JP2009242819A - Steel, steel for die and die using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel capable of improving hardenability and securing annealability, a steel for a die and a die using the same. <P>SOLUTION: The steel comprises, by mass, 0.02-2.50% Si, 0.20-4.00% Mo, 0.15-0.55% C, V, Cr, Mn, Ni, Cu and the balance being Fe and unavoidable impurities, provided that the amounts of C, V, Cr, Mn, Ni and Cu satisfy the relation (1): 0.729-1.01C≤V≤1.718-1.65C [%] and relation (2): 7.382-2.79X≤Cr≤10.395-3.69X [%] (wherein X=Mn+0.38(Ni+Cu) [%]; and 0.30≤X≤2.10 [%]). 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, mold steel and a mold using the same, and more specifically, quenching of steel used for molds and molding members such as injection molding, die casting and hot forging of plastics and rubber. The technology relates to the improvement of heat resistance and ensuring annealing.

  Molds and molding members 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 and inserts, and tools such as plates and sleeves used for forging, die casting, low pressure casting, and injection molding (hereinafter, these are also simply referred to as “molds”). 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.
As these solutions,
(1) Extension of mold life,
(2) Reduction of finishing man-hours and
(3) Ensuring machinability,
As these concrete methods,
(A) High impact value of steel material,
(B) softening of the annealed material, and
(C) Warpage reduction during quenching can be mentioned (see Table 1).

Costs can be reduced, resources can be saved, and the environmental burden can be reduced by extending the mold life as described in (1) above. However, the cost can be reduced because the number of products manufactured per mold increases and the product unit price decreases. At the same time, the production interruption time required for mold replacement is shortened, and the number of products produced per hour increases. Resource saving is possible because the number of mold exchanges is reduced, the number of molds produced is reduced, and the amount of rare elements used is reduced. Furthermore, the environmental load can be reduced because the number of mold exchanges is reduced, the number of molds produced is reduced, the melting process and the heat treatment process are reduced, and the amount of greenhouse gas emissions is reduced.
In order to extend the mold life, it is effective to increase the impact value of the steel material (a) (see Table 1). This is because the mold life generally has a good correlation with the impact value, and a mold with a high impact value tends to have a long life. By the way, there are so many manufacturers that specify the impact value of the mold itself and the mold steel when ordering and receiving the mold.

  The cost can be reduced by securing the machinability of (2) above. If machinability is ensured, rough machining (cutting before quenching with a rough shape as a mold) becomes easy and production is possible. This is because efficiency is improved and replacement of expensive cutting tools is reduced, so that the manufacturing cost and product cost of the mold are reduced. In order to ensure machinability, it is effective to soften the annealed material (b) (see Table 1). In many cases, the steel subjected to roughing is annealed (that is, the mixed structure of the austenite phase and residual carbides is slowly cooled, the carbides are enlarged in the process, and at the same time, the matrix phase is transformed into the ferrite phase, cooling This is because the subsequent tissue is softened (annealed). The hardness after annealing is closely related to the machinability, and the softer the tendency is, the higher the machinability is. Therefore, from the viewpoint of machinability, a steel material that becomes soft after annealing is desired.

  Cost reduction can be achieved by reducing the number of finishing processes in (3) above, because the cost can be reduced if the number of finishing processes (cutting after quenching to give the final shape as a mold) is small. is there. And the reduction of the curvature at the time of hardening of the above-mentioned (c) is effective for reduction of the number of finishing processes (refer to Table 1). This is because the finishing process is a cutting process after quenching, so that if the “warpage” caused by quenching can be reduced, the finishing process can also be reduced, and cost reduction can be achieved by reducing the number of finishing processes. By the way, as shown in Fig. 1, when the warp is large, if the finish finish is small, the hardened skin cannot be completely removed, and the die-cast product's cast skin will be roughened. Cannot be used for die casting. Therefore, in a mold that is likely to be warped, it is necessary to make the finishing process sufficiently large, but this causes an increase in cost due to an increase in the number of finishing processes. In the same figure, a simple plate is illustrated, but in a complex-shaped mold, the warping direction differs depending on the part, and thus it is more difficult to ensure the final shape.

In order to increase the impact value of the steel material (a), “rapid quenching (quenching is a process of transforming a high-temperature austenite phase into a structure mainly composed of martensite in the process of cooling to a room temperature. Is effective). This is because the phase can be transformed at a low temperature by "rapid quenching", and the quenching material becomes finer as the structure transformed at a lower temperature, and the impact value becomes higher as the microstructure becomes finer.
On the other hand, “slow quenching” is effective in reducing the warpage during quenching (c). This is because “slow quenching” can reduce the temperature difference between the surface and the inside of the mold, reduce the stress generated during quenching, and reduce warpage. Usually, the coolant used for quenching the mold is oil of about 100 ° C, but by using this at room temperature air, the quenching speed can be reduced, resulting in “slow quenching”.
Therefore, the specific methods to achieve “high impact value of steel” and “reducing warpage during quenching” are “rapid quenching” and “slow quenching”, respectively. Can not do it.

JP 2006-104519 A

  In recent years, in the mold industry, including die casting, “reducing the number of parts” and “simplifying processes” for the purpose of cost reduction due to “responding to larger size” of molds are accelerating. As a result, the individual parts tend to be integrated and enlarged, and therefore the mold is also enlarged. The larger the mold is, the lower the temperature difference between the surface and the interior of the mold becomes as small as possible, and the cooling rate during quenching becomes slower, resulting in slower quenching. As a result, the steel material undergoes phase transformation at a high temperature, the structure becomes coarse, and the impact value of the steel material becomes small. Therefore, the metal mold having a large cross section has a problem that the life is particularly likely to be shortened, and a steel material capable of extending the mold life by increasing the impact value is demanded by the industry even for a metal having a large cross section.

  However, in the case of a large cross-section mold, if an attempt is made to achieve a high impact value by rapid quenching, warpage during quenching increases. This is because the larger the mold size, the greater the stress that acts, and the greater the warpage after quenching. Therefore, in the case of a large-section mold, it is necessary to enlarge the finishing process (see FIG. 1), and the cost increase due to an increase in the number of finishing processes tends to be a problem. Therefore, there is a problem that warpage tends to increase particularly with large cross-section molds, and even with large cross-section molds, there is a demand from the industry for steel materials that can reduce the number of finishing processes by reducing warpage after quenching. ing.

Thus, as a specific method for increasing the impact value of the steel material while reducing warpage during quenching, “improving hardenability” can be mentioned (see Table 1). This is because if the hardenability is good, a fine martensite structure can be obtained even when slow quenching is performed, and the impact value becomes high.
In order to improve “hardenability”, it is effective to make steel components more appropriate, and it can be said that Cr, Mn, Ni, and Cu should be positively added.

Further, in order to soften the annealed material (b), it is necessary to “ensure annealing properties”, that is, to sufficiently soften (become spheroidized annealed) by annealing under industrial conditions. . However, even if Cr, Mn, Ni, and Cu are added positively to improve the hardenability, the annealability is deteriorated.
Therefore, it is difficult to achieve both “improving hardenability” and “ensuring annealability”, and steels that achieve both of these, including the steel disclosed in Patent Document 1, have been conventionally known. Absent. In particular, it is required in the industry to “improve hardenability” and “ensure annealing” in a large-section die adapted to “enlargement correspondence” of the die.

