KR102495092B1 - Steel for mold, and mold - Google Patents

Abstract

In the present invention, 0.28 mass%≤C≤0.65 mass%, 0.01 mass%≤Si≤0.30 mass%, 1.5 mass%≤Mn≤3.0 mass%, 0.5 mass%≤Cr≤1.4 mass%, 1.9 mass%≤Mo+ W/2≤4.0 mass%, 0.2 mass%≤V≤1.0 mass%, 0.01≤N≤0.10 mass%, the balance being Fe and unavoidable impurities. And in the state after tempering, the amount of (Mo, W) carbides having a diameter of 0.2 μm or less is 1.2% by mass or more, and the ratio (mass ratio) of the amount of (Mo, W) carbides to the amount of Cr carbides is 11 or more, , the hardness change is less than 15 HRC.

Description

Steel for mold and mold {STEEL FOR MOLD, AND MOLD}

The present invention relates to a steel for a mold and a mold, and more particularly, to a steel for a mold exhibiting high softening resistance and a mold using the steel for a mold.

"Die-casting" refers to a casting method for manufacturing castings with high dimensional accuracy by press-fitting molten metal into a mold cavity under high pressure. Die casting is used to manufacture castings made of metals with relatively low melting points (eg, Al, Zn, Mg or alloys thereof).

"Hot-stamping" refers to a forming method in which a sheet material heated to a high temperature is press-formed using a mold, and at the same time as forming the sheet material, the sheet material is rapidly cooled (quenched) with a mold cooled with cooling water. say Hot stamping is used for press forming of a steel sheet having low cold formability and large springback (eg, ultra-high-strength steel sheet).

"Tailored die quenching" means that a plate material heated to a high temperature is press-molded using a mold whose design surface is partially heated by a heat source, and at the same time as forming the plate material, the plate material is formed with the design surface maintained at a low temperature. It refers to a forming method in which a part of the quenching (quenching) is quenched. Tailored die quenching is used for press forming of parts requiring strength distribution (for example, parts constituting the frame of an automobile).

BACKGROUND ART In a method of processing a workpiece at a high temperature such as die casting, hot stamping, and tailored die quenching, a mold is subjected to a heat cycle during use. Therefore, high softening resistance and heat check resistance are required for this type of mold.

So, in order to solve this problem, various proposals have been made conventionally.

For example, in Patent Document 1, (a) in mass%, C: 0.30% to 0.45%, Si: more than 0.3% and 1.0% or less, Mn: 0.6% to 1.5%, Ni: 0.6% to 1.8%, Cr: 1.4 % to less than 2.0%, Mo+W/2: more than 1.0% and 1.8% or less, V+Nb/2: 0.2% or less, the balance consisting of Fe and unavoidable impurities, (b) between each component A high-toughness hot tool steel in which a predetermined relational expression holds is disclosed.

In the same literature, the following three points: (A) softening resistance is improved by optimizing the balance between each component, (B) formation of coarse nitride by reducing the amount of N to 0.015% by mass or less It is described that this is suppressed and toughness is improved, and (C) machinability and softening resistance are improved by optimizing the size and number per unit area of carbides.

In Patent Document 2, (a) in mass%, C: 0.34% to 0.40%, Si: 0.3% to 0.5%, Mn: 0.45% to 0.75%, Ni: 0% to less than 0.5%, Cr: 4.9% to less than 0.5% 5.5%, Mo and W alone or in combination (Mo+1/2W): 2.5% to 2.9% and V: 0.5% to 0.7%, the balance consisting of Fe and unavoidable impurities, (b) quenching thereof The cross-sectional structure of the city includes mass structure and acicular structure, mass structure (A%) is less than 45 area%, acicular structure (B%) is less than 40 area%, and retained austenite (C%) is 5% to 20% by volume of hot tool steel is disclosed.

In this document, the following three points: (A) The structure after quenching has a large effect on the toughness and high-temperature strength, (B) In order to suppress the decrease in toughness, the lumpy structure and the acicular structure are each a predetermined area ratio It is described that it is desirable to reduce to the following, and (C) that the toughness improves when austenite is adequately retained, although the smaller the retained austenite, the better for suppressing the deterioration of the strength characteristics.

In Patent Document 3, (a) by weight, C: 0.25% to 0.55%, Si: 1.2% or less, Mn: 1.5% or less, Ni: 2.0% or less, Cr: 6.0% to 8.0%, 1/2W+ Mo: 5.0% or less, and Cr/Mo≤3 is satisfied, the balance is composed of Fe and unavoidable impurities, (b) the area ratio of undissolved carbide having a grain size of 0.1 μm or more on the texture surface is 1% or more, ( c) Disclosed is a nitrided mold for warm-temperature working, including a nitride layer on at least a contact surface with a workpiece, and having a hardness of 1,100 HV or more at a position of 25 μm from the surface of the nitride layer.

In this document, the following two points: (A) One of the main causes of wear of a nitrided mold is that the surface layer is softened by the heat generated by friction between the mold surface and the high-temperature workpiece, causing plastic flow. Phosphorus point, and (B) by increasing the surface hardness by nitriding treatment and simultaneously increasing the amount of unsolidified carbide, it is disclosed that wear accompanied by heat generation can be synergistically improved. there is.

Further, in Patent Document 4, C: 0.60% to 0.85%, Si: 0.50% to 1.50%, Mn: 1.70% to 2.30%, Cr: 0.70% to 2.00%, Mo: 0.85% to 1.50%, and V by weight ratio. : 0.10% or less, the remainder being Fe and unavoidable impurity elements.

