JP5444938B2 - Steel for mold - Google Patents

Description

  The present invention relates to a mold steel suitable for being applied to molds for plastic or rubber injection molding, die casting, low pressure casting, forging and the like.

In recent years, there is a growing need for lower product costs for plastic and rubber injection molded products, die-cast products, low-pressure cast products, forged products, etc., and as a means to that end, extension of mold life (I), for molds Steel cost reduction (II) is a useful means in combination with cost reduction of products such as cast products (III).
Here, the extension of the mold life of (I) is a major factor affecting the mold cost.
If the mold life is long, the number of products that can be manufactured by one mold increases, and the ratio of the mold cost per product inevitably becomes small.
On the other hand, if the mold life is short, the ratio of the mold cost per product becomes large.

By the way, in recent years, from the viewpoint of improving productivity and shortening the delivery time, the product molding has been advanced in a high cycle. Taking die casting as an example, a casting machine corresponding to a high cycle has been developed.
However, this high cycle becomes a factor for shortening the mold life.
This point will be described below by taking die casting as an example.

  Die-casting is a casting method in which molten metal is filled into a cavity (molding space) formed in a mold and solidified to be taken out. The shape of the cast product can be freely controlled by the shape of the cavity, resulting in high productivity. Has features. In particular, die casting of Al alloy has grown greatly in response to the development of the automobile industry.

  This die casting manufacturing process is performed in the order of hot water supply → injection → solidification → mold opening → product removal → mold release agent application → mold clamping → hot water supply. In this case, the release agent application is performed by contact with a high-temperature metal. This is an important process with the meaning of film treatment to lower the elevated mold surface temperature and at the same time facilitate the removal of the cast product.

In such die casting, it is important to increase the cooling rate of the mold in order to increase the cycle of product production.
If the production of die-cast products is simply made into a high cycle, the next casting cycle will be reached without the cooling time of the mold being sufficiently secured (before the mold temperature has fallen), and the mold will inevitably The surface temperature of becomes higher.

The increase in the temperature of the mold surface causes a decrease in the solidification rate of the cast product, and is therefore a serious problem that directly leads to deterioration of the casting quality.
In addition, the cast product taken out from the mold at a higher temperature has a large thermal shrinkage due to subsequent cooling, which adversely affects the dimensional accuracy. Furthermore, products removed at high temperatures are prone to deformation during subsequent cooling.

Therefore, in order to correct such a problem, it is very important to increase the cooling rate of the mold in the high cycle.
However, under such a high cycle, the tensile thermal stress applied to the mold increases, resulting in the promotion of a heat check on the mold surface.

In this heat check, cracks are caused by thermal stress (especially tensile stress due to subsequent forced cooling on the mold surface heated to a high temperature by filling the molten metal) on the minute notch on the surface caused by wear, melting damage, corrosion, etc. Is a phenomenon that occurs and progresses, and is a thermal fatigue phenomenon associated with heating and cooling.
When such a heat check occurs, it is transferred to the product surface, and depending on the product, the quality is significantly impaired or becomes zero.
In this sense, the heat check generated on the mold surface is a major factor that determines the mold life.

Another important factor that determines the life of the mold is the problem of mechanical fatigue strength of the mold.
For example, when a coarse foreign substance exists in the steel structure constituting the mold, cracks are generated starting from that and the mold life is shortened.
Furthermore, even when the impact value of the mold is low or the high temperature strength is low, the life of the mold is reduced.
Therefore, as countermeasures, it is effective to reduce the thermal stress applied to the mold under rapid heating / cooling, to improve the mechanical fatigue strength of the mold, and to improve the impact value and the high temperature strength.

Here, in order to reduce the thermal stress applied to the mold, it is useful to increase the thermal conductivity of the mold.
If the thermal conductivity of the mold is increased, the difference between the mold surface temperature and the internal temperature during heating and cooling can be reduced, and the thermal stress generated on the mold surface can be reduced. In addition, the mold cooling rate can be increased.

On the other hand, for lowering the cost of die steel, it is useful to improve the hot workability during hot working and avoid cracks during working to increase the yield.
If the hot workability is poor, cracks occur during hot working, which deteriorates the yield and increases the cost required for the mold steel.
Further, those having poor hot workability require reheating (reheating) several times during the hot work. In this case, the hot work process becomes complicated and productivity is lowered. This also causes an increase in cost required for mold steel.

It is also important to reduce the amount of expensive rare elements to be added and to ensure machinability in order to reduce the cost of mold steel.
If the machinability is poor, the work and time required for manufacturing the mold by machining the mold steel is increased, which leads to an increase in cost.

Next, as a matter of course, it is also important to reduce the cost of the product (III).
This cost reduction of the product itself can be realized by shortening the production cycle and further reducing the defect rate.
For example, in the case of a cast product, the production cycle time can be shortened by rapidly decreasing the mold temperature after filling the cavity with the molten metal and rapidly solidifying the molten metal.
Here, increasing the thermal conductivity of the mold is an effective means for rapidly decreasing the mold temperature of the mold.
Although the explanation is omitted, the above situation is the same in the fields of plastic and rubber injection molding, low pressure casting and forging.

  As a prior art of the present invention, the following Patent Document 1 discloses an invention about “pre-hardened steel for die-cast molds”, in which the composition of pre-hardened steel for die-cast molds having a Rockwell hardness of 30 HRC or more and 40 HRC or less is shown. “In terms of mass content, 0.15% or more and 0.35% or less of C, 0.05% or more and less than 0.20% of Si, 0.05% or more and 1.50% or less of Mn, 0.020% or less of P, and 0.013% or less of S, Cu of 0.10% or less, Ni of 0.20% or less, Cr of 0.20% or more and 2.50% or less, Mo of 0.50% or more and 3.00% or less, V and Nb of 0.05% or more and 0.30% or less, and 0.020% It is disclosed that the composition contains Al of 0.040% or less, 0.003% or less of O, and 0.010% or more and 0.020% or less of N, with the balance being substantially Fe.

The one disclosed in Patent Document 1 is that the upper limit of Si, Cr, Mn content is Si: less than 0.20%, Cr: 2.5% or less, Mn: 1.50% or less. In this Patent Document 1, the point of V + Nb = 0.05 to 0.30% is disclosed.
However, all of the materials disclosed in the examples have a Nb content of 0.03% or more, and those disclosed in Patent Document 1 are different from the present invention in terms of the Nb content.

  Patent Document 2 below discloses an invention about “pre-hardened steel for die casting molds”, in which the composition of the pre-hardened steel is “weight%, C: 0.10 to 0.30%, Si: 0.20 to 0.35%, Mn: 0.50 to 2.00%, P: 0.02% or less, S: 0.013% or less, Cu: 0.10% or less, Ni: 0.20% or less, Cr: 1.00 to 3.00%, Mo: 0.20 to 1.00%, one of V and Nb Species or two: 0.05 to 0.30%, s-Al: 0.03% or less, O: 0.003% or less, N: 0.020% or less, and inclusions: 0.10% or less, with the balance being substantially Fe ” This point is disclosed.