  This invention is made | formed in view of the said situation, The objective is to provide the steel which can improve hardenability and an annealing property, steel for metal mold | die, and a metal mold using the same. is there. 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.

Steel according to the present invention is an essential constituent element,
% By mass
(1) Si: 0.02 to 2.50%,
(2) Mo: 0.20 to 4.00% and
(3) C: contains 0.15-0.55%, and
The gist is that the balance containing C, V, Cr, Mn, Ni, and Cu satisfying the following formula (1) to the following formula (2) in mass% is composed of Fe and inevitable impurities.
(4) 0.729-1.01C ≦ V ≦ 1.718-1.65C [%] ... Formula (1),
(5) 1.382−2.79X ≦ Cr ≦ 10.395−3.69X [%] (where X = Mn + 0.38 (Ni + Cu) [%], 0.30 ≦ X ≦ 2.10 [%])) Formula (2).

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, in mass%, Si: 0.02 to 2.50%, Mo: 0.20 to 4.00%, and C: 0.15 to 0.55%, and further, in mass%, the formulas (1) to ( 2) Since the balance containing C, V, Cr, Mn, Ni, and Cu satisfying 2) is made of Fe and inevitable impurities, it is possible to improve hardenability and secure annealing. .

  In particular, since the expression (1) is satisfied, the coarse VC can be reduced and the fine VC can be increased, and the impact value can be increased.

  Moreover, since Formula (2) is satisfy | filled, there exists an effect that a hardenability improvement and securing of annealing property are possible. Usually, since the annealability is deteriorated by improving the hardenability, the annealed state of the steel material having high hardenability is hardened. For this reason, machinability deteriorates and the productivity for processing into a metal mold | die becomes low. However, since the steel according to the present invention has the above-described configuration, it is possible to balance the contradicting factors of hardenability and spheroidizing annealing property, and despite improving the hardenability, the steel is equivalent to the conventional steel. Since the annealing property is secured, the hardness in the annealed state is the same as that of the conventional steel, and the productivity for processing into a mold does not decrease.

Therefore, if the steel according to the present invention is applied to a mold, it is possible to achieve a life extension by high toughness and a reduction in processing man-hour by low strain. As a result, there is an effect that cost reduction, resource saving, and environmental load reduction are realized.
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.
Furthermore, since the metal mold | die which concerns on this invention consists of the steel for metal mold | die concerning this invention, there exists an effect similar to this.
Furthermore, the mold according to the present invention is:
(1) In the case of a mold having a large cross section and a large weight (150 mm or more in terms of thickness, 100 kg or more in weight),
(2) Even in the case of molds quenched with equipment with low cooling capacity,
With high impact value and long enough mold life. This is because the mold steel according to the present invention has high hardenability, so that a fine structure mainly composed of martensite can be obtained even at a slow quenching speed.
In addition, since the mold according to the present invention does not require quenching as hard as conventional steel in quenching, the distortion of the mold is reduced, and the effect of reducing cost and achieving a single delivery time by reducing the number of finishing processes is achieved. is there.

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) Si: 0.02 to 2.50%.
Si is an essential constituent element that does not form carbides in steel and dissolves in the matrix. Since Si greatly improves the machinability of steel, it is desirable that the content is large from the viewpoint of reducing machining costs. On the other hand, if added excessively, it becomes easy to crack during hot working. From the balance between machinability and hot workability, the Si content was 0.02 to 2.50% by mass. A particularly suitable Si content range is 0.20 to 1.50%, which ensures machinability and is difficult to crack even during hot working.

(2) Mo: 0.20 to 4.00%.
Mo is an essential constituent element useful not only for improving hardenability but also 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, excessive addition reduces the impact value. From the balance between the softening resistance and the impact value, the Mo amount was 0.20 to 4.00% in mass%. A particularly preferable amount of Mo is 0.40 to 3.50%, which has high softening resistance and little reduction in impact value.

(3) 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%. In order to ensure the characteristics as a die material used for die casting, low pressure casting, hot forging, injection molding, etc., the C content was 0.15 to 0.55% by mass.

(4) 0.729-1.01C ≦ V ≦ 1.718-1.65C [%] Formula (1).
V is also an essential constituent element, but an appropriate amount of V is an amount that satisfies the formula (1) and varies depending on the amount of C. Hereinafter, the reason why the V amount is in a range satisfying the expression (1) will be described.
In order to provide a steel with a high impact value even at slow quenching speeds,
(1) The amount of coarse inclusions (in mold steel, it is often carbide mainly composed of V and C),
(2) crystal grain size (size of austenite crystal grains when quenched to 1020 to 140 ° C.), and
(3) quenching speed,
There are three items.
In order to increase the impact value, it is better that (1) is smaller, (2) is smaller, and (3) is faster. The aim of the present invention is to obtain a high impact value even if (3) is slow, but a high impact value cannot be expected unless (1) and (2) are optimized. Therefore, prior to the response to (3), the optimization of (1) and (2) will be described.

  Usually, the interface between steel and inclusions has no continuity of crystal structure, so that an initial crack is likely to occur when an external force is applied. The reason why the impact value decreases if coarse inclusions (diameter ≧ 10 μm in terms of a perfect circle) exist as inclusions is that large initial cracks (length ≧ 10 μm) are easily created at the interface with the base steel. This is because it occurs and grows into a long crack and propagates through the steel (see FIG. 2). Therefore, it is desirable that there are no coarse inclusions. Coarse inclusions that are problematic in mold steel are often carbides mainly composed of V and C. Hereinafter, the inclusion is described as VC (vanadium carbide).

Coarse VCs that become inclusions precipitate during casting (when the steel solidifies). When the amount of precipitation is not large, it is dissolved and rendered harmless in the homogenization heat treatment step. However, when the number is too large and the size is large, complete solid solution by the homogenization heat treatment is difficult and remains in a considerable proportion. This becomes a starting point of destruction and lowers the impact value.
The VC that precipitates during casting increases in number and size as the amount of V increases. Therefore, from the viewpoint of inclusions, it is desirable that the V amount be small.

The reason why the impact value decreases as the austenite crystal grain size at the time of quenching increases is that large cracks are likely to occur at the grain boundaries of martensite. The size of the crack is closely related to the martensite crystal grain size. If the crystal grain size is large, a large crack is likely to be formed with a single load (see Fig. 3), and penetration of the steel material due to their propagation is also likely. It will be easy.
The crystal grain size of martensite has a positive correlation with the crystal grain size of austenite (see FIG. 4). Since a plurality of martensite crystal grains are generated in one austenite crystal grain, the crystal grain diameter of martensite increases as the austenite crystal grain diameter increases. As a result, it becomes easy to produce a big crack (refer FIG. 3). Therefore, from the viewpoint of impact value, it is desirable that the austenite crystal grain size is small.