In this document, the following three points: (A) Compared with the conventional case, by reducing the amount of C and the amount of Cr, the amount of carbides is reduced, the grain size of carbides is refined, and the engravability and cold cold hobbing property is improved, (B) when the amount of V is regulated to 0.10% or less, hardenability is improved, formation of stripe-like carbide can be prevented, and engravability is improved; and (C) that Si has an effect of imparting temper softening resistance at about 200°C.

The high-toughness hot tool steel described in Patent Document 1 has a low thermal conductivity because of the large amount of Si. In addition, the amount of Mo+W/2 is small, and the softening resistance is also low.

In the hot tool steel described in Patent Document 2, since the amount of Cr is excessive, Cr carbides precipitate before carbides containing Mo and/or W (hereinafter also referred to as “(Mo, W) carbides”). As a result, although the amount of Mo+W/2 is appropriate, the amount of (Mo, W) carbides precipitated is reduced, and high softening resistance cannot be obtained.

Similar to the hot tool steel described in Patent Literature 2, the nitriding mold for warm working described in Patent Literature 3 also has an excessive amount of Cr, so high softening resistance cannot be obtained.

In addition, the air-quenched cold tool steel described in Patent Document 4 has a small amount of Mo+W/2, and thus has low temper softening resistance in a high temperature range exceeding 200°C.

Japanese Unexamined Patent Publication No. 2016-166379 Japanese Unexamined Patent Publication No. 2008-095190 Japanese Unexamined Patent Publication No. 2001-073087 Japanese Unexamined Patent Publication No. 06-256895

An object of the present invention is to provide a steel for a mold having high softening resistance.

Another object of the present invention is to provide a steel for a mold having high initial hardness and/or high thermal conductivity.

Another object of the present invention is to provide a mold using such mold steel.

In order to solve the above problems, the steel for a mold according to the present invention has the following configuration:

(1) The steel for the mold,

0.28 mass% ≤ C ≤ 0.65 mass%;

0.01 mass% ≤ Si ≤ 0.30 mass%;

1.5 mass% ≤ Mn ≤ 3.0 mass%;

0.5 mass% ≤ Cr ≤ 1.4 mass%;

1.9 mass% ≤ Mo+W/2 ≤ 4.0 mass%;

0.2 mass% ≤ V ≤ 1.0 mass%;

0.01 ≤ N ≤ 0.10 mass%, the balance being Fe and unavoidable impurities.

(2) The steel for the mold is in a state after quenching and tempering,

The amount of (Mo, W) carbides having a diameter of 0.2 μm or less is 1.2 mass% or more,

The ratio (mass ratio) of the amount of (Mo, W) carbides to the amount of Cr carbides is 11 or more,

Hardness change is less than 15 HRC.

The steel for the mold is in the state after quenching and tempering,

Initial hardness greater than 52 HRC and/or

It is preferable that the thermal conductivity at room temperature is 30 W/(m·K) or more.

The mold according to the present invention has the following configuration:

(1) The mold includes a mold steel according to the present invention,

(2) the mold,

The amount of (Mo, W) carbides having a diameter of 0.2 μm or less is 1.2 mass% or more,

The ratio (mass ratio) of the amount of (Mo, W) carbides to the amount of Cr carbides is 11 or more,

Hardness change is less than 15 HRC.

The mold,

Initial hardness greater than 52 HRC and/or

It is preferable that the thermal conductivity at room temperature is 30 W/(m·K) or more.

By making the amount of Cr relatively small and the amount of Mo+W/2 relatively large, a large amount of fine (Mo, W) carbides are deposited, and the precipitation of Cr carbides is suppressed. As a result, softening resistance is improved.

1 is a schematic diagram of hat-bending.
Fig. 2 is an external photograph of the surface of a punch worn by hat-bending.
Fig. 3 is an external photograph of the surface of a punch seized by hat-bending.

EMBODIMENT OF THE INVENTION Hereinafter, embodiment of this invention is described in detail.

[One. Steel for mold]

[1.1. main constituent elements]

The steel for a mold according to the present invention contains the following elements, the remainder being Fe and unavoidable impurities, and may be an ingot material or powder for additive manufacturing. The types of additional elements, their composition ranges and reasons for limitation are as follows.

(1) 0.28 mass% ≤ C ≤ 0.65 mass%:

C is an element required to obtain high hardness. When the amount of C is small, the amount of dissolved C and the amount of carbides become small, and a high hardness of 52 HRC or more cannot be obtained. Therefore, the amount of C needs to be 0.28% by mass or more. The amount of C in the ingot material is preferably 0.50% by mass or more, and more preferably 0.55% by mass or more.

On the other hand, when the amount of C is excessive, since the amount of coarse carbides increases and the amount of retained austenite also increases, a high hardness of 52 HRC or more cannot be obtained. In addition, when the amount of C is excessive when manufacturing a mold using the additive manufacturing method, the mold is easily broken during additive manufacturing. Therefore, the amount of C needs to be 0.65% by mass or less. The amount of C in the powder for additive manufacturing is preferably 0.40% by mass or less, and more preferably 0.35% by mass or less.

