JP6459539B2 - Mold steel and mold - Google Patents

Description

  The present invention relates to a mold steel and a mold, and more particularly to a mold steel and a mold that are difficult to coarsen during quenching and have excellent thermal conductivity.

Conventionally, die-cast molds and parts are required to have hardness to ensure erosion resistance and toughness to ensure crack resistance. These molds and parts are used in a quenched and tempered state. Has been. In most cases, the heating conditions for quenching are a temperature of 1030 ° C. and a holding time of 1 to 3 hours at that temperature.
During quenching heating, the steel becomes a mixed structure of austenite and residual carbide (sometimes austenite single phase). Thereafter, austenite is transformed into a structure mainly composed of martensite by cooling, and hardness and toughness are imparted by combination with tempering.

  Here, considering toughness, it is desirable that the austenite grain size number during quenching is larger. In other words, a structure transformed from fine austenite is desirable for the mold and parts. This is because cracks are less likely to propagate when the crystal grains of these molds are finer, and the effect of suppressing cracks is greater.

The austenite grain size during quenching is determined by the heating temperature and holding time. The austenite grain size number becomes large (crystal grains become fine) when the heating temperature is low and the holding time is short. For this reason, in quenching, care must be taken so that the heating temperature does not become excessively high and the holding time does not become excessively long.
Further, in order to prevent coarsening of crystal grains, a technique of dispersing residual carbides in austenite may be employed. In this case, a component steel in which the amount of C and the amount of carbide forming elements are optimized is used.
The residual carbide has an effect of suppressing the movement of the austenite grain boundary, and as a result, coarsening of the austenite crystal grain is prevented (a large grain size number is maintained).

By the way, in quenching, “mixed loading” in which a large mold and a small mold are heated together is generally used. The reason for the mixed loading is that the productivity is not increased if the molds are processed one by one.
FIG. 3 schematically shows the transition of the furnace temperature during heating and the temperature of the mold charged in the furnace in the case of mixed loading. As described above, the holding time at the quenching temperature needs 1 to 3 hours, but if the holding time of the furnace temperature is set so that a large mold satisfies this condition, the holding at the quenching temperature is held in a small mold whose temperature rises quickly. The longest time is close to 5 hours, and the crystal grains become coarse (the crystal grain size number becomes small).

  In recent years, in order to shorten the cycle time of die casting and to reduce seizure, high thermal conductivity steel having excellent cooling efficiency is increasingly used for die casting molds and parts. The thermal conductivity of SKD61, which is a general-purpose steel for die casting molds, is 23 to 28 W / m / K, whereas the high thermal conductivity steel has a thermal conductivity of 29 W / m / K or more. Such steel is reduced in Cr (Cr <3%) in order to increase the thermal conductivity.

However, such steel has little or almost no carbide remaining when quenched at 1030 ° C. Therefore, in order to prevent coarsening of the crystal grains during quenching (in order to make the austenite grain size number ≧ 5), the quenching temperature must be lowered to 1020 ° C. or lower, and only the mold of the steel is used. Since the quenching temperature is different from other molds, it is necessary to perform quenching individually. In other words, only one steel mold is charged into a large furnace for processing, and the productivity becomes very low.
Some high thermal conductivity steels contain almost no Cr and have a thermal conductivity exceeding 42 W / m / K. However, such steels have low high-temperature strength and are used in die casting molds and parts. That is not recommended. Further, the hardenability is very poor, and it is difficult to obtain the necessary hardness for the mold by heat treatment.

To summarize the above, if there is a low Cr steel having a thermal conductivity of 29 to 42 W / m / K and a steel having an austenite grain size number of 5 or more even if held at 1030 ° C. for 5 hours, the following three points Can be realized simultaneously.
(1) Improved quenching productivity (because it can be mixed with a large mold that is quenched at 1030 ° C.).
(2) Die-casting cycle time reduction and seizure reduction (because of high thermal conductivity, cooling efficiency increases).
(3) Prevention of cracking of die-cast mold (because toughness can be increased by refining austenite crystal grains during quenching).
However, at present there is no such steel, and there is a very strong need in the industry for a high thermal conductivity steel that is difficult to coarsen during quenching.

