JP6601051B2 - Steel powder - Google Patents

Description

This invention relates to the end flour steel excellent in both thermal conduction performance and corrosion resistance.

In general, injection molds such as resin and rubber, die-casting dies, and hot press (also called hot stamping or die quenching) dies are made by melting steel to make ingots, then forging and rolling to form blocks or flats. It is manufactured by making a square bar and machining it into a mold shape, followed by heat treatment such as quenching and tempering.
In these molds, it is generally performed to cool the mold by providing a cooling circuit therein and circulating cooling water therethrough.

In such a mold, increasing the cooling efficiency with cooling water leads to a reduction in cycle time, that is, an improvement in productivity due to a high cycle of product manufacturing (molding).
As a method of increasing the cooling efficiency, it is conceivable that the cooling circuit has a complicated and winding shape inside the mold, and the cooling capacity is increased by the overall shape and layout of the cooling circuit. In a method of cutting and manufacturing by machining, it is technically impossible to form the cooling circuit in such a complicated shape.

Under such circumstances, in recent years, a technique for modeling a mold by an additive manufacturing method (three-dimensional additive manufacturing method) has attracted attention.
The additive manufacturing method is a processing method that materializes the three-dimensional model data by adhering materials. In the additive manufacturing method, first, the shape expressed by the three-dimensional CAD data is a number of surfaces orthogonal to predetermined axes. The cross-sectional shape of the flakes generated by slicing is calculated, and the flakes are actually produced, stacked, and pasted together to materialize the computer-expressed shape.

In this additive manufacturing method, there are a case where 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 region is irradiated with thermal energy, for example, laser beam or electron beam to melt and solidify or sinter the powder layer, And the whole shape is modeled by stacking this one layer further.

On the other hand, in additive manufacturing using a plate as a material, individual parts (plates) generated by slicing 3D shape data in CAD are actually manufactured by machining, etc., and the parts are stacked and diffusion bonded. The whole three-dimensional shape is formed with.
For example, the following patent document 1 and patent document 2 disclose an example in which a mold is manufactured by this kind of additive manufacturing method.

  Specifically, the following Patent Document 1 discloses an invention relating to “a metal powder for powder sintering lamination, a method for producing a three-dimensional shaped article using the same, and a three-dimensional shaped article to be obtained”, and precipitation hardening therein. Irradiating the powder material of the mold metal component with a light beam to sinter or melt and solidify the powder at a predetermined location to form a solidified layer, and further form a solidified layer on the solidified layer thus obtained The point which manufactures a three-dimensional shape molded article by repeating is disclosed.

  Further, the following Patent Document 2 discloses an invention relating to “a mold insert, a mold insert manufacturing method, and a resin mold”, in which a insert having a spiral cooling path is manufactured. Based on the slice data, a plurality of metal plates are processed to form grooves that form cooling paths, the grooved metal plates are laminated in a predetermined order, and diffusion-bonded together. The point of shape processing is disclosed.

  The layered manufacturing method as described above is to form the entire shape by stacking materials, and can be easily processed and formed even with complicated cooling circuits that are twisted in length and width that can not be achieved by cutting, Even if the cooling circuit is not intentionally brought closer to the molding surface of the mold, the cooling efficiency can be more effectively improved than that of a mold produced by cutting by conventional machining.

As described above, the cooling capability has been improved by providing a curved and complicated three-dimensional cooling circuit inside a mold or the like, but the cooling efficiency is already at its limit, and it is difficult to further improve the cooling efficiency. Factors that hinder the improvement of cooling efficiency include (1) low thermal conductivity, (2) promotion of water-cooled hole cracking, and (3) low corrosion resistance. Another problem separate from the cooling efficiency in the mold is (4) frequent heat check. The problems (1) to (4) will be described below.
First, (1) the problem of low thermal conductivity is described. Generally, 18Ni maraging steel and SUS420J2 series powder are used for additive manufacturing, but these steels have low thermal conductivity. Therefore, no matter how efficiently the cooling circuit is arranged, there is a limit to improving the cooling efficiency because heat transfer in the mold (between the design surface and the cooling circuit) is not fast.

  Next, (2) the problem of promoting water-cooled hole cracking is described. In order to increase the cooling efficiency, the cooling circuit may be brought close to the design surface, but if it is too close, a crack from the cooling circuit easily develops on the design surface due to superposition of stress increase and penetration distance reduction. Therefore, there is a limit to the proximity of the cooling circuit to the design surface, and thus there is a limit to improving the cooling efficiency. Further, 18Ni maraging steel and SUS420J2 system have low thermal conductivity, so that the temperature gradient in the mold becomes large, so that the thermal stress on the inner surface of the cooling circuit is increased and the water cooling holes constituting the cooling circuit are liable to crack. Also in this sense, the low thermal conductivity material makes it difficult to bring the cooling circuit and the design surface close to each other, and has become a bottleneck for improving the cooling efficiency.