(2) 0.01 mass% ≤ Si ≤ 0.30 mass%:

When the amount of Si is excessive, the thermal conductivity decreases. If the thermal conductivity is lowered, the surface temperature of the mold is excessively increased when processing a high-temperature workpiece, and seizure is likely to occur. Therefore, the amount of Si needs to be 0.30% by mass or less. The amount of Si is preferably 0.15% by mass or less, and more preferably 0.10% by mass or less.

(3) 1.5 mass% ≤ Mn ≤ 3.0 mass%:

When the amount of Mn is too small, quenchability is lowered. Therefore, the amount of Mn needs to be 1.5% by mass or more. The amount of Mn is preferably 1.55% by mass or more.

On the other hand, when the amount of Mn is excessive, the thermal conductivity decreases. Therefore, the amount of Mn needs to be 3.0% by mass or less. The amount of Mn is preferably 2.0% by mass or less, and more preferably 1.8% by mass or less.

(4) 0.5 mass% ≤ Cr ≤ 1.4 mass%:

When the amount of Cr is too small, hardenability falls. Therefore, the amount of Cr needs to be 0.5% by mass or more. The amount of Cr is preferably 0.6% by mass or more, and more preferably 0.7% by mass or more.

On the other hand, when the amount of Cr is excessive, a large amount of Cr carbide is generated during tempering, and the amount of (Mo, W) carbide decreases. As a result, softening resistance is lowered. In addition, when the amount of Cr is excessive, the thermal conductivity is lowered.

In addition, when a high-temperature workpiece is processed using a mold containing excess Cr, a high-hardness Cr oxide is generated on the mold surface, and the oxide is entrained to easily wear the mold. In addition, Cr oxide is non-uniformly formed on the surface of the mold, and seizure of the workpiece (eg, coated steel sheet) is likely to occur.

Therefore, the amount of Cr needs to be 1.4% by mass or less. The amount of Cr is preferably 1.3% by mass or less, and more preferably 1.2% by mass or less.

(5) 1.9 mass% ≤ Mo+W/2 ≤ 4.0 mass%:

Mo+W/2 contributes to high hardness and high softening resistance. When the amount of Mo+W/2 is too small, a high hardness of 52 HRC or higher cannot be obtained due to a decrease in the amount of secondary precipitation carbides. Also, the amount of (Mo, W) carbides is reduced, and the softening resistance is lowered. Therefore, the amount of Mo+W/2 needs to be 1.9% by mass or more.

On the other hand, when the amount of Mo+W/2 is excessive, the amount of coarse (Mo, W) carbides increases and the amount of solid solution C decreases. As a result, high hardness of 52 HRC or more and high softening resistance cannot be obtained. In addition, since the amount of expensive Mo and/or W increases, the melting cost also becomes high. Therefore, the amount of Mo+W/2 needs to be 4.0% by mass or less. The amount of Mo+W/2 is preferably 3.0% by mass or less, and more preferably 2.7% by mass or less.

(6) 0.2 mass% ≤ V ≤ 1.0 mass%:

V contributes to suppression of coarsening of crystal grains during quenching. When the amount of V is small, the number of pinning particles (VC particles) is small, and crystal grains are coarsened during quenching. As a result, toughness is low. Therefore, the amount of V needs to be 0.2% by mass or more. The amount of V is preferably 0.3% by mass or more, and more preferably 0.4% by mass or more.

On the other hand, when the amount of V is excessive, the amount of coarse carbides that do not improve the hardness increases. Also, since the amount of dissolved C and the amount of (Mo, W) carbides are reduced, the hardness is lowered and high softening resistance cannot be obtained. Therefore, the amount of V needs to be 1.0% by mass or less. The amount of V is preferably 0.85% by mass or less, and more preferably 0.8% by mass or less.

(7) 0.01% by mass ≤ N ≤ 0.10% by mass:

When the amount of N is small, the amount of fine nitrides serving as nuclei of carbides becomes small, and a predetermined amount of (Mo, W) carbides cannot be obtained. Therefore, the amount of N needs to be 0.01% by mass or more.

On the other hand, when the amount of N is excessive, a large amount of coarse nitride is generated, and fine (Mo, W) carbide cannot be obtained. Therefore, the amount of N needs to be 0.10% by mass or less. The amount of N is preferably 0.05% by mass or less.

(8) Inevitable impurities:

In the present invention, when the following elements are less than the following upper limits, they are treated as unavoidable impurities.

(a) Al < 0.005% by mass;

(b) P < 0.05 mass %;

(c) S < 0.01% by mass;

(d) Cu < 0.30% by mass;

(e) Ni < 0.30% by mass;

(f) O < 0.01% by mass;

(g) Co < 0.10% by mass;

(h) Nb < 0.10% by mass;

(i) Ta < 0.10% by mass;

(j) Ti < 0.10% by mass;

(k) Zr < 0.01% by mass;

(l) B < 0.001% by mass;

(m) Ca < 0.001% by mass;

(n) Se < 0.03% by mass;

(o) Te < 0.01% by mass;

(p) Bi < 0.01% by mass;

(q) Pb < 0.03% by mass;

(r) Mg < 0.02% by mass, and

(s) REM < 0.01% by mass.

[1.2. Sub-constituting element]

The steel for a mold according to the present invention may further contain one or more of the following elements in addition to the main constituent elements. The types of elements to be added, their component ranges, and reasons for limitation are as follows.