The factor that determines the austenite crystal grain size during quenching is the amount of fine VC (diameter in a circle) ≦ 1 μm. This is because fine VC has an action of suppressing the growth of austenite crystal grains, and the effect becomes greater as the amount of fine VC increases.
Grain growth always involves the movement of grain boundaries, but considerable energy is required for grain boundaries to move beyond one fine dispersed particle such as VC. Therefore, as more VC is present at the grain boundary, the energy required for the growth of the crystal grain becomes larger, so that the movement of the grain boundary becomes difficult, and as a result, the crystal grain is kept small (see FIG. 5).
The amount of fine VC that suppresses the movement of grain boundaries increases as the amount of V increases. That is, from the viewpoint of keeping the austenite crystal grain size small, it is desirable that the V amount be large.

From the above, in order to ensure a high impact value, it is only necessary to (1) reduce the amount of coarse VC that becomes inclusions, and (2) increase the amount of fine VC that suppresses the movement of crystal grain boundaries. In other words, (1 ′) the amount of V is small from the viewpoint of inclusions, and (2 ′) the amount of V is increased from the viewpoint of the austenite crystal grain size. Therefore, it can be seen that a high impact value cannot be secured even if V is too small or too large, and that balancing is important.
Therefore, the conditions for simultaneously reducing the coarse VC amount and increasing the fine VC amount will be described.

  First, a rough reduction in VC amount will be described. In the experiment, 18 steel types shown in Table 2 were used. The amount of Si used in die casting molds is 0.3-1.2%, Mn is 0.3-0.9%, Cr is 5.0-5.5%, and Mo is 0.8-1.5%. The amount of Si was unified at about 0.60%, the amount of Mn was about 0.45%, the amount of Cr was about 5.2%, and the amount of Mo was unified at about 1.0%. P and S etc. were the contents of the impurity level.

  As shown in Table 2, 18 steel grades with C content varied from 0.18 to 0.55% and V content from 0.69 to 1.45% were cast into a 6-ton ingot and hot-worked (forging and rolling) after homogenization heat treatment at 1250 ° C. To give a round bar with a diameter of 60 mm. A test piece was prepared from this round bar, soaked at 1030 ° C. for 30 min, quenched at a rate of 20 ° C./min, tempered to 45 HRC, and the impact value was investigated. The fracture surface after the test was observed, and the presence or absence of coarse VC (diameter when converted to a perfect circle ≧ 10 μm) was investigated.

FIG. 6 shows the survey results. The steel in which coarse VC was not observed was “◯ (passed)”, and the steel in which coarse VC could be confirmed was “× (failed)”. The impact value of the steel type in which coarse VC was recognized was 25 J / cm ² or less. As expected, the increase in C and V encourages the creation of coarse VCs. On the other hand, it was found that the region where coarse VC is difficult to generate can be approximated by the following equation (1A).
V <= 1.718-1.65C [%] ... Formula (1A).

  Next, miniaturization of austenite crystal grains by increasing the amount of fine VC will be described. In the experiment, 20 steel types shown in Table 3 were used. The amount of Si used in die casting molds is 0.3-1.2%, Mn is 0.3-0.9%, Cr is 5.0-5.5%, and Mo is 0.8-1.5%. The amount of Si was unified at about 0.60%, the amount of Mn was about 0.45%, the amount of Cr was about 5.2%, and the amount of Mo was unified at about 1.0%. P and S etc. were the contents of the impurity level.

As shown in Table 3, 20 steel grades with varying C content of 0.15-0.54% and V content of 0.16-0.61% were cast into a 6-ton ingot and hot-worked (forging and rolling) after homogenization heat treatment at 1250 ° C. To give a round bar with a diameter of 60 mm. A test piece was prepared from this round bar, soaked at 1030 ° C for 30 minutes and then quenched at a rate of 20 ° C / min. This was corroded to reveal a metal structure, and the austenite grain size before martensitic transformation was evaluated. .
In general, when the austenite crystal grain size exceeds 100 μm in terms of a perfect circle (when it is less than about 4 in terms of crystal grain size), the impact value decreases significantly. Therefore, a steel with a particle diameter of 100 μm or less when converted into a perfect circle is “O” (passed), and a steel with a particle diameter exceeding 100 μm when converted into a perfect circle is “× (failed)”. Is shown in FIG. It was observed that steel with finer crystal grains tended to have more VCs with a diameter of 0.1 to 1 μm.

From this, it can be seen that the decrease in C and V promotes the coarsening of crystal grains. The reason is that as described above, fine VCs having a diameter of 0.1 to 1 μm are reduced. On the other hand, it was found that the region in which the austenite crystal grain size is 100 μm or less in terms of a perfect circle can be approximated by the following equation (1B).
0.729-1.01C <= V [%] ... Formula (1B).

From the above investigation results, in the present embodiment, the component range of the C amount and the V amount in which the reduction of the coarse VC amount and the austenite crystal grain refinement (agreement with the increase of the fine VC) are simultaneously achieved are as follows:
C: 0.15-0.55%
0.729−1.01C ≦ V ≦ 1.718−1.65C [%] (1)
It was set as the range which satisfy | fills.

  A comparison of the specified range of this embodiment with JIS steel is shown in FIG. In the standard of conventional steel, a region where the coarse VC amount is excessive or a region where the fine VC amount is excessive is included in the standard range. Therefore, it is difficult to stabilize the characteristics of the steel material. In the present embodiment, since the range of the truly necessary V amount is determined according to the C amount, the characteristics of the steel material can be stabilized.

Furthermore, if the suitable range of V is shown from a viewpoint of stabilizing a characteristic, it is 0.35-1.0% by mass%. If it is less than 0.35%, there is an increased risk that the austenite structure at the time of quenching becomes mixed grains. The mixed grain refers to a structure in which coarse crystal grains and fine crystal grains are mixed, and is not desirable from the viewpoint of impact value. The mixed grain structure is likely to occur when the fine VC (see FIG. 5) that suppresses the movement of the crystal grain boundary is slightly less than the appropriate amount. This is because VC distribution is uneven and crystal grains grow in a region where VC is small.
On the other hand, if the amount of V exceeds 1.0%, coarse VC (see FIG. 2) tends to occur when cast with a large ingot. The reason for this is that the larger the ingot, the lower the solidification rate, and the slower the solidification rate (there is enough time to allow the elements to diffuse and coarsen inclusions), the larger the VC size. Considering casting with a large ingot of 10 tons or more, the preferable upper limit of V amount is 1.0%.

As described above, (1) the amount of coarse inclusions and (2) the crystal grain size have been described among the three items to be considered in order to increase the impact value.
Then, next, (3) Quenching speed and annealing properties will be described together with the reason why the Cr content is in a range satisfying the formula (2).

(5) 1.382−2.79X ≦ Cr ≦ 10.395−3.69X [%] (where X = Mn + 0.38 (Ni + Cu) [%], 0.30 ≦ X ≦ 2.10 [%])) Formula (2).
In order to obtain a high impact value even if the quenching speed is low, it is essential to improve the hardenability. Therefore, attention was focused on elements that improve hardenability. Cr, Mn, Ni, and Cu are essential as elements for improving hardenability. On the other hand, it is also an element that degrades annealing. In the present invention for optimizing the balance between hardenability and annealability, the importance of optimizing the range of Cr, Mn, Ni, and Cu is particularly high.
First, the range of these component elements will be described.