  Furthermore, (3) the problem of low corrosion resistance is described. Since 18Ni maraging steel has low corrosion resistance, water-cooled holes are likely to rust. Since rust, which is an oxide, has a very low thermal conductivity, it becomes a barrier for heat exchange between the cooling water and the mold, and hinders improvement in cooling efficiency. When rust becomes prominent, the cooling circuit becomes narrow due to rust, and the cooling efficiency also decreases due to a decrease in the flow rate of the refrigerant. In severe cases, the cooling circuit may be clogged with rust, and in this case, the bent cooling circuit will be useless at all.

  Finally, (4) the problem of frequent heat checks is described. Since 18Ni maraging steel and SUS420J2 series have low thermal conductivity, the temperature gradient in the mold increases, so the thermal stress on the design surface also increases and heat check is likely to occur. When the high temperature strength is low, the heat check problem becomes more apparent.

In summary, the problems of molds and parts having a three-dimensional cooling circuit result in low thermal conductivity and low corrosion resistance. Due to the low thermal conductivity, there is a limit to improving the cooling efficiency. In addition, cracks in the water-cooled holes are promoted, and heat checks occur frequently. In addition, because of low corrosion resistance, the cooling efficiency decreases due to rusting (worst is the clogging of water cooling holes), making it more difficult to improve cooling efficiency.
In other words, the above problem can be solved if there is a mold or component that is formed by layering using steel powder that has both high thermal conductivity and high corrosion resistance.

However, regardless of whether or not the mold is formed by the additive manufacturing method, conventionally used JIS SKD61, SUS420J2, maraging steel and other mold steels have high temperature strength but are dissolved in the matrix. Since many elements such as Si, Cr, Ni, and Co are easily contained, the thermal conductivity is low, and it is difficult to increase the cooling efficiency from the viewpoint of thermal conductivity.
That is, steel for molds that can realize sufficient performance in terms of corrosion resistance and heat conduction performance in addition to high-temperature strength when it becomes a mold has not been provided.

  In addition, as another prior art to the present invention, the following Patent Document 3 discloses an invention concerning “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: 0.20-0.90%, W: 0.01-1.65%, Co: 0.03-0.89%, Ni: 0.01-1.65%, The balance consists essentially of Fe and unavoidable impurities, N of the unavoidable impurities is controlled to 0.009% or less, Ti is controlled to 0.003% or less, and B is controlled to 0.012% or less to clean non-metallic inclusions. The degree of JIS dA is 0.005% or less and d (B + C) is 0.020% or less, and the directionality of the martensite structure after heat treatment Hot work tool steels is disclosed in the range of 17-33%.

  Further, as another prior art, the following Patent Document 4 discloses an invention relating to “a steel for crushing blade and a method for producing a crushing blade”, in which C: 0.3 to 0.5%, Si: 0.00. 2 to 0.5%, Mn: 0.1 to 1.0%, Cr: 4.0 to 6.0%, one or two of Mo and W being Mo + 1 / 2W: 0.8 to 2 .5%, one or two of V and Nb, containing V + 1 / 2Nb: 0.3-1.0% as a basic component, the balance being steel for crushing blades composed of Fe and inevitable impurities Is disclosed.

  Further, as another prior art, the following Patent Document 5 discloses an invention relating to “hot forging die and manufacturing method thereof”, in which C: 0.32 to 0.42%, Si: 0.3% Hereinafter, Mn: 0.3 to 1.5%, Ni: 0.5% or less, Cr: 4.0 to 6.0%, V: 0.2 to 1.0%, Mo + 1 / 2W: 0.8 A hot forging die containing ˜2.0% and N: 0.005 to 0.04%, the balance being Fe and inevitable impurities is disclosed.

  Further, as another prior art, the following Patent Document 6 discloses an invention relating to a “hot working die”, in which C: 0.30% or more and less than 0.50%, Si: 0.10 0.5%, Mn: 0.30 to 1.0%, P: 0.02% or less, S: 0.005% or less, Cr: 4.0 to 8.0%, Mo: 0.2% or more 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, with the balance being Fe and impurities A hot working mold having both a chemical composition of Ni and W as impurities of less than 0.7% and a tensile strength of 900 MPa or more, with a hardening depth of 200 μm at least on the surface in contact with the workpiece And a hardness at a position where the depth of the nitride layer is 30 μm or more. Discloses a hot working mold having a Vickers hardness of 900 or less.