(9) 0.005 mass% ≤ Al ≤ 1.5 mass%:

(10) 0.01 mass% ≤ Ti ≤ 0.5 mass%:

(11) 0.01% by mass ≤ Nb ≤ 0.5% by mass:

(12) 0.01% by mass ≤ Zr ≤ 0.5% by mass:

(13) 0.01 mass% ≤ Ta ≤ 0.5 mass%:

Al, Ti, Nb, Zr, and Ta all contribute to suppression of grain coarsening during quenching. All of these elements form precipitates that function as pinning particles. As a result, crystal grains become fine grains and toughness is improved. In order to obtain such an effect, it is preferable that the content of each of these elements is equal to or more than the lower limit.

On the other hand, when the content of each of these elements is excessive, precipitates aggregate and do not function as pinning particles. Therefore, the content of each of these elements is preferably equal to or less than the above upper limit.

The steel for a mold may contain any one of these elements, or may contain two or more of these elements.

(14) 0.01 mass% ≤ Co ≤ 1.0 mass%:

Co contributes to improvement of high temperature strength. In order to obtain these effects, the amount of Co is preferably 0.01% by mass or more. The amount of Co is preferably 0.3% by mass or more, and more preferably 0.5% by mass or more.

On the other hand, when the amount of Co is excessive, the melting cost increases and the thermal conductivity decreases. Therefore, the amount of Co is preferably 1.0% by mass or less. The amount of Co is more preferably 0.9% by mass or less.

(15) 0.30% by mass ≤ Ni ≤ 1.0% by mass:

(16) 0.30 mass% ≤ Cu ≤ 1.0 mass%:

Both Cu and Ni retard the formation of pearlite and contribute to the improvement of hardenability. In order to obtain such an effect, the amount of Ni and the amount of Cu are each preferably 0.30% by mass or more.

On the other hand, when the amount of Ni or Cu is excessive, the melting cost increases and the thermal conductivity decreases. Also, when the amount of Ni is excessive, the amount of retained austenite increases and the hardness decreases. Therefore, the amount of Ni and the amount of Cu are each preferably 1.0% by mass or less.

The steel for the mold may contain Ni or Cu, or both.

(17) 0.01 mass% ≤ S ≤ 0.15 mass%:

(18) 0.001% by mass ≤ Ca ≤ 0.15% by mass:

(19) 0.03 mass% ≤ Se ≤ 0.35 mass%:

(20) 0.01 mass% ≤ Te ≤ 0.35 mass%:

(21) 0.01% by mass ≤ Bi ≤ 0.50% by mass:

(22) 0.03 mass% ≤ Pb ≤ 0.50 mass%:

S, Ca, Se, Te, Bi and Pb all contribute to improvement of machinability. In order to obtain such an effect, the content of each of these elements is preferably equal to or greater than the lower limit.

On the other hand, when the content of each of these elements is excessive, inclusions are likely to be formed. Inclusions function as a starting point of fracture and cause a decrease in toughness. Therefore, the content of each of these elements is preferably equal to or less than the above upper limit.

The steel for a mold may contain any one of these elements, or may contain two or more of these elements.

[1.3. characteristic]

[1.3.1. heat treatment conditions]

Since the composition of the steel for a mold according to the present invention is optimized, it exhibits high properties in the state after quenching and tempering.

Here, the expression "state after quenching and tempering" means: (a) after soaking at 1,030 ° C ± 20 ° C for 45 minutes ± 15 minutes, quenching at a cooling rate of 9 ° C / min to 30 ° C / min; , (b) It refers to the state after tempering at 540°C to 600°C for 1 hour followed by air cooling twice.

In addition to optimizing the components, the amount of Cr carbides and the amount of (Mo, W) carbides can be controlled by performing quenching and tempering under the conditions described above.

Since crystallized carbides may be produced after melt casting in some cases, it is preferable to perform a soaking treatment at 1,200° C. or higher after melt casting and before hot forging. In particular, when the amount of C is in the range of 0.55% by mass to 0.65% by mass, it is preferable to perform the soaking treatment. When the soaking treatment is performed, coarse crystallized carbides can be reduced, fine (Mo, W) carbides can be deposited in large quantities after quenching and tempering, and softening resistance is improved.

When the quenching temperature is too low, the amount of solid solution elements becomes small, and the hardness and softening resistance become low. Therefore, the quenching temperature is preferably 1,010°C or higher.

On the other hand, when the quenching temperature is too high, the amount of solid-solution elements is too large, the retained austenite increases, and the hardness is lowered. Also, crystal grains are coarsened. Therefore, the quenching temperature is preferably 1,050°C or less.

The holding time at the quenching temperature is preferably 45 minutes ± 15 minutes from the viewpoint of making the quenching temperature uniform.

The cooling rate is preferably 9° C./min or more to make the structure full martensite, and preferably 30° C./min or less to suppress quenching cracks.

When the tempering temperature is too low, the amount of Cr carbides increases and the amount of (Mo, W) carbides decreases. Therefore, as for tempering temperature, 540 degreeC or more is preferable.

On the other hand, when the tempering temperature is too high, the carbide is coarsened, and the effect of improving the initial hardness and softening resistance is reduced. Therefore, as for tempering temperature, 600 degreeC or less is preferable.

The holding time at the tempering temperature is preferably about 1 hour so as to obtain the highest hardness in terms of tempering parameters including temperature and time.

The number of times of tempering is preferably twice because it is necessary to temper the structure transformed from retained austenite to martensite during the first tempering.