  The one disclosed in Patent Document 2 significantly reduces machinability by lowering the S content of Cr-Mo steel pre-hardened steel to 0.013% or less and lowering the inclusion content to 0.10% or less. The heat check resistance is improved without any change, but the one disclosed in Patent Document 2 is different from the present invention in the Si content, and the Nb content is disclosed as an example. In this case, the content is a high content of 0.07% or more, which is different from the present invention in this respect.

Furthermore, the following Patent Document 3 discloses an invention about “hot tool steel”, and the description of claim 1 discloses that Nb is contained in an amount of 0.01 to 0.15%.
However, also in this Patent Document 3, any of those disclosed as examples has a high Nb content of 0.04% or more, which is also different from the present invention.
Further, none of those disclosed in Patent Documents 1 to 3 disclose the technical idea of the present invention.

JP 2005-307242 A JP 2003-138342 A JP-A-6-88163

  The present invention is based on the circumstances as described above. In addition to reducing the cost of the product itself, the heat check is reduced, seizure is reduced, the mold fatigue life is increased by securing the mechanical fatigue strength and impact value, and the rare elements are reduced. To provide a mold steel that can reduce the cost required for the mold by reducing the cost of the mold steel by ensuring the machinability, and that can reduce the product cost comprehensively and effectively by using them as a whole. It was made for the purpose.

  Thus, Claim 1 relates to a mold steel, which is expressed in mass% 0.15 ≦ C ≦ 0.30, 0.01 ≦ Si <0.19, 1.50 <Mn ≦ 1.78, 1.01 <Cr <3.05, 0.48 <Mo <0.88, 0.01 ≦ V <0.21, 0.004 ≤ Nb <0.03, and the composition of balance Fe and inevitable impurities.

  A second aspect of the present invention is characterized in that, in the first aspect, 0.30 ≦ W ≦ 4.00 is further contained in mass%.

  A third aspect of the present invention is characterized in that in any one of the first and second aspects, 0.30 ≦ Co ≦ 3.00 is further contained in mass%.

  A fourth aspect of the present invention is as defined in any one of the first to third aspects, wherein 0.004 ≦ Ta ≦ 0.100, 0.004 ≦ Ti ≦ 0.100, 0.004 ≦ Zr ≦ 0.100, 0.004 ≦ Al ≦ 0.050, 0.004 ≦ N ≦ 0.050 in mass%. Of these, it further comprises at least one or more.

  Claim 5 further comprises at least one of 0.15 ≦ Cu ≦ 1.50, 0.15 ≦ Ni ≦ 1.50, 0.0010 ≦ B ≦ 0.0100 by mass% in any one of claims 1 to 4. It is characterized by.

  A sixth aspect of the present invention provides the method according to any one of the first to fifth aspects, wherein 0.010 ≦ S ≦ 0.50, 0.0005 ≦ Ca ≦ 0.2000, 0.03 ≦ Se ≦ 0.50, 0.005 ≦ Te ≦ 0.100, 0.01 ≦ Bi ≦ 0.30 in mass%. It is characterized by further containing at least one of 0.03 ≦ Pb ≦ 0.50.

According to a seventh aspect of the present invention, there is provided a die casting steel according to any one of the first to sixth aspects.
In the present invention, the “mold” includes not only the mold body but also mold parts such as pins that are assembled and used. Furthermore, the metal mold | die which consists of steel of this invention and the surface-treated thing is contained.

Effects and effects of the invention

In the present invention, Nb is contained for crystal grain refinement, but the present invention has one feature that the content range is regulated to 0.004% or more and less than 0.03%.
That is, conventionally, a relatively large amount of Nb is added for crystal grain refinement, but in the present invention, Nb of a predetermined amount (lower limit) or more is added to ensure the effect of crystal grain refinement, The amount of Nb added is restricted to a low level.

When the amount of Nb added is increased as in the past, when the molten steel is solidified to form an ingot, an industrially large ingot, for example, a large ingot of 5 t (ton) or 10 t, Nb carbonitriding during the solidification process. The product (NbCN) crystallizes coarsely.
In the case of a small ingot of about 50 kg or 100 kg, the solidification speed is fast, so that Nb becomes small and crystallizes and is not easily crystallized.

However, in the case of industrially large ingots, the rate of solidification is slow, so that Nb becomes coarse carbonitrides, that is, foreign substances, and crystallizes in the steel structure.
The crystallized Nb carbonitride has an extremely high melting point, and once crystallized, it does not dissolve in the subsequent heat treatment and remains in the structure as it is.

  Thus, if the Nb mononitride is fine, it exhibits a pinning effect (pinning effect), but the coarse Nb carbonitride does not function as a pinning and instead becomes a foreign substance, which is the starting point of cracking. Thus, the mechanical fatigue life of the mold is reduced.

The present inventor can suppress the crystallization of the coarse carbonitride of Nb when solidifying the molten steel into an industrial size ingot by setting the amount of Nb added to less than 0.03%, thereby enabling the die machine to The knowledge that the fatigue life can be effectively increased was obtained. The present invention has been made based on such findings.
This coarse Nb carbonitride also reduces the impact value of the mold.
Therefore, according to the present invention, the impact value, that is, the toughness of the mold can be increased by suppressing the crystallization of coarse Nb carbonitride.

Nb also acts to increase the recrystallization temperature of the steel when added to the steel.
If the recrystallization temperature of the steel rises, it will be difficult for the steel to recrystallize during hot working and deformation resistance will increase, making it difficult to work. Become.

This deteriorates the yield of the mold steel, which in turn increases the material cost of the mold steel.
Or, in order to prevent cracks during hot working, it is necessary to repeatedly perform reheating in the middle of hot working.
In this case, the hot working process is complicated and the production speed is lowered, which similarly increases the material cost of the steel for molds.

  In the present invention, since the amount of Nb added is suppressed to less than 0.03%, it is possible to effectively suppress an increase in the recrystallization temperature of the steel, thereby avoiding cracks during hot working, or during hot working. The number of reheating (reheating) can be reduced, the productivity can be increased, and the material cost of the steel for mold can be reduced.

In the present invention, the heat conductivity of the mold is increased by restricting the addition amount to less than 0.19% while securing the machinability of the steel by making the addition amount of Si 0.01% or more.
Thus, by increasing the thermal conductivity of the mold, the responsiveness of the mold to heating / cooling can be enhanced. For example, the mold can be cooled well during cooling during die casting, which can increase the solidification rate of the cast, refine the cast structure, and eliminate or suppress the formation of nests. As a result, the production cycle can be increased and the manufacturing cost of the cast product can be reduced.

In addition, the life of the mold can be extended by increasing the thermal conductivity.
By increasing the thermal conductivity of the mold, the temperature difference between the mold surface and the interior during heating and cooling can be reduced, thereby reducing the tensile thermal stress on the mold surface. Generation of cracks can be suppressed, so that heat check resistance can be improved and the mold life can be extended.

