer was as short as 7 [seconds], which is a preferable result. Wear was observed, and it was judged that securing the life of the mold was difficult. This is shown in FIG. In the groove M that forms a part of the runner 22, near the corner k where the direction of the flow of the molten metal changes abruptly, dripping skin is observed due to wear.

  In each of the 30 invention examples, the die timer was as short as 10 seconds or less, and the target was achieved. This is because the thermal conductivity is as high as 24.0 [W / m / K] or more, and heat exchange is easily performed. In addition, since the spool core 12 had sufficient high-temperature strength, there was no significant wear on the spool core 12 after 10,000 shot casting. There were no cracks from the water cooling holes of the cooling circuit 14 in both the comparative example and the invention example.

Next, it was verified whether the die timer could be set to 10 seconds or less even in Comparative Examples 1 to 3. Specifically, in order to promote heat exchange, a spool core 12 in which the distance between the water cooling hole and the surface was reduced to 10 mm was produced, and a test was performed under the same conditions as the test in Table 2. Table 3 shows the results.
The die timer was shortened to the same level as the invention example in Table 2. A mold structure in which water cooling holes are brought close to the surface is extremely effective for shortening the die timer.

  On the other hand, before the 10,000-shot casting was completed, cracks from the water-cooled holes penetrated the surface, and the life was extended. This is because the thermal stress increased in addition to the shortening of the crack penetration distance. Even if the die timer is shortened, it is difficult to improve the productivity of die casting (since it takes a long time to change the mold). Although 10,000 shots were not achieved, the wear was not remarkable as in the case of the test in Table 2.

  As can be seen from the above, in the invention example, the die timer can be shortened while preventing the abrasion and the water cooling hole cracks and ensuring the mold life. In the comparative example, if the die life is ensured, the die timer becomes longer, and if the die timer is shortened, the die life cannot be ensured. The reason why the invention example can achieve both the securing of the mold life and the shortening of the die timer is that both the high-temperature strength and the thermal conductivity are high.

  Further, the high thermal conductivity, which is a feature of the invention, is also effective in securing the mold life by suppressing the heat check. In the die casting, a heat cycle of heating by the molten metal and cooling by spraying the release agent is given to the mold surface. However, the thermal stress generated by the heating and cooling becomes low in the invention example having high thermal conductivity. As a result of reducing the thermal fatigue due to the thermal stress, in the example of the invention, the heat check (crack on the mold surface) hardly occurs and the mold life is secured.

  In a series of casting tests, when the surface of the mold after 10,000 shots was observed, a minute heat check had already occurred in Comparative Examples 1, 2 and 3 having low thermal conductivity. If the casting of several thousand shots is continued, the heat check may become apparent and the mold may reach the end of its life. In contrast, in the invention example, no heat check was observed even after 10,000 shots, and a smooth mold surface was maintained. It is expected that the mold life can be ensured even if the casting of tens of thousands of shots is continued.

The embodiment of the present invention has been described above in detail, but this is merely an example.
The steel of the present invention, which has both high thermal conductivity and high-temperature strength, is suitable not only for a die-casting die but also for a resin injection-molding die. It also exhibits high performance as a mold for hot pressing (also called hot stamping or die quench) of steel sheets. At this time, even if the steel of the present invention is applied to mold manufacturing by ordinary machining and heat treatment instead of additive manufacturing, the mold life is ensured and cycle is improved compared to the conventional steel mold of the same shape made by the same manufacturing method. It is effective for shortening.
Further, it is also effective to combine the metal mold of the present invention with surface modification (shot blast, sand blast, nitriding, PVD, CVD, plating, etc.).
Further, the steel of the present invention can be used as a welding material in a state of a rod or a wire. Specifically, using the welding material for mold steel according to the present invention, a mold manufactured by additive manufacturing, or a mold manufactured by machining a material obtained by processing an ingot by machining. Alternatively, welding repair can be performed. In this case, the chemical composition of the mold to be repaired may be different from the range of the steel of the present invention, or may be within the range of the steel of the present invention. In any case, the part repaired by the welding material of the steel of the present invention has high high-temperature strength and high thermal conductivity exhibited by the components of the steel of the present invention.
In addition, the present invention can be implemented in a form in which various changes are made without departing from the spirit thereof.

10 Die casting mold 12 Spool core 14 Cooling circuit

Claims (9)

By mass%
0.15 <C ≦ 0.30
0.20 <Si <0.52
5.32 <Cr <5.72
-0.05814 × [Cr] +0.4326 <Mn <-0.2907 × [Cr] + 2.4628..Formula (1)
(However, [Cr] in the formula (1) represents the content% of Cr)
0.72 <Mo <1.60
0.20 <V ≦ 0.58
The balance consists of steel powder with the composition of Fe and unavoidable impurities,
Mold steel, which is used as a material for forming a mold by an additive manufacturing method. By mass%
0.10 <Al <1.20
The steel for molds according to claim 1, further comprising: By mass%
0.30 <Cu ≦ 1.5
The mold steel according to any one of claims 1 and 2, further comprising: By mass%
0.0001 <B ≦ 0.0050
The mold steel according to any one of claims 1 to 3, further comprising: By mass%
0.003 <S ≦ 0.250
0.0005 <Ca ≦ 0.2000
0.03 <Se ≦ 0.50
0.005 <Te ≦ 0.100
0.01 <Bi ≦ 0.50
0.03 <Pb ≦ 0.50
The mold steel according to any one of claims 1 to 4, further comprising at least one of the following. By mass%
0.004 <Nb ≦ 0.100
0.004 <Ta ≦ 0.100
0.004 <Ti ≦ 0.100
0.004 <Zr ≦ 0.100
The mold steel according to any one of claims 1 to 5, further comprising at least one of the following. By mass%
0.10 <W ≦ 4.00
0.10 <Co ≦ 3.00
The mold steel according to any one of claims 1 to 6, further comprising at least one of the following. The thermal conductivity at 25 ° C. evaluated by the laser flash method is 24.0 W / m / K or higher for a test piece for measuring thermal conductivity made from a steel block having the above composition manufactured by melting. The mold steel according to any one of claims 1 to 7, characterized in that: A mold manufactured by an additive manufacturing method using the steel powder according to claim 1.