[1.3.2. (Mo, W) amount of carbide]

In the present invention, the expression “amount of (Mo, W) carbides” refers to the mass ratio of fine carbides containing Mo and/or W in steel (with a diameter of 0.2 μm or less). The term "diameter" refers to the equivalent diameter of a circle.

Fine (Mo, W) carbides contribute to improvement of initial hardness and softening resistance. In general, the higher the amount of (Mo, W) carbide, the better the initial hardness, and/or the higher the softening resistance. To obtain such an effect, the amount of (Mo, W) carbides in the state after quenching and tempering needs to be 1.2% by mass or more.

[1.3.3. The ratio of the amount of (Mo, W) carbides to the amount of Cr carbides]

The expression “ratio of the amount of (Mo, W) carbide to the amount of Cr carbide (hereinafter also referred to as “carbide ratio”)” refers to a mass of Cr carbide having a diameter of 0.2 μm or less ( It refers to the mass ratio of Mo, W) carbides (amount of fine (Mo, W) carbides/amount of fine Cr carbides).

Fine (Mo, W) carbides and fine Cr carbides both contribute to the initial hardness. However, when the amount of Cr carbide is relatively excessive, since C is taken away by Cr carbide, the amount of (Mo, W) carbide decreases and high softening resistance cannot be obtained. Therefore, the carbide ratio in the state after quenching and tempering needs to be 11 or more.

[1.3.4. initial hardness]

"Initial hardness" refers to the Rockwell hardness (C scale) measured at room temperature immediately after quenching and tempering.

Since the steel for a mold according to the present invention has a relatively small amount of Cr and a relatively large amount of Mo+W/2, when quenching and tempering are performed under appropriate conditions, fine ( Mo, W) A large amount of carbide is precipitated. As a result, high initial hardness is obtained. By optimizing the manufacturing conditions, the initial hardness in the state after quenching and tempering can be 52 HRC or more. Further optimizing the manufacturing conditions, the initial hardness may be 53 HRC or higher, or 54 HRC or higher.

[1.3.5. Hardness change (softening resistance)]

"Hardness change" refers to the absolute value expressed by the following formula (1). Hardness change represents the magnitude of softening resistance, and the closer the hardness change is to zero, the higher the softening resistance.

Hardness change = |H _b - H _a | (One)

However, H _a represents the Rockwell hardness (C scale) measured at room temperature after quenching and tempering under the above conditions and further holding at 600 ° C. for 130 hours,

H _b is the Rockwell hardness (C scale) measured at room temperature immediately after quenching and tempering under the above conditions.

Since the steel for a mold according to the present invention has a relatively small amount of Cr and a relatively large amount of Mo+W/2, when quenching and tempering are performed under appropriate conditions, fine (Mo, W) in the matrix A large amount of carbide is precipitated. Fine (Mo, W) carbides not only improve initial hardness, but also contribute to improvement in softening resistance. By optimizing the manufacturing conditions, the hardness change can be less than 15 HRC. Further optimizing the manufacturing conditions, the hardness change can be 12 HRC or less, or even 10 HRC or less.

[1.3.6. thermal conductivity]

Generally, Si is added for the purpose of a deoxidizer or improvement of hardness, improvement of machinability, improvement of oxidation resistance, improvement of temper softening resistance at about 200°C, and the like. However, when the Si content is excessive, the thermal conductivity is remarkably lowered.

The steel for a mold according to the present invention has high thermal conductivity because the amount of Si is kept to a minimum. Specifically, when manufacturing conditions are optimized, the thermal conductivity at room temperature can be 30 W/(m·K) or more. If the manufacturing conditions are further optimized, the thermal conductivity at room temperature can be 35 W/(m·K) or more.

[2. mold]

The mold according to the present invention has the following configuration:

(1) The mold includes a mold steel according to the present invention,

(2) the mold,

The amount of (Mo, W) carbides having a diameter of 0.2 μm or less is 1.2 mass% or more,

The ratio (mass ratio) of the amount of (Mo, W) carbides to the amount of Cr carbides is 11 or more,

Hardness change is less than 15 HRC.

The mold,

Initial hardness greater than 52 HRC and/or

It is preferable that the thermal conductivity at room temperature is 30 W/(m·K) or more.

[2.1. Steel for mold]

The mold according to the present invention is made of the mold steel according to the present invention. Since the details of the steel for a mold are as described above, the description thereof is omitted.

[2.2. characteristic]

The mold according to the present invention is obtained by quenching and tempering the mold steel according to the present invention under predetermined conditions. Therefore, in the state after quenching and tempering, the mold,

(a) the content of (Mo, W) carbides having a diameter of 0.2 μm or less is 1.2% by mass or more;

(b) the ratio (mass ratio) of the amount of (Mo, W) carbides to the amount of Cr carbides is 11 or more,

(c) Hardness change is 15 HRC or less.

In addition, the mold

(d) an initial hardness of at least 52 HRC and/or

(e) It is preferable that the thermal conductivity in room temperature is 30 W/(m·K) or more.

Since the details of the characteristics of the mold are as described above, the description thereof is omitted.

[2.3. Usage]

The mold according to the present invention is particularly suitable as a mold for performing hot working. Applications of the mold according to the present invention include, for example, a mold for die casting, a mold for hot stamping, a mold for tailored die quenching, and the like.

[3. Mold manufacturing method]

The mold according to the present invention can be manufactured in a variety of ways.