That is, according to the present invention, it is possible to increase the life of the mold by suppressing the formation of coarse Nb carbonitride and increasing the thermal conductivity of the mold.
Further, by making the mold responsiveness to heating and cooling high, the production cycle time can be shortened and the productivity can be increased.

In addition, since the mold temperature during casting can be lowered, the “wetting” between the molten Al and the mold is deteriorated, and seizure of the cast product to the mold is difficult to occur during casting.
When such seizure occurs, it becomes difficult to take out the product and maintenance is required, which becomes a factor of reducing productivity.
However, by reducing the mold temperature, the occurrence of seizure can be suppressed, the productivity can be increased, and the occurrence of a product defect rate can be reduced.

In the present invention, the amount of addition of Mo and V, which are expensive rare elements, is restricted to be small, and this can reduce the material cost of the steel for molds.
And by these, the cost of the product produced using a metal mold | die can be reduced effectively.

Next, each chemical component in the present invention and reasons for limitation thereof will be described in detail below.
0.15 ≤ C ≤ 0.30
If it is less than 0.15%, it is difficult to obtain a required hardness of 34 HRC or more. If it exceeds 0.30%, it becomes hard, and the machinability in the quenched and tempered state deteriorates.
Moreover, the amount of carbides and carbonitrides become excessive, and fatigue strength and impact value are deteriorated. A preferable range is 0.18 ≦ C ≦ 0.27, which is excellent in balance between hardness, fatigue strength, and impact value. More preferably, 0.20 ≦ C ≦ 0.24.

0.01 ≤ Si <0.19
If it is less than 0.01%, the machinability is remarkably deteriorated, and it becomes very difficult to process into a mold shape. When it is 0.19% or more, the decrease in thermal conductivity is large. A preferable range is Si ≦ 0.15, in which high thermal conductivity is obtained and industrial machinability can be secured. When particularly high thermal conductivity is required, Si <0.10 is a more preferable range.

These are shown in FIGS. 1 and 2. FIG.
Fig. 1 shows the distance of cutting until the tool reaches the end of life when cutting 0.30C-1.52Mn-3.04Cr-0.80Mo-0.11V-0.01Nb-Si steel with respect to the amount of Si. . The test piece was a square material of 55 mm × 55 mm × 200 mm, and cutting was repeated in the direction of 200 mm in length, and when the maximum amount of side flank wear of the cutting tool reached 300 μm (cumulative cutting distance), the life was determined. The larger the cutting distance, the better the cutting.
When Si is less than 0.01%, the cutting distance is extremely small. The cutting distance increases as Si increases. However, when Si exceeds 0.19%, there is a tendency that the more Si is, the better the cutting is, but the improvement effect is not significant compared to the low Si side.

In addition, the material for machinability evaluation was created in the following procedure.
First, molten steel melted and refined in vacuum was cast into a 2 ton ingot. The ingot was subjected to a homogenization heat treatment by heating at 1250 ° C. for 18 hours. Thereafter, a block having a cross section of 250 mm × 250 mm was manufactured by hot forging of the ingot. The block in the high-temperature state after forging was allowed to cool to room temperature, reheated to 820 ° C. after heating for 6 hours at 670 ° C., and slowly cooled at 10 ° C./Hr after holding for 4 hours. When the block reached 620 ° C., the slow cooling was stopped, and the block was removed from the furnace and allowed to cool to room temperature.
Through such a series of treatments, a block having a soft hardness of about 90 HRB and a uniform structure was manufactured. FIG. 1 shows the results of making a 55 mm × 55 mm × 200 mm test piece using this block as a material and examining the machinability.

A round bar of φ11 mm × 50 mm cut out from the same part of the same material used in the test of FIG. 1 was heated to 920 ° C., quenched, tempered and tempered to 38 HRC. Further, a test piece for measuring thermal conductivity of φ10 mm × 2 mm was prepared from this round bar. FIG. 2 shows the thermal conductivity measured at room temperature by the laser flash method with respect to the amount of Si.
Higher thermal conductivity is preferable because of excellent cooling ability when it becomes a mold.
The thermal conductivity is influenced by other elements such as Mn and Cr as described later, but the cooling capacity is dramatically improved as compared with SKD61 (thermal conductivity 24 W / m / K). In order to obtain a thermal conductivity of m / K or more, in the present invention, the Si content is less than 0.19%.

1.50 <Mn ≤ 1.78
If it is 1.50% or less, the hardenability is insufficient, and it is difficult to ensure hardness and impact value. If it exceeds 1.78%, it will be difficult to maintain high thermal conductivity. A preferable range is 1.50 <Mn ≦ 1.65, which can ensure hardness and impact value and can obtain high thermal conductivity.

These are shown in FIGS. 3 and 4. FIG.
A 100 mm × 100 mm × 60 mm block made from 0.30C-0.15Si-3.04Cr-0.81Mo-0.12V-0.01Nb-Mn steel was heated to 920 ° C., rapidly cooled and tempered, and tempered to 38 HRC. Further, a 10 mm × 10 mm × 55 mm JIS No. 3 impact test piece was prepared from the center of the block, and the impact value was measured at room temperature.

  FIG. 3 plots the impact value against the amount of Mn. A larger impact value is preferable because it is less likely to break when it becomes a mold. Steel with low Mn content is hard to be hardened to the center of the block (poor hardenability), so it tends to be a coarse structure and has a low impact value. Since the hardenability is improved by increasing the amount of Mn, the impact value increases. When the Mn content exceeds 1.50%, the impact value is highly stable.

The material for impact value evaluation was prepared by the following procedure.
First, molten steel melted and refined in a vacuum was cast into a 2t ingot. The ingot was subjected to a homogenization heat treatment by heating at 1250 ° C. for 18 hours. Thereafter, a block having a cross section of 250 mm × 250 mm was manufactured by hot forging of the ingot.

  The block in the high-temperature state after forging was allowed to cool to room temperature, reheated to 820 ° C. after heating for 6 hours at 670 ° C., and slowly cooled at 10 ° C./Hr after holding for 4 hours. When the block reached 620 ° C., the slow cooling was stopped, and the block was removed from the furnace and allowed to cool to room temperature. Through such a series of treatments, a block having a soft hardness of about 90 HRB and a uniform structure was manufactured.

  Using this block as a raw material, a square bar of 11 mm × 11 mm × 60 mm was prepared from the vicinity of the center of the cross section, and this square bar was heated to 920 ° C., rapidly cooled, tempered, and tempered to 38 HRC. Further, FIG. 3 shows the result of making a 10 mm × 10 mm × 55 mm JIS No. 3 impact test piece from this square bar and examining the Charpy impact value in accordance with JIS Z 2242.

