JP2009242820A - Steel, steel for die and die using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel capable of securing hardenability, heat conductivity and softening resistance, a steel for a die and a die using the same. <P>SOLUTION: The steel comprises, by mass, 0.15-0.55% C, 0.02-0.60% Si, 0.30-4.00% Mo, 0.03-1.00% V, Cr, Mn, Ni, Cu and the balance being Fe and unavoidable impurities, provided that the amounts of Cr, Mn, Ni and Cu satisfy the relation (1): 4.963-3.43Y≤Cr≤14.597-5.97Y [%] (wherein Y=Mn+0.36(Ni+Cu) [%]; and 0.10≤Y≤2.40 [%]). The steel for a die and the die using the same are also provided. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to steel, steel for molds, and molds using the same, and more specifically, firing of molds used in molds and molding members for plastic and rubber injection molding, die casting, hot forging, and the like. The present invention relates to a technique for suppressing the occurrence of cracks due to thermal shock through improvement of permeability, high thermal conductivity, and high softening resistance.

  Molds are indispensable in plastic and rubber injection molding, die casting, hot forging, and the like. Examples of these include forging dies and punches, die casting dies and pins and inserts, low pressure casting dies and pins and inserts, and injection molding (of resin and glass-based mixtures) Injection) molds, pins, inserts, and tools such as plates and sleeves used for forging, die casting, low pressure casting, and injection molding. Patent Document 1 discloses an example of steel suitable for use in these molds.

  The mold manufacturing process generally includes the steps of “melting-smelting-casting-homogenizing heat treatment-hot working-annealing-roughing-quenching / tempering-finishing- (surface treatment) -finishing”. Therefore, in the mold manufacturing, it takes a very long time and a lot of energy and resources from melting to completion.

In the manufacturing industry that uses or manufactures dies (die casting, plastic and rubber injection molding, hot forging, etc.), cost reduction, resource saving, and environmental load reduction are always issues. These solutions include mold life extension.
The cost can be reduced by extending the mold life. The number of products manufactured per mold increases, the unit price of the product decreases, and the manufacturing interruption time required for mold replacement is shortened. This is because of the increase. Incidentally, the mold replacement is performed when it is determined that the mold life is reached. For example, in die casting of aluminum wheels for automobiles, when a crack is transferred to a product, it is determined that the mold life is reached. This is because the skin becomes extremely rough and the commercial value as an aluminum wheel is lost.
The reason why the life of the mold can be saved by extending the mold life is that the number of molds produced is reduced and the amount of rare elements represented by V is reduced because the number of mold exchanges is reduced.
Further, the environmental load can be reduced by extending the mold life because the number of mold exchanges is reduced, the number of molds is reduced, the melting process and the heat treatment process are reduced, and the amount of greenhouse gas emissions is reduced.

And to extend the mold life,
(A) High impact value of steel material,
(B) thermal stress reduction (low thermal expansion coefficient, high thermal conductivity), and
(C) It is effective to increase the resistance to softening of the steel material (the softening resistance refers to the difficulty in decreasing the hardness when the steel material tempered to a predetermined hardness is heated under specific conditions).

  The reason why the die life can be extended by increasing the impact value in (a) above is that the die life generally has a good correlation with the impact value, and a die with a high impact value tends to have a long life. Because. And the steel material with a high impact value has also high resistance with respect to the propagation of the crack used as the judgment factor of a metal mold | die lifetime. Therefore, there are so many manufacturers that specify the impact value of the mold itself and the mold steel when ordering and receiving the mold.

  This high impact value has good hardenability, that is, it is transformed into a martensitic structure by cooling (quenching) at a speed higher than the critical cooling speed (quenching speed at the boundary between martensite and bainite). Cost. JIS SKD61 (see FIG. 1) with a critical cooling rate of 12 ° C./min is well known as a JIS steel with a sufficiently low critical cooling rate. JIS SKD61 is applied to dies such as die casting and hot forging, and has a relatively high softening resistance described later.

  The reduction in thermal stress in (b) above allows the mold life to be extended due to the heating and cooling cycle, which reduces the thermal stress that is the driving force for the generation and development of cracks. This is because it reduces the load. The phenomenon that the cracks lead to the life of the mold is called "thermal fatigue". "Satisfactory thermal fatigue" is a particular problem. While the mold is heated by contact with high-temperature materials such as molten metal and forging materials, it is cooled rapidly during the spraying process in which a mold release agent is applied (high-pressure spraying or high-pressure spraying). This is because stress acts, and this serves as a driving force for the generation and development of cracks, thereby extending the life of the mold.

As a method of reducing this thermal stress,
(A) Use of steel with a low coefficient of thermal expansion, and
(A) Use of steel materials with high thermal conductivity can be mentioned.
As a representative of the conventional steel with a small coefficient of thermal expansion (a) above, it contains 30% to 45% nickel ("%" means "% by mass" unless otherwise specified) in the following description) Invar alloys are known. Further, JIS S40C and JIS SCM440 are known as representatives of the conventional JIS steel having a large thermal conductivity (b). These thermal conductivities are 40 to 50 W / m / K, and a load reduction of 30% or more is expected compared to JIS SKD61, and it is considered that thermal fatigue of the mold is greatly reduced.

The reason why the mold life can be extended by increasing the softening resistance in the above (c) is that the hardness is not easily reduced by heating, and it can withstand long-term use. This point is also apparent from the thermal fatigue behavior of steel materials with high and low softening resistance in FIG. As shown in the figure, steel materials with high softening resistance have little decrease in hardness during use, so they can maintain higher yield strength than load stress, while steel materials with low softening resistance have a large decrease in hardness during use. As a result, the thermal stress becomes remarkable as a result of the load stress exceeding the yield strength. (There is a clear positive correlation between hardness and yield strength in many steel materials. Can also be associated).
That is, since the hardness of a steel material having a low softening resistance is easily reduced by heating, it is difficult to maintain strength for combating thermal fatigue. Die-casting and hot-forging dies are heated to 400 ° C or higher by contact with high-temperature objects such as molten metal and forging materials. As a result, if the die surface softens, the strength to resist thermal fatigue Cannot be maintained.

  As a technique for increasing the softening resistance, representative elements of the softening resistance are Mo and V, so that both of them are contained in a predetermined amount. The above JIS SKD61 is known as a steel with high softening resistance, that is, high heat resistance, and contains 1.2% Mo and 0.9% V.

JP 2005-307242 A

However, the pre-hardened steel of Patent Document 1 has a long life, but has a problem that the critical cooling rate is large (hardening is difficult) and the hardness does not appear even after quenching and tempering.
The above JIS SKD61 has a low critical cooling rate (easy to be baked) and a relatively high softening resistance, and is frequently used as a die for die casting, but has a problem of low thermal conductivity.
Moreover, even if the above-mentioned Invar alloy has a small thermal expansion coefficient, the material cost is very high, and there is a problem that the strength required as a mold cannot be obtained, and it cannot be used as a mold in the first place.
Further, the above JIS S40C and JIS SCM440 have a problem that both hardenability and softening resistance are poor even though the thermal conductivity is large.

  Accordingly, there remains a problem to use as a die such as die-casting, the impact value of the steel material (a) is increased, the thermal stress is reduced (b), and the steel material (c) is high. No steel has been known in the past that has realized softening resistance at the same time. From the above, it can be said that high impact value is achieved by ensuring hardenability, thermal stress reduction is realized by high thermal conductivity, and high softening resistance is realized by optimization of component elements.

  This invention is made | formed in view of the said situation, The objective provides the steel which can ensure hardenability, thermal conductivity, and softening resistance, metal mold steel, and a metal mold using the same. There is. In particular, the object is to provide steel that can be applied to a large-section mold, steel for mold, and a mold using the same. Through these, the present invention aims to extend the life of the mold and simultaneously realize cost reduction, environmental load reduction, and resource saving.

In order to solve the above-mentioned problems, the present inventor has found the basic characteristics required for the steel according to the present invention by conducting various studies and experiments.
First, the present inventor confirmed the influence of the quenching speed on the impact value of JIS SKD61 in order to find a quenching speed for realizing a high impact value. In the experiment, the test piece was kept at 1030 ° C. for 30 minutes, then quenched at a predetermined speed, and subsequently tempered to 45 HRC by tempering. FIG. 3 shows the impact value of the test piece as a result of the quenching speed. It can be seen that the impact value decreases rapidly when the quenching speed is less than 12 ° C / min. On the other hand, FIG. 1 is a CCT (continuous cooling transformation) diagram of JIS SKD61. This figure shows how many degrees Celsius transforms into bainite and martensite when the 1030 ° C austenite phase is cooled. When the cooling rate is as high as 30 ° C / min, a martensite structure is formed. However, when the cooling rate is 3 ° C / min, the structure is mainly bainite, and the structure changes greatly at 12 ° C / min. According to the figure, the critical cooling rate of JIS SKD61 is 12 ° C / min.

According to FIG. 3 and FIG. 1, when the cooling rate is higher than the critical cooling rate (in this case, 12 ° C./min), “baking” is entered, that is, the impact value is increased due to martensite, but cooling is slower than this. Then it becomes bainite and the impact value decreases. The point where the structure and impact value change greatly with this value as a boundary is common to almost all steel types.
Therefore, if the steel type has a critical cooling rate of 60 ° C / min, the impact value and the structure greatly change at 60 ° C / min.

As shown in FIG. 3 and FIG. 1, to obtain a high impact value that is essential for extending the mold life, it is required to obtain a martensite structure, that is, to burn. Here, when the critical cooling rate of JIS SCM440 is investigated, it exceeds 2000 ° C./min. That is, when trying to quench JIS SCM440, a quenching speed exceeding 2000 ° C./min is required. Here, the correlation between the mold thickness and the quenching speed is shown in FIG. It can be seen that thin pins with a diameter of 2 to 3 mm are fine, but JIS SCM440 does not burn in large sizes such as molds used for die casting. Also, JIS S40C is harder than JIS SCM440. Therefore, it is difficult to secure a long mold life even if the mold is made with JIS S40C or JIS SCM440.
Considering that most of the pins and inserts used for die casting have a thickness and diameter of 30 mm or more, the critical cooling rate of the steel material needs to be 60 ° C./min or less. The target value for the critical cooling rate of steel was set to "60 ° C / min or less".
Here, it supplements about the "impact value" used by the description so far. In order to investigate the impact value, a JIS No. 3 test piece was used and evaluated at room temperature. The impact value is a value obtained by dividing the absorbed energy by the cross-sectional area. A larger value indicates that the material is more difficult to break. The current JIS does not have a shock value standard. The item has disappeared due to the previous revision. However, it is still used industrially as an important characteristic value at present, and as described above, there are manufacturers that use the impact value as the acceptance standard for molds. Therefore, in the present invention, the impact value is used as a scale for evaluation according to such industrial practice. The following is the same as the impact value evaluation method and handling.

Secondly, the present inventor conducted the following verification / experiment in order to reduce thermal stress, which is a cause of thermal fatigue. First, it is known that the thermal stress can be approximated by equations (2) and (3).
Thermal stress = constant × temperature difference × Young's modulus × thermal expansion coefficient ÷ Poisson's ratio Equation (2),
Temperature difference = f (thermal conductivity) Formula (3).
The present inventor has found the possibility that the thermal fatigue of the mold can be remarkably reduced by reducing the thermal stress by controlling these physical property values as follows. Some physical properties of steel materials are very sensitive to the effects of components and others are not. With regard to equation (2), Young's modulus and Poisson's ratio are very unlikely to be affected by the components. Therefore, even if the steel material components are changed greatly, these physical property values do not change so much. On the other hand, the thermal expansion coefficient is a physical property value that is considerably affected by the components. In order to reduce thermal stress, it is desirable to reduce the thermal expansion coefficient.
Next, regarding the thermal conductivity shown in the formula (3), the thermal conductivity is also a physical property value that is affected by the components. Increasing the thermal conductivity reduces the temperature difference, reduces thermal stress and reduces thermal fatigue.

As a result of examining the thermal expansion coefficient and the thermal conductivity, the present inventor examined and focused on the thermal conductivity. Regarding the thermal expansion coefficient, the material cost is very high, and there is a problem that the required strength as a mold cannot be obtained.
FIG. 5 shows the result of estimating the correlation between thermal conductivity and thermal stress by the finite element method. The object of analysis is rapid cooling of a cylindrical specimen. Specifically, the maximum value of the thermal stress in the cooling process is simulated by giving an ambient temperature of 20 ° C and a heat transfer coefficient of 5000W / m ² / K to the surface of a φ60mm cylinder heated to 500 ° C and quenching. Evaluated. Steel with a thermal conductivity of 22 W / m / K corresponds to JIS SKD61, which is frequently used for dies such as die casting and hot forging. The thermal stress is shown as 1.0 when this thermal stress is 1.0. According to FIG. 5, it can be seen that the thermal stress sharply decreases when the thermal conductivity is 28 W / m / K or more.
Therefore, the present inventor has set the target value of the thermal conductivity of the steel material to “28 W / m / K or more” in order to reduce thermal stress.

Thirdly, the present inventor conducted various studies from the viewpoint of component elements in order to achieve high softening resistance. JIS SKD61, which is frequently used in dies such as die casting and hot forging, has Mo and V as the representative elements responsible for softening resistance. It was judged that the content of V must exceed JIS SCM440. This is because JIS SCM440 has low Mo content and does not contain V, and therefore has low softening resistance (JIS S40C has lower softening resistance than JIS SCM440).
Therefore, the present inventor decided to make “Mo amount 0.3% or more”. On the other hand, since V is not a positively added element of JIS SCM440, the content cannot be determined by comparison with JIS SCM440. Accordingly, when focusing on non-heat treated steel, since 0.03% or more is treated as being actively added, the present inventor decided to set the V amount to 0.03% or more.

As described above in the first to third, the present inventor, as a target value of basic characteristics to be required for steel materials applied to a die such as die casting and hot forging,
(A) To ensure hardenability, the critical cooling rate is set to 60 ° C / min or less,
(B) To ensure high thermal conductivity, the thermal conductivity is set to 28 W / m / K or more,
(C) The present invention was completed by setting the Mo amount to 0.3% or more and the V amount to 0.03% or more in order to ensure high softening resistance.

In order to solve the above problems, the steel according to the present invention as a steel satisfying the above basic characteristics, as an essential constituent element,
% By mass
(1) C: 0.15-0.55%
(2) Si: 0.02 to 0.60%,
(3) Mo: 0.30 to 4.00%, and
(4) V: 0.03 to 1.00% contained,
The gist is that the balance containing Cr, Mn, Ni, and Cu satisfying the following formula (1) by mass% is made of Fe and inevitable impurities.
(5) 4.963−3.43Y ≦ Cr ≦ 14.597−5.97Y [%] (However, Y = Mn + 0.36 (Ni + Cu) [%], 0.10 ≦ Y ≦ 2.40 [%]) (1)

The steel according to the present invention is a selective additive element,
Furthermore, in mass%,
(6) W: 0.01 to 4.00%,
(7) Co: 0.01-2.00%
(8) Nb: 0.002 to 0.500%,
Ta: 0.002 to 0.500%,
Ti: 0.002 to 0.500% and
Zr: one or more selected from the group consisting of 0.002 to 0.500%,
(9) Al: 0.005 to 1.500%, and
N: One or more selected from the group consisting of 0.005 to 0.300%,
(10) B: 0.0002 to 0.0200%, and
(11) S: 0.005 to 2.000%,
Ca: 0.0005 to 0.5000%,
Se: 0.005-0.500%,
Te: 0.005-0.500%,
Bi: 0.005-0.500%, and
Pb: one or more selected from the group consisting of 0.005 to 0.500%,
It is desirable to contain at least one of them.

The steel according to the present invention may be used as mold steel. Therefore, the mold steel according to the present invention is made of the steel according to the present invention. Specific applications of the steel for molds according to the present invention include forging molds and punches, die casting molds and pins and inserts, low pressure casting molds and pins and inserts, and injection molding. Tools such as molds, pins and inserts, and plates and sleeves used for forging, die casting, low pressure casting, and injection molding.
The mold according to the present invention as described above may have a weight of 100 kg or more, and may further include a layer having a component or hardness different from that of the base material on the surface thereof.

  The steel according to the present invention contains, by mass%, C: 0.15 to 0.55%, Si: 0.02 to 0.60%, Mo: 0.30 to 4.00%, and V: 0.03 to 1.00%. Since the balance containing Cr, Mn, Ni, and Cu satisfying formula (1) is made of Fe and inevitable impurities, it is possible to ensure hardenability, thermal conductivity, and softening resistance. .

In particular, since the formula (1) is satisfied, the hardenability is good, the impact value can be increased, and the thermal conductivity can be increased, so that the thermal stress can be reduced.
Since the steel for molds according to the present invention is made of the steel according to the present invention, it has the same effect.

  Since the mold according to the present invention is made of the mold steel according to the present invention, high thermal conductivity and high softening resistance are possible, and the thermal stress generated by the heating and cooling cycle is low, resulting from the thermal stress. Generation and propagation of cracks is suppressed. Therefore, the metal mold | die which concerns on this invention has the effect that lifetime becomes long and can implement | achieve cost reduction, environmental impact reduction, and resource saving, such as injection molding of plastics and rubber | gum, die casting, and hot forging.

  Hereinafter, with reference to drawings, the component composition of the steel which concerns on one Embodiment of this invention, and its reason for limitation are demonstrated. In the following description, “%” means “mass%” unless otherwise specified.

(1) C: 0.15-0.55%.
C is an essential constituent element indispensable for adjusting the strength of steel. If C is too small, the strength required for the mold material cannot be obtained. On the other hand, the hardness of steel that has become martensite by quenching tends to saturate when C exceeds 0.55%. On the other hand, if C exceeds 0.55%, coarse carbides are likely to be generated, which greatly deteriorates the mechanical fatigue characteristics. In order to ensure the characteristics as a die material used for die casting, low pressure casting, hot forging, injection molding, etc., the C amount in this embodiment is 0.15% to 0.55% in mass%.

(2) Si: 0.02 to 0.60%.
For cost reduction, it is extremely important not only to extend the mold life but also to ensure the machinability of the steel material. This is because the rough shape as a mold is given by roughing and the final shape is completed by finishing. If the machinability of the steel material is poor, not only the machining efficiency is lowered, but also expensive cutting tools are frequently replaced. As a result, if the machinability is poor, the manufacturing cost of the mold rises, which in turn increases the product cost.
In order to improve the machinability of steel, addition of Si is effective. In this embodiment, Si is an essential constituent element. The Si in the steel being cut adheres to the surface of the cutting tool, protects the tool surface, and reduces its wear. For this reason, the machinability of Si-added steel is often relatively good. The correlation between the Si content and the machinability of the steel is roughly as follows.
Very bad: Si ≦ 0.20%
Bad: 0.20 <Si ≦ 0.35%
Good: 0.35 <Si ≦ 0.50%
Very good: 0.50 <Si%
On the other hand, Si is also an element that greatly reduces the thermal conductivity of steel. In consideration of the thermal conductivity and machinability, the upper limit of the Si amount is set to 0.60% in this embodiment.

A characteristic of Si as seen from the manufacturing process is that it cannot be reduced by refining. For this reason, in the production of a low Si material, very expensive raw materials having a low Si content must be used, and an increase in material cost is inevitable. If Si is less than 0.02%, the effect of increasing the thermal conductivity is saturated and the manufacturing cost is significantly increased. Therefore, in this embodiment, the lower limit of the Si amount is set to 0.02%.
A particularly preferable Si amount range is 0.35% or less in terms of mass% if importance is attached to thermal conductivity, but may be exceeded if importance is attached to machinability. What is necessary is just to select according to a use.

(3) Mo: 0.30 to 4.00%.
Mo is an essential constituent element that is useful as an element for forming carbides and increasing the strength of steel by dispersion strengthening. Moreover, the effect of increasing the softening resistance is very large. Here, the softening resistance of 0.4C-3Cr steel containing Mo is as shown in FIG. After quenching from 1030 ° C, it is the hardness at room temperature when held at 600 ° C for 1 hour, but it is maximum at around 4%. Excessive addition promotes the agglomeration of carbides, and rather reduces softening resistance. On the other hand, if the amount added is too small, the strength required for the mold cannot be secured. From such a viewpoint, in the present embodiment, the Mo amount is set to 0.30% to 4.00% by mass%. A particularly preferable amount of Mo is 0.40% to 3.50%.

(4) V: 0.03-1.00%.
V is an essential constituent element that is useful as an element for forming carbides and increasing the strength of steel by dispersion strengthening. Moreover, the effect of increasing the softening resistance is very large. On the other hand, when added excessively, it becomes easy to produce very coarse carbides in the solidification process, which significantly reduces mechanical fatigue characteristics and impact value. Here, the impact value of 0.4C-3Cr steel containing V is as shown in FIG. It is an impact value at room temperature when it is tempered to 45 HRC by holding 1 Hr at 580 to 620 ° C. after quenching from 1030 ° C., but it shows a sharp drop when it exceeds 1%. This corresponds to the formation of coarse carbides as a result of the decrease in solidification rate because the material was manufactured from a large ingot. In industrial size ingots, it is very difficult to avoid this phenomenon. Although not shown, coarse carbides may halve fatigue strength. The reason why the impact value is low even when V is less than 0.35% is that the crystal grains become coarse. This is because fine V carbide (VC) suppresses the growth of crystal grains, but the fine VC decreases as the amount of V decreases. However, the coarsening of the crystal grains does not significantly affect the fatigue strength.
For the above reason, the V amount in the present embodiment is 0.03% to 1.00% by mass. JIS SKD61, JIS SKD8, JIS SKH51, or the like is used for die casting or hot forging, but the upper limit of the V amount is also lower than these, which is a feature of this embodiment. A particularly preferable amount of V is 0.20% to 0.80% in which softening resistance and fatigue strength are ensured and the impact value is not significantly reduced.

(5) 4.963−3.43Y ≦ Cr ≦ 14.597−5.97Y [%] (where Y = Mn + 0.36 (Ni + Cu) [%], 0.10 ≦ Y ≦ 2.40 [%]) (1).
Cr, Mn, Ni, and Cu are essential constituent elements as elements for improving hardenability. The appropriate Cr amount is an amount that satisfies the formula (1), and varies depending on the Mn amount, Ni amount, and Cu amount. In the present embodiment for ensuring hardenability, it is important to optimize the amounts of these elements.
Hereinafter, the reason why the Cr amount is in a range satisfying the formula (1) will be described.

First, “good hardenability” is synonymous with “inhibition of bainite transformation”. In order for bainite transformation not to occur at a lower quenching speed and to become martensite (quenching), it is effective to increase the stability of the austenite phase and suppress the diffusion of carbon dissolved in the austenite. is there. The reason is
(A) When the austenite phase becomes unstable, bainite transformation is likely to occur.
(B) The bainite transformation is accompanied by carbon diffusion.
Mn, Ni, and Cu are well known as elements that increase the stability of the austenite phase. On the other hand, Cr is well known as an element that suppresses the diffusion of carbon dissolved in austenite. Thus, the mechanism for enhancing the hardenability differs between Mn, Ni, Cu and Cr. Therefore, the improvement of hardenability will be described in two groups of Mn, Ni, Cu and Cr.

  The hardenability can be evaluated by the hardened layer depth of the hardened material. For example, when investigating the hardness distribution of a water-quenched round bar, the deeper the hardened layer, that is, the harder the steel material is to the inside of the round bar, the better the hardenability. In addition, since inexpensive Mn is frequently used to stabilize the austenite phase, the effects of Ni and Cu were converted to Mn and evaluated. Specifically, an experiment was carried out in which the effect of improving hardenability was quantified from the depth of the hardened hardened layer of the steel material in which Mn, Ni and Cu were changed and expressed in terms of Mn.

  Table 1 shows the chemical components of the steel materials used in the experiment. In order to evaluate the influence of Mn / Ni / Cu, the other components were made common. "..." means that the impurity level was 0.01% or less. A round bar with a diameter of 200 mm made of these 18 steel types is poured into stirring water, quenched, and the hardness distribution of the cut cross section is measured, and the depth of the hardened and hardened region is defined as the “quenching depth”. did. Table 1 also shows the quenching depth. It can be judged that the hardenability is better as the quenching depth is larger.

Based on Table 1, the result of evaluating the effect of improving hardenability by stabilizing the austenite phase with respect to Mn, Ni, and Cu will be described. First, two steel types that contain almost no Ni and Cu are compared. When steel 1 and steel 5 are compared, the quenching depth per 1% of Mn is 26.5 mm larger. The range of Mn is 0.1 to 0.3%.
Next, the effect of Ni and Cu is calculated by comparing the steel type with Mn of about 0.1% and Mn + Ni + Cu of 0.32% or less with Steel 1. When Steel 1 and Steel 2 are compared (both are about 0.1% Mn), the quenching depth is increased by 0.6 mm with an increase of 0.1% of Ni, which is 6.00 mm when converted to 1% of Ni. That is, the effect of adding Ni can be calculated to be 0.226 times “6.00 ÷ 26.5” with respect to Mn.
Furthermore, when steel 1 and steel 3 are compared (both 0.1% Mn), the quenching depth is increased by 0.7 mm with an increase of 0.11% of Cu, which is 6.36 mm when converted to 1% of Cu. In other words, the effect of Cu addition can be calculated as 0.240 times “6.36 ÷ 26.5” with respect to Mn.
From the above, it was found that the effect of Ni and Cu is 0.233 times (0.226 to 0.240 times) of Mn.
Finally, in order to confirm whether each effect can be added in the combined addition of Ni and Cu, the results of Steel 4 are verified.
Steel 4 has about 0.1% of Ni and Cu added to steel 1. If hardenability is added, the effect of increasing the quenching depth of steel 4 relative to steel 1 is
(0.11 + 0.10) × 0.233 × 26.5 = 1.30mm
It can be said that. Here, when 1.30 mm is added to 47.4 mm of steel 1, it becomes 48.70 mm, which is almost the same as 48.4 mm of steel 4. From the above, it was confirmed that the effect of Ni and Cu is 0.233 times that of Mn. The range of Mn is 0.1 to 0.3%.

Next, verification will be performed when the amount of Mn / Ni / Cu added is large.
First, two steel types that contain almost no Ni and Cu are compared. When steel 5 and steel 9 are compared, the quenching depth per 1% of Mn is 13.93 mm larger. The range of Mn is 0.3-0.9%.
Next, the effect of Ni and Cu is calculated by comparing a steel type with Mn of about 0.3% and Mn + Ni + Cu of 0.91% or less with steel 5. When steel 5 and steel 6 are compared (both are approximately 0.3% Mn), the quenching depth is increased by 1.7 mm due to a 0.3% increase in Ni, which is 5.67 mm when converted to 1% of Ni. That is, the effect of adding Ni can be calculated as 0.407 times “5.67 ÷ 13.93” with respect to Mn.
Furthermore, when steel 5 and steel 6 are compared (both are about 0.3% Mn), the quenching depth is increased by 1.7 mm with an increase of 0.29% of Cu, which is 5.86 mm when converted to 1% of Cu. In other words, the effect of Cu addition can be calculated as 0.421 times “5.86 ÷ 13.93” with respect to Mn.
From the above, it was found that the effects of Ni and Cu are 0.414 times (0.407 to 0.421 times) of Mn.
Finally, in order to confirm whether each effect can be added in the combined addition of Ni and Cu, the results of Steel 8 are verified.
Steel 8 has about 0.3% of Ni and Cu added to steel 5. If hardenability is added, the effect of increasing the quenching depth of steel 8 relative to steel 5 is
(0.30 + 0.31) × 0.414 × 13.93 = 3.52mm
It can be said that. Here, when 3.52 mm is added to 52.7 mm of steel 5, it becomes 56.22 mm, which is almost the same as 55.7 mm of steel 8.
From the above, it was confirmed that the effect of Ni and Cu is 0.414 times that of Mn. The range of Mn is 0.3 to 0.9%.

Subsequently, the effect of addition of Ni and Cu in the range of Mn up to 2.4% was estimated by the same method.
First, two steel types that contain almost no Ni and Cu are compared. When steel 10 and steel 14 are compared, the quenching depth per 1% of Mn is 15.56 mm larger. The range of Mn is 0.27 to 0.81%.
Next, the effect of Ni and Cu is calculated by comparing a steel type with Mn of about 0.27% and Mn + Ni + Cu of 0.84% or less with steel 10. When steel 10 and steel 11 are compared (both 0.27% Mn), the quenching depth is increased by 1.5 mm with a 0.26% increase in Ni, which is 5.77 mm when converted to 1% of Ni. That is, the effect of adding Ni can be calculated as 0.371 times “5.77 ÷ 15.56” with respect to Mn.
Furthermore, when steel 10 and steel 12 are compared (both 0.27% Mn), the quenching depth is increased by 1.4 mm with a 0.27% increase in Cu, which is 5.19 mm when converted to 1% of Cu. In other words, the effect of Cu addition can be calculated as 0.334 times “5.19 ÷ 15.56” with respect to Mn.
From the above, it was found that the effect of Ni and Cu is 0.353 times (0.371 to 0.334 times) of Mn.
Finally, in order to confirm whether each effect can be added in the combined addition of Ni and Cu, the results of steel 13 are verified.
In Steel 13, about 0.27% of Ni and Cu are added to Steel 10, respectively. If hardenability is added, the effect of increasing the quenching depth of steel 13 relative to steel 10 is
(0.27 + 0.28) × 0.353 × 15.56 = 3.02mm
Should be. Here, when 3.02 mm is added to 52.1 mm of steel 10, it becomes 55.12 mm, which is almost the same as 54.8 mm of steel 13.
From the above, it was confirmed that the effect of Ni and Cu was 0.353 times that of Mn. The range of Mn is 0.27 to 0.81%.

Next, we will proceed with verification when the amount of Mn / Ni / Cu added is large.
First, two steel types that contain almost no Ni and Cu are compared. When steel 14 and steel 18 are compared, the quenching depth per 1% of Mn is increased by 10.06 mm. The range of Mn is 0.81 to 2.41%.
Next, the effect of Ni and Cu is calculated by comparing a steel type with Mn of about 0.8% and Mn + Ni + Cu of 2.43% or less with steel 14. When steel 14 and steel 15 are compared (both are about 0.8% Mn), the quenching depth is increased by 3.6 mm with a 0.82% increase in Ni, which is 4.39 mm when converted to 1% of Ni. That is, the effect of adding Ni can be calculated as 0.436 times “4.39 ÷ 10.06” with respect to Mn.
Furthermore, when steel 14 and steel 16 are compared (both are approximately 0.8% Mn), the quenching depth is increased by 3.9 mm with a 0.81% increase in Cu, which is 4.81 mm when converted to 1% of Cu. That is, the effect of adding Cu can be calculated as 0.479 times “4.81 ÷ 10.06” with respect to Mn.
From the above, it was found that the effect of Ni and Cu is 0.458 times (0.436 to 0.479 times) of Mn. Finally, in order to confirm whether each effect can be added in the combined addition of Ni and Cu, the results of steel 17 are verified.
Steel 17 has about 0.81% of Ni and Cu added to steel 14. If hardenability is added, the effect of increasing the quenching depth of steel 17 relative to steel 14 is
(0.80 + 0.82) × 0.458 × 10.06 ＝ 7.46mm
It can be said that. Here, when 7.46 mm is added to 60.5 mm of steel 14, it becomes 67.96 mm, which is almost the same as 67.8 mm of steel 17.
From the above, it was confirmed that the effect of Ni and Cu is 0.458 times that of Mn. The range of Mn is 0.8 to 2.4%.
In summary, the effect of Ni and Cu is 0.233 times Mn in the range of 0.10 to 0.30% Mn, 0.414 times Mn in the range of 0.30 to 0.90% Mn, 0.353 times Mn in the range of 0.27 to 0.81% Mn, In the range of 0.81 to 2.43% Mn, it is 0.458 times Mn.
Considering the above findings on average, it can be concluded that the effect of Ni and Cu is 0.36 times that of Mn in the range of 0.1 to 2.4% Mn.

Here, when the quenching depth is shown with respect to Mn + Ni + Cu (see FIG. 8), a positive correlation is observed, but the variation is large, and it can be seen that the effects of Ni and Cu cannot be accurately evaluated. On the other hand, when the quenching depth is shown with respect to Mn + 0.36 (Ni + Cu), the variation is small and a very clear positive correlation can be confirmed (see FIG. 9). That is, the effects of Ni and Cu are accurately evaluated.
From the above, the validity of the effect of Ni and Cu for improving hardenability was confirmed to be 0.36 times that of Mn. The Mn + 0.36 (Ni + Cu) of the steel used for the study is 0.10 to 2.43, which is almost consistent with the range of practical steel. Therefore, in the steel according to this embodiment,
Y = Mn + 0.36 (Ni + Cu), and the parameter Y range was 0.10 to 2.40.

  Using the parameters obtained in this way, a steel component system with good hardenability was studied. Table 2 shows the components of the steel materials used for the evaluation. Elements not shown were 0.01% or less. Steel used in die casting molds has a C content of 0.3-0.5%, a Si content of 0.3-1.2%, a Mo content of 0.8-1.5%, and a V content of 0.3-1.2%. , C amount is about 0.40%, Si amount is about 0.60%, Mo amount is about 1.2%, V amount is about 0.6%. With reference to the steel used for the die casting mold, the parameters Y = 0.1 to 1.4% and Cr = 0.3 to 5.3% were set.

The CCT (continuous cooling transformation) diagram was created for these 25 steel types, and the results of examining the critical cooling rate are shown in FIG. Steels with a critical cooling rate of 60 ° C / min or less were evaluated as “◯ (passed)”, and steels with a critical cooling rate exceeding 60 ° C / min were evaluated as “x (failed)”. It has been found that the hardenability is improved by increasing Cr and parameter Y, and the region where the critical cooling rate is 60 ° C./min or less can be approximated by equation (1A).
Cr ≧ 4.963−3.43Y [%] (1A)

Subsequently, the range in which the thermal conductivity was 28 W / m / K or higher was examined. Here, the thermal conductivity is generally correlated with a component that decreases as the amount of alloy element added increases. Therefore, the thermal conductivity tends to decrease as the amount of Cr, Mn, Ni and Cu increases. Therefore, as in the case of hardenability, the thermal conductivity was evaluated using Cr and parameter Y.
Table 3 shows the components of the steel materials used in the investigation. Elements not shown were 0.01% or less. The amount of C in steel used for die casting molds is 0.3-0.5%, the amount of Mo is 0.8-1.5%, and the amount of V is often 0.3-1.2%. Accordingly, the amount of C is about 0.40%, The amount of Mo was unified at about 1.2% and the amount of V was unified at about 0.6%. On the other hand, the amount of Si was 0.2% or less. This is because, as described above, low Si is effective when priority is given to thermal conductivity. Further, the parameters Y were set in the range of 0.1 to 2.4% and Cr = 0.29 to 14.5%.

FIG. 11 shows the results of investigating the thermal conductivity of these 25 steel types. Assuming a situation where it is actually used as a mold, the specimens were tempered to 43-46HRC by quenching and tempering. A laser flash method was used to measure the thermal conductivity, and the measurement temperature was 25 ° C. In the figure, steels with a thermal conductivity of 28 W / m / K or higher were marked with “◯ (pass)”, and those with no thermal conductivity were marked with “x (failed)”.
The tendency of thermal conductivity to decrease with increasing Cr and parameter Y is remarkable, and the validity of the evaluation method was confirmed. And it turned out that the area | region where heat conductivity is 28 W / m / K or more can be approximated by Formula (1B).
Cr ≦ 14.597−5.97Y [%] Formula (1B)

  Here, if the suitable range of Mn is shown, it is 0.2 to 1.8%. If the amount of Mn is less than 0.2%, a large amount of expensive rare elements such as Ni, Cu, and Cr must be added to ensure hardenability, which greatly increases the cost of the material. From the viewpoint of material cost and hardenability, it is desirable to actively use inexpensive Mn. On the other hand, if Mn exceeds 1.8%, the annealing property may be adversely affected. Annealing is a process in which the mixed structure of the austenite phase and residual carbide is slowly cooled, the carbide is enlarged in the process, and at the same time, the matrix phase is transformed into a ferrite phase, and the structure after cooling is softened. If the material is not soft, it is very difficult to perform a cutting process that gives the shape of the mold. Steel that softens to a certain range of hardness by annealing is called “good annealing” and is preferred from the viewpoint of cutting. Here, since the annealability is contrary to the hardenability, if a hardenability improving element such as Mn is positively added, the annealability often deteriorates. For these reasons, the upper limit of Mn is set to 1.8%. Of course, if annealing is not a problem, the amount of Mn may be increased to 2.4% which is the upper limit of the parameter Y.

  A preferable range of Ni is 0.3% or less if the material cost is important, and 0.3 to 1.2% if the ductility of the steel material is important. The ductility of a steel material whose hardness is adjusted by changing the tempering condition is usually monotonously increased with respect to the decrease in hardness. However, in steel materials of a specific component system, the ductility may show a minimum value with respect to the hardness. Since the fatigue strength of steel may show a positive correlation with ductility, it is desirable to avoid a decrease in ductility. Appropriate addition of Ni (0.3 to 1.2%) is effective for avoiding the decrease in ductility. When the added amount exceeds 1.2%, the effect of ensuring ductility tends to saturate and the profit is poor. In addition, 0.3 to 1.2% is preferable from the viewpoint of improving hardenability, but as described above, inexpensive Mn should be actively used for such purposes.

The preferred range of Cu is 0.3% or less if the material cost and hot workability are important, and 0.3 to 1.0% if the hardening characteristics and sizing characteristics of the steel are important. The material cost is the same as Ni. Cu has the property of concentrating on the austenite grain boundaries when heated at high temperatures. Cu has a low melting point of about 1080 ° C. For this reason, when hot working such as forging or rolling is performed in the austenite region at 1100 ° C. or higher, the Cu-containing steel is easily cracked from the austenite grain boundary in the molten state. When hot working is reduced to 1050 ° C or lower, cracking due to Cu grain boundary enrichment is improved, but the deformation resistance of the material increases, making it difficult to form, reducing the production efficiency by lengthening the machining process. Yes (increased material costs).
On the other hand, Cu precipitates during tempering to increase the strength of the steel material. For this reason, when obtaining predetermined | prescribed hardness, Cu containing steel can reduce C amount rather than Cu non-containing steel. As a result, since the carbide is reduced, the hardness of the annealed material is reduced, and the machinability is improved. The tempered Cu-free steel mold secures hardness by decomposition of retained austenite and precipitation of carbides, but the dimensions change due to structural changes at that time. For this reason, in the worst case, the final shape of the mold may not be secured. Such a problem is unlikely to occur in Cu-containing steels whose dimensions do not change even after tempering. Therefore, Cu addition is effective from the viewpoint of machinability, tempering hardness, and mold dimensions of the annealed material, and a preferable range of the Cu content is 0.3 to 1.0%. When the added amount exceeds 1.0%, the above effect tends to saturate and the profit is poor. In addition, from the viewpoint of improving hardenability, 0.3 to 1.0% is preferable, but as described above, inexpensive Mn should be actively used for such purposes.

From the above investigation results, in the present embodiment, the component range excellent in the balance between hardenability and thermal conductivity is:
4.963−3.43Y ≦ Cr ≦ 14.597−5.97Y [%] (Y = 0.10 to 2.40 [%], Y = Mn + 0.36 (Ni + Cu) [%])) Equation (1)
It was set as the range which satisfy | fills.

A comparison between the steel according to the present embodiment and the conventional steel is as shown in FIG. It can be confirmed that the steel according to the present embodiment does not completely overlap with any of the conventional steels, and has selected a region excellent in the balance between hardenability and thermal conductivity. If the high Si side is selected, machinability can be secured.
The component range of the steel according to this embodiment shown in FIG. 12 is relatively close to JIS SNCM439 and JIS SNCM630, although there is a difference in the upper and lower limits of the Si amount.
However, the steel according to the present embodiment is greatly different in softening resistance from JIS SNCM439 and JIS SNCM630. JIS SNCM630 has higher softening resistance than JIS SNCM439, which is due to containing 0.5% Mo. On the other hand, JIS SNCM630 does not contain V. For this reason, JIS SNCM630 is inferior in softening resistance to the steel according to this embodiment containing V even if the Mo amount is the same (0.5%) as that of the steel according to this embodiment.
Even if the Mo amount is 0.3%, the steel according to the present embodiment contains V, and therefore has better softening resistance than JIS SNCM630.
As described above, the steel according to the present embodiment has an excellent balance of hardenability, thermal conductivity, and softening resistance. If the high Si side is selected, machinability can be secured.

(6) W: 0.01 to 4.00%.
W as a selective additive element forms carbides and strengthens the steel by dispersion strengthening. Like Mo and V, it forms carbides that are difficult to agglomerate, so it has a great effect of increasing softening resistance. However, excessive addition leads to saturation of characteristics and an increase in material cost, and is not profitable. From the balance between the softening resistance and the manufacturing cost, the amount of W in this embodiment is 0.01% to 4.00% in mass%.

(7) Co: 0.01 to 2.00%.
Co as a selective additive element dissolves in the ground iron and increases the high-temperature strength. On the other hand, excessive addition only causes saturation of high temperature strength and an increase in production cost. From the balance between the high temperature strength and the manufacturing cost, the Co amount in this embodiment is set to 0.01% to 2.00% by mass.

(8) Nb: 0.002 to 0.500%,
Ta: 0.002 to 0.500%,
Ti: 0.002 to 0.500% and
Zr: One or more selected from the group consisting of 0.002 to 0.500%.
Nb as a selective additive element forms NbC, NbN, or the like, and prevents coarsening of austenite crystal grains during quenching. However, excessive addition causes saturation of characteristics and an increase in material cost. Moreover, NbC and NbN crystallized relatively coarsely reduce the fatigue strength and impact value when the steel according to the present embodiment is a mold. The Nb content in the present invention is set to 0.002% to 0.500% in mass% from the balance between the effect of preventing the coarsening of crystal grains, the manufacturing cost, the fatigue strength, and the impact value.
Ta as a selective additive element has the same effect as Nb. For the same reason as in the case of Nb, the amount of Ta in the present invention is set to 0.002% to 0.500% in mass%.
Ti as a selective additive element has the same effect as Nb. For the same reason as in the case of Nb, the amount of Ti in the present invention is 0.002% to 0.500% by mass%.
Zr as a selective additive element has the same effect as Nb. For the same reason as in the case of Nb, the amount of Zr in the present invention is set to 0.002% to 0.500% by mass%.

(9) Al: 0.005 to 1.500%, and
N: One or more selected from the group consisting of 0.005 to 0.300%.
Al is an element that forms AlN and prevents coarsening of crystal grains during quenching. However, excessive addition not only leads to saturation of characteristics and an increase in manufacturing cost, but also increases alumina as an inclusion, and the impact value when the steel according to the present embodiment becomes a tool or a mold is reduced. Decrease greatly. In the present embodiment, the mass% is set to 0.005% to 1.500% from the balance between the effect of preventing the coarsening of crystal grains and the impact value. A preferred range is from 0.010% to 0.800%.
N is an element that forms AlN and prevents coarsening of crystal grains during quenching. Moreover, it dissolves in the matrix and increases the strength of the steel. From the viewpoint of crystal grain size and strength, it is desirable that N is large. However, a special melting process is required for remarkably high nitrogen, which increases material costs. Furthermore, excessive addition only causes saturation of characteristics, and is not profitable. In the present embodiment, the mass% is set to 0.005% to 0.300% from the balance between the effect of preventing the coarsening of crystal grains and the manufacturing cost. A preferred range is 0.010% to 0.200%.

(10) B: 0.0002 to 0.0200%.
B as a selective additive element is an element that segregates at the austenite grain boundary and suppresses precipitation of the ferrite phase, and remarkably enhances the hardenability of the steel. However, excessive addition leads to saturation of characteristics and poor profit. In the present embodiment, the range in which a sufficient hardenability improving effect is obtained in practice is 0.0002% to 0.0200% by mass%.

(11) S: 0.005 to 2.000%,
Ca: 0.0005 to 0.5000%,
Se: 0.005-0.500%,
Te: 0.005-0.500%,
Bi: 0.005-0.500%, and
Pb: One or more selected from the group consisting of 0.005 to 0.500%.
S as a selective additive element is an element that forms MnS and improves the machinability of steel. However, excessive addition causes saturation of characteristics and an increase in manufacturing cost. Further, MnS crystallized relatively coarsely greatly reduces the impact value when the steel according to the present embodiment becomes a tool or a mold. In the present embodiment, the mass% is set to 0.005 to 2.000% from the balance of the effect of improving the machinability, the manufacturing cost, the material, and the like.
Ca as a selective additive element is an element that improves machinability. It is also an element that improves the hot workability of steel by changing the form of non-metallic inclusions in the steel. However, excessive addition causes saturation of characteristics and an increase in manufacturing cost. In this embodiment, the mass% is set to 0.0005% to 0.5000% from the balance of the effect of improving the machinability and hot workability, the manufacturing cost, the material, and the like.
Se as a selective additive element is an element that improves machinability. However, excessive addition results in a large amount of Se compound, which decreases the hot workability and impact value of steel. In this embodiment, the mass% is set to 0.005% to 0.500% from the balance of the effect of improving the machinability, the manufacturing cost, the material, and the like.
Te as a selective additive element is an element that improves machinability. However, excessive addition results in a large amount of Te compound, which reduces the hot workability and impact value of the steel. In this embodiment, the mass% is set to 0.005% to 0.500% from the balance of the effect of improving the machinability, the manufacturing cost, the material, and the like.
Bi as a selective additive element is an element that improves machinability. However, excessive addition increases the number of Bi particles dispersed in the steel, reducing the hot workability and impact value of the steel. In this embodiment, the mass% is set to 0.005% to 0.500% from the balance of the effect of improving the machinability, the manufacturing cost, the material, and the like.
Pb as a selective additive element is an element that improves machinability. However, excessive addition increases the amount of Pb particles dispersed in the steel, reducing the hot workability and impact value of the steel. In this embodiment, the mass% is set to 0.005% to 0.500% from the balance of the effect of improving the machinability, the manufacturing cost, the material, and the like.

Inventive steel and comparative steel were prepared, and die casting was actually used to evaluate the hardenability, thermal conductivity, and die life (mechanical fatigue / thermal fatigue).
(Component composition of steel)
Tables 4 to 7 show chemical components of the steel materials used for the evaluation. Tables 4 and 5 are for invention steels, and Table 5 is a continuation of Table 4. Tables 6 and 7 are for the comparative steel, and Table 7 is a continuation of Table 6. Comparative Example 13 is JIS SKD61, Comparative Example 14 is JIS SKD4, Comparative Example 15 is JIS SKD7, and Comparative Example 16 is JIS SCM440. Moreover, "..." in a table | surface shows that it was not adding positively but was an impurity level.

Inventive steels 1 to 3 are levels for evaluating the influence of C. Invention steels 11 to 13 are levels for evaluating the influence of Si in combination with invention steel 2. Invention steels 21 to 29 and invention steels 2A to 2D are levels for evaluating the influence of parameter Y (Mn, Ni, Cu) together with invention steel 2. Inventive steels 31 and 32 are levels for evaluating the influence of Cr together with inventive steel 2. Inventive steels 41 to 44 are levels for evaluating the influence of Mo together with inventive steel 2. Invention steels 51 and 52 are levels for evaluating the influence of V in combination with invention steel 2.
Further, the inventive steels 61 to 63 are at a level for evaluating the influence of W together with the inventive steel 2. Inventive steels 71 to 73 are levels for evaluating the influence of Co together with inventive steel 2. Invention steels 81 to 86 are levels for evaluating the influence of Nb and the like in combination with invention steel 2. Inventive steels 91 to 93 are levels for evaluating the influence of Al and N in combination with Inventive steel 2. Inventive steels A1 to A3 are levels for evaluating the influence of B together with inventive steel 2. Invention steels B1 to B6 are levels for evaluating the influence of S and Bi together with invention steel 2.
On the other hand, in Comparative Steels 1 to 12, the amount of the specific element is excessive and excessive with respect to the specifications of the invention steel. Comparative steels 13 to 15 are general-purpose JIS steels used for die casting. The comparative steel 16 is JIS SCM440.

(Material cost)
The cost of the alloying elements was calculated for these 67 steel grades, and the material cost was evaluated as pass (○) if it was equal to or less than JIS SKD61, and reject (x) if not. Table 8 shows the evaluation results.

(Hardenability-critical cooling rate)
The 67 steel types shown in Tables 4 to 7 were each cast into a 3.6 ton ingot, subjected to homogenization heat treatment at 1250 to 1300 ° C., and then subjected to hot forging and hot rolling to obtain a round bar having a diameter of 82 mm. Subsequently, the round bar was softened by annealing at 700 ° C. × 24 Hr.
A test piece was cut out from the center of this round bar, cooled at various rates after soaking at 1030 ° C. for 60 min, the phase transformation behavior was investigated, and the critical cooling rate was evaluated. The meaning of the critical cooling rate is as described above. If the critical cooling rate was 60 ° C / min or less, it was judged as pass (◯), otherwise it was rejected (x), and the presence or absence of hardenability that could be used as a mold was evaluated. Table 8 shows the evaluation results.

(With or without strength that can be used as a mold)
In addition, after tempering the above-mentioned quenched specimen under the condition of 550 ° C. × 2 Hr, the hardness at room temperature is measured. If it is 43 HRC or higher, it is accepted (◯), otherwise it is rejected (×), and gold It was evaluated whether it was strong enough to be used as a mold. Table 8 shows the evaluation results.

(Thermal conductivity)
Furthermore, the test piece cut out from the center of the round bar was quenched at 100 ° C / min after soaking at 1030 ° C for 60min, and then tempered to 40-43HRC, and the thermal conductivity at 20 ° C was 28W. If it is more than / m / K, the thermal conductivity was evaluated as acceptable (◯), otherwise it was rejected (×). Table 8 shows the evaluation results.

(Discussion-Steel)
Invented steel is “◯” in all items, and it was found that it was a sound material with no problems in terms of basic characteristics.
However, when Invention Steel 2 (0.61V), Invention Steel 51 (0.12V) and Invention Steel 52 (0.93V) were compared, the superiority of Invention Steel 2 (0.61V) was confirmed from the viewpoint of structure. Invented steel 52 with a large amount of V was found to have larger carbides (the main components were V and C) than the inventive steels 02 and 51. The ingot is as large as 3.6 tons. Carbides are not so large as to affect fatigue strength, and there are no practical problems, but caution is required when the ingot is larger. Further, the inventive steel 51 with less V tended to have a larger austenite grain size than the inventive steels 02 and 52. The reason is thought to be that there are few fine VCs that suppress the movement of crystal grain boundaries. However, there is no practical problem because the crystal grain size is not so large that the impact value is drastically reduced and is not a mixed grain structure.
On the other hand, the comparative steel has “x” in at least one item, which indicates that it has some problems.
From the above, the basic performance of the steel material was confirmed.

Next, actual type verification will be described.
(Mold making from round bar)
A mold having a diameter of 80 mm and a length of 110 mm was produced from a round bar having a diameter of 82 mm. However, in Tables 4 to 7, steel materials that had problems with material cost and hardness were excluded because they were unlikely to be put to practical use. Specifically, comparative steel 1 that does not achieve 43HRC, comparative steel 3, comparative steel 5, comparative steel 10, comparative steel 14 and high steel 14 with high raw material costs. Eight types of comparative steel 7 and comparative steel 15-16, which do not enter, were excluded.

  A mold having a diameter of 80 mm and a length of 110 mm was quenched after soaking at 1030 ° C. for 60 minutes. The quenching method is to immerse in 150 ° C. oil to which forced convection is applied. After quenching, it was tempered to 43HRC by tempering.

(Casting using mold)
These molds were incorporated in the vicinity of the gate where heat fatigue is likely to occur even in the die-cast mold. The pouring gate means an inlet through which the molten metal flows into the cavity, and since the molten metal is in contact with the hot molten metal for a long time, the temperature rise of the mold becomes obvious, and thus thermal fatigue becomes remarkable.
The casting conditions are as follows: the hot water material is ADC12, the hot water temperature (in the melting and holding furnace) is 680 ° C., and the cycle time is about 60 seconds. After 10000 shot casting, thermal fatigue characteristics were judged as acceptable (◯) if no large crack was found in the insert, otherwise rejected (x).
Then, after casting 10,000 shots, the vicinity of the surface of the insert was cut out, the hardness was investigated, and the superiority or inferiority of the softening resistance was determined.
Table 9 shows the results of the casting test.

(Discussion-actual type verification)
According to the results of the casting test shown in Table 9, all of the molds except the comparative steel 2 and the comparative steel 12 were able to finish the casting of 10000 shots. The inventive steel had slight cracks due to thermal fatigue. The reason for this is that the thermal stress generated is low due to the high thermal conductivity. In addition, since the inventive steel has high softening resistance and almost no decrease in hardness (sag), it can be determined that thermal fatigue has been reduced by maintaining the yield strength (see FIG. 2). It was judged that the damage caused by casting of 10,000 shots was minor and could be used further.

  On the other hand, all the comparative steels were markedly cracked by thermal fatigue, and it was determined that further use was impossible. The reason for the severe damage is due to the high thermal stress generated due to the low thermal conductivity. Moreover, since the comparative steel 9, the comparative steel 11, and the comparative steel 13 have a large decrease in hardness (sagging), it can be determined that thermal fatigue was promoted by a decrease in yield strength (see FIG. 2).

From the above,
(A) Critical cooling rate of 60 ° C / min or less,
(B) Thermal conductivity is 28 W / m / K or more,
(C) It was confirmed that the thermal fatigue of the mold can be reduced by applying steel satisfying the basic characteristics of Mo amount of 0.3% or more and V amount of 0.03% or more.

Initially, 10000 shots were planned for casting, but the molds of comparative steel 2 and comparative steel 12 were cracked in about 2000 shots, and the test was stopped. Coarse carbides mainly composed of V were confirmed at the crack starting point. The reason for the extremely short die life is that coarse carbides have become the starting point for mechanical fatigue cracks.
A feature of the comparative steel 2 and the comparative steel 12 is that C and V exceed the upper limit of the invented steel. Therefore, a large amount of coarse carbides mainly composed of V was generated. From the viewpoint of mechanical fatigue properties, the upper limit of C and V is set in the invention steel, but its validity has been confirmed. As described above, large carbides were found in Invention Steel 52, but not as large as Comparative Steel 12. For this reason, it can be said that the invention steel 52 did not cause the problem of mechanical fatigue.

  Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment. The present invention can be variously modified and applied without departing from the spirit of the present invention.

  The steel according to the present invention, the mold steel, and the mold using the same can be applied to molds and molding members such as plastic or rubber injection molding, die casting, and hot forging. In particular, it can be applied to molds and molding members that are used under conditions where the amplitude of the surface temperature is large. Steel according to the present invention, steel for molds and molds using the same are manufactured with high impact value, reduced thermal stress, and high resistance to softening, and use or manufacture molds and molding members. In the industry, cost reduction, resource saving and environmental load reduction can be realized, which is extremely beneficial to the industry.

It is a CCT diagram of JIS SKD61. It is a graph which shows the correlation of softening resistance and thermal fatigue. It is a graph which shows the quenching speed dependence of the impact value of JIS SKD61. It is a graph which shows the correlation of the size of a metal mold | die, and quenching speed. It is a graph which shows the correlation of thermal conductivity and thermal stress. It is a graph which shows the correlation of Mo amount and softening resistance. It is a graph which shows the correlation of V amount and an impact value. It is a graph which shows the relationship between a quenching depth and an element (Mn + Ni + Cu) amount. It is a graph which shows the relationship between a quenching depth and the amount of elements (Mn + 0.36 (Ni + Cu)). It is a graph which shows the relationship between the amount of Cr and the amount of elements (Mn + 0.36 (Ni + Cu)) which shows the boundary where a critical cooling rate becomes 60 degrees C / min. It is a graph which shows the relationship between the amount of Cr and the amount of elements (Mn + 0.36 (Ni + Cu)) which shows the boundary from which thermal conductivity is 28 W / m / K. It is the graph which compared and showed the component range (Cr amount and Si amount with respect to the amount of elements (Mn + 0.36 (Ni + Cu)) of invention steel and JIS steel).

Claims (9)

% By mass
C: 0.15-0.55%
Si: 0.02 to 0.60%,
Mo: 0.30 to 4.00% and
V: 0.03 to 1.00% contained,
A steel characterized in that the balance containing Cr, Mn, Ni, and Cu satisfying the following formula (1) in mass% is made of Fe and inevitable impurities.
4.963−3.43Y ≦ Cr ≦ 14.597−5.97Y [%] (Y = Mn + 0.36 (Ni + Cu) [%], 0.10 ≦ Y ≦ 2.40 [%]) (1) Furthermore, in mass%,
The steel according to claim 1, containing W: 0.01 to 4.00%. Furthermore, in mass%,
Co: 0.01 to 2.00% of steel, The steel of Claim 1 or 2 characterized by the above-mentioned. Furthermore, in mass%,
Nb: 0.002 to 0.500%,
Ta: 0.002 to 0.500%,
Ti: 0.002 to 0.500% and
The steel according to any one of claims 1 to 3, comprising one or more selected from the group consisting of Zr: 0.002 to 0.500%. Furthermore, in mass%,
Al: 0.005 to 1.500% and
N: The steel according to any one of claims 1 to 4, comprising one or more selected from the group consisting of 0.005 to 0.300%. Furthermore, in mass%,
B: The steel according to any one of claims 1 to 5, containing 0.0002 to 0.0200%. Furthermore, in mass%,
S: 0.005 to 2.000%,
Ca: 0.0005 to 0.5000%,
Se: 0.005-0.500%,
Te: 0.005-0.500%,
Bi: 0.005-0.500%, and
The steel according to any one of claims 1 to 6, comprising one or more selected from the group consisting of Pb: 0.005 to 0.500%.   Mold steel comprising the steel according to any one of claims 1 to 7.   A mold made of the mold steel according to claim 8.

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