For example, the mold according to the present invention,

(a) forming an ingot by melting and casting a raw material blended to have a predetermined component;

(b) performing a soaking treatment for dissolving crystallized carbides;

(c) hot forging the ingot;

(d) performing a heat treatment for softening (eg, spheroidizing annealing) on the hot forged product;

(e) cutting and crudely processing the softened steel,

(f) quenching and tempering the crudely processed product under predetermined conditions;

(g) Performing finishing on the heat treated product.

can be manufactured through

The method and conditions of each process are not particularly limited, and the optimal method and conditions can be selected according to the purpose.

Alternatively, the mold according to the present invention,

(a) using an atomization method, to produce a powder made of steel for a mold according to the present invention,

(b) laminated molding of the obtained powder;

(c) tempering the shaped product;

can be manufactured through

Further, if necessary, finishing is performed after tempering.

In the case of manufacturing a mold using the additive manufacturing method, by optimizing the conditions for additive manufacturing, quenching can be performed simultaneously with additive manufacturing.

Other matters regarding the method and conditions of each process are not particularly limited, and the optimal method and conditions can be selected according to the purpose.

[4. Action]

Since the amount of additive components is optimized in the steel for a mold according to the present invention, when an appropriate heat treatment (soaking treatment, quenching treatment, and tempering treatment) is performed, the amount of Cr carbide and the amount of (Mo, W) carbide are within an appropriate range. can be controlled in As a result, a mold having a change in hardness of 15 HRC or less and excellent wear resistance can be obtained. In addition, if the components and heat treatment conditions are optimized, the initial hardness can be 52 HRC or more, and / or the thermal conductivity can be 30 W / (m K) or more, so that even if hot working is performed, the increase in the surface temperature of the mold is suppressed. As a result, softening of the mold is further suppressed.

In addition, since the amount of Cr is relatively small, even when the mold according to the present invention is applied to hot stamping or the like, generation of high-hardness Cr oxide can be suppressed. As a result, wear of the mold due to the high-hardness Cr oxide and seizure of the oxide film on the surface of the workpiece to the mold surface can be suppressed. In addition, since the thermal conductivity of the mold is high, the effect of shortening the cycle time is obtained.

Further, when other component elements such as Al, Co, Cu, and Ni are added, softening resistance, high-temperature strength, high-temperature hardenability and machinability can be further improved.

For example, when the initial hardness of a mold for hot stamping is low, the mold is worn out and damaged during hot stamping. Also, since the temperature of the mold rises during molding, when the softening resistance is low, the hardness is low and wear is promoted.

In particular, in the case of a mold for tailored die quenching, the mold is partially heated and the cooling rate of the portion in contact with the heating region is reduced to a cooling rate that does not cause martensitic transformation, thereby preventing quenching of this portion. Therefore, since a part of the mold is exposed to high temperatures for a long time, the mold is required to have high softening resistance. Also, when the thermal conductivity of the mold is low, the surface temperature of the mold becomes high, which leads to promotion of softening and wear. When the thermal conductivity is low, the rate of removing heat from the heated steel sheet to the mold is slow, and it takes time to quench the steel sheet. In addition, after forming and taking out the steel sheet, the time required for cooling the mold becomes longer. If the next steel plate is formed while the mold is not sufficiently cooled, the steel plate may not be sufficiently cooled and quenching may be insufficient.

In contrast, the steel for a mold according to the present invention not only has high initial hardness and high softening resistance, but also has high thermal conductivity when manufacturing conditions are optimized. Therefore, when the steel for a mold according to the present invention is applied to, for example, a mold for hot stamping or a mold for tailored die quenching, the molding time and the time required for cooling the mold can be shortened, and the cycle time can be shortened.

Example

(Examples 1 to 26 and Comparative Examples 1 to 13)

[One. Preparation of sample]

[1.1. Examples 1 to 21 and Comparative Examples 1 to 13 (ingot material)]

Ingots were formed by melting each steel having the chemical composition shown in Tables 1 and 2. The obtained steel ingot (excluding Comparative Example 13) was subjected to a soaking treatment at 1,240° C. for 20 hours under a homogeneous heating condition. Next, the ingot was hot forged to prepare a bar material having a cross section of 55 mm × 55 mm square. Quenching and tempering were performed on this bar.

Quenching was performed by soaking at 1,030°C for 60 minutes and then cooling radially at a cooling rate of 20°C/min to 30°C/min.

Tempering was performed by repeating the treatment of soaking at 540°C to 600°C for 1 hour and then cooling in air twice. As the tempering temperature, the temperature with the highest hardness was selected for each sample.

[1.2. Examples 22 to 26 (laminated molding material)]

Each powder having the chemical composition shown in Table 1 and Table 2 was prepared. Lamination molding was performed using the obtained powder to produce a bar having a cross section of 55 mm × 55 mm square.

Next, tempering of the bar was performed. Tempering conditions were the same as those of the ingot material.

[2. Test Methods]

[2.1. amount of carbide]

A test piece for measuring the amount of carbide of 10 mm × 12 mm × 20 mm was obtained from the bar after quenching and tempering. The total amount (mass) of carbide contained in this test piece was measured by electrolytic extraction. In addition, the electrolytically extracted carbides were subjected to automatic analysis by a scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX) method, and the composition and quantity (number) of each carbide having a diameter of 0.2 μm or less was measured. Based on the results obtained, the ratio of the amount of (Mo, W) carbides and the amount of (Mo, W) carbides to the amount of Cr carbides was calculated.