  Furthermore, as another prior art, the following patent document 7 discloses an invention about “hot forging die steel”, in which 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 : 1.0-2.0%, V: 0.1-0.5% is contained, and the balance is made of steel for hot forging dies composed of Fe and inevitable impurities.

Further, as another prior art, the following Patent Document 8 discloses an invention relating to “hot tool steel”, in which C: 0.25 to 0.40%, Si: 0.50% or less, Mn: 0 .30-1.00%, P: 0.015% or less, S: 0.005% or less, Ni: 0.50-2.00%, Cr: 2.70-5.50%, Mo: 1. Contains 0.00-2.00%, V: 0.40-0.80%, B: 0.0005-0.0100%, Al: 0.015-0.10%, N: 0.015% or less The balance is an alloy steel composed of Fe and inevitable impurities, the fracture toughness value (KQ) at room temperature is 250 kgf / mm ^3/2 or more, and the proof stress (0.2% PS) at high temperature (600 ° C.) is 60 kgf. A hot work tool steel having a / mm ² or greater is disclosed.

  However, all of those disclosed in Patent Documents 3 to 8 have a C content of 0.25% or more, and those disclosed in these Patent Documents differ from the present invention in the C content. Is. In addition, these patent documents do not mention the point that the powder is used as a material for the layered manufacturing method.

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 producing a mold by applying a layered manufacturing method, it was made for the purpose of providing a flour powder possible steels achieve a high corrosion resistance and high thermal conductivity Is.

Thus, claim 1 relates to a steel powder, and in mass%, 0.10 ≦ C <0.25, 0.005 ≦ Si ≦ 0.600, 2.00 ≦ Cr ≦ 6.00,
−0.0125 × [Cr] + 0.125 ≦ Mn ≦ −0.100 × [Cr] +1.800... Formula (1) (where [Cr] represents Cr content mass%)
0.01 ≦ Mo ≦ 1.80,
−0.00447 × [Mo] + 0.010 ≦ V ≦ −0.1117 × [Mo] +0.901... Formula (2) (where [Mo] represents the content percentage of Mo in Mo in Formula (2))
0.0002 ≦ N ≦ 0.3000, and the balance has a composition of Fe and inevitable impurities.

  According to a second aspect of the present invention, in the first aspect, the composition further contains 0.10 <Al ≦ 1.20 by mass%.

  According to a third aspect of the present invention, in any one of the first and second aspects, at least one of 0.30 <Ni ≦ 3.50 and 0.30 <Cu ≦ 2.00 is further contained in mass%. And

  According to a fourth aspect of the present invention, in any one of the first to third aspects, 0.0001 <B ≦ 0.0100 is further contained in mass%.

  According to a fifth aspect of the present invention, in any one of the first to fourth aspects, 0.003 <S ≦ 0.250, 0.0005 <Ca ≦ 0.2000, 0.03 <Se ≦ 0.50 in mass%. It further contains at least one of 0.005 <Te ≦ 0.100, 0.01 <Bi ≦ 0.50, 0.03 <Pb ≦ 0.50.

A sixth aspect of the present invention is the same as in any one of the first to fifth aspects, wherein the weight percentage is 0 . It further contains at least one of 004 <Ta ≦ 0.100, 0.004 <Ti ≦ 0.100, 0.004 <Zr ≦ 0.100.

Those of claim 7, in any one of claims 1 to 6, characterized in that it further contains 0.10 <W ≦ 5.0 0 mass%.

In the present invention, the “mold” includes not only the mold body but also mold parts such as a spool core that are assembled and used. Furthermore, the metal mold | die which consists of steel of this invention and the surface-treated thing is contained.

  The present invention is an alloy that lowers the thermal conductivity of high alloy steels such as maraging steel and stainless steel under the circumstances where steel powder having both high thermal conductivity and high corrosion resistance has not been provided. While reducing the content of the component, if the Cr content is excessively reduced, the corrosion resistance will deteriorate, so that 2.00 ≦ Cr ≦ 6.00 without excessively reducing Cr, and appropriately balancing these alloy components It is possible to achieve high thermal conductivity while maintaining high corrosion resistance.