As a prior art to the present invention, the following Patent Document 1 discloses an invention relating to “steel for molds”, in which the thermal conductivity of steel is effectively secured to a desired value or more and machinability is also achieved. In addition, for the purpose of providing a mold steel that can sufficiently secure an impact value, the composition of the steel is “mass%, 0.35 <C ≦ 0.50, 0.01 ≦ Si <. 0.19, 1.50 <Mn <1.78, 2.00 <Cr <3.05, 0.51 <Mo <1.25, 0.30 <V <0.80, 0.004 ≦ N ≦ “0.040, balance Fe and inevitable impurities” is disclosed.
However, the one disclosed in Patent Document 1 has a Cr content exceeding 2.0%, and the one disclosed in Patent Document 1 is different from the present invention in the Cr content.

Patent Document 2 listed below discloses an invention relating to “die steel with excellent thermal fatigue characteristics”, in which heat check and water-cooled hole cracking are suppressed by increasing thermal fatigue characteristics and softening resistance. For the purpose of providing a mold steel capable of extending the life, the composition of the steel is “mass%, C: 0.1 to 0.6, Si: 0.01 to 0.8, Mn: 0.1-2.5, Cu: 0.01-2.0, Ni: 0.01-2.0, Cr: 0.1-2.0, Mo: 0.01-2.0, Further, one or more of V, W, Nb and Ta are combined: 0.01 to 2.0, Al: 0.002 to 0.04, N: 0.002 to 0.04, O: The point of “0.005 or less, remaining Fe and inevitable impurities” is disclosed.
However, although what is disclosed in Patent Document 2 focuses on thermal conductivity, there is no disclosure of examples of chemical compositions that satisfy the claims of the present invention, and prevention of crystal grain coarsening during quenching Is different from the present invention in that it is not mentioned.

Patent Document 3 below discloses an invention about “steel for mold excellent in spheroidizing annealing and hardenability”, in which hardenability is good and a required impact value is obtained, and the mold life is improved. For the purpose of providing a mold steel that can be applied to a large mold having a mass of 500 kg or more, which has a long life, good spheroidizing annealing property, and is easy to cut, the mold steel is changed to “mass% And C: 0.2-0.6%, Si: 0.01-1.5%, Mn: 0.1-2.0%, Cu: 0.01-2.0%, Ni: 0. 01-2.0%, Cr: 0.1-8.0%, Mo: 0.01-5.0%, Total of one or more of V, W, Nb and Ta: 0.01 ~ 2.0%, Al: 0.002 to 0.04%, N: 0.002 to 0.04%, balance Fe and inevitable impurities " Point is disclosed.
However, what is disclosed in this Patent Document 3 is that, in the range of chemical components specified in the claims, although the components partially overlap with the steel for molds of the present invention, Cr within the range specified in the present invention. The material containing is not disclosed in the examples. In addition, what is disclosed in Patent Document 3 does not mention the relationship between the additive component and the thermal conductivity, and further the relationship between the additive component and prevention of crystal grain coarsening during quenching. Is different.

Further, in Patent Document 4 below, for the purpose of providing a steel for a mold material which has a high softening resistance despite its low rare metal content and is excellent in thermal fatigue characteristics and wear resistance, the composition of the steel for the mold material is disclosed. "In mass%, C: 0.15-0.55%, Si: 0.01-2.0%, Mn: 0.01-2.5%, Cu: 0.01-2.0% , Ni: 0.01 to 2.0%, Cr: 0.01 to 2.5%, Mo: 0.01 to 3.0%, and at least one total amount selected from the group consisting of V and W : 0.01 to 1.0%, with the balance being Fe and inevitable impurities ”.
However, what is disclosed in Patent Document 4 does not include those containing Mn, Cr, Mo, V within the range defined by the present invention in the examples. In addition, there is no mention of thermal conductivity or prevention of grain coarsening during quenching, and the one disclosed in Patent Document 4 is also different from the present invention.

Furthermore, the following Patent Document 5 discloses an invention about “tool steel”, in which the composition of the steel is “weight%, C: 0.26 to 0.55%, Cr: less than 2.%, Mo: 0 to 10%, W: 0 to 15%, Ti, Zr, Hf, Nb and Ta alone or in total 0 to 3%, V: 0 to 4%, Co: 0 to 6%, Si: 0 to 1 .6%, Mn: 0 to 2%, Ni: 0 to 2.99%, S: 0 to 1%, the balance being Fe and unavoidable impurities.
However, the one disclosed in Patent Document 5 has a thermal conductivity at room temperature larger than 42 W / m / K, and the thermal conductivity range is different from the present invention. In addition, those containing Mo, V and Mn within the range defined in the present invention are not disclosed in the examples, and are different from the present invention.

JP 2011-94168 A JP 2008-56982 A JP 2008-121032 A JP 2008-169411 A JP 2014-1111835 A

  The present invention is based on the circumstances as described above, and can prevent the coarsening of austenite crystal grains during quenching heating, can increase the toughness after quenching and tempering, and has excellent heat conduction performance. It was made for the purpose of providing a mold.

  Thus, Claim 1 relates to a mold steel, and is 0.35 <C <0.55, 0.01 <Si <0.20, 1.50 <Mn <1.80 in mass%. , 1.80 <Cr <2.00, 0.90 <Mo <1.30, 0.61 <V <1.20, 0.0004 <N <0.0500, the balance being Fe and inevitable A mold steel having a composition of impurities, having a hardness of 25 to 55 HRC, a prior austenite grain size number of 5 or more before transformation, and a thermal conductivity at 25 ° C. measured using a laser flash method λ [W / m / K] is 29 ≦ λ ≦ 42.

Normally, in the steel for molds, the following components can be included as inevitable impurities within the following ranges.
P ≦ 0.05, S ≦ 0.003, Cu ≦ 0.30, Ni ≦ 0.30, Al ≦ 0.10, W ≦ 0.30, O ≦ 0.0100, 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.

  According to a second aspect of the present invention, in the first aspect of the present invention, at least one of 0.30 <W ≦ 5.00 and 0.10 <Co ≦ 4.00 is further contained by mass%.

  According to a third aspect of the present invention, in any one of the first and second aspects, 0.004 <Nb ≦ 0.100, 0.004 <Ta ≦ 0.100, 0.004 <Ti ≦ 0.100 in mass%. It further contains at least one of 0.004 <Zr ≦ 0.100.

  According to a fourth aspect of the present invention, in any one of the first to third aspects, the composition further contains 0.10 <Al ≦ 1.50 in mass%.

  According to a fifth aspect of the present invention, in any one of the first to fourth aspects, at least one kind of 0.30 <Cu ≦ 3.00, 0.30 <Ni ≦ 4.00 is further contained by mass%. And

  A sixth aspect of the present invention is characterized in that in any one of the first to fifth aspects, 0.0001 <B ≦ 0.0050 is further contained in mass%.

  In a seventh aspect of the present invention, in any one of the first to sixth aspects, 0.003 <S ≦ 0.050, 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.

An eighth aspect of the present invention relates to a mold, and is made of the steel according to any one of the first to seventh aspects.
In the present invention, the “mold” includes not only the mold body but also mold parts such as pins 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 as described above includes C, Mo and V for ensuring hardness, Si and Cr for maintaining high thermal conductivity, Mn for ensuring hardenability, and austenite at the time of quenching. In order to make the crystal grains fine (increase the crystal grain size number), C, V, and N are respectively optimized.
Such steel according to the present invention has higher thermal conductivity than SDK61, which is a general-purpose steel for die casting molds, and can prevent toughening of austenite crystal grains during quenching heating, so that the toughness after quenching and tempering is reduced. The crack resistance when used for a metal mold or a part is high.
In particular, the steel of the present invention has an austenite grain size number of 5 or more even when quenched at 1030 ° C. for 5 Hr despite being a Cr <2.00% low Cr steel (preventing grain coarsening). Therefore, it can be mixed with a large mold that is quenched at 1030 ° C., and the productivity of quenching can be increased.
Furthermore, the steel of the present invention can have a hardness of 55 HRC at the maximum after quenching and tempering.

When the steel of the present invention as described above is applied to a die casting die or part, it is possible to prevent the die or part from cracking and to shorten the die casting cycle time.
When the plastic is applied to a mold or a part for injection molding, the same effect as in the case of die casting can be obtained. It can also be applied to dies such as warm forging, sub-hot forging and hot forging.
Furthermore, the steel of the present invention can be made into a rod shape or a line shape and used as a welding repair material for dies or parts. Or it can also apply to the metal mold | die and components manufactured by the additive manufacturing of a powder or a board. In this case, if a complicated internal cooling circuit is provided in the mold or component, the effect of the high thermal conductivity of the present invention is further exhibited.

Next, the reasons for limiting each chemical component in the present invention will be described below.
“Chemical composition of claim 1”
0.35 <C <0.55
When C ≦ 0.35, it is difficult to stably obtain a hardness of 25 HRC or more when the quenching speed is low and the tempering temperature is high. If 0.55 ≦ C, coarse carbides increase, which becomes the starting point of cracks, and the toughness decreases. On the other hand, when 0.55 ≦ C, the retained austenite increases, and it becomes coarse bainite by tempering, so the toughness decreases. Further, when 0.55 ≦ C, the weldability decreases.
A preferable range of C is 0.36 <C <0.54, which is excellent in the balance of various characteristics, and more preferably 0.37 <C <0.53.

0.01 <Si <0.20
When Si ≦ 0.01, the machinability during machining is remarkably deteriorated. On the other hand, when 0.20 ≦ Si, the thermal conductivity is greatly reduced.
A suitable Si range is 0.02 <Si <0.19, which is excellent in the balance of various characteristics, and more preferably 0.03 <Si <0.18.

1.50 <Mn <1.80
When Mn ≦ 1.50, the hardenability is insufficient and the toughness is reduced due to the mixing of bainite. When 1.80 ≦ Mn, the annealability is deteriorated, and the heat treatment for softening is complicated and takes a long time to increase the manufacturing cost. Further, when 1.80 ≦ Mn, the thermal conductivity is greatly reduced.
A preferable range of Mn is 1.52 <Mn <1.78 excellent in the balance of various properties, and more preferably 1.54 <Mn <1.76.

1.80 <Cr <2.00
When Cr ≦ 1.80, the hardenability is insufficient. On the other hand, when Cr ≦ 1.80, the corrosion resistance is extremely deteriorated. On the other hand, when 2.00 ≦ Cr, the decrease in thermal conductivity is large.
A preferable Cr range is 1.81 <Cr <1.99, which is excellent in the balance of various characteristics, and more preferably 1.82 <Cr <1.98.

0.90 <Mo <1.30
When Mo ≦ 0.90, the contribution of secondary curing is small, and when the tempering temperature is high, it is difficult to stably obtain a hardness of 25 HRC or more.
On the other hand, if 1.30 ≦ Mo, the annealability deteriorates. If 1.30 ≦ Mo, the material cost increases.
A preferable range of Mo is 0.92 <Mo <1.28, which is excellent in the balance of various characteristics, and more preferably 0.94 <Mo <1.26.

0.61 <V <1.20
When V ≦ 0.61, the effect of increasing the austenite grain size number at the time of quenching (maintaining fine austenite crystal grains) is poor.
The effect of V on the austenite grain size number is shown in FIG. 0.44C-0.07Si-1.61Mn-1.90Cr-1.11Mo-0.018N as a basic component, steels with different V contents are held at 1030 ° C for 5 hours, quenched and martensitic transformed to corrode. The prior austenite grain boundaries before transformation were revealed in the liquid and the grain size number was evaluated. The determination was in accordance with JIS G0551.

As shown in FIG. 1, it may be high C and high N, and in a region where the amount of V is large, the crystal grain size number is stably large (crystal grains are fine). This is because there are a large amount of residual VC particles that suppress the movement of the austenite grain boundaries. As V decreases, the residual VC also decreases, so the crystal grain size number becomes smaller (the crystal grains become coarser), and when V ≦ 0.61, the grain size number 5 or more cannot be maintained.
When 1.20 ≦ V, the effect of maintaining fine crystal grains is saturated, and the cost is increased. On the other hand, when 1.20 ≦ V, coarse crystallized carbides (those that precipitate during solidification) increase, which becomes the starting point of cracks, and the toughness decreases.
A preferable V range is 0.62 <V <1.15, which is excellent in the balance of various characteristics, and more preferably 0.80 <V <0.98.
When 0.80 <V, the crystal grain size number is 6 or more, and the toughness is extremely stabilized.

0.0004 <N <0.0500
When N ≦ 0.0004, the effect of increasing the austenite grain size number at the time of quenching (maintaining fine austenite crystal grains) is poor. The effect of N on the austenite grain size number is shown in FIG. 0.46C-0.12Si-1.62Mn-1.95Cr-1.12Mo-0.98V as a basic component, different N content steel is held at 1030 ° C for 5 hours, quenched and martensitic transformed to corrode. The prior austenite grain boundaries before transformation were revealed in the liquid and the grain size number was evaluated. The determination was in accordance with JIS G0551.

As shown in FIG. 2, it may be high C, high V, and in a region where the amount of N is large, the crystal grain size number is stably large (crystal grains are fine). This is because there are a large amount of residual VC particles that suppress the movement of the austenite grain boundaries. As N decreases, the residual VC also decreases, so the crystal grain size number becomes smaller (the crystal grains become coarser), and when N ≦ 0.0004, the grain size number 5 or more cannot be maintained. Thus, N has a great influence on the amount of residual VC.
When 0.0500 ≦ N, the effect of maintaining fine crystals is saturated. On the other hand, if 0.0500 ≦ N, the refining time and cost required for N addition increase, leading to an increase in material cost. Furthermore, when 0.0500 ≦ N, coarse nitrides increase, which becomes the starting point of cracks, and the toughness decreases.
A preferable range of N is 0.0010 <N <0.0400, which is excellent in the balance of various properties, and more preferably 0.0030 <N <0.0350.
When 0.0010 <N, the grain size number is 6 or more, and the toughness is extremely stabilized.

“Chemical composition of claim 2”
Since the steel according to the present invention has a smaller total amount of Mn and Cr than SKD61, which is a general-purpose steel for die casting molds, the hardenability is not so high. For this reason, when the quenching speed is low and tempering is performed at a high temperature, it is difficult to ensure a hardness of 25 HRC or more. In such a case, W or Co may be selectively added to ensure strength. W increases strength by precipitation of carbides. Co increases the strength by solid solution in the base material, and at the same time contributes to precipitation hardening through a change in carbide form. In particular,
0.30 <W ≦ 5.00
0.10 <Co ≦ 4.00
It is sufficient to contain at least one kind (one element).
If any element exceeds a predetermined amount, saturation of characteristics and significant cost increase are caused.

“Chemical composition of claim 3”
If the quenching heating temperature becomes high or the quenching heating time becomes long due to unexpected equipment troubles, etc., there is a concern about the coarsening of crystal grains. In such a case, Nb—Ta—Ti—Zr can be 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
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, resulting in a decrease in toughness.

“Chemical component of claim 4”
Similarly, in order to suppress coarsening of austenite crystal grains, 0.10 <Al ≦ 1.50
Can be contained. Al combines with N to form AlN and has the effect of suppressing the growth of austenite grain boundaries.
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 for nitriding for higher wear resistance.
However, when Al exceeds a predetermined amount, the thermal conductivity and toughness are reduced.

“Chemical component of claim 5”
As described above, the steel of the present invention is not so hard. For this reason, when the quenching speed is low and tempering is performed at a high temperature, it is not only difficult to secure a hardness of 25 HRC or more, but also there is a concern about a decrease in toughness. In such a case, Cu or Ni may be selectively added to improve the hardenability. In particular,
0.30 <Cu ≦ 3.00
0.30 <Ni ≦ 4.00
What is necessary is just to contain at least 1 sort (s) of these. Cu also has the effect of increasing hardness by aging precipitation. If any element exceeds a predetermined amount, the thermal conductivity decreases and the material cost increases remarkably.

“Chemical component of claim 6”
Addition of B is also effective as a measure for improving hardenability. In particular,
0.0001 <B ≦ 0.0050
Containing.
In addition, since B loses the effect of improving hardenability when BN is formed, 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 the elements listed in claim 3. Even if the element of claim 3 is present at the impurity level, it has an effect of fixing N, but depending on the amount of N, it may be added within the range specified in claim 3. Even if B is combined with N in steel to form BN, if surplus B is present alone in the steel, it enhances hardenability.
B is also effective in improving machinability. In order to improve machinability, BN may be formed. BN is similar in nature to graphite and lowers cutting resistance while improving chip friability. In addition, when B and BN exist in steel, hardenability and machinability are improved at the same time.

“Chemical component of claim 7”
For improving the machinability, it is also effective to selectively add S-Ca-Se-Te-Bi-Pb. In particular,
0.003 <S ≦ 0.050
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.
If any element exceeds a predetermined amount, machinability is saturated, hot workability is deteriorated, impact value and specular polishing are deteriorated.

  According to the present invention as described above, a steel for molds and a mold which can increase the toughness after quenching and tempering by preventing coarsening of austenite crystal grains at the time of quenching heating and have excellent heat conduction performance. Can be provided.

It is the figure which showed the influence of V amount which acts on a crystal grain size number. It is the figure which showed the influence of N amount which acts on a crystal grain size number. It is the schematic diagram which showed the temperature transition of the metal mold | die at the time of quenching heating.

About 19 types of steel materials shown in Table 1, the prior austenite grain size number, the hardness after quenching and tempering, and the thermal conductivity were investigated. The comparative steel 1 in the table is a die-cast die general-purpose steel JIS SKD61. The comparative steel 2 is steel whose V amount deviates from the lower limit from the range of the present invention.
These 19 kinds of steel materials were produced by the following procedure. First, molten steel was cast into a 50 kg ingot and then homogenized at 1240 ° C. And it was finished in the shape of a bar with a rectangular cross section of 60 mm × 45 mm by hot forging. Subsequently, normalizing was performed by heating to 1020 ° C and quenching, and tempering by heating to 670 ° C. Further, after heating to 820 ° C. to 900 ° C., forced cooling to 600 ° C. at 15 ° C./Hr, allowing to cool to 100 ° C. or less, and subsequently annealing to 680 ° C. was performed. Specimens were cut from this steel bar and used for various investigations.

<Old austenite grain size number>
A small block of 15 mm × 15 mm × 25 mm cut out from the above-mentioned material was used as a test piece. The block was heated to 1030 ° C. and held for 5 hours, and then cooled at a rate of 20 ° C./min to cause martensite transformation. Then, the prior austenite grain boundaries before transformation were revealed with a corrosive solution, and the grain size number Evaluated. The determination is in accordance with JIS G0551, and the results are shown in Table 2.
The comparative steel 2 is at a level in which the V amount of the present invention deviates from the lower limit, and the number of VC particles pinning the austenite grain boundary during quenching is small, so the particle size number is less than 5. In Comparative Steel 4, ferrite is precipitated, and accurate evaluation of the austenite grain size before transformation is difficult and is a reference value. However, the particle size number was clearly less than 5. On the other hand, the grain number number of each invention steel stably exceeds 7.

<Hardness and thermal conductivity>
A small block in which the coarsening characteristics of the crystal grains were investigated was tempered at 625 ° C. × 2 Hr, and the HRC hardness was measured at room temperature. Further, a small disk-shaped test piece having a diameter of 10 mm × 2 mm was prepared from the block. The thermal conductivity of this test piece at 25 ° C. was measured by a laser flash method.
That is, a laser beam emitted from a laser oscillator is irradiated at a right angle to the above test piece at 25 ° C., and then the amount of heat radiated from the back of the test piece is measured with an infrared detector to obtain specific heat and thermal diffusivity, Finally, the thermal conductivity λ (= specific heat × thermal diffusivity × density) was calculated. These results are shown in Table 3.
Regarding the hardness after quenching and tempering, ferrite is precipitated in the comparative steel 4, and the hardness of 25 HRC or more necessary for the mold cannot be obtained. Other steel types are 42HRC or more.
On the other hand, the thermal conductivity does not satisfy the target value of 29 W / m / K or higher for the comparative steel 1 and the comparative steel 3 having a large amount of Cr. The thermal conductivity of other steel types is stably 29 W / m / K or more.

The above survey results are summarized in Table 4.
As for the prior austenite grain size number when heated at 1030 ° C. × 5 Hr, 5 or more was rated as “◯” and less than 5 was marked as “x”.
About the hardness after quenching and tempering, the range of 25-55 HRC was made into (circle), and other than that was set to x.
Regarding the thermal conductivity, the range of 29 to 42 W / m / K was marked with ◯, and the others were marked with ×.

For comparative steel, there is a cross in any item. Since the comparative steel 2 and the comparative steel 4 have a small crystal grain size number (the crystal grains are large), the same phenomenon occurs when the mold is actually quenched, and there is a concern that the mold in use may be cracked in die casting. The
In addition, the comparative steel 4 has a high thermal conductivity of 43 W / m / K and a low hardness, and there is a concern about durability when this steel is used as a die casting mold.
Moreover, the comparative steel 1 and the comparative steel 3 have difficulty in thermal conductivity, and when this steel mold is applied to die casting, it is expected that it is difficult to shorten the cycle time.
On the other hand, each invention steel has no problem in the grain size number and the thermal conductivity. Therefore, even when the invention steel is applied to an actual mold, it is difficult to break and it is expected that the cycle time can be shortened.
Except for the comparative steel 4, both the comparative steel and the invented steel were obtained with 42 HRC or more, and the hardness range was sufficient as a mold.

Although the embodiment of the present invention has been described in detail above, this is merely an example.
The steel of the present invention, which is excellent in hardness, toughness, and heat conduction performance, is suitable not only for die casting molds but also for molds for plastic injection molding. It can also be applied to dies such as warm forging, sub-hot forging and hot forging.
Further, the steel of the present invention is also effective in combination with surface modification (shot blasting, sand blasting, nitriding treatment, PVD treatment, CVD treatment, plating treatment, etc.).
Further, the steel of the present invention can be made into a rod shape or a wire shape and used as a welding repair material for dies or parts. Or it can also apply to the metal mold | die and components manufactured by the additive manufacturing of a powder or a board. In this case, if a complicated internal cooling circuit is provided in the mold and parts, the effect of the high thermal conductivity of the steel of the present invention can be further exerted.
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 (8)

% By mass 0.35 <C <0.55
0.01 <Si <0.20
1.50 <Mn <1.80
1.80 <Cr <2.00
0.90 <Mo <1.30
0.61 <V <1.20
0.0004 <N <0.0500
And the balance is Fe and inevitable impurities composition steel, the hardness is 25-55 HRC, the prior austenite grain size number before transformation is 5 or more, laser flash method A mold steel having a thermal conductivity λ [W / m / K] measured at 25 ° C. of 29 ≦ λ ≦ 42 measured at 25 ° C. In Claim 1, 0.30 <W ≦ 5.00 by mass%
0.10 <Co ≦ 4.00
A mold steel characterized by further containing at least one of the following. In any one of Claim 1, 2, 0.004 <Nb <= 0.100 in the mass%.
0.004 <Ta ≦ 0.100
0.004 <Ti ≦ 0.100
0.004 <Zr ≦ 0.100
A mold steel characterized by further containing at least one of the following. In any one of Claims 1-3, 0.10 <Al <= 1.50 in the mass%.
Furthermore, it contains steel for metal mold | die characterized by the above-mentioned. In any one of Claims 1-4, it is 0.30 <Cu <= 3.00 in the mass%.
0.30 <Ni ≦ 4.00
A mold steel characterized by further containing at least one of the following. In any one of Claims 1-5, 0.0001 <B <= 0.0050 in the mass%
Furthermore, it contains steel for metal mold | die characterized by the above-mentioned. In any one of Claims 1-6, 0.003 <S <= 0.050 in the 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
A mold steel characterized by further containing at least one of the following.   A mold made of the steel according to any one of claims 1 to 7.

Images