The case where the amount of (Mo, W) carbide is 1.2% by mass or more and the ratio of the amount of (Mo, W) carbide to the amount of Cr carbide is 11 or more is evaluated as "A", and other cases are evaluated as "B" evaluated as

[2.2. initial hardness]

A test piece for initial hardness measurement was obtained from the residual material of the bar after quenching and tempering, and the cross section (cross section perpendicular to the axial direction of the bar) was plane-polished. The Rockwell hardness (C scale) was measured at room temperature using the polished surface as a test surface. Tempering was performed at various temperatures in the range of 540° C. to 600° C., and cases where the maximum hardness was 52 HRC or higher were evaluated as “A”, and cases other than that were evaluated as “B”.

[2.3. Resistance to softening (hardness change)]

The remnants of the bar after quenching and tempering were heated at 600°C for 130 hours. After cooling to room temperature, a test piece for hardness measurement was obtained from the residue, and the cross section (cross section perpendicular to the axial direction of the bar) was plane-polished. The Rockwell hardness (C scale) was measured at room temperature using the polished surface as a test surface. A case where the value obtained by subtracting the hardness after heat treatment from the initial hardness was 15 HRC or less was evaluated as "A", and a case other than that was evaluated as "B".

[2.4. thermal conductivity]

A test piece for thermal conductivity measurement of 10 mm (diameter) × 2 mm was obtained from the remnants of the bar after quenching and tempering. Thermal conductivity at room temperature was measured using a laser flash method. A case where the thermal conductivity at room temperature was 30 W/(m·K) or more was evaluated as "A", and a case other than that was evaluated as "B".

[2.5. Wear test]

A punch of 30 mm × 60 mm × 50 mm was prepared from the residue of the bar after quenching and tempering. As the tempering temperature, the temperature with the highest initial hardness was selected. Using this punch, hat bending was performed on an uncoated steel sheet heated to 920°C, and wear evaluation on the punch was performed.

1 shows a schematic diagram of hat-bending. First, on the die 12 including the protrusion 12a, the movable parts 12b and 12b disposed on the left and right sides of the protrusion 12a, and the springs 12c and 12c supporting the movable parts 12b and 12b. The steel plate 14 was placed on (Fig. 1 (A)). Next, the plate 16 disposed above the protrusion 12a and the punches 18 and 18 disposed on the left and right sides of the plate 16 were lowered to bend the steel plate 14 in four places (Fig. B)). Hat-bend was an accelerated test, and the clearance was -15%.

Figure 2 shows a photograph of the appearance of the surface of a punch worn due to hat-bending. Hat-bending was performed under the above conditions, and a case where wear as shown in Fig. 2 occurred within 90 shots was evaluated as "B", and a case other than that was evaluated as "A".

[2.6. baking test]

A punch was prepared in the same manner as in the abrasion test. Hat bending of an Al-plated steel sheet heated to 920 DEG C was performed using this punch. The clearance was -30%.

Fig. 3 shows a photograph of the appearance of the surface of a punch in which burns have occurred due to hat-bending. Hat-bending was performed under the above conditions, and a case where burnout as shown in Fig. 3 occurred within 90 shots was evaluated as "B", and other cases were evaluated as "A".

[3. result]

A result is shown in Table 3 - Table 6. From Tables 3 to 6, the following can be seen.

(1) Comparative Example 1 has low initial hardness, large change in hardness, and low wear resistance. This is considered to be because the amount of C is small and the amount of fine carbides is small.

(2) Comparative Example 2 has low initial hardness, large change in hardness, and low wear resistance. This is considered to be because the amount of C is excessive and the amount of coarse carbides is large.

(3) Comparative Example 3 had a large change in hardness, low thermal conductivity, low abrasion resistance, and low seizure resistance. This is considered to be because the thermal conductivity is low because the amount of Si is high, and the amount of fine (Mo, W) carbides is small because the amount of Cr is high.

(4) Comparative Example 4 has a large change in hardness, low thermal conductivity, low abrasion resistance, and low seizure resistance. This is considered to be because the amount of fine (Mo, W) carbides is small because the amount of C and Cr is high, and the amount of Mn is excessive.

(5) Comparative Example 5 has a large change in hardness, low thermal conductivity, low abrasion resistance, and low seizure resistance. This is considered to be because the amount of fine (Mo, W) carbides is small because the amount of Cr is high.

(6) Comparative Example 6 has a large change in hardness, low thermal conductivity, low abrasion resistance, and low seizure resistance. This is considered to be because the amount of Cr was more excessive compared to Comparative Example 5.

(7) Comparative Example 7 has a large change in hardness, low thermal conductivity, low abrasion resistance, and low seizure resistance. This is considered to be because the amount of Cr was more excessive compared to Comparative Example 5.

(8) Comparative Example 8 has low initial hardness, large change in hardness, and low wear resistance. This is considered to be because the amount of Mo+W/2 is small.

(9) Comparative Example 9 has low initial hardness, large hardness change, low thermal conductivity, low abrasion resistance, and low seizure resistance. This is considered to be because the amount of Mo+W/2 is excessive and the carbide is coarse.

(10) Comparative Example 10 has low initial hardness, large hardness change, and low wear resistance. This is considered to be because the amount of V is excessive, and the amount of solid solution C and the amount of fine (Mo, W) carbides are small.

(11) Comparative Example 11 had a large change in hardness and low abrasion resistance. This is considered to be due to the small amount of N.