The steel powder of the present invention is suitable for a powder material used when a mold is manufactured by modeling by the additive manufacturing method.
In the additive manufacturing method using powder, when heat energy is applied to a layer in which powders are arranged and hardened, the powder is melted and 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. In this case, quenching is carried out rapidly under a fast cooling rate. That is, quenching is performed sequentially and simultaneously in the process of powder lamination.

  Thus, since quenching is performed under a fast cooling rate, quenching is favorably performed at the time of additive manufacturing even if the content of the hardenability improving component is suppressed to a low level as a steel component in advance. According to the present invention, it is possible to obtain a hardness of 30 to 50 HRC required for a mold as it is by layered shaping.

When the steel powder of the present invention as described above is applied to a die-casting die or component by the additive manufacturing method, it is possible to improve the cooling efficiency, suppress the heat check, and prevent the water cooling holes from cracking.
Moreover, high performance can be exhibited even when applied to molds and parts used for injection molding of resin and rubber, forging, and hot pressing of steel sheets.
In addition, it is not necessary to manufacture the whole said metal mold | die and components by additive manufacturing. For example, only a part having a curved three-dimensional cooling circuit may be manufactured by the additive manufacturing method using the steel powder of the present invention based on a member created by a normal manufacturing method (machined from a block of melted material). .

Next, the reasons for limiting each chemical component in the present invention will be described below.
In addition, all the values of each chemical component are mass%.
1) <Chemical component of claim 1>
0.10 ≦ C <0.25
When C <0.10, the hardness of 30 HRC or more necessary for the mold cannot be obtained when there is tempering after the layered manufacturing. On the other hand, when 0.25 ≦ C, the thermal conductivity decreases. In addition, when 0.25 ≦ C, the hardness of the layered manufacturing exceeds 50 HRC, and the risk of large cracks increases when using the layered manufacturing as it is.
A preferable range of C is 0.11 ≦ C <0.24 which is excellent in the balance of various characteristics, and more preferably 0.12 ≦ C <0.23.

0.005 ≦ Si ≦ 0.600
When Si <0.005, the machinability is significantly deteriorated. On the other hand, when 0.600 <Si, the thermal conductivity is remarkably reduced. A suitable Si range is 0.010 ≦ Si ≦ 0.550, which is excellent in the balance of various properties, and more preferably 0.020 ≦ Si ≦ 0.200.

2.00 ≦ Cr ≦ 6.00
If Cr <2.00, the corrosion resistance is insufficient, which causes rusting and cracking of the water cooling circuit. Further, when Cr <2.00, the martensitic transformation point becomes high, the structure becomes coarse, and hardness and toughness are insufficient. On the other hand, when 6.00 <Cr, the thermal conductivity decreases. A preferable Cr range is 2.05 ≦ Cr ≦ 5.90, which is excellent in the balance of various properties, and more preferably 2.10 ≦ Cr ≦ 5.70.

−0.0125 × [Cr] + 0.125 ≦ Mn ≦ −0.100 × [Cr] +1.800 Formula (1)
When Mn <−0.0125 × [Cr] +0.125, the transformation point becomes high, the structure becomes coarse, and hardness and toughness are insufficient. On the other hand, if -0.100 × [Cr] +1.800 <Mn, the thermal conductivity decreases.
The tendency of the structure to become coarse and lack hardness and toughness is particularly remarkable when Cr is low. The decrease in thermal conductivity is particularly remarkable when Cr is high.

0.01 ≦ Mo ≦ 1.80
When Mo <0.01, the high temperature strength is insufficient. Further, when Mo <0.01, it is difficult to secure a hardness of 30 HRC or more when there is a heat treatment at the Ac1 point or less after the layered manufacturing.
On the other hand, when 1.80 <Mo, the fracture toughness value is greatly reduced. A preferred range is 0.05 ≦ Mo ≦ 1.70. A more preferable range is 0.10 ≦ Mo ≦ 1.60.

−0.00447 × [Mo] + 0.010 ≦ V ≦ −0.1117 × [Mo] +0.901 .. Formula (2)
When V <−0.00447 × [Mo] +0.010, the high temperature strength is insufficient. Moreover, when there is a heat treatment at the Ac1 point or less after the layered manufacturing, it is difficult to ensure a hardness of 30 HRC or more. Further, when V <−0.00447 × [Mo] +0.010, the crystal grains become coarse and the toughness decreases when there is quenching by heating at Ac3 point or higher after the layered manufacturing.
On the other hand, when −0.1117 × [Mo] +0.901 <V, the above effect tends to be saturated, and the cost is significantly increased.