FIG. 9 is a CCT (continuous cooling transformation) diagram of JIS SKD61. This figure shows how many times the austenite phase at 1030 ° C. is transformed into bainite and martensite when 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.
Thus, the quenching speed that becomes the boundary between martensite and bainite is called “critical cooling speed”. That is, the critical cooling rate of JIS SKD61 is 12 ° C / min. Further, cooling at a rate higher than the critical cooling rate (that is, martensite) is generally expressed as “burning”.
The structure of steel has a close relationship with the temperature of phase transformation. In general, the finer the structure that has undergone phase transformation at a low temperature. Similarly, in the case of JIS SKD61, bainite obtained at around 400 ° C is a coarse structure, but martensite obtained at around 300 ° C has a very fine structure. It is also known that the tissue has a strong correlation with the impact value.

FIG. 10 shows the effect of quenching speed on the impact value of JIS SKD61. The hardness of the specimen is 45 HRC. When the quenching speed is less than 12 ° C./min, the impact value decreases rapidly, which is closely related to the phase transformation behavior of FIG. That is, when the cooling rate is higher than the critical cooling rate (in this case, 12 ° C./min), “baking” occurs and fine martensite is formed, but when cooling is slower than this, coarse bainite is formed. As can be seen from FIGS. 9 and 10, the critical cooling rate is a boundary condition in which the structure and the impact value change greatly.
Although the critical cooling rate varies depending on the steel type, the fact that the structure and impact value change greatly with this value as a boundary is a feature common to almost all steel types. For example, if the steel type has a critical cooling rate of 6 ° C / min, the impact value and the structure greatly change at 6 ° C / min.

Here, FIG. 11 shows the cooling rate when a JIS SKD61 mold used for die casting is quenched. As the mold thickness increases, the weight increases and it becomes difficult to cool. For this reason, a negative correlation is recognized between the mold thickness and the quenching speed.
In general, what is treated as a “large-section type” is a mold having a size that makes it very difficult to obtain a high impact value. Usually, it often refers to a mold having a thickness exceeding 150 mm, and a mold having a weight exceeding 100 kg.
In FIG. 11, paying attention to the critical cooling rate of 12 ° C./min of JIS SKD61, the thickness of the mold corresponding to this is about 110 mm. As shown in FIGS. 9 and 10, when the quenching speed becomes smaller than this, in JIS SKD61, “hardening” does not occur and the impact value decreases rapidly. That is, in the JIS SKD61 mold having a thickness exceeding 110 mm, there is a concern that the impact value may decrease.
Since a high impact value cannot be obtained, a mold with a thickness of more than 150 mm is treated as a “large cross-section type”, which can be said to be a reasonable judgment from the critical cooling rate of JIS SKD61. Further, the “large cross-sectional mold” can also be regarded as “a mold having a size that does not cause burning”.

Here, paying attention to a thickness of 150 mm or more, which is treated as a “large-section type”, the quenching speed of such a large mold is 6 ° C./min or less. Considering FIGS. 9 and 10, if a steel type having a critical cooling rate of 6 ° C./min or less is developed, a high impact value can be expected even for a mold having a thickness exceeding 150 mm. In particular, in recent years, the number of large cross-section types has increased, and the need for a steel type that can obtain a high impact value is increasing.
Therefore, in order to obtain a high impact value even with a large cross-section type, the target value for the development of steel grade was set as “critical cooling rate is 6 ° C / min or less”. This is because a steel with a low critical cooling rate can obtain martensite even at a lower quenching speed, that is, it can be said to be “steel with good hardenability” because “quenching” occurs even at a lower quenching speed.
Here, it supplements about the "impact value" used by the description so far. In order to investigate the impact value, JIS 3 test pieces were 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 an acceptance standard for molds. Therefore, also in this embodiment, the impact value is used as a scale for evaluation in accordance with such industrial practice. The following is the same as the impact value evaluation method and handling.

By the way, “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
(1) When the austenite phase becomes unstable, bainite transformation is likely to occur,
(2) This is because the bainite transformation involves 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 improving the hardenability is different between Mn, Ni, Cu and Cr.
Therefore, improvement of hardenability was examined by dividing into two groups of Mn, Ni, Cu and Cr.

The hardenability can be evaluated by the depth of the hardened layer of the hardened material. For example, when examining the hardness distribution of a water-quenched round bar, the deeper the hardened layer, that is, the steel material that is hardened to the inside of the round bar, it can be judged that the hardenability is better.
In addition, since inexpensive Mn is frequently used to stabilize the austenite phase, it was decided to evaluate the effect of Ni and Cu in terms of Mn.
Based on the above policy, an experiment was performed in which the effect of improving hardenability was quantified based on the depth of the hardened hardened layer of steel with Mn / Ni / Cu changed, and expressed in terms of Mn.

  Table 4 shows 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. A round bar with a diameter of 100 mm made of these 10 steel types was poured into stirring water and quenched, the hardness distribution of the cut cross section was measured, and the depth of the hardened and hardened region was defined as the “quenching depth” . Table 4 also shows the quenching depth. It can be judged that the hardenability is better as the quenching depth is larger. Below, the effect of improving the hardenability by stabilizing the austenite phase will be described based on the evaluation results for Mn / Ni / Cu.

First, two steel types that contain almost no Ni and Cu are compared. Comparing Steel 1 and Steel 5, the quenching depth per 1% of Mn is a ratio that increases by 6.0 mm. The range of Mn is 0.3-0.9%.
Next, the effect of Ni and Cu is calculated by comparing the steel type with Mn of 0.3% and Mn + Ni + Cu of 0.9% or less with Steel 1. When Steel 1 and Steel 2 are compared (both 0.3% Mn), the quenching depth is increased by 0.5 mm with an increase of about 0.3% of Ni. This is 0.08% when converted to Mn, which is “0.5 ÷ 6.0”. That is, the effect of adding Ni can be calculated as 0.27 times “0.08 ÷ 0.3” with respect to Mn.
Furthermore, when steel 1 and steel 3 are compared (both 0.3% Mn), the quenching depth is increased by 0.6 mm with an increase of about 0.3% of Cu. In terms of Mn, this is “0.6 ÷ 6.0” or 0.10%. That is, the effect of Cu addition can be calculated as 0.33 times “0.10 ÷ 0.3” with respect to Mn.
From the above, it was found that the effect of Ni and Cu is about 0.30 times (0.27 to 0.33 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 0.3% of Ni and Cu added to steel 1. The addition of 0.3% is estimated to be 0.09% of “0.3 × 0.30” in terms of Mn. If the effect of increasing the quenching depth is added, the addition of Ni and Cu is “0.3 + 0.09 + 0.09”, which corresponds to 0.48% Mn. That is, it is equivalent to increasing Mn of steel 1 by 0.18%.
When the Mn range was 0.3 to 0.9%, the quenching depth was increased by 6.0 mm per 1% of Mn. That is, when Mn is increased by 0.18%, the quenching depth is “0.18 × 6.0”, which is 1.08 mm larger. If 1.08mm is added to the quenching depth of Steel 1, it will be 24.18mm with "23.1 + 1.08", which is almost the same as 24.3mm, which is the actual result of Steel 4.
From the above, it was confirmed that the effect of Ni and Cu was 0.3 times that of Mn. The range of Mn is 0.3-0.9%. Next, the case where the amount of Mn / Ni / Cu added is large will be verified.