(12) Comparative Example 12 had a large change in hardness and low abrasion resistance. This is considered to be because the amount of N is excessive.

(13) Comparative Example 13 has low initial hardness, large hardness change, and low wear resistance. This is considered to be because coarse crystallized carbides remain because the soaking treatment is not performed, and the amount of solid solution C and the amount of fine (Mo, W) carbides are small.

(14) All of Examples 1 to 26 had an initial hardness of 52 HRC or more, a hardness change of 15 HRC or less, and a thermal conductivity of 30 W/(m·K) or more at room temperature. In addition, they are also excellent in both abrasion resistance and seizure resistance. Therefore, when the alloys of the embodiments are applied to, for example, a mold for hot stamping, wear resistance can be improved.

As mentioned above, although the embodiment of the present invention has been described in detail, the present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the gist of the present invention.

This application is based on Japanese Patent Application No. 2019-218621 filed on December 3, 2019 and Japanese Patent Application No. 2020-172572 filed on October 13, 2020, the contents of which are incorporated herein by reference. included

The steel for a mold according to the present invention can be used as a material for a die casting mold, a hot stamping mold, a tailored die quenching mold, and the like.

Claims (10)

As a steel for molds,
0.28 mass% ≤ C ≤ 0.65 mass%;
0.01 mass% ≤ Si ≤ 0.30 mass%;
1.5 mass% ≤ Mn ≤ 3.0 mass%;
0.5 mass% ≤ Cr ≤ 1.4 mass%;
1.9 mass% ≤ Mo+W/2 ≤ 4.0 mass%;
0.2 mass% ≤ V ≤ 1.0 mass%;
0.01 ≤ N ≤ 0.10 mass %, and
Optionally,
Al ≤ 1.5% by mass;
Ti ≤ 0.5% by mass;
Nb ≤ 0.5% by mass;
Zr ≤ 0.5% by mass;
Ta ≤ 0.5% by mass;
Co ≤ 1.0% by mass;
Ni ≤ 1.0% by mass;
Cu ≤ 1.0% by mass;
S ≤ 0.15% by mass;
Ca ≤ 0.15% by mass;
Se ≤ 0.35% by mass;
Te ≤ 0.35% by mass;
Bi ≤ 0.50% by mass, and
Pb ≤ 0.50% by mass;
The balance consists of Fe and unavoidable impurities,
The steel for the mold is in the state after quenching and tempering,
The amount of (Mo, W) carbides having a diameter of 0.2 μm or less is 1.2 mass% or more,
The ratio (mass ratio) of the amount of (Mo, W) carbides to the amount of Cr carbides is 11 or more,
A steel for a mold having a change in hardness represented by the following formula (1) of 15 HRC or less.
Hardness change = |H _b - H _a | … … (One)
(However, H _a represents the Rockwell hardness (C scale) measured at room temperature after holding at 600 ° C. for 130 hours after the above quenching and tempering, and H _b is measured at room temperature immediately after the above quenching and tempering is the Rockwell hardness (C scale) According to claim 1,
0.005 mass% ≤ Al ≤ 1.5 mass%;
0.01 mass% ≤ Ti ≤ 0.5 mass%;
0.01 mass% ≤ Nb ≤ 0.5 mass%;
0.01 mass % ≤ Zr ≤ 0.5 mass %, and
0.01 mass% ≤ Ta ≤ 0.5 mass%
Steel for a mold further comprising at least one element selected from the group consisting of. According to claim 1,
0.01 mass% ≤ Co ≤ 1.0 mass%
Steel for a mold further comprising a. According to claim 1,
0.30 mass% ≤ Ni ≤ 1.0 mass% and
0.30 mass% ≤ Cu ≤ 1.0 mass%
Steel for a mold further comprising at least one element selected from the group consisting of. According to claim 1,
0.01 mass% ≤ S ≤ 0.15 mass%;
0.001 mass% ≤ Ca ≤ 0.15 mass%;
0.03 mass% ≤ Se ≤ 0.35 mass%;
0.01 mass% ≤ Te ≤ 0.35 mass%;
0.01 mass % ≤ Bi ≤ 0.50 mass % and
0.03 mass% ≤ Pb ≤ 0.50 mass%
Steel for a mold further comprising at least one element selected from the group consisting of. According to any one of claims 1 to 5,
Steel for molds having an initial hardness of 52 HRC or more in the state after quenching and tempering. According to any one of claims 1 to 5,
Steel for a mold having a thermal conductivity of 30 W/(m·K) or more at room temperature in the state after quenching and tempering. A mold comprising the steel for a mold according to any one of claims 1 to 5,
The mold,
The amount of (Mo, W) carbides having a diameter of 0.2 μm or less is 1.2 mass% or more,
The ratio (mass ratio) of the amount of (Mo, W) carbides to the amount of Cr carbides is 11 or more,
A mold having a hardness change of 15 HRC or less represented by the following formula (1).
Hardness change = |H _b - H _a | … … (One)
(However, H _a represents the Rockwell hardness (C scale) measured at room temperature after holding at 600 ° C. for 130 hours after the above quenching and tempering, and H _b is measured at room temperature immediately after the above quenching and tempering is the Rockwell hardness (C scale) According to claim 8,
Molds with initial hardness greater than 52 HRC. According to claim 8,
A mold having a thermal conductivity of 30 W/(m K) or more at room temperature.

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