0.0002 ≦ N ≦ 0.3000
When N <0.0002, it is difficult to ensure a hardness of 30 HRC or more. Further, when N <0.0002, the effect of improving the corrosion resistance is poor. Further, when N <0.0002, the crystal grains become coarse when quenching is performed after additive manufacturing.
On the other hand, if 0.3000 <N, the effect of increasing the strength and improving the corrosion resistance shows a saturation tendency, and at the same time, the refining cost increases remarkably. If 0.3000 <N, nitrogen may escape from the melted part during additive manufacturing. If it becomes like this, a hole will be formed in a layered modeling part, and characteristics, such as toughness, will not be satisfied. A preferred range is 0.0003 ≦ N ≦ 0.2500. A more preferable range is 0.0004 ≦ N ≦ 0.2000.

In addition, in the steel of this invention, the component shown below may be normally contained in the following quantity as an unavoidable impurity.
P ≦ 0.05
S ≦ 0.003
Cu ≦ 0.30
Ni ≦ 0.30
Al ≦ 0.10
W ≦ 0.10
O ≦ 0.05
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
REM ≦ 0.10

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

3) <Chemical component of claim 3>
The steel of the present invention may be quenched after the additive manufacturing. When the hardenability is poor, ferrite, pearlite, and coarse bainite are precipitated during quenching, and various properties are deteriorated. To deal with such concerns, Cu and Ni may be selectively added to enhance the hardenability. In particular,
0.30 <Ni ≦ 3.50
0.30 <Cu ≦ 2.00
What is necessary is just to contain at least 1 sort (s) of these.
Regardless of the presence or absence of quenching, when there is a heat treatment to a temperature below the Ac1 point, Ni has the effect of binding to Al and precipitating intermetallic compounds to increase the hardness. Cu also has the effect of increasing the hardness by aging precipitation when heated to a temperature below the Ac1 point. The preferred range is
0.50 ≦ Ni ≦ 3.00
0.50 ≦ Cu ≦ 1.80
It is. If any element exceeds a predetermined amount, thermal conductivity and toughness are lowered.

4) <Chemical component of claim 4>
Addition of B is also effective as a measure for improving hardenability. In particular,
0.0001 <B ≦ 0.0100
Containing.
Note that when B forms BN, the effect of improving the hardenability is lost, and therefore B needs to be present alone in the steel. Specifically, it is sufficient that nitride is formed with an element having an affinity for N stronger than B, and B and N are not bonded. Examples of such elements include Nb, Ta, Ti, Zr and the like. Even if these elements are present at the impurity level, they have an effect of fixing N, but depending on the amount of N, they may be added within the scope of claim 6 described later. Even if B is combined with N in the steel to form BN, if surplus B exists alone in the steel, it enhances hardenability.
B is also effective in improving machinability and grindability. Molds and parts made of the steel of the present invention may be subjected to cutting and grinding after additive manufacturing. In order to improve machinability and grindability, BN may be formed. BN is similar in nature to graphite and lowers cutting and grinding resistance while improving chip crushability.
In addition, when B and BN exist in steel, hardenability, machinability and grindability are improved at the same time.

5) <Chemical component of claim 5>
Since the steel of the present invention has a small amount of Si, the machinability is somewhat poor. As a workability improvement measure, 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
What is necessary is just to contain at least 1 sort (s) of these.
When any element exceeds a predetermined amount, the impact value is lowered.

6) <Chemical component of claim 6>
When quenching is performed after additive manufacturing, if the quenching heating temperature is increased or the quenching heating time is increased due to an unexpected equipment trouble or the like, various characteristics may be deteriorated due to coarsening of crystal grains. In case such a, T a, Ti, and selectively adding Zr, it is possible to suppress the coarsening of austenite grains by fine precipitates which these elements are formed. Specifically ,
0.004 <Ta ≦ 0.100
0.004 <Ti ≦ 0.100
0.004 <Zr ≦ 0.100
What is necessary is just to contain at least 1 sort (s) of these.
If any element exceeds a predetermined amount, carbides, nitrides and oxides are excessively generated, and the impact value is lowered.

7) <Chemical component of claim 7>
In order to increase the strength of the steel according to the present invention with C <0.25 and low C as the steel for the mold, W may be added selectively.
W increases the strength by fine precipitation of carbides . In concrete terms,
0.10 <W ≦ 5.0 0
The may be contained.
However , exceeding a predetermined amount causes saturation of characteristics and a significant increase in cost . Good optimal range,
0.30 ≦ W ≦ 4.5 0
It is.

  According to the present invention as described above, a steel powder capable of realizing high thermal conductivity and high corrosion resistance and a mold manufactured using the same can be provided when a mold is manufactured by applying the additive manufacturing method. can do.