A round bar of φ11 mm × 50 mm cut out from the same part of the same material used in the test of FIG. 3 was heated to 1030 ° C., quenched, tempered and tempered to 38 HRC. Further, a test piece for measuring thermal conductivity of φ10 mm × 2 mm was prepared from this round bar. FIG. 4 shows the thermal conductivity measured at room temperature by the laser flash method with respect to the amount of Mn.
Compared with SKD61 (thermal conductivity 24 W / m / K), in order to obtain a thermal conductivity of 33 W / m / K or more, which dramatically improves the cooling capacity, the Mn content is 1.78% or less in the present invention.

1.01 <Cr <3.05
If it is 1.01% or less, the hardenability is insufficient, and sufficient hardness and impact value cannot be obtained. On the other hand, if the Cr content is 3.05% or more, it is difficult to maintain high thermal conductivity. The preferred range is 1.50 ≦ Cr ≦ 2.50, which is excellent in the balance of hardness, impact value and thermal conductivity. More preferably, 1.75 ≦ Cr ≦ 2.25.

FIG. 5 and FIG. 6 show these states.
A 100 mm × 100 mm × 60 mm block made from 0.30C-0.15Si-1.78Mn-0.79Mo-0.10V-0.01Nb-Cr steel was heated to 920 ° C., quenched, tempered and tempered to 38 HRC. Further, a 10 mm × 10 mm × 55 mm JIS No. 3 impact test piece was prepared from the center of the block, and the impact value was measured at room temperature. FIG. 5 plots the impact value at room temperature against the Cr content.

  Steel with low Cr content is hard to harden to the center of the block (poor hardenability), so it tends to be a coarse structure and has a low impact value. Particularly when Cr is 1.01% or less, the impact value is remarkably reduced. On the other hand, since the hardenability is improved by increasing the Cr content, the impact value increases. When Cr is 1.01 to 3.05%, the impact value is highly stable.

The material for impact value evaluation was prepared in the same procedure as the case where the influence of Mn described above was examined.
A round bar of φ11 mm × 50 mm cut out from the same part of the same material used in the test of FIG. 5 was heated to 920 ° C., rapidly cooled, tempered and tempered to 38 HRC. Further, a test piece for measuring thermal conductivity of φ10 mm × 2 mm was prepared from this round bar. FIG. 6 shows the thermal conductivity measured at room temperature by the laser flash method with respect to the Cr content.
Compared with SKD61 (thermal conductivity 24 W / m / K), in order to obtain a thermal conductivity of 33 W / m / K or more, which dramatically improves the cooling capacity, the Cr content is made less than 3.05% in the present invention.

0.48 <Mo <0.88
If it is 0.48% or less, sufficient high-temperature strength cannot be obtained. If it is 0.88% or more, it becomes brittle and the impact value decreases. From such a viewpoint, a preferable range is 0.60 ≦ Mo ≦ 0.85.

This is shown in FIG. 7 and FIG.
A φ15 mm × 50 mm round bar made from 0.22C-0.08Si-1.73Mn-1.04Cr-0.11V-0.01Nb-Mo steel was heated to 920 ° C., rapidly cooled, tempered and tempered to 38 HRC. Furthermore, a test piece for measuring deformation resistance of φ14 mm × 21 mm was prepared from this round bar, the test piece was heated to 600 ° C. at 5 ° C./s and held for 100 s, and then compressed in the axial direction at a strain rate of 10 s ^−1. The deformation resistance (high temperature strength) was measured. FIG. 7 shows the deformation resistance at this time with respect to the amount of Mo.
The higher the deformation resistance, the higher the strength. Deformation resistance (high temperature strength) decreases rapidly when the Mo content is 0.48% or less, and avoiding this range is considered essential for ensuring wear resistance.

The material for evaluating deformation resistance was prepared by the following procedure.
First, molten steel melted and refined in a vacuum was cast into a 2t ingot. The ingot was subjected to a homogenization heat treatment by heating at 1250 ° C. for 18 hours. Thereafter, a block having a cross section of 250 mm × 250 mm was manufactured by hot forging of the ingot.

  The block in the high-temperature state after forging was allowed to cool to room temperature, reheated to 820 ° C. after heating for 6 hours at 670 ° C., and slowly cooled at 10 ° C./Hr after holding for 4 hours. When the block reached 620 ° C., the slow cooling was stopped, and the block was removed from the furnace and allowed to cool to room temperature. Through such a series of treatments, a block having a soft hardness of about 90 HRB and a uniform structure was manufactured.

  Using this block as a raw material, a round bar of φ15 mm × 50 mm was shaved from near the center of the cross section, this round bar was heated to 920 ° C., rapidly cooled, tempered and tempered to 38 HRC. Further, FIG. 7 shows the result of making a test piece of φ14 mm × 21 mm from this round bar and examining the deformation resistance.

A square bar of 11 mm × 11 mm × 60 mm was prepared from the same part of the same material used in the test of FIG. 7, and this square bar was heated to 920 ° C. to be cooled rapidly, and returned to temper to 38 HRC. Further, a JIS No. 3 impact test piece of 10 mm × 10 mm × 55 mm was prepared from this square bar, and the Charpy impact value was investigated. FIG. 8 plots the impact value against the Mo amount.
Up to about 0.4% Mo, the high Mo material has a higher impact value, but embrittlement cannot be ignored if the Mo amount is 0.88% or more. When considered as general-purpose steel, an increase in cost becomes an industrial problem when the Mo content is 0.88% or more.

0.01 ≤ V <0.21
If it is less than 0.01%, the softening resistance is not sufficient. If it is 0.21% or more, the amount of coarse carbide becomes excessive and the impact value deteriorates. A preferable range is 0.05 ≦ V ≦ 0.15, which is excellent in the balance between softening resistance and fatigue strength.

  This is shown in FIG. FIG. 9 shows the softening resistance at 600 ° C. of 0.22C-0.09Si-1.52Mn-1.95Cr-0.80Mo-0.01Nb-V steel tempered to 38HRC. The test piece was a square bar of 10 mm × 10 mm × 30 mm, held at 600 ° C. for 30 hours, cooled to room temperature, measured for hardness, and evaluated for the amount of decrease ΔHRC from the initial hardness of 38 HRC. In FIG. 9, ΔHRC is plotted against the V amount. A smaller ΔHRC is preferred because of excellent heat resistance. When the amount of V is less than 0.01%, the effect of improving the softening resistance is hardly recognized.

V improves softening resistance only when V dissolves in austenite during quenching. The fine V carbides precipitated by tempering are difficult to coarsen, so the decrease in hardness is delayed. Therefore, for the improvement of the softening resistance, it is sufficient to add an amount of V sufficient to dissolve at the time of quenching.
Generally, the quenching temperature of general-purpose steel is 1000 ° C. or less, but the amount of V dissolved in this temperature range is less than 0.21%.
Therefore, a V amount of less than 0.21% is sufficient. When considered as general-purpose steel, an increase in cost becomes an industrial problem when the V content is 0.21% or more.

In addition, the raw material for softening resistance evaluation was created in the following procedures.
First, molten steel melted and refined in a vacuum was cast into a 2t ingot. The ingot was subjected to a homogenization heat treatment by heating at 1250 ° C. for 18 hours. Thereafter, a block having a cross section of 250 mm × 250 mm was manufactured by hot forging of the ingot. The block in the high-temperature state after forging was allowed to cool to room temperature, reheated to 820 ° C. after heating for 6 hours at 670 ° C., and slowly cooled at 10 ° C./Hr after holding for 4 hours.

When the block reached 620 ° C., the slow cooling was stopped, and the block was removed from the furnace and allowed to cool to room temperature. Through such a series of treatments, a block having a soft hardness of about 90 HRB and a uniform structure was manufactured. Using this block as a raw material, a square bar of 11 mm × 11 mm × 62 mm was shaved from the vicinity of the center of the cross section, the square bar was heated to 920 ° C., rapidly cooled, tempered, and tempered to 38 HRC.
Further, FIG. 9 shows the results of making a 10 mm × 10 mm × 30 mm test piece from this square bar and examining the softening resistance.

0.004 ≦ Nb <0.03
If it is less than 0.004%, the effect of suppressing the growth of crystal grains during quenching is poor. If it is 0.03% or more, coarse carbonitride is formed, and impact value and mechanical fatigue strength are lowered. In particular, the fatigue strength is significantly reduced.
If the content is 0.03% or more, the recrystallization temperature rises excessively, so in hot working of the mold material, cracking, an increase in processing force, and an extension of the processing time are caused, resulting in an increase in material cost.
A preferable range is 0.007 ≦ Nb ≦ 0.027, which is excellent in the effect of suppressing crystal grain growth, can ensure high fatigue strength, and does not inhibit hot workability.

These are shown in FIGS. 10, 11, and 12. FIG.
An 11 mm × 11 mm × 60 mm square bar made from 0.22C-0.08Si-1.53Mn-1.91Cr-0.80Mo-0.11V-Nb steel was heated to 980 ° C. and rapidly cooled after holding for 30 min. Further, it was tempered at 575 ° C. and tempered to 38 HRC.
FIG. 10 shows the austenite crystal grain size (based on JIS G 0551) evaluated by observation by corroding the surface of the square bar with a chemical to reveal a metallographic structure with respect to the amount of Nb. A larger particle size is preferred because it is excellent in toughness.

  Generally, it is desirable that the grain size at the time of quenching exceeds 4. However, if the Nb content is less than 0.004%, the crystal grain size is as small as 4 or less, and the empirically recommended value cannot be satisfied. When the Nb content is 0.03%, the crystal grain size is as large as 7.6, and the crystal grain is considerably fine. Further, as the amount of Nb increases, the particle size increases, but the degree of increase is not so significant.

The material for tissue observation was prepared by the following procedure.
First, molten steel melted and refined in a vacuum was cast into a 2t ingot. The ingot was subjected to a homogenization heat treatment by heating at 1250 ° C. for 18 hours. Thereafter, a block having a cross section of 250 mm × 250 mm was manufactured by hot forging of the ingot. The block in the high-temperature state after forging was allowed to cool to room temperature, reheated to 820 ° C. after heating for 6 hours at 670 ° C., and slowly cooled at 10 ° C./Hr after holding for 4 hours.

  When the block reached 620 ° C., the slow cooling was stopped, and the block was removed from the furnace and allowed to cool to room temperature. Through such a series of treatments, a block having a soft hardness of about 90 HRB and a uniform structure was manufactured. Using this block as a raw material, a square bar of 11 mm × 11 mm × 60 mm was prepared from the vicinity of the center of the cross section, and the square bar was heated to 920 ° C. to rapidly cool, and returned to temper to 38 HRC. Further, FIG. 10 shows the result of investigating the austenite grain size by revealing the metal structure of the square bar surface.

FIG. 11 shows a 10 mm × 10 mm × 55 mm impact test piece (JIS No. 3) prepared from the square bar used in the investigation of FIG. 10, and plots the Charpy impact value measured at room temperature against the Nb amount.
A higher impact value is preferable because it is less likely to break against an unexpected impact load. The change in impact value roughly corresponds to FIG. 10 showing the change in crystal grain size.
The impact value increases with refinement. However, when Nb is 0.03% or more, the impact value decreases. This is because cracks starting from coarse NbCN tend to occur. Nevertheless, 0.2% Nb steel has a slightly higher impact value than 0.004% Nb steel.

Figure 12 is a full length creates a rotating bending fatigue test piece of 150mm in parallel portion is 6mm × 18 mm from the same site of the same material as used in the test of FIG. 10, the fatigue strength of 10 ⁴ cycles was evaluated at room temperature (time intensity ) Is plotted against the amount of Nb. Here, in the rotating bending fatigue test, one end of the test piece is supported, a weight is hung on the other end, a load is applied, and the support portion of the test piece is rotated about the axis of the test piece to rotate. The fatigue strength was evaluated based on the stress value when the specimen reached failure at 10 ⁴ . Higher fatigue strength is preferable because it is less likely to crack against repeated loads.

Since the fatigue test load style is close to that of a practically used mold, the superiority or inferiority of the fatigue strength corresponds well with the actual life of the mold. Similar to FIG. 11 showing the change of the impact value, the fatigue strength shows a maximum value with respect to the Nb amount.
The reason for this is the same as in the case of the impact value. However, the deterioration of the characteristics when Nb is 0.03% or more is more remarkable than the case of the impact value. That is, the influence of the Nb amount is large. When Nb is 0.004% or more and until 0.03% (less than), high fatigue strength exceeding 964 MPa is exhibited, whereas when Nb content exceeds Nb, the degree of strength decrease is large.

  When the fracture surface after the fatigue test was observed, coarse foreign matter was observed at the crack initiation point of the high Nb material with low fatigue strength. As a result of analysis by X-ray, Nb, C, N Was detected. That is, this foreign material was found to be NbCN. Coarse NbCN is extremely detrimental to mechanical fatigue strength.

On the other hand, coarse NbCN was not observed in the low Nb material having low fatigue strength. The foreign matter present at the starting point of the crack was small, and as a result of elemental analysis by X-ray, it was an oxide or a sulfide.
It is thought that the major factor that reduced the fatigue strength of the low Nb material was that the crack progressed easily because the crystal grain size was small.

In addition, it is as follows when the preparation procedure of a fatigue strength test piece is supplemented.
First, a round bar of φ22 mm × 160 mm was made from the vicinity of the center of the raw material, this round bar was heated to 920 ° C., rapidly cooled, tempered and tempered to 38 HRC.
Furthermore, FIG. 12 shows the result of examining the fatigue strength by preparing a rotating bending fatigue test piece having a parallel portion of φ6 mm × 18 mm and a total length of 150 mm from this round bar.

From FIG. 10 to FIG. 12, it is clear that 0.004 ≦ Nb <0.03 is desirable. In particular, when considering the mechanical fatigue strength of steel products (in this case, molds) manufactured from industrial-sized ingots, it is important to select a smaller amount of Nb than usual (the invention example disclosed in the prior patent document). It can be said.
Nb also has an effect of increasing softening resistance and wear resistance, but such an effect can hardly be expected when added in a small amount as defined in the present invention. Therefore, the addition of Nb according to the present invention is not intended to improve softening resistance or wear resistance.

[Mechanical fatigue and Nb]
Since stress is repeatedly applied to the mold, the fracture behavior, i.e., fatigue strength, when periodic stress is applied over a long cycle is very important. Fatigue strength decreases when there are many coarse foreign substances that become crack starting points. In the present invention, coarse NbCN is a typical foreign material.

  NbCN is formed during solidification and hardly dissolves in subsequent heat treatments. Therefore, in order to ensure fatigue strength, it is essential that coarse NbCN is not formed during solidification. Since the size of NbCN increases as the solidification rate decreases, in order to avoid the formation of coarse NbCN, the molten steel injected into the ingot must be rapidly solidified.

In a small experimental ingot (several kg to several hundred kg), NbCN does not become so large because of the high coagulation rate.
On the other hand, in an industrial size (several t to several tens of t) ingot with a low solidification rate, NbCN becomes extremely coarse. Therefore, a decrease in fatigue strength when it becomes a mold becomes a problem.

  Nb has the effect of suppressing the growth of crystal grains and improving the mechanical properties such as impact value, but if the appropriate Nb addition amount is not selected, fatigue strength will be reduced, and an appropriate mold life will be obtained. It is not possible. Therefore, the optimization of the amount of Nb added is an extremely important issue when considering extending the mold life. When an ingot of a large industrial size is assumed, it is important to select a smaller amount of Nb than usual (prior art example).

[Hot workability and Nb]
In hot working in which steel is formed into a mold material, recrystallization occurs repeatedly in the process. Recrystallization is a phenomenon responsible for strain release, and the material that releases strain is soft and highly ductile. Therefore, the recrystallized material is less likely to cause problems such as difficulty in processing due to high deformation resistance and cracking due to low deformation capability in the subsequent hot working process. By recrystallization, hot workability can be ensured.

Nb is an element that remarkably suppresses recrystallization. When Nb is added, the recrystallization temperature of the steel increases. That is, the Nb-containing steel does not recrystallize unless the temperature is higher than that of the Nb-free steel.
The surface temperature of the material during the hot working process is lowered by contact with the tool or heat radiation to the atmosphere. When the surface temperature is equal to or lower than the recrystallization temperature, the above-described problems (difficult to work with high deformation resistance, easy to crack with low deformation ability) tend to occur.
Therefore, the material whose temperature has dropped is returned to the heating furnace and reheated to a temperature exceeding the recrystallization temperature. When it reaches a predetermined temperature, it is removed from the furnace and hot working is continued again. Reheating is a treatment that restores hot workability.

Here, the Nb-containing steel does not recrystallize unless it is at a higher temperature than the Nb-free steel, so that the number of times of reheating by inevitably returning to the furnace increases. That is, the process time is extended, the productivity is lowered, and the material cost is likely to increase.
Further, when the temperature is lowered, the Nb-containing steel has a lower deformability than the Nb-free steel, so that cracking is likely to occur. That is, the yield tends to deteriorate and the material cost tends to increase.

In other words, Nb has the effect of suppressing the growth of crystal grains and improving mechanical properties such as impact value, but if an appropriate Nb addition amount is not selected, the recrystallization temperature is excessively increased, Securing processability (low deformation resistance and high deformability) becomes difficult.
Therefore, optimizing the amount of Nb added is an extremely important issue when considering the cost reduction of mold steel.

  Usually, recrystallization of Nb-free steel occurs in a temperature range of 780 ° C. or higher. When 0.03% or more of Nb is added to this steel, recrystallization does not occur unless the temperature is 1000 ° C. or higher. That is, even when the temperature of ordinary steel is lowered to about 800 ° C., it is possible to somehow ensure hot workability. However, in the case of 0.03% Nb steel, hot workability cannot be secured unless the temperature is 1000 ° C. or higher. Further, in a steel having an Nb content exceeding 0.05%, recrystallization does not occur unless the temperature is 1050 ° C. or higher.

In general, since hot working is performed at 1000 ° C. or higher, recrystallization in this temperature range is essential for ensuring hot workability, that is, maintaining low deformation resistance and high deformability.
Therefore, the effect of Nb on deformation resistance and deformability was investigated. The target was 1000 ° C. close to the lower limit temperature of hot working and 50% compression rate close to the maximum working amount of hot working.

This is shown in FIGS. 13 and 14. FIG.
A test piece for measuring deformation resistance of φ14 mm × 21 mm was prepared from the same part of the same material used in the test of FIG. 10, this test piece was heated to 1000 ° C. at 5 ° C./s, held for 100 s, and strain rate 10 s. ^The deformation resistance was measured by processing at ^-1 . The processing amount is set to 50% compression rate at which the height of the test piece is half of the initial state.

  FIG. 13 shows the deformation resistance at this time with respect to the Nb amount. Since the resistance to hot working is low, the lower the deformation resistance, the easier the working and the better. The deformation resistance changes abruptly when the Nb content is around 0.03%, and it can be said that the deformation resistance is low on the lower Nb side and the workability is good. In addition, for each steel type having a different Nb amount, 10 test pieces were processed, and an average value of 10 pieces was plotted as the deformation resistance.

Moreover, when the test piece which received 50% processing was allowed to cool to room temperature and the surface was observed after cooling, cracks were sometimes observed. FIG. 14 shows how many of the 10 test pieces processed for each steel type are cracked as the crack generation rate, which is shown with respect to the Nb amount. The lower the crack occurrence rate, the higher the yield and the better. The cracking incidence is 50% with 0.03% Nb material, which means that half of them are cracked.
On the lower Nb side, the crack generation rate is low, and it can be said that the workability is good (yield is good). In a situation where more than half are broken, the decrease in yield is remarkable industrially, so the Nb content is considered to be less than 0.03%. This value is consistent with FIGS.

In the present invention, the impurity component can be regulated as follows.
W <0.30%
Co <0.30%
Ta <0.004%
Ti <0.004%
Zr <0.004%
Al <0.004%
N <0.004%
Cu <0.15%
Ni <0.15%
B <0.0010%
S <0.010%
Ca <0.0005%
Se <0.03%
Te <0.005%
Bi <0.01%
Pb <0.03%
Mg <0.005%
O <0.0080%

In the present invention, since strength is increased by precipitation of carbides, W can be added in the following range as necessary.
0.30 ≤ W ≤ 4.00
If it is less than 0.30%, the effect of increasing the strength is small, and if it exceeds 4.00%, the effect is saturated and the cost is significantly increased.

In the present invention, since strength is increased by solid solution in the base material, Co can be added in the following range as necessary.
0.30 ≤ Co ≤ 3.00
If it is less than 0.30%, the effect of increasing the strength is small, and if it exceeds 3.00%, the effect is saturated and the cost is significantly increased.

In the present invention, if the effect of crystal grain refinement by addition of Nb is not sufficient, at least one of the following can be added as necessary for the purpose of further refinement.
0.004 ≤ Ta ≤ 0.100
0.004 ≤ Ti ≤ 0.100
0.004 ≤ Zr ≤ 0.100
0.004 ≤ Al ≤ 0.050
0.004 ≤ N ≤ 0.050
If any element is less than a predetermined amount, the effect of improving strength and toughness is small. On the other hand, when the amount exceeds a predetermined amount, carbides, nitrides and oxides are excessively generated, and on the contrary, the toughness is lowered. This is the same as Nb.

In the present invention, in order to improve hardenability, at least one of the following can be added as necessary.
0.15 ≤ Cu ≤ 1.50
0.15 ≤ Ni ≤ 1.50
0.0010 ≤ B ≤ 0.0100
If any element is less than a predetermined amount, the effect of improving the hardenability is small. On the other hand, if the amount exceeds a predetermined amount, the effect is saturated and the actual profit is poor. Furthermore, for Cu and Ni, excessive addition reduces the thermal conductivity.

In the present invention, in order to improve machinability, at least one of the following can be added as necessary.
0.010 ≤ S ≤ 0.500
0.0005 ≤ Ca ≤ 0.2000
0.03 ≤ Se ≤ 0.50
0.005 ≤ Te ≤ 0.100
0.01 ≤ Bi ≤ 0.30
0.03 ≤ Pb ≤ 0.50
If any element is less than a predetermined amount, the machinability improvement effect is small. On the other hand, if the amount exceeds a predetermined amount, the hot workability is remarkably deteriorated, so that many cracks occur in the plastic working and the productivity and the yield are lowered.

  In addition, the steel of the present invention is particularly effective when used in a die-casting mold having a severe heat load. That is, the die casting mold has a large temperature amplitude on the mold surface, and thermal fatigue tends to accumulate. Moreover, since the temperature of the mold surface becomes high, the problem of easy seizure tends to occur. For this reason, the steel of the present invention has a great effect when used in a die casting mold.

It is the figure which showed the relationship between Si content and machinability. It is the figure which showed the relationship between Si content and thermal conductivity. It is the figure which showed the relationship between Mn content and an impact value. It is the figure which showed the relationship between Mn content and thermal conductivity. It is the figure which showed the relationship between Cr content and an impact value. It is the figure which showed the relationship between Cr content and thermal conductivity. It is the figure which showed the relationship between Mo content and the intensity | strength in 600 degreeC. It is the figure which showed the relationship between Mo content and an impact value. It is the figure which showed the relationship between V content and softening resistance. It is the figure which showed the relationship between Nb content and the crystal grain size at the time of quenching. It is the figure which showed the relationship between Nb content and an impact value. It is the figure which showed the relationship between Nb content and mechanical fatigue strength. It is the figure which showed the relationship between Nb content and deformation resistance. It is the figure which showed the relationship between Nb content and a crack generation rate.

Next, embodiments of the present invention will be described in detail below.
The 48 steel types shown in Tables 1 and 2 were melted in a vacuum and cast into 6t ingots.
After these ingots were homogenized at 1260 ° C. × 24 Hr, blocks having a cross section of 220 mm × 560 mm were manufactured by hot forging. The block in a high-temperature state after completion of forging is allowed to cool to room temperature, subsequently tempered at 700 ° C., then reheated to 900 ° C. and held for 4 hours, and then gradually cooled at 10 ° C./Hr to about 90 HRB. Softened.

This block was heated to 920 ° C., and after 2 hours of holding, it was poured into water (quenching). Further, this block was tempered to about 38 HRC by tempering at 560 ° C. to 580 ° C. × 1 Hr to 8 Hr. Appropriate conditions were selected for the tempering temperature and time depending on the components so that 38 HRC was obtained.
Note that the comparative steel A01 had a carbon content of about 31 HRC.
A die cast mold of about 200 kg was manufactured from the conditioned block by machining. At the same time, test pieces for evaluating various properties were cut out from the vicinity of the center of the conditioned block.

Tables 3 and 4 show the test results of the test pieces cut out from the vicinity of the center of the block. The evaluation methods for impact value, thermal conductivity, high temperature strength, and mechanical fatigue strength are as described above.
Comparative steel A15 corresponds to JIS SKD61.
As for the impact value, 63 J / cm ² or more is judged as “◯”. As for the thermal conductivity, a value of 33 W / m / K or more at which the cooling ability is dramatically improved as compared with JIS SKD61 (Comparative Steel A15, thermal conductivity 24 W / m / K) is judged as “◯”. The high-temperature strength is “◎” when 830 MPa or more, “×” when 770 MPa or less, and “◯” otherwise.
The mechanical fatigue strength is 964 MPa or less as “x” and the others as “◯”.
For hot workability, if the amount of cut increases more than necessary due to the occurrence of cracks, or if the production efficiency decreases due to the need for multiple reheats, the case where cracks are not a problem or A case where no reheat is required and the production efficiency has not decreased is indicated as “◯”.

Inventive steel 33 shows good characteristics in all items, and the evaluation is “◯”. This is proof that the balance of chemical components is appropriate.
In the comparative steel, there is a problem in any of the characteristics other than the two steel types A03 and A11. Since the comparative steel A12 of 0.59% V has many carbides that are the starting points of crack generation, the impact value is lowered. However, no significant adverse effects on mechanical fatigue are observed.
The comparative steel A15 of 0.94% V has a significant decrease in impact value due to an increase in carbides as compared with the comparative steel A12, and has a great influence on fatigue strength.

Although the 0.080% Nb comparative steel A14 has an impact value, the fatigue strength is significantly reduced.
This is because coarse Nb carbonitrides are the starting point for crack initiation. Comparative steel A14 also has poor hot workability, which is the result of Nb significantly delaying recrystallization.
Moreover, it was a test piece cut out from the vicinity of the center of a large block where the quenching speed becomes small. Comparative steel A05 and comparative steel A07 with low Cr and Mn and low hardenability have low impact values.

  The comparative steel A09 and the comparative steel A10 have low impact values because the Mo amount is not appropriate. The thermal conductivity was low in the steel grades with a lot of Si and Cr. Further, the high temperature strength is low in the comparative steel A01 in which the predetermined hardness was not obtained and in the comparative steel A09 that reached the predetermined hardness but had a small Mo content.

Tables 5 and 6 show the damage situation when the die casting mold is assembled and cast in the machine.
The cast aluminum alloy is ADC12, and the temperature of the melting and holding furnace is 680 ° C. The weight of the die-cast product is about 5 kg, and one cycle is 47 s. Heat check, wear, and softening (hardness) were evaluated on the mold surface after 10,000 shot casting.