First, two steel types that contain almost no Ni and Cu are compared. When steel 6 and steel 10 are compared, the quenching depth per 1% of Mn is increased by 3.9 mm. The range of Mn is 0.7 to 2.1%.
Next, the effect of Ni and Cu is calculated by comparing steel grades with Mn 0.7% and Mn + Ni + Cu 2.1% or less with steel 6. Comparing Steel 6 and Steel 7 (both 0.7% Mn), the quenching depth is increased by 1.2mm with an increase of about 0.7% of Ni. In terms of Mn, this is “1.2 ÷ 3.9” or 0.31%. That is, the effect of adding Ni can be calculated as 0.44 times “0.31 ÷ 0.7” with respect to Mn.
Comparing Steel 6 and Steel 8 (both 0.7% Mn), the quenching depth is 1.3mm larger with about 0.7% increase in Cu. In terms of Mn, this is “1.3 ÷ 3.9” or 0.33%. That is, the effect of Cu addition can be calculated as 0.47 times “0.33 ÷ 0.7” with respect to Mn.
From the above, it was found that the effect of Ni and Cu is about 0.46 times (0.44 to 0.47 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 9 are verified.

Steel 9 has 0.7% of Ni and Cu added to steel 6. The addition of 0.7% is estimated to be 0.32% of “0.7 × 0.46” in terms of Mn. If the effect of increasing the quenching depth is added, the addition of Ni and Cu is “0.7 + 0.32 + 0.32,” which corresponds to 1.34% Mn. That is, it is equivalent to increasing Mn of steel 6 by 0.64%.
When the range of Mn was 0.7 to 2.1%, the quenching depth per 1% of Mn was a ratio that increased by 3.9 mm. That is, when Mn is increased by 0.64%, the quenching depth is “0.64 × 3.9”, which is 2.50 mm larger. When 2.50 mm is added to the quenching depth of steel 6, “25.8 + 2.50” is 28.30 mm, which is almost the same as 28.2 mm, the actual result of steel 9.
From the above, it was confirmed that the effect of Ni and Cu was 0.46 times that of Mn. The range of Mn is 0.7 to 2.1%. In summary, the effect of Ni and Cu is 0.30 times that of Mn in the range of 0.3 to 0.9% Mn, and 0.46 times that of Mn in the range of 0.7 to 2.1% Mn.
Considering the above findings on average, it can be concluded that the effect of Ni and Cu is 0.38 times that of Mn in the range of 0.3 to 2.1% Mn.

Here, when the quenching depth is shown with respect to Mn + Ni + Cu (see FIG. 12), a positive correlation is recognized, 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 + 0.38 (Ni + Cu), the variation is small and a very clear positive correlation can be confirmed (see FIG. 13). 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 0.38 times that of Mn. The Mn + 0.38 (Ni + Cu) of the steel used for the study is 0.31 to 2.11, which almost matches the range of practical steel.
Therefore, in the steel according to the present embodiment, X = Mn + 0.38 (Ni + Cu) and the range of the parameter X is 0.30 to 2.10.

  Using the parameters determined in this way, a steel component system with good hardenability was studied. Table 5 shows the components of the steel materials used for the evaluation. 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%. The parameters X = 0.31 to 2.00% and Cr = 1.50 to 8.03% were selected with reference to the steel used for the die casting mold. P and S etc. were the contents of the impurity level.

CCT (continuous cooling transformation) diagrams were created for these 19 steel types, and the results of investigating the critical cooling rate are shown in FIG. Steels with a critical cooling rate of 6 ° C / min or less were rated as “◯ (passed)”, and steels with a critical cooling rate exceeding 6 ° C / min were rated as “x (failed)”. It was found that the hardenability was improved by increasing Cr and parameter X, and the region where the critical cooling rate was 6 ° C./min or less could be approximated by equation (2A).
7.382−2.79X [%] ≦ Cr (2A)

As described above, Cr, Mn, Ni, and Cu may be positively added to produce a steel with good hardenability. On the other hand, the annealability deteriorates due to the positive addition of Cr, Mn, Ni, and Cu. Therefore, it is not easy to achieve both hardenability and annealability, and there is no such practical steel.
This time, the component system was examined from the viewpoint of annealing.

Annealing is a treatment in which the mixed structure of the austenite phase and residual carbide is slowly cooled, and 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. An annealed structure in which carbides have grown greatly is softer, which is preferable from the viewpoint of mold machining.
The general conditions for annealing are:
・ Heating temperature = 800-950 ° C
・ Cooling rate = 15 ℃ / Hr
・ End temperature of annealing = 600 ℃
-Cooling after completion = air cooling. The hardness after annealing is preferably 91HRB or less. This is because if the hardness exceeds this, it is difficult to machine the mold.

As described above, the characteristic of steel with good hardenability is that the diffusion of carbon dissolved in austenite hardly occurs. This feature is not desirable in annealing. This is because carbon diffusion is indispensable for the enlargement of carbides that occurs during annealing, and steel in which this is suppressed has small carbides and insufficient softening.
In addition, steel with good hardenability was also characterized by high austenite phase stability. This is also not preferable in annealing. The transformation from the austenite phase to the ferrite phase during annealing is suppressed, and as a result of the austenite phase remaining at the end of the annealing temperature (usually 600 ° C), bainite transformation occurs during the subsequent air cooling to room temperature, This is to significantly increase the hardness of the annealed material.
Considering the mechanism of such a structure change, it can be said that the parameter X = Mn + 0.38 (Ni + Cu) can be applied in the evaluation of the annealability as well as the hardenability.

  Based on the above circumstances, an attempt was made to evaluate the annealing properties using Cr and parameter X. Table 6 shows the components of the steel materials used in the investigation. 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%. The parameters X = 0.32 to 2.02% and Cr = 3.52 to 10.96% were selected with reference to the steel used for the die casting mold. P and S etc. were the contents of the impurity level.

These 23 steel types were annealed at 15 ° C./Hr (900 ° C. → 15 ° C./Hr→600° C. → cooling), and the results of examining the hardness are shown in FIG. Steel with an HRB hardness of 91 or less was rated as “◯ (passed)”, and steel with an HRB hardness exceeding HRB91 was rated as “x (failed)”. As a general trend, it can be seen that annealing increases with increasing Cr and parameter X. Although the results of the elements that improve the hardenability deteriorate the annealability are the same as before, the validity of using the parameter X as an index of hardenability was confirmed.
It was found that the annealing property was improved by the reduction of Cr and parameter X, and the region where the annealing property was good (HRB 91 or less) can be approximated by the equation (2B).
Cr ≦ 10.395-3.69X [%] ... Formula (2B)

From the above investigation results, in this embodiment, the component range excellent in the balance between hardenability and annealability is:
7.382−2.79X ≦ Cr ≦ 10.395−3.69X [%] (X = 0.30-2.10 [%], X = Mn + 0.38 (Ni + Cu) [%]) (2)
It was set as the range which satisfy | fills.