Using 34 kinds of steel powder shown in Table 1, a mold and a test piece were manufactured by an additive manufacturing method, and various tests were performed. Specifically, tests for evaluating hardness, thermal conductivity, mold surface temperature, heat check, and water-cooled hole cracking were performed.
In addition, the steel powder described in Table 1 may contain elements not shown in the table within a range of amounts specified as impurities.
In Table 1, the comparative steel 1 is JIS SKD61, the comparative steel 2 is 18Ni maraging steel, the comparative steel 3 is martensitic stainless steel SUS420J2, and the comparative steel 4 is mechanical structure steel SCM435. Each comparative steel has at least two elements outside the specified range of the present invention.

These 34 kinds of steel powders were produced by the gas atomization method. The powder has a shape close to a sphere, and when a histogram of the diameter is taken, the powder of 100 μm or less is 80% or more of the whole (flaky or bowl-shaped powder is also mixed a little).
Preferable as a powder for additive manufacturing is a fine powder having an average diameter of 400 μm or less, and when taking a histogram of diameters, 80% or more of the total has a diameter of 400 μm or less.

The powder was layered with an electron beam on an SKD61 block mold (which is the base) to produce a mold (mold body). The total weight of the manufactured mold is about 18 kg. The laminated modeling part is provided with a bent cooling circuit, and the distance between the cooling circuit and the design surface is 15 mm.
Comparative steel 1, comparative steel 3, and comparative steel 4 were too hard and very low toughness if they were layered, so they were tempered in the range of 300 to 650 ° C. for 1 hour and adjusted to a hardness suitable for the mold. .

  The mold was assembled in a die-casting machine with a clamping force of 135 tons, and a cast product having a mass of 630 g was cast as 30000 shots as a casting test. The mold surface temperature (maximum temperature) at the 10th and 30000th shots at this time was evaluated. Moreover, the heat check of the design surface was observed after 30000 shot casting. Furthermore, the mold after the evaluation of the heat tic was cut, and the degree of corrosion and cracking of the water cooling holes in the cooling circuit was observed. In addition, about 30 degreeC industrial water was poured into the cooling circuit inside a metal mold | die.

In addition, a test piece for measuring thermal conductivity was cut out from a small bar manufactured by additive manufacturing separately from the mold, and the thermal conductivity was measured at 25 ° C. by a laser flash method.
The results of these evaluation tests are shown in Table 2.

  As shown in Table 2, the hardness of the mold formed by layered manufacturing using each steel of the invention is 41 to 49 HRC, and the hardness is suitable for the mold as it is by layered modeling. The comparative steel also has an appropriate value of 36 to 45 HRC by tempering.

<Die surface temperature>
If the surface temperature (maximum temperature) of the mold is generally 410 ° C. or lower, defects (seizure, defective casting structure, extended cycle time, heat check) hardly occur.
According to Table 2, it is 1 to 3 of the comparative steel that the surface temperature of the mold has already reached an unfavorable temperature exceeding 410 ° C. at the 10th shot at the beginning of casting. / K or less. In these comparative steels, there are concerns about defects due to overheating of the mold.
On the other hand, all of the inventive steels 1 to 30 having a high thermal conductivity of 29 W / m / K or higher do not exceed 410 ° C. at the 10th shot. Empirically, if the thermal conductivity is 28 W / m / K or more, high cooling efficiency can be obtained, but overheating is certainly suppressed in these invention steels.

Corrosion resistance greatly affects the mold surface temperature at the 30000th shot. This is because if the water cooling holes are rusted, the cooling efficiency is lowered due to the heat exchange hindrance by the rust and the reduction of the cooling water amount (the water cooling holes become narrow due to rusting).
From this point of view, the comparative steel 2 and comparative steel 4 with very low Cr content have a higher maximum mold temperature at the 30000th shot than at the 10th shot, and rust is generated in the water cooling holes. Show what you did.
In Comparative Steel 4, the mold surface temperature at the 10th shot was 394 ° C., but it exceeded 410 ° C. at the 30000th shot.
On the other hand, since the comparative steel 3 has a very high Cr content and very good corrosion resistance, the mold surface temperature at the 30000th shot is not different from the mold surface temperature at the 10th shot. However, the comparative steel 3 has a temperature of 410 ° C. or higher from the 10th shot, and it is clear that high corrosion resistance is not sufficient and it is not possible to effectively suppress overheating of the mold unless it has high thermal conductivity.