  The heat check is evaluated by visual observation and maximum crack depth. First, the part where the heat check is most prominent is selected by visual observation. Next, the part is cut out so that a region from the mold surface to the inside can be observed. Then, paying attention to the deepest crack observed in the cut surface (heat check that has advanced into the mold), the distance from the opening (mold surface) to the tip of the tip crack is measured. This is the maximum crack depth.

If the maximum depth of the crack is 0.3 mm or less, the opening of the crack is also narrow, so that it is difficult to insert aluminum, so the surface quality is not a problem with this product. Therefore, the evaluation is “◯”. If the maximum crack depth is 1 mm or more, the opening of the crack is wide, and therefore aluminum can be easily inserted, so that the surface quality of the product becomes a problem, resulting in a defective product.
Furthermore, this deep crack has a high risk of promoting large cracks in the mold. Therefore, the evaluation is “x”. When the maximum crack depth exceeds 0.3 mm and is less than 1 mm, the evaluation is “Δ”.

Wear due to injected Al is determined by the degree of influence on the dimensional standard of the product. If the wear was not significant and the product did not deviate from the dimensional standard, “◯” was indicated. If the product was worn out to a local extent and the product deviated from the dimensional standard, “X” was indicated.
For the softening, the surface hardness of the portion where the heat check is most prominent was measured.
In addition, the presence or absence of large cracks penetrating the mold was evaluated. When such a crack occurred, “x” was indicated, and when it did not occur, “◯” was indicated. Large cracks may be caused by mechanical fatigue, or when the heat check progresses to the inside, they may merge.
Therefore, it becomes remarkable in the steel material with low mechanical fatigue strength and easy to generate heat check.

  Further, it was evaluated whether or not significant seizure occurred until the casting of 10,000 shots was completed. If the seizure is mild and the maintenance of the mold is sporadically less than several times, “O”, if the casting must be interrupted to remove seizure every 200 to 2000 shots, × ”.

  The evaluation of machinability was judged from the work efficiency when the die casting die was actually cut and the wear state of the cutting tool. Cutting steel with poor machinability tends to cause local abnormal wear and chipping on the cutting tool, resulting in lower work efficiency due to frequent replacement of the cutting tool and increased cost due to the use of a large number of cutting tools. Will be forced.

This time, the machinability is evaluated by comparing with the case of cutting JIS SKD61, which is most often used as a die casting mold. “○” if the wear state and machining efficiency of the cutting tool are equivalent to JIS SKD61, “X” if the cutting tool is frequently replaced and the machining efficiency is less than half that of JIS SKD61. Although the property (cutting efficiency) decreased, there were few practical problems.
Invented steel 33 has no major problems, and all items excluding machinability are evaluated as “◯”.
Further, it was judged that machinability is not a problem in practical use. That is, it is clear that the 33 type steels are mold steels excellent in both mold performance and cost.

Since the metal mold using the inventive steel has high thermal conductivity, overheating is suppressed and aluminum seizure is drastically reduced. Moreover, the reason why the heat check is difficult to occur is that the thermal stress is small because the thermal conductivity is high. The wear caused by the aluminum injected at high speed is very small and corresponds to the high temperature strength. Since there are very few coarse foreign substances that are the starting points of crack generation, no large cracks have occurred. The softening of the mold surface that facilitates the heat check is very slight, and it corresponds to the high resistance to softening. Although the amount of rare elements such as Cr, Mo and V is small, the invention steel exhibited excellent mold performance.
Moreover, as a result of investigating the structure of the cast product, it was confirmed that it was made finer than before. This is due to the increase in the solidification rate because the mold temperature decreased. Furthermore, it became clear that the casting cycle can be shortened by 2 to 3 sec because of faster solidification.

  On the other hand, the comparative steel 15 type has some problems. As a general tendency, seizure and heat check are remarkable in the steel types having low thermal conductivity. Steel types with low impact values and softening resistance tend to promote heat check. Large cracks are likely to occur in steel types with very low impact values, or steel types with relatively high impact values but low fatigue strength.

  Since the comparative steel A01 has a low high-temperature strength, heat check and wear are remarkable. Since the comparative steel A02, the comparative steel A04, the comparative steel A06, the comparative steel A08, and the comparative steel A15 have low thermal conductivity, heat check and seizure are remarkable. In particular, in comparative steel A15, large cracks corresponding to low mechanical fatigue strength also occurred.

Since the comparative steel A03 had extremely low Si, the machinability was poor, and the mold manufacturing cost was increased. Since the comparative steel A05, the comparative steel A07, the comparative steel A12, and the comparative steel A13 have low impact values, the heat check is remarkable.
Although the comparative steel A09 has high thermal conductivity, wear and heat check are remarkable due to low high-temperature strength. Comparative steel A10 has high temperature strength and thermal conductivity, and is difficult to soften, but has a low impact value, so heat check is remarkable. Moreover, it cannot be recommended from the viewpoint of material cost and resource saving.
Since the comparative steel A11 has a small amount of V, softening of the mold surface is remarkable, and as a result, heat check is promoted.
Although the comparative steel A14 had an impact value, the mechanical fatigue strength was low, and a large crack corresponding to it was generated.

  Although the embodiment of the present invention has been described in detail with reference to a die-casting die as an example, this is merely an example, and the present invention can be implemented in various modifications without departing from the spirit of the present invention. That is, for low pressure casting, plastic and rubber injection molding, and forging dies, the surface of the dies is repeatedly heated and cooled to expose the dies to heat load. Applicable.

Claims (7)

In mass%
0.15 ≤ C ≤ 0.30
0.01 ≤ Si <0.19
1.50 <Mn ≤ 1.78
1.01 <Cr <3.05
0.48 <Mo <0.88
0.01 ≤ V <0.21
0.004 ≤ Nb <0.03
Mold steel having a composition of the balance Fe and inevitable impurities. In Claim 1, in mass%
0.30 ≤ W ≤ 4.00
A mold steel further comprising In any one of Claims 1 and 2,
0.30 ≤ Co ≤ 3.00
A mold steel further comprising In any one of Claims 1-3, In mass%
0.004 ≤ Ta ≤ 0.100
0.004 ≤ Ti ≤ 0.100
0.004 ≤ Zr ≤ 0.100
0.004 ≤ Al ≤ 0.050
0.004 ≤ N ≤ 0.050
Steel for molds further comprising at least one of them. In any one of Claims 1-4, In mass%
0.15 ≤ Cu ≤ 1.50
0.15 ≤ Ni ≤ 1.50
0.0010 ≤ B ≤ 0.0100
Steel for molds further comprising at least one of them. In any one of Claims 1-5 in mass%
0.010 ≤ S ≤ 0.50
0.0005 ≤ Ca ≤ 0.2000
0.03 ≤ Se ≤ 0.50
0.005 ≤ Te ≤ 0.100
0.01 ≤ Bi ≤ 0.30
0.03 ≤ Pb ≤ 0.50
Steel for molds further comprising at least one of them.   The die-casting steel for die-casting in any one of Claims 1-6.

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