  Here, if the suitable range of Mn is shown, it is 0.4 to 1.3% by mass%. If the amount of Mn is less than 0.4%, 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 1.3%, the risk of high temperature temper embrittlement increases. High temperature temper embrittlement is a phenomenon in which the impact value of a steel material tempered at 500 to 600 ° C. is lowered, which is not desirable from the viewpoint of mold life. At the time of refining, measures are taken to reduce the content of P (phosphorus) that reveals high-temperature temper brittleness, but depending on the raw materials used, it may be difficult to reduce P. Therefore, 1.3% is desirable as the upper limit of Mn. Of course, if the P content can be reduced, there is no problem even if the Mn content is increased to 2.1% which is the upper limit of the parameter X.

  A suitable range of Ni is 0.3% or less in terms of mass% if importance is placed on the material cost, and 0.3 to 1.2% in terms of mass% if importance is attached to the ductility of the steel material. 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 desirable from the viewpoint of improving hardenability, but as described above, inexpensive Mn may be positively used for such purposes.

A suitable range of Cu is 0.3% or less by mass% if importance is attached to the material cost and hot workability, and 0.3 to 1.0% by mass% if importance is given to the hardening characteristics and sizing characteristics of the steel material. 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, the final shape of the mold may not be secured for the same reason as shown in FIG. In Cu-containing steel, such problems are unlikely to occur, and the finishing cost can be reduced to reduce the processing cost. Therefore, Cu addition is effective from the viewpoint of machinability, tempering hardness, and finish machining of the annealed material, and its preferred range is 0.3 to 1.0% by mass. When the added amount exceeds 1.0%, the above effect tends to saturate and the profit is poor. From the viewpoint of improving hardenability, 0.3 to 1.0% is desirable, but as described above, inexpensive Mn should be actively used for such purposes.

If the steel which concerns on this embodiment is compared with the existing steel, it will be as shown in FIG. JIS SKD4, JIS SKD7, JIS SKD6, JIS SKD61, JIS SMn443, and JIS SKH51 are greatly separated from the steel according to this embodiment. That is, it is proved that it is very difficult to apply to a large section type.
On the other hand, Steel A, Steel B, Steel C, and Steel D are brand steels marketed by steel manufacturers as “steel with good hardenability”. Steel A and steel C are close to the steel according to this embodiment, but still have a hardenability. In addition, steel D, which frequently suffers from the problem of annealing in actual production due to its too good hardenability, is positioned as it has been.
The steel according to the present embodiment is in a very narrow range, but by selecting specific components, the hardenability and the annealability are balanced at a high level.

(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 W amount is set to 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 high-temperature strength and production cost, the Co content was 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. From the balance between the effect of preventing the coarsening of crystal grains, the manufacturing cost, the fatigue strength, and the impact value, the Nb content is set to 0.002 to 0.500% by mass%.
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 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 Ti amount is set to 0.002 to 0.500% in 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 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 die. Decrease greatly. From the balance between the effect of preventing the coarsening of the crystal grains and the impact value, the Al amount is set to 0.005 to 1.500% in mass%. A preferable range of the Al amount is 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 the material cost. Furthermore, excessive addition only causes saturation of characteristics, and is not profitable. From the balance between the effect of preventing the coarsening of crystal grains and the manufacturing cost, the N amount is set to 0.005 to 0.300% by mass%. A preferable range of the N amount 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 boundaries and suppresses precipitation of the ferrite phase, and remarkably increases the hardenability of the steel. However, excessive addition leads to saturation of characteristics and is not profitable. As a range in which a sufficient hardenability improving effect can be obtained in practical use, the B amount is set to 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 leads to 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. From the balance of the effect of improving machinability, manufacturing cost, material, etc., the S amount is set to 0.005 to 2.000% by mass%.
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 leads to saturation of characteristics and an increase in manufacturing cost. From the balance of the effect of improving machinability and hot workability, manufacturing cost, material, etc., the Ca content is 0.0005 to 0.5000% in mass%.
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. From the balance of the effect of improving machinability, manufacturing cost, material, etc., the Se amount was 0.005 to 0.500% in mass%.
Te as a selective additive element is an element that improves machinability. However, excessive addition results in a large amount of Te compound, which decreases the hot workability and impact value of steel. From the balance of the effect of improving machinability, manufacturing cost, material, etc., the amount of Te was set to 0.005 to 0.500% in mass%.
Bi as a selective additive element is an element that improves machinability. However, excessive addition will increase Bi particles dispersed in the steel, reducing the hot workability and impact value of the steel. From the balance of the effect of improving machinability, manufacturing cost, material, etc., the Bi amount was set to 0.005 to 0.500% by mass.
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. From the balance of the effect of improving machinability, manufacturing cost, material, etc., the Pb amount is set to 0.005 to 0.500% in mass%.

Invention steels and comparative steels were produced and evaluated for warpage, impact value, annealing, and mold life.
(Component composition of steel)
Table 7 and Table 8 (Table 8 is a continuation of Table 7) show the component composition of the steel materials used for the evaluation, and the upper limit value and the lower limit value of Formula (1) and Formula (2). The comparative steel 13 is JIS SKD61, the comparative steel 14 is JIS SKD4, and the comparative steel 15 is JIS SKD7.

"..." in the table indicates that it was not actively added but was at 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 24 are levels for evaluating the influence of parameter X (see formula (2)) (Mn, Ni, Cu) in combination 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 or insufficient with respect to the standard of the invention steel. Comparative steels 13 to 15 are general-purpose JIS steels used for die casting.

(Material cost)
The alloy element costs were calculated for these 57 steel grades, and if it was equal to or less than JIS SKD61, it was judged as acceptable (◯), otherwise it was judged as unacceptable (x), and the material cost was evaluated. Table 9 shows the evaluation results.

(Hot workability)
The 57 steel types shown in Table 7 (and Table 8) were each cast into a 10-ton ingot and subjected to hot forging after homogenization heat treatment at 1250 to 1300 ° C. to obtain a square material having a height of 300 mm and a width of 700 mm. During hot forging, the level at which cracking was likely to occur was determined to be unacceptable (x), otherwise it was determined to be acceptable (O), and hot workability was evaluated. Table 9 shows the evaluation results.

(Annealing)
The square bar after hot forging was annealed to 15 ° C / Hr after heating to 870 ° C. At this time, if the hardness was 91HRB or less, it was determined to be acceptable (O), otherwise it was determined to be unacceptable (x), and the annealing property was evaluated. Table 9 shows the evaluation results.

(Maintaining fine austenite grain size)
Here, a test piece was cut out from the center part of the square bar, quenched after soaking at 1030 ° C. for 60 minutes, and the austenite grain size before transformation was investigated. Select the magnification and number of field of view of the structure so that the total is 4 mm ² or more. If the average value of the austenite crystal grains converted to a perfect circle (handled as austenite crystal grain size) is 100 μm or less, it passes ( ○), otherwise, it was judged as rejected (x), and the effect of suppressing coarsening of crystal grains due to fine VC was evaluated. Table 9 shows the evaluation results.

(With or without coarse VC)
In addition, if the above test piece does not have a coarse VC with a diameter of 10 μm or more, it will be judged as pass (○), otherwise it will be judged as fail (×), and cracks based on the inclusion interface will occur. Evaluation was made. Table 9 shows the evaluation results.

(Hardenability)
In addition, a specimen was cut out from the center of the square bar, cooled at various rates after soaking at 1030 ° C. for 60 min, and phase transformation behavior was investigated to evaluate the martensitic critical cooling rate. When the martensite critical cooling rate was 6 ° C./min or less, it was judged as pass (◯), otherwise it was judged as fail (×), and the hardenability was evaluated. Table 9 shows the evaluation results.

(With or without strength that can be used as a mold)
Furthermore, tempering the above-mentioned quenched specimen under the condition of 550 ° C x 2Hr. If the HRC hardness is 43 or more, it is judged as pass (○), otherwise it is judged as fail (x), and it can be used as a mold. It was evaluated whether it was equipped. Table 9 shows the evaluation results.

(Discussion-Steel)
As shown in Table 9, the invention steel was “◯” in all items, and it was found that the basic steel was a sound material with no problems at all.
However, when Inventive Steel 2 (V: 0.61), Inventive Steel 51 (V: 0.37) and Inventive Steel 52 (V: 0.98) were compared, it was found that the presence or absence of coarse VC and the maintenance of fine austenite grain size The superiority of Steel 2 (V: 0.61) was confirmed. Although there is no coarse (10 μm ≦ diameter) VC, the inventive steel 52 with a lot of V tended to have a larger VC than the inventive steels 02 and 51. Although there is no practical problem, it is considered that the solidification rate was reduced because the ingot was as large as 10 tons. In addition, although the austenite crystal grain size is 100 μm or less, the inventive steel 51 with less V tends to have a larger austenite crystal grain size than the inventive steels 02 and 52. However, it is not a mixed grain structure. There is no practical problem, but it is thought to be caused by a small amount of fine VC.

On the other hand, the comparative steel has “x” in at least one item, which indicates that it has some problems. In particular, in relation to the invention steel 51 and the invention steel 52, the comparative steel 11 (V: 0.26) in which the V amount deviated from the lower limit of the invented steel and the comparative steel 12 in which the V amount deviated from the upper limit of the invented steel ( V: 1.26) shows that there are problems with coarse VC and austenite grain sizes, respectively.
From the above, the basic performance of the steel material was confirmed.

Next, actual type verification will be described.
(Manufacture of mold from square bar)
Following the investigation of the basic performance, a mold was produced from the annealed squares (height 300 mm x width 700 mm). However, the steel materials having problems in Table 9 were excluded because the possibility of practical use was low. Specifically, five steel types were excluded: comparative steel 1 that could not achieve 43HRC, comparative steel 3, comparative steel 10 and comparative steel 14 with high material costs, and comparative steel 4 with poor hot workability.
Three types of molds with different weights of 30 kg, 100 kg, and 450 kg were selected. These molds were quenched after soaking at 1030 ° C. for 60 min. There are two quenching methods: (1) a combination of cooling and blast cooling, and (2) 150 ° C. oil cooling. Subsequently, after tempering to 43HRC by tempering, the warpage of the mold was evaluated.

(Mold warpage)
As shown in FIG. 17 and the following equation (3), the warpage evaluation method is based on the percentage of “drip” on the back of the mold corresponding to the length. This value should be small. Usually, when the warpage is 0.2% or more, the trouble that the hardened skin remains as shown in FIG. 1 is likely to occur. Accordingly, if the warpage can be suppressed to about 0.1%, the surplus of finishing can be reduced, which is advantageous for cost reduction and single delivery.
Warpage = 100d ÷ L ... Formula (3)
In addition, since the steel of the invention steel exerts a great effect when the quenching speed is low, 150 ° C. oil cooling, which is a rapid cooling, was not performed.
Table 10 shows the evaluation results for the 30 kg mold, Table 11 shows the evaluation results for the 100 kg mold, and Table 12 shows the evaluation results for the 450 kg mold.

(Completion of mold)
In addition, after the evaluation of the warp, finishing was performed to complete the mold.

(Casting using mold)
These molds were assembled in a die casting machine and actually cast. 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. The time (number of shots) when a large crack was found in the mold was determined as the mold life. Then, a material was cut out from the center of the mold after the service life, and the impact value was investigated in order to associate with the service life.
Table 10 shows the evaluation results for the 30 kg mold, Table 11 shows the evaluation results for the 100 kg mold, and Table 12 shows the evaluation results for the 450 kg mold.

(Discussion-actual type verification)
In the case of the 30 kg mold shown in Table 10, since the mold is small, rapid cooling is possible and the temperature difference in the cross section is not easily attached. For this reason, according to the results of Table 10, the overall tendency is that the warp is small and the impact value is high. Looking at the invented steel, the impact value exceeds 40 J / cm ² and a life of approximately 80,000 shots is secured.
Therefore, it can be seen that the inventive steel is also suitable for a small mold.

With comparative steels that are quenched by a combination of natural cooling and blast cooling, it is difficult to ensure a life of 80,000 shots, except for some. With reference to Table 9, the level that could secure 80,000 shots is steel with good hardenability (Comparative Steel 6 and Comparative Steel 8) having a problem in annealing.
Particularly, the shortest life is the comparative steel 12 and the comparative steel 11, and referring to Table 9, it can be seen that the steel material has a large amount of coarse VC and the steel material has a coarse crystal grain. The other comparative steels have a life of about 18000 shots, which is about 1/4 of the invented steel with a life of about 80,000 shots.

  In addition, the comparative steel quenched with 150 ° C. oil has the same tendency as the case of quenching with a combination of natural cooling and blast cooling. As the quenching speed is increased, the impact value is high and the life is extended as a whole, but the level that can secure 80,000 shots is as follows. 6. Comparative steel 8) only. However, as shown in Table 10, warpage is large due to rapid cooling, which is not desirable level although it is 0.2% or less.

  From the above, it was confirmed that the invention steel that was quenched slowly has a longer mold life and less warpage than the quenched comparative steel in a small mold. The mold life corresponds to the impact value. If Table 9 is referred, it can be judged that the reason why the impact value of the inventive steel is high is that there are few coarse VCs exceeding 10 μm, many fine VCs of 1 μm or less, and high hardenability. In addition, the invented steel also has an annealing property.

In the case of the 100 kg mold shown in Table 11, rapid cooling is considerably difficult even if it is a small class in the large section mold. Moreover, since a large cross-sectional temperature difference is likely to occur, warpage also increases. Therefore, according to the results in Table 11, the warpage is large and the impact value is low compared to the case of the 30 kg mold. Looking at the invented steel, the impact value is 35 J / cm ² or more, and a life of about 40,000 shots is secured.
Therefore, it can be seen that the inventive steel is also suitable for relatively large molds.

For comparative steels hardened by a combination of natural cooling and blast cooling, it is difficult to ensure a life of 40,000 shots, except for some. The level that could secure 40,000 shots is steel with good hardenability (Comparative Steel 6 and Comparative Steel 8) that has a problem in annealing, as shown in Table 9.
Particularly, the shortest life is the comparative steel 12 and the comparative steel 11, and referring to Table 9, it can be seen that the steel material has a large amount of coarse VC and the steel material has a coarse crystal grain. The other comparative steels have a life of about 9000 shots, which is about 1/4 of the invented steel with a life of about 40,000 shots.

  In addition, the comparative steel quenched with 150 ° C. oil has the same tendency as the case of quenching with a combination of natural cooling and blast cooling. As the quenching speed increases, the impact value is generally high and the life is prolonged, but the level that can secure 40,000 shots is as follows. 6. Comparative steel 8) only. Moreover, as shown in Table 11, the warpage is large due to rapid cooling, exceeding 0.2%.

  From the above, it was confirmed that the invention steel that was quenched slowly has a longer mold life and less warpage than the quenched comparative steel in a relatively large mold. The mold life corresponds to the impact value. Referring to Table 9, it can be judged that the reason why the impact value of the inventive steel is high is that there are few coarse VCs exceeding 10 μm, many fine VCs of 1 μm or less, and high hardenability. In addition, the invented steel also has an annealing property.

In the case of the 450 kg mold shown in Table 12, it is a typical large cross-sectional mold, and rapid cooling is very difficult. Further, since a large temperature difference in the cross section is added, warpage is also increased. According to the results in Table 12, the warpage is large and the impact value is low as compared with the case of the 100 kg mold. Looking at the invented steel, the impact value is about 30 J / cm ² and the life of about 20000 shots is secured.
Therefore, it can be seen that the inventive steel is also suitable for the large section type.

With comparative steels quenched by a combination of natural cooling and blast cooling, it is difficult to ensure a life of 20000 shots, except for some. The level that could secure 40,000 shots is steel with good hardenability (Comparative Steel 6 and Comparative Steel 8) that has a problem in annealing, as shown in Table 9.
Particularly, the shortest life is the comparative steel 12 and the comparative steel 11, and referring to Table 9, it can be seen that the steel material has a large amount of coarse VC and the steel material has a coarse crystal grain. The other comparative steels have a life of about 6000 shots, which is about 1/3 of the invented steel with a life of about 20000 shots.

  In addition, the comparative steel quenched with 150 ° C. oil has the same tendency as the case of quenching with a combination of natural cooling and blast cooling. As the quenching speed increases, the impact value is high and the life is extended as a whole. However, the level at which 20000 shots could be secured is as follows. 6. Comparative steel 8) only. Moreover, as shown in Table 12, the warpage is very large due to rapid cooling, which is about 0.25%.

  From the above, it was confirmed in the large-section mold that the invention steel that was quenched slowly has a longer mold life and less warpage than the quenched comparative steel. The mold life corresponds to the impact value. If Table 9 is referred, it can be judged that the reason why the impact value of the inventive steel is high is that there are few coarse VCs exceeding 10 μm, many fine VCs of 1 μm or less, and high hardenability. In addition, the invented steel also has an annealing property.

From the above, it is difficult for the comparative steel to secure the mold life even by rapid quenching, and it is not possible to obtain a merit corresponding to the disadvantage of increasing the warpage. In addition, comparative steels with poor annealing (that is, with good hardenability) can ensure the mold life even with slow quenching, but the machinability of the annealed material deteriorates, so that the mold manufacturing cost increases. In addition, comparative steels with good annealing properties (i.e., poor hardenability properties), when slowly quenched, have a small warpage, but the mold life is significantly shortened.
On the other hand, the invention steel can secure the mold life even if it is slowly quenched, and the reduction of warpage can be achieved at the same time. Moreover, since the annealing property is ensured, the machinability of the annealed material is not deteriorated, and the die manufacturing cost is not increased.

As explained above, the inventive steel is
(A) There are few coarse inclusions mainly composed of V and having a diameter of 10 μm or more.
(B) Many fine VCs with a diameter of 1 μm or less,
(C) High hardenability,
(D) Annealing property is ensured,
It has the feature.
Therefore, the steel of the present invention is effective in reducing the warpage of the mold and extending the life.

  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.

  Steel according to the present invention, steel for molds and molds using the same are molds and molding members for plastic and rubber injection molding, die casting, hot forging, etc., in particular, large molds and heavy molds And can be applied to molding members. The steel according to the present invention, the mold steel and the mold using the same are designed to have a high impact value, ensure machinability and reduce the number of finishing processes, and use a mold or a molding member. In the manufacturing industry, it is possible to realize cost reduction, resource saving and environmental load reduction, which is extremely beneficial to the industry.

It is a figure for demonstrating the relationship between the curvature of a metal mold | die, and processing white. It is a figure for demonstrating the generation | occurrence | production and progress of a crack which started from coarse VC. It is a figure for demonstrating the influence of the martensite crystal grain diameter which has on generation | occurrence | production and progress of a crack. It is a figure for demonstrating the correlation of a martensite crystal grain size and an austenite crystal grain size. It is a figure for demonstrating the correlation with an austenite crystal grain diameter and fine VC amount. It is a graph which shows the existence boundary of coarse VC. It is a graph which shows the relationship between C amount and V amount which show the existing boundary of an austenite crystal grain diameter of 100 micrometers. It is a graph for aiming at optimization of C amount and V amount. It is a CCT diagram of JIS SKD61. It is a graph which shows the influence of the quenching speed on the impact value of JIS SKD61. It is a graph which shows the relationship between quenching speed and die size. It is a graph which shows the relationship between a quenching depth and the element (Mn * Ni * Cu) amount. It is a graph which shows the relationship between a quenching depth and the amount of elements (Mn + 0.38 (Ni + Cu)). It is a graph which shows the relationship between the amount of Cr and the amount of elements (Mn + 0.38 (Ni + Cu)) which shows the boundary where a critical cooling rate becomes 6 degreeC. It is a graph which shows the relationship between the amount of Cr and the amount of elements (Mn + 0.38 (Ni + Cu)) which shows the boundary which becomes 91HRB by annealing of 15 degreeC / Hr. It is a graph which shows the relationship between the amount of Cr and the amount of elements (Mn + 0.38 (Ni + Cu)) which shows the optimization of the range of Cr amount and parameter X (X = Mn + 0.38 (Ni + Cu)). It is a figure for demonstrating the evaluation method of curvature.

Claims (9)

% By mass
Si: 0.02 to 2.50%,
Mo: 0.20 to 4.00% and
C: contains 0.15-0.55%, and
A steel characterized in that the balance containing C, V, Cr, Mn, Ni, and Cu satisfying the following formulas (1) to (2) in mass% is composed of Fe and inevitable impurities.
0.729−1.01C ≦ V ≦ 1.718−1.65C [%] ... Formula (1),
7.382−2.79X ≦ Cr ≦ 10.395−3.69X [%] (where X = Mn + 0.38 (Ni + Cu) [%], 0.30 ≦ X ≦ 2.10 [%]) (2). 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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