On the other hand, invention steels 1 to 30 that achieve both high corrosion resistance and high thermal conductivity maintain a low temperature of 410 ° C. or lower even at the mold surface temperature of the 30,000th shot.
In addition, the temperature difference from the 10th shot tends to be larger as the amount of Cr is relatively low like the inventive steels 5, 8, 26, which indicates that rust is slightly generated in the water cooling holes. However, since the cooling efficiency is high with high thermal conductivity, the decrease in cooling capacity due to rust is not so significant. It was confirmed that high corrosion resistance and high thermal conductivity were necessary to maintain the mold temperature stably.

<Heat check>
Next, the heat check of the design surface of the mold after 30000 shots was observed. Conditions under which heat check is likely to occur are when the high-temperature strength of the mold is low (low initial hardness, low softening resistance) and high thermal stress is applied (low thermal conductivity).
The comparative steel 1 had high high-temperature strength, and the heat check was moderate among the comparative steels because of its relatively high thermal conductivity. This undesirable state is denoted by Δ.
Since comparative steel 2 has low high-temperature strength (low initial hardness) and low thermal conductivity, an extremely severe heat check was generated and evaluated as x.
Although the comparative steel 3 had high high-temperature strength, it produced a remarkable heat check due to its low thermal conductivity, and was evaluated as x (however, it was slightly milder than the comparative steel 2).
Since the comparative example 4 has low high-temperature strength, a heat check similar to that of the comparative steel 3 occurred even when the thermal conductivity was high, and was evaluated as x.

On the other hand, invention steels 1 to 30 have both high-temperature strength and high thermal conductivity, so the heat check is very slight, and they were evaluated as ◯.
This time, the casting test was completed with 30000 shots, but the heat check was so small that tens of thousands of shots could be cast. It was confirmed that high thermal conductivity was necessary to suppress the heat check.

<Rust and cracking of water-cooled holes>
Next, the metal mold | die cast 30000 shot was cut | disconnected, and the rust and the crack of the water cooling hole of the cooling circuit were confirmed.
Corresponding to the result of the mold surface temperature, the rust was markedly rusted in the comparative steel 2 and the comparative steel 4. In the comparative steel 3 of stainless steel, almost no rust was generated, and the rust of the comparative steel 1 was mild. Comparative steel 1 was not stainless steel, but Cr had a high corrosion resistance of 5% and a considerable corrosion resistance.
As for the inventive steel, the higher the Cr content, the less the rust.

On the other hand, water-cooled hole cracking is likely to occur when the corrosion resistance is low (the Cr content is small) and the thermal conductivity is low (the thermal stress is high).
Although comparative steel 1 had relatively high corrosion resistance and few corroded portions as starting points of cracks, cracks with a depth of about 5 mm were developed due to low thermal conductivity, and this was evaluated as Δ. Although it is not so deep that a crack penetrates the design surface, it is not a preferable state.
Since the comparative steel 2 had low corrosion resistance and low thermal conductivity, it grew into a crack exceeding 10 mm. The distance between the design surface and the water cooling holes is 15 mm, which is a very dangerous state in which water leakage due to penetration of cracks in the design surface is a concern. Of course, the evaluation is x.
Since the comparative steel 3 has very high corrosion resistance, there are almost no corroded portions that are the starting points of cracks, and almost no cracks were observed. It can be seen that water-cooled hole cracking can be suppressed if the generation of cracks is suppressed, although the thermal conductivity is low.
The comparative steel 4 has high thermal conductivity but low corrosion resistance, so the occurrence of cracks cannot be suppressed. As a result, the cracks have progressed by about 5 mm and were evaluated as Δ.

In contrast, the invention steel is characterized by high corrosion resistance and high thermal conductivity. For this reason, the water-cooled hole crack was mild and the depth of the crack was about 1 mm at the maximum. Evaluation is (circle). The casting test was completed with 30000 shots this time, but the cracks in the water-cooled holes were shallow enough that tens of thousands of shots could be cast.
From the results shown in Table 2, it was confirmed that compatibility between high corrosion resistance and high thermal conductivity is effective in improving the cooling performance in the mold, suppressing heat check, and reducing water-cooled hole cracking.

<Distance between water-cooled hole and design surface>
By the way, the comparative steel 3 having high corrosion resistance has a stable cooling performance that the mold maximum temperature at the 10th shot and the 30000th shot does not change. Therefore, a mold having a water-cooled hole close to a position 10 mm from the design surface was manufactured using the steel powder of comparative steel 3, and an evaluation test was performed under the same conditions as the casting test of Table 2. Since the thermal stress on the design surface is reduced by making the water cooling hole close to the design surface, a heat check improvement effect can also be expected. The results are shown in Table 3.

As shown in Table 3, the mold maximum temperature in the 10th shot was 397 ° C., and the temperature was lowered similarly to the invention steel in Table 2 (the distance between the water-cooled hole and the design surface was 15 mm). The method of bringing the water cooling holes close to the design surface is very effective for lowering the temperature. Further, even at the 30000th shot, it remains at 397 ° C., and the cooling ability is very stable. Furthermore, the heat check was improved from x in Table 2 to Δ as expected.
However, water-cooled hole cracking deteriorated from ○ in Table 2 to ×. When the design surface and the water-cooled hole are brought close to each other as in this example, the thermal stress on the design surface decreases, but the thermal stress on the surface of the water-cooled hole increases conversely. For this reason, even if there are few corroded portions that are the starting points of cracks (even with high corrosion resistance), it is considered that the progress of cracks has been accelerated. Since the crack depth exceeds 5 mm and the distance between the design surface and the water-cooled hole is 10 mm, it is a very dangerous state in which water leakage due to penetration of the crack into the design surface is a concern.
As described above, when the water cooling holes are brought close to the design surface in order to improve the cooling ability, the water cooling hole cracks become obvious.

As described above, if the corrosion resistance is poor even if the thermal conductivity is high, the cooling capacity is greatly reduced by rust, and the water-cooled hole cracking is likely to occur because the corroded portion is the starting point of the crack. On the other hand, if the thermal conductivity is low even if the corrosion resistance is increased, water cooling hole cracking is promoted as well as the cooling ability is lowered, and the heat check resistance is also deteriorated.
For this reason, even if only one of thermal conductivity and corrosion resistance is increased, it is difficult to simultaneously realize three items of lowering the temperature (improving cooling ability), suppressing heat check, and reducing water-cooled hole cracking.
In contrast, the invention steel has both high corrosion resistance and high thermal conductivity, so these three items can be realized simultaneously.

Although the embodiment of the present invention has been described in detail above, this is merely an example. Although the example of application to a die casting die has been described above, the steel of the present invention having both high thermal conductivity and high corrosion resistance is a die having a circuit in which a circuit for flowing a temperature control refrigerant is formed. It can be suitably applied to parts. Specifically, it exhibits high performance when applied to molds and parts for injection molding, forging, and hot pressing of steel plates.
Furthermore, if the steel of the present invention is used as a welding material in the form of a rod, wire, or wire, the same hardness can be obtained as it is as it is, and the properties of high thermal conductivity and high corrosion resistance can be obtained. Can be used. Welding is also a kind of additive manufacturing. Of course, reheating for the purpose of adjusting the hardness and removing strain and stress may be performed after welding as in the case of ordinary welding materials.
It is also effective to combine the steel mold of the present invention with surface modification (shot blasting, sand blasting, nitriding, PVD, CVD, plating, etc.).
In addition, this invention can be implemented in the aspect which added the various change in the range which does not deviate from the meaning.

Claims (7)

0.10 ≦ C <0.25 in mass%
0.005 ≦ Si ≦ 0.600
2.00 ≦ Cr ≦ 6.00
−0.0125 × [Cr] + 0.125 ≦ Mn ≦ −0.100 × [Cr] +1.800 Formula (1)
(However, [Cr] in formula (1) represents the mass% of Cr)
0.01 ≦ Mo ≦ 1.80
−0.00447 × [Mo] + 0.010 ≦ V ≦ −0.1117 × [Mo] +0.901 .. Formula (2)
(In the formula (2), [Mo] represents the mass% of Mo)
0.0002 ≦ N ≦ 0.3000
A steel powder characterized in that the balance has a composition of Fe and inevitable impurities. 0.10 <Al ≦ 1.20 in mass%
The steel powder according to claim 1, further comprising: In mass% 0.30 <Ni ≦ 3.50
0.30 <Cu ≦ 2.00
The steel powder according to claim 1, further comprising at least one of the following. 0.0001 <B ≦ 0.0100 in mass%
The steel powder according to any one of claims 1 to 3, further comprising: 0.003 <S ≦ 0.250 by mass%
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 steel powder according to claim 1, further comprising at least one of the following. Mass% 0.004 <Ta ≦ 0.100
0.004 <Ti ≦ 0.100
0.004 <Zr ≦ 0.100
The steel powder according to claim 1, further comprising at least one of the following. Mass% 0.10 <W ≦ 5.0 0
The steel powder according to claim 1 